an innovative electro-optical nose for artificial...

PhD Dissertation

International Doctorate School in Information andCommunication Technologies

DIT - University of Trento

An Innovative Electro-Optical Nose for

Artificial Olfaction Applications

Arianna Tibuzzi

Advisor:

Prof. Giovanni Soncini

Universita degli Studi di Trento

February 2005

Abstract

Electronic noses, array of chemical cross-responsive gas sensors able to de-

tect and classify complex compounds mixtures (”odors”), are being increas-

ingly employed in food and agriculture industry, industrial process control,

environmental monitoring and as non invasive diagnostic instruments in

medical applications. Nevertheless in order to achieve a mass diffusion in

the market, a cheaper and more portable system needs to be developed. In

this dissertation work I studied, fabricated and tested a prototype of an

electro-optical nose based on a matrix of silicon integrated photodiodes and

phototransistors, employed as optical sensor transducers, coated by metal-

loporphyrins as sensing layers able to change the peak amplitude and peak

wavelength in their adsorption spectrum on exposure to volatile organic

compounds (VOCs). Since the most significant variation occurs around

440nm, new silicon photodetectors with enhanced responsivity in the blue

spectral range have been designed. Due to the employment of integrated de-

vices and on its room temperature operation, such a system offers important

advantages with respect to the existing electronic noses: low cost, weight

and size; further integration of sensors and signal processing electronics on

the same chip; low power consumption.

Keywords

[electronic nose,silicon photodiode,BJT phototransistor,metalloporphyrin,gas

sensor,ethanol]

Contents

1 Introduction 1

1.1 The Context . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 The Human Olfactory System . . . . . . . . . . . . 2

1.1.2 Natural and Artificial Olfaction . . . . . . . . . . . 2

1.1.3 Electronic Nose Applications . . . . . . . . . . . . . 5

1.2 The Problem and the Proposed Solution . . . . . . . . . . 7

1.2.1 Working in the Soret Band . . . . . . . . . . . . . . 8

1.3 Innovative Aspects . . . . . . . . . . . . . . . . . . . . . . 8

1.4 Structure of the Thesis . . . . . . . . . . . . . . . . . . . . 9

2 State of the Art 11

2.1 Electronic Nose Sensors Technology . . . . . . . . . . . . . 11

2.2 MO Metal Oxide . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.1 KAMINA: Chemical Gas Detector Sensor (SPECS

Inc.) . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3 CP Conductive and non-Conductive Polymers . . . . . . . 13

2.4 SAW and BAW Surface and Bulk Acoustic Wave . . . . . 14

2.5 FET Field Effect Transistor . . . . . . . . . . . . . . . . . 14

2.6 QMB Quartz Micro Balance . . . . . . . . . . . . . . . . . 15

2.6.1 LIBRA NOSE (TechnoBioChip) . . . . . . . . . . . 15

2.7 FO Fiber Optic . . . . . . . . . . . . . . . . . . . . . . . . 17

2.8 Optical Technologies . . . . . . . . . . . . . . . . . . . . . 18

i

2.8.1 SMELLSEEING: A Colorimetric Electronic Nose (Chem-

Sensing Inc.) . . . . . . . . . . . . . . . . . . . . . 18

2.8.2 Optical NoseTM and BeadArrayTM (Illumina Inc.) 19

3 An Electro-Optical Nose E-ON 23

3.1 Metalloporphyrins . . . . . . . . . . . . . . . . . . . . . . . 23

3.1.1 Optical Properties . . . . . . . . . . . . . . . . . . 25

4 Silicon Integrated Photodetector Transducers 33

4.1 Metalloporphyrins Deposition Methods . . . . . . . . . . . 33

4.2 Conventional Photodiodes . . . . . . . . . . . . . . . . . . 35

4.3 Finger Photodiodes . . . . . . . . . . . . . . . . . . . . . . 39

4.3.1 Modelling . . . . . . . . . . . . . . . . . . . . . . . 40

4.3.2 Design and Fabrication . . . . . . . . . . . . . . . . 47

4.3.3 Electrical Characterization . . . . . . . . . . . . . . 53

4.3.4 Optical Characterization . . . . . . . . . . . . . . . 56

4.4 Finger BJT Phototransistors . . . . . . . . . . . . . . . . . 61

4.4.1 Electrical Characterization . . . . . . . . . . . . . . 62

4.4.2 Modelling . . . . . . . . . . . . . . . . . . . . . . . 66

4.4.3 Optical Characterization . . . . . . . . . . . . . . . 69

5 The E-O Nose System 81

5.1 Package: the Nose Nostril . . . . . . . . . . . . . . . . . . 82

5.2 The Nose Box . . . . . . . . . . . . . . . . . . . . . . . . . 83

5.2.1 Read-Out Circuit . . . . . . . . . . . . . . . . . . . 85

6 Sensors Experimental Testing 89

6.1 First campaign of Measurements . . . . . . . . . . . . . . . 92

6.2 Second campaign of Measurements . . . . . . . . . . . . . 96

6.2.1 Finger photodiodes . . . . . . . . . . . . . . . . . . 96

ii

6.3 Third campaign of Measurements . . . . . . . . . . . . . . 102

6.3.1 Finger phototransistors . . . . . . . . . . . . . . . . 102

6.4 Parasitic Porphyrin Resistance . . . . . . . . . . . . . . . 109

7 Conclusion 119

Bibliography 125

A Photodetectors Layout 129

B Nostril Packaging & Bonding 131

B.1 Sensors Board . . . . . . . . . . . . . . . . . . . . . . . . . 131

B.2 Sensors package . . . . . . . . . . . . . . . . . . . . . . . . 134

iii

List of Tables

1.1 Mimicking the Human Olfactory System . . . . . . . . . . 4

1.2 EN application fields . . . . . . . . . . . . . . . . . . . . . 6

4.1 Wavelength indexes for the spectral responsivity peaks. . . . 51

4.2 Measured Dark Current and Breakdown Voltage for all the

fabricated photodiodes. . . . . . . . . . . . . . . . . . . . . 55

4.3 Maximum measured responsivity values for all photodiodes

in the blue spectral range at 5V reverse bias. . . . . . . . . 57

4.4 Measured responsivity values for N+PD40 and N+PDstand

for increasing reverse bias (ionization multiplication). . . . 61

4.5 Maximum β current gain and correspondent base current for

all phototransistors. . . . . . . . . . . . . . . . . . . . . . . 63

6.1 Conversion from percentages of Ethanol vapor in the flow to

ppm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

v

List of Figures

1.1 Schematic image of the human olfactory system. . . . . . . 3

1.2 The multidimensional sensor space of the EN output. . . . 5

1.3 Plot of the first 2 principal components providing a good

clustering and separation of different olive oils. . . . . . . . 6

2.1 Image of a quartz crystal with gold electrodes. . . . . . . . 16

2.2 Equivalent circuit of a QMB. . . . . . . . . . . . . . . . . 16

2.3 Image of Tor Vergata Libra Nose, commercially available

from Technobiochip. . . . . . . . . . . . . . . . . . . . . . . 17

2.4 Zoom of the sensors nostril, showing the 8 QMB inserted in

8 oscillating circuits. . . . . . . . . . . . . . . . . . . . . . 17

2.5 Photograph of the latest Tor Vergata nose upgrade: En-Qube. 17

2.6 Schematics of the Colpitts sinusoidal oscillator employed to

extract the quartz frequency variation. . . . . . . . . . . . . 17

2.7 Comparison of Zn(TPP) spectral shifts upon exposure to

ethanol and pyridine (py). a) In methylene chloride so-

lution; b) on the reverse phase support. In both a) and

b), the bands correspond, from left to right, to Zn(TPP),

Zn(TPP)(C2H5OH) and Zn(TPP)(py), respectively. . . . . 19

vii

2.8 Color change profiles of a metalloporphyrin sensor array. A)

Color change profiles of the metalloporphyrin sensor array

as a function of exposure time to nbutylamine vapor; B)

Color change profiles for a series of vapors: the degree of

ligand softness increases from left to right, top to bottom. . 20

2.9 Left: optical fiber bundle; Middle: microscopic etched well

at the end of each individual fiber; Right: one bead is drawn

in each well. . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.1 Block diagram of the electro-optical nose system with the two

types of sensors employed: (a) photodiodes, and (b) BJT

phototransistors. . . . . . . . . . . . . . . . . . . . . . . . 24

3.2 Basic molecular structure of a porphyrin. The central metal

can host many of the metal of the periodic table, while at the

R, R’ position lateral groups can be linked. . . . . . . . . . 25

3.3 Characteristic absorption spectrum of metalloporphyrins. . 26

3.4 Picture of the experimental set-up used for spectroscopic mea-

surements of metalloporphyrins absorption spectrum varia-

tions with VOCs. . . . . . . . . . . . . . . . . . . . . . . . 27

3.5 Transmission spectra of Zn-T(heptyloxy)PP (a) with no TEA

and (b) with no EtOH and both after 2’,4’,12’ from injection.

The spectrum variations are reversible. . . . . . . . . . . . 28

3.6 Transmission spectra of Mn-T(hexadodecyloxy)PP with no

EtOH and after 2’,4’,12’ from injection. The spectrum vari-

ation is reversible. . . . . . . . . . . . . . . . . . . . . . . . 29

3.7 Molecular structure of a T(heptyloxy)PP, a porphyrin with

added alkyl chains. . . . . . . . . . . . . . . . . . . . . . . 30

viii

3.8 Sensor response time versus the length of the alkyl chain of

the sensitive molecule. The response time is defined as the

time necessary to reach a steady sensor signal after analyte

introduction into the sensor chamber. . . . . . . . . . . . . 31

3.9 UV-VIS spectra of Co-TPP (-), Co-TPP-7 (.-.), CoTPP-12

(...), CoTPP-18 (- -). . . . . . . . . . . . . . . . . . . . . 31

4.1 One of the conventional photodiodes employed in the first

matrix after deposition of Mn-TPP-Cl by air brush. . . . . 34

4.2 One of the conventional photodiodes employed in the first

matrix after deposition of Zn-T(heptyloxy)PP by evaporation. 35

4.3 Image of part of the silicon photodiodes matrix. . . . . . . 36

4.4 Responsivity spectra of several types of fabricated photodi-

odes. The responsivity of the one employed in the E-ON

project is marked with a star. . . . . . . . . . . . . . . . . 36

4.5 Absorbed optical power by silicon for 440nm light vs distance

from Si incident surface according to the Lambert-Beer Law:

P0 is the incident optical power, α is the absorption coeffi-

cient, R1 and R2 are the reflectivities without and with the

SiO2 ARC (Anti Reflective Coating) on top. . . . . . . . . 38

4.6 First packaged matrix of silicon photodiodes . . . . . . . . 38

4.7 Drawing of the package metal line and pads and the sensor

matrix. The capital letters show the locations of the spray-

coated photodiodes. . . . . . . . . . . . . . . . . . . . . . . 38

4.8 Cross-section of the two types of finger-junction photodiodes

designed: a)P+PDxx, p+-finger anode in the n-substrate,

and b) N+PDxx, n+-finger cathode in a p-well implanted in

the n-substrate. . . . . . . . . . . . . . . . . . . . . . . . . 39

ix

4.9 2D plot, produced by SILVACO, of the distribution of elec-

trons concentration in a finger photodiode with 10µm inter-

digit distance. . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.10 2D plot, produced by SILVACO, of the distribution of holes

concentration in a finger photodiode with 10µm interdigit

distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

4.11 Electric field, electrons and holes concentrations plots result-

ing from 2 X-cuts at (a) 10nm and (b) 100nm (distance from

Si surface) of the 2D structure in Fig. 4.10. . . . . . . . . 43


ing from 2 X-cuts at (a) 200nm and (b) 300nm (distance

from Si surface) of the 2D structure in Fig. 4.10. . . . . . 44


ing from an Y-cut between the second and third finger of the

2D structure in Fig. 4.10 . . . . . . . . . . . . . . . . . . . 45

4.14 2D plots of electron density in a finger (left side) and stan-

dard photodiode (right side) with no bias applied (results

obtained with ISE-TCAD after running DESSIS and TEC-

PLOT for viewing): the blue color represents a high holes

concentration (p-well), green represents the junction area

and holes depletion region. . . . . . . . . . . . . . . . . . . 46

4.15 Simulated responsivity spectra of standard and finger pho-

todiodes with different interdigit distance (results obtained

with ISE-TCAD after running DESSIS and OPTIK for op-

tical generation). Only the latters show high peaks in the

blue region, around 400nm, with a maximum value for the

maximum interdigit distance, d=40µm. . . . . . . . . . . . 47

x

4.16 Simulation results of the dark current dependence on (a) S0,

surface recombination velocity, for finger photodiodes and

(b) τg, carriers lifetime for conventional photodiodes. . . . 48

4.17 Image of the layout of PD20, finger photodiode with d=20µm

interfinger distance. . . . . . . . . . . . . . . . . . . . . . . 50

4.18 Micrographs of two finger photodiodes after Zn-T(heptyloxy)PP

evaporation. . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4.19 Micrograph of the standard photodiode after Zn-T(heptyloxy)PP

evaporation. . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4.20 Schematic cross section of the finger photodiode (cut in the

middle of the area in Fig. 4.17) at the end of the fabrication

process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.21 Measured IV curves for the two types of photodiodes, (a)

N+PDxx, and (b) P+PDxx and comparison with the stan-

dard fully implanted photodiode. . . . . . . . . . . . . . . . 54

4.22 Comparison between the IV curves of the two types of pho-

todiodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

4.23 Breakdown voltage versus impurity concentration for one-

sided abrupt doping profile with cylindrical and spherical

junction geometries, where rj is the radius of curvature. . . 56

4.24 Finger photodiodes spectral responsivity at 5V reverse bias

for increasing interdigit distance and standard photodiode:

a) for n+-finger cathode in a p-well photodiode, N+PDxx;

b) for p+-finger anode in the n-substrate photodiode, P+PDxx. 58

4.25 PD40 spectral responsivity for increasing reverse bias volt-

age: a) for n+-finger cathode in a p-well photodiode, N+PD40

(Vbreakdown=22.5V); b) for p+-finger anode in the n-substrate

photodiode, P+PD40 (Vbreakdown=53V). . . . . . . . . . . . 59

xi

4.26 Spectral responsivity for increasing reverse bias voltage for

the standard photodiode N+PDstand, n+ fully implanted in

the p-well. . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

4.27 Cross-section of the npn BJT with the finger-shaped Emit-

ter/Base junction. . . . . . . . . . . . . . . . . . . . . . . 62

4.28 Experimental Gummel plots (Ib and Ic vs Vb) for 3 different

BJTs: BJT0 standard, BJT10 and BJT40. . . . . . . . . . 63

4.29 BJT current gain Beta vs collector current Ic for increasing

interfinger distance and the standard BJT. . . . . . . . . . 64

4.30 Dependence of the fingers perimeter on the fingers number. 65

4.31 Dependence of the base current on the fingers perimeter. . 66

4.32 Dependence of the current gain on the fingers perimeter. . 66

4.33 2D plots, generated by ISE-TCAD Tools with the software

Tecplot, representing the holes current density in the vicinity

of the E/B junction for the BJT0 (top left), BJT10 (top

right) and BJT40 (bottom left). . . . . . . . . . . . . . . . 67

4.34 Holes density along X resulting from an Y-cut at 1µm dis-

tance from Si surface (just below the finger implants) of the

three structures in Fig 4.33. . . . . . . . . . . . . . . . . . 68

4.35 SRH recombination distribution in the three structures of

Fig 4.33: the prevailing yellow color in BJT40 around the

E/B junction stands for a smaller recombination and con-

sequently a smaller Ir component of total Ib. . . . . . . . . 69

4.36 Finger phototransistors spectral responsivity at Vcc=5V for

increasing interdigit distance and standard BJT, with small

light beam (only E/B junction contribution). . . . . . . . . 70

xii

4.37 Comparison between spectral responsivities of finger photo-

diodes and phototransistors with respectively Vrev=5V and

Vcc=5V for the standard devices and the finger photodetec-

tors with d=10µm and d=40µm. . . . . . . . . . . . . . . . 71

4.38 Comparison between spectral responsivities of finger photo-

diodes and phototransistors with respectively Vrev=5V and

Vcc=5V for: (a) PD10 and PT10; (b) PD20 and PT20; (c)

PD30 and PT30; (d) PD40 and PT40. . . . . . . . . . . . 73

4.39 Phototransistors spectral responsivity at Vcc=5V and 15V

for standard BJT and (a) PT10 with small light beam (only

E/B junction contribution); (b) PT10 and PT40 with large

light beam (also B/C lateral junction contribution). . . . . 75

4.40 Spatial responsivity of BJT10 at different wavelengths when

scanned horizontally by a 200µm beam with a 23µm step. . 78

4.41 Zoom of the curves at 350 and 400nm in Fig. 4.40. . . . . 78

5.1 Picture of the assembled sensors matrix after die and wire

bonding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

5.2 Image of the package used as nose nostril to lodge the sen-

sor matrix: (a) first prototype; (b) second prototype, for a

differential measurements configuration. . . . . . . . . . . . 82

5.3 Two images of the first chamber where the sensor matrix was

placed, provided with holes for electrical connections access

and gas flow. . . . . . . . . . . . . . . . . . . . . . . . . . 83

xiii

5.4 Picture of the top of the open nose box. The circuit board

is on top left with the three operational amplifiers and their

respective RC feedback networks; the batteries container (for

op amps power supply) is on bottom left, close to the LED

intensity selector, which in turn is near the 3 outputs BNCs

(vertical); the castle with the sensors and LED board in on

the right (only the backside of the LED board is visible) with

the 2 plastic tubes for gas flow coming in and out; on top of

the picture, on the left the 3 connectors for voltage biasing of

the 3 sensors rows are visible, together with the 3 selectors

(vertical) of the sensor in each row. . . . . . . . . . . . . . 84

5.5 Schematic image of the castle: left, explosion of the three

boards: the one mechanically fixed to the metal box, from

which all electrical connections to the sensors matrix and

the LED source start; middle, removable part of the castle,

made up by the sensors board and the LED board (Fig. 5.6);

right, complete close castle. . . . . . . . . . . . . . . . . . . 85

5.6 Picture of the top part of the castle: the LED board is at-

tached to the sensors board through a series of connectors

and screws, easily removable. The two pieces of hose for gas

flow are visible on the lateral sides of the sensors package,

going in and out. . . . . . . . . . . . . . . . . . . . . . . . 86

5.7 Schematic of the circuit configuration for output signal ex-

traction of (a) photodiodes and (b) phototransistors sensors. 87

5.8 Electrical scheme of a single signal extraction channel. The

4 BJTs are placed in the matrix package (Fig. 5.2(b)) on the

sensors board (Fig. 5.1), while the I-V converter is mounted

on a separated circuit board (Fig. 5.4). . . . . . . . . . . . 87

xiv

6.1 Picture of the experimental set up: first version of the E-

ON metal box, coupled to a manual monochromator, which

in turn is coupled to a white halogen lamp. The gas bench

is completely visible, with the 4-channels flow meter, the

ethanol bubbler and the hoses. . . . . . . . . . . . . . . . . 90

6.2 Output voltage versus time of 7 measurement cycles at de-

creasing EtOH concentrations with a standard photodiode-

based sensor, spray-coated with Co-T(hexadodecyloxy)PP. . 93

6.3 Output voltage versus time of 7 measurement cycles at de-

creasing EtOH concentrations with a standard photodiode-

based sensor, spray-coated with Zn-T(butyloxy)PP. . . . . . 93

6.4 Four voltage output variations for the same sensor at 33%

EtOH concentration for four different emission wavelengths

around the metalloporphyrin transmission peak. . . . . . . 94

6.5 Output voltage versus time of 2 measurement cycles at 20%

and 5% EtOH concentrations, by employing a photodiode de-

tector spray-coated by Zn-T(heptyloxy)PP and a light source

with 426nm emission wavelength. . . . . . . . . . . . . . . 95

6.6 Output voltage versus time of 4 measurement cycles at 2.5%,

5%, 10% and 20% EtOH concentrations by employing a pho-

todiode detector evaporated by Zn-T(heptyloxy)PP and a light

source with 440nm emission wavelength. . . . . . . . . . . 95

6.7 Output voltage versus time of 4 measurement cycles at 10%,

5%, 2%, 1% and 0.5% EtOH concentrations by employing

the finger-photodiode sensor evaporated by Zn-T(heptyloxy)PP.

The light source set-up employed is the white lamp+monochromator. 96

6.8 N+PD10 sensor response to increasing ethanol concentra-

tion for 3 different reverse bias conditions. . . . . . . . . . 97

xv

6.9 4 measurement cycles for N+PD10 sensor at 50%, 40%,

30% and 20% ethanol concentration (a) before and (b) after

applying the differential drift cancellation. . . . . . . . . . 99

6.10 4 measurement cycles for N+PD10 sensor at 10%, 5%, 2%

and 1% ethanol concentration, after applying the differential

drift cancellation. . . . . . . . . . . . . . . . . . . . . . . . 100

6.11 Zoom of the voltage output increase on exposure to 20%

EtOH concentration (Fig. 6.9). CoTPP rise is faster than

ZnTPP rise. . . . . . . . . . . . . . . . . . . . . . . . . . . 101

6.12 6 measurement cycles for BJT10 sensor at 20%, 10%, 5%,

2%, 1% and 0.5% ethanol concentration (a) before and (b)

after applying the differential drift cancellation. . . . . . . 103

6.13 6 measurement cycles for BJT10 sensor at 20%, 10%, 5%,

2%, 1% and 0.5% ethanol concentration, after 8 months

from the first experiments (Fig. 6.12(b)). . . . . . . . . . . 104

6.14 Repeatability test: 3 measurement cycles are repeated for the

same EtOH concentration, at 10%, 5%, 2%, 1% and 0.5%. 105

6.15 Response curve for photodiode and phototransistor sensor:

the latter exhibits higher response and higher sensitivity at

low concentrations (the line is steeper). . . . . . . . . . . . 106

6.16 4 measurement cycles for all types of phototransistor sen-

sors at 10%, 5%, 2% and 1% ethanol concentration, con-

ducted with the latest nose box set-up. In these plotted results

BJT10 (a) and BJT20 (b) were coated with Zn-T(heptyloxy)PP;

BJT30 (c) and BJT40 (d) were coated with Co-T(hexadodecyloxy)PP.107

6.17 Response curve for sensor N+PD10 and all phototransis-

tor sensors, BJT10, BJT20, BJT30 AND BJT40, with Zn-

T(heptyloxy)PP coating. . . . . . . . . . . . . . . . . . . . 108

xvi

6.18 Response curve for the phototransistor sensors BJT30 AND

BJT40, with Co-T(hexadodecyloxy)PP coating. . . . . . . . 109

6.19 (a) Layout of one of the finger photodetector; (b) schematic

cross view of the critical electrical path that gives place to

the parasitic parallel resistor. . . . . . . . . . . . . . . . . . 111

6.20 Measurement cycles at a wide range of ethanol concentra-

tions, from 50% till 0.5%, for different photodiode reverse

bias voltage: (a) 5V, (b) 10V. . . . . . . . . . . . . . . . . 113

6.21 Different zooms of the plot in Fig. 6.20(a): (a) the ”spike” is

in fact a change in the response variation (from decreasing

to increasing output voltage); (b) low EtOH concentrations

cycles: from 20% recovery phase, the optical sensing mech-

anism becomes dominant and the parasitic resistor disappears.114

6.22 Measurement cycles at the same EtOH concentrations em-

ployed in Fig. 6.20(a) and Fig. 6.20(b) with the LED off in

order to test only the conductivity increase of metallopor-

phyrin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

6.23 Parasitic response to 40% EtOH concentration of the sensor

BJT40 coated with (a) Co-T(hexadodecyloxy)PP and (b) Zn-

T(heptyloxy)PP, at 20C. . . . . . . . . . . . . . . . . . . . 116

6.24 Parasitic response to 40% EtOH concentration of the sensor

BJT40 coated with (a) Co-T(hexadodecyloxy)PP and (b) Zn-

T(heptyloxy)PP, at 10C. . . . . . . . . . . . . . . . . . . . 117

A.1 Images of the layout of four photodiodes. PD20 layout is

shown in Fig. 4.17. . . . . . . . . . . . . . . . . . . . . . . 130

B.1 Schematic top view of the photodiodes sensors matrix board

with the wall isolating the bottom row for differential mea-

surements. . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

xvii

B.2 Schematic top (left) and backside (right) view of the photo-

transistors sensors matrix board. . . . . . . . . . . . . . . . 133

B.3 Schematic design of the matrix package: (a) first prototype;

(b) second prototype, for a differential measurements config-

uration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

B.4 Measures of the matrix package: (a) first prototype; (b) sec-

ond prototype, for a differential measurements configuration. 134

xviii

Chapter 1

Introduction

1.1 The Context

Human sense can be divided in two main categories: physical senses (tac-

tile, sight, hear) and chemical senses (smell and taste). The latter operate

at unconscious level and the perceptions are not fully expressed. For ages,

the human nose has been an important tool in assessing the quality of

many products. In 1982 the possibility to mimic the human olfaction, by

the development of an ”objective” means for using ”subjective” informa-

tion confined in the smell, became real: Persaud and Dodd introduced the

concept of the Electronic Nose [1]. Since then, several efforts have been car-

ried on to find out new technologies and suitable data analysis instruments

to face this big interdisciplinary scientific challenge. The development and

build-up of an electronic nose bring together several skills belonging to

different scientific fields: electronics, physics, chemistry, biology, computer

science, mathematics and even medicine and telecommunications in some

cases. This derives from being a complex system that integrates microelec-

tronic devices with chemical sensing layers and signal processing electronics

with pattern recognition algorithms and data analysis techniques, with a

big variety of applications.

The contribution given by this dissertation work belongs mainly to the

1

CHAPTER 1. INTRODUCTION

electronic research area, more specifically dealing with the design, fabrica-

tion and testing of a new class of chemical gas sensors to be assembled in

a matrix to build up the integrated nostril of an innovative electro-optical

nose.

1.1.1 The Human Olfactory System

In order to develop an electronic nose, it is useful to examine the phys-

iology behind olfaction since biological olfactory systems contain many

of the desired properties for electronic noses. The mammalian olfactory

system uses a variety of chemical sensors, known as olfactory receptors,

combined with an automated pattern recognition system incorporated in

the olfactory bulb and higher portions of the brain. Fig. 1.1 illustrates

the major components and function of the mammalian olfactory system.

When we inhale, odors reach the nasal chamber. The odorant molecules

interact with the olfactory receptors, distributed in the epithelium; there

are approximately ten million receptor cells in the human nose and each

receptor is sensitive to a great number of compounds. They have to pro-

vide a bio-chemical transduction and amplification, they are not specific

and they are redundant. After the chemical stimulation, they produce an

electrical stimulus (the response time is in the order of seconds), that is

transmitted to the neurons of the olfactory bulb. The neurons in the bulb

form a network able to perform a first step processing of the information.

A second network of neurons located in the olfactory cortex is responsible

for the final processing which makes us experience consciously the odor

perception [2] [3] [4] [5].

1.1.2 Natural and Artificial Olfaction

The fundamental olfaction components are:

2

1.1. THE CONTEXT

Figure 1.1: Schematic image of the human olfactory system.

• Sampling and Delivering system

• Measurement chamber

• Sensors

• Signal processing

• Pattern recognition

The comparative Table 1.1 summarizes the main features of the natural

and artificial olfactory systems [6].

The architecture of an Electronic Nose (EN) mimics the biological ol-

factory components. It consists of an array of chemical cross-responsive

sensing elements, packaged to make up the measurement chamber, an

electronic interface providing signal conditioning and pre-processing and

a pattern recognition system able to create a database of signatures of the

different odorants. Each odorant or volatile compound presented to the

sensor array produces a characteristic pattern. By exposing many distinct

odorants to the sensor array, the database is built up and then used to

3


Table 1.1: Mimicking the Human Olfactory System

NATURAL OLFACTION ARTIFICIAL OLFACTION

Receptors

• Non selective

• Ultra High Redundancy (108)

• Biochemical Transduction

• Signal: pattern of spikes

Sample Delivery

• Actuation of sniffing

• Two sources of odor (outside and

mouth)

Signal Processing

• Data synthesis

Data Analysis

• Ultra Wide Database

• Drift compensation

• High integration with other senses

Sensors

• Non selective

• Low Redundancy (10)

• Chemical Transduction

• Signal: steady signal

Sample Delivery

• Continuous sniffing

• One source of odor (outside)

Signal Processing

• One sensor-one signal

Data Analysis

• Limited Database

• Poor Drift compensation

• Integrability with other instruments

train the pattern recognition system. The goal of this training process

is to configure the recognition system to produce unique classifications or

clusterings of each odorant so that an automated identification can be im-

plemented.

4

1.1. THE CONTEXT

Data Analysis

A multisensor system output belongs to a multidimensional space (Fig. 1.2)

and a great care must be taken with Electronic Nose data [5]. In the sensor

space a single measure is a n-dimensional vector and only the employment

of suitable data analysis techniques (Principal or Independent Components

Analysis PCA-ICA, Artificial Neural Networks ANN) allow to visualize this

space as a 2D plot, where odors are mapped and represented in clusters

according to their similarities and differences (e.g. in Fig. 1.3).

Figure 1.2: The multidimensional sensor space of the EN output.

1.1.3 Electronic Nose Applications

The EN can be employed in every environment where the atmosphere com-

position or a characteristic odor gives relevant information. Table 1.2 re-

ports the main application areas, among which the medical ones represent

an important field in which recently electronic noses have been employed

as non invasive fast instruments for disease diagnostics [7].

5


Figure 1.3: Plot of the first 2 principal components providing a good clustering and sepa-

ration of different olive oils.

Table 1.2: EN application fields

INDUSTRIAL FOOD &

AGRICULTURE

MEDICAL

• Leather industry

• Olfactory Impact

• Packaging

• Automotive industry

• Space aircraft

• Perfumes

• Tobacco

• Coffee

• Wines

• Drinks

• Vegetables quality

after harvest

• Food quality

• Fish products

• Flavor enhancement

• Diabetes

• Hepatic diseases

• Bacterial infections

Skin

Breath channels

Urine

• Lung cancer

• Schizophrenia

6

1.2. THE PROBLEM AND THE PROPOSED SOLUTION

1.2 The Problem and the Proposed Solution

The several existing electronic noses are mostly based on the technologies

described in Chapter 2 and some of them are bulky, some are power con-

suming, some are operated at high temperature, some are too sensitive

to environmental parameters and become unstable in long time periods.

The motivation and goals of the EN prototype presented here match the

following needs:

• low cost, weight and size

• low power consumption

• possibility of integration of transducers and signal processing electron-

ics on the same chip

• room temperature operation

The main issue is the realization of a cost effective portable ”sniffing” de-

vice.

Moreover, lately, some experiments of odors transmission between two re-

mote places have been conducted at the University of Rome Tor Vergata

and in a future scenario the availability of a portable nose would allow to

register and transmit odors in any kind of situation and environment [8].

The solution proposed in this dissertation consists of building up a very

small nose nostril made up of a matrix of integrated transducers photode-

tectors coated with a thin layer of several different metalloporphyrins, in

order to convert the change in their absorption spectrum, related to VOCs

concentration in air, into a photogenerated current and a final output volt-

age.

7


1.2.1 Working in the Soret Band

On exposure to VOCs (Volatile Organic Compounds) metalloporphyrins

modify their absorption spectrum by shifting their peak towards higher/lower

wavelengths and by changing the peak amplitude value. The most signif-

icant peak variation occurs in the most intensive absorption band, the so

called Soret Band, around 440nm, therefore the sensors to be employed

have to be operated in the blue spectral region to maximize the response

to gases.

The main problem related to working with silicon photodetectors in this

spectral region is the short penetration of blue photons into the silicon sub-

strate, and consequently low responsivity and low conversion efficiency. In

order to optimize the matching between metalloporphyrin optical response

and transducer photocurrent response, novel silicon integrated photodiodes

and phototransistors have been investigated and successfully developed.

Moreover, during the thesis work both the synthesis and the deposition of

metalloporphyrins have been optimized in order to achieve a more porous

layer with a highly resolved absorption peak and a uniform deposited sens-

ing layer with a controlled and reproducible thickness. These improvements

have significantly contributed to the optimization of the sensor response.

1.3 Innovative Aspects

In the EN technology the attention and interest towards optical systems

and optical working mechanisms has grown only recently, especially after

the work done by Suslick on the ”Smell-Seeing” system (Chapter2) [9].

The approach reported in this work is based on the same sensing layer,

metalloporphyrin, and on same optical properties adopted and considered

by Suslick, taking advantage of the significant peak changes induced in

metalloporphyrins absorption spectrum by VOCs ligand binding. Differ-

8

1.4. STRUCTURE OF THE THESIS

ently, while Suslick developed an easy colorimetric technique thus minimiz-

ing the need for extensive signal transduction hardware by using a simple

commercial scanner, the EN proposed here employs metalloporphyrins as

optical filters able to modulate incident blue light according to the VOC

concentration in air, mostly due to the peak amplitude variations rather

than peak wavelength shifts (color change). The transduction is built-in

within the sensor by evaporating metalloporphyrins on the active area of

silicon photodetectors. Such a system is extremely miniaturized, low cost

and simple also with respect to similar technologies in which metallopor-

phyrins are employed with optical fibers [10].

In this dissertation, novel silicon photodetectors with enhanced responsiv-

ity in the blue spectral region have been designed and modelled starting

from a previous work by Ghazi [11]. Differently from Ghazi’s photodiodes,

two types of finger photodiodes have been implemented here, p+-anode in

n-substrate-cathode and n+-cathode in a p-well anode (implanted in the

n-substrate).

For first time finger shaped junction npn BJT have been developed with

current gains much higher than the ones exhibited by standard BJTs.

1.4 Structure of the Thesis

This dissertation has a Chapters structure in which every Chapter is di-

vided into Sections and Subsections, where necessary. After this first chap-

ter, which intends to introduce briefly the context, the motivations and the

novel sensors matrix adopted, Chapter 2 on the State of the Art gives a

much closer overview of the existing sensor technology for EN development

and highlights a few significant examples of commercial or innovative ENs.

Chapter 3 is completely devoted to the Electro-Optical nose, from the

schematics of the complete system to its working principle and chemical

9


description of metalloporphyrins and their optical properties.

The silicon photodetectors are extensively described in the following Chap-

ter 4, divided into 3 main Sections through which standard photodiodes,

finger-shaped photodiodes and finger-shaped phototransistors are presented

and compared, illustrating all the experimental results of their electrical

and optical characterization.

Chapter 5 rises above the sensor/device level up to a higher system level

by presenting the complete Electro-Optical nose system, from the nostril

packaging to the output signal extraction circuitry.

Chapter 6 collects all the experimental results recorded during successive

measurements campaigns carried on at different alternate times, according

to testing phases following upgrades in design and fabrication process, but

here reported all together for sake of clarity and easiness of comparison.

It is divided into 3 main sections according to the type of photodetector

employed in the sensor matrix: standard photodiode, finger photodiode,

finger phototransistor. The experimental set-up is also described in detail.

At the end of this chapter an extensive description and study of a parasitic

effect observed in the experimental results is included, being relevant for

research in metalloporphyrins and their interaction with gases.

In the last Chapter 7, conclusions are drawn and future development and

improvements are suggested.

After the Bibliography, Appendixes deal with more detailed information

about the sensor layout design and matrix packaging and bonding, infor-

mation extracted from laboratory and internal reports related to the thesis

project.

10

Chapter 2

State of the Art

2.1 Electronic Nose Sensors Technology

Sensors to be employed as receptors in an EN must meet key design pa-

rameters, such as sensitivity, speed of operation, cost, size, manufactura-

bility, the ability to operate in diverse environments, and the ability to be

automatically and quickly cleaned. The sensors must be able to adsorb

large numbers of molecules of a particular species to produce a measur-

able change. After the odorant is identified, the process must be reversed

through a cleaning process. The choice of chemical sensors to meet these

requirements is large and includes metal-oxide semiconductors (MOS),

conductive polymers (CP), conducting oligomers (CO), surface and bulk

acoustic wave (SAW, BAW) devices, quartz crystal microbalance (QMB),

chemical field effect transistors (ChemFET), fiber optic (FO) sensors, and

discotic liquid crystal (DLC) sensors. In addition, GCs and spectrome-

ters can also be used alone or in combination with the above mentioned

chemical sensors [12] [13].

11

CHAPTER 2. STATE OF THE ART

2.2 MO Metal Oxide

A metal-oxide semiconductor (MOS) sensor is a resistive device made from

a metal-oxide film (e.g., tin oxide, SnO2 [14]) deposited onto two differ-

ent types of substrates, taking into account the AppliedSensors technology

[15]: alumina substrates (thick-film sensors) and Si-micromachined sub-

strates (micro sensors). Both substrates are provided with electrodes that

enable measurement of the resistance of the sensing layer, and heaters pro-

viding for the heating of the sensing layer which needs to be operated at

high temperature, 200-400oC. The odorant molecules undergo a reduction

reaction on the film surface producing a conductivity change in the sensor.

To remove the odorant molecules, an oxidation reaction must take place.

Heaters within the sensors aid in the oxidation process.

The advantages of metal oxides include low cost, longevity, low response

to humidity, and electronic simplicity. The disadvantages include the ne-

cessity to operate at high temperatures, restrictive selectivity, high power

requirements, and modest sensitivity (5-500ppm).

2.2.1 KAMINA: Chemical Gas Detector Sensor (SPECS Inc.)

An interesting example of EN based on metal-oxide semiconductor tech-

nology is the KAMINA nose, developed at the Karlsruhe Research Center

in Germany [16]. It employs a SnO2 : Pt sensitive thin film segmented

by Pt electrodes to create a microarray of 38 sensors, heated up by 4 Pt

heaters on the backside of the substrate. By supplying the heaters with

different voltages a temperature gradient is generated along the sensor seg-

ments and the temperature distribution along the chip is controlled via 2

Pt thermoresistors on the front side of the chip.

The implementation of a temperature gradient improves gas discrimination

and sensors sensitivity and selectivity, by matching each sensitive layer with

12

2.3. CP CONDUCTIVE AND NON-CONDUCTIVE POLYMERS

its best performing temperature. Other solutions reported in literature for

generating a temperature difference along a microarray are fabricating a

PolySi variable microheater on a SiO2 released membrane [17] and plac-

ing the microheater on top of a SiO2 layer with a gradually decreasing

thickness (thinner means hotter) [16].

2.3 CP Conductive and non-Conductive Polymers

A conductive polymer (CP) sensor is a semiconducting polymer film coated

to adsorb specific species of molecules. When the odorant molecules in-

teract with the coating, the conductivity of the sensor changes. On the

contrary, in the commercially available ”Cyranose 320” Electronic Nose

(from CyranoTM Sciences [18]), each individual detector of the sensor ar-

ray is a composite material consisting of a non-conducting polymer homo-

geneously blended throughout conductive carbon graphite. The detector

materials are deposited as thin films on an alumina substrate, each across

two electrical leads thus creating conducting chemoresistors. The output

from the device is an array of resistance values. When a composite is

exposed to a vapor-phase analyte, the polymer matrix ”swells up” while

absorbing the analyte. The increase in volume causes an increase in resis-

tance because the conductive carbon-black pathways through the material

are broken. When the analyte is removed, the polymer ”sponge” off-gasses

and ”dries out”.

The advantages of conductive polymers are wide selectivity, high sensitiv-

ity (0.1-100ppm), stability and operation at ambient temperatures. The

biggest disadvantage is a strong sensitivity to humidity.

13


2.4 SAW and BAW Surface and Bulk Acoustic Wave

Surface and bulk acoustic wave (SAW, BAW) sensors are piezoelectric

quartz crystals coated with selective coatings which adsorb species of mole-

cules [19]. The adsorbed molecules increase the mass of the sensor changing

its resonance frequency. By measuring this shift (also in term of phase de-

lay), a concentration of odorant can be derived.

The advantages of SAWs and BAWs include high selectivity, high sensitiv-

ity, stability over wide temperature ranges, low response to humidity and

good reproducibility. The disadvantage is the complexity in the interface

electronics.

2.5 FET Field Effect Transistor

A chemical field effect transistor (ChemFET) is based on a field effect

transistor with a catalytic metal as gate contact [8] [15]. The gate volt-

age controls the current through the MOSFET. The interaction of gases

with the catalytic gate, which adsorbs odorant molecules, induces dipoles

or charges generation, which adds up to the gate bias thus changing the

current through the transistor.

For the MOSFET sensor, gate and drain are connected and the sensor op-

erates as a two-terminal device. The voltage (around 2V) at a constant

current (100A) is monitored. The gas response is recorded as a voltage

change in the sensor signal. The operation temperatures are 150-200oC

and 200-600oC for devices respectively based on silicon and silicon carbide

as semiconductor.

The advantages include high sensitivity (ppm), high selectivity and ease

of integration with other electronics. The disadvantages include lack of

suppliers and the necessary penetration of the odorant molecules into the

14

2.6. QMB QUARTZ MICRO BALANCE

transistor gate.

2.6 QMB Quartz Micro Balance

Quartz microbalance (QMB) sensor technology is based on measuring the

frequency of quartz crystals coated by a sensing layer. The frequency is

influenced by bulk absorption of analyte molecules into the sensing ma-

trix because it is a function of the graviting mass. The sensitivity and

selectivity of the QMB sensors can be varied through the selection of dif-

ferent coatings, having different functional groups in the side chains. In a

Thickness Shear Mode Resonator, made up by an AT-cut quartz crystal

(Fig. 2.1), a linear relationship between variation of mass and variation of

resonance frequency exists (Sauerbrey Law), for small amount of mass:

∆f = − f 20

2νSρcA∆m = − f 2

0

2νSρcA×mmol × nabsorbed(pi) (2.1)

The fundamental frequency is in the range 5-30MHz and the typical sen-

sitivity is of the order of 10 ng/Hz.

The equivalent circuit of a QMB sensor consists of a serial connection of

an inductance, a capacitor and a resistor (Fig. 2.2). The additional shunt

capacitor CO refers to stray capacitance due to soldering and housing ef-

fects.

2.6.1 LIBRA NOSE (TechnoBioChip)

The design of the innovative electro-optical nose that is presented in this

thesis project taps on the long experience of the Sensors Group of the

University of Rome Tor Vergata, partner in this research. They have been

15


working on the electronic nose since 1991 and they have been studying met-

alloporphyrins as sensing films since 1994; they have developed a very suc-

cessful commercialized Libra Nose [20], based on QMB technology. Fig. 2.3

shows the whole cylindrical nose, made up of two main parts: the odorant

pumping system and valves at the bottom, the nostril at the top (Fig. 2.4).

The nostril, which represents the measurement chamber, consists of eight

QMBs inserted in eight Colpitts oscillator circuits (Fig. 2.6), in order to

read-out the frequency shift of each of them when exposed to the odor-

ant. The output signals are then sent to a software via a serial connection,

able to display them simultaneously and to register them. Successively the

data array will be processed by PCA algorithms by using Matlab pattern

recognition tools and matrix calculus.

Each quartz has a resonance frequency of 20MHz and it is spray-coated by

a distinct metalloporphyrin, a not selective sensing layer able to quickly

adsorb odorant molecules and to release them at room temperature.

Fig. 2.5 shows the latest Libra version, called EN-Qube, where the gas

pumping system has been optimized in order to reduce the instrument

weight and size.

Figure 2.1: Image of a quartz crystal with

gold electrodes.

Figure 2.2: Equivalent circuit of a QMB.

16

2.7. FO FIBER OPTIC

Figure 2.3: Image of Tor Vergata Libra Nose, commercially available from Technobiochip.

Figure 2.4: Zoom of the sensors nostril,

showing the 8 QMB inserted in 8 oscillating

circuits.

Figure 2.5: Photograph of the latest Tor Ver-

gata nose upgrade: En-Qube.

Figure 2.6: Schematics of the Colpitts

sinusoidal oscillator employed to extract

the quartz frequency variation.

2.7 FO Fiber Optic

A fiber optic (FO) sensor is a conventional optical fiber coated with a

fluorescent coating which interacts with the odorant molecules [21]. An

17


optical pulse is applied to the sensor and is adsorbed by the coating. The

interaction of the odorant molecules and fluorescent dyes produces a fre-

quency shift in the returned fluorescent signal. The returned signal is then

analyzed to determine the properties of the odorant molecules.

2.8 Optical Technologies

This approach was introduced by Walt in 1996 by employing fluorescent

polymers sensing layer with optical fibers [21] and started in Europe with

the work by D’Amico and Di Natale on metalloporphyrins [22], on which

this thesis project is based. The latest significant examples, reported in

the following sections, can be found in U.S.A. research institutes and com-

panies.

2.8.1 SMELLSEEING: A Colorimetric Electronic Nose (Chem-

Sensing Inc.)

This simple approach is based on sensor array detection and utilizes the

colorimetric response from a library of immobilized vapor-sensing metal-

containing dyes, metalloporphyrins [9]. They provide a way of reporting

the presence and concentration of odors by changes in color (Fig. 2.7).

Once a two-dimensional display (6x6) is arranged, a digital image before

and after exposure to the analyte is registered and a final difference map is

produced as ”molecular fingerprint” (Fig. 2.8). Suslick’s SmellSeeing has

been addressed as ”intriguing” by Lundstrom in [23] because the possibil-

ity of identifying different smells by eye could be used to monitor levels of

insecticides in the environment or to sniff out bacteria causing infections.

The replacement of pattern recognition routines and computer-made de-

cisions with the eyes of an experienced operator will have advantages in

many other situations.

18

2.8. OPTICAL TECHNOLOGIES

Figure 2.7: Comparison of Zn(TPP) spectral shifts upon exposure to ethanol and pyridine

(py). a) In methylene chloride solution; b) on the reverse phase support. In both a)

and b), the bands correspond, from left to right, to Zn(TPP), Zn(TPP)(C2H5OH) and

Zn(TPP)(py), respectively.

2.8.2 Optical NoseTM and BeadArrayTM (Illumina Inc.)

In this approach, bead sensors are fabricated by either adsorbing or co-

valently immobilizing fluorescent dyes in a polymer microsphere matrix

[24]. Responses are generated by measuring intensity changes, spectral

shift and time-dependent variations associated with the fluorescent sensors.

The bead array is assembled on an optical fiber bundle of the diameter of

1.5mm (Fig. 2.9, left), consisting of about 50000 individual fibers (Fig. 2.9,

middle), successively dipped into a chemical solution and then into a pool

of coated beads (Fig. 2.9, right).

19


Figure 2.8: Color change profiles of a metalloporphyrin sensor array. A) Color change

profiles of the metalloporphyrin sensor array as a function of exposure time to nbuty-

lamine vapor; B) Color change profiles for a series of vapors: the degree of ligand softness

increases from left to right, top to bottom.

20

2.8. OPTICAL TECHNOLOGIES

Figure 2.9: Left: optical fiber bundle;

Middle: microscopic etched well at the

end of each individual fiber; Right: one

bead is drawn in each well.

21

Chapter 3

An Electro-Optical Nose E-ON

The system architecture of the electro-optical nose developed in this dis-

sertation is schematically presented in Fig.3.1: the blue light source is

a LED with emission peak around 440nm, in the Soret band, where the

absorption spectrum of metalloporphyrins exhibits a maximum peak; met-

alloporphyrins are deposited directly on the active region of the silicon

integrated photodiodes, which collect the photons coming from the LED

and generating electron-hole pairs. These charges produce a current flow

when generated in the depletion region of the p-n junction, where an elec-

tric field is present. The output current is then converted into a voltage

signal and processed to have a good S/N (Signal/Noise) ratio. A multi-

plexer can be employed to monitor and register the voltage output of each

photodiode in the array with a certain switching rate and, after an Analog

to Digital conversion, data are sent to a Pattern Recognition software for

the final mapping and response.

3.1 Metalloporphyrins

Tetra Phenyl Porphyrins (TPP) are among the most important class of

chemical families [25] [22]. They have been selected by Nature for impor-

tant biological functions such as oxygen transport in blood and photosyn-

23

CHAPTER 3. AN ELECTRO-OPTICAL NOSE E-ON

(a) Photodiodes

(b) Phototransistors

Figure 3.1: Block diagram of the electro-optical nose system with the two types of sensors

employed: (a) photodiodes, and (b) BJT phototransistors.

thesis in plants. A great number of features makes porphyrins eligible as

good sensing material able to detect the volatile organic compounds present

in the environment. Porphyrins in fact are rather stable compounds and

their properties can be finely tuned by simple modifications of their basic

molecular structure: this is planar, formed by four pyrrolic rings and me-

thine bridges making up an aromatic system of 18 π -electrons. This basic

structure can be modified complexing a metal atom at the center of the

cycle and/or adding peripheral groups (Fig.3.2). Eight metals are usually

considered for the electronic nose sensing films: Zn, Co, Cu, Mn, Ru, Rh,

Fe, Sn.

The coordinated metal, the peripheral substituents, the conformations

of the macrocyclic skeleton influence the coordination and the related sens-

ing properties of these compounds. All together these characteristics in-

24

3.1. METALLOPORPHYRINS

Figure 3.2: Basic molecular structure of

a porphyrin. The central metal can host

many of the metal of the periodic table,

while at the R, R’ position lateral groups

can be linked.

crease the versatility of these molecules and different transducers have been

proposed for porphyrin-based chemical sensors, all showing outstanding

properties of these materials in terms of stability, chemical sensitivity and

reproducibility. The adsorption properties of solid state porphyrins are

characterized by large sensitivities and wide selectivities: both of these

features are particularly appealing for electronic nose applications.

The metalloporphyrins employed in this dissertation work are the same

used in the LIBRA nose at the Univeristy of Rome Tor Vergata (Subsec-

tion 2.6.1, Chapter 2) and they have been synthesized at the Department

of Chemical Sciences and Technologies of the same university.

3.1.1 Optical Properties

The optical features shown by porphyrins and related compounds make

these molecules particularly appealing for optical sensing purposes. The

absorption and luminescence properties derive from the aromatic charac-

ter of these materials and are related to electronic transitions within their

π-aromatic system. The characteristic absorption spectrum of a metallo-

porphyrin is reported in Fig. 3.3, where the main peak in the Soret band

(blue region) and the secondary peaks in the Q-bands (green-red region)

25


Figure 3.3: Characteristic absorption

spectrum of metalloporphyrins.

are presented.

Furthermore, due the presence of ππ interactions between macrocycles,

solid state aggregation of porphyrins can result in broadening, splitting and

shifts of the bands to respect solution spectra. Interactions with VOCs can

induce a decrease of these interactions, leading to additional modifications

of the spectra.

Variations of the absorption spectrum of the metalloporphyrins employed

in this thesis have been experimentally investigated at the Electro-Optical

Laboratory at the Department of Information and Communication Tech-

nology of the University of Trento, Povo, by conducting spectroscopic mea-

surements with the spectrometer Avaspec-2048. The light source employed

was a tungsten halogen lamp (HL-2000-FHSA, Avantes, 17mW maximum

power), coupled via optical fiber to a glass cuvette containing the solid

state metalloporhyrin deposited on a small glass by evaporation (same

thickness as the material evaporated afterwards on the sensors). The cu-

vette was kept isolated from ambient light by a special black housing. The

output light was coupled to the spectrometer via another optical fiber. The

described set-up is shown in the picture of Fig. 3.4. Fig. 3.5 and Fig.3.6 re-

port three of the measured transmission spectra of Zn-T(heptyloxy)PP and

Mn-T(hexadodecyloxy)PP, before and after the injection, by a common

26


Figure 3.4: Picture of the experimental set-up used for spectroscopic measurements of

metalloporphyrins absorption spectrum variations with VOCs.

syringe, of saturated vapors of triethylamine (TEA) and ethanol (EtOH):

three measures at three different times after exposure (2’, 4’, 12’) have

been performed with no significant distinction in the resulted spectrum.

The absorption process by the porphyrins is totally reversible, at room

temperature and 1atm pressure. In Fig. 3.5 a big variation of the peak

amplitude is registered, while in Fig. 3.6 also a shift of the peak wave-

length towards lower values is shown.

The metalloporphyrins used in the previous measurements and eventu-

ally deposited on the sensors have been modified in their structure with a

synthesis procedure that inserts alkyl chains of variable length at the pe-

ripheral sites; depending on the number n of the CH2 groups in the chains,

metalloporphyrins are named T(butyloxy)PP if n=4, T(heptyloxy)PP if

n=7 (Fig. 3.7), T(hexadodecyloxy)PP if n=17.

27


(a) Zn-T(heptyloxy)PP and TEA

(b) Zn-T(heptyloxy)PP and EtOH

Figure 3.5: Transmission spectra of Zn-T(heptyloxy)PP (a) with no TEA and (b) with no

EtOH and both after 2’,4’,12’ from injection. The spectrum variations are reversible.

28


Figure 3.6: Transmission spectra of Mn-T(hexadodecyloxy)PP with no EtOH and after

2’,4’,12’ from injection. The spectrum variation is reversible.

29


Alkyl chains increase the mutual distance between porphyrin rings, thus

reducing the mutual interactions and giving rise to a less compact film with

improved morphological properties able to provide a faster absorption of

the VOC molecules [26]. Fig. 3.8 reports the sensor response time versus

the length of the alkyl chain for similar metalloporphyrins studied by Di

Natale and Paolesse in [26]: response time is inversely proportional to the

length of the corresponding alkyl chain (e.g. for n=4 tr=850s, for n=7

tr=700s).

Moreover these modified sensing layers exhibit a more resolved absorption

spectrum, with an increase of sharpness of the Soret band with the length

of the chain, like shown in Fig. 3.9. This property affect the sensor perfor-

mance by allowing for higher resolution and sensitivity in gas detection.

Figure 3.7: Molecular structure of a

T(heptyloxy)PP, a porphyrin with added

alkyl chains.

N HNHN

NNHNH

O(CH2)7CH3O(CH2)7CH3

H3C(H2C)7OH3C(H2C)7O O(CH2)7CH3O(CH2)7CH3

O(CH2)7CH3O(CH2)7CH3

30


Figure 3.8: Sensor response time versus the length of the alkyl chain of the sensitive

molecule. The response time is defined as the time necessary to reach a steady sensor

signal after analyte introduction into the sensor chamber.

Figure 3.9: UV-VIS spectra of Co-TPP (-), Co-TPP-7 (.-.), CoTPP-12 (...), CoTPP-18

(- -).

31

Chapter 4

Silicon Integrated Photodetector

Transducers

In the thesis work different types of silicon integrated photodetectors have

been employed as signal transducers and they have been all fabricated in

the Microfabrication Laboratory of ITC-Irst, Trento. They are responsible

for converting the chemical interaction between metalloporphyrin and VOC

molecules into an electric signal, easy to extract, store and analyze.

The main part of the thesis work has dealt with the optimization of the

photodetectors, that make up the heart of the E-ON, with the goal of taking

the biggest advantage of the spectral absorption change of porphyrins.

Since such a variation is not particularly high, the need for ad hoc designed

photodetectors has been relevant, together with the study and development

of a good deposition method for metalloporphyrins coating.

4.1 Metalloporphyrins Deposition Methods

While employing the first type of standard photodiodes, metalloporphyrins

have been deposited by spray-coating through a thin metal shadow mask

with an air-brush, mixing them in a liquid chloroform solution. Since chlo-

roform, the solvent, is very volatile, it evaporates immediately on touching

33

CHAPTER 4. SILICON INTEGRATED PHOTODETECTOR TRANSDUCERS

the chip target and only the porphyrin solid thin film remains on the sen-

sor. This method does not provide uniformity of the film thickness over

the photodiode area and reproducibility. Moreover common film thickness

is high, around 1µm. Fig. 4.1 shows a picture of one of the sensors after

metalloporphyrin deposition.

Figure 4.1: One of the conventional photodiodes employed in the first matrix after depo-

sition of Mn-TPP-Cl by air brush.

One of the most significant improvements of the sensors response has

been achieved by switching to deposition by evaporation: metalloporphyrin

powder is put in a small quartz cylinder, surrounded by a wire in which a

current flow provides Joule effect heating. Everything is placed in a reactor

with high vacuum and the porphyrin powder evaporates to hit the photo-

diodes dies attached to a metal target right in front. Fig. 4.2 shows the

picture of a sensor evaporated with Zn-T(heptyloxy)PP (Zn-TPP-7), one

of the two sensing layers employed in the successive experimental testing

(the second metalloporphyrin is Co-T(dodecyloxy)PP, Co-TPP-12). The

coating appears much more uniform with a controllable and thin thickness

(150nm).

34

4.2. CONVENTIONAL PHOTODIODES

Figure 4.2: One of the conventional photodiodes employed in the first matrix after depo-

sition of Zn-T(heptyloxy)PP by evaporation.

Chapter 6 reports relevant experimental differences in sensors perfor-

mance between photodiodes coated with the first and the second method

(Fig. ??).

4.2 Conventional Photodiodes

The first sensor matrix of the E-ON has been assembled by employing

”off-the-shelf” silicon conventional p-n photodiodes previously fabricated

in ITC-Irst. The picture in Fig. 4.3 shows part of the matrix. The photo-

diode active area is 710µm×710µm, while the single die, included lateral

aluminum contacts, measures 1010µm×750µm. Data sheet reports a re-

verse dark current of 100pA and reverse breakdown voltage of 52V.

Their responsivity spectrum is reported in Fig. 4.4: the maximum peak

of 0.65A/W occurs at 880nm and the spectral bandwidth is 600-1000nm

at 50% peak value. This conventional responsivity curve provides only

0.14A/W in the blue spectral region, around 430nm, in the Soret band,

which is the desired sensor working region. The absorbed optical power in

35


Figure 4.3: Image of part of the silicon photodiodes matrix.

Figure 4.4: Responsivity spectra of several types of fabricated photodiodes. The responsivity

of the one employed in the E-ON project is marked with a star.

silicon is ruled by the Lambert-Beer Law:

P = (1−R)P0e−αx (4.1)

where

P=absorbed optical power at distance x in the silicon photodiode from the

36

4.2. CONVENTIONAL PHOTODIODES

incident surface

P0=incident optical power

α=absorption coefficient with α440nm=2.7×104cm−1

R=reflectivity calculated by the following formula:

R =((n− 1)2 + e∗2)((n + 1)2 + e∗2)

(4.2)

where

n=4.823 real part of Si refractive index at 440nm

e∗=imaginary part of Si refractive index at 440nm

Fig. 4.5 reports the calculated curve of the absorbed optical power for

440nm incident light. Short wavelength photons, like the blue ones, have

a short penetration into the silicon substrate and to achieve an efficient

collection a depletion region (p-n junction) and electric field should be

create as close as possible to the silicon surface. For instance, 45% of the

incident optical power is absorbed within the first 200nm below the surface.

For a conventional p-n photodiode made in standard CMOS process almost

all of the incident photons are absorbed at a typical depth of 0.2-0.4µm

and the majority of the photogenerated carriers recombine in the p+-anode,

with a consequent low responsivity for blue light.

This is one of the main reasons of the bad experimental performance of

these first sensors, forced to work in a region with very low photon-electron

conversion efficiency.

Fig. 4.6 shows a picture of the first 3×7 matrix, already packaged: different

metalloporhyrins have been sprayed alternately on the 21 dies, to make the

selective deposition easier. In this way only 8 sensors in the matrix were

completely coated and electrically connected (like sketched in the drawing

in Fig. 4.7).

37


Figure 4.5: Absorbed optical power by silicon for 440nm light vs distance from Si incident

surface according to the Lambert-Beer Law: P0 is the incident optical power, α is the

absorption coefficient, R1 and R2 are the reflectivities without and with the SiO2 ARC

(Anti Reflective Coating) on top.

Figure 4.6: First packaged matrix of silicon

photodiodes

Figure 4.7: Drawing of the package metal line

and pads and the sensor matrix. The capital

letters show the locations of the spray-coated

photodiodes.

38

4.3. FINGER PHOTODIODES

Figure 4.8: Cross-section of the two types of finger-junction photodiodes designed:

a)P+PDxx, p+-finger anode in the n-substrate, and b) N+PDxx, n+-finger cathode in

a p-well implanted in the n-substrate.

4.3 Finger Photodiodes

In order to improve the sensor performance and overcome the undesired

low spectral responsivity explained in the previous section, new photodi-

odes have been studied and designed with space-charge regions just below

the incident surface. A finger cathode has been adopted as a solution to

move the p-n junction as close as possible to the surface: space-charge

regions between neighboring n+-fingers merge together creating a series of

photosensitive areas at the sides of the n+ implants (Fig. 4.8). A similar

design strategy was reported by Ghazi et al. [11], who inspired the develop-

ment of this new photodetector. Main differences between the two works

are mentioned in Subsection 4.3.2.

39


4.3.1 Modelling

Modelling was carried on with two softwares, SILVACO (ATHENA) and

ISE-TCAD, and different photodiodes geometries have been analyzed by

varying the distance between two fingers (10, 20, 30 and 40µm) while keep-

ing the finger width (5µm) and the device area constant.

Fig. 4.9 and 4.10 represent the 2D plots produced by SILVACO of the

electrons and holes concentrations, respectively, in the n+-fingers/p-well

photodiode with an interdigit distance of 10µm. Only 4 finger-implants

have been reproduced and one p-well contact on the right. The first plot

shows a series of electrons depletion regions between the fingers in the p-

well, in correspondence of a concentration of 1016 holes/cm3 (yellow region)

in Fig. 4.10, with an increase and decrease of electrons and holes densities,

respectively, when approaching the Si surface. Right below the surface

the colors legends show 108 electrons/cm3 and 104 holes/cm3, thus proving

an inversion of majority carriers and the presence of a p-n junction and a

consequent space charge region within the first 200nm below the surface

between the fingers implants. This simulation result confirmed the merg-

ing occurring between depletion regions around adjacent fingers.

In order to find some coordinates and identify the surface depletion region,

Fig. 4.11 and 4.12 report electric field and electrons and holes concentra-

tions along 4 different horizontal sections of the 2D structure in Fig. 4.10

at 10, 100, 200 and 300nm below the surface, and Fig. 4.13 shows a vertical

section between the second and third finger.

In the horizontal X-cuts plots, the curves follow the fingers shape and in the

regions between the fingers at 10nm the electrons concentration is higher

than holes, while at 100nm holes become to increase and to overcome elec-

trons till reaching the p-well concentration value of 1016 /cm3 at 300nm.

Fig. 4.13 confirms the creation of the majority carriers (holes) depletion

40


Figure 4.9: 2D plot, produced by SILVACO, of the distribution of electrons concentration

in a finger photodiode with 10µm interdigit distance.

region very close to the Si surface by showing the intersection between the

carriers concentrations curves at about 125nm: holes increase and reach

the p-well concentration at about 400nm and the electric field linearly de-

creases to zero through the surface space charge region of about 350nm

width.

41


Figure 4.10: 2D plot, produced by SILVACO, of the distribution of holes concentration in

a finger photodiode with 10µm interdigit distance.

In the simulations performed with ISE-TCAD, particular care was taken

in the comparison between the conventional photodiode and the n+-fingers/p-

well photodiode with an interdigit distance of 40µm. Simulation results

were again successful in proving the effectiveness of the adopted solution,

confirming the creation of the surface depletion region and the consequent

responsivity increase in the blue spectral range. Fig. 4.14 shows the main

42


(a) x=10nm

(b) x=100nm

Figure 4.11: Electric field, electrons and holes concentrations plots resulting from 2 X-cuts

at (a) 10nm and (b) 100nm (distance from Si surface) of the 2D structure in Fig. 4.10.

43


(a) x=200nm

(b) x=300nm

Figure 4.12: Electric field, electrons and holes concentrations plots resulting from 2 X-cuts

at (a) 200nm and (b) 300nm (distance from Si surface) of the 2D structure in Fig. 4.10.

44


Figure 4.13: Electric field, electrons and holes concentrations plots resulting from an Y-cut

between the second and third finger of the 2D structure in Fig. 4.10

difference between a finger and a conventional photodiode, represented in

cross-section respectively with a zoom of half of the finger and half of the

n+-implant contact. Electron density is related to a color legend: red rep-

resents a high electron concentration, that is 1020/cm3 inside the finger

implant; blue represents a lack of electrons, corresponding to the p-well

zone; green represents the junction depletion layer. On the finger right

side, holes concentration decreases in the p-well thus creating a surface

depletion region < 0.1µm, with a minimum concentration of 104/cm3 at

y=0 (Si surface), while in the standard device the junction area starts only

under 0.5µm.

Fig. 4.15 reports the responsivity spectra for all the implemented de-

vices, with four different interdigit distances of 10, 20, 30, 40µm and 5µm

finger width, calculated after running cycles of electrical and optical sim-

ulations at different incident light wavelengths. The finger photodiodes

exhibit three main peaks at 420nm, 480nm and 560nm, with amplitude in-

45


Figure 4.14: 2D plots of electron density in a finger (left side) and standard photodiode

(right side) with no bias applied (results obtained with ISE-TCAD after running DESSIS

and TECPLOT for viewing): the blue color represents a high holes concentration (p-well),

green represents the junction area and holes depletion region.

creasing with interdigit distance, while the standard device presents lower

responsivity values with a maximum around 560nm. The best result is

obtained for an interdigit distance of 40µm because in this device the pho-

todetector active area is mostly covered by the surface depletion region

rather than by numerous n+-finger implants (maximum value of d/w ra-

tio). In the finger photodiodes the carriers surface recombination at the

SiO2/Si interface is fundamental while in the standard device the reverse

current is dominated by generation in the bulk. These two phenomena

are ruled by two parameters, respectively, S0, the surface recombination

velocity, and τg, the carriers lifetime, which are present in the expression

46


Figure 4.15: Simulated responsivity spectra of standard and finger photodiodes with differ-

ent interdigit distance (results obtained with ISE-TCAD after running DESSIS and OP-

TIK for optical generation). Only the latters show high peaks in the blue region, around

400nm, with a maximum value for the maximum interdigit distance, d=40µm.

of the reverse current with a surface and bulk contribution. Simulations

run with different values of S0 (4, 6, 10, 30, 60cm/s) and τg (10−4, 10−3,

5×10−3, 10−2, 2×10−2s) for both the devices confirmed the strong depen-

dence of the finger photodiodes from S0 and the weak dependence from

τg, and vice versa for the standard detector. The simulation results are

plotted in Fig. 4.16.

4.3.2 Design and Fabrication

The design of the new photodiodes layout followed the simulations and ac-

cording to the modelling results, 4 different geometries of finger-photodiodes

have been designed together with a conventional fully implanted photodi-

ode for reference.

47


(a) finger photodiode: dependence on S0

(b) standard photodiode: dependence on τg

Figure 4.16: Simulation results of the dark current dependence on (a) S0, surface re-

combination velocity, for finger photodiodes and (b) τg, carriers lifetime for conventional

photodiodes.

The description of the devices is listed below with the main differences

between this work and Ghazi’s previous work:

48


• two types of photodiode have been implemented here: p+-finger an-

ode in the n-silicon substrate cathode (named P+PD10/20/30/40),

and n+-finger cathode in a p-well anode implanted in the n-substrate

(named N+PD10/20/30/40) (Fig. 4.8), while Ghazi studied and fab-

ricated only the former type;

• in both works, in order to find the optimized interdigitated structure,

finger photodiodes with different numbers of fingers for a constant area

of the photodiodes have been processed to study different combina-

tions of d/w ratio (d=interfinger distance, w=finger implant width).

Here the finger width is kept constant (in both cases it has been cho-

sen minimal according to the design rules, 5µm here and 1.2µm by

Ghazi) and the interfinger distance varies, while Ghazi changed both

the geometrical parameters, keeping the ratio between 4 and 5; the

d/w ratios considered here are the following:

40/5=8

30/5=6

20/5=4

10/5=2

with the highest being the most performing in the blue/UV spectral

range (Fig. 4.24(a)), as expected by Ghazi’s considerations [11].

• the creation of the surface depletion region is here significantly affected

by the positive charge trapped in the SiO2 passivation film on top of

the photodiode, with a concentration of about 5×1011cm−2;

• Ghazi’s goal was optimizing the finger photodiode for the UV/blue

and red spectral ranges at the same time, by varying the doping con-

centration of the epitaxial layer, and he was concerned with both

responsivity and response time, while here the target spectral range

49


Figure 4.17: Image of the layout of PD20, finger photodiode with d=20µm interfinger

distance.

is the blue one (and only one doping concentration has been used for

the epitaxial layer) and response speed is not an issue (due to the slow

chemical interactions in the sensor). In this thesis work more care has

been taken in the study of the responsivity dependence on interfin-

ger distance d and on the photodiode type (finger-shaped anode or

finger-shaped cathode?).

Fig. 4.17 shows the image of the layout of one of the finger photodiode,

with 20µm interfinger distance. The analogue layouts of the other devices,

PDstand, PD10, PD30, PD40, are collected in Appendix A. For a detailed

description of the layout, dies names and identification on wafer, see the

internal technical report [27]. The fabrication process is based on the

conventional planar silicon process technology: it is a 6 masks run and

low resolution optical photolitography has been used. The active area of

the photodiode measures 714µm× 744µm (p-well area). The Si substrate

is 600µm thick and has a high resistivity, with a donors concentration

of 1011/cm3; the implanted p-well is 2µm thick and boron doped with a

50


maximum concentration of 4×1016/cm3 at 0.9µm; the n+-fingers have been

implanted in the p-well with 1020/cm3 atoms of arsenic. Aluminum metal

lines contacting the fingers have been designed in order to allow a fast

collection of the carriers through the whole active region and two pads

give access to anode and finger-cathode (Fig. 4.17). The final LTO (Low

Temperature Oxide) layer deposited on top of the photodiodes is 1µm

thick and is not optimized as ARC (Anti Reflective Coating) because its

thickness has been set by the characteristics of other devices on the same

wafer and employed for passivation. Its exact thickness (1068nm) has been

calculated from the spectral responsivity peaks experimentally found from

the optical measurements described in Subsection 4.3.4 and by employing

the following known formula:

tox =1

2nox(

1

λr− 1

λr+1)−1 (4.3)

and

mr =tox4nox

λr

where

tox=top oxide thickness

nox=oxide refraction index=1.46

λr and λr+1=wavelengths of two adjacent peaks in the responsivity spec-

trum

mr=wavelength index, whose values have been calculated in Table 4.1.

Table 4.1: Wavelength indexes for the spectral responsivity peaks.

λr λpeak [nm] Calculated mr

λ11 560 11.1

λ13 480 13.0

λ15 420 14.9

λ17 370 16.9

51


(a) PD10, d=10µm (b) PD40, d=40µm

Figure 4.18: Micrographs of two finger photodiodes after Zn-T(heptyloxy)PP evaporation.

Figure 4.19: Micrograph of the

standard photodiode after Zn-

T(heptyloxy)PP evaporation.

At the end of the fabrication in the Clean Room of ITC-Irst, two sets of

dies have been completely coated with 170nm of two different metallopor-

phyrins, Zn-T(heptyloxy)PP and Co-T(dodecyloxy)PP, with two succes-

sive evaporations in the Clean Room of IMM-CNR, Tor Vergata Research

Area, in Rome (Fig. 4.18 and 4.19).

Fig. 4.20 shows a schematic cross section of the finger photodiode at the

end of the fabrication process, after metalloporphyrin evaporation.

52


Figure 4.20: Schematic cross section of the finger photodiode (cut in the middle of the

area in Fig. 4.17) at the end of the fabrication process.

4.3.3 Electrical Characterization

Electrical testing has been conducted over all the photodiodes types by

measuring IV curve with a variable reverse bias of the finger-shaped p-n

junction. Fig. 4.21(a) and 4.21(b) show the IV curves for N+PDxx and

P+PDxx devices respectively together with the standard photodiode curve.

Average measured dark current and breakdown voltage for N+PDxx and

P+PDxx devices are respectively around 2pA 22.5V, and 13pA 53V. More

detailed experimental values are reported in Table 4.2 (all measurements

have been conducted with the positive output probe on the anode/p-well

contact).

Both simulations and experimental results show dark currents values

higher for the finger photodiodes than the standard devices: this is due to

the surface current that generates within 1µm around the finger implant

and adds up to the bulk current, proportionally to the fingers perimeter.

Fig. 4.22 shows a direct comparison between the two types of photodiodes

N+PDxx and P+PDxx.

53


(a) N+PDxx, n+fingers implanted in a p-well on n-Si substrate

(b) P+PDxx, p+fingers implanted in the n-Si substrate

Figure 4.21: Measured IV curves for the two types of photodiodes, (a) N+PDxx, and (b)

P+PDxx and comparison with the standard fully implanted photodiode.

According to Eq. 4.4,

VB =εSE2

max

2q(NB)−1 (4.4)

the breakdown voltage is inversely proportional to impurity concentra-

tion and from the plot in Fig. 4.23 from Sze [28], it is possible to ver-

54


Table 4.2: Measured Dark Current and Breakdown Voltage for all the fabricated photodi-

odes.Photodiode Name Reverse Dark

Current [pA]

Reverse Break-

down Voltage

[V]

P+PDstand -7 63

P+PD10 -15 55

P+PD20 -13 54

P+PD30 -16 53

P+PD40 -13 52.5

N+PDstand +2.6 at 0V, 1 at -1V 23.5

N+PD10 -1.5 22.5

N+PD20 -3 22.5

N+PD30 -2.5 22.5

N+PD40 +0.13 at 0V, -3.5 at

1V

22.5

Figure 4.22: Comparison between the IV curves of the two types of photodiodes.

55


Figure 4.23: Breakdown voltage versus impurity concentration for one-sided abrupt dop-

ing profile with cylindrical and spherical junction geometries, where rj is the radius of

curvature.

ify the measured VB for the N+PDxx, about 22V (Boron concentration=

4×1016atoms/cm3), smaller than 53V of the undoped substrate photodiode

P+PDxx. The same plot reports the strong dependence of the breakdown

voltage on the junction radius, which dramatically decreases Vb, especially

for spherical junctions at low impurity concentrations (for P+PDxx with

such a low n-substrate concentration, 1011/cm3, 53V couldn’t be acceptable

without taking into account the radius of curvature).

4.3.4 Optical Characterization

Optical testing of the photodiodes has been conducted at ITC-Irst with a

series of spectral responsivity measurements at different reverse bias volt-

ages. Fig. 4.24 report the two experimental curves of the spectral responsiv-

ity for the two types of photodiodes at 5V reverse bias. Both plots exhibit

peaks in the blue region, located at 370, 420, 480, 560nm, with responsivity

value around the blue peak higher for the finger-photodiode with respect

to the standard reference. For a clear comparison, maximum responsivity

56


Table 4.3: Maximum measured responsivity values for all photodiodes in the blue spectral

range at 5V reverse bias.

Photodiode

Name

Max Respon-

sivity [A/W] @

5Vrev, 420nm

Photodiode

Name

Max Respon-

sivity [A/W] @

5Vrev, 420nm

N+PDstand 0.14 P+PDstand 0.15

N+PD10 0.20 P+PD10 0.21

N+PD20 0.21 P+PD20 0.23

N+PD30 0.21 P+PD30 0.23

N+PD40 0.22 P+PD40 0.24

values at 5V reverse bias are reported in Table 4.3 for both N+PDxx and

P+PDxx devices. At 420nm and 5V reverse bias the measured responsivity

values for N+PD40(d=40µm) and P+PD40 and the conventional photodi-

ode are respectively 0.22, 0.24 and 0.14A/W, which means that the finger

junction provides an increment of 60%. Responsivity values at low reverse

bias voltages do not vary significantly between N+PDxx and P+PDxx de-

vices neither with increasing interdigit distance, even if a slightly higher

value is observed for 40µm (Fig. 4.24). At high reverse bias voltages, only

in the N+PDxx devices (Fig. 4.25), an impact ionization phenomenon oc-

curs due to the heavy doping of the n+ fingers and the p-well (Fig. 4.25(a))

and it causes a photocurrent multiplication, responsible for the exceptional

increase in responsivity, which becomes even higher than the value corre-

sponding to 100% quantum efficiency (Table 4.4). Since this effect is based

on the doping levels, it has been observed also for the standard photodiode

N+PDstand (Fig. 4.26 and Table 4.4).

57


(a)

(b)

Figure 4.24: Finger photodiodes spectral responsivity at 5V reverse bias for increasing in-

terdigit distance and standard photodiode: a) for n+-finger cathode in a p-well photodiode,

N+PDxx; b) for p+-finger anode in the n-substrate photodiode, P+PDxx.

58


(a)

(b)

Figure 4.25: PD40 spectral responsivity for increasing reverse bias voltage: a) for n+-

finger cathode in a p-well photodiode, N+PD40 (Vbreakdown=22.5V); b) for p+-finger anode

in the n-substrate photodiode, P+PD40 (Vbreakdown=53V).

59


Figure 4.26: Spectral responsivity for increasing reverse bias voltage for the standard pho-

todiode N+PDstand, n+ fully implanted in the p-well.

The plot in Fig. 4.24(a) consistently matches the simulation results of

Fig. 4.15, with slightly lower experimental responsivity values (e.g. for

d=40µm at 420nm: 0.22A/W measured vs 0.27A/W simulated).

The significant difference in the N+PDxx and P+PDxx devices spectra is

due to the presence of the p-well in the former, with a thickness of 2µm.

Only a part of the spectrum wavelengths can be absorbed by the p-well

that eventually will cut off photons beyond red. Therefore these detectors

are expected to have the responsivity peaks shifted towards the green/blue

region, while the P+PDxx, directly implanted in the n-substrate, which

constitutes a much thicker cathode, are expected to present the classical

responsivity curve, increasing with wavelength, but with higher responsiv-

ity values in the blue region (due to the finger-shaped p+implant).

Mentioning again Ghazi’s work, as experimental results concern, he found

responsivity peaks in the blue range at 400nm of 0.23A/W and in the red

60

4.4. FINGER BJT PHOTOTRANSISTORS

Table 4.4: Measured responsivity values for N+PD40 and N+PDstand for increasing re-

verse bias (ionization multiplication).

Reverse Voltage [A/W] N+PD40

@420nm

(Q.E.=0.34A/W)

[A/W]

N+PDstand

@420nm

[A/W]

N+PDstand

@490nm

(Q.E.=0.40A/W)

5V 0.22 0.14 0.20

10V 0.23 0.15 0.23

15V 0.27 0.17 0.32

18V 0.34 0.20 0.42

20V 0.44 0.24 0.54

spectrum at 638nm of 0.49A/W, with ARC, while here the correspondent

peaks are located at 420nm with a maximum value of 0.24A/W (P+PD40)

and at 700nm with 0.45A/W, without ARC. Ghazi compared this high re-

sponsivity value with a reference photodiode performance, with the same

antireflecting coating, registering an increment by a factor of 2.8, but the

reference photodiode was not fabricated with the same technology process,

while all the comparisons shown in this thesis take into account refer-

ence conventional photodiodes fabricated in the same run and wafer of

the finger-shaped photodiodes, in order to minimize the parameters that

may affect a reliable comparison. At 420nm the responsivity value for the

reference device P+PDstand is 0.15A/W with an increment factor of 1.7,

without ARC optimization.

4.4 Finger BJT Phototransistors

After performing several measurements campaigns with the finger pho-

todiodes and Ethanol vapor at different concentrations (for details and

experimental results, see Chapter 6) in order to test the sensors response

and sensitivity, I decided to use the same N+PDxx photodetectors as npn-

61


Figure 4.27: Cross-section of the npn BJT with the finger-shaped Emitter/Base junction.

BJTs Bipolar Junction Phototransistors. The n-Si substrate is contacted

from the backside and it serves as the n−-collector, while the p-well and

the n+-fingers constitute base and emitter respectively, like presented in

Fig. 4.27.

4.4.1 Electrical Characterization

Output Characteristics and Gummel plots have been experimentally mea-

sured for the new BJTs at ITC-Irst. The most significant results are the

following: a larger current gain in finger-type devices with respect to the

standard one has been observed and, among the finger BJTs, an increasing

beta for larger interfinger distance has been recorded. Fig. 4.28 presents

a comparative Gummel plot of the base Ib and the collector Ic currents

vs the base bias voltage Vb for the standard BJT and two finger-BJTs,

named BJT10 and BJT40 (interdigit distance 10µm and 40µm respec-

tively). These three devices have been taken into particular account in

the testing and simulation phases because a comparison among them can

include also the trend observed for BJT20 and BJT30. While the collector

current remains constant for all devices, a significant decrease in the base

current is registered for a larger interfinger distance and in general the high-

62


Figure 4.28: Experimental Gummel plots (Ib and Ic vs Vb) for 3 different BJTs: BJT0

standard, BJT10 and BJT40.

est base current is exhibited by the standard transistor. This trend highly

affects the beta internal current gain of the BJTs, which consequently

increases for increasing interfinger distance and for the finger-emitter tran-

sistor with respect to the standard one (Fig. 4.29). Table 4.5 reports the

beta maximum values and the correspondent base currents for all five types

of devices.

BJT base current is therefore affected by the interdigit distance or by the

Table 4.5: Maximum β current gain and correspondent base current for all phototransis-

tors.DUT β Ibase [A]

BJT0 72 3.85× 10−7

BJT10 109 3.00× 10−7

BJT20 132 2.50× 10−7

BJT30 148 2.16× 10−7

BJT40 158 2.00× 10−7

63


Figure 4.29: BJT current gain Beta vs collector current Ic for increasing interfinger dis-

tance and the standard BJT.

number of emitter n+-fingers in the p-well area. This dependence has been

further studied by calculating the total fingers perimeter for each device

and plotting the base current Ib and the current gain β as a function of

the perimeter itself, that in turn grows with the fingers number (Fig. 4.30)

according to a second order polynomial function

Y = A + B1X + B2X2

with the following fitting coefficients:

Fitting Coeffients for P/]-fingers plot

Parameter Value Error

A −14094.28139 4593.50752

B1 168.44193 30.28672

B2 −0.06679 0.04292

Fig. 4.31 and 4.32 show the functions Ib and β versus the fingers perime-

ter: the former has been fitted by a second order polynomial fit

Y = A + B1X + B2X2

64


Figure 4.30: Dependence of the fingers perimeter on the fingers number.

and the latter by a Holliday function (with no weighting)

Y =a

1 + bX + cX2

with the following coefficients for the two expressions:

Fitting Coeffients for Ib/P plot


A 1.59351× 10−7 1.85084× 10−9

B1 3.25792× 10−12 1.3131× 10−13

B2 −1.2597× 10−17 1.87431× 10−18

Fitting Coeffients for β/P plot


a 183.04446 2.69711

b 0.00001 1.311× 10−6

c 2.146× 10−12 1.7305× 10−11

The β dependence and proper fitting has been found by taking into

account the following relationships:

Ib = f(A + B1X + B2X2),

β =Ic

Ib=

const

f(A + B1X + B2X2)=

a

1 + bX + cX2

65


Figure 4.31: Dependence of the base current

on the fingers perimeter.

Figure 4.32: Dependence of the current gain

on the fingers perimeter.

where X=number of fingers, and Ic is considered constant as observed

from the experimental results (Fig. 4.28).

4.4.2 Modelling

In order to further study the behavior observed from experiments, electri-

cal simulations of the three types of transistors BJT10, BJT40 and BJT0

standard have been performed by ISE-TCAD, which allowed to generate

2D plots of the structures, representing the hole current density, dominant

component of the base current. Fig. 4.33 collects the three plots corre-

spondent to the standard, BJT10 and BJT40 devices, where five fingers

and two fingers have been implemented for BJT10 and BJT40, in order

to keep the cross section of 82µm width and 600µm height constant, thus

allowing a correct straight comparison. According to the color legend and

gradient, all the three structures show a holes current density decreasing

with the distance from the p-well base contact on the right, but this gradi-

66


X

Y

0 25 50 75

-1

0

1

2

3

4

5

6

holes current densityd10beta4_mdr.grd - d10beta4_000010_des.dat

X

Y

0 25 50 75

-1

0

1

2

3

4

5

6

holes current densityd40beta3_mdr.grd - d40beta3_000010_des.dat

X

Y

0 25 50 75

-1

0

1

2

3

4

5

6

Abs(hCurrentDensity0.316740.2006480.1271060.08051930.05100730.03231210.0204690.01296670.008214150.005203490.003296310.002088140.001322790.0008379630.0005308330.0003362710.0002130210.0001349448.54845E-055.41527E-053.43046E-052.17313E-051.37663E-058.72067E-065.52437E-063.49957E-062.21691E-061.40437E-068.89636E-07

holes current densitystandbeta4_mdr.grd - standbeta4_000010_des.dat

Figure 4.33: 2D plots, generated by ISE-TCAD Tools with the software Tecplot, repre-

senting the holes current density in the vicinity of the E/B junction for the BJT0 (top

left), BJT10 (top right) and BJT40 (bottom left).

ent is affected by the finger-implant configuration and is higher for larger

interdigit distance. This is confirmed quantitatively by the three plots in

Fig. 4.34, resulting from a cross cut at Y=1µm of the structures presented

in the previous Fig. 4.33: for sake of comparison, taking as reference level

the holes current density value 10−2, the three curves fall below this value

at X=78µm for BJT0, at X=75µm for BJT10 and at X=69µm for BJT40,

meaning that the current decreasing gradient is faster for a higher interdigit

distance.

The different base currents can be also affected by the SRH recombina-

tion, that is smaller for BJT40, like shown in Fig. 4.35, which leads to a

smaller contribution of the Ibr recombination current in the neutral base

67


Figure 4.34: Holes density along X resulting from an Y-cut at 1µm distance from Si

surface (just below the finger implants) of the three structures in Fig 4.33.

region, component of the total base current, like recalled in the expression

below [29]:

Ib = Ipe + Ire − Igc + Ibr (4.5)

where

Ipe =hole component of the emitter current

Ire =recombination current in the depletion region of the emitter/base

junction

Igc =generation current in the depletion region of the collector/base junc-

tion

68


X

Y

20 40 60 80

-1

0

1

2

SRH Recombinationd40beta3_mdr.grd - d40beta3_000010_des.dat

X

Y

20 40 60 80

-1

0

1

2

SRH Recombinationd10beta4_mdr.grd - d10beta4_000010_des.dat

X

Y

20 40 60 80

-1

0

1

2

srhRecombinatio7.06367E+184.08033E+182.357E+181.36152E+187.86482E+174.54311E+172.62433E+171.51594E+178.75684E+165.05839E+162.92198E+161.68788E+169.75003E+155.6321E+153.25338E+151.87932E+151.08559E+156.27089E+143.62237E+142.09246E+141.20869E+146.98177E+134.03261E+132.32872E+131.34395E+13

SRH Recombinationstandbeta4_mdr.grd - standbeta4_000010_des.dat

Figure 4.35: SRH recombination distribution in the three structures of Fig 4.33: the pre-

vailing yellow color in BJT40 around the E/B junction stands for a smaller recombination

and consequently a smaller Ir component of total Ib.

Ibr =recombination current in the neutral base region

4.4.3 Optical Characterization

A spectral responsivity measurements campaign has been conducted at

ITC-Irst also for the phototransistors in order to study the role of the E/B

and B/C junctions in the blue photons collection and to make a compari-

son with the photodiodes. BJTs have been kept with floating base and two

collector voltages have been applied, 5V and 15V. Three measurements se-

ries have been done: in the first series a small light beam (300µm diameter)

has been used and it lighted only the central area of the phototransistor,

69


therefore only the E/B junction took part to the photons collection; in the

second series a larger light beam (1250µm diameter) has been employed

in order to light all the phototransistor die, thus including also the B/C

lateral junction surrounding the p-well perimeter; finally, ”spatial” respon-

sivity measurements have been performed at 8 different light wavelengths

(350, 400, 450, 500, 550, 600, 650, 700nm) by scanning the device horizon-

tally (23µm step) with a small beam of 200µm diameter.

Even if the E/B finger-shaped junction is almost forward biased in the

BJTs case (opposite of the reverse bias applied to photodiodes), it affects

the blue photons collection as shown by the spectral responsivity curves in

Fig.4.36, measured at 5V collector bias voltage. They still differ one from

another, even if the trend is not the same observed for the photodiodes:

PT10 exhibits the highest responsivity at 420nm, while PT40 the lowest.

From the direct comparison of three of the photodiodes with the corre-

Figure 4.36: Finger phototransistors spectral responsivity at Vcc=5V for increasing inter-

digit distance and standard BJT, with small light beam (only E/B junction contribution).

70


Figure 4.37: Comparison between spectral responsivities of finger photodiodes and photo-

transistors with respectively Vrev=5V and Vcc=5V for the standard devices and the finger

photodetectors with d=10µm and d=40µm.

spondent phototransistors in Fig. 4.37, standard, d10 and d40 devices, the

responsivity peaks occur almost at the same wavelengths, with a possi-

ble 5-10nm shift (in both directions), and the main difference is the curve

trend that for BJTs keeps increasing for increasing wavelengths. This is

due the bulk contribution to the collection of higher wavelengths photons,

totally absent for the photodiodes, whose n-substrate is kept floating (cut-

off wavelength set by the 2µm p-well thickness). The phototransistors have

a more resolved (less broad) and higher peaks in the blue spectral region,

like clearly shown in the four comparative plots of Fig. 4.38, meaning that

the former can better sense small changes, especially wavelength shifts in

71


metalloporphyrins absorption spectrum (Fig. 3.5 and Fig. 3.6, Chapter 3).

For this reason phototransistors are expected to be more performing than

photodiodes in the experimental response to ethanol. The registered incre-

ment in responsivity is not constant for all types of phototransistors and

it is calculated below for each BJT at 420nm:

∆Resp(PT0-N+PDstand)=(0.20-0.14)=0.06A/W

∆Resp(PT10-N+PD10)=(0.24-0.20)=0.04A/W

∆Resp(PT20-N+PD20)=(0.25-0.21)=0.04A/W

∆Resp(PT30-N+PD30)=(0.24-0.22)=0.02A/W

∆Resp(PT40-N+PD40)=(0.23-0.22)=0.01A/W

72


(a) (b)

(c) (d)

Figure 4.38: Comparison between spectral responsivities of finger photodiodes and photo-

transistors with respectively Vrev=5V and Vcc=5V for: (a) PD10 and PT10; (b) PD20

and PT20; (c) PD30 and PT30; (d) PD40 and PT40.

73


The improvement in responsivity introduced by the phototransistor de-

teriorates with increasing interdigit distance, in fact PT40 responsivity is

almost equal to its correspondent photodiode, N+PD40, while the greatest

variation has been observed for the standard BJT. This can be explained

by recalling the two main mechanisms of formation of the surface depletion

region between neighboring finger implants: the positive charge trapped

in the passivation oxide layer and the reverse bias of the n+-fingers/p-well

junction, applied to the photodiodes during optical testing. In addition,

interdigit distance is also important for the merging of neighboring deple-

tion regions around adjacent fingers to occur. In the phototransistors case

the second mechanism ceased to be, in fact the E/B junction can be con-

sidered slightly forward biased, and the first mechanism only can’t provide

the same amount of surface depletion layer for the efficient collection of the

blue photons and the interdigit distance start to play a fundamental role:

if the fingers are too far (from calculations, d=20µm seems like a thresh-

old distance), responsivity decreases for a lack of active surface collection

region, the merging doesn’t occur. This effect is in competition with the

improvement brought by the phototransistor device and tend to almost

compensate it for PT30 and PT40.

Fig. 4.39 shows the comparison between the spectral responsivity curves at

5V and 15V collector voltage for measurements done with small (Fig. 4.39(a))

and large (Fig. 4.39(b)) light beam.

74


(a)

(b)

Figure 4.39: Phototransistors spectral responsivity at Vcc=5V and 15V for standard BJT

and (a) PT10 with small light beam (only E/B junction contribution); (b) PT10 and PT40

with large light beam (also B/C lateral junction contribution).

75


A straight comparison between the responsivity measurements of the

first and second series, with small and large beam, is not reliable and can’t

be done due to the lack of homogeneity of the two beams employed and

some set-up differences. Nevertheless, three relevant results and qualitative

differences regarding the blue spectral region in the two cases of Fig. 4.39

can be highlighted:

1. an increase of the responsivity occurs only in the second case; the in-

crements from 5V to 15V are calculated below for each BJT at 420nm:

∆Resp(PT015V cc-PT05V cc)=(0.22-0.20)=0.02A/W

∆Resp(PT1015V cc-PT105V cc-)=(0.24-0.19253)=0.05A/W




Below follow the increments calculated taking into account the shift

of the peak from 420 to 425nm at 15V and making the difference be-

tween the two peak values:

∆Resp(PT015V cc,425nm-PT05V cc,420nm)=(0.24-0.20)=0.04A/W

∆Resp(PT1015V cc,425nm-PT105V cc,420nm-)=(0.25-0.19253)=0.06A/W




2. only in the second case a relevant change in the standard BJT re-

sponsivity spectrum occurs: it becomes similar to the other spectra,

exhibiting peaks at the same wavelengths and with the similar ampli-

76


tude, for both collector voltages;

3. with respect to the spectra measured for the same devices with the

small beam in the first case, in the second case the responsivity spectra

exhibit closer peaks, 360, 390 and 420nm in the blue region, with

respect to 365 and 425nm of the first plot.

All the results can be explained by considering that by employing the large

light beam also the B/C depletion region around the p-well perimeter is

lighted: the first and the second result prove that it provides a lateral sur-

face space-charge region able to collect blue photons, which widens for in-

creasing collector voltage (increasing junction reverse bias) thus providing

higher responsivity values at Vcc=15V. As for the amount of these incre-

ments, no differences are expected among BJTs with different interfinger

distance, since the p-well perimeter is constant for all types and conse-

quently the extension of this lateral active region is constant for all: from

the calculation reported above, the same increment has been found in both

cases (0.05 and 0.06A/W).

This lateral B/C active region, being the p-well perimeter the same also

for the standard BJT, is completely responsible for the appearing of the

peaks in the blue region of its spectrum (second result), in spite of not

having finger-shaped E/B junction.

The third result is related to a fabrication detail: the thickness of the oxide

layer on top of the p-well area (1068nm) is smaller than the thickness of the

oxide layer on the rest of the die and therefore also on the lateral collector

Si-substrate, due to a photolitography step. According to Eq. 4.3,

tox ∝ λrλr+1

λr+1−λr∝ 1

∆λ

if the oxide thickness becomes larger, ∆λ becomes smaller and adjacent

responsivity peaks become closer to each other.

The third and last series of optical measurements done with a 200µm beam

77


Figure 4.40: Spatial responsivity of BJT10 at

different wavelengths when scanned horizon-

tally by a 200µm beam with a 23µm step.

Figure 4.41: Zoom of the curves at 350 and

400nm in Fig. 4.40.

scanned horizontally along the BJT10 die gave further information on the

B/C lateral depletion region contribution and on the most responsive ar-

eas of the photodetector. Fig. 4.40 reports all the results for the 8 beam

wavelengths used: 350, 400, 450, 500, 555, 600, 650, 700nm. The pho-

tocurrent on the Y-axis has not been normalized to the current gain and

it is the straight output value read by the HP4145 analyzer. The zoom for

the smaller curves at 350 and 400nm in Fig. 4.41 shows the clear contri-

bution of the lateral B/C junction to the blue photons collection: the two

highest peaks (right and left) occur at the p-well border with the collector.

Such lateral peaks become less relevant for higher wavelengths, except for

600 and 700nm, where in the former case they even mark the maximum

current values. This might be explained by the different oxide thickness

outside the p-well zone that gives place at different maximum transmission

peaks. All the curves present a series of small peaks and valley due to

the geometry of the metal lines (emitter contacts) shown in the layout of

Fig. 4.17: the zone in the curve with minimum signal correspond to the

78


Al lines. On the other hand all the curves present a central maximum due

to the absence of finger implants in the central area of the device layout

(Fig. 4.17): in the BJT10 case, this area is 250µm wide and the peak in

the middle of the curve in Fig. 4.40 is about the same. That central part of

the photodetector is the most responsive, where the surface space charge

region is more extended.

79

Chapter 5

The E-O Nose System

At the end of the fabrication process, the photodetectors have been evap-

orated with different metalloporphyrins (with the method described in

Chapter 4, Section 4.1, Fig. 4.2), cut into single die and bonded to a black

plastic board with three lines of gold metal contacts and pads to attach

4 sensors per row, 12 per package (Fig. 5.1; see Appendix B for bonding

schematics and pin-out details, Fig. B.1 for photodiodes and Fig. B.2 for

phototransistors). Every photodiode has two contacts, p-well/anode and

n+-fingers cathode, while phototransistors need only one single contact

per die, the n+-fingers emitter, because the collector is contacted from the

backside of the chip and is common for all the sensors in the same row,

while the p-well base is left floating.

Figure 5.1: Picture of the assembled sen-

sors matrix after die and wire bonding.

81

CHAPTER 5. THE E-O NOSE SYSTEM

(a) (b)

Figure 5.2: Image of the package used as nose nostril to lodge the sensor matrix: (a) first

prototype; (b) second prototype, for a differential measurements configuration.

5.1 Package: the Nose Nostril

In order to assemble the nose nostril, an ”ad hoc” package has been built

to provide all the necessary features: transparent glass top cover to let

the blue light pass and two lateral holes for gas flowing (to attach the

small hoses). It measures 15mm×15mm×8.5mm. Fig. 5.2 shows the two

versions of the package: the second one (Fig.5.2(b)) differs from the first

(Fig. 5.2(a) and Fig. 4.6 in Chapter 4) for the position of the lateral hole,

which has been moved up, and for the presence of a wall to isolate the

sensors bottom line from gas flowing and take advantage of a differential

measurement configuration, in which one of a couple of sensors with the

same metalloporphyrin is not exposed to VOCs. This configuration is

extremely useful for chemical sensors to cancel baseline drift, sensing layer

aging and any environmental dependence (temperature, relative humidity,

etc...). Details on the geometry and all dimensions of the two packages can

be found in Appendix B.

82

5.2. THE NOSE BOX

(a) Top view with the LED in front of the packagedsensors matrix

(b) Side view with electrical wires coming out

Figure 5.3: Two images of the first chamber where the sensor matrix was placed, provided

with holes for electrical connections access and gas flow.

5.2 The Nose Box

For the first experimental measurements campaign, the assembled matrix

was lodged in a cylindrical PVC black chamber (Fig.5.3), containing just

the sensors package and the blue LED source, placed in front of it, while the

read-out circuit board was kept separated and connected to the chamber

via electrical wires.

This first set-up did not provide a box for the full system and conse-

quently a good SNR for the output voltage neither a good isolation from

environmental light. The first metal box, perfectly isolating the optical

sensors from environmental light and electronic noise (when connected to

ground) is shown in Fig. 6.1 in Chapter 6 [30]: it contained also the read-

out circuit board, it had two lateral holes for the hoses carrying the gas in

and out and it had a large hole at one side to couple to a monochromator

output, which replaced the LED source in some measurements campaigns.

The final optimized box, making up the complete E-ON, LED included, is

shown in Fig. 5.4 and is fully described in detail in the internal Technical

report [31]. Every board in this final flexible system has been optimized to

83


Figure 5.4: Picture of the top of the open nose box. The circuit board is on top left with

the three operational amplifiers and their respective RC feedback networks; the batteries

container (for op amps power supply) is on bottom left, close to the LED intensity selector,

which in turn is near the 3 outputs BNCs (vertical); the castle with the sensors and LED

board in on the right (only the backside of the LED board is visible) with the 2 plastic

tubes for gas flow coming in and out; on top of the picture, on the left the 3 connectors

for voltage biasing of the 3 sensors rows are visible, together with the 3 selectors (vertical)

of the sensor in each row.

be easy to mount and removable, especially the sensors and the LED source

boards, in order to quickly change the sensors matrix and proceed with the

testing of another set of sensors (coated with different metalloporphyrins)

or change the LED type (another wavelength emission, another brand,...)

(Fig. 5.6). These two boards are mounted on a third one, mechanically

fixed to the box, making up a so-called ”castle” unit (Fig. 5.5). The

LED mounted is a Pink Super Bright LED, with emission wavelength of

440nm; Candle Power: 1.1 (1100 mcd); Viewing Angle: 15 degrees (from

84

5.2. THE NOSE BOX

Figure 5.5: Schematic image of the castle: left, explosion of the three boards: the one

mechanically fixed to the metal box, from which all electrical connections to the sensors

matrix and the LED source start; middle, removable part of the castle, made up by the

sensors board and the LED board (Fig. 5.6); right, complete close castle.

LedsDirect.com).

5.2.1 Read-Out Circuit

The circuit for the sensor signal extraction is made up by three equal I-V

converters, one per sensors row in the matrix, allowing the real time mon-

itoring of one sensor per row. Each circuit reads out the photogenerated

current (photodiode reverse current or BJT emitter current) and converts

it into an output voltage by employing a low input bias current low noise

operational amplifier (AD 549JH). The feedback network is flexible and

variable according to the sensors to test: if photodiodes are employed,

the feedback network provides a transimpedence gain of 200MΩ and a low

pass filter for white noise reduction with 1.4Hz cut-off frequency; if photo-

transistors are employed, the output signal needs a smaller amplification,

thanks to the BJT internal gain, and the feedback network provides a

gain of 470KΩ and 1Hz cut-off frequency. Both the feedback networks are

mounted on the board and selected with a switch for each row. Fig. 5.7

85


Figure 5.6: Picture of the top part of the castle: the LED board is attached to the sensors

board through a series of connectors and screws, easily removable. The two pieces of hose

for gas flow are visible on the lateral sides of the sensors package, going in and out.

shows the PSPICE schematics of both the circuit configurations.

Fig. 5.8 presents the electrical scheme of a single signal extraction chan-

nel for the phototransistors case: the four BJTs have the collector in com-

mon, connected to the power supply (5V), the base floating and the emitter

is connected to the inverting input of the op amp when the switch is closed

(a manual selector per row performs the switching task). The op amp is

dually powered by two 9V batteries placed in a special container in the

nose metal box.

86

5.2. THE NOSE BOX

(a) Photodiodes sensors (b) Phototransistors sensors

Figure 5.7: Schematic of the circuit configuration for output signal extraction of (a) pho-

todiodes and (b) phototransistors sensors.

Figure 5.8: Electrical scheme of a single signal extraction channel. The 4 BJTs are placed

in the matrix package (Fig. 5.2(b)) on the sensors board (Fig. 5.1), while the I-V converter

is mounted on a separated circuit board (Fig. 5.4).

87

Chapter 6

Sensors Experimental Testing

Numerous experimental measurements to assess the sensors performance

have been conducted in the Electro-Optical Laboratory at the Dept. of

Information and Communications Tech. of the University of Trento. The

VOC used in the tests is Ethanol, because it is an easy to find, not danger-

ous and very volatile compound. This means that it does not bind easily

to the porphyrin molecules or to any other sensing layer, therefore it can

be considered as a challenging VOC in testing a prototype. It is poured in

a liquid phase in a bubbler and brought from its saturation concentration

in air to different decreasing concentrations by means of a 4-channels mass

flow controller, able to mix sature ethanol vapor to a neutral carrier, dry

air. Dry air is stored in a tank outside the lab building for security reasons

and a pipeline system carries it to the gas bench inside. All the experi-

mental set-up is shown in the picture of Fig. 6.1, where the first type of

metal box is presented and the light source set-up is made up by a white

lamp coupled with a manual monochromator (see Section 6.1).

The three sensors output voltages are monitored on the PC by the

NI 4350 high-precision voltage meter acquisition card and the software

Labview 6.1, which is the basis also for the flow meter remote control

application. Measurements are done in continuous mode (cc).

89

CHAPTER 6. SENSORS EXPERIMENTAL TESTING

Figure 6.1: Picture of the experimental set up: first version of the E-ON metal box, coupled

to a manual monochromator, which in turn is coupled to a white halogen lamp. The gas

bench is completely visible, with the 4-channels flow meter, the ethanol bubbler and the

hoses.

In order to better understand the ethanol concentrations referred to in

the following measurements results, the Antoine Equation is here recalled,

which allows to find the saturation pressure in air of any compound at a

certain temperature [32]:

lg(p/p0) = A−B/(T + C) (6.1)

where

p0=ambient pressure=1bar('1atm)

p=vapor or saturation pressure [bar]

T=temperature=293K

Antoine Coefficients for Ethanol calculated by NIST:

A=5.37229; B=1670.409; C=-40.191

90

Table 6.1: Conversion from percentages of Ethanol vapor in the flow to ppm.

percentage % ppm

100 58196

50 29098

40 23280

30 17460

20 11640

10 5820

5 2910

2 1164

1 582

0.5 291

Therefore

p=0.058196bar

p×106=58196ppm(@100% EtOH flow)

The conversion to ppm, parts per million, is important because ppm is the

standard unit of measure used for VOCs concentrations in sensors science.

In the experimental measurements performed in this thesis different con-

centrations have been used and Table 6.1 reports the value in ppm of the

corresponding ethanol percentages in the total flow (100sccm, square cubic

cm) that have been used in the experiments. In the rest of the text and in

the measurements plots, percentages will be used just for shortness, even

if ppm should be always taken into account.

The measurements results plotted in this chapter have been chosen among

the most representative and significant out of numerous experimental re-

sults collected in three years sensors testing. They are presented chronolog-

ically, starting with the first type of standard photodiodes used as sensors

till the latest E-ON system, based on phototransistors.

91


6.1 First campaign of Measurements

The first sensors employed have been the standard photodiodes already

available at ITC-Irst (Section 4.2, Chapter 4), spray-coated with simple

metalloporphyrins, with the molecular structure of Fig. 3.2, Chapter 3.

Performances of the sensors were very poor and the minimum detectable

response corresponded to an ethanol concentration of 67%, also due to the

employment of the first PVC chamber described in Section 5.2, Chapter 5

(Fig. 5.3).

The first improvement came from the chemistry of the sensing layer, whose

molecular structure was modified like described in Section 3.1.1, Chap-

ter 3. Fig. 6.2 and 6.3 show the first results obtained by using sensors

spray-coated with Co-T(hexadodecyloxy)PP and Zn-T(butyloxy)PP, and

a blue LED with an emission wavelength of 470nm, operated in a contin-

uous mode. Both the plots present seven measure cycles performed with

ethanol at decreasing concentrations: 100%, 80%, 60%, 50%, 40%, 20%,

10%. Even if they didn’t show a significant response in term of output

voltage variation for 10% ethanol, an extremely relevant improvement was

achieved.

The second optimization came from the blue light source, when another

set-up replaced the LED: a halogen white lamp HL-2000-FHSA (Avantes,

360-1700nm range, 17mW bulb output) coupled to a manual monochro-

mator, aligned in front of the sensors nostril (Fig. 6.1). Even if bulkier and

noisier than the simple small LED, such a set-up proved the importance of

adjusting the light source on the emission wavelength correspondent to the

maximum variation of metalloporphyrins transmission spectrum. Even a

few nm can make a difference, like shown by the results plotted in Fig. 6.4

for four wavelengths. The highest response signal is obtained at 426nm.

Fig. 6.5 shows two measurements cycles at 20% and 5% EtOH concentra-

92

6.1. FIRST CAMPAIGN OF MEASUREMENTS

Figure 6.2: Output voltage versus time of 7 measurement cycles at decreasing

EtOH concentrations with a standard photodiode-based sensor, spray-coated with Co-

T(hexadodecyloxy)PP.

Figure 6.3: Output voltage versus time of 7 measurement cycles at decreasing EtOH con-

centrations with a standard photodiode-based sensor, spray-coated with Zn-T(butyloxy)PP.

tions conducted with the same sensor previously used, spray-coated with

Zn-T(heptyloxy)PP, but here with a light source emission of 426nm. A

good response is obtained down to 5% EtOH.

The third important improvement came from metalloporphyrins deposi-

93


Figure 6.4: Four voltage output variations for the same sensor at 33% EtOH concentration

for four different emission wavelengths around the metalloporphyrin transmission peak.

tion method (Section 4.1, Chapter 4): the better results achieved with the

evaporated sensing layer (Fig. 6.6) proved that the porphyrins spectral be-

havior and in general their optical properties strictly depend on the state

of aggregation of the deposited film, on its uniformity and thickness.

94

6.1. FIRST CAMPAIGN OF MEASUREMENTS

Figure 6.5: Output voltage versus time of 2 measurement cycles at 20% and 5% EtOH

concentrations, by employing a photodiode detector spray-coated by Zn-T(heptyloxy)PP

and a light source with 426nm emission wavelength.

Figure 6.6: Output voltage versus time of 4 measurement cycles at 2.5%, 5%, 10%

and 20% EtOH concentrations by employing a photodiode detector evaporated by Zn-

T(heptyloxy)PP and a light source with 440nm emission wavelength.

95


6.2 Second campaign of Measurements

The introduction of the optimized fingers-photodetectors with enhanced

responsivity in the Soret band allowed for further important improvement:

the minimum detectable ethanol concentration achieved is 0.5% correspon-

dent to less than 300ppm. In addition to this, the adoption of the nose

metal box described in Section 5.2, Chapter 5 (Fig. 6.1 and 5.4) proved to

be fundamental in improving both the sensor response and the S/R ratio.

6.2.1 Finger photodiodes

Fig. 6.7 shows 4 measurements cycles at decreasing ethanol concentrations,

down to 0.5%, performed with the finger-photodiode N+PD10. A zero re-

verse bias has been applied in this case. Only the N+PDxx have been

experimentally tested with ethanol (not P+PDxx). Measurements at dif-

Figure 6.7: Output voltage versus time of 4 measurement cycles at 10%, 5%, 2%, 1%

and 0.5% EtOH concentrations by employing the finger-photodiode sensor evaporated by

Zn-T(heptyloxy)PP. The light source set-up employed is the white lamp+monochromator.

96

6.2. SECOND CAMPAIGN OF MEASUREMENTS

Figure 6.8: N+PD10 sensor response to increasing ethanol concentration for 3 different

reverse bias conditions.

ferent reverse bias voltages have been also conducted to find out the best

sensor working point. Fig. 6.8 reports the calculated N+PD10 sensor re-

sponse for three different reverse bias conditions, 0, 1 and 5V, and the

highest voltage output variations are observed for the unbiased sensor.

97


The previous measurements have been conducted by employing the

lamp+monochromator light source set-up, while the results plotted in

Fig. 6.9 and 6.10 have been measured by using the latest nose system

and the pink LED at 440nm. The nostril mounted on the sensors board

contained a row (bottom) of finger-photodiodes sensors coated with Co-

T(hexadodecyloxy)PP (”CoTPP” for shortness in the figures) and a row

(top) with Zn-T(heptyloxy)PP sensors (”ZnTPP” for shortness in the fig-

ures). The package used for this matrix is not the one with the isolation

wall for differential configuration measurements, but the useful differential

cancellation is allowed by the middle row containing uncoated photodiodes.

Fig. 6.9(a) reports two series of measurement cycles for both the sensors

affected by baseline drift, while Fig. 6.9(b) shows the same cycles after

applying the differential drift cancellation with the uncoated devices.

98


(a) Before differential cancellation

(b) After differential cancellation

Figure 6.9: 4 measurement cycles for N+PD10 sensor at 50%, 40%, 30% and 20% ethanol

concentration (a) before and (b) after applying the differential drift cancellation.

99


Figure 6.10: 4 measurement cycles for N+PD10 sensor at 10%, 5%, 2% and 1% ethanol

concentration, after applying the differential drift cancellation.

The low concentrations measurement cycles are shown in Fig. 6.10, al-

ready adjusted by the drift cancellation.

From these experimental results it is clear that the most responsive sens-

ing layer is Co-T(hexadodecyloxy)PP, as expected from its chemical struc-

ture, because the alkyl chains synthetized at its peripheral sites are longer

(n=17) than the chains in Zn-T(heptyloxy)PP (n=7), thus providing an im-

proved morphological layer and more resolved spectrum (Subsection 3.1.1,

Fig. 3.9, Chapter 3).

In the same chapter and subsection of the thesis a comparison among re-

sponse time for different alkyl chains lengths has been shown. Even if a

direct absolute comparison can’t be done between the response times shown

in Fig. 3.8 and the performance of the two metalloporphyrins used in these

experiments, due to the different thickness, deposition method (spray-

coating vs evaporation) and transduction mechanism (QMB vs photodetec-

tors), Co-T(hexadodecyloxy)PP proved to be faster than Zn-T(heptyloxy)PP,

100


Figure 6.11: Zoom of the voltage output increase on exposure to 20% EtOH concentration

(Fig. 6.9). CoTPP rise is faster than ZnTPP rise.

like expected from the plot of Fig. 3.8. The zoom of Fig. 6.11 shows that

the derivative of the CoTPP slope is higher (steeper) than the derivative

of ZnTPP. Response time for both sensors at 20% EtOH concentration has

been calculated in order to make a comparison with the response times in

Fig. 3.8 for spray-coated films deposited on Quartz Microbalance sensors

[26]. For uniformity, also here response time is considered as the time in-

terval necessary to achieve 90% of the complete signal transition.

tr(ZnTPP)=80s with tot transition=3.44mV, ∆Vout,90%=53.7mV

tr(CoTPP)=120s (n=17) with tot transition=9.56mV, ∆Vout,90%=47.9mV

These numbers, especially the latter, having the same coordinated metal,

can be compared with the response time in literature [26] and they result

much smaller: even taking the longest alkyl chain metalloporphyrin tested

by Di Natale, CoTPP-18 (n=18), the minimum response time is 400s. This

101


is a relevant result because it proves the success of optical transduction in

being faster (about 4 times decrease in response time) than QMB transduc-

tion, like expected from optical sensors, though taking into account that

the sensing layer here used has been evaporated, so the morphology and

thickness are different.

6.3 Third campaign of Measurements

The last measurements campaign was performed with the BJT-based sen-

sors and a final comparison between photodiodes and phototransistors per-

formance is presented at the end of this section.

6.3.1 Finger phototransistors

The first measurements with phototransistors sensors have been conducted

by employing the lamp+monochromator light source set-up. Fig. 6.12

shows 6 measurement cycles at decreasing ethanol concentration down to

0.5% (291ppm) by employing the sensor BJT10. Again the improvement

gained with the differential configuration is highlighted.

102

6.3. THIRD CAMPAIGN OF MEASUREMENTS

(a) Before differential cancellation

(b) After differential cancellation

Figure 6.12: 6 measurement cycles for BJT10 sensor at 20%, 10%, 5%, 2%, 1% and 0.5%

ethanol concentration (a) before and (b) after applying the differential drift cancellation.

103


Figure 6.13: 6 measurement cycles for BJT10 sensor at 20%, 10%, 5%, 2%, 1% and 0.5%

ethanol concentration, after 8 months from the first experiments (Fig. 6.12(b)).

The same measurement with the same sensor was repeated after 8

months and the results are plotted in Fig. 6.13. The sensor exhibits a de-

teriorated response and a smaller sensitivity at the lowest concentrations:

0.5% can’t be properly distinguished from 1%, but this could be also due

to the different total flow employed, 100sccm in this case and 200sccm in

the previous experiment. Since the mass flow controller accuracy is 0.5%,

a 100sccm flow is not very reliable at very low ethanol concentrations.

After this reproducibility experiment, Fig. 6.14 shows a repeatability

measurement conducted by repeating the flowing of the same ethanol con-

centration for three times successively with the same sensor, for 5 ethanol

concentrations (10%, 5%, 2%, 1% and 0.5%). Even if the measure is still

affected by baseline drift, the output signal variations for the same concen-

tration are constant for the three cycles.

The response curves of the two sensors N+PD10 and BJT10 have been

104


Figure 6.14: Repeatability test: 3 measurement cycles are repeated for the same EtOH

concentration, at 10%, 5%, 2%, 1% and 0.5%.

calculated and plotted together in Fig. 6.15: the phototransistor exhibits

higher response values and higher sensitivity at low concentrations.

Fig. 6.16 collects four measurements plots for all types of finger- pho-

totransistors, tested with the latest set-up and nose system (metal box

with incorporated pink LED). Sensors with different coatings are shown

because BJT10 coated with CoTPP resulted damaged and BJT20 coated

with CoTPP didn’t work properly and exhibited a very noisy response. For

these two sensors only the devices coated with Zn-T(heptyloxy)PP have

been taken into account. For BJT30 and BJT40 only measurements with

Co-T(hexadodecyloxy)PP coating are shown, but both the coatings have

105


Figure 6.15: Response curve for photodiode and phototransistor sensor: the latter exhibits

higher response and higher sensitivity at low concentrations (the line is steeper).

been tested and results with the response curves are reported in Fig. 6.17

and Fig. 6.18.

106


(a) (b)

(c) (d)

Figure 6.16: 4 measurement cycles for all types of phototransistor sensors at 10%, 5%, 2%

and 1% ethanol concentration, conducted with the latest nose box set-up. In these plotted

results BJT10 (a) and BJT20 (b) were coated with Zn-T(heptyloxy)PP; BJT30 (c) and

BJT40 (d) were coated with Co-T(hexadodecyloxy)PP.

107


Figure 6.17: Response curve for sensor N+PD10 and all phototransistor sensors, BJT10,

BJT20, BJT30 AND BJT40, with Zn-T(heptyloxy)PP coating.

The plotted response curves for all sensors coated with Zn-T(heptyloxy)PP,

included the photodiode N+PD10, refer to the latest measurements cam-

paign in which all the presented sensors were bounded in the same matrix

and mounted in the complete nose box. The comparison between N+PD10

and BJT10 is very important and reliable in this case because exactly the

same device has been tested in the two cases but with different bias condi-

tion and output signal probe. Again, like in Fig. 6.15, the most performing

sensor is the phototransistor based sensor with higher response and higher

sensitivity at low ethanol concentrations, even if a low S/N ratio is ob-

served with respect to the low noise photodiodes measurements.

Among the BJT sensors, again BJT10 exhibits the best response and sen-

sitivity, while the worst is BJT40. This result matches with the results

of the spectral responsivity measurements, in which the highest and the

lowest responsivity values are registered for BJT10 and BJT40 respectively

108

6.4. PARASITIC PORPHYRIN RESISTANCE

Figure 6.18: Response curve for the phototransistor sensors BJT30 AND BJT40, with

Co-T(hexadodecyloxy)PP coating.

(with both the small and large light beam set-up, Fig. 4.36 and Fig. 4.39,

Chapter 4).

6.4 Parasitic Porphyrin Resistance

This section is dedicated to the illustration and explanation of a par-

asitic phenomenon that has been observed in the results of all experi-

mental measurements with ethanol concentration higher than 20%. The

semiconductor-like behavior of porphyrins has been investigated in litera-

ture [33] and D’Amico and Di Natale [22] demonstrated porphyrins conduc-

tivity dependence on the presence of volatile compounds, such as amines

and organic acid and reducing gases, by performing tests at room tempera-

ture with films deposited by solvent casting onto interdigitated electrodes.

109


Also at high ethanol concentrations in air, the metalloporphyrin film cease

to be an insulator to become conductive. Since the film is deposited here

on the whole sensor die, contact pads included, like schematically shown in

Fig. 6.19, macroscopically a resistance in parallel to the p-n junction pho-

todiode and to the B/E junction in the phototransistor is observed. This

could be considered a second sensing mechanism in the same transducer

but in this work it is called parasitic because undesired and unexpected.

In fact the response generated by the conductivity variation is opposite in

sign to the optical response.

For ethanol concentrations of 50%, 40%, 30%, the observed parallel resis-

tance, even if still high, drives a current comparable to the output current

of the photodevices and makes up the most significant sensor response, be-

ing responsible for a variation in the response curve of the optical sensor.

In order to better explain this parasitic effect, Fig. 6.20(a) reports the

measurement cycles where it was observed for first time with the finger-

photodiode sensor. It occurs only when the photodiode is biased and re-

sults in Fig. 6.20(a) and Fig. 6.20(b) have been measured at 5V and 10V

reverse voltage respectively. Only in this biased case the current through

the parasitic resistor generates, adding up to the photocurrent and causing

the output signal to increase.

Two zooms of the plot in Fig. 6.20(a) are presented in Fig. 6.21 to show

the variation in the output voltage curve versus time. At the beginning the

signal response is only due to the optical mechanism: the output voltage

value decreases on increasing of the absorption peak amplitude of metallo-

porphyrin, when it starts to absorb ethanol molecules.

110


A A’A A’

(a) Layout

(b) AA’ cross section

Figure 6.19: (a) Layout of one of the finger photodetector; (b) schematic cross view of the

critical electrical path that gives place to the parasitic parallel resistor.

111


The second response mechanism, metalloporphyrin conductivity increase,

starts to manifest after a few seconds (Fig. 6.21(a)), with a sudden change

in the voltage curve: the first zoom proves that the strange signals that

look like noisy spikes in fact are not, but are the consequence of the sud-

den response variation. The voltage output increases slowly as long as the

conductivity of the sensing layer increases on further ethanol absorption.

Therefore this second sensor response is opposite to the optical response

and at first it compensates it but quickly becomes the dominant effect, thus

producing a rise in the signal, proportional to the reverse bias applied. Only

at 20% ethanol concentration the parasitic response starts to be overcome

by the optical response and when air flows to clean the nostril, the out-

put voltage doesn’t decrease like in the previous cycles, but it increases,

like expected from the recovery phase of the optical sensing mechanism

(Fig. 6.21(b)). The successive 6 cycles at 15%, 10%, 5%, 2%, 1% and 0.5%

are totally dominated by the optical response, being the absorbed ethanol

molecules not enough to allow a conductivity rise in metalloporphyrin, and

the output signal decreases on exposure to ethanol.

Fig. 6.22 reports dark measurements at 5V and 10V reverse bias: here

the only sensing mechanism is the conductivity increase because no optical

interaction can be observed. No spikes, no sudden changes in the voltage

curve are registered because no other competitive response is present in the

darkness set-up. For low ethanol concentrations, no significant response are

registered because of a lack of a good amount of ethanol molecules in air.

Only 50%, 40% and 30% responses are clearly visible while the 20% vari-

ation is very small, even if higher for 10V, proportionally to the reverse

bias.

112


(a) Vrev=5V

(b) Vrev=10V

Figure 6.20: Measurement cycles at a wide range of ethanol concentrations, from 50% till

0.5%, for different photodiode reverse bias voltage: (a) 5V, (b) 10V.

113


(a) Spike zoom

(b) 20% EtOH: the optical sensing mechanism appears

Figure 6.21: Different zooms of the plot in Fig. 6.20(a): (a) the ”spike” is in fact a

change in the response variation (from decreasing to increasing output voltage); (b) low

EtOH concentrations cycles: from 20% recovery phase, the optical sensing mechanism

becomes dominant and the parasitic resistor disappears.

114


Figure 6.22: Measurement cycles at the same EtOH concentrations employed in

Fig. 6.20(a) and Fig. 6.20(b) with the LED off in order to test only the conductivity

increase of metalloporphyrin.

More investigations have been conducted lately with the phototransis-

tor sensors and the pink LED set-up, focusing on BJT40 only, with both

coatings and with measurements performed at two different temperatures,

20C and 10C. The latter have also proved that the observed parasitic re-

sistor in parallel is not due to the ethanol itself, which could condense on

the device when flowing.

Fig. 6.23 and Fig. 6.24 show the variation of the output voltage on exposure

to 40% ethanol concentration: the parasitic effect is dominant.

115


(a) CoTPP

(b) ZnTPP

Figure 6.23: Parasitic response to 40% EtOH concentration of the sensor BJT40 coated

with (a) Co-T(hexadodecyloxy)PP and (b) Zn-T(heptyloxy)PP, at 20C.

116


(a) CoTPP

(b) ZnTPP

Figure 6.24: Parasitic response to 40% EtOH concentration of the sensor BJT40 coated

with (a) Co-T(hexadodecyloxy)PP and (b) Zn-T(heptyloxy)PP, at 10C.

117


Voltage increase rate, proportional to the conductivity increase rate, has

been calculated for both the sensors at both operating temperatures:

@ 20C: CoTPP: 7.7µV/s, ZnTPP: 7.2µV/s

@ 10C: CoTPP: 3.9µV/s, ZnTPP: 5.3µV/s

These rates show that metalloporphyrin conductivity depends on temper-

ature and on the alkyl chain length. It decreases if temperature decreases,

exhibiting a NTC (Negative Temperature Coefficient) character, accord-

ing to the porphyrin semiconductor behavior, described in literature [33].

While at room temperature the rates of the two sensing layers are com-

parable, at 10C, Co-T(hexadodecyloxy)PP conductivity increases slowly

with respect to Zn-T(heptyloxy)PP: this could be explained by consider-

ing the longer alkyl chains at the peripheral sites of the aromatic ring.

Longer chains means farther molecules (more porous film) and more dif-

ficult interactions among them. Further investigations on this interesting

dependence will be conducted in the future in order to better understand

how the alkyl chain affects the porphyrin conductivity and how it can im-

prove its insulator character when needed.

The described phenomenon is not considered an issue for the E-ON system

because it appears only at high VOC concentrations, in a range far from

the usual operating range of the sensors. Moreover it is possible in the fu-

ture to selectively evaporate metalloporphyrins only on the photodetector

active area and shadowing the aluminum contacts by employing a metal

mask.

118

Chapter 7

Conclusion

The research work reported and summarized in this dissertation thesis con-

tributed to the field of Electronic Nose for artificial olfaction applications,

which nowadays are gaining more and more importance due to the employ-

ment of electronic noses in medical diagnosis, environmental and industrial

process control and standardization, quality of food assessment.

Europe is leader in electronic nose research and this work has been devel-

oped in collaboration with the University of Rome Tor Vergata, where the

LIBRA nose was born almost 10 years ago and a long time work on gas

sensors matrix and metalloporphyrins sensing films has been done. The

intent of this thesis was to develop the sensors system of a new Electro-

Optical Nose, smaller, lighter, faster, less power consuming and cheaper

than the existing electronic noses.

Since metalloporphyrins had proved to work particularly well as sensing

films for VOCs (Volatile Organic Compounds) in gas sensors array, they

have been chosen as responsible of the first chemical interaction with gas

molecules, and silicon integrated photodetectors, photodiodes at the begin-

ning and phototransistors later, have been employed as signal transducers.

For first time in literature metalloporphyrins have been directly deposited

on the active area of the photodevices and served as blue light modulation

119

CHAPTER 7. CONCLUSION

filter, due to the change in their spectral absorption peak in the Soret band

(440nm) on exposure to VOCs.

Modified metalloporphyrins with alkyl chains in the peripheral sites of their

molecular structure have been evaporated on the sensors and they proved

to be responsible for a high improvement of the sensors response, provid-

ing a more porous layer with a more resolved spectral absorption peak.

Sensors response became also faster and together with the evaporation de-

position, which provided more uniform and controllable films, an ethanol

concentration in air of 1455ppm (parts per million) could be detected in

the first experiments (ethanol saturation vapor in air=58196ppm).

The most innovative solution proposed and studied in this thesis is the

adoption of silicon photodetectors with enhanced responsivity in the blue

spectral range. A fingers-shaped n+-p junction has been designed in order

to move the depletion region very close to the light incident surface to effi-

ciently collect the blue photons, with small penetration in the Si substrate

(100-200nm). Depletion regions around neighboring fingers merge together

by providing a continuous surface space charge region. Two types of pho-

todetectors have been fabricated and electrically and optically tested: 5µm

wide n+-finger implants in a p-well in the undoped n−-Si substrate and 5µm

p+-finger implants in the n−-Si substrate. Both the structures proved suc-

cessful in improving spectral responsivity around 440nm with a series of

peaks in the blue region, with slightly increasing amplitude for increas-

ing interfinger distance (10, 20, 30, 40µm). The maximum responsivity

value for the finger-photodiodes with 5V reverse bias applied is 0.24A/W

at 420nm, with an increment factor of 1.7 over the responsivity value of

the fabricated reference device (0.15A/W), without taking advantage of

any ARC (Anti Reflective Coating) optimization.

npn BJT phototransistors have been also developed with the same pho-

todevice structure by using the n+-fingers as the emitter, the p-well as the

120

base and the n−-substrate as the collector (contacted from the backside).

Such a simple upgrade of the original device brought some important ad-

vantages, like a higher spectral responsivity in the blue range and the beta

current gain which internally amplifies the photocurrent generated in the

floating base.

Electrical testing of the BJTs demonstrated higher current gains for in-

creasing interfinger distance, with a maximum of 158 for BJT40, more

than double with respect to 70 of the standard BJT.

The finger-implants significantly affect the spectral responsivity of both

photodiodes and phototransistors and the base current of the latter.

For BJTs an important contribution of the p-well perimeter has been

demonstrated in the collection of blue photons: the depletion region of the

lateral B/C junction contributes to the surface collection and it widens with

higher applied collector voltage, thus increasing the spectral responsivity.

This effect is registered only for the phototransistors and in the future a

new design for the p-well geometry will be implemented: a fingers-shaped

perimeter. In fact the fingers-E/B junction, if not reversely biased like in

the photodiodes case, doesn’t contribute much to the blue responsivity in-

crease, while implanting a p-well with fingers all around its perimeter will

provide an extended surface depletion region all around the B/C lateral

junction.

As regards sensing performance, the employment of the finger-photodiodes

and phototransistors improves the response till a minimum detectable sig-

nal of 291ppm. BJT-based sensors proved to be more performing than

photodiodes sensors, even if with a low S/N ratio, with higher response

and higher sensitivity to ethanol.

Repeatability and reproducibility have been experimentally tested during

8 months with a small deterioration of the sensor response in time, proba-

bly due to the aging of the sensing layer.

121

CHAPTER 7. CONCLUSION

Investigations of the role of metalloporphyrin conductivity variation with

ethanol have been also conducted as soon as a second sensing mecha-

nism appeared in the measurements results for high ethanol concentrations

(29000-12000ppm). It was demonstrated that metalloporphyrin conduc-

tivity is dependent on temperature and on the length of the alkyl chains

inserted at its peripheral groups, opening a new research on porphyrin con-

ductivity modulation and control through alkyl chains synthesis.

The final system developed in this dissertation is not yet a real electro-

optical nose, it is a prototype which proved the success of this optical

transduction approach and served to optimize the matching between the

sensing layer maximum response to gases and the photodetctor transduc-

tion.

Future work must concentrate on the experimental testing with a broader

range of VOCs and metalloporphyrins deposited, with more care on envi-

ronmental parameters dependence (e.g. relative humidity response).

All the data analysis part is still missing and it is going to be taken in

charge by the group of prof. Di Natale at the University of Tor Vergata.

The system box is not completed yet because the circuit board should con-

tain a multiplexer to allow the real-time extraction of all the sensors signals

in the matrix, and not only one per row. Moreover, in order to avoid the

mass flow controller, it will be necessary to mount valves and pumps in

the nose box itself. The same system adopted for the LIBRA nose can be

used for this.

This thesis work proved to be successful in providing a miniaturized nos-

tril completely optimized for VOCs sensing with metalloporphyrins and

in addition, interesting phototransistors with high spectral responsivity in

the blue range, which can be employed in different application fields, such

as new optical storage systems (DVD-ROMs and DVRs), which require

shorter wavelength laser diodes and read-out sensors with good sensitivity

122

in the UV/blue spectrum.

123

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128

Appendix A

Photodetectors Layout

The layout of the fingers photodetectors is made up by 5 mask layers:

1. p-well implant litho

2. p+ohmic contacts to p-well litho

3. n+-fingers implant litho

4. metal litho

5. contact holes litho

The layout images of all types of photodetectors designed are reported in

Fig. A.1 and Fig. 4.17, Chapter 4. For further details on the layout and

dies labeling on wafer, see the internal Technical Report [27].

129

APPENDIX A. PHOTODETECTORS LAYOUT

(a) PDstand (b) PD10

(c) PD30 (d) PD40

Figure A.1: Images of the layout of four photodiodes. PD20 layout is shown in Fig. 4.17.

130

Appendix B

Nostril Packaging & Bonding

For complete description of the E-ON box, pin out of all the boards and

schematics, see the internal Technical Reports [30] and [31], from which

the pictures below have been taken.

B.1 Sensors Board

Fig. B.1 represents the schematics of the die and wire bonding of the pho-

todiodes N+PDxx, 4 for each row. The sensors in the same row have the

p-well anode (red square contact) in common (only one sensor per row at a

time is monitored), short-circuited by a bridging wire, and the n-substrate

left floating. The p-well is connected to ground or the voltage generator

(negative reverse bias voltage), while the n+-fingers cathode (black square

contact) of each sensor is separately connected to the inverting input of

the op amp.

Fig. B.2 represents the schematics of the die and wire bonding of the

phototransistors: PTxx stands for BJTxx, the top row contains the 4 types

of finger sensors coated with Co-T(hexadodecyloxy)PP, the bottom row 4

finger sensors coated with Zn-T(heptyloxy)PP, the middle row contains 4

uncoated sensors for reference. The red square contact on each die rep-

resents the p-well/base contact, while the black square represents the n+-

131

APPENDIX B. NOSTRIL PACKAGING & BONDING

Figure B.1: Schematic top view of the photodiodes sensors matrix board with the wall

isolating the bottom row for differential measurements.

fingers/emitter contact. All the BJT bases are left floating, while the col-

lector is common for all sensors in the same row and is contacted from the

substrate backside. On the right of the figure, the sensors board backside

shows the bonding pads for soldering the wires to the circuit board. The

emitter is connected to the inverting input of the op amp and the collector

to the voltage generator (5V).

132

B.1. SENSORS BOARD

Figure B.2: Schematic top (left) and backside (right) view of the phototransistors sensors

matrix board.

(a) (b)

Figure B.3: Schematic design of the matrix package: (a) first prototype; (b) second pro-

totype, for a differential measurements configuration.

133

APPENDIX B. NOSTRIL PACKAGING & BONDING

B.2 Sensors package

Fig. B.3 and Fig. B.4 show the technical designs of the two nostril packages

employed during the thesis work.

(a) (b)

Figure B.4: Measures of the matrix package: (a) first prototype; (b) second prototype, for

a differential measurements configuration.

134

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