Journal of Non-Crystalline Solids 338–340 (2004) 744–748

www.elsevier.com/locate/jnoncrysol

IR bolometers based on amorphous silicon germanium alloys

M. Garc�ıa *, R. Ambrosio, A. Torres, A. Kosarev

Instituto Nacional de Astrof�ısica, �Optica y Electr�onica, INAOE, P.O. Box 51&216, Puebla Z.P. 72000, M�exico

Abstract

In this work, we report the fabrication and characterization of bolometers based on amorphous silicon germanium alloys (a-

Si1� xGex:H,F). The fabrication of microbolometers with thermally isolated a-SiGe thin film as sensing layer, is described. Mem-

brane-supported and bridge-supported detectors have been fabricated. Sensing layer is deposited by low frequency plasma enhanced

chemical vapor deposition technique. Since this deposition is carried out at relative low temperature TD ¼ 573 K, the read-out IC

fabricated on silicon substrate is not affected, providing compatibility with silicon IC technologies. Responsivity and detectivity were

measured under illumination at different bias voltages. The thermal time constant, the thermal conductance and current noise are

determined by means of the voltage–time curves. Responsivity and detectivity are observed to depend on the physical properties of

the materials forming the device and on the device geometry. Obtained results of pixel resistance, thermal resistance and responsivity

demonstrate a-SiGe:H,F like a material suitable for realizing high-performance microbolometers.

� 2004 Elsevier B.V. All rights reserved.

PACS: 73.61.Jc; 85.60.)q; 85.60.Dw; 85.85.+j

1. Introduction

The use of micromachining techniques has greatly

increased the detectivity of thermal infrared (IR)

detectors like bolometers. These techniques enable the

thermal insulation of the device to be increased, maxi-

mizing the temperature increase due to the absorption of

IR radiation. Bolometers present several advantages

over semiconductor diodes, such as uncooled operationclose to ambient temperature and providing cheap

detector technologies in compact systems.

Resistive bolometers are formed by a temperature-

dependent resistor and an IR absorber. The resistor

should have a large temperature coefficient of resistance,

which is defined as TCR ¼ ð1=RÞðdR=dT Þ, so a small

temperature increase results in a significant change in

the bolometer resistance. In order to increase sensitivity,a good thermal insulation of the detector is needed.

Membrane-supported detector and bridge-supported

detectors have been developed in order to get such

thermal insulation. Typical membrane material are sili-

con nitride or silicon oxide. Metals like platinum and

* Corresponding author.

E-mail address: luz@inaoep.mx (M. Garc�ıa).

0022-3093/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2004.03.082

nickel, semiconductors like vanadium oxide, supercon-ductors and non-crystalline material have been used as

sensing element. However metals present a very low

TCR, vanadium oxide is not a standard material in IC

technology and superconductors need special tempera-

ture conditions.

For room temperature operation, amorphous silicon

(a-Si:H) has been used in commercial applications [1–3],

however it presents a very high resistivity. In order toreduced it, boron doping is used but this results in lower

activation energy (Ea) and consequently in lower TCR

[4]. Previous studies of a-Si1�xGex:H,F (x ¼ 0; . . . ; 1)thick films have demonstrated this material as a good

candidate as the active material of uncooled micro-

bolometer, due to its relatively high activation energy,

Ea ¼ 0:4 eV, and therefore, a high TCR and mod-

erate resistivity at room temperature, rRT ¼ 2:5 · 10�3

X�1 cm�1 [5,6,9]. Additionally, the complete process of

fabrication of a bolometer using these films is performed

at low temperature (T ¼ 573 K). This temperature is

compatible with standard processes of silicon IC fabri-

cation, which makes this process an excellent candidate

for the fabrication of 2-D arrays of microbolometers.

In this work, the process of fabrication of membrane-

supported and bridge-supported microbolometers using

M. Garc�ıa et al. / Journal of Non-Crystalline Solids 338–340 (2004) 744–748 745

a-Si1�xGex:H,F (x ¼ 0; . . . ; 1) as the sensing material isdescribed. Characterization of the fabricated structure is

presented and the measured characteristics are com-

pared with other recent published results.

Fig. 1. Diagrams of the membrane-supported detector structure: (a)

Back-side view through SiO2 membrane on a Si substrate. There are

two structures of different size on the diaphragm. (b) Top-view of an

a-Ge:H,F microbolometer on the membrane. (c) Cross-section of the

membrane-supported microbolometer.

Fig. 2. Diagrams of the bridge-supported detector structure: (a) Top-

view of an a-SiGe bridge-supported microbolometer. (b) Cross-section

of the microbolometer.

2. Fabrication process description

Two different configurations for thermal isolationwere used in fabricated devices: membrane-supported

structure and bridge-supported structure. The fabrica-

tion process of both structures is briefly summarized in

the following. For the membrane-supported structure,

the initial substrate is a p-type crystalline silicon wafer,

in which a 1 lm-thick SiO2 layer was thermally grown.

Later, a 0.1 lm-thick a-Si3N4 layer was deposited on the

polished side of substrate by mean of low-pressurechemical vapor deposition (LPCVD). A mask is used to

define the diaphragm by removing the c-silicon by wet

chemical etching in a bulk-micromachining process.

Once the diaphragm is formed, aluminum (Al) is e-beam

deposited and etched for patterning the connection lines,

pads and contacts for the sensing layer. On top of this,

the active element material deposition is performed. The

sensing layer itself is made by depositing 0.1 lm of a-Ge:H,F by low frequency (LF) PECVD at temperature

of TD ¼ 573 K, a pressure PD ¼ 0:6 Torr, a power

W ¼ 350 W. A mixture of GeF4 and H2 was used for the

deposition of a-Ge:H,F. After this, a 0.19 lm thick a-

Si3N4 layer was deposited by LF-PECVD on top of the

sensing layer. Finally, the sensing and the a-Si3N4 layers

were etched to define the active area of the device. Fig.

1(a) and (b) show the top and bottom view of the fab-ricated diaphragm-supported structure and the cross

section is depicted in Fig. 1(c).

The fabrication process of bridge-supported detector

consists of depositing the active and supporting layer

onto a sacrificial layer, which is etched away to form a

bridge. First, a 1 lm-thick SiO2 layer was deposited on

the p-type substrate. Deposition of an Al sacrificial layer

of 2.5 lm thick is performed. A lithographic step todefine the bumps which will define the bridge dimension

is made. Later, a deposition of a 0.7 lm-thick layer of a-

Si3N4:H by PECVD is made, this will be the mechanical

support and will provide thermal isolation to the sensing

layer. Reactive ion etching (RIE) was performed on a-

Si3N4:H for patterning the bridge and the sacrificial

layer was removed by wet etching. At this point the

bridge structure is formed. Active layer was formed bydeposition of a 0.6 lm-thick layer of a-Si1� xGexH,F

(x ¼ 0:9) by LF-PECVD at temperature TD ¼ 573 K, at

pressure PD ¼ 0:6 Torr, and a frequency fD ¼ 110 kHz.

Deposition was made from a mixture of SiH4 +GeH4.

The activation energy of sensing layer is Ea ¼ 0:29 eV,

and the electrical conductivity is rRT ¼ 2 · 10�3

X�1 cm�1. A 0.13 lm-thick a-Si3N4:H layer was depos-

ited by LF-PECVD on top of the sensing layer. Etching

the a-SiGe:H,F/Si3N4 by RIE was the following step

to define the active area of the sensor. The next step was

the opening of contacts through the a-Si3N4 layer by

RIE. Finally Al was e-beam deposited and patterned on

the active area of the sensor forming the electrical con-

tacts to the sensor. The top view of a device is shown inFig. 2(a). The active area of the sensing layers was

100 · 100 lm2, as shown in Fig. 2(a). A sketch of the

fabricated bridge structure is shown in Fig. 2(b).

746 M. Garc�ıa et al. / Journal of Non-Crystalline Solids 338–340 (2004) 744–748

3. Characterization of the bolometers

Among the figures of merit which define the perfor-

mance of bolometers are the responsivity and the de-

tectivity. The responsivity is defined as the signal

generated per unit incident power. For the case of bo-

lometers, the generated signal is an electrical signal,

voltage or current, and the incident power (Pinc) was

taken as the input signal. The output signal, when deviceis voltage-biased, was obtained from the difference in

current produced by the incident power on the device:

Iout ¼ Iir � Idark; ð1Þand responsivity is expressed as

RI ¼IoutPinc

: ð2Þ

In this way, responsivity of four devices of both struc-

tures were calculated. The values of responsivity of the

devices at different voltages are presented in Fig. 3.

Thermal behavior of a bolometer is characterized by

its thermal conductance (G) and its thermal capacitance

(C), and both parameters determine the response time

through the thermal time constant (s) of the bolometer,which is defined as the following relationship:

s ¼ CG: ð3Þ

Measurements of voltage–time curves for determining

these thermal characteristics were performed in vacuum

at pressure Pm ¼ 3 mTorr. Thermal time constant was

measured by applying a current pulse to the bolometer

0.1

1

10

Res

pons

ivity

R (

mA

/ W

)

8765432Voltage V ( V )

7654321

Noise I noise ( pA

)

Responsivity Inoise

12

46

102

46

100

Res

pons

ivity

R (

mA

/ W

)

20151050Voltage V ( V )

70605040302010

Noise I noise ( pA

)

ResponsivityI noise

(a)

(b)

Fig. 3. Responsivity of several devices in function of applied voltage.

The IR incident power Pinc ¼ 107 nW from a black body radiator

at T ¼ 1123 K. (a) Responsivity of membrane-supported detector.

(b) Responsivity of bridge-supported detector.

and measuring the voltage drop using a oscilloscope anddetermining the time to reach the steady state. The smeasured for the membrane-supported bolometer was

500–800 ms, and that one for the bridge-supported

structure was 300–500 ms.

The thermal conductance was measured by applying

a current pulse. The voltage drop on the bolometer (Vb)and the current (Ib) flowing through it were measured. A

power W ¼ IbVb is dissipated in the bolometer. For abias pulse of duration much longer than the s, the steadystate is reached, and then W and G are related by

W ¼ GðTb � T0Þ; ð4Þwhere Tb is the bolometer temperature at the steady state

and T0 is the room temperature. Tb is estimated from [7]

Tb ¼T0

1þ kT0Ea

ln RðTbÞRðT0Þ

h i : ð5Þ

The resistance of the bolometer RðT0Þ is calculated as

Vb=Ib, when the bias pulse is very narrow, in such a waythat the structure is not heated, and RðTbÞ is calculatedin the same way when steady state is reached. G was

estimated from equation (4). The thermal conductance

measured in this way was G ¼ 2 · 10�7 W/K for the

membrane-supported structure and G ¼ 5 · 10�7 W/K

for the bridge-supported structure. Since equation (3),

thermal capacitance can be estimated from calculated

values of s and G. Then, for the membrane-supported

106

2

3

456

107

Det

ectiv

ity D

* ( c

m H

z1 / 2

/ W

)

8765432

104

2

4

68

105

Det

ectiv

ity D

* ( cm

Hz1

/ 2 /

W )

20151050

Voltage V ( V )

Voltage V ( V )(a)

(b)

Fig. 4. Detectivity as a function of applied voltage for several a-SiGe

bolometers: (a) Detectivity of membrane-supported detector. (b) De-

tectivity of bridge-supported detector.

Table 1

Comparison of uncooled microbolometers

Characteristics Units Syllaios et al. [1] Iborra et al. [9] Liang et al. [10] Present work

Membrane-

supported

structure

Bridge-supported

structure

Temperature-sensing

material

a-Si:H,B GexSi1� xOy Poly-Si–Ge a-Si1�xGex:H,F

x ¼ 1 (a-Ge)

a-Si1�xGex:H

x ¼ 0:9

Activation energy, Ea eV 0.22 0.185–0.316 )0.145 0.4 0.29

TCR, a K�1 0.028 0.024–0.04 0.014–0.022 0.051 0.037

Pixel area, Ab lm2 48 · 48 100· 100 40 · 40 60 · 60 100· 100Pixel resistance, Rb X 3 · 107 (1.2–6.4)· 105 3.5 · 105 109 3· 107Thermal resistance KW�1 4 · 107 5 · 105 – 5 · 106 2· 106Thermal time constant, sth ms 11 1.8–2 16.6 100–800 300–500

Responsivity, RV VW�1 106 50–380 (at 100

nA bias)

1.6 · 104 4.2 · 106 1.9· 106 at

I ¼ 550 nA

Responsivity, RI AW�1 – – – 6.2 · 10�3 8.6· 10�2 at

V ¼ 7 V

Spectral response lm 5–14 – – 2–14 2–14

Detectivity, D� cmHz1=2 W�1 – 4.8 · 1075.6 · 106

8.3 · 108 2.6 · 106 1· 105

M. Garc�ıa et al. / Journal of Non-Crystalline Solids 338–340 (2004) 744–748 747

structure C ¼ 1:2 · 10�7 J/K, and for the bridge-sup-

ported structure C ¼ 2 · 10�7 J/K.

Detectivity relates several parameters of detectors in

order to compare performances of different devices, and

it is defined as

D� ¼ R� ffiffiffiffiffiAd

p

Inoise=ffiffiffiffiffiffiDf

p ; ð6Þ

where R is the detector responsivity, Ad is the detector

area (60 · 60 lm2 for membrane-supported structure

and 100 · 100 lm2 for the bridge-supported structure),

Inoise is the noise current, and Df represents the band-

width frequency. The calculated detectivity, from exper-imental responsivity measurements for each one of the

structures at different biasing voltage, is presented in

Fig. 4.

The noise current depends on the voltage bias and

has values ranging from 1–5 pA for a voltage range

from 2 to 8 V for the membrane-supported. Noise

current of the bridge-supported structures is one order

magnitude higher than that of the membrane-supportedstructure. The obtained characteristics are compared

with those presented by Syllaos et al. [1], Iborra et al.

[7] and Liang et al. [8], the comparison is presented in

Table 1.

Pixel resistance of membrane-supported detector has

not been optimized, but its TCR is the highest of the

compared devices. Higher conductivity is observed when

doping the a-Si:H sensitive layer, but it also yields alower TCR. Our a-Si1�xGex bridge-supported structure

overcomes this disadvantage, it presents the same pixel

resistance as in [1], and a high TCR since doping is not

used. Additionally, the obtained thermal resistance of

both structures is comparable to the one reported in

other papers. Detectivity of studied microbolometers is

lower than the reported values, this is due to the high

noise of our structure.

At this point of the work we have shown that the

sensing material here proposed will offer a better alter-

native for the fabrication of 2-D monolithic arrays for

IR image detection than that used for comparison in this

work. A future work is the reduction of the noise and

resistance of the pixel by improving the quality of theohmic contacts to our materials.

4. Conclusions

Microbolometers with thermo-sensing layer made of

a-SiGe-based films have been fabricated in two different

configurations: the membrane-supported structure andthe bridge-supported structure. Activation energy and

conductivity of sensing layer provide high TCR. The

process of fabrication is fully compatible with the IC

standard process of fabrication. Performance charac-

teristics of the fabricated devices have been studied and

present values comparable with those reported for sim-

ilar materials, which present a-SiGe based films as a

good candidates for IR uncooled microbolometers.Work in order to reduce noise and improve thermal

isolation is needed.

Acknowledgements

The authors would like to thank Mr Mauro Landa

for his help during the technical preparation of the

samples. M.G. also acknowledges CONACYT for sup-

port granted through scholarship #128942.

748 M. Garc�ıa et al. / Journal of Non-Crystalline Solids 338–340 (2004) 744–748

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