1.chemical sensors 2.chemical actuators 3.bioelectric ...ece434/winter2008/434_5.pdfexample: isfet...

(C) Andrei Sazonov 2005, 2006 1

Chemical and Biochemical Microsystems

1.Chemical Sensors

2.Chemical Actuators

3.Bioelectric Devices

4.Example: Electronic Nose


Generally, chemical microsystems are used to interact with and measure composition and/or concentration of reagents in the ambient.

Application areas:- environmental monitoring, e.g., air pollution or water quality control;- security systems, e.g., combustible gas analyzers, explosives detectors;- pharmaceutical industry, i.e., drug discovery, drug delivery (immunoassays);- medical diagnostics, i.e., blood analysis, HIV and hepatitis testing, DNA tests (Alzheimer’s, heart failure, stroke, sepsis);- food industry, i.e., quality monitoring (pH meters);- medical implants (Cochlea implants, neural probes).

Issues:

1) Direct exposure to the environment – by definition, there always is a part of chemical microsystem exposed to the environment. It causes signal drift and enhanced noise;

2) Selectivity (any kind of ions can be adsorbed whereas we usually need sensitivity to one species only);

3) Chemical stability and biocompatibility (if chemical reactions occur, sensor corrodes whereas multiple sensing operations over long time would be preferred in most cases).


Example: ISFET (Ion Sensitive Field Effect Transistor).

Applications: - pH meters (water quality testing, acidity testing for the food industry – milk, cheese, juices, soft drinks, wine, …).

Principle: - the channel area of a FET is exposed to the ambient (solution).The concentration of H+ ions in the solution changes the FET drain current at fixed Vds. The feedback adjusts Vgs to keep the Id constant. Hence Vgs is proportional to [H+] and therefore to pH.

Fabrication: - surface micromachining (c-Si or glass).

p-Si substrateS D

SiO2

passivation

Vds

Vgssuspendedgate

ISFETreference

FET

gate


Chemical sensors

Passive (chemiresistors, chemicapacitors)

Work function based

(ISFET, CHEMFET)

Electrochemical (pH-meters)

Acoustic wave based (SAW)

Biosensors


Chemical sensors.

Sensing mechanism:

Sensor Signal

Environment

parameter


Passive sensors.

1. Chemiresistor.

Resistance of the sensitive layer between two electrodes is modified depending on the concentration of the analyzed chemical in the environment. To obtain high sensitivity, the electrodes are interdigitated.

Example 1: NH3 and NO2 sensor. Au electrodes (50 interdigitated pairs) 25 μm wide, 7.25 mm overlap length, 25 μm interelectrode gap. Deposited on top of SiO2coated c-Si substrate. Coated with 45 layers of polymer (phtalocyanine), each layer 2.5 nm thick.

Sensitivity: 0.5-2 ppm.Response time: 1 min.

Substrate (c-Si, glass)

SiO2

Chemically sensitive layer (polymer)

Metal (Au)


Metal electrodes should form ohmic contacts with selective layer. Gold is the best option.

Signal: can be DC or AC. In AC mode (about 1 kHz), signal-to-noise ratio can be improved. The signal drift can be eliminated by using passivated reference resistor.

Applications: gas sensors (NH3, NO2, Hg vapor).

Problems: poor selectivity, non-linearity, long response time.


SnO2 gas sensor detects CO and H2.

The concentration range detectable: > 1000 ppm in air.Respective resistance ratio (Rs/R0): < 1 (air level ~ 5).

Sensor is non-selective.

Si membrane

SiO2

Poly-Si heater

SnO2 Metal contacts

PCO, kPa

ΔG, Ohm-1

105

104

102 103 104

Example 2: Metal-oxide gas sensor.

Gases adsorbed on the surface of conductive metal oxides (SnO2, ZnO, TiO2) change the resistance.

Generally, adsorbed oxygen atoms trap electrons reducing the resistance:O2 + 2e- → 2O-.

Combustible gases react with oxygen to form H2O and release electrons; resistance decreases:

H2 + O- → H2O + e-,2H + O- → H2O + e-,CO + O- → CO2 + e-.

Thus, the change in the resistance depends on the change in the concentration of oxygen (CO) or combustible gases.


2. Chemicapacitor.

Dielectric constant of the sensitive layer varies with analyte concentration.Design may be similar to that for chemiresistor – interdigitated electrodes, polymer

layer on top, insulating substrate.

The response is non-linear.

Applications: humidity sensing, CO, CO2, CH4.

Example: integrated humidity/temperature sensor.

C, pF

Pp, kPa

100 Hz

1 kHz

10 kHz100 kHz

0.1

0.01

0.05

1 2

c-Si substrate, p-n junction diode as temperature sensor, metal capacitor with spin coated polymer dielectric, which absorbs the moisture.

The signal is AC (1kHz range). The response time is 1-2 s.


3. Work function based sensors.

The work functions at the interfaces of metal-insulator-semiconductor structure can be modified.

ADFET.

FET with very thin gate oxide (< 5 nm) and air gap. Gas molecules adsorb on the oxide surface and modulate drain current.

Drawbacks: - no selectivity;- high noise level;- poor stability (SiO2 degrades);- properties drift due to native oxide growth.

Air gap and suspended gate design improves noise and selectivity (sensitive layer can be attached to the gate).

p-Si substrate

S DG

SiO2

sensitive layer

optional


Pd gate MOS.Hydrogen adsorbs well onto Pd surface, decomposes:

H2 → 2H,diffuses through Pd and adsorbs on Pd/SiO2 interface.

p-Si substrate

S D

SiO2

Pd gate

+

- - - - -

EcEFEv

H

Pd SiO2 p-Si

This decreases flat-band voltage and thus reduces VT.

Application: hydrogen sensors.

mVVVT 6006.010*85.8

10*2*10*67.1*10*5*10*6.112

519819−=−=

−=Δ −

−−−

ΔVT – threshold voltage shift;μ – dipole moment of interfacial hydrogen

(μ = qd, d – oxide thickness);N – surface density of adsorption sites (for

Pd, N = 1.67x1019 m-2);θ – fraction of surface sites covered (0…1).

Example: calculate ΔVT for 25μm x 500μm MOS with 50nm oxide and 10 ppm H2concentration.

Solution:

0εθμNVT

−=Δ


Ion sensitive MOS – ISFET and CHEMFET.In ISFET, gate dielectric is directly exposed to the environment.

Therefore, drain current is modulated by ion concentration on the oxide surface. The gate electrode is located nearby to provide constant Vgs. The gate current is modulated by both Vgs and ions, and by keeping Id constant, we get linear relationship between Vgs and pH.

CHEMFET responds to specific ions. CHEMFET = ISFET + ion-specific membrane (organic).

p-Si substrateS D

SiO2

passivation organic layer

p-Si substrateS D

SiO2

passivation

ISFET CHEMFET

ISFETreference

FET

gate


3 mm

3 mm15 μm

S

D

ΔVT, mV

pH

0

+100

−100

0 5 10

The output signal may be non-linear; linearization is required. Sensitivity: 50-100 mV/pHLinear range: 1-13 pHPrecision: 0.05 pH (2.5-5 mV)ISFET

reference FET

gate


Electronic Nose – concept, design, fabrication and applications.

1. Why do we need E-nose?

2. How to make E-nose?

3. Chemical sensor array – a core of E-nose.

4. Fabrication of E-nose.

4. Applications:- food processing;- explosives detection;- alcohol detection;- hazardous chemical detection.


Why do we need e-nose?1) To avoid unnecessary casualties (environment control in areas with potential chemical hazard);2) To increase the productivity in chemical industry, food processing, etc.);3) For security purposes (airport security, subway security, etc.).

What is an e-nose?Electronic nose is a microsystem that recognizes a compound or a combination of compounds in a gaseous environment.

E-nose:1) Sensitive;2) Versatile.


“Electronic nose”: principle of operation

Issue: each chemical sensor is usually either non-selective or selective to one compound only.

Solution approach: Use of array of sensors, each of which is coated with different sensitive layer. Each sensitive layer may be sensing several chemicals (say, sensor 1 may be sensitive to CO2, H2O, NH3; sensor 2 – to H2O; sensor 3 – to CO, CO2; sensor 4 – to NH3, H2O; etc.) The pattern recognition program analyzes the response of the array and extracts the information on the nature and the

concentration of unknown chemical.

1 2 3 …nsensors:

analyteanalyte

Chemical 1

Chemical 2

Chemical 3

Chemical nPattern recognition Output signal


Components of an e-nose:

1) Chemical sensor array;2) Micropump + microfluidic channels for sampling control;3) Computer/PDA with pattern recognition software;3) Robotic vehicle (optional) for autonomous/remote operation.


Chemical sensor array:• An array of various chemiresistors (typically –polymers filled with metal nanopartilces);

• Polymer film swells as it absorbs chemical – resistance increases;

• Resistance is proportional to chemical concentration;

• Each polymer has its individual known response to each of chemicals to be detected;

• By comparing responses of all resistors (resistance pattern) with reference data (pattern recognition program), the chemical and its concentration are detected.


E-nose parameters:

Fabrication: surface micromachining (metal electrodes spin coated with polymer films 10nm-1μm thick);Sensitivity:depending on the chemical and the environment, could be as low as 0.1ppb. Typically – 1 ppm.Response time:Depends on the polymer film thickness and on the sampling algorithm. Varies from < 0.1s to 100s.Response features:• Like a mammal nose, e-nose is sensitive to differential signal (changes in the concentration) –ambient odor is in the background);• In case of mixed odor, individual concentrations can be subtracted by using pattern recognition program.

E-nose response diagram.


4. Acoustic wave sensors.Acoustic waves propagated through the sensor area are changed by the adsorption

of the analyte.

Acoustic wave generation: usually piezoelectric.

Applications: detection of chemicals.

Drive/sense electrodes

Example: Surface acoustic wave sensor.

In SAW sensor, acoustic waves generated by voltage pulses applied to interdigitatedelectrodes are propagated along the surface and sensed by another set of electrodes. The sensor made of piezoelectric material operates at the resonant frequency. Ambient molecules bound to the surface shift this frequency:

Δf = kf02Δm/A,k – constant;

f0 – resonant frequency;Δm – mass of surface-bound molecules;

A - active area.


5. Biosensors.Any sensor that involves biologically derived molecules.

Advantage: selectivity.Drawback: irreversibility.

Biomolecule layer

Sensor (ISFET, SAW,…)

Analyte

Electrical output

Biosensor


Immobilization:

- membrane entrapment (semipermeablepolymer membrane – e.g., polyimide membrane with 200nm pores is permeable to viruses or DNA but not to bacteria);

- physical adsorption (sensor surface to favor adsorption of specific species – e.g., proteins are attached well SiO2 but not to hydrogen plasma treated oxide);

- matrix entrapment (porous encapsulation matrix is formed around the biomaterial);

- covalent bonding (the surface contains the bonds to which specific biomaterial binds –e.g., antibody coated Si).


Example: ISFET with antibody coating.

Principle: On the sensor surface, a layer of biomolecules is immobilized. Biomolecules are

usually enzymes – proteins well known as metabolism catalysts (oxidants, hydrolytes, etc.). Enzymes bind with specific “target” compound covalently. Enzymes can be engineered for specific targets (e.g., HIV virus or glucose molecule), thus making highly selective sensor.


Example: Fluorescent Immunoassays.

Principle: Immunoassay is a technology to identify and quantify organic and inorganic compounds. Specifically designed antibodies highly specific to target compound bind with it producing the output signal. Immunoassays are simple and quick to use.

ELISA = Enzyme Linked ImmunoSorbent Assay

Detection limit: 1 ppt to 1ppm.

absorption

fluorescence


Example: DNA microarray (“DNA chip”).

Principle: On top of the substrate (transparent), various single stranded DNA parts labeled with

fluorescent dyes are immobilized. Since DNA binds only with complementary pairs, standard DNA microarrays containing a variety of DNA parts are used to identify unknown DNA fragments. Fluorescence occurs only in case of complementary bonding.

Fluorescent microarraymicromachined on top of LED.

Printed fluorescent DNA microarray.


ENFET.

This device uses covalent bonding of a molecule to which a specific receptor antibody then adsorbs.

p-Si substrateS D

SiO2

passivation enzyme layer

reference

electrode

Enzyme coated CHEMFET gate is able to bind antibodies. After that, target is added; the target bound by antibody changes pH proportionally to the amount bonded, which is sensed by CHEMFET.


Example: glucose level monitoring device.

A Pt film has an enzyme called glucose oxydase (an oxydant) immobilized on its surface. Thin porous polymer membrane protects it. Glucose diffusing from solution through membrane is oxydized to gluconic acid, which in turn converts enzyme to its reduced form (oxygen removal).

Si

SiO2

Pt

Enzymelayer

Metalcontacts

Polymer membrane

Blood oxygen then reacts with enzyme, and products include oxydized enzyme, water and free electrons. Conductivity increase, therefore, is proportional to glucose level.


Chemical actuators

Polymeric

Electrochemical Thin film batteries


Chemical actuators.

Chemical actuators – devices in which controlled chemical reactions result in generation of another for of energy (mechanical, heat, etc.).

1. Electrochemical actuators.Electrochemical reactions resulting in the change of phase (gas generation, solid

electroplating) cause mechanical movement.

Example: electrolytic gas generation powered membrane actuator.

Application: low power relays and switches.Fabrication: 2 bonded bulk micromachined Si wafers form sealed chamber. Upper wafer has corrugated SiNx membrane 1 μm thick deposited on top. Lower wafer has Cu and Pt sputtered electrodes. Cu electrode is covered with polymer impermeable to O2. Before sealing, the chamber was filled with CuSO4 + H2O. By applying a current, we electrolyze the solution. Released O2 pushes up the membrane.


Polymer actuators.

Polymer electrolyte gel contracts when a voltage is applied across it in an electrolyte (saline) solution.

Applications: robotics, bioactuators (drug delivery, biorobots).

Example: polypyrrole (PPy) electrochemomechanical switch.

Fabrication: Si substrate, thin Cr layer deposited, patterned, then Au layer evaporated, spin coated by rigid polymer BCB and patterned. Then contractablePPy layer deposited electrochemically and patterned with Au.

Operation: if the voltage is applied between the electrodes, gel contracts and BCB layer moves up.

Operating voltage: -1 V …+ 0.35 V.

Response time: 0.5 s… 10 s.

Dimensions: 10 μm … 100 μm.


3. Thin film batteries.

Here, chemical potential energy is transformed into electrical energy. Anode is more electropositive than cathode. Thus, a chemical potential gradient

establishes between them. Electrolyte allows only ionic flux (oxidation-reduction reaction), not electronic. Electrons thus must flow through external circuit.

substratecathode

electrolyteanodeions electrons External

circuitIf the reaction is reversible, the battery is rechargeable.

120 mAh/cm2. Modern lithium thin film battery:- a-V2O5 cathode;- Li-P-ON electrolyte;- Li-Al anode.

Fabrication:1. Vanadium anode and cathode sputtering (in Ar).2. V2O5 sputtering (in Ar).3. LiPON sputtering (Li3PO4 target in N2).4. Li-Al evaporation.5. Passivation (inorganic/organic multilayer).


Neural probes and neural electrodes.

Also called as bioelectric interfaces, these devices transduce signals between electronic systems and living tissues.

Applications:- To measure electric signals/impedance of neuron tissue;- To electrically stimulate neurons/tissue.

Include:Metallic electrodes;Substrate;Insulation.

array type penetrating type regeneration type

Issues:- Small signals (


Fabrication process – penetrating electrodes (directed normally to viewer):


Fabrication process – regeneration electrodes:

1.chemical sensors 2.chemical actuators 3.bioelectric ...ece434/winter2008/434_5.pdfexample: isfet...

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