Chemical and Biochemical Microsystems

Transcription

1 Chemical and Biochemical Microsystems 1. Chemical Sensors 2. Chemical Actuators 3. Bioelectric Devices 4. Example: Electronic Nose (C) Andrei Sazonov 2005,

2 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). (C) Andrei Sazonov 2005,

3 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 V ds. The feedback adjusts V gs to keep the I d constant. Hence V gs is proportional to [H + ] and therefore to ph. Fabrication: - surface micromachining (c-si or glass). suspended gate V gs passivation S D p-si substrate V ds gate SiO 2 reference (C) Andrei Sazonov FET ISFET 2005,

4 Passive (chemiresistors, chemicapacitors) Electrochemical (ph-meters) Chemical sensors Acoustic wave based (SAW) Work function based (ISFET, CHEMFET) Biosensors (C) Andrei Sazonov 2005,

5 Chemical sensors. Sensing mechanism: Environment Sensor parameter Signal (C) Andrei Sazonov 2005,

6 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: NH 3 and NO 2 sensor. Au electrodes (50 interdigitated pairs) 25 μm wide, 7.25 mm overlap length, 25 μm interelectrode gap. Deposited on top of SiO 2 coated c-si substrate. Coated with 45 layers of polymer (phtalocyanine), each layer 2.5 nm thick. Sensitivity: ppm. Response time: 1 min. Chemically sensitive layer (polymer) Metal (Au) SiO 2 Substrate (c-si, glass) (C) Andrei Sazonov 2005,

7 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 (NH 3, NO 2, Hg vapor). Problems: poor selectivity, non-linearity, long response time. (C) Andrei Sazonov 2005,

8 Example 2: Metal-oxide gas sensor. Gases adsorbed on the surface of conductive metal oxides (SnO 2, ZnO, TiO 2 ) change the resistance. Generally, adsorbed oxygen atoms trap electrons reducing the resistance: O 2 + 2e - 2O -. Combustible gases react with oxygen to form H 2 O and release electrons; resistance decreases: H 2 + O - H 2 O + e -, 2H + O - H 2 O + e -, CO + O - CO 2 + e -. Thus, the change in the resistance depends on the change in the concentration of oxygen (CO) or combustible gases. SnO 2 gas sensor detects CO and H 2. The concentration range detectable: > 1000 ppm in air. Respective resistance ratio (R s /R 0 ): < 1 (air level ~ 5). Sensor is non-selective. SnO 2 Metal contacts 10 5 ΔG, Ohm -1 SiO 2 Poly-Si (C) Andrei heater Sazonov 2005, Si membrane P CO, kpa

9 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, CO 2, CH C, pf 100 Hz Example: integrated humidity/temperature sensor khz c-si substrate, p-n junction diode as temperature sensor, metal capacitor with spin coated polymer dielectric, which absorbs the moisture khz 100 khz P p, kpa The signal is AC (1kHz range). The response time is 1-2 s. 1 2 (C) Andrei Sazonov 2005,

10 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 (SiO 2 degrades); - properties drift due to native oxide growth. S optional G D sensitive layer SiO 2 Air gap and suspended gate design improves noise and selectivity (sensitive layer can be attached to the gate). p-si substrate (C) Andrei Sazonov 2005,

11 Pd gate MOS. Hydrogen adsorbs well onto Pd surface, decomposes: H 2 2H, diffuses through Pd and adsorbs on Pd/SiO 2 interface. H + S p-si substrate Pd SiO 2 p-si E c E F E v D SiO 2 Pd gate This decreases flat-band voltage and thus reduces V T. Application: hydrogen sensors. μnθ ΔV T = ε 0 ΔV T threshold voltage shift; μ dipole moment of interfacial hydrogen (μ = qd, d oxide thickness); N surface density of adsorption sites (for Pd, N = 1.67x10 19 m -2 ); θ fraction of surface sites covered (0 1). Example: calculate ΔV T for 25μm x 500μm MOS with 50nm oxide and 10 ppm H 2 concentration. Solution: *10 V (C) Andrei *5*10Sazonov *1.67* , *2*10 Δ 2006 V mv 11 T = = 0.06 = *10

12 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 V gs. The gate current is modulated by both V gs and ions, and by keeping I d constant, we get linear relationship between V gs and ph. CHEMFET responds to specific ions. CHEMFET = ISFET + ion-specific membrane (organic). gate reference FET ISFET passivation passivation organic layer S p-si substrate D S p-si substrate D ISFET SiO 2 CHEMFET SiO 2 (C) Andrei Sazonov 2005,

13 ΔV T, mv S 3mm μm 3mm 0 D 100 ph reference FET gate ISFET The output signal may be non-linear; linearization is required. Sensitivity: mv/ph Linear range: 1-13 ph Precision: 0.05 ph (2.5-5 mv) (C) Andrei Sazonov 2005,

14 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. (C) Andrei Sazonov 2005,

15 What is an e-nose? Electronic nose is a microsystem that recognizes a compound or a combination of compounds in a gaseous environment. 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.). E-nose: 1) Sensitive; 2) Versatile. (C) Andrei Sazonov 2005,

16 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 CO 2, H 2 O, NH 3 ; sensor 2 to H 2 O; sensor 3 to CO, CO 2 ; sensor 4 to NH 3, H 2 O; etc.) The pattern recognition program analyzes the response of the array and extracts the information on the nature and the concentration of unknown chemical. analyte analyte sensors: n Chemical 1 Chemical 3 Pattern recognition Chemical 2 Chemical n Output signal (C) Andrei Sazonov 2005,

17 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. (C) Andrei Sazonov 2005,

18 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. (C) Andrei Sazonov 2005,

19 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. (C) Andrei Sazonov 2005,

20 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. Example: Surface acoustic wave sensor. In SAW sensor, acoustic waves generated by voltage pulses applied to interdigitated electrodes 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 = kf 02 Δm/A, k constant; f 0 resonant frequency; Δm mass of surface-bound molecules; A - active area. Drive/sense electrodes (C) Andrei Sazonov 2005,

21 5. Biosensors. Any sensor that involves biologically derived molecules. Advantage: selectivity. Drawback: irreversibility. Analyte Biomolecule layer Sensor (ISFET, SAW, ) Biosensor Electrical output (C) Andrei Sazonov 2005,

22 Immobilization: - membrane entrapment (semipermeable polymer 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 SiO 2 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). (C) Andrei Sazonov 2005,

23 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. (C) Andrei Sazonov 2005,

24 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 (C) Andrei Sazonov 2005, fluorescence

25 (C) Andrei Sazonov 2005,

26 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 microarray micromachined on top of LED. Printed fluorescent DNA microarray. (C) Andrei Sazonov 2005,

27 ENFET. This device uses covalent bonding of a molecule to which a specific receptor antibody then adsorbs. 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. passivation enzyme layer S p-si substrate D SiO 2 (C) Andrei Sazonov 2005,

28 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). Enzyme layer Pt Metal contacts Polymer membrane SiO 2 Si Blood oxygen then reacts with enzyme, and products include oxydized enzyme, water and free electrons. Conductivity increase, therefore, is proportional to glucose level. (C) Andrei Sazonov 2005,

29 Chemical actuators Electrochemical Thin film batteries Polymeric (C) Andrei Sazonov 2005,

30 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 SiN x membrane 1 μm thick deposited on top. Lower wafer has Cu and Pt sputtered electrodes. Cu electrode is covered with polymer impermeable to O 2. Before sealing, the chamber was filled with CuSO 4 + H 2 O. By applying a current, we electrolyze the solution. Released O 2 pushes up the membrane. (C) Andrei Sazonov 2005,

31 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 contractable PPy 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 V. Response time: 0.5 s 10 s. Dimensions: 10 μm 100 μm. (C) Andrei Sazonov 2005,

32 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. ions electrons anode electrolyte cathode substrate External circuit If the reaction is reversible, the battery is rechargeable. Modern lithium thin film battery: -a-v 2 O 5 cathode; - Li-P-ON electrolyte; - Li-Al anode. 120 mah/cm 2. Fabrication: 1. Vanadium anode and cathode sputtering (in Ar). 2. V 2 O 5 sputtering (in Ar). 3. LiPON sputtering (Li 3 PO 4 target in N 2 ). 4. Li-Al evaporation. 5. Passivation (inorganic/organic multilayer). (C) Andrei Sazonov 2005,

33 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. Issues: - Small signals (<1 mv); - High impedance ( khz). Therefore, signal has to be amplified on-site (as close to the probe as possible). Microelectrodes: (C) Andrei Sazonov 2005, array type penetrating type regeneration type

34 Fabrication process penetrating electrodes (directed normally to viewer): (C) Andrei Sazonov 2005,

35 Fabrication process regeneration electrodes: (C) Andrei Sazonov 2005,