1. Distributed Microsystems Laboratory Integrated Interface Circuits for Chemiresistor Arrays Carina K. Leung * and Denise Wilson, Associate Professor Department of Electrical Engineering University of Washington *Now with Intel, Dupont Washington

2. Integrated Interface Circuits for Chemiresistor Arrays Outline Project Description (High Density Chemiresistor Arrays) Chemiresistor Background Project Context Circuit Approach 1: Differential Measurement of Resistance Circuit Approach 2: Resistance-to-Frequency Conversion Comparison of Approaches Summary Acknowledgements

3. Project Description Popular approach to chemical sensing (“traditional”) Small number (highly selective) sensors in an Application targeted to 1-2 analytes In an “understood” background Another approach to chemical sensing (“olfactory”) Large number (broad, overlapping selective) sensors in an Application targeted to many analytes And their (many) interferents In a cluttered and complicated background Candidates for high density arrays of chemical sensors are few: Require small size, linear operation, broad selectivity, compatibility with integration, and room temperature operation

4. Chemiresistor Background Composite polymer chemiresistors Conductive Element (such as carbon black) combined with Chemically sensitive element (polymer) Basic operation Polymer “swells” in response to target analytes Conductive particles move farther apart (conductivity increases) Linear response at low concentrations R-R o = R o (k) [C] R o = baseline resistance (large and highly variable) [C] = analyte concentration Superposition can be applied to multiple analytes presented simultaneously

5. Project Context High resolution Sensor Arrays Require Integration Circuits produced in CMOS Gold post-deposited electrochemically Sensor coating “sprayed” on gold 1-2 layers of metal required for sensor Challenge: Design processing circuits that Ignore large, variable baseline resistance Amplify very small changes in polymer resistance on top of large baselines Conform to VLSI footprint that addresses: Electrode Geometry Required sensor density Circuit performance

6. Circuit Approach #1 Differential Approach On-chip chemiresistor divided into: –One chemically sensitive resistor –One or (three) reference resistors Passivated (responsive to zero analytes) or Exposed, not functionalized (responsive to all analytes) Resistive “Bridge” is part of sensor Remaining circuits are designed for maximum gain under constrained footprint (= sensor platform)

7. Circuit Approach #1 Differential Approach Resistive Bridge output transferred to: –Differential Amplifier –Comparator with ramping input for serial A/D conversion Design constraints: –Differential Amplifier: maximum gain in small footprint –Comparator: fully serial (simple) A/D conversion acceptable because of slow sensor response time

8. Circuit Approach #1 Differential Approach Circuit Gain – 20 (Differential Amplifier) –-20 (Comparator) Sensor Performance: –Bridge approach eliminates effect of broad range in baseline on circuit gain –However, additional bias resistors add more noise (electrical and transduction) Translation: –25  V detection limit –Independent of baseline –0.01% (  R) detection limit and resolution

9. Circuit Approach #2 Resistance to Frequency Conversion Sensor platform contains three terminals: –Outer ring terminals shorted together outside sensor platform to enable circuits to fit underneath –Allows a single resistor per platform for chemical sensing –More “active” area (fill factor) than previous approach. –Electrode geometry more readily optimized for best noise performance.

10. Circuit Approach #1 Resistance to Frequency Conversion Operation: –Sensor resistance charges C o –As the capacitor charges, it trips the Schmitt trigger, causing the feedback to discharge the capacitor –The frequency of the charge/discharge cycle becomes smaller with increasing resistance (smaller current) Hysteresis reduces impact of noisy sensor response

11. Circuit Approach #2 Resistance to Frequency Conversion Sensitivity: –Baseline (730k  ) =.12%/  –Baseline (9.26k  ) = 4.1%/  Resolution/Detection Limit: –Change in resistance from baseline –Baseline (730k  ) =.07% –Baseline (9.26k  ) =.02%

12. Comparison Both circuits fit underneath sensor platform (.04 mm 2 area) Fill Factor: Approach #1: 25% Approach #2: close to 100% (with exceptions for metal routing) Sensitivity: –Approach #1: 400 (V/V) –Approach #2: between.12%/  and 4.1%/  Resolution/Detection limit: –Approach #1:.01% change in resistance –Approach #2: between.02% and.07% Other: –Approach #2: more resilience to fluctuations in response due to built in hysteresis.

13. Summary We have designed and fabricated two circuits for processing the response of composite polymer chemiresistors. Performance enables sub-ppm detection of many common analytes, while having having zero impact on sensor area.

14. Acknowledgements The authors would like to thank Nathan Lewis and his graduate group at the California Institute of Technology for data and technical assistance, as well as a subcontract through CalTech on ARO Grant DAAG55-98-1-0266.