Distributed Microsystems Laboratory

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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
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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

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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

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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-Ro Ro (k) C
• Ro baseline resistance (large and highly
  variable)
• C analyte concentration
• Superposition can be applied to multiple analytes
  presented simultaneously

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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

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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)

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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

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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
• 25mV detection limit
• Independent of baseline
• 0.01 (DR) detection limit and resolution

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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.

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Circuit Approach 1

• Resistance to Frequency Conversion
• Operation
• Sensor resistance charges Co
• 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

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Circuit Approach 2

• Resistance to Frequency Conversion
• Sensitivity
• Baseline (730kW) .12/W
• Baseline (9.26kW) 4.1/W
• Resolution/Detection Limit
• Change in resistance from baseline
• Baseline (730kW) .07
• Baseline (9.26kW) .02

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Comparison

• Both circuits fit underneath sensor platform (.04
  mm2 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/W and 4.1/W
• 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.

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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.
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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.