Distributed Microsystems Laboratory Integrated Interface Circuits for Chemiresistor Arrays Carina K....
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Transcript of Distributed Microsystems Laboratory Integrated Interface Circuits for Chemiresistor Arrays Carina K....
Distributed Microsystems Laboratory
Integrated Interface Circuits for Chemiresistor Arrays
Carina K. Leung* and Denise Wilson, Associate ProfessorDepartment of Electrical Engineering
University of Washington*Now with Intel, Dupont Washington
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
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
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
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
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)
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
Vdd
Rbias
RBaseline
+-
+-
Out
Vdd
Rbias
RSensor
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:– 25V detection limit
– Independent of baseline
– 0.01% (R) detection limit and resolution
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.
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
RSensor
C’
C
SchmittTrigger
Out
Co
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%
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%/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.
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.
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.