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Integrated Sensors
© University of Glasgow, 2009
David Cumming
Department of Electronics and Electrical Engineering
University of Glasgow
Outline
• Microelectronics
• Medical Devices
• Sensing-system-on-chip
© University of Glasgow, 2009
• Sensing-system-on-chip
• Extracellular ion imaging
Moore’s Law
• Dimensions get smaller
• Number of transistors per chip increases
• Operating voltage
© University of Glasgow, 2009
• Operating voltage decreases
• Power consumption decreases
• Memory size increases
Established – Cheap - Available
More than Moore
• Moore’s Law – indicates likely progress of
microelectronics in traditional sectors
• In More than Moore we push microelectronic
technologies into new sectors, and combine with
known strengths
© University of Glasgow, 2009
known strengths
– Sensors, biomedical devices, assay and instrumentation on a chip
OR
– SENSING-SYSTEM-ON-CHIP
Earliest work
• Components
– 1 pressure sensor
– 1 transistor
– 1 battery
© University of Glasgow, 2009
– 1 battery
– 10 ft string
• Enabling technology
– the transistor
R. S. Mackay Berkeley, 1959
Can do much more now
• Pressure – gut motility
• Video - imaging
• Temperature - hyperthermia
© University of Glasgow, 2009
• Temperature - hyperthermia
• Dissolved oxygen - post trauma
• pH – acid reflux
• Tumour markers - cancers
• Glucose/fatty acids – foods and digestion
Laboratory-on-a-Chip
• Miniaturisation of analytical systems
• Microfluidics
• High speed
DO NOT SCALE
© University of Glasgow, 2009
• High speed
• Multi-process
• Multi-sensor– Temp
– pH
– DO
– Conductivity (σ)
2 mm
Small ASIC
• Single chip comprising
• Microcontroller
• ROM and RAM
• 8 Instrument channels
© University of Glasgow, 2009
• 8 Instrument channels
• pH, T, DO, σσσσ
• Data conversion
• Wireless encoder
5 mm
0.6 µm 2 poly 2 metal
CMOS from AMS via
Europractice
Package it up
• Separate components on small PCB
• Connections (interconnect) take up lots of space for little functional reward
• Batteries are large (!)
Sensors
© University of Glasgow, 2009
• What can be done with a single chip?
– Using CMOS that is widespread
– Reduce size
– Reduce power
• Sensing-System-on-Chip
ASIC/SoCBatteries
Ion Sensitive Field Effect Transistor
H+
H+
H+
H+
H+
H+
H+
H+
RE
G
© University of Glasgow, 2009
� ISFET threshold voltage proportional to pH
silicon dioxide
S
P substrate
DN well
sensing area
silicon
H
H+
H
H+
A CMOS Compatible ISFET
H+
H+
H+
H+
H+
H+
RE
G
silicon nitride
H+
sensing area
H+
H+
H+
H+
H+
RE
G
© University of Glasgow, 2009
silicon dioxide
S
P substrate
DN well
sensing area
silicon
H+
H+
H+
H+
H+ silicon dioxide
D S
silicon dioxide
silicon nitride
silicon oxynitride
� Floating gate
ISFET Materials/Characteristics
• pH sensitive materials
– Surface charge varies with pH
– Silicon nitride gives a Nernstian response
– By happy coincidence SiN is widely used passivation layer on CMOS
© University of Glasgow, 2009
passivation layer on CMOS
• CMOS-compatible
• Sensitivity ~ 47
mV/pH
Proton imaging
• Image sensors commonplace
– Cameras, CMOS or CCD pixel arrays
– Capture photons
• Biological imaging with photons widespread
© University of Glasgow, 2009
• Biological imaging with photons widespread
• Can we be more direct?
– Imaging with ions
• Build CMOS imaging arrays for protons
• Functionalise post-fabrication
Starting point
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• The ISFET
• It’s a proton sensor (proton = H+ → pH)
• Need array version
Interface Electronics
• Column of pixels– Addressable elements
– S/D follower
• Readout circuitry– Decoding and
multiplexing
© University of Glasgow, 2009
Implementation
• Foundry technology.– Standard 0.35 µm CMOS process– Access via Europractice IC service
• Cross-section through ISFET.
© University of Glasgow, 2009
Polysilicon
Metal 1Metal 2Metal 3
Via holes
Passivation
W/L = 1/0.35 µm
Threshold Voltage Modification
• Test circuit • Effect of UV exposure• Pixel layout
© University of Glasgow, 2009
• Apply bias to change conduction band level in ISFET bulks
• UV radiation excites electrons to overcome energy barrier of oxide
• Population on floating electrodes achieves equilibrium with those in bulks after 10 hr
Surface Topography
• 16 x 16 array• 2 x 2 array
© University of Glasgow, 2009
• Frame border
• Pixel: 11.6 x 11.6 µm2
• Spacing: 0.6 µm
• Step height: 600 nm
• Pixel: 3 x 6 µm2
• Spacing: 10 µm
Glycolysis Bioassay
• Sensor array• Example pixel
© University of Glasgow, 2009
• Decrease in pH due to lactate (before addition of glycolysis inhibitor) and CO2 diffusing out of cells into medium
• Increase in pH due to vanishing [H+] in micro-environment between cells and sensor array surface