Derek Wright

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Tutorial 5

• Derek Wright
• Wednesday, February 16th, 2005

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Sensors and Image Systems

• Physical Principles of Sensors
• Optical Imaging Systems
• IR Imaging Arrays
• Electronic Nose
• Tactile Sensors and Arrays

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Sensor Basics

• Sensors are transducers
• Transducers convert one form of energy to another
• Alternator in your car turns mechanical into
  electrical
• Engine converts chemical to thermal to mechanical
• Eyes convert light into electrical

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Sensor Basics

• Sensors either
• Directly convert one form to another
• Use one form to change (modulate) another
• Direct Conversion
• Solar panel Light ? Electricity
• Thermocouples Heat ? Electricity
• Modulating
• Thermoresistive, Optoresistive Changing
  resistance must be have current driven through it
  to measure

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Biological Sensor Arrays - Eyes

• The eye is a biological form of a sensor array
• It consists of an array of transducers (rods and
  cones)
• The signals are transmitted by neurons along axons

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Optical Imaging Systems

• Array structures allow multidimensional
  measurement to occur
• Optical Imaging Systems
• Charge Coupled Devices (CCDs)
• CMOS Cameras
• X-ray Imagers

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Charge Coupled Devices

• Incident photons cause creation of electron-hole
  pairs
• Electrons move to insulator boundary under bias
  for storage
• Charge is shifted out of a row or column by a
  shifting potential
• Cannot be integrated on the same substrate as
  accompanying electronic circuits

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CCD Operation
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CCD Operation
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CCD Operation

• http//micro.magnet.fsu.edu/primer/java/photomicro
  graphy/ccd/shiftregister/index.html
• http//www.extremetech.com/article2/02C39732C154
  652C00.asp

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CMOS Cameras

• Can be created with standard CMOS processes
• Can be integrated with accompanying electronic
  circuits
• An incident photon creates an electron-hole pair
  in a reverse biased diode
• Configured to cause charge to drain off of a
  capacitor
• Photon absorption ? capacitor voltage decrease

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CCD vs. CMOS Cameras

• CCD has a better Fill Factor (FF)
• Better image quality and photon capture
• Lower noise
• CCD only outputs the analog charge
• Must be converted to digital by another chip
• CMOS has on-chip integration
• Results in high-speed and low-power
• Reduces flexibility, but decreases cost

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X-ray Imagers

• Amorphous thin film techniques can produce
  large-area x-ray detectors
• Two types
• Indirect
• Direct
• On-pixel amplification means fewer x-rays needed
  to make an image ? Safer!

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p-i-n Structure
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h?
?Ev
i a-SiCH
n a-SiCH
Al
p a-SiCH
ITO
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X-ray Imagers

• Indirect Method
• A top layer of phosphor turns the x-ray into a
  visible discharge
• Visible photons are then detected by amorphous
  silicon (a-Si) p-i-n photodiodes

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X-ray Imagers

• Direct Method
• Amorphous selenium (a-Se) absorbs x-rays
• A layer of a-Se with a huge E-field is used
• It converts x-rays into electron-hole pairs
• E-field separates them into current

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IR Imagers

• Two detection methods
• Quantum (photon ? e-h pairs)
• Thermal (photon ? temp?)
• Useful in night vision systems
• Police use them in Ontario to find pot grow houses

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IR Imagers

• Quantum Detection
• Photons have an energy hf hc/?
• If this energy is bigger than the bandgap of a
  detector material, e-h pairs will be created
• IR has lower energies than visible, so the
  bandgap has to be reduced
• Detector bandgaps can be tuned from 0eV up
• These detectors must operate at very low
  temperatures
• Restricted to special uses

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IR Imagers

• Thermal Detection
• IR photons will turn into heat when they hit
  certain materials
• The heat can be detected and imaged
• A pyroelectric material will generate a voltage
  or current proportional to the IR power shining
  on it

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Microcalorimetric Sensors

• A heated chamber is kept at a constant
  temperature
• An incoming gas flow is burned
• When the gas burns it releases heat energy
• The released heat results in less heat from the
  chamber to keep a constant temperature
• Released heat energy can be measure by how much
  less the chamber needs to heat the gas flow

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Microcalorimetric Sensors
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Electrochemical Cells

• Use a catalyst to convert molecules to be
  measured into ions
• Two modes of operation
• Amperometric The ions are moved through a
  catalyst and electrolyte to create a current
• Potentiometric The ions charge a capacitor and
  appear as a voltage

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Electrochemical Cells
Amperometric
Potentiometric
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Acoustic Wave Devices

• Tiny free-standing beams are created through
  micromachining
• They have a mechanical resonance frequency (?)
• They are coated in a polymer that adsorbs the
  specific molecules to be observed
• More molecules stick ? mass? ? ??

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Acoustic Wave Devices
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Gas-Sensitive FETs

• A small channel lets gas pass between the gate
  and the substrate (channel)
• The underside of the gate can be coated with a
  material to adsorb certain gasses
• When the gasses adsorb into the coating, it
  changes the threshold voltage

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Resistive Semiconductor Gas Sensors

• O2 can act as a p-type dopant in silicon
• It attacks point defects
• The number of point defects increases with
  temperature
• The Si must be heated
• The more O2 in the silicon, the higher the
  conductivity

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Resistive Touchscreens

• Two flexible resistive layers are separated by a
  grid of spacers
• When the two layers are pressed together the
  resistance can be measured between several points
• This determines where the two resistive layers
  contacted

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Resistive Touchscreens
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Capacitive Touchscreens

• A conductive layer is covered with a dielectric
  layer
• The finger represents the other plate of the
  capacitor
• A kHz signal is transmitted through the
  conductive plate, the dielectric, and the finger
  to ground
• The current from each corner is measured to
  determine the touch location

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Capacitive Touchscreens
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Ultrasound Touchscreens

• Ultrasonic sound waves (gt40 kHz) are transmitted
  in both the horizontal and vertical directions
• When a finger touches the screen, the waves are
  damped
• Receivers on the other side detect where the
  sound was damped
• Multiple touch locations are possible

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Ultrasound Touchscreens
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Fingerprint Sensors

• An array of tiny capacitive sensors
• Works similarly to the capacitive touchscreen
• Finger works as one plate of a capacitor
• Chip works as the other
• Sensors are small enough to determine if a
  fingerprint ridge is touching it
• An image is produced

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Fingerprint Sensors
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Thank You!

• This presentation will be available on the web.