Tutorial 5

Derek Wright

Wednesday, February 16th, 2005

Sensors and Image Systems

• Physical Principles of Sensors

• Optical Imaging Systems

• IR Imaging Arrays

• Electronic Nose

• Tactile Sensors and Arrays

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

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

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

Optical Imaging Systems

• Array structures allow multidimensional measurement to occur

• Optical Imaging Systems:– Charge Coupled Devices (CCDs)– CMOS Cameras– X-ray Imagers

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

CCD Operation

CCD Operation

CCD Operation

• http://micro.magnet.fsu.edu/primer/java/photomicrography/ccd/shiftregister/index.html

• http://www.extremetech.com/article2/0%2C3973%2C15465%2C00.asp

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

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

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!

p-i-n Structure

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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

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

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

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

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

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

Microcalorimetric Sensors

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

Electrochemical Cells

Amperometric

Potentiometric

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

Acoustic Wave Devices

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

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

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

Resistive Touchscreens

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

Capacitive Touchscreens

Ultrasound Touchscreens

• Ultrasonic sound waves (>40 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

Ultrasound Touchscreens

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

Fingerprint Sensors

Thank You!

• This presentation will be available on the web.