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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/photomi crography/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 Ec h Ev 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.