A CMOS CAPACITIVE PRESSURE SENSOR CHIP FOR FINGER PRINT DETECTION Yung-Shih Hsiung and Michael S-C Lu Department of Electrical engineering and Institute of Nano engineering & Micro System NTHU Reporter-Vivek Hegde Course Lectured By- Dr.Cheng-Hsien Liu
Why CMOS capacitive pressuresensor? Biometric uses whorl ,loop and arc patterns of finger print Ridge. In optical-Photographed image is stored for subsequent identification Piezoresitive uses wheat stone bridge and sensors are more sensitive to temperature
ConceptDetect Different pressure induced by ridges and valleys of finger printBecause of difference in the contact area on the surface ,capacitance alsovaries.
Architecture Circular Membrane radios is 20 µm and four support beams on the sides are 8 μ m long and 2 μ m wide. The total size of a sensing pixel by inclusion of the surrounding anchor area is 65×65 μ m2
ArchitectureConstant Current Source is used to Charge Sensing CapacitorCapacitor potential is Periodically reset by transistor Switch.8x32 Signals are Sequentially measured through shared buffer via Decoder Control.O/P Signal is processed by External 14-Bit A/D ConverterCapacitance Change with respect to the uniform pressure applied to the membrane
ArchitectureArchitecture of Sensor array and the Schematics of Current Sourcein-pixel capacitive sensing circuit Constant Current Source
Experiment Simulated and measured force- displacement curves.
Measured output waveforms from differentsensing membranes before pressure wasapplied
Capacitance change vs applied pressureMeasured value and Simulated values are close to each other
Measured capacitance changes of the array by: (a) applying a pressure on theright side of the chip;(b) pressing part of the finger tip on the chip. Shows successful detection of ridges and valleys
ConclusionCapacitive Pressure Sensor array is presented for finger print detectionSensor development take advantage of multiple metal layer and fast electronics provided by the sub micro meter CMOS processPost CMOS fabrication can be conducted at the chip levelIs almost insensitive to temperature variations