1. 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

2. Outline
Why CMOS Capacitive pressure Sensor
Concept
Device Fabrication
Architecture
Experiment
Conclusion

3. Why CMOS capacitive pressure
sensor?
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

4. Concept
Detect Different pressure induced by ridges and valleys of finger print
Because of difference in the contact area on the surface ,capacitance also
varies.

5. Device fabrication

6. 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

7. Architecture
Constant Current Source is used to Charge Sensing
Capacitor
Capacitor 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 Converter
Capacitance Change with respect to the uniform pressure
applied to the membrane

8. Architecture
Architecture of Sensor array and the Schematics of Current Source
in-pixel capacitive sensing circuit
Constant
Current
Source

9. Experiment
Simulated and measured force-
displacement curves.

10. Measured output waveforms from different
sensing membranes before pressure was
applied

11. Capacitance change vs applied pressure
Measured value and Simulated values are close to each other

12. Measured capacitance changes of the array by: (a) applying a pressure on the
right side of the chip;(b) pressing part of the finger tip on the chip.
Shows
successful
detection
of ridges
and
valleys

13. Conclusion
Capacitive Pressure Sensor array is presented for finger print
detection
Sensor development take advantage of multiple metal layer
and fast electronics provided by the sub micro meter CMOS
process
Post CMOS fabrication can be conducted at the chip level
Is almost insensitive to temperature variations

14. Question ?