DESIGN AND MICROFABRICATION OF A CMOS-MEMS PIEZORESISTIVE
ACCELEROMETER AND A NANO-NEWTON FORCE SENSOR
by
Mohd Haris Md Khir
Adviser: Hongwei Qu, Ph.D.
This thesis work consists of three aspects of research efforts:
I. Design, fabrication, and characterization of a CMOS-MEMS piezoresistive
accelerometer
2. Design, fabrication, and characterization of a CMOS-MEMS nano-Newton force
sensor
3. Observer-based controller design of a nano-Newton force sensor actuator system
A low-cost, high-sensitivity CMOS-MEMS piezoresistive accelerometer with
large proof mass has been fabricated. Inherent CMOS polysilicon thin film was utilized
as piezoresistive material and full Wheatstone bridge was constructed through easy
wiring allowed by three metal layers in CMOS thin films. The device fabrication process
consists of a standard CMOS process for sensor configuration and a deep reactive ion
etching (DRIE) based post-CMOS microfabrication for MEMS structure release. Bulk
single-crystal silicon (SCS) substrate was included in the proof mass to increase sensor
sensitivity. Using a low operating power of 1.67 m W, the sensitivity was measured as
30.7 mV/g after amplification and 0.077 mV/g prior to amplification. With a total noise floor of 1.03 mg!-!Hz, the minimum detectable acceleration is found to be 32.0 mg for a
bandwidth of I kHz which is sufficient for many applications.
The second device investigated in this thesis work is a CMOS-MEMS capacitive
force sensor capable ofnano-Newton out-of-plane force measurement. Sidewall and
fringe capacitance formed by the multiple CMOS metal layers were utilized and fully
differential sensing was enabled by common-centroid wiring of the sensing capacitors.
Single-crystal silicon (SCS) is incorporated in the entire sensing element for robust
structures and reliable sensor deployment in force measurement. A sensitivity of 8 m V /g
prior to amplification was observed. With a total noise floor of 0.63 mg!-IHz, the
minimum detection acceleration is found to be 19.8 mg, which is equivalent to a sensing
force of 449 nN.
This work also addresses the design and simulation of an observer-based
nonlinear controller employed in a CMOS-MEMS nano-Newton force sensor actuator
system. Measurement errors occur when there are in-plane movements of the probe tip;
these errors can be controlled by the actuators incorporated within the sensor. Observerbased
controller is necessitated in real-world control applications where not all the state
variables are accessible for on-line measurements.
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