Late Announcement: Ph.D. Oral Examination - Alvin Barlian

[A. Alvin Barlian]

Microfabricated Piezoresistive Shear Stress Sensors for Underwater  
Applications

A. Alvin Barlian

Department of Mechanical Engineering, Stanford University

Ph.D. Oral Examination

Thursday, June 12, 2008 (Packard 101), 9AM



Abstract

Shear stress at the solid-fluid interface is a frequently studied  
parameter in fluid dynamics because of its relevance to many  
engineering applications, such as those in aerodynamic and  
hydrodynamic design. Shear stress measurements are also critical in  
biomedical and environmental science research. For example, shear  
stress data lead to improved understanding of fluid flow physics in  
cardiovascular systems and coral reef ecologies.
We present the design and characterization of a piezoresistive  
floating-element shear stress sensor. Conventional and oblique-angle  
ion-implantation techniques were used to form piezoresistors on the  
top and sidewall surfaces of the tethers. Hydrogen anneal technology  
was used to smooth sidewall scallops commonly seen in the Deep  
Reactive Ion Etching (DRIE) process and to reduce the noise in  
sidewall piezoresistors. A microfabricated piezoresistive cantilever  
was used to characterize the in-plane sensitivity of the sensor, while  
Laser Doppler Vibrometry was used to characterize its out-of-plane  
sensitivity.
The SiO2/Si3N4/SiO2 triplex layer and Parylene C were used as  
passivation schemes in two underwater experiments. The first  
experiment used a cylindrical water tank sitting on a rotating table  
to produce solid body rotation. The second experiment used a gravity- 
driven water flume to create a uniform, fully-developed flow over the  
sensor. Polymer flip-chip flexible interconnects were fabricated and  
used for the packaging of the sensor in the second experiment.
Piezoresistors formed using the oblique-angle ion-implantation  
technique required a thermal annealing step to activate dopants, hence  
increasing junction depth and reducing sensitivity. A novel sidewall  
epitaxial piezoresistor fabrication process, using selective  
deposition, is demonstrated for in-plane sensing applications. Early  
findings on electrical and mechanical characteristics are presented.

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