Late Announcement: Ph.D. Oral Examination - 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.