Electrical and Computer Engineering ETDs

Publication Date

Spring 5-1997

Abstract

Surface micromachined pressure sensors were designed, modeled, fabricated, and tested. They employed a piezoresistive transduction mechanism and were based upon circular diaphragms, which vary from 50 to 1000 μm in diameter and 1 to 2 μm in thickness. The piezoresistors were placed in Wheatstone bridge configurations to provide simple signal amplification and first order temperature compensation.

Of the different micromachining techniques, surface-micromachining has the advantage of being the most similar to integrated circuit manufacturing. Hence an existing IC equipment set can be used to create mechanical structures. Furthermore, the monolithic integration of a mechanical device with control electronics is simpler using surface micromachining.

Two distinct architectures of piezoresistive pressure sensors have been fabricated and tested: non-planar and planar. The non-planar sensors have topographic features on the order of 2 μm which complicate such fabrication processes as photolithography, metallization, plasma etch, and ion implantation. These complications are largely absent in the planar devices, which have reduced topography by the application of chemical mechanical polishing.

An important fabrication step in common surface-micromachined processes is the release etch, whereby a sacrificial oxide is etched in an aqueous HF solution to create free standing structures. Ideally there is a high selectivity of the HF etch rates between the sacrificial oxide and structural material. However, since the structural material will etch at a finite rate, there is a danger that the it will be completely removed during a long release etch. To address this problem, work was done to optimize both the chemistry of the etchant and the geometry of the structure. Also, closed form models were developed to describe the behavior of the release etch for a variety of useful geometries.

In order to predict the output of a pressure sensor, it is necessary to understand the mechanical behavior of a micromachined diaphragm. Thin plate, small deflection theory does not accurately predict sensor response. A model based on thick plate, large deflection theory was developed which includes thin film stress effects. This model is a better representation of actual sensor characteristics.

Keywords

MEMS, microelectromechanical systems, pressure sensors, surface micromachining, release etch, built-in stress

Document Type

Dissertation

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Don L Kendall

Second Committee Member

Kenneth Jungling

Third Committee Member

Joseph L Cecchi

Fourth Committee Member

James H Smith

Fifth Committee Member

Jeffry J Sniegowski

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