Date on Master's Thesis/Doctoral Dissertation

8-2000

Document Type

Master's Thesis

Degree Name

M. Eng.

Department

Electrical and Computer Engineering

Committee Chair

Walsh, Kevin Michael, 1955-

Subject

Microfabrication

Abstract

In July 1995, National Science Foundation Award # 9551869 funded the development of a new inter-disciplinary microfabrication course under the primary leadership of Dr. Kevin Walsh at the University of Louisville. Along with this award, the completed construction of a new building in 1996 that contained a class 1000/100 cleanroom laboratory facilitated the development of the course. Moreover, curricula had to be completed to provide students with practical, hands-on experience in building Micro Electro-Mechanical Systems (MEMS) devices using processes and methodologies introduced in the course. Dr. Walsh wanted to include a mass airflow sensor in his portfolio of total possible devices students could build in the cleanroom lab for the course. This document describes the design of a bulk-micromachined, monolithic, mass airflow sensor with a thermally-isolated, thin-film, dielectric, microbridge/diaphragm design. In addition, several fabrication methodologies were explored, as well as a means to test and evaluate the sensors for this undergraduate class laboratory. The mass airflow sensor architecture chosen was based upon a closed-loop-control,microelectronic thermal (hot-wire) anemometer design, which was first developed and presented by Johnson, Higashi, et. al. at Honeywell in the mid 1980s [2]. Two separate photomask sets were developed using L-Edit™ software (by Tanner Research), with each set including multiple geometric variations of a dual/triple microbridge/cantilever flow sensor structure to be suspended over a precision, anisotropically-etched pit, integrated onto a (100) silicon substrate. Four primary structural fabrication strategies were explored to produce the thin-film material for the flow sensors: (1) RF planar magnetron sputter-deposited 1 m m -thick silicon nitride microbridges/cantilevers; (2) anodically-bonded-and-machined 20-30 m m -thick borosilicate glass diaphragms; (3) spin-on-glass microbridges/cantilevers; and (4) low-stress, 0.5 m m -thick, LPCVD silicon nitride microbridges/cantilevers. Four resistor metallizations were separately evaluated: permalloy (Ni81Fe19), chromium, titanium, and platinum. A process was developed and documented to successfully fabricate flow sensors with low stress LPCVD silicon nitride microbridges/cantilevers. DC planar magnetron sputterdeposited platinum thin-film resistors (with a ~120 nm-thick RF planar magnetron sputterdeposited chromium adhesion layer), with nominal thicknesses of ~56 – 70 nm, were delineated by photolithographic imaging techniques. The resistors had measured Temperature Coefficients of Resistance (TCR) in the range of 1.93 – 2.25 x10-3 W /W /°C at 25 - 125 °C. Anisotropic KOH etching of the (100)-oriented silicon substrate was utilized to release the flow sensor microbridge/cantilever microstructures. After designing and building a flow sensor test machine capable of controlled volumetric air flow rates of up to ~15 SLPM (0.54 m/s), nominal sensor sensitivities (SV) of up to 0.67 mV/SLPM (20.4 mV/(m/s)) were measured. The sensitivities varied somewhat depending upon resistor values set in the flow sensor heater-driver circuit and the insertion depth of the devices within the flow channel.

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