Date of Award

Winter 2001

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Computational Analysis and Modeling

First Advisor

Weizhong Dai

Abstract

Laser-induced chemical vapor deposition (LCVD) is an emerging new technique with many practical applications. To optimize the system for fabricating a microstructure with a pre-specified geometry in pyrolytic LCVD, a three-dimensional mathematical model is developed for predicting temperature distributions and laser dwell times across the substrate scanned by the laser beam. A microstructure is fabricated layer by layer, and for each layer the laser beam moves from one pixel to the next. The complicated correlations among temperature distribution, deposit growth rate, and laser dwell time are investigated. A purely heterogeneous reaction is assumed and any gas-phase transport is ignored.

A finite difference scheme and an iterative numerical algorithm were developed for solving the model. The numerical computation is stable and convergent. The normal growth at each pixel is computed from the geometry of the deposit and the temperature distribution is obtained when the laser beam is focused at different pixels. From the temperature and normal growth, the dwell time for every pixel of each deposit layer is predicted.

The processes for fabricating a convex and a concave microlens with a prespecified geometry in pyrolytic LCVD with a Gaussian laser beam were simulated. Nickel and graphite were selected as materials for deposit and substrate, respectively. Factors such as intensity of the laser beam and geometry of the microstructure are discussed. The temperature distributions when the laser beam is focused at different pixels on the surface of deposit were obtained and analyzed for each layer. The dwell time distribution, which determines the laser scanning pattern, is predicted. The process for fabricating a microlens is quite different from that of a rod. The maximum temperature on the surface of the deposit decreases with an increase in the deposit thickness. This result indicates that when the temperature reaches a certain threshold, growth will stop unless the laser intensity is increased.

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