AVS 47th International Symposium
    MEMS Thursday Sessions
       Session MM-ThA

Paper MM-ThA8
Comparison of In-situ Boron-doped and In-situ Phosphorus-doped Polysilicon Films for Microelectromechanical Systems

Thursday, October 5, 2000, 4:20 pm, Room 309

Session: Material Science of MEMS
Presenter: J.J. McMahon, Case Western Reserve University
Authors: J.J. McMahon, Case Western Reserve University
J.M. Melzak, Case Western Reserve University
C. Zorman, Case Western Reserve University
J. Chung, Case Western Reserve University
M. Mehregany, Case Western Reserve University
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In order to produce thick polysilicon films for MEMS applications, a single step, high deposition rate, APCVD process was developed and used to deposit in situ boron-doped, undoped, and in situ phosphorus-doped polysilicon films at susceptor temperatures ranging from 625C to 850C. For doped and undoped films alike, the deposition rate increases with increasing susceptor temperature, with boron-doped films exhibiting the highest deposition rate at each temperature setting. The highest deposition rates occurred at 850C, with boron-doped films being deposited at a rate of 733A/min, undoped films at a rate of 716A/min, and phosphorus-doped films at a rate of 622 A/min. Spreading resistance and SIMS measurements performed on the boron-doped samples indicate that the boron concentrations are generally constant throughout the thickness of the films, with the highest boron concentrations found in films deposited at 625C. Wafer curvature measurements on boron-doped films indicate that the films are in compression over the entire temperature range, with the magnitude of the stress decreasing with increasing temperature. In contrast, the undoped films are in tension at lower temperatures, but become compressive as the deposition temperature is increased. TEM analysis shows that film microstructure strongly depends on the deposition temperature, with equiaxed grains forming at low substrate temperatures and columnar grains forming at higher temperatures. Surface micromachined cantilevers, strain gauges, and lateral resonators were successfully fabricated from 5.0 micron-thick doped films grown at 850C. The extended paper will detail the process used to deposit the doped and undoped films, present a summary of the characterization study, and highlight the performance of micromachined structures and devices.