Silicon carbide process development for microengine applications : residual stress control and microfabrication

The high power densities expected for the MIT microengine (silicon MEMS-based micro-gas turbine generator) require the turbine and compressor spool to rotate at a very high speed at elevated temperatures (1300 to 1700 K). However, the thermal softening of silicon (Si) at temperatures above 900 K limits the highest achievable operating temperatures, which in turn significantly compromises the engine efficiency. Silicon carbide (SiC) offers great potential for improved microengine efficiency due to its high stiffness, strength, and resistance to oxidation at elevated temperatures. However, techniques for microfabricating SiC to the high level of precision needed for the microengine are not currently available. Given the limitations imposed by the SiC microfabrication difficulties, this thesis proposed Si-SiC hybrid turbine structures, explores key process steps, identified, and resolved critical problems in each of the processes along with a thorough characterization of the microstructures, mechanical properties, and composition of CVD SiC. Three key process steps for the Si-SiC hybrid structures are CVD SiC deposition on silicon wafers, wafer-level SiC planarization, and Si-to-SiC wafer bonding. Residual stress control in SiC coatings is of the most critical importance to the CVD process itself as well as to the subsequent wafer planarization, and bonding processes since residual stress-induced wafer bow increases the likelihood of wafer cracking significantly. Based on CVD parametric studies performed to determine the relationship between residual stresses in SiC and H2/MTS ratio, deposition temperature, and HCl/MTS ratio, very low residual stress (less than several tens of MPa) in thick CVD SiC coatings (up to -50 pm) was achieved.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2004

Author dc:creator

• Choi, Dongwon, 1973-

Advisor dc:contributor.advisor

• S. Mark Spearing.

• Materials Science and Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

en_US

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/28348

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/28348