 The NASA aerospace program, in particular, requires breakthrough instrumentation
inside the combustion chambers of engines for the purpose of, among other things, improving computational fluid dynamics code validation and active
engine behavioral control (combustion, flow, stall, and noise). This environment can be as high as 600 0C, which is beyond the capability of silicon and
gallium arsenide devices. Silicon-carbide- (SiC-) based devices appear to be the most technologically mature among wide-bandgap semiconductors with the
proven capability to function at temperatures above 500 0C. However, the contact metalization of SiC degrades severely beyond this temperature because
of factors such as the interdiffusion between layers, oxidation of the contact, and compositional and microstructural changes at the metal/semiconductor
interface. These mechanisms have been proven to be device killers. Very costly and weight-adding packaging schemes that include vacuum sealing are
sometimes adopted as a solution.
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Specific contact resistance of Ti/TaSi2/Pt on an n-type SiC epilayer measured after a 500 0C treatment in air. The inset
shows the corresponding I-V characteristics after 640 hr, showing excellent linear behavior.
Specific contact resistance of Ti/TaSi2/Pt on an n-type SiC measured after a 600 0C treatment in air. The inset shows the
corresponding I-V characteristics after 200 hr, showing excellent linear behavior.
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Research work at the Sensors and Electronics Branch of NASA Glenn Research Center's
Instrumentation and Controls Division has demonstrated thermally stable I-V characteristics and contact resistance of Ti(100nm)/TaSi2(200nm)/Pt(300nm)
ohmic contact metalization on n-type 6H-SiC after 500 0C and 600 0C treatment in air for over 600 hr (top figure) and over 200 hr (bottom figure),
respectively.
Auger electron spectroscopy and high-resolution trans-mission electron microscopy
were used to analyze the metal and semiconductor interfaces to understand the prevailing reactions. The thermal stability of the ohmic contact in air is
believed to be due to the formation of silicides and carbides of titanium after being initially annealed at 600 0C in N2 for 30 min. Most importantly,
the oxidation of silicon species is proposed to be the critical diffusion barrier mechanism that prevents further oxygen penetration into the
metalization.
These results had an immediate impact on ongoing research in SiC sensors and
electronics projects. Real operating environment testing of SiC devices with this metalization scheme is planned. This contact scheme is expected to
have the following attributes:
- Ohmic contact with reasonably low contact resistance relative to the bulk epilayer
- Long-term contact stability in the harsh environment
- Compatibility with SiC large-scale integrated fabrication technology
- Good wirebond strength
- Compatibility with high-temperature interconnect and packaging technology
Author: Dr. Robert S. Okojie
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