Jing-Jia Huang¹, Christian Militzer^1,2, Charles A. Wijayawardhana^1,2, Henrik Pedersen¹, Urban Forsberg^1
1Linköping University, Linköping, Sweden
²SGL Carbon GmbH, Bonn, Germany
Graphite is considered as an ideal material for many high temperature applications from its high thermal stability, heat conductivity and relatively low thermal expansion coefficient. But due to its weak resistance against harsh chemical environments, its application in semiconductors requires a protective layer, for example, silicon carbide (SiC) or tantalum carbide (TaC), to protect the graphite parts from etching. The protective layer also acts as a diffusion barrier which prevents any residual impurities from the graphite to disturb the semiconductor processing. The conventional technique to prepare such SiC protective layers on graphite is chemical vapor deposition (CVD). However, in CVD processes, the features on substrates, such as drilled holes or trenches, often suffer from uneven coating thickness due to the gradual precursor depletion along the feature depth.
We report new conformal CVD approaches to obtain SiC coatings that are uniform in thickness throughout the trenches. The fundamental concept of a conformal CVD process is to reduce the surface reaction probability of precursors, either by decreasing the deposition temperature or by adding a growth inhibitor. The CVD concepts we have adopted were originally proposed in the work of Abelson and Girolami where thin films, primarily for electronic applications, were deposited at low temperatures (200 – 300 ℃) and low pressures (10 – 10^-8 Pa). We extended their concepts further to depositions that are performed at elevated temperatures (950 – 1200 ℃) and a moderate pressure (1 kPa), which are commonly utilized in CVD processes for protective coatings. We even achieved a superconformal deposition, that is, a faster growth at the bottom of the trench than at the opening, in our experiments with precursor pulsed CVD. This phenomenon could be attributed to an enhanced inhibition effect at the trench opening.
Our findings in improving the coating conformality of SiC at elevated temperatures and a moderate pressure could potentially benefit numerous CVD processes for protective coatings using chlorinated precursors because these coatings are often deposited under such conditions. This would allow uniform coatings deep down in holes for gas foil rotations or pyrometer reading spots, and in trenches in wafer carrier pockets.