Silane Overpressure Annealing Process

The basic principle behind the silane overpressure annealing process is as follows. When the SiC wafer is annealed at high temperatures ( 1,600°C), Si atoms on the surface of the SiC wafer may be exchanged with Si atoms provided by the silane gas in the vapor phase. Chemical process calculations were performed for various pressures and silane flow rates as a function of temperature. These calculations, 

This modei is based on a very simpie premise if the partiai pressure of Si atoms above the SiC surface during anneaiing is greater than the vapor pressure Si in the SiC matrix, then out-diffusion of Si from the iattice wiii be suppressed. To theoreti-caiiy obtain the most suitabie conditions for Siiane overpressure anneaiing, a modei was constructed by J.T. Woian at the University of South Fiorida to caicuiate both the vapor pressure of Si in SiC and the partiai pressure of Si as a function of anneaiing temperature and gas pressure [85]. 

Caicuiations performed by Woian et ai. of the caicuiated vaiues of vapor pressure of SiC for temperatures of interest for impiant anneaiing have been reported in the iiterature and the reader is referred to these caicuiations for compieteness [85]. From these caicuiations it was determined that the required vapor pressure of SiC at iower temperatures ( 1,600°C) is very smaii, as expected. The reiationship between vapor pressure, Vj, and temperature, T, is exponentiai. Fience an exponentiai curve fit was appiied for vapor pressure caicuiations for temperatures iess than 2,500°C. To optimize the process, the siiane fiow rates were caicuiated for various anneaiing temperatures and process pressures. The partiai pressure of siiane and the vapor pressure of SiC obtained in Section 4.3.2 under atmospheric pressure (AP) conditions are used here as the reference vaiues. Aii of the caicuiations were performed based on initiai anneaiing experiments conducted at 1,600°C with siiane (20 seem) and argon (6 sim) as the process gases. (These experimentai conditions wiii be discussed next.) 

A silane-based CVD reactor suitable for performing high-temperatnre anneals in an Si- rich ambient was used for these experiments [86]. The samples were placed on a SiC-coated graphite susceptor and an RF induction coil used to heat the susceptor to temperatures on the order of 1,600-1,800°C. Silane and argon were the two process gases used, where Ar not only serves as a dilutant gas but also as a carrier gas to transport silane molecules to the crystal surface. All the implant annealing experiments were performed at atmospheric pressure. 

The annealing process was experimentally optimized using Silane in Ar [3% Silane in 97% nltra-high pnrity (UHP) Ar)]. Initial experiments were conducted with palladium-purified hydrogen as the carrier gas. Unfortunately, this method 

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