)Engineering Solution Int. Ltd., Howth Junction Business Park, Dublin-5, Ireland.
)Deposited film uniformity is a critical issue for APCVD because of high pressure, high flow rate, high gas- phase reaction rates, strong dependence of gas-phase rate and dependence of surface sticking probabilities on temperature. This combination of factors makes it very difficult to achieve uniform deposition rate across the substrate area. It’s also difficult to separate different factors influencing deposition process in experiment.
The goal of the simulation was investigation of individual influence of radiation (heating) intensity distribution over susceptor and the dynamics of working gas in deposition chamber on the film uniformity. Special attention is paid to heat-mass transfer in lower layer above the chamber bottom and influence of unevenness of the susceptor surface on deposition uniformity.
Typical construction of the reactor is modeled, with semispherical quartz dome and rotating susceptor of 50cm diameter in the bottom and three wafers on it. The precursor- trichlorosilane (TCS) is used in mixture with hydrogen.
 Fig.1 Typical profile of surface temperature over susceptor obtained in the calculation |
Heat transfer calculation includes mass transport, radiation
transport and thermal conductivity. A k-
approximation is used
to account for turbulent flow. TCS has obvious dominant
decomposition reaction SiCl3H -> SiCl2
+ HCl (1), which was used as a
basic one in the calculations. Simple kinetic scheme with four
processes involved is used for simulation of the film growth:
(2) SiCl2 + HCl -> SiCl3H,
(3) SiCl2 -> Si + Cl2,
(4) Cl(2)+ H(2) -> HCl
Test comparison of this scheme with other calculation and experiment [1] gives good results.
The calculation shows that the surface temperature is determined mainly by the distribution of radiation intensity over the susceptor, which is easily controllable. The film growth is substantially controlled by chemical kinetics in a narrow surface layer and ultimately by the surface temperature. This may allow the opportunity of correcting non-homogeneities, caused by chemical and mechanical processes in the upper part of chamber by means of redistribution of the surface temperature (see Fig.1). However, only symmetrical regular non-homogeneities of ring-type can be corrected by this simple means.
The unevenness of the susceptor surface (elements of wafer holders, for example) can cause irregular asymmetric non-homogeneities of the deposited film. The calculation shows that an obstacle of 1-2mm height conditions substantial non-homogeneity of deposition at rotation speed 1-2 r/sec. For instance, the edge of 150mm wafer extends 2mm above the susceptor surface causes up to 2% of local non-homogeneity.
