뒤집힌 단일웨이퍼 반응기에서 클로로실란으로부터 실리콘 화학증착

김철진, 김의정

울산대학교 화학공학부

Si CVD from Chlorosilanes in an Inverted Single-Wafer Reactor

Chul Jin Kim, Eui Jung Kim

School of Chemical Engineering, University of Ulsan

INTRODUCTION

Chlorosilanes are the most widely used source materials for the deposition of silicon in the semiconductor industry. Therefore, both theoretical and experimental studies have been carried out to explore their deposition characteristics.[1-3] Recent numerical studies on the CVD processes show that deposition rates and film uniformities, which are a major measure of CVD performance, depend strongly on deposition conditions as well as reactor geometry.

Polland and Newman[4] developed a one-domensional model for the silicon deposition from SiCl4 in excess of hydrogen on a rotating disk. Narusawa[5] studied numerically deposition from chlorosilanes in a horizontal reactangular reactor. He determined reaction constants for gas-phase reactions involving SiCl4, SiHCl3, SiCl2, HCl, and H2 by comparing their predictions with experimental data taken by Aoyama et al.[3] His predicted results suggest that the effect of buoyancy-driven secondary flow may cause nonuniform deposition.

In a vertical stagnation flow reactor configuration, the substrate faces upwards and the primay direction of gas flow is downwards, perpendicular to the substrate. Transport phenomena studies in these reactors have shown that natural convection effects limit the actual area of uniform growth across the susceptor. Buoyancy-driven recirculation between parallel horizontal surafces is inhibited when the hotter surface is at the top, which is the inverted reactor configuration. The probability of particle contamination is lower for this reactor configuration.

In this study, we investigated numerically the CVD of silicon films from silicon tetrachloride and hydrogen in an inverted single-wafer reactor. We adopted gas-phase reaction mechanism scheme employed by Narusawa[5]. We considered here an inverted reactor configuration to reeduce natural convection effect leading to unform deposition. The effect of reactor geometry and operating conditions such as deposition temperature, pressure, reactant flow rate is rigorously explored.

MODEL DEVELOPMENT

The mathematical model developed for the Si CVD in an inverted single-wafer reactor includes a set of partial differential equations with suitable boundary conditions describing fluid flow, heat transfer, mass transport of chemical species. The thermal diffusion is included in the model because there is a large temperature gradient in the cold-wall CVD reactor.

In this work, we consider the following two gas-phase reactions suggested by Narusawa

[eqn: rm SiCl_4 ~+~ H_2 ~LRARROW & SiHCl_3 ~+~HCl# rm SiHCl_3 ~LRARROW & SiCl_2 ~+~ HCl]

We assumed that silicon is deposited from surface reactions of SiHCl3 and SiCl2 with the latter being the dominant deposition species.

The reactor configuration considered here is depicted in Fig. 1. The reactor is assumed to be axisymmetric, with no variations or velocity components in the angular direction. To solve numerically a set of governing equations along with boundary conditions in two-dimensional cylindrical coordinates, we employ in this work control-volume-based finite difference methods. The velocity field is first obtained from the discretized momentum equations in the z and r directions, employing SIMPLER (semi-implicit method for pressure-linked equations- revised) algorithm [6]. The energy equation is then solved. Finally, the species balance equations are solved based on velocity and temperature fields. The iterative procedure is followed until a satisfactory convergence is achieved. The residuals of the equations are typically less than 10-7 after 1000 iterations. All calculations are carried out on a Solbourne computer (Axil 311).

RESULTS AND DISCUSSION

The mathematical model described above is used to investigate the deposition of Si films from SiCl4 diluted in the hydrogen in an inverted stagnation flow reactor. The computational domain typically consists of 90 grid cells in the axial direction and 26 grid cells in the radial direction. To check the cell independence of the results, calculations are also made on a 180[eqn: TIMES]52 cell for one representative situation. The calculated deposition rates on the fine cell are found to differ less than 5% from those obtained on the standard cell.

The base conditions for the present analysis are: total flow rate = 5000 sccm, SiCl4 flow rate = 25 sccm, deposition pressure = 760 torr, wafer temperature = 1400K, inlet temperature = 300K, reactor diameter = 20 cm, and  distance between the inlet and the wafer = 12 cm. These conditions are based on experimental conditions given by Aoyama et al.[3] The wall temperature is assumed to be 400K.

According to calculated results, concentrations of HCl, SiCl4, SiHCl3, and SiCl2 vary substantially near the wafer surface over the range of operating conditions investigated in this work. The distributions of predicted deposition rates are shown in Fig. 2. We see that the deposition rate decreases as the total flow rate is increased. This may be because the deposition species are generated insufficiently in the gas phase with increasing total flow rate resulting from the reduced residence time. The deposition rate increases slightly as one goes from the wafer center to the wafer edge. In a common CVD process, the deposition rate is increased with increasing total flow rate since the convective mass transfer is improved. Fig. 2 indicates that thermal diffusion reduces the predicted deposition rates at a low velocity of 3.2 cm/s. However, at a high velocity of 32 cm/s, thermal diffusion does not play an important role in predicting deposition rates.

CONCLUSION

A new mathematical model has been developed for CVD of silicon from silicon tetrachloride in an inverted single-wafer cold-wall reactor. We employed the mechanism scheme for gas-phase reactions suggested by Narusawa[5]. The effect of various operating conditions is systematically explored. Predicted results show that concentrations of reactive species vary substantially near the wafer surface over the range of operating conditions considered here. Thermal diffusion reduces the predicted deposition rates at a low flow rate, but it does not play an important role at a high velocity.

REFERENCE

1. W.H. Shepherd, J. Electrochem. Soc., 112, 988 (1965).

2. J. Bloem, W.AW. Claassen, and W.G.J.N. Valkenburg, J. Crystal Growth, 57, 177 (1982).

3. T. Aoyama, Y. Inoue, and T. Suzuki, J. Electrochem. Soc., 130, 203 (1983).

4. R. Pollard and J. Newman, J. Electrochem. Soc., 127, 744 (1980).

5. U. Narusawa, J. Electrochem. Soc., 141, 2072 (1994).

6. S.V. Patankar, "Numerical Heat Transfer and Fluid Flow", Hemisphere Publishing Corp., Washington, DC (1980).

Figure 1. Schematic diagram of the inverted single-wafer reactor.

Fig. 2. Deposition rate profiles across the wafer surface. The solid curve is without thermal diffusion and the dashed curve is with thermal diffusion.