디에틸실란/산소로부터 이산화규소 저압화학증착의 속도론적 연구

김철진, 김의정

울산대학교 화학공학부

A Kinetic Study on LPCVD of SiO2 from Diethylsilane/Oxygen

Chul Jin Kim, Eui Jung Kim

School of Chemical Engineering, University of Ulsan

INTRODUCTION

As current trends in the high performance integrated circuits are pointing toward increasing levels of integration for interconnect technology, low temperature deposition of dielectric films is increasingly important. If one chooses aluminum as an interconnection metal, the dielectric film deposition temperature must be less than 400℃ because of hillock growth on aluminum  surfaces leading to fatal device reliability problems. Silicon dioxide films have  been widely employed as interlayer dielectrics due to their superior chemical and electrical properties. Further, silicon dioxide films produced by chemical vapor deposition have excellent confomality over severe topography.

Diethylsilane (DES) has recently gained attention as a new silicon source material suitable for low temperature deposition. Huo and Foo [1] obtained high-quality SiO2 films from DES and oxygen by a low pressure chemical vapor deposition (LPCVD) at low temperatures below 500℃. They reported   that the deposited films have a low residual carbon concentration less than about 1% and the film conformality exceeds 83%. Patterson and Ozturk [2] observed in their experiments that there is a maximum in deposition rate at around 475℃ and copious amount of SiO2 particulates are generated at temperatures above 500℃ resulting in no deposition on the wafers. They   also observed that below 0.55 torr the deposition rate is so small and above 0.95 torr ignition of the reactants in the gas phase occurs creating  considerable SiO2 particulates. Levy et al. [3] showed that the deposition rate ceased abruptly for conditions where total pressure was less than 0.35 torr and where the O2/DES ratio was higher than 2.35. They obtained a poor uniformity across the wafer of about 20%.

In this study, we investigated numerically the LPCVD of SiO2 films from DES and oxygen in a horizontal hot-wall reactor. We propose a new kinetic mechanism to explain the inversion maximum in deposition rate with increasing temperature, which is observed by Patterson and Ozturk [2]. 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 DES/oxygen LPCVD in a hot-wall reactor includes a set of partial differential equations with suitable boundary conditions describing fluid flow, heat transfer, mass transport of chemical species.

Based on the observations made by Patterson and Ozturk [2] and Martin et al. [4] we propose a new simple kinetic mechanism for the DES/oxygen CVD: DES decomposes to intermediate species in the gas phase when colliding with oxygen. We assume diethylsilanone(SIL), (C2H5)2Si=O, as a strong candidate for the intermediate as Martin et al. [4] speculated. Some of the intermediate molecules decompose completely to SiO2 in the gas phase before they reach the wafer surface. Undecomposed intermediate molecules adsorb on the wafer surface and react with two adjacent adsorbed oxygen atoms to yield SiO2 films. We assume that the oxygen molecule dissociates upon adsorption with each oxygen atom occupying one vacant site. We also assume that the deposition of SiO2 films is inhibitted by gas products such as ethylene(ETH) and water. However, the inhidition effect is found not to be critical in this analysis.

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 [5]. The energy equation is then solved. Finally, the species balance equations are solved based on velocity and temperature fields.

RESULTS AND DISCUSSION

The mathematical model described previously is used to investigate the deposition of SiO2 films from DES/O2 in a hot-wall horizontal reactor. We consider in this work the situation where five wafers are placed in the middle of the reactor. Simulated results show that there is no virtual difference in film deposition rate and uniformity between wafers.

In this work, the experimental results reported by Patterson and Ozturk [2] are utilized to determine the reaction rate constants and are compared with the model predictions. Rate constants for silanone decomposition and surface reactions are determined by matching the predicted deposition rates from the model with experimental data. The model predictions are found to be in good agreement with  experimental results.

The predictions show that below 0.5 torr the deposition rate is decreased rapidly as the pressure is reduced. Patterson and Ozturk [2] reported that the deposition rate obtained is so small below 0.55 torr. Levy et al. [3] also observed in their experiments that there is no measurable deposition rate at pressure less than 0.35 torr. The reason for this appears to be that the frequency of collisions  with activating species decreases rapidly at very low pressures. An effective model must account for this phenomenon and our model predicts this trend well.

The good  agreement between the predictions from the model we developed and the experimental deposition rate data enables us to perform a systematic study of the influence of process conditions to get a deeper insight in the effects of these variables on film growth rate and uniformity. The predictions show that at high wafer temperatures high flow rates are preferable for obtaining unform films with high deposition rate. The deposition rate becomes a maximum at the O2/DES ratio of around 2. The model suggests that one would get most uniform films at 0.6 torr where the deposition rate is close to a maximum. As the wafer temperature is increased up to 475℃, the depostion rate is increased at the expense of the film uniformity.

CONCLUSION

A mathematical model has been developed for LPCVD of SiO2 films from DES/O2 in a horizontal hot-wall reactor. A new kinetic mechanism which includes parasitic gas phase reactions is proposed to explain the inversion maximum with increasing temperature. The reaction constants associated with the kinetic mechanism proposed are determined by matching the simulated  results with the experimental data available. The model is found to explain successfully the trend of the experimental data. The predicted results show that high flow rates are preferable for obtaining unform films with high deposition rate. There exists an inverse in deposition rate at the O2/DES ratio of around 2.

REFERENCE

1. D.T.C. Huo, M.F. Yan, and P.D. Foo, "SiO2 films  by low pressure chemical vapor deposition using diethylsilane: Processing and characterization", J. Vac. Sci. Technol. A, 9, 2602 (1991).

2. J. D. Patterson and M. C. Ozturk, "Low pressure chemical vapor deposition

of silicon dioxide below 500℃by the pyrolysis of diethylsilane in oxygen", J. Vac. Sci. Technol. B, 10, 625 (1992).

3. R.A. Levy, J.M. Grow, and G.S. Chakravarthy, "Low-Pressure Chemical Vapor Deposition of Silicon Dioxide Using Diethylsilane", Chem. Mater., 5,	1710, (1993).

4. J.G. Martin, H.E. O'Neal, and M.A. Ring, "Mechanisms of Silicon Dioxide Deposition from the Low Pressure Chemical Vapor Deposition of Diethylsilane/Oxygen Mixtures", J. Electrochem. Soc., 142, 3873 (1995).

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

Figure 1. Deposition rate as a function of pressure. Experimental data(●) by Patterson and Ozturk [2]. Numerical predictions are solid lines.