Doctoral Thesis

07/30/03

Bahasa Indonesia: tekan disini

Particle Trapping Behavior in Plasma-Enhanced Chemical Vapor Deposition Reactor and Its Effects on Wafer Contamination

By Heru Setyawan

Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University

Supervisor:

Professor Kikuo Okuyama

Abstract

The main objectives of this study are to investigate particle behavior in plasma-enhanced chemical vapor deposition reactor and its effects on wafer contamination. Particles in the PECVD reactor were visualized by laser light scattering (LLS) technique combined with video imaging. The contamination level on the wafer surfaces was inspected by scanning electron microscopy (SEM). Two approaches were used in this study, namely using a model experiment and using an actual plasma deposition process. In the model experiment, particles are introduced into the reactor from the outside. Because of the difficulty in preparing test aerosols at low pressure, the test aerosols with known diameter were produced at atmospheric pressure and then introduced into a low-pressure reactor through a capillary or other pressure reducer. This needs better understanding of the rarefied gas flow in a tube or an aperture. Tetraethylorthosilicate (TEOS)/oxygen plasma was used as the test case for the actual PECVD process.

This thesis is organized as follows. The background and objectives of the study are described in Chapter 1. In addition, review on previous studies relating to the present study is presented.

Rarefied gas flow through a circular tube of finite length is presented in Chapter 2. The conductance of nitrogen gas was measured for flow in short and long tubes. The pressure ratios across the tube were from low to high and the flow is in continuum to transition regimes. The flow in the continuum to transition regimes at low Knudsen number was analyzed numerically using continuum approach. Free molecular approach was used to analyze the flow in the transition regime at high Knudsen number using direct simulation Monte Carlo method (DSMC) method. A semi-empirical equation was derived by substituting the Poiseuille term in the Bernouli formula with Hank-Weissberg’s equation under the condition of isothermal expansion. The proposed equation is valid in the range from continuum to transition regimes at low Knudsen number.

Chapter 3 describes the visualization and numerical simulation of fine particle transport in the low-pressure parallel-plate reactor without plasma. A system was constructed that permits particle motion in the reactor to be visualized. The effects of pressure and temperature on particle transport in the reactor were examined. A three-dimensional numerical simulation was performed using the commercially available computational fluid dynamics code Fluent. A detailed configuration of the reactor, including the showerhead structure was considered in the simulation. It is found, both experimentally and by numerical simulation that, when the wafer-substrate plate is not heated, the effect of pressure on particle trajectory in the space between plates is not significant. However, the particle trajectory is apparently influenced by pressure at elevated temperature. It seems that the thermophoresis dominates the mechanism of particle transport. The experimentally observed phenomena were satisfactorily reproduced by simulation.

The distribution and transport of fine particles trapped in the radio-frequency (rf) PECVD rector is described in Chapter 4. Structured clouds of particles were observed at localized regions between the holes below the showerhead. Typically, at a high rate of gas flow, particles emerging from the showerhead holes overshoot the equilibrium position of the particle trap, and the particle clouds in the trap were small and thin (winding mode). At a low rate of gas flow, the particles are directly attracted to the trap locations, and large particle clouds (lumping mode) were observed. The particle number concentration of trapped particles tends to increase with increasing rf power and decrease with increasing particle size. When the gas flow rate is increased, a sharp decrease occurs at a certain flow rate.

In Chapter 5, the effects of particle trapping behavior on wafer contamination are described. A method was proposed to determine the occurrence time of particle contamination by making use of the capability of thermophoresis to shield the wafer from particle deposition. It was found that the particle contamination occurred during the postplasma when the injected particles were trapped in the sheath region below the powered electrode. When the injected particles were not trapped due to a strong inertial effect of particle, the contamination occurred during plasma operation. There is a regime of operation condition in which the lowest level of contamination occurs. Most particles were found to retain their negative charge in the postplasma. The charge on these particles was determined from particle motion.

Chapter 6 describes particle formation and growth in TEOS/O2 rf plasma as well as the particle trapping behavior The particles are found to reside around the sheath near the electrodes. The particles trapped around the sheath near the powered electrode (showerhead) are located in localized regions between the showerhead holes. The particles form a lumping cloud at low gas flow rate, change to a line shape with increasing the flow rate, and finally the LLS technique cannot detect them any longer when the flow rate becomes high. These are also observed for the case of model experiment. The particle trapping behavior described above has shown clearly to influence particle contamination on the wafer. It seems that the particles grow through coagulation. High gas flow rate and high substrate temperature tend to suppress particle formation and growth. On the other hand, the particle formation and growth is enhanced with increasing the rf power.