Ind. Eng. Chem. Res. 2005, 44, 7907-7915
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APPLIED CHEMISTRY Quantitative Analysis on the Growth of Negative Ions in Pulse-Modulated SiH4 Plasmas Dong-Joo Kim and Kyo-Seon Kim* Department of Chemical Engineering, Kangwon National University, Chuncheon, Kangwon-Do 200-701, Korea
The evolutions of chemical species (SiH4, SiHx, SiHx+, and polymerized negative ions) in the pulse-modulated SiH4 plasmas during the plasma-on and -off times were analyzed by solving the model equations of chemical species. During the plasma-on time, the SiHx concentration increases with time, mainly by the generation reaction from SiH4, but during the plasma-off time, it decreases because of the reaction with H2. During the plasma-on time, the concentrations of negative ions increase with time by the polymerization reactions of negative ions, and those become almost zero in the sheath regions because of the electrostatic repulsion. During the plasma-off time, the concentrations of negative ions decrease with time by the neutralization reactions with positive ions, and some negative ions can diffuse toward the sheath regions because there is no electric field inside the reactor. Our theoretical analysis shows quantitatively that the pulse-modulation plasma technique can be an efficient method to reduce the polymerized negative ion clusters, which are not good precursors for a high-quality thin film and which can also be the sources of particle contamination. Introduction The SiH4 plasma processes can be used to synthesize the hydrogenated amorphous silicon (a-Si:H) thin films, which are widely used as the solar cell, image sensor, electrophotographic drum, and thin film tansistor (TFT).1-4 In the a-Si plasma chemical vapor deposition (PCVD) processes, the amorphous silicon thin films grow slowly with a deposition rate of 1-3 Å/s, and it is desirable to increase the deposition rate without damaging the device performance. In general, increasing the deposition rate by just raising the power or reactor pressure in the conventional continuous-wave plasma discharge results in the generation of particles in the range from nanometers to microns, followed by the particle contamination to deteriorate the performance of microelectronic devices.1-3 The pulse-modulated plasma discharges have been considered as a relatively simple method to control the deposition characteristics of thin films,4-12 and several studies have reported that various films of amorphous silicon,1-2,13 polycrystalline silicon,1,2 titanium nitride,14 amorphous hydrogenated silicon carbide,15 diamond,16 silicon dioxide,17 and fluorocarbon9-12 were deposited by using the pulse-modulated plasmas. Timmons and colleagues10-12 synthesized the fluorocarbon thin film of low dielectric constant from pentafluorostyrene monomer as the next-generation materials for integrated circuits by adjusting the duty cycles in the pulsedplasma processes. Madan and Morrison1 reported that the amorphous/polycrystalline silicon thin films were * To whom correspondence concerning this article should be addressed. Tel.: 82-33-250-6334. Fax: 82-33-251-3658. Email:
[email protected].
synthesized at the maximum deposition rates of 15 Å/s in the pulse-modulated PCVD. Itagaki et al.18 used the pulsed electron cyclotron resonance plasmas to synthesize the a-Si:H thin films and showed that the deposition rate and the quality of thin films can be changed by the pulse modulation and that high-quality thin films can also be grown at a deposition rate of 14 Å/s without substrate heating. The effects of the polymerization of nanosized clusters on the particle generation and the quality of thin films in plasma processes have also been studied by several researchers. Watanabe and colleagues4-8 reported that the deposition of a few nm of particles leads to a deterioration in the quality of the a-Si:H thin films and also that the SinHx (n ≈ 4) are critical clusters for particle nucleation in the SiH4 plasmas. They also suggested that the growth of nanosized particles in plasma reactors can be suppressed efficiently by using the pulse-modulated plasma technique.5,6 Howling and colleagues19-24 showed experimentally that negative ions can be polymerized to form high-mass chemical species in SiH4 plasmas. Choi and Kushner25 suggested that the negative ion clusters can stay in the plasmas for a long time by the repeatable electron attachment and neutralization reactions of high-mass clusters and that they can grow to the particles. Kirimura et al.13 analyzed the effects of the change of plasma parameters on the particle growth and also on the properties of a-Si:H thin films in pulsed-plasma processes; they synthesized the a-Si:H films of high quality at high deposition rates by the amplitude modulated plasmas. To synthesize the high-quality thin films at high deposition rates by the pulsed-plasma processes, it is quite important to analyze the plasma chemistry for particle generation and growth in the gas phase. The
10.1021/ie0503803 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/10/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 21, 2005
Figure 1. Schematic of the silane pulsed-plasma reactor for modeling.
particle dynamics combined with the plasma chemistry in the continuous-wave plasma processes have been analyzed systematically on the experimental/theoretical basis.4-8,26-32 But the quantitative analysis on the change of plasma chemistry related with the particle generation during the pulsed plasmas has not been made yet. This study focuses on the numerical investigation of the quantitative evolution of negative ions, which are considered to be the precursors for particle generation, in the pulsed SiH4 plasmas during the plasma-on and -off times. We also showed how the concentrations of several chemical species, which are related with the polymerization of negative ions, change during the plasma-on and -off times. The model equations of chemical species include the terms of plasma chemical reactions, fluid convection, diffusion, and electrical migration. Theory To analyze the plasma chemistry in the SiH4 pulsedplasma processes, the fluid approach in plasma modeling was adopted instead of the kinetic approach based
on the particle-in-cell/Monte Carlo simulation or the hybrid Monte Carlo/fluid mode, which will need the rigorous computational work. The gas flow inside the plasma reactor is in the transition regime (Knudsen no. ∼ 0.02 for the standard conditions). We might need to use the Boltzmann equation or specialized Monte Carlo equation to predict the flow in the transition regime. We believe the continuum equations (Knudsen no.
