AVS 45th International Symposium
    Plasma Science and Technology Division Wednesday Sessions
       Session PS-WeA

Paper PS-WeA2
Gas Phase and Surface Kinetics in Plasma Enhanced Deposition of Silicon Nitride: Effect of Gas Dilution on Electron Energy Distribution, Radical Generation, and Film Composition

Wednesday, November 4, 1998, 2:20 pm, Room 318/319/320

Session: Plasma-Surface Interactions I
Presenter: T.M. Klein, North Carolina State University
Authors: T.M. Klein, North Carolina State University
C.S. Yang, North Carolina State University
A.I. Chowdhury, North Carolina State University
G.N. Parsons, North Carolina State University
Correspondent: Click to Email
Gas diluents, including N@sub@2, He, H@sub 2@, Ar, etc., are often used in silicon nitride plasma CVD to improve film density and electronic properties. We have formed silicon nitride films by parallel plate rf PECVD using SiH@sub 4@/NH@sub 3@ and SiH@sub 4@/N@sub 2@ source gases, and studied the effects of H@sub 2@, He, and N@sub 2@ dilution with substrate temperatures between 350 and 25°C. The plasma was characterized using optical emission and mass spectroscopy, and the thin films were characterized using infrared transmission, IV and CV measurements. We find that dilution can control the Si-H and N-H bond concentrations, and can improve the electrical performance in silicon nitride formed at very low temperature (<125°C). However, H contents are very high (>35 at. %) at low temperature, resulting in high etch rates. Using SiH@sub 4@ and N@sub 2@ source gases, N/Si ratios > 1.3 can be achieved with hydrogen content < 20 at.%. However, optimized N/Si in the film is not obtained by simply increasing N@sub 2@ in the gas phase. At 12W and 250°C, increasing the N@sub 2@ gas fraction from 16% to 37%, leads to a decrease the N/Si in the film from 1.37 to 1.27. Using mass spectroscopy, the silane consumption fraction is greater than 90%, and significant change in silicon incorporation is not expected with N@sub 2@ dilution. We have modeled the effect of N@sub 2@ dilution on the electron energy distribution in the plasma using available software to solve the Boltzmann equation, and developed a simple gas kinetic model to estimate relative N and SiH@sub x@ radical concentrations. The model shows the high energy (>10eV) electron density decreases with N@sub 2@ dilution, consistent with the experiments. The model can predict the effects of gas residence time and He and N@sub 2@ dilution during nitride deposition from SiH@sub 4@/NH@sub 3@ mixtures. These results demonstrate a detailed understanding of gas dilution effects in plasma deposition, and indicate that optimized material properties can be predicted from detailed gas reaction analysis.