Invited Paper PS+TC-WeM3
Atmospheric Pressure Plasma Processes for Preparation of Si-Based Thin Films

Wednesday, October 31, 2012, 8:40 am, Room 24

Thin-film deposition processes using atmospheric-pressure (AP) plasmas have attracted considerable attention as the low-cost deposition methods, because they can realize low-temperature and high-rate deposition without the necessity for ultrahigh vacuum. Recently, we have demonstrated that good-quality Si films with almost no ion damage can be prepared by using a 150-MHz very high-frequency AP plasma. Based on this result, we are studying applications of AP plasmas in semiconductor processing, e. g., i) atmospheric-pressure plasma chemical vapor deposition (AP-PCVD) of semiconductor and insulator thin films, ii) AP plasma oxidation of Si and Al, and iii) atmospheric-pressure plasma enhanced chemical transport (APECT) processes for Si purification and deposition. In this work, we will report the experimental results on the formation and characterization of Si-based thin films prepared by these AP plasma processes.

Firstly, we have studied the epitaxial Si growth by AP-PCVD. The purpose of the study is to develop a totally low-temperature semiconductor device fabrication process. Epitaxial Si films have been prepared on 4-inch-(001) Si wafers by AP-PCVD using a porous-carbon electrode. Defect-free growth of epitaxial Si is confirmed in the temperature range of 470–570°C by transmission electron microscopy and a selective etching method. A high carrier generation lifetime (≈ 2.0 ms) is observed in the Si film grown at 570°C with a reasonably high growth rate (≈ 0.4 μm/min). In situ H₂ plasma cleaning of the substrate surface is effective for eliminating O and C concentration peaks at the film/substrate interface. Heavy B doping with a carrier concentration of about 10²⁰ cm⁻³ is achieved using B₂H₆ as a doping gas at 570°C. The relation between the mobility and carrier concentration in p- and n-doped Si films can be well fitted by the reported curves for bulk Si single crystals. These results demonstrate that the electrical quality of Si epitaxial films grown by AP-PCVD is sufficiently good for semiconductor device applications.

One of the other topics is on the AP plasma oxidation of Si. AP plasma oxidation is supposed to be most compatible for a chamber-less and open-air process, which is attractive as a high-throughput oxidation process for photovoltaic applications. SiO₂/Si structures have been prepared by AP plasma oxidation at 400°C using gas mixtures of O₂ and He. Various characterization results reveal that the properties of AP plasma oxides are similar to those of high-temperature thermal oxides, and that the SiO₂/Si structure has a low interface state density of the order of 10¹⁰ cm^-2eV^-1.