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Electron beam recrystallization of chemically vapor deposited polysilicon films
Thin Solid Films, 63 (1979) 195 © ElsevierSequoiaS.A., Lausanne--Printedin the Netherlands
195
ELECTRON BEAM RECRYSTALLIZATION OF C H E M I C A L L Y VAPOR DEPOSITED POLYSILICON FILMS* S. J. SOLOMON, A. C. GREENWALD, W. E. NEAL AND A. R. KIRKPATRICK
Spire Corporation, Bedford, Mass~O1730 ( U.S.A.)
A two-step treatment consisting of pulsed electron beam heating followed by swept d.c. electron beam heating was used to recrystallize chemically vapor deposited (CVD) polysilicon deposited onto foreign substrates. The resulting crystallites are significantly broader than their thickness (20-50 Ixm) and hence are useful for thin film silicon solar cells. The two-step thermal process was chosen because reported single-step treatments have not produced silicon crystallites of the necessary aspect ratio. The pulsed beam treatment consists of exposing the full sample surface to an electron beam generated by discharging a capacitance through a plasma diode. The resultant electron beam has current densities in the 103 A cm-2 range and pulse widths of the order of 100 ns. In this heating mode, the temperature profile in the sample is determined by the electron absorption profile of the sample. As a consequence, the thickness heated is determined by the electron energy and the temperature is determined by the beam current. The resulting one-dimensional thermal gradient produces the geometry necessary for directional crystallization. If the beam current is sufficient to melt the CVD polysilicon to a depth that approaches the film thickness, the formation of columnar crystallites with rather small cross sections at the liquid-solid interface is uncontrolled. The d.c. electron beam treatment converts the columnar crystallites into crystallites with cross sections useful in solar cells. This treatment is carried out in a conventional electron beam welder modified to optimize the current density and produce the required scan velocities. Two d.c. heating modes are possible. In the fast scan mode, analogous to pulsed beam heating, the thermal gradient produced in the sample is unidirectional. In the slow scan mode, analogous to conventional electron beam heating, the thermal gradient is three-dimensional. While the latter d.c. beam configuration is more easily realized, the former is more desirable since it does not heat the material below the melt interface. This work was supported by the U.S. Department of Energy under contract EG-77-C-01-4106.
*Abstract of a paper presented at the InternationalConferenceon MetallurgicalCoatings,San Diego, California,U.S.A.,April 23-27, 1979.
195
ELECTRON BEAM RECRYSTALLIZATION OF C H E M I C A L L Y VAPOR DEPOSITED POLYSILICON FILMS* S. J. SOLOMON, A. C. GREENWALD, W. E. NEAL AND A. R. KIRKPATRICK
Spire Corporation, Bedford, Mass~O1730 ( U.S.A.)
A two-step treatment consisting of pulsed electron beam heating followed by swept d.c. electron beam heating was used to recrystallize chemically vapor deposited (CVD) polysilicon deposited onto foreign substrates. The resulting crystallites are significantly broader than their thickness (20-50 Ixm) and hence are useful for thin film silicon solar cells. The two-step thermal process was chosen because reported single-step treatments have not produced silicon crystallites of the necessary aspect ratio. The pulsed beam treatment consists of exposing the full sample surface to an electron beam generated by discharging a capacitance through a plasma diode. The resultant electron beam has current densities in the 103 A cm-2 range and pulse widths of the order of 100 ns. In this heating mode, the temperature profile in the sample is determined by the electron absorption profile of the sample. As a consequence, the thickness heated is determined by the electron energy and the temperature is determined by the beam current. The resulting one-dimensional thermal gradient produces the geometry necessary for directional crystallization. If the beam current is sufficient to melt the CVD polysilicon to a depth that approaches the film thickness, the formation of columnar crystallites with rather small cross sections at the liquid-solid interface is uncontrolled. The d.c. electron beam treatment converts the columnar crystallites into crystallites with cross sections useful in solar cells. This treatment is carried out in a conventional electron beam welder modified to optimize the current density and produce the required scan velocities. Two d.c. heating modes are possible. In the fast scan mode, analogous to pulsed beam heating, the thermal gradient produced in the sample is unidirectional. In the slow scan mode, analogous to conventional electron beam heating, the thermal gradient is three-dimensional. While the latter d.c. beam configuration is more easily realized, the former is more desirable since it does not heat the material below the melt interface. This work was supported by the U.S. Department of Energy under contract EG-77-C-01-4106.
*Abstract of a paper presented at the InternationalConferenceon MetallurgicalCoatings,San Diego, California,U.S.A.,April 23-27, 1979.