EFFECTS OF PLASMA AND OR 193 NM EXCIMER-LASER IRRADIATION IN CHEMICAL-VAPOR DEPOSITION OF BORON FILMS FROM B2H6+HE

Plasma-enhanced chemical-vapor deposition (PECVD) with the surface irradiated by a 193 nm, 50 Hz pulsating laser was performed to synthesize boron films from B2H6 + He at a pressure of 200 Pa, where the plasma was employed to generate precursor radicals for the growth while the irradiation was intended for photochemical enhancement of the surface processes such as migration and growth reactions. In addition to the PECVD, PECVD without the irradiation as well as pyrolytic CVD with and without the irradiation were done so as to clarify the effects of the plasma and the laser in the CVD. Micromorphological boron columns were found to grow toward the laser light, and this indicated directly that the surface growth reactions were enhanced photochemically at the laser energy density of 170 mJ/cm2 per pulse. Heating of the surface by the irradiation at this energy density was estimated to be negligible. In the pyrolytic CVD, where the surface migration of the precursor radicals was considered to be relatively hindered according to measured activation energies, a morphological change was found to indicate irradiation-enhanced migration at the laser energy density of 3 mJ/cm2 per pulse. Semiempirical molecular orbital calculations predicted that borane molecules (BH3) should be the dominant precursor in the pyrolytic CVD while the counterpart could be BH2 radicals in the PECVD: This prediction supports a hypothetical photoinduced growth reaction such as BH2* + nhv = B(s) + H2 (n = 1,2, ...) where BH2* signifies a BH2 radical chemisorbed to a site for growth reaction and B(s) does a boron atom incorporated into the solid structure. This prediction was also consistent with the experimental result that crystalline boron films have grown only in the PECVD while the pyrolytic CVD yielded only amorphous growth at substrate temperatures between 690 and 890-degrees-C. The predicted precursor BH3 for the pyrolytic CVD suggested its photoinduced migration mechanism similar to the photolysis of diborane.