Identification of Growth Precursors In Hot Wire CVD of Amorphous Silicon Films
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Identification of Growth Precursors In Hot Wire CVD of Amorphous Silicon Films
H. L. Duan, G. A. Zaharias and Stacey F. Bent Department of Chemical Engineering Stanford University Stanford, CA 94305
ABSTRACT
A soft ionization laser-based technique using 10.5 eV photon energy has been used to probe radical growth precursors in the hot wire chemical vapor deposition (HW-CVD) of a-Si:H. Using a Re filament, it is shown that Si, SiH3, and Si2H6 are the major silicon-containing species formed from the hot wire dissociation of silane, and SiH2 is at most a very minor product. However, chamber history is found to influence the radical species produced; i.e. SiH3 and Si2H6 are largely related to the chamber wall and filament conditions. The gas species produced by W and Re filaments at wire temperatures between 1000oC and 2000oC have been studied and compared. Heating the filament to higher temperatures increases the flux of Si, SiH3 and Si2H6 in a similar fashion for both filament materials. Above 1800oC, the Si intensity saturates, while SiH3 and Si2H6 show monotonic increase without saturation up to 2000oC.
INTRODUCTION The hot-wire chemical vapor deposition (HW-CVD) of amorphous and polycrystalline silicon from silane has been the subject of intensive research, in part because the technique has been shown to produce films of superior quality in comparison to the more conventional growth method of plasma-enhanced (PE) CVD.1,2 Although gas phase radicals are known to play an important role in the film growth, reports differ regarding the identity of precursors produced by the hot filament. According to a study by Doyle et al., the primary hot-wire products are Si and H radicals, together with a small portion of SiH3.3 Nozaki et al reported that under collision-free conditions Si was a major product of hot-wire decomposition and that production of SiH and SiH3 was negligible.4 At the higher pressures commonly used in film deposition, however, they observed that the density of SiH3 becomes even larger than that of Si due to gas-phase reactions. Nozaki et al also note that in the absence of silane, pure hydrogen gas results in SiH3 production via etching of Si by H on the chamber walls.5 In contrast, in addition to Si, Inoue et al observed significant SiH2 and SiH3 species below 1700K, which became negligible above 1700K.6 Additionally, Holt et al observed SiH2 as a hot-wire product under non-collisional conditions using threshold ionization mass spectrometry.7 As these various results demonstrate, detecting free radicals under HW-CVD conditions can be a complicated task. One advantage of the SPI technique employed here is its ability to detect all relevant radicals simultaneously, thereby adding useful information to the growing understanding of the hot-wire chemistry of silane.
A3.1.1
Filament lifetime is one of the critical technical issues impacting the viability of the HWCVD technique. It has been shown that the usable lifetimes of different filament materials vary under similar deposition conditions. One likely
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