Measurements of SiH 3 and SiH 2 Radical Densities in RF Silane Plasmas using Laser Spectroscopic Techniques
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MEASUREMENTS OF SiH3 AND SiH2 RADICAL DENSITIES IN RF SILANE PLASMAS USING LASER SPECTROSCOPIC TECHNIQUES
T. GOTO Department of Electronics, Nagoya University, Nagoya 464, Japan
ABSTRACT In the RF (13.56 MHz) silane plasma, recently the SiH3 radical density was measured using infrared diode laser absorption spectroscopy, and the correlation between the SiH3 radical density and the growth rate of hydrogenated amorphous silicon thin film was investigated. The SiH2 radical density was also measured using modified laser induced fluorescence spectroscopy and intracavity laser absorption spectroscopy. Those measurement methods and main results are reviewed here.
1. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) thin films are produced by RF discharge silane plasma usually. In that RF plasma, although ions, neutral radicals, molecules and electrons exist, the non-emissive neutral radicals in the electronic ground states are possibly the main precursors of the thin film. Especially among them, SiH3 and SiH2 radicals have been regarded as the important precursors, but there has been controversy about the contribution of the SiH3 and Sill2 radicals to the formation of a-Si:H thin film[ 1-31. To clarify how the SiH3 and SiH2 radicals contribute to the thin film formation quantitatively, information on the densities of those radicals under actual CVD conditions has been demanded. However there have been no methods to measure the SiH3 and SiH2 radicals before. With such a background, we developed the in-situ measurement method of the radical density in the plasma using infrared diode laser absorption spectroscopy (IRLAS) and succeeded in measuring the SiH3 radical density in the dc pulsed silane plasma[4-6] and also in the RF silane plasma[7,8] for the first time. On the basis of those results, the contribution of the SiH3 radical to the thin film formation was discussed[7,9]. Moreover we succeeded in detecting the SiH2 radical in the RF silane plasma using modified laser induced fluorescence spectroscopy (denoted by MLIF here) for the first time[10]. Since the SiH2 radical is reactive with the SiH4 molecule, its density is rather low (on the order of 1I9 cm-- or even lower). In addition, the SiH2 radical populated to the electronic excited state by laser irradiation is predissociated. Therefore the SiH2 radical has never been detected with LIF while it was measured with the intracavity laser absorption spectroscopy (ICLAS)[I I]. In this review paper, the measurement method and main results of the SiH3 radical by IRLAS are described in Sec.2, and those of the SiH2 radical by MLIF and ICLAS are described in Sec.3. On the basis of those measured results, the contribution of the SiH3 and SiH2 radicals to the a-Si:H thin film formation are discussed briefly in each section. For the comparisons, the measured results on SiH and Si are also introduced briefly in Sec.4.
Mat. Res. Soc. Symp. Proc. Vol. 297. @1993 Materials Research Society
2. MEASUREMENT OF THE SiH3 RADICAL BY IRLAS
2.1 Measurement method and experimental F
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