Real-Time Monitoring of Hydrogen Elimination Processes in Pulsed-Gas PECVD Using in Situ Mass Spectroscopy
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structural transition [1-7]. However, the vastly varied experimental conditions that have been used have not yielded a universal mechanism. In situ mass spectroscopy has been used to monitor hydrogen elimination mechanisms in post-deposition deuteration of sputtered hydrogenated amorphous silicon (a-Si:H) [8]. Similar postdeposition deuteration experiments have been performed on plasma or hot-wire assisted CVD a-Si:H [9,10]. In this article, we will discuss results obtained using in situ mass spectroscopy to monitor the hydrogen elimination mechanisms during atomic hydrogen exposure in a cyclic process to form microcrystalline silicon. These results will be used to obtain kinetics of critical hydrogen elimination mechanisms; abstraction and etching. EXPERIMENT The rf plasma deposition system [11] and the mass spectroscopic chamber [12] have been discussed in detail elsewhere. Films have been formed by varying the extent of atomic hydrogen (deuterium) exposure during pulsed-gas deposition. In this process, a thin layer of amorphous silicon is deposited during tON and then the growth surface is exposed to hydrogen (deuterium) plasma during tOFF. The expected flow of gases in the chamber during pulsing (without plasma) is shown in 755
Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
Fig. 1. Trace amounts of HD contamination is present in the D2 bottle supplied and will be seen during tOFF. The flow of gases after shutting off the valves decrease slowly to zero due to the residence time in the chamber. This cycle is repeated to obtain thick films. The mass spectrometer has been set up to probe the products and unused reactants exiting the chamber. The mass spectrometer is connected to the deposition system using a 75 pm orifice that enables the mass spectroscopic chamber to be maintained at 4x10- 6 torr during deposition at typical pressures of 0.6 torr. Simple flow analysis using fundamentals of mass transport show that flow in the deposition chamber is laminar, diffusive and well-mixed, ensuring that the species probed using the mass spectrometer is representative of those in the deposition chamber. For mass spectroscopic studies, deuterium is used in place of hydrogen. Products from surface reactions during atomic deuterium exposure are monitored using mass spectroscopy. Mass spectroscopic data used to determine reaction kinetics have been obtained using tON= 6 0 s and tOFF= 9 0 s. These long cycle times have been used to minimize signal fluctuations due to pulsing of gases. The temperature was varied between room temperature (25*C) and 150'C and rf powers of 10 W and 50 W were used. Other conditions for deposition were, a pressure of 0.6 torr in the deposition chamber, 2% silane diluted in helium with a flow rate of 100 sccm and a hydrogen (deuterium) flow of 60 sccm. RESULTS Fig. 2 shows infrared spectra of films deposited at a substrate temperature of
0O-
Z0
.6
O,0
< .0 U
0
2500 2000 1500 1000 500 Wave number (cm-1)
100 200 300 400 500 600 time (s)
Fig. 1: Expected flow of gases in
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