Deposition of Highly Conductive n + Silicon Film for a-Si:H Thin Film Transistor
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Coming 7059 glass (15 in by 15 in, coated with 2000A PECVD SiNx) was loaded on the top electrode facing downward and was heated to 390'C from the backside. The bottom electrode was driven by a 60 MHz rf power supply. Feed gases were introduced from one side of the plate and exhausted to the opposite side. The feed stream was composed of SiH4 (containing 1% PH3) in the flow rate range of 30 to 200 sccm and hydrogen at 3500 or 4000 sccm. The pressure was between 0.5 and 1.0 Torr. The plasma power was in the range of 375 to 450 W. The intrinsic silicon film was deposited from pure SiH 4 and H2. The deposition rate was calculated by dividing the film thickness with the deposition time. The resistivity was determined from the 4-point probe. P and H concentrations were measured with a high resolution secondary ion mass spectroscope (SIMS) using implanted wafers as standards. The film's morphology was detected with Raman scattering and the crystallinity was determined with x-ray diffraction (XRD). RESULTS AND DISCUSSION Process Effects on Deposition Rate and Film's Resistivity Figure I shows the film deposition rate as a function of process parameters, such as the SiH 4 flow rate, the PH 3 content, the H2 flow rate, and the plasma power. Several conclusions can be summarized from this figure: 1) the deposition rate increases with the increase of the SiH4 flow rate, 2) the existence of PH 3 in the feed stream enhances the deposition rate, 3) the deposition rate decreases with the increase of the H2 flow rate, and 4) the deposition rate increases with the plasma power. Variations of deposition rate with these parameters are monotonously. 500 E 400-
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SiH 4 (1% PH3 ) Flow Rate (sccm) Figure 1. n' Film Deposition Rate as a Function of Deposition Parameters Since SiH 4 and PH 3 fragments, such as SiHx and PHx, are precursors of the n' film, it is natural that the deposition rate increases with the increase of the SiH 4 (l%PH 3) concentration in the feed stream or the power. The mechanism of the enhancement of silicon deposition rate by including PH 3 in the feed stream is not understood currently. However, it has been observed that when the plasma deposited amorphous silicon is doped with P, its epitaxial growth rate is higher than that of the undoped film [10,11]. Since the increase of deposition rate is large, e.g. more than 30%, it is unlikely that the increase of deposition rate is caused by the small amount of additional P atoms in the film, which is less than 3%. The film's electrical properties, such as the conductivity, may be responsible for the deposition rate difference. The same theory has been 892
used to explain the enhancement of silicon etch rate in the chlorine plasma by doping the film with P [12]. However, more detailed studies are required to verify it. The lowering of deposition rate at the high hydrogen concentration condition ca
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