Large Area Deposition of Amorphous and Microcrystalline Silicon by Very High Frequency Plasma

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Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society

centrally placed on the reactor floor (grounded electrode). Large windows positioned at the end of 10 cm long extension tubes were introduced into our reactor for plasma diagnostics, and grids were placed at the reactor wall to preserve electrical continuity and plasma confinement. The interelectrode RF voltage distribution across the electrode surface in the plasma zone was measured in the absence of plasma with a passive RF voltage probe connected to a floating oscilloscope as shown in Fig 1(b). In situ FTIR absorption spectroscopy was used to measure the silane gas concentration and the degree of dissociation D defined by: D = (nSiH4 -- SiH4 ) n~iH 4 , where SiH iand 4 are respectively the SiH4 density with and without plasma at constant total pressure. This technique is based on the absorption measurement of the Q branch of the V3 ro-vibrational band at 2100 cm-' and is described in detail in [8]. A microwave resonant cavity technique was used to measure the electron density. For this purpose a low power microwave signal of variable frequency in the 10 GHz range was coupled into the plasma reactor by means of a rectangular X band wave guide. The response of the cavity was recorded by another antenna and a diode placed at the end of a rectangular wave guide and processed by a digitizing oscilloscope. Generally, cavity measurements are performed using a single resonant mode and the electron density measured is weighted over the density and the electric field distribution in the cavity. In our case, due to the large cavity dimensions and the low Q factor, it was not possible to isolate a single resonant mode. The observed peaks in the frequency spectrum were therefore formed by superposition of several resonant modes. However, since the electron density can be considered to be uniform in the horizontal plane and the wave guides were oriented to excite and detect only the fundamental vertical mode, Af/f (where Af is the frequency shift due to the plasma) for the different modes is the same. Therefore the shift of any one peak can be used to determine the electron density. The film thickness uniformity was measured by an ex situ global interferometry technique [9] and the in situ deposition rate was measured at one point using a laser interferometer. Plasma parameters relevant to the deposition of amorphous silicon were chosen for the frequency study, namely, a 100 sccm flow of silane with a pressure of 0.2 Torr and a reactor temperature of 200'C. The excitation frequency was varied from 13.56 MHz to 70 MHz and the power dissipated in the plasma, estimated by the subtractive method, was kept constant at 80 W. (a) wave guide:

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