Low Temperature Deposition of Polycrystalline Silicon thin Films by Hot-Wire CVD
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The reactor configuration is shown in Figure 1. Hydrogen and disilane were introduced into the reactor as indicated in the figure: only hydrogen passed the filament assembly, disilane entered the reactor through a dispersal ring bypassing the filament. The distance between filament cartridge and the susceptor was 4 cm and the dispersal ring was half way between them. The reactor was pumped by a rotary vane pump with a roots blower. The pumping capacity set the lower limit of the deposition pressure for a certain gas throughput. In this study, the deposition pressure was controlled by the gas flow rates. The deposition conditions are summarized in Table I.
Substrate• Dispersal Ring I:• i r
Filament
'
AC PowerI
~Supply MFC
Si H
2 6 t-- H2
Figure 1. Schematic diagram of the reactor configuration. The films were deposited on oxidized c-Si with 2500A Si0 2 or Coming 7059 glass substrates. The thickness of the deposited films were measured by a Leitz MVP-SP system based on interference analysis and a Sloan Dektak IIA; for Dektak measurement, a step was created by using a mechanical mask. To calculate the deposition rate from the film thickness data, the incubation time before the film start to grow has been included. TEM samples were thinned by grinding on sand papers and then ion milling. Then they were examined at 200 kV in a JEOL2000 transmission electron microscope. Surface morphology of the films were examined at 5 kV in a Hitachi S800 field emission scanning electron microscope. The Raman analysis was done on a Spex 1877 TRIPLEMATE spectrometer equipped with a Spex SpectrumOne liquid nitrogen cooled CCD detector system. The 488 nm line of an Ar+ laser was used as the excitation source. X-ray diffraction (XRD) was done on a Rigaku Rotaflex rotating anode x-ray diffraction unit. Table I. Deposition Conditions 1700 0C 310 - 450 oC 20 - 70 mTorr 0.53 - 10 sccm 18 - 97 sccm
Filament temperature (Tf) Substrate temperature (Ts) Reactor pressure (Pd) Disilane flow rate (FR(Si 2H 6)) Hydrogen flow rate (FR(H 2))
70
RESULTS AND DISCUSSION Deposition Rate Deposition rate is significantly influenced by the disilane flow rate. The result is shown in Figure 2. For a fixed hydrogen flow rate, the deposition rate increases linearly with disilane flow rate, and reaches 1500 A/min at a disilane flow rate of 10 sccm. At this point, the deposition rate does not show any sign of saturation implying that higher growth rate are achievable at higher disilane flow rates. This result indicates that the deposition rate may be limited by the gas phase concentration of the film forming species. However, as disilane flow rate increases, at the same time film crystallinity decreases as shown by the XRD result in Figure 2. The XRD intensity(IxRD) is the sum of the corrected intensities of the three main peaks, (111), (220) and (311), present in the XRD spectra. To obtain the corrected intensity, the raw XRD intensity is first normalized by the film thickness and then divided by the intensity ratio measured on a randomly oriented cryst
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