Properties of Catalytic CVD SiN x for Antireflection Coatings
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with heated catalyzer placed near substrates, and so that, films are deposited at low temperatures without help from plasma nor photochemical excitation. We have already succeeded in depositing high quality SiN, thin films by the Cat-CVD method using a Sill 4 and NH 3 gas mixture [3]. These films showed good properties for the application as device passivation films, especially for GaAs devices such as high electron mobility transistor (HEMT) [4]. High quality amorphous silicon and poly crystalline silicon films with low hydrogen content have also been already obtained by this method. Applying this technique, we have succeeded in surface nitridation of Si at 200'C using NH 3 gas [5]. The method is also useful for surface cleaning [6] and nitridation of GaAs [7]. EXPERIMENT The Cat-CVD apparatus is schematically illustrated in Fig. 1. A tungsten (W) wire is coiled and spread with an area of 70 x 70 mm2 at the distance about 40 mnm from the substrates keeping it parallel to the substrate surface. It is expected to be easily scaled up for applications to large area deposition simply by spreading the catalyzer widely. The wire is heated at 1800'C by supplying the electric power directly on it and the temperatures are monitored by both an infrared thermometer and the electrical resistivity. The temperature of the substrate is usually measured by a thermocouple (TC) attached on the substrate holder beside samples. The substrate holder temperature, T.h, includes the effects of both the thermal radiation and the
161 Mat. Res. Soc. Symp. Proc. Vol. 555 ©19 99 Materials Research Society
Fig. 1. Schematic diagram of a Cat-CVD apparatus.
heated gas flow. The real sample surface temperatures, Ts,, are also monitored by attaching TC to the surface of silicon wafer directly with ceramic bonds. The difference between Th and Tsr is several tens degrees. Si(100) substrates used for the deposition were degreased and cleaned by RCA method [6]. Then, they were dipped in 2% diluted HF for 2 min. After the cleaning process, they were 7 immediately loaded into the Cat-CVD chamber. The Cat-CVD chamber was evacuated to 5x10 Torr. A gas mixture of Sill4 and NH 3 with 99.999% purity was used as the deposition gas. The gas pressure during deposition was 3 - 8 mTorr. The deposition conditions are summarized in Table I. Table I. Deposition conditions for Cat-CVD SiN5 films Parameter Silane flow rate FR(SiH4 ) Ammonia flow rate FR(NH3) Substrate temperature Th Catalyzer temperature T., Power supply to catalyzer Reactor pressure
Set point 1.1 sccm 10 - 60 sccm 300°C 18000C 670 W 3 - 8 mTorr
The refractive index and thickness of the films were determined by ellipsometetry method. A helium-neon laser with a wavelength of 632.8 nm was used in the measurements. The depth profile of atomic composition of the films is obtained by ex-situ X-ray photoelectron spectroscopy (XPS) using Ar ion sputtering. The etching rate of Ar ion sputtering was 15
A/min. The composition of the films was estimated by areal intensities of N(ls), O(ls) and Si(2p) spect
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