CAT-CVD Process and its Application to Preparation of Si-Based Thin Films
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2. FUNDAMENTALS OF CAT-CVD SYSTEM (2-1) Deposition Apparatus An example of Cat-CVD apparatus is schematically illustrated in Fig. 1. In this case, deposition gases are introduced from a shower head placed at top of the chamber and flow down to substrates through a heated catalyzer. A tungsten (W) wire is used as a catalyzer, and it is spread in parallel to the substrates. The catalyzer is heated by supplying electric power directly on it, The temperature of catalyzer (T0 a,) is measured by an electronic infrared thermometer through a quartz window at a chamber wall and it is also checked by the temperature dependence of electric resistivity of W. It is easily known that the temperature of thermometer is in good agreement with that obtained by the resistivity for the emissivity of about 0.4. The temperature of substrates, actually the substrate-holder temperature (Tjh), is measured by a thermocouple attached there. Typical deposition conditions are summarized in Table I. In the Table, FR(X), PW 1,,, Scat, L and Pg refer to the flow rate of gas X, the electric power supply to heat up the catalyzer, the surface area of the catalyzer, the distance between the catalyzer and substrates and the gas pressure during deposition, respectively. (2-2) Contamination from Catalyzer In Cat-CVD system, other refractory metals apart from W can be used as a catalyzer, however, W appears the best choice among other metals, since the melting temperature is as high as 3400 'C and the vapor pressure at temperatures lower than 1900 'C is as low as 10-" Tort. This low vapor pressure suggests that the contamination in the films due to spontaneous evaporation of W during deposition should be less than 1015 cm-3 when Tcat is kept at about 1700 'C and the films are formed with deposition rates larger than 2 A/s. This value of W contamination is acceptable for the present and even future IJLSI industry. The W contamination is usually observed when it is oxidized. When W is heated at the temperature from about 1000 °C to 1400 'C in poor vacuum or in residual water vapor, the surface of W is easily oxidized, and since the melting temperature of this oxidized W is lower than 1500 'C, the W is likely to contaminate the deposited films. One of key requirements in Cat-CVD is to avoid this oxidation of W by introducing reduced gas such as H, gas during heating of W. The other requirement is to avoid the formation of silicide due to the reaction Showerhead
Infrared Thermometer
Deposition gases
Catalyzer
wafer Robot
Turbo pump
Thermocouple
Wa77!Robot Substrate holder
Fig. 1. Schematic diagram of Cat-CVD apparatus.
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Table I. Typical deposition conditions for Cat-CVD a-Si, poly-Si and SiN, films. a-Si Tcat FR(Sil 14) FR(1t2) FR(NH3) FR(112)/FR(Sil]4) FR(NI 13)/FR(Sil 14) PWcat Scat L Pg Tsh
poly-Si
1500 -1900 "C 10 - 20 sccm 0 - 40 sccm
1500-1900 °C 0.5 10 sccm 0- 200 sccm
SiNx 1500-1900 0 C 0.5- 5 sccm 50 200 sccm
0 2
0- 100 10-
100- 600 5-30
W cm 2
4-8 cm 0.001 - 0.1 Torr 150 - 300 "C
800
1500W
10-60
cm 2
4-6 cm 0.001 - 3 Tor
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