Axial Concentration Profile of H 2 Produced in the CVD of Si 3 N 4 .
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AXIAL CONCENTRATION PROFILE OF H 2 PRODUCED IN THE CVD OF Si 3N4.
STEPHEN 0. HAY AND WARD C. ROMAN United Technologies Research Center, E. Hartford, CT 06108
ABSTRACT Silicon nitride (Si 3N4) has been demonstrated to be an effective high temperature anti-oxidant when deposited in its c--crystalline form. The Materials Technology Laboratory at UTRC has developed a pilot-scale chemical vapor deposition (CVD) reactor capable of depositing Q-Si 3N 4 from ammonia (NH 3) and silicon tetrafluoride (SiF 4) at 1.8 torr and 1440 C. Coherent anti-Stokes Raman spectroscopy (CARS) has been applied to measure H2 produced in this reactor. Axial concentration measurements have been performed both in the presence and absence of SiF 4. Previous CARS measurements demonstrated the importance of surface (Si 3N4) catalyzed decomposition of NH 3: 2NH 3 -+ 3H2 +
N2
as a competing reaction to: 4NH 3 + 3SiF 4 -- Si 3N4 + 12HF in the CVD reactor under deposition conditions. The observed hydrogen concentration profiles confirm these measurements and allow quantitative comparison between the competing reactions. NH 3 decomposition is suppressed 20% by the addition of SiF 4 in a 6:1 (NH 3:SiF 4) molar ratio. No decomposition is observed in the absence of Si 3N4.
Introduction Silicon nitride (Si 3N4) is an effective high temperature anti-oxidant [1] when deposited in its z-crystalline form. The UTRC Materials Technology Laboratory has developed a thermal CVD process [2] for cs-Si 3N4 deposition from ammonia (NH 3) and silicon tetrafluoride (SiF 4) at 1.8 torr and 1440 C. In order to validate the modeling performed [3] on the UTRC reactor design, it is desirable to measure species concentration profiles within an operational reactor. Our objective is to apply non-intrusive optical diagnostics within the UTRC reactor environment for in situ species concentration and temperature measurements. To accomplish this objective, a CVD reactor with multiple optical access ports was constructed. This access allows in situ measurements to be made at reference operating conditions at several positions downstream of the reactant injectors. CARS was selected as the diagnostic to be applied for several reasons. First, this technique requires only limited line of sight optical access to the reactor. Second, the resultant signal is coherent. This allows background discrimination of intense black body background by simple Mat. Res. Soc. Symp. Proc. Vol. 250. @1992 Materials Research Society
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beam propagation through a distance, R (the background drops off as 1/R 2 while the coherent signal is attenuated only due to diffractive and scattering losses which are small). Third, CARS is applicable to all species possessing a Raman active vibrational mode and is therefore theoretically capable of being used to observe the ground state of any molecular species. And finally, a rovibrational CARS spectra can be interpreted to yield both a species concentration and a rotational temperature. In a thermal reactor, such as the one that the UTRC method utilizes, rotati
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