Silicon is rapidly etched by the gas-phase halogen fluorides ClF3, BrF3, BrF5, and IF5, in analogy to XeF2 etching silicon. Nearly complete selectivity over SiO2 is achieved in all cases. By contrast, ClF and Groups III and V fluorides such as NF3, BF3, PF3, and PF5 do not spontaneously etch either Si or SiO2 under the same experimental conditions. These relatively inexpensive interhalogens can be applied to pattern silicon and more generally to remove silicon or polysilicon layers without a plasma. Low-temperature plasmaless gasification of substrates by these fluorine-containing interhalogens is an economically attractive alternative to fluorine-based plasma etching.
Plasma emission actinometry has been used to study the mechanism by which small additions of oxygen (∼0.5%) enhance the rate of diamond deposition in a dilute (4%) CH4/H2 discharge at high temperature (900–1300 K). Increasing amounts of CH4 in the feed depress [H], while increasing the O2 concentration, up to ∼5%, produces a fivefold increase in atomic hydrogen in the discharge zone. Invoking a mechanism where diamond growth competes with the formation of an amorphous/graphitic inhibiting layer, these results and earlier studies suggest that oxygen (1) increases [H] which selectively etches amorphous/graphitic carbon, (2) accelerates reaction of this layer with molecular hydrogen, and (3) may itself act as a selective etchant of nondiamond carbon. As a result, the number of active diamond growth sites is increased and enhanced growth rates are observed. We also have grown diamond by alternating a CH4/He discharge with a H2/O2/He discharge and results are consistent with this mechanism. Instantaneous growth rates are very high (45 μm/h) and codeposition of nondiamond allotropes is less. The H2/O2/He discharge removes the inhibiting carbon layer more effectively, while the CH4/He plasma presumably is richer in diamond-forming precursors. Thermochemical calculations suggest that these precursors are likely to be C2H2, C2H, C2, or C atoms rather than methyl radicals (CH3) as others have postulated.
Silicon gasification by XeF2 is compared with F-atom etching under conditions typical of those used in plasma etching. Temperatures ranged from −17 to 360 °C and XeF2 pressures were between 0.05 and 2 Torr. Silicon etching by XeF2 shows a sharply different etch rate/temperature dependence than the Si/F or Si/F2 reaction systems; there is no detectable reaction between XeF2 and SiO2 in contrast to the F-atom/SiO2 system. These data indicate that physisorption can limit silicon etching by XeF2 and show that basic studies which use XeF2 as a model compound for the etching of silicon and SiO2 by F atoms should be interpreted with caution.
Silicon carbide thin films have been deposited on Si( 100) from the single-source reagents silacyclobutane (SCB, c-C3H&iH2) and methylsilane (MeSiH3). Thermal decomposition kinetics were studied in a low-pressure (50 mTorr), cold-wall CVD reactor over the temperature range 600-1 150 OC. Partial pressure analyses based on mass spectrometry indicate the dominant decomposition channels for SCB and MeSiH3 are consistent with the formation of the isomeric intermediates H2Si=CH2 and HSiCH3, respectively. Deposition rates using the two reagents were comparable at all temperatures, reaching 80 A/s at 1050 OC. It is apparent from the lattice vibrational spectrum and X-ray diffraction data that S i c deposited from MeSiH3 is more crystalline. These structural differences suggest a distinct difference in the surface reaction mechanism which is inconsistent with isomeric equilibration. SCB deposits carbon-rich material (C/Si = 1. l), becoming stoichiometric above 1000 OC, while that deposited from MeSiH3 is Si-rich (C/Si = 0.8) at all temperatures. The hydrogen content of SCB films decreased from 6 to 1 atom 8 with increasing temperature. Results were similar with MeSiH3, but with half as much hydrogen. Modeling of heat and mass transfer at the substrate shows that decomposition occurs exclusively at the surface, allowing surface reaction probabilities to be measured as a function of temperature.Activation energies for SCB and MeSiH3 decomposition are 41 and 53 kcal/mol, respectively. A mechanism for SiC/CVD is proposed that is consistent with the observed kinetics and products of SCB and MeSiHs decomposition.
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