The plasma-enhanced chemical vapor deposition process for SiN=H~ films has been in use for over two decades, but the chemistry of the process has yet to be explained. In the Present work, the composition of a 13 MHz NH~-SiH4 parallel plate glow discharge plasma was analyzed by line-of-sight sampling from the film deposition plane into a triple-quadrupole mass spectrometer, which can resolve compositional ambiguities at a given mass number by utilizing collision-assisted secondary cracking. At low RF power, disilane was the main plasma product even when NHjSiH4 was 25/1, whereas at higher power (0.1 W/cm ~ of cross section) disilane was eliminated and tetra-aminosilane, Si(NH~)4, and the triaminosilane radical, Si(NH2)3, became dominant. The concentration of these aminosilanes closely tracked deposition rate, and they are believed to be the principal SiN=Hy film precursors. Films deposited with Si(NH2)~ maximized and disilane suppressed in the plasma were excess in N and contained no Si--H bonding, consistent with the precursor composition. Silane utilization was near unity. The composition and properties of films deposited under these "amino-saturated" plasma conditions were examined vs. substrate temperaiure, Ts. With increasing Ts, there occurred a densification, a loss of H and excess N in a 3/1 ratio, and an increase in tensile stress, suggesting surface and subsurface chemical condensation of the adsorbed precursors via 3Si(NH2)4 -~ Si3N4 + 8NH3 ~. Postdeposition flash desorption showed NH3, not H2, to be the main volatile product of condensation. These results demonstrate that plasma chemistry can be manipulated to control film properties in a predictable manner.
We have grown Ar+ ion beam sputtered Si epitaxially on Si(100) at substrate temperatures, T, between 390 and 480 K. At 480 K and 0.65 nm/s deposition rate, epitaxy is sustained at 1 μm of film thickness. At lower T, we observed an abrupt transition to amorphous growth at a critical thickness, he, which exhibited an Arrhenius dependence on T, as has previously been observed in molecular beam epitaxy (MBE) [D. J. Eaglesham, H. J. Gossmann, and M. Cerullo, Phys. Rev. Lett. 65, 1227 (1990)]. Our slope, d(ln he)/d(1/T), was 3 times steeper than in MBE, resulting in much thicker he at the higher T. The steep slope shows that the high kinetic energy of the sputtered Si is not enhancing surface diffusion enough to overcome thermal surface diffusion. We propose instead that the arriving kinetic energy is preventing void formation and thereby decreasing the rate at which statistical surface roughness, Δh, increases with film thickness. In both deposition processes, we propose that the collapse of epitaxy occurs when Δh exceeds the thermal surface diffusion length.
Conventional preparation of Si for epitaxy involves high-temperature heating to remove surface oxide or to remove H passivation left by HF cleaning. Attempted MBE on H-passivated Si(100) has resulted in amorphous films below 640 K. We show that the use of ion beam sputtered Si allows the growth of thick (300 nm) epitaxial layers on H-passivated Si(100) at high rate (0.65 nm/s) and at 483 K, well below the H desorption temperature. This is because the sputtered Si has translational kinetic energy of tens of eV, well above the Si-H bond strength of a few eV, so that the passivating H does not block the epitaxial bonding sites as it does in MBE. The ability to grow epitaxially on H-passivated Si not only reduces the maximum Si processing temperature required, but also keeps the substrate passivated against contamination until the onset of deposition.During deposition, the LEED pattern changes gradually from the 1 × 1 of the H-passivated Si to the 2 × 1 reconstruction of bare or partially hydrogenated Si(1 00), yet SIMS shows that little of the disappearing H accumulates at the substrate interface or in the bulk of the film. Thermal desorption spectra show that its bonding remains similar to that on the substrate surface. Thus, it appears to float to the surface of the depositing film and become gradually knocked off of the surface over the course of a few hundred nm of deposition. Cross-sectional TEM lattice imaging shows near-perfect ordering of the epilayer and a barely distinguishable substrate interface.
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