Atomic layer etching (ALE) of SiO 2 was studied by alternating exposure of a 5 nm-thick SiO 2 film on Si substrate to (1) a plasma beam emanating from a cC 4 F 8 inductively coupled plasma (ICP), to grow a fluorocarbon (FC) film composed mainly of CF 2 , and (2) an energetic (130 eV) Ar + ion beam extracted from a separate Ar ICP. In situ x-ray photoelectron spectroscopy was used to analyze the chemical composition of the near-surface region, and to quantify the thickness of the FC and SiO 2 films. A very thin (3-6 Å), near self-limiting thickness CF 2-rich FC film was found to deposit on the SiO 2 surface with exposure to continuous or pulsed power C 4 F 8 plasma beams, under conditions that generated a large relative flux of CF 2. Following this, a FC film of similar composition grew at ~10 times slower rate. Exposure of the thin film to the Ar + beam led to removal of 1.9 Å SiO 2. An estimated yield of 1.3 SiO 2 molecules-per-Ar + was found for a single ALE step. The rate of 1.9 Å/cycle persisted over multiple ALE cycles, but a carbon-rich residual film did build up. This film can be removed by a brief exposure to an O 2-containing plasma beam.

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Silicon nitride (SiN, where Si:N ≠ 1:1) films low pressure-chemical vapor deposited on Si substrates, Si films on Ge on Si substrates, and p-Si samples were exposed to plasma beams emanating from CH3F/O2 or CH3F/CO2 inductively coupled plasmas. Conditions within the plasma beam source were maintained at power of 300 W (1.9 W/cm3), pressure of 10 mTorr, and total gas flow rate of 10 sccm. X-ray photoelectron spectroscopy was used to determine the thicknesses of Si/Ge in addition to hydrofluorocarbon polymer films formed at low %O2 or %CO2 addition on p-Si and SiN. Polymer film thickness decreased sharply as a function of increasing %O2 or %CO2 addition and dropped to monolayer thickness above the transition point (∼48% O2 or ∼75% CO2) at which the polymer etchants (O and F) number densities in the plasma increased abruptly. The C(1s) spectra for the polymer films deposited on p-Si substrates appeared similar to those on SiN. Spectroscopic ellipsometry was used to measure the thickness of SiN films etched using the CH3F/O2 and CH3F/CO2 plasma beams. SiN etching rates peaked near 50% O2 addition and 73% CO2 addition. Faster etching rates were measured in CH3F/CO2 than CH3F/O2 plasmas above 70% O2 or CO2 addition. The etching of Si stopped after a loss of ∼3 nm, regardless of beam exposure time and %O2 or %CO2 addition, apparently due to plasma assisted oxidation of Si. An additional GeOxFy peak was observed at 32.5 eV in the Ge(3d) region, suggesting deep penetration of F into Si, under the conditions investigated.

A CH3F/CO2 inductively coupled plasma (ICP), sustained in a compact plasma reactor, was investigated as a function of power (5–400 W) and feed gas composition, at a pressure of 10 mTorr, using optical emission spectroscopy and rare gas actinometry. Number densities of H, F, and O increased rapidly between 74% and 80% CO2, ascribed to the transition from polymer-covered to polymer-free reactor walls, similar to that found previously in CH3F/O2 ICPs at 48% O2. Below 40% O2 or CO2, relative emission intensity ratios were almost identical for most key species in CH3F/O2 and CH3F/CO2 ICPs except for higher OH/Xe (a qualitative measure of OH and H2O densities) over the full range of CH3F/O2 composition. The number density of H, F, and O increased with power in CH3F/CO2 (20%/80%) plasmas (polymer-free walls), reaching 4.0, 0.34, and 1.6 × 1013/cm3, respectively, at 300 W. The CO number density increased with power and was estimated, based on self-actinometry, to be 8.8 × 1013/cm3 at 300 W. The CO2 number density was independent of power below 40 W (where very little decomposition occurred), and then decreased rapidly with increasing power, reaching 2.8 × 1013/cm3 at 300 W, corresponding to 83% dissociation. Films deposited on p-Si, 10 cm from the open, downstream end of the plasma reactor, were analyzed by x-ray photoelectron spectroscopy. Between 10% and 40% CO2 or O2 addition to CH3F, film deposition rates fell and O content in the films increased. Faster deposition rates in CH3F/CO2 plasmas were ascribed mainly to a larger thermodynamic driving force to form solid carbon, compared with CH3F/O2 plasmas. Oxygen content in the films increased with increasing CO2 or O2 addition, but for the same deposition rate, no substantial differences were observed in the composition of the films.

CH 3 F/O 2 inductively coupled plasmas at 10 mTorr were investigated using optical emission spectroscopy. A 'self-actinometry' method was developed to measure the absolute number density of CO that formed in reactions following dissociation of CH 3 F and O 2 in the plasma. In this method, small amounts of CO were added to the plasma, leading to small increases in the CO emission intensity. By carefully accounting for small perturbations to the plasma electron density and/or electron energy distribution, and by showing that very little of the CO added to the plasma was decomposed by electron impact or other reactions, it was possible to derive absolute number densities for the CO content of the plasma. With equal fractions (0.50) of CH 3 F and O 2 in the feed gas, the CO mole fraction as a function of plasma power saturated at a value of 0.20-0.25. As O 2 in the feed gas was varied at a constant power of 100 W, the CO mole fraction went through a maximum of about 0.25 near an O 2 feed gas fraction of 0.5. The relative CO number densities determined by 'standard' actinometry followed the same functional dependence as the absolute mole fractions determined by self-actinometry, aided by the fact that electron temperature did not change appreciably with power or feed gas composition.