Si Etch Rate Research Articles

PLASMA PARAMETRS AND SILICON ETCHING KINETICS IN CF4 + C4F8 + O2 MIXTURE: EFFECT OF CF4/C4F8 MIXING RATIO

In this work, we investigated the influence of fluorocarbon component ratio in the CF4 + C4F8 + O2 gas mixture on electro-physical plasma parameters, steady-state densities of active species and silicon etching kinetics under typical reactive-ion process conditions. The combination of plasma diagnostics (double Langmuir probes, optical emission spectroscopy) and plasma modeling confirmed known peculiarities of plasma chemistry of individual fluorocarbons in the presence of oxygen as well as provided an extended analysis of both fluorine and oxygen atom kinetics in the three-component gas mixture. It was shown that the substitution of CF4 by C4F8 at the constant fraction of O2 a) causes the weak disturbance in electrons- and ions-related plasma parameters (electron temperature, electron density, ion energy flux); b) provides drastically increasing density of polymerizing CFx (x = 1, 2) radicals; and c) results in monotonically decreasing F atoms density. The latter is due to simultaneous changes in both F atom formation rate and their loss frequency, especially in C4F8-rich plasmas. From experiments, it was found that Si etching rate is by more than 85% controlled by its chemical component (in a form of ion-stimulated heterogeneous reaction Si + xF → SiFx) and decreases with increasing C4F8 fraction in a feed gas. The change in effective reaction probability contradicts with the growth of polymer deposition rate and film thickness (as it follows from changes in gas-phase plasma characteristics) but may reflect the weakening of surface passivation by oxygen atoms. For citation: Efremov A.M., Bobylev A.V., Kwon K.-H. Plasma parametrs and silicon etching kinetics in CF4 + C4F8 + O2 mixture: effect of CF4/C4F8 mixing ratio. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 6. P. 29-37. DOI: 10.6060/ivkkt.20246706.6982.

Control of selective SiGe etching by enhanced formation of hydroxyl radicals and by surface passivation in peracetic acid solution

The selective etching of SiGe over Si is a crucial process in the fabrication of advanced 3D semiconductor devices. During the etching process, highly selective SiGe etching is required to obtain precise etch profiles. In this study, the enhancement of the selective SiGe etching in a peracetic acid (PAA) etchant solution by adding hydroxylamine and pentyl acetate was investigated. The addition of hydroxylamine increased the SiGe etching rate and SiGe/Si etching selectivity owing to the more vigorous oxidation of the SiGe surface produced by hydroxyl radicals. The SiGe/Si etching selectivity was also increased through the addition of pentyl acetate owing to the larger decrease in the Si etching rate when compared to that in SiGe. This is due to the stronger passivation effect of pentyl acetate on the Si surface, which suppresses its oxidation more effectively than on the SiGe surface. The SiGe etching rate and SiGe/Si etching selectivity were significantly increased from 223.3 nm/min and 125 to 310.8 nm/min and 557, respectively, through the simultaneous addition of hydroxylamine and pentyl acetate to the PAA etchant solution. Consequently, a mechanism for the selective etching of SiGe on vertically stacked SiGe/Si trench structures using the PAA etchant solution was proposed.

Comparison of distributions of etching rate and calculated plasma parameters in dual-frequency capacitively coupled plasma

Abstract We measured the etching rates of SiO2 and Si in dual-frequency excited CF4 plasma and compared the etching rates with the radical species distribution calculated by plasma simulation. The etching rates of SiO2 and Si at the wafer edge became higher than those at the wafer center, and the etching rate of Si distributed uniformly without bias voltage. In the simulation, the calculation model was calibrated on the basis of the measured (electron density). Assuming that CF3 + contributes to SiO2 and Si etching, and F contributes to Si etching without bias voltage, the radical fluxes flowing into the wafer were calculated by plasma simulation. The fluxes of CF3 + and CF2 became higher at the edge, and the F flux distribution was uniform without the bias voltage. It was shown that the distributions of the etching rate in the experiment and radical flux that contributes to etching in the simulation were in agreement.

Selective Etching of Si versus Si1-xGex in Tetramethyl Ammonium Hydroxide Solutions with Surfactant.

We investigated the selective etching of Si versus Si1−xGex with various Ge concentrations (x = 0.13, 0.21, 0.30, 0.44) in tetramethyl ammonium hydroxide (TMAH) solution. Our results show that the Si1−xGex with a higher Ge concentration was etched slower due to the reduction in the Si(Ge)–OH bond. Owing to the difference in the etching rate, Si was selectively etched in the Si0.7Ge0.3/Si/Si0.7Ge0.3 multi-layer. The etching rate of Si depends on the Si surface orientation, as TMAH is an anisotropic etchant. The (111) and (010) facets were formed in TMAH, when Si was laterally etched in the <110> and <100> directions in the multi-layer, respectively. We also investigated the effect of the addition of Triton X-100 in TMAH on the wet etching process. Our results confirmed that the presence of 0.1 vol% Triton reduced the roughness of the etched Si and Si1−xGex surfaces. Moreover, the addition of Triton to TMAH could change the facet formation from (010) to (011) during Si etching in the <100>-direction. The facet change could reduce the lateral etching rate of Si and consequently reduce selectivity. The decrease in the layer thickness also reduced the lateral Si etching rate in the multi-layer.

Molecular dynamics simulation of Si trench etching with SiO2 hard masks

Molecular dynamics simulations were performed to demonstrate nanometer-scale silicon (Si) trench etching with silicon dioxide (SiO2) hard masks by chlorine (Cl+) ion beams possibly with low-energy chlorine (Cl) radicals. Although the sputtering yield of SiO2 is typically much lower than that of Si, the etch rates of SiO2 and Si can be comparable because of the lower Si atomic density of SiO2. This implies that the erosion of the mask can significantly affect etched structures. This study has demonstrated that although the fluxes of incident ions and radicals are uniform in space and constant in time, the individuality of incident ions and radicals causes atomic-scale surface roughness, which cannot be neglected for nanometer-scale etched structures. Furthermore, some transient effects of surface etching, such as initial swelling of the Si surface due to incorporation of Cl atoms and preferential sputtering of oxygen, can affect the profiles of etched structures. The insufficiency of the local mechanical strengths of nanometer-scale materials also enhances their erosion, leading to the formation of nanometer-scale roughness on the sidewalls of masks and etched structures.

Comparative study of CF4 + O2 and C6F12O + O2 plasmas for reactive‐ion etching applications

AbstractThis work represents a comparative study of gas‐phase parameters and reactive‐ion etching kinetics for Si and Si and SiO2 in CF4 + O2 and C6F12O + O2 plasmas. An interest to the C6F12O gas is because it combines the low global warming potential (low environmental impact) and weakly studied dry etching performance. It was shown that the investigated gas systems showed similar effects of processing conditions on electron‐ and ion‐related plasma parameters as well as on the densities of F and O atoms. Etching experiments showed identical behaviors of Si and SiO2 etching rates as well as very similar SiO2/Si etching selectivities. The notable feature of the C6F12O + O2 plasma is only the systematically lower absolute etching rates that correlate with lower in F atom densities.

Using block-copolymer nanolithography as a tool to sensitively evaluate variation in chemical dry etching rates of semiconductor materials with sub-5 nm resolution

Ion implantation is a robust and established method to customize the electronic properties of Si. However, fabricating doped, ultrafine semiconductor nanostructures can be challenging. Ion implantation has well-established effects on the dry etch rates of Si, which becomes increasingly consequential as the target dimension shrinks below a few tens of nanometers. While dry etching arrays of block copolymer-templated nanoscale holes (pitch = 37.5 nm, diameter ∼25 nm) into p-type, n-type, and undoped Si, we observed that the lateral etch rate was notably larger for the n-type regions than p-type or undoped regions. By doing image analyses on high resolution electron micrographs of the nanostructured hole arrays, we were able to extract the porosity and average radii of the holes with subnanometer sensitivity and compare the relative etch rates between different doping conditions. We found that degenerately doped n-type silicon consistently etches between approximately 17% and 27% faster in the lateral direction than p-type Si, resulting in significantly larger porosity and, consequently, less mechanical stability. Here, we demonstrate that top-down dimensional analysis of a densely packed porous nanostructure is a robust method for assessing extremely small differences in the lateral, chemical etch rate of doped Si to a degree of sensitivity that was previously unachievable. The minute, dense-packed nature of block copolymer self-assembled nanostructures is shown to be ideal for this application. This proposed method could be useful for designing fabrication processes for heterogeneous nanostructures, as slight dry etch rate variations that may be within process tolerance at the micrometer-scale appear to have nontrivial consequences at the nanometer scale.

A novel and reliable approach for controlling silicon membrane thickness with smooth surface

Despite self-supporting ultrathin Si membranes are already being widely applied, its thickness control in micron scale still remains a big challenge for delicate systems such as micro-opto-electro-mechanical systems and silicon-based X-ray diffractive optics. Instead of using the conventional wet etch rate of Si to monitor the membrane thickness, this work has developed a reliable approach to control the membrane thickness in the precision of micrometer order by using an optical detection hole with high precision depth, which decides the membrane thickness in the Si thinning process. The wet etch rate is first optimized to ensure the fundamental condition for such a thickness control. Furthermore, the membrane surface roughness is greatly reduced by optimizing the additive ethanol concentration in the KOH etchant, based on the investigation results for the origin of the membrane surface roughness. Using the proposed etching and thickness-monitoring method, self-supporting micron thick Si (100) membranes with the surface roughness of <15 nm have been successfully fabricated.

Etching of smoothing/without undercutting deep trench in silicon with SF6/O2 containing plasmas

In this article, the effects of He and Ar which are added into SF6/O2 containing plasmas were investigated in deep Si etching. External controllable parameters such as the He or Ar gas flow were used to study the changes of Si etch rate, Si undercut, oxide selectivity and feature shape correlation. The opening width of feature shape is from 0.22 μm to 0.42 μm. The result shows that the Si undercut decreases and the profile of Si trench is more vertical with the increasing He gas flow in all structures. But the Si etch rate and oxide selectivity will decrease. It is noteworthy that Ar and He have similar effects in reducing undercut. After detailed data analysis, it was found that there were some correlations between Si undercut and normalized residence time in both plasmas. The mechanism of this phenomenon is that the F radicals are diluted due to the addition of a large amount of He and Ar. With the decrease of the F radicals, the lateral etching ability decreases and undercut decreases. The concentration of O and F radicals decreases at almost the same rate when He or Ar gas is mixed, the sidewall profile of trench has no change. Due to the additional vertical bombardment of He+ and Ar+ which produces in the plasma with the addition of the He and Ar, the sidewalls of the Si trench become more vertical. Based on this, an optimized trench etching process is proposed and the etch rate is 1.25 um min−1, the selectivity for oxide mask is 9.5, and the undercut is less than 50 nm.

Gas‐phase chemistry and reactive‐ion etching kinetics for silicon‐based materials in C4F8 + O2 + Ar plasma

AbstractIn this study, we investigated the effects of C4F8/O2 and Ar/O2 component ratios in C4F8 + O2 + Ar gas system on plasma parameters, gas‐phase chemistry, and etching kinetics for Si, SiO2, and Si3N4 under the condition of inductively coupled radiofrequency (13.56 MHz) plasma. The use of plasma diagnostics by Langmuir probes, together with the zero‐dimensional plasma modeling, allowed one to compare two different gas mixing regimes in respect to (a) electrons‐ and ions‐related plasma parameters; (b) steady‐state densities of plasma active species; and (c) formation and decay kinetics for F atoms and polymerizing radicals. It was shown that variations in both C4F8/O2 and Ar/O2 mixing ratios result in nonmonotonic (with maximums at ∼10% to 15% O2) Si, SiO2, and Si3N4 etching rates as well as in opposite changes in corresponding effective reaction probabilities. The model‐based analysis of etching mechanisms allowed one to suggest that the net effect from all heterogeneous (i.e., appeared on the etched surface) interaction pathways may be controlled either by the fluorocarbon radical flux (in the case of Ar/O2 mixing ratios at constant fraction of C4F8) through the polymer film thickness or by the O atom flux (in the case of C4F8/O2 mixing ratios at constant fraction of Ar) through the balance of adsorption sites.

ПАРАМЕТРЫ ГАЗОВОЙ ФАЗЫ И РЕЖИМЫ РЕАКТИВНО-ИОННОГО ТРАВЛЕНИЯ Si И SiO2 В БИНАРНЫХ СМЕСЯХ Ar + CF4/C4F8

The comparative study of plasma electro-physical parameters, steady-state gas phase compositions and reactive-ion etching kinetics for Si and SiO2 in binary CF4 + Ar and C4F8 + Ar gas mixtures were studied under conditions of 13.56 MHz inductive RF discharge. As fixed input parameters, we used the total pressure of feed gas (6 mTorr) as well as power levels supplied by plasma excitation source (700 W) and bias source (200 W). The investigation approach combined plasma diagnostics experiments with double Langmuir probe and 0-dimensional (global) model for the chemistry of neutral species. It was shown that investigated gas mixtures exhibit quite close properties in respect to both ions-related parameters and electron gas while are characterized by sufficient differences in kinetics of atoms and radicals. The features of C4F8 + Ar gas under the given set of processing conditions are the higher density of polymerizing radicals, the lower density of F atoms as well as the weaker sensitivity the last parameter to the change in Ar fraction in a feed gas. Etching experiments indicated that a) an increase in Ar fraction in CF4 + Ar and C4F8 + Ar gas mixtures results in qualitatively different changes in Si and SiO2 etching rates; and b) obtained dependencies of etching rates on Ar fraction in both gas mixtures contradict with the behavior of F atom flux. Obviously, such situation corresponds to the change in reaction probability of F atoms with the treated surface. It was suggested that an increase of Ar fraction in the low-polymerizing CF4 + Ar plasma activates the heterogeneous chemical reaction through the intensification of ion-stimulated desorption of etching products and/or surface amorphization. The similar effect for the high-polymerizing C4F8 + Ar plasma may be related to decreasing fluorocarbon film thickness that provides the better access of F atoms to the etched surface.

Dry Etching Performance and Gas-Phase Parameters of C6F12O + Ar Plasma in Comparison with CF4 + Ar

This research work deals with the comparative study of C6F12O + Ar and CF4 + Ar gas chemistries in respect to Si and SiO2 reactive-ion etching processes in a low power regime. Despite uncertain applicability of C6F12O as the fluorine-containing etchant gas, it is interesting because of the liquid (at room temperature) nature and weaker environmental impact (lower global warming potential). The combination of several experimental techniques (double Langmuir probe, optical emission spectroscopy, X-ray photoelectron spectroscopy) allowed one (a) to compare performances of given gas systems in respect to the reactive-ion etching of Si and SiO2; and (b) to associate the features of corresponding etching kinetics with those for gas-phase plasma parameters. It was found that both gas systems exhibit (a) similar changes in ion energy flux and F atom flux with variations on input RF power and gas pressure; (b) quite close polymerization abilities; and (c) identical behaviors of Si and SiO2 etching rates, as determined by the neutral-flux-limited regime of ion-assisted chemical reaction. Principal features of C6F12O + Ar plasma are only lower absolute etching rates (mainly due to the lower density and flux of F atoms) as well as some limitations in SiO2/Si etching selectivity.

Study on silicon removal property and surface smoothing phenomenon by moderate-pressure microwave hydrogen plasma

The Si removal property achieved by a localized microwave hydrogen plasma was investigated. The relationship between plasma generation parameters (input power, H2 gas flow rate, and initial surface roughness) and the removal property (etching rate and surface roughness) was revealed. Stress relief and removal of the affected Si layer by H2 plasma etching were also demonstrated. Moreover, this H2 plasma etching technique presents no etching-rate dependence on the crystal plane orientation. Therefore, even though a multi-crystalline Si sample was etched, a mirror surface could be revealed. In addition, a 100-μm-thick Si sample could be thinned to 5.7 μm by H2 plasma etching without breaking the sample. The surface roughness of the etched Si sample depends on the etching rate. A critical etching rate exists, which determines whether the etched Si surface becomes rough or smooth. When the etching rate is significantly lower than the critical etching rate, large surface roughness is generated owing to the formation of {111} platelet defects induced by H atoms in Si crystal. By contrast, when the etching rate is higher than the critical value, the roughness formation is suppressed, and the Si surface becomes smooth despite the high H flux supplied to the Si surface. The suppression of roughness formation is discussed by comparing the etching rate with the diffusion rate of H atoms into Si. It is important to maintain the Si etching rate higher than the H diffusion rate to obtain a smooth etched Si surface.

Effects of crystal planes on topography evolution of silicon surface during nanoscratch-induced selective etching

Diamond machining-induced structural defects can usually degrade the optical and electrical performance of silicon (Si)-based devices. Investigating topography evolution of machined Si surfaces during wet etching is of significance for detecting and eliminating these defects and thereby improving device performance. In this paper, effects of Si crystal planes on surface topography evolution during nanoscratch-induced selective etching in the commonly used etchants, i.e., potassium hydroxide (KOH), tetra-methyl-ammonium hydroxide (TMAH), and a mixture of hydrofluoric acid (HF) and nitric acid (HNO3), were investigated using a single-asperity diamond tip. It is noted that nanoscratch-induced subsurface deformation of Si(100) and Si(110) can resist KOH and TMAH solutions etching, and thereby in situ form protrusive hillocks, which can be ascribed to the amorphous Si from the scratching. Higher etching rate of Si(110) planes appears to be responsible for the formation of higher protrusive hillocks. Nevertheless, nanoscratch-induced selective etching on Si(111) surface in TMAH solution is quite distinct from that in KOH solution. The difference in activated atomic steps on Si(111) surface may cause scratched regions to promote KOH etching but resist TMAH etching. It is also found that in case of HF/HNO3 mixtures etching, nanoscratch-induced subsurface deformation on Si surface can promote the etching and thereby form sunken grooves, regardless of crystal plane orientations. Further analysis indicates that Si wafers with lower hardness and elastic modulus are beneficial for forming more serious subsurface damages during the scratching, yielding deeper sunken grooves through subsequent selective etching. These findings provide a useful reference for defect detection on Si surface and developing the nanofabrication based on scanning probe microscope.

Optimization of Reactive-Ion Etching (RIE) parameters to maximize the lateral etch rate of silicon using SF6/N2 gas mixture: An alternative to etching Si in MEMS with Au components

In this work, the effects of the N2 addition to the SF6 plasma used in the isotropic silicon etching of Micro-electromechanical systems (MEMS) with Au components are investigated. A four-variables Doehlert design was implemented for optimizing the etching parameters (power, pressure, gas flow rate, and N2/SF6 ratio) to maximize the lateral etch rate of Si using SF6/N2 gas mixture. The optimized etch condition founded for a lateral etch rate of 1.8 µm/min was: power = 143 W, chamber pressure = 86 mTorr, flow rate = 22 sccm, and N2/SF6 ratio = 0.1. Furthermore, it was demonstrated that the established etching process avoids the structure damage of Au components.

On the Control of Plasma Chemistry and Silicon Etching Kinetics in Ternary HBr + Cl2 + O2 Gas System: Effects of HBr/O2 and Cl2/O2 Mixing Ratios

The influences of both HBr/O2 (at constant Cl2 fraction) and Cl2/O2 (at constant HBr fraction) ratios in HBr + Cl 2 + O2 gas mixture on bulk plasma characteristics, active species densities and etching kinetics of silicon were studied. The results indicated that an increase in O2 content in a feed gas at constant Cl2 fraction in a processing gas (1) produces the stronger impact on plasma chemistry by the influence on the kinetics of electron-impact and atom-molecular reaction; and (2) provides the wider adjustments for both halogen atom flux and ion flux with the opposite tendencies with those for variable Cl2/O2 mixing ratio. The experiments demonstrated that the transition toward more oxygenated plasmas in both cases lowers the Si etching rate as well as result is decreasing effective reaction probability and etching yield. These effects may be associated with decreasing amount of adsorption sites for Cl/Br atoms as well as increasing sputtering (ion-stimulated desorption) threshold for reaction products due to the formation of the low-volatile silicon oxy-chlorides and-bromides in heterogeneous SiClx + O/OH and SiBrx + O/OH reactions.

A Novel Route to High-Quality Graphene Quantum Dots by Hydrogen-Assisted Pyrolysis of Silicon Carbide.

Graphene quantum dots (GQDs) can be highly beneficial in various fields due to their unique properties, such as having an effective charge transfer and quantum confinement. However, defects on GQDs hinder these properties, and only a few studies have reported fabricating high-quality GQDs with high crystallinity and few impurities. In this study, we present a novel yet simple approach to synthesizing high-quality GQDs that involves annealing silicon carbide (SiC) under low vacuum while introducing hydrogen (H) etching gas; no harmful chemicals are required in the process. The fabricated GQDs are composed of a few graphene layers and possess high crystallinity, few defects and high purity, while being free from oxygen functional groups. The edges of the GQDs are hydrogen-terminated. High-quality GQDs form on the etched SiC when the etching rates of Si and C atoms are monitored. The size of the fabricated GQDs and the surface morphology of SiC can be altered by changing the operating conditions. Collectively, a novel route to high-quality GQDs will be highly applicable in fields involving sensors and detectors.

Effect of cations on silicon anisotropic etching process in solutions containing TMAH and TMAH with tensioactive compounds

Almost everything has been written on silicon anisotropic etching in alkaline solutions, however, the difference between etching in pure KOH and TMAH as well as in these solutions containing tensioactive compounds has not been completely elucidated yet. This paper present the results of anisotropic etching of Si(100) and Si(110) in the solutions containing two etching agents, KOH and TMAH. Due to this it is possible to control the effect of concentration of each agent on the course of etching and clarify the effect of cation type (inorganic and organic) on the course of the process. The effect of TMA+ ions on etch rate of Si(100) has already been proven both during etching in pure TMAH as well as after addition of tensioactive compounds to this solution. It results in a change in anisotropy ratio (R(110)/R(100)) in TMAH containing solutions in comparison to solutions based on KOH. As the commonly used KOH concentrations (5–10 M) are larger than those of TMAH 25 % (2.7 M), the concentration range of the studied TMAH solution has been extended to 50 %. The paper describes the properties of those concentrated TMAH solutions, both pure and containing surfactants.

HCl + GeH4 etching for the low temperature cyclic deposition/etch of Si, Si:P, tensile-Si:P and SiGe(:B)

We have investigated a few years ago the cyclic deposition/etch of Si(:P) in the Sources/Drains regions of MOS transistors. Blanket growth occurred at 550 °C while materials on dielectrics were selectively etched at 600 °C with GeH4-catalyzed HCl. Thanks to the presence of Ge atoms close to the surface, Si etching was indeed much faster with HCl+GeH4 than with HCl. Performing those etches at 550 °C instead of 600 °C would be advantageous, as the thermal budget would be lower (3D integration), the throughput higher and amorphous Si(:P) etching easier. We have therefore quantified the impact of temperature, pressure and flows on the HCl+GeH4 etch rate of monocrystalline Si, (doped or tensile) Si:P ([P] = 1020 or 1.5 × 1021 cm−3) and Si0.7Ge0.3(:B) ([B] ∼2 × 1020 cm−3) for channel and raised sources and drains applications. Increasing the etch pressure and reducing the H2 flow resulted in linear increases of the Si Etch Rate (ER). Decreasing at 650 °C the HCl flow yielded (i) definitely higher Si ER at 20 Torr and (ii) explosively higher Si ER at 90 Torr. We have then quantified, with those reduced flows, the impact of temperature on etch rate. 90 Torr etch rates were always higher than those at 20 Torr. We had at 20 Torr exponential etch rate decreases as the temperature decreased, with SiGe:B ER higher than SiGe ER, which were much higher than Si ER. The former was due to surfaces richer in Ge (and B) atoms, which acted as catalysts for H and/or Cl desorption. We otherwise had Si:P ER which were higher than Si ER, which were themselves higher than t-Si:P ER. Going over to 90 Torr yielded reasonable SiGe(:B) (Si, Si:P and t-Si:P) ER at temperatures as low as 500 °C (550 °C).

Role of sulfur in catalyzing fluorine atom fast etching of silicon with smooth surface morphology

Chemical reaction probabilities, defined as the number of silicon atoms removed per incident fluorine atom, have been investigated in mixtures of NF3 and SF6 plasmas in an inductively-coupled plasma reactor. Fluorine atom densities were measured by optical emission actinometry, and isotropic etching rates were measured by the degree of undercutting of SiO2-masked silicon by cross-sectional scanning electron microscopy. In addition, atomic force microscopy was used to examine surface morphology of etched Si surfaces. The F atom reaction probabilities derived from isotropic etching rates indicate an ∼30-fold higher reaction probability in SF6 plasmas compared with values in NF3 plasmas. Surfaces etched in SF6 plasmas were much smoother than those etched in NF3 plasmas. The addition of only 10% SF6 to an NF3 plasma produced a much higher reaction probability (∼5-fold) than in a pure NF3 plasma. This surprising enhancement of reaction probabilities for F with Si in SF6 plasmas is further investigated, based on the mechanism of adsorbed sulfur acting as a catalyst to greatly enhance the etching rate of Si. Dilute sulfur solutions in isopropyl alcohol were allowed to evaporate on the masked Si samples, depositing sulfur in relatively high concentrations near mask edges in ∼2 μm diameter periodic “strings of beads.” The sulfur-dosed sample etched several times faster at the center of each bead than a sample not exposed to sulfur that was placed side by side. The catalytic effect of sulfur is ascribed to an enhanced F sticking coefficient and/or decreased desorption rate on a surface covered with sulfur.