Reactive Sticking Coefficient Research Articles

Scaling of silicon nanoparticle growth in low temperature flowing plasmas

Low temperature plasmas are an emerging method to synthesize high quality nanoparticles (NPs). An established and successful technique to produce NPs is using a capacitively coupled plasma (CCP) in cylindrical geometry. Although a robust synthesis technique, optimizing or specifying NP properties using CCPs, is challenging. In this paper, results from a computational investigation for the growth of silicon NPs in flowing inductively coupled plasmas (ICPs) using Ar/SiH4 gas mixtures of up to a few Torr are discussed. ICPs produce more locally constrained and quiescent plasma potentials. These positive plasma potentials produce an electrostatic trap for negatively charged NPs, which can significantly extend the residence time of NPs in the plasma, which in turn provides a controllable period for particle growth. The computational platforms used in this study consist of a two-dimensional plasma hydrodynamics model, a three-dimensional nanoparticle growth and trajectory tracking model, and a molecular dynamics simulation for deriving reactive sticking coefficients of silane radicals on Si NPs. Trends for the nanoparticle growth as a function of SiH4 inlet fraction, gas residence time, energy deposition per particle, pressure, and reactor diameter are discussed. The general path for particle synthesis is the trapping of small NPs in the positive electrostatic potential, followed by entrainment in the gas flow upon reaching a critical particle size. Optimizing or controlling NP synthesis then depends on the spatial distribution of plasma potential, the density of growth species, and the relative time that particles spend in the electrostatic trap and flowing through higher densities of growth species upon leaving the trap.

Shape, Size Evolution, and Nucleation Mechanisms of GaAs Nanoislands Grown on (111)Si by Low-Temperature Metal–Organic Vapor-Phase Epitaxy

The shape, size evolution, and nucleation mechanisms of GaAs nanoislands grown at 400 °C on As-stabilized (111)Si by metal–organic vapor phase epitaxy are reported for the first time. GaAs crystallizes in the zincblende phase in the very early nucleation stages until a continuous epilayer is formed. GaAs nanoislands grow (111)-oriented on Si as truncated hexagonal pyramids, bound by six equivalent {120} side facets and a (111) facet at the top. Their diameter and height appear to increase linearly with the deposition time, yielding a constant aspect ratio of ∼1/4. The nanoisland density (before coalescence) stays constant with time at ∼2 × 1010 cm–2, suggesting that the nucleation occurs at specific Si surface sites (defects) during the very early growth stages, rather than being due to the continuous formation of new nuclei. To understand the molecular-level mechanisms driving the low-temperature MOVPE growth of GaAs on Si, we applied a deposition–diffusion–aggregation (DDA) nucleation model, which predicts a linear evolution of the overall GaAs growth rate with surface coverage, in good agreement with experimental observations, under the assumption that direct impingement of trimethylgallium (Me3Ga) molecules onto the nanoisland surface dominates the material nucleation and growth rate; the contribution of Me3Ga adsorbed onto the As-stabilized (111)Si is negligible, pointing out the reduced reactivity of the Si surface (As passivation). Our DDA model allows estimation of the effective reactive sticking coefficient of Me3Ga onto GaAs, which is equal to 2.82 × 10–5: the small value is compatible with the Me3Ga large steric hindrance and the competitive role of methyl radicals in surface adsorption at low temperature.

Collision-Energy Dependence of the Uptake of Hydroxyl Radicals at Atmospherically Relevant Liquid Surfaces

A new experimental approach to the study of collisions of hydroxyl radicals with liquid surfaces is described, incorporating a molecular-beam source of OH (or, in practice, OD, for technical reasons) radicals. This allowed the collision-energy dependence of the scattering to be examined. The incident and scattered OD molecules were detected by laser-induced fluorescence. The representative branched, long-chain alkane, squalane (2,6,10,15,19,23-hexamethyltetracosane), and its partially unsaturated analogue, squalene (2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene), were compared with perfluoropolyether as an inert reference liquid. Dynamical aspects of the scattering necessary to quantify the OD survival probability, and hence its complement, the reactive sticking coefficient, were determined. Results were obtained at average laboratory-frame kinetic energies of 7.2 and 29.5 kJ mol–1; they are compared with previous independent measurements using a photolytic source of OH with an average kinet...

KINETICS OF LOW PRESSURE AMMONIA OXIDATION OVER RH(111)

The kinetics of the NH3 + O2 reaction over a Rh(111) single crystal catalytic surface was explored in the 10-6 mbar pressure range at temperatures between 300-900 K. Selectivity towards N2 and NO products, and reactive sticking coefficients were monitored in situ using differentially pumped quadrupole mass spectroscopy (QMS).

Kinetics of the deposition step in time multiplexed deep silicon etches

The time multiplexed deep silicon etch (TMDSE) process is the etch process of choice to make MEMS devices and through wafer vias. It has been used to produce deep trenches and vias at reasonable throughputs. Significant issues remain for the TMDSE process as well as room for improvement even though it has been both experimentally studied and modeled by a wide variety of researchers. This is because it is a highly complex process. Aspect ratio dependencies, selectivity, and the ability to use photoresist masks (instead of SiO2) are examples of remaining issues. The presently obtainable etch rates do not indicate efficient use of the etchant species. In this article, the authors focus on the deposition step in the TMDSE process. While prior research has generally assumed that the deposition step can be adequately modeled as being controlled by a reactive sticking coefficient, they have experimentally examined the deposition step of the process and found that the film growth is dominantly ion-enhanced. The results shown here were obtained in C4F8 plasmas but are also consistent with results found in CHF3 and C4F6 plasmas. As a result, the deposited film thickness can be larger at the bottom of a high aspect ratio feature than at the top sidewall, which is exactly the opposite of the desired profile. The very nature of the deposition mechanism leads to mask undercut at the same time as feature closing/etch stop.

The O2 + NH3 Reaction Over Rh(110): Steady State Kinetics and Oscillatory Behavior

The kinetics of ammonia oxidation with oxygen over a Rh(110) surface were studied in the pressure range 10−5–10−4 mbar. Nitrogen was found to be the preferred product at low partial pressures ratios $$ {\text{p}}_{{{\text{o}}_{ 2} }} :{\text{p}}_{{{\text{NH}}_{ 3} }} $$ , while the NO pathway was favored with oxygen rich gas mixtures and at high temperature. The reactive sticking coefficient of O2 reaches up to 0.05 under steady state conditions. Pronounced hysteresis effects in the reaction rates were found in T-cycling experiments. Sustained oscillations in the reaction rates occurred under isothermal conditions at T = 620 K at a total pressure of 4 × 10−5 mbar.

Robust gas detection at sub ppm concentrations

Gas sensors often suffer from signal drift and long response and recovery times, giving rise to problems to evaluate the steady state gas response. It is shown that these problems can be reduced by modulating the test gas concentration and utilizing the change in the slope of the sensor signal as the sensor response feature. The case of low hydrogen concentration detection is studied using a light pulse technique and a PdPt-MIS field-effect device. For the difference in slope method it was possible to reduce the measurement time from hours in the steady state measurements to 1 + 1 min (test gas + reference gas exposure time) and still achieve a detection limit of about 40 ppb for a step change in hydrogen concentration. Such measurements could be made in spite of a drifting baseline caused, e.g. by previous hydrogen exposures. A theoretical model of how a step change in the hydrogen partial pressure affects the difference in slope is given. The model also predicts a non-reactive hydrogen sticking probability (i.e. the probability that an incoming hydrogen molecule from the gas phase will contribute to the response) of about 1 × 10 −8 at 100 ppb H 2, which is about 2 orders of magnitude lower than the reactive sticking coefficient.

Dynamics of porous silicon formation by etching in HF + V2O5 solutions

Formation of porous silicon by etching of silicon wafers with vanadium pentoxide (V2O5) dissolved in hydrofluoric acid (HF) has been studied with infrared spectroscopy and electron microscopy. V2O5 creates in solution, which initiates the reaction by injecting holes into the silicon valence band. Much is known about the mechanism of etching that leads to flat Si surfaces; however, the transition to pore formation is not well understood. The rate of film growth depends linearly on the V2O5 concentration in aqueous solutions but has a nonlinear dependence on the formal HF concentration. Addition of ethanol greatly decreases the etch rate and changes the pore morphology from a mixture of {100} + {110} planes to predominantly {100} planes. A plot of thickness versus etch time evolves from a quadratic to a linear dependence, whereas the surface area depends linearly on the etch depth. These observations are consistent with a model in which pores with a uniform diameter nucleate randomly then lengthen linearly in time. The pore density increases at short times and then reaches a saturation value. The probability that the collision of a ion with the surface leads to etching of a Si atom (reactive sticking coefficient) is ∼3 × 10−8.

CO Oxidation over a Pt(100) Surface in a Ternary Gas Mixture of CO, NO, and Oxygen

Abstract Catalytic CO oxidation in a ternary gas mixture of CO, NO and O2 has been studied in the 10–5–10–4 mbar range using a Pt(100) sample as catalyst. Measurements of the reactive sticking coefficients of CO and NO are used to decompose the overall CO oxidation rate into the contributions from oxidation by NO and from oxidation by O2. The experiments are complemented by simulations with a realistic 7-variable model. The surface phase transition hex ↔ 1×1 and the inhibition of NO dissociation and O2 adsorption at high adsorbate coverages lead to significant hysteresis effects.

Kinetic Evidence for the Influence of Subsurface Oxygen on Palladium Surfaces Towards CO Oxidation at High Temperatures

Transient state kinetics of the catalytic oxidation of CO with O(2) on Pd-surfaces has been measured under isothermal conditions by using a molecular beam approach. Systematic studies were carried out as a function of reaction temperature and CO+O(2) composition. With sufficient kinetic evidence, we have demonstrated the positive influence of subsurface oxygen towards CO-adsorption and oxidation to CO(2) at high temperatures (600-900 K) on Pd-surfaces, and the likely electronic nature of the surface changes with oxygen in the subsurface. These studies also provide a direct proof for CO-adsorption with a significantly reactive sticking coefficient at high temperatures on Pd-surfaces exhibiting a significant subsurface O-coverage.

Effect of ferroelectric polarization on the adsorption and reaction of ethanol on BaTiO 3

The effect of ferroelectric polarization on the adsorption and reaction of ethanol on BaTiO 3 was studied using temperature-programmed desorption (TPD). Consistent with previous studies for methanol, the reactive sticking coefficient for ethanol on the surfaces of the ferroelectric, tetragonal phase of BaTiO 3 was found to be dependent on the orientation of the ferroelectric domains with a higher sticking coefficient on the c + surface relative to the c − surface. TPD peak shapes and the relative product yields were also found to be polarization dependent suggesting that ferroelectric polarization may affect the intrinsic reactivity of the surface.

Seasoning of plasma etching reactors: Ion energy distributions to walls and real-time and run-to-run control strategies

Wafer-to-wafer process reproducibility during plasma etching often depends on the conditioning of the inside surfaces of the reactor. Passivation of reactor surfaces by plasma generated species, often called seasoning, can change the reactive sticking coefficients of radicals, thereby changing the composition of the radical and ion fluxes to the wafer. Ion bombardment of the walls may influence these processes through activation of surface sites or sputtering, and so the spatial variation of ion energies on the walls is important. These seasoning processes may occur during a single etching process or on a wafer-to-wafer basis. The seasoning of plasma etching reactors will be discussed using results from a computational investigation of p-Si etching in chlorine plasmas. The transport of etch products, passivation of walls, and sputtered products from walls are accounted for, as well as differentiating the ion energy distributions to different surfaces. A real-time, closed-loop control of etch rate to counter the effects of seasoning was achieved using the bias voltage as an actuator.

Low Pressure Chemical Vapor Deposition of In Situ-Doped n- and p-Type Si1 − xGex Films at 425 ° C

The deposition of in situ-doped n- and p-type polycrystalline films by low pressure chemical vapor deposition (LPCVD) at low temperature is characterized. The results demonstrate the dominance of surface reaction effects for low temperature deposition. For n-type films, is required to avoid phosphorus (P) poisoning effects and to achieve controllable deposition. P incorporation increases with Ge concentration within the as-deposited films, likely due to dimer suppression by . For p-type films, is the preferred Si precursor gas because it yields over one order of magnitude higher B concentration as compared with . This difference is attributed to the differences in reactive sticking coefficient (RSC) and in the amount of Si produced by the precursor gas. B incorporation decreases with increasing Ge concentration within the as-deposited films, due to competition for adsorption sites during deposition. depletion was observed to affect the dependence of Ge composition on partial pressure, but not the deposition rate, which has a greater dependence on the RSC.

A General Correction to Surface Reaction Models Based on Reactive Sticking Coefficients

AbstractFollowing the work of Coltrin et al. (Surface Chemkin III, Reaction Design, San Diego 1996), many CVD models express the rate of surface reactions as the molecular flux into the surface multiplied by a reactive sticking coefficient and a correction factor that was derived by Motz and Wise (J. Chem. Phys. 1960, 32, 1893). According to Motz and Wise, this factor is only valid for small reactive species concentrations, a condition that is not generally fulfilled in CVD processes. Nevertheless, it is widely used in CVD modeling.In the present study, the direct simulation Monte Carlo (DSMC) method was used to determine the deviation from this theory for more general CVD process conditions. The results show that the deviation increases with increasing reactive sticking coefficient and reactant mass fraction, and a significant deviation from the theory of up to 45 % was found. A fitted correlation is presented to predict this deviation using the sticking coefficient and the reactant mass fraction for cases with a reactant mass fraction near the surface up to (approximately) 99 %.

Rotary reactor for atomic layer deposition on large quantities of nanoparticles

Challenges are encountered during atomic layer deposition (ALD) on large quantities of nanoparticles. The particles must be agitated or fluidized to perform the ALD surface reactions in reasonable times and to prevent the particles from being agglomerated by the ALD film. The high surface area of nanoparticles also demands efficient reactant usage because large quantities of reactant are required for the surface reactions to reach completion. The residence time of the reactant in a fluidized particle bed reactor may be too short for high efficiency if the ALD surface reactions have low reactive sticking coefficients. To address these challenges, a novel rotary reactor was developed to achieve constant particle agitation during static ALD reactant exposures. In the design of this new reactor, a cylindrical drum with porous metal walls was positioned inside a vacuum chamber. The porous cylindrical drum was rotated by a magnetically coupled rotary feedthrough. By rotating the cylindrical drum to obtain a centrifugal force of less than one gravitational force, the particles were agitated by a continuous “avalanche” of particles. In addition, an inert N2 gas pulse helped to dislodge the particles from the porous walls and provided an efficient method to purge reactants and products from the particle bed. The effectiveness of this rotary reactor was demonstrated by Al2O3 ALD on ZrO2 particles. A number of techniques including transmission electron microscopy, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy confirmed that the Al2O3 ALD film conformally coats the ZrO2 particles. Combining static reactant exposures with a very high surface area sample in the rotary reactor also provides unique opportunities for studying the surface chemistry during ALD.

Low-pressure methanol oxidation over a Cu(110) surface under stationary conditions: (I) reaction kinetics

The steady-state kinetics of methanol oxidation over Cu(110) have been studied in the 10 −7 to 10 −3 mbar range. H 2, H 2O, and CO 2 are found in addition to the main product formaldehyde. The reactive sticking coefficient of methanol reaches 0.2. A pronounced hysteresis is observed when the temperature is cycled. Our data indicate the presence of two rate maxima, one at ≈ 400 – 520 K and the other at ≈ 900 K . The amplitude of the low-temperature reactivity peak decreases with increasing total pressure, vanishing beyond 10 −3 mbar. High oxygen partial pressures led to a poisoning of the reaction, which was apparently caused by the inhibitory effect of high oxygen coverage on methanol adsorption.

Kinetics of ammonia oxidation on stepped platinum surfaces. I. Experimental results

The kinetics of ammonia oxidation with oxygen have been investigated in the 10 −5 and 10 −4 mbar range on Pt(5 3 3) and Pt(4 4 3). Only N 2 and NO but no N 2O were detected as reaction products. The dependence of product formation on temperature and on the partial pressures of the reactants has been studied under steady state conditions. The reactive sticking coefficients were determined under reaction conditions with s reac of ammonia reaching nearly 0.2. The Pt(5 3 3) surface was found to be catalytically more active than Pt(4 4 3) by a factor of 2–4.

Analysis of closed loop control and sensor for a reactive sputtering drum coater

The reactive sputtering process is characterized by a hysteresis of reactive gas concentration and reactive gas flow [Schiller, Thin Solid Films 118, 255 (1984); Affinito and Parsons, J. Vac. Sci. Technol A. 2, 1275 (1984)]. The precise control of the reactive sputtering process requires maintaining operation at points on the hysteresis that ensures the desired high sputter flux and deposited thin film stoichiometry. The hysteresis is highly nonlinear in these preferred operating regimes. The practical challenge of meeting this in a reactive sputtering batch coating system requires a control system that can compensate for changes in reaction rate for various sputtered metals, reactive gas sticking coefficients, and system pumping speed. Closed loop control algorithms that rapidly bring the reactive sputtering system to the desired hysteresis steady state operating point are desired for multilayer applications such as thin film interference filters that require many target starts (and stops). Conventional methods of starting the reactive sputtering process such as temporally ramping target power, voltage or current and target shutters are undesirable for precision thin film interference filters. We present a reactive gas sensor and analysis of the proportional-integral-derivative closed loop control algorithm for an industrial reactive sputtering batch drum coater. The key enabler of the control algorithms is a reactive gas sensor employed on these drum coaters that exhibits a temporal response of several hundred milliseconds. We discuss how elements from control theory applied to the reactive sputtering process can determine what points on the hysteresis will be the most difficult to control and the limits to implementation for an industrial system.

Kinetics of phosphine adsorption and phosphorus desorption from gallium and indium phosphide ( [formula omitted

The kinetics of phosphine adsorption and phosphorus desorption from gallium and indium phosphide (0 0 1) has been determined using reflectance difference spectroscopy to monitor the phosphorus coverage in real time. Assuming a Langmuir adsorption mechanism, phosphine exhibited an initial reactive sticking coefficient at 500 °C of 8.7 ± 1.0 × 10 −2, 3.5 ± 1.0 × 10 −2 and 1.0 ± 0.2 × 10 −3 on the GaP (2 × 4), GaP (1 × 1) and InP (2 × 4) reconstructions, respectively. The sticking coefficient increased with temperature on the gallium phosphide surfaces, exhibiting an activation energy of 0.5 ± 0.2 eV, while on indium phosphide, no temperature dependence was observed. The desorption of phosphorus from the GaP (2 × 1) surfaces was first-order in coverage with rate constants of 5.0 × 10 15 (s −1) exp(−2.6 ± 0.2 (eV)/kT). These results may be used to estimate the feed rate of phosphine relative to the group III precursors during the metalorganic vapor-phase epitaxy of gallium and indium phosphide.

Enhanced etching of Si( [formula omitted]) by neutral oxygen cluster beam

We present the study of high temperature (800–1250 °C) oxidation of Si(1 0 0) by large neutral oxygen clusters (600 and 4000 molecules per cluster). The coexistence of passive and active oxidation areas on the surface under cluster beam impact allows studying simultaneously both reactive etching via the reaction 2Si+O 2→2SiO(g) and onset of SiO 2 formation. The reactive sticking coefficient of oxygen in the active oxidation area is significantly higher than that for molecular oxygen and exhibits a strong dependence on the cluster flux. We suggest that the enhancement of active oxidation results from multiple collisions of cluster molecules with the surface. While in the case of the larger clusters the flux dependence of the etch rate may be partly explained by etching-induced surface misorientation, for the smaller clusters it is obviously a clusters-related effect, presumably related to the capture of oxygen scattered from the surface by oncoming clusters. The analysis of the critical conditions for oxide growth indicates that SiO 2 nucleation is also enhanced for the smaller clusters.