Controlled Fluxes of Silicon Nanoparticles to a Substrate in Pulsed Radio-Frequency Argon–Silane Plasmas

It has been hypothesized that high-energy impact of very small silicon nanoparticles on a substrate may lead to epitaxial growth of silicon films at low substrate temperature. A possible means for producing such energetic nanoparticle fluxes involves pulsing an RF silane-containing plasma, and applying a positive DC bias to the substrate during the afterglow phase of each pulse so as to collect the negatively charged particles generated during the RF power on phase. We here report numerical modeling to provide a preliminary assessment of the feasibility of this scheme. The system modeled is a parallel-plate capacitively-coupled RF argon–silane plasma at pressures around 100 mTorr. Simulation results indicate that it is possible to achieve a periodic steady state in which each pulse delivers a controlled flux of nanoparticles to the biased substrate, that average particle sizes can be kept below 2–3 nm, that impact energies of the negatively-charged nanoparticles that are attracted by the applied bias can be maintained in the ~1 eV/atom range thought to be conducive to epitaxial growth without causing film damage, and that the volume fraction of neutral nanoparticles that deposit by low-velocity diffusion can be kept well below 1 %. The effects of several operating parameters are explored, including RF voltage, pressure, the value of the applied DC bias, and RF power on and off time during each pulse

An experimental and numerical study of particle nucleation and growth during low-pressure thermal decomposition of silane

Abstract This paper discusses an experimental and numerical study of the nucleation and growth of particles during low-pressure (∼1:0 Torr) thermal decomposition of silane (SiH 4 ). A Particle Beam Mass Spectrometer was used to measure particle size distributions in a parallel-plate showerhead-type semiconductor reactor. An aerosol dynamics moment-type formulation coupled with a chemically reacting uid ow model was used to predict particle concentration, size, and transport in the reactor. Particle nucleation kinetics via a sequence of chemical clustering reactions among silicon hydride molecular clusters, growth by heterogeneous chemical reactions on particle surfaces and coagulation, and transport by convection, di usion, and thermophoresis were included in the model. The e ect of pressure, temperature, ow residence time, carrier gas, and silane concentration were examined under conditions typically used for low-pressure (∼1 Torr) thermal chemical vapor deposition of polysilicon. The numerical simulations predict that several pathways involving linear and polycyclic silicon hydride molecules result in formation of particle "nuclei," which subsequently grow by heterogeneous reactions on the particle surfaces. The model is in good agreement with observations for the pressure and temperature at which particle formation begins, particle sizes and growth rates, and relative particle concentrations at various process conditions. A simpliÿed, computationally inexpensive, quasi-coupled modeling approach is suggested as an engineering tool for process equipment design and contamination control during low-pressure thermal silicon deposition.

SARS-CoV-2 susceptibility and COVID-19 disease severity are associated with genetic variants affecting gene expression in a variety of tissues

Variability in SARS-CoV-2 susceptibility and COVID-19 disease severity between individuals is partly due to genetic factors. Here, we identify 4 genomic loci with suggestive associations for SARS-CoV-2 susceptibility and 19 for COVID-19 disease severity. Four of these 23 loci likely have an ethnicity-specific component. Genome-wide association study (GWAS) signals in 11 loci colocalize with expression quantitative trait loci (eQTLs) associated with the expression of 20 genes in 62 tissues/cell types (range: 1:43 tissues/gene), including lung, brain, heart, muscle, and skin as well as the digestive system and immune system. We perform genetic fine mapping to compute 99% credible SNP sets, which identify 10 GWAS loci that have eight or fewer SNPs in the credible set, including three loci with one single likely causal SNP. Our study suggests that the diverse symptoms and disease severity of COVID-19 observed between individuals is associated with variants across the genome, affecting gene expression levels in a wide variety of tissue types

A first update on mapping the human genetic architecture of COVID-19

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Theory of nucleation from the gas phase by a sequence of reversible chemical reactions

A general theory is presented for the problem of condensed-phase particle nucleation from gas-phase precursors via a sequence of reversible chemical reactions, where no supersaturated vapor exists. We consider a system in which nucleation is initiated by the reaction between an ''initiating species'' and a '' growth species.''Subsequent steps in cluster growth involve reversible reactions between a cluster and the growth species, producing a larger cluster and a volatile byproduct, which may be considered a ''suppressing species.'' Following the mathematical formalism of homogeneous nucleation theory, a steady-state nucleation rate is derived in the form of a summation over discrete cluster sizes. The resulting nucleation rate is linearly proportional to the product of the concentrations of the initiating species and the growth species, while the ratio of the concentrations of the growth species to the suppressing species, relative to a suitably defined equilibrium value, is seen to play a similar role as the vapor saturation ratio in homogeneous nucleation

Coagulation of nanoparticles in a plasma

Abstract: Nanoparticles can form via chemical nucleation from gas-phase species during plasma processing of silicon films. Nanoparticle-plasma interactions are studied by simulating finite-rate charging and transport of particle nuclei in a low-pressure processing plasma. Results show little change in electron temperature and concentration during the early stages of particle nucleation. However, the ion density profile changes drastically as ions accumulate near the reactor center. Increased ion concentration corresponds to the growing concentration of negatively charged particles, which are shown to have the highest production rate in the reactor center where they are trapped. A significant number of neutral particles are deposited on reactor walls or onto a deposition substrate by diffusion. Positively charged particles impact the substrate at low concentrations but with high energies, which may affect film morphology during plasma-enhanced chemical vapor deposition (PECVD)