Conformality of atomic layer deposition in microchannels: impact of process parameters on the simulated thickness profile

Unparalleled conformality is driving ever new applications for atomic layer deposition (ALD), a thin film growth method based on repeated self-terminating gas-solid reactions. In this work, we re-implemented a diffusion-reaction model from the literature to simulate the propagation of film growth in wide microchannels and used that model to explore trends in both the thickness profile as a function of process parameters and different diffusion regimes. In the model, partial pressure of ALD reactant was analytically approximated. Simulations were made as function of kinetic and process parameters such as temperature, (lumped) sticking coefficient, molar mass of the ALD reactant, reactant’s exposure time and pressure, total pressure, density of the grown material, and growth per cycle (GPC) of the ALD process. Increasing the molar mass and the GPC, for example, resulted in a decreasing penetration depth into the microchannel. The influence of the mass and size of the inert gas molecules on the thickness profile depended on the diffusion regime (free molecular flow vs. transition flow). The modelling was compared to a recent slope method to extract the sticking coefficient. The slope method gave systematically somewhat higher sticking coefficient values compared to the input sticking coefficient values; potential reasons behind the observed differences are discussed.

Hysteresis effect during reactive sputtering

In this work, we studied the effect of constant parameters of the sputtering system on the width of the hysteresis loop during reactive sputtering. The sticking coefficient of the reactive gas to the surface, the chamber pumping speed, the target area, etc. are taken as parameters. The comparative study was carried out by numerical solution of systems of algebraic equations describing the chemisorption and physicochemical models of metal target reactive sputtering in a single reactive gas. The calculations were performed for sputtering a tantalum target in an Ar + O2 mixture. The studied dependences were non-linear in all cases.

Finite size scaling in the heterogeneous reaction 2A + B2 → 2AB

In the present paper we extend the exact solution previously obtained for the heterogeneous catalytic reaction 2A + B2 → 2AB on small domains, to arbitrary lattice sizes () and calculate the average number of reactive steps necessary to poison the lattice first, . We determine as a function of through Monte Carlo simulations previously contrasted with the exact solution in lattices. We show that follows a power law with , without appreciable transient behaviors, and a scale factor () dependent on the two parameters of the model, the sticking coefficient probability and the desorption probability . The dependence of on both and is determined.

Ripple Formation during Oblique Angle Etching

Chemical removal of materials from the surface is a fundamental step in micro- and nano-fabrication processes. In conventional plasma etching, etchant molecules are non-directional and perform a uniform etching over the surface. However, using a highly directional obliquely incident beam of etching agent, it can be possible to engineer surfaces in the micro- or nano- scales. Surfaces can be patterned with periodic morphologies like ripples and mounds by controlling parameters including the incidence angle with the surface and sticking coefficient of etching particles. In this study, the dynamic evolution of a rippled morphology has been investigated during oblique angle etching (OAE) using Monte Carlo simulations. Fourier space and roughness analysis were performed on the resulting simulated surfaces. The ripple formation was observed to originate from re-emission and shadowing effects during OAE. Our results show that the ripple wavelength and root-mean-square roughness evolved at a more stable rate with accompanying quasi-periodic ripple formation at higher etching angles (θ > 60°) and at sticking coefficient values (Sc) 0.5 ≤ Sc ≤ 1. On the other hand, smaller etching angle (θ < 60°) and lower sticking coefficient values lead to a rapid formation of wider and deeper ripples. This result of this study can be helpful to develop new surface patterning techniques by etching.

H2 chemistry in interstellar ices: the case of CO ice hydrogenation in UV irradiated CO:H2 ice mixtures

Context. In dense clouds, hydrogenation reactions on icy dust grains are key in the formation of molecules, like formaldehyde, methanol, and complex organic molecules (COMs). These species form through the sequential hydrogenation of CO ice. Although molecular hydrogen (H2) abundances can be four orders of magnitude higher than those of free H-atoms in dense clouds, H2 surface chemistry has been largely ignored; several laboratory studies show that H2 does not actively participate in “non-energetic” ice chemistry because of the high activation energies required. Aims. For the example of CO ice hydrogenation, we experimentally investigated the potential role of H2 molecules on the surface chemistry when energetic processing (i.e., UV photolysis) is involved. We test whether additional hydrogenation pathways become available upon UV irradiation of a CO:H2 ice mixture and whether this reaction mechanism also applies to other chemical systems. Methods. Ultra-high vacuum (UHV) experiments were performed at 8–20 K. A pre-deposited solid mixture of CO:H2 was irradiated with UV-photons. Reflection absorption infrared spectroscopy (RAIRS) was used as an in situ diagnostic tool. Single reaction steps and possible isotopic effects were studied by comparing results from CO:H2 and CO:D2 ice mixtures. Results. After UV-irradiation of a CO:H2 ice mixture, two photon-induced products, HCO and H2CO, are unambiguously detected. The proposed reaction mechanism involves electronically excited CO in the following reaction steps: CO + hν→CO*, CO* + H2→HCO + H where newly formed H-atoms are then available for further hydrogenation reactions. The HCO formation yields have a strong temperature dependence for the investigated regime, which is most likely linked to the H2 sticking coefficient. Moreover, the derived formation cross section reflects a cumulative reaction rate that mainly determined by both the H-atom diffusion rate and initial concentration of H2 at 8–20 K and that is largely determined by the H2 sticking coefficient. Finally, the astronomical relevance of this photo-induced reaction channel is discussed.

An Energy-Based Fouling Model for Gas Turbines: EBFOG

Fouling is a major problem in gas turbines for aeropropulsion because the formation of aggregates on the wet surfaces of the machine affects aerodynamic and heat loads. The representation of fouling in computational fluid dynamics (CFD) is based on the evaluation of the sticking probability, i.e., the probability a particle touching a solid surface has to stick to that surface. Two main models are currently available in literature for the evaluation of the sticking coefficient: one is based on a critical threshold for the viscosity, and the other is based on the normal velocity to the surface. However, both models are application specific and lack generality. This work presents an innovative model for the estimation of the sticking probability. This quantity is evaluated by comparing the kinetic energy of the particle with an activation energy which describes the state of the particle. The sticking criterion takes the form of an Arrhenius-type equation. A general formulation for the sticking coefficient is obtained. The method, named energy-based fouling (EBFOG), is the first “energy”-based model presented in the open literature able to account any common deposition effect in gas turbines. The EBFOG model is implemented into a Lagrangian tracking procedure, coupled to a fully three-dimensional CFD solver. Particles are tracked inside the domain, and equations for the momentum and temperature of each particle are solved. The local geometry of the blade is modified accordingly to the deposition rate. The mesh is modified, and the CFD solver updates the flow field. The application of this model to particle deposition in high-pressure turbine vanes is investigated, showing the flexibility of the proposed methodology. The model is particularly important in aircraft engines where the effect of fouling for the turbine, in particular the reduction of the high pressure (HP) nozzle throat area, influences heavily the performance by reducing the core capacity. The energy-based approach is used to quantify the throat area reduction rate and estimate the variation in the compressor operating condition. The compressor operating point as a function of the time spent operating in a harsh environment can be in this way predicted to estimate, for example, the time that an engine can fly in a cloud of volcanic ashes. The impact of fouling on the throat area of the nozzle is quantified for different conditions.

An Energy Based Fouling Model for Gas Turbines: EBFOG

Fouling is a major problem in gas turbines for aero-propulsion. The aerodynamics and heat load of the blades are severely affected by this phenomenon with local geometrical variations due to deposition and erosion. Currently two major models are available in literature for the evaluation of fouling effects in CFD: the first one is based on a critical threshold for the viscosity, whereas the second is characterized by the normal velocity to the surface. Both models aim to define a likelihood coefficient which estimates the probability a particle has to stick to a surface, known as sticking coefficient. However current models lack of generality being application specific. This work presents an innovative model for the estimation of the sticking probability. The fouling effect is defined as function of particle velocity, temperature and size through an energy based approach. Expressing the energy involved in the impact through an Arrhenius’ type equation a general formulation for the sticking coefficient is obtained. The method, named EBFOG (Energy Based FOulinG), is the first “energy” based model presented in the open literature that can account any common deposition effect in gas turbines. The EBFOG model is implemented into a Lagrangian tracking procedure, coupled to a full three-dimensional CFD solver. Particles are tracked inside the domain and the velocity, size and temperature of each ones are calculated. The local geometry of the blade is modified accordingly to the deposition rate, the mesh is modified and the CFD solver updates the flow field. The application of this model to particle deposition in high pressure turbine vanes is investigated showing the flexibility of the proposed methodology. Such model is particular important in aircraft engines where the effect of fouling for the turbine, in particular the reduction of the HP nozzle throat area, influences heavily the performance: the energy based approach is thus used to quantify the area modification and estimate the variation of the compressor performance. The compressor map as a function of the operating hours in a severe environment can be in this way predicted to estimate, for example, the time that an engine can fly in a cloud of volcanic ashes. For this reason the impact of the fouling on the throat area of the nozzle is quantified for different conditions.