David Humbird

Atomistic simulations of spontaneous etching of silicon by fluorine and chlorine

Updated interatomic potential energy functions for Si–F and Si–Cl are used in molecular dynamics simulations of spontaneous etching of Si. Steady halogen uptake and spontaneous silicon etching are predicted as F and Cl atoms impact initially crystalline Si. At 300 K, the simulated etch probability (silicon atoms etched per incident F atom) is 0.03 for F atoms and 0.005 for Cl. The major etch products are SiF4 and Si2F6 for F etching and SiCl4 for Cl. Etching is not observed with F2 or Cl2. At 300 K and below, the simulation predictions are within the range of reported experimental measurements of the surface coverage, etch reaction probability, and etch product distribution. Etch products that remain weakly bound to the surface are detected in significant quantities. At higher temperature (T>450 K), SiF2 and SiCl2 become the dominant etch products as the temperature is increased, in agreement with experiment, but the simulation underpredicts the etch reaction probability. The atomistic mechanisms of etch ...

Improved interatomic potentials for silicon–fluorine and silicon–chlorine

Improved sets of empirical interatomic potentials for silicon-fluorine and silicon-chlorine are presented. The Tersoff-Brenner potential form has been reparameterized using the density-functional theory (DFT) cluster calculations of Walch. Halogenated silicon cluster energetics computed with DFT are, on average, within several tenths of an eV of the energies of the corresponding clusters with the reparameterized empirical potential for both Si-F and Si-Cl. Using the reparameterized Tersoff-Brenner potentials, molecular-dynamics simulations of F and Cl atom exposure to undoped silicon surfaces are in excellent agreement with published data on etch probability, halogen coverage at steady state, and etch product distributions.

Surface chemistry associated with plasma etching processes

Abstract We present our progress towards an accurate simulation model of plasma etching of silicon. A study of the interactions of energetic argon ions with silicon surfaces using molecular dynamics (MD) simulations is reported. A dynamic balance between ion-induced damage and recrystallization of the surface is detected. By manipulating ion energy, argon ions are able to both create disordered regions near the surface, and recrystallize these disordered regions. Silicon atoms in this amorphous region are readily mixed by argon ions. Limited mixing in the crystalline layer is observed. Fluorine adsorbed on the silicon surface does not mix into the layer with argon ion impact. When an energetic F+ impacts a silicon surface, the uptake and apparent sub-surface mixing of F is much greater than Ar+-induced mixing. However, a closer examination shows that the F impacts have primarily increased the Si surface area by creating crevices and cracks, and that the F remains mainly on the surface of this layer. A similar situation results when SiF3+ impacts the surface.

Fluorocarbon plasma etching of silicon: Factors controlling etch rate

Molecular dynamics simulations of CF2, F, and Ar+ impacting silicon surfaces were conducted to understand the mechanisms controlling steady-state etching in typical fluorocarbon (FC) plasmas. The simulations reveal the central importance of a mixed amorphous silicon carbide (a-Si:C) top layer that forms due to ion impact and ion-induced mixing. This layer of a-Si:C forms to a depth that depends on ion energy and the composition of the radicals impacting the surface. With only thermal CF2 and 200 eV Ar+, the a-Si:C layer stops the etching of the underlying Si. Adding as little as 1 F per ion reduces the thickness and increases the permeability of this layer, resulting in steady etching of the underlying Si. A mixed Si–C layer forms whenever C sticks to the Si surface in the presence of energetic bombardment. The FC polymer and bare Si etch much faster than the a-Si:C layer, suggesting that the competition to form and destroy this layer is key in controlling the Si etch rate in FC plasmas under some conditi...

Molecular dynamics simulations of Ar+-induced transport of fluorine through fluorocarbon films

Recent experimental studies of fluorocarbon (FC) plasmas etching various substrates suggest that ions will transport initially bound fluorine (F) through overlying FC films, thereby defluorinating these films and inducing fluorination reaction with the underlying substrate material. Simulations of thermal CF2 on Si with simultaneous bombardment by energetic Ar+ demonstrate this defluorination phenomenon, showing that F is separated from adsorbed CF2 and mixed into the underlying Si, initiating etching. Additionally, this creates dangling bonds on the surface where CF2 may adsorb. Thus, our simulations show that F and C uptake is enhanced by energetic rare gas ion impact, the number of Si–F bonds is greatly increased, and the resultant Si etch rate is higher than expected from physical sputtering alone. The results are compared to experimental measurements made under similar conditions, and the mechanisms of ion-induced F transport are identified.

Silicon etch by fluorocarbon and argon plasmas in the presence of fluorocarbon films

Molecular dynamics simulations have been conducted to study the mechanisms of silicon etch in the presence of fluorocarbon species (CF and C4F4), F atoms and Ar+ ions. The specific goal of the study was to find conditions in which steady Si etching occurs in the presence of a fluorocarbon (FC) film. Results indicate that if incident species are not properly chosen for the simulation, either steady etching is observed with no FC film present, or a FC film is present (often continuously growing in thickness) with no steady etching of the underlying film. With the proper set of incident species, C∕F ratio, neutral/ion flux ratio, and ion energy, we observed steady Si etching in the presence of a steady FC film. We also observed that the thicker the FC film, the lower the etch yield. A sufficiently thick film results in no etching and a continuous deposition. Simulation results are in qualitative agreement with analogous experimental measurements. The key is to find FC species that will stick with a high prob...

Mechanism of silicon etching in the presence of CF2, F, and Ar+

Molecular dynamics simulations of CF2, F, and Ar+ impacting silicon surfaces reveal the spontaneous formation of segregated layers of Si-C and SiFx, formed due to Ar+ ion impact and ion-induced mixing. The mechanisms of steady-state etching under these conditions involve a leading front of SiFx that fluorinates the Si substrate, followed by a region or zone of Si-C. The SiFx and Si-C layers move through the substrate Si during steady-state etching. Si is generally etched from the surface of the Si-C layer by an ion impact. Carbon reaction with Si in the Si-C zone raises the total atomic density in the Si-C layer to nearly three times the value observed in undisturbed Si and reduces the Si etch rate by limiting ion mixing. Etching stops completely if the Si-C layer becomes so impervious that ions cannot reach the SiFx front. The importance of the depth profile of ion energy deposition in sustaining etching is very clearly observed in the simulations.

Molecular dynamics simulations of Si-F surface chemistry with improved interatomic potentials

Molecular dynamics results using an improved Tersoff?Brenner style interatomic potential for Si?F (denoted by TB?HG) are presented. In simulations of F/Ar+, F+, and etching of silicon, the TB?HG potential predicts different behaviour from that of Stillinger and Weber (SW). With the SW potential, F atoms do not mix into Si surfaces, creating instead a roughened surface with F on the outside. With the TB?HG potential, F atoms are able to mix into Si, leading to higher F uptake and Si etch rate in all cases. The TB?HG potential is compared to the modified SW potential of Weakliem et al (SW?WWC) in simulations of F+ etching of Si. The quantitative values of steady-state F uptake and Si etch rate for the (SW?WWC and TB?HG) potentials are nearly identical, but surface structure and etch product distributions are qualitatively different. Evidence of spurious energetic barriers in the SW potential form is given.

Controlling surfaces in plasma processing: role of ions via molecular dynamics simulations of surface chemistry

A study of the interactions of energetic ions with various surfaces using molecular dynamics simulations is reported. Silicon atoms in the amorphous region are readily mixed by argon ions. Limited mixing in the crystalline layer is observed. Fluorine adsorbed on the silicon surface does not mix into the layer with argon ion impact. When an energetic F+ impacts a silicon surface, the uptake and apparent sub-surface mixing of F is much greater than Ar+-induced mixing. However, a closer examination shows that the F impacts have primarily increased the Si surface area by creating crevices and cracks, and that the F remains mainly on the surface of this layer.

Ion-induced damage and annealing of silicon. Molecular dynamics simulations*

A study of the interactions of energetic argon ions with silicon surfaces using molecular dynamics simulations is reported. A dynamic balance between ion-induced damage and recrystallization of the surface is detected. By manipulating ion energy, argon ions are able to both create disordered regions near the surface, and recrystallize these disordered regions.