First-Principles Study of Chemisorption of Oxygen and Aziridine on Graphitic Nanostructures

Using ab initio plane wave pseudopotential calculations, we study the energetics and structure of adsorbed linear arrays of oxygen and aziridine on carbon nanotubes, graphitic ribbons, and graphene sheets. Chemisorption of arrays of O or NH causes splitting of the CC bond and local deformation of the graphitic structures. The (3,3) nanotube cross section assumes a teardrop-like shape, while graphene sheets warp into a new local geometry around the chemisorbed molecules. The interior of a (3,3) nanotube is less prone to oxidation than the exterior because of steric effects. A zigzag (6,0) nanotube is less reactive and thus chemically more stable than an armchair (3,3) nanotube. The results suggest a partial explanation for the experimentally observed selective etching of metallic carbon nanotubes

Pattern-Dependent Charging in Plasmas: Electron Temperature Effects

The differential charging of high-aspect-ratio dense structures during plasma etching is studied by two-dimensional Monte Carlo simulations. Enhanced electron shadowing at large electron temperatures is found to reduce the electron current density to the bottom of narrow trenches, causing buildup of large charging potentials on dielectric surfaces. These potentials alter the local ion dynamics, increase the flux of deflected ions towards the sidewalls, and result in distorted profiles. The simulation results capture reported experimental trends and reveal the physics of charging damage

The influence of mask thickness on charging damage during overetching

Feature-scale charging simulations during gate electrode overetching in high-density plasmas reveal that the thickness of the insulating mask plays a critical role in charging damage. When thinner masks are used, the electron irradiance of the conductive part of the sidewalls increases, causing the charging potentials of the polysilicon lines to decrease, thus reducing the probability for catastrophic tunneling currents through the underlying oxide. Simultaneously, changes in the charging potential distribution at the bottom SiO2 surface cause a significant perturbation in the local ion dynamics which, in turn, adversely affects notching. Notches are predicted to form everywhere in a line-and-space structure, even when the lines are electrically isolated. The results suggest that the trend toward thinner (hard) masks—to keep the aspect ratio low as device dimensions shrink—should reduce oxide failure but at the cost of more severe notching

Prediction of multiple-feature effects in plasma etching

Charging and topography evolution simulations during plasma etching of dense line-and-space patterns reveal that multiple-feature effects influence critically the etch profile characteristics of the various lines. By including neighboring lines, the simulation predicts a peculiar notching behavior, where the extent of notching varies with the location of the line. Feature-scale modeling can no longer be focused on individual features alone; "adjacency" effects are crucial for understanding and predicting the outcome of etching experiments at reduced device dimensions

Simulation of current transients through ultrathin gate oxides during plasma etching

Monte Carlo simulations of electron tunneling through a 3 nm gate oxide during etching of dense patterns of gate electrodes in uniform high-density plasmas reveal two current transients, which occur: (a) when the open area clears, and (b) when the polysilicon lines just become disconnected at the bottom of trenches. The first charging transient is fast (controlled by charging) and may be followed by a steady-state current which lasts until the lines get disconnected. The second charging transient lasts longer; the magnitude of the tunneling current generally decreases as the sloped polysilicon sidewalls become straighter. Most of the damage occurs at the edge gate when the open areas are covered by field oxide; however, the edge gate suffers no damage when the 3 nm oxide extends into the open areas

Hollow cathode sustained plasma microjets: Characterization and application to diamond deposition

Extending the principle of operation of hollow cathode microdischarges to a tube geometry has allowed the formation of stable, high-pressure plasma microjets in a variety of gases including Ar, He, and H2. Direct current discharges are ignited between stainless steel capillary tubes (d = 178 µm) which are operated as the cathode and a metal grid or plate that serves as the anode. Argon plasma microjets can be sustained in ambient air with plasma voltages as low as 260 V for cathode-anode gaps of 0.5 mm. At larger operating voltage, this gap can be extended up to several millimeters. Using a heated molybdenum substrate as the anode, plasma microjets in CH4/H2 mixtures have been used to deposit diamond crystals and polycrystalline films. Micro-Raman spectroscopy of these films shows mainly sp3 carbon content with slight shifting of the diamond peak due to internal stresses. Optical emission spectroscopy of the discharges used in the diamond growth experiments confirms the presence of atomic hydrogen and CH radicals

Aspect-ratio-dependent charging in high-density plasmas

The effect of aspect ratio (depth/width) on charge buildup in trenches during plasma etching of polysilicon-on-insulator structures is studied by Monte Carlo simulations. Increased electron shadowing at larger aspect ratios reduces the electron current to the trench bottom. To reach a new charging steady state, the bottom potential must increase, significantly perturbing the local ion dynamics in the trench: the deflected ions bombard the sidewall with larger energies resulting in severe notching. The results capture reported experimental trends and reveal why the increase in aspect ratio that follows the reduction in critical device dimensions will cause more problems unless the geometry is scaled to maintain a constant aspect ratio

The influence of surface currents on pattern-dependent charging and notching

Surface charge dissipation on insulator surfaces can reduce local charging potentials thereby preventing ion trajectory deflection at the bottom of trenches that leads to lateral sidewall etching (notching). We perform detailed Monte Carlo simulations of pattern-dependent charging during etching in high-density plasmas with the maximum sustainable surface electric field as a parameter. Significant notching occurs for a threshold electric field as low as 0.5 MV/cm or 50 V/µm, which is reasonable for the surface of good insulators. The results support pattern-dependent charging as the leading cause of notching and suggest that the problem will disappear as trench widths are reduced