Masahito Mori

A Model Analysis of Feature Profile Evolution and Microscopic Uniformity during Polysilicon Gate Etching in Cl2/O2 Plasmas

A phenomenological model has been developed to simulate the feature profile evolution of polycrystalline silicon (poly-Si) gate etching in Cl2/O2 plasmas. The model takes into account the deposition of etch products, surface oxidation, and the forward reflection of energetic ions on feature sidewalls. To describe the formation of multilayer SiClx or SiClxOy on feature surfaces during etching, the substrates consist of a number of small cells or lattices of atomic size in the computational domain; this model provides a nanometer-scale representation of the feature geometry and the chemical constituents therein. The inelastic or nonspecular reflection of incoming ions from feature surfaces and the penetration of ions into substrates are incorporated into the model by calculating the trajectory of ions through successive binary collisions with substrate atoms. Etching experiments were performed to evaluate and improve the accuracy of the model. To analyze the effects of the control variables of a plasma reactor on profile evolution, the simulated profiles for different gas flow ratios and incident ion energies were compared with the etched profiles obtained in the experiments. The numerical results reproduced the behaviors of profile anomalies such as sidewall tapering and microtrenches at the corner of the feature bottom, upon varying the incident fluxes of O neutrals and etch by-products, and the incident energy of ions. Moreover, the simulated profiles exhibited passivation layers deposited on feature sidewalls, which is a similar geometry to those obtained in the experiments.

Atomic-Scale Cellular Model and Profile Simulation of Si Etching: Analysis of Profile Anomalies and Microscopic Uniformity

Reactive ion etching (RIE) has been used in the manufacture of semiconductor integrated circuit devices. However, the formation mechanisms of profile anomalies and microscopic uniformity have been poorly understood until now. In this paper, we focus on the analysis of formation mechanisms of profile anomalies and microscopic uniformity during plasma etching of Si in Cl2 plasmas, using our own atomic-scale cellular model (ASCeM). The numerical results indicated that high neutral-to-ion flux ratios result in microtrench formation. Moreover, RIE lag tends to occur at low neutral-to-ion flux ratios (<50), whereas inverse RIE lag occurs at high neutral-to-ion flux ratios in typical low-pressure and high-density plasmas. In particular, the etch rates for narrow patterns (<70 nm) increase significantly with increasing neutral-to-ion flux ratio. The synergistic effects between ion-enhanced etching and neutral shadowing in microstructural features play a significant role in the formation of profile anomalies.

Uniform lateral etching of tungsten in deep trenches utilizing reaction-limited NF3 plasma process

The reaction-limited etching of tungsten (W) with NF3 plasma was performed in an attempt to achieve the uniform lateral etching of W in a deep trench, a capability required by manufacturing processes for three-dimensional NAND flash memory. Reaction-limited etching was found to be possible at high pressures without ion irradiation. An almost constant etching rate that showed no dependence on NF3 pressure was obtained. The effect of varying the wafer temperature was also examined. A higher wafer temperature reduced the threshold pressure for reaction-limited etching and also increased the etching rate in the reaction-limited region. Therefore, the control of the wafer temperature is crucial to controlling the etching amount by this method. We found that the uniform lateral etching of W was possible even in a deep trench where the F radical concentration was low.

Effect of temperature on deposition layer formation in HBr/N2/fluorocarbon-based plasma

The effects of wafer temperature on etching rate and surface composition were investigated to clarify the surface reaction mechanism under HBr/N2/fluorocarbon-based gas plasma for developing a process for three-dimensional NAND flash devices. The etching rates of both polycrystalline silicon (poly-Si) and SiO2 were found to increase at a wafer temperature of 20 °C as compared with those at 60 °C. Comparing the gas combination of fluorocarbon/N2 and HBr/N2 mixtures, the temperature dependence of SiO2 etching rates was considered to relevant to the sticking probability of fluorocarbon polymers. To determine the cause of the temperature dependence of the poly-Si etching rate, surface composition was evaluated by thermal-desorption-spectroscopy and laser-sputtered-neutral-mass-spectrometry analyses. Ammonium bromide was confirmed in the deposition film at a wafer temperature of 20 °C. The observed increase in poly-Si etching rate at lower temperatures was possibly caused by increased amounts of nitrogen, hydrogen, and bromine fixed to the surface with the formation of ammonium bromide.

Role of surface-reaction layer in HBr/fluorocarbon-based plasma with nitrogen addition formed by high-aspect-ratio etching of polycrystalline silicon and SiO2 stacks

The etching of polycrystalline silicon (poly-Si)/SiO2 stacks by using VHF plasma was studied for three-dimensional NAND fabrication. One critical goal is achieving both a vertical profile and high throughput for multiple-stack etching. While the conventional process consists of multiple steps for each stacked layer, in this study, HBr/fluorocarbon-based gas chemistry was investigated to achieve a single-step etching process to reduce process time. By analyzing the dependence on wafer temperature, we improved both the etching profile and rate at a low temperature. The etching mechanism is examined considering the composition of the surface reaction layer. X-ray photoelectron spectroscopy (XPS) analysis revealed that the adsorption of N–H and Br was enhanced at a low temperature, resulting in a reduced carbon-based-polymer thickness and enhanced Si etching. Finally, a vertical profile was obtained as a result of the formation of a thin and reactive surface-reaction layer at a low wafer temperature.

Negative Impact of Etched Si Area on Selectivity and Positive Impact of Photoelectric Current on Etched Profile in Gate Etching with Different Wafer Bias Frequencies

The effect of wafer-bias frequency on the dummy-gate fabrication of fin-shaped field-effect transistor (Fin-FET) was investigated. The clear difference in the selectivity of polycrystalline silicon to SiO2 between 400 kHz and 13.56 MHz decreased when the etched Si area increased. On the other hand, a higher frequency increased such selectivity when Si area decreased. These results can be explained by the effect of by-product deposition. As for the etched profile, the amount of side etching was much larger at 13.56 MHz than at 400 kHz. It was reported that this phenomenon is caused by local charging. It was also suggested that the charging should be suppressed by reducing the ratio of ion saturation current to photoelectric current. Therefore, in this study, we investigated the effect of such current ratio on side etching. The result confirmed that a reduction in current ratio induced by increasing gas pressure decreases the amount of side etching.