Taku Iwase

Effect of oxygen addition to an argon plasma on etching selectivity of poly(methyl methacrylate) to polystyrene

Abstract. The effect of oxygen addition to an argon plasma on the etching selectivity of poly(methyl methacrylate) (PMMA) to polystyrene (PS) (hereafter “PMMA/PS etching selectivity”) was investigated. The PMMA/PS etching selectivity was evaluated by using inductively coupled plasmas composed of argon and oxygen. The etching selectivity in the case of argon plasma was estimated to be 3.9, which is higher than that of oxygen plasma, which is 1.7. The time dependence of etching depth shows that the etching rate of PMMA is reduced to less than one half of its initial value after the etching depth exceeds 15 nm. X-ray photoelectron spectroscopy of the PMMA surface revealed that the reduction of etching rate is caused by a depletion of oxygen concentration by argon-ion bombardment. To compensate the oxygen-concentration depletion, 1% oxygen was added to the argon plasma. As a result, the reduction of PMMA etching rate was suppressed, and constant etching rate was obtained even when etching depth exceeded 50 nm. The mixed argon-oxygen plasma was used to fabricate a PS mask pattern with a full pitch in the range of 25.5 to 77 nm.

Characteristics of selective PMMA etching for forming a PS mask

The characteristics of poly(methyl methacrylate) (PMMA) etching of self-assembled poly(styrene-block-methyl methacrylate) (PS-b-PMMA) thin film for forming a polystyrene (PS) mask were investigated. In this investigation, first, the etching selectivity of PMMA to PS under argon- and oxygen-plasma processes was evaluated. Higher selectivity was obtained in the case of argon plasma (3.9) compared to that of oxygen plasma (1.7). Second, to investigate the argon process in detail, the time dependence of etching depth was evaluated. It was found that PMMA etching rate decreases by more than half after etching to a depth of around 15 nm. To investigate the mechanism of this decrease in PMMA etching rate, the surface composition of PMMA was measured by X-ray photoelectron spectroscopy (XPS). The XPS result revealed that the reduction of etching rate is caused by a depletion of oxygen by argon ions, and the depleted oxygen attaches to the PMMA film in air exposure. In accordance with these results, to compensate the decrease in oxygen concentration, oxygen was added to the argon plasma at a composition of 1%. As a result of this oxygen addition, constant PMMA etching rate was confirmed, even beyond etching depth of 50 nm. It is thus concluded from these results that a PS lamellar mask pattern with a pitch from 25.5 to 70 nm could be successfully formed by using selective PMMA etching.

Reducing hole-size variation and defect ratio after pattern transfer by using self-assembled polymer with spherical structure

This study presents a method to reduce hole-diameter variation and defect ratio in patterning of a self-assembled block copolymer (BCP) for imprint-mold fabrication. The BCP material used is PMMA-b-poly(methyl acrylate) polyhedral oligomeric silsesquioxane (PMAPOSS) in which PMMA spheres with 18.3-nm-pitch are aligned in the hexagonal close-packed positions in the PMAPOSS matrix. When the self-assembled BCP film was etched in the conventional dry-development process, the hole-diameter variation and the amount of hole defects (defect ratio), defined as “no-opening defects” or “connecting holes,” increased. Variation of PMMA sphere diameter and/or position in the perpendicular direction to the substrate plane was assumed to be the main cause of the increase in hole-diameter variation and defect ratio after BCP development. To optimize the etching conditions for BCP development, a new model representing the relationship between defect ratio and relative standard deviation of PMMA sphere diameter and/or posit...

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.

Development of High-Efficiency Half-Ducted Propeller Fan for Packaged Air Conditioners

We developed a high-efficiency half-ducted propeller fan to reduce the electric power consumption of the outdoor unit of a packaged air conditioner by using a design tool combining computational fluid dynamics (CFD) with multi-objective optimization techniques based on a genetic algorithm (GA). The baseline fan was a half-ducted propeller fan with three blades of a currently available product. Blade shape was defined using 16 design variables including inlet and outlet blade angles, setting angles, blade length, sweep angles, dihedral angles, and so on. An in-house program was used to automatically generate the grids for CFD calculation. The objective functions were static pressure efficiency and fan noise level for optimization. The fan noise was calculated with an aerodynamic noise prediction model that used the relative inlet and outlet velocities of the fan blades from the CFD results. We found there was a trade-off relationship between the static pressure efficiency and the fan noise. We then selected the optimized fan that had the same noise level as the baseline fan but with an improved static pressure efficiency. The blade tip of the optimized fan was curled toward the suction side direction. Finally, we confirmed through experiments that the static pressure efficiency of the optimized fan was increased by 1.6% compared to the baseline fan.Copyright

Calculation of Aerodynamic Noise for Centrifugal Fan of Air-Conditioner

Flow fields in a centrifugal fan for an indoor unit of an air-conditioner were calculated with finite element method-based large eddy simulation (LES) with the aim of predicting fan performance and aerodynamic noise in this study. The numerical simulation code employed throughout the LES was called FrontFlow/blue (FFB). We compared 10M grid [coarse grid] and 60M grid [fine grid] calculation results for investigation of influence of grid resolution. In the fine grid, the number of grid elements in blade-to-blade direction, and of region between the shroud and the bell mouth increased in particular. By calculating with the fine grid, calculated distributions of absolute velocities at blade exit reasonably agreed with experimental results. Because of this, maximum absolute velocity by fine grid near hub decreased as compared to those by coarse grid. Calculated sound pressure level by fine grid was therefore smaller than that by coarse grid, and the overestimation of sound pressure was suppressed by calculating with fine grid. This decrease of the absolute velocity was a first factor for the improvement of calculation accuracy. Moreover, number of captured streaks on the blade, hub, and shroud surfaces by fine grid increased as compared to those by coarse grid. As a result, size of streak by fine grid became smaller than that by coarse grid. Static pressure fluctuations by fine grid on the blade, hub, and shroud surfaces therefore reduced as compared to those by coarse grid. Aerodynamic noise was related to static pressure fluctuations according to Curle’s equation. This reduction of static pressure fluctuations was therefore a second factor for improvement of calculation accuracy.© 2013 ASME

Development of High Efficiency Propeller Fan for Heat-Pump Unit Using CFD and Numerical Optimization

We developed a high-efficiency propeller fan to reduce electric power consumption of the fan motor for outdoor heatpump units, and we developed a designing tool combining computational fluid dynamics (CFD) with multi-objective optimization techniques based on the genetic algorithm (GA). In CFD, a numerical model is calculated using commercial software based on steady state, Reynolds-averaged Navier-Stokes (RANS) and k-e turbulent flow model. The objective functions are fan efficiency and fan noise for optimization. Fan efficiency is calculated directly from the CFD results, and fan noise is calculated using an aerodynamic noise prediction model using the relative inlet and outlet velocities of the fan blades from the CFD results. We fabricated a high-efficiency propeller fan characterized with curled trailing edge tips from Pareto optimal solutions. The experimental results from the performance of the fan showed the developed fan was more efficient than conventional fan.© 2011 ASME

Technique for Designing Forward Curved Bladed Fans Using Computational Fluid Dynamics and Numerical Optimization

We developed a technique for designing forward curved bladed fans using computational fluid dynamics (CFD) and numerical optimization. The target is a forward curved bladed fan including an impeller with blades and a volute casing. In our research, we developed a two-step calculation method with a blade-to-blade grid model and a full grid model. Using these models individually according to purpose, we reduced the design time by one quarter. An automatic grid-generation program that was developed in-house generated the grids for the CFD calculation. The fan performance was calculated using commercial CFD software based on an incompressible Reynolds-averaged Navier-Stokes (RANS) solver. For numerical optimization, we used a simulated annealing algorithm (SA) to determine the optimized design variables. Using the developed technique, we attempted to minimize the total pressure loss of an impeller and a suction cone. We could obtain the optimized design variables: the gap between the impeller and suction cone, the inside diameter of the shroud rim cover and the number of blades. Our results demonstrated that the optimized fan design had smaller shaft power than the initial design, especially at the low flow rate. Clearly therefore, our technique is capable of designing an energy saving fan in a short time. Moreover, it was found that that the leak flow between the impeller and suction cone of the optimized fan was suppressed. The change in these design variables contributed to this suppression.Copyright

Development of Autonomous Design Technique for Axial Fans Using Numerical Optimization

We designed an axial fan for servers using computational fluid dynamics (CFD) and numerical optimization. The performance of the fan, namely static pressure rise and efficiency, was calculated using commercial CFD software based on an incompressible Reynolds-averaged Navier-Stokes (RANS) solver. An automatic program developed in-house was used to generate the grids for CFD calculation. Numerical optimization—using a simulated annealing algorithm (SA)—was used for determining the optimized shape of the fan. After optimizing the fan, initial and optimized fan designs were made for experiments using rapid prototyping, and their performances, based on such things as efficiency and noise level, were measured. Results demonstrated that the optimized fan design achieved higher efficiency than the initial design. Multi optimization was also developed for maximizing the fan efficiency and minimizing the casing height. An additional finding was that there was a trade-off between the fan efficiency and casing height.Copyright