Yasushi Toma

Fast Atom Beam Etching of Glass Materials with Contact and Non-Contact Masks.

Fast atom beam (FAB) etching of multicomponent glass and silica glass was performed using a contact mask (electron beam resist) and two non-contact masks (typically 5-µ m-diameter particles and a copper mesh with a 5 µ m line width and 20 µ m line spacing). FAB etching of a multi component glass substrate with the micro-particle mask successfully fabricated a precisely projected, 1.0-µ m-high outline pattern on the substrate. FAB etching of a silica glass substrate with the copper-mesh mask, which was separated from the substrate by about 100 µ m, successfully produced a projected, 34-nm-high outline pattern on the substrate. A combination of electron beam lithography with FAB etching on silica glass successfully fabricated nano-scale ultrafine patterns whose aspect ratio was higher than 7 (50 nm line width and 360 nm height). In all three fabrications, the side walls and etched surfaces were very smooth and were perpendicular to each other.

Removal Rate Simulation of Dissolution-Type Electrochemical Mechanical Polishing

A new method for simulating the Cu removal rate in electrochemical mechanical polishing (ECMP) based on the dissolution-type polishing mechanism was developed. The effect of a protective layer on the Cu removal rate was considered in this method because the protective layer is a key element in the dissolution-type polishing mechanism. This method was used to simulate the removal rate in a rotary-type ECMP system. The simulations accurately provided the dependence of the Cu removal rate on the aperture ratio. Furthermore, the dependence of the Cu removal rate on the aperture ratio was described with respect to changes in the average protective layer amount with time. Regarding the dependence of the Cu removal rate on the aperture diameter, however, a discrepancy was observed between the simulation and experimental results because this method did not take into account the effect of the aperture diameter on the electrolyte-filling ratio in apertures.

Development of Electrochemical Machining System in Ultrapure Water and Application to Planarization Machining Process.

An electrochemical machining system using ultrapure water has been developed, designed for planarization machining and free-curvature-shape machining. The machining system is composed of three units: an electrochemical machining apparatus, an ultrapure water refining and circulation unit, and a high-pressure ultrapure water supply unit. The machining apparatus is equipped with an XYθ table and a Zφ machining electrode. A cylindrical or a spherical electrode is used for the machining electrode. The machining chamber is continually filled with refined ultrapure water by the refining and circulation unit. During machining experiments, process by-products, bubbles, and the like around the machining point can be efficiently removed by the high-pressure ultrapure water supply unit. Using this machining system, planarization experiments have been performed on copper plates, which are known to be easily etched, via application of a positive voltage. The effects of various machining parameters (current density, rotating speed of the electrode, etc.) on the roughness of the etched surface are investigated. It was found that the use of a high-pressure ultrapure water supply nozzle is effective in the planarization process. Furthermore, it was also found that there is an ideal set of conditions with respect to the combination of current density and rotating speed of the electrode. Copper plate surface roughness (Ra) was successfully reduced from 164 nm to 10 nm by planarization machining, with the optimal set of parameters.

A Study on Electrochemical Machining Method in Ultrapure Water. Chemical Reaction Process of Si(001) Anode Surface.

In order to reveal the mechanism of the electrochemical machining process in ultrapure water, first-principles molecular-dynamics simulations of Si(001) surfaces interacting with OH molecules were carried out on the basis of the Kohn-Sham local-density-functional formalism. A plane-wave basis set was used, and the cut-off energy is 327eV(24Ry). A norm-conserving pseudopotential was also used. We adopt the standard molecular-dynamics method for the optimization of the ionic system and the preconditioned conjugate-gradient (CG) method for the quenching procedure of the electronic degrees of freedom. We determined the optimized ionic configurations and electronic distributions for OH chemisorbed Si(001) surfaces and clarify the elemental process of the chemical reactions. It was confirmed that the Si surface atom cannot bond with four OH molecules, so it cannot be etched as an Si(OH)4 molecule. In this simulation, it was also confirmed that two OH molecules react with each other producing an H2O molecule and an oxygen atom. The oxygen atom bonds with two Si surface atoms at the surface bridge site or back-bond. We concluded that the Si surface atom cannot be etched with Si(OH)4 molecule but oxidized by oxygen atom produced by two OH molecules. These results agree with the experimental results of anodic oxidation of Si surface.