Bottom surface smoothing of high aspect ratio hole by guiding large-area electron beam with magnet

In a large-area electron beam (EB) irradiation method, high energy density of EB with about 60 mm in diameter can be used for melting metal surface instantly. Surface roughness of mold steels with planar shape decreases in a short time by the large-area EB irradiation. However, it is difficult to smooth a bottom surface of high aspect ratio hole, since the EB tends to concentrate on the entrance edge and/or upper inside wall of hole, and the energy density of the EB may decrease on the bottom surface of hole. Then, the method for guiding the EB to the bottom surface of high aspect ratio hole is required. The EB may concentrate on the bottom surface by control of magnetic field near the workpiece with the magnet, since the EB tends to move spirally along the magnetic force lines. In this study, guiding the large-area EB on the bottom surface of high aspect ratio hole is proposed by setting the magnet under the workpiece in order to smooth the bottom surface. Magnetic force lines near the workpiece are calculated by using the electromagnetic field analysis, and the possibility of guiding the EB to the bottom surface of high aspect ratio hole is investigated. Experimental and analytical results clarified that the bottom surface of high aspect ratio hole can be smoothed by multi-step EB irradiation with moving the horizontal offset distance of magnet.