In a method for the machine polishing of workpieces, the workpiece is immersed into a container filled with abrasive and is moved relative to the abrasive container. The GaN based semiconductor layer is removed by etching except in a region on the SiO2 film, and a p electrode is then formed on the top surface of the GaN based semiconductor layer on the SiO2 film, to join the p electrode on the GaN based semiconductor layer to an ohmic electrode on a GaAs substrate. The main body is attached to the electronic device. The multiplexer receives the clock signal and a low-frequency signal outputted from the signal generating circuit. By combining the etching that increases the resist thickness with the conventional etching of resist in which the resist thins during etching, features having high aspect ratios can be etched. A spring is within the case that compresses and expands in response to oscillatory motion of a bar that extends through an opening in the case.