Siダイアフラム式可変焦点ミラーの収差低減

抄録

A thin Si diaphragm, which can be easily deformed into a concave shape by applying an electrostatic force, can be used as a dynamic focusing mirror. A diaphragm mirror which has a uniform thickness produces large aberrations because its deformation is not optically ideal. To reduce the aberrations, an optimum thickness profile is calculated by FEM analysis and the profile is created using a unique processing method; initially, photoresist on a Si substrate is exposed to a specific energy profile through a mask made by dither method. The photoresist is then formed a specific curved shape. Optical Fourier transform is also applied during the exposure to smooth it. Finally, it is transcribed to the Si substrate by dry etching in CF₄ /O₂ plasma. The processed shape matches the calculated one with only several sub-microns deviations. The resolving power of the optimum thickness mirror is much higher than the uniform thickness mirror in the wide range of the focal length.