Scanning ion beam etching: A method for the fabrication of computer-generated hologram with nanometric accuracy for aspherical testing

Abstract

The phase type computer generated hologram (CGH) plays an important role in aspherical surface optical testing, especially for low reflectivity material mirror surfaces, owing to its high diffractive efficiency. With the increasing demand for high precision CGHs, traditional methods such as reactive ion etching and focusing ion beam, have difficulty realizing the fabrication of a large aperture and high accuracy phase type CGH. In this study, fabrication of a high precision CGH has been demonstrated via a new method called scanning ion beam etching (SIBE), which is suitable for most common optical materials and the fabrication of high performance CGH. On the basis of the movable radio frequency ion source, we determine an etching depth calculation method to realize high accuracy etching regardless of whether the removal function is a perfect Gaussian distribution. Experiments are conducted to verify the etching depth accuracy. Moreover, the linewidth, sidewall angle, roughness, and etching uniformity results of the CGH are superior to the fabrication of CGH via the conventional method. As a proof of concept, a phase type CGH with an aperture of 80 mm is fabricated, and the wavefront accuracy is 6.2 nm, which can meet the processing requirements of a high precision aspheric mirror. Thus, a SIBE method is developed for high performance CGH, which may aid in the development of high accuracy diffractive optics.