Fabrication Of Echelette Gratings By Holographic Reactive Ion Beam Etching

Echelon gratings which are kinds of blazed gratings with grooves of zigzag structure and apex angle of 90°, are usually separated into echelle gratings and echelette gratings. Echelette gratings’line densities is higher which vary from hundreds lines/mm to thousands lines/mm, and their orders with blazing effects are lower, compared with echelle gratings. Echelette gratings are usually applied to echelette spectroscopies for the detection of light with broad wavelength in astronomical telescopes. With the development of extremely large telescopes and the technology of spectrometry, it’s very necessary to study the advanced production process of echelette gratings with high efficicency, large aperture and high-quality of wavefront.Transmission blazed gratings made on the transparent substrate, especially the structures combined transmission gratings with prisms which have the advantages of simple optical paths and can effectively save spaces, are favored in recent years. Diamond ruling is currently used for the fabrication of reflection blazed gratings. However, holographic ion beam etching, a method for the fabrication of blazed gratings which combines the high diffraction efficiencies and little scatter light and ghost and also has the potential of manufacturing for large aperture, is good at making a transmission grating on the surface of the transparent substrate like quartz.Therefore, in this paper we adopt holographic reactive ion beam etching to the fabrications of echelette gratings with low line densities. First, grating patterns of photoresist on quartz were made by holographic lithography. Then the photoresist which is usually served as mask in titled etching was replaced by native substrate grating mask by the means of vertical ion beam etching. And finally we change the incident angles of ion beam to make gratings with different blazed angles.Significant phenomena of redeposition during titled etching were observed in shadows of native substrate masks which haven’t been reported in previous papers. The effects of the redeposition were discussed and results show that the redeposition will increase the blazed angles by slowing down the rate of substrate exposed to the reactive ion beam. The relation curve between ion beam incident angles and blazed angles under the influence of re deposition were given. In order to look for the possibilities of changing the apex angles of the grating grooves etched with small beam incident angles, the continuing etching with the same ion beam incident angles after native substrate grating masks etched up were studied and by this method the apex angle of gratings etched with the beam incident angle of 30° was changed from 44° to 78°.By the method mentioned above three kinds of echelette gratings were successfully made. First kind of gratings has a line density of 360 lp/mm with a blazed angle of 16.8°, and the other two kinds with the same line density of 400 lp/mm have different blazed angles of 34.7℃ and 43°, respectively. The transmission diffraction efficiencies of these three kinds of echelette gratings were measured by changing the wavelengths of incident light with the same incident angle. The results show that the measured efficiencies of gratings fabricated by this method are above 75% of theoretical values mostly. Therefore in this paper the range of gratings fabricated by holographic ion beam etching were broadened under the premise of gratings’ efficiencies, which provide a good reference for the fabrication of gratings with larger periods and blazed angles.