Study On High Diffraction Efficiency Immersion Grating

Using high-resolution imaging spectrometer to monitor the concentration of CO2 and other greenhouse gases has gradually become the focus of research in various countries.The high efficiency immersion grating is the key component of high-resolution imaging spectrometer,so it is of great significance to study the immersion grating.In this paper,the fabrication of high diffraction efficiency immersion grating by holographic ion beam etching is studied.The main work is as follows:At first,this paper has introduced the basic concept of immersion prism grating and the progress both at home and abroad.We also have introduced the fabricating method of the grating.Next,we have introduced the Finite Difference Time Domain(FDTD)theory.We have designed four kinds of immersion gratings,and the grating groove include rectangular and trapezoid.We chose the diffraction efficiency as the evaluation function to optimize the two groove parameters of the grating ratio of width and groove depth and obtained the possible ranges of the ratio of width and of the groove depth.Considering the influence of the slant of the side wall of the grating in the actual production,we have analyzed the diffraction efficiency of immersion gratings with different trapezoidal sidewall angles from 80°to 88°.For the trapezoidal groove type immersion grating used in the strong CO2 band with trapezoidal sidewall angle in the range of 80° to 88°,when the ratio of width is 0.53-0.65 and the groove depth is 810-1150nm,its diffraction efficiency is higher than 70%.These optimized parameters provided guidance for the subsequent experimental production.Thirdly,we have carried out experiments to explore some important etching parameters of quartz,K9 glass and S9912 positive photoresist,such as the etching uniformity,etching rate and etching selection ratio.Furthermore,we also investigated the effects of exposure light intensity,exposure time and development time on ratio of width and groove depth of photoresist mask in the holographic exposure process.The photoresist mask meeting the requirements of etching was made,and the changes of mask and width ratio in the ion beam etching process were analyzed.Finally,through a large number of experiments,we made a strong CO2 band immersion grating sample,and analyzed the influence of the abnormal etching phenomena such as bottom fillet,bottom depression and lateral wall etching on the diffraction efficiency.