Fabrication Of Soft-x-ray Transmission Gratings

As one of important dispersive elements usually used in x-ray region, gold transmission gratings find increasing applications in plasma diagnostic in Inertial Confinement Fusion, and in High Energy Transmission Grating Spectrometer (HETGS) used in x-ray astrophysics. It is complicated to produce a gold transmission grating by holographic lithography, especially for self-standing transmission gratings with high frequency. So it is necessary to have a systematic and detailed study on the fabrication of gold transmission gratings.The main purpose of this thesis is to master the fabrication process of soft X-ray transmission gratings and to fabricate some self-standing gold gratings with high frequency. The subject matter of the thesis are as the follows:(1) In-depth study on the fabrication process of a pattern transfer mask by interferometric lithography. A monitoring apparatus for latent image during exposure and for developing diffraction gratings in a photo-resist has been set up and the relationship between the intensity of latent image and exposure dose and development has been investigated, by which the optimum exposure dose and development time of photo-resist on surface having slight variations in optical properties can be simulated.(2) Experimentally study on the ion beam milling and electroplating which usually are used to transfer the mask patterns into gold during fabrication of gold transmission gratings. It is demonstrated that redepostion is a problem during process of milling gold, but experimental results indicates it can be cleared away by inclining and rotating the sample or using thinner mask instead of thicker photoresist mask. As far as electroplating, it is known that some influence factor such as pH, temperature of solution and current density have an impact on the quality of gold deposited by electroplating. Optimum parameters have been given by comparing results which are obtained by electroplating a large number of samples.(3) The distribution of intensity of incident irradiation in photoresist during exposure has been figured out, and it is shown that the pattern in photoresist on surface with high reflectivity will suffer from standing wave as a result that the incidence irradiation interference with the reflective light from the photoresist-substrate interface. The higher the reflectivity, the worse the effection of standing wave, and it is shown that the standing wave will have a bad effects on the profile and the duty cycle of photoresist grating and decrease the tolerance of the difference between intensity of the two incident beams. So the reflectivity of substrate must be decreased for perfect photoresist patterns transfer mask, especially for mask with finer period and small and limited linewidth. The anti-reflection coating has been used to decrease intensity of reflected beam from substrate and it is proved to be a good way.(4) Some gold transmission phase grating with 1μm period, 200nm height, 0.55 duty cycle and a size of 5mm×15mm have been produced to achieve higher efficiencies than opaque gratings in the range of 5-12nm region. Efficiencies have been measured in the Spectral Radiation Standard and Metrology Experiment Station of National Synchrotron Radiation Laboratory (NSRL) and the efficiency of the +1st order is about 16%.(5) Study on the bi-level resists process in which photoresist patterns are imaged and developed in resist over an antireflection coating. It contains a durable material (i.e. durable layer), typically a metal such as chromium, needed to be shadow-evaporated to cap the resist structure to transfer the resist pattern, reactive-ion etching (RIE) the antireflection coating (ARC) and controlling of the duty cycle of plating mask, the related problem with electroplating and the fabrication of mesh-support structure. At last, a mesh-supported gold transmission grating with 290nm period and a size up to 10mm×15mm is finished. The total obstruction of support structures is about 35%. The diffraction efficiency has been measured and the efficiency of the 1st order is about 4%-6%.