Fabrication Of Soft X-ray Freestanding Blazed Transmission Gratings

As a new type of transmission grating, the freestanding blazed transmission grating combines the advantages of traditional transmission gratings with those of reflection gratings, and has a wide application prospect in Inertial Confinement Fusion, x-ray astrophysics and X-ray phase contrast imaging. It is complicated to produce a freestanding blazed transmission grating, so it is necessary to have a systematic and detailed study on the fabrication of freestanding blazed transmission gratings.The main purpose of this thesis is to master the fabrication process of the freestanding blazed transmission grating. The subject matter of the thesis areas the follows:(1) How to produce the grating structures (bars and grooves) by the anisotropic wet etching silica-wafer? In order to achieve the fabrication of the freestanding blazed transmission gratings by anisotropic wet etching, a high accuracy of the orientation alignment, a large etch rate anisotropy between (110) and (111) crystal plane and a large duty cycle of the etching mask were necessary. Potassium hydroxide (KOH) etching of the nanostructure was investigated to improve etch anisotropy and uniformity. And it was shown that an increase of the anisotropy was expected at high concentrations and low etching temperatures. In addition, a surfactant (HT-541) and a megasonic system were applied to detach the hydrogen bubbles and improve etch uniformity.(2) How to achieve alignment of grating pattern and the special crystal orientation. Grating pattern alignment is one of the most dominant factors that determine the success of the grating fabrication. The process can be divided into four steps. Firstly, pre-etch an alignment pattern to determin the true{111} direction of a<110> wafer. Secondly, align the reference grating with the true{111} direction via a light microscope. Thirdly, align another reference grating with the holographic interference fringes by the moire fringe method. Finally, align the two reference gratings by the diffraction spots. Due to the different alignment patterns, there are two methods of alignment:fan pattern alignment and hexahedron alignment. Compared to the hexagonal alignment, the fan pattern alignment is more precise.(3) Increasing the duty cycle of the wet etch mask. A big duty cycle of the wet etch mask can improve process latitude. With the lift-off technique, an overhang of the photoresist grating was got, and the photoresist grating with a small duty cycle was transferred to a metal grating with a big duty cycle. In addition, depositing aluminum with a large angle by vacuum thermal evaporation can also increase the linewidth. (4) A freestanding transmission grating with a period of1μn, duty cycle of0.13, thickness of10μm and fractional area of60%and a freestanding transmission grating with a period of500nm, duty cycle of0.34, thickness of5μm and fractional area of62%are successfully fabricated by holographic lithography and anisotropicwet etching. The size of a single die is15mm x15mm divided into four5mm x5mm windows. The diffraction efficiency of the fabricated grating with1μm period is measured at the National Synchrotron Radiation Laboratory in the range of5-50nm region. The wavelength scanning results show a strong blazing effect in the direction of specular reflection from mirror-like grating sidewalls, as expected. The measured and normalized diffraction efficiency is consistent with the theoretical prediction within38%-53%due to grating structural imperfections.