Fabrication Of Nano-scale Gratings By Laser Interference Lithography

Grating has been studied as a kind of optical element for more than200years. In recent60years, photolithography has been widely used in fabrication of integrated circuits (gratings). The combination of short wavelength light sources and innovations such as immersion lithography and phase shift masks has pushed the feature size (linewidth of grating) down into22nm.As a new kind of optical element, nano-scale grating plays an important role in photonic crystal, biosensing, solar cell and data storage.Although traditional photolithography has been widely used in nano-scale grating fabrication, the cost of the lens system and photomask limits its application in industry. Laser interference lithography is a much simpler and cheaper laboratory-scale methodology. It is a powerful technique to define large-area, nano-scale, periodically patterned structures by interference of dual coherent lights instead of lens and photomask.This paper is mainly about the applications of laser interference lithography in terms of micro-and nano-fabrication. The aim of the study is to develop a method to fabricate high-quality nano-scale gratings in large area at low cost and high throughput as molds for nanoimprint lithography.This paper is divided into four parts. The first part is the introduction, mainly about the new progress of nano-fabrication recently. The novel methods include nano-imprint, laser interference lithography, x-ray lithography, electron beam lithography, focused ion beam and so on. By comparison, we find that laser interference lithography is a powerful laboratory-scale nano-fabrication tool.The second part is description of optic theory related to interference lithography. A fundamental introduction and theory analysis of interference lithography is presented. The coherent condition of light, period equation derivation of interference fringes and the influence of laboratory environment on the fringe contrast are described.Next part is results and discussion of experiment. Gratings with period about440nm、600nm、2μmand dot arrays are fabricated in2-inch silicon or silica substrate using laser interference lithography. Then structures are transferred into substrate by induction couple plasma (ICP) etching. In this part, we analyze the impact of sinusoidal topography to structure transferring. The sinusoidal topography makes "lift off" much more difficult. Then we adopt three different kinds of methods to get steep side wall such as toluene processing, ammonia processing and tri-layer stack system. All those methods can simplify the procedure of lift off and fabricate high aspect ratio nanostructures.Also a hybrid mold is fabricated by combining nano-imprint, laser interference lithography and photolithography. The hybrid mold includes two different structures: nano-scale diffraction grating and micro-scale free-standing grating. There are two kinds of diffraction gratings,200-nm-period and440-nm-period gratings, fabricated by nano-imprint lithography and photolithography, respectively. The10-μm-period grating is fabricated as free-standing grating. The diffractive grating and standing grating can be obtained in one step of imprint. The process simplifies the procedure and decreases the defects of the free-standing grating. At last, conclusion is made.