Research On Fabrication Of Photonic Crystals By Nanoimprint Lithography And Its Application

As one of the most promising nanostructure processing technologies, nanoimprint lithography (NIL) has the advantages of high-resolution, low-cost and large-scale production. Therefore, NIL was classified as one of the important representatives of next generation lithography (NGL) for the 32nm, 22m and 16nm nodes in the International Technology Roadmap for Semiconductors (ITRS). Since 1995, Chinese scientist, Professor Stephen Chou, put forward the concept of"nanoimprint"for the first time in the journal of Appl. Phys. Lett., it has aroused more and more attention and the research boom from the academic and industrial circles. Foreign manufacturers of semiconductor equipment, materials, and technology providers have begun to set foot in this area of research work. In the last more than a decade, NIL has developed with some types of imprinting methods, such as Hot Embossing Lithography (HEL), UV-based Nanoimprint Lithography(UV-NIL) andμ-Contact Print(μCP), etc.. Nevertheless, as a contact processing technology of nanostructure fabrication, some key issues of the NIL technology, such as nano-scale material properties, high precision imprinting process, demolding, are still being investigated up to now, and in particular compatibility with the current semiconductor industrial process is still not unavailable, which is different from the non-contact process of optical lithography. That is the reason why NIL is still at the level of laboratory research and only several semiconductor Integrated Circuit (IC) pioneers are in the attempt to carry out industrial tests.In this paper, the aim is to fabricate two-dimensional photonic crystal (2D-PC) structure and develop its applications. The key technologies in the process of UV-NIL were deeply investigated. Researches on large-area and low-cast UV-NIL stamp fabrication technologies, functional nanoimprint resist preparation and stamp surface anti-stick modification were carried out to solve the bottlenecks in the process. For the application, the optimized 2D-PC structure for improving the light extraction efficiency of LED were theoretically designed and finally fabricated via combing UV-NIL and semiconductor manufacturing processes. These research works are necessary for mastering the core intellectual property rights, and promoting the industrialization of NIL technologies. Paper mainly include the following contents.First, for the disadvantages of high-cost and low-efficiency of fabricating UV-NIL stamp directly by EBL, a new process was invented to fabricate 2D-PC nano-arrays with the critical dimension of below 100 nm by transferring a concave pattern to a convex nanosize stamp via wet etching technology, which is low-cost and large-area process available. Meanwhile, A new kind of UV-NIL resist with photoluminescent properties was prepared by modifying the imported nanoimprint resist, AMONIL, with ZnO quantum dots (QDs). The new functional UV-NIL resist not only enhancing the photoluminescent properties of ZnO QDs but also preserving the nanoimpint capabilities, which explores the promising applications in the optoelectronic devices.The key technologies in UV-NIL process including substrate and stamp cleaning, stamp anti-stick modification, preparation of the resist film, high precision nanoimprint process controlling and pattern transferring to the substrate were studied in detail. A vapor deposition set was developed for the stamp surface anti-stick treatment. The contact angle of the stamp surface was enhanced from 33°before modified to 113°after modified, which resolved the problem of demoulding difficulty in the UV-NIL process. Ultimately, the 2D-PC structure was fabricated by UV-NIL successfully with the accuracy within 10 nm.Finally, for the actual application to enhance the light extraction efficiency of LED, using the equivalent refractive index theory and antireflective optical film theory, the 2D-PC structures on the surface of gallium nitride (GaN) of LED epitaxial wafer were designed. A set of 2D-PC parameters was put forward. With the effective integration of UV-NIL and LED semiconductor manufacturing process, PC-LEDs were manufactured successfully. After chip packages, the initial test results indicated that PC-LEDs had clearly improvement than that without PC structures in the flux, light efficiency and light radiation with the highest increases by 22.3%, 36.65%, 14.23% respectively. Comparing the experimental results with the theoretical designs, it was shown that the 2D-PC structure with high duty ratio (2r / a) was conducive to the light energy limited into the LED chip to be coupled out to the free-space, thereby enhancing the efficiency of light output. Different periods and etching depths of 2D-PC structures also have influences on the light extraction efficiency under the same duty cycle. The smaller the period, the higher the etching depth was required. This conclusion has great significance for the further theoretical optimization and process improvement.Part of the studies in this paper was carried out cooperating with the LED chip manufacturer. The device production process was completed through the industrial line and the packaging and testing were in accordance with the current standard. Therefore, implementation process has completely compatibility with the existing semiconductor technology. Accordingly, the present research work in this paper show more practical significance for the industrial applications of NIL in the development of the nanoelectronic semiconductor industry to the aim of smaller, colder, faster nanodevices.