Preparation And Evaluation Of The Nano-patterned Sapphire Substrates Fabricated By Imprint Method

Sapphire is widely used in many fields such as GaN epitaxial substrate and opticalwindow materials due to its excellent performance. Compared with the conventionalsapphire substrate, the patterned sapphire substrate, which is defined as periodicsubmicron structures fabricated on the sapphire surface, can improve the light extractionefficiency in LED field and overcome the low transmittance of conventional sapphireoptical window. Wet etching and dry etching are common methods to fabricatepatterned sapphire substrate, but both of them are time-consuming and high cost. In thisstudy, nanoimprint, a method to fabricate patterned sapphire substrate, is studied indetail and the patterned substrate is evaluated.The nanoimprint method to fabricate patterned sapphire substrate can be devidedinto several steps, including sputtering aluminum, preparing the PDMS soft seal,fabricating the photoresist, UV imprinting, etching and annealing. In this article, severalcharacterization methods such as XRD, SEM and AFM are used to analyze theinfluence of working pressure, sputtering power and substrate temperature on the filmmorphology, deposition rate and crystal quality of aluminum during magnetronsputtering process, the impact of spin coating speed, pressure, exposure intensity duringnanoimprint, and the high temperature annealing parameters in process of aluminumfilm transform to sapphire. Finally, Tracepro software and Filmstar software are used toinvestigate the effect of patterned sapphire substrate on the LED light extraction andoptical window transmittance.The test results show that with parameters of0.2Pa,60W,70℃during magnetronsputtering process, we can get the optimized film （Ra=1.346nm）. PDMS soft seal isfabricated by spin coating on AAO template. Resist thickness, pressure and exposureintensity are the important factors during nanoimprint. As the spin-coating speedincreasing, thickness of the photoresist declined sharply and then flatten. Photoresist iscoated at3000rpm for60s, imprinted with pressure of300mbar for300s, exposured for45s, then we get the patterned photoresist with thickness, depth, adhesive residue are210nm,65nm and30nm respectively. Next, the pattern is transferred to aluminum byreactive ion etching and ion beam etching by controlling the gas flow rate, ion beam,neutral current, air pressure, and etching angle (3sccm,60mA,75mA,1.6×10^-2Pa,30°).With the parameter of epitaxial temperature at450℃and1000℃, the FWHM ofdiffraction peak （0006） is0.0133°. Numerical calculation results show that increasing the area ratio and the depth ofthe pattern can improve light extraction efficiency. We also found that the performanceof the inclined sidewall substrate was better than the perpendicular sidewall substrate.The light extraction efficiency can reach as high as61.8%when the sidewallinclination is45°. In areas of optical window, with the area ratio increases,antireflection effect increase and then decrease. Light transmittance can up to91.53%when the area ratio, depth are0.296,90nm respectively.