Research On Improving The Efficiency Of Laser Lift-off Technique

In this article how to improve the efficiency of laser lift-off technique by studying the temperature distribution in GaN/sapphire structure induced by the laser heating was analyzed. By analyzing GaN thermal decomposition process in theory, the relationship between pressure and decomposition temperature of GaN was analyzed. Then one-dimensional heat flow model in GaN/sapphire structure irradiated by pulse laser was established and how the main process parameters affect the efficiency of laser lift-off was analyzed. By means of two-dimensional simulation based on COMSOL the temperature distribution in GaN was obtained.Then the experiments that GaN/sapphire was irradiated by KrF excimer laser under different conditions were carried out.The main works and results are as follows:(1) By analyzing the one-dimensional heat flow model and GaN thermal decomposition process in theory, the main parameters including ambient temperature, ambient pressure, laser energy density, pulse width and pulse frequency which have a significant impact on the temperature field was obtained.Then the threshold laser energy density was defined by the decomposition temperature of GaN.We achieve that both high temperature and low pressure help to reduce the threshold laser energy density.And the threshold laser energy densities under the normal temperature and atmospheric pressure, the high temperature and atmospheric pressure, the normal temperature and low pressure are340mJ/cm2,322mJ/cm2å'Œ237mJ/cm2, respectively.(2) The two-dimensional model based on COMSOL was established, then the temperature field in GaN was calculated and analyzed. We achieved that the temperature distribution was non-uniform both in the width direction and height direction. And the temperature decreased rapidly with a large gradient at the edge of the heat source. This non-uniform temperature distribution results in stress, bending, and even damage in the film.(3) The experiment showed the threshold energy density reduced by25mJ/cm2when the sample was heated to70℃.The results proved that under high ambient temperature laser lift-off with high efficiency can be realized.The experiment that how low pressure affected the threshold energy density with laser irradiation from the sapphire side had trouble because of the difficult escaping of N2. We proposed another experiment with laser irradiation from the GaN side.Then we achieved that the threshold energy density reduced by20mJ/cm2when the sample was under low pressure with single pulse laser irradiation.Then we investigated the decomposition depth of sample with multi-pulse laser irradiation. The results showed that the decomposition depth of GaN under low pressure increased by10.2%,19.0%and24.3%,while the number of laser pulses was10,20and30.So low pressure helped to lower the threshold energy density, it meant that we can magnify the facula to achieve fast processing.