Abstract
GaN light emitting diodes (LED) are often grown on sapphire substrates due to the restriction of epitaxy condition. The low thermal conductivity of sapphire causes the efficiency of LED to seriously degrade. In order to solve this problem, sapphire substrates must be removed and other substrate materials with better thermal conductivity must be utilized. Among the sapphire lift-off methods, the laser lift-off is the most convenient and efficient method. The high temperature caused by the laser decomposes GaN at the GaN/sapphire interface, thus allowing the sapphire substrate to be removed. The LED wafers are comprised of different materials created by decomposition and epitaxy. If the differences of growth temperature between materials are large, then a coefficient of thermal expansion (CTE) mismatch between materials will lead wafer warpage after it returns to room temperature. Otherwise, the laser tacjectory may influence stress distribution between the free and still attached parts of the wafer and therefore has an effect on mechanics behavior and wafer warpage. In this thesis, the finite element method (FEM) software, ANSYS®, is employeed to analyze the laser thermal conductivity and wafer warpage behavior. First, for the thermal analysis, a 2” wafer model is established to simulate the temrperature field after the laser lift-off process. The laser is applied through an equivalent heat flux, and the results are then compared with experiment data to prove the feasibility of equivalent heat flux. To understand the mechanics of wafer warpage, research is performed on the effect of the lift-off procedure and material thickness on wafer warpage. The initial stress of wafer from the manufacturing process is calculated using process modeling. The effect of the lift-off procedure on warpage is simulated using 2-D and 3-D models, respectively. Using the structure-thermal couple system of ANSYS®, the former results of the thermal analysis are substituted into the structure analysis and the rising temperature by the laser, which showed less effect on wafer warpage. The wafer warpage curve is 2nd order polynomial, therefore the warpage results of a 2” wafer can be calculated from smaller models to reduce calculation time. Different warpages from various lift-off procedures can be observed using the 3-D structure model. Moreover, a lift-off with a spiral trajectory can reduce the warpage by 7.7%. Finally, from this research, the best lift-off procedure can be indentified, and this study is expected to become a reference for future research.