dc.description.abstract | The aim of this work is using finite element analysis (FEM) to study the effects of thermal load and rotation speed on the structural integrity of a substrate holder module in an MOCVD reaction chamber. Several loading conditions are considered, including thermal load alone and thermal load plus rotation speeds of 10 rpm, 100 rpm, 500 rpm, 1000 rpm, and 1500 rpm. In addition, the wafer bow and residual stress of GaN growth on silicon or sapphire are systematically studied for various scenarios. The effects of size and material of wafer, thickness of film and substrate, buffer layer, and temperature gradient are characterized. Moreover, in order to validate the FEM model constructed in the current study, experimental results of a previous study are applied to assessing the credibility of the numerical methods by comparison of the simulation results with the experimental measurements of wafer bow. The variation trends of wafer bow and curvature radius in simulation agree well with those in experiment such that the constructed model is validated. Therefore, the constructed model is effective in assessing the effect of various parameters acting on a film-substrate system.
As the calculated critical stress is less than the strength of material, no structural failure is predicted for all the components in the given substrate holder module under all of the given loading conditions. The variation of critical stress with rotation speed in all of the components is small. Given a similar heat source in the MOCVD reaction chamber, temperature of the upper components such as susceptor, substrate holders, and wafers is higher in the case of sapphire wafer than that in the case of silicon wafer. The temperature gradient of upper components is greater for the silicon wafer case. A greater temperature gradient in the film-substrate system generates a greater wafer bow and residual stress. Therefore, the temperature uniformity is an important parameter for the epitaxial process. The sign of residual stress is different between a GaN film grown on a sapphire wafer and a silicon wafer (compressive for sapphire wafer and tensile for silicon wafer). For growing a GaN thin film, GaN thin film, sapphire wafer is better than silicon wafer in terms of lessening cracking in film.
No matter GaN is grown on sapphire wafer or silicon wafer, wafer bow increases and residual stress in the film decreases with an increase in thickness of film. Increasing the thickness of wafer can effectively reduce wafer bow, which is also a method commonly used in industry, but the residual stress in the film is increased. Given a wafer thickness, the size of bow is increased with wafer diameter, which is one of the major challenges in growth of a large-size epitaxial wafer. The magnitude of residual stress in a thin film can be reduced when a thick buffer layer is added between film and wafer. For a lower residual stress, the reliability of a thin film can be improved by the addition of buffer layer. | en_US |