| As the representative of third generation semiconductors,gallium nitride(Ga N)has excellent properties such as wide band gap,high electron drift velocity,low dielectric constant and good thermal conductivity,thus has broad applications in the fields of power electronics,microwave communication,photovoltaic inverter and lighting.Single crystal Ga N is a typical hard-brittle and difficult-to-machine material.Current fabrication of single crystal Ga N substrates uses similar processes to those for silicon wafers,including wire sawing,grinding/lapping and chemical mechanical polishing(CMP).Due to the limitations of lapping process using free abrasives,it is difficult to effectively improve the material removal rate.It is thus not suitable to be used to get the substrate ready for the final CMP process.Therefore,a machining process technology is urgently needed to realize the economical,high efficiency and ultraprecision process of single crystal Ga N substrate.Based on the fact that Ga N is prone to be removed follow the layer-by-layer manner,nano-grinding is proposed to replace the lapping process in this thesis.It is thus critical important to understand the deformation induced by the fixed abrasive and removal mechanism of Ga N for suppressing cracks and achieving low damage removal in grinding.Therefore,nanoscratch and nano-grinding experiments are carried out to explore the deformation and material removal mechanism of single-crystal Ga N.It would provide a theoretical foundation for the subsequent optimization of the nano-grinding process.The main research work of this thesis is summarized as follows:Nano-scratch experiment using variable loads was carried out on single-crystal Ga N,and it was found that the elastic-plastic transformation load threshold of single-crystal Ga N was relatively high,and the subsequent finite element simulation results of indentation and scratches further confirmed this finding.Meanwhile,deformation in ductile regime is realized when the normal loading is small,and it is dominated by lateral and cross slips,dislocation,lattice distortion and grain rotation.The thickness of subsurface damage layer is related to the ratio of the tangential force to the normal force during scratching.Variable rate scratching were conducted and the results show that the increase of scratching rate does not substantially change the plastic deformation mechanism of the scratched surface and subsurface of single crystal Ga N.Meanwhile,scratching rate has insignificant effect on the elastic-plastic transformation.However,high-speed scratching makes the scratched surface and subsurface exhibit more plastic deformation characteristics,meanwhile decrease the depth of plastic deformation layer.Therefore,it seems that high scratching rate is beneficial for the plastic removal of single crystal Ga N.In order to choose the suitable grinding wheel,and to explore the influence of abrasive particle size and grinding parameters(grinding wheel speed and feeding rate)on the grinding removal mechanism of single crystal Ga N,nano-grinding experiments using grinding wheels with different hardness were carried out.The results show that:(1)The surface quality of ground Ga N using the medium hardness grinding wheel is slightly better than using the soft grinding wheel,but much better than using the high hardness grinding wheel;(2)The surface and subsurface of single crystal Ga N after grinding by diamond grinding wheel with different abrasive grain sizes show significantly different material removal characteristics.The surface after grinding with 2000# grinding wheel is a typical brittle-plastic coexistence surface,with a layer-by-layer removal manner;After grinding with 6000# grinding wheel,the surface shows obvious form of plastic removal;(3)The phenomenon of layer-by-layer removal is enhanced by increasing the rotational speed of the grinding wheel;while the feeding rate varying from 0.1μm/s to 0.5 μm/s has insignificant effect... |
PDF Full Text Request