Effects Of Ultrasonic Vibration On Sapphire Chemical Mechanical Polishing Studied By Molecular Dynamics

Sapphire has good optical,thermal,dielectric and mechanical properties and is an ideal substrate material for gallium nitride,which is currently used in semiconductor devices,aerospace,military engineering and other applications.In all these applications,extremely high surface quality is required.However,sapphire has high brittleness,very high hardness and low fracture toughness.Its surface quality accuracy is difficult to control and processing efficiency is low.Therefore,it is not easy to obtain efficient and high quality processing by traditional chemical mechanical polishing(CMP)technology.Thus,it is necessary to develop a more efficient polishing process and reveal its polishing mechanism to achieve faster and higher quality sapphire surface flattening.Ultrasonic vibration-assisted chemical-mechanical polishing(UV-CMP)is a novel and efficient polishing technique,which has been proven to be very useful for processing hard and brittle materials.Compared with conventional CMP,UV-CMP can effectively improve the material removal rate and processing quality of sapphire polishing.However,its removal mechanism is still unclear and the internal damage of the workpiece is difficult to be observed.In order to reveal the material microscopic deformation and removal mechanism at the atomic scale during UVCMP of sapphire,this thesis investigated the ultrasonic vibration-assisted polishing process of sapphire with silica abrasives using molecular dynamics(MD)method.The effects of vibration frequency,ultrasonic amplitude,polishing speed,depth and abrasive size on the polishing results were explored in terms of polishing force,temperature,surface morphology,number of removed atoms,subsurface damage,stress distribution and average friction coefficient.This thesis firstly established a simulation model for sapphire UV-CMP using silica abrasives,then investigated the effects of different vibration frequencies and ultrasonic amplitudes on the sapphire UV-CMP process by MD simulation method.The results show that with increasing the vibration frequency,the tangential and normal forces gradually decrease,the workpiece subsurface temperature rises,and the number of removed atoms increases.The thickness of the subsurface damage layer and the area of the stress area both increase and then decrease,and the subsurface damage of the sapphire workpiece is minimum at 20 GHz vibration frequency.By increasing the ultrasonic amplitude,the tangential and normal forces reduce more rapidly,the average friction coefficient decreases gradually,the temperature increases significantly,and the number of removed atoms rises significantly.The subsurface damage layer thickness and the stress atomic area spread deeper into the subsurface.When the ultrasonic amplitude is 5 (?),the workpiece subsurface damage layer thickness is the smallest.In addition,in order to investigate the effects of various polishing speeds,polishing depths and abrasive radii on sapphire UV-CMP results,the simulation was carried out to demonstrate the impacts of the above polishing parameters on the sapphire material removal mechanism.The results show that as the polishing speed increases,the workpiece subsurface temperature rises,the number of removed atoms grows more slowly,and the sapphire subsurface defect layer thickness and stress region area first decrease and then increase.Larger polishing depths lead to higher polishing forces and subsurface temperatures,and poorer subsurface quality despite more removed atoms.At bigger abrasive sizes,the normal force becomes higher,the number of removed atoms is larger,the subsurface temperature is higher,but the average friction coefficient is smaller,the subsurface stress region area and the damage layer thickness first decrease and then become larger.The removal efficiency and polishing quality are both good when the polishing speed,polishing depth and abrasive radius are 150 m/s,15 (?) and 40 (?) respectively.