Design,Preparation And Experimental Study Of Multi Gradient Polishing Pad For Sapphire

Sapphire has stable chemical properties,high hardness and brittleness.It is often used in various fields as substrate material and window material.In the process of chemical mechanical polishing of sapphire,the polishing pad has a great influence on its polishing effect.At present,researchers mostly focus on the material removal rate and surface quality,but there are few studies on the material removal uniformity of sapphire wafer.Therefore,in this paper,a multi gradient polishing pad was prepared to improve material removal uniformity of wafer in chemical mechanical polishing,and the relevant theoretical and experimental research were carried out.The main work and research results of this paper were as follows:(1)The theoretical model and simulation model of the elastic contact between polishing pad and wafer were established,and the theoretical model of the elastic contact was solved by semi-inverse method.The effects of the number of the gradient rings,the thickness of the elastic layer,the number of polishing pad layers and the downward displacement of wafer on the contact stress were explored by ANSYS simulation.It was found that when the number of the gradient rings was greater than or equal to 3,the contact stress distribution on the wafer surface began to approach the ideal state gradually.Increasing the thickness of the elastic layer and reducing the wafer downward displacement would reduce the contact stress.The appearance of the upper layer in the double-layer polishing pad would have a negative impact on the variation trend of the contact stress on the wafer surface.(2)In order to verify the uniform removal effect of multi gradient polishing pad on sapphire wafer,firstly,based on the simulation analysis and elastic modulus test results,the number of gradient rings and the materials of each gradient ring were determined,and the multi gradient polishing pad was prepared.Secondly,on the nanopoli-100 intelligent ultraprecision plane polishing machine,the sapphire was processed with polyurethane polishing pad and multi gradient polishing pad respectively.The removal effects of the two polishing pads under three process parameters were explored,and the effects of each process parameter on the processing effects of the two polishing pads were compared.The results showed that the uniformity of the wafer material removal on the multi gradient polishing pad was better than that on the polyurethane polishing pad.When the wafer was processed with the multi gradient polishing pad,the larger polishing disc speed would reduce the chemical corrosion rate in the process of chemical mechanical polishing,and then affected the material removal rate.(3)In order to further explore the influence of process parameters on the polishing effect of multi gradient polishing pad,a three factor four level orthogonal test was designed,taking polishing pressure,polishing disc speed and polishing time as variables.The range analysis method was used to explore the influence of the three variables on the material removal rate.The results showed that the influence of polishing pressure on the material removal rate was greater than that of polishing disc speed and polishing time.With the increase of the rotating speed of the polishing disc,the material removal increased first and then decreased.(4)By analyzing the relationship between the surface flatness of sapphire wafer and the total amount of material removal,it was found that the surface flatness was positively correlated with the total amount of material removal.Therefore,taking the material removal rate as the goal,the BP neural network model was established,and the adaptive differential evolution algorithm was used to optimize the process parameters.