Investigation On Tribology Behavior Of IC-1000 Polyurethane Pad And Its Influencing Mechanism

As the only processing technic which can achieve the global flatness,chemical mechanical polishing（CMP）is often used as the step of substrate process.However,with the surface wear and matrix compression of pad in CMP process,the process performance of pad changes greatly,which can induce the within-wafer-nonuniformity and wafer-to wafer-nonuniformity and low qualification rate of semiconductor equipment ultimately.Therefore,exploring the friction and wear behavior of pad and its mechanism has a significant meaning in the stabilization of wafer processing quality and increase of semiconductor equipment production efficiency.Firstly,in order to improve the simulating reliability of the tribology behavior between wafer and pad,the new movement trajectories of worktable were proposed and optimized.MATABL was used to simulate the wafer-pad relative motion trajectory under different motion modes,and the superiority of the new worktable movement trajectories were verified.The results of finite element analysis indicated that the new worktable movement trajectory not only does the deformation degree and stress concentration degree of the pad sample effectively reduce,but also improves the distribution uniformity of friction heat on the wafer surface.The surface morphology of pad under different motion modes was observed.It was found that with the increase of relative trajectory density,the average surface height and porosity of pad decreased,the concentration degree of asperity height and the wear uniformity of pad surface increased.But when the period ratio of rectangular motion to rotary motion raised up to 8:1,the pore walls collapsing,which caused by excessive wear,induced the porosity increased accidently and the average height of pad surface decreased sharply.Besides,the friction coefficient（COF）between wafer and pad was affected by both material compliance and flatness of pad surface.In order to explore the optimal duration of single use of pad,the surface morphology evolution trend of pad during continuous CMP process was studied by polishing experiments,and the mechanism of its performance degradation was discussed.The white light interferometer and scanning electron microscope（SEM）were performed to observed surface micro-texture of pad samples.Through the images processing,the surface roughness,the asperity height normal distribution curves and the porosity of pad surface can be obtained.Next,the friction and wear tester was employed to performer the simulating of wafer-pad tribology behavior,and the optimized motion mode was adopted for the worktable.The experiment results showed that the use circle of pad can be roughly divided into four stages:early period-the first 10 minutes polishing process,the roughness,the surface average height and the porosity of pad surface declined rapidly,and the degree of height concentration dropped fist and then rose;middle period-10-20 minutes polishing process,the roughness kept relatively stable,and the decrease rate of porosity and surface average height slowed down;late period-20-30minutes polishing process,the pad surface worn quickly,surface roughness declined rapidly again,the pores were almost completely blocked and the flatness of pad surface rose;end of period:40-60 minutes polishing process,pad surface had undergone excessive wear,the pore walls gradually collapsed,pad surface roughness and porosity first rase then descended,the surface average height changed slightly,and the degree of height concentration drop sharply then increased.During the first 20 minutes,the COF between wafer and pad first decreased then increased under the combined effects of debris accumulation,flatness of pad surface and the increase of contact area between wafer and pad.During the 20-40 minutes,the COF was relatively stable.During the 40-60 minutes,COF fluctuated obviously due to the effect of pore wall collapsing,surface material cracking and rolling.Furthermore,in the condition of continuous polishing processing,the effect of friction and wear behavior of pad on the material removal rate（MRR）and average roughness（Ra）of wafer was investigated by polishing experiments.Besides,the material removal rate model was expanded by introducing the parameterα·charactering the effect of friction and wear behavior of pad on wafer processing efficiency.The results showed that during the first 10 minutes polishing process,MRR increased continuously,which was due to the enhanced mechanical action caused by the accumulation of"debris"on pad surface.The decreasing mechanical action and chemical action differences resulted the improvement of wafer surface quality.Among the10-20 minutes polishing process,as the flatness of pad increased,the contact area between wafer and pad kept rising.And the mechanical action and chemical action approach the equilibrium point gradually.The average MRR rose rapidly and Ra of wafer surface continued to decrease over time in this stage.During the 20-30 minutes,severe pore blocking lead to a decrease in chemical oxidation rate.So that a slight decrease happened to the MRR.Unbalanced mechanical action and chemical action degrade the surface quality of wafer.In 40 minutes,the collapse of pores wall,and the cracking and rolling of the pad surface materials improved the mechanical action and chemical action,and the MRR slight increase.However,the poor pad surface condition decreased the quality of wafer surface after processing.After 40 minutes,the glazing degree of the pad surface became worse,and the degradation of pad performance decreased the MRR and increased the Ra of wafer surface slowly.Combined MRR and Ra as evaluation indexes,the optimal use time of polishing pad was 20 minutes.Additionally,according to the data analysis results,it can be seen that before the MRR of wafer reached the highest point in the actual CMP processing,that is,before 20 minutes,the change trend of experimental MRR was opposite to that of theoretical MRR,and was the same as that ofθ=α·（1/β+1/γ-1）^-1.After the MRR reached the highest point,experimental MRR and theoretical MRR had the same trend of change,but the trend is opposite was opposite withθ.Combinedθand theoretical MRR,the changing trend of MRR in actual wafer processing can be better predicted.