Effect Of The Micro-scale Contact Status On The Material Removal Process During The Chemical Mechanical Polishing(CMP) And Cross-scale Modeling Of The CMP Process

Chemical mechanical polishing(CMP)technology has been proved to be effective in achieving atomic-level ultra-smooth surfaces,as well as stringent local and global planarization under the synergistic effect of chemical and mechanical actions.It has been extensively used in manufacturing fields,such as semiconductor manufacturing,optical device processing,and other fields.Since chemical and mechanical synergistic effect during the CMP process is complex,and the chemical and mechanical actions are themselves affected by many factors,quantitatively simulating and accurately predicting of the material removal process during CMP is thus difficult.Up to now,fundamental understandings of the material removal mechanisms during CMP have been far lagged behind the development needs.CMP technology is more of a semi-empirical technique,and more based on large-scale experiments to optimize experimental parameters.To meet the challenges of emerging materials as well as the diversified requirements in the future,a quantitative prediction method for the CMP material removal process that considers chemical-mechanical synergistic effect is under urgent demand.The material removal process during CMP is generally governed by cross-scale interactions among nano/micro/macro system elements,where the nano-scale abrasive particles cause atomic-level tribochemical wear via contact of micro-scale pad asperities,and further generating ultra-smooth surface via macro-scale motion.In view of the cross-scale feature of the CMP system and the quantitative prediction requirements of the material removal process,this thesis proposed a novel modeling strategy.Here a single-pad-asperity was taken as the basic unit,and the macro-scale material removal process was regarded as a superposition of stochastic actions of masses of pad asperities.Through experimental observations and theoretical analysis of the material removal process,an accurate semi-empirical model of material removal rate(MRR)in micro-scale was established.Based on this model and the theory of probability,a cross-scale material removal model was further established.The researches in this thesis not only contribute to the study of chemical mechanical synergistic mechanisms and realize quantitative prediction of MRR,but also provide theoretical guidance for stability analysis of the CMP process.The major research works and conclusions of this thesis are as follows:(1)Researches on the mechanical properties and the micro-contact characteristics of the polishing pads were carried out.Through the compression mechanics test,the nonlinear compression behavior of the polishing pad was explored.An asperity-substrate layer series model was also established to analysis the effect of polishing pad microstructures on the compression behavior.Elastic modulus and hardness of pad asperities were measured using a micro-scale indentor.Finally,a polishing pad micro contact status measurement system was developted.Based on optical principle and digital image processing techniques,the system was able to collect the real contact images at pad/workpiece and extract characteristic parameters for the study of the micro-scale material removal mechanism.(2)A novel modeling and experimental method was proposed and carried out for studying micro-scale chemical mechanical synergistic mechanisms,where a polymer tip was used to mimic a single-pad-asperity.Through the Hertz contact theory and the crosss-scale modeling,a single-pad-asperity material removal model was established.Then,a series of polishing tests under polymer tip contact was conducted on monocrystalline silicon,BK7 glass,and fused silica glass surface,respectively.Through theoretical analysis and experimental validation,chemical mechanical synergistic mechanisms in the micro-scale was revealed,and the applicability and accuracy of the modeling and experimental method were also verified.These researches establish foundation for the following research in the cross-scale modeling of the CMP process,where the single-pad-asperity model is taken as the basic unit.(3)Researches on the material removal process and a MRR prediction method under a known contact status were carried out.Based on the real contact area images,a multi-pad-asperity material removal model was established.Then,a series of reciprocating polishing tests were carried out combined with the in-situ contact area measurement method.Through the analysis of the experimental results and the model prediction results,effect of the micro contact status of“concentrated”distribution and“dispersed”distribution on the micro-scale chemical mechanical synergistic effect was revealed.The accuracy of the multi-pad-asperity material removal model was also validated in terms of MRR and cross-sectional profiles.On the other hand,a real-time coefficient of friction(COF)analysis method was integrated with the reciprocating polishing tests to investigate the micro-scale frictional behaviors.The results showed that the micro-scale stick-slip phenomenon dominated the material removal process at the scratch speed of 1 mm/s to 20 mm/s.To explain the micro-scale stick-slip phenomenon and its effect on the atomic-level material removal process,a Stribeck friction model was adopted and disscussed.(4)Based on classical tribology and probability theory,a cross-scale model incopertaing nano/micro/macro interactions was established.By defining effective contact spots n_e,chemical reaction capacity parameterβ,and mechanical removal capacity parameterγ,the model was able to decouple the chemical mechanical synergistic effect during the CMP process.Experimental observations and theoretical analysis results indicated that the model proposed in this thesis was effective in explaining the difference in material removal behavior caused by different micro contact statuses and material physi-chemical properties in the perspective of microscale chemical-mechanical synergy.Finally,a novel CMP process stability analysis method based on the micro contact status of the polishing pad was proposed,which provided theoretical basis and technical support for the stability control of the CMP process in the IC manufacturing industry.