Study On Component Optimization Of Single Crystal SiC Chemical Mechanical Polishing Slurries Based On Friction And Wear Mechanism

As a high-efficiency and high-precision polishing technology,Chemical mechanical polishing（CMP）is widely used in the polishing of glass,ceramics,metals and semiconductor materials.During CMP processing,surface roughness,surface morphology,and material removal rate（MRR）are usually used to evaluate the processing effect.These evaluation methods are affected by the accuracy of the instrument and the inspector,and cannot directly and accurately reflect the change of the wafer surface during the CMP.Friction usually occurs in the contact area between the polishing disc,the abrasive and the wafer surface,which depends on the roughness,structure,mechanical properties and chemical properties of the contact surface.Some researchers measure the coefficient of friction（COF）during the CMP process to reflect the relationship between MRR and COF,which may be a feasible method for verifying the mechanical and chemical effects of the CMP process.Therefore,this dissertation topic tried to use the friction and wear method to evaluate the effect of the polishing slurries,and the contact state between the wafer and the slurry was reflected through the change of the COF curve,and the material removal effect was reflected by the difference in the wear of SiC,which laid a foundation for the optimization of the CMP slurry process.Firstly,the mechanism of the main components of the polishing slurries on SiC was studied.The friction experiments showed that the SiC surface had obvious scratches and larger COF value only the silica sol existed.The SiC surface formed a loose and porous chemical reaction layer only the oxidant presented,which made the SiC surface rough and the COF curve fluctuate.When the chemical reaction worked in combination with the abrasive mechanical removal,the COF curve tended to stabilize but wear rate increased.The CMP experiment results were consistent with the friction test results.Secondly,the polishing slurries based on Fenton reaction were optimized.In the friction test of Fenton reagent with several metal ions as catalysts in different acidic environments,on the other hand,metal ions(Fe²⁺,Fe³⁺,Al³⁺and Cr³⁺)can catalyze the decomposition of H₂O₂to produce·OH,such as Fe²⁺;on the other hand,metal ions will capture·OH,which will affect the efficiency of the Fenton reaction.The final result was:Fe²⁺>Fe³⁺>Al³⁺>Cr³⁺.However,the Cl^-,SO₄^2-,NO₃^-and H₂PO₄^-can capture·OH in the Fenton system,and then inhibiting the oxidation efficiency of the Fenton reaction.In addition,Cl^-,SO₄^2-and H₂PO₄^-would also complex with Fe²⁺,Fe³⁺,etc.,which make the oxidation efficiency worse.Therefore,the best acidic environment was HNO₃.Thirdly,based on the above friction test results,the acidic environment of the Fenton polishing slurry was optimized.The results showed that the p H adjustment of different acids had little effect on the surface roughness of SiC,but had a greater effect on MRR,which was represented by HNO₃>H₂SO₄>HCl>H₃PO₄.Under the condition of HNO₃,the MRR was the highest at 138.76 nm/h,and the R_a was the lowest at 1.136 nm.The polishing effect under acidic conditions was better than that of weakly acidic or neutral environments.Adjusting the p H to 3 by HNO₃ can obtain the highest MRR（141.85 nm/h）and the lowest R_a（1.295 nm）.Then,orthogonal tests were carried out and the results showed that the best polishing solution was 15 wt%H₂O₂ concentration,15 wt%silica sol concentration,40 r/min polishing disc speed,80 r/min workpiece speed and 0.05 Mpa polishing pressure to achieve good material removal rate and surface roughness.The BP neural network model established based on the results of orthogonal experiments showed that the fitting errors of the model for R_a and MRR are both within 1%.For the prediction results,the maximum error of R_awas 18.56%,the minimum was 0.01%,and the maximum error of MRR was 8.09%,the minimum was 0.01%.Then,in order to simulate the CMP test as much as possible,a pin-disc friction tests were used to explore the influence of different abrasives on the surface morphology of SiC and to investigate its surface creation mechanism in conjunction with power spectral density（PSD）analysis.The COF obtained with diamond abrasives was the largest and the surface profile was wavy;the surface profile obtained with SiC abrasives was uneven and the COF was larger;a more uniform surface profile could be obtained with Ce O₂ abrasives;and the COF with silica sol was significantly lower than the other three groups of tests.The PSD analysis of the surface wear profile of SiC showed that the frequency band of 0.63 mm^-1～1000 mm^-1 had the greatest influence on the surface roughness R_a,while the influence of the frequency band after 1000mm^-1 was smaller.For diamond,SiC and Ce O₂ abrasives,the influence of the frequency band after 1000 mm^-1 on the R_a value can be ignored,while for silica sols should be considered.Finally,combining all the above tests,the surface creation mechanism of single crystal SiC by different abrasives were analyzed.It is concluded that diamond abrasives exhibit a strong mechanical scratching effect and therefore have the largest material removal and surface roughness peaks.SiC abrasives can also produce significant material removal from single-crystal SiC,while SiC abrasive edges were passivated by·OH oxidation and therefore the surface roughness peaks were reduced.Ce O₂ can promote the Fenton reaction,but the effect was weak,while Ce O₂ abrasive particles can be adsorbed on the surface of SiC wafer by chemical tooth action,but this adsorption was beneficial to the material removal on the wafer surface.Silica sol can facilitate the removal of SiC material as a solid phase reaction occurs during the friction process,while in an acidic environment,the silica sol particles were adsorbed on the surface of SiC wafers under the action of electrostatic gravity to form a barrier layer,resulting in minimal surface roughness.