Research On The Influence Factors And Mechanism Of Copper Electropolishing In Phosphoric Acid

Copper with the low resistivity and high resistance to electric mobility hasbeen widely used as IC interconnecting material. Recently, theelectrochemical-mechanical polishing technology has been developed because thetraditional chemical-mechanical polishing process can not meet the strictrequirements on polishing quality. Although the solution based on phosphoric acidis generally used in the electropolishing of copper, however the mechanism ofpolishing using such solution is still unclear. Besides, the additives for adjusting theviscosity and other parameters need to be investigated further.The intermediate products from the copper/phosphoric acid solution in theanodic and the electrochemical behavior of copper are studied by rotating discelectrodes （RRDE）. Two current peaks on the ring electrode at0.26V and0.04Vare detected, which are corresponding to the reaction of Cu/Cu⁺in active dissolutionzone and Cu/Cu₂O in passivation, in the anodic polarization process of copper onthe disc electrode, and both of reactions are controlled by the diffusion process.The electrochemical characteristics of copper in different anode polarizationpotential zones are analysized by electrochemical impedance spectrum （EIS）. thatthe results illustrate that Cu⁺have not only caused the inductance of arc near0.275V, at the same time also caused a capacitive reactance arc consisting a negativeresistance near0V. The results of the ohm impedance-time （R_s-t） and ohmimpedance-potential （R_s-E） curves show that the two curves can characterize theformation of the viscous layer and oxide layer on the interface between copper andphosphate acid. The R_s-E curve can be divided into six regions. In activedissolution region, the changes of R_s is caused by generation of viscous phosphatefilm on the surface of copper not by generation of copper oxide. In transition regionbetween active dissolution and passivation, the change of R_sis caused by the changeof viscous phosphate film and the generation of copper oxide as a n typesemiconductor. In passivation region, the reaction is controlled by the diffusion ofthe ligand, and the current density remains unchanged. Therefore, R_sis mainlycaused by the thickening of copper oxide. When the copper oxide starts turing into ptype semiconductor at0.6V, R_sincreases faster. The larger anodic polarization potential and the higher concentration ofphosphoric acid would be advantageous to the leveling of microscopic andmacroscopic structures. The surface roughness of copper after electropolishing hasthe lowest value at0.2V and55wt%H3PO4. The passivation state will be enhanced,the passivation current will decline and the ohmic resistance will increase., Thecarrier density and flat potential of the oxide film of copper declined with theincrease in the concentration of phosphoric acid which indicates that the degree ofexcess of metal in copper oxide film decreases.In Tafel curve test, the different pressures of50g/cm²and200g/cm²areapplied on the copper electrode. It is found that the rotation of electrode greatlyinfluences the cathode process, and the corrosion rate of copper and the speed ofrotation are in line with Levich relations. These results illustrate that the corrosionin phosphoric acid solution is controlled by the diffusion of cathode process underthe stable potential and the applied force would be benefit to accelerating theprocess of cathode depolarization （the diffusion of oxygen in the solution） and thecorrosion.In this paper, the effects of surfactants including SDS, PEG and PVP on copperelectropolishing are investigated. The results show that the anodic dissolution of Cuin phosphoric acid solution at different applied potentials is inhibited when SDS isadded. In both active and passive potential regions, the inhibiting effect due to theadsorption of SDS on Cu is oberved. At high anodic potentials, O2is generated fromCu surface, and tiny O2bubbles aggregate on the whole electrode surface. Thecompact O2layers act as the physical barrier between Cu and electrolyte solutionpreventing Cu from further dissolution, which enhance the inhibiting efficiency.Therefore, SDS can be used as an additive in phosphoric acid electrolyte to achieveexcellent planarization efficiency for Cu electrochemical mechanical planarization.PEG and PVP also show a good inhibiting efficiency on copper in the zone ofoxygen evolution.The effects of inhibitors of BTA and TTA on copper electropolishing arestudied too. It is found that the corrosion inhibiting efficiency of BTA is80%nearopen potential and drop with the increase of potentials in the passivation region.BTA shows the better effect on copper electropolishing than that of TTA.