Electrochemical-mechanical Polishing Law And Mechanism Of New Interconnect Metal Cobalt For Integrated Circuits In Electrochemical Mechanical Polishing

With the progress of science and technology,the characteristic size of integrated circuits has been reduced to 5 nm and below.Cu as an interconnect metal for integrated circuits faces problems,such as easy diffusion of Cu atoms,poor filling of gaps with barrier layers,increased interconnect resistance,and rising RC delay.Therefore,it is urgent to find a new interconnect metal to replace Cu.Due to the advantages of low resistivity,good thermal stability,great barrier gap filling,and the difficulty in diffusion of Co atoms,cobalt starts to replace Cu as a new generation of interconnection metal material.As the interconnection metal,cobalt still needs to be flattened during processing.However,Co has high hardness and it is difficult to be fabricated.With the reduction of feature size,the downward pressure applied in polishing process is becoming smaller and smaller.Traditional chemical mechanical polishing（CMP）is not easy to achieve high material removal rate（MRR）of cobalt.Electrochemical mechanical polishing（ECMP）is considered to be one of the most promising processing technologies that can achieve efficient machining of cobalt.However,there are few studies on cobalt electrochemical mechanical polishing,and the mechanism of polishing process is not clear.In this paper,a polishing slurry suitable for the electrochemical mechanical polishing of cobalt was developed.The process parameters during Co ECMP were optimized.Finally,through polishing experiments,electrochemical experiments,X-ray photoelectron spectroscopy（XPS）,energy dispersive spectroscopy（EDS）and atomic force microscopy（AFM）,the mechanism of various factors in Co ECMP was revealed.The research results of this paper are as follows:The appropriate concentration of each component in the polishing slurry is determined by static electrochemical measurement using corrosion current（Icorr）,corrosion voltage（Ecorr）,surface roughness and material removal rate（MRR）.Finally,the appropriate concentration of polishing slurry is 20wt%Si O₂+1.5wt%H₂O₂+7 m M BTA.When the slurry is used for electrochemical mechanical polishing,the material removal rate of cobalt can reach 105.66 nm/min.After polishing for 10 min and 120min,the surface roughness of cobalt is Ra 1.35 nm and 0.418 nm,respectively.The process parameters in the of cobalt electrochemical mechanical polishing are optimized.The results show that the optimal polishing conditions are:polishing voltage2V,polishing pressure 2psi,the speed of polishing head 110 rpm and the speed of polishing disc 105 rpm.Under these polishing conditions,the MRR of cobalt reaches150.07 nm/min,and the surface roughness after polishing is Ra 0.46 nm.In cobalt electrochemical mechanical polishing,with the increase of polishing voltage,the MRR of cobalt and the surface quality after polishing first increase and then decrease.With the rising pressure and polishing speed,the MRR of cobalt ECMP and the surface quality after polishing will also increase.The mechanism of mechanical,chemical,and electrical factors in the polishing process of cobalt ECMP is studied.The results indicate that mechanical action plays a major role in the polishing process,while chemical or electrical action alone has relatively small influences on the polishing of cobalt.However,chemical and electrical action assisted mechanical action can greatly improve the polishing efficiency of cobalt.XPS and EDS results imply that the oxides generated on the surface of cobalt during polishing are mainly Co O,Co（OH）₂,and Co₃O₄.These oxides will react with the complexing agent in the slurry to form Co（II）-BTA complex.Co（II）-BTA is continuously generated and removed,which is the main material removal method of cobalt ECMP.