Investigation Of Chemical Mechanical Polishing Slurry For Copper

With the rapidly development of very large scale integration circuits?VLSI?,copper has become the most important circuit interconnecting material because of high electro-migration resistance and low electrical resistivity.Hence,it is crucial to achieve the ultralow roughness of copper surfaces.Chemical mechanical polishing?CMP?is the most popular and effective process to achieve global planarization of surfaces,in which the polishing slurry plays an important role.In previously reported slurries,amount of additives were employed to ensure good surface quality,such like oxidizing agent,abrasive particles,complexing agent,corrosion inhibitor et al.Besides,most of the slurries are not friendly to operators and environment,containing hazardous chemicals such as strong acid,strong alkali and benzotriazole?BTA?.In this paper,a novel CMP slurry for copper is developed according to theory analysis and experimental study.The novel slurry consists of hydrogen peroxide?H₂O₂?,silica sol,chitosan oligosaccharide?COS?and deionized?DI?water.After the optimization of process parameters,such as speed,pressure and polishing pad,the novel slurry achieves ultralow surface roughness 0.444 and 5.468 nm for Ra and PV values respectively,over a measurement area of 70×50?m².This result is better than most of copper surfaces polished with previously reported slurries while the polishing time is only 7 minutes.Another strength of the novel slurry is environmental friendliness.In the novel slurry,H₂O₂ is a very clean oxider accompanied by the production of water and oxygen during the reaction.Silica sol containing silicon dioxide?SiO₂?and water has no pollution to environment or harm to operators.COS is also an innoxious additive used widely in the field of medicine and food.So the novel slurry is clean and eco-friendly.In the slurry,H₂O₂ is an oxidant and oxides the copper surface during the reaction.Silica sol plays a role of abrasive and removes the surplus material mechanically.It has hardly effect to chemical reaction.As for COS,it has the effect on complexing with Cu²⁺,reducing the slurry's surface tension,stabilize the slurry's pH value and so on.The novel slurry's mechanism was also investigated in the paper by X ray photoelectron spectroscopy?XPS?,electrochemical experiment and Fourier infrared spectrometry?FTIR?.There are four processes,oxidation,dissolution,complexation and removal,in one material removal cycle of CMP.Firstly,cupric/cuprous oxide is generated on the copper surface on the effect of H₂O₂.Then,the cupric/cuprous oxide is dissolved slowly by hydrogen ions,which are mainly from the weak ionization of COS.Next,the COS molecules form complex with Cu²⁺,which reduces the number of Cu²⁺and accelerates the dissolution of cupric/cuprous oxide.In addition,the complex could be adsorbed on the copper surface.For one thing,the adsorption layer separates the surface from chemical reaction and protects the concave surface from corrosion;for another,the adsorption layer is soft enough and could be remove lightly by the SiO₂ nanospheres.Finally,the nanospheres remove the mixed layer mechanically.