Fundamentals of CMP polishing of metallic and hard materials

Chemical mechanical polishing has distinguished itself for the past 2 decades in the planarization of metallic materials and dielectrics in semiconductor manufacturing. Recently this technique has become increasingly popular in polishing materials such a silicon carbide, sapphire, diamond, etc., which are mechanically hard and chemically inert. So the mechanisms for achieving a high degree of finish for both these materials systems are quite different. The fundamentals of CMP in metallic and hard materials system to understand the mechanisms and the ability to improve the polishing quality will be discussed. Further, the talk will focus on the tribochemical /electrochemical phenomena in metallic systems and chemical reactions in hard materials systems. The CMP of metallic systems and hard materials needs different scientific methods to achieve planarization according to the requirements. The methods for metal polishing will employ chemistry which will etch the metal, but only under applied pressure and friction conditions. Also in some methods, the metals can form a hard surface, such as forming oxides, which are removed with the use of abrasive particles. In the case of hard materials, CMP is achieved by reaction chemistry at the surface of the material to form soft layer. The soft layer is then peeled off using abrasives that are harder than the soft layer, but softer than the material that under goes polishing. Hence the slurry system and polishing process for the metallic systems and hard materials systems are different. Metallic systems needs ultra-gentle process to avoid defectivity and the removal rates are determined using surface passivation agents. The requirement in polishing metallic systems also demands stringent control over removal rates and defectivity because of their application in nano-structured devices. In the case of hard materials, removal rates are controlled by the rate of soft layer formation on the substrates with the chemistry used. The - oft layer formation with the conventional chemistry has limitation due to inertness of hard materials and results in lower removal rates. In recent years there has been significant development in polishing of hard materials with slurry and process engineering. This talk will focus on the distinct differences polishing of metallic and hard materials systems and the progress made in the last 10 years on the CMP of both the materials systems.