Model Friction Studies of Chemical Mechanical Planarization Using a Pin-on-Disk Tribometer

Abstract

A pin-on-disk attachment for a commercial rheometer was used as a model system for chemical–mechanical planarization. The system enables real-time measurements of friction and the pin height changes as a function of velocity and load. We found the pin-on-disk tribology geometry was able to achieve a range of down pressures relevant to CMP processes (24.5 kPa at Fn = 1.5 N). It was possible to measure changes in the pin height, or gap, on the order of 1 µm during the test, which was useful to determine the amount of wear on the polishing pad and whether the system moves into hydrodynamic lubrication. The pad conditioning, or break-in process with water using a stainless steel disk with embedded diamond teeth, was conducted right before the tribology test, as in CMP processes, to give a consistent texture and remove surface impurities. Control studies of pure water at various pH’s showed the friction coefficient data and gap changes between the pad and the silicon wafer vary with the pH and ionic strength of the solution. CMP polishing was also studied with commercial slurries, pads and silicon wafers, showing that COF and pad wear can be measured reliably in the tribometer and compares well to measurements made on a 200 mm CMP commercial polisher for both water and the slurry. Overall, the pin-on-disk attachment was found to provide good insights of the tribology of a full CMP system.