M.S. Thesis Presentation by Rob Hight
Wednesday, April 10, 2002

(Dr. Steven Danyluk, advisor)

"Interfacial Fluid Pressure and Pad Viscoelasticity during Chemical Mechanical Polishing "


Chemical Mechanical Polishing (CMP) is an indispensable process used for integrated circuit (IC) fabrication. With constantly decreasing feature size on IC dies, controlled material removal and planar wafer surfaces are increasingly important during device fabrication; CMP is utilized to achieve both of these ends. CMP involves sliding a silicon wafer across a soft pad that is flooded with slurry. Therefore, a combination of chemical and mechanical mechanisms contributes to material removal. This thesis addresses the mechanical contributions to the process. Specifically, the fluid pressure at the interface of CMP will be related to the viscoelastic nature of the polishing pads.

Prior research has shown that sub-ambient fluid pressures exist at the pad-to-wafer interface. The results of this work suggest that the pad asperities are viscoelastically compressed during the initial minutes of pad/wafer contact. For a given downforce and velocity, the magnitude of the interfacial fluid pressure increases monotonically until the pad asperities are fully compressed and a steady state pressure is reached. The effect of disrupting steady state conditions by altering velocity was studied, and compression tests confirmed the viscoelastic nature of the polymeric polishing pad. Experimental results were related to an existing physical model that explains the effects of viscoelastic creep on interfacial fluid pressure.

Coupled thermal-electric finite element models have been developed to understand the thermal contours developed across these springs, and to determine the maximum temperature reached.  In addition, the role of scale effects on the thermal conductivity of the spring material is studied and incorporated in the model.  The electrical contact resistance between the micro-spring and the bonding pad, and its effect on the maximum temperature is outlined.  The results are compared with those obtained from experimental data.

Contact models and submodels have been developed to simulate the establishment of contact, sliding and indentation resulting in plastic deformation of the pad.  Comparisons of the resulting scrub mark are made with experimental measurements using Focused Ion Beam images to determine the role of friction in the contact mechanics.  The stresses at the spring tip and base are also determined.  Finally, mechanical fatigue experiments are also conducted to determine the fatigue life of the springs.