Correlating Coefficient of Friction and Shear Force to Platen Motor Current in Tungsten and Interlayer Dieletric Chemcial Mechanical Planarization

Abstract

This thesis presents a series of studies related to a new way of analyzing data from previous polishing tests pertaining to tribological, thermal, kinetic and slurry utilization aspects of chemical mechanical planarization (CMP) processes. The purpose of our current studies is to expand upon the correlation of real-time shear force (SF) and coefficient of friction (COF) data with that of real-time platen motor current (PMC) in order to determine if PMC is a viable alternative to SF and COF when studying polishing mechanisms. The first contribution to this thesis is the investigation of the correlations between shear force and platen motor current, as well as those between COF and PMC for various tungsten and interlayer dielectric (ILD) chemical mechanical planarization cases where the processes were intentionally made to be highly non-steady-state. The study initially focuses on non-steady-state conditions because we believe the relationships among shear force, COF, and platen motor current to be clearer as opposed to steady-state conditions. Shear force, normal force and PMC data were collected from twelve different previously obtained Stribeck+ curves at an acquisition frequency of 1,000 Hz and analyzed in order to determine if there were any emerging trends. For the 12 cases, involving 8 pre-polished blanket CVD tungsten and 4 silicon dioxide blanket wafers, it was discovered that PMC closely mirrored shear force as evidenced by a high average correlation coefficient (0.955) and coefficient of determination (0.916) obtained from all runs. For COF vs. PMC, the average correlation coefficient and coefficient of determination for all cases were 0.758 and 0.608, respectively. These average values were dragged down by 5 cases in which the dominant tribological mechanism was found to be “boundary lubrication” where COF changed minimally with the pseudo-Sommerfeld number.

The second contribution to this thesis has been the investigation of the relationship between SF and PMC, in parallel with the relationship between coefficient of friction and PMC for various tungsten and interlayer dielectric (ILD) chemical mechanical planarization cases at non-steady-state conditions. This study is intended to determine whether or not PMC can be used as a reliable indicator instead of SF and COF at steady-state conditions. For the 12 cases studied, 72 distinct steps are analyzed. It was determined that PMC somewhat mirrored SF and PMC for long time (i.e. 10 secs or longer) intervals after data averaging and applying a trend matching algorithm. SF and PMC trends matched only about 64% of the time (ranges were between 45 to 85%) for all 72 steps, while PMC and COF trends matched 62% of the time ranged between 42 and 85%. PMC-SF and PMC-COF correlations were fairly poor at short time (i.e. 1 sec) intervals as evidenced by much lower percent match values. Such poor correlations proved that at short-time intervals, PMC was not sensitive enough to capture important information regarding myriad fluid dynamics and tribological phenomena and the instantaneous stick-slip occurrences encountered in CMP.