Methods for Efficient Slurry Utilization and Tribological Stability Analysis in Chemical Mechanical Planarization
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PublisherThe University of Arizona.
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EmbargoRelease after 25-May-2018
AbstractThis thesis presents a series of studies pertaining to tribological, thermal, kinetic and slurry utilization aspects of chemical mechanical planaraization processes. The purpose of this work is to both develop a better method of characterizing the tribological mechanisms during polishing, as well as propose methods by which slurry utilization efficiency can be increased in order to minimize environmental hazards and operational costs associated with polishing without compromising the desired polish outcomes. The first study was conducted using a modified version of the generic Stribeck curve using real-time shear and down force data collection at 1,000 Hz. This investigation served to provide a better understanding of the tribological and thermal mechanisms associated with polishing copper and tungsten blanket wafers on an industrially relevant soft pad. A multitude of gradual yet continuous changes in sliding velocity and polishing pressure were applied during polishing. Results indicated that polishing on the soft pad produced stable coefficient of friction (COF) values entirely within the "boundary lubrication" regime, while copper polishing on a hard pads produced a tremendous spread of data and resulted in both “boundary lubrication” and "mixed lubrication" regimes. In addition, the average pad surface temperature showed a linear relationship with the product of the COF, sliding velocity, and downward pressure for all copper and tungsten polishes on both soft and hard pads. Another study in this thesis investigated slurry availability and the extent of slurry mixing for three different slurry injection schemes. An ultraviolet enhanced fluorescence technique was employed to qualitatively measure slurry film thicknesses atop the pad surface during polishing. This study investigated standard pad-center point slurry dispensing and a slurry injection system (SIS) that covered only the outer half of the wafer track. Results indicated that the radial position of slurry injection and fluid interactions with the SIS greatly influenced slurry mixing and availability atop the pad. Silicon dioxide removal rates were also found to increase as slurry availability increased. Using a combination of standard pad-center slurry dispensing and a half-wafer track SIS resulted in similar silicon dioxide removal rates as standard pad-center slurry dispensing but at a 40% lower slurry flowrate. The final study in this thesis investigated the effects of ultrapure (UPW) water dilution of a ceria-based slurry on silicon dioxide removal rates. Results showed that pre-mixing the slurry and UPW increased the removal rate with dilution up to a slurry to UPW ratio of 1:7.5 due to the increasing presence of Ce3+ via the reduction of Ce4+ by UPW. Further dilution yielded a plateau in the removal rate trend as additional UPW reduced the coefficient of friction (COF) and the temperature during polishing, causing the benefits of increased ceria-silica binding to be offset by mechanical limitations. Mixing the slurry directly at point-of-use at the dispense nozzle resulted in a removal rate trend that was highly similar to pre-mixing, however, removal rates were higher at every dilution ratio. A novel slurry injection system (SIS) was employed at various rotation angles as measured from the leading edge. The SIS angles produced different retaining ring bow wave thicknesses, which led to varying extents of dilution and, by extension, removal rates. The SIS at -8° produced the highest removal rates of all angles. A third dilution ratio test was performed using point-of-use mixing through the SIS at the optimum angle of -8°, which resulted in a similar removal rate trend as pre-mixing and pad-center dispense point-of use mixing, but with dramatically higher removal rates at each dilution ratio. The ability to attain higher removal rates could potentially allow integrated circuit (IC) manufacturers to either reduce polishing times or reduce slurry consumption, subsequently reducing slurry waste and creating a more environmentally benign semiconductor manufacturing process.
Degree ProgramGraduate College