CMP Slurry Filtration
Updated: Oct 20, 2020
Chemical mechanical polishing/planarization (CMP) is typically used to manufacture wafers for the semiconductor industry. It is a process that removes materials by a combination of chemical and mechanical (or abrasive) actions to achieve highly smooth and planar material surfaces.
As seen in the diagram below, dispenser delivers slurry to the wafer surface for polishing. To achieve an extremely smooth & normal wafer surface for photolithography, the slurry should contain abrasive components in a controlled particle size range.
However, the slurry may contain large particles (> 1μ) as a result of shipping or handling issues, drying, and interactions within the slurry distribution systems. These large particles can increase the level of scratches on the semiconductor wafer surface, which will only be found after the CMP process has been completed, deeming them defective. Slurry filtration can decrease the level of defects caused by large slurry particles, and increase yields.
There are currently two preferred methods of slurry filtration:
1) Loop or recirculation filtration
Remove large particles from the slurry as it is recirculated through a distribution loop which delivers the slurry to the tools.
For these applications, 0.5 to 5 micron filters are typically used and flow rates of 1 to 3 GPM are recommended.
2) Point-of-use (POU) filtration.
Captures large particles at or near the point at which the slurry is dispensed onto the polishing pad.
Flow rates for POU applications should not exceed .5 to 2.5 GPM.
Both methods of filtration have trade offs with respect to the volume of slurry that can be filtered before filter failure and the size of the particles that can be filtered. POU filtration is preferred because of the ability to use submicron filters at low flow rates, which tends to decrease the occurrence of premature filter plugging and allows for the potential filtration of < 1.0μ particles. It has been shown to be beneficial in reducing wafer defects and increasing yields in CMP processes.
Traditionally, non-pleated filters have been used in both the POU and loop filter positions. These cylindrical depth filters can exhibit limitations in efficiency, dirt holding capacity (service life), flow rate, and pressure drop. Conversely, single layer pleated filters offer better classifying performance, but can exhibit shorter filter life, suggesting that thin pleats have little support and bunch together in this type of high solids application.
For the best of both, we recommend the QXL™ Series filters as the top choice for CMP slurry filtration. These hybrid pleated/depth filters combine the best features of depth filters (ability to remove a range of particle sizes due to graded pore structure, stable media configuration) with the advantages of pleated filters (high surface area for improved flow rates, lower pressure drops, and longer on-stream life.)
For bulk slurries prior to day tank, use the QXL 3 to 10 micron depending on slurry type. Use finer filter for oxide type and coarser for metal type slurries. Absolute rated melt blown MBA filters are another option when traditional depth filters are desired.
For recirculating loops, use the hybrid pleated depth QXL 3 to 10 micron filters to remove large particles and agglomerates.
For point-of-use, the QXL 0.45 micron filter will assure abrasive particle size is tightly controlled.