CMP Slurry Filtration
Chemical Mechanical Planarization (CMP) is a polishing process used to manufacture wafers for the semiconductor industry. It requires the use of a polishing tool and polishing slurry, manufactured by companies such a Cabot, Fujimi, Rohm and Haas, Hitachi Chemical and Ferro Industries. The slurry in the tool is delivered to the wafer surface and may contain large particles/agglomerates (> 1 μ) as a result of shipping/handling issues, drying, and interactions within the slurry distribution systems. These large particles can, in turn, increase the level of defectives (scratches) on the semiconductor wafer surface found after the CMP process has been completed.
One of the solutions for decreasing the level of defects caused by large slurry particles is through the use of slurry filtration. There are currently two preferred methods of slurry filtration: 1) loop or recirculation filtration and 2) point-of-use (POU) filtration. Loop or recirculation filtration attempts to 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.
POU filtration attempts to capture 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 tradeoffs 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 over loop filtration 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 < 1.0 μ particles. POU slurry filtration 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.
Graver recommends the QXL™ Series filters as the best 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.)
Objective of CMP Slurry Filtration (Chemical Mechanical Planarization):
||Slurry should contain abrasive components in a controlled particle size range to achieve an extremely smooth & normal wafer surface for photolithography
||Remove particulate and agglomerates while leaving abrasive components
||Customer objective: reduce scratches and increase yields
Graver Product Selection
||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 reciculating 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.
- CMP oxide slurries
- For silica removal
- pH 10-12
- Abrasive size 0.02 - 0.08 um
- Agglomerates form from exposure to air
- CMP metal slurries
- For metal removal
- pH 2-4
- Tends to settle; requires fast recirculation to keep abrasives suspended
- Agglomerates need to be removed via filtration