Chemical mechanical planarization (CMP) is used during the semiconductor manufacturing process at each layer of the wafer to remove excess materials and create a smooth surface. This is done through the interaction of a pad and slurry on a polishing tool. 

The main functions of the retainer ring are : 

• To hold the wafer in place during polishing. 
• To ensure that each die on the outer edge of the wafer can be effectively planarized. The ring applies a larger force on the pad than the wafer and is able to push out the undulations present in the PU-pad conformation. This allows a uniform polishing all across the wafer.
• To permit the flow of the polishing slurry into/out from underneath the wafer through millimeter-sized channels carved into the ring.

CMP retainer rings require excellent chemical resistance, high mechanical strength and thermal stability. In CMP, there is both a mechanical and chemical actions. The retainer ring plastic must have good wear resistance as it is subjected to: 

• Adhesive component involving the polyurethane pad
• Abrasive component associated with slurry particles.
• Chemical attack from the chemicals in the slurry 

The wear debris should not to be hard, sticky, or anything that might harm the silicone wafer, like a black speck. So neat polymers are commonly used. Low ionic contamination levels and good processability in extrusion are also important.