Introduction of Plastic Material for Semiconductor CMP Retaining Ring

With the continuous development of semiconductor technology, in order to increase the output of chips and reduce the production cost, the diameter of the wafer becomes increasingly higher. After the wafer diameter increases, warpage and deformation are prone to occur during processing. For wafers with a diameter of more than 150mm, it is easy to form an "over-grinding" phenomenon at the edge of the wafer, which reduces the polishing quality and wafer utilization. Adding a retaining ring off the wafer can effectively solve this problem.

The CMP retaining ring is mainly used to fix the wafer during the CMP polishing process, and flatten the polishing pad on the edge and the polishing pad below the wafer to the same height, which can effectively solve the problem of "over-grinding"; in addition, the polishing liquid is contacted by the retaining ring. The multiple grooves on the surface are transported to the interface between the polishing pad and the wafer, increasing the non-uniformity of the wafer removal rate.

As shown in the figure below, during the CMP process, the wafer is fixed in the CMP retaining ring, the lower surface of the wafer faces the polishing pad, and the polishing liquid flows through the CMP retaining ring to the interface between the polishing pad and the wafer. When the ring rotates, the wafer is also driven to rotate, so that the lower surface of the wafer rotates relative to the surface of the polishing pad to be ground and polished.

During this process, the CMP retaining ring is polished together with the wafer, and is in contact with the polishing liquid and polishing pad. The CMP retaining ring should have high wear resistance, corrosion resistance, low vibration characteristics, performance stability, and high purity. Therefore, the material requirements are:

The polishing liquid is generally a mixture of fine-grained abrasives and chemical additives, which will wear, scratch, and corrode the CMP retaining ring during the polishing process.

The CMP retaining ring needs to withstand mechanical loads and should have good elasticity, toughness and strength.

Dimensional variation increases dust generation and affects wafer surface accuracy.

The material of the retaining ring should have low outgassing properties and low particle release.

The relative wear rates of different retaining rings vary with CMP processing conditions and slurry characteristics. Therefore, the material of the retaining ring needs to be selected according to the process parameters of the CMP process (such as polishing pressure), the specific conditions of the polishing liquid, polishing pad, etc., as well as the production cost.

The choice of CMP retaining ring material affects the cost of the CMP process, including retaining ring life and retaining ring wear, vibration, or surface roughness. The CMP retaining ring has high requirements on the performance of the material. The selected materials are PPS, PEEK, PET-P, PAI, etc. These materials are generally modified and have better performance.

CMP retaining rings are usually made of standard special engineering plastic PPS. PPS has high temperature resistance, high mechanical strength, stiffness, wear resistance, creep resistance, dimensional stability, superior chemical resistance and hydrolysis resistance. After modification, the performance is improved, and it can better meet the requirements of CMP process.

Carbon fiber reinforced PEEK composites are recognized as ideal materials for the preparation of CMP retaining rings. PEEK material has excellent abrasion resistance, chemical resistance, mechanical strength, high temperature resistance and other characteristics, can meet the performance requirements of the retaining ring, and has a longer service life in oxide and tungsten polishing liquids than commonly used. It has double time of wear resistance life than PPS retaining ring.

The PET used for the CMP retaining ring is a semi-crystalline polyethylene terephthalate material, known as PET-P plastic, which has excellent mechanical strength, good creep resistance, good sliding and friction properties, resistance to wear, dimensional stability, chemical resistance and other properties. PET-P has a long service life in oxide polishing liquids, 5 times that of PPS, and lower cost.

Polyamideimide (PAI) can be used as CMP retaining ring, PAI is an amorphous high temperature engineering thermoplastic resin with many excellent properties, including high strength, stiffness, wear resistance, creep resistance, high temperature resistance , good chemical resistance, etc. The wear resistance and cost of PAI are relatively the best, but the PAI material has high hygroscopicity, which may cause dimensional changes, and is sensitive to exposure to saturated steam.

The manufacturing process of a chip is very complex, requiring thousands of steps, and each stage of processing faces difficulties, including extremely high temperatures, exposure to highly aggressive chemicals, extremely harsh cleanliness requirements, etc. The material requirements are also very strict.

Our twin screw extruder and downstream lines can produce these raw materials qualified for this application.