1. Where Do Temperature Changes Occur in CMP Equipment?
Temperature changes between the wafer and polishing pad during grinding: Mechanical friction between the wafer, retain ring, and polishing pad generates heat, causing the temperature at the interface of the wafer and pad to rise during Chemical Mechanical Polishing (CMP).
Temperature changes in the cleaning unit’s mega tank: To ensure effective wafer cleaning, the cleaning solution in the mega tank is typically maintained within a specific temperature range, critical for removing residual polishing slurry and particles post-CMP.
2. How Do Temperature Changes Impact the CMP Process?
Impact of temperature changes between the wafer and polishing pad.As grinding progresses, the temperature on the pad surface rises. Elevated temperatures accelerate chemical reaction rates, thereby speeding up the chemical reactions between the wafer and polishing slurry—directly influencing the material removal rate (MRR).
Simultaneously, temperature increases alter the physical properties of the polishing pad, such as hardness and compressibility. Typically, pad hardness decreases, which tends to reduce the mechanical grinding rate. However, the accelerated chemical reactions may counteract this effect. The net change in MRR depends on the dominant mechanism of the CMP process: in mechanically driven CMP, the overall grinding rate will decrease.
Additionally, changes in pad hardness and compressibility affect dishing (a common planarization defect). Higher temperatures usually soften the pad and increase its compressibility, worsening dishing. From the perspective of pad conditioning, temperature-induced changes in pad hardness alter the conditioner’s cut rate on the pad and the pad’s surface roughness. These changes, in turn, affect MRR, dishing, and pad lifetime. Generally, increased or uneven pad roughness leads to greater dishing and poorer MRR uniformity.
Impact of temperature changes in the mega tank.Higher temperatures in the mega tank accelerate chemical reactions between the cleaning solution and surface particles or residues on the wafer. They also reduce the surface tension of the cleaning solution or water, enhancing the efficiency of megasonic shock waves on the wafer surface—thereby improving particle removal rates.
However, temperatures cannot be excessively high; the optimal range depends on the type of cleaning solution and wafer. Typically, temperatures are maintained between 30–50°C to balance cleaning efficacy and material compatibility.
3. How to Mitigate the Impact of Temperature Changes on CMP?
Physical cooling: CMP equipment typically incorporates a loop of cooling water pipes beneath the platen. Circulating cooling water dissipates heat generated during grinding, stabilizing the platen temperature and preventing excessive heating of the polishing pad.
Adjusting slurry flow rate: Increasing the polishing slurry flow rate helps transfer heat from the wafer-pad interface. However, this may affect MRR, requiring comprehensive evaluation based on process requirements. Notably, higher slurry flow can also reduce temperature-induced dishing by maintaining more uniform pad properties.
Reducing grinding time: Through process integration and CMP capability assessments, minimize the total material removal amount. Using consumables with higher MRR can also shorten grinding time, reducing heat accumulation.
Lowering rotational speed: Under the premise of meeting process requirements, reduce the rotational speeds of the polishing head and platen to decrease frictional heat generation.
Thermal exchangers: Thermal exchangers are typically installed in the mega tank’s cleaning solution pipeline to stabilize the cleaning fluid temperature, ensuring consistent post-CMP cleaning performance.