Cu Layer thickness monitoring in CMP process by using eddy current sensor

The method of how to determine the thickness of Cu layer on wafer in CMP process is a key technology in advanced CMP unit, especially in the stress free polishing (SFP) process. In this new technique, the traditional CMP process is adopted to remove a large portion of Cu layer till the thickness of Cu layer is reduced to about 200 nm~300 nm, avoiding the damage of Cu interconnects and low-k materials. The in-situ measurement system of Cu layer thickness is required for this new technique. In this report, we have developed a measurement system based on eddy current method alone for in-situ Cu layer thickness detection. The sensor was installed in the polishing platen and covered by polishing pad, where the hole is not required. After optimization, the sensitivity of sensor can reach 1 mV/nm under larger lift-off distance mainly caused by polishing pad. When the polishing began, differential signal process was adopted to remove the drift of system, and the curve of signal with polishing time was received. In calibration process, three or more wafers with different initial thickness of Cu layer were polished with different polishing time. The initial and end of the Cu layer thickness were determined by the four-probe method. So the relationship of the voltage of the sensor and the thickness was obtained. It was shown that the repeatability error of the system is less than 3.0%. Keywords: CMP, Eddy current, Cu layer, Thickness, Sensitivity