Presented at the International Semiconductor Technology Conference, March 2007

ABSTRACT
Chemical mechanical polishing (CMP) is a critical process step for producing high performance and high yield integrated circuits (IC) with copper interconnects. CMP polishing rates are dependent on many variables including the electroplate profile, slurry chemistry, pad wear, inter-level dielectric material and pattern density. These effects become more pronounced at advanced technology nodes where CMP dishing and erosion may have a significant impact on the resistance of the interconnect structures. To maintain the specified interconnect resistance, it is important to monitor the copper thickness of both bond pads and line array structures. These thicknesses may vary significantly across the wafer and from wafer to wafer, so a high-throughput inline metrology technique is desirable. The picosecond ultrasonic metrology technique has been implemented for CMP monitoring at the 90 nm technology node and is being qualified for 65 nm processes. Picosecond ultrasonics is a non-contact, nondestructive technique that uses an ultrafast laser light pulse to generate a sound wave in the top layer film on the tested areas. This sound wave travels into the structure until it meets an interface. At the interface an echo is generated. The echo then travels back to the surface, where it is detected. The thickness of the film can therefore be determined by multiplying the one-way trip time of the echo through the film and the speed of sound in the material. This technique has been extensively used to characterize various metal films, including the sub-micron line array structures used as CMP test structures. This article will further describe the picosecond ultrasonic technique and its applications for characterizing CMP processes.

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