Presented at ADMETA, October 2008

ABSTRACT
Picosecond Ultrasonic Metrology (PULSE™) has been successfully adopted in copper process monitoring by logic device manufacturers. In this article, we present the capability of this technique to measure in the in-die cell area of a memory chip. Using picosecond ultrasonic technology, we measured in-die Cu thickness of Electroplated (EP) Cu and post-CMP Cu wafers. Cu thickness measurements were also made on solid pads located in the scribe lines for comparison. EP Cu data
showed that Cu was thicker on the in-die cell area demonstrating the well known overburden effect. For CMP wafers, Cu thickness at cell area was significantly different from the thickness at pad box. PULSE results confirm the necessity and benefits for monitoring the Cu thickness in the in-die cell area versus relying on pad measurements for process monitoring.

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