Optical Etch‐Rate Monitoring Using Active Device Areas: Lateral Interference Effects

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Abstract
The technique of optical (laser) etch‐rate monitoring is proving useful for integrated circuit manufacture and process development. This technique detects the change in film thickness during etching by monitoring the reflectance of the film on the integrated circuit wafer. When the reflectance is monitored over a region of the wafer containing both etched and unetched areas, the results are affected by lateral interference, which is interference between the light reflected from the different regions. This lateral interference has only a minor effect when less than 30% of the region being monitored is covered by an etch‐resistant layer (typical for metal‐type lithography levels), but it can cause severe distortions when a larger fraction of the region is covered by the resist. The effects of lateral interference have been examined using numerical simulations. The results are in good agreement with experimental etch‐rate monitor signals from the etching of patterns with resist coverage ranging from 10 to 80% of the total area.

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