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Subsurface scanning probe metrology for overlay through opaque layers

The performance of image-based and diffraction-based overlay metrology depends on the quality of optical signal returning from the buried mark. A desirable class of hard mask materials are the metal-based hard masks. The challenge with metal-based hard masks is that they are typically opaque in the visible range. To enable overlay metrology, a costly process integration scheme replaces the opaque material over the overlay target, while another detects residual topography propagating through the film. Here we discuss the progress towards the fully automated Scanning Probe Metrology tool that combines surface and subsurface channel information in a single image to measure overlay through opaque films.