Implementation of quad detection scheme in coherent Fourier scatterometry for inspection of patterned structures

Coherent Fourier scatterometry (CFS) is a powerful scanning technique for inspecting defects on structured surfaces, relying on split detectors to measure asymmetry in the far-field scattered light. The split signal, a differential signal derived by subtracting signals from opposing halves of the detector, effectively detects asymmetries along the scan direction. However, this approach is inherently limited when inspecting patterned structures, as it loses information orthogonal to the scan direction. This results in signals that vary depending on the orientation of the patterns, complicating the characterization of certain defects. To overcome this limitation, we introduce a quad detector-based CFS scheme. By utilizing four independent photodetectors and processing their signals to generate integrated, split, and quad outputs, we capture complete far-field information. A Fourier filtering step removes detector-specific offsets, enabling robust signal analysis. Unlike the split-detector approach, this method provides defect and nanostructure inspection independent of the shape and orientation of the underlying patterns. We present the results of implementing this scheme to inspect defects on patterned surfaces. The quad detector signal reveals the edges of defects and demonstrates the versatility of this approach across different surface features. This advancement enhances the capability of CFS for defect inspection, offering a comprehensive and reliable solution for patterned structures where traditional split-detector methods fall short.