Topographic measurement of encapsulated microstructures via active wavefront-compensatory coherence scanning interferometry.

In this Letter, we present what we believe to be a novel technique for robust, high-precision measurement of buried microstructures in wafer-level packaging, termed active wavefront-compensatory coherence scanning interferometry (AWC-CSI). By combining high numerical aperture (NA) optics with a deformable mirror for active aberration correction and a dispersion-matched compensation plate in the reference arm, AWC-CSI significantly enhances signal fidelity and signal-to-noise ratio under encapsulating layers. Experimental validation on packaged micro-electromechanical systems (MEMS) devices and resolution targets demonstrated an average topographic lateral resolution of approximately 0.95 μm under encapsulation layers ranging from 100 μm to 300 μm in thickness, with a system optical resolution of 0.89 μm. These results confirm its effectiveness for high-precision inspection and its potential for advanced packaging process optimization.