Aberration calibration and correction with nano-scatterers in digital holographic microscopy for semiconductor metrology
Overlay metrology measures pattern placement between two layers in a semiconductor chip. The continuous shrinking of device dimensions drives the need to explore novel optical overlay metrology concepts that can address many of the existing metrology challenges. We present a compact dark-field digital holographic microscope that uses only a single imaging lens. Our microscope offers several features that are beneficial for overlay metrology, like a large wavelength range. However, imaging with a single lens results in highly aberrated images. In this work, we present an aberration calibration and correction method using nano-sized point scatterers on a silicon substrate. Computational imaging techniques are used to recover the full wavefront error, and we use this to correct for the lens aberrations. We present measured data to verify the calibration method and we discuss potential calibration error sources that must be considered. A comparison with a ZEMAX calculation is also presented to evaluate the performance of the presented method.