Aberration calibration and correction with nano-scatterers in digital holographic microscopy for semiconductor metrology

Abstract

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.

Diffraction-based overlay metrology from visible to infrared wavelengths using a single sensor

“…This technique is explained in detail in one of our earlier papers. 10 As shown in this paper, excellent imaging performance can be achieved using this method even using a severely aberrated optic. Since the lens does not have to be well-corrected for aberrations a simple, single lens element can be used.…”

“…In addition, the stability of the measurements can be increased by enclosing the beam to avoid air turbulence and by increasing the region with good fringe contrast by reducing the bandwidth of the source and by path length matching the illumination beam to the reference beam coming from the opposite side of the sensor. Another interesting addition to the sensor is the aberration correction technique, 10 which will increase the image quality and remove the need for mechanical refocusing. Finally, many other technical improvements are envisioned to make the sensor more suited to an industrial setting.…”

“…The aberrations of this single lens can be computationally corrected using established backpropagation algorithms. 10 Details of our df-DHM concept have been published previously 11 and here we extend our setup with an image sensor that offers a large wavelength range with high quantum efficiency but with somewhat elevated read noise levels 12 (Sony IMX990). However, the coherent detection in df-DHM raises even weak metrology signals well above the noise levels of this image sensor.…”

Pupil apodization in digital holographic microscopy for reduction of coherent imaging effects

“…Using the plane wave propagation model for a single-lens imaging system as described by Goodman in [13] we can show that the complex field E i in image plane is given by [12]:…”

“…This has already been demonstrated in [8], where a weak measured signal of a silicon wafer at 1030 nm wavelength is coherently mixed with a reference beam resulting in a "noise clean" amplified holographic image. Finally, the retrieved complex field in DHM allows us to computationally correct for aberrations in the imaging lens [12]. A single imaging lens might seem undesired as it adds a quadratic wavefront to the complex image on the camera, but it is required as it enables imaging on a larger wavelength range.…”