Coherent Fourier scatterometry combined with synthetic optical holography for nanoparticle detection

Abstract

As demand for chips increases and critical dimension keeps shrinking, the inspection of wafer becomes one of the critical challenges in the high volume production of chips. Coherent Fourier scatterometry (CFS) is a scanning bright field technique that is capable to detect nanoparticles on Si surfaces. The optical readout of CFS consists of a two pixel split detector that gives a voltage signal based on the intensity difference between the two halves. While an area of the wafer is raster scanned, the flat surface with no particles gives a zero voltage and on the particle an asymmetrical (non-zero) signal will be detected.

In this thesis, we demonstrate the use of synthetic optical holography (SOH) as a new method to improve the sensitivity of CFS. By adding a reference mirror with a piezo stage to the setup, we can interfere the scattered field with a plane reference wave. And by moving the mirror in steps, we can change the phase of the reference wave for every line of the raster scan, such that the 2D scan represents a digital off-axis hologram. Applying the standard digital holography reconstruction process, we retrieve a signal that equals the complex far field difference scaled by the reference field amplitude.

Overall, we see consistent signal-to-noise ratio (SNR) improvement over the conventional CFS. For the detection of a polystyrene latex (PSL) particle with a diameter of 60 nm (λ/10) on a silicon wafer, this new implementation leads to a SNR of 10 dB, which is about 4 dB over the filtered conventional CFS signal. For measurements of a dust particle using a very low amount of incident power (0.0014 mW on wafer), a SNR gain of more than 6 dB is achieved compared to filtered conventional CFS, due to the attenuation of low frequency electronic noises. Therefore, the implementation of SOH improves the sensitivity for detecting small nanoparticles and allows low power applications, such as biological imaging.