A highly sensitive laser focus positioning method with sub-micrometre accuracy using coherent Fourier scatterometry

Abstract

We report a novel method of focus determination with high sensitivity and submicrometre accuracy. The technique relies on the asymmetry in the scattered far field from a nanosphere located at the surface of interest. The out-of-focus displacement of the probing beam manifests itself in imbalance of the signal of the differential detector located at the far field. Up–down scanning of the focussed field renders an error S-curve with a linear region that is slightly bigger than the corresponding vectorial Rayleigh range. We experimentally show that the focus can be determined not only for a surface with high optical contrast, such as a silicon wafer, but also for a weakly reflecting surface, such as fused silica glass. Further, for the probing wavelength of 405 nm, three sizes of polystyrene latex spheres, namely 200, 100, and 50 nm in diameter, are tested. Higher sensitivity was obtained as the sphere diameter became smaller. However, due to the fact that the scattering cross-section decreases as the sixth power of the nanosphere diameter, we envision that further size reduction of the studied sphere would not contribute to a drastic improvement in sensitivity. We believe that the proposed method can find applications in bio/nano detection, micromachining, and optical disk applications.

Machine learning techniques applied for the detection of nanoparticles on surfaces using coherent Fourier scatterometry

“…The effect of the defocus produces unbalance of the signal as well as a drop in the SNR drop, as it has been demonstrated in Ref. [23]. nm.…”

“…The effect of the defocus produces unbalance of the signal as well as a drop in the SNR drop, as it has been demonstrated in Ref. [23]. When analysing the surface in a raster scanning fashion, one needs to choose the proper ∆y displacement step between the parallel lines of scanning.…”

“…2(b)). The zero-crossing of this pulse refers to the perfect alignment between the centre of the nanoparticle and the focused spot [23] (green point in Fig. 2(b)).…”

“…When the probe is focused on the interface, by scanning a spherical nanoparticle on the surface will render the so-called balanced pulse (positive and negative lobes of equal intensities, see Figure 2 B)). The zero-crossing of this pulse refers to the perfect alignment between the centre of the nanoparticle and the focused spot [23] (green point in Figure 2 B)). The effect of the defocus produces unbalance of the signal as well as a drop in the SNR drop, as it has been demonstrated in Ref.…”