Aperture design for a dark-field wafer defect inspection system

The dark-field defect inspection system occupies 70% of the market in the field of unpatterned wafer inspection. But the detection limit is still restrained by the haze signals. Signal-to-noise ratio (SNR) enhancement could effectively decrease the detection limit by decreasing the influence of the haze signals on the defect signals. The existing method of optimizing the inspection conditions, including beam path and collection channel, can enhance the SNR, but the effect is restrained by the system structure. The empirically designed aperture has been attempted to be used by blocking the scattering signals in a certain azimuth angle range. However, the performance is restrained, as a signal with a large SNR exists in the blocked scattering signals. In this paper, we propose a novel (we believe) aperture design method in the light of scattering field analysis to reduce the influence of the haze signals caused by the wafer surface roughness on the particle signals. On the basis of the bidirectional reflectance distribution function, apertures are designed according to the ratio field of the particle signal to haze and verified by the scattering model developed based on the tools of the National Institute of Standards and Technology. Additionally, incident conditions are optimized according to their influence on the SNR. It is noteworthy that the aperture designed under specific conditions cannot be used for all particles. Three aperture combination schemes are proposed in this paper, which can ensure the scattering characteristics such as intensity and sensitivity to meet the system requirements while improving the contrast. Simulation results verify that the detection limit decreases from 48 to 22 nm by introducing a well-designed aperture, with the case of p-polarized incident light when the threshold is 3 and the incident angle is 72°. Multiaperture schemes have better performance over others, especially the one-to-one scheme.