Diffraction-based overlay metrology using angular-multiplexed acquisition of dark-field digital holograms

Abstract

In semiconductor device manufacturing, optical overlay metrology measures pattern placement between two layers in a chip with sub-nm precision. Continuous improvements in overlay metrology are needed to keep up with shrinking device dimensions in modern chips. We present first overlay metrology results using a novel off-axis dark-field digital holographic microscopy concept that acquires multiple holograms in parallel by angular multiplexing. We show that this concept reduces the impact of source intensity fluctuations on the noise in the measured overlay. With our setup we achieved an overlay reproducibility of 0.13 nm and measurements on overlay targets with known programmed overlay values showed good linearity of R2= 0.9993. Our data show potential for significant improvement and that digital holographic microscopy is a promising technique for future overlay metrology tools.

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

“…In our current setup, we use free-space coupling to project illumination beams on the wafer and the reference beams on the camera. However, we have shown previously 11 that we can also use fiber coupling which we will implement as one of the next steps in this investigation.…”

“…We propose to measure overlay (OV) of μDBO gratings using a dark-field digital holographic microscope. Our df-DHM microscope has already been extensively reported previously 11 and is schematically shown in Fig. 1.…”

“…From this hologram, the complex electric field of the light at the camera plane can be recovered computationally as described in Ref. 11.…”

“…One other feature of our df-DHM concept is the parallel acquisition of the þ1 st and −1 st order dark-field images of the μDBO overlay targets. 11 As a result of this unique feature of DHM, we can use the full numerical aperture (NA) of the imaging lens, which helps to improve the spatial resolution, which is beneficial for small metrology targets. Moreover, a large imaging NA also offers a larger wavelength range over which a metrology target can be measured.…”

“…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.…”