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.

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.

Background: Integrated circuits are fabricated layer by layer. It is crucial to their performance that these layers are well aligned to each other, and any undesired translation of a layer is called overlay. Thus far, overlay measurements have been limited to visible wavelengths, but the use of materials that are opaque to visible wavelengths necessitates measurements using infrared light.Aim: We set out to demonstrate that an overlay sensor based on digital holographic microscopy can perform such overlay measurement at infrared wavelengths, while maintaining functionality at visible wavelengths.Approach: This was done by constructing a breadboard setup that is capable of measuring overlay at wavelengths ranging from 400 to 1100 nm.Results: Using the setup, we demonstrated good linearity between an applied amount of overlay and the measured amount. In addition, we demonstrated that the setup is only sensitive to structures at the top of the wafer. Measurements are therefore unaffected by the fact that Si is transparent at 1100 nm.Conclusions: These results demonstrate the viability of an overlay sensor that is sensitive to visible and infrared light, allowing more freedom in choice of materials for integrated circuits.

Measuring overlay between two layers of semiconductor devices is a crucial step during electronic chip fabrication. We present dark-field digital holographic microscopy that addresses various overlay metrology challenges that are encountered in the semiconductor industry. We present measurement results that show that the point-spread function of our microscope depends on the position in the field-of-view. We will show that this novel observation can be explained by a combination of the finite bandwidth of the light source and a wavelength-dependent focal length of the imaging lens. Moreover, we will also present additional experimental data that supports our theoretical understanding. Finally, we will propose solutions that reduce this effect to acceptable levels.