Abstract
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.
Highlights
Over the years, optical overlay metrology has seen significant innovations that were needed to keep up with the demanding overlay requirements of the semiconductor industry
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
Another big step forward was the introduction of diffraction-based overlay metrology where an overlay target consists of overlapping grating-pairs.[2,3,4]
Summary
Optical overlay metrology has seen significant innovations that were needed to keep up with the demanding overlay requirements of the semiconductor industry. Image-based overlay metrology using box-in-box metrology targets has been the work horse on essentially all layers.[1] A big improvement in metrology precision and robustness came with the introduction of advanced imaging metrology targets where the box structures were replaced by gratings. Another big step forward was the introduction of (micro) diffraction-based overlay metrology (μDBO) where an overlay target consists of overlapping grating-pairs.[2,3,4] These overlapping gratings are optically coupled, and as a result of this coupling, a small shift between the gratings (overlay) creates a small but measurable intensity variation in the diffracted light. Overlay measurements have been limited to visible wavelengths, but the use of materials that are opaque to visible wavelengths necessitates measurements using infrared light
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