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