Phase imaging of EUV masks using a lensless EUV microscope

In extreme ultraviolet (EUV) lithography, controlling the reflection phase of the mask pattern is important for enlarging the process window and for compensating for phase defects. And, there are shadowing effect owing to the oblique illumination which modifies reflection phase of absorber patterns. A phase imaging microscope is required to determine this actinic phase distribution. To this end, we have developed a coherent EUV scatterometry microscope (CSM) based on coherent diffraction imaging (CDI). The CSM consists of a coherent EUV source and a charge-coupled device (CCD) camera, which records the diffraction images from the mask pattern directly. The system is lensless and makes use of the inverse computations based on the intensity of the scattered radiation, instead of the image-forming optics, to retrieve the frequency-space phase data. This allows the aerial-image phase data to also be reconstructed. Using the CSM system, one can obtain the intensity and phase images of the sample pattern. In this study, we also reconstructed the phase images of line-and-space patterns that were free of the shadowing effect as well as of patterns in which shadowing occurred. In the case of the latter, shadowing could be observed clearly in the phase image. Finally, the phase image of a programmed phase defect was also reconstructed and its phase value evaluated quantitatively. Thus, the CSM system is powerful tool for developing phase-controlled masks.