<bold>Background:</bold> Reliable photomask metrology is required to reduce the risk of yield loss in the semiconductor manufacturing process as well as for the research on absorber materials. Actinic pattern inspection (API) of EUV reticles is a challenging problem to tackle with a conventional approach. For this reason, we developed RESCAN, an API platform based on coherent diffraction imaging.</p><p>
<bold>Aim:</bold> We want to verify the sensitivity of our platform to absorber and phase defects.</p><p>
<bold>Approach:</bold> We designed and manufactured two EUV mask samples with absorber and phase defects, and we inspected them with RESCAN in die-to-database mode.</p><p>
<bold>Results:</bold> We reconstructed an image of an array of programmed absorber defects, and we created a defect map of our sample. We inspected two programmed phase defect samples with buried structures of 3.5 and 7.8 nm height.</p><p>
<bold>Conclusions:</bold> We verified that RESCAN, in its current configuration, can detect absorber defects in random patterns and buried (phase) defects down to 50 × 50 nm<sup>2</sup>.</p>
RESCAN is an actinic patterned EUV mask metrology tool based on coherent diffraction imaging. An image of the reticle is reconstructed from recorded diffraction patterns using a phase retrieval algorithm. As semiconductor manufacturing has moved to EUV lithography to meet the next technology node, accurate photomask metrology with resolution in the nanometer range is crucial for high production yield. To find the optimal reconstruction strategy to achieve the highest resolution, sensitivity and reconstruction speed in RESCAN, we compared three algorithms. We demonstrate that, for the current setup, the best approach is the difference map algorithm.
The EUV photomask is a key component of the lithography process for semiconductor manufacturing. A critical defect on the mask could be replicated on several wafers, causing a significant production yield reduction. For this reason, actinic patterned mask inspection is an important metrology component for EUV lithography. The RESCAN microscope is a lensless imaging platform dedicated to EUV mask defect inspection and metrology. The resolution of the tool is about 35 nm, which is similar to that of state-of-the-art EUV microscopes. To improve the resolution of RESCAN, we designed an upgraded optical layout for the illumination system and we developed a coherent diffraction imaging-compatible method to synthesize a custom pupil structure. This new scheme will enable a lensless EUV microscope with a resolution down to 20 nm and thereby allow mask review capabilities for future technology nodes with EUV lithography.
The production of modern semiconductor devices is based on photolithography, a process through which a pattern engraved on a mask is projected on a silicon wafer coated with a photosensitive material. In the past few decades, continuous technological progress in this field allowed the industry to follow Moore’s law by reducing the size of the printed features. This was achieved by progressively increasing the numerical aperture of the projection system and reducing the wavelength. The latest lithography platforms for semiconductor manufacturing employ Extreme Ultra Violet (EUV) light at a wavelength of 13.5 nm. The metrology for the optics and the components of such platforms is not fully mature yet. Specifically, the inspection of the EUV photomask is still an open issue as no commercial solutions are currently available. Here we describe a lensless approach to this problem, based on coherent diffraction imaging at EUV that overcomes the main technological issues linked to the conventional mask inspection approach.
Reliable photomask metrology is required to reduce the risk of yield loss in the semiconductor manufacturing process. Actinic pattern inspection (API) of EUV reticles is a challenging problem to tackle with a conventional approach. For this reason we developed an API platform based on coherent diffraction imaging. Aim: We want to verify the sensitivity of our platform to absorber and phase defects. Approach: We designed and manufactured two EUV mask samples with absorber and phase defects and we inspected them with RESCAN in die-to-database mode. Results: We reconstructed an image of an array of programmed absorber defects and we created a defect map of our sample. We inspected two programmed phase defect samples with buried structures of 3.5 nm and 7.8 nm height. Conclusions: We verified that RESCAN in its current configuration can detect absorber defects in random patterns and buried (phase) defects down to 50 × 50 nm<sup>2</sup>.
The aim of this research is to explore the limits of the basic Ptychography algorithm (FPA) at deep ultra violet (DUV) wavelength of 193 nanometers and for binary and phase shift masks. Furthermore, imaging at high numerical apertures involves polarization effects, which are not covered in the scalar phase retrieval algorithms of FPA. The impact of these effects on FPA is investigated for a test chart with feature sizes close to the resolution limit. The quality of the images before and after applying FPA was measured using different error criteria.
The Normalized Image Log Slope (NILS) is the criterion which is most sensitive to the lithographically important change in the edge sharpness of features. The Michelson contrast provides a global assesment of the image contrast. The Mean squared Error (MSE) provides an overall assessment of the image quality with respect to a known object. When FPA is used to recover high resolution images of a phase shift mask, it is found out that the edge sharpness is increased but the overall contrast is declined. Additionally, the printability of side lobes contributed to increase of MSE. After using a rigorous method to compute the mask diffraction spectrum instead of the conventional Fourier transform imaging, it is confirmed that thin object assumption is not at all accurate for high numerical aperture DUV imaging applications. For the first time, the polarization effects at large NAs are introduced to FPA and the output is evaluated. Here, we verified that polarization can be used to increase the edge sharpness at a specific direction.