Background: 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.
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 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 nm2.
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 nm2.
Background: One of the challenges for extreme ultraviolet (EUV) lithography is the mitigation of mask three-dimensional effects arising from the oblique incident angle and the mask topography. As the scanners’ numerical aperture and the pattern aspect ratio increase, these effects become more prominent. A potential solution to reduce them consists in replacing the current TaBN absorber for a higher-k material. Aim: We demonstrate the potential of a mask inspection platform to evaluate the impact of different absorber materials on actinic defect inspection. Approach: We evaluate the performance of a reflective-mode EUV mask scanning microscope (RESCAN), our actinic lensless inspection tool, with three different absorber materials (hydrogen silsesquioxane, TaBN, and Ni). We study the effect of these materials on the image formation and compare the defect maps. Results: The Ni absorber mask exhibits a better contrast compared to the TaBN one, even though the thickness of the layers differs only by 10 nm. Programmed defects are localized and detected with a high signal-to-noise ratio (SNR). Conclusions: The gain in contrast for the Ni absorber being significant, the SNR is higher for a smaller defect in a TaBN absorber photomask. RESCAN allows the evaluation of the performance of absorber materials in defectivity and image formation on small samples.
Background: The purpose of EUV pellicles is to protect the surface of EUV lithography masks from particle contamination. It is important to ensure that the optical characteristics of the pellicle membrane do not critically affect the reticle image quality. Aim: We want to verify the possibility to integrate pellicle inspection and characterization capabilities in reflective-mode EUV mask scanning microscope (RESCAN), our actinic mask inspection platform based on coherent diffraction imaging. Approach: We studied the impact of a few selected EUV pellicle prototypes on the quality and the contrast of the reticle image obtained with RESCAN. Results: We measured the scattering distribution of the pellicles, and we correlated it with the mask image contrast and fidelity. We also detected the presence of a 6.5-μm-diameter fiber on the pellicle surface. Conclusions: We demonstrated that RESCAN is suitable for through-pellicle actinic mask inspection and can be also used to characterize and monitor the pellicle quality.
As extreme ultraviolet (EUV) lithography is entering the high-volume manufacturing (HVM) phase, the ability to identify printable defects on EUV reticles becomes increasingly important to achieve the required wafer yield. However, no commercially available tool exists today for actinic patterned mask inspection (APMI). RESCAN is an APMI tool based on scanning coherent diffraction imaging (SCDI) under development at the Paul Scherrer Institut. In the last years, using RESCAN, we have demonstrated actinic identification of absorber defects on mask down to 36 nm size, and through-pellicle defect inspection. In this paper, we address a very critical but hitherto not reported feature of an APMI tool, namely the identification and characterization of phase defects on a patterned mask. Phase defects could be due to imperfections on the blank substrate leading to modification of the multilayer topology or due to particles embedded within the multilayer itself. Independent of the origin, the wave exiting the multilayer surface will have domains of phase variations as it propagates in the three-dimensional reticle stack. Mapping the exit wave that leave the EUV reticle both in amplitude and phase would be of paramount importance towards accurately predicting the EUV aerial images. Exploiting the amplitude and phase maps provided by SCDI, we use RESCAN for phase contrast imaging and to characterize programmed phase defects in a hybrid absorber-phase sample in a lens-less scheme, demonstrating the capability of the method and the tool.
The purpose of EUV pellicles is to protect the surface of EUV lithography masks from particle contamination. Currently several pellicle prototypes are being developed. It is important to ensure that the optical characteristics of the pellicle membrane do not critically affect the reticle image quality. We present here a study of the impact of a few selected EUV pellicle prototypes on the quality and the contrast of the reticle image obtained with an actinic lensless microscope.
For EUV photomasks, high-k absorber materials represent a potential strategy to effectively mitigate mask 3D effects which are getting more prominent as the scanners’ NA increases. The performance of RESCAN, our actinic lensless imaging microscope is evaluated through three different absorber materials (HSQ, TaBN, and Ni) together with the imaging properties of the materials themselves. Defect maps for each material are analyzed and compared.
RESCAN is a metrology platform, currently under development at Paul Scherrer Institut to provide actinic inspection capability for EUV reticles. It is a lensless microscope and its defect detection protocol is based on coherent diffraction imaging. One of the key features of an actinic pattern inspection tool is the ability to operate on reticles protected by an EUV pellicle. Thanks to the absence of imaging optics in close proximity of the sample, there are no geometrical constraints preventing the inspection of a pellicle-protected reticle in RESCAN. Nevertheless, the defect detection sensitivity depends on the quality of the reconstructed images and it is therefore important to assess if and how these are affected by the presence of an EUV pellicle. We report here the results of an evaluation of the effects of different types of EUV pellicles on the reconstructed images. We observed that high-absorption silicon nitride pellicles significantly reduce the imaging quality whereas in the case of the CNT-based pellicles the imaging performance was not affected. We also observed no damage of the CNT-based pellicle. To our knowledge, this work is the first successful attempt to perform mask inspection through EUV pellicles.
In this paper, we present a method for accurate EUV mask inspection of arbitrarily shaped absorber patterns using lensless imaging methods. With our reflective-mode EUV mask scanning lensless imaging microscope (RESCAN), we have imaged a mask with programmed defects and present here the computed defect map for both die-to-die and die-to-database pattern inspection. The signal-to-noise ratio in both cases was high enough to clearly isolate the defect from the structures (~13 for die-to-die and ~7 for die-to-database inspection).
To reach the high-throughput required by industry, we implemented an extended ptychographic algorithm that allows for continuous scanning of the sample and subsequent deconvolution of the distortions in the incident illumination that are due to the fast stage movement. We will show how this algorithm was implemented on a multi-GPU platform for maximum performance that will eventually allow us to reach the final goal of 7 hours scan time for a full mask.
The re ective-mode EUV mask scanning lensless imaging microscope (RESCAN) is being developed to provide actinic mask inspection capabilities for defects and patterns with high resolution and high throughput, for 7 nm node and beyond. Here we, will report on our progress and present the results on programmed defect detection on random, logic-like patterns. The defects we investigated range from 200 nm to 50 nm size on the mask. We demonstrated the ability of RESCAN to detect these defects in die-to-die and die-to-database mode with a high signal to noise ratio. We also describe future plans for the upgrades to increase the resolution, the sensitivity, and the inspection speed of the demo tool.