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.
Actinic mask defect inspection is an essential process step for the implementation of extreme ultraviolet (EUV) lithography in high-volume manufacturing. The main challenges for any mask defect inspection platform are resolution, sensitivity, and throughput. The reflective-mode EUV mask scanning lensless imaging microscope (RESCAN) is being developed to provide actinic patterned mask inspection capabilities for defects and patterns with high resolution and high throughput for node 7 and beyond. Namely, the goal of the RESCAN project is to develop a tool capable of inspecting an EUV reticle in 7 h and detect mask defects down to a size of 10 nm×10 nm. The lensless imaging concept allows overcoming the resolution limitations due to the numerical aperture and lens aberrations of conventional mask imaging systems. With the increasing availability of computational power and the refinement of iterative phase reconstruction algorithms, lensless imaging became a powerful tool to synthesize the complex amplitude of the reticle image providing us also with extremely valuable information about phase and mask three-dimensional effects. Here, we present a brief description of the current prototype of the RESCAN platform and illustrate a few experimental examples of programmed defect detection.
With extreme ultraviolet (EUV) lithography getting ready to enter high volume manufacturing, there is an imminent need to address EUV mask metrology infrastructure. Actinic defect inspection of patterned EUV photomasks has been identified as an essential step for mask qualification, but there is no commercial tool available right now. We address this gap with the RESCAN tool, a defect inspection platform being built at Paul Scherrer Institut (PSI), co-developed in collaboration with Nuflare Inc, Japan. RESCAN uses Scanning Scattering Contrast Microscopy (SSCM) and Scanning Coherent Diffraction Imaging (SCDI) for fast defect detection and fine defect localization. The development of a stand-alone tool based on these techniques relies on the availability of (1) a bright coherent EUV source with a small footprint and (2) a high frame-rate pixel detector with extended dynamic range and high quantum efficiency for EUV. We present two in-house projects at PSI addressing the development of these components: COSAMI and JUNGFRAU. COSAMI (COmpact Source for Actinic Mask Inspection), is a high-brightness EUV source optimized for EUV photons with a relatively small footprint. JUNGFRAU (adJUstiNg Gain detector FoR the Aramis User station) is a silicon-based hybrid pixel detector, developed in house at PSI and prototyped for EUV. With a high frame rate and dynamic range at 13.5 nm, this sensor solution is an ideal candidate for the RESCAN platform. We believe that these ongoing source and sensor programs will pave the way towards a comprehensive solution for actinic patterned mask inspection bridging the gap of actinic defect detection and identification on EUV reticles.
One of the major obstacles towards the implementation of extreme ultraviolet lithography for upcoming technology nodes in semiconductor industry remains the realization of a fast and reliable detection methods patterned mask defects. We are developing a reflective EUV mask-scanning lensless imaging tool (RESCAN), installed at the Swiss Light Source synchrotron at the Paul Scherrer Institut. Our system is based on a two-step defect inspection method. In the first step, a low-resolution defect map is generated by die to die comparison of the diffraction patterns from areas with programmed defects, to those from areas that are known to be defect-free on our test sample. In a later stage, a die to database comparison will be implemented in which the measured diffraction patterns will be compared to those calculated directly from the mask layout. This Scattering Scanning Contrast Microscopy technique operates purely in the Fourier domain without the need to obtain the aerial image and, given a sufficient signal to noise ratio, defects are found in a fast and reliable way, albeit with a location accuracy limited by the spot size of the incident illumination. Having thus identified rough locations for the defects, a fine scan is carried out in the vicinity of these locations. Since our source delivers coherent illumination, we can use an iterative phase-retrieval method to reconstruct the aerial image of the scanned area with – in principle – diffraction-limited resolution without the need of an objective lens. Here, we will focus on the aerial image reconstruction technique and give a few examples to illustrate the capability of the method.
Actinic mask defect inspection is an essential process step for the implementation of EUV Lithography in high-volume manufacturing. The main challenges for any mask defect inspection platform are resolution, sensitivity, and throughput. The reflective-mode EUV mask scanning lensless imaging microscope (RESCAN) is being developed to provide actinic patterned mask inspection capabilities for defects and patterns with high resolution and high throughput, for node 7 and beyond. Namely, the first goal of the RESCAN project is to develop a tool capable of inspecting an EUV reticle in about 7 hours and detect mask defects down to a size of 10 nm. The lensless imaging concept allows to overcome the resolution limitations due to the numerical aperture (NA) and lens aberrations of conventional actinic mask imaging systems. With the increasing availability of computational power and the refinement of iterative phase reconstruction algorithms, lensless imaging became a powerful tool to synthesize the complex amplitude of the actinic aerial image providing us also with extremely valuable information about phase and mask 3D effects. Here, we present a brief description of the current prototype of the RESCAN platform and illustrate a few experimental examples of programmed defect detection.
Actinic mask inspection for EUV lithography with targeted specifications of resolution, sensitivity, and throughput
remains a big hurdle for the successful insertion of EUVL into high volume manufacturing and effective solutions are
needed to address this. We present a method for actinic mask inspection based on scanning coherent scattering
microscopy. In this method, the mask is scanned with an EUV beam of relatively small spot size and the scattered light is
recorded with a pixel detector. Customized algorithms reconstruct the aerial image by iteratively solving the phaseproblem
using over-determined diffraction data gathered by scanning across the specimen with a finite illumination. This
approach provides both phase and amplitude of actinic aerial images of the mask with high resolution without the need to
use high NA (numerical aperture) lenses. Futher, we describe a reflective mode EUV mask scanning lensless imaging
tool (RESCAN), which was installed at the XIL-II beamline and later at the SIM beamline of the Swiss Light Source and
show reconstructed aerial images down to 10 nm (on-wafer) resolution. As a complementary method, the a-priori
knowledge of the sample is employed to identify potential defect sites by analyzing the diffraction patterns. In this
method, the recorded diffraction patterns are compared with the die or database data (i.e. previously measured or
calculated diffraction data from the defect-free mask layout respectively) and their difference is interpreted as the defect
signal. Dynamic software filtering helps to suppress the strong diffraction from defect-free structures and allows
registration of faint defects with high sensitivity. Here, we discuss the basic principles of these Fourier domain
techniques and its potential for actinic mask inspection with high signal-to-noise ratio and high throughput.
For the successful implementation of extreme ultraviolet (EUV) lithography in the upcoming technology nodes, a major challenge to overcome is the stable and reliable detection and characterization of mask defects. We have recently presented a reflective mode EUV mask scanning lensless imaging tool (RESCAN) which was installed at the XIL-II beamline of the swiss light source and showed reconstructed aerial images of test patterns on EUV masks. RESCAN uses scanning coherent diffractive imaging (SCDI) methods to obtain actinic aerial images of EUV photomasks and was designed for 80 nm onmask resolution. Our SCDI algorithm reconstructs the measured sample by iteratively solving the phase problem using overdetermined diffraction data gathered by scanning across the specimen with a finite illumination. It provides the phase and amplitude aerial images of EUV photomasks with high resolution without the need to use high numerical aperture (NA) lenses. Contrary to scanning microscopy and full-field microscopy, where the resolution is limited by the spot size or NA of the lens, the achievable resolution with our method depends on the detector noise and NA of the detector. To increase the resolution of our tool, we upgraded RESCAN with a detector and algorithms. Here, we present the results obtained with the tool that is capable of up to 40-nm onmask resolution. We believe that the realization of our prototype marks a significant step toward overcoming the limitations imposed by methods relying on imaging optics and shows a viable solution for actinic mask metrology.
Actinic mask inspection for EUV lithography with targeted specification of sensitivity and throughput is a big challenge and effective solutions are needed. We present a novel method for actinic mask inspection, i.e. scanning scattering contrast microscopy. In this method the EUV mask is scanned with a beam of relatively small spot size and the scattered light is recorded with a pixel detector. Since the mask layout is known, the scattering profile of a defect-free mask at the detector can be calculated. The signal between the measured and calculated signal provides the deviation between the real mask and its ideal counterpart and a signal above a certain threshold indicates the existence of a defect within the illumination area. Dynamic software filtering helps to suppress strong diffraction from defect free structures and allows registration of faint defects with high sensitivity. With the continuous scan of the whole mask area, a defect map can be obtained with high throughput. Therefore, we believe that this method has the potential of providing an effective solution for actinic mask inspection. Here we discuss the basic principles of the method, present proof-of-principle experiments, describe the basic components of a feasible stand-alone tool and present early results of the performance estimations of such a tool.
For the successful implementation of extreme ultraviolet (EUV) lithography in the upcoming technology nodes, a major challenge to overcome is the stable and reliable detection and characterization of mask defects. We have recently presented a reflective mode EUV mask scanning lensless imaging tool (RESCAN) which is installed at the XIL-II beamline of the Swiss Light Source and showed reconstructed aerial images of test patterns on EUV masks. RESCAN uses scanning coherent diffractive imaging (SCDI) methods to obtain actinic aerial images of EUV photomasks and was designed for 20 nm on-wafer resolution. Our SCDI algorithm reconstructs the measured sample by iteratively solving the phase-problem using over-determined diffraction data gathered by scanning across the specimen with a finite illumination. It provides phase and amplitude aerial images of EUV photomasks with high resolution without the need to use high NA (numerical aperture) lenses. Contrary to scanning microscopy and full-field microscopy, where the resolution is limited by the spot size or NA of the lens, the achievable resolution with our method depends on the detector noise and NA of the detector. To increase the resolution of our tool, we upgraded RESCAN with a new detector and algorithms. Here we present the results obtained with the new tool that is capable of up to 10 nm on-wafer resolution. We believe that the realization of our prototype marks a significant step towards overcoming the limitations imposed by methods relying on imaging optics and shows a viable solution for actinic mask metrology.
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