BackgroundExtreme ultraviolet (EUV) attenuated phase-shifting masks are complex structures with stringent requirements for manufacturing precision and materials properties, and they have been object of extensive research lately.AimWe aim to characterize the optical constants (n and k) and the thickness of the layers in the mask stack with a nondestructive method.ApproachUsing a spectral reflectometry approach with EUV and soft X-ray illumination at various incidence angles, different layer’s properties in a photomask blank can be selectively probed. The optical constants and the thicknesses of the layers can be obtained by fitting a suitable model to the experimental reflectance.ResultsThe optical constants of the Pt-W alloy absorber and the thicknesses of the top three layers of the sample stack were accurately characterized.ConclusionsStacked layer’s properties can be selectively probed with the instrument (REGINE) we developed. The properties of the topmost layer can be characterized by assessing the probing depth, before investigating deeper layers with a suitable choice of illumination wavelength and angle of incidence.
Non-destructive nano-metrology is a fundamental tool for semiconductor device manufacturing. Practically, a combination of several metrology techniques is needed throughout the fabrication process of semiconductor chips, from EUV photomask inspection to patterned wafer metrology. EUV light not only facilitates the production of smaller features in lithography but also emerges as a powerful approach for the metrology required to characterize and analyze these intricate features in future technology nodes, thanks to its short wavelength, relevant penetration depth, and high reflectivity with relatively high grazing angles compared to that of x-rays. The reflective grazing incidence nanoscope for EUV (REGINE) at the Swiss Light Source is a prototype platform for reflectometry, scatterometry, and coherent diffraction imaging. It operates in the soft x-ray and EUV region (80 to 200 eV photon energy), and covers a grazing incidence angle range of 0° to 28.6°. REGINE provides the possibility to perform grazing incidence reflectometry and scatterometry for semiconductor metrology. In this study, we present the results of optical constants (n and k) determination and layer thickness characterization with reflectometry procedure for an EUV attenuated phase shifting mask blank, and the analysis of scatterometry results of line-space grating profile reconstruction and overlay measurement.
Nanoscale non-destructive metrology is a key requirement in several steps of the manufacturing process of modern semiconductor devices. In particular, with the introduction of EUV lithography into the high-volume manufacturing, enabling further shrinking of feature sizes, metrology for future technology nodes will become increasingly challenging. Depending on the specific requirements and constraints of the metrology tasks, the choice of the measurement methods is critical, and sometimes limited. Conventional metrology techniques must be constantly adapted to keep up with the device scaling roadmap. To explore new and easily scalable methods for semiconductor wafer metrology, we developed the EUV reflective grazing-incidence nanoscope (REGINE). REGINE is a lensless nanoscope prototype that combines reflectometry, scatterometry and coherent diffraction imaging (CDI) in a single instrument. Being a lensless imaging system, it is compact, cost-effective, and free of lens-induced aberrations. In this work, we will present the REGINE system and the latest results of reflectometry, scatterometry and EUV pellicle transmission experiments.
Non-destructive metrology for photomasks and wafers has always been an important requirement for semiconductor lithography, and with the advent of EUVL, enabling further shrinkage of semiconductor devices, the challenges in this field have increased significantly. Coherent diffraction imaging (CDI) is a promising alternative to standard imaging for EUV photomask actinic inspection. EUV light can also be used for wafer inspection to benefit from the resolution improvement allowed by its short wavelength. In order to perform lensless imaging for patterned wafers, however, we need to probe the sample surface at grazing incidence to ensure a sufficiently high reflectance. The EUV reflective grazing incidence nanoscope (REGINE) at the Swiss Light Source was develped to perform grazing incidence lensless imaging for patterned wafers. REGINE is a tool that combines CDI, scatterometry and reflectometry in the photon energy range between 80 to 200 eV, and at the grazing incidence angle of 1 to 28 degrees. In this work, we will present the latest characterization of our system, and preliminary results.
EUV lithography is currently being used in semiconductor high volume manufacturing, however, performing non-destructive metrology at the nanoscale for different types of structures and materials is still an extremely challenging task. The EUV reflective grazing incidence nanoscope (REGINE) is a new synchrotron end-station dedicated to non-destructive EUV metrology at the nanoscale for surface/layered structures with different material compositions. REGINE is being developed at the Swiss Light Source synchrotron and aims to enable grazing-incidence coherent diffraction imaging (CDI), scatterometry, and reflectometry in the energy range of 80 to 200 eV. In this work, we present the concept of the REGINE tool and the results of its commissioning experiments.
Background: With the adoption of extreme ultraviolet (EUV) lithography in the semiconductor manufacturing, actinic EUV mask metrology has become a crucial technology to ensure the required defect sensitivity and throughput for high-volume manufacturing. Reflection-mode EUV scanning microscope (RESCAN) is a lensless actinic microscope dedicated to EUV mask metrology based on coherent diffraction imaging (CDI) as an alternative approach for EUV mask metrology and inspection.Aim and Approach: In CDI, the complex-amplitude image of the sample is obtained through its measured diffraction. Though this approach can overcome the disadvantages and limitations of conventional imaging systems, the quality of the recorded diffraction data is crucial for the reliable reconstruction of a high-resolution image. Ultimately, the signal-to-noise ratio of the recorded diffraction data depends on several parameters, such as the sample’s reflectance, the quantum efficiency of the detector, its full well capacity, and the intensity of the illumination.Result: We investigate the optimal photon flux for RESCAN and provide a systematic study on the relation between the image resolution and the illumination intensity for CDI-based imaging at EUV wavelength.Conclusions: The insights provided will be helpful for the optimization of CDI for EUV imaging, in particular for increasing the throughput of EUV mask inspection with low power sources.
The reflective-mode EUV mask scanning lensless imaging microscope (RESCAN) is a synchrotron-based platform dedicated to EUV mask inspection and review at the Swiss Light Source. It is based on coherent diffraction imaging (CDI), which allows retrieving both the phase and the amplitude information of the mask surface. RESCAN has been successfully tested on masks with programmed phase and amplitude defects. A metrology method that can not only detect defects and contamination, but can also determine the material of the defects, can be a powerful tool to help identify the root cause of the defects. Here, we explore the possibility of leveraging the ability of RESCAN to detect the complex amplitude of the sample to distinguish surface defects of different materials. We fabricated a sample with random logic-like absorber patterns and pillar defects on top of a Mo/Si multilayer. We show our experimental results that show the defects of different materials exhibit specific contrast and phase values. This method can be used not only to detect the masks defects but also to identify the defect materials to a limited extent.
With the transition of EUV lithography to high volume manufacturing, EUV mask metrology has become a critical requirement. At PSI, we are developing RESCAN, a lensless actinic microscope dedicated to EUV mask inspection. RESCAN is based on coherent diffraction imaging (CDI), a method that reconstructs the complex image of the sample through its diffraction spectrum measured with a CCD detector. While this approach can overcome the cons and limitations of traditional optical imaging systems, in CDI, the quality of the recorded diffraction data is crucial for the reliable reconstruction of a high-resolution image. Ultimately, the signal-to-noise ratio of the recorded diffraction data depends on several parameters, such as the reflectance of the sample, the quantum efficiency of the detector, its full well capacity, and the intensity of the illumination. This paper investigates the optimal photon flux for RESCAN and analyzes the relation between the image quality and the EUV illumination intensity for a CDI-based imaging tool dedicated to EUV mask inspection and review.
EUV photomasks define the lithographic layers of chips, which are binary structures of relatively low versatility in contrast to specimen in biology or materials science. This knowledge can be used in EUV photomask imaging and inspection methods to improve the speed or sensitivity. We present here a total variation-based phase retrieval algorithm similar to previous methods by Chang et al. and Enfedaque et al. for EUV mask imaging and metrology. Total variation (TV) regularization exploits the binary structure of the reticles, enforcing a sparse sample gradient. We compare the TV regularized algorithm, PtychoADMM, to a standard phase retrieval approach, the difference map (DM). For simulated data containing Poisson noise, we do not observe a benefit from using the TV based PtychoADMM algorithm. The reconstructed image quality is similar, while PtychoADMM being a computationally more demanding method. In future, we will investigate if TV can recover information where the standard DM approach fails, e.g. for relaxed measurement requirements like a lower signal to noise ratio or less probe overlap in the ptychography scan.
High-resolution imaging at short wavelengths from extreme ultraviolet to hard X-rays has many applications in a plethora of fields from astronomy to biology and semiconductor metrology. Unfortunately, efficient optics for these wavelengths are difficult to manufacture or have limited resolution. For this reason, in the past few years, coherent diffraction imaging (CDI) applications become widely used. In CDI, the object is illuminated by a coherent beam and the diffraction intensity is collected by a 2D pixel detector. In this process, the phase information of the diffracted light is lost. A phase retrieval algorithm is then used to reconstruct the object’s complex amplitude. Ptychography is a scanning version of coherent diffraction imaging and it is based on an iterative reconstruction algorithm that relies on the quality of the recorded diffraction intensity to converge. To obtain diffraction patterns with a high signal-to-noise ratio, a beam stop is used in many ptychography setups to avoid over-saturation and blooming effects on the detector. While using a beam stop in a ptychography setup has become common practice, the limits of affordable data loss due to beam stop have not been systematically investigated. Pixel masking is the conventional method to recover the lost frequencies. In this method, when enforcing the Fourier domain constraint, the invalid pixels are ignored. In the missing data region, the algorithm is allowed to keep the guess from the previous iteration. The illumination conditions of the ptychography experiment play a critical role in the signal recovery procedure. The diffraction pattern on the detector is the convolution of the Fourier transform of the object and the illumination. An illumination with a finite numerical aperture encodes the object information over a larger detector area. This makes the reconstruction algorithm more robust to pixel loss. We provide simulation and experimental results to demonstrate this theory.
Anamorphic mask design is necessary to overcome the geometrical limitation imposed by the reflective geometry used in EUV lithography projection systems and will be used in high-NA EUVL scanners. In this study, we demonstrate the capability of anamorphic imaging using CDI. We created anamorphic EUV masks with programmed defects and inspected it with RESCAN, a lensless microscope dedicated to EUV mask inspection.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.