Editor-in-chief Harry Levinson looks back on the recent Advanced Lithography + Patterning Symposium.

Background: The successful introduction of extreme ultraviolet (EUV) lithography to high volume manufacturing has increased the interest to push this technology to its ultimate limits. This will require photoresist materials, which enable a better tradeoff between resolution, linewidth roughness and sensitivity, and the adaptation of optical resolution enhancements that were originally developed for deep ultraviolet (DUV) lithography.

Aim: We review published research on attenuated phase shift masks (attPSM) for EUV with special emphasis on modeling and fundamental understanding of the imaging characteristics of alternative absorber materials. The overview on previous work is intended to summarize typical observations and learning on obtained results and to serve as a reference for further research on this important topic.

Review approach: The review starts with a summary of related work on attPSM for DUV lithography. It is shown that the understanding and mitigation of mask topography (or mask 3D) effects is key for the analysis and optimization of attPSM for EUV lithography. Observations from several research groups and application of dedicated modeling approaches help to understand the physical mechanisms behind observed lateral image shifts and pitch-dependent shifts of the best focus position.

Results: The imaging physics of attPSM for EUV lithography differs significantly from attPSM imaging in DUV lithography. The “double diffraction” of EUV light from the absorber, the reflection characteristics of the multilayer blank, and the guidance of light through the openings in a low-refractive-index (low-n) absorber introduce important effects that need to be considered in the design and use of attPSM in EUV lithography. It is important to use the optical properties (n and k) and the thickness of the absorber as predictive design parameters of attPSM for EUV lithography (instead of phase and reflectivity). The refractive index of the absorber material is important for binary masks as well. The discussion of low-n absorbers includes both “traditional” attPSM for EUV and low reflectivity absorbers, which exploit the guidance of light inside patterned layers.

Conclusions: In-depth modeling investigations of attPSM and first experiments suggest that absorbers with a refractive index around 0.9 (low-n materials) can help to push high NA EUV lithography into the low k₁ regime. Comprehensive optimization of source and mask is required to exploit the advantages of low-n absorbers. Further enhancements can help to push EUV imaging to its ultimate limit.

Guest Editors Erik Hosler and Brennan Peterson introduce the Special Section on Next Generation Light Source, Materials, and Metrology/Inspection Equipment.

The semiconductor industry continuously moves toward smaller features and more complex, three-dimensional (3D) structures to enable next-generation devices. Thus there is a high need for 3D metrology techniques that would provide feedback to the advancing nanoscale fabrication processes. We introduce an approach for evaluating, calibrating, and developing new metrology of nanopatterns through comparing the new metrology data to 3D ground truth data, obtained by accurate scanning transmission electron microscopy (STEM) tomography 3D characterization. We demonstrate this approach by evaluating 3D height maps of sub-20-nm patterns obtained using multi-secondary electron detector scanning electron microscopy (multi-perspective SEM) and photometric stereo algorithm. We demonstrate the importance of full 3D characterization, including cross-sectional structure, height fluctuations, and line edge roughness to truly probe the pattern’s average structures and their 3D variations. Although STEM tomography is a high-resolution and accurate method, it is considered demanding in terms of sample preparation and cannot be used as an efficient in-line metrology method. However, we show that it can be used as a reliable 3D characterization technique to obtain the necessary 3D data for the development of new 3D metrology tools and demonstrate this approach by evaluating the accessible, in-line, multi-perspective SEM.

In-line Raman spectroscopy for compositional and strain metrology throughout front-end-of-line (FEOL) manufacturing of next-generation gate-all-around nanosheet field-effect transistors is presented. Thin and alternating layers of fully strained pseudomorphic Si_{(1  −  x)}Ge_x and Si were grown epitaxially on a Si substrate and subsequently patterned. Intentional strain variations were introduced by changing the Ge content (x  =  0.25, 0.35, 0.50). Polarization-dependent in-line Raman spectroscopy was employed to characterize and quantify the strain evolution of Si and Si_{(1  −  x)}Ge_x nanosheets throughout FEOL processing by focusing on the analysis of Si-Si and Si-Ge optical phonon modes. To evaluate the accuracy of the Raman metrology results, strain reference data were acquired by non-destructive high-resolution x-ray diffraction and from destructive lattice deformation maps using precession electron diffraction. It was found that the germanium-alloy composition as well as Si and Si_{(1  −  x)}Ge_x strain obtained by Raman spectroscopy are in very good agreement with reference metrology and follow trends of previously published simulations.

Rules are derived to obtain specifications on radiance, power, lifetime, and cleanliness of the source for an actinic patterned mask inspection system. We focus on the physical processes and technological aspects governing the requirements of radiation sources for reticle inspection. We discuss differences and similarities to scanner with respect to magnification, system etendue, and image recording. Source radiance requirements are estimated from a perspective of targeted throughput and defect detection sensitivity. The derivations consider the influence of photon shot noise on signal detection and conservation laws of light etendue and radiant flux. We describe the scaling laws for required radiance with targeted sensitivity index, optical contrast, field size, and system throughput. In addition, we address the limits on the required brightness and minimum repetition rate set by mask damage threshold. Finally, system and source cleanliness requirements and criticality of the source availability and lifetime are discussed. The analysis can be applied to other microscopy-based metrology and inspection applications.

Monitoring the high aspect ratio etch profiles in state-of-the-art three-dimensional NAND memory fabrication processes has pushed metrology technologies to new limits. Here, we discuss how a mid-infrared ellipsometric measurement can yield angstrom level discrimination in critical dimension changes of memory channel hole (CH) profiles across such a memory chip. Using finite-difference time-domain and rigorous coupled-wave analysis simulations, we demonstrate how dispersion mitigated mid-infrared beam penetration into these memory structures permits parameter decorrelation and the measurement of the full CH profile.

Over the past several years, stacked nanosheet gate-all-around (GAA) transistors captured the focus of the semiconductor industry and have been identified as the lead architecture to continue logic complementary metal-oxide-semiconductor scaling beyond 5 nm node. The fabrication of GAA devices requires specific integration modules. From very early processing points, these structures require complex metrology to fully characterize the three-dimensional parameter set. As the technology progresses through research and development cycles and is poised to transition to manufacturing, there are many opportunities and challenges that still remain for in-line metrology. Especially valuable are measurement techniques that are non-destructive, fast, and provide multi-dimensional feedback, where reducing dependencies on offline techniques has a direct impact on the frequency of cycles of learning. More than previous technologies, then, nanosheet technology may be when some offline techniques transition from the lab to the fab, as certain critical measurements need to be monitored in real time. Thanks to the computing revolution the semiconductor industry enabled, machine learning has begun to permeate in-line disposition, and hybrid metrology systems continue to advance. Of course, metrology solutions and methodologies developed for prior technologies will also still have a large role in the characterization of these structures, as effects such as line edge roughness, pitch walk, and defectivity continue to be managed. We review related prior studies and advocate for future metrology development that ensures nanosheet technology has the in-line data necessary for success.

Background: The chemically amplified resist (CAR) has been the workhorse of lithography for the past few decades. During the evolution of projection lithography to extreme ultraviolet lithography (EUVL), a continuous reduction in feature size is observed. Also, a reduction in resist film thickness (FT) is required to prevent large aspect ratios that lead to pattern collapse. A further reduction in resist FT, into an ultrathin film regime (<30  nm resist FT), is expected when advancing to high NA EUVL. This brings along associated challenges with (1) resist critical dimension scanning electron microscope (CDSEM) metrology and (2) resist patterning performance.

Aim: Assessment of metrology challenges and patterning limits of a CAR working in this ultrathin film regime. Deconvoluting the metrology and patterning effect on the determination of the unbiased line width roughness (uLWR).

Approach: Patterning a CAR at different nominal resist FTs on two different underlayers to quantify the changes in CDSEM image quality and resist patterning performance with the resulting uLWR changes.

Results: The CDSEM image signal-to-noise ratio (SNR) depends on resist FT and the underlayer. The uLWR increases with a reduction in resist FT but scales differently on the two underlayers.

Conclusions: A relationship between CDSEM image SNR and uLWR is found. The SNR and uLWR scaling difference on the two underlayers, as well as the uLWR dependency on SNR was determined to be a metrology effect. The general uLWR increase for a reduced resist FT was determined to be a patterning effect.

Background: In the extreme ultaviolet (EUV) lithography process the performance of the photoresist is a crucial factor regarding the quality and critical dimensions of the fabricated structures.

Aim: The characterization of the latent image structures in photoresists during the process steps before the development of the resist is key to understand the relation between the material of the resists, the selection of process parameters, and the resulting quality of fabricated structures.

Approach: Spectroscopic EUV reflectometry is a nondestructive metrology technique that measures the broadband reflectance of samples in the EUV spectral range and under grazing incidence angles. The technique offers a combination of high sensitivity to nanoscale structural parameters of periodic structures as well as a high sensitivity to the material composition samples, enabling the characterization of latent images of periodic structures.

Results: Measurements of the reflectance of an EUV-exposed and unexposed photoresist reveal the contrast in optical constants after the resists are treated with a post-exposure bake as well as shrinkage of the resist layer thickness. Based on this data, simulative studies on latent images of periodic grating structures are conducted showing the possibility to extract information on the structure parameters including the latent image profile and surface topography.

Conclusion: Spectroscopic EUV reflectometry shows to be sensitive to the contrast of exposed and unexposed photoresist which commends the technique to be adequate for the characterization of latent images in photoresists.

Background: High-throughput three-dimensional metrology techniques for monitoring in-wafer uniformity (IWU) and in-cell uniformity (ICU) are critical for enhancing the yield of modern semiconductor manufacturing processes. However, owing to physical limitations, current metrology methods are not capable of enabling such measurements. For example, the optical critical dimension technique is not suitable for ICU measurement, because of its large spot size. In addition, it is excessively slow for IWU measurement.

Aim: To overcome the aforementioned limitation, we demonstrate a line-scan hyperspectral imaging (LHSI) system, which combines spectroscopy and imaging techniques to provide sufficient information for spectral and spatial resolution, as well as high throughput.

Approach: The proposed LHSI system has a 5-μm spatial resolution together with 0.25-nm spectral resolution in the broad-wavelength region covering 350 to 1100 nm.

Results: The system enables the simultaneous collection of massive amounts of spectral and spatial information with an extremely large field of view of 13  ×  0.6  mm². Additionally, throughput improvement by a factor of 10³ to 10⁴ can be achieved when compared with standard ellipsometry and reflectometry tools.

Conclusions: Owing to its high throughput and high spatial and spectral resolutions, the proposed LHSI system has considerable potential to be adopted for high-throughput ICU and IWU measurements of various semiconductor devices used in high-volume manufacturing.

The development of ways to increase the intensity of extreme ultraviolet (EUV) light sources for future EUV lithography is important to realize high throughput fine patterning. The energy-recovery linac (ERL) free-electron laser (FEL), which is an accelerator based light source, is a candidate for this. We clarify the design concept of the ERL-FEL for EUV light sources for future lithography, delivery systems of the FEL light to multiscanners, and future development items of the accelerator technologies and a possibility of the beyond EUV.

Background: In extreme ultraviolet lithography, maximum printable feature density is limited by stochastic defectivity. One of the methods to reduce it is the optimization of the aerial image via source mask optimization. To guide this optimization, we need to know which aerial image metric predicts defectivity. Feature variability is linked to defectivity.

Aim: Find which aerial image metric best predicts variability and defectivity.

Approach: We construct seven pupils that vary aerial image metrics [normalized image log slope (NILS), depth-of-focus (DoF), maximum intensity] in a controlled way. We measure variability and defectivity after development and after two different etch processes and correlate them to aerial image metrics.

Results: Stochastic critical dimension uniformity (CDU) is best predicted by NILS. Systematic CDU is determined by mask roughness through spatial frequency, with low frequencies differentiating outer and inner sigma pupils. Defectivity is best predicted by maximum intensity for missing holes and minimum intensity for merging holes. NILS and maximum intensity are strongly correlated. DoF has minimal impact.

Conclusions: Optimization toward maximum intensity reduces defectivity. Outer sigma pupils reduce variability. Increasing numerical aperture in next-generation EUV lithography is expected to reduce missing hole defects.

Background: A unique extreme ultraviolet (EUV-) scanner with a high numerical aperture (NA) of 0.55 was designed to enable printing of resolution lines with 8 nm half-pitch in a single exposure. The introduction of a central obscuration in the optics design reduces the angular load on the multilayer mirrors, enabling a high transmission and throughput. The central obscuration area has been minimized for best imaging, overlay, and transmission.

Aim: The wavefront is only available in the non-obscured area. This raises the question of how to describe such a wavefront.

Approach: We discuss the choice of fringe-Tatian basis functions to represent the wavefront for an obscured pupil. To make this choice, one needs to balance mathematical correctness while maintaining a simple and intuitive description.

Results: We provide a detailed analysis for selecting basis functions that are adequate to describe measured wavefronts on the non-obscured part of the pupil. This statement is supported by imaging simulations. A fast and stable evaluation of the chosen basis functions is presented. An adapted definition of the wavefront root-mean-square deviation for these functions is proposed; it has the advantage of being simple and independent of the number of basis functions used.

Conclusions: Because of the benefits of the proposed representation, the community is encouraged to use the same formalism.

Background: Scanning electron microscopy (SEM)-based line edge roughness (LER) measurement suffers from an error due to noise in SEM image. Noise correction methods have been developed to obtain unbiased roughness results, however, there are still concerns in the viewpoint of measurement precision.

Aim: To develop an unbiased roughness analysis for highly precise LER measurement.

Approach: Combining the conventional unbiased roughness analysis with a profile-averaging method, where a line pattern is repeatedly measured and then the obtained profiles are aligned and averaged. The experimental result measured by SEM was verified using atomic force microscopy (AFM)-based LER metrology, which has higher reliability than SEM.

Results: The experimental evaluation showed that the proposed method can obtain roughness parameters more precisely than the conventional method.

Conclusions: When the noise in the line edge profile by SEM is too large, it is necessary to reduce the noise beforehand and then perform roughness analysis in order to obtain precise roughness results. The proposed method enables to measure LER with the highest precision using SEM. Additionally, the AFM-based LER metrology was demonstrated as a feasible technique to evaluate the performance of SEM-based LER metrology.

Lithography process hotspot is a traditional design and quality issue for the integrated circuit manufacturing due to the gap between exposure wavelength and critical feature size. To efficiently detect the hotspot regions and minimize the necessity of conducting expensive lithography simulation experiments, various pattern-based methods have been proposed in the past years. Recent solutions have been focused on implementing deep learning strategies because of the unique strength in imagery classification tasks by employing the artificial neural networks. However, solving the technical bottlenecks such as imbalanced learning, identifying rare hotspots and effective feature extraction remains challenging. For this research, we introduce a hotspot detection method based on a convolutional neural network classifier and enhanced it by the imagery feature extraction and a generative adversarial network data augmentation system. Experimental results show competitive performance compared with the existing works.

Extending 0.33NA extreme ultraviolet (EUV) single patterning to pitch 28 nm will enable significantly shorter process flow for N2 node and cost-efficiency of metal layers patterning. At the same time, EUV single patterning becomes very challenging in terms of stochastic defectivity and process window. To enable EUV single patterning at pitch 28 nm with good process window and patterning fidelity (low defectivity and line edge roughness), three mask candidates are considered: a standard binary Ta-based absorber mask, a high extinction (high-k) absorber mask, and a low-n attenuated phase-shift mask (attPSM). The patterning performance of these three mask candidates is compared by means of source mask optimization. The patterning performance of the candidate masks is assessed using an imec N3 (foundry N2 equivalent) random logic M1 layout. The impact of mask tonality (bright field versus dark field) and insertion of sub-resolution assist features (SRAFs) on pattern fidelity and process window is evaluated. Considering all the aspects, simulations indicate that the low-n attPSM has the best patterning performance both for dark-field mask with SRAFs and bright-field mask without SRAFs.

As device scaling continues, development of photoresists with low pattern roughness and high sensitivity has become challenging. One obstacle that delays the material development process is the discrepancy between postdevelop and postetch pattern roughness, where a photoresist with high postdevelop patterning performance shows poor postetch pattern roughness. Herein, we demonstrate that pattern roughness after a nontrim etch process can be accurately estimated by a multiple regression analysis of a power spectral density (PSD) variable of postdevelop roughness and an etch resistance parameter of resins. The nontrim etch process here refers to an etch condition which leads to increased pattern roughness. Unbiased line width roughness (LWR) shows the highest correlation with postetch LWR among postdevelop roughness PSD variables. An etch resistance parameter also correlates well with postetch roughness. A multiple regression analysis reveals that the contributions of postdevelop unbiased LWR and etch resistance to the postetch LWR are 59% and 41%, respectively. Based on the calculated contributions, postetch LWR is estimated with a high accuracy (R²  >  0.93). This estimation method allows for an efficient material screening at a lithography level without assessing postetch patterning performance, thus the process of material development could be accelerated.

We describe the rationale for selecting graphite as a substrate material suitable for manufacturing curved high-aspect ratio metallic x-ray gratings and experimentally validate that its properties satisfy requirements relevant for clinical phase-contrast and dark-field x-ray imaging. Selection criteria applied to two candidate materials graphite and polyimide were compliance to bending, mechanical tenacity of the attachment of the lamellar grating structure to the substrate, the substrate material’s x-ray robustness, and the compatibility with the x-ray LIGA process used to manufacture the grating structures. In contrast to other standard materials such as silicon wafers with titanium layer, graphite wafers could be bent to smaller radii and are natively electrically conductive. While polyimide wafers allowed for even smaller bending radii, we found their high risk of grating structure detachment to be a strong detractor. Minimum achievable bending radii were 55 and 70 mm for pure graphite wafers and graphite wafers with mounted grating structure, respectively. Electron microscopy of graphite surface and cross-sections revealed a fine porous structure conducting to a very stable metal–wafer interface. Grating structures with heights of more than 200  μm were bonded to graphite wafers and their integrity confirmed in flat as well as in bent state using microfocus x-ray imaging. We conclude that graphite is a very well-suited substrate material for manufacturing curved x-ray gratings.