Holistic litho-etch development to address patterning challenges towards high NA EUV

Extreme ultraviolet (EUV) lithography is crucial to enabling technology scaling in pitch and critical dimension (CD). Currently, one of the key challenges of introducing EUV lithography to high volume manufacturing (HVM) is throughput, which requires high source power and high sensitivity chemically amplified photoresists. Important limiters of high sensitivity chemically amplified resists (CAR) are the effects of photon shot noise and resist blur on the number of photons received and of photoacids generated per feature, especially at the pitches required for 7 nm and 5 nm advanced technology nodes. These stochastic effects are reflected in via structures as hole-to-hole CD variation or local CD uniformity (LCDU). Here, we demonstrate a synergy of film stack deposition, EUV lithography, and plasma etch techniques to improve LCDU, which allows the use of high sensitivity resists required for the introduction of EUV HVM. Thus, to improve LCDU to a level required by 5 nm node and beyond, film stack deposition, EUV lithography, and plasma etch processes were combined and co-optimized to enhance LCDU reduction from synergies. Test wafers were created by depositing a pattern transfer stack on a substrate representative of a 5 nm node target layer. The pattern transfer stack consisted of an atomically smooth adhesion layer and two hardmasks and was deposited using the Lam VECTOR PECVD product family. These layers were designed to mitigate hole roughness, absorb out-of-band radiation, and provide additional outlets for etch to improve LCDU and control hole CD. These wafers were then exposed through an ASML NXE3350B EUV scanner using a variety of advanced positive tone EUV CAR. They were finally etched to the target substrate using Lam Flex dielectric etch and Kiyo conductor etch systems. Metrology methodologies to assess dimensional metrics as well as chip performance and defectivity were investigated to enable repeatable patterning process development. Illumination conditions in EUV lithography were optimized to improve normalized image log slope (NILS), which is expected to reduce shot noise related effects. It can be seen that the EUV imaging contrast improvement can further reduce post-develop LCDU from 4.1 nm to 3.9 nm and from 2.8 nm to 2.6 nm. In parallel, etch processes were developed to further reduce LCDU, to control CD, and to transfer these improvements into the final target substrate. We also demonstrate that increasing post-develop CD through dose adjustment can enhance the LCDU reduction from etch. Similar trends were also observed in different pitches down to 40 nm. The solutions demonstrated here are critical to the introduction of EUV lithography in high volume manufacturing. It can be seen that through a synergistic deposition, lithography, and etch optimization, LCDU at a 40 nm pitch can be improved to 1.6 nm (3-sigma) in a target oxide layer and to 1.4 nm (3-sigma) at the photoresist layer.

As minimum feature size shrinks to a metal pitch of 21 nm, the current extreme ultra violet (EUV) lithographic tool with a numeric aperture (NA) of 0.33 will face resolution limit for some critical layers. High NA (0.55) EUV with anamorphic optics or EUV double patterning (DP) at 0.33 NA are being considered for the next generation of lithographic technology. Both the high NA EUV system and EUV DP will enhance resolution relative to current EUV single patterning (SP). Nevertheless, in order to be able to compare EUV DP and High NA EUV processes, important lithographic factors including image contrast, mask three dimension (M3D) effects, process variation band, stochastic effects and local critical dimension uniformity need to be investigated to understand their contributions to process variations. This study was carried out using rigorous lithographic model simulations in Sentaurus Lithography, where strong M3D effects in EUVL are computed physically. We have simulated patterns with both isomorphic and anamorphic optical proximity corrections (OPC) using the rigorous model. The study focuses on 3nm node Via layer designs. These vias need to connect to metal features which have pitches of 21 nm. Simulation results using 0.33 NA SP, 0.33 NA DP, and 0.55 NA anamorphic SP are presented. The benefit of using an alternative mask absorber and a thinner resist as well as the impact of stochastic effects have also been explored. Although a 0.55 NA EUV is expected to produce a superior image to 0.33 NA EUV and to have less impact from overlay errors and stochastic effects, an analysis of process margins of 0.33 NA EUV SD and DP versus 0.55 NA anamorphic systems helps to better understand the benefits, challenges and optimal insertion point for introducing High-NA EUV.

Extreme ultraviolet (EUV) lithography with 13.5 nm wavelength is the main option for sub-10nm patterning in the semiconductor industry. We report improvements in resist performance towards EUV high volume manufacturing. A local CD uniformity (LCDU) model is introduced and validated with experimental contact hole (CH) data. Resist performance is analyzed in terms of ultimate printing resolution (R), line width roughness (LWR), sensitivity (S), exposure latitude (EL) and depth of focus (DOF). Resist performance of dense lines at 13 nm half-pitch and beyond is shown by chemical amplified resist (CAR) and non-CAR (Inpria YA Series) on NXE scanner. Resolution down to 10nm half pitch (hp) is shown by Inpria YA Series resist exposed on interference lithography at the Paul Sherrer Institute. Contact holes contrast and consequent LCDU improvement is achieved on a NXE:3400 scanner by decreasing the pupil fill ratio. State-of-the-art imaging meets 5nm node requirements for CHs. A dynamic gas lock (DGL) membrane is introduced between projection optics box (POB) and wafer stage. The DGL membrane will suppress the negative impact of resist outgassing on the projection optics by 100%, enabling a wider range of resist materials to be used. The validated LCDU model indicates that the imaging requirements of the 3nm node can be met with single exposure using a high-NA EUV scanner. The current status, trends, and potential roadblocks for EUV resists are discussed. Our results mark the progress and the improvement points in EUV resist materials to support EUV ecosystem.

An Extreme Ultra-Violet (EUV) photo mask is a complex engineering marvel. It is integral to semiconductor manufacturing, as it holds the design layout information intended for the device. The mask complexity allows many potential points of variation during the mask manufacturing process. Some of these deviations from the ideal mask remain benign to wafer patterning but others impact the wafer pattern. To date, reported studies have characterized the impact of local mask defects in the absorber pattern and within the multilayer mirror of an EUV mask [1], as well as various non-local mask deficiencies such as multilayer mirror surface roughness and contamination growth [2]. The results revealed relationships between mask quality limitations and stochastic failures on the wafer. The studies to date were based on line-and-space patterns to reduce optical complexity and were completed using NA 0.33. The study proposed will extend the research effort into two-dimensional (2D) structures such as contacts, via, and tip-to-tip containing line/space structures, to further understand the impact of mask defects in complex imaging configurations. The study will also investigate these 2D structures patterned with high-NA EUV lithography. As a start, one-dimensional structures will be imaged with high-NA EUV using conditions from previous studies to provide a continuous baseline. The introduction of 2D structures brings optical and computational complexities into the study. The optics contain more complex information than found in the line/space patterns used in previous studies. This may lead to additional impacts not observed in previous work. In addition, due to the complexities of these patterns, the computational effort becomes larger, which may necessitate improved algorithms or learning methodologies. Additionally, in the anamorphic high-NA system the choice of pattern orientation in the 4x or 8x direction is thought to be based on critical feature type such as tip-to-tip or minimum CD, but the introduction of stochastic mask analysis may influence this preference. This work is expected to contribute to a better understanding of which mask quality aspects must be tightened for high-NA EUV.

Extreme ultraviolet (EUV) materials are deemed as critical to enable and extend the EUV lithography technology. Currently both chemically amplified resist (CAR) and metal-oxide resist (MOR) platforms are candidates to print tight features on wafer, however patterning requirements, process tonality (positive or negative), illumination settings and reticle tonality (dark or bright) play a fundamental role on the material performance and in consequence on the material choice. In this work we focus on the patterning of staggered pillars using a single EUV exposure, and this by looking at the lithographic and etching performance of CAR and MOR platforms, using metrics as process window, local critical dimension uniformity (LCDU), pillar edge roughness (PER), pillar placement error (PPE) and (stochastic) nano-failures. As a bright field reticle shows a lower aerial image contrast to print pillars compared to the aerial image of contact holes using a dark field reticle, we also investigate alternative patterning solutions such as the tone reversal process (TRP) to pattern pillars from contact holes.

Implementation of extreme ultra violet lithography (EUVL) for printing features of dimensions around 32nm and below has faced multiple challenges that have been overcome mostly through tool and process engineering. Currently, the main concern for implementation of EUVL in high volume manufacturing is related to stochastic defects, such as line bridges or breaks, which may occur randomly and have an impact on device performance.1 Photon shot noise, in addition to the inherent process variability and material non-homogeneities can lead to defects on the final patterned structures. As the pitch and critical dimension (CD) approach 32nm pitch and below, the dependency of stochastics defects increases exponentially. This paper fits within imec’s EUV stochastics and defectivity program, which investigates the key parameters for the generation and mitigation of defects from both a theoretical and practical approach. Initially, a reference process has been established and a methodology has been developed to capture and characterize bridge-type defects after pattern transfer into relevant layers for metrology and applications. Optical inspection and scanning electron microscopy (SEM) review are used to collect information on the number and types of defects present over large patterned areas (in the order of cm2). With a well-defined inspection technique, experiments related to fundamentals as well as materials and processes are continuously run. In the past years, imec has been leading the fundamental work on stochastic defects.2,3 De Bisschop et al. identified the “defect cliff”, or the exponential dependency of defect density as a function of space CD for multiple parameters, such as resist formulation and exposure/imaging conditions. More recently, the focus has changed to the investigation of the “defect floor”, which as the name suggests, is a flat response of defects to CD. The defect floor suggests that defectivity cannot be further improved, making one of the most relevant problems to solve for implementation of EUV in high volume manufacturing (HVM). Our results show a decomposition of the potential contributors to explore the effect of processing conditions related to scanner, track and pattern transfer on final defect density for a chemically amplified resist and attempt a comparison to a metal-oxide resist. Goal is to understand the contributors to the defect floor and identify an effective route to avoid/minimize defect formation.

Understanding, characterization and management of 3D mask effects, including non-telecentricity, contrast fading and best focus shifts, become increasingly important for the performance optimization of future extreme ultraviolet (EUV) projection systems and mask designs. Novel imaging configuration and central obscuration in high NA EUV projection systems introduce additional imaging effects. A simplified coherent imaging model, rigorous mask diffraction simulations, images for individual source areas and a hybrid mask model are employed to analyze the root causes of observed imaging artifacts. Based on this, several image enhancement strategies including modifications of mask and source are devised and investigated for lines/spaces.

Extreme ultraviolet (EUV) lithography is considered to be the most effective strategy for realize 7 nm generation manufacturing and beyond. A key factor for the realization of EUV lithography is the choice of EUV resist material that is capable of resolving below 15-nm half pitch with high sensitivity. Chemical Amplified Resist (CAR) using positive-tone development (PTD) is still one of the strongest candidates for EUV lithography. Recently, some researchers have reported concerns on the limitations in the performance of CAR materials. Consequently, there is a critical need for new chemistry and development of new resist materials. However, new resist materials still have lots of concerns for manufacturing, such like a non-CAR materials including metal resist. Therefore, CAR materials are still most important items for EUV lithography manufacturing. We’ve been developing negative-tone imaging (organic solvent development) with EUV exposure (EUV-NTI) for a long time. EUV-NTI has advantages for line-width roughness (LWR) due to their low swelling and dissolving smoothly. New EUV-NTI performances will be shown and also process condition progress will be updated. Also, the basic study will be reported, which is high absorption unit including materials for improving stochastic effect. We report herein recent progress of CAR materials, both positive-tone and negative-tone for EUV lithography.

Extreme ultraviolet (EUV) lithography is considered to be the most effective strategy for realize 7 nm generation manufacturing and beyond. With recent rapid progress on the source power improvement, process and material explorations are more and more accelerated to achieve HVM requirements. Therefore, a key factor for the realization of EUV lithography is the choice of EUV resist material that is capable of resolving below 15-nm half pitch with high sensitivity. It has been thought that the main challenge of EUVL is satisfying Resolution (R) – LWR (L) – Sensitivity (S) requirements simultaneously. On the other hand, recent reports indicate defectivity caused by stochastic effect have to consider as well as RLS-tradeoff improvement. For 7 nm node application, traditional chemically amplified resist (CAR) system is first candidate because of its well-studied property through KrF and ArF manufacturing process for many years, e.g. stability, metal contamination and post-litho process compatibility. However its low EUV absorption property enhancing photon-shot-noise (PSN) effect is one major disadvantage of conventional CAR system, especially for low dose resist process construction. In the past, several researches aiming high EUV absorption CAR system were reported, but material design and its lithographic performance still need detailed investigation. The present study aims to clarify how we can overcome RLS-tradeoff and reduce defects. Accordingly, we investigated the PSN effect using progress dependency study and material design requirements to achieve high EUV absorption resists. Lithographic results of high EUV absorption resists based on the study were also described.

Initial readiness of extreme ultraviolet (EUV) patterning has been demonstrated at the 7-nm device node with the focus now shifting to driving the “effective” k1 factor and enabling the second generation of EUV patterning. In current EUV lithography, photoresist thicknesses <30 nm are required to meet resolution targets and mitigate pattern collapse. Etch budgets necessitate the reduction of underlayer thickness as well. Typical spin-on underlayers show high defectivity when reducing thickness to match thinner resist. Inorganic deposited underlayers are lower in defectivity and can potentially enable ultrathin EUV patterning stacks. However, poor resist-inorganic underlayer adhesion severely limits their use. Existing adhesion promotion techniques are found to be either ineffective or negatively affect the etch budget. Using a grafted polymer brush adhesion layer, we demonstrate an ultrathin EUV patterning stack comprised of inorganic underlayer, polymer brush, and resist. We show printing of sub-36-nm pitch features with a good lithography process window and low defectivity on various inorganic substrates, with significant improvement over existing adhesion promotion techniques. We systematically study the effect of brush composition, molecular weight, and deposition time/temperature to optimize grafting and adhesion. We also show process feasibility and extendibility through pattern transfer from the resist into typical back end stacks.

Extreme ultraviolet (EUV) patterning appears feasible using currently available EUV exposure tools, but some issues must still be resolved for EUV patterning to be used in production. Defects in EUV mask blanks are one such major issue, as evidenced by the research focused on defect printability. Inspection tools are needed to detect phase defects on EUV mask blanks that could possibly print on the wafer. Currently available inspection tools can capture defects on the mask, but they also need to be able to classify possible printable defects. Defect classification for repair and mitigation of printable defects is very difficult using DUV inspection tools; however, if the actinic inspection tool (AIT) could gather defect information from more multilayer stacks, it may be able to separate printable defects from unprintable defects. If unprintable defects could be eliminated, the defect information could be used for mask pattern shifts to reduce printable defects. Fewer defects would need to be repaired if there were a better chance of capturing printable defects using an actinic inspection tool. Being able to detect printable defects on EUV blanks is therefore critical in mask making. In this paper, we describe the characterization of native phase defects in the manufacturing of EUV mask blanks using the state-of-the-art mask metrology equipment in SEMATECH's Mask Blank Development Center (MBDC). Commercially available quartz substrates were used and Mo/Si multilayers were deposited on the substrates to characterize phase defects. Programmed defects of various dimensions were also prepared using e-beam patterning technology on which multilayers were deposited. Transmission electron microscopy (TEM) was used to study multilayer profile changes, while SEMATECH's AIT was used to image defects and predict their printability. A defect library for native defects and printability of programmed phase defects is introduced. Finally technical challenges for EUV defect printability will be discussed.

Metal containing resists (MCR) are one of the candidates for Extreme Ultraviolet (EUV) resists aiming to achieve the resolution, linewidth roughness and sensitivity (RLS) requirements of advanced design nodes. MCRs intrinsically have high etch resistance owing to their metal content. Therefore, low resist thickness (~18nm) is sufficient to transfer resist patterns into an underlying hard mask. Also, the thin resist reduces susceptibility to pattern collapse during development because of low aspect ratio. However, thus far little attention has been paid to optical metrology and inspectability (overlay, defect inspection, scatterometry, etc.) of these resists, which is another critical requirement to move MCR toward high volume manufacturing (HVM). In this paper, we investigate overlay metrology and overlay correction with MCR. Even though the optical contrast for MCR is slightly lower than for chemically amplified resist (CAR) it seemed sufficient for high-quality overlay metrology. However, the measurement precision for MCR is deteriorated compared to that for CAR, resulting in significantly higher residuals. The root cause of the deteriorated measurement precision was found in grains in the optical image after MCR development. Interestingly, the after etch performance of CAR and MCR is identical. We demonstrate that with sufficient sampling, appropriate correctables can be extracted from the MCR overlay results. Finally, we discuss how the increased image noise can be compensated by the applied sampling scheme.

Extreme ultraviolet (EUV) lithography has emerged as a promising candidate for the manufacturing of semiconductor devices at the sub-14nm half pitch lines and spaces (LS) pattern for 7 nm node and beyond. The success of EUV lithography for the high volume manufacturing of semiconductor devices depends on the availability of suitable resist with high resolution and sensitivity. It is well-known that the key challenge for EUV resist is the simultaneous requirement of ultrahigh resolution (R), low line edge roughness (L) and high sensitivity (S). In this paper, we investigated and developed new chemically amplified resist (CAR) materials to achieve sub-14 nm hp resolution. We found that both resolution and sensitivity were improved simultaneously by controlling acid diffusion length and efficien-cy of acid generation using novel PAG and sensitizer. EUV lithography evaluation results obtained for new CAR on Micro Exposure Tool (MET) and NXE3300 system are described and the fundamentals are discussed.

For high volume manufacturing (HVM) utilizing extreme ultraviolet (EUV) lithography, practical resist outgassing qualification system is required. Witness sample (WS) testing systems using electron beam (EB) or low power EUV light have been proposed as candidates, however some issues remain on how these alternative light sources, in comparison to high power EUV, will affect resist chemical reactions and ultimately resist outgassing. In this paper, we have investigated resist induced optics contamination by utilizing two types of WS test systems of high power EUV light and EB sources. A correlation between these light sources is discussed, especially focusing on the resulting chemical phenomena depending on resist material properties.

This paper demonstrates a new simulation-based methodology for optimizing critical dimension (CD) bias for contact holes (CH) arrays using several different extreme ultraviolet (EUV) resists that were fully calibrated and verified with physical resist models. The bias for CH was optimized using local CD uniformity (CDU) 3-sigma as a cost function. The CD sigma variations of near-neighbor contact holes were simulated as a function of dose-to-size and mask bias, averaged over a large number of stochastic trials. There is a distinct bias for minimum CD sigma accompanied by an increase in the process window. The results are confirmed with wafer data. We will discuss the results in terms of EUV photon shot noise coupled with resist parameters. The simulation results will be used to predict a parameter space for EUV resist that can optimize line edge roughness (LER)/resolution/process window and CDU. Finally, various tradeoffs will be presented that will enable the process to perform in a high volume manufacturing environment.