The materials challenge behind High-NA EUV readiness

As the semiconductor industry moves toward high-numerical aperture (NA) extreme-ultraviolet (EUV) lithography, much of the attention has focused on exposure tools, optics, patterning performance, and the promise of continued transistor scaling. One of the most significant advances in lithography, High-NA systems enable the tighter patterning requirements needed for future logic and memory devices. Lithography tools alone, however, will not determine the success of the next scaling era.

By Li Yang, John Collins, Christopher Caroff, and Thao Nguyen, Gelest, a Mitsubishi Chemical Group company

Increasingly, semiconductor manufacturers are recognizing that materials readiness has become one of the most critical enablers of High-NA EUV adoption.

The industry’s ability to achieve the required levels of resolution, line-edge roughness, process window, defectivity, and manufacturability will depend not only on advances in optics and patterning schemes, but also on the underlying materials that make those processes possible.

Among the most promising candidates for future High-NA EUV processes are metal oxide resists (MORs) and dry resist platforms. These advanced materials offer potential advantages in EUV sensitivity, pattern fidelity, and process control. However, they also introduce new challenges involving precursor chemistry, impurity control, analytical characterization, thermal stability, and manufacturing scalability.

As feature dimensions continue to shrink and process windows narrow, seemingly insignificant variations in precursor chemistry can have outsized effects on lithographic performance. The path to High-NA EUV therefore requires a broader view—one that encompasses not only lithography systems, but the entire materials ecosystem supporting them.

Why materials matter more for High-NA
The increased numerical aperture of next-generation EUV systems promises improved imaging resolution, but it also increases sensitivity to process variability defects that could limit yield. Likewise, variations in material composition that were once insignificant can affect pattern formation, roughness, critical dimension control, and process repeatability.

This means the requirements for resist materials extend beyond basic photosensitivity. They must be more precisely engineered to provide consistent performance across increasingly demanding manufacturing environments while maintaining the stability and purity necessary for high-volume production.

This challenge has helped drive interest in MOR systems, particularly tin-based chemistries, which offer attractive properties for EUV applications. One reason tin has emerged as a leading candidate for MOR development is its strong interaction with EUV radiation. Because EUV photons are expensive to generate, efficient absorption is a critical requirement for photoresist materials. Tin can efficiently capture incoming energy at the 13.5 nm wavelength used in EUV lithography, generating the reactions necessary for pattern formation. This makes it particularly attractive for next-generation MOR architectures.

Although spin-on MOR approaches have demonstrated promising lithographic performance, they also reveal a key challenge: moisture sensitivity. Moisture present in solvents can react with precursors, introducing embedded defects and impacting film quality that become increasingly problematic as dimensions shrink. These limitations have contributed to growing interest in dry resist approaches.

Dry resist precursors and vapor-phase processing
Dry resist technologies represent a fundamentally different approach to resist formation. Rather than relying on spin-coated liquid materials, dry resist systems use chemical vapor deposition (CVD) techniques to form resist layers directly on the wafer. One of the major advantages of this approach is the ability to precisely engineer precursor chemistry. The selection of ligands - often referred to as leaving groups - plays a central role in determining water reactivity, chemical stability, deposition characteristics, and storage requirements.

These design choices create a complex optimization problem. Greater reactivity may improve deposition performance but can reduce storage stability. Successful precursor development therefore requires balancing multiple competing requirements simultaneously.

Dry resist systems offer additional benefits including tighter film thickness control, reduced particle generation and potentially broader process windows. Combined with dry development techniques, they have demonstrated the potential for improved roughness performance and reduced exposure dose requirements relative to conventional chemically amplified resists. However, these benefits can only be realized if the underlying precursor materials meet extremely demanding purity standards.

As semiconductor geometries shrink, the tolerance for contamination continues to decrease. For advanced CVD processes, precursor purity becomes a first-order manufacturing concern. Even trace levels of impurities can alter deposition behavior, introduce defects, or affect lithographic performance.

Tin chemistry presents unique challenges in this regard. Several synthetic routes exist for preparing tin-based precursors, including metathesis reactions and redistribution pathways. Unfortunately, these processes can generate a range of closely related impurity species that may be difficult to detect using conventional analytical methods.

From a manufacturing perspective, identifying and controlling these impurities is essential. Understanding impurity profiles helps researchers develop purification strategies, improve synthesis processes, and deliver materials suitable for high-volume semiconductor production. The challenge is not simply achieving high purity once; it is achieving it repeatedly, consistently, and at manufacturing scale.

Historically, semiconductor materials have relied heavily on analytical techniques such as gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS), high-performance liquid chromatography (HPLC), inductively coupled plasma mass spectrometry (ICP-MS), and thermal analysis methods.

For many organometallic tin precursors, traditional chromatography presents significant limitations. Some tin compounds decompose before vaporization under atmospheric conditions, making GC-based analysis difficult or impossible. Likewise, many of these materials are intentionally designed to be highly reactive with water, creating challenges for HPLC methods that would require extremely dry mobile phases and analytical environments. These limitations necessitate alternative approaches capable of providing both structural information and impurity detection with high sensitivity.

The Growing Importance of ¹¹⁹Sn NMR
One analytical technique that has emerged as particularly valuable for tin precursor development is Tin-119 (¹¹⁹Sn) nuclear magnetic resonance (NMR) spectroscopy. Unlike traditional chromatography methods, ¹¹⁹Sn NMR can provide detailed insight into precursor composition without requiring destructive testing or complex sample preparation. The technique offers parts-per-million sensitivity and high spectral resolution, enabling researchers to identify impurity species that may otherwise remain undetected.

For precursor developers, this capability is increasingly important. As lithography processes become more sensitive to material variation, the ability to accurately characterize trace impurities becomes essential for process optimization, manufacturing readiness, and product performance. Researchers have demonstrated that analytical parameters must themselves be carefully selected and controlled; variations in measurement conditions can negatively influence the ability to consistently detect low-concentration impurities.

In this sense, advanced characterization is a competitive differentiator. The industry’s ability to understand materials at a deeper level directly impacts its ability to manufacture them reliably. Characterization alone is not enough; High-NA EUV materials must ultimately transition from laboratory-scale research to industrial-scale manufacturing. This introduces another set of requirements centered on thermal stability, supply chain robustness, and production consistency.

Process window and thermal stability
For CVD precursors, thermal stability directly influences storage, transportation, and process performance. Materials must remain stable under operational conditions while maintaining the volatility and reactivity required for vapor-phase delivery and deposition.

Recent work with next-generation tin precursors demonstrates how detailed stability studies can help identify optimal synthesis and manufacturing conditions. By understanding how process variables influence precursor stability, suppliers can improve consistency and reduce batch-to-batch variation.

This focus on manufacturing readiness is becoming increasingly important as semiconductor manufacturers evaluate materials for future production nodes. Qualification decisions are no longer based solely on lithographic performance; suppliers must also demonstrate the ability to provide secure, scalable, and repeatable production capabilities suitable for supporting high-volume manufacturing.

Designing the next generation of precursors
The search for improved MOR materials continues. Researchers are actively exploring new precursor designs intended to improve sensitivity, reduce exposure dose requirements, and enhance patterning performance.

One area of investigation involves understanding how bond dissociation energies influence ligand removal during EUV exposure. The ability to efficiently cleave EUV-reactive ligands can affect resist sensitivity and overall lithographic performance. Although bond dissociation energy represents only one factor among many, it serves as a useful early-stage indicator when screening new molecular precursors.

Recent development efforts have already produced multiple generations of candidate precursors exhibiting improved lithographic characteristics, including reductions in exposure dose and improvements in pattern quality metrics. In addition to tin, researchers are investigating other high-absorption materials that could contribute to next-generation resist platforms as the industry advances toward the 2 nm era and beyond.

Collaboration vital to success
Perhaps the most important lesson emerging from High-NA EUV development is that no single company or technology can solve these challenges alone.

Lithography suppliers, materials developers, semiconductor manufacturers, research organizations, and equipment providers must work together to accelerate innovation cycles and align technology roadmaps.

The interdependence between lithography performance, materials chemistry, and manufacturing capability is becoming increasingly apparent. Progress in one area often depends on advances in another.

For materials suppliers, closer engagement with device manufacturers provides valuable feedback that can guide precursor development. For semiconductor manufacturers, early collaboration helps ensure materials are optimized for future process requirements before large-scale deployment begins.

The transition to High-NA EUV lithography represents more than a lithography upgrade; it is an ecosystem challenge that demands coordinated innovation across multiple disciplines.

The next wave
The semiconductor industry often measures progress through the capabilities of its most visible tools. High-NA EUV scanners undoubtedly represent a remarkable engineering achievement and will play a central role in future scaling.

The ability to fully realize the benefits of High-NA EUV lithography may depend just as much on less visible innovations occurring within the materials supply chain. High-purity precursors, advanced analytical methods, scalable manufacturing processes, and collaborative development models are all emerging as critical elements of future lithography success.

As the industry prepares for the next generation of semiconductor manufacturing, materials readiness has become every bit as important as optical readiness. Gelest, for example, has constructed a 50,000-square-foot facility at its U.S.-based headquarters that will increase the company’s production of precursor chemicals for use in dry-resist EUV lithography - broadening its ability to support advanced markets such as microelectronics, medical devices and automation. Addressing today the need to position themselves for the optical future will enable companies like Gelest to define the success of High-NA EUV lithography going forward.