How EUV Resist Outgassing Puts Optics at Risk
The subtle release of gas from materials, known as outgassing, poses a significant challenge to the production of the advanced chips that power our modern devices.
Imagine a manufacturing process where the very materials used to create microscopic patterns on computer chips could gradually destroy the multimillion-dollar equipment itself. This is the paradoxical challenge of extreme ultraviolet (EUV) lithography, the technology that enables the production of the advanced chips in our smartphones, laptops, and AI systems. At the heart of this challenge lies a phenomenon called "resist outgassing" – where the light-sensitive materials (photoresists) used to pattern silicon wafers release volatile chemicals when exposed to EUV light 2 .
Cost of a single EUV scanner
Potential cost of contamination-related downtime
These released chemicals can form thin, contaminating layers on the most expensive and precise components of an EUV tool: its mirrors. With a single EUV scanner costing over $150 million, and contamination-related downtime potentially costing manufacturers $500,000 per day, controlling this invisible threat has become a critical battleground in semiconductor manufacturing 8 . This article explores how scientists are working to understand and control resist outgassing to keep the mirrors clean and the future of Moore's Law on track.
Extreme ultraviolet lithography represents a revolutionary leap in semiconductor manufacturing. Using light with a wavelength of just 13.5 nanometers (compared to 193 nm for previous technologies), EUV can create incredibly fine patterns on silicon chips 4 . This capability is essential for continuing the miniaturization of transistors that follows Moore's Law.
Organic compounds form carbon layers on mirror surfaces, reducing their reflectivity 8 .
Certain elements, particularly from the photoacid generators (PAGs) in chemically amplified resists, can create permanent contamination that requires costly tool disassembly and cleaning 3 .
The economic impact is substantial – even minor contamination can reduce production yields by 15-20%, translating to millions in lost revenue per production line 8 .
Because EUV light is strongly absorbed by all materials, including air, the entire process must occur in a vacuum 4 . This vacuum environment creates the perfect conditions for outgassed molecules to travel freely and adhere to critical optics.
The semiconductor industry primarily works with two broad categories of EUV photoresists, each with distinct outgassing behaviors:
These complex materials contain a polymer base along with photoacid generators (PAGs) – compounds that release acid when exposed to EUV radiation 7 .
These materials encompass various alternative chemistries, including metal-oxide resists and other innovative approaches 3 .
Research has revealed that in CAR systems, the PAG cation is a key component contributing to contamination 3 . When exposed to high-energy EUV photons, these materials can undergo complex breakdown processes, including the recently discovered EUV-induced breakdown of perfluoroalkyl substance (PFAS) photoacid generators 7 .
Interestingly, some non-CAR materials like inorganic resists can actually have beneficial outgassing – one study found that resist outgassing containing water and oxygen could remove carbon contamination 3 .
The specific chemical products released during resist outgassing depend significantly on the energy of the incident radiation 5 . Research comparing outgassing from deep-ultraviolet (DUV), vacuum UV (VUV), EUV, and beyond-EUV (BEUV) irradiation has revealed that:
For PMMA (a model resist material), the partition of outgassing to the main-chain scission pathway was 61.5% upon DUV/VUV irradiation 5 .
Side-chain cleavage accounted for 83% and 71% of species outgassed due to EUV and BEUV exposure respectively 5 .
The distribution of outgassed species shows no significant dependence on resist film thickness in the 50-125 nm range, but is strongly energy-dependent 5 .
A groundbreaking 2025 study deployed table-top EUV photoemission spectroscopy to observe chemical changes occurring during exposure in a model chemically amplified photoresist 7 . This innovative approach allowed researchers to track chemical dynamics in real-time, revealing previously unobserved reaction pathways.
Researchers studied a variant of the Environmentally Stable Chemically Amplified Photoresist (ESCAP) platform 7 .
Using a coherent table-top EUV source based on high-harmonic generation 7 .
Advanced simulation framework to interpret changes in the EUV photoemission spectrum 7 .
The combined experimental and theoretical approach revealed a surprising discovery: the EUV-induced breakdown of PFAS photoacid generators – critical components that drive the acid-catalyzed deprotection mechanism in CAR materials 7 .
| Resist Type | Detected Outgassing Products | Contamination Risk |
|---|---|---|
| CAR Systems | PAG fragments, PFAS breakdown products (HF, CF₃, H₂SO₂), benzene, biphenyl, diphenylsulfide 2 7 | High risk of non-cleanable contamination 3 |
| Non-CAR Systems | Aromatic compounds (in poly-olefin sulfones) 3 | Varies by material; some pose contamination risk |
| Inorganic Resists | Water, oxygen 3 | Beneficial - can remove carbon contamination |
This previously unobserved PFAS breakdown pathway could be a significant source of chemical stochastic defect generation in EUV lithography, potentially reducing device yield in high-volume manufacturing processes 7 .
Accurately measuring resist outgassing has been a persistent challenge for researchers. Various methods have been developed, each with strengths and limitations:
| Method | Key Measurements | Advantages | Limitations |
|---|---|---|---|
| Residual Gas Analysis (RGA) | Identifies and quantifies specific gaseous species 3 5 | Provides chemical specificity | May lack sensitivity for some compounds |
| Pressure Rise Method | Overall pressure increase in vacuum chamber 2 | Simple principle, direct measurement | Results vary between setups; requires compensation values 2 |
| Witness Sample Testing | Direct measurement of contamination on test surfaces 3 | Measures actual contamination potential | Time-consuming; may not capture all contamination mechanisms |
| GC-MS Method | Specific organic compounds like isobutene, benzene 2 | High specificity for identifiable compounds | Cannot detect certain compounds like CO₂ without modifications 2 |
Early comparisons between the pressure rise and GC-MS methods showed that results varied greatly depending on the evaluation method utilized 2 . This highlighted the need for standardized quantification approaches, leading researchers to develop compensation values based on factors affecting each method to improve reliability 2 .
The semiconductor industry has developed multiple approaches to address the challenge of resist outgassing:
| Strategy Category | Specific Approaches | Mechanism of Action |
|---|---|---|
| Resist Material Innovations | Low-outgassing polymer platforms, hybrid CAR/non-CAR systems, metal-oxide resists 6 8 | Reduces volatile components at source; designs materials with cleaner breakdown pathways |
| Tool Protection Systems | Dynamic gas lock (DGL) membranes, hydrogen ambient, in-situ cleaning technologies 5 8 | Creates barrier between resist and optics; enables continuous cleaning of contaminated surfaces |
| Process Optimization | Controlled pre-bake conditions, specialized development solutions, optimized exposure parameters 6 | Removes residual solvents and weakly bound molecules before exposure; optimizes reaction pathways |
The challenge of resist outgassing in EUV lithography exemplifies the incredibly complex interplay of materials science, chemistry, and engineering required to continue semiconductor miniaturization. What might seem like a minor issue – the release of tiny amounts of gas from thin films – has profound implications for the multi-billion-dollar semiconductor industry.
Future developments will likely focus on in-situ cleaning technologies that can restore contaminated optical surfaces without requiring system disassembly, and the creation of holistic contamination management ecosystems where contamination is controlled at every stage from resist development through exposure and post-processing 8 .
The ongoing battle against resist outgassing highlights a fundamental truth in advanced technology: as we push further into the nanoscale world, understanding and controlling seemingly minor phenomena becomes increasingly critical to maintaining progress. The future of Moore's Law may well depend on our ability to keep EUV mirrors clean from resist outgassing – an invisible but formidable adversary in chip manufacturing.