The battle of immersion‑DUV was not yet over; the "curse" of the optical lens still hung like a sword of Damocles. Yet the march of technology allowed no pause. Driven and led by Xiuxiu, a more forward‑looking, more challenging pre‑research project quietly launched within the R&D center—exploration of **Extreme Ultraviolet (EUV) Lithography Technology**.
If DUV lithography was art of fine engraving in the micro‑world, EUV lithography meant venturing into an utterly different, more extreme and unfamiliar domain where physical rules themselves were distinct. The team's focus began shifting partly toward this ultimate Everest of the semiconductor industry for strategic advance. And the first step in climbing this peak was not directly attacking the most dazzling light source or most precise optical system, but building a seemingly basic yet crucial environment—**ultra‑high vacuum**.
At the team's first EUV‑special‑topic technical seminar, Xiuxiu stood before a whiteboard and wrote two key numbers: **13.5 nm**. This was the working wavelength used in EUV lithography.
"Everyone, I know you still vividly remember the challenges in the DUV field," Xiuxiu's gaze swept over each core member present, their faces bearing fatigue from overcoming difficulties, also alert toward unknown territory. "But EUV is different from everything we know. Its first, most fundamental difference shows up in this wavelength."
She paused, letting the number sink in.
"The DUV we're familiar with uses deep‑ultraviolet light of 193 nm wavelength. Light in this band, propagating through air, though also absorbed and scattered, does so relatively lightly, allowing us to perform lithography in atmospheric or near‑atmospheric pure‑air environments."
"But **13.5 nm extreme‑ultraviolet light is a completely different story.**" Xiuxiu's tone grew exceptionally serious. "Photons at this wavelength have extremely high energy, almost matching the inner‑electron transition energy levels of most atoms. What does that mean?"
She drew a simple schematic on the whiteboard showing a beam of EUV light propagating through air.
"It means that when 13.5 nm photons pass through air filled with nitrogen, oxygen, water vapor, and other trace components, they interact with these gas molecules extremely efficiently—**not simple scattering, but strong absorption!**"
She cited specific data: "Experiments show that light of 13.5 nm wavelength at standard atmospheric pressure attenuates to less than one percent of its initial intensity after propagating just **a few millimeters**! In other words, **air is almost an opaque 'wall' to EUV light!**"
Low gasps sounded in the conference room. A few millimeters? That meant if they attempted EUV lithography in air, light from the source would be completely "eaten" by air before reaching the wafer surface.
"This isn't merely an energy‑loss issue," Xiuxiu continued explaining deeply. "The absorbed photon energy converts into internal energy of gas molecules, heating and even ionizing the gas, generating unwanted plasma. This introduces huge noise and instability, possibly contaminating and damaging precision optical components and the wafer."
She looked around, giving an incontrovertible conclusion: "Therefore, **the entire optical path of EUV lithography—from source generation, through collector mirror, illumination system, mask, to projection objective finally focusing on the wafer—must be in an extremely clean, high‑vacuum environment.** This is not a choice; it is a **prerequisite** for EUV technology to exist!"
"What we need to establish is not ordinary low vacuum, but an exceptionally demanding **ultra‑high vacuum (UHV)** environment, typically requiring **10^‑7 Pa or lower pressure.**" She wrote this staggering number: 10^‑7 pascals, over ten billion times thinner than the air we breathe!
"This vacuum environment imposes several almost harsh requirements." Xiuxiu began dissecting specific challenges:
"First, **vacuum attainment and maintenance**. We need large‑pumping‑speed turbomolecular pumps, ion pumps, even cryopumps combined to pump the massive, internally complex EUV lithography chamber to such extreme vacuum within reasonable time. And we must solve **leak‑rate** problems; any tiny leak could prevent vacuum maintenance. Water vapor, hydrocarbons, etc., would quickly adsorb on cold optical surfaces, forming a 'carbon‑contamination layer,' drastically reducing EUV‑mirror reflectivity."
"Second, **material outgassing**. In ultra‑high vacuum, any material—metals, seals, cable insulation, even lubricants—slowly release internally adsorbed or dissolved gas molecules (mainly water vapor, hydrogen, carbon monoxide). This 'outgassing' becomes a continuous contamination source, degrading the vacuum environment and contaminating optical elements. We must strictly select low‑outgassing‑rate materials and subject all components to prolonged **baking for degassing**."
"Third, **dynamic sealing and transfer**. The lithography process requires wafers and masks to enter and exit the vacuum chamber, involving complex load‑locks and precision robots. How to achieve high‑speed, damage‑free wafer transfer while guaranteeing ultra‑high vacuum is a huge engineering challenge. Vibration from moving parts, friction‑induced gas generation—all are devilish details needing precise control."
"Fourth, **contamination control and monitoring**. We need to place high‑sensitivity residual‑gas analyzers inside the vacuum chamber for real‑time vacuum‑quality monitoring, tracing contamination sources. Simultaneously, consider how to clean contaminated optical elements in‑line or offline without breaking vacuum."
Xiuxiu laid out the extreme demands of the EUV vacuum environment clearly and coolly; each challenge resembled a small snow‑capped mountain needing crossing. The conference room fell silent; expressions grew graver than before. They realized EUV wasn't merely a technology upgrade but a complete engineering‑paradigm shift. They must move from a "macroscopic" world where they could touch and debug, into a world of ultimate "nothingness" where they manipulated light in utter isolation.
Pre‑research work unfolded in this somber atmosphere. Xiuxiu led a lean team to build the first EUV proof‑of‑principle vacuum vessel. The process brimmed with setbacks. The first welded stainless‑steel chamber, after 48‑hour continuous pumping, stalled at 10^‑5 Pa, refusing to go lower. They took a helium‑mass‑spectrometer leak detector, probing inch by inch like surgeons, finally locating a nanometer‑scale leak at a seemingly perfect weld.
After replacing materials, new problems emerged. A batch of ceramic insulators developed micro‑cracks after baking, outgassing rate exceeding limits. A sealing ring in the transfer mechanism lost elasticity at low temperature, causing load‑lock repeated failures.
Every day they wrestled with these invisible, intangible yet stubborn "detail demons." Progress was agonizingly slow. Team members took shifts monitoring vacuum‑pump groups and data loggers; eyes bloodshot from long screen and gauge staring.
Another deep night; Xiuxiu alone in the lab. The huge vacuum vessel lay like a silent metal beast crouching in the lab center, emitting the low hum of molecular pumps. On screen, the vacuum‑level curve crawled slowly, struggling downward, still distant from target. Weariness and pressure washed over her will like icy tide‑waves. She felt a deep‑marrow exhaustion—not just physical, but mental—facing a barrier set by physical laws, that sense of human limitation.
She leaned over the console, intending just brief rest, but unwittingly succumbed to sleepiness.
Some time later she felt someone gently pat her shoulder. Startled awake, she looked up; in blurry vision appeared an unexpected figure.
Mozi.
He stood beside her, wearing a dark wool overcoat, travel‑weary, holding a paper bag printed with a well‑known Hong‑Kong‑style teahouse logo, emitting tempting food aroma.
"You… how did you get here?" Xiuxiu rubbed her eyes, almost thinking she still dreamed. This was a high‑security R&D campus; how did he enter?
"Had a meeting in a nearby city, finished and came to see." Mozi's tone as ever calm, as if appearing at a national‑security lab deep night were perfectly normal. He didn't explain details, just set the paper bag on the console. "Brought you some supper—shrimp dumplings, century‑egg lean‑pork congee. Eat while warm."
Xiuxiu stared at him, then at the still‑steaming food; an indescribable warmth surged abruptly, breaching her strained defenses. In this isolated deep night, in this lab full of frustration, his arrival brought a sudden, warm, real light—brief yet immensely comforting.
"Vacuum level… still won't rise." Instinctively, like reporting work, she uttered the current biggest trouble, voice carrying unnoticed grievance.
Mozi didn't look at the data on screen; his gaze rested on Xiuxiu's tired yet persistent face.
"I know it's hard." His voice low, steady. "Fighting physical laws is always the hardest battle. Air absorbs light—that's a rule of the universe. What you're doing is carving out a space that defies 'common sense' within those rules."
He pointed at the massive vacuum vessel. "Inside there—isn't that the 'pure land' for light you're creating? Though still many 'impurities,' each leak eliminated, each order‑of‑magnitude pressure drop brings you closer to that pure land."
"Don't just see how far remains from target," he looked at her, eyes deep. "Look how far you've come from atmospheric pressure. From 10^5 Pa to 10^‑5 Pa—you've crossed ten orders of magnitude. The remaining road, though difficult, you've proven this path is walkable."
His words, no flowery rhetoric, yet like a precise key unlocking Xiuxiu's will locked by pressure and fatigue. Yes, they had taken solid, astonishing strides on the path against physical laws. Each discovered leak, each resolved outgassing source, was a tiny victory worth celebrating.
She opened the still‑warm congee bowl; food aroma diffused through lab air smelling of machine oil and electronics, bringing a strange, everyday comfort.
"Thank you." She said softly, this time containing more complex emotion.
"Eat quickly." Mozi nodded slightly. "I'll wait till you finish before leaving."
Xiuxiu lowered her head, eating congee in small mouthfuls; felt cold body and mind warming bit by bit. Mozi stood quietly nearby, gaze occasionally sweeping over complex vacuum equipment and monitoring screens, no excessive inquiry, just silent companionship.
In this deep night preparing the "vacuum cradle" for EUV light, his sudden arrival and simple encouragement were like an unexpected, warm beam shining into Xiuxiu's world full of technical challenges. It couldn't directly raise vacuum level, couldn't solve material outgassing. Yet it infused her with something invisible but vital—a sense of being understood, supported, believed in. She knew after finishing this congee, she would continue facing those cold devices and stubborn problems. But at least now, she wasn't alone groping in endless darkness. That support from another world—silent yet firm—would become a precious mental reserve for her continuing assault on this realm of "nothingness."