The domestic R&D park seemed forever locked in a tense race against time; boundaries between day and night blurred there. The initial victory Xiuxiu's team had achieved in improving the DUV light source had not lasted long—like a stone dropped into a deep pool, after the ripples dissipated, deeper, hidden rocks beneath snarled into view.
What they were currently attacking was the light‑source system required for the next‑generation, higher‑performance DUV lithography machines. This system's demand for **laser stability** reached an almost exacting level; any tiny frequency drift or intensity jitter, amplified by the complex optical system, would etch unacceptable defects onto the silicon wafer. The core component ensuring the laser operated in near‑absolutely stable condition was a highly integrated **laser‑frequency stabilizer** containing a precision optical cavity and electronic control unit.
The team's original plan was to procure this core part from a long‑term partner—a top‑tier Swiss precision‑instrument company. That company's stabilizer was world‑famous for its unparalleled **frequency stability** and long‑term reliability, the go‑to choice for lithography giants like ASML. Technical‑solution demonstration, budget application, even preliminary technical alignment were complete, awaiting only contract signing and payment.
But a piece of paper from across the ocean—an ever‑expanding technology‑control list—dropped like an invisible iron gate. That Swiss company regretfully notified them that due to that stabilizer model containing controlled high‑sensitivity piezoelectric‑ceramic materials and specific‑architecture integrated circuits, it could no longer be exported to Chinese related entities. All previous communication and effort instantly vaporized.
When the news arrived, the atmosphere in the team lab plunged to freezing point. The morale that had coalesced after the preliminary success deflated like a pricked balloon. Young engineer Wang, responsible for liaison with the Swiss company, was especially dismayed and indignant, fist hitting the table hard: "This is bullying! We pay, they're willing to sell, why should…"
Engineer Li frowned deeply; the wrinkles on his face seemed to have deepened overnight. He gathered the core team for a meeting; smoke hung in the conference room, the mood heavy enough to wring water.
"Everyone knows the situation," Engineer Li's voice was hoarse. "The external‑import route is temporarily blocked. We must find a way to solve the laser‑frequency‑stability problem ourselves. Engineer Xiu, you're responsible for the light‑source system; share your thoughts."
All eyes focused on Xiuxiu. She felt the weight of expectation, and a trace of scrutiny—wanting to see how this "returnee" expert would face truly hard problems.
Xiuxiu took a deep breath, forcing herself to calm down. She knew right now any emotional complaint was useless; only cool technical analysis could find a way. She went to the whiteboard, picked up a marker, and began sorting out the problem's technical core.
"Everyone, the bottleneck we face lies in achieving extremely high **laser frequency stability**," she wrote those words on the board. "To understand why this stabilizer is so critical, and the difficulty of our independent R&D, we need to deeply understand the principles behind it."
She drew a simple laser‑schematic. "A laser's frequency is not absolutely constant. Temperature fluctuations, mechanical vibrations, even tiny current variations cause changes in the laser‑resonator cavity length, leading to output‑laser‑frequency drift. That's fatal for lithography."
"Then how to 'lock' this 'mischievous' laser onto the extremely stable frequency we need?" Xiuxiu switched her thinking, starting to construct a conceptual model. "This relies on a precision **feedback‑control system**. You may think of it as an extremely sensitive, fast‑reacting 'automatic‑adjustment steward.'"
She drew a classic feedback‑control‑loop block diagram on the board.
"The 'eyes' of this system are a **reference standard**." Xiuxiu labeled the diagram's input. "In the highest‑precision stabilizers, this reference is usually a **Fabry‑Pérot optical interference cavity**. This is a precision cavity formed by two mirrors with extremely high parallelism and reflectivity. Its length is extremely stable (typically made of ultra‑low‑expansion materials like Zerodur or silicon carbide, placed in constant‑temperature, vibration‑isolated environments). When laser light passes through this cavity, only when the laser frequency strictly equals one of the cavity's resonance frequencies will the transmitted light be strongest. This stable optical cavity is like an extremely precise 'frequency ruler.'"
"The system's 'brain' is an **error‑signal detection circuit**." She pointed to the diagram's processing unit. "It monitors in real time the intensity of light transmitted through the reference cavity, comparing it with preset ideal values. Once it detects laser‑frequency deviation from target (manifested as transmitted‑intensity change), it immediately produces a corresponding **error signal**. This signal's magnitude and polarity (positive/negative) precisely reflect the degree and direction of frequency deviation."
"Finally, the system's 'hands' are an **actuator**." Xiuxiu labeled the output. "This error signal, amplified, is applied to a **frequency adjuster** on the laser—usually a **piezoelectric‑ceramic transducer (PZT)** attached to the laser's resonator mirror (or directly adjusting other laser‑frequency control elements like a grating). When voltage is applied, the PZT undergoes extremely tiny deformation, precisely altering the resonator length, 'pulling' the laser frequency back to the correct target value."
She connected the entire loop, forming a closed circuit. "**Detect error -> generate signal -> execute correction**. This cycle repeats at extremely high speed (typically MHz or higher bandwidth), dynamically, in real time, opposing all internal/external disturbances trying to cause laser‑frequency drift. It's like a superb acrobat holding a long pole, maintaining balance of the object atop by minute, swift bodily adjustments."
"The stabilizer we cannot import," Xiuxiu summarized, "its core difficulties are: first, that reference optical cavity as 'ruler'—its material's ultra‑low thermal‑expansion coefficient, mirror surfaces' extremely high flatness and reflectivity, plus overall mechanical and thermal‑stability design—accumulates decades of materials science and precision‑processing; second, that error‑signal detection circuit as 'brain' requires extremely high signal‑to‑noise ratio and response speed, involving high‑speed low‑noise electronic‑device design and algorithm optimization; third, the piezoelectric ceramic as 'hand' needs extremely high linearity, resolution, and response speed—its material and drive technology are bottlenecks too."
She candidly pointed out domestic gaps in these niche areas versus top levels: "We might make functionally similar parts, but achieving breakthroughs in **long‑term reliability, size, power consumption, and most importantly—meeting next‑generation lithography's ultimate‑stability requirements** demands deep interdisciplinary, cross‑process collaborative research; this is absolutely not something quickly completed short‑term."
The conference room fell silent. Xiuxiu's clear, deep analysis, like a scalpel, precisely dissected the problem's complexity, also letting everyone more profoundly recognize the towering mountain before them. This wasn't solvable just by fervor and overtime; it required solid underlying‑technology accumulation and systematic breakthroughs.
"Then… is there really no other way?" young engineer Wang asked unwillingly, voice tinged with despair.
Xiuxiu remained silent; she too was rapidly searching her mind for all possible alternative solutions and potential technology sources, but each path seemed blocked by that invisible control barrier. A deep sense of powerlessness, mixed with concern for team morale, surged in her like tidewater.
Late at night, Xiuxiu stayed alone in the lab. Outside, all sounds vanished, only the machinery's low hum accompanying her. She repeatedly reviewed the embargoed stabilizer's technical documents, trying to glean inspiration that could be borrowed or circumvented—but the deeper she went, the more she felt that suffocating sensation of being precisely "pinched." Their technological advantage, built upon extremely deep industrial foundation and long‑term R&D investment, was absolutely not surmountable overnight.
Intense pressure and fatigue made her dizzy. Leaning against the cool lab bench, she unconsciously picked up her phone, finger pausing over a name—Mozi.
Even she felt surprised. In this isolated, helpless moment, the person she thought of wasn't domestic colleagues or mentors, but this stranger from a seemingly unrelated field, with only one brief phone call. Perhaps because that earlier discussion about "order" displayed a kind of macro‑problem‑solving thinking transcending specific fields? Perhaps simply because she needed someone completely detached, yet seemingly possessing unique energy and perspective, to confide this heaviness that couldn't be voiced to teammates?
After much hesitation, that urgency to seek a sliver of breakthrough possibility finally overcame hesitation over abruptness. She dialed that saved‑but‑never‑called number.
The phone rang a long time; just as Xiuxiu prepared to hang up, it connected.
"Hello?" Mozi's voice came, with a clarity characteristic of late‑night work, but no displeasure at being disturbed.
"Mo… Mr. Mozi? This is Xiuxiu," her voice slightly dry from fatigue and tension. "I'm very sorry to bother you so late."