Twenty Degrees Too High
I waited for my mentor right up to the last moment before the Inform was supposed to begin. He never came.
"Haa..."
The seniors' sighs grew a shade louder. I felt as if I were sitting on a bed of thorns, and I hadn't even done anything wrong.
"Let's just do the Inform. We've got to get going too."
"Ah, yes."
My senior picked up the Inform notebook and started the handover. I took careful notes. Nothing out of the ordinary — except for a single tool.
"WN-15. Ever since Dongcheol booted the tool yesterday over an error, the thickness on the wafers in process keeps coming out low. It's under a Down Code right now."
"...Uh, how many lots actually ran?"
"Ten lots."
God.
This was a bigger mess than I'd thought. As a rule, when product wafers are scrapped, an abnormality notice goes out if five or more wafers in a single lot are thrown away. That means special handling — they pick the cause apart piece by piece. And yet—
Ten lots?
A lot usually holds twenty-five wafers. That meant at least two hundred and fifty wafers had come out with low deposition thickness. An accident. A major one.
"Downstream's trying to salvage them, but whether the wafers live or die... no idea."
"...Yes."
"We still haven't found the cause. Nothing stands out in the data, and we confirmed every valve is operating normally. Just in case, we rebooted it again. Same result."
That was the end of the Inform.
"Thank you for your work."
"Yeah. Seokhyeon, don't push yourself. Just hold the status quo. Don't go poking around on your own and get hurt."
"Understood. Thank you for worrying about me."
"All right. We're off."
"Yes. Get some rest."
The two seniors left the meeting room. Alone, the place felt unbearably quiet.
This isn't going to be easy.
Managing a production line alone, one month into the job? That was genuinely not easy. A shift was supposed to run two to a team at the minimum. But my mentor hadn't come in, so there was nothing I could do about it.
Even so, I have to try.
I stood. A shift starts with the daily.
The daily was over almost before I noticed. I sat down at the engineer-only computer set up inside the line and, with practiced hands, brought up the tool management system — the kind that let you take in every tool's status at a glance. The only one lit was WN-15.
Hmm...
The previous shift had told me to do nothing but hold the status quo. Even so, sitting here truly doing nothing made my hands itch. You never knew. I might find another fault. With the ability I had, I thought I could manage it well enough.
Should I go take a look?
I got up and headed for WN-15. From the outside, nothing looked special. It was identical to every other WN tool.
Might as well look it over.
I started circling the tool, searching for the same kind of glimmer I'd caught when I found the fault on WN-3. But—
...Nothing?
Top and bottom. I went over every inch of it and saw no glimmer. Nothing you could call an anomaly, either.
That can't be right.
Was there, in fact, no fault at all? Maybe the dummy data had only dipped for a moment. The thought occurred to me. With no fault I could actually see, it was the natural conclusion.
Let me run it one more time.
It might only be hunting off the dummy wafers. So I immediately requested another dummy-data check. A short while later the dummy came out, and I looked at the data, but—
"It's still low?"
The thickness was still coming out low. An invisible fault was still in there. It felt wrong — the data off, and no fault anywhere I could see.
Did the process conditions change?
They'd said the FDC data was normal when I took the Inform, but I needed to see it myself. People look from different angles. What one person misses, another catches. With that resolve, I pulled up the FDC data. And—
"..."
I despaired. Not a single anomaly. I slumped back in the chair on the line.
...What now?
I couldn't even get a feel for how to start a trace on something like this. Was this the wall that came with too little experience? Was I just going to have to hand the trace to the next shift like this?
"I don't want that."
Passing an Inform with nothing to show for it was no different from not having worked at all. And I hated being the one who let that happen.
"...Wait."
I jolted upright out of the chair. A thought, out of nowhere.
What I can see might not be the whole of it.
From the outside, all a tool showed you was valves and pipe. But a tool doesn't run on those alone. A PC and a swarm of controllers work together to execute the process.
I was being an idiot.
How had I failed to think of that? I got up again and hurried back to WN-15 — to a part of the tool I had never once looked at on the job. The section shut behind a cover, where the controller and a cluster of other devices lived. A little more experience, and I would have gone straight there.
Nothing to be done about it.
The experience I was getting now still counted. Someday I'd use it to find something. I lifted the cover at once, and there it was. A glimmer. But—
"...What is that?"
I didn't know the name of the device that was glittering. A new hire, one month in. Up against the wall again.
Haa.
A tool had never been a simple thing. A tool was nothing but the sum of countless parts, and I didn't know more of them than I knew. If a senior had been here, I could have asked. I got up without lingering on it. At a time like this, there was one thing to do.
Read the manual.
I sat at the computer and opened the department shared folder, where the tool manuals were kept. I found the illustrated one, and there was the glittering part I'd just seen.
The controller that managed the tool's temperature. So why was this thing glittering?
If the temperature's low, of course the thickness comes out low.
The problem was that, on the FDC data, the temperature was moving exactly to spec. Which was as good as saying there was nothing wrong with the temperature. And still my eyes kept shouting that the Temp Controller was the problem.
"..."
I wavered. What if my eyes were wrong? What if I was just going on seeing things that weren't there? No. That wasn't it. I shook my head. If it were, I never would have fixed WN-3.
I trust my eyes.
It was the ability I had. Trusting them was the same as trusting myself. Then why was the Temp Controller glittering? A thought struck me. What if—
It's pulling the wrong temperature value?
What if the tool's real temperature and the temperature the controller was reading weren't the same? A simple assumption: the thermometer had failed.
Then it makes sense.
FDC data takes the values coming off the tool as they are. It's no different from taking whatever value the Temp Controller happens to be reading. So we'd had no choice but to trust bad data and keep on tracing. Of course we couldn't find the cause. And to prove it—
I have to swap it.
I had to replace the Temp Controller. I pulled an identical model from the parts cabinet and swapped the controller body. The structure wasn't complicated, so it came easily enough.
From here on is what matters.
Please let it be what I thought. Heart unsteady, I sat back down at the computer and checked the FDC data. The result?
There it is.
Marker: Temp Controller body replaced
Temperature: −20
There was an anomaly. At the exact moment I'd swapped the controller body, the temperature data on the FDC had dropped by twenty. That meant the old temp controller had been reading the temperature twenty degrees too high.
So that was it.
When my mentor booted the tool, the load had landed on the Temp Controller. The reason was plain enough. On a boot, the temperature usually shot from zero up to the Recipe value in an instant. Every setpoint was issued all over again, and a controller could fault from that. The run had gone on with the temperature lower than it should have been, and the thickness had come out low.
Let's run the dummy again.
If my guess was right — no. It would be right. The data was saying so. The temperature had been different. The dummy data came back.
I clenched my fist tight. You set a hypothesis, walk the steps, and arrive at a result. And when that result lines up with the hypothesis, you feel a catharsis nothing else can touch.
God, that feels good.
Could anything feel better than this? I'd fixed a tool again. Again! With no one but myself. I was about to ask them to run the first lot right away, but—
Nearly two. Time for Inform.
I'll leave the first lot to the next shift.
I gathered up the Inform notebook and headed for the meeting room. When I got there, Senior Uhyeon and Senior Jihan were already in their seats. Maybe because of WN-15, both of them looked grim.
"Good afternoon, seniors."
"Oh. Seokhyeon. Hey."
"Hey, Seokhyeon."
We traded a light greeting, and I sat down across from them. Just the thought of giving the Inform on WN-15 had me foolishly buoyed.
"Something good happen?"
Senior Uhyeon asked.
"Ah, no."
Was it that obvious?
"Never mind that. What about Dongcheol?"
"Ah, he didn't come in today."
"...Yeah? So you worked alone today?"
"Yes. I did."
"Hmm."
Senior Uhyeon nodded slowly.
"A one-month newbie running the line by himself. That's not easy."
No. I think so too, Senior.
"God, tell me about it. Even if they ordered me, I absolutely couldn't."
"Right. You couldn't have."
"Hey, senior!"
The two of them let out a short laugh. The heavy air seemed to break a little.
"Then I'll start the Inform."
"All right. Let's hear it."
Senior Uhyeon opened the floor. My first Inform, done alone. I was nervous for no good reason.
"First... nothing out of the ordinary. On WN-15, after replacing the Temp Controller, we confirmed normal dummy data. If you could cover the first lot, that should be all."
I looked at the two of them, full of expectation. Both looked thrown. Senior Jihan spoke first.
"Uh... you fixed it?"
"Yes."
"Whoa..."
Surprise, plain as day. It felt good. Then Senior Uhyeon.
"You know what I'm going to ask, right?"
I nodded. Of course I'd known he would ask why. So I walked them through the whole trace, from how I'd started it to how I'd finished.
"That's something. So cases like this happen too."
Senior Uhyeon said it as if the thing hadn't turned up even in more than ten years of a senior's experience.
"Anyway, got it. Less on our plate thanks to you. Appreciate it."
"Not at all."
I smiled and dipped my head. Senior Uhyeon smiled too, and so did Senior Jihan. And that was the end of the Inform. Another day, done!
...Or so I thought.