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How to Build a Computer, Part 1 of N: Silicon
As the illustrious @JohnWalker no longer treads these halls, I figured there was an opportunity to thrust my metaphorical booties into his clodhoppers. I’ve been kicking the idea of this series around for a long time. Broadly speaking it covers everything you need to know to build a computer. Everything. Today, we’re going to learn how to make silicon wafers.
He’s Gone Silicon
Start with silicon. Except we can’t start with silicon; you’ve got to get it from somewhere. Start with sand. (I can hear someone in the back asking where I get the sand. Well, they mine it in Wisconsin these days, but mostly to ship to the North Dakotan oil fields. I don’t know where the computer industry gets its sand, but if you can’t find any you aren’t trying.) Sand, chemically, is SiO2. Close, but we need less O. To render it into pure silicon you heat it up to about a fifteen hundred degrees commie in the presence of coke. Uh, the fuel, not the nose candy. This is the chemical reaction:
SiO2 + 2C -> Si + 2CO
Or possibly this one. Probably both; my sources gave me differing answers.
SiC + SiO2 = Si + SiO + CO
This gets it fairly pure, but you need really really pure. (So pure that only one really won’t cut it.) With the 96% pure stuff you get out of that reaction, you make tetrachlorosilane. That’s one silicon atom with four chlorine atoms stuck to it. You could also make trichlorosilane, which has one hydrogen atom instead of a chlorine. In either case you’ve got a silicon compound that’s readily made into a vapor. With that you can use fractional distillation (like with petroleum, or pappy’s still out back) to increase the purity. This stuff has got to be not just 99.9% pure, but 99.9999999% pure.
Okay, but you can’t make devices out of a gas. You want to reduce that, using something called the Siemens process. You take a rod of (also very pure) silicon. Heat it up to about a thousand degrees C. Then mix your trichlorosilane with hydrogen gas. One more chemical equation.
2SiHCl3 + 2H2 -> 2Si + 6HCl
You get hydrochloric acid out, and silicon. Great! But you can’t just make wafers out of that. I mean you could, but then you’d end up with solar panels, not microchips. What you’ve got now is referred to as ‘polysilicon’ because the crystal structure is all over the place. To make proper transistors you need it to be monosilicon, which is all one big silicon crystal. So we melt the stuff.
One Big Crystal Coming Right Up!
We make a single huge crystal of silicon out of the melt by means of the Czorchalski growth method. Say it with me. “Cho-RAL-ski”. That was pitiful. Try it again. “Cho-RAL-ski”. Eh, close enough. You melt your silicon in a big crucible made of silica. That way when the crucible mixes with the molten silicon, all you’re getting is more silicon. And oxygen. There are reasons why you might want oxygen in your otherwise pure wafer. You want to keep the temperature of your melt just above the melting point. Then you reach down with a seed crystal into the melt and slowly pull it back up.
The surface tension of the silicon will cause the liquid to rise up above the melt. The greater surface area causes it to cool off. By carefully controlling the temperatures you can get the molten silicon to freeze onto the crystal. Because it’s already a single crystal all your solidifying silicon will stick itself into the lattice and extend that crystal structure. The diameter of the cylinder you get depends on how quickly you pull the crystal back out.
There’s another way to get your monocrystal. It’s called the float zone method. You take a column of your very pure polysilicon. You run an infrared heating coil up and down it. The coil heats the silicon in the space directly below it to the melting point. The silicon has enough surface tension to keep the column together. Again, when the molten silicon hardens next to your seed crystal it takes up the already existent lattice and reforms into a single crystal. (So where do they get the seed crystals? Out of previous runs. And where did they get the first one? Good question; I don’t know.)
Breaking Down your Boule
What you get after that is called a ‘boule’. Don’t ask me why. You grow your boule a little wider than you need so you can grind it down afterwards. That lets you control the size much more finely than Czorchalski (“Cho-RAL-ski”) growth does. These things grow anywhere from 75mm to 300mm (roughly three inches to one foot in diameter). They’re also capable of making 450 mm wafers, but the benefits of the larger size aren’t sufficient to upgrade all the machines in your wafer fab to handle the larger size. If it’s one of the smaller ones (up to 150 mm, or six inch wafers) then you grind the edge of your boule to tell the customer what kind of wafer it is. On the larger ones they just grind a notch out of the edge, and scribe that information on with a laser.
You take your long cylinder of silicon and slice it like salami. You use a diamond wire saw to cut it into wafers. The wider you made your boule the thicker you need to make your wafers; they have to support their own weight. You’ve got wafers now, but they’re not ready to ship yet. These things need to be real flat. The smaller the features you’re putting on it the flatter it needs to be. No matter how straight you aligned your saw it isn’t straight enough. You use a process called “lapping” to smooth it out. It’s a planarization process that uses mechanical polishing and some sort of slurry. While you’re at it you should grind that edge down; it makes it harder to break your wafer afterwards.
Lapping your wafer left it with a bunch of micron-sized scratches in the surface. Etch that sucker down with some really nasty acid. acid. It’s still not smooth enough. You can smooth the wafer surface through something called chemical mechanical planarization. Still not flat enough! Nah, I’m kidding. We’re good. Clean ’em and ship ’em.
Now that we’ve got silicon wafers, we can start to build transistors. Exciting! Join us next week when we dive into the intentional and accidental defects in the silicon in “Doping: Semiconductors the Lance Armstrong Way”, or “Gettering R Dunnering”.
This is part one of my ongoing series on building a computer, the Richard Dean Anderson way. You may find all of them under the tag How to Build a Computer. This week’s post has been brought to you by the Society for the Promotion of Chippewa Valley Sand. When you go pound sand, make sure it’s Chippewa Valley Sand.Published in Science & Technology
Also, IBM didnt really care.
They where the mainframe guys, they sold big iron with big support and consulting contracts that generated revenue for years – if not decades after the original purchase. IBM had little interest in the PC market, much like Xerox-PARC, they didnt foresee the growth that PCs would eventually spark.
Would it have been the same if the original Mac (Lisa) had a Xerox nameplate instead of Apple?
When I was fifteen, a friend of mine taught me how to get great electronic parts cheap, astonishingly cheap, things I never dreamed existed: take the subway to Courtlandt Street in lower Manhattan, where there was a weird, funny ancient subculture of second hand radio parts shops, armed forces tech surplus, and other businesses on the dubious end of retail. The old guys would keep up a continuous patter: “Pick ’em up, gentlemen, pick ’em up! That’s a JAN (Joint Army-Navy) oscilloscope, gentlemen! Thousands of transistors! Servicemen, hams! Pick ’em up!”
It was a dream world for a kid who was into electronics. And it came to an end just after I found it, because the whole neighborhood was being excavated for a new project, the World Trade Center. The remnants of the old radio parts scene moved a little bit north to Canal Street. True veterans considered it to be an inferior successor to Courtlandt Street, but it still had its charms, and well into the Eighties it was the smart place to get a cheap VCR, or a 1955 Geiger counter, or an aerial camera.
Of the three movies, “Pirates of Silicon Valley” with Anthony Michael Hall, “Jobs” with Ashton Kutcher, and “Steve Jobs” with Michael Fassbender, I think Kutcher’s portrayal was the best. It seemed to match his personality and physical type perfectly — from the first shot lazing around at college (barefoot) to the later dominant bully and at last to the man in full.
Personally, I despised Jobs and especially Apple’s early business model. But, he learned and got better and he gave us some amazing technology. Pixar and the iPod were his greatest achievements.
That’s right, it is important to convert all commies into freedom.
(You don’t have to for my sake. I move between the 2 often enough to have a pretty good idea in both systems.)
Wacker has been selling down thier shares in Siltronics, but they created the initial business. The fabrication of silicon wafers has been even more codified since the late 80’s early 90’s. Several Asian nations have made substantial efforts, since that period, to “buy into” (i.e. undercut through subsidized national investment). The Germans still make the high end material, but the market is rapacious, tends not reward that quality. However if you are looking for base material for focal plane sensors to put on the front end of your laser/IR guided missiles, they are the go to folks for many of the LA base aerospace contractors.
Pronunciation clue, “W” in German are our “V” so we would pronounce it more like Vulcker.
I will quibble here, the “Kelvin” scale was a by product of the work of The Scottish Enlightenment, which in addition to providing the current everyday working models for Energy & Work (Thermodynamics), it was also the boiling pot that gave us the ideas for our founding leaders to craft the Declaration and Constitution.
So stuff the commie units gig. It is far easier to manage the prediction of the Universe in SI units, vs the pre enlighten Ye Ole English units….. you can keep your Slugs, Ton’s, Grains’s, Fluid Ounce’s (vs a mass related Oz’z), Pint’s, Quart’s, Gallon’s, Rod’s, Furlong’s, Perch’s, Acre’s, Fantom’s, Cable’s, and Lord know whatever other units I once had to convert from….
Don’t forget slinches.
I find that for most everyday purposes the Imperial units are superior. The fact that Farenheit degrees are about half as thick as Celsius ones makes it easier to give accurate estimates of the temperature outside, that distances are easier to state in inches, feet and miles. Maybe it’s my bias, but I spent a couple years in my impressionable youth living in a metric country and even then I could tell you my height in feet easier than in centimeters.
Part of the thing is that nobody asks you to do conversions in ordinary life. “It’s a quarter mile down the road. Quick — what’s that in inches?” When I’m working physics it’s SI units all the way. Then I’m worrying about such things.
@larrykoler – Personally, I despised Jobs and especially Apple’s early business model. But, he learned and got better and he gave us some amazing technology. Pixar and the iPod were his greatest achievements.
It was interesting to me how the openness changed. I had one of the early Apple II’s and it came with the “Red Book” which had schematics and the original firmware source code (mostly written by Wozniak). That made it possible for lots of peripherals to be designed and sold by third party vendors. You could get almost anything including other computer boards to plug in. By the time the Lisa and newer models came out, the openness was gone. IBM with the PC became the open platform and took off.
Before the Apple II came out, my company built a specialized computer to be used in the 1000+ Meldisco shoe stores in Kmarts. It had a 6502 processor (same as the Apple), a pretty large memory and a battery backup. There was a numeric keypad plus small display and a Kimball tag reader which was used to read the inventory tags on the shoes. It also had a 1200 baud acoustic modem which was used to transfer sales data every night to the main office. I wrote the code and participated in the hardware design and sweated bullets over the modem. (“What do you mean a square wave isn’t good enough?”)
It was pretty ugly, but Meldisco saw that as a feature, since the staff would be less inclined to walk off with them.
I was no Woz, but if anything, the hardware was equivalent to the Apple II and in some ways better. The difference was that my company just saw the one use of what they had, even after the Apple II came out. That was what Jobs brought to the table* an imagination of what could be done and the force of will to get it done
* I met Steve Jobs and Wozniak once at the New Jersey computer club show. They were both standing behind a card table with early Apple products on it along with probably 50 other long haired guys selling similar stuff at the same type tables. But Jobs managed to build a major company.
Kelvin isn’t in question here, it was in Celsius — which is a silly half-measure towards a unit based on basic scientific/natural laws. Kelvin at least anchors itself at a reasonable point.
I will try to forget slinches.
The iPod’s an mp3 player, right?
Gah! I remember 1200 baud modems, but not acoustic ones.
The thing on the right is called a “telephone.”
You’re welcome, whippersnapper.
Sorry I missed it.
I remember telephones, and acoustic modems, but I wouldn’t have guessed they were 1200 baud.
1200 would be a fast one… The first one I wanted was 300 baud.
If you havent heard the term before, Baud is the term for bits per second. In contrast, I think the slowest connection the cable company sells today is 10 Mb/second or 10 MegaBaud.
Yeah, I would have thought acoustic and 300 baud would be about the same point on the timeline. I wonder if anyone made a 1200. If only there was some resource I could use to look something like that up.
Ah, but that was then.
This is now.
The Hayes Smartmodem 1200A came out in 1985. Acoustic couplers had just about reached their limit, but if your phone was wired through RJ-11 connectors, you could plug straight into the line for the higher speeds.
Everyone I know in Australia, New Zealand, England, and Canada who’s not a scientist measures distance, area and volume in freedom. For weight, cooking and everything but their weight is in pounds and ounces.
They all, and I mean all, measure their personal weight in stones. Which at 14 pounds per is about the dumbest unit I’ve personally encountered.
I think stones are almost exclusively a UK thing, Canadians are pretty divided between pounds and kilograms (mostly along age limits).
I used to think I was fat, then I bought an electronic scale and at the push of a button, there was a reassuring number on the screen.
Everytime O’Brian gives Aubrey’s weight in stones I have to go look it up.
My short attention span requires me to see a YouTube video of you making these wafers. Don’t make it too long, though.
A plus would be a YouTube video of whoever figured out how to do it, complete with blooper reel.
I was going to ask if that rod mentioned in the OP had to be exactly 16.5 feet. (“You take a rod of (also very pure) silicon.”)
Unfortunately my short attention span prevents me from watching much in the way of YouTube videos.
Probably where the phrase “dumb as a rock” comes from.
I remember when 300 baud was the high speed device. But when I google it, I learn that the old TI Silent 700 terminal was switch-selectable between 110 and 300 baud, so maybe I’m not remembering correctly just what the options were.