# How to Build a Computer 7: Patterning

We left off last time discussing circuits and logic and how to make your transistors do something useful. Fun stuff, but I wanted to swing back through a bit more of the manufacturing details. Let’s say I’m trying to make this circuit:

Don’t be fooled by the clever marketing; this is an OR gate with a NOT gate stuck on its nose. Wake up sheeple!

Yup, that’s a bunch of lines on a piece of paper. I want to manufacture these; to sell them and make money. So I can’t just make one, I need to make a lot. Okay, build the circuit. Let’s say I lay that out on a wafer, this is about what it’d look like.

Is it just me or do the blue and red lines make this look like a hockey rink? It’s probably me.

Couple things going on here that we haven’t seen before. The red line at the top is positively charged, the blue line on the bottom is negatively charged. I assume those are hooked up to a power source somewhere off screen. The circuit wouldn’t do much if they weren’t. The black lines are our traces, metal wires deposited on the wafer. The great big accordion thing on the right isn’t just there because I got bored drawing the circuit. That’s a resistor. When you get down to the nano scale you don’t need to make custom devices for resistors; the wires are small enough that you can get all the resistance you need just by making your wires long enough.

The green blocks are transistors; they’re a new type we haven’t talked about before. It’s called a “Field Effect Transistor”, or FET for short. These things deserve an explanation all of their own; I’ll blithely skip them today. If you run a current through the wire in the middle it’ll switch the transistor on and off.

The great big “A” “B” and “O” are bond pads; a field of conductor that’s much larger than your traces. That’s where someone would solder connections to your chip. The pads need to be big enough to let that poor schlub do his job. Those blueish-grey pads are jumps in the wire. After this layer is down you’ll put a layer of insulator on, etch out holes over those pads, deposit a metal connection between layers (called a ‘via’ in case you’re taking notes, or possibly building Roman roads), and finally you’d put a bit more metal down to connect without touching the wire in the middle. Confused? Here’s what it’d look like on a completed wafer:

One of the neat things about working with silicon wafers; you need an insulator you can add oxygen and grow one. Unlike certain other lazy elements I could name.

Huh, that’s a lot more complex than just drawing wires on paper. And I bet it’s only going to get more difficult. The boss man says “That’s a nice job wiring up that NOR gate. Now make me seventeen million of them, just like it.” Hope you weren’t planning on cutting out early on Friday.

Before I answer that question take a moment to think about it. Wiring your circuits by hand won’t cut it; Eckert and Mochley made a good living calculating ballistics tables for the army but I can’t afford that. You’ve got to apply a supremely complicated pattern quickly and precisely. How?

The answer lies in a friendly chemical called photoresist. Photo-what now? Photoresist. You know how film works, right? Light hits it, it makes a chemical change in the film, and leaves a pattern. Only we’re trying to make a computer chip, not film Sunshine, Bikinis, and Murder Most Foul. Although now that you mention it … Right, back on topic.

The word “photoresist” covers a number of chemicals with broadly similar properties. Positive photoresist reacts to the ultraviolet light by weakening its chemical bonds; the stuff that gets exposed dissolves off where the light hits. Negative photoresist strengthens it’s chemical bonds when the light hits it, becoming tougher. The stuff that isn’t exposed dissolves away.

To pattern the above circuit I’d use a mask that looks like this:

Hey wait a minute; that’s just that same picture he showed up above with all the colors stripped off! What kind of two-bit operation is this?

Briefly, the process goes like this:

2. Coat it uniformly with photoresist.
3. Align it with a mask with your features blacked out.
4. Shine a whacking big ultraviolet light on it.
5. Run your wafer through a developer solution. The photoresist dissolves, leaving your pattern intact.
6. Etch it or plate metal on it or whatever it was you were going to do. Shoot ions at it?
7. Strip the remaining photoresist off.

This is part seven of my ongoing series on building a computer, the way your frontier forefathers did it. You may find previous parts here: 1 (silicon) 2 (crystallography) 3 (doping) 4 (diodes and transistors), 5 (fundamental chemistry) 6 (simple transistor circuits) or all of them under the tag How to Build a Computer. This week’s post has been brought to you by Wisconsin Mining & Manufacture. Providing comfort and luxury for so little cost that even a housewife can live like a queen. Wisconsin Mining & Manufacturing: Your pals for the atomic age!

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## There are 23 comments.

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1. Thatcher
Percival
@Percival

Those wafer layouts make me cranky. I want to be able to tell which is the collector, which is the emitter, and which is the base without thinking about it — mainly because it takes away from my quality “where is that hardware dweeb and why am I looking at this” time.

2. Member
The Reticulator
@TheReticulator

Hank Rhody, Possibly Mad: The answer lies in a friendly chemical called photoresist. Photo-what now? Photoresist. You know how film works, right? Light hits it, it makes a chemical change in the film, and leaves a pattern. Only we’re trying to make a computer chip, not film Sunshine, Bikinis, and Murder Most Foul. Although now that you mention it… Right, back on topic.

Shouldn’t that be spelled photo#resist?

3. Coolidge
DonG
@DonG

Tell us the story about CMP, Uncle Hank.

4. Contributor
Gary McVey
@GaryMcVey

If I’d written Sunshine, Bikinis, and Murder Most Foul in 1979, I’d be a wealthy, retired screenwriter today, instead of a middle class, retired film festival director.

Even when Hank Rhody jokes about another field of endeavor, he instinctively knows how to do it right. Gentlemen and ladies, I look forward to Part 8.

5. Moderator
Randy Weivoda
@RandyWeivoda

Hank Rhody, Possibly Mad: This week’s post has been brought to you by Wisconsin Mining & Manufacture. Providing comfort and luxury for so little cost that even a housewife can live like a queen. Wisconsin Mining & Manufacturing: Your pals for the atomic age!

They put the zap in your transformers.

6. Contributor
Gary McVey
@GaryMcVey

If you listened to a radio with copper wiring, flew on a Douglas airliner, or took the luxury express to Duluth, you put money in Rhody pockets today.

7. Member
Boss Mongo
@BossMongo

I am incapable of understanding any of this.

I like the little A/B airplane thingy at the bottom of your sketch, though.

8. Member
Joseph Eagar
@JosephEagar

I’ve always thought it would be fun to make a primitive semiconductor fab.  The only thing that scares me is the etching.  It’s not exactly easy to etch quartz or raw silicon, and the chemicals are rather frightening (fluorine acids are just frightening).

9. Member
Joseph Eagar
@JosephEagar

I am incapable of understanding any of this.

I like the little A/B airplane thingy at the bottom of your sketch, though.

OMG.  You have insulted the holy NOR Gate.  The Gate from which all logic spawns! The one true Gate! How could you! Apologize to the holy NOR Gate!

10. Member
Judge Mental
@JudgeMental

OMG. You have insulted the holy NOR Gate. The Gate from which all logic spawns! The one true Gate! How could you! Apologize to the holy NOR Gate!

I thought that was Bill.

11. Contributor
Gary McVey
@GaryMcVey

For some time, the field of artificial intelligence, or more accurately the branch of it that simulates the operation of living neurons instead of a classic Von Neumann-style processor, had trouble executing the XOR function. As in, “Really couldn’t do it”. That hardware hangup caused a surprising amount of moaning and existential doubt in the software wing of the then-embattled AI community. Eventually they figured out the architecture of multi-level perceptrons and I guess the XOR problem was left in the rear view mirror.

12. Contributor
Gary McVey
@GaryMcVey

I’ve always thought it would be fun to make a primitive semiconductor fab. The only thing that scares me is the etching. It’s not exactly easy to etch quartz or raw silicon, and the chemicals are rather frightening (fluorine acids are just frightening).

When I was a kid, there was a brief fad of electronics magazines printing stark black and white circuit layouts for some of their monthly build-at-home projects, to be clipped and used as negatives in circuit printing. The magazine plans were scaled exactly to standard sizes of widely available photoresist/photoetch printed circuit boards. These didn’t have the chemistry of standard photographic darkrooms; they were easier to use and more benign for the user.

Obviously even an ambitious amateur’s homebrewed printed circuit board of 1964 is vastly larger and cruder than the electronics lab, the fabricator that you’re speaking about, but remember that the age of vacuum tubes and hand-wired, multicolored “spaghetti” under an aluminum chassis was still in charge; a kid being able to turn out a two-transistor radio was hot stuff.

13. Member
Joseph Eagar
@JosephEagar

I’ve always thought it would be fun to make a primitive semiconductor fab. The only thing that scares me is the etching. It’s not exactly easy to etch quartz or raw silicon, and the chemicals are rather frightening (fluorine acids are just frightening).

When I was a kid, there was a brief fad of electronics magazines printing stark black and white circuit layouts for some of their monthly build-at-home projects, to be clipped and used as negatives in circuit printing. The magazine plans were scaled exactly to standard sizes of widely available photoresist/photoetch printed circuit boards. These didn’t have the chemistry of standard photographic darkrooms; they were easier to use and more benign for the user.

Obviously even an ambitious amateur’s homebrewed printed circuit board of 1964 is vastly larger and cruder than the electronics lab, the fabricator that you’re speaking about, but remember that the age of vacuum tubes and hand-wired, multicolored “spaghetti” under an aluminum chassis was still in charge; a kid being able to turn out a two-transistor radio was hot stuff.

It’s interesting to look at the history of this stuff, especially the early days when they had to make computer chip circuit masks by hand. They’d start big and shrink down:

Kind of interesting.

14. Contributor
Hank Rhody, Possibly Mad
@HankRhody

I am incapable of understanding any of this.

I like the little A/B airplane thingy at the bottom of your sketch, though.

Lemme try it again. You’re familiar with the movie Predator, right? The squad is your wafer. Push them all into the river; they get coated with cold mud, or photoresist. Then the lady sees some of them and tells them to go wash off. The lady is the exposer, and the washing off process is the developer step.

Then the Predator comes by. He’s the etchant, and he etches the heck out of anything he can see. He etches the squad away because they aren’t protected by the photoresist. Even Jesse Ventura, who didn’t have time to bleed. Schwarzenegger, on the other hand, still has is layer of cold mud on (he was out on patrol when the lady came by.) He survives the etching process.

The mini nuke on the Predator’s arm, that’s the stripper. It doesn’t matter if your photoresist was exposed or not, that stripper is going to strip it away. The analogy is imperfect becuause the stripper shouldn’t also strip everything underneath away, but you get the idea.

15. Member
Joseph Eagar
@JosephEagar

I am incapable of understanding any of this.

I like the little A/B airplane thingy at the bottom of your sketch, though.

Lemme try it again. You’re familiar with the movie Predator, right? The squad is your wafer. Push them all into the river; they get coated with cold mud, or photoresist. Then the lady sees some of them and tells them to go wash off. The lady is the exposer, and the washing off process is the developer step.

Then the Predator comes by. He’s the etchant, and he etches the heck out of anything he can see. He etches the squad away because they aren’t protected by the photoresist. Even Jesse Ventura, who didn’t have time to bleed. Schwarzenegger, on the other hand, still has is layer of cold mud on (he was out on patrol when the lady came by.) He survives the etching process.

The mini nuke on the Predator’s arm, that’s the stripper. It doesn’t matter if your photoresist was exposed or not, that stripper is going to strip it away. The analogy is imperfect becuause the stripper shouldn’t also strip everything underneath away, but you get the idea.

It might be simpler to just say you’re 3D printing layers of silicon, quartz, and some metal circuits to tie things together.  There’s also some special ink that gets thrown in to make some bits of silicon act differently than others to make transistors.

16. Contributor
Hank Rhody, Possibly Mad
@HankRhody

Tell us the story about CMP, Uncle Hank.

One of these days.

CMP is in a bit of a rough spot; it’s not something I’ve had to work with, and it’s something that was only touched on lightly in my coursework. I’m going to have to do a good bit of learnin’ before I can write about it. Going to happen on quite a bit of this stuff.

17. Contributor
Hank Rhody, Possibly Mad
@HankRhody

Joseph Eagar (View Comment):
It might be simpler to just say you’re 3D printing layers of silicon, quartz, and some metal circuits to tie things together. There’s also some special ink that gets thrown in to make some bits of silicon act differently than others to make transistors.

Possibly, but it wouldn’t give me an excuse to rewatch Predator, now would it?

18. Moderator
Randy Weivoda
@RandyWeivoda

Tell us the story about CMP, Uncle Hank.

One of these days.

CMP is in a bit of a rough spot; it’s not something I’ve had to work with, and it’s something that was only touched on lightly in my coursework. I’m going to have to do a good bit of learnin’ before I can write about it. Going to happen on quite a bit of this stuff.

Maybe CMP is just something I’ve never heard of, but is it possible you guys mean CP/M?

19. Contributor
Hank Rhody, Possibly Mad
@HankRhody

Tell us the story about CMP, Uncle Hank.

One of these days.

CMP is in a bit of a rough spot; it’s not something I’ve had to work with, and it’s something that was only touched on lightly in my coursework. I’m going to have to do a good bit of learnin’ before I can write about it. Going to happen on quite a bit of this stuff.

Maybe CMP is just something I’ve never heard of, but is it possible you guys mean CP/M?

Chemical Mechanical Planarization. You lay down enough wires on your wafer and it starts to get real bumpy. Causes problems. To deal with that the general plan is to grow a large layer of oxide and then polish it down to a flat surface again. The bumpiness gets smoothed out and you can start your next section on a flat surface, connected to the previous parts via vias.

20. Contributor
Gary McVey
@GaryMcVey

The lady is the exposer, the lady came by, that’s the stripper… exposed or not, that stripper is going to strip it away… strip everything underneath away, but you get the idea.

Okay, that’s a concept I understand.

21. Thatcher
Percival
@Percival

The lady is the exposer, the lady came by, that’s the stripper… exposed or not, that stripper is going to strip it away… strip everything underneath away, but you get the idea.

Okay, that’s a concept I understand.

Is this stripper a redhead named Tiffany?

Asking for a friend.

22. Member
Michael Minnott
@MichaelMinnott

The schematic looks like the gain section of an early distortion pedal for guitars.

Sooo…how do I add buffered inputs/outputs and add a decent tone control?

23. Contributor
Hank Rhody, Possibly Mad
@HankRhody

The schematic looks like the gain section of an early distortion pedal for guitars.

Sooo…how do I add buffered inputs/outputs and add a decent tone control?

That was in last weeks’ schematics:

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