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Contributor Post Created with Sketch. How to Build a Computer 34: Etching

 

We’re moving back from the series on measurement to the whole process of making computer bits out of silicon. Way back, starting with Computers 7, I started a series on patterning; how you can take an idea and draw it small enough that you can apply that pattern to these really tiny circuits. I went over, step by step, each thing you need to do to create the pattern. I skipped entirely the bit where I tell you what, exactly, you do with one of those patterns when you’ve got them. This is the first of a couple of articles that fit, in manufacturing terms, between Computers 15 (Developing), and 16 (Stripping). You develop your pattern on with photoresist, this is how you make it permanent.

We’ll start with etching. Broadly speaking ‘etching’ covers any process where you start with more material and end up with less material. I mean aside from gambling. Let’s say you’ve got your silicon wafer, you want to etch some of that silicon away. To do this we start by burning your wafer. …Okay, perhaps that’s poor phrasing. Put the flamethrower down and I’ll describe what I mean. To protect your silicon wafer from the damage the etching process would do to it we’re going to want to mask it, with a silicon dioxide layer. Heat your wafer up in the presence of oxygen and this happens:

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Contributor Post Created with Sketch. How to Build a Computer 31: Sputtering

 

Today we answer an important question: “How do I coat things in metal; even things that don’t want to be coated in metal?” You want to plate gold onto you Sacajawea dollar, that’s easy enough. You can use electricity to get one metal to stick to another. You want to cover Jill Masterson you use gold paint. But let’s say you’ve got a little plastic doohickey you want to look at under an SEM. Plastic famously refuses to conduct electricity. So how do you defeat the charging problems? (The charging problems that we mentioned last time. You were paying attention, weren’t you?) The answer is you sputter coat it. And this week I’ll be explaining what that means.

Also in the SEM lab; you can tell by the example images they’ve stuck into the window.

Start with an Argon plasma. Hmm… maybe let’s start a little earlier than that. A plasma is a gas where the atoms have an electron stripped off. Also, the stripped off electrons. You’ve got to keep your plasma at a pretty high energy, otherwise your atoms recapture their electrons and you end up with a boring ol’ gas. If you’re the Sun then you can make a plasma by heating up these gases to an enormous temperature. On Earth that’s less convenient, so we use electricity.

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 30: SEMsational

 

This is a continuation of last time’s discussion on Electron Microscopy. In that one, we covered the question of why you’d want one of these and gave a summary of how you’d work one. Take some electrons, throw it at your sample, and watch what bounces off for information. Sounds so simple when we put it that way, right? This week we’re talking about what happens when you actually buckle down to do it in practice.

Taken from Chem lab, when there weren’t any chem techs around to stop me.

Okay, just looking at the thing isn’t doing me much good. What’s going on there, and why? Start from the top. That bottle on the left? That’s for liquid nitrogen, used in the x-ray detector. (Neat! Why do we want to detect x-rays? That’s a subject for a future column.) The cylinder on top marked “GEMINI” is your column; the electron gun is in the top, and the rest of it contains the magnets for focusing and directing the electron beam. The cube-ish box it’s sitting on is your sample chamber; the front pulls out to reveal the stage where you’d put your puck holding the samples. The dark grey table surface is granite, used to lend stability to the whole apparatus. The cabinet it’s sitting on contains electronics and the vacuum pumps. Now let’s get to how all that works together.

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 23: Magnets

 

Hard Disk Drives record data using a technology long known to baffle juggalos. The read/write head uses magnets to store information on those disks. How? Why? What does that even mean? Let’s jump in.

What makes a magnet a magnet? Moving electricity. When you get down to the atomic level atoms are magnetic because their electrons are spinning. Glomp a bunch of those atoms together (like sticking magnets one to another) and you have a grain. Get enough grains lined up in the same direction and you have a permanent magnet.

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Contributor Post Created with Sketch. How to Build a Computer 36: Chemical Vapor Deposition

 

No matter how much fun you’re having etching silicon, applying and stripping photoresist, or implanting ions, sooner or later you’re going to have to actually put down some lines. Gotta build a circuit eventually. Chemical Vapor Deposition (CVD) is one of the main ways this gets done. Let’s have a look at what we’re doing, shall we?

If I had known I was going to use this picture at least three times I might have put a little more effort into the sketching.

What you’re looking at is a jump over a wire. You have two wires that need to cross but not touch each other, you gotta do something like this. Let’s go over the process to get there:

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Contributor Post Created with Sketch. How to Build a Computer 35: Anisotropic Etching

 

Last time we talked about how to make tiny little holes in silicon using harsh acids. Wet etching is fine and all, but sometimes you just can’t make a feature small enough. You’re limited by the aspect ratio. That is, how wide it is versus how tall it is. A post hole has a high aspect ratio because it’s much deeper than it is wide. A strip mine is a pretty low aspect ratio hole. The difficulty with making high aspect ratio holes in your silicon is that your etchant is going to etch down, yes, but it’s also going to etch towards the sides.

Before we get into dry etching there’s one more trick for making an anisotropic (uh, it etches downward quicker than it goes sideways. Literally the word means not-the-same-in-all-directions.) wet etch. What happens if you do your etching with a strong base instead of a strong acid? As it turns out, and for no reason, I’ve managed to determine, a strong base will etch one crystal face preferentially.

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Contributor Post Created with Sketch. How to Build a Computer 33: Atomic Force Microscopy!

 

Atomic Force Microscopy is a refinement of that long and hallowed scientific tradition: poke it with a stick and see what happens. Picture, if you will, a blind man walking across the street. He taps the ground with his cane, profiling the height of the surface. That tells him where the curbs are; he doesn’t trip because he knows when to step up and step down. Now picture that blind man in a skate park, full of ramps and contours. He could, by painstaking effort, tap his cane up and down the entire area of the skate park and build up a picture in his mind where all the half-pipes lay, even though he can’t see ’em himself. Now picture him in that same skate park, doing kick-flips and grinding like a pro. Because that sounds awesome.

Three square microns of (highly ordered pyrolitic) graphite. A friend of mine measured this as part of a school project we worked on. This is after a metaphorical baseball bat to the head of mathematical smoothing.

Atomic Force Microscopy builds up a portrait of the surface of a thing by rubbing a tiny, tiny needle across it, and reading it like you’d read the grooves on a record. Heck, you could probably play it like a record too, only it’d sound all staticky because nobody bothered to lay down music on that spot to begin with. (Although…)

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 10001: Binary

 

We’ve just come off a long saunter through the manufacturing process. We’ll go back soon enough I promise you, but I figured that we could stand a changeup. We’ll be visiting the wild and wonderful world of binary today. Despite what you may have been told there will be math.

The 10 Types of People in This World

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Contributor Post Created with Sketch. How to Build a Computer 32: X-Rays

 

I think x-rays have had their dramatic potential shortchanged by the way they’re actually useful. You hear “gamma rays” and your mind is drawn to the Incredible Hulk and how he gained his bright purple shorts. Cosmic Rays? Space madness! But when your mind turns to x-rays you start thinking “dentistry.” Much less exciting.

Right. Computers. Today we’re going to spend one more post on Electron Microscopy, and another way these things are useful. This one is actually pretty straightforward from topics we’ve already covered. I’m sure y’all have been taking notes, and know immediately that I’m referring to Computers 5: Fundamental Chemistry, where I described the process of prodding electrons into giving up photons. I’ll save you the reread, even though jokes about New Jersey never get old. Here are the useful bits:

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 15: Developing

 

No, this isn’t a story that’s still in progress, this is a process step. ‘Developing’ in this context means you’re chemically removing half your photoresist (either the stuff that was exposed in a positive photoresist or the stuff that wasn’t in a negative photoresist.) And no, it’s not about guys making software either. Look, if you’re going to stop us every single time the chance for a bad joke comes up… actually that’s pretty much the game plan. Carry on.

An exposed photoresist molecule. The cheese is carbon, the sausage is oxygen, and I’m thinking it’s high time to conduct some more chemistry, if you know what I mean.

Recall the cheese and sausage photoresist molecule from earlier: That group on the right, that’s a carboxylic acid group. Keyword there being ‘acid’. Add a base to it and you’ll fizz away the resist like Sean Connery’s chest wound in The Last Crusade. Exactly like it. I think they passed off a science-fair baking soda volcano like it was Hollywood magic. But no matter. Developing.

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 14: Alignment

 

Last time we saw how you physically expose a panel. That is, how you shoot it with ultraviolet light to get a pattern into the stuff so that you can do things to that pattern later on. Today the plan is to talk about all the ways this can go wrong. We’ll start with the big one: alignment. If you’ll recall the profile of the jumping trace we looked at a couple weeks ago:

Hooray for a well-stocked media library!

See that trace on the top? Suppose you were to shift it over to the right. Eventually, you’d lose contact with your left via and you’ve got a hole in your wire. Busted circuit, sorry, can’t sell that one. Now imagine you’re shifting it forwards or backward; sooner or later you lose contact with your via and again you start making scrap. Or twist it side to side. Or shift it and twist it. Suddenly you’re wondering how they get these things on there at all. Don’t worry, it gets worse. Suppose both the vias and the top trace are aligning to the bottom traces. The vias get printed in an okay spot, but a little south of where they ought to be. Still in tolerance. The trace gets placed in its own okay spot, but a little north of where it ought to be. See where I’m going with this? The compounding of the two errors is enough to, again, cost you money. The problems compound when you have a second phototool on the bottom to align as well.

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 12: Exposing

 

We know how photoresist works and how to get it on your material. Or my material rather; most of you aren’t going to be running laminators but no matter. How about transferring a pattern to the photoresist? That’s what we’re going over today. To change the photoresist you’ve got to hit it with an ultraviolet photon. Here, let me demonstrate:

A chunk of photoresist with a bunch of common engineering tools on it. Especially the fez.

I set that out on my cubicle desk several hours before snapping this photo. The small amount of ambient ultraviolet light in the white office light has been bombarding it for hours. Now let’s take all those funny-shaped tools off and see what’s underneath

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 28: Video Edition!

 

Coming to you taped from the Wastes of Wisconsin Winter we present a special video edition of how to build a computer. In this post I take apart a hard drive and look at the bits piece by piece. Thrills, chills, blood and laughter, folks this film has it all! And at a price so low I’m practically giving it away.

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 29: Electron Microscopy

 

For the next couple of posts, we’ll be sauntering through the science of measurement. To put it simply, computer bits are really, really small. So as you wander through the world of building them how do you know you’ve made the thing right? Well, let’s start simple. You can just look at ’em. I could go on a great big tear about optical microscopy which is still an important subject, and relevant. The problem with it is that I just don’t find the subject very interesting. Still, you get some neat images.

This is my fingerprint, photographed on the background of one of them hard drive platters I ripped out of that drive in the video. FBI please ignore.

To understand why you need the electron microscope it helps to spend some time with an optical microscope. The majority of the time I spent looking at parts I spent looking through an optical microscope, not on the SEM. Largely because Chem Lab owned the SEM, and they get all fidgety when someone else touches their stuff. Briefly though, I think I can demonstrate the usefulness of an electron microscope with two images.

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 9: Photoresist

 

We’ve just got off a quick overview on organic chemistry. Now we’re getting back to photoresist. The point of photoresist, if you’ll recall, is to take a pattern so you can print stuff on your wafer. To do that it has to be a chemical that responds to ultraviolet light. And I mean more “responds to” than get a mild sunburn; it’s got to chemically change so you can transfer the pattern of light into a pattern of stuff.

It’s a polymer made of benzene rings. Someone’s showing off.

The word “Photoresist” covers a great deal of variation, but the nature of the job it has to do requires certain commonalities. For starters, rather than all one substance, it’s a mix of three different things. You’ve got a photoactive compound, naturally. You’ve also got a resin, for stability. And then you’ve also got a solvent, for instability. The solvent keeps your resist liquid so you can apply it evenly. The resin keeps it solid once it’s on, so that it doesn’t move around as you’re working with it. That picture up there is of a resin. A thing called meta-cresol novolac. Can’t tell you why.

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 27: Data Recovery

 

We’ve covered the physical aspects of a hard disk drive, tonight we’ll touch on the way data is organized on the drive, by covering those two most important topics; keeping secrets and ferreting other people’s out.

In this case describing the times this joke has been used since it was last funny.

We’ll start by deleting files: Let’s say that I’ve got a backlog of old and worn-out memes to purge. That’s no problem, you just move them from the exquisitely detailed and organized archive of these things into the trash can, but that doesn’t actually erase anything. Bill Gates, knowing that we mere mortals are flawed and prone to regret, keeps your trashed files around in case your stale jokes may, someday in the future, be called for again. But we’re stronger than that. So we empty the trash folder (or, pro-tip; on a Windows box if you hold down ‘Shift’ as you delete a file the file doesn’t go to the trash at all; it empties automatically.)

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