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If you’re making a modern computer chip you’re going to need to put layer after layer of traces down. You’re also going to need layer after layer of insulator between your metal in order to not short circuit everything. But if you start growing oxides and depositing metals and such and so forth your wafer is going to end up wrinklier than yer grandma’s keister. That’s going to cause problems. Your layers won’t have uniform thicknesses anymore. Particularly the photoresist which means your chances at making a decent pattern degrade. Assuming you don’t end up with fatal defects from underdeveloping at the very least you lose feature size precision. Oh, and you end up with the occasional smug columnist making you visualize yer grandma’s keister. Heh.
Surely You Wouldn’t Be Bringing This Up if You Didn’t Have a Handy Gadget Solution?
Well you’re in luck Mr. Douglas. What you need is a Chemical Mechanical Planaraization rig. Or Chemical Mechanical Polishing. CMP. Whatever. (It makes a difference, but people often use the terms interchangeably.) Now, you could just deposit some borophosphosilicate glass on there and heat it up, expect the glass reflow, cover that topography like the snows o’ winter covering a shallow grave. In the olden days that was good enough; got the job done. These days the devices are smaller, the lines sharper. If you want real flatness, if you need to get down to a fifty angstrom step height over the surface of the entirety of your wafer, you want the real McCoy. CMP; accept no substitutes.
I know y’all have been waiting eagerly with your wafer in the chamber, the temperature pumped down and your native oxide layer stripped off for me to finish this two-parter. Well, wait no longer! Okay, maybe wait some as you have to find a tank of dichlorosilane to hook up so you have something to epitax onto your wafer. Di-what now?
Dichlorosilane! Or Tri- or Tetra; really anything from SiH4 to SiCl4 works, though I’m told industry generally works with SiCl2H2. Alright, you pump in dichlorosilane gas and react it on the wafer and it puts silicon on top of your silicon. Neat, huh? That’s it! Join us fortnight next for —
Hello and welcome back to How to Build a Computer. If any of y’all are worried about my long absence, well, let that be a lesson to you: The bearded nogoodnik with the dimensional transportalponder does not have your best interests in mind. Sadly, the story is much less interesting than that; I ran out of processes that I either learned about in school or worked with on the job. I’m much less happy regurgitating textbooks than I am imparting actual experience. For instance, I don’t even know if “epitax” is a real verb, but I’m going to use it like such because it’s fun to say.
With the preliminaries out of the way, let’s take a look at the wonderful world of Epitaxy. From the Greek root it looks like we’re talking about a tax atop your other taxes, but however timely and relevant such a word might otherwise be that’s not what we’re working on. What we’re building here is a crystal on top of your other crystal. Recall way back from the start how wafers are sliced out of boules that are composed of one giant crystal. There’s some advantage to remember that that’s not a perfectly flat surface. Here, let me demonstrate:
Last time, if you’ll recall, I discussed the basic idea of a chemical vapor deposition system, and described how you’d use it to deposit silicon onto your wafer. Today we’re following rather directly from that post, where we answer some important questions. Questions like “What if I don’t want to put down silicon? What other things can you offer me?” Well, for starters
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.
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.
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:
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.
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:
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.
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.
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.
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.
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’ve discussed what it means to actually store information on a hard disk drive, how you magnetize it and how you pull that information off. Neat stuff, but a bit heavy on the abstract physics. Today we’re going to zoom out a bit and look at the mechanical bits of how hard drives work. Here, let me start you off with a picture. Take a look at these two hard drives (conveniently cracked open for viewing purposes), one I borrowed from the boss man, and the other I picked up off the “Free Stuff” shelf when they moved the engineering department. Tell me which you think stores more data:
None of the above. Neither of is ever going to run again. Look at that dust!
Our story starts with Lord Kelvin, one of the great old school physicists. You can read about his career from anonymous’s old Saturday Night Science. Actually, at the point he enters this story I don’t think Kelvin had made lord yet; he was just some bloke named Thompson. This Thompson fellow was playing around with magnets and electricity and that sort of thing. What he discovered is that you can change the resistance of a wire with a magnetic field. And furthermore that that change in resistance depends on the angle between the wire and the magnetic field.
Let’s take that a little more slowly. Change in resistance when you’re in a magnetic field? Okay, I can buy that; there’s all this nonsense about wires and magnets and whatnot that I’ve been blathering about up until this point. Angle? The resistance in your wire will vary a great deal whether it’s parallel or antiparallel to the magnetic field on your disk. (Antiparallel means parallel, but facing the other direction. The northbound lane on a highway is antiparallel to the southbound lane.) If your wire is running current right-to-left and your magnetic field is pointed left-to-right then your wire’s resistance is at it’s highest because of your antiparallel configuration.
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.
We’re going to take a jaunt entirely out of sequence here, moving from circuits and silicon into larger scale components. Today we’re talking about hard disk drives. Why? Because it’s a fun and interesting technology, because I know a thing or two about it from first-hand experience, but mostly because I’ve got a book to return. And so we’ll take a quick dive into the world of hard disk drives to see what, as the bear over the mountain intended, we can see.
An example HDD. Entirely too dusty to be functional.