Tag: How to Build a Computer

Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 20: Digital Watches

 

Because I’m the sort of simian who still thinks that digital watches are pretty neat, I figured I’d work through a practical example. We know how to represent binary numbers, and we know how to express logic with gates. That’s enough knowledge to be dangerous. From there I worked out how to light the lights on a digital watch face. Here, let me show you. But first a quick disclaimer. This isn’t my area of expertise; odds are there are plenty of ways to do this better or more efficiently. I can say at least that this one works.

Okay, let’s talk numbers. Recall from the discussion of binary that you can express any normal number as a series of ones and zeros. So, for example, you could draw One (that is, the presence of bread) in the 4’s place, one in the 2’s place, one in the 1’s place. 4 + 2 + 1 = 7

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 19: Logic Gates

 

Welcome back to How to Build a Computer. You recall where we’re at, right? Hah! Trick question. As if I’d stick to a rational sequence. Today we’re going over some of the details in how you go from electrical circuits doing whatever it is that electrical circuits do and turn that into logic. We’re talking Logical Gates

Logic gates! Each one more logical than the last.

Logic gates are transistor circuits that you can use to modify a signal. Let’s take the NOT gate as an example. If the input is on, the output is off. If the input is off, the output turns on. Whatever you put in, you get not-that coming out. Simple enough. Except the part where you’re creating energy out of nothing; what’s up with that? Well, not pictured you’ve got a five-volt source and a grounded drain. When you’re creating energy out of nothing you’re actually stealing it from that source. Those details make laying out your circuits more complicated but can generally be ignored when you’re drawing logic gates. Heck, as long as I’m recycling my drawings have another:

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Contributor Post Created with Sketch. Member Post

 

Listen, I’m going to be straight with you. This one is mostly for my fun. I mean, they’re all up largely because I like to hear the sound of my own voice. But this one, this one is a bit superfluous. This is the quantum mechanical explanation for how semiconductors work. I’ve already described the […]

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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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Today we’re talking about strippers. By which I mean the machines and the process of stripping photoresist off of your material once the pattern has been applied. Now, the main problem with the strip operation, before you even get to the chemistry involved, is the number of bad jokes that are available. You don’t start […]

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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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Recommended by Ricochet Members Created with Sketch. The Texas Two-Step

 

In a recent installment of @hankrhody ‘s excellent (and delicious) series on building a computer, he wrote about how to do a binary search. In the comments, I made an oblique reference to a better way to do that kind of matching, referring to something I called the Texas Two-Step.

Now don’t get me wrong; in many situations Hank’s solution is an excellent choice, particularly if you want to do a single, real-time lookup. For a single or only a few lookups, Hank’s way is hard to beat. But what if you want to shuffle the whole deck?

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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. Member Post

 

Screw it, I’m taking the week off. [First] [Previous] [Next] More

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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 11: The Binary Search Algorithm

 

We’re taking a break from the manufacturing process to cover some ideas in programming. Algorithms, what that means and why. Sounds fancy, doesn’t it? It ain’t as bad as it sounds. Let’s jump right in:

What’s An Algorithm?

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We’ve seen how photoresist works, now we’re going to see how that actually works in the real world. Before you can print things with the patterns you draw in your photoresist you’ve got to draw those patterns. Before you can draw those patterns you’ve got to stick your photoresist on there. Today we’re going to […]

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 8: Organic Chemistry

 

I started with a discussion of the magic of photoresist, however (say it with me!) it got long-winded and I cut it down to the organic chemistry review. Next week photoresist. This week we’re going over some basic organic chemistry. Sounds fun, right? It’s going to be even more fun than that! You wait and see. We’re going to start small though, with methane.

You smell something? No? It’s probably just me.

This is a carbon atom, often found in the presence of ranch dressing. It’s surrounded by four olives, or hydrogen atoms. The pimento is only there for flavor. Carbon though is much like Silicon in that it comes with four electrons in its outer shell. In terms of orbitals that works out to 1s2 2s2 2p2. Carbon is in something of a unique position; you can make long and interesting chains of molecules with it. You can try making chains of other molecules, say, oxygen, but the results get… explosive. Anyway, a single carbon atom makes the simplest possible carbon chain. Let’s make it a more complicated.

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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 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.

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 6: Simple Transistor Circuits

 

The problem with simple transistor circuits is that any circuit with a transistor in it isn’t all that simple. And frankly, I don’t know how much you know about circuits; I’m guessing it ranges from “nothing at all” to “teach your grandmother to suck eggs why don’t you.” At the risk of boring the latter crowd we’re going to give this a slow and superficial treatment. Let’s start with a circuit that’s just about as simple as I can make it. So simple it doesn’t even have a transistor in it!

I’d make this circuit more interesting but I don’t know the symbol for ‘electric chair’.

 

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 5: Fundamental Chemistry

 

I know I promised simple transistor uses last time. Thinking about it though, I’d rather go into a bit more detail about the electron golfing I described earlier. It’s a neat analogy, but it doesn’t cover some things you can do with diodes. Interesting things. Therefore we’re gonna dive in for a deeper understanding of chemistry, atoms, and cartoons. Let’s look at a model of an atom using common household objects:

You all have your Ricochet mugs, right?

An atom consists of protons and neutrons in the middle and electrons outside. The common picture of an atom has those electrons whizzing in neat, well-defined orbits. That’s wrong. It’s closer to electrons having spaces they hang out in. We’re on the level of quantum mechanics here, so odd stuff happens. It’s not actually possible to tell where an electron is; it’s small enough that you can only give probabilities. Hmm… let me try it again.

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 4: Diodes and Transistors

 

We all love blasting things with ions, and most of us could spend all day shooting at wafers, but eventually someone is going to ask you to build something useful. What am I doing with all this mess of silicon anyhow? Here’s where we see the use of all that stuff. What do you suppose happens when you put a p-doped chunk of silicon next to an n-doped chunk of silicon?

One last cookie photo, then I’m going on a diet. Swearsies.

Diodes to Kill For

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer 3: The Hows of Doping

 

First you gotta find a dealer. Right, not that kind of doping. Today we’re going to discuss how to how you mix your dopant atoms into your silicon wafer so you can make transistors.

How Do I Dope My Wafer?

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Contributor Post Created with Sketch. Recommended by Ricochet Members Created with Sketch. How to Build a Computer Part 2 of N: Crystallography

 

Last week we saw how to turn sand into silicon. This week I was planning on showing you how to turn silicon into a semiconductor. I mean more of one than it already is. Unfortunately my brief notes on crystallography went long. This week we’ll discuss crystals, next week we’ll do doping, and the week after that we’ll finally get to transistors. Unless I wax even more loquacious, which is the way the smart money is betting.

In a crystal every atom is slotted neatly into an ordered lattice, and every spot in the lattice has an atom in it. With some exceptions. Actually those exceptions are most of what we’re going to talk about today. Let’s assume this is a perfect silicon crystal:

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