Tag: Crystallography

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. 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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