# Tag: Crystallography

## How to Build a Computer 39: Epitaxy II: The Reckoning

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 —

## How to Build a Computer 38: Epitaxy

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:

## 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.

## 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?

## 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: