Tag: Plasma

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 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.