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.
Start with an Argon plasma. Hmm… maybe let’s start a little earlier than that. A plasma is a gas where the atoms have an electron stripped off. Also, the stripped off electrons. You’ve got to keep your plasma at a pretty high energy, otherwise your atoms recapture their electrons and you end up with a boring ol’ gas. If you’re the Sun then you can make a plasma by heating up these gases to an enormous temperature. On Earth that’s less convenient, so we use electricity.
Imagine, if you will, a baseball player in a pickle. He’s caught between third and home; when he gets closer to home the third baseman throws the ball to the catcher. He can’t get the run, so he heads back towards third and safety. The catcher throws the ball to third, and all of a sudden it ain’t safe there anymore. He heads back towards home, and the third baseman throws to the catcher, cutting him off once again. That’s essentially how you generate a plasma.
Let’s say you’ve got an ion (represented by the runner) in an electric field. The positive ion heads away from the positive source and heads towards the negative source. Then you flip the polarity. The positive side becomes negative, the negative side becomes positive. The catcher has thrown the ball to third base. The runner takes to his heels and heads the other way. Once you get your ion traveling fast enough in a gas it’ll knock the other atoms, breaking off electrons, and converting more atoms of the gas into plasma. (Here our baseball analogy breaks down. It’s my understanding that when this sort of thing happens in an actual game a fight breaks out, the dugouts clear, and there’s a lot of shoving before things are resolved.)
There’s a question to answer about that process: how do you get your initial ion? You don’t get any acceleration out of neutral atoms. You need to get that initial spark from somewhere, but where? The answer lies in cosmic rays. Yes, really. You turn your electricity on, get your field cycling, and within a very short period of time, a cosmic ray will happen along and ionize a molecule inside your plasma chamber. From there your cycling field affects that one ion and does the rest.
How fast do you cycle your electric field? 13.56 megahertz (million cycles per second.) That’s… that’s an oddly specific number. The way the story goes, there was a pair of researchers inventing this stuff, and they had built a plasma generator where they could tune the frequency to test different results. Then the men in suits showed up at their door. You see, whenever you oscillate an electric charge you produce radio waves, and these two geniuses were making radio waves up and down the spectrum, creating a whole buncha static and knocking out communications at a nearby airport. The FCC guys had words for them. “What frequency are you on now? Okay good; that one’s yours. You keep that. And don’t change it!” Or so the story goes.
Okay, we can make a plasma. Make it out of argon too. Now that you’ve got your argon atoms dancing, what do you do with them? Strike ’em against a target. Get yourself a high-purity block of gold (and if I knew how to do that trick I wouldn’t be wasting time with you guys). Give your gold a negative charge, and then don’t switch it away before the argon atoms can hit it. Your accelerated argon atom will come crashing down like a stockbroker on Black Monday. When it strikes, it’ll knock a gold atom off of the surface, and that gold atom will go spinning through your chamber.
That, incidentally, is why you want to use argon atoms. It’s a noble gas; it won’t hang around on your metal surface, getting into chemical compounds. Ions are, by their nature, pretty reactive. You do the same thing with, say, oxygen on copper and you’ll end up with a copper oxide layer on your copper target. Argon doesn’t hang around. It’s also relatively heavy; you’re more likely to get a good smack out of it than, say, neon. You could do better with krypton or xenon but those are more expensive.
Okay, we left a gold atom hanging in free space like Lois Lane. What does it do? Well, it keeps going until it hits something and sticks. In Miss Lane’s case, that’s usually Superman. In the case of the gold atom it’s any old thing it finds in the plasma chamber. If you put your SEM puck out of the way of the plasma field but still within line-of-sight to the gold target, you’ll get a thin layer of gold atoms stuck to your SEM puck. Your newly pimpin’ puck has an electrically conductive layer on top of whatever you wanted to look at. Under a normal microscope, that’d distort the colors. Under an SEM you’re only getting black and white anyway, and the conductive layer is really useful.
At the tech school, we used gold to sputter coat things. At work, we used an iridium target. Neat, huh. Why iridium? In the process, we plate gold onto our parts (makes an excellent electrical contact.) If you want to know if a certain something-or-another has gold on it, you can’t very well sputter it with gold first. And so the chem lab uses iridium. With a degree of inevitability, this results in some less-than-brilliant engineer trying to track down the source of iridium contamination on the floor. To avoid this chem lab usually switches the label from iridium to … phosphorus I think; been a while since I’ve had to worry about the precise element. Naturally, this causes more problems, but not for chem lab.
But how do you know what elements you have in the first place? That’s the subject of the next post. Join us fortnight next for “Generating hard radiation for fun and profit,” or “X-ray specs!”
This is part 31 of my ongoing series on building a computer, the ‘Life’s a Beach’ way. You may find previous parts under the tag How to Build a Computer. This week’s post has been brought to you by what-the-heck-am-I-doing-when-there’s-sand-and-waves-and-girls-out-there?