Building a Swimming Pool Using MEMS (ish)

[From the last episode: We looked at the normal way to build a pool, and then looked generically at what’s different when you use a MEMS approach.]

What follows echoes (but is not identical to) a piece I did for EE Journal way back in 2011. Because that audience is largely focused on designing circuits, MEMS was novel even for them, so my hope was that an analogy like this would help them as well.

So here goes. This will take a little more imagination, since it won’t be the obvious way to do it – unless you’re a MEMS designer. I’ve indicated the kind of step being performed with [D] for depositionA means by which materials can be added to a silicon wafer., [E] for etch, and [L] for litho*. We’re going to include one extra type of step that isn’t so complicated: heating something up ([H]). At the chipAn electronic device made on a piece of silicon. These days, it could also involve a mechanical chip, but, to the outside world, everything looks electronic. The chip is usually in some kind of package; that package might contain multiple chips. "Integrated circuit," and "IC" mean the same thing, but refer only to electronic chips, not mechanical chips. level, this means placing it in an oven; it’s no big deal; we didn’t spend any time looking at it. For a backyard… well… that’s where you’re imagination kicks in.

1. Start with bare ground.

2. [D] Cover the backyard with metal spiky things– perhaps shaped like jacks – so that the spikes will poke up about 6 inches (12 cm) above the ground. (You’ll see what this is for shortly.)

3. [D] Pour on a layer of rubber 2 inches (4 cm) thick. Let it harden so that it holds the spiky things in place. The spikes will now be sticking up 4 inches (8 cm).

4. [D] Cover the backyard with a layer of wax 5 inches (10 cm) deep. The goal here is to cover the spikes so that there’s a smooth surface and the spikes can’t poke through anything.

5. [L] Lay a piece of tough plastic down over the entire backyard, leaving a holeIn a material with specific places where electrons should be (like silicon), if an electron moves out of its designated spot, what’s left is called a hole. A hole effectively has a positive charge, and, as electrons move from hole to hole, it looks like the hole moves (even though, strictly speaking, it doesn’t – it just gets filled or emptied by a moving electron). where the pool will go. This will act as a mask. Make sure it’s sealed at the edges so that nothing can get to the stuff underneath it.

6. [E] Flood the entire backyard with something that will melt the wax. It will melt only in the pool area, since the rest is protected by plastic.

7. [E] Flood the backyard with… hmmm… turpentine? Something that will melt rubber, but not the plastic layer. This will remove the rubber only in the hole in the plastic; the plastic mask will protect the rest of the rubber so that it will stick around.

8. [E] Flush away everything that you can, which will remove the spiky things – but only where the pool will be. The plastic mask will protect the rest of the yard.

9. [L] Remove the plastic layer (equivalent to removing a mask). You now have a waxy, rubbery spiky layer over everything except where you want the pool. That area is now plain dirt.

10. [E] Remove the rest of the wax from around the pool. Because we’ve removed the maskA piece of glass with a pattern on it that is used to block light or let light pass through in different places. Used for photolithography., we no longer have to worry about the spikes poking through the mask.

11. [E] Using a giant air or water gun, blast the entire yard with the gun facing straight down. The rubber will protect most of the yard, but you will gouge a hole into the ground where you want the pool. Go until you reach the depth you want. (You’ll need to have a way to flush away the dirt as it comes out…) Note that, for simplicity, we’re making this pool a uniform depth.

12. [D] Spray the entire yard with a layer of concrete as thick as you want the pool walls – say, 3 inches (6 cm). You’ll get a layer over everything, but in the pool, it will stick to the sides and bottom to create the concrete shell of the pool. The spikes will be sticking up 1 inch (2 cm) in the area away from the pool (but not in the pool – that’s critical!).

13. [E] The problem now is that you have concrete everywhere. You want to remove it from the yard, but not from the pool. This is where the spikes come it. Flood the yard with an acid that will dissolve the metal spikes, but nothing else. Because the spikes go through the concrete and rubber around the pool, the acid will drill little holes around there as it follows the spikes down below the concrete and rubber.

14. [D] Cover it all with a thin layer of something goopy like epoxy; let it harden. This will seal the little holes for now, trapping air inside the rubber and concrete. (It will cover the pool area too, but we don’t care.)

15. [H] Take a big torch to the yard, or have some other way of heating the entire yard. The air in the holes will need to expand, but it can’t because the epoxy is blocking it. (Wax wouldn’t work here because the heat would melt it.) The expanding air will squeeze the rubber, but it can’t deform the concrete because it’s brittle. So the concrete will form lots of little cracks under the pressure. But this will happen only around the pool, not in the pool (since there were no spikes in the pool).

16. [E] Now flood the yard with something to etch away the epoxy. The holes will be open again.

17. [E] Now flood the yard with whatever you used to melt the rubber. It will flow into the little holes, melting the rubber underneath. But, again, this won’t happen in the pool.

18. [E] Flush everything away. The concrete around the pool is now all cracked up, so it will disintegrate and flow away – as will the melted rubber underneath. You’ll be back to dirt around the pool, and yet the concrete lining the pool will remain.

That’s 17 steps instead of just 3 for doing it the normal way. And I had to get pretty creative when figuring out how to get rid of unwanted concrete. That’s what the spikes were about, and there were a lot of extra steps needed to handle those spikes.

This is the kind of thing MEMS designers have to do. They have very few tools available (there are no micro-backhoes), so it can be a lot of extra work.

Hopefully the video will help clarify any confusion that will remain. If this hurts your brain, don’t worry; it hurt mine while thinking it through. It is truly not an obvious way to do things. But it’s a powerful way.