[From the last episode: We will now turn our attention to the technologies that allow both sensorsA device that can measure something about its environment. Examples are movement, light, color, moisture, pressure, and many more. and actuatorsA way of controlling some device electronically. It might turn the device on or off or change a setting or property or do any other thing that the device is capable of. to be miniaturized.]

We will, for the next few weeks, be looking at the basics of building computer chipsAn 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.. Except that we’ll be taking this in a direction other than computer chips. So why use the same technology?

We can summarize the answer in one word: siliconAn element (number 14 in the periodic table) that can be a semiconductor, making it the material of preference for circuits and micro-mechanical devices.. Silicon has become the darling material of technology. Silicon Valley has been exported around the world into the numerous Silicon <state your geological feature here> places: Silicon Forest (Oregon), Silicon Glen (Scotland), etc.

Silicon’s popularity comes originally from the fact that it’s a so-called semiconductor. Most materials fall into one of two categories:

• Those that can readily conduct electricity. These are called conductorsA material through which electricity can flow. Metals are a good, familiar example.. Metal wires (or metal in any shape) are a good example.
• Those that cannot conduct electricity. These are called insulatorsA material through which electricity cannot readily flow. Plastic is a good, familiar example.. Plastic and (dry) wood are examples.

Between Conductor and Insulator

But there are a few materials that can be made to conduct or not depending on how they’re made and how they’re electrically set up. These are the semiconductorsA material that, under some circumstances, can conduct electricity and, in other circumstances, cannot.. That ability to control them is great for circuits, since circuits are all about controlling the flow of electricity.

So that’s great for circuits, but we’re not here to talk about circuits. What does this have to do with sensors and actuators?

Some clever people figured out how to adapt the steps made to use a circuit in a way that lets them build mechanical devices at a ridiculously small scale. We’re talking dimensions in microns (abbreviated μm*), or a millionth of a meter. For perspective, a human hair is 17 to 181 μm in thickness.

Silicon isn’t the cheapest material out there; plastic is cheaper. And you can make circuits out of the right plastics, but they’re really slow and not nearly as small as silicon circuits. But, compared to the many other materials that could compete well with silicon, silicon is cheap. People think of it as sand (although, if you want to be precise about it, sand is the oxideA substance created when oxygen binds to some other element. When that happens with iron, we get rust. When that happens with silicon, we get materials like sand and quartz. Often, the oxides of conductors and semiconductors will be insulators. of silicon).

And there’s a huge industry already in place that can make things out of silicon. If mechanical devices were the only thing being made (that is, no circuits), would we choose silicon? Maybe; maybe not. But the circuits came first, so sensors and actuators get to take advantage of all of the manufacturing infrastructure that’s already in place. Reusing and adapting existing processes, equipment, and factories is always cheaper than inventing new ones.

It’s Wafer Thin

Silicon chips are made on wafersIn the context of making circuits, sensors, and actuators, a thin, round slice of pure silicon. Multiple devices will be made on it; it will then be sliced up to separate the individual chips.. A single wafer will contain many chips – perhaps hundreds or more, depending on the size of the wafer and of the chip. You can clearly see, on the image below, the individual chips arranged in a grid. Precise saws (or, occasionally, lasers) cut along the lines separating the chips (called scribe lines). The bigger the wafer, the more chips it can hold. Because the whole wafer gets built at once, you’re making all of the chips at the same time. The more you can do at the same time, the cheaper the chips are.

In fact, wafers can range in size, although smaller ones usually are from older technologies. As we’ve learned how to make them better, they’ve gotten bigger. Today’s mainstream wafers are 300 mm across – about 12 inches. (There has been talk about doing 18” wafers, but the benefits of going bigger than 12” aren’t as clear.)

Wafers are created by carefully slicing long ingots of pure silicon that are made pretty much like rock candy: by letting molten silicon crystalize around a core as it’s slowly pulled out of the melt. Once sliced, the wafer is polished to a high sheen, and then the fun starts.

Making Things on Wafers

You can more or less divide the steps used to make things on silicon wafers into three categories:

• DepositionA means by which materials can be added to a silicon wafer., where material is added to the wafer;
• EtchingA means by which materials can be removed from a silicon wafer., where material is removed from the wafer;
• PhotolithographyA way of creating patterns on a silicon wafer for selective deposition or etching. You can think of it as taking a picture of the pattern and having it show up on the wafer. In the industry, photolithography is often abbreviated as lithography., which allows for creating patterns that will define the features of whatever is being built.

These three steps are used both for making circuits (which we won’t look at for now) and for making mechanical devices. We’ll cover each of them over the next three weeks. We’re not going to get into rigorous detail; it doesn’t take an engineering degree to figure out what these look like. (It does take one to figure out how actually to do it, however.)

* The μ stands for “micro”; the m for “meter.” Yes, this should – and can – be pronounced “micrometer”; “micron” is historical and more common, however.

 

(Wafer photo credit: By German Wikipediabiatch, original upload 7. Okt 2004 by Stahlkocher de:Bild:Wafer 2 Zoll bis 8 Zoll.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=928106)

(Ingot photo credit: CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=314282)