(This video is from EngineerGuy series #4)
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I think this is one of the coolest features of today’s smartphones. It knows up from down! Build into the circuitry is a tiny device that can detect changes in orientation and tell the screen to rotate. Let me show you what it looks like in an old iPhone.
There it is: It’s an accelerometer. I’ll tell you how this kind of chip works and how its made, but first, some basics of accelerometers.
They have two fundamental parts: A housing attached to the object whose acceleration we want to measure, and a mass that, while tethered to the housing, can still move. Here its a spring with a heavy metal ball. If you move the housing up the ball lags behind, stretching the spring. If we measure how much that spring stretches we can calculate the force of gravity. You can easily see that three of these could determine the orientation of a 3-dimensional object. While lying with the z-axis perpendicular to gravity only the ball on the x-axis spring shows extension. Turn this on it side so that z-axis point up and only the accelerometer along the spring on that axis stretches.
So, how does this phone and this chip measure changes in gravity? While more complex than the simple ball and spring model it has the same fundamental elements. Inside the chip engineers have created a tiny accelerometer out of silicon.
It has, of course, a housing that’s fixed to the phone, and a “comb-like” section can move back and forth. That’s the seismic mass equivalent to the ball. The spring in this case is the flexibility of the thin silicon tethering it to the housing. Clearly if we can measure the motion of this central section we can detect changes in orientation. To see how that’s done examine three of the fingers on the accelerometer.
The three fingers make up a differential capacitor. That means that if the center section moves than current will flow. Engineers correlate the amount of flowing current to acceleration.
This accelerometer fascinates me, but even more amazing is how they make such a thing. It would seem nearly impossible to make such an intricate device as the tiny smartphone accelerometer. At only 500 microns across no tiny tools could craft such a thing. Instead, engineers use some unique chemical properties of silicon to etch the accelerometer's fingers and H-shaped section. To get an idea of how they do this, let me show you how to make a single cantilevered beam - like a diving board - in a solid chunk of silicon.
Empirically, engineers noticed that if they pour potassium hydroxide on a particular surface of crystalline silicon it would eat away at the silicon until it forms a pyramidal-shaped hole. This occurs because of the unique crystal structure of silicon. To make a pyramidal hole in silicon engineers cover all but a small square with a mask impervious to the KOH. Now, it only etches within the square shape cordoned off by the mask. The KOH dissolves silicon faster in the vertical than in the horizontal direction. This why it makes a pyramidal hole. Now, to make a cantilevered beam engineers follow these steps. First, mask the surface except for a u-shaped section. At first the KOH will cut two inverse pyramids side-by-side. As the etching continues the KOH begins to dissolve the silicon between these holes. If we wash it away at just the right point - before it dissolves the silicon just underneath the mask - it will leave a small cantilever beam hanging over a hole with a square bottom.
Engineers make smartphone accelerometer using these same methods, but as you can picture it takes a series of detailed masks to create the intricate structure of a smartphone accelerometer. While complex, a key point is that the whole process can be automated. This is absolutely essential in the miniaturization of technology; engineers now make all sorts of amazing things at this tiny scale: microengines with gears that rotate 300,000 times a minute; nozzles in ink-jet printers, and my favorite, micromirrors that focus light in semiconductor lasers.
I’m Bill Hammack, the Engineer guy.
This video is based on a chapter in the book Eight Amazing Engineering Stories. The chapters features more information about this subject. Learn more about the book at the address below.