[From the last episode: Building 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. starts with a 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. wafer. From there you add and remove material with the assistance of 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..]

This week’s piece is about what we called depositionA means by which materials can be added to a silicon wafer. in last week’s edition. “Deposition” actually means something more specific when processing silicon 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., so, for the moment, I’m using the word more generally – as a way to describe how we add materials to a wafer.

You could think of this as adding materials to a multimedia art project. You might add paint – using a brush or spattering or airbrushing; you might add found materials; you might add something that blocks paint, removing that material later. This all fits into the general notion of deposition here: you’re depositing stuff on something.

Ingredients in the Recipe

There are lots of ways to add materials, and some of the ways reflect what materials are being added. And you might add materials for lots of different reasons.

• You might want to add more silicon to the wafer.
• You might want to add oxygen so that you can convert silicon (a semiconductorA material that, under some circumstances, can conduct electricity and, in other circumstances, cannot.) to silicon’s 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., silicon dioxide (SiO₂ – an insulatorA material through which electricity cannot readily flow. Plastic is a good, familiar example.).
• You might add other specialized oxides with beneficial properties.
• You will want to add photoresist for use in defining patterns. This is important for photolithography.
• You might want to add various specialized compounds like silicon nitride (SiN) for various reasons.
• You will want to add metals to connect circuits; you can also add them as structural elements in mechanical chips.
• You might want to embed atoms of some kind (other than silicon) below the surface of the wafer to change the electrical characteristics of the silicon.
• In mechanical chips, you may want to use so-called piezoelectric materials. These are special materials that, when bent or otherwise stressed, change their electrical characteristics (or vice versa: when put into an electric field, they may bend or otherwise deform). This is extremely useful for converting mechanical changes to and from electrical signals.

Many Ways to Add Materials

There are multiple techniques for adding materials, and they depend on what the material is and how you want it applied.

• If you want to add more silicon and keep the crystalline nature of the wafer, then you can literally grow more silicon on top of the wafer. Done right, the added silicon will nestle into the existing crystal structure so that you can’t see the seam between the original wafer and the new silicon. This process is called epitaxy.
• Deposition (in its more specific sense) is a catch-all for simply taking a material and coating the wafer with a thin layer. For example, if you want more silicon but you don’t care about it being an extension of the wafer’s crystal (or about it being a crystal at all), you can add a layer of polycrystalline silicon (commonly abbreviated as poly). There are lots and lots of materials that can be applied this way, usually starting as a gas.
• Adding metals is often done using a technique called sputtering. You do this by taking some solid block of the metal and then bombarding it with ions or some other tiny particles. This literally causes metal to spatter and splash out of the solid in tiny droplets that land on the wafer.
• If you want to embed atoms deep into the silicon, you can take ions of the material and literally fire them at the wafer, embedding the ions below the surface. This is called ion implantation.
• If you want to create silicon dioxide (useful in circuits for separating different conducting regions with an insulator; useful in mechanical devices simply as a material), then, by adding oxygen, you get a reaction with the silicon that’s already on the wafer, and the silicon turns into silicon dioxide.
• Finally, there’s a version of deposition that allows extremely close control over how much material you add when you want something extremely thin. It’s called atomic-layer deposition, and the idea is that you can add a single layer of atoms (plus or minus) of some material, repeating until you have the thickness you want.

We’ll see why we might want to add some of these materials later. Before we do that, we need to understand the ways in which we can remove materials. If it seems like this is all a little vague, you’re right; it is – for now. Once we have a picture of deposition, etchingA means by which materials can be removed from a silicon wafer., and photolithography, we can bring it together, and it will make more sense. So… hold that thought.