What is epitaxy? How a perfectly ordered layer of silicon grows on a wafer

Epitaxy (in the industry, often just "epi") grows a new crystal layer on a wafer: the new atoms follow the arrangement of the wafer underneath, one layer at a time, so the new layer comes out just as orderly. Most often it's a layer of silicon on a silicon wafer, but other materials can be grown this way too. The word comes from Greek: epi means "upon" and taxis means "arrangement".

What does "orderly" mean here?

The wafers chips are made on are single-crystal silicon: all the silicon atoms in the wafer face the same direction and line up in a neat, regular grid (for where wafers come from, see "Why are wafers round?").

Picture a wall built from identical bricks. The bottom rows are laid perfectly square, and every row after that follows the joints of the row below, so the whole wall is just as neat. Epitaxy works the same way: the wafer is the bottom rows, and the new atoms follow its grid upward. When it's done, the new layer and the wafer are one continuous crystal.

How is epitaxy different from ordinary film deposition?

In most of the films on a chip, the atoms don't have to follow the layer below (for how ordinary films are grown, see "What is thin film deposition?"):

	How the atoms are arranged	Like	Example
Amorphous	No fixed pattern	A pile of bricks dumped on the ground	Insulating layers such as silicon dioxide and silicon nitride
Polycrystalline	Many small pieces, each orderly, all facing different ways	Many small walls pushed together, their joints not lining up	The polysilicon often used for transistor gates
Epitaxial (single crystal)	One direction through the whole layer, continuing the layer below	Following the joints below all the way up	The silicon layer on an epi wafer

Silicon grown on exposed single-crystal silicon, under the right conditions, continues the grid and becomes an epitaxial layer. Grown on an oxide layer, it has no grid to follow and turns polycrystalline. The most common way to grow an epitaxial layer is also chemical vapor deposition (CVD), where gases react on the hot wafer surface; the conditions just have to give the atoms time to line up.

How is an epitaxial layer grown, step by step?

Take a layer of silicon on a silicon wafer.

Step 1: Clean it

• What: chemicals wash particles, organic residue and metals off the surface, then the wafer is rinsed and dried (see "What is wafer cleaning?").
• Why: the new atoms attach to the topmost row of the surface, so the surface is their template.
• If not: wherever something is stuck, the atoms can't attach properly and line up wrong. The mistake also grows upward, like a crooked brick in a wall: every row laid on top of it is crooked too, all the way to the top.

Step 2: Remove the native oxide

• What: when silicon meets air and water, its surface grows an oxide only a few atoms thick on its own. A quick dip in very dilute hydrofluoric acid (HF) dissolves it. Once the wafer is in the tool, it's often heated briefly in hydrogen to drive off whatever is left.
• Why: the oxide isn't arranged in a grid. It's like a thin coat of cement over the brick wall: the new atoms can't see the joints underneath.
• If not: the layer comes out with far more defects, or not orderly at all. So a cleaned wafer can't wait long, or the oxide grows back. That's Q-time.

Step 3: Into the chamber, and heat it up

• What: the wafer goes into the sealed chamber of an epitaxy tool and is heated to around 1,000 °C, higher in some processes.
• Why: once atoms land on the surface, they need enough heat to move, travel a short way and find a spot that lines up with the layer below.
• If not: too cold, and the atoms stick wherever they land, so the layer turns polycrystalline or even amorphous. But high heat also makes atoms implanted earlier spread sideways (the thermal budget), so some steps use a lower temperature.

Step 4: Flow in a silicon gas and grow it layer by layer

• What: a gas such as silane, or a silicon compound containing chlorine, flows in, often carried by hydrogen (see "What gases does a chip fab use?"). The gas breaks down on the hot surface and leaves silicon atoms behind. One layer of atoms fills in, then the next, usually until the layer is somewhere from under a micrometer to a few micrometers thick (a micrometer is a thousandth of a millimeter).
• Why: the atoms arrive one by one, so each has a chance to settle into a spot lined up with the layer below.
• If not: if the temperature, the amount of gas and the time aren't balanced, the thickness comes out wrong and the atoms don't have time to line up.

Step 5: Dope it as it grows

• What: at the same time, a little gas containing phosphorus or boron is mixed in. The phosphorus or boron atoms take their places in the grid along with the silicon, and the layer becomes n-type or p-type (see "What is doping?").
• Why: the doping in this layer can be completely different from the wafer underneath, for example heavy below and light on top.
• If not: if the amount is off, the switches built on top are off too. Atoms grown into the layer can't be taken out, so the gas flow has to be set right first.

Step 6: Measure the thickness and check for defects

• What: the layer's thickness and doping are measured, and light scans the whole wafer for particles and spots that grew wrong (see "What is wafer defect inspection?"). Thickness is often measured with infrared light: the two layers are doped differently, so some infrared light also reflects off the boundary between them, and the two reflections give the thickness.
• Why: the transistors are built right in this layer. It's the foundation of the whole chip.
• If not: with a crooked foundation, hundreds of later steps are wasted, and it may only show up months later, when the electrical tests are run.

Why grow this extra layer?

• A clean layer of silicon for transistors: the wafer's own doping is the same all the way through. An epitaxial layer can be grown cleaner and with fewer defects, and its doping can be set separately. Transistors are only built in the thin layer at the very top (see "How is a transistor made?").
• Power devices: many switches that handle high voltages and large currents (they're in chargers and electric cars) send current from top to bottom. The heavily doped wafer underneath lets current flow easily; on top sits a thicker, lightly doped epitaxial layer that holds back the high voltage when the switch is off. The thicker and more lightly doped it is, the higher the voltage it can hold, but the higher its resistance when the switch is on.
• Other materials: materials like gallium nitride are hard to grow as large crystals, so they're usually grown on silicon, silicon carbide or sapphire wafers. That's called heteroepitaxy. The two materials space their atoms differently, like stacking bricks of two different sizes, so defects form easily at the boundary (see "What are SiC and GaN?"). The light-emitting layers of blue and white LEDs are grown this way too.
• The source and drain of advanced transistors: in advanced transistors, the source and drain areas are often etched away a little and then regrown by epitaxy, with heavily doped silicon or a material such as silicon germanium, so current flows more easily. Here the layer grows only where silicon is exposed.

What is an epi wafer?

A wafer that carries an epitaxial layer is called an epi wafer (epitaxial wafer). It looks just like an ordinary wafer, less than a millimeter thick; the difference is only a thin layer at the very top. Some wafers arrive at the fab with the layer already grown; the source-and-drain kind is grown in the fab when the process reaches that step.

In fab24hr

The game has no epitaxy tool and no epi wafers. The closest is chapter 2 of the main story, The life of a chip, "Station by Station". On Day 1 after wafer start, you're the R&D integration engineer, taking the first 12 wafers of a fictional hearing-aid chip, "Glowmere", to the wet bench WET-R1 on the R&D line. You clean them one bath at a time and finish with a quick dip in dilute HF, which removes the native oxide and leaves bare, clean silicon. In the game, though, the next stop is the furnace FURN-R1 for a thin pad oxide, not epitaxy, and the silicon nitride laid down at CVD-R1 on Day 2 is an ordinary film that doesn't follow the grid.

A day in the fab has thin film roles too: tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab" or "R&D line", swipe to the process engineer or the R&D process engineer, and tap "Thin film" under the name. The companies, chip, people and tool numbers in the game are fictional; this article follows public, general sources on epitaxy, simplified. For other jargon, see Chip fab jargon in plain English.