What does a furnace do in a chip fab? Oxidation and annealing explained

A furnace, often called a diffusion furnace, is a chip fab's high-temperature oven: it lines up a whole batch of wafers in a rack and slides them into a long, very hot tube, usually somewhere between 800 and 1,200 °C. It has two main jobs: thermal oxidation, where silicon and oxygen grow a layer of silicon dioxide in the heat, and annealing, where heat lets atoms that were knocked out of place settle back in. Some thin films are grown in furnaces too.

What does a furnace look like?

From the outside, a furnace is usually a tall upright cabinet. Inside is a long tube made of quartz, surrounded by heaters.

The wafers go into a quartz rack called a boat, one wafer per slot, like a dish rack, a hundred or more at a time. The whole boat rises slowly into the tube from below.

Loading so many at once makes sense because one run, from heating up through the process to cooling down, takes hours and can approach a full day. The price is that when a run goes wrong, the whole batch goes wrong.

Many fabs call this the diffusion area. Changing how well parts of the silicon conduct used to be done in hot furnaces, by letting other atoms diffuse in on their own; today that is mostly done by ion implantation. For the other areas, see What do the seven process areas of a chip fab do?.

What is thermal oxidation?

Silicon left in air grows a very thin layer of silicon dioxide on its own, a bit like iron rusting. The difference is that silicon's "rust" is a layer of clear glass: silicon dioxide is the main ingredient of glass and quartz. It doesn't conduct, and it grows very evenly, which makes it a very useful insulator in a chip (see What is a semiconductor?).

At room temperature, though, that layer stops at one or two nanometers. Oxygen has to get through the layer already there to reach the silicon underneath, and at room temperature it doesn't have enough energy. Heat the wafer in a furnace and the oxygen gets through, so the oxide keeps growing. That's thermal oxidation.

New silicon dioxide forms underneath, at the boundary between the oxide and the silicon, and it uses up the wafer's own silicon. Of 100 nanometers of oxide, about 44 nanometers used to be silicon, so nearly half of it has eaten its way into the wafer.

In many processes, the most important layer grown this way is the very thin insulating layer under a transistor's switch (see What is a transistor?). Oxide can also act as a shield: places that shouldn't receive implanted atoms are covered with it first.

What's the difference between dry and wet oxidation?

There are two ways to supply the oxygen:

	Dry oxidation	Wet oxidation
What goes in	Pure oxygen gas	Water vapor (steam)
How fast it grows	Slow	Much faster
The layer it makes	Denser, with a cleaner boundary to the silicon	A little looser
Usually used for	The thinnest, most critical layers	Layers that need to be thick

Water vapor speeds up oxidation, so even a tiny bit of extra moisture in the tube has to be kept out, or the oxide comes out thicker than planned.

What is annealing?

Annealing was a metalworking word long before fabs: heat the metal, then let it cool slowly so its disordered structure can sort itself out. Fabs borrowed the word. It means heating wafers for a while so the atoms can move a little and settle back where they belong.

The most common case is right after ion implantation. Silicon atoms sit in a perfectly neat arrangement, like oranges packed in a crate. Implantation fires other atoms in at high speed (for how, see What is ion implantation?). They knock into everything on the way, and the neat rows get messed up. Most of the implanted atoms end up wedged in the gaps, not sitting in any orange's place.

Atoms stuck in the gaps don't help conduct electricity. With heat, the damage slowly sorts itself out, and the implanted atoms take the place of silicon atoms in the arrangement. Only then do they start to conduct. Engineers call this activation. In plain words: the atoms have taken up their posts.

Whether an anneal did its job is often checked with sheet resistance: the more atoms at their posts, the better the layer conducts, and the lower the sheet resistance.

Annealing is a trade-off. Higher temperatures and longer times help more atoms take their posts, but the longer the heat lasts, the more the implanted atoms spread sideways and blur what was built. So some anneals are still done in a furnace, a whole batch at once, and others have moved to rapid heating, one wafer at a time, for only seconds.

Can a furnace grow thin films too?

Yes. In thermal oxidation the silicon itself reacts with oxygen. A furnace can also take gases into the tube and, at high temperature and low pressure, let them react on the wafer surface and lay down a new material, such as silicon nitride or polysilicon. That's a kind of chemical vapor deposition, done a whole batch at a time. For the different ways to grow films, see What is thin film deposition?.

Why does the temperature have to be so even?

How fast oxide grows depends a lot on temperature: a little hotter, a little faster. A load of a hundred-plus wafers forms a long stack, and if the top runs hotter than the bottom, the top wafers come out thicker. It's like a home oven, where cookies on the top rack and the bottom rack often brown differently.

So a furnace splits its heating into several zones, each with its own heater and thermometer (a thermocouple), adjusted separately. The very top and bottom of the boat see different temperatures and gas flow from the middle, so a few blank wafers that never ship (dummy wafers) fill those ends, and the product stays in the middle.

Each load also carries a few monitor wafers in different positions, for example at the top, middle and bottom, and their oxide thickness is measured after the run. If the top and bottom differ more than usual, someone has to find out whether the temperature or the gas flow is off.

Heating and cooling have to be slow, too. When a wafer heats unevenly, the edge and the middle expand by different amounts, and the silicon crystal gets pulled out of line, leaving fine straight marks at the edge called slip lines. That's why the boat goes in and comes out of the furnace slowly.

What is the thermal budget, and why count every heating step?

In the heat, atoms slowly move around, the way a drop of ink spreads faster in hot water than in cold. That's a problem for a chip. The atoms implanted earlier were placed at a planned position and depth, and every later heating step, whether oxidation, annealing or film growth, moves them a little more, so the shapes slowly blur.

So engineers keep an account called the thermal budget: the "temperature times time" a wafer spends in heat over its whole life. Like a fixed budget, once it's spent you can't take it back, and running the same hot step one more time gives a different result.

The account has to be planned from start to finish: the conditions for an early step must already allow for all the heat that comes later (see What does a process integration engineer (PIE) do?). The smaller the structures, the more a tiny bit of movement shows, and the tighter the budget. Once metal wiring is on the wafer, it can never go back into a furnace at over a thousand degrees; only a few hundred degrees are allowed.

In fab24hr

In fab24hr, tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab", swipe to the process engineer and tap "Furnace" under the name: you're the furnace process engineer. Which of the four shifts you get depends on the time you open it. You look after the Diffusion section's two furnaces, FURN-01 and FURN-02. On the late-night shift, FURN-02 trips halfway through a run, with a hundred wafers stopped in the middle: you find which heating zone dropped first and which steps of the recipe didn't finish, then decide between a full rerun (a whole extra step's worth of thermal budget) and putting the lots on hold, measuring the monitor wafers and running only what's left. On the morning shift, you measure the three monitor wafers from the top, middle and bottom of the boat. On the afternoon shift, FURN-01's top-bottom gap has grown, and there's still an anneal load tonight: calibrate first, or run as planned. On the evening shift, you look at the boat's slots before loading it.

The equipment side of the same story is the furnace equipment engineer (production fab › equipment › furnace), who owns the thermocouples, heaters and boats. On the R&D line, the furnace R&D process engineer sets the conditions for the very thin oxide under the transistor's switch and for the anneal. All of these roles are marked Beta. The companies, people, tool numbers, lot numbers and figures in the game are fictional; what a furnace does and how oxidation and annealing work come from public, general sources. For other jargon, see Chip fab jargon in plain English.