What is wafer cleaning, and why are wafers cleaned so many times?

Wafer cleaning means washing invisible dirt off the wafer surface with chemicals between the steps of making a chip, then rinsing it with ultrapure water and drying it. Four kinds of things get washed off: tiny particles, organic residue, metals, and a very thin layer of oxide that silicon grows on its own in air. A chip is built up layer by layer, every step leaves a little something behind, and the next step needs a clean surface, so wafers are cleaned before and after almost every major step. That makes cleaning one of the most repeated steps in the whole flow. Fabs usually call it wet clean.

What is there to wash off a wafer?

A wafer looks as shiny as a mirror, but its surface often isn't clean. What gets washed off falls into roughly four kinds:

• Particles: dust, debris worn off tool parts, fine grit left from polishing. The finest lines on a chip are a few tens of nanometers wide or less, so one particle landing on them can break a line or join two lines together. The cleanroom keeps particles out of the air (see How clean is a cleanroom?), but wafers still pick them up going in and out of tools and sitting in chemicals.
• Organic residue: what's left of photoresist, and traces of organic vapor in the air. They leave an invisible film on the surface, and the next layer doesn't grow well or stick properly.
• Metals: iron, copper and the like, which can come from chemicals or tool parts. Left on the surface, they make a very thin insulating layer break down more easily; at high temperature they move into the silicon, and transistors become prone to leakage.
• Native oxide: when silicon meets air and water, it grows an oxide layer only a few atoms thick on its own. Some steps keep it as protection; others have to remove it first, so the next layer sits directly on clean silicon.

Why clean before and after almost every step?

Every step leaves something behind, and every next step has its own worries:

• After etch and resist strip: etch residue and leftover photoresist must not travel to the next step (for how etching works, see What is etching?).
• After CMP: the slurry is full of tiny abrasive particles, so the wafer always gets cleaned after polishing (see What is CMP?).
• Before the furnace: metals and organics on the surface get "baked in" by the heat, and the oxide that grows comes out worse. That's why a wafer gets a thorough clean before a high-temperature step like the furnace (see What does a furnace do in a chip fab?).
• Before thin film: if the surface isn't clean, the new film doesn't grow evenly.

A chip has dozens of layers, and each layer takes the wafer around several areas of the fab (see What do the seven process areas of a chip fab do?). Before a wafer is finished, it goes through a wet clean tool many, many times.

A cleaned wafer can't wait long, either. Clean silicon sitting in air slowly grows its oxide back and picks things up from the air. So there is often a time limit between cleaning and the next step, called Q-time (see What is Q-time, and why can't wafers wait?).

What are wafers washed with? A common standard sequence

Just as grease and rust stains on laundry need different treatments, different dirt on a wafer needs different chemicals. The most common standard wet clean was worked out decades ago and is still the basis of many recipes today. The order is: organics and particles first, then metals.

1. First, an alkaline mix: ammonia, hydrogen peroxide and water, used hot. Hydrogen peroxide is an oxidizer that breaks down organics; at the same time the surface is etched very lightly, so particles stuck to it come loose and are carried away.
2. Rinse with ultrapure water to wash off the first mix.
3. Second, an acidic mix: hydrochloric acid, hydrogen peroxide and water, which dissolves metals off the surface.
4. Rinse again with ultrapure water, then dry.

When the native oxide has to go, a step of very dilute hydrofluoric acid is added. Thicker organics such as photoresist are often treated first with sulfuric acid and hydrogen peroxide, a very strong oxidizer. In the fab, the two mixes are commonly called SC-1 and SC-2.

Tank after tank, or one wafer at a time?

• Batch: a whole lot of wafers goes through a row of tanks together, one tank after another: chemical, rinse, the next chemical... then dry. Cleaning many wafers at once gets a lot of them through in a short time. The downside is that one bath cleans many lots, so some of what it washes off stays in the tank, and some lands back on wafers. That's why a bath is changed completely after a set number of lots.
• Single wafer: one wafer at a time. The wafer spins in a small enclosed space while nozzles spray chemicals and ultrapure water onto its surface, then it spins dry at high speed. What comes off one wafer is less likely to reach the next, and conditions are easier to control; the cost is that cleaning one wafer at a time takes more tools.

Fabs use both, depending on what matters most at that step. Either way, the last step is a rinse with ultrapure water and then drying: whatever is in the water can stay on the wafer, so the water has to be almost nothing but water molecules (see How pure is ultrapure water?). If the water doesn't dry evenly, it also leaves ring-shaped water marks.

Why do concentration, temperature and time all matter?

How hard a chemical cleans depends on three things: how concentrated, how hot, and how long. A bit stronger, hotter or longer cleans harder, but also eats more of the surface.

The trouble is that the chemicals change on their own. In a heated bath, hydrogen peroxide slowly breaks down and ammonia escapes bit by bit; the more lots a bath has cleaned, the more dirt it holds. So concentration, temperature and soak time all go into the recipe, which the tool follows; chemicals are topped up on schedule, and the bath is changed when it reaches its count; and monitor wafers run on a schedule to measure how much was removed and how many particles were added, to see whether the tool is changing.

Can cleaning too long damage a wafer?

Yes. Cleaning is usually a very light etch as well: while the chemicals take off the dirt, they also eat a tiny bit of the surface. Soak too long, or use too strong a mix, and the layer underneath that shouldn't be touched gets thinner, or the silicon surface turns rough. What's eaten away can't be put back.

Each clean removes only a tiny amount, but a wafer is cleaned many times and lines keep getting finer, so it adds up and has to be counted. The goal isn't "longer is cleaner" but just enough: the dirt comes off, and the layer underneath ideally doesn't change at all. That's also how cleaning differs from wet etch: both soak the wafer in chemicals, but wet etch wants to remove a layer of material, while cleaning only wants the dirt gone.

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 "Wet clean" under the name: you're the wet clean process engineer. You look after WET-05, a row of tanks that cleans lots headed for the furnaces and lots coming off CMP. Four shifts:

• Late-night shift, "Not Till Ten": you're on-call at home. Furnace FURN-02 goes down in the middle of the night, and the two lots cleaned the evening before have a Q-time of just four hours: send them to the other furnace, or wait for morning and clean them again? Just after three, WET-05's bath also reaches its change count.
• Morning shift, "Climbing Particles": the particle count after cleaning keeps climbing. Stop the tool now and fix it, or lengthen the soak time to get by for now?
• Afternoon shift, "Metal You Can't See": the surface looks clean, but metal is high. The evening furnace load goes in all together: put the two lots on hold and add a metal-removal step to the re-clean, or keep the original recipe?
• Evening shift, "The Stronger Bath": the freshly changed bath is stronger, and the soak time lengthened that morning has to go back to standard. CMP also sends over a few wet wafers with exposed metal lines, and you decide how to clean them.

Swipe to the equipment engineer and tap "Wet clean" to look after WET-05's hardware; the R&D line has a wet clean role too. The fab, people, tool numbers and times in the game are fictional; what cleaning removes and why concentration, temperature and time matter come from public, general sources. For other jargon, see Chip fab jargon in plain English.