What is etching? How are dry and wet etching different?

Etching removes the layer of material that the photoresist leaves uncovered on the wafer, following the pattern that litho printed in the resist. Think of the resist as a stencil stuck to the wafer: covered areas stay, exposed areas get cut away. Once the resist is stripped off, the circuit pattern is really part of the wafer. In fabs it's usually just called "etch."

How does etching relate to litho?

Litho prints, etch cuts. Litho puts the pattern into the resist, and the material underneath hasn't been touched yet; how that printing works is in What is photolithography?. Then etch actually removes the exposed material.

A chip is built up one layer at a time, and many layers go through the same loop: deposit a layer of material, print the pattern with litho, cut it with etch. So a wafer goes through etch tools many times before it's finished.

What is wet etching?

Wet etching means soaking the wafer. It goes into (or is sprayed with) a chemical that dissolves a certain material, and the exposed layer slowly dissolves away. Hydrofluoric acid, for example, dissolves silicon dioxide, the main ingredient of glass.

The upside is simplicity: the equipment is simple, the etch is fast, and one bath can hold many wafers at once. The chemicals are also usually picky, dissolving only the layer they're meant to.

The problem is direction. Most etching chemicals have no sense of direction. As they eat down, they also eat sideways, digging in under the edge of the resist. The sideways cut is roughly as deep as the downward one, and the edge comes out rounded, like a bowl scooped out with a spoon.

Why can't fine patterns use wet etching?

Picture cutting a line whose width is about the same as the thickness of the layer. By the time a wet etch cuts down through the layer, it has also eaten about one layer-thickness into each side of the line. Most of the line is gone, or all of it.

With large patterns, a little extra off the edges doesn't matter. University course notes put wet etching's useful range at features of a few micrometers or more (it also depends on how thick the layer is), and the finest lines on today's chips are a few tens of nanometers or less, dozens of times smaller. So fine patterns mostly go to dry etching. Wet etching is still used a lot, mostly where no pattern has to be followed, such as removing a whole layer or cleaning the surface.

What is dry etching, and what is plasma?

Dry etching uses gas instead of liquid. The wafer goes into a sealed space pumped down to near vacuum, called a chamber. Certain gases flow in, and high-frequency electric power turns the gas into plasma.

Plasma is gas that electricity has pulled apart: electrons are torn off the atoms, leaving a cloud of charged particles plus lots of very reactive fragments eager to react with something. The glow in a neon sign is plasma, and an etch chamber lights up with a glow too.

Plasma cuts with two forces at once:

• Chemistry: the reactive fragments react with the surface material, turning it into a gas that is pumped away.
• Impact: an electric field pulls the charged particles downward, so they slam straight into the wafer and knock material loose, a bit like sandblasting in a vacuum.

The particles come from straight above, so the bottom of a hole keeps getting hit while the sidewalls barely do. That's why dry etching mostly goes down and can cut straight, deep walls. The price is a much more complex tool with many more conditions to control.

Side by side	Wet etching	Dry etching
What does the cutting	Liquid chemicals	Plasma
Which way it cuts	In every direction	Mostly straight down
The edge it leaves	Rounded, like a bowl	Straight
Mostly used for	Large patterns, removing whole layers, cleaning	Fine patterns

How wide, how straight, and when to stop?

An etch step cares about three things: the line width it cuts has to match what litho printed; the sidewalls have to be straight, not hollowed out underneath; and it has to cut all the way through this layer without damaging the one below.

How does the tool know it has broken through? Many tools watch the plasma's light as they etch. Material being knocked off mixes into the plasma and makes certain colors of light stronger; once the layer is gone and the material below shows, those colors fade. When the tool sees that turn, it knows it has reached the bottom. This is called endpoint detection.

At that moment, though, not every spot has broken through. Across one wafer, the film isn't equally thick everywhere and the etch isn't equally fast everywhere. So the tool keeps etching a while longer to make sure every hole reaches the bottom. That's called over-etch. Too long damages the layer below; too short leaves some holes closed. The narrower and deeper the holes, the harder it is to get right.

What is etch selectivity?

Think of erasing a pencil mark: the pencil comes off fast, but the paper barely gets thinner. Etching wants the same thing: the layer being cut should go fast, and anything you want to keep should go slowly. The ratio between how fast the two materials are removed is called selectivity.

With high selectivity, the extra time spent on over-etch takes only a little off the layer below; with low selectivity, a few extra seconds can cut right through it. The resist gets etched too, and with too little selectivity it's gone before the etch is done. In general, etching by chemical reaction is pickier about materials, and etching by impact alone is not very picky. Dry etching combines the two forces to get both straight walls and pickiness.

Can a bad etch be redone?

Not really. If litho goes wrong, the resist can be stripped and the pattern printed again as long as the wafer hasn't gone to etch. That's called rework. Material that etch has removed can't be put back. An etch that didn't go far enough can sometimes be finished; one that went too far and damaged the layer below can't be undone, and in bad cases the wafer is scrapped.

Why do etch tools need watching every day?

Because conditions inside the chamber slowly drift. With every wafer, some of the etched material sticks to the chamber walls and builds up into a layer. That layer soaks up or gives off things from the plasma, and the etch speed shifts with it. The same recipe (the settings the tool runs) can etch differently this month than next. So the tool is looked after in fixed steps:

1. Regular PM (maintenance): open the chamber, clean it and replace worn parts.
2. Right after a clean, the walls are too clean, and the first wafers etch differently from usual. So a few wafers that aren't customer product run first to bring the walls back to a steady state. That's called seasoning the chamber.
3. Next, test wafers run to measure the etch speed and count added particles. Only when both are in spec does the tool go back to production. That's a qual run.
4. In normal production, line width is also measured after etch and plotted on a control chart to see if it's slowly drifting to one side.

When something's off, the tool lights up; for how to read the lights, see What do the red, yellow and green lights on a tool mean?. For who looks after the output and who looks after the tool, see What's the difference between a process engineer and an equipment engineer?.

In fab24hr

In fab24hr, etch is one of the leads. 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 "Etch" under the name: you're the etch process engineer. On the late-night shift, the phone rings at 02:47: Chamber A of ETCH-03 has thrown an alarm, and a wafer is stuck inside halfway through its etch. On the morning shift, the chamber has been opened, and you decide how many season wafers to run and how long to pump down and bake, so the etch rate and particles both come back into spec. On the afternoon shift, you finish that wafer with a rework etch, but the manual run screen opens with every wafer selected; if you don't slip, it's the new hire on ETCH-05 next door who does. On the evening shift, the helium flow on ETCH-03 creeps up bit by bit and the yellow light comes on.

Chapters 6 to 9 of "The life of a chip" are this process engineer's day. Chapter 5, "The Last Box", is on the R&D line: before handing the process over to the production fab, the etch R&D process engineer sets the alarm limit for helium flow. The companies, people, tool numbers and figures in the game are fictional; how etching works and why it needs watching come from public, general sources. For the jargon, see Chip fab jargon in plain English.