Etching

In plain language

After lithography, the resist on the wafer is a stencil. Etching removes the film in the open areas of that stencil. It can be done in a liquid chemical bath, which is called wet etching, or with an electrically charged gas called a plasma, which is called dry etching.

Wet etching is simple and gentle, but the liquid eats sideways as fast as it eats downward, so it cannot cut narrow lines. In a plasma etch, charged particles are driven straight down onto the wafer, so the cut goes down and not sideways. Nearly all the fine patterns on a chip are etched with plasma.

Image

Figure 1. An isotropic etch undercuts the mask. An anisotropic etch copies the mask with vertical sidewalls.

Core lesson

Four numbers that describe an etch

• Etch rate is how fast the film is removed, usually in nanometers per minute.
• Selectivity is the etch rate of the film divided by the etch rate of another material, such as the mask or the layer beneath.
• Anisotropy describes how much faster the etch goes down than sideways.
• Uniformity describes how evenly the etch runs across the wafer.

An etch must stop on the layer beneath the film without damaging it. High selectivity to that layer makes this possible. High selectivity to the resist lets a thin resist survive a long etch.

Wet etching

Each film has its own etchant. Silicon dioxide is etched in hydrofluoric acid, which barely attacks silicon. Silicon nitride is etched in hot phosphoric acid. Aluminum is etched in a mixture of phosphoric, acetic, and nitric acids.

These etches are isotropic, which means they proceed at the same rate in every direction. The film is undercut beneath the mask by about as much as its own thickness. That is acceptable when the features are much wider than the film is thick, and it rules out wet etching for fine lines. Wet chemistry is still used widely for cleaning, for removing whole films, and for some micromechanical devices. Silicon etched in potassium hydroxide is a special case, because the etch rate depends strongly on the crystal plane and gives flat, angled walls.

Plasma etching

A plasma is made by applying radio-frequency power to a gas at low pressure. The power strips electrons from some molecules and breaks others apart. The result is a mixture of ions, which carry a charge, and radicals, which are neutral and very reactive. Fluorine radicals from a gas such as carbon tetrafluoride react with silicon to form silicon tetrafluoride, which is itself a gas and is pumped away. A plasma etch only works if the product is a gas. Copper forms no suitable gas, which is why copper wiring is made in a different way, as Module 9 explains.

Radicals alone etch in all directions. Ions alone knock atoms off the surface slowly and with little selectivity. Together they etch far faster than either does alone, and only where the ions strike. Ions arrive from straight above, so they hit the bottom of a feature and miss its sidewalls. Etching that uses both is called reactive ion etching.

Many recipes add a gas that deposits a thin protective film on all surfaces. The ions clear this film from the bottom while it stays on the sidewalls, which keeps the walls vertical.

Limits and newer methods

Narrow, deep features etch more slowly than wide ones, because ions and radicals have trouble reaching the bottom. Energetic ions can also damage the layers they uncover. Tools watch the light given off by the plasma to detect the moment the film has cleared, which is called the endpoint.

For very deep holes, one method alternates short etch steps with short steps that coat the sidewalls. It can cut hundreds of micrometers into silicon with vertical walls. For the most delicate features, atomic layer etching removes one atomic layer in each cycle.

Going deeper

These sections add the equations behind the core lesson, each with a worked example. They assume first-year algebra and can be skipped without losing the thread of the course.