8.3.2 Sputter etching: argon ion etching of gold
One commercial process for cutting inkjet printer nozzles uses sandblasting. Not surprisingly, the surface finish is rather poor and there are issues with particles contaminating the devices. However, it is a physical process very like this that we need if we are to achieve a vertical etch profile.
The key is directed bombardment by highly energetic particles. When processing on the microscale, these particles are not sand grains but ions accelerated towards the surface by an electric field. Ar+ ions are most commonly used, as argon is chemically inert, easily ionised, relatively cheap and its heavy ions, like heavy hammers, are more effective for chipping away at a surface. The process is harder than you might think. An equivalent task would be breaking down a cemented brick wall by hurling bricks at it.
In the simplest apparatus, such as the reactive ion etcher shown in Figure 36, an electrical discharge is created in a vacuum chamber by applying a voltage to a platen where the wafer sits. With a gap of a few cm to the ceiling, the best gas pressure is around 100 Pa and a plasma can be sustained with a few hundred volts. Ions are pulled onto the platen and therefore onto the wafer with energy (in electron volts) similar to the applied voltage. Since 1 eV of ion energy is equivalent to a temperature of 11 000 K (this is not a typographical error!), these ions are quite capable of dislodging atoms and sputtering the material away.
A plasma is an ionised gas. The plasmas used in semiconductor processing are near to room temperature. They consist mainly of ordinary gas molecules doing nothing special, but a small proportion of them has been excited electrically, so that they are ionised. These ions can be made to move very fast by means of an electric field. The kinetic energy this gives them allows them to physically and chemically attack surfaces in ways otherwise possible only at very high temperatures.
Usually at least one layer in the device structure is insulating, so you can't pass a DC current through it. However, RF currents are still transmitted so it is usual to operate plasma-processing apparatus at MHz frequencies: 13.56 MHz is a convenient standard frequency set aside by statute for industrial use.
Sputter etching is not subtle, but it can be the only choice for materials (like gold and platinum) that are resistant to chemical attack. However, there are some big problems:
Etching tends to be slow. You are lucky to knock off more than one atom for each ion hitting the wafer, and the ion flux is limited by the current that your power supply can deliver. Typically, sputter etch rates are measured in tens or at best hundreds of nm min−1.
Sputter etching is not selective, so you need a mask that is either thicker or more resistant to sputtering by having stronger bonds than the material you want to etch. Mask deposition and lithography then become as big a problem as the etch itself.
The sputtered atoms don't go away. They were recently part of a solid, and they will stick on the first surface they hit to form a solid once again. Not only do the chamber walls get coated (there is a good living to be made recovering precious metals from sputter chambers), but the material will also be redeposited up the sides of the etched hole and all over the face of the wafer. This makes the problems of drying residues pale into insignificance.