Physical deposition methods

The growth behavior in molecular beam epitaxy (MBE) corresponds to that of silicon vapor phase epitaxy. However, in this case the silicon is deposited onto the wafer in a different way.

The process takes place under ultra-high vacuum (UHV, 10^-8 Pa); the wafer to be processed is held upside down at the top of the process chamber and heated to about 600–800 °C so that the native oxide volatilizes.

Silicon is evaporated using an electron beam in a crucible and then deposits onto the wafer. Dopants can be evaporated in an effusion source and thus reach the wafer together with the silicon vapor. Through targeted temperature control and the use of shutters, the particle beam can be precisely dosed. This process also makes it possible to grow layers of different materials on top of one another that would otherwise not be possible due to differing atomic sizes. This allows a layer of silicon and germanium to be produced, which is needed for high-frequency applications in bipolar technology.

This is precisely where the strength of the process lies: because the atoms arrive individually and without a chemical reaction, the composition can be changed from one atomic layer to the next. For layer sequences made of compound semiconductors, as needed in laser diodes and high-frequency devices, this is indispensable. In silicon manufacturing, on the other hand, MBE plays no role – there it is too slow.

However, the effort required to generate the UHV in this epitaxy process is very high. To reach this pressure, which is less than one-trillionth of normal atmospheric pressure, several pumps must be combined and the process chamber pumped down for a long time. Only one wafer can be processed at a time, and growth amounts to only about 1 µm per hour.

In evaporation, metallic layers, such as aluminum, can be produced on the wafers. The material is placed in a crucible made of a high-melting-point metal such as tantalum and heated there until it transitions into the gaseous state. The metal vapor strikes the wafer perpendicularly, so that edges are not covered well; the resulting layer is polycrystalline. As an alternative to melting in a crucible, the metal can also be evaporated using an electron beam. Compared to thermal evaporation, this electron beam method makes it possible to achieve a very precise deposition rate. Since edge coverage is not very good, these two processes are mostly used for full-area backside coating for later electrical contacting.

In the manufacturing of integrated circuits, evaporation has largely been displaced by sputtering, which provides denser layers and better edge coverage. It has been retained where poor edge coverage is precisely what is desired: in the lift-off process, a resist mask is first patterned, then the metal is evaporated, and afterward the resist together with the metal lying on top of it is removed. Because the sidewalls of the resist mask are barely covered, the film breaks cleanly at the edge. This makes it possible to pattern metals that are difficult to etch.