Why just turning up the pump speed isn’t so simple.
Process designers tend to not think very much about the waste gases from their processes. The chamber exhaust sends the effluent gases to the fab scrubbers, and that is pretty much that.
Except when it’s not. It turns out that the design of the ALD process can make life significantly more challenging for the chamber exhaust pumps.
In atomic layer deposition, the first precursor gas, such as HfCl, forms a self-limiting monolayer on the wafer surface. After that first precursor is removed from the process environment, an oxidizing gas, such as water or ozone, reacts with the monolayer to form hafnium oxide. Depending on the chamber design, the wafer (or wafers, in a batch system) can rotate between process zones, one filled with precursor A and the other with precursor B. Or, the wafer can remain stationary while the chamber is flooded with the two precursors in turn.
Either way, the exhaust system is responsible for making sure the two precursors only interact on the wafer surface, not on the chamber sidewalls and not in the exhaust system itself. This can be challenging, as Mike Boger of Edwards Vacuum explained. Where does the excess precursor go? For example, to reduce cycle time in a two-zone system, equipment designers would like to flow precursor gas to both zones continuously, simply shunting the excess to the exhaust system. Even in a single-zone system, the low volatility of precursors like HfCl makes it desirable to keep the gas source going at all times.
Remember, though, that the precursor gases are co-reactive. If precursor A has not been fully evacuated when precursor B flows into the process chamber, the resulting hafnium oxide can land on the wafer in the form of particles. If the pump is full of precursor A when precursor B starts flowing in, the likely result is a pump full of hafnium oxide dust. Not only can the pump seize, but back pressure can force the oxide dust back toward the process chamber. If the ambient temperature inside the pump is too high, the precursor can break down; too low, and it can condense and still be present when the other reactant flows in.
According to Boger, the key to preventing these issues is to increase pump speed and separate the two precursor materials. Equipment manufacturers needed some convincing, though, as increasing the chamber evacuation time necessarily increases the process cycle time. Installing additional pumps helps, but only to an extent.