4. Precursor steam generation and dosing

A key element of any ALD system is the precursor vapor generation and pulse dosing system. The function of this system is to generate a well-controlled vapor of the various precursor molecules in what is often a separate volume. In the case of temporal ALD, the system will inject an intermittent flow (in the form of a sequence of pulses) into the reactor. In the case of spatial ALD, the system generates a continuous flow in one of the precursor zones.

A first step is the generation of steam, either in its pure form or mixed with a carrier gas. For most ALD systems, it is desirable to have a pressure of at least 1 Torr in the precursor gas lines. If the precursor pressure is too low, it can generate a reflux phenomenon, i.e. a flow of gas from the reactor to the precursor line, instead of from the precursor line into the reactor. Not only does this (obviously) prevent...