Controlling gas flows for sputtering

Controlling gas flows for sputtering

Sputtering is a technique used to deposit thin film coatings onto materials, often to give them specific optical, photovoltaic, or mechanical properties. Precisely controlling the gas flows into a sputtering vacuum chamber works to ensure the highest possible sputter yields.

In the process of sputtering, a plasma gas, typically argon, is used to fill a vacuum chamber which contains a target lattice and a substrate. The number of atoms released from the target lattice that can coat the substrate can best be controlled by controlling the amount of the plasma gas within the vacuum chamber. In reactive sputtering specifically, reactive gases such as oxygen or nitrogen are also added to the chamber. This is most often done to coat substrates with compound atoms such as titanium oxide.

Partial pressure control prevents target poisoning

Setup with three Alicat controllers with downstream valves to control gas flows in a sputtering vacuum chamber

Three Alicat mass flow controllers monitor gas partial pressures in a sputtering vacuum chamber

Target poisoning occurs when the sputtered coating forms on the target lattice in addition to on the substrate. This is a costly scenario, because the sputter yield is significantly lower than desired. The reduced rate of deposition on the substrate will require more time and materials spent on the batch, and the potential for a lost batch.

Maintaining the proper partial pressures of the reactive gases in the vacuum chamber are the best method of preventing target poisoning. As such, it is important to be monitoring and adjusting the flows and pressures of these gases in real-time, to compensate immediately for any changes that occur. Using equipment and devices which are themselves fairly insensitive to ambient temperature and pressure changes, and will compensate for process temperature and pressure changes, will also contribute to successful sputtering.

Using gas flow devices with fast response times and fine flow resolutions are therefore critical, as millisecond response time will lead to much higher yields than responses which take several seconds. For longer cathode arrays and sputtering applications which use multiple gas sources, multiple mass flow controllers can be controlled from one serial connection or through analog input/output to monitor and respond to all process conditions through a single system.

Flow devices which are controlled by process parameters or via industrial communication protocols such as EtherCAT can also be tuned specifically for a set of process parameters using in-field PID tuning. This ensures that the device will respond as appropriate to a given set of process conditions, and is another means of ensuring that gas flows are precisely controlled into the sputtering chamber and respond quickly and completely to any changes in process conditions.