Aluminum nitride is an increasingly popular material because of its piezoelectric properties. Applications include surface acoustic wave (SAW) sensors and strain gauges. Our deposition expert, Rob Belan, recommends several process conditions which can contribute to making good, reactively sputtered, AlN thin films including:
1. The substrate and sputtering gun should share the same center axis. “On axis” deposition of AlN is the best way to get oriented thin films.
2. The N2 to argon gas mixture should not exceed 35%, as sputtering from pure N2 is not optimal.
3. The N2 needs to be cracked into monotonic nitrogen so that it is chemically active and more likely to combine with the Al adatoms in the sputter plasma. Sample RF bias plasma or ion gun plasmas are also credible ways to crack N2.
4. Quite often a seed layer, such as molybdenum, is required to help nucleate the depositing film.
5. All nitrides are particularly sensitive to oxygen contamination. Accordingly, the deposition system should be capable of achieving a base pressure, before the introduction of any process gases, on the order of 10-7 Torr. If oxides have been deposited in the system or it is not leak tight, likely there will be significant O2 inclusions in the films.
Rob also recommends a pulsed DC power supply for the sputter cathode. That will enable a smooth and stable reactive deposition of AlN from an aluminum metal target. Because of the extreme sensitivity of AlN to oxygen contamination we also recommend an aluminum source which is 99.999% pure and also characterized for oxygen content. Trace elemental analysis should be similar to the following:
Category: Deposition Materials
Sub-Category: Sputtering Targets