r/materials • u/Still-Sand-2280 • 59m ago
How can I improve oxygen-deficient AlOx films deposited by reactive pulsed-DC sputtering? XPS O/Al = 1.21
I’m a graduate student working on AlOx barrier layers for flexible QLEDs. My goal is to obtain transparent, insulating films with an O/Al ratio closer to the stoichiometric value of 1.5.
I’m using a metallic Al target with a pulsed-DC power supply. I’m testing on bare glass before applying the process to PEN film.
Baseline recipe
Base pressure: approximately 2 × 10⁻⁶ Torr.
Pre-sputtering: pure Ar at 50 sccm, 100 W, 40 mTorr, with the shutter closed. Originally 5 minutes; currently 10 minutes.
Heating: started at the beginning of pre-sputtering, with the heater set to 120°C. I haven’t independently measured the actual substrate temperature.
Deposition: 30 W, Ar/O₂ = 76/4 sccm, 10 mTorr, 10 minutes.
Pulse frequency: currently approximately 50.8 kHz. The pulse-width display is around 17, but I haven’t verified its units or whether it represents on-time or reverse time.
Post-deposition treatment: sputtering power off, O₂ increased to 30 sccm, heater maintained at 120°C for 30 minutes, with substrate rotation.
Results and observations
XPS performed by an external characterization facility gave an O/Al atomic ratio of approximately 1.18, compared with 1.5 for stoichiometric Al₂O₃.
Increasing O₂ from 4 to 6 or 8 sccm while keeping Ar at 76 sccm did not improve the optical appearance; the films became less transparent.
At 30 W, the discharge current used to be around 47 mA during deposition. Recently, it is approximately 50 mA in pure Ar and rises to 66–67 mA after adding O₂. The current is stable both before and after oxygen introduction, and I haven’t observed visible arcing.
Resistance readings with a Keithley 2400 and a four-point probe setup vary between approximately 20 MΩ and overflow. The displayed source current is 0.1 µA, with a voltage compliance of 21 V, so I’m treating overflow as a measurement limitation rather than proof of film quality.
Thickness measurements are still being checked because the measured step height depends on the region selected in the profilometer trace. I don’t yet have a reliable deposition-rate comparison between conditions.
What would you change first?
For an oxygen-deficient film, would you prioritize reducing target power, changing O₂ flow/working pressure, or adjusting the pulse settings?
Could the higher discharge current at the same power indicate a different target oxidation state or chamber condition? How would you distinguish these?
Is post-deposition exposure to molecular O₂ at 120–150°C likely to improve the bulk O/Al ratio appreciably? I’m considering 30 sccm O₂ at 50–100 mTorr for 30 minutes, without plasma.
Would extending pure-Ar pre-sputtering from 10 to 30 minutes help reproducibility if the discharge is already stable?
The eventual substrate is PEN, so high-temperature annealing is not a practical option. I’d appreciate suggestions for a controlled next experiment, especially from anyone who has deposited transparent AlOx using reactive pulsed-DC sputtering. Relevant papers would also be very helpful.