Monday, July 28, 2014

Proper Material Set Up is Key

Material set up is probably the most important thing you can do for your process to run smoothly. Everything about your deposition will derive from it.

I was asked to characterize a coating chamber with materials that were already in place. The materials had been premelted in the electron gun prior to my arrival. I started with the easy stuff first and everything was going well until I came to the Ta2O5.

I started evaporation manually to set power for soak and rate parameters. I set the initial O2 bleed based on rate and the size of the coating chamber. But when I ran an automatic process for a Ta2O5 layer to find the tooling factor and index, I saw some rate and pressure fluctuations that indicated a problem with the material set up.

I opened the chamber and removed the boule of Ta2O5 to find it hadn't been premelted thoroughly. The crucible was filled with Ta2O5 tablets and the top was melted leaving voids beneath the melted surface that is exposed to the vacuum where evaporation occurs.

Bad premelt of Ta2O5

If your material isn't premelted into a solid boule prior to evaporation the problems that will arise include inconsistent rate, spitting, inconsistent refractive index of deposited film, and inconsistent oxidation of deposited film. And of course, bad coatings - performance and quality.

Make sure the materials you start with are well prepared and your films will be well deposited.

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