Friday, December 19, 2014

'Tis the Season

Coaters Tech has developed what I like to call a Winter Filter®. An unthermal coating.

Practical uses for this filter are many. But making it requires skill and ingenuity.

Only the best coating designers using unique materials and a complete understanding of those material properties will be able to design this coating.

Only the best coaters will be able to deposit this filter. Constant monitoring of coating parameters is a must. Deposition temperature cannot exceed -6°C.

The substrate is a very clean lawn. Preferably short grass although it will work with grass as long as 5.0cm. No leaves or twigs can be on the lawn when preparing it for coating. Any leaves or twigs will cause defects and dislocations in the crystal structure.

Material selection for the coating comprises of several crystal morphologies. From big and fluffy to small, round, dense pellets. I was able to use only 3 different types to make this coating.

Production of this filter gets easier the further north your lab is located. But not too far north. Too far north and adhesion starts to become a problem.

What applications do you think you could you use this filter for?



Tuesday, November 11, 2014

Ophthalmics v Precision Optics

Coaters Tech clients include both precision optics labs and ophthalmics labs. I've worked in both industries extensively. Every once in awhile I am asked what the difference between the two are.

Both use exactly the same equipment to coat lenses. Leybold, Satisloh, Shincron... Electron guns, ion guns, turbo, diffusion and cryo pumps, vacuum gauges...

Both use the same materials for coating. TiO2, SiO2, ITO, Al2O3, etc.

Both require a high degree of precision and engineering.

Both conform to strict optical performance parameters.

Both go through stringent quality testing. Hardness, abrasion, environmental testing, etc.

The difference? No one buys a $300 camera lens and tosses it on the coffee table or sofa where it gets abused and maybe even put in the baby's mouth.

You wouldn't put this $300 camera lens on the sofa.
Coating ophthalmics is a difficult business. The same equipment and materials are used for the coatings but eye glasses are taken for granted. A $500+ pair of glasses is usually taken care of for a couple of months but is then left on the front seat of the car where they may get sat on.

Can you imagine doing that with a new camera lens?



Why would you put these $500 prescription Ray Bans on the sofa or car seat? (Photo credit www.ray-ban.com)

Monday, November 03, 2014

The Optical Interference Matrix

Designing thin films is engineering an optical interference matrix. Phase shift at an interface produces interference. At the air to substrate interface and the layer to layer interface a phase shift takes place. When it comes down to it it's as simple as that and you should start to think of your designs as such.

I've heard many people claim that AR coatings need to have the last deposited layer be a material that is of "low" index.
What does that mean?
Designation of high or low or medium index is relative. It's relative to the substrate, the medium in which the optical system operates in, and the other material(s) used in the coating.

A simple example is a single layer AR coating.
On glass having an index of 1.9 a single layer of MgF2 will result in near 0%R at the design wavelength (1.38 being very close to the square root of 1.9).
On Ge a single layer AR could be made of ZnS having a refractive index of 2.2 resulting in a reflection of ~1.3%R (2.2 being close to the square root of 4).
The refractive index of ZnS is much higher than the index of MgF2, but much lower than the refractive index of Ge.

Comparing these two simple examples we can see that there is the same phase shift occurring at the air to film interface and the film to substrate interface regardless of the actual refractive index.

Going further with multi-layer AR coatings we can see with the design;
85.08L 43.3H 46.2L 27.73H (where nL=1.46 and nH= 2.2) on glass the result is an anti-reflection thin film at 532nm (a frequency doubled 1064 laser). (click to enlarge image)



Now this design may be impractical but it illustrates my point. An AR coating ending in a "high" index layer.
But it's important to keep in mind that the phase shift at the air to film interface is going to be the same no matter what the last deposited layer is (the medium to film boundary layer). The other layers in this multi-layer example are the rest of the optical interference matrix engineered to result in low reflection at the desired wavelength(s).

Looking at your design as an optical interference matrix this way increases your choices of materials. You can start to use harder / environmentally resistant materials as boundary layers, a third material to induce a phase shift change in the matrix at some point in your multi-layer, or choosing materials where equipment or deposition methods may be a limiting factor.

To see more about this and more examples watch Episode 9 of Coaters Tech videos.

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.

Friday, July 18, 2014

Coaters Tech Videos on YouTube

All Coaters Tech videos are available on YouTube to watch at your convenience. Coaters Tech videos are geared towards everyone including engineers, technicians, operators and management.

You can find them all here - https://www.youtube.com/user/coaterstech/videos



Tuesday, July 08, 2014

Memories of a Coating Tech

Having worked in the coating lab for many years when I get together with other coaters there's always some good stories to tell.


Coating failure!
There's the obvious; forgot to put the shutter in, but have you ever forgotten to actually put the parts in? I did a full 3 hour deposition once and when I was done I turned to make room on the flow bench and son of a...! The optics I was supposed to have just coated were still sitting tooled up ready to go in the chamber! I had to stay late without booking the OT to get them coated like I was supposed to.

I once lobbied hard to use a different material for a prototype. I gave every good reason to set up and characterize the material in the chamber and run the tests. I got permission to do so and the results were as I expected and engineering was happy with the results. The day after testing concluded I received the $20K+ single optic to coat for the prototype device. Somewhere in the middle of a long process the new material I had successfully suggested and put into process exploded and spit molten material all over the part. That was not good! The damage to the polished surface was too much to polish out and the part had to be remade. Luckily management was very understanding. But we never used that material again.

Have you ever made a silly mistake? Or a very costly mistake? Let me know. Change the names to protect the innocent or make your tale anonymous if you like.