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Thread: Sputtering coatings

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    Default Sputtering coatings

    Hey Everyone!

    It's been a while since I've made a thread here.
    As i've been delving deeper and deeper into the technical side of lasers: questions kept growing.

    As I've been doing more and more laser experiments out of my own the interest in a ion sputter coater build has grown.. they are not easy builds but they are do-able.. the information is out there and build files have started to spring up on the internet more and more.

    However that brings me to a big question: The sputtering targets.

    I know dielectric mirrors are made using sputtering and also output couplers which is a semi-transparent dielectric mirror (and I also believe dichroic filters too)
    I keep hearing the formulas of these coatings are secrets held closely within optics companies and while I can understand this for optics with really exotic narrow band coatings.. I find it quite hard to believe that general formulas aren't more publicised.

    For example I've been reading papers of optical cavities made where the lab sputter coated their mirrors in house.. meaning: They used some kind of target aquired from somewhere.

    Can anyone shed more light on the compositions that are used and how its determined in advance what target is useful for what wavelength range?
    and last not at least: how do they achieve certain transmittance percentages with sputtering?? are they slightly contaminating the argon enviroment with oxygen resulting in a semi transparent dielectric mirror because of oxidization?? or is it coating thickness??

    So why would one go through this effort over just ordering them?: Testing different focal lengths or different reflectance/pass through properties of output couplers or recoating third party purchased non linear crystals that are bought uncoated.
    Maybe these labs have enormous budgets and have a seperate facility they collaborate with and the papers do not shed light on these things: but I'd be interested to know!!
    Last edited by masterpj; 11-07-2021 at 13:06.

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    Most sputtering targets in the US are made by a company in Pittsburgh, Pennsylvania.

    So my question for you are simple.:

    1. Single or multi wavelength mirrors
    2. Gas or Solid State..



    If its a single wavelength, if your not trying for 99.9995 I will email you an excellent paper on thermal evap and using a laser or light source at the desired center wavelength as the coating thickness monitor..

    Its good enough to make HENE. Hr and Oc. Ie quarter wave stacks.

    What its not good for is making simultaneous dual source sinusoid variations in refractive index to make those ultra-broadband mirrors. Ie Rugate coatings.

    Try one of these calculators to see if you want to leap into this.
    https://www.filmetrics.com/reflectance-calculator

    Keep adding quarter waves of alternative index until you spot the error in the simple textbook explanation.

    The usual way to calculate an OC if in doubt is to add a variable loss into the cavity. To measure the gain. A tilted uncoated plate does this nicely and follows the Law of Malus.
    Last edited by mixedgas; 11-07-2021 at 20:36.
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    The calculator makes sense!

    1: single wavelength. However it can pass a bit more around the band so it doesnt have to be extremely narrow band
    2. Solid State and no more like 0.1 to 0.05

    The coating while shining a laser seems to be interesting.. How does this work when you are working under vacuum? Some kind of apperture window where the losses of those stages are factored in and then while the coating is happening the measurements are being taken as things are happening?

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    I wanted to make sure you had a way of seeing what you were getting into before trying this. Hence the calculator.

    There are neat techniques with integer fraction coating layer thickness. I've never done them but before you build or buy a coater you need to see if what you have available will work.

    The thermal materials do not have huge index changes, hence the "Try before you buy".

    How many layers did you have to put down in the calculator before you remotely saw what you wanted on the reflection graph?

    I usually end up around 30 with thermal materials such as SiO2 and MgF for a high reflector.

    Steve
    Last edited by mixedgas; 11-08-2021 at 07:52.
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    I'll probably be using SiO2 most of the time and maybe in a rare ocassion doing a nonlinear crystal.. gladly most companies barely or dont charge at all for doing a coating on the crystals you order from them.. used crystals you buy online you probably have to repolish to remove the coating to recoat them.

    Also you mentioned thermal evaporation.
    I do know there are 2 methods regarding deposition on optical surfaces.. one I have seen involves heating a material in I believe a vacuum enviroment (not sure if its filled with argon) where the material evaporates because of immense heating caused by current passing through it... which is said to be the easiest method but only works with certain metals where the composition wont be changed off when heated. I know a requirement on that is high current

    and then there is the sputtering of a high voltage with a magnetic field situation which requires a high voltage.

    With thermal evap are you reffering to heating of a material by passing a high current through it in vacuum?

    Will test with layers now

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    Sputtering : Glow discharge with material to be sputtered at cathode. Ours runs at 3Kv @ 20 - 40 mA and. A few hundred milli-Torr of air or Argon depending on material. We usually are coating a sample with aluminum Or platinum to make it just conductive enough for an electron microscope to scan it. Training consists of loading a dead fly in the chamber, and putting down a few tens of nanometers of metal using vary low current. The metalized fly is pretty much undamaged, and the next step is for the user to load it into the electron microscope.

    Magnetron sputtering Adds a big magnet behind the sputtering cathode. Makes the electrons and some ions oscillate and spiral, beforehitting the target. More material transport, better uniformity, current to tens of smps..A wide variety of gasses are used.

    E-beam My favorite Several amps of electrons are flipped by a magnet onto a target in a cup of ceramic or refractive metal. Ours could select any of four cups in a turret.
    I miss that machine. Ours constantly consumed vast amounts of LN2 for the cryopumping.

    Thermal: A tungsten or molybdenum coil or refractive metal boat holds the material to be evaporated. Tens to hundreds of amps heats the source at s very low voltage.
    Exception: Aluminum and Nickle will nicely come off a heater not much bigger then a 100 watt light bulb filament.

    Simple sputtering and thermal have been in use since the mid thirties. Both are within the range of a determined Hobbyist on small scale.

    I miss working in that department, sometimes.

    Midwest Tungsten is often selling overstock cools and boats.

    Kurt Lesker corporation has a great web page.

    With the exception of Platinum we usually made our own targets for sputtering.

    Last time I needed to aluminize at work it took me two hours to build the chamber, 20 minutes to pump down, a few seconds to outgas the heater coil, and all of about 1 second to coat a silicon wafer to mirror finish. That Professor had a roughing pump / turbopump combo that
    Was about the size of a breadbox. We had stainless steel pipes with knife edge flanges laying around. Only "difficult" part was the feed through for the current, and they show up on Ebay.
    Last edited by mixedgas; 11-08-2021 at 11:11.
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    Ah my bad!
    At the beginning of my post I was indeed reffering to magnetron sputtering.
    However thermal was said to be easier to achieve but the issue is that the material is off course evaporated and thus hot.

    I heard as well it's not too crazy to make your own targets and the ones you buy usually last quite long!

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    Depending on what your doing heating may be needed. Especially for optics, It can improve adhesion. You will certainly need to drive any adherent gas films off the substrate. If your coating more than one optic, they are generally mounted on a heated spinning platen.
    Heating in this case may just be a halogen lamp in the chamber.
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    Quote Originally Posted by turbohead View Post
    yes watched those and they are great but they dont talk about the targets and their function

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