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Ellipsometry goniometer orientation

  1. Oct 31, 2012 #1
    Hi Folks,

    I'm building an ellipsometer-like system similar to the one shown in attached picture, but with much longer and thinner rotation bars carrying viewing optics assembly (VOA) and incident optics assembly (IOA), respectively; the system would require that the sample holder plane can rotate independently from the common rotation axis of the VOA and the IOA bars; in other words, the system is like a clock with 3 hands/bars that can rotate independently. For now I can fix the IOA bar to a certain position to simplify the design, so the problem is how to align the rotation axis of the sample holder and the VOA.

    I can't afford those expensive motorized multi-axis rotation stages from Aerotech or Huber; I plan to use a servo motor to drive a gear box; the output axis of the gear box to drive the VOA bar. So my question is: how to align the sample holder plane rotation axis so it coincides with the VOA bar rotation axis, but the two can rotate independently?

    Thanks very much for reading and any suggestions!


    Attached Files:

  2. jcsd
  3. Oct 31, 2012 #2


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    Disclaimer: I design optical metrology instruments like this for a living, so I'm familiar with their design processes.

    Do you have any experience in designing a system like this? It's pretty complex, and I get the feeling you don't quite understand what you're in for... You need to nail down things like your accuracy and precision requirements, required degrees of freedom, system alignment plan, etc. etc. Getting axes to move independently is easy; you just need a separate actuator for each one!

    When you say "longer and thinner rotation bars" do you understand the tradeoffs w.r.t stiffness and length/section? To maintain stiffness and a high natural frequency, you'll actually want thicker if you go longer. The system in the picture (a Woollam M2000 Ellipsometer) is a high-end piece of instrumentation and it shows in it's specifications; weight-optimized athermallized structures, high-end motion axes, etc. Are you basically wanting to duplicate these capabilities?

    Regarding your motor drives, you'll also need to look at your adjustment requirements there as well, especially if you're talking about trying to achieve high resolution with sub-par components. Odds are you can't just use the output drive of a gearbox as a rotation stage; you need mechanical support! What samples are you measuring? How precisely do they have to be held? What kind of hysteresis can you tolerate in the rotation axis, let alone backlash? How will you achieve linear positioning without using "expensive Aerotech stages"?

    There's a lot to think about, that's for sure.
  4. Nov 2, 2012 #3

    Thank for your reply. I do have experience with similar systems and detailed specs for accuracy, torque, moment of inertia and deg of freedom etc., just wanted to give a very brief description without flooding lots specs.

    What I am trying to do is a much more simplified version of the device in your link. In my original post picture, I can fix the IOA bar at a certain angular position and only rotate the VOA bar using a servo/gearbox (of course I considered system support a must); the angular accuracy is not so critical, 0.5 deg would suffice. My question is, how to make the sample holder rotation axis coincide with the VOA bar rotation axis?

    I attach a picture of a system I used before: a cylindrical pivot (not visible in picture) is attached to the base plate and both the VOA bar and the sample holder support plate can rotate freely and independently around this pivot cylinder. This is a totally manual system. What I'm trying to build is using a motor to drive the VOA bar yet leave the sample holder plate independent. The reason I want the sample holder plate to rotate around the same axis as the VOA bar is for system alignment: by using a reflector I can easily check if the incident light can precisely enter the viewing optics aperture.

    Linear positioning is not necessary in the picture attached here, it is guaranteed as the VOA bar is co-axial with the sample holder plate. However now I'm using a gearbox axis to drive the VOA bar, I must use some linear/rotation stage to position the sample holder plate. How to make sure the sample plate rotation axis co-axial with the gearbox output axis?


    Attached Files:

  5. Nov 5, 2012 #4


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    What is the runout tolerance for the sample holder plate w.r.t. the gearbox? You say you can tolerate an angular accuracy of +/-0.5 deg for the IOA, so how much does the centration of of the sample holder matter? I'm guessing you can probably just bolt to the output shaft with a precision bore.
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