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Scanning electron microscope

  1. May 16, 2003 #1
    I know that optical devices are limited in the detail that can be seen because of the size of the wavelength of visible light. What is the limit, the smallest thing that can be seen with an optical device? Is the level of detail directly proportional to the wavelength? So what is the wavelength used by the SEM? Is this the device that can show the highest detail?
  2. jcsd
  3. May 16, 2003 #2
    Hello Will,

    I posted this once upon a time back in PF2. So far as I know this was/is the most powerful optical microscope ever built (The Universal Microscope);

    Last edited by a moderator: May 16, 2003
  4. May 16, 2003 #3
    So, what is the resolution of this microscope? Wavelength? Half wavelength? Quater?
  5. May 16, 2003 #4

    SEM's today use electrons with anywhere from 2 - 50 KeV of energy and you can get ~5 nm resolution, or better, with them quite easily. The highest resolution devices are scanning transmission microscopes.

  6. May 16, 2003 #5
    Is Rife microscope a SEM?
  7. May 16, 2003 #6
    Have you clicked on the link I provided?
  8. May 16, 2003 #7
    Yes. It does not say what is the resolution of Rife microscope (say, 0.1 micron for blue light, or half wavelength for visible, or anything like this).

    So, the question - what is best resolution of optical microscope achieved so far - is still open.
  9. May 16, 2003 #8
    In the 1870s, a man named Ernst Abbe explained why the resolution of a microscope is limited. He said that since the microscope uses visible light and visible light has a set range of wavelengths. The microscope can't produce the image of an object that is smaller than the length of the light wave.

    More details here;
  10. May 16, 2003 #9


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    Basically, as light waves hit really small objects, they experience diffraction, and also interference. This limits the resolution you get from it. You can decrease the wavelength of you em waves to get better resolution, but then you have another problem - you have weak signal intensity because then the waves tend to go THROUGH the object, or THROUGH the receiver.
    By using electrons, you can get better resolution because the debroglie wavelength of electrons are much smaller than the wavelength of standard em radiation. Hence, you experience less diffraction, less interference and hence better quality.
  11. May 16, 2003 #10
    On a good day, the SEM in our lab can resolve 10 nm. Of course, we can use it to write lines typically ~40 nm thick on a PMMA surface.

  12. May 17, 2003 #11
    What is PMMA? Do you write lines to make a mask for photolithography?
  13. May 17, 2003 #12
    PMMA is polymethyl methacrylate, it's an organic resist. When PMMA is exposed to electrons, bonds between molecules is broken. So the exposed areas have a lower molcular weigth than the unexposed areas. You then use MIBK(Methyl Isobutyl Ketone) and IPA to devlop, or remove the exposed areas of PMMA. Then you can evaporate metal unto the wafer, then remove rest of the PMMA, leaving only the pattern you exposed made with a thin layer of metal.
  14. May 17, 2003 #13
    Cool. How do you remove PMMA from under metal (gold, I presume)?
  15. May 17, 2003 #14
    IPA breaks it down and removes it. Acetone also works, but not as well.

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