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An interesting experimental phenomenon related to electric field

Discussion in 'General Electronics Discussion' started by Chengjun Li, Mar 18, 2015.

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  1. Chengjun Li

    Chengjun Li

    84
    7
    Oct 21, 2014
    Hi,

    I find an interesting related to electric field when doing experiment.
    I put a chip which has the following simplified circuit inside Scanning Electron Microscopy(SEM), an electron beam is irradiating on R1.
    [​IMG]
    The AC source is isolated from earth by using a capacitor C1 and resistor R3 in the middle.

    I found that the electron beam is oscillating with a frequency the same with the AC source.
    So I think there may be electric field resulted which deflects the electron beam periodically.

    Then I did the following experiment.
    [​IMG]
    The only change I made is cutting off the connection between point A and B.
    I found this time electron beam is no longer oscillating.


    My question is that 1. how does the oscillating electric field produced?
    2. why in the second situation, there is no electric field?

    I know that electric field is produced due to potential gradient in space.
    [​IMG]
    I think in the second situation Point B should have a potential of 0V since it is connected to earth, Point A has a potential not equal to 0V, there is potential difference between them, why there is no electric field be produced between point A and B?
     
  2. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    The electron beam will be deflected by the magnetic field, that is in fact how the beam is focussed.

    I would think that there will be little effect of potential differences, you will be using about 20kV and an odd volt may not do much.

    Disconnecting the circuit will stop the current and stop the resulting magnetic field.
     
  3. Chengjun Li

    Chengjun Li

    84
    7
    Oct 21, 2014
    Thanks for your reply.
    The largest voltage I use is 15Vpp. Do you mean that this voltage is too small to produce a large enough electric field to deflect electron beam?
     
  4. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    The beam will be produced with very high voltage, tens of kV. Electrostatic deflection can occur and non conducting specimens are coated with aluminium or gold to give uniform potential at the surface.

    Magnetic fields can deflect the beam. How do you focus the SEM? Try a magnet near to a cathode ray tube TV and see the colours change.
     
  5. Chengjun Li

    Chengjun Li

    84
    7
    Oct 21, 2014
    I understand the electron beam is focused using a magnetic lens which is similar to a solenoid with current running through it to produced a magnetic field. Actually I have two years experience with SEM, before I always apply DC voltage to our device, and everything is fine. This is the first time i apply AC voltage, and i observed something abnormal.

    My question now is I want to totally understand how is the electron beam being deflected. Was it due to electric field or magnetic field or both? How does the electric field produced? And magnetic field?

    May I say that you prefer it is due to the magnetic field? And you think it is not due to the electrostatic field because electron beam has a large energy(20keV) and is not easily be deflected by electric field?

    I think your answer is reasonable. But I still have a small question with that explanation.

    The previous statement of the problem makes several simplification because I was worrying about people will be confused about what I said if not do that. Sin you are familiar with SEM, I think I'd better to describe the problem in detail.

    We fabricate a chip, a MEMS chip, mainly used for mechanical testing of nanostructure. The inner design of the chip is shown below.
    [​IMG]
    R1(250Ω) mentioned in first thread is the area which the arrow points at. Enlarged image is shown below.
    [​IMG]
    R1 mechanically function as a thermal actuator, it will oscillate under AC voltage.The aim of the experiment is to measure oscillation amplitude under different frequency. The part on the left is a fixed structure, it should always be stationary.

    R2(500kΩ):
    one end of R1 is also connected to other structures as I highlighted in yellow color. I simplified them as R2. All the pins are electrically connected(green line) using conductive tape, except the two pins through which the AC voltage is applied to. I connect the upper left corner pin to ground when doing experiment.
    [​IMG]

    R3(1MΩ): The function generator we use to provide AC voltage is Agilent 33250A
    [​IMG]
    On the manual, it says "Except for its remote interface connectors and trigger connector,the 33250A is isolated from chassis (earth) ground. This isolation helps to eliminate ground loops in your system and also allows you to reference the output signal to voltages other than ground."

    Below is the R1 under electron beam.
    [​IMG]
    As I mentioned the left part is always stationary. But when I applied the AC voltage, I found the left part is also oscillating.

    [​IMG]
    I think the only reason is electron beam is deflected periodically.

    If the electron beam is deflected due to magnetic field and magnetic field is due to AC current running through R2, then my question is AC current also runs through R1 and it is larger than current running through R2, then why the electron beam is not deflected in the second situation in the first thread?

    Thanks.
     
  6. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    I do not think I can comment further. You need an expert on elecro-optical science to analyse the effect of beam bending with two cases of current and voltage.

    I used early SEMs fitted with x-ray detectors for analysis. Very fine spacial resolution was not important.
     
  7. Chengjun Li

    Chengjun Li

    84
    7
    Oct 21, 2014
    Thank you all the same. The idea of magnetic field is something I didn't pay attention before. Maybe that's the answer.I will keep working on it.
     
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