NPGS  : Line Frequency Noise

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This image shows 60 Hz interference that was being picked up on the BNC cables between the NPGS PC and the SEM. Notice how the interference is worse on the lines with a positive slope and is almost unobservable on the lines with a negative slope. This results from an equal interference being picked up on the XY BNC cables. On the SEM used to write this pattern, the direction of the (+X, +Y) input voltage was to the upper left, which causes the interference to wiggle the beam in that direction. Consequently, lines parallel to that direction show little interference, while lines perpendicular to that direction show the maximum interference.

It is also worth noting that this pattern was written on an SEM without any beam blanker.

This image shows a very severe case of line frequency noise. The effect of such interference may range from as shown here to almost perfect sinusoidal waves as shown above. In this case, the noise pickup was apparently in the microscope electronics, since the problem changed dramatically depending on the magnification range being used. For more information on the effect of magnification ranges, click Bad Magnification.

The most common noise problem for SEM lithography is from sources radiating at line frequency. Solutions may involve moving the noise source (for example, relocating a power bus that is too near the SEM column or simply turning off fluorescent lights), shielding the SEM sample chamber and/or column or the source of the field with mu-metal, or using an active field cancelling system. It has been observed that when an external noise source is affecting lithography, then it can also be seen when using the microscope for inspection.  Knowing this can be very helpful when trying to locate and remove a source of noise. In general, it is always best to identify the source(s) of the magnetic field(s) before using the relatively expensive solutions of shielding or active field cancellation.

One very simply problem that can lead to very large magnetic fields is improper wiring. For example, normally the current that runs on the "hot" conductor should return on the "neutral" conductor in the same electrical conduit. In this case, the currents are equal and opposite and the fields will typically cancel nearly completely just a short distance away (<1 meter). However, if any equipment on the circuit allows the current to connect to some other ground, rather than return on the neutral wire, a very large magnetic field can be produced. While the resulting field may be very obvious, the source of the problem , i.e., where the wiring fault exists, may actually be on the other end of the building. A field that is nearly uniform throughout a room is often caused by a wiring fault.

Another source of magnetic fields that can often be easily fixed is a breaker box. Even if the wiring within a breaker box has equal and opposite currents, if the hot and neutral wires are not in close proximity to each other, the resulting field my extend for some distance (> 1 meter). If the wiring can be arranged to keep the hot and neutral wires as close together as possible, then the external field can be significantly reduced.

In very few cases, the noise source is internal to the microscope. In such cases, it is recommended to use a spectrum analyzer to check for ripple on DC power supplies and to look for ground loops.

A site survey of the room to characterize the magnetic fields and vibration levels can also be very informative. However, it must be understood that any site survey only applies to the time period of the survey and that external sources may change at any time. The specifications given by most SEM manufacturers are as follows:
  • Stray AC less than 3x10-7 Tesla(p-p)* = 3 mGauss(p-p)* or 0.1 uT(rms)** = 1 mGauss(rms)**
  • Vibration less than 2x10-6 meters(p-p) over 5 Hz

*When using a coil to make a calibrated peak to peak measurement of a magnetic field, be aware that most calibrated coils will have a voltage to magnetic field conversion factor that is calibrated at 50 or 60 Hz. If the field being measured has significant components at other frequencies, the appropriate corrections must be made to the conversion factor in order to get a valid reading. Alternately, a relative measurement can be made using a coil of wire and a multimeter or oscilloscope.

**A digital gauss meters that makes frequency independent RMS readings can be purchased for about $100 USD (see the Extech Model #480823 at Transcat, (800) 800-5001).

In any case, such an instrument is extremely useful in tracking down sources of magnetic fields.

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