The JEOL system is calibrated by staff every 7 - 21 days, with separate calibration necessary for each condition file (current). These are typically stable for 1-3 weeks. As part of this process, column shift and tilt is adjusted (similar to an SEM), current is measured (and adjusted via the column zoom lenses, if necessary), wobble is checked and minimized (via the objective aperture, again similar to an SEM), focus and astigmatism is adjusted (via the objective lens strength and stigmator correction values), and the DAILYCAL file is run to ensure everything passes.

Data is shown for the current FEG and previous FEG. These are replaced approximately every 2 years.

Some particularly relevant measurements are shown below:

• Current: specified as nominal values. As part of the exposure, the machine will measure the current (by default, every 20 minutes) and automatically adjust the exposure dose based on this. As such, drifts in current will not directly affect the writing quality, but may affect user experience on the system.

  • How is this set?

    • Current is a result of the particular characteristics of the emitter at a given point in its lifetime (which may increase or decrease current at the sample), the staff set values of shift and tilt (which should maximize it), and most importantly, the values of the zoom lenses in the column. Current is adjusted by adjusting the zoom lenses. This is not done frequently due to the needed time for accurate adjustment, and the minimal impact on write quality that current drift has for users. Current settings are adjusted whenever a particular condition file is getting close to the specification limit.

  • How is this measured?

    • The system moves to the Faraday cup and takes, takes a number of instantaneous measurements of the current, and averages them together.

  • Upper specification limit: the nominal current itself

    • Via the dose equation, too high of a staff set current may cause the machine to need a clock speed higher than 125 MHz if a user chooses a typical clock speed very close to the limit (e.g. typically >=120 MHz). This would result in an error message when trying to create the magazine file. From the user standpoint, this could only be remedied by increasing the minimum dose (not usually desirable, as this may overdose your pattern) or increasing the shot pitch (which may or may not result in different pattern results, but is also generally not desirable because of the unknown effect on your results). As a result, staff endeavor to keep the actual current below the nominal current at all times.

  • Lower specification limit: 10% less than the nominal current

    • Too low of a current (within reason) will not in anyway harm the pattern, but the write will take longer than expected. If the write is limited by the beam on time, 10% lower current (90% of the specified current) will take ~11% longer to write (100%/90% ≅ 1.11). This is considered a reasonable error, but longer than ~11% may unexpectedly prolong the write, and thus staff will endeavor to keep the actual current not lower than 10% lower than the nominal current.

• Calibration measured beam size: The beam size in the X and Y directions measured by the system on a “clean” spot of the AE mark used by staff for calibration. This will always be larger than the true size of the beam, and is only useful in the context explained here.

  • How is this set?

    • This is set by having proper alignment by staff of the blanking aperture, shift, tilt, focus, and astigmatism. Additionally, as a particular mark is used to for measurement of the beam size, it will get “contaminated”, artificially inflating the measured beam size. Staff will slightly move the measured position every week, but there is still some low level inflation of the measured beam size that may not be real due to this mark contamination.

  • How is this measured?

    • The beam is scanned over a metal knife edge on top of a current detector, which does math to obtain the size of the beam, assuming it is Gaussian. There are scans in both the X and Y direction. The Gaussian assumption is a good assumption at low beam currents (< 30 nA) and an increasingly poorer assumption at high beam currents (> 30 nA), so this value is only given for low currents. Note that the values here will be lower than values given when the focus program is run as part of DAILYCAL because a less clean mark is used for that (which in no way means the beam is actually larger, nor does it negatively affect the write quality). A less pristine mark position is used for writes vs. calibrations because the machine will still properly focus even if the mark is slightly contaminated. The mark positions are moved as needed by staff to ensure a clean enough mark is used at all times.

  • Upper specification limit: theoretical beam size plus an inherent scattering factor due to the measurement technique plus an allowable blur.

  • Lower specification limit: none.

• Astigmatism:

  • How is this set?

    • The focus program is run in a different mode, which measures the beam size at different focus and stigmator values, and find the minimum size across these different values.

  • How is this measured?

    • The astigmatism value returned is the number of DAC points between the X and Y best focus values per a curve fit.

  • Upper specification limit: + 20 DAC points.

  • Lower specification limit: - 20 DAC points.

High currents (>30 nA) have their focus and astigmatism set manually by staff rather than through the built-in calibration routines on the system. Similarly, the built-in measurement programs do not give reliable values for beam size or astigmatism. As a result, only beam current (which is measured similarly and reliably compared to low currents) is posted here.

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