This is a preliminary draft set of procedures for checking various instrument calibrations. Some of these methods will be familiar to some of you, others may not. We are seeking feedback and suggestions for improving these various calibrations.
These initial procedures were compiled by Mike Matthews, John Donovan, Aurelien Moy and Scott Boroughs and we welcome input from all others.
In this document we will discuss methods to check the calibration of our EPMA instruments. Some of these instrument calibration parameters are dependent on the manufacturer building the instrument properly, for example, that the column is perpendicular to the sample stage and centered with respect to the various WDS and EDS spectrometers. Others calibration parameters may change as the instrument and its detectors age over the lifetime of the instrument, for example, the WDS dead time calibrations.
1. Sample, stage, beam and other geometric calibrations
a. Check that the sample is level with respect to the light optics. Using a flat sample with a top referenced sample holder, move to three different stage positions approximately 1 to 2 cm apart, ideally forming a roughly equilateral triangle, in order to record the Z stage optical focus (and X/Y stage positions), and calculating the sample tilt from this data. If these three Z stage optical focus positions are always within the optical depth of field, the sample can be considered level with respect to the column.
b. Check that the beam is central down the column by “rocking the beam focus”. For example, acquire a low mag image (SEM: at a short working distance) with all dynamic focus features turned off, and check that the area of defocus is concentric around the center of the image.
c. Check that the stage is perpendicular to the electron beam and note: one cannot check that the stage is perpendicular to the electron beam using the light optics because if the stage is tilted, the optical focus will remain in focus over the stage top surface at all X/Y positions. Therefore, we must utilize an alternative method, for example, check that while electron imaging, an object (a small hole for example) remains centered as the Z stage axis is adjusted up and down.
2. Spectrometer alignment and effective take-off angle for WDS and EDS spectrometers
a. Check that all spectrometers yield similar k-ratios within measurement statistics. Using the method of “simultaneous k-ratios”, for example measuring Si Ka on SiO2 (primary standard) and Mg2SiO4 (secondary standard) on all TAP or PET Bragg crystals, one should obtain the same k-ratios within measurement statistics. For LiF or PET Bragg crystals we can utilize Ti Ka on TiO2 (primary standard) and SrTiO3 (secondary standard) for example.
3. Dead time, picoammeter calibrations and Bragg diffraction symmetry checks. Details for the following three calibration procedures can be found in this recent publication: (
https://doi.org/10.1093/micmic/ozad050).
a. Check that each spectrometer yields the same k-ratios at different count rates (beam currents) by utilizing the method of “constant k-ratios” as described here by measuring k-ratio intensities for both the primary and secondary standard at different beam currents, e.g., both Ti metal and TiO2 at 10 nA, both at 20 nA, both at 30 nA, etc. up to say, 200 nA.
b. Once the dead times are constant within measurement statistics over a range of beam currents (count rates), the picoammeter linearity can also be checked by utilizing only a single primary standard at one beam current, say 10 nA, and calculating k-ratios using a secondary standard measured over the same range of beam currents. Note that both 3a and 3b tests can be performed using the same dataset!
c. Finally, using simultaneous k-ratio measurements (on multiple spectrometers tuned to the same emission line), check that all spectrometers yield similar k-ratios. There are several geometric/alignment issues that can mask each other: for example are the spectrometers properly centered around the column? Are the Bragg crystals diffracting symmetrically. Both of these can yield different effective takeoff angles for each spectrometer/crystal combination. But if these effective takeoff angles are determined with reasonable accuracy, these various effective takeoff angles could be stored in one’s spectrometer calibration file for use in quantitative matrix corrections.
4. Electron beam landing energy calibration
a. As we all know, the Duane-Hunt limit can be utilized to check the accuracy of the electron beam accelerating voltage, however, it is best to test this limit “blind”, by having a colleague acquire the EDS spectra with sufficient statistics, but not at a nominal beam energy, for example: 15.2 keV or 14 .9 keV or 20.5 keV. Then, without knowing the purported beam energy in advance, one attempts to determine the Duane-Hunt limit for the spectrum in question. Always use a well grounded conductive (pure metal such as Cu or Au) sample and count a sufficiently long time to accurately determine the Duane-Hunt limit, and do not be misled by a few continuum coincidence events that will yield photons greater than the operating voltage. These continuum coincidence events can be reduced by counting at a relatively low beam current, for example 10 or 20 nA, and counting for a sufficiently long time, e.g., 1000 seconds.
5. Miscellaneous test and calibration procedures. An example of these procedures can be found here: https://epmalab.uoregon.edu/reports/Additional%20Specifications%20New.pdfa. Check spectrometer reproducibility testing using the “half-intensity” method. First determine the spectrometer position which yields one-half the maximum peak intensity (on either side of the peak) for each pair of elements on each crystal at each end of the spectrometer range. Then move the spectrometer to each of these “half-intensity” positions and record the intensities. Repeat 100 times. The peak intensities shall vary by less than 0.6% (+/- 0.3%) with 99% confidence levels from the previous set and the one-half the maximum intensities shall vary by less than 1.2% (+/- 0.6%) at 99% confidence levels without a backlash or re-peak procedure.
b. Repeat the procedure with a crystal flip in between each “half-intensity” position. Verify that the intensities measured vary less than 2% (+/- 1%) with 99% confidence levels from the previous set without a backlash or re-peak procedure.
c. Check beam stability over time. Beam current stability should be 0.1% or less per hour (+/- 0.05%) and 0.6% or less per 12 hours (+/- 0.3%) and 1.0% or less in 24 hours (+/- 0.5%) as measured at 15KeV and 10nA while repeatedly inserting the faraday cup approximately once per minute.
d. Check for stray beam using an aperture mounted in a sample holder. Stray beam measured using a 100 micron W or Mo aperture target in a Ti target block to produce W or Mo Lα and Ti Kα k-ratios (both EDS and WDS) less than 0.0001 (0.01wt% or 100ppm) using a 100 nA beam and at operating voltages from 5 KeV to 30 KeV.
e. The auto focusing reproducibility must be tested by performing the following test: 100 repeated auto-focuses that reproduce the stage Z position within 1 um each time on a static flat polished carbon coated Cu sample (dark blue color).
Please keep your software updated for best results!