Automated application of drift correction to sample studied under electron microscope
Methods and systems for calibrating a transmission electron microscope are disclosed. A fiducial mark on the sample holder is used to identify known reference points so that a current collection area and a through-hole on the sample holder can be located. A plurality of beam current and beam area measurements are taken, and calibration tables are extrapolated from the measurements for a full range of microscope parameters. The calibration tables are then used to determine electron dose of a sample during an experiment at a given configuration.
1. A method for measuring electron dose in a sample with a transmission electron microscope (TEM), the method comprising:
locating a fiducial mark on a TEM holder tip, wherein the TEM holder tip includes a through-hole located at a predetermined distance from the fiducial mark and a current collection area located at a predetermined distance from the fiducial mark;
calibrating the TEM for measuring beam area across a range of possible beam areas to generate a calibration table for beam area for the TEM;
calibrating the TEM for measuring beam current across a range of possible beam currents to generate a calibration table for beam current for the TEM; and
measuring electron dose on the sample during an experiment using the calibrated TEM having a defined configuration, wherein the measured electron dose is determined using the calibration table for beam area and the calibration table for beam current.
2. The method of claim 1 , wherein calibrating the TEM for measuring beam area across the range of possible beam areas comprises:
locating the fiducial mark on the TEM holder tip;
translating the TEM to the through-hole of the TEM holder tip based on the location of the fiducial mark;
taking multiple beam area measurements of the TEM, with the multiple beam area measurements corresponding to multiple beam magnifications of the TEM; and
extrapolating the multiple beam area measurements to generate the calibration table for beam area for the TEM.
3. The method of claim 1 , wherein calibrating the TEM for measuring beam current across a range of possible beam currents comprises:
locating the fiducial mark on the TEM holder tip;
translating the TEM to the current collection area of the TEM holder tip based on the location of the fiducial mark;
collecting current using a Faraday cup on the TEM holder tip;
taking multiple beam current measurements of the TEM from the collected current, with the multiple beam current measurements corresponding to multiple configurations of the TEM; and
extrapolating the multiple beam current measurements to generate the calibration table for beam current for the TEM.
4. The method of claim 1 , wherein the defined configuration of the TEM includes spot size, an aperture setting, an intensity or brightness setting, or an accelerating voltage.
5. The method of claim 1 , further comprising correlating measured beam current to beam current reported by a fluorescent screen or camera across a range of TEM configurations to determine a correction factor such that a true beam current value can be determined for a value of fluorescent screen current or camera current for the defined configuration.
6. The method of claim 1 , further comprising reducing an electron dose rate when a critical value for an electron dose rate or a cumulative electron dose has been reached.
7. The method of claim 6 , wherein the electron dose rate is reduced by changing an aperture setting, changing the spot size, changing a beam intensity, or changing the beam current.
8. A microscope control system for measuring electron dose in a sample with a transmission electron microscope (TEM), the system comprising:
a processor configured for:
calibrating the TEM for measuring beam area across a range of possible beam areas to generate a calibration table for beam area for the TEM;
calibrating the TEM for measuring beam current across a range of possible beam currents to generate a calibration table for beam current for the TEM; and
measuring electron dose on the sample during an experiment using the calibrated TEM having a defined configuration, wherein the measured electron dose is determined using the calibration table for beam area and the calibration table for beam current.
9. The microscope control system of claim 8 , wherein calibrating the TEM for measuring beam area across the range of possible beam areas comprises:
translating the TEM to a through-hole of a TEM holder tip based on a location of a fiducial mark on the TEM holder tip;
taking multiple beam area measurements of the TEM, with the multiple beam area measurements corresponding to multiple beam magnifications of the TEM; and
extrapolating the multiple beam area measurements to generate the calibration table for beam area for the TEM.
10. The microscope control system of claim 8 , wherein calibrating the TEM for measuring beam current across a range of possible beam currents comprises:
translating the TEM to a current collection area of a TEM holder tip based on a location of a fiducial mark on the TEM holder tip;
taking multiple beam current measurements of the TEM using readings from an ammeter that reads current collected using a Faraday cup on the TEM holder tip, with the multiple beam current measurements corresponding to multiple configurations of the TEM; and
extrapolating the multiple beam current measurements to generate the calibration table for beam current for the TEM.
11. The microscope control system of claim 8 , wherein the defined configuration of the TEM includes spot size, an aperture setting, an intensity or brightness setting, or an accelerating voltage.
12. The microscope control system of claim 8 , wherein the processor is further configured for correlating measured beam current to beam current reported by a fluorescent screen or camera across a range of TEM configurations to determine a correction factor such that a true beam current value can be determined for a value of fluorescent screen current or camera current for the defined configuration.
13. The microscope control system of claim 8 , wherein the processor is further configured for reducing an electron dose rate when a critical value for an electron dose rate or a cumulative electron dose has been reached.
14. The microscope control system of claim 13 , wherein the electron dose rate is reduced by changing an aperture setting, changing the spot size, changing a beam intensity, or changing the beam current.
15. A transmission electron microscope (TEM) holder tip for measuring electron beam current, the TEM holder tip comprising:
a through-hole for allowing an electron beam to pass through the TEM holder tip;
a current collection area for capturing beam current of the electron beam; and
a fiducial mark positioned a predetermined distance from the collection area and a predetermined distance from the through-hole.
16. The TEM holder tip of claim 15 , wherein the beam current is measured using an ammeter.
17. The TEM holder tip of claim 16 , wherein a path from the current collection area to the ammeter comprises a low-resistance material and is electrically shielded to prevent interference.
18. The TEM holder tip of claim 15 , wherein the TEM holder comprises a material having a low atomic number and low electrical resistivity for minimizing electron backscatter.
19. The TEM holder tip of claim 15 , wherein the TEM holder tip comprises an aperture for minimizing electron backscatter.
20. The TEM holder tip of claim 15 , wherein the current collection area is electrically isolated from a body of the TEM holder tip to avoid leakage.
21. The TEM holder tip of claim 15 , wherein the through-hole, current collection area, and fiducial mark are defined in the same layer of the TEM holder tip to avoid a tolerance stack.
22. A method for measuring electron dose in a sample with a scanning transmission electron microscope (STEM), the method comprising:
locating a fiducial mark on a STEM holder tip, wherein the STEM holder tip includes a current collection area located at a predetermined distance from the fiducial mark;
calibrating the STEM for measuring beam current across a range of possible beam currents to generate a calibration table for beam current for the STEM; and
measuring electron dose on the sample during an experiment using the calibrated STEM having a defined configuration, wherein the measured electron dose is determined using the calibration table for beam current.
23. The method of claim 22 , wherein calibrating the STEM for measuring beam current across a range of possible beam currents comprises:
locating the fiducial mark on the STEM holder tip;
translating the STEM to the current collection area of the STEM holder tip based on the location of the fiducial mark;
collecting current using a Faraday cup on the STEM holder tip;
taking multiple beam current measurements of the STEM from the collected current, with the multiple beam current measurements corresponding to multiple configurations of the STEM; and
extrapolating the multiple beam current measurements to generate the calibration table for beam current for the STEM.
24. The method of claim 22 , further comprising reducing an electron dose rate when a critical value for an electron dose rate or a cumulative electron dose has been reached.
25. The method of claim 24 , wherein the electron dose rate is reduced by reducing dwell time, reducing raster area, or using sparse scanning techniques.