Measuring electron beam diameter

Posted on March 17, 2014 by Randy

This procedure will allow you to determine the electron beam diameter diameter on scanning auger electron spectrometers using the line scan feature of AugerMap software and the magnification standard (or, a straight edged sample). Although written for scanning auger electron spectrometer systems, the principle is the same for SEMs – scanning electron microscopes – measure the slope across an edge at a known magnification.

1. Insert the beam size standard into the system and perform an elastic peak alignment on the top surface of the sample to ensure that the sample is at the correct Z position with respect to the analyzer. Do not tilt the sample.

Set elastic peak

2. Next, change the beam voltage to 10KV and adjust the electron gun parameters for a good image at 5000X or higher magnification.

3. if so equipped, adjust the Condenser and Objective steering plates for minimum movement. (If an SEM, rock the lenses).

4. Ensure that the image is in the best possible focus.

5. Lower the magnification to the lowest possible setting and move the beam size standard to the center hole to find the grid and adjust the Z position until the grid is in focus. The sample is now at the correct focal point on the analyzer. This is a good trick that you can use on any scanning auger system – if you first set the elastic peak and then the focus, as long as you mechanically bring the sample back into focus after moving to a different location on the sample (and do not change the focus knob or settings in the software), the sample to analyzer distance is still correct.

6. Set the magnification to 100KX and fine-tune the focus and stigmators as required. The sample can be moved on either the X or Y axis until you are able to line up on one grid line.

7. Obtain an SED video map and use the quantization feature if necessary.

8. Select the line scan feature in AugerMap and draw a line horizontally across the grid line.

9. For AugerMap1, Add region – SED line.

10. Select Video input; point one ms per step, number of sweeps one, and resolution 256 points per line. For AugerMap II, select SED video map.

AugerMap 2 SED dialog box

Auger Map version 1 SED dialog box

11. Acquire the line scan.

12. After the line scan is completed it will show a display of signal intensity on the Y-axis vs. distance scanned on the X-axis. At 100KX magnification, this distance is 1uM, or, 10,000 angstroms. By determining the slope of the beam diameter as it crosses the edge of the sample, the beam diameter can be determined.

13. Print out the line scan and use a ruler to determine the top 20% and bottom 20% of the line scan. Draw a line across the top 20% and bottom 20% of the line scan.

14. Next, draw a line on the slope of the beam diameter as it drops down or up between the 20% lines.

15. Determine the distance between the slope of the line at the top of the scan and the bottom of the scan. This distance represents the beam diameter in relationship to the full scan (10,000 angstroms at 100KX).

16. The example below shows in detail how this measurement is calculated.

Beam Size Measurement

Perform this measurement anytime that you wish to know the size of the electron beam diameter for any given set of conditions. If the beam size is too large to see one grid line, you can reduce the magnification to 50KX, in which case full scale on the X-axis would be equal to 20,000 angstroms.

Ion Pump Rejuvenation Procedure

Posted on March 12, 2014 by Randy

After prolonged periods of sputtering with Argon gas, the ion pumps can become saturated, resulting in occasional “belches” of Argon during which the ion pumps overheat and release large amounts of gas. These belches usually result in a snowball effect that can dump the system. Rejuvenating the ion pumps once every few months (more often if you do a lot of sputtering) will help to prevent the belch problem from recurring.

To rejuvenate the ion pumps with O2:

1. Turn off all filaments, including the ionization tube (DIG).

2. Set the ion pump control panel meter to the 200mA current settings and set the

ion pump to the run (protected) mode.

3. Slowly bleed in O2 until there are 40mA of current shown on the ion pump panel meter. You will need to change ranges on the panel meter as the current is increased.

4. Adjust the leak valve as needed to maintain 40mA of current for 20 to 30 minutes.

5. Close the leak valve. It takes about one day for the vacuum to return to its previous level.

For more information on rebuilding ion pumps, search for Ion Pump in the RBD TechSpot blog search box.

For more information on ion pump theory, here is a link to an informative paper – https://cds.cern.ch/record/454179/files/p37.pdf

And, from Wikipedia:

An ion pump (also referred to as a sputter ion pump) is a type of vacuum pump capable of reaching pressures as low as 10⁻¹¹ mbar under ideal conditions.An ion pump ionizes gas within the vessel it is attached to and employs a strong electrical potential, typically 3kV to 7kV, which allows the ions to accelerate into and be captured by a solid electrode and its residue.

The basic element of the common ion pump is a Penning trap. A swirling cloud of electrons produced by an electric discharge are temporarily stored in the anode region of a Penning trap. These electrons ionize incoming gas atoms and molecules. The resultant swirling ions are accelerated to strike a chemically active cathode (usually titanium). On impact the accelerated ions will either become buried within the cathode or sputter cathode material onto the walls of the pump. The freshly sputtered chemically active cathode material acts as a getter that then evacuates the gas by both chemisorption and physisorption resulting in a net pumping action. Inert and lighter gases, such as He and H₂ tend not sputter and are absorbed by physisorption. Some fraction of the energetic gas ions (including gas that is not chemically active with the cathode material) can strike the cathode and acquire an electron from the surface neutralizing it as it rebounds. These rebounding energetic neutrals are buried in exposed pump surfaces.

Both the pumping rate and capacity of such capture methods are dependent on the specific gas species being collected and the cathode material absorbing it. Some species, such as carbon monoxide, will chemically bind to the surface of a cathode material. Others, such as hydrogen, will diffuse into the metallic structure. In the former example, the pump rate can drop as the cathode material becomes coated. And, in the latter, the rate remains fixed by the rate at which the hydrogen diffuses.

There are three main types of ion pumps, the conventional or standard diode pump, the noble diode pump and the triode pump.

Ion pumps are commonly used in ultra-high vacuum (UHV) systems, as they can attain ultimate pressures less than 10⁻¹¹ mbar. In contrast to other common UHV pumps, such as turbomolecular pumps and diffusion pumps, ion pumps have no moving parts and use no oil. They are therefore clean, need little maintenance, and produce no vibrations. These advantages make ion pumps well-suited for use in scanning probe microscopy and other high-precision apparatus.

High Optical Quality Torr Scientific Vacuum Window used on ALBA

Posted on February 10, 2014 by Mel Thomas

Located in Barcelona, ALBA is a Synchrotron Light Source producing radiation for experiments in many fields of science, www.cells.es. The quality of the radiation delivered to users depends on the quality of the electron beam and it is therefore necessary to continuously monitor the beam status in a non-invasive way. At ALBA, the visible part of the synchrotron radiation itself is used to measure distribution of electrons in the machine. The used synchrotron radiation is guided far from the machine tunnel up to an “extraction mirror”. In order to maintain the vacuum in the tunnel, the radiation routed up to the mirror is in-vacuum too. A Torr Scientific viewport is used for extracting the light. The goal this year is to measure the transverse beam size by using the interference pattern produced by the synchrotron radiation passing through a double slits aperture. The performance of this kind of experiment requires the preservation of the quality of the light. Torr Scientific therefore provided a special viewport with a flatness better than lambda/10 and a curvature better than 1 arc sec. Accelerator Division Researcher Laura Torino
said ‘the special viewport we ordered fulfills all the requirements outstandingly; even better than what we were asking. Thank you for the great job you did’. This ALBA project is sponsored by oPAC.