The rotationally-parabolic mirror (RPM) is designed to capture light from the luminescing sample and focus it down the pipe into the spectrometer system. Alignment is critical because the light path is over a meter long, but the mirror itself is only millimeters in size. To get a parallel beam of light into the spectrometer means the focus has to be positioned correctly to within a few microns.
Because the RPM is in the way, the BSE detector won’t work at all (and should be removed). The SE and VP detectors will work at decreased efficiency, getting a signal that manages to escape from under the mirror. Adjust brightness and contrast accordingly.
Once the alignment is done, all focusing should be done with the Z-axis control, not the electron beam focus knobs. That keeps the sample and mirror correctly aligned.
Schematic showing the parabolic mirror collection system. The problem in any CL system is collecting as much light as possible, ideally from a large solid angle directly above the sample. From directly above, the emitted light is not affected so much by the sample itself, by absorption, scattering, and total internal reflection. The RPM is the best in that respect, but consequently mirror alignment is critical.
Fiber optic light pipes are also used. They collect less light, have their own absorption characteristics, and they must collect light from the side, but alignment isn’t so critical. The VP detector is of this type, and works as a poor-man’s CL detector.
This shows the RPM inserted into the chamber. The bottom of the mirror needs to be very close to the sample surface. If the sample holder touches the mirror, don’t worry. The stage will stop and nothing will be damaged. Simply lower the stage and try again.
Move the stage to a sample that luminesces brightly and evenly, like willemite (14) on the CL block.
Raise the sample surface to about 1 mm below the mirror bottom. If you are using the sample holder for six round mounts, you can raise the stage to a Z-axis position of 23.0 mm.
Set magnification to 1000-3000x and focus the SEM image using the SEM controls.
Press the RPM-out button, shown here, and select In chamber from its menu. The mirror should move to its last saved position. It takes about a minute, so be patient.
Choose the 1800 lines/mm grating: Menu Acquisition / Instrument setup / grating / choose 1800 from menu. You can actually use any grating, but this one has the best reflectivity at the zero position.
Set the grating position to zero: Menu Aquisition / Acquisition parameters / Spectro (nm) / set value to zero. The grating acts like a mirror at this setting.
Choose the CCD detector: Menu Acquisition / Instrument setup / Detector / choose Syncerity, and / Exit mirror / choose CCD.
Set the: Menu Acquisition / Acquisition parameters / RTD time to a value of 0.1. This is the signal acquisition time during real-time tasks like mirror alignment. This may have to be adjusted to higher or lower values, depending on how much signal you see.
In the SEM scanning tab, select the Spot check box. This puts the electron beam at a single spot, rather than rastering across a rectangle. the electron spot is indicated by a “+” on the screen. Drag it to the exact center of the SEM field. You can tell where the center is if you make the image crosshairs visible.
Go to: Menu Maintenance / RPM adjustments. This is where you set the mirror X and Y position.
Y is set to optimize signal symmetry (left to right on the screen).
X is set to maximize signal intensity.
Pick the Spectra tab and press the Real time button.
You should see a graph with a peak near an X-axis value of zero. Adjust the graph Y-axis scale to get the peak top approximately in the middle.
In the SEM system, slowly raise and lower the stage (Z-axis) to maximize peak height. Adjust the graph Y-axis scale, if necessary.
If the peak gets so big it saturates the detector (red line at 65,000 counts per cycle), reduce the RTD time as described above. If the peak is too small, increase the RTD time.
Adjust the Menu Maintenance / RPM adjustments / RPM_Y Offset value to make the peak symmetrical left to right. The Inc. value is the increment for the Offset value. At first set this to 10 or 20. For final adjustments, set it to 2.
Adjust the Menu Maintenance / RPM adjustments / RPM_X Offset value to maximize peak height. At first set the Inc. value to 10 or 20. For final adjustments, set it to 2.
Cycle through adjusting Z to maximize peak height, Y for left-right symmetry, and X for peak height. Ultimately the peak should be narrow, symmetrical, and high.
There is no particular peak height to strive for. In addition to mirror alignment, peak height depends on electron beam settings, luminous efficiency of the sample, detector efficiency at the wavelengths the sample emits, and the Menu Acquisition / Acquisition parameters / RTD time value.
Press the red STOP ALL button. You have to press this button to stop any kind of acquisition, including real-time, before trying to change most settings and display modes. It also halts spectrum and image acquisitions you don’t want. It is a very popular button.
Go to the CCD image tab. A 2-d image window should display, representing the 1024 x 256 pixel CCD array. Press the Start button. A colored spot should appear in the middle. This is the image of the luminescing spot, focused onto the CCD array.
Go through the Z, Y, and X adjustments again, as above, to make the spot as small and symmetrical as possible, using small Inc. values.
Warning: If your peak, above, was wide and squarish or with squarish shoulders, and you see here a big rectangular or arch-shaped field, the mirror Z-axis position is way off. The squarish shoulders and rectangle are shadows of the slit
Now that the mirror is centered, press the two Set working position buttons. That saves your new X and Y positions so, when the RPM is next retracted and inserted, it will move back to approximately the same position. Also write down the stage Z-axis position.
DO NOT press the Restore default position buttons. Those are only in case of emergency, if the RPM positions get hopelessly messed up.
The system should now be ready to collect spectra and images. Remember that any focusing you do, from now on, should be with the SEM Z-axis control, NOT the focus controls on the SEM keyboard.
