Electron Mirrors
Greg Pilgrim - University of Rochester Department of Chemistry
OPT 407 Spring 2011

Introduction and Background
Electron mirrors are the result of charge buildup on an electrically isolated sample. That sample becomes a mirror when an electron beam with less energy (a lower accelerating voltage) than is stored on the mirror is directed at it.  The beam electrons are repelled by the negative charge on the sample and sent in another direction where they interact normally with another “sample”.  That sample can be another actual sample on the stage or the inside of the SEM chamber.  Such a mirror adds a degree of freedom to the imaging process allowing one to (for example) image around corners.  That's all well and good but significant problems arise when trying to use electron mirrors for useful imaging.  Specifically the shape of the mirror, any imperfections on its surface, and even the added working distance from inserting the mirror into the beam path will cause distortions in the image. (Figure 1) Here attempts are made to qualify and quantify that distortion.

EscherInset
Figure 1: A micrograph acquired via an electron mirror of the stage and SEM chamber interior.  The mirror used was a Teflon sphere and the image displays a characteristic “fish eye” distortion.  (Inset: a photograph of the stage with stubs for comparison.)


Experimental Apparatus
Three different types of electron mirrors were used to generate micrographs: Teflon (Polytetraflouroethylene also known as PTFE) spheres, Teflon “holes” (dilled out spheres) and a High Density Polyethylene (HDPE) “plate” (a flat piece of plastic).  Several of these mirrors can be loaded at once. In this manner one mirror can be imaged by way of another.  All the mirror types were charged for several minutes at a low scan speed (~1 scan/2 seconds) with a 20kV electron beam.  The power of the beam was then dropped to a lower accelerating voltage (8eV 9eV or 10eV as noted on the micrographs).  As soon as the voltage has dropped and stabilized a new (reflected) image appears on the raster.  It should be noted that although all the mirror materials used are insulators, mirrors can be formed with conductors as well as long as they are isolated from ground.  Most micrographs were collected with the SE2 detector due to it's sensitivity compared to the in lens.  The in lens detector does not us a Faraday cage but rather only collects electrons directly incident on the lens.  Imaging was also done using the back scatter detector which will be discussed later.
Trippy2
Figure 2: The small ball mirror imaged via the large ball mirror.  Colorized is for artistic effect.

Size Distortion and Focal Points
A standard sample of 300 mesh copper TEM grid was imaged using each type of mirror.  The grids were placed on a special 45 degree stub as the reflected electron beam does not lend itself to imaging samples on flat stubs.  The resulting grid images were then measured using ImageJ and compared to a image taken without a mirror in the beam path.  Significant differences in scaling were observed (Figure 3).  In addition to issues with scale focal points are also an issue.  Increases in working distance as seen in these experiments serve to improve depth of field however the sample geometry works against it.  The plate image (Figure 8) illustrates this particularly well.  The right side (as viewed) of the image is in focus, but the rest of it is out.  This is because the samples were oriented such that the plate-reflected beam came across the breadth of the grid stage rather than hitting it at normal.  An attempt was also made to use 1um gold sputter coated polystyrene balls as a standard.  It ended poorly.  (Figure 9)  Stigmation correction is also an issue.  Many times the stigmators would be maxed out and the stigmation still not corrected.

Type Grid Width (um) Increase in Working Distance (mm)
None 17.551 0.000
Large Ball 0.486 12.13
Small Ball 0.279 29.41
Plate 1.390 21.24
Hole 0.267 12.13
Figure 3: Table of measured grid widths and working distance increases associated with each mirror type.

Figure 4Figure 6
Figure 4: Grid with no mirror                                                            Figure 5: Grid via large ball

Plate
Figure 6: Grid via small ball                                                            Figure 7: Grid via plate

Figure 7Balls
Figure 8: Grid via hole                                                                    Figure 9: Polystyrene Balls (1 um)


Working Distance
Electron mirrors serve to massively increase the working distance of the electron beam.  Figure 1 shows a normal Sample/Beam interaction with the working distance highlighted.  Figure 2 displays the increase in that working distance when a mirror is inserted into the path of the beam.
NormalBeamMirrorBeam
Figure 10: Normal (non mirror) working distance     Figure 11: Mirror working distance

Increases in working distance improve depth of field but at the cost of resolution.  The loss in resolution comes from the increased effect of spherical aberrations as seen in Figure 12 below.  Example A has a spherical aberration causing the beam to have multiple focal points.  Example B does not have a spherical aberration and therefore has only one focal point.  This extended working distance is what makes imaging at higher resolutions nigh on impossible.
Focal pointFigure13
Figure 12: Effect of working distance on focal point                    Figure 13: Sample mirror geometry illustrating differences in working distance.

Interaction volume
Since, as previously mentioned the grid is placed on a 45° slant the interaction volume of the reflected beam (10kV for this experiment) is tilted as well.  In the actual experiment the beam and interaction volume would be rotated 90° clockwise. (Figure 14)  As the copper is very thin most of the interaction is actually in the carbon tape layer with a bit leaking into the stage.  There is no interaction volume on the mirror as the electrons repel the beam before it can reach the bulk.

Interaction Volume
Figure 14: Interaction volume of a 10kV beam with 600A of
copper and 5800A of carbon tape on an aluminum stub


Application - Mirrors for BSD troubleshooting
A practical use for electron mirrors within the confines of scanning electron microscopy is aiming a mirror at the backscatter detector.  This detector (at least in the Zeiss Auriga model) has four components (Figure 15) that when activated change color (Figure 16) when viewed through that same backscatter detector.  If the detector isn’t working one of these sections will fail to light up.  By imaging the detector this way a technician can tell which component is malfunctioning and replace it.  
Figure16Figure 17
Figures 16 and 17: The back scatter detector.  Figure 17 was taken by combining the signal from the SE2 detector with that of the back scatter detector.

Conclusions
While electron mirrors are stable enough to use for extended periods of time and have some practical application the distortions associated make imaging challenging.  This is especially true at high magnification where the combination of increased working distance and uncorrectable mirror astigmatism combine to destroy resolution.  More stable (perhaps powered, with stigmators) and finely machined mirrors could help alleviate these issues but for the time being electron mirrors remain a curiosity rather than a widely useful imaging tool.

Acknowledgments
Thanks first and foremost to Brian McIntyre for his guidance and patience with this project
To Greg Savich for allowing me to continue with his work and for his help getting me going
To Zack Lapin and Mike Theisen for their time and assistance as TAs in OPT 407


References
1) Rasband, W.S., ImageJ, U. S. National Institutes of Health, Bethesda,
Maryland, USA, http://imagej.nih.gov/ij/, 1997-2011.
2) Savich, Gregory, http://www.optics.rochester.edu/workgroups/cml/opt307/spr08/greg/ 2008
3) Smith College Microscopy, http://131.229.114.77/microscopy/sem.html
4) F. CROCOLO, C. RICCARDI, Passive Mirror Imaging through a Solid-State Back-Scattered Electron Detector, Microscopy Today, March 2008.
5) M Milani et. al. ˝Rear window˝ : looking at charged particles hitting a charged target in a FIB/SEM Microscopy: Science, Technology, Applications and Education 2010

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