Demonstration and Analysis of Electron Mirror Effects
in the Scanning Electron Microscope
OPT407: Practical Electron Microscopy
Institute of Optics
OPT407: Practical Electron Microscopy
Background and Introduction
|Experimental Procedure||Results and Discussion||
Conclusions and Acknowledgements
BACKGROUND AND INTRODUCTION
The mirroring effect occurs off of a surface insulated from ground when a charge builds up on the sample and imaging of the surface is attempted at an accelerating voltage lower than the potential stored on the sample surface . When a lower potential electron approaches the charged surface, the force of repulsion between the electron in flight and the higher potential electron on the mirror causes a change in the flight path of the electron sending it back into the SEM chamber. Consider an analogy of two balls of differing mass. Let one ball have a significantly larger mass than the other, and let this ball be at rest. If the ball of lighter mass is propelled at the heavier ball, the resulting elastic collision will launch the lighter ball off of and away from the heavy ball, while the heavy ball remains mostly unaffected by the incoming ball. The light ball is analogous to an electron of lower potential being launched towards the electron mirror surface, and the heavy ball represents the electron of higher potential on the surface of the mirror. If the lower potential electron had a potential near that of the mirror electron, the interaction of two particles would result in the displacement of both electron, and the mirroring effect caused by the reflection of the low potential electron would be destroyed. This demonstrates the importance of charging the mirror with a potential that is higher than the desired imaging potential. In the case of an electron mirror, however, there is no collision between the electrons, but rather as the lower potential electron approaches the mirror, the electron meets a virtual mirror surface that acts as a potential barrier preventing the electron from getting any closer to the mirror. As a result, the electron is repelled, or in this case reflected at this virtual mirror surface. Thus there is virtually no direct contact between the physical mirror surface and the electrons of the lower potential imaging beam, but rather an interaction involving the repulsive forces between electrons.
As it turns out, it is incredibly simple to repeatedly produce this mirror effect. One must simply use an appropriate and appropriately insulated mirror surface and charge the mirror at a high accelerating voltage. Once a mirror is created, it can be used as an imaging tool at a lower accelerating voltage.
2. Charging of Insulated Samples
The evacuated chamber of the SEM is an ideal location to build up a charge on a sample. If a sample is not grounded, it can accumulate a charge if it is bombarded with high energy electrons. In the case of the SEM, if a high accelerating voltage electron beam is incident upon a region of an ungrounded sample, the electrons will accumulate evenly on the sample since they have no other place to go. If the sample surface is conductive and properly attached to the SEM sample stage, the electrons incident on the sample from the electron beam will flow out of the sample and to ground; however, if this conductive surface is not attached to the sample stage in a way that guarantees a path to ground, a charge will build up on the surface. Furthermore, simply using a sufficiently thick insulating material, such as a piece of plastic, will guarantee that there is no path to ground. As the electron beam is allowed to raster over the surface of this insulator, a charge builds up that has no way of leaving the surface especially in the absence of atmosphere. This material simply needs to be thick enough to guarantee that the electron interaction volume does not reach a grounded surface and so that electrons cannot pass through the insulator or arc to the grounded sample stage.
3. Electron Mirror Basics
In order to create an electron mirror, all that is required is a sample insulated from ground . Charge this sample, and an electron mirror can be created. An un-coated piece of plastic serves as a good example, although a metal surface isolated from ground will provide satisfactory results. Virtually any shape insulator can be used to create a mirror; however, the best possible images are obtained using flat or spherical surfaces. These surfaces, especially when smooth, will produce the images with the least amount of distortion.
Particularly, spherical surfaces excel at creating good images at any point in the SEM chamber encompassed within the hemisphere above the mirror surface. Flat surfaces provide the best images only for features exactly parallel to the surface of the mirror, thus they are somewhat less versatile than spherical surfaces. Spherical mirrors can image well in any direction simply by changing where on the sphere the electron beam is rastered. If the beam rasters across the top of the sphere, the features directly above the mirror, like the final lens of the microscope, will be imaged; on the other hand, if the side of the sphere is rastered, features to the side of the mirror will be imaged. Further adding to the versatility of a spherical mirror, at high magnifications, the area of the sphere that is used during the raster is relatively small compared to the radius of the sphere, thus as long as the sphere is sufficiently large, the surface of a spherical mirror will approach a flat surface for high magnifications. This further decreases distortion effects and allows for analyses requiring high magnification to be performed while using an electron mirror.
1. Mirror Materials
Plastic samples are inexpensive and easy to find. As a result, they make an ideal insulating material to synthesize an electron mirror. Samples of Polyethylene Terephthalate (PET), High Density Polyethylene (HDPE), Polyvinyl Chloride (PVC), Low Density Polyethylene (LDPE), Polypropylene (PP), Polystyrene (PS), and Polytetraflouroethylene (PTFE) which is also known as Teflon™. Flat samples of all of these plastic materials were collected as well as Teflon™ and polystyrene spheres.
It is important to recall that an electron mirror does not have to be an insulator as long as it's well insulated from ground. To demonstrate this point, a surface of Gold-Palladium was sputtered onto the top hemisphere of a PTFE sphere and a small square of Gold-Palladium was sputtered onto a larger, flat sample of LDPE to demonstrate the mirroring effect off of a metallic surface.
2. Charging the Mirror
Charging the electron mirror is a relatively simple process and has been reported in the literature . Once the sample of interest has been placed in the SEM and the sample chamber evacuated, the electron beam can be engaged. Beginning with an accelerating voltage of 20kV and utilizing a secondary electron detector, the intended mirror surface is located. The magnification should be adjusted such that the entirety of the mirror surface is located in the frame of the image. At this point, astigmatism and focus do not need to be adjusted beyond the point where the mirror surface can be located. The electron beam raster is changed to a slow scan mode. A total scan time between 30 and 60 seconds per frame was found to be sufficient to guarantee ample charging when the frame was scanned approximately three or four times. After the mirror is scanned multiple times at the slow scan rate, the scan rate is decreased and the accelerating voltage is immediately dropped to 3.0kV. As the contrast adjusts to the new imaging conditions at the lower accelerating voltage, an image of the upper hemisphere of the SEM sample chamber appears. Only features above the mirror surface can be imaged. This image can be focused to show relevant features of the SEM chamber and surrounding components.
3. Detector Considerations
The choice of detector drastically effects the type of output experienced when using an electron mirror. The SEM considered here is configured with an Everhart-Thornley secondary electron detector, an in-lens secondary electron detector, and a back-scattered electron detector.
The back-scattered electron detector and the in-lens secondary electron detector are similar in that neither uses a biased faraday cage to attract electrons from the detector. As a result, for both of these detectors, only electrons directly incident on the detecting surface are collected. Unfortunately, these electrons only image the detector surface or features extremely close to the detector, so an electron mirror can only be used practically to determine whether or not the active area of the detector is functioning as it should. An image obtained from either of these detectors will show only the shape of the detector's active area as a white spot on a field of black.
The chamber secondary electron detector in the SEM is an Everhart-Thornley detector which utilizes a biased faraday cage to attract and collect electrons generated anywhere in the chamber. Under normal imaging conditions, secondary electrons are only generated on the region of the sample exposed to the electron beam. When an electron mirror is used, the electron beam is rastered over the entire hemisphere above the mirror surface, thus the chamber detector can collect electrons from anywhere in the sample chamber provided that the surface is rastered with the electron beam. This property makes the biased secondary electron detector the most useful detector for imaging with an electron mirror.
As a demonstration, imaging with all three detectors was performed. Furthermore, the signals obtained from the in-lens secondary electron detector and the back-scattered electron detector were mixed with the signal from the chamber detector to show both the active region of the detector as well as the chamber features surrounding the detector.
4. Advanced Techniques
The ability to image a complete two pi region of space within the sample chamber allows for a significant, novel technique for imaging in an electron microscope. Using an appropriate electron mirror, particularly a spherical PS or PTFE mirror, sample features outside of the direct path of the electron beam can be imaged, even at high magnifications. Using a mirror to simply redirect the path of the electron beam at an angle as high as 90 degrees, hidden sample features can be imaged relatively easily.
As a demonstration of the immense promise of this technique, a letter "G" was scribed onto the inside face of a copper cap used in plumbing. The cap was mounted on its side in an orientation such that the round face of the cap was perpendicular to the surface of the sample stage. A PTFE and PS sphere were placed next to the copper cap so that the spherical mirror surface could send the electron beam into the copper cap (fig. 1).
Figure 1: Mirror arrangement for imaging the inside of a copper cap. The inside face of the cap is hidden from the direct path of the electron beam even at high sample stage tilt angles.
When the inside of the copper cap was oriented in the SEM chamber so that the inside surface faced the chamber detector, the electron mirror could be used to image the features and the "G" inside the cap at surprisingly high magnification (see results and discussion).
Results and Discussion
1. Imaging the Chamber
Following the procedures outlined above, electron mirrors were created using various plastic samples as well as plastic samples sputter coated with metal. The best results were obtained from PVC, PS, Teflon™, and a Teflon™ sphere coated with gold-palladium on one hemisphere. An electron mirror image of the inside of the SEM sample chamber obtained using a polystyrene sphere can be found in figure 2.
Figure 2: Image of the interior of the SEM sample chamber taken with a polystyrene sphere. The image encompasses a full hemisphere of the chamber.
The micrograph of figure 2 shows many of the relevant components of the microscope including detectors, the final lens, illuminator LEDs, and the sample stage. This micrograph can be compared to a standard camera image of the chamber to clarify the locations of relevant features and show regions with the most distortion (fig. 3).
Figure 3: The micrograph on the left is a colorized version of figure 2. The image on the right is an image of the sample chamber with identical colorization. Matching colors indicate the same features in each image.
In figure 3, the regions with matching color represent the same feature in each image; the micrograph on the left is identical to figure 2, and the image on the right is a standard camera image of the chamber. The blue regions correspond to final lens of the SEM system. Notice particularly that end of the cone is much larger than it should be compared to other features surrounding it. This occurs because the final lens protrudes through the cavity of the chamber and approaches the sample. As a result, the end of the final lens is much closer to the mirror surface than the features on the wall of the chamber, thus the lens appears larger. Furthermore, notice that there is very little distortion in the shape of the end of the lens. This is expected because the lens is directly above the mirror. The green areas are the illuminating LEDs and the red region represents the housing of the back-scattered electron detector.The yellow regions represent the chamber detector of the microscope which is the detector used to collect the image. The purple regions are the tilt mechanism of the sample stage and the orange regions are the sample stage itself. Notice the high level of distortion in the sample stage in the electron micrograph. In general, the regions furthest from the center of the mirror image are the most highly distorted features. This distortion can be lessened by moving the feature to the center of the mirror, or identically, moving the mirror so that the center of the raster pattern impinges on the feature of interest.
Returning to figure 2, notice that the image is brightest around the region of the chamber detector and features within a straight path from the detector surface. The active region of the detector, the area within the Faraday cage, is the brightest of all. This occurs because we are using that detector to image the chamber. The resulting micrograph appears to have a light source at the detector that is illuminating the chamber. This phenomenon is general for any biased detector. A biased detector will act as an apparent source in an electron mirror micrograph since the electrons with a direct path back to the detector have the highest probability of reaching the detector . Regions blocked from the detector by some other feature will appear darker as though the blocking feature is casting a shadow.
It is important to consider distortion in the image more precisely. The shape of the mirror will affect the type of distortion in the image. These differences can be seen in images obtained from a flat surface and a spherical surface (fig. 4).
Figure 4: The micrograph on the left was obtained using a flat piece of PVC imaging a feature that was not perpendicular to the mirror surface. The micrograph on the right is a flat sample on a sample stub imaged with a PS sphere. These images suffer from different types of distortion.
The first micrograph of figure 4 was collected using a flat PVC electron mirror and is an image of the chamber detector. Again, notice the bright area indicating the active area of the detector. The second micrograph used a small polystyrene sphere as a mirror and images a sample stub with the flat PVC electron mirror on top. As a point of interest, the flat PVC sample is not charged thus is not acting as a mirror. If the PVC was charged, significant charging would appear on the surface and the image would be distorted around it. Notice that the distortion in the flat mirror image is mainly linear and in one direction. This is type of distortion associated with a flat mirror imaging a feature that is not perpendicular to its surface. If the detector were perpendicular to the mirror, it would appear circular as is expected. The angle at which the object is located relative to the mirror normal imposes a difference in the distance the sample rasters in the x direction when compared to the y raster. The x raster stretches over a longer region than the y raster is allowed to thereby stretching the image.
The micrograph obtained from the spherical surface has a different type of distortion. Flat features of an image are stretched into circular ones. Notice the base of the sample stage as well as the shape of the sample stub. Both should be flat across the top, but in the micrograph of figure 4, they appear to have a curvature. This is a result of the e-beam rastering over a curved mirror. The spherical surface of the mirror sends the electron beam non-uniformly across the sample stub resulting in a curved surface.
Utilizing proper mirroring techniques and a larger spherical surface help to minimize the effect of these distortion phenomena. Keeping the surface of interest as close to perpendicular to the mirror as possible will also help correct for distortion.
2. Detector Comparisons
To demonstrate the difference between using a biased detector and an unbiased detector, images from the chamber detector are compared to identical ones obtained using the back-scattered electron detector and the in-lens detector. The signals from the two detectors were mixed on the microscope to show both imaging effects concurrently.
The in-lens detector was considered with the chamber detector. The results of this analysis can be found in figure 5.
Figure 5:Micrographs of the end of the final lens and the in-lens detector taken with the chamber detector, the in-lens detector, and a mixed signal of both detectors respectively. It is believed that the in-lens detector is imaging the bore of the final lens because of the proximity of the these features to the in-lens detector.
The image obtained using only the in-lens detector consists of a bright annulus on a field of black as shown in the center image of figure 5. As expected, this detector cannot image the entirety of the final lens like the chamber detector does in the first image of figure 5. It is believed that the in-lens detector is imaging the bore of the final lens which is hidden from the chamber detector. The mixed signal image in figure 5 verifies that the in-lens is seeing features that cannot be seen by the chamber detector.
An identical analysis was performed with the back-scattered electron (BSE) detector. It is known for a fact that the image from this detector represents the active area of the detector (fig. 6).
Figure 6: Micrographs of the bottom of the back-scattered electron detector taken with the chamber detector, the back-scattered electron detector, and a mixed signal of both detectors respectively. The back-scattered electron image shows the curved scintillator of the detector.
Again, the image obtained from the unbiased BSE detector, the center image of figure 6, shows a bright region on a field of black. This bright region represents the curved scintillator of the BSE detector. Such an analysis can be used to verify whether or not a detector is operating correctly without having to remove the detector from the SEM . The combination of this BSE image with the chamber detector micrograph shown in the left image of figure 6 yields a unique perspective of the BSE detector showing its active region collecting electrons as shown in the third image of figure 6.
3. Imaging Hidden Features
As discussed in the experimental procedure, the ability to image features outside of the direct path of the electron beam is a significant new technique. Following the procedure outlined above, the "G" inside a copper cap was imaged using a Teflon™ sphere as the mirror surface (fig. 7).
Figure 7: Micrograph of the letter G inscribed on the inner face of a copper cap. Notice the edges of the micrograph show the edge of the cap. The G is hidden on the back side of the copper object.
The "G" is clearly discernable in figure 7, but appears flipped. This inverting of the image is simply due to the raster pattern of the SEM system. The direction of the raster correlated with the orientation of the mirror and the "G" result in the microscope scanning the cap from right to left, inverting the image from left to right. The cross of the "G" was considered at higher magnifications (fig. 8).
Figure 8: High magnification images of the cross of the G inside the copper cap. The scrape features caused by the knife used to write the G can clearly be seen. The image on the right indicates the electron mirror can be used at high accelerating voltages as well.
The images of figure 8 show that the electron mirror is a very stable reflecting surface with virtually no distortion at high magnification. At these magnifications, both the astigmatism and focus can be corrected on the microscope. Also, even though the mirror was charged at a 20kV accelerating voltage, imaging up to 15kV was achieved. The ability to use a mirror at such a high voltage greatly increases its usefulness as an analytical tool. A cautionary word must be made about the magnifications listed on the micrographs. The radius of the mirror affects the final magnification of the image and it is not yet well understood whether or not a correction factor needs to be applied at these high magnifications. A standardization process may be necessary to obtain an accurate value of the magnification. Nevertheless, such an analysis is a significant achievement and opens the door to a number of new applications and possibilities.
4. Potential Applications
There are a number of potential applications for the electron mirror. The ability to image features outside of the path of the beam allow for features within non-destroyable samples to be image provided that there is a cavity sufficient to insert an electron mirror. Furthermore, multiple sides of an object can be imaged without having to vent the chamber, move the sample, and then evacuate again. This simply requires an appropriate sample-mirror geometry where the sample is free to rotate independently of the mirror so the region of the object facing the mirror changes. This is a significant new way to save expensive microscope time.
Furthermore, it may prove possible to use electron mirror techniques to analyze the surface quality of a sphere. If the image obtained from a sphere can be compared to an image of a known standard, like a reference sphere, the distortion in the mirror image can be analyzed to determine the surface features of the surface in question.
The electron mirror creates more opportunities for the field of electron beam lithography. The unique properties of the electron mirror may allow complex rotational lithography patterns to be written inside the surface of a cylinder as an example simply by translating the mirror in an appropriate way. One can easily imagine how an electron mirror could be used to write patterns on five of six sides of a cube without having to remove the cube from the microscope.
It is likely that many more applications for the electron mirror will emerge as the technology is explored, but there is certainly immense promise for this technique in many fields related to electron microscopy.
The electron mirror is more than a simple curiosity. In the SEM system, an electron mirror can be used to image features at fairly high magnification that otherwise are not in the path of the electron beam. This significant advantage opens the door for a number of analytical applications including imaging inside features of an object without the need of breaking the sample and causing irreversible damage as well as imaging multiple sides of an object without having to vent and re-evacuate the sample chamber, thereby saving often critical and expensive microscope time. Furthermore, an electron mirror can be used as a simple analytical tool to determine whether or not the detectors in the microscope are functioning properly. These applications in conjunction with the relative ease of creating and using a good quality mirror make the electron mirror an important tool for imaging in the SEM. It is likely that many more applications for the electron mirror will be developed including, but not limited to, using the mirror to analyze the quality of a flat or spherical surface and writing complex e-beam lithography patterns within otherwise unreachable regions of a sample.
Sincerest thanks to Brian McIntyre for many hours in front of the SEM; for being a useful source of ideas, criticism, and good humor; and for being an integral part of my problem solving thought process as I attempted to decipher the details in this project. Thanks to Andreas Liapis for being the theorist my experimentalist mind sometimes requires.
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