1 Analysis of Specimen Orientation Methodology for Energy Dispersive X-Ray Spectroscopy Writer: Alexander Huey Advisor: Dr. Chunfei Li Abstract: When using a scanning electron microscope (SEM) equipped with a spectrometer, it is difficult to know if the specimen being observed is in the optimal position to get the best readings from the equipment. Previous data collected by my advisor, the Principal Investigator (PI), and Clarion University (CU) has determined a method of specimen orientation through rotation and tilt of the specimen table to determine its optimal positioning. This was done by constructing a macroscale copy of the equipment found inside a SEM and using it to measure the ratio of the specimen’s projected lengths versus the rotation of the specimen table. To confirm the methodology and interpretations of data developed in previous experiments, new data has been collected to test against previous trials. The newest trial has found evidence in support of previous findings, with similar data being produced. This is an indication that the developed method of orienting a specimen inside a SEM may fill a gap in research dedicated to the use of lab equipment. Suggestions for further research and apparatus redesign will be offered along with the conclusions. 2 1. Introduction: What is the function of a scanning electron microscope? A SEM allows for the topological analysis of a specimen down to the order of a few micrometers, that is, 1/1000000 of a meter. In the figure to the right, a breakdown of a SEM can be seen along with an example of the type of image that can be achieved. Inside of an SEM, it is possible to equip a spectrometer. This allows for the spectral analysis of an object in tandem with the topological analysis. While the topological analysis provides a clear image of a specimen, the spectral analysis, known as Electron Dispersive Spectroscopy (EDS), provides a clear elemental composition of the specimen. Both of these functions are commonplace in labs across the globe, but there can be issues with their effectiveness that depend upon the orientation of the analyzed specimen. The goal of this research is to analyze the method of specimen orientation, proposed by PI, that may be completed on site and will ensure the effectiveness of the equipment. 2. Importance: This area of research is relatively untouched, however its benefits can be appreciated by every scientist that uses this equipment on a regular basis. One major benefit of EDS is that it is non-destructive. This means that a very small amount (< 1/1000000000 grams) of material is required and the material will not be altered or harmed after its use inside the SEM. In years preceding spectral analysis, to determine the composition of a material, it would have been burned or submerged in a chemical bath that would give the same results as EDS, but leave the specimen chemically altered or destroyed altogether. This can be beneficial, for example, in the case of jewelry. EDS can be used to check the authenticity of jewelry without rendering it useless in the process. The non-destructive3 nature of EDS makes it indispensable to agencies like NASA that often deal with materials from outer space that are not easily obtainable but must be studied to understand their place in the cosmos. 3. Predetermined Method PI and CU have determined a method of specimen orientation that they tested theoretically and experimentally before this trial. The main idea behind getting an effective reading from a SEM/EDS is that the surface of the specimen being analyzed must be perpendicular to the electron beam emitted from the SEM. The top image shows an example of how a “specimen” (in our case a 10 cm wedge) may be oriented in such a way to achieve this goal. The electron beam, denoted by the red arrow, can be made perpendicular to the surface of the specimen by using the rotation and tilt functions available in the SEM. The steps taken can be seen in parts a, band c. From a to b, the table is rotated so that the interception line of the Specimen is parallel to the tilting Axis. From here, the table may be rotated to bring the specimen’s surface perpendicular to the electron beam, as can be seen from b to c. However, not all specimens will be as uniform as a wedge and a set of steps must be designed to guarantee that the surface of the specimen can be brought perpendicular to the beam. To do this, a method of specimen orientation was created by PI to determine a “special rotation angle.” This SRA is where the specimen is in position to be tilted so that it is in the optimal spot for EDS (part b of the top figure). The method to determine the SRA relies on the projected lengths of two points on the specimen, and how these projected lengths change based on tilting the specimen. The bottom left figure shows how the distances between the same two points can change based on tilting the specimen. Part b of the figure shows the distance between two points on the specimen without any tilting inside the SEM. Parts a and c show how the projected lengths can either be lengthened or 4 shortened depending on which direction the specimen is tilted. The figure on the bottom right helps to demonstrate why these lengths change. In the image, the true length (A to B) is not what is seen from the top-down view inside the SEM. Instead, the projected length (A’ to B’) is seen. As the specimen is tilted, the true length does not change, however the projected length does. This is a key point of the research, as the method of specimen orientation must be completed with only the functions found inside the SEM. This means that only tilt and rotation can be used to orient the specimen, and it must be completed only with information derived from the top down view, as would be the case when using a SEM. The two new figures on the right show the theoretical results based on simulations ran by PI (top) and experimental results used by gathering actual data in a SEM (bottom). The first graph is a comparison of ratios of the projected lengths versus the stage rotation angle. The importance of this graph is that the local minimum values (found at 0, 180 and 360 degrees) are where our SRA lies. So, if we have our wedge set at 0 degrees or 180 degrees rotation, its interception line will be parallel to the tilting axis and therefore can be oriented so that it is in the optimal position for the imaging functions of the SEM or EDS analysis. The bottom graph detailing the experimental results tell a similar story, but they do not necessarily guarantee the results that are hoped for by the method of orientation. To examine the method closer, an apparatus was designed on the macroscale to imitate the functions found inside the SEM. Using the macroscale apparatus, it is possible to mimic these functions and take data that could help confirm the orientation methodology. 5 4. The Experiment The apparatus used in the experiment is pictured above. It consists of a rotary table (equipped with rotation and tilt functions similar to those found in the SEM), our “specimen” placed on top of the rotary table, and two wires pinned to the specimen that hang down and allow a measurement of the projected distances between two points on the specimen. The idea of the experiment is to take a measurement of the projected length between the two points at 0 degrees rotation and 0 degrees tilt, then measure the projected length again at 0 degrees rotation and 10 degrees tilt. From there, the apparatus is lowered back to 0 degrees tilt and moved to 20 degrees rotation. From there, the process is repeated again until projected lengths at 0 and 10 degrees tilt are measured at 0 degrees rotation, 20 degrees rotation, 40 degrees rotation and so on until the specimen has been rotated a full 360 degrees back to its original position. At this point, the measurements at each rotation angle and tilting position can be graphed to show similar results to the experimental and theoretical results found by PI. 6 5. Results The top graph to the right shows the data points of the experiment. The x-axis is the rotation angle of the rotary table, and the y-axis is the ratio of the projected lengths of the points on the specimen [(Tilt at 0 degrees - Tilt at 10) / Tilt at 0 degrees]. This is very similar to the data found by PI in the initial experiment done inside the SEM. The bottom graph shows the same data, but applied in a way that can resemble results produced from the theoretical interpretation of the experiment. Both graphs determine the same thing, that a “special rotation angle” exists at 0 degrees, 180 degrees, and 360 degrees. Looking back at the first figure in part 3, it makes sense that this will be the case. Zero degrees rotation and 180 degrees rotation place our specimen at a point where it can be tilted to be perpendicular to the electron beam. 360 degrees rotation will obviously work as well, because at that point the specimen has been brought back to what is considered its starting point, zero degrees rotation, and it has been established that this is one of the special rotation angles. This confirms that the method of specimen orientation did in fact determine the special rotation angle. Two things should be noted. First, these results were achieved only by using the information available as it would be when using a real SEM. This is important because the goal of the experiment is to successfully orient a non-uniform specimen inside a SEM using only the functions and views available inside the equipment. Second, these findings do not represent every specimen, only ours in particular. However, this method should be applicable to all specimens, even those not as uniform as the wedge used in this experiment. 6. Conclusions and Recommendations The methodology determined by PI and CU provide clear and comprehensible results that allow a specimen to be placed in its optimal positioning for SEM imaging and EDS analysis. To further research, another experiment should be conducted in which digital imaging techniques could be applied to a more sophisticated apparatus. This will further the idea of “mimicking” the functions found inside the SEM and give more exact measurements regarding how the projected lengths change based on rotation and tilt. 7