PARAMETERIZING AN OXFORD - The University of Texas at Arlington

Siemens Research Report

PARAMETERIZING AN OXFORD Nd:YAG 355 nm DIODE-PUMPED UV LASER TO FABRICATE MICROCHANNELS IN THE SURFACE OF GLASSY CARBON 2013

Table of Contents Abstract / Executive Summary ............................................................................................................... ii Introduction .......................................................................................................................................... 1 Materials and Methods .......................................................................................................................... 2 Illustrations and Discussion.................................................................................................................... 4 Results ................................................................................................................................................... 8 Conclusions & Future Work .................................................................................................................. 9 References ........................................................................................................................................... 10

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Abstract Glassy carbon (GC) is a material known for its resistance to traditional ablation techniques, which makes fabrications of micro channels difficult. Previous experiments were conducted in which glassy carbon was directly ablated by a combination of XeCl and ArF excimer lasers and a diode-pumped Nd:YAG laser [1]. In this experiment, an Oxford 355 nm Nd:YAG diode-pumped UV laser was parameterized to ablate glassy carbon (GS20, Tokai Carbon, Japan) by testing various focal points on sheets of aluminum. The aluminum was ablated in repeating successions with each pass elevating the focal point of the laser by 0.1 mm until the ablations were as smooth as possible. Microchannels were successfully fabricated on the surface of the glassy carbon.

Executive Summary Glassy carbon is a very strong material that resists most cutting methods, which causes difficulty when attempting to mold microchannels on its surface. A previous experiment was conducted that produced successful microchannel molds by using a combination of laser cutting techniques. In this experiment, sheets of aluminum were used to adjust the focal point of an Oxford Nd:YAG laser to the height needed to properly mold the surface of glassy carbon. The laser made several passes on the surface of the aluminum, with each pass increasing in height by 0.1 mm. The setting of the proper focal point allowed the proper laser parameters to be placed, and the surface of glassy carbon was successfully micromolded.

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Introduction Fabricating microchannels on the surface of a metal is a particularly difficult process for a laser. A laser’s processing, power, pulse rate, focal point, and much more all play important roles in adjusting the geometry of the microchannels. It is not as simple as requesting a rectangle and pressing a button. Lasers only have the ability to ablate V-shapes into a metal, which makes the ablations of specific shapes rather difficult. This means that measurements and parameters must be very precise to cut properly. Every cutting job requires different parameters, which means that every cutting job requires different measurements in order to ensure precision cutting. This issue is very clear when it comes to certain materials, such as glassy carbon [2]. The Oxford 355 nm Nd:YAG diode-pumped UV laser has a very small beam wavelength. The advantage of having a small wavelength is that the laser will be able to produce a smaller beam focus point (not to be confused with focal point), which means the laser is capable of smaller and more precise cuts. As previously stated, the laser cuts in a V-shape, the technique of which runs at high speeds and temperatures in order to create the ball of plasma it uses to cut through the given material. The laser runs using desired programs made by the user to do the specific job desired. The laser uses planar X, Y, and Z coordinates which pinpoint the width, length and depth of the desired cutting pattern. These coordinates are used in conjunction with basic programming to create a cutting job for the laser. The width of the typical microchannel can range from 100- 500 nm, depending on the job requirements. A large channel is much easier to cut than a small channel because a large channel takes less effect from the Vshaped cutting motion.

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Materials and Methods The first step in starting this research project was to research and learn about past laser ablation research experiments that involved ablating glassy carbon with different types of lasers. This was done to research and discover the laser parameters used in successful ablation from other experiments. Once the sample of glassy carbon arrived to the lab, the next step was to examine the material to see its structure and surface texture using an EDX S- 3000 N scanning electron microscope. This microscope fires electrons at the material being examined at different angles, which produces an image on the monitor. This electron microscope can fire electrons at the object from up to three directions, depending on how the user would like to view it. After closely examining the material, the next step was to use the gathered research to create the best program. A previous experiment had success ablating glassy carbon, using an Oxford 355 nm Nd:YAG diodepumped UV laser alongside other lasers [1]. The parameters from that experiment were used as a starting point to set the best parameters for ablation of the surface of a sample of aluminum. Each time the laser made a pass on the aluminum, the Z-coordinate increased by one micron. After each ablation run was completed, the sample was taken under a high-power microscope to examine the ablations and determine which two microchannels were the smallest and neatest. The sample was taken back to the laser, which made more ablations on it. The second time, the Z-coordinate increased with each pass by one tenth of a micron. The same process of taking the sample to the microscope and determining which two microchannels were the best was used once again. This process was repeated a total of four times, with each repetition decreasing the Z-coordinate change by another factor of ten; the third time the Z-coordinate was changed by one hundredth of a micron, and the fourth time it was changed by one thousandth of a micron. The focal point determined to be the best was at a Zcoordinate of 82.301 microns. The focal point remained programmed into the laser for the glassy carbon ablation. The sample of GC was placed on the laser tray and prepared for ablation testing. Microchannels were 2

successfully ablated into the surface of the glassy carbon, but they were jagged, due to its structure. Further research needs to be conducted to determine how to create smooth microchannels in the surface of glassy carbon for the purpose of inserting microchips.

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Illustrations and Discussion The following figures show the aluminum used when trying to find the focal point. When compared to glassy carbon, aluminum is a very soft metal that is easily ablated. The program that was used to find the focal point was used by incrementing Z-coordinates and changing power. Four experiments were conducted with this program at four different laser power percentages. Trial 1 was run with the laser at 100 %, as shown in Figure 1. This was too much power for the fragile aluminum sample. After the ablation was finished, the aluminum was taken to a microscope, which has a camera hooked up to a computer that allows pictures to be taken at high magnification. Trial 2 was run with a much lower power at 25%, which was still too powerful for the aluminum. In Trial 3 the power was lowered to 15%, which was less destructive than the first two, but it was still too much power for the aluminum sample. Trial 4 was run at 5% power. This trial was the best out of all four because it showed a better picture of how wide the cuts were made without destroying the aluminum surface completely. The computer was then used to make measurements of the length of each gap. An average of 4 measurements was taken for each line to ensure there was exact data. All information was then input into Excel. Excel was used to find the average of the width of every line. The data collected on Excel marked exactly what the best focal point was. As previously stated, most lasers cut in a V-shape. The 355 nm Nd:YAG laser is no exception. Figure 4 is an illustration on how exactly the laser cuts.

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Figure 1: First Laser Run Aluminum Ablation After the laser made its first passes with increasing focal point height for each pass, the aluminum sample was taken under a microscope. In this image are the microchannels created by the laser. The two lines 8th and 9th from the left side were determined to be the smallest and neatest, so the respective Z-coordinates for those microchannels were entered into the laser programming. The Z-coordinate increment was then changed to one tenth of a micron and the laser went on another run.

Figure 2: Fourth Laser Run Aluminum Ablation

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In this image are the microchannels created by the laser with focal point measurements down to the thousandth of a micron. The first two lines were determined to be the neatest and smallest, but they were considered too similar to differentiate, so a program on the microscope was used to measure the width of the microchannels. This is shown in the following figure.

Figure 3: Estimated Measurements of Microchannels on Aluminum Sample Surface The microscope has its own program that can be used to manually estimate width in any microscopic image projection. This program was used to measure the microchannels. The measurements were compared to one another and the best microchannel was selected. The focal point for that microchannel was used to successfully ablate the glassy carbon and mold microchannels in its surface.

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Figure 4: “Single V-shape laser channel and multiple, displaced V-shaped channels to form an approximate rectangular profile” [2] This diagram shows the shape of the “V” channels created by the lasers and how they are combined to approximate a specific shape. In this case, the shape is a rectangle.

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Results This experiment was conducted by researching, finding necessary materials, testing equipment, and testing laser parameters. The research present did not provide sufficient information to complete the experiment. Much time was spent using the trial and error method to discover the necessary parameters for the laser. This experiment provides many previously unknown parameters to a specific laser to allow for easier and simpler ablation of glassy carbon.

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Conclusions & Future work The experiment leaves opportunity for many future experiments and research in which the fabrication of micro channels on the surface of glassy carbon may be even further simplified. This experiment helps to further understand ablation techniques of glassy carbon and other materials, which will allow micro engineering to grow and increase success. Further research must be conducted to determine how to create smooth microchannels on the surface of glassy carbon in order to insert microchips into them.

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References [1] Kuhnke, M., Lippert, T., Ortelli, E., Scherer, G. G., & Wokaun, A. (2004). Microstructuring of glassy carbon: comparison of laser machining and reactive ion etching. Thin Solid Films, 453-454, 36-41. doi: http://dx.doi.org/10.1016/j.tsf.2003.11.156 [2] Process Report- University of Texas Arlington, E-355. (2008). Unpublished raw data, Industrial Engineering, University of Texas at Arlington, Arlington, Texas.

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