MEMS Indium FEEP Thruster: Manufacturing Study and First Prototype Results

MEMS Indium FEEP Thruster: Manufacturing Study and First Prototype Results M. Tajmar * ARC Seibersdorf research, A-2444 Seibersdorf, Austria Miniaturization of Field-Emission-Electric-Propulsion (FEEP) thrusters have been proposed already more than a decade ago. MEMS manufacturing techniques of FEEP arrays promise a reduction in weight and the possibility to scale the required thrust level by the size of the thrusters chip. Unfortunately, miniaturization of the indium FEEP thruster developed at ARC Seibersdorf research (ARCS) is not as straightforward as it is for e.g. for colloid thrusters. Indium does not wet silicon; therefore traditional silicon etching alone is not suitable to produce emitter electrode structures. In this paper we give an overview of a manufacturing study performed at ARCS showing prototypes produced using laser, photochemical and micro-edm manufacturing using metal that is compliant with indium propellant. Also a first silicon based emitter prototype is shown. We also report the assembly of a complete MEMS indium FEEP thruster module and first prototype testing results. The ARCS prototype has an array of 21x21 thrusters on an area of only 25 mm 2. I. Introduction IELD emission thrusters such as the ARCS In-FEEP thruster are an enabling technology for future science F missions in space that allow ultraprecise satellite attitude control and positioning 1. The thruster core constists of a Liquid-Metal-Ion-Source (LMIS) with indium as propellant. If a sufficient electric potential is applied between the LMIS and an extractor electrode, ions are directly pulled out of the liquid metal and are then accelerated out of the thruster by the same field (see Fig. 1). In general, FEEP thrusters offer unmatched characteristics in the area of electric propulsion, such as an electrical efficiency > 95% and high propellant densities (liquid indium metal, see Fig. 2) compared to other technology on the market (e.g. best efficiencies for other technologies are about 60%). However, a single emitter can only produce thrusts in the µn range. Another disadvantage is the rather high power-to-thrust ratio of about 70 W/mN, due to the high specific impulse of the thruster. This narrows the application to fundamental science missions. The characteristics of such a single thruster element are listed in Table 1. Figure 1. In-FEEP Thruster Principle with Needle-Type Emitter. Parameter Values Thrust 0.1 10 µn (Peaks up to 35 µn) Thrust Resolution < 0.01 µn Thrust Noise 0.01 µn/hz 0.5 * Minimum Impulse Bit < 5 nns Total Impulse > 600 Ns ** Specific Impulse > 5,000 s Power-to-Thrust Ratio 70 W/mN Singly Charged Fraction 98% Electrical Efficiency 95% *** * Depending on power supply used, lower values possible ** Using the present reservoir size of 30 g, larger sizes are possible *** Comparing the current to the emitter with the current in the ion beam (minus extractor and plume shield current losses) Table 1. In-FEEP Single Element Performance Parameters. * Principal Scientist, Space Propulsion, Email: martin.tajmar@arcs.ac.at, Member AIAA. 1

As the µn thrust from a single element is too low even for fundamental science missions such as LISA Pathfinder or µscope, which require maximum thrusts between 100-150 µn, clustering of thruster elements is necessary. Miniaturization and clustering using MEMS technology promise a strong reduction in volume and weight compared to conventional technology. MEMS clustering was already applied to colloid thrusters, a low thrust technology with similar electrode structures 2-5. Moreover, if MEMS clustering could be realized on a large scale, thrusts up to the mn range might be possible. That could transform the unique properties of FEEP thrusters into a larger market. A patent for such a concept was filed by ARCS 6. Density [g/cm 3 ] 8 7 6 5 4 3 2 1 0 Lithium (e.g. MPD Thrusters) Supercritical Xe (e.g. Ion or Hall Thrusters) Indium In-FEEP Thruster Figure 2. Propellant Density Comparison. This paper gives an overview of the so-called µfeep MEMS concept and manufacturing study to investigate the miniaturization of the ARCS In-FEEP thruster. Moreover, first preliminary test results of the µfeep thruster are presented. II. MEMS Concept Throughout its development, several emitter types have been considered for FEEP thrusters 7 : needles, capillaries, slits, rings, etc. For integration with the propellant reservoir, a capillary-type emitter seems to be the most straightforward choice. For instance, the needle is usually mounted on the bottom of the propellant reservoir. Depending on the propellant tank size, the needle can be several centimetres long. This is not easily compatible with a MEMS approach. On the Figure 3. µfeep Assembly. other side, the capillary is only mounted on the top of the propellant tank the size of the tank itself does not matter in this case. Therefore, a capillary-type MEMS emitter concept was developed (see Fig. 3 and 4). Figure 4. MEMS Electrode Structure. Figure 5. SRI Microvolcano Structure (Courtesy of SRI International) 8,9. 2

A similar type of structure was developed by SRI as a field ionization source for hydrocarbon fuels 8,9 (see Fig. 5). Mitterauer 10 first suggested using this ionization source for FEEP thruster applications. The structure was made out of silicon with a micron sized chromium layer deposited on the bottom part. Also Centrospazio did a study on miniaturizing slit-type FEEP thrusters using microfabrication techniques 11 and actually produced one prototype (slit only without propellant wetting). Their prototype was purely made out of silicon. Unfortunately, indium does not wet silicon. Therefore, the microstructure has to be manufactured directly out of metal, which has strong limitations in the achievable aspect ratios, or to make the structure out of silicon and use metal deposition with an indium compatible material (no dissolution) and a layer that is thick enough to not dissolve in indium after long term operation. The right aspect ratios of the microcapillaries are very important to focus the electric field and enable a good build-up of the Taylor-cone, and to tailor the electrical characteristics to high impedances in order to achieve a homogenous clustering throughout all emitters 12. III. Manufacturing Study In this section, several manufacturing techniques are reviewed and results on the prototyping of emitter microstructures are presented that could be used for a µfeep thruster. A. Laser Micromanufacturing First, an emitter prototype trial was done using an excimer laser (532 nm wavelength) in a stainless steel sample at EXITECH. Fig. 6 shows the sample, the main problems are the artefacts between the capillaries and the capillary surfaces. Also it was not clear if the holes were really drilled through. According to the manufacturer, material re-deposition was one of the main difficulties. They suggested to try shorter laser wavelengths with higher pulse energy and to use a fluid assist system for removing the deposited material. The next trial was done using a femto-second laser at the Laser Zentrum Hannover (LZH). Both stainless steel and tantalum plates were used for the microstructuring. Fig. 7 shows a complete microstructure with 21x21 emitters on a 5x5 mm area under an SEM microscope. The results in stainless steel were much better compared to tantalum. In general, the hole structure in stainless Figure 6. Emitter Prototype using EXITECH s Excimer Laser. steel was quite good whereas holes tantalum were unsymmetric. Vertical and horizontal grooves were used to form a sort of capillary structures, however, the grooves were not wide enough. This was a limitation from the positioning table used by LZH. Material deposition on the top of the hole structure is evident. No purge gas was used during manufacturing. By checking also the bottom side of the plates, we could confirm that all holes were actually drilled through. In order to try different laser systems, also a NeYAG laser was used for micromanufacturing of a stainless steel plate. Fig. 8 shows the emitter produced by the company FOBA. The structure in this case had a much better shape, but the accuracy was not good. A lot of material deposition can be seen. A hole through one capillary was tried - but was not found under the microscope. Following the lessons learned, a prototype was manufactured using an excimer laser with a purge gas and a µm accurate positioning table from the company FEIGL. A special program was made to manufacture round shaped capillaries both in stainless steel and tantalum. The results are shown in Fig. 9 with an SEM picture and a 3D profiler analysis. Capillaries with a height of 90 µm and holes with a diameter of 10 µm could be reproducibly achieved. 3

Figure 7. Emitter Prototype using LZH Femtosecond Laser. Figure 8. Emitter Prototype using FOBA NeYAG Laser. Figure 9. Emitter Prototype using FEIGL Excimer Laser (left SEM, right 3D Profiler Picture). 4

Figure 10. Emitter Prototype using Koese Micro-EDM Machining. Figure 11. Emitter Prototype using Photochemical Etching. 5

B. Micro EDM Manufacturing Figure 12. SANDIA Emitter Prototype using Chemical Etching in Silicon. Micro EDM was used to form single pins with a height of 100 µm and later on to drill 50 µm holes into the stainless steel plates. The structures manufactured by Koese Engineering were very clear, however, the machine had not a sufficiently accurate wire positioning system; therefore the holes were not always drilled in the middle of the pin (see Fig. 10). The results were very promising, however, it is a slow and expensive production method as each pin has to be manufactured separately in a slow process. After improving the wire positioning technique, the microstructured emitter would represent the optimum structure made out of bulk metal using micromanufacturing techniques. C. Photochemical Etching In order to combine the scalability from chemical etching, the time to manufacture one capillary or thousands is in principle the same, and the advantage of using bulk metal as the basic structure, photochemical etching of stainless steel and tantalum plates was tried. We chose Etchform as our partner for these trials as they have extensive etching experience also with tantalum and work on space applications for various ESA programmes. The main difficulty in this approach was to produce a film on the metal sheet than withstood the etching time as we were trying to get the best possible aspect ration of the capillaries. Several prototypes were manufactured. It was also difficult to place the film accurately on the metal sheet with respect to the outer dimensions. It turned out that etching of tantalum in these small dimensions was impossible, therefore only stainless steel were pursued. Several types were produced each with a different etching time. Typ I etched a depth of 74 µm, Typ II etched a depth of 87 µm and Typ III etched down to 150 µm. However, also the structure vanished at Typ III. Hence, only realistic depths of about 80 µm are possible. Fig. 11 shows such a prototype for a pin structure, the achieved accuracy was about 5 µm. For this prototype, laser or EDM would be necessary as a second step to manufacture the holes. Several trials were done to also etch the holes through the capillaries. However, as the aspect ratio for photochemical etching is only about 1:1, this was impossible. D. Silicon Etching In cooperation with SANDIA, silicon capillary microstructures were manufactured using Deep-Reactive-Ion- Etching. Fig. 12 shows an example of the emitter. Obviously, the achieved accuracy and aspect ratio is far superior to the metal based trials above. Unfortunately, due to the fact that indium does not wet silicon, such emitters can not be used as is. SANDIA and ARCS are working together to improve the silicon-based emitter concept with coatings and optimized geometries. First results are expected to be available soon. 6

IV. Assembly and Test of µfeep Thruster Using the results from the laser manufacturing, two complete µfeep assemblies were manufactured. Fig. 13 shows a stainless steel emitter manufactured by LZH, a glass spacer and a doped silicon extractor electrode which was gold sputtered. In order to improve the positioning of the electrodes, also rectangular µfeep prototypes were manufactured by FEIGL. A module housing with heaters, thermal and electrical insulation was manufactured and assembled with the FEIGL µfeep emitter (see Fig. 14). The prototype was mounted in a vacuum chamber and heated up until indium was liquid. Then a positive voltage was applied on the emitter structure while the extractor was grounded. Opposite of the µfeep thruster was a collector electrode to collect the emitted ions. Fig. 15 shows the first prototype results. At a threshold voltage of about 300 V, a small current appeared on the collector proving that ions are actually emitted. The current-voltage characteristic was quite steep up to 2 kv. The voltage suddenly dropped and the impedance decreases from 974 MΩ to 61 MΩ - this is exactly a factor of 16. Therefore it seems that 16 times more capillaries were firing at that time. Obviously the achieved currents of 4-5 µa are very small which are largely due to the not optimized geometry. Moreover, it was only possible to adjust the emitter and extractor electrode under an optical microscope which was not sufficient to resolve the center of the capillaries under the extractor holes. However, up to the knowledge of the author, these results are the first reported actual test data on a micromanufactured FEEP thruster proofing that such a concept can actually work. Figure 13. Complete µfeep Prototype using LZH Femtosecond Laser (From left to right: Emitter, Glass Spacer Electrode, Doped Silicon Extractor Electrode with Gold Deposition). Figure 14. Complete µfeep Prototype using FEIGL Excimer Laser (left) and Complete Module Configuration (right). 7

Figure 15. First Firing Data of FEIGL Excimer Laser µfeep Thruster. V. Conclusion A design and manufacturing study was carried out to micro-manufacture FEEP thrusters out of bulk metal and silicon. In the metal based concept, capillaries with a height of about 100 µm and holes down to 10 µm could be realised. The manufacturing accuracy of micro EDM seems to be the most promising approach; however, this manufacturing technique is expensive and slow for large arrays. Silicon etching achieved the best accuracies and aspect ratios but needs further manufacturing steps such as a metal layer deposition which is difficult for the capillary holes. For the first time, a complete laser manufactured µfeep thruster was assembled and tested. It consisted of an array of 21x21 thrusters on an area of 5x5 mm. The thruster could be fired; however, only low currents up to 5 µa were recorded on the collector due to manufacturing and positioning shortcomings. This manufacturing study has to be considered as only a first step into micromanufactured FEEP thrusters. Optimizing of the geometry to enable a homogenous cluster operation as well as improvements in the accuracies (for metal based) and metal coating (for silicon based) have to be done for further improvement. According to the unique properties and demonstrated proof-of-principle, such a work program seems to be promising. Acknowledgments The silicon based µfeep emitter concept was pursued together with SANDIA. The contributions from R. Lenard and S. Kravitz were greatly appreciated. This work was partly funded by the ASAP program from the Austrian Space Agency. 8

References 1 Tajmar, M., Genovese, A., and Steiger, W., "Indium FEEP Microthruster Experimental Characterization", AIAA Journal of Propulsion and Power, Vol. 20, No. 2, 2004, pp. 211-218 2 M. Paine and S. Gabriel, "A Micro-Fabricated Colloidal Thruster Array", AIAA Paper 2001-3329, 2001 3 L. Velasquez Garcia and M. Martinez-Sanchez, "A Microfabricated Colloid Thruster Array", AIAA Paper 2002-3810, 2002 4 J. Stark, M. Paine, B. Kent, B. Stevens and M. Sandford, M. Alexander, "Colloid Propulsion - A Re-evaluation with an Integrated Design", AIAA Paper 2003-4851, 2003 5 J. Stark, B. Stevens, M. Alexander, "Fabrication and Operation of Micro-fabricated colloid Thruster Arrays", AIAA Paper 2003-4852, 2003 6 Tajmar, M., and Semerad, E., "Skalierbare Indium Flüssigmetall Ionenquelle", Austrian Patent Application, A 1932/2002, 2002 (PCT/AT03/00370) 7 Tajmar, M., Genovese, A., Buldrini, N., and Steiger, W., "Miniaturized Indium-FEEP Multiemitter Design and Performance", AIAA Paper 2002-5718, 2002 8 Spindt, C.A., "Microfabricated Field-Emission and Field-Ionization Sources", Surface Science, 266, 1992, pp. 145-154 9 Spindt, C.A., "Microelectronic Field Ionizer and Method of Fabricating the Same", US Patent 4926056, 1990 10 Mitterauer, J., "Miniaturized Liquid Metal Ion Sources (MILMIS)", Surface Science, 246, 1991, pp. 107-112 11 Marcuccio, S., Lorenzi, G., and Andrenucci, A., "Development of a Miniaturized Field Emission Propulsion System", AIAA Paper 98-3919, 1998 12 Tajmar, M., "Indium Capillary Liquid-Metal-Ion-Source Operation in the Flow Resistance Regime", Journal of Physics D: Applied Physics, submitted (2004) 9