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Wafer Scale Integration Enabling Space Science Danielle M. Wesolek, M. Ann Garrison Darrin, Robert Osiander

The Johns Hopkins University Applied Physics Laboratory 11100 Johns Hopkins Road

Laurel, MD 20723 240-228-4952

ann.darrin@jhuapl.edu

AbstractWafer scale integration enables the ability to miniaturize space craft instrument builds. We use the term wafer scale to indicate a packaging concept where the wafer is the substrate eliminating the use of individually packaged microstructures. The key here is the combination of advanced integration and miniaturization. The Johns Hopkins University Applied Physics Laboratory (JHUAPL) in collaboration with the US Air Force Academy is building a wafer integrated plasma spectrometer (WISPER) for mapping missions. The fabrication of the WISPER instrument suite uses a number of Micro Electro Mechanical Systems (MEMS) and micro electronics fabrication technologies. JHUAPL has successfully demonstrated these approaches in the fabrication of the single instrument currently in orbit on the FalconSat-3 mission. This instrument is a Flat Plasma Spectrometer (FlaPS) which includes a sensor-head array, printed circuit board with amplifier array electronics, power supply, and chassis which occupies a volume of approximately 400 cm3 in a 0.5kg, 700mW package. The sensor head array is fabricated and assembled at the wafer-level and stacked in a planar geometry. Together with common electronics to control the array, our design takes advantage of emerging micro-fabrication techniques including deep reactive ion etching, laser machining, and wire electrical discharge machining, as well as advanced electronic die assembly and packaging methods. Innovations in packaging combined with etch process steps from the micro-fabrication industry allow for novel instrument builds which will open up the realm of nano sats and cubesats in mapping missions. This paper reviews the combination of manufacturing methods, material combinations and packaging techniques that can be applied to miniaturizing space instrumentation. This work is representative of the next generation of space instruments that will be enablers for smaller and more cost effective missions.12

TABLE OF CONTENTS

1. INTRODUCTION......................................................1 2. MICRO-MACHINED PLASMA SPECTROMETERS ...2 3. FLAPS.....................................................................3 3. WISPER................................................................3 4. CONCLUSIONS .......................................................5 ACKNOWLEDGEMENTS .............................................5 REFERENCES .............................................................5 BIOGRAPHY ...............................................................6 1 1 1-4244-1488-1/08/$25.00 2008 IEEE 2 IEEEAC paper #1337, Version 1, Updated Jan. 15, 2008

1. INTRODUCTION Small satellite missions can be achieved by taking full advantage of ongoing technology development efforts leading to miniaturization of engineering components, development of micro-technologies for sensors and instruments. The development of fabrication and assembly techniques from the world of microelectronics create application specific integrated micro-instruments using microelectronics for data processing, signal conditioning, power conditioning, and communications. These micro- and nano-technologies have led to the concepts of nano- and pico-satellites, constructed by stacking wafer-scale integrated instruments together with solar cells and antennas on the exterior surface.

Further milestones in the cost-effective Earth observation mission developments are the availability and improvement of small launchers, the development of small ground station networks connected with rapid and cost-effective data distribution methods, and cost-effective management and quality assurance procedures.

Space sensor webs are one outcrop of this technology. [1]

More generally cost-effective missions are supported by four contemporary trends: Advances in electronic miniaturization and associated performance capability; The recent appearance on the market of new small launchers (e.g. through the use of modified military missiles to launch small satellites); The possibility of independence in space (small satellites can provide an affordable way for many countries to achieve Earth Observation and/or defense capability, without relying on inputs from the major space-faring nations); Ongoing reduction in mission complexity as well as in those costs associated with management; with meeting safety regulations, etc. [2] The FlaPS and the WISPER which are described in this paper are indicative of this significant movement to miniaturized highly integrated instruments. Three examples are selected below and are obviously not an all inclusive list but indicative of these trends in miniaturization and integration. A number of NASA instrument suites, ranging from those currently operating in

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space to those that are in early study phases demonstrate this trend to greater integration and miniaturization. [3] Examples of this trend in instrument build are seen in the University of Calgarys work on the Miniature Enhanced Geometry Electrostatic Analyzer (MEGEA). The MEGEA concept is a spiral 270 Electrostatic Analyzer (EA). Adopting the spiral shape results in an instrument that is a factor of 2.7 smaller by volume than a conventional cylindrical EA with a comparable input cross section and accompanied by a 10% improvement in the energy resolution. The reduction in size is a first step to miniaturizing such instruments for in-situ measurements of space plasma properties. [4] Another excellent example is the miniature integrated camera and spectrometer (MICAS) aboard deep space 1 developed by the Jet Propulsion Laboratory. MICAS is an integrated multi-channel instrument that includes an ultraviolet imaging spectrometer (80-185 nm), two high-resolution visible imagers (10-20 rad/pixel, 400-900 nm), and a short-wavelength infrared imaging spectrometer (1250-2600 nm). These technologies provided a novel systems approach enabling the miniaturization and integration of four instruments into one entity, spanning a wavelength range from the UV to IR, and from ambient to cryogenic temperatures with optical performance at a fraction of a wavelength. The specific technologies incorporated were: a built-in fly-by sequence; lightweight and ultra-stable and monolithic silicon-carbide construction. The MICAS provided a highly integrated four instrument package with a mass less than 10 kg, instrument power less than 10 W, and total instrument cost of less than ten million dollars.[5] The final example cited is the Jet Propulsion Laboratory Electron Luminescence X-ray Spectrometer (ELXS). The ELXS is a compact (less than or equal 1 kg) electron-beam based microinstrument that can determine the chemical composition of samples in air via electron-excited x-ray fluorescence and cathodoluminescence. The enabling technology is a 200-nm-thick, MEMS-fabricated silicon nitride membrane that encapsulates the evacuated electron column while yet being thin enough to allow electron transmission into the ambient atmosphere. [6]

2.MICRO-MACHINED PLASMA SPECTROMETERS Previous measurements of the plasma environment in the ionosphere reveal such a high degree of complexity in the plasmas in space that single-point measurements are not sufficient to adequately describe this phenomena. Therefore, there is a critical need for instruments suitable for missions that can make simultaneous multi-point measurements of ion and electron velocity distribution. Deploying constellations of ever-smaller spacecraft, including nanosats and picosats, for characterizing the ionosphere and magnetosphere will require new miniaturized plasma spectrometers with a high ion throughput to instrument volume ratio. Since the cost of

putting a normal size satellite into orbit is so high, the future of space science depends on our ability to make increasingly smaller spacecraft carrying scientific instruments capable of measurements of at least the same quality as conventional spectrometers. The suite of plasma spectrometers introduced in this paper use a new approach to energy analysis, providing an imaging spectrometer of very small mass with the aperture/sensitivity performance of spectrometers weighing 10 to 100 times as much. The basic approach to the spectrometer is to select ion rays with small divergence in a collimator, and then deflect the particles by a uniform electric field between parallel plate electrodes defined in the electrostatic energy analyzer. The arrangement of the electrodes, shown in Figure 1, is such that particle trajectories of a given energy are deflected by the potential difference across the electrodes according to the speed and direction with which they enter the analyzer region. Energy separation is possible without focusing by controlling the angular divergence of the particles entering the analyzer. [7]

Figure 1: Schematic of a single cell for the plasma spectrometer The advantage of this approach is that it scales very well with size. For a given applied voltage Vapplied the energy Kp for peak ion/electron transmission (charge q) is

appliedfp VqPK = (1) where fP is the plate factor given by:

( )2)41( dLfPf = . (2)

Here, L is the ESA depth, d is the electrode gap, and f is the fraction of d that is open for ion passage. A typical plate factor is in the order of 10 for dimensions of L about 2 mm and d about 0.5 mm, which allows for peak energies in the range of 100 eV for an applied Voltage of 10 V only. The dimensions of the entrance aperture and the electrode distance determine the energy resolution as well as the fill

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factor. Each application requires a different design of the plasma spectrometer, which can be accomplished within a single wafer, maintaining the dimension L, and allowing different spectrometer cells (pixels) with different plate factors on the same detector. In addition, modern micromachining techniques such as electric discharge machining allow slanted collimator and analyzer electrodes on the same wafer, which offers a range of different directions in one detector. Applications of this spectrometer design are the Flat Plasma Spectrometer (FLAPS) and the Wafer integrated Plasma Spectrometer (WISPER).

3.FLAPS The Flat Plasma Spectrometer (FLAPS) experiment was designed to investigate low latitude plasma depletions using an approach that provides energy-angle analysis of the ions, necessary for measuring non-Maxwellian spectra on-orbit. It is postulated that non-Maxwellian spectra may be responsible for generation of small-scale plasma depletions in the ionosphere. This drives a requirement for an ion spectrometer capable of providing fine energy resolution (E/E < 5%). The uppermost top layer of the spectrometer is a particle collimator that selects particles within an angular cone defined by the geometry of the microfabricated tunnels. The field of view is approximately 1.5, and each sensor head (pixel) is designed to have a unique look direction in the range of 8 from the surface normal. The layer below the collimator is the energy selector. Within this layer, an electric field steers ions through a series of parallel plate electrodes, and those within a specific energy passband would exit the energy selector and traverse the next stage of the sensor head. This next stage is comprised of a layer of chevron stacked Microchannel Plates (MCPs), which provide 106 charge multiplication. Finally, the current is collected by set of anodes connected to the Pre-Amplifier Discriminators (PADs), which link the signals to amplifier array electronics for signal processing. [8]. The Flat Plasma Spectrometer (FlaPS), designed using advanced micro-fabrication techniques, was developed as an instrument demonstration capable of making fine resolution measurements of the kinetic energy spectra and angular distributions of ions in a space environment in the range of 0eV to 15eV with a 8 FOV and geometric factor of 5x10-5 cm2-sr per pixel. A FlaPS instrument, including sensor-head array, printed circuit board with amplifier array electronics, power supply, and chassis has been designed and built to occupy a volume of approximately 400 cm3 in a 0.5kg, 750mW package. The sensor head, which consists of an array of five identical spectrometer sensor heads (pixels), each with a different fixed field-of-view is fabricated and assembled at the wafer-level and stacked in a planar geometry.

Figure 2: The FlaPS instrument payload prior to its integration to the FalconSAT-3 micro satellite.

The FlaPS instrument, shown in figure 2, was integrated onboard the US Air Force Academy FalconSAT-3 micro satellite, and launched from an Atlas V on March 8, 2007. During the mission, FlaPS will provide data that will enable validation of the ionosphere plasma bubble and radio wave scintillation nowcasting and forecasting system associated with Communication/Navigation Outage Forecasting System (C/NOFS), as well as to contribute to the characterization of the effects of non-Maxwellian charged particle distributions on the formation, propagation, and decay of plasma bubbles. Satellite operations are being managed from the USAFA ground station and cadet crew. To date, FlaPS remains in a commissioning orbit where initiation and check-out procedures are being conducted. Experimentation will begin during normal operations mode within the coming months.

3. WISPER The Wafer Integrated Spectrometer (WISPER) instrument is currently being developed with the ability to detect the presence of both ion and cold-gas thrusters for the identification of proximity spacecraft activity. This ability will be demonstrated during the FalconSAT-5 mission, manifest for launch in the fall of 2009, where WISPER will assist in providing RF and plasma measurement correlation by characterizing a perturbed plasma environment created by an on-board ion source. Given the different application and the wider energy ranges as compared to the mission for the FlaPS instrument, the WISPER instrument concept, shown in Figure 3, is similar to FlaPS, but designed with 7 pixels, each with a different fixed FOV, look angle, active area, and energy range defined by the plate factor.

Wafer Scale Integration

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All pixels for the WISPER instrument are fabricated into the same wafers, CuBe for the analyzer, Si for the collimator, and SOI for the entrance and exit aperture, which all will be assembled at the wafer-scale level, utilizing MEMS batch fabrication and assembly technologies. This can be expanded to any number of pixels in the case of WISPER the size is limited by the dimensions of the micro-channel plates and will reduce fabrication and assembly cost. In addition, it can be adapted to any mission by designing plate factors, FOV, and look angle for each pixel. For WISPER, the electrostatic analyzers in each module (pixel) have geometrically designed plate factors as well as an extended scanning plate voltage range that enable detection of energetic ions in the range of 0eV to 2000eV. The total volume occupied by the WISPER instrument including sensor head array, control electronics and high voltage power supply is 10cm3, and packaged to weigh a total of 0.5kg while consuming 0.9W of power. The electronics in the WISPER instrument use the micro-channel plates in a gain mode, which, at higher particle throughput, could easily be replaced by higher gain amplifiers in the front-end electronics. This mode of operation is demonstrated by a similar instrument, the Miniaturized ElectroStatic Analyzer (MESA), also manifest for launch on Falconsat-5. Operation without the micro-channel plates eliminates the need for a high-voltage power supply, thus reducing the form factor and power requirements of the instrument dramatically.

Figure 3: WISPER instrument concept showing multiple pixel arrays assembled and packaged at the wafer-scale.3 Due to scale, aperture holes in Si and SOI 4 3 The anode, amplifier electronics, high voltage, and scanning power supplies are not shown.

wafers are not shown.

Fabrication of a Wafer Integrated Spectrometer The conceptual design of the WISPER electrostatic analyzer plate is shown in Figure 4. In this view, particles enter the electrode region from above the plate, and pass through the 2.5mm plate depth, where, similar to FLAPS, ion deflection occurs with the path trajectory defined as a function of the applied electrode plate voltage. The view does not show the entrance and exit apertures which, together with the applied voltage, determine the detected energy. Each electrode is defined by a cut in the BeCu layer using wire electrode discharge machining (EDM). In this process, a single wire pass is made to etch the inter-digitated structure of the electrodes. The separation between positive and ground electrodes is made after the entire wafer stack (shown in Figure 3) is assembled, and bonded to preserve the structural integrity of the sensor array during assembly. Upon separation of the ESA layer, electrical isolation is established. The key to enabling a broad range of ion energy detection lies within the variation in electrode distance (d in Fig. 1). Shown in the top view of the ESA, Figure 4, the spacing of the inter-digitated electrodes varies among pixels. By changing the electrode distance, and therefore the plate factor for each pixel, each of the seven pixels measure a different energy range of ion energies, from 0 to 2000eV. Different look angles can be accomplished by angled EDM micromachining of the ESA electrodes, and aligning the entrance and exit apertures in the collimator to achieve the desired FOV. The slanted side-wall electrodes are shown in the four corner pixel locations in Figure 4. Three of the seven pixels are designed to capture at total filed-of-view of 5 in azimuth and elevation. The remaining four pixels are micro-machined at angles of 5 to capture a FOV of 2.5 to 7.5 in azimuth and elevation, resulting in a total FOV of 15 for these four pixels.

Collimator (Stacked Si Wafers)

ESA Entrance/Exit Apertures (SOI Wafers)

ESA electrode plate

MCP & assembly fixture

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Figure 4. Top view of the Electrostatic Analyzer (ESA) design picturing 7 sensor pixels. Electrical isolation is established when the protruding tabs are cut after wafer-scale assembly.

In addition to the design of the electrostatic analyzer, the design and alignment of the entrance and exit apertures for both the ESA and the collimator are of equal importance since, together with the ESA, they define the energy as well as the energy resolution and the total throughput. An array of square apertures (10 m2 x 400 m in depth), fabricated using advanced MEMS fabrication techniques, is stacked at the wafer-scale level above and below the ESA electrode plate. The aperture plates are fabricated using Deep Reactive Ion Etching (DRIE) of silicon-on-insulator wafers. This processing technique permits micro-scale features to be designed with photolithographic precision on each side of the wafer while providing the necessary buried oxide layer for electrical isolation. The collimator is also fabricated with this technique, and resembles the aperture layer in design, with the exception of the entrance hole sizes, and center-alignment locations. It is assembled at the wafer scale (i.e. one layer/wafer containing all pixel collimators) at the upper-most portion of the sensor-stack, and serves as the particle direction filter, i.e. only particles within 15 in azimuth and elevation are allowed to pass. Figure 5 shows a SEM of the collimator.

Figure 5: Silicon wafer collimator fabricated using DRIE processing.

4. CONCLUSIONS Key drivers to miniaturization of microelectronics are the reduced cost and increased redundancy due to multiple identical instruments. These drivers combine with the current significant trends to integrate more and more components and subsystems into fewer and fewer chips, enabling increased functionality in ever- smaller packages. Micro Electro Mechanical Systems (MEMS) and other sensors and actuator technologies allow for the possibility of miniaturizing and integrating entire systems and platforms. This combination of reduced size, weight, and cost per unit with increased functionality has significant implications for Air Force missions, from global reach to

situational awareness. Examples may include the rapid low-cost global deployment of sensors, launch-on-demand tactical satellites, distributed sensor networks, and affordable unmanned aerial vehicles (UAVs). Collective arrays of satellites that function in a synchronized fashion promise significant new opportunities in capabilities and robustness of satellite systems. For example, the weight and size reduction in inertial measurement units (IMUs) composed of MEMS accelerometers and rate gyros, Global Positioning System (GPS) receivers for navigation and attitude determination, and MEMS-based microthruster systems are enablers for small spacecraft, probes, space robotics, nano satellites, and small planetary landers. The benefits include decreased parts count per spacecraft, increased functionality per unit spacecraft mass, and the ability to mass produce micro, nano, and pico satellites for launch-on-demand tactical applications (e.g., inspector spacecraft) and distributed space systems. Micro launch vehicles enabled by micromachined subsystems and components such as MEMS liquid rocket engines, valves, gyros, and accelerometers could deliver one or two kilograms to low Earth orbit. Thus, it will be possible to place a payload (albeit a small one) as well as fully functional micro satellites into orbit for $10,000 to $50,000 rather than the $10 million to $50 million required today. [9] MEMS devices for space science applications will and are being developed and ultimately flown in optimized MEMS-based scientific instruments and spacecraft systems on current and future space missions.

ACKNOWLEDGEMENTS

The authors would like to acknowledge their collaborators from NASA GSFC, Dr. Fred Herrero and the USAFA Dr. Linda Krause and Dr. Geoff McHarg on FlaPS and on WISPERS, Dr. Kenny Clark from NRL and Lt.Col. Michael Dearborn, Dr. Richard Balthazor, Dr. Geoff McHarg, and Maj. Anne Clark from the USAFA. Also, the contributions of Mr. Bliss Carkhuff, Mr. Howard Feldmesser and Dr. Larry Paxton at JHUAPL are greatly appreciated.

REFERENCES [1] Sandau, Rainer; Paxton, Larry; Esper, Jaime (2007): Trends and Visions for Small Satellite Missions. In: Sandau, Rainer; Rser, Hans-Peter; Valenzuela, Arnoldo [Hrsg.]: Small Satellites for Earth Observation - Digest of the 6th International Symposium of the International Academy of Astronautics, Berlin, April 23-26, 2007, Wissenschaft und Technik Verlag Berlin, S. 19 - 22, 6th Symposium on Small Satellites for Earth Observation, Berlin, 2007-04-23 - 2007-04-26, ISBN 3-89685-571-9. [2] International Study Cost Effective Earth Observation Missions IAA Commission IV Study Group Commission IV: System Operation & Utilisation, International Academy

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of AstronauticsOctober, 2005. [3] Trends in instrument systems for deep space exploration Dorsky, Leonard I. (Jet Propulsion Laboratory, California Institute of Technology) Source: IEEE Aerospace and Electronic Systems Magazine, v 16, n 12, December, 2001, p 3-12. [4] Towards the miniaturization of a space-borne electrostatic energy analyzer: The Miniature Enhanced Geometry Electrostatic Analyzer (MEGEA), Amerl, Peter V. (Institute for Space Research, Dept. Physics and Astronomy, University of Calgary); Yau, Andrew W. Source: Proceedings - 2005 International Conference on MEMS, NANO and Smart Systems, ICMENS 2005, Proceedings - 2005 International Conference on MEMS, NANO and Smart Systems, ICMENS 2005, 2005, p 139-142. [5] Advanced technologies demonstrated by the miniature integrated camera and spectrometer (MICAS) aboard deep space 1Rodgers, David H. (Jet Propulsion Laboratory, California Institute of Technology); Beauchamp, Patricia M.; Soderblom, Laurence A.; Brown, Robert H.; Chen, Gun-Shing; Lee, Meemong; Sandel, Bill R.; Thomas, David A.; Benoit, Robert T.; Yelle, Roger V. Source: Space Science Reviews, v 129, n 4, April, 2007, p 309-326. [6] MEMS-based micro instruments for in-situ planetary exploration George, T. (Jet Propulsion Laboratory); Urgiles, E.; Toda, R.; Wilcox, J.Z.; Douglas, S.; Lee, C.-S.; Son, K.; Miller, D.; Myung, N.; Madsen, L.; Leskowitz, G.; El-Gammal, R.; Weitekamp, D. Source: Proceedings of SPIE - The International Society for Optical Engineering, v 5836, Smart Sensors, Actuators, and MEMS II, 2005, p 188-199. [7]Herrero, F. A and D.J. Chornay, Miniature Imaging Plasma Spectrometer: A New Approach with Large Geometric Factor and Wide Field of View, 2002 IEEE Aerospace Conference, March 2002, Big Sky, Montana.

[8} Microsatellite missions to conduct midlatitude studies of equatorial ionospheric plasma bubbles L. Habash Krause, C.L. Enloe, R.K. Haaland and P. Golando US Air Force Academy, Department of Physics, 2354 Fairchild Drive, USAF Academy, CO 80840, USA Received 19 October 2002; revised 27 February 2004; accepted 1 March 2004. Available online 21 November 2005.

[9] Implications of Emerging Micro- and Nanotechnologies Committee on Implications of Emerging Micro- and Nanotechnologies; Air Force Science and Technology Board Division on Engineering and Physical Sciences, 2002.

BIOGRAPHY Danielle M. Wesolek Danielle M. Wesolek received the B.S. degree in physics

from Allegheny College, Meadville, PA in 1999, and is currently enrolled at The Johns Hopkins University where she will receive an M.S. in Material Science and Engineering. She joined the staff at The Johns Hopkins University Applied Physics Laboratory in Laurel, MD in 2000 as a system analyst evaluating SLBM Trident weapon system

performance. In 2001 she joined the Research and Technology Development Center of the JHUAPL, and is currently a MEMS research and development engineer. Her current research includes MEMS sensor development for RF, space, and thermal control applications with emphasis on multiple aspects of micro-system development, including design, fabrication, and characterization. M. Ann Garrison Darrin Ms. Darrin is the Group Supervisor for the Research &

Technology Development Center Aerospace and Materials Sciences Group at the Johns Hopkins University Applied Physics Laboratory. She has over 20 years experience in both government (NASA, DoD) and private industry in particular with technology development, application, transfer and insertion into space flight missions. She holds an M.S. in Technology

Management and has authored several papers on technology insertion along with co-authoring several patents. Ms. Darrin was the Division Chief at NASA Goddard Space Flight Center for Electronic Parts, Packaging and Material Sciences from 1993-1998. She has extensive background in aerospace engineering management, microelectronics and semiconductors, packaging, and advanced miniaturization; is a recognized expert in 3-D packaging, advanced interconnect, and engineering management; and has been instrumental in several applied technology breakthroughs. Robert Osiander Dr. Osiander received his Ph. D. in Physics from the

Technical University in Munich, Germany, in 1991. He is presently Assistant Group Supervisor of the Sensor Science Group in the Research & Technology Development Center at the Johns Hopkins University Applied Physics Laboratory. His current research interests include Micro-Electro-Mechanical Systems (MEMS), Nanotechnology, and Terahertz

Imaging and Spectroscopy.. He was the PI on MEMS Shutters for Spacecraft Thermal Control, one of NASAs

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New Millennium Space Technology Missions launched in 2007., and is presently the PI on WISPERS, a MEMS plasma spectrometer for FalconSAT, and an optically transduced MEMS Gyroscope. Dr. Osiander was the co-recipient of the 2002 APL Invention of the Year Award. His bio includes 8 US Patents and more than 80 publications, including Editor of MEMS and Micro-structures in Aerospace Applications.