Arrays of MEMS Langmuir probes that are flush-mountable (Figure 1) can serve as a sensorial skin on a spacecraft for fine spatial and temporal resolution of plasma phenomena. The technology can also provide diagnostics for other applications such as tokamaks and nanosatellite scientific payloads ^[1] . The benefits are innumerable for deeper understanding of plasma physics, which is in great need of these microprobes ^[2] . For instance, multiplexed microprobes that are flush-mounted on all the faces of a 3-D “tip” can allow for simultaneous capture of a detailed “whole picture” of plasma behavior in different axes at a given timescale. In addition, two or more different sensory configurations, e.g., single-, double-, triple-probe methods, etc., can be adapted into the same flat die, profiting at the same time from their individual data acquisition strengths. Protruded probes cannot offer these advantages. Another area of deployment is in the observation of electron phase-space holes, self-consistent nonlinear plasma structures that are formed from strong current- or beam-driven turbulence and found in magnetic reconnection regions, which are magnetic field topology modifiers responsible for the explosive release of magnetic energy in magnetospheric storms, solar flares, and laboratory plasmas ^[3] . Fast micro-Langmuir probes that work at high frequencies are indispensable for studying these plasma fluctuations. We developed a system of flush-mounted MEMS Langmuir probes and apparatus with fast timescale; i.e., shorter time compared to the timescale of reconnection events in the Versatile Toroidal Facility at MIT (Figure 2); and wide bandwidth extending across regions of magnetosphere-photosphere, i.e., considering both electron and ion plasma frequencies associated with these regions.

1. E. S. Field, “Batch-fabricated planar arrays of MEMS Langmuir probes for spacecraft reentry plasma diagnostics and nanosatellite scientific payloads,” Master’s thesis, Massachusetts Institute of Technology, Cambridge, 2011.
2. P. Pribyl, W. Gekelman, M. Nakamoto, E. Lawrence, F. Chiang, J. Stillman, J. Judy, N. Katz, P. Kintner, and P. Niknejadi, “Debye size microprobes for electric field measurements in laboratory plasmas,” Review of Scientific Instruments, vol. 77, no. 073504, pp. 1-8, July 2006.
3. W. Fox, M. Porkolab, J. Egedal, N. Katz, and A. Le, “Observations of electron phase-space holes driven during magnetic reconnection in a laboratory plasma,” Physics of Plasmas, vol. 19, no. 032118, pp. 1-12, Mar. 2012.

Development of miniature vacuum pumps that can be integrated with electronic or MEMS components is necessary for producing advanced equipment such as portable analytical instruments ^[1] and high performance sensors ^[2] . The proposed approach graphically illustrated in Figure 1 is based on electron impact ionization (EEI) or field ionization (FI) of the gas molecules using nano-scale sharp silicon tips. The ionized gas molecules are then evacuated from the chamber using a strong electric field to accelerate the ions and implant them permanently into a getter medium made of Ti or Al. In the EEI mode of the operation, a positive voltage is applied between the gate and the emitter to extract electrons that are used to ionize the background gas. In the FI regime, the Si sharp tips are biased at a positive voltage with respect to the gate to extract electrons from the outer shell of the gas molecules in a quantum tunneling process. The former process occurs at electric fields in the range of   3 – 6 ×10⁷ V/cm while the later process initiates at electric fields above 10⁸ V/cm ^[3] . Despite the larger required voltage, the operation in the FI regime is mandatory since the back-streaming of the positive ions during EEI mode of operation will damage the field emitter (FE) tips at mTorr-pressure range. Although state-of-the-art field emitters have been reported ^{[4] [5] [6]} , the focus of this work is to improve the reliability of the FE or FI devices for extended operation times and large currents necessary for pumping application. Since these devices demand application of large voltages between the gate and the tip of the FE/FI, wear or breakdown of the insulating dielectric is a major issue. Finite element modeling (shown in Figure 2) has been conducted to optimize the design of the device for pumping application. A new fabrication process is also being developed for high-yield fabrication of an array with more than 300K Si FEs/FIs.

1. Z. Ouyang and R. G. Cooks, “Miniature mass spectrometers,” Annual Review of Analytical Chemistry, vol. 2, pp. 187-214, Feb. 2009.
2. M. Esashi, “Wafer level packaging of MEMS,” Journal of Micromechanics and Microengineering, vol. 18, no. 7, pp. 073001:1-13, May 2008.
3. R. Gomer, Field Emissions and Field Ionization, New York: Springer-Verlag, Dec. 1992.
4. M. Ding, G. Sha, A. I. Akinwande, “Silicon field emission arrays with atomically sharp tips: turn-on voltage and the effect of tip radius distribution,” IEEE Transaction on Electron Devices, vol. 49, no. 12, pp. 2333-2342, Dec. 2002.
5. L. F. Velásquez-García, S. A. Guerrera, Y. Niu, and A. I. Akinwande, “Uniform high-current cathodes using massive arrays of Si field emitters individually controlled by vertical Si ungated FETs – part 1: Device design and simulation,” IEEE Transactions on Electron Devices, vol. 58, no. 6, pp. 1775-1782, June 2011.
6. L. F. Velásquez-García, S. A. Guerrera, Y. Niu, and A. I. Akinwande, “Uniform high-current cathodes using massive arrays of Si field emitters individually controlled by vertical Si ungated FETs – part 2: Device fabrication and characterization,” IEEE Transactions on Electron Devices, vol. 58, no. 6, pp. 1783-1791, June 2011.

The need to measure particle energies arises in many applications, from calibrating electron sources for electron guns in precision microscopes to determining the efficiency of space-based ion beam thrusters.  Retarding potential analyzers (RPAs) are capable of filtering particles based on their energy and have been used as early as the late 1950s and early 1960s for such purposes ^[1] .  However, these devices maintain limited application due to stringent dimensional constraints driven by plasma Debye length.  Cold dense plasmas require minute apertures and tight spacing tolerances between biasing grids that are difficult to enforce using conventional means.  We suggest microelectromechanical system (MEMS) batch-fabrication techniques in order to achieve unprecedented alignment accuracy of successive electrodes while incorporating the necessary micron-scale features.  Assembly to a precision of a few tens of microns has been demonstrated with a hybrid RPA (see Figure 1a) ^[2] .  Figure 1b shows the fully MEMS-fabricated sensor inspired by in-plane assembly of high-voltage devices, which will have tolerances on the order of 1μm ^[3] .

Augmenting the optical transparency of RPAs provides a more direct path for particles to the collector plate.  Signal strength is thus improved as the effective collection area is increased.  Preliminary results and comparisons between MEMS-fabricated electrodes and conventional stainless steel mesh have revealed an ameliorated signal quality.  Figure 2 shows a greater than two-fold improvement in peak signal strength with the micromachined grids over the conventional RPA ^[2] .  Currents captured by the various grids and simulations suggest the possibility of ion beam focusing and interception of ions prior to reaching the collector.  Alteration of the internal dynamics of the sensor provides a cleaner signal that may lead to a better interpretation of the measurements than with models that incorporated the stochastic behavior of charged species through randomly oriented electrode apertures.

1. W. C. Knudsen, “Evaluation and demonstration of the use of retarding potential analyzers for measuring several ionospheric quantities,” Journal of Geophysical Research, vol. 71, no. 19, pp. 4669–4678, Oct. 1966.
2. E. V. Heubel, A. I. Akinwande, and L. F. Velásquez-García, “MEMS-enabled retarding potential analyzers for hypersonic in-flight plasma diagnostics,” in Proceedings of the 15th Solid-State Sensors, Actuators, and Microsystems Workshop, Hilton Head Is., SC, June 2012, pp. 324–237.
3. B. Gassend, L. F. Velásquez-García, and A. I. Akinwande, “Precision in-plane hand assembly of bulk-microfabricated components for high-voltage MEMS arrays applications,” Journal of Microelectromechanical Systems, vol. 18, no. 2, pp. 332–346, Apr. 2009.