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Microplasma: Physics and Applications

Microplasma: Physics and Applications. Jeffrey A. Hopwood Northeastern University Boston, MA 02115 Presented to the Plasma Science Committee of the National Academies, September 27, 2003. Outline. Motivation: Applications Plasma Display Panels Micro Chemical Analysis Systems

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Microplasma: Physics and Applications

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  1. Microplasma: Physics and Applications Jeffrey A. Hopwood Northeastern University Boston, MA 02115 Presented to the Plasma Science Committee of the National Academies, September 27, 2003 Draft Version (9/24/2003)

  2. Outline • Motivation: Applications • Plasma Display Panels • Micro Chemical Analysis Systems • Micro Propulsion • Microplasma Devices and Physics • DC, RF, microwave • Challenges and Opportunities Draft Version (9/24/2003)

  3. Plasma Display Panels Draft Version (9/24/2003)

  4. blue red green Plasma Display Panels (PDPs) Structure From S.S. Yang, et al, IEEE Trans. Plasma Sci. 31, 596 (2003). Draft Version (9/24/2003)

  5. initiate breakdown (~ 300 volts) sustain plasma (~ 180 volts) + + + + + + Plasma Display Panels (PDPs) Basic Operation Sustain Electrode + + + + Bus Electrode h ~ 200 m l ~ 400 m d ~ 60 m From S.S. Yang, et al, IEEE Trans. Plasma Sci. 31, 596 (2003). Draft Version (9/24/2003)

  6. Plasma Physics of PDPs • Ne + Xe (1-10%) • Ne is a buffer gas (Eiz-Ne>> Eiz-Xe) • but neon decreases diffusion losses of Xe+ • UV production from • Xe (1s4) • Xe (1s5) • Xe2* …optically thin, desired state • produced by three-body collision: Xe* + Xe* + M  Xe2* + M • Power  ions (large sheath voltage) • excitation is only ~15% of total system power } …optically thick, inefficient: Xe*+ e  Xe+ Draft Version (9/24/2003)

  7. State-of-the-art DiagnosticsK. Tachibana, et al ., IEEE Trans. Plasma Sci. 31, 68 (2003)3-D temporally-resolved emission and diode laser absorption Draft Version (9/24/2003)

  8. State-of-the-art DiagnosticsK. Tachibana, et al ., IEEE Trans. Plasma Sci. 31, 68 (2003)3-D temporally-resolved emission and diode laser absorption Address electrode Sustain electrode side view front view near-IR emission from Xe(2p) Draft Version (9/24/2003)

  9. Modelingexample from S.S. Yang, et al, IEEE Trans. Plasma Sci. 31, 596 (2003). Draft Version (9/24/2003)

  10. Issues in PDP • Efficiency • currently <1 lumen/watt • goal: 5 lumens/watt • incandescent lamp ~ 25 lm/w; fluorescent~100 lm/w) • gases, pressure, electrodes, geometry • something more creative? control of the eedf (Rauf-Kushner)? • Phosphor degradation/MgO degradation • due to energetic Xe+ bombardment • RF sustain voltages (LG Electronics) • electrons are trapped in cell; improves eff. (~2 lm/w) • but…complex microstructure, electronics, EMI • Manufacturing is dominated by Asia Draft Version (9/24/2003)

  11. Micro Chemical Analysis Draft Version (9/24/2003)

  12. J. Eijkel, Stoeri, Manz, Anal. Chem 71, 2600 (1999) Micro Chemical Analysis • Emission Spectrometry • possibly coupled with another separation technique (e.g., GC) • Issues • pumping • stability/repeatability • lifetime/contamination • power/heat Draft Version (9/24/2003)

  13. Micro Chemical Analysis - ion mobility spectrometry - from R.A. Miller, et al, Sensors and Act. Workshop, Hilton Head, 2000) Microplasma’s potential role • micro-ionizers • optical emission detection • high ionization or excitation efficiency  improved signal-to-noise (presently, DL~100ppb) Draft Version (9/24/2003)

  14. Micro Chemical Analysis- Issues - • Very limited success in micropump development • must operate at atmospheric pressure • No practical method for storage of inert gases • must operate with air or other ambient • Long term stability of physical and chemical proc. • no erosion of the microstructure • contamination/fouling • Low power (< 1W), to be portable • Low temperature (only ambient cooling) Draft Version (9/24/2003)

  15. Funding • DARPA BAA 03-40 “Micro Gas Analyzers” • “DARPA seeks innovative proposals in the area of microelectromechanical systems (MEMS) implementations of MGA, with the ultimate objective being the realization of tiny separation analyzer-based chemical warfare agent (CWA) sensors capable of orders of magnitude reductions in analysis time, detection limit, and power consumption, over equivalent bench top systems, while maintaining true and false alarm rates on par with bench top gas chromatograph/mass spectrometer (GC/MS) systems. By harnessing the advantages of micro-scale miniaturization, the MGA program is expected to yield chip-scale gas analyzers with unprecedented performance characteristics.” • NSF (XYZ-on-a-chip) Draft Version (9/24/2003)

  16. Micro Propulsion Draft Version (9/24/2003)

  17. Micro Propulsion for Nanosats(autonomous satellites with a mass <10 kg) from D.L. Hitt, et al, Smart Mater. Struct. 10 (2001) 1163–1175 Draft Version (9/24/2003)

  18. Field Emission Electric Propulsion Taylor cone; E~109 V/m (Cs) another microplasma opportunity? Source: http://www.centrospazio.cpr.it/FEEPPrinciple.html Draft Version (9/24/2003)

  19. Micro Pulsed Plasma Thruster(micro-PPT) from Keidar, et al., AIAA Joint Propulsion Conference, Huntsville, AL, 20-23July2003. Draft Version (9/24/2003)

  20. Funding • DARPA BAA 03-41 “Micro Electric Propulsion” • DARPA is soliciting proposals for the development of novel, high performance, highly flexible micro-thruster and micro-propulsion systems based upon Micro Electromechanical Systems (MEMS) micro-colloidal propulsion technology. However, DARPA will also consider proposals based upon alternate technologies able to satisfy the program goals. These goals are to (1) demonstrate a thruster system capable of varying its specific impulse in real time across a range from 500 sec. to 10,000 sec. utilizing a single propellant, (2) operate said thrusters with electrical thrust efficiencies in excess of 90% over significant portions of this range, (3) demonstrate said thruster with a thruster specific mass less than 0.3 g/Watt, and (4) demonstrate said thruster in a propulsion system capable of delivering total mission delta-Vs for a 100 kg satellite in excess of 10 km/s. • NASA/JPL Draft Version (9/24/2003)

  21. Other Microplasma Applications Draft Version (9/24/2003)

  22. Medical Applications RF Plasma Needle • 1 atm, He (+ air, N2, Ar) • d ~ 0.1 – 1 mm • 13.56 MHz, < 1 W • 250-500 Vp-p • Trot < 100 C, non-equil. • Plasma surgery, dentistry • Apoptosis, not necrosis E. Stoffels, et al., Eindhoven University of Technology from Plasma Sources Science and Technology (2002) Draft Version (9/24/2003)

  23. Materials Processing from R. M. Sankaran and K. P. Giapis, J. Appl. Phys. 92, 2406 (2002). Draft Version (9/24/2003)

  24. Microplasmas • DC • RF capacitively coupled • RF inductively coupled • microwave Draft Version (9/24/2003)

  25. DC microplasmas Draft Version (9/24/2003)

  26. Sputtered material Review of DC Microplasma Sources DC helium plasma on a chip. Plasmas were created in volumes as small as 50 nL. Discharge voltage ~ 800V; Starting voltage ~ 6 kV; Lifetime ~ 2 hours. Eijkel, Stoeri, and Manz, Dept. of Chemistry, Imperial College, UK “An atmospheric pressure dc glow discharge on a microchip and its application as a molecular emission detector,” J. Anal. At. Spectrom., pp.297-300, (2000). Higher pressure  Collisional sheathes  Reduced sputter erosion Draft Version (9/24/2003)

  27. Anode Cathode Cathode - 250m DC Micro Hollow Cathode Discharges • Electron confinement within hollow cathode • thermionic emission? • Lower voltage than simple capillary: 300-400 V • Increased lifetime, but still has electrode erosion • Tgas~ 2000 K • Refs: K. Schoenbach, Old Dominion University • G. Eden, University of Illinois Draft Version (9/24/2003)

  28. Exploiting Electrode Erosion:DC Micro Plasma with Liquid Electrodes Pb Liquid Electrode Spectral Emission Chip Wilson and Gianchandani, University of Michigan from IEEE Trans. on Electron Dev. 49, 2317 (2003). Draft Version (9/24/2003)

  29. Wilson, et al. DC Microplasma Modeling meas. model Draft Version (9/24/2003)

  30. DC Microplasma Modeling Strong Spatial Potential Gradient: E=300k-400kV/m Electron Energy Distribution has a High Energy Tail Draft Version (9/24/2003)

  31. RF capacitively coupled microplasmas Draft Version (9/24/2003)

  32. RF Micro Plasma Sources 13.56 MHz Capacitively Coupled Plasma: M. Blades, U. British Columbia from Journal of Analytical Atomic Spectrometry (2002) • 1 atm, Helium only • 1 mm plasma channel • ~ 20 watts Draft Version (9/24/2003)

  33. RF inductively-coupled microplasmas Draft Version (9/24/2003)

  34. Coil H ERF S Plasma ERF Capacitive vs. Inductive • ERF is perpendicular to boundary • High voltage sheaths • ~100’s V at 13 MHz • Sputter erosion by positive ions • Low ionization efficiency • Power  sputtering • ERF is parallel to the boundary • Low voltage sheaths (~10’s V) • Little sputter erosion by ions • Higher ionization efficiency • Power  ionization, excitation Draft Version (9/24/2003)

  35. Optical Fiber Serpentine coil Al2O3 or glass Gas Inlet Plasma Jet 175 MHz RF input …Hopwood, Northeastern U. 700 MHz RF input Inductively Coupled Plasmasfor emission spectrometry …Horiike, U. Tokyo Draft Version (9/24/2003)

  36. Hybrid package Glass wafer Interdigitated capacitor 5 mm coil Microfabricated ICP Draft Version (9/24/2003)

  37. Microfabrication Process SEM of Interdigitated Capacitor Structure with 10 micron thick Au Draft Version (9/24/2003)

  38. ICP Frequency Scaling Choosing a frequency that maximizes the efficiency of ionization experiment parabolic least squares fit ~ w2 Draft Version (9/24/2003)

  39. Maximize frequency: Coil H Glass wafer Seal Plasma E Glass tube(Chamber) Vacuum Region Frequency Scaling Model Power efficiency= RS / (RS+RC) … RS = 2k2LPLCRP / (RP2 + 2LP2) RS 2k2LPLC/RP if RP2>> 2LP2 …mICP RS  k2LCRP /LP if RP2 << 2LP2 ...large ICP Draft Version (9/24/2003)

  40. Frequency Scaling of Miniature ICPs Electron inertia limits further improvements as w>>ne-n. w>>ne-n Draft Version (9/24/2003)

  41. 3nen< w mICP Efficiency vs. Pressure (nen)constant frequency, f = 493 MHz Efficiency, % Draft Version (9/24/2003)

  42. Frequency Limitation Increasing coil resistance Electron Density @ 690 and 818 MHz 690 MHz 818 MHz Draft Version (9/24/2003)

  43. Coil’s cross section Coil Resistance (FEM model)- rf current crowds to the inner and outer radii of the coil (skin effect, RC ~ f 1/2) - the crowding is asymmetric toward the center (proximity effect, RC ~ f 2). LIMITS ICP OPERATION TO f < 1 GHz Draft Version (9/24/2003)

  44. mICP Frequency Scaling Draft Version (9/24/2003)

  45. Microwave frequencymicroplasmas Draft Version (9/24/2003)

  46. Why microwave microplasma? • Microwave breakdown • Sheath scaling • Vsheath ~ 1/f 2 • Low cost cellphone power amplifier chips • 1-3 W at ~850 MHz or 1700 MHz Meek J.M. and Craggs J.D., “Electrical Breakdown of Gases”, Wiley, New York, 1978 pp 697 Draft Version (9/24/2003)

  47. 2.45 GHz, ~10 W • 90 mm x 33 mm glass over copper • 1 mm x 1 mm chamber cross section • Argon at 1 atm • 200 hour lifetime • A M Bilgic, et al, Univ. of Dortmund • Plasma Sources Sci. Technol. 9, 1 (2000) Microwave Microplasmas Capacitively-coupled Micro-Strip Plasma (MSP) Draft Version (9/24/2003)

  48. Split-Ring Microstrip Resonator A gap-excited microwave discharge Pressure: 100 mTorr - 1 atm (argon) @ ~0.5 watts 100 mTorr - 100 torr (air) @ 1 watt Lifetime: >100 hrs @ 1 W @ 1 atm (argon) - no erosion of 9 um-thick copper electrodes F. Iza and J. Hopwood, IEEE Trans. Plasma Sci., Aug 2003 Draft Version (9/24/2003)

  49. Glass tube Glass tube Glass tube Microstrip Microstrip Microstrip Split-Ring Resonator Microplasmain argon @ 1watt @ 900 MHz 9 torr 20 torr ~ 5 mm 760 torr 100x500 mm Trot ~ 100 C Draft Version (9/24/2003)

  50. Microwave Capacitive Coupling microplasma ~ 100 um Eg ~ 1000kV/m Eg ~ 200kV/m -Vosinwt 500 um gap 100 um gap +Vosinwt er=10.8 Cross section view Rp Top view 1/wCS 1/wCS No sputter erosion: * DC gap voltage = 0 * Vsheath ~ 1/new2 * collisional sheaths Draft Version (9/24/2003)

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