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Operating Analytical Plasmas at the Sub-Watt Level

Operating Analytical Plasmas at the Sub-Watt Level. Presented at the PITTCON 2006 March 13, 2006 Jeff Hopwood, Northeastern University Boston, Massachusetts, USA. OUTLINE. Motivation: microplasma applications for sensors Background: types of microplasmas Microwave-excited microplasmas

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Operating Analytical Plasmas at the Sub-Watt Level

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  1. Operating Analytical Plasmas at the Sub-Watt Level Presented at the PITTCON 2006 March 13, 2006 Jeff Hopwood, Northeastern University Boston, Massachusetts, USA

  2. OUTLINE • Motivation: microplasma applications for sensors • Background: types of microplasmas • Microwave-excited microplasmas • Design • Diagnostics • Operation in 1 atm. Air • Conclusion

  3. Motivation (95 references)

  4. - Verionix - Alcatel …several research groups power -Ramsey Group -Cruz et al, Sandia light - Eiceman Group - R Miller (Sionex) gas sample optical spectrometer mplasma Motivation for microplasma:a miniature source of photons, ions and electrons Optical Emission Spectrometry Ion Mobility Spectrometry Mass Spectrometry

  5. BackgroundOther Microplasma Concepts • DC microplasma • Ion erosion • DBD: high voltage, surface contamination(?) • RF capacitively coupled microplasma • Low efficiency (electron and ion loss, esp. mplasma) • RF inductively coupled microplasma • Ok, for low pressure nm~w 0.01<p<10 torr at 0.5 GHz …but free-standing coils are very lossy at f>1 GHz + …Manz Group

  6. A realistic Analytical Microplasma should be… • low power • low voltage • long lifetime (repeatable) • low temperature • high intensity: high density of hot electrons • pressure • industrial process monitor: rough vacuum, possibly corrosive gases • environmental sensors: atmospheric air

  7. Low Power: minimize power losses • Sputtering • Ion-solid collisions • Minimize/eliminate electrode voltages • Gas Heating • Electron-neutral collisions (esp. N2*) • Minimize non-ionizing and non-excitation collisions  hot electrons • Resistive Heating (circuitry)

  8. massive ions do not respond to microwave electric fields… (w > wpi) …electrons are partially confined within the plasma: Average displacement < 5 mm + + + + Microwave frequencycoplanar CCP The microwave excitation must be symmetric, otherwise a self-bias will accelerate ions (similar to a plasma etcher). +/- -/+

  9. Line Dielectric Ground Plane Discharge gap Half-wave Split Ring Resonator (SRR):Surface Current Simulation at 1.0 W 900 MHz I fro V V / I = 50W GAP fro 50 ohm INPUT e=10 ro=10mm INPUT GAP

  10. Electric Field Intensity (900 MHz) 25 mm discharge gap Egap > 12 MV/m

  11. Portable Microplasma System Split Ring Resonator Power Amp (GSM Band Cell Phone, 4 watts) VCO (900MHz) not shown: 6 v battery, power level control

  12. Demonstration1 atm air3 watts - cell phone amplifier chip

  13. Demonstration1 atm airgas detection spectrum isopropyl alcohol

  14. Demonstration1 atm airgas detection spectrum (breath)

  15. Optical Diagnostics

  16. Optical Emission in Air at 1 atm Trot

  17. 0-2 Trot = 450 K Trot = 400 K 1-3 2-4 Trot = 350 K 3680 3700 3720 3740 3760 3780 3800 3820 Wavelength (Å) Glass tube Microstrip Rotational Temperature, Trot (0.1% N2 in argon at 1 atm., 1 watt) not an arc! F. Iza and J. Hopwood IEEE TPS (2004)

  18. Gas temperature at 1 atm air Air Microplasma Gap = 25 um Argon microplasma Gap = 500 um

  19. Langmuir Probe:Floating Potential

  20. Microwave Capacitive Coupling DC Microplasma Kolobov, et al JAP (2002) (Vf measured with a 25 um gold wire inside a 500 um gap) Rp 1/wCS 1/wCS typical CCP increasing ne and Cs no sputter erosion F. Iza et al IEEE TPS (2003)

  21. Lifetime Evaluation 25 mm discharge gap after 50 hrs. operating in open room air microstrip line (Au) 25 mm gap carbon deposition Al2O3 substrate Al2O3 substrate microstrip line(Au) false color optical micrograph

  22. Microwave Capacitive Coupling with SRR microplasma ~ 25 mm +Vosinwt -Vosinwt er=10 Rp 1/jwCS 1/jwCS Zp: the plasma impedance Zp = RP + 1/jw(2Cs) • Minimal sputter erosion: • DC gap voltage = 0 • 1/jwCs < Rp @ high pressure • collisional sheaths cross section

  23. Plasma Impedance:Electron densitySheath voltages

  24. Power Plasma Impedance 0.50W - 13000 – j5500 ohms 0.75W - 6500 – j2400 ohms 1.00W - 4800 – j2000 ohms 1.25W - 4100 – j1800 ohms 1.50W - 3200 – j1350 ohms est. size ne 2x1014 cm-3 Electron Density Diagnostic nein argon, g=120mm F. Iza and J. Hopwood, Plasma Source Sci. Technol (2005)

  25. Argon, 1 atm

  26. RP= 4.8kW 45sinwt -45sinwt microstrip 1/jwCS = -j1000W 1/jwCS = -j1000W Cs = 0.17 pF Electrode Bulk Plasma Sheath1 Sheath2 Voltage Distribution within the Plasma1 watt (@65% efficiency), 1 atm. argon 900 MHz SPICE simulation

  27. E/p = 1.9 v cm-1 torr-1 =gap voltage/(gap*pressure) E/p = 7.6 v cm-1 torr-1 Voltage Distribution vs. Frequency 100 MHz Vp= 22 vpk Vs= 88 Vpk KEY: Electrode: 90v Bulk Plasma Sheath1 Sheath2 1800 MHz Vp= 87 vpk Vs= 16 vpk

  28. Internal E-Field Conclusions: The applied voltage drops… …across the sheath region in DC and RF microplasma low E/p in bulk plasma, but large sheath fields  sputtering and low ionization efficiency …across the bulk plasma in microwave microplasma • high E/p in bulk plasma • high electron energy • efficient ionization • low sheath voltages eliminate sputter erosion

  29. The Role of E/p:a pathway to low power operation

  30. Boltzmann Equation Solution using BOLSIG Large gap High pressure Small gap Low pressure

  31. ? Ave. Electron Energy in Atmospheric Pressure Airvs. gap size* *BOLSIG calculation in 78%N2+21%O2+1%Ar

  32. Partitioning of Power (helium) Helium ionization Helium excitation Elastic collisions

  33. Partitioning of Power (Ar) argon ionization argon excitation Elastic collisions

  34. N2 excitation O2 excitation N2 ionization O2 ionization Partitioning of Power (air)

  35. >0.25 watts 10x >2.5 watts X X

  36. Intensity vs. Power g=200um g=25um 10x

  37. Frequency Scaling 0.9 GHz 1.8 GHz 1.8 GHz MSRR 350 mW 100 mm discharge gap 2006 AVS Symposium Rodriguez, Xue, and Hopwood

  38. Frequency Scaling • Increased frequency: • Decreased sheath impedance • Increased E/p in the plasma • Important if E/p <10 V/cm/torr …where ionization efficiency is very sensitive to E/p

  39. Conclusion • Microstrip Split-Ring Resonators provide • Simple, low cost atmospheric microplasma • High intensity (ne ~ 1014 cm-3 per watt, Ar) • Minimal ion erosion • Symmetric excitation, VDC = 0 • Vplasma>>Vsheath due to 1/jwCs<Rp • w >> wpi • Sub-watt operation in air: large E/p • Small gap (<25mm) increases E = (Vplasma) /gap • High frequency operation (>1.8GHz) reduces sheath impedance and increases Vplasma = Velectrode - Vsheath

  40. Acknowledgments • Students • Jun Xue (scaling) • Istvan Rodriguez (cell phone power amp electronics, freq scaling) • John Nwagbaraocha (EM modeling) • Dr. Felipe Iza…now at POSTECH, S. Korea (ring resonator studies) • Chris Doughty, Steven Coy, David Fenner • This work was supported by the National Science Foundation under Grants No. DMI-0078406, CCF-0403460, Verionix, Inc., and Sionex, Inc.

  41. Operating Analytical Plasmas at the Sub-Watt Level Presented at the PITTCON 2006 March 13, 2006 Jeff Hopwood, Northeastern University Boston, Massachusetts, USA

  42. Electromagnetic Modeling Q = 140 Zin = 50 W

  43. Semiballistic Electron Heating: Low power, non-equilibrium operation in atmospheric pressure air wide gap low electric field +electron collisions  Maxwellian distribution 1 atm Air: back-of-the-envelope calc. le ~ 5 mm (hot e- at Tg=700 K) gap = 25 mm Vgap ~ 80 volts (peak) Ee = qVgap(le/g) ~ 16 eV (peak) Minimum ionization cost: 66 eV/electron (Stoletov constant in air) narrow gap high electric field +few electron collisions  Hot electrons …bypasses low-energy molecular excitation (N2)

  44. Meek J.M. and Craggs J.D., “Electrical Breakdown of Gases”, Wiley, New York, 1978 pp 697 Microwave Breakdown

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