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Microplasma-on-a-chip Excited by UHF and Microwave Frequency

Microplasma-on-a-chip Excited by UHF and Microwave Frequency. Presented at the 2006 International Conference on Reactive Plasma and Symposium on Plasma Processing January 24 – 27, 2006 Jeff Hopwood, Northeastern University Boston, Massachusetts, USA. OUTLINE.

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Microplasma-on-a-chip Excited by UHF and Microwave Frequency

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  1. Microplasma-on-a-chip Excited by UHF and Microwave Frequency Presented at the 2006 International Conference on Reactive Plasma and Symposium on Plasma Processing January 24 – 27, 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 NM-TuM3 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. 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). +/- -/+

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

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

  11. Demonstration1 atm air3 watts - cell phone amplifier chip

  12. Demonstration1 atm airgas detection spectrum isopropyl alcohol

  13. Demonstration1 atm airgas detection spectrum (breath)

  14. Optical Emission in Air at 1 atm

  15. 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.) not an arc! F. Iza and J. Hopwood IEEE TPS (2004)

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

  17. Modeling

  18. Electric Field Intensity Egap > 12 MV/m

  19. Microwave Capacitive Coupling with SRR microplasma ~ 25 mm +Vosinwt -Vosinwt er=10 Rp 1/jwCS 1/jwCS • Minimal sputter erosion: • DC gap voltage = 0 • 1/jwCs < Rp @ high pressure • collisional sheaths cross section

  20. 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

  21. 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)

  22. 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)

  23. 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

  24. 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

  25. 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 …across the bulk plasma in microwave microplasma • high E/p in bulk plasma • high electron energy • efficient ionization • low sheath voltages eliminate sputter erosion

  26. 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-lying molecular excitation of N2

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

  28. Optical Emission in Air at 1 atmModel: E/p ~ 50, <e> ~ 4.5 eVStrong emission in the UV (El ~ 3-4 eV)

  29. >0.25 watts >2 watts X X

  30. 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

  31. Frequency and Gap Comparisons in He/N2 0.8 W More Hb excitation with 50 um gap Less N2(BA) Excitation and more He*at 1800 MHz

  32. 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 • Vsheath<<Vplasma due to 1/jwCs<Rp • w >> wpi • Air operation at 1 atm. • Requires semiballistic electron heating to minimize non-ionizing collisional losses

  33. 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.

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