Transcript Slide 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 OUTLINE • Motivation: microplasma applications for sensors • Background: types of microplasmas • Microwave-excited microplasmas – Design – Diagnostics – Operation in 1 atm. Air • Conclusion Motivation (95 references) Motivation for microplasma: a miniature source of photons, ions and electrons Optical Emission Spectrometry Ion Mobility Spectrometry Mass Spectrometry - Verionix - Alcatel …several research groups power gas sample -Ramsey Group -Cruz et al NM-TuM3 light mplasma optical spectrometer - Eiceman Group - R Miller (Sionex) Background Other Microplasma Concepts • DC microplasma – Ion erosion + …Manz Group • 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 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 Microwave frequency coplanar CCP The microwave excitation must be symmetric, otherwise a self-bias will accelerate ions (similar to a plasma etcher). massive ions do not respond to microwave electric fields… (w > wpi) + + +/- -/+ + + …electrons are partially confined within the plasma: Average displacement < 5 mm Microwave Breakdown Meek J.M. and Craggs J.D., “Electrical Breakdown of Gases”, Wiley, New York, 1978 pp 697 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 INPUT ro=10mm Dielectric Ground Plane Discharge gap GAP Portable Microplasma System Split Ring Resonator Power Amp (GSM Band Cell Phone, 4 watts) VCO (900MHz) not shown: 6 v battery, power level control Demonstration 1 atm air 3 watts - cell phone amplifier chip Demonstration 1 atm air gas detection spectrum isopropyl alcohol Demonstration 1 atm air gas detection spectrum (breath) Optical Emission in Air at 1 atm Rotational Temperature, Trot 0-2 (0.1% N2 in argon at 1 atm.) Tgas Trot = 400 K Glass tube 1-3 Trot = 450 K 2-4 Microstrip Trot = 400 K Trot = 350 K not an arc! 3680 3700 3720 3740 3760 Wavelength (Å) 3780 F. Iza and J. Hopwood IEEE TPS (2004) 3800 3820 Gas temperature at 1 atm 800 Trot (n=0) Trot (n=1) 700 Trot (K) Trot (n=2) Argon + 0.1% N2 600 500 400 Argon microplasma Gap = 500 um Air Microplasma Gap = 25 um 300 0.0 0.5 1.0 1.5 2.0 Power (W) 2.5 3.0 3.5 Modeling Electric Field Intensity Egap > 12 MV/m Microwave Capacitive Coupling with SRR microplasma ~ 25 mm +Vosinwt -Vosinwt er=10 cross section Rp 1/jwCS 1/jwCS Minimal sputter erosion: • DC gap voltage = 0 • 1/jwCs < Rp @ high pressure • collisional sheaths 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 false color optical micrograph microstrip line(Au) Microwave Capacitive Coupling Rp 1/wCS 25 DC Microplasma 1/wCS 1250 mW 1000 mW 750 mW 500 mW 250 mW 150 mW Floating Potential (V) typical CCP 20 15 increasing ne and Cs 10 Kolobov, et al JAP (2002) no sputter erosion 5 0 F. Iza et al IEEE TPS (2003) -5 0.1 1 10 100 1000 Pressure (torr) (Vf measured with a 25 um gold wire inside a 500 um gap) Electron Density Diagnostic ne in argon, g=120mm Power est. size ne 2x1014 cm-3 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 F. Iza and J. Hopwood, Plasma Source Sci. Technol (2005) Voltage Distribution within the Plasma 1 watt (@65% efficiency), 1 atm. argon RP= 4.8kW 45sinwt -45sinwt 1/jwCS = -j1000W 1/jwCS = -j1000W microstrip Cs = 0.17 pF 100V Electrode Bulk Plasma Sheath1 Sheath2 50V 900 MHz 0V -50V -100V 0s 0.5ns V(Va:+,Vb:-) 1.0ns V(Db:2,Db:1) 1.5ns 2.0ns 2.5ns -V(Da:1,Da:2) V(Csa:1,Csb:2) Time 3.0ns 3.5ns 4.0ns 4.5ns 5.0ns SPICE simulation Voltage Distribution vs. Frequency 100V 50V 100 MHz Vp= 22 vpk Vs= 88 Vpk 0V -50V -100V 0s 5ns V(Va:+,Vb:-) 10ns V(Db:2,Db:1) V(Va:+,Vb:-) 0.5ns V(Db:2,Db:1) 15ns 20ns -V(Da:1,Da:2) 25ns V(Csa:1,Csb:2) Time 30ns 35ns 40ns 45ns 50ns 100V 50V 1800 MHz Vp= 87 vpk Vs= 16 vpk 0V -50V -100V 0s 1.0ns -V(Da:1,Da:2) V(Csa:1,Csb:2) Time 1.5ns 2.0ns 2.5ns KEY: Electrode: 90v Bulk Plasma Sheath1 Sheath2 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 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 104 EL (eV) Vgap ~ 80 volts (peak) 103 Ee = qVgap(le/g) ~ 16 eV (peak) N2 102 Ar Minimum ionization cost: 66 eV/electron (Stoletov constant in air) 101 1 2 3 4 5 6 7 8 910 Te (eV) narrow gap high electric field +few electron collisions Hot electrons (bypasses low-lying molecular excitation of N2 Ave. Electron Energy in Atmospheric Pressure Air vs. gap size* 120um 50um 25 um 10 um 10 ? Ave. Electron Energy (eV) 8 6 4 *BOLSIG 2 calculation in 78%N2+21%O2+1%Ar 0 0 20 40 60 80 100 E/p (Volt cm-1 Torr-1) 120 140 Optical Emission in Air at 1 atm Model: E/p ~ 50, <e> ~ 4.5 eV Strong emission in the UV (El ~ 3-4 eV) Comparision of Ionization Efficiency in Air, Argon and Helium 120um 50um 25 um 10 um Ionization Efficiency (%) 100 >0.25 watts 10 >2 watts 1 Air Argon Helium X 0.1 0 X 20 40 60 80 100 E/p (V cm-1 torr-1) 120 140 Frequency Scaling 350 mW 100 mm discharge gap 1.8 GHz 0.9 GHz 1.8 GHz MSRR 2006 AVS Symposium Rodriguez, Xue, and Hopwood Frequency and Gap Comparisons in He/N2 1400 1200 0.8 W 7065 A 5875.6 A 1000 H 6678 A More H excitation with 50 um gap Intensity 900 MHz, 50um gap 800 600 900 MHz, 250um gap 400 Less N2(BA) Excitation and more He* at 1800 MHz 200 1800MHz, 100 um gap 0 4500 5000 5500 6000 Wavelength (A) 6500 7000 7500 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 >> w pi • Air operation at 1 atm. – Requires semiballistic electron heating to minimize non-ionizing collisional losses 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, CCF0403460, Verionix, Inc., and Sionex, Inc.