Microplasma: Physics and Applications

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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

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Outline

• Motivation Applications
• Plasma Display Panels
• Micro Chemical Analysis Systems
• Micro Propulsion
• Microplasma Devices and Physics
• DC, RF, microwave
• Challenges and Opportunities

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Plasma Display Panels
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Plasma Display Panels (PDPs) Structure
From S.S. Yang, et al, IEEE Trans. Plasma Sci.
31, 596 (2003).
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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).
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Plasma Physics of PDPs

• Ne Xe (1-10)
• Ne is a buffer gas (Eiz-Negtgt 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
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State-of-the-art DiagnosticsK. Tachibana, et al
., IEEE Trans. Plasma Sci. 31, 68 (2003)3-D
temporally-resolved emission and diode laser
absorption
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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)
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Modelingexample from S.S. Yang, et al, IEEE
Trans. Plasma Sci. 31, 596 (2003).
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Issues in PDP

• Efficiency
• currently lt1 lumen/watt
• goal 5 lumens/watt
• incandescent lamp 25 lm/w fluorescent100
  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)
• butcomplex microstructure, electronics, EMI
• Manufacturing is dominated by Asia

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Micro Chemical Analysis
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Micro Chemical Analysis

• Emission Spectrometry
• possibly coupled with another separation
  technique (e.g., GC)
• Issues
• pumping
• stability/repeatability
• lifetime/contamination
• power/heat

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Micro Chemical Analysis - ion mobility
spectrometry -
from R.A. Miller, et al, Sensors and Act.
Workshop, Hilton Head, 2000)

• Microplasmas potential role
• micro-ionizers
• optical emission detection
• high ionization or excitation efficiency ?
  improved signal-to-noise (presently, DL100ppb)

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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 (lt 1W), to be portable
• Low temperature (only ambient cooling)

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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)

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Micro Propulsion
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Micro Propulsion for Nanosats(autonomous
satellites with a mass lt10 kg)
from D.L. Hitt, et al, Smart Mater. Struct. 10
(2001) 11631175
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Field Emission Electric Propulsion
Taylor cone E109 V/m
(Cs)
another microplasma opportunity?
Source http//www.centrospazio.cpr.it/FEEPPrinci
ple.html
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Micro Pulsed Plasma Thruster(micro-PPT)
from Keidar, et al., AIAA Joint Propulsion
Conference, Huntsville, AL, 20-23July2003.
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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

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Other Microplasma Applications
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Medical Applications RF Plasma Needle

• 1 atm, He ( air, N2, Ar)
• d 0.1 1 mm
• 13.56 MHz, lt 1 W
• 250-500 Vp-p
• Trot lt 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)
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Materials Processing
from R. M. Sankaran and K. P. Giapis, J. Appl.
Phys. 92, 2406 (2002).
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Microplasmas

• DC
• RF capacitively coupled
• RF inductively coupled
• microwave

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DC microplasmas
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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
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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

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Exploiting Electrode ErosionDC 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).
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DC Microplasma Modeling
meas.
model
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DC Microplasma Modeling
Strong Spatial Potential Gradient
E300k-400kV/m Electron Energy Distribution has
a High Energy Tail
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RF capacitively coupled microplasmas
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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

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RF inductively-coupled microplasmas
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Capacitive vs. Inductive

• ERF is perpendicular to boundary
• High voltage sheaths
• 100s V at 13 MHz
• Sputter erosion by positive ions
• Low ionization efficiency
• Power ? sputtering

• ERF is parallel to the boundary
• Low voltage sheaths (10s V)
• Little sputter erosion by ions
• Higher ionization efficiency
• Power ? ionization, excitation

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Inductively Coupled Plasmasfor emission
spectrometry
Horiike, U. Tokyo
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Microfabricated ICP
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Microfabrication Process
SEM of Interdigitated Capacitor Structure
with 10 micron thick Au
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ICP Frequency Scaling
Choosing a frequency that maximizes the
efficiency of ionization
experiment
parabolic least squares fit w 2
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Frequency Scaling Model
Power efficiency RS / (RSRC) RS
?2k2LPLCRP / (RP2 ?2LP2)
RS ? ?2k2LPLC/RP if RP2 gtgt ?2LP2 mICP
RS ? k2LCRP /LP if RP2 ltlt ?2LP2
...large ICP
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Frequency Scaling of Miniature ICPs
Electron inertia limits further improvements as
wgtgtne-n.
wgtgtne-n
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mICP Efficiency vs. Pressure (nen)constant
frequency, f 493 MHz
Efficiency,
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Frequency Limitation
Electron Density _at_ 690 and 818 MHz
690 MHz
818 MHz
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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 lt 1 GHz
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mICP Frequency Scaling
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Microwave frequencymicroplasmas
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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
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Microwave Microplasmas
Capacitively-coupled Micro-Strip Plasma (MSP)
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Split-Ring Microstrip Resonator A gap-excited
microwave discharge
Pressure 100 mTorr - 1 atm (argon) _at_ 0.5
watts 100 mTorr - 100 torr (air) _at_ 1
watt Lifetime gt100 hrs _at_ 1 W _at_ 1 atm (argon)
- no erosion of 9 um-thick copper electrodes
F. Iza and J. Hopwood, IEEE Trans. Plasma Sci.,
Aug 2003
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Split-Ring Resonator Microplasmain argon _at_ 1watt
_at_ 900 MHz
9 torr
20 torr 5 mm
760 torr 100x500 mm Trot 100 C
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Microwave Capacitive Coupling
No sputter erosion DC gap voltage 0
Vsheath 1/new2 collisional sheaths
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Microwave Capacitive Coupling
typical CCP
no sputter erosion
(Vf measured with a 25 um gold wire)
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Comparison mICP vs.Split-Ring Resonator
2.5x
Same power, pressure, gas, plasma dimensions,
electron temperature, and plasma potential
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Low loss split-ring resonator minimizes skin and
proximity losses at high frequencies
Microstrip resonator
Free-standing mICP coil Q 37, h 30
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Low Rotational Temperature (0.1 N2 in argon at
1 atm.)
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An Unresolved Issue
wide gap ? low electric field electron
collisions ? Maxwellian distribution

• Low power (100 mW) operation in atmospheric air,
  not argon
• energy loss to molecular states
• Micromachined discharge gap
• increased power density
• gap width a few electron mean free paths ( 1
  um)
• semi-ballistic electron heating

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Conclusion

• Many exciting applications for microplasma
• displays, chemical analysis, medical, materials
• In general, there are strong physical arguments
  for using high frequency power sources
• technologically, 2 GHz _at_ 1 watt is trivial
• Very little detailed plasma physics is known
• PDPs are best understood (but not optimized)

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Acknowledgments

• Graduate Students
• Felipe Iza (0.8 GHz ICP and 0.9 GHz ring
  resonator)
• Olga Minayevanow at Old Dominion U. (ICP-AES)
• Yu Yin...now at Teradyne (0.1 - 0.5 GHz ICP)
• Undergraduates
• Peter Grimes
• Jason Messier
• David Williamson (sponsored by Raytheon)
• This work was supported by the National Science
  Foundation under Grants No. ECS-9701916,
  DMI-9980777 and DMI-0078406 .