1. ELECTRON CURRENT EXTRACTION
FROM RF MICRO-DIELECTRIC BARRIER DISCHARGES *
Jun-Chieh Wanga), Napoleon Leonib), Henryk Bireckib),
Omer Gilab), and Mark J. Kushnera)
a)University of Michigan, Ann Arbor, MI USA
b)Hewlett Packard Research Labs, Palo Alto, CA USA
AVS 57th International Symposium & Exhibition
October 17-22, 2010, Albuquerque, Mew Mexico
* Work supported by Hewlett Packard Research Labs
Good afternoon, I am Jun-Chieh Wang.
Welcome to my presentation today.
During my talk, I am going to present some interesting findings from our study
titled “electron current extraction from rf micro-dielectric barrier discharges”
This work is supported by Hewlett Packard Research Labs

2. University of Michigan Institute for Plasma Science & Engr.
AGENDA
Introduction to micro-Dielectric Barrier Discharges (mDBD)
Current extraction
Description of model
mDBD Scaling
mDBD sustained in N2 (1 atm)
Electron current extraction vs rf frequencies
Charge per pulse and pulse number vs frequencies
Electron current extraction vs dielectric constant
Concluding Remarks
I will first begin with an introduction to micro-dielectric barrier discharges
and current extraction
followed by a description of the model we use,
and the simulation result of mDBD sustained in Nitrogen at atmosphe’ric pressure.
We compare the current extraction, charge per pulse and pulse number Versus different driving frequencies
Also the current extraction versus dielectric constant
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p1

3. DIELECTRIC BARRIER DISCHARGES
The plasma in DBDs is sustained between parallel electrodes of which one (or both) is covered by a dielectric.
When the plasma is initiated, the underlying dielectric is electrically charged, removing voltage from the gap.
The plasma is terminated when the gap voltage falls below the self-sustaining value and so preventing arcing.
On the following half cycle, a more intense electron avalanche occurs due to the higher voltage across the gap from previously charged dielectric.
The plasma in DBD is sustained between parallel electrodes where
one or both of the electrodes are covered by dielectrics
which separate the electrodes from the gas.
When the plasma is initiated and impinges on the dielectrics,
the dielectrics charge and reduce (remove) the voltage across the gap.
When the voltage falls below the self-sustaining value,
the plasma is quenched, thereby preventing the formation of an arc.
When the polarity changes, a more intense electron avalanche occurs
due to the higher voltage across the gap from the previously charged dielectric,
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p2

4. University of Michigan Institute for Plasma Science & Engr.
MICRO-DBDs
Microplasmas (10s to 100s m) are interesting for planar current sources due to the ability of fabricating large arrays.
Non-arcing, micro-DBDs (mDBDs) using rf voltages are attractive for these arrays due to inexpensive mass production or area-selective modification.
These tens to hundreds “of” microns of micro-plasma are interesting for planar current sources
due to its ability to fabricate large arrays.
So the non-arcing micro-DBDs using rf voltages are attractive for these arrays due to inexpensive mass production.
Here is an application.
Based on the principle of mDBDs,
this micro-plasma stamp is applied to area-selective surface modification at atmosphe’ric pressure.
The plasma is formed in the cavities which are generated between the micro-plasma stamp
and the substrate to be treated by compression.
“Microplasma stamps” based on mDBDs are applied to area-selective surface modification at atmospheric pressure.
Ref: J. Phys. D: Appl. Phys. 41, (2008)
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Institute for Plasma Science & Engr.
AVS_Oct2010_p3

5. University of Michigan Institute for Plasma Science & Engr.
SCALING OF MICRO-DBDs
At atmospheric pressure, plasma formation and decay times can be a few ns whereas the rf period is 10s to 100s ns – the mDBD may need to be re-ignited with each cycle.
Electron extraction from the mDBD may require a third electrode and so the structure, electron emitting properties and dynamics are important to the operation.
We have computationally investigated the extraction of electron current from mDBDs:
Geometry
Frequency
Gas mixture
However, at atmospheric pressure, the plasma formation and decay times
can be “only” few ns whereas the rf period is 10s to 100s “of” ns;
which means the mDBDs need to be re-ignited with each cycle.
Furthermore, electron extraction from the mDBDs may require a third electrode,
so the structure, electron “emission” and plasma dynamics
are important to the operation.
In our study, we have computationally investigated the extraction of electron currents
from the mDBDs by varying frequencies, gas mixture, and geometry
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p4

6. MODELING PLATFORM: nonPDPSIM
Poisson’s equation:
Transport of charged and neutral species:
Charged Species:  = Sharfetter Gummel
Neutral Species:  = Diffusion
Surface Charge:
Electron Temperature (transport coefficient obtained from Boltzmann’s equation
The model we “are using” in the investigation, nonPDPSIM, is shown here.
The fundamental equations are
poisson’s equation for electric potential,
“the” continuity equation for the transport of charged and neutral species
and “the” surface charge balance equation.
The electron temperature is obtained by solving the electron energy equation
..is “the” energy loss due to inelastic collision. xxx in div.() is the flux density of electron energy,
kappa is “the” electron thermal conductivity.
The equations are integrated using Newton-Raphson method.
“The” Scharfetter-Gummel(銷非特兒 辜摸) form is used for electron and ion flux.
The electron transport coefficient and rate coefficient for bulk electrons as a function of electron temperature
are obtained by solving Boltzmann equation for the electron energy distribution.
Although the electron energy distribution is not a Maxwellian, we still refer Te=2/3 * average electron energy.
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p5 6 6

7. MODELING PLATFORM: nonPDPSIM
Radiation transport and photoionization:
Electron Monte Carlo Simulation tracks sheath accelerated secondary electrons produced by ion and UV bombardment of surfaces.
After the charge density and potential have been updated,
Radiation transport is addressed using a Green’s function approach.
We also applied Monte Carlo Simulation to track the sheath accelerated secondary electrons
produced by ions or UV bombardment at surface.
(…Andy has other so show, but I didn’t put them here)
When you have photons produced, they propagate with decay or absorption exponentially,
So the photon intensity reduces.
G is the probability of survival of the emitted photon and divergence of its flux
between emission and ionization. A is the Einstein coefficient.
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p6 7 7

8. University of Michigan Institute for Plasma Science & Engr.
mDBD GEOMETRY
The mDBD is a “sandwich” cylindrical geometry.
rf electrode buried in printed-circuit-board
Grounded electrode separated from rf by dielectric sheet – 35 m hole.
Biased extraction electrode across a gap of 500 m
rf: 1.4 kV at MHz.
Extraction: 1-2 kV, ballasted (Rb= 100 kΩ) to prevent arcing or run away current
The model geometry is shown here.
The mDBD is a sandwich structure and “has” cylindrical symmetry.
The rf electrode is buried in “the” printed circuit board.
A grounded electrode “is” separated from “the” rf electrode by a dielectric sheet,
with “an” opening of 35 microns in the center.
A biased extraction electrode is separated by a gap of 500 microns.
The voltage applied to “the” rf electrode is 1.4 kilo-volt at 2.5 to 25 MHz.
A one or 2 kilo-volt voltage and 100 kilo-ohm ballast resistor is connected to
“the” bias electrode to extract current and “to” prevent arcing or run-away current.
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p7

9. 20 MHz N2 (1 atm): SURFACE CHARGE, [e]
 (-2x1016~2x1016 cm-3)
 [e] (2x1015 cm-3, 5 dec)
 [e] (2x1015 cm-3, 5 dec)
AVS2010_pdpsim_total_nezoom_ne_20MHz.lay
case147; time#800~900=>400~450 ns;1353x673;linear,log,log; 20MHz(period=50)
Here is the simulation results of atmospheric nitrogen plasma.
The animation on the left is the total charge density, in the middle and one the right are electron density.
The rf votage of 1.4 kilo-volt at 20 MHz is shown in the bottom of the animation.
On the top is “a” 2 kilo-volt bias voltage and 100 kilo-ohm ballast resistor.
At the “positive” peak of the rf vooltage, the grounded electrode acts as a cathode
to enable the avalanche to occur in the mDBD cavity.
Electrons charge the dielectric negatively.
As the rf voltage decreases to zero, no net potential “is applied”
other than that produced by the negative charge on the dielectric.
electrons are extracted from the cavity,
Before” the negative peak of rf voltage,
the electron plume “has already reached” its maximum,
Sufficient ions have been collected on the dielectric “which” reduces the net voltage drop
“The” electron plume begins to diminish.
As the rf voltage returns to zero, net voltage across the gap is sufficiently small
due to the positive charges the dielectric,
electron plume is attracted to the dielectric and extinguished.
This electron flux neutralize the positive charged dielectric.
Vrf= 1.4 kV, 20 MHz, 2 kV bias and ballast resistor 100 kΩ
Vrf=+1.4 kV: Avalanche in mDBD cavity. Electrons charge dielectric negatively.
Vrf= 0: Biased electrode extracts electrons from cavity.
Vrf=-1.4 kV: Positively charged dielectric reduces voltage drop.
Vrf= 0: E-plume extinguishes. e-flux neutralizes positively charged dielectric.
MIN MAX
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p8
Animation Slide-GIF

10. 20 MHz N2 (1 atm): E/N, Te, Se+ Ssec
E/N (40~4000 Td)
 Te (0.1~15 eV)
[Se+ Ssec](3x1024 cm-3 s-1,3dec)
AVS2010_pdpsim_EN_TE_SEIMPACT+SEMCS_20MHz.lay
case147; time#800~900=>400~450 ns;1353x671;linear,linear,log
We can see the plasma “is” nearly extinguished every cycle “from the previous slide”,
hence it requires re-initiation “each time”.
The animations here are E/N on the left, electron temperature “in” the middle
and electron sources due to electron impact ionizations in bulk plasma
and from secondary electron on the right.
When the electrons are extracted or recombined, larger E/N occurs at “the”
zero crossing of rf voltage due to the previously charged dielectric,
higher E/N increases the electron temperature and electron impact ionization.
(at low f, higher E/N happens not only at zero-crossing=>so we have multi-pulses)
We found the this bulk ionization is commensurate with the ionization due to secondary beam electrons.
When the electrons are created or attracted to the cavity,
the conductive plasma shields out the electric field
and lower the E/N which reduce the ionization in the cavity.
Ionization reaction, threshold energy for ionizaiton:
E + N2 > N2^ + E + E; threshold energy=15.5 ev
E + N2V > N2^ + E + E; ev
E + N2* > N2^ + E + E ; ev
Larger E/N occurs at Vrf zero-crossing due to previously charging of dielectric. Te and Se follow E/N.
Ionizations due to secondary “beam” electrons at surface seeded by positive ions and UV photons from long lived N2 excited states are commensurate with bulk ionization.
Conductive plasma shields out electric field and lowers E/N which reduces ionization in the cavity.
MIN MAX
University of Michigan
Institute for Plasma Science & Engr.
Animation Slide-GIF
AVS_Oct2010_p9

11. ELECTRON CURRENT EXTRACTION vs FREQUENCY
20MHz
10MHz
Peak currents at top electrode tend to increase with increasing frequency.
Single current pulse at higher frequencies, multiple pulses at lower frequencies.
Longer periods allow charging and discharging of dielectric, launching new pulses.
rf Voltage
Top Electrode
Current
20 MHz: Case_147/AVS2010_pdpsim_cir_con_20MHz_v02,600*533
10 MHz: Case_149AVS2010_pdpsim_cir_con_10MHz_v02,600*533
5 MHz: Case_160_ /AVS2010_pdpsim_cir_con_5MHz_v02,600*533
2.5 MHz:Case_159_ /AVS2010_pdpsim_cir_con_2.5MHz_v02.lay, 600*533
case_159_ /AVS2010_pdpsim_cir_con_case_147_149_160,0805_159,0904.tif
I-V characteristics were shown for rf frequencies of MHz in atmospheric nitrogen plasma.
A 1.4 kV rf voltage is applied to rf electrode,
“a” 2kV bias potential and 100 k-ohm ballast resistor are applied to “the” top electrode.
Traces are shown for the rf voltage, and current on the top electrode.
We found that the peak current collected on the top electrode tend to increase with increasing frequency.
Also, at higher frequency, single current pulse was observed,
while at lower frequency we observe multiple pulses.
The trend is likely attributed to the “overshoot” of quasi-steady state conditions
that are enabled by the larger dV/dt of the high frequency wave form.
Higher frequency produce higher current and potential drop on top electrode.
If it is the ballast resistor that make multiple pulses, higher frequency with larger current and potential drop on top
Will have greater effect, but it is not.
Although the top electrode is applied with a 2KV dc voltage, the ballast resistor
produces small dips in the applied voltage across the gap.
5MHz
2.5MHz
Vrf = 1.4 kV, MHz,2 kV bias, kΩ ballast resistor
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p10

12. ELECTRON CURRENT EXTRACTION vs FREQUENCY
2.5MHz, Simulation
2.5MHz, Experiment
Top Electrode
Current
AVS2010_pdpsim_cir_con_2.5MHz_compare_v02.tif
AVS2010_time_voltage_current_v02.tif
The figures shown on the left and right are the triple pulses predicted by simulation and measured by experiments.
The simulation shows good agreement with the measure result under similar operating conditions.
Simulation shows good agreement with the measured result.
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p11

13. 2.5 MHz N2 (1 atm): SURFACE CHARGE, [e]
 (-3x1016~3x1016 cm-3)
 [e] (5x1015 cm-3, 8 dec)
 [e] (5x1015 cm-3, 8 dec)
AVS2010_pdpsim_total_nezoom_ne_2.5MHz.avi
case_159_ ; time#600~800=>1200~1600 ns;1353x671;linear,log,log
Here is the simulation results of atmospheric nitrogen plasma.
The animation on the left is the total charge density, in the middle and on the right are electron density.
The rf votage of 1.4 kilo-volt at lower frequency, 2.5 MHz, is shown in the bottom
On the top is “a” 2 kilo-volt bias voltage and 100 kilo-ohm ballast resistor.
At the “positive” peak of the rf voltage, the dielectric was charged negatively,
the residual electrons are almost extinguishes.
Before the rf voltage decreases to zero, avalanche occurs,
the first electron plume has been extracted. E/N increase.
Before” the negative peak of rf voltage, 2 pulses are extracted due to
the oscillated potential results from multiple charging of the dielectric.
As the rf voltage returns to zero, The electron flux start to neutralize the positive charged dielectric.
---.
2 pulses extracted due to the avalanche-charging process
Vrf= 1.4 kV, 2.5 MHz, 2 kV bias and ballast resistor 100 kΩ
Vrf=+1.4 kV: Dielectric charged negatively. Residual electrons almost extinguishes.
Vrf=0: Avalanche has occurred. 1st E-plume has been extracted. E/N increases.
Vrf=-1.4 kV: 2 pulses extracted due to the multiple dielectric charging process.
Vrf= 0: E-flux neutralizes positively charged dielectric.
MIN MAX
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p12
Animation Slide-GIF

14. University of Michigan Institute for Plasma Science & Engr.
CHARGE PER PULSE
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_159_
AVS2010_146_147_148_149_159( )_160( )_coulomb_per_pulse_vs_pulse_number.qpc
Case_149\presentation\icops2010_146_147_148_149_coulomb_per_pulse.TIF
The figure shows here is the charge per pulse versus pulse number.
Due to the positive ions “accumulating” in the gap over time,
the extraction field is perturbed.
“This is” the reason why charge per pulse is gradually increasing and oscillating
This perturbed electric field and charge collected on top electrode reach a steady state
state when the increasing positive ions balance the decay due to recombination per pulse
and the transport of positive ions.
The first pulse is not shown here, that’s because the charge is affected by the initial condition.
If you put a probe at some point in the gap, the field does not necessary increase,
but the potential increase.
I believe the charges gradually increase and reach the steady state is due to the over all averaged field.
Positive ion accumulation in gap over time perturbs the extraction field.
The perturbed electric field and charge collected on top electrode gradually reach a steady state when the increasing positive ions balance the decay due to recombination per pulse and the transport of positive ions.
Vrf = 1.4 kV, MHz,2 kV bias, kΩ ballast resistor
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Institute for Plasma Science & Engr.
AVS_Oct2010_p13

15. AVERAGED CHARGE and PULSE NUMBER
vs FREQUENCY
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_159_
GEC2010_146_147_148_149_159( )_160( )_average_coulomb_per_pulse_vs_frequency.qpc
GEC2010_146_147_148_149_159( )_160( )_current_per_cycle
Frequency(MHz) average(c) number of pulses are averaged e e e e e e
(dielectric constant=20)
The figures shows here on the left and right are average charge per pulse and pulse number per cycle.
We found that the averaged charge per pulse collected on the top electrode is not a strong function of frequency.
Triple and double current pulses per cycle at lower frequency is a consequence of multiple dielectric charging and avalanche process.
(multiple avalanche process)
The current width is about half of the period
(measured from the beginning of the current).
(base to base)
Averaged charge per pulse collected on top electrode is not a strong function of frequency.
Triple and double current pulse per rf cycle at lower frequency is a consequence of multiple dielectric charging and avalanche process.
University of Michigan
Institute for Plasma Science & Engr.
Vrf = 1.4 kV, MHz,2 kV bias, kΩ ballast resistor
AVS_Oct2010_p14

16. CURRENT EXTRACTION vs DIELECTRIC CONSTANT
25MHz
20MHz
εr=10 20
Charging of the insulator in part controls current collection on top electrode.
Larger dielectric constant allows larger current collection, though not linear with .
case_146/GKRS2010_circuit_leg_A1_141&146_142&147_143&148_145&149.tif
Current pulses for dielectric constant=10 and 20 for the dielectric sheet are shown here at frequencies of 25 to 10 MHz.
the charging of the insulator in part controls the current collection on the top electrode,
so larger dielectric constant allows larger current extraction from the gap,
though it’s not a linear dependence.
a transient in current peaks on the first pulse is due to the initial conditions
and consequences of suddenly applying voltage(10%of period),
followed by current increasing until it reaches a quasi-steady state.
There are pulse-to-pulse variations that is due to the “statistical”
nature of MCS.
The pulse to pulse variation in current and plasma characteristics is due to random statistical variations
in the Monte Carlo algorithms used to track energetic secondary electrons.
15MHz
10MHz
Vrf = 1.4 kV, MHz,2 kV bias, kΩ ballast resistor
Univ. of Michigan
Inst. Plasma Sci. & Engr.
AVS_Oct2010_p15

17. mDBD N2 (1 atm): IONIZATION SOURCE IN THE GAP
Vrf=1.4 kV, 25 MHz
[Se] (1025 cm-3 s-1, 7dec)
Re-ignition is by avalanche by small residual electrons and photo-electrons from surfaces.
+1 kV extraction does not produce sufficient net electron impact ionization across gap. Plasma is ignited in the mDBD cavity and extracted.
+2 kV produces finite electron impact sources in the gap. Charges extracted from cavity seed the avalanche in the gap.
1 kV bias
2 kV bias,
ballast resistor 100 kΩ
(case_97;GKRS2010_Seimpact_gap;timeframe= 560(280ns)~640(320ns);size=449x673)
(case_146; GKRS2010_Seimpact_gap;timeframe=640(320ns)~720(360ns);size=450x674)
10^18~10^25, 7decs
We have seen the ionization in the cavity, “but” do we also have similar
ionization sources in the gap?
The figure shows here on the left and right are electron impact ionization source
A 1.4 jilo-volt voltage at 25 MHz is applied to the rf electrode.
For 1 kV on “the” top electrode, it is not enough to produce sufficient net electron impact ionization across the gap.
plasma is ignited in the cavity and extracted.
For 2 kV, it produce finite electron impact ionization in the gap.
Charges are extracted from the cavity and seed the avalanche in the gap
A blank region between ionization in pic on the right Is because high electron density
shield out the electric field so reduce the ionization.
MIN MAX
University of Michigan
Institute for Plasma Science & Engr.
Animation Slide-GIF
AVS_Oct2010_p16

18. University of Michigan Institute for Plasma Science & Engr.
CONCLUDING REMARKS
Two modes of operation:
Low extraction voltage: Little ionization in gap and charge is extracted from cavity.
High extraction voltage: Charge from cavity seeds avalanche in gap.
Peak currents collected at top electrode (2.5-25MHz) tend to increase with frequency.
More current pulses per rf cycle at lower frequency.
Average charge per pulse collected on top electrode is not a strong function of rf frequency ( MHz)
The current increases with increasing dielectric constant of insulator.
In summary:
2 modes of operation are investigated:
for the low extraction voltage, little ionization is in “the” gap and charge is extracted from the cavity.
for the high extraction voltage, charges from the cavity seed the avalanche in gap.
Second, the peak currents collected at “the” top electrode at 2.5 to 25 MHz tend to increase with frequency.
Third, more current pulses per rf cycle are observed at lower frequency
Moreover, average charge per pulse collected on top electrode is not a strong funciton of rf frequency.
Finally, the current increases with increasing dielectric constant of insulator.
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p17

20. University of Michigan Institute for Plasma Science & Engr.
MULTIPLE APERTURES
Multiple apertures enable customizing of extracted charge. The addressable nature of the apertures enable subtle differences in excitation rates.
Dielectric
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_181
AVS_2010_mark_geometry_whole.lay
1500*411
AVS_2010_mark_geometry_single.tif
600*501
Screen electrode
Dielectric
Discharge electrode
Dielectric
rf electrode
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p15

21. MULTIPLE APERTURES: IN PHASE EXCITATION
With simultaneous extraction of electron current, the collected flux can be controllably uniform provided plumes are below the “space charge” limit of affecting their neighbors.
0 V
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_181
AVS_2010_mark_ne.sty
Time #:80~160, = 40~80 ns, 600*533
AVS_2010_mark_pdpsim_flux1dint_FE_80ns.lay
600*533
-1950 V
-2000 V
DC:-2 kV
AC:1.4 kV, 25 MHz
 Integrated Electron Flux
[e] (1x1015 cm-3, 4 dec)
N2, 1 atm,
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p15

22. MULTIPLE APERTURES: DELAYED EXCITATION
Delaying the center aperture by ¼ cycle enables its propagation into a volume with slightly positive space charge remaining from neighbors – produces more extracted charge .
0 V
D:\UIGELS_D_JERRY\RESEARCH\Kushner_Lab\My_Project\HP\plt\case_182
AVS_2010_mark_ne.sty
Time #:80~160, = 40~80 ns, 600*533
AVS_2010_mark_pdpsim_flux1dint_FE_80ns.sty
600*533
-1950 V
-2000 V
DC:-2 kV
AC:1.4 kV, 25 MHz
 Integrated Electron Flux
10 ns delay
[e] (1x1015 cm-3, 4 dec)
N2, 1 atm,
University of Michigan
Institute for Plasma Science & Engr.
AVS_Oct2010_p15