Microhollow Cathode Discharge & Cathode Boundary Layer Discharge

Wei-Dong Zhu

Saint Peter’s College Jersey City, New Jersey, USA

Summer Institute on Complex PlasmasAugust 5, 2008, Hoboken, New Jersey

Outline

• Introduction: what leads us to small size?• Microhollow Cathode Discharge (MHCD)

-Two scaling laws- Excimer formation- Plasma Plume

• Cathode Boundary Layer Discharge:• Visible and Vacuum Ultra Violet Emissions• Self-organization• Expending the active area: parallel operation• Attempts to explain self-organization

• Summary

Types of plasmas•

High-temperature plasmas (HTP)

Ti

≈

Te

≥107

Ke.g., fusion plasmas

•

Low-temperature plasmas (LTP)Thermal LTP

Ti

≈

Te

≈

Tg

≤

2 x 104

Ke.g. arc plasma at normal pressure

Non-thermal LTPTi

≈

Tg

≈

300 KTi

<< Te

≤

105

Ke.g. low-pressure glow discharge

high-pressure non-thermal plasma

Hippler R./ Kersten H./ Schmidt M./ Schoenbach K. H.“Low Temperature Plasmas, Fundamentals, Technologies and Techniques”

(2001, 2008)

Low Pressure Non-Equilibrium Plasmas

-1mTorr to a few Torr

-

Semi-conductor industry has achieved great success using plasma processing (e.g. in the manufacture of microchips)

-

Essentially all plasmas used in semiconductor processing are low-pressure plasmas.

There are many applications where the vacuum enclosure required for a low-pressure plasma is an obstacle for its technological use

Air Pressure Non-Equilibrium Plasmas

Atmospheric pressure Non- Equilibrium Plasmas

No vacuum is involved at normal pressure, but ran into some new challenges such as glow to arc transition (GAT).

Arc Discharge: thermal plasma

-

Gas temperature can reach as high as 2x104 K- Low voltage drop at cathode- High cathode current density

The first Atmospheric Non- Equilibrium Plasmas: 1857

“Ozone Production in an Atmospheric-Pressure Dielectric Barrier Discharge” (1857)Werner V. Siemens

Atmospheric-Pressure

More to see in next talk by Dr. J. Lopez

Achieve Non-Equilibrium at Atmospheric PressureA few solutions to achieve stable, glow discharge at atmospheric

pressure

Transient plasmas: In atmospheric plasmas, for efficient gas heating at least 100-1000 collisions are necessary. Thus, if the plasma duration is shorter than 10-6

– 10-5

s, gas heating is limited. Of course, for practical purposes such plasma has to be operated in a repetitive mode, e.g., in trains of microsecond pulses with millisecond intervals.

Micro-plasmas: Gas heating occurs in the plasma volume, and the energy is carried away by thermal diffusion/convection to the outside. If the plasma has a small volume and a relatively large surface, gas heating is limited.

Dielectric Barrier Discharges: These plasmas are typically created between flat parallel metal plates, which are covered by a thin layer of dielectric or highly resistive material. The dielectric layer plays an important role

in suppressing the current: the cathode/anode layer is charged by incoming positive

ions/electrons, which reduces the electric field and hinders charge transport towards the electrode. DBD also has a large surface-to-volume ratio, which promotes diffusion losses and maintains a low gas temperature.

Reducing the Distance between Anode and Cathode

J.P. Boeuf, E. Marode, J. Phys. D 15, (1982), 2169

Reducing the electrode distance to the order of the cathode fall reduces the discharge mainly to cathode fall and negative glow where the electron energy distribution is highly non-thermal: beam electrons.

electron energy distributions that are strongly non-

Maxwellian

Micro Hollow Cathode Discharge(MHCD)

A HC discharge device consists of a cathode with hollow structure (hole, aperture, etc.) in it and an arbitrarily shaped anode.

Two scaling laws

1. Paschen Breakdown Law (p, d)The product of pressure (p) and the anode-cathode separation (d), “p·d”

and the break down voltage obeys the well-known Paschen breakdown law, which applies to all discharges and determines the required breakdown voltage for given values of p, d, and the operating gas.

Hollow Cathode (HC) Discharge

Normally running at low pressure

A HC discharge device consists of a cathode with hollow structure (hole, aperture, etc.) in it and an arbitrarily shaped anode.

Two scaling laws

2. White-Allis Law (p, D)

-It relates the discharge sustaining voltage V to the product of pressure (p) and the cathode opening dimension (D), “p·D”.

-

If the product p . D is between a lower and a higher limit (e.g. 0.1 to 10 Torr . cm), the discharge develops in stages, each with a distinctive I-V characteristics.

Hollow Cathode (HC) Discharge

Lower Limit:the mean free path for ionization cannot exceed the hole diameter.

Higher Limit:the distance between “opposite”

cathodes cannot exceed the combined lengths of the two cathode fall regions plus the glow region.

Micro Hollow Cathode Discharge (MHCD)

MHCD: Pushing HC towards atmospheric pressure.Sometimes referred to as high pressure HC.

Critical dimensions at atmospheric pressure:d: <500 μmD: 10 –

300 μm (assuming at room temperature)

Human Hair: 60 –

100 μmMost of the experimental studies are in rare gases and rare gas halide mixtures, with an increasing interest on atmospheric pressure air .

Sandwich Structure:Electrode –

Dielectric -

Electrode

Electrode Geometries, Materials, and Fabrication Techniques

Electrode Material: molybdenum, silver, stainless steel, nickel platinum, copper

Dielectric Material: mica, aluminum oxide wafer, silicon oxide film, even polymer in some cases

Fabrication Techniques: 1)Openings: drilled mechanically, milled ultrasonically, drilled with excimer laser, wet chemical etching2)Packaging: high temp glue, direct plasma spray, micro-fabrication

MHCD as UV radiation source –

Xenon as an example

•

High electron energy (E>8.4 eV) is required for the generation of excimer precursor: Xe (2P3/2

)6s.

XeXeXeXeXe +→++ *2

*

•

High pressure is needed to provide a favorable environ-

ment for three body processes to form excimers.

*XeeXe →+

2nd

excimer continuum at 172 nm

K.H. Schoenbach, A. El-Habachi, W. Shi and M. Ciocca, Plasma Sources Sci. Technol. 6 (1997) 468

Calculated I-V characteristic (solid squre) and peak gas temperature (cross) vs current. The dashed lines sid and siid are characteristics calculated for discharges sustained by electron emission from inside the hollow cathode or from the outer surface of the cathode, respectively.

J. P. Boeuf, L. C. Pitchford and K.H. Schoenbach, Appl. Phys. Lett. 86, 071501 (2005)

MHCD as UV radiation source –

Xenon as an example

Pressure dependence of the VUV emission spectrum of MHCD in xenon. V: 215 -

235 V and I: 3 -

3.3 mA

Measured and calculated efficiency of the xenon excimer radiation vs pressure. I: 3–3.5 mA

A. El-Habachi and K. H. Schoenbach, Appl. Phys. Lett., Vol. 73, No. 7, 17 August 1998

MgF2

window and vacuum is used to reduce the absorption of UV by air!!

Plasma ParametersElectron Temperature:

-

Evaluated via emission spectroscopy (line intensity) shows electron temperature ~ 1 eV in argon MHCD.

-

The fact that MHCDs are efficient source of excimer radiation suggest that large concentrations of high-energy electrons (in excess of the excitation energy of rare gas atoms) must be present.

Electron Density:-

Evaluated via Stark broadening and shift of argon lines at 801.699 and 800.838 nm, and the hydrogen Balmer-β

line at 486.1 nm-

Evaluated via a heterodyne infrared interferometry-

Electron density is in the range of 1013

– 1015

cm-3

depending on the mode of operation (pulsed or DC) and on operating current

Gas Temperature:-

Evaluated via optical emission spectroscopy (by fitting experimental data of the rotational (0,0) band of the second positive N2

system to modeling data) or absorption spectroscopy (Doppler Broadening of argon lines )

-

ranging from 400 K to 2000 K (higher for molecular gas, lower for low atomic weight rare gas)

Plasma-Reactors (e.g., Dr. R. Besser, Dr. K.H. Schoenbach)

Other Applications of MHCD

C. Jiang, A.-A. H. Mohamed, R. H. Stark, J. H. Yuan, and K.l H. Schoenbach, IEEE TRANSACTIONS ON PLASMA SCIENCE, 33, 2005

Other Applications of MHCDPlasma display cell (e.g., Dr. G. Eden)

More to see in Dr. G. Eden’s talk

When a potential is placed across the electrodes and a gas is directed through the openings, the MHCD assembly is capable of generating a non-thermal, atmospheric pressure plasma micro jet.

Plasma micro jet

[A. Mohamed, et al., US patent application 20060028145]

The non-thermal plasma micro jet is usually in the dimensions of sub-

millimeter in diameter and less than 1 centimeter in length.

MHCD Plasma Plume

Metal

Insulator

5 k

100 - HV

Gas

Flo

w

MHCD Plasma Plume

•

J. K. Kolb, A.-A.H. Mohamed, R.O. Price, R. J. Swanson, A. Bowman, R.L. Chiavarini, M. Stacey and K.H. Schoenbach, Appl. Phys. Lett.

2008, in press•

H.Q. Feng, P. Sun, Y.F. Chai, G.H. Tong, J. Zhang, W. Zhu and J. Fang, IEEE Trans. Plasma Sci., 2008, transcript in revision

Direct CurrentGas: Air, Oxygen, Nitrogen, Noble GasFlow Rate: 0.3 –

3 slmPower: ~ 10 –

15 W

Opening diameter: ~0.8 mmLength of plume: ~ 1cm

A B C D E F-5

0

5

10

15

20

25

30

35

40

Surv

ival

Rat

e (%

)

Type of Bacteria

Treated Area Untreated Area

100 - HV

1.0 cm

Ø 9.0 cm

Petri Dish

2.0 cm

2.0 cm

E. Coli

Bacteria

A Escherichia coli

B Staphylococcus aureus

C Micrococcus luteus

D Bacillus megaterium

E Bacillus subtilis

F Bacillus natto

Plasma Plume Treatment of Bacteria

More to see in Dr. K. Becker’s Talk

Plasma-Liquid Interaction

Gas

Liquid

The plasma activated species (in the case of air: O, O3

, NOx

(very small amount), OH…) is directly injected into the liquid.

Comparing to other Plasma- Liquid Interaction Techniques

Spark Discharge Corona discharge Gliding arc discharge

H2

O2

production is used as an indication of the oxidative ability

Pictures courtesy of Nachiket Vaze (Drexel University)

Discharge Voltage (kV) Power (W) H2

O2 (g kW−1h−1) References

Gliding arc 12 250,500 0.0652, torch mode

Burlica R0.544, normal mode

Pulsed 40 30 < 100 Hz 0.843

Anpilov,

A.

M.25 200 < 100 Hz 0.905

Pulsed 46 22.8, 60 Hz 3.64 Grymonpr´e D R

Pulsed streamer corona 40 140, 60 Hz 0.137 Joshi A A

Glow 1.55 90 4 Ivannikov A AGlow 0.5–0.8 10–160 — Hickling AContact glow 0.4–0.5 1.6–3.6 — Sengupta S KGlow 3 30 5.44 Titova U VDC diaphragm 2.8 456 0.74 Stara

Z

Pulsed streamer corona 25–40 50 Hz — Sharma A K

Capillary 1 mm 3.01 76.7 4.1 A Yu Nikiforov

Capillary 5 mm 5.4 18.3 7.8 Direct Injection of PAS 0.4 -

0.5 8 ~ 40 This work

[Table partially from “A Yu Nikiforov and Ch Leys 2006 Plasma Sources Sci. Technol.

16 (2007) 273–280”]

Hydrogen Peroxide Production

Degradation of Rhodamine WT

200 ml of de-ionized water contaminated with Rhodamine WT (pink dye) (200 ppb). Rhodamine WT was reduced from 200 ppb to ~80 ppb in 1

hour.

Cathode Boundary Layer Discharge (CBLD)

Cathode Boundary Layer (CBL) Discharges

Materials:Electrodes: MolybdenumDielectric: Alumina

Dimensions:Electrode Thickness: 100 μm to 250 μmDielectric Thickness: 100 μm to 250 μm Opening Diameter: 300 μm to 4.5 mm

CBL discharges are DC driven and have:

High excimer

intensity (~ 1 W/cm2)

Efficiency of 3-7%

Low operating voltage

(<400 V)

Simple structure

and 2-D flat panel feasibility

K.H. Schoenbach, M. Moselhy, and W. Shi, Plasma Sources Scie. Technol. 13, 177 (2004)

Experimental Setup

•

Gas: Xenon (Scientific grade)

•

Spectral filter: 170 nm with FWHM of 26.8 nm

•

Sample: Either mechanically assembled at Old Dominion University or plasma sprayed at University of Minnesota

PRINT

HELP

ALPHA

SHIFT

ENTERRUN

DG ER FI

AJ BK CL

7M 8N 9O

DG DG DG

DG T 3U

0V .WX Y Z

TAB

% UTILIZATION

HUB/MAU NIC

2BNC4Mb/s

Self-organization in the visible

2.32 0.42 0.12 0.1

3.9 1.1 0.67 0.49

6.7 2.7 1.75 0.64

7.6 5.1 3.4 1.9

75

200

400

760

Pres

sure

(Tor

r)

Current (mA)

2.32 0.42 0.12 0.1

3.9 1.1 0.67 0.49

6.7 2.7 1.75 0.64

7.6 5.1 3.4 1.9

75

200

400

760

Pres

sure

(Tor

r)

Current (mA)

visible

Self-organization in VUV

1 0.4 0.13 0.09

4 1 0.62 0.53

7 2.5 1.6 0.42

7.6 5 3.4 1.7

75

200

400

760

Pres

sure

(Tor

r)

Current (mA)

1 0.4 0.13 0.09

4 1 0.62 0.53

7 2.5 1.6 0.42

7.6 5 3.4 1.7

75

200

400

760

Pres

sure

(Tor

r)

Current (mA)

VUV (172nm)

current: 0.37 mACurrent density: 85 mA/cm2

75Torr xenond = 290μm

Cathode Boundary Layer Discharge between Molybdenum Cathode and Indium Tin Oxide

(transparent) Anode

cathode

anode (with Indium Tin Oxide film on glass slide)

750μm

290μm

direction of observation

75 Torr xenon; cathode diameter: 750μm

Reducing the Current Leads to Self- organization

0.165 mA 0.090 mA

0.036 0.052 mA0.064 mA

0.136 mA

0.108 mA0.146 mA0.373 mA

mA

Single Filament

0.60 mA 0.09 mA

0.07 mA 0.05 mA

50 Torr

0.60 mA 0.09 mA

0.07 mA 0.05 mA

50 Torr

with reduced pressure or cathode diameter

0.041mA

0.20mA 0.085mA

0.062mA

, D = 750μm 62Torr , D = 300μm

0.0 0.2 0.4 0.6 0.8 1.0 1.2

270

300

330

360

390

420

450

Vol

tage

[V]

Current [mA]

75 Torr

I-V Characteristics (close-up)

Instability at transition from abnormal to (sub)normal discharge leads to formation of filaments.

1

2

3

4

5

6

75 Torr XenonD = 750μmd = 250μm

1

2

34

56

7

7

unstable

Information on Cathode Fall Voltage and Secondary Emission Coefficient from

Measurements of Cathode Fall Length

camera

Schematic diagram of setup

camera

quartzTungsten

camera

Schematic diagram of setup

camera

quartzTungsten

camera

Schematic diagram of setup

camera

quartzTungsten

I = 0.092 mAI = 0.179 mAI = 0.224 mA

Cathode Fall Thickness

Measured normal cathode fall thickness depending on pressure

The observed cathode fall thicknesses, as a function of current I

. The self-organization is exhibited when I

< 0.2 mA

γ

(secondary emission coefficient) is calculated using information on the thickness of the cathode for normal glow operation:

Apdn

)/11ln(76.3 γ+=

Secondary Emission Coefficient

40 60 80 100 120 1400

50

100

150

200

γ=0.01

γ=0.03

γ=0.07

γ=0.2

Calculated Dataγ=0.03

Experimental Data

Thic

knes

s of

cat

hode

fall

[ μm

]

Pressure [Torr]

0.1 1

270

300

330

360

390

420

450

Vol

tage

[V]

I [mA]0.1 1

270

300

330

360

390

420

450

Vol

tage

[V]

I [mA]

Normal glow

↓

γ

= 0.03

A is the inverse of the product of pressure and mean free path for ionization. For xenon, A = 26 (cm Torr)−1

UV-Optical Power and Efficiency decreases when the discharge transits into the abnormal glow mode

Current (mA)0.1 1 10

Pow

er (m

W)

0

5

10

15

20

25

75 Torr200 Torr400 Torr760 Torr

Xenon

Current (mA)0.1 1 10

Effi

cein

cy (%

)

0

1

2

3

4

5

6

75 Torr200 Torr400 Torr760 Torr

Xenon

Moselhy M and Schoenbach K H, 2004 J. Appl. Phys. 95 1642-49

Rate coefficients are taken from D. Eckstrom et.al., J. Appl. Phys. 64, 15 (1988).

Calculation of Excimer

Density

Xe2* Xe*

Xe+

Xe

electronic collisionheavy particle collisionradiation

**

* 2)(XeXe

Xe nNKnnKKnNKdt

dngasexcimeredestepegasex ××−××+−××=

exderadgasexcimer nnKnKnNKdt

dnXeXeXe

Xe ××−×−××= − *2

*2

*

2*

2

{ }2)()(

3

*2

gasexcimerstepdexdeerad

edegas

NKKKnKnKnKNKn

e

excimer

Xe ×++××+×××

=−

1014 1015 10160

1x1014

2x1014

3x1014

4x1014

5x1014

6x1014

Exci

mer

den

sity

[1/c

m3 ]

Electron density [1/cm3]

75 Torr, 400K 150 Torr, 600K 250 Torr, 800K

Higher Pressure Is Preferable for Excimer

Formation

Excessive electrons (slow electrons) inhibit excimer formation.

Current (mA)0.1 1 10

Pow

er (m

W)

0

5

10

15

20

25

75 Torr200 Torr400 Torr760 Torr

Xenon

Optimization of the operating point

Visible Pictures

VUV Pictures (172 nm)

•

The peak of the excimer

emission coincides with the turning of the I-V curve, which is at a current of about 0.8 mA

•

In order to operate multi-holes in parallel, the I-V characteristic has to be positive. Around 1 mA/hole seems to be an optimum region.

0.4 0.8 1.2 1.6 2.0220

240

260

280

300

320

0.0

0.5

1.0

1.5

2.0

2.5

Voltage

Volta

ge (V

)

Current (mA)

Excimer Emission

Exc

imer

Em

issi

on In

tens

ity (

W/c

m2 )

250 Torr

Excimer

Emission and Discharge Voltage over an Extended Time

0 5 10 15 20 25

230

240

250

260

270

280

1500

2000

2500

3000

3500

4000

Volta

ge (V

)

Time (hrs)

VoltageRefill

Excimer Emission

Exc

imer

Em

issi

on In

tens

ity (A

.U.) •

The excimer

emission intensity decays to about 50% in 20 hours.

•

The decrease in emission is always correlated to an increase in discharge voltage.

•

Exchanging the gas in the chamber returns the excimer

emission to almost its initial value.

•

Impurity due to leaking or outgassing

from the components in the chamber

•

Sealing up the device

0 4 8 12 16 20800

1000

1200

1400

1600

1800

2000

2200

OH

Emis

sion

Inte

nsity

(A.U

.)

Time (hrs)

Refill

Scaling

Parallel operation of multiple openings –

Multi-CBL structure

Maintain the sandwich structure and scale up in one direction –

Micro-slit structure

Parallel operation without individual ballasting

• Cathode: Mo ~0.25 mm thick

• Dielectric: Al2

O3

~0.25 mm thick

• Anode: Mo ~0.25 mm thick

• Hole diameter: ~0.75 mm

• Center to center distance: ~1.5 mm

Visible Picture of parallel operation of 9 holes

(Operating gas: xenon (scientific grade)

Base pressure: ~1 mTorr; Working pressure: 200 Torr

Cathode voltage: -398 V; Discharge current: 6 mA)

Without individual ballasting or other initial ignition assistance, the multi-hole samples tend to be more easily ignited and sustained at lower pressure (<~200 Torr)

Parallel operation without individual ballasting

0 1 2 3 4 5 6 7

380

400

420

440

460

480 Increasing

Volta

ge (-

V)

Current (mA)

Decreasing

Current-Voltage characteristics of an eight circular-opening CBL discharge (visible images of the discharge are embedded at corresponding currents indicated by arrows).

Subsequent individual discharges occurred at voltages much lower than that which was required to ignite the discharge in the first opening:

-

pre-ionization of the gas in the surrounding openings by the VUV light emitted from previously ignited opening(s) and/or possibly the diffusion of long-living positive and negative ions towards neighboring opening(s)

VUV emission- Only partial image (limited by the magnification of the system)- Low excimer emission (due to the lower pressure: 100 Torr)- At higher pressures (>150 Torr), stable operation with discharges in all openings is not easily achievable. - Also noticeable is that the excimer emissions from different openings differ in intensity, which may have been caused by the difference in shape and diameter between individual openings due to the mechanical drilling process.

Partial VUV image of an 8 circular-opening CBL discharge working in parallel

(100 Torr, 446 V and 5.07 mA)

Self-organization

Xenon (100 Torr) Xenon (250 Torr)

(ignition assisted with mechanical switch)

Parallel operation of self-organization can be achieved by tuning down current steadily after all holes are ignited

Operation of Micro-Slit CBL Structure

Visible picture of a Micro-slit discharge

(Working pressure: 250 Torr

(Xenon)

Cathode Voltage: 224 V

Discharge current: 9.3mA)

Slit width: ~ 0.2 mm

Slit length: ~ 1.3 cm

Dielectric thickness: ~ 0.1 mm

More on Self Organization

Self-organization (Visible images) of a micro- slit CBL discharge: (a)50 Torr; (b)150 Torr; (c)245 Torr; (d)354 Torr and (e)homogeneous discharge at 100 Torr (249V and 4 mA)(The images are at different magnification for a better demonstration purpose)

(a)

(b)

(c)

(d)

(e)

Current-Voltage characteristics of the Discharge from a Micro-Slit CBL Structure

Region I:

Plasma trying to fill the micro-slit

Region II:

Slit filled with plasma, self-organization patterns persist

Region III:

Evenly distributed plasma

The best operating condition to achieve high excimer

emission intensity is at the current indicated by the lower boundary (between region I and II).

230

280

330

380

430

0 2 4 6 8 10 12

Current (mA)

Vol

tage

(-V

)

50 Torr 75 Torr100 Torr 125 Torr150 Torr 175 Torr200 Torr 225 Torr250 Torr 300 TorrIII

II

I

Explanation of self-organizationN Takano and K H Schoenbach: Coulomb Forces[Plasma Sources Sci. Technol. 15 (2006) S109–S117] Coulomb forces are assumed to be the reason for self-organization. The cathode fall is determined by a large positive space charge. Any filament developing in this region will therefore extend repulsive forces to neighbouring

cathode fall filaments. These repulsive forces are assumed to be balanced by electrostatic forces from positive surface charges at the surrounding dielectric spacer.

Hamiltonian (the sum of the electrostatic energies of the filaments) of a system which consists of n filaments

The potential ϕι

of a filament at a distance ri

from the centre is

Explanation of self-organization

M. S. Benilov: Bifurcation analysis-

International Conference on Plasma Science (ICOPS), Travers City, Michigan, June, 2006; -

Physical Review E 77, 036408 2008

Summary•

Stable (dc), high pressure glow discharges. Structure reduced to cathode fall, negative glow, and anode fall

•

CBL discharges in xenon show excimer

emission intensity up to 2 W/cm2

with internal efficiency up to 7%.•

Excimer

emission peaks at the transition from normal

glow to self-organization.•

Excimer

emission intensity decreases to about 50% in

20 hours without refilling and returns almost to the initial value after exchanging the gas.

•

Single opening CBL discharge can be expanded to multi-CBL structure without individual ballasting or to micro-slit CBL structure.

•

Self-organization patterns were observed in both micro- CBL discharge and micro-slit CBL discharge.

Acknowledgement

•

Dr. K.H. Schoenbach•

Dr. K. Becker•

Dr. J. Lopez•

Dr. J. Kolb•

Dr. J. Heberlein•

Dr. J. Zhang•

Dr. R. Besser•

Dr. N. Takano•

Mr. R. Price•

Mr. P. Sun•

Ms. H. Feng