SANapl03.pdf

Argon excimer emission from high-pressure microdischarges

in metal capillaries

R. Mohan Sankaran and Konstantinos P. Giapis

Division of Chemistry and Chemical Engineering, California Institute of Technology,

Pasadena, California 91125

Mohamed Moselhy and Karl H. Schoenbach

Physical Electronics Research Institute, Department of Electrical and Computer Engineering, Old Dominion

University, Norfolk, Virginia 23529

Received 25 August 2003; accepted 13 October 2003

We report on argon excimer emission from high-pressure microdischarges formed inside metal

capillaries with or without gas flow. Excimer emission intensity from a single tube increases linearly

with gas pressure between 400 and 1000 Torr. Higher discharge current also results in initial

intensity gains until gas heating causes saturation or intensity drop. Argon flow through the

discharge intensifies emission perhaps by gas cooling. Emission intensity was found to be additive

in prealigned dual microdischarges, suggesting that an array of microdischarges could produce a

high-intensity excimer source. ©

2003 American Institute of Physics.

DOI: 10.1063/1.1632034

Microhollow cathode discharges

MHCDs

are high-

pressure microdischarges formed in holes

inside diameter

i.d.

100

drilled in thin metal-dielectric-metal films.

These discharges can operate stably at pressures exceeding 1

atm while sustaining a large concentration of high-energy

electrons.

These two characteristics make microdischarges

attractive as a source of excimer radiation which requires

three-body collisions of excited atomic states. Indeed, exci-

mer emission has been observed in microdischarges of Ne,

Ar and Xe,

ArF,

XeCl,

and XeI.

Limited enhancement in

excimer intensity from single MHCDs has been achieved

increasing gas pressure. Larger gains in emission intensity

could be obtained more readily by increasing the total

plasma volume, a difficult task with these inherently planar

devices. Significant gains in excimer intensity could enable

fabrication of cw excimer microlasers.

6,8

Since high-energy electrons in MHCDs are generated in

the cathode fall, a thicker cathode electrode with a deeper

hole should automatically provide larger cathode area. Metal

capillaries offer an easy solution to fabricating such a device

with more internal

cathode

area for plasma expansion. Mi-

crodischarges in capillaries can still be operated in the hol-

low cathode mode and have been used as microreactors for

materials processing.

We report here results on their perfor-

mance as sources of excimer radiation.

The experimental set-up for striking microdischarges in

capillary tubes bears similarities with that of MHCDs, as

shown in Fig. 1. The electrodes consist of a stainless steel

tube

5 cm long, i.d.

178

and a stainless steel grid,

operated as the cathode and anode, respectively. A sapphire

washer

i.d.

203

separates the two electrodes by 381

m. The cathode tube is fitted into a water-cooled copper

tube

not shown

. Discharges were operated using a positive

dc power supply; breakdown voltages were less than 2 kV

for all conditions studied. The plasma voltage was monitored

using a digital oscilloscope and the current was measured

with a resistor in series with the discharge. Spectral measure-

ments in the vacuum-ultraviolet

VUV

were performed us-

ing a 0.2 m McPherson scanning monochromator

Model

302

, with a grating of 600 grooves/mm blazed at 150 nm.

The setup was placed in a vacuum chamber and aligned with

the inlet of the vacuum monochromator ( MgF

window

that emission spectra could be collected through the grid

anode. Before each experiment, the discharge chamber was

evacuated using a turbomolecular pump, then back-filled

with high-purity argon to the desired pressure. Gas was sup-

plied via a mass flow controller either through a side-port in

the vacuum chamber

cross-flow

or through the cathode

tube

back-flow

Emission spectra for argon discharges in a cross-flow of

gas are shown in Fig. 2 as a function of ambient pressure. At

high pressures

400 Torr

, the spectra are dominated by the

argon excimer emission continuum peaking near 128 nm. As

the pressure is raised from 400 to 1000 Torr, the intensity of

the excimer emission increases. Also evident in the spectra

are lines corresponding to atomic oxygen at approximately

130.5 nm and atomic carbon at 156, 160, and 165 nm. These

lines are attributed to impurities in the microdischarge from

outgassing of materials used in the device set-up.

Increas-

ing the gas cross-flow did not reduce the contamination. The

excimer intensity was also found to increase with discharge

current from 2 to 10 mA; beyond this range excessive tube

FIG. 1. Schematic of the microdischarge setup

not to scale

. The device

consists of a stainless steel capillary tube

cathode

, a metal grid

anode

and a sapphire spacer washer. VUV spectra were collected from the anode

side. Connection to the dc power supply was through a current limiting

resistor (

100 k

APPLIED PHYSICS LETTERS

VOLUME 83, NUMBER 23

8 DECEMBER 2003

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heating ensued

Fig. 3

. At 1000 Torr, the optical power

which was obtained by integrating the spectra from 115 to

155 nm, increased with current up to 6 mA and then satu-

rated

Fig. 3

. At lower pressures, a maximum was reached at

smaller discharge currents followed by a drop in emission,

which is attributed to heating ensued.

Cooling of the discharge should be more efficient when

gas is flown through the cathode tube

back-flow

. Figure 4

shows emission spectra obtained from an argon microdis-

charge as a function of the flow rate. The discharge was

operated at an ambient pressure of 1000 Torr and a current of

4 mA. More intense excimer emission was observed at larger

flow rates. Unlike the emission spectra for cross-flow of the

gas

Fig. 2

, lines from oxygen and carbon contamination

were indiscernible even for the lowest flow rate. The absence

of emission lines from contaminants is attributed to reduced

outgassing due to more efficient cooling. The concomitant

increase in the intensity of the excimer emission continuum

with flow rate corroborates the role of gas heating and its

adverse effect on excimer emission.

Our experiments in single capillary microdischarges sug-

gest that gains in excimer emission intensity are limited de-

spite the dependence on discharge current, gas pressure and

flow rate. The positively sloped current-voltage dependence

Fig. 3

indicates operation in the abnormal glow discharge

mode. Although the tube geometry provides abundant cath-

ode surface for the plasma to expand, the desired higher

discharge current must still pass axially through the tiny tube

cross-section. We speculate that the ensuing intense heating

the tube tip can turn red-hot

enhances electron emission at

the tube end thus limiting plasma expansion.

An alternative scheme for intensifying the on-axis exci-

mer emission while avoiding heating is to operate aligned

arrays of identical microdischarges. The same voltage drop

across each tube would sustain a fixed current through the

array. The current would be adjusted for maximum excimer

emission while minimizing heating problems for a single mi-

crodischarge. We tested this idea by introducing a tube seg-

ment

5 mm long

between the cathode and the anode of a

single microdischarge

see inset, Fig. 5

. The supply voltage

was increased to permit two discharges to form: two identi-

cal plasma potentials must be maintained. Since the current

was kept low, tip heating was not excessive and the plasma

expansion was not impeded. The total discharge volume was

thus doubled. To determine the difference in excimer emis-

sion between a single-tube and dual-tube setup, the two dis-

charges were operated sequentially. First, a single discharge

was formed in tube 1

grid is anode

; second, a single dis-

charge was formed in tube 2

tube 1 is anode

; finally, both

discharges were formed simultaneously

grid is anode, tube 1

is floated

. In Fig. 5 we compare spectra for the three cases,

collected through a 0.2

10 mm slit located 6 mm away; the

ambient pressure and discharge current were 1000 Torr and 4

mA, respectively. We find that discharge 1 is about five times

more intense than discharge 2. Assuming that these dis-

charges are an assembly of identical point sources extending

FIG. 2. VUV emission spectra of argon microdischarges in a cathode tube

i.d.

178

as a function of the ambient gas pressure. The discharge

current was held constant at 4 mA. OI and CI refer to lines corresponding to

atomic oxygen and carbon states

impurities

FIG. 3. VUV optical power and discharge voltage as a function of current

for a single microdischarge in argon at a pressure of 1000 Torr. Gas was

supplied in a cross-flow configuration.

FIG. 4. Argon excimer emission spectra of microdischarges with gas flown

through the cathode tube at the indicated flow rates. Ambient gas pressure

and discharge current were kept constant at 1000 Torr and 4 mA, respec-

tively.

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Appl. Phys. Lett., Vol. 83, No. 23, 8 December 2003

Sankaran

et al.

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2–3 mm into each tube, geometric considerations suggest

that the intensity difference should be a factor of three. Since

the front discharge

closer to the monochromator

can be

viewed off axis, we expect more light to be collected from

discharge 1 through the rectangular slit. The excimer inten-

sity of the dual discharge setup is approximately equal to the

sum of the intensities from the individual discharges for

pressures between 600 and 1350 Torr

not shown

Excimer emission has been studied in high-pressure ar-

gon microdischarges formed in metal capillary tubes. Emis-

sion intensity was found to increase with discharge current,

ambient pressure, and argon flow rate through the discharge.

However, the intensity gains were limited by gas heating.

The additivity of excimer intensity from prealigned dual

microdischarges—operated simultaneously at a current such

that the intensity of a single discharge is maximum—

suggests a scheme for fabricating a source of intense excimer

radiation that could lead to a dc excimer microlaser.

We gratefully acknowledge partial support of this work

by NSF

CTS-0317397

K. H. Schoenbach, A. El-Habachi, W. Shi, and M. Ciocca, Plasma Sources

Sci. Technol.

, 468

1997

J. W. Frame, D. J. Wheeler, T. A. DeTemple, and J. G. Eden, Appl. Phys.

Lett.

, 1165

1997

P. Kurunczi, H. Shah, and K. Becker, J. Phys. B

, L651

1999

A. El-Habachi and K. H. Schoenbach, Appl. Phys. Lett.

,22

1998

K. H. Schoenbach, A. El-Habachi, M. Moselhy, W. Shi, and R. H. Stark,

Phys. Plasmas

, 2186

2000

A. El-Habachi, W. Shi, M. Moselhy, R. H. Stark, and K. H. Schoenbach,

J. Appl. Phys.

, 3220

2000

J. W. Frame, P. C. John, T. A. DeTemple, and J. G. Eden, Appl. Phys. Lett.

, 2634

1998

B. A. Vojak, S.-J. Park, C. J. Wagner, and J. G. Eden, Appl. Phys. Lett.

1340

2001

R. M. Sankaran and K. P. Giapis, J. Appl. Phys.

,2406

2002

M. Moselhy, R. H. Stark, K. H. Schoenbach, and U. Kogelschatz, Appl.

Phys. Lett.

, 880

2001

J. D. Ametepe, J. Diggs, D. M. Manos, and J. M. Kelley, J. Appl. Phys.

7505

1999

FIG. 5. Argon excimer emission spectra from single-tube and dual-tube

microdischarges operated in a sequence described in the text. The inset

illustrates the setup schematic. The single-tube discharge was operated at a

current of 4 mA and voltage of 210 V; simultaneous operation of the dual-

tube was obtained at a current of 4 mA and voltage of 420 V.

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Appl. Phys. Lett., Vol. 83, No. 23, 8 December 2003

Sankaran

et al.

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