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Diagnostics of microdischarge-integrated plasma sources for display and materials processing

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2005 Plasma Phys. Control. Fusion 47 A167

(http://iopscience.iop.org/0741-3335/47/5A/012)

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INSTITUTE OF PHYSICS PUBLISHING PLASMA PHYSICS AND CONTROLLED FUSION

Plasma Phys. Control. Fusion 47 (2005) A167–A177 doi:10.1088/0741-3335/47/5A/012

Diagnostics of microdischarge-integrated plasmasources for display and materials processing

K Tachibana, Y Kishimoto, S Kawai, T Sakaguchi and O Sakai

Department of Electronic Science and Engineering, Kyoto University, Kyoto-daigaku Katsura,Nishikyo-ku, Kyoto 615-8510, Japan

Received 29 October 2004Published 14 April 2005Online at stacks.iop.org/PPCF/47/A167

AbstractTwo different types of microdischarge-integrated plasma sources havebeen operated at around the atmospheric pressure range. The dischargecharacteristics were diagnosed by optical emission spectroscopy (OES), laserabsorption spectroscopy (LAS) and microwave transmission (MT) techniques.The dynamic spatiotemporal behaviour of excited atoms was analysed usingOES and LAS and the temporal behaviour of the electron density was estimatedusing the MT method. In Ar and Xe/Ne gases, waveforms of the MT signalfollowed the current waveform in the rise period and lasted longer according tothe recombination losses. However, in He the waveform followed the densityof metastable atoms, reflecting the production of a large amount of electronsby the Penning ionization process with impurities. The estimated peak electrondensity in those plasma sources is of the order of 1012 cm−3, and the metastableatom density can reach 1013 cm−3. Thus, it is suggested that these sourcescan be potentially applied to convenient material processing tools of large areaoperated stably at atmospheric pressure.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Technologies to generate large area plasmas at around the atmospheric pressure range are ofmuch interest for various materials processing applications, such as surface treatments andcoatings [1–3]. Two types of such methods exist, although in both cases dielectric barrierdischarge (DBD) schemes are employed. In one of them, two plane electrodes covered byinsulating material are placed in parallel facing each other, with a typical gas gap of a few mm(parallel-plate configuration). However, careful tuning of the driving frequency and the voltagewaveform of the power source is required in order to obtain a uniform discharge over a wideelectrode area [4, 5]. The other approach is to use an integrated structure of microdischarges,which has the potential to be operated in a wider range of external parameters, such as powersources, gas conditions and so on. We have been investigating the second method over the lastfew years [6].

0741-3335/05/SA0167+11$30.00 © 2005 IOP Publishing Ltd Printed in the UK A167

A168 K Tachibana et al

720

µm

Figure 1. (a) Structure of unit cell of a counter-electrode type microdischarge-integrated panel and(b) waveforms of driving pulse trains.

In this paper, we describe two different types of microdischarge-integrated plasma sources,which can be applied to large area materials processing tools as well as flat panel displays.For such applications, however, it is important to characterize the generated plasmas inorder to optimize the functions depending on the respective purposes. Although there aredifficulties in making appropriate diagnostics due to the smallness of the target space, wehave been performing a series of measurements on the excited Xe atoms in a unit cell ofplasma display panels (PDPs) by using spectroscopic microscopy techniques [7–9]. Here, wealso apply this method for the measurement of He(2 3S1) metastable atoms. As for plasmaparameters, there are reported results of Thomson scattering measurements applied to thediagnostics of PDP-like plasmas [10]. However, we try an alternative, easier method, whichuses the transmittance characteristics of microwaves in the range from 50 to 75 GHz for acrude estimation of electron densities, since fairly high densities are expected in such types ofmicrodischarges.

By using both results, a comparison of the characteristics of our plasma sourceof the microdischarge-integrated type with the conventional parallel-plate source will begiven. Another interesting issue, concerning the possibilities of using these sources in newapplications to electromagnetic-wave controlling devices in the mm to sub-mm wavelengthranges will also be mentioned.

2. Experimental apparatus and procedures

2.1. Device structures and driving schemes

Figure 1(a) shows the structure of our test panel with a set of parallel electrodes buried in theceramic ribs. The electrical connection of these electrodes is taken alternatively at both ends,so that the two electrodes face each other across the gas gap in a discharge cell. This structurehas been realized by a method called the thick-film ceramic sheets (TFCS) technique [11],

Diagnostics of microdischarge-integrated plasma sources A169

Figure 2. (a) Cross-sectional view of a mesh-type microdischarge-integrated panel together withthe driving voltage waveform and (b) an example of a discharge image operated in N2 at atmosphericpressure.

where thick ceramic sheets are stacked layer by layer. The exposed surfaces of the electrodesare covered by dielectric material for the discharge operation in a DBD scheme. The unit cellis 400 µm in width (effective electrode gap), 350 µm in depth and 720 µm in length. Thedevelopment of this panel was originally intended for the improvement of luminous efficiencyin PDPs by using a new cell structure with a different electrode configuration and larger celldepth. But it can also be applied easily to the plasma source for materials processing since agas flow through the electrode assembly is possible with this structure. In this study, however,we only suggest the possibility and use a panel constructed as a PDP for its characterization;it has a mesh structured TFCS electrode assembly sandwiched by front and rear glass platesand shielded around with a glass flit. A gas mixture of Ne and Xe (10%) was filled at a totalpressure of 450 Torr. The driving waveform is shown in figure 1(b). A positive pulse trainwith a fixed width of 10 µs is applied to one of the paired electrodes with a period of 24 µsand another pulse train delayed by 180˚ was applied to the other. The pulse amplitude Vs wasfixed at a typical value of 270 V within the variable range from 240 to 300 V.

Figure 2(a) shows the other type of electrode structure. In this case, a metal-mesh sheet250 µm in thickness was coated with a ceramic (Al2O3) film of thickness 150 µm using aplasma spray technique. With a mesh opening of 500 × 2000 µm, the net aperture of the unithole became 200 × 1700 µm. Two insulated metal electrodes are stacked on top of each otherwith every opening aligned coaxially, so that the DBD occurs between the front and rear platesalong the inner surface of the openings. The driving waveform is also shown in the samefigure. In this case a bipolar pulse power supply was used, which provided a pair of positiveand negative pulses, Vp and −Vp, with a fixed width of 4 µs and an intermission time of 1 µs.The repetition rate was typically set at 20 kHz. The minimum firing voltage was about 500 Vand 1200 V in He and N2, respectively, at atmospheric pressure. Figure 2(b) shows an opticalimage observed in a discharge with N2 at atmospheric pressure.

A170 K Tachibana et al

2.2. Laser absorption measurement

The set-up and procedure for laser absorption spectroscopy (LAS) was the same as what wehave been using for the PDP diagnostics [7–9]. Briefly, the wavelength of a diode laser wastuned to the absorption line peak and the temporal behaviour of absorbance was observed bya digital oscilloscope utilizing the average function. We used the (2 3PJ –2 3S1) transition at1082 nm for the measurement of He(2 3S1) metastable atoms [12], where the two fine structurecomponents with J = 1 and 2 were overlapped due to the very large pressure broadeningeffect. Anyway, the absolute density of He(2 3S1) atoms can be derived from the integratedarea of the absorption coefficient over the line profile [7]. For the measurement of Xe(1s5)metastable atoms (in Paschen notation), we used the (2p6–1s5) transition at 823 nm. In thiscase, the complicated hyperfine components were broadened and overlapped completely, butthe procedure was the same as that given above for the absolute density derivation.

2.3. Microwave transmission measurement

The light source for the microwave transmission (MT) measurements was a mm-wave sourcemodule driven by a synthesized swept signal generator, which provided the output frequencyfrom 50 to 75 GHz. The emitted microwaves through a pyramidal horn antenna were ledthrough the electrode assembly (plasma source) and the transmitted power signal was collectedby another horn antenna and detected by a Schottky diode detector, which had a nominalbandwidth of 10 MHz. The repetitive signal was averaged by a digital oscilloscope to obtainthe temporal transmittance signal. Due to the finite sizes of the opening in both typesof plasma sources, the transmittance without plasma is affected by the orientation of theelectrode assemblies. In order to obtain a larger intensity level, we placed each panel suchthat the direction of its larger opening length was perpendicular to the electric vector of theTE01 mode of the emitted microwaves. In order to see the effect of diffraction through holesof sizes smaller than the wavelength of microwaves, the distance of the detecting horn antennafrom the plasma source was changed from 3 to 20 cm in the measurement of the PDP typepanel, but no significant difference was observed in the characteristic feature.

3. Experimental results and discussion

3.1. PDP type structure with counter electrodes

The discharge behaviour, as recorded by a CCD camera with a gate width of 10 ns, is shownin figure 3(a), in which the optical emission from excited Xe atoms (mostly from 2p levels) inthe near-IR range was observed. It is seen that the discharge starts from the anode side, andthen expands towards the cathode side forming a relatively large negative glow region, whilea narrow anode glow is formed on the opposite side. The emission follows approximatelythe current waveform of about 100 ns in half-width. The spatiotemporal behaviour of Xe(1s5)metastable atoms measured by LAS is shown in figure 3(b). The characteristics of the spatialbehaviour are similar to the emission images, but as far as the temporal behaviour is concerned,the metastable atoms remain much longer since the lifetime of Xe(1s5) atoms is determinedby the relatively slow rate of three-body processes to form Xe∗

2 excimers; Xe(1s5) + 2Xe orXe + Ne → Xe∗

2 + Xe or Ne. The peak density reaches 3 × 1013 cm−3 after the current isterminated.

The characteristic feature of the MT signal is shown in figure 4 together with the currentwaveform and the total number of Xe(1s5) atoms in a cell. It is seen that the intensityof microwaves was attenuated synchronously by the discharge, suggesting the effects of

Diagnostics of microdischarge-integrated plasma sources A171

215 ns

205 ns

210 ns

220 ns

225 ns

180 ns

160 ns(a)

170 ns

190 ns

200 ns

250 ns

230 ns

240 ns

260 ns

280 ns

Anode

Cathode

Figure 3. (a) Spatiotemporal behaviour of near-IR emission observed by a gated CCD camera ina unit discharge cell of counter-electrode type panel filled with Ne–Xe (10%) gas at 450 Torr ina half-cycle when the upper side is working as the anode and the lower side as the cathode, and(b) corresponding data of Xe(1s5) metastable atom density measured by LAS.

generated plasma electrons. (Arguments for the mechanism will be given later.) The riseof the attenuation signal follows the current waveform but the tail lasts much longer thanthe current. This implies that the lifetime of electrons governed by the volumetric and surfacerecombination processes is longer than the current fall time.

3.2. Metal-mesh electrode structure

Since the electrode assembly is naked in contrast to the PDP structure, it was placed in a vacuumchamber under the flow of He or Ar gas up to 3 liter min−1 (SLM). The pressure in the chamber

A172 K Tachibana et al

(b)

Figure 3. (Continued).

was kept at atmospheric pressure. The discharge behaviour in He observed by the CCD camerais shown in figure 5 as a temporal sequence. The discharge tends to fill the volume withinthe coaxial hole. The density of He(2 3S1) metastable atoms measured by the LAS methodwas of the order of 1012 cm−3, but it was influenced drastically by the change in gas flow rate.It suggests that the quenching rate by Penning ionization processes with impurities governsthe lifetime of the metastable atoms. Actually, in pure He gas, the lifetime is estimated tobe about 6 µs at atmospheric pressure from a literature value of 2.5 × 10−34 cm6 s−1 for thethree-body process: He(2 3S1) + 2He → He∗

2 + He [13]. Therefore, the measured value ofaround 2 µs implies an impurity level of the order of 100 ppm if we take a reported value of7 × 10−11 cm3 s−1 for the Penning reaction with N2 [14].

Diagnostics of microdischarge-integrated plasma sources A173

Figure 4. Waveforms of the discharge current, the total number of Xe(1s5) atoms in a unit celland the microwave attenuation signal measured in a counter-electrode type panel in a half-cycle ofdischarge.

Figure 5. Spatiotemporal discharge images in a unit coaxial hole of a mesh-type panel in He atatmospheric pressure.

A174 K Tachibana et al

Figure 6. Waveforms of the discharge current, the LAS absorption signal and the microwaveattenuation signal at 70 GHz measured in a mesh-type panel in one cycle of He discharge atatmospheric pressure.

Waveforms of the current, the LAS signal of metastable atoms and the MT signal areshown together in figure 6; they were obtained at Vp = 0.95 kV and a gas flow rate of 2 SLM.The large and sharp peak in the current waveform at the rise and fall of the applied square wavevoltage is due to the displacement current, and the conduction current with a smaller amplitudeand a longer tail appears at the shoulder. It is seen that the metastable atom density rises veryslowly after the end of the discharge current and lasts much longer with a lifetime of a fewmicroseconds as stated above. This shows that the He(2 3S1) metastable atoms are producednot only by the direct electron-collision excitation from the ground state atoms but also by otherprocesses, such as the dissociative recombination of He+

2 followed by the radiative cascade inthe afterglow phase. Anyway, as stated above, the lifetime is determined predominantly bythe Penning ionization process with impurities. Therefore, it is suggested that a majority ofelectrons are produced in the afterglow by the Penning processes as long as the metastableatoms remain in accordance with the observed waveform of the microwave transmittance signalshown in the figure. This conclusion is consistent with previously published reports on the one-dimensional simulations of DBDs in He [4, 15], where a contribution of the Penning ionization(occurring mainly in the afterglow phase) comparable to the direct ionization (occurring in theearly phase with a sharp rise in current) was predicted, although the comparison depends onthe impurity level.

A similar set of data taken in an Ar discharge is shown in figure 7. (In this case, the LASmeasurement has not been done yet.) It is noted here that the rise in MT signal follows thecurrent waveform. This is completely different from the results in He, but rather close to theresult in the panel with the Ne–Xe mixture given above. This may be attributed to the smallerexcitation energy for Ar(1s5) metastable atoms of 11.6 eV, with which no Penning ionizationoccurs if the main source of impurities is N2 or O2.

3.3. Analysis for electron densities

In all the tested cases, the attenuation of microwaves was up to a few per cent. If this is dueto the plasma electrons, we can estimate the transmittance T through a plasma slab with finitethickness d using the following equation [16]:

T =∣∣∣∣ET

E0

∣∣∣∣2

= |cosh{(α + jβ)d} + (Zr − jZi) sinh{(α + jβ)d}|−2

Diagnostics of microdischarge-integrated plasma sources A175

Figure 7. Waveforms of the discharge current and the microwave attenuation signal at 66 GHzmeasured in a mesh-type panel in one cycle of Ar discharge at atmospheric pressure.

with

k2 ≡ (β − jα)2 = k20

{1 − ω2

pe

ω2

[1 + j (νm/ω)

1 + (νm/ω)2

]},

Zr = 1

2

(β

k0

) [ |ε| + 1

ε

], Zi = 1

2

(α

k0

) [ |ε| − 1

ε

],

k0 = ω

c, ε = 1 −

(ωpe

ω

)2 1

1 − j (νm/ω),

where E0 and ET are the electric field strengths of incident and transmitted microwaves, α

and β are the imaginary and real parts of the wave vector k, ω is the angular frequency of theincident microwaves, ωpe is the electron plasma frequency and νm is the momentum transfercollision frequency. Here, in He we assume νm = 5.6 × 10−16υN0, where υ is the meanthermal velocity of electrons in units of cm s−1 given by the electron temperature Te and N0

is the density of ground state atoms at 1 atm in cm−3, and calculate the transmittance T in themesh-type configuration with effective d as 0.5 mm (about half the thickness of the electrodeassembly). The result is shown in figure 8(a) as a function of the electron density ne with Te

as a parameter, where ω/2π is fixed at 60 GHz. The resonant electron density correspondingto ωpe/2π = 60 GHz becomes 4.4 × 1013 cm−3. It is seen that the value of ne giving the samevalue of T depends on Te through νm. Although Te is unknown, we try to assume a moderatevalue of 0.5 eV in the afterglow phase where ne peaks, and calculate T as a function of ω withne as a parameter as shown in figure 8(b). This choice of Te is ambiguous, but the dependenceof the final results of ne on Te is rather small since νm is only proportional to the square rootof Te. The experimental data obtained in the mesh-type panel at several different appliedvoltages are also plotted in the figure. If we take the full thickness of the electrode assemblyas d (=1.1 mm), the estimated ne becomes about a half of the value assigned to each curve inthe figure. Anyway, in this crude estimation, the electron density is of the order of 1012 cm−3

in He discharge at atmospheric pressure. This result is consistent with a Langmuir probemeasurement performed with the same panel, where a cylindrical probe 100 µm in diameterand 2 mm in length was placed in parallel to the electrode assembly at a distance of 1 mm,

A176 K Tachibana et al

50 60 70 80 900.94

0.95

0.96

0.97

0.98

0.99

1.00

ne = 5.0×1012 cm-3

ne = 3.5×1012 cm-3

Vp = 0.80 kV

Vp = 0.90 kV

Vp = 0.95 kV

Tra

nsm

ittance

Frequency (GHz)

ne = 1.5×1012 cm-3

1010 1011 1012 1013 1014 10150.0

0.2

0.4

0.6

0.8

1.0

Tra

nsm

ittance

Electron Density (cm–3)

Collisionless T

e = 0.1 eV, ν

m = 280 GHz

Te = 1.0 eV, ν

m = 900 GHz

Te = 10 eV, νm = 2.8 THz

(a) (b)

Figure 8. Transmittance characteristics calculated (a) as a function of ne with Te as a parameterat a constant frequency of 60 GHz and (b) as a function of frequency with ne as a parameter atTe = 0.5 eV in a He plasma at 1 atm. Experimental data taken at several applied voltages are alsoshown.

while a reference metal-plate electrode was placed at a distance of 2 mm [6]. When we usedAr gas, the attenuation became much larger (in some cases more than 5%), as seen in figure 7,suggesting an electron density close to or over 1 × 1013 cm−3. In the PDP-type panel, theattenuation was also large with an even smaller thickness (see figure 4).

However, through the MT experiments we found peculiar phenomena in the transmittance.For instance, the attenuation depends strongly on the frequency as shown in figure 8(b) andthe transmittance shows a fairly large fluctuation with a slight change in the frequency. Sincethe pitch of microdischarge cells in our experiments is close to or smaller than the wavelengthof the microwaves, the electrode assembly becomes something like a grid polarizer. Thus,the transmittance or attenuation characteristics are actually complicated due to the diffractionand reflection of electromagnetic waves. A more detailed consideration is required for furtherarguments.

According to our preliminary measurements in a typical parallel-plate DBD system, it wasseen that the electron density and the metastable atom density in our microdischarge-integrateddevices are at least one order of magnitude larger when the input power density per unit volumeis kept at the same level. Considering that the difference in the volume of both plasmas isabout a factor of 10, the net production efficiency of electrons and excited species might becomparable per volume. However, the figure of merit of our device is at the point where the gasflow can be conducted through the electrode assembly, which might be favourable for manyapplication purposes.

Another possible application of our sources is in the controlling devices of electromagneticwaves in the mm to sub-mm wavelength range. If we can realize a plasma with an appropriatedensity, it can be used as a shutter or filter in a one-dimensional arrangement, as suggested bya simulation [17], or hopefully as a photonic crystal in a two-dimensional arrangement.

4. Conclusions

Optical emission and absorption methods as well as a MT method were applied to diagnose twodifferent types of microdischarge-integrated plasma sources. From these measurements, weshowed that relatively large densities of electrons and excited atoms can be produced with thosesources. The combination of these characteristics with the figure of merit in the gas feeding

Diagnostics of microdischarge-integrated plasma sources A177

method through the electrode assembly, could give rise to devices having much potential forapplication in material processing tools of large area operated at atmospheric pressure. Asfor the characterization of plasma parameters, such as electron density and temperature, mucheffort is required for more quantitative analyses, especially towards the realization of plasmasources for electromagnetic wave controlling devices.

Acknowledgment

This work has been partially supported by a Grant-in-Aid for Scientific Research from theMinistry of Education, Culture, Sports, Science and Technology.

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