J. Light & Vis. Env. Vol.3J, No.J, 2007 5

Pa per A Compact Lamp Constructed by Piezoelectric Transformer

Kenji TERANISHl* and Haruo ITOH**

* Researdl Felbw of the Japan Soc~'ety for the Promotion of Scien~ (Chiba Institute of Technology) - Graduate Sibool of Engineering, Chiba Insti'tute of l~nology

Received December 2, 2005, Accepted January 24, 2007

ABSTRACT A compact excimer lamp eonstructed with a piezoelectric transforrner (PT) has been developed in our laboratory. This excimer lamp is designed based on the excitaton of a dielectric barrier discharge (DBD) in a He･Xe mixture generated by the PT, which is driven with low applied voltages of several tens of volts (i5~5¥/). Vacuum urtr:aviolet (VUV) radiated tom tfle lamp is detected by a phototube having a spectral response region ranging ftom i60 to 320 nm in conjuncton with an optical filter to discriminate the VUV and UV regions. The VUV radiaton in the wavelength range 160-200 nm is recognized in this experiment. This may be derived 'rrom the 1 72-nm light emission by the transitons of the Xe2 to the ground state. Investigatons into the radiated VUV intensity are carried out by varying the gas pressure and the gap length. The optimal condition for the present excimer lamp is determined.

KEYWORDS: piezoelectric transformer, excimer lamp, dielectric bamer dlscharge He~Xe mlxture Xe excimer, atrnospherlc pressure

1 . Intuduction reactors to be accomplished because the PT plays roles of Excimer lamps are equipment designed as light sources both a high voltage source and a discharge electrode. capable of radiating high intensity and narrow band uv Previously, a PTbased generator was proposed and light or vacuum (VUV) lightD ~ . To obtain the ozone generation characteristics were investigated by excimer formation in the lamps in a simple mauner and feeding the generator with oxygen and airl5). In the case of with great efficiency, dielectric barrier discharges (DBDS) oxygen-fed ozone generation, ozone was obtained at a in noble gases or a / mixture are widely con~ntration of 3.1-20.3 g/Nm3 and maximurn ozone yield utitize d, and availability of the light wavelength dep ends efiiciency to the extent of 223g/kWh was achievedl9). on the kind and composition of working molecules. It is These values are competitive enough to be compared to comprehended that particularly xenon ex(imer lanrps, the recently reported DBD-type ozone generators which provide more powerful VUV radiation at 172 nrn, is operated in room temperautcl9). liable to destroy molecular bonds for ahnost all agents. For An excimer lamp designed based on the PT has also this reason, the lamps have been widely used for the been exaJnined to be entrusted to further investigation for purpose of industrial application, such as dry cleaning and application of the reactor. This paper describes the photochemical vapor deposition in semiconductor experixnental investigations with a view to detecting VUV processing, surface modification, and fabrication of oxide radiation emitted from the excimer lamp, using a film3). In re~nt years, experimental studies and phototube combined with an optional glass filter. computational simulations have been actively perfouned Characteristics of the VUV radiation are also observed on the characteristics of the xenon DBD tog:ether with with respect to different types of gas pressure and gap geometry opttmization for electrodes and excitation lengths to determine the optirnal driving conditions for the regime for xenon excimer lamps with a view to obtaining ex(imer lamp . higher intcnsity and achieving efflcient operation~)~lD. Vaiious types of plasma reactors incorporating 2. Experimental piezoelectric transformers (PT) 12)･19) have been developed 2.1 PTbased excixLer lamp in our laboratory. In these reactors, discharges are A schematic diagram of the PTbased excimer lamp in generated directly on the PT surface, inducing high configuration and drcuit is shown in Fig. 1(a) . The lamp voltage caused by the piezoelectric effect. Such plasma contains a Rosen-type PT made of Pb(Zr, Ti)03 measuring generation methods allow the design of compact plasma 60X13xl mm. A quartz glass plate includirrg mesh wires on its back is employed as the dielectric electrode having *Present a~liation: Department of Electrical and Computer Engineering, the metallic back electrode allowing the generated VUV to lchinoseki National College ofTechnology

5 The llluminating Engineering Institute of Japan 6 J. Light & Vis. Env. Vol.31, N0.1, 2007

Gas nlet ~

:*t:y..;~~4 ;;T' ~;+･* ;~ Piezoeiectric ; Back e ectrode transformer + ~~:*~ ,*~,,~ Piezoelectr[c transformer Spacer DBD (Mesh wires) Optical filter (UV-250) Dielectric layer (Quartz) ~ VUV >*~ ,~~; Primary voitage vp Support ~ ~ ~ ~*:*~i i~.* ~:s (~~~ *; . tL =~~.~~ ~ l¥jr "'" ,_. ~~ r ;~;_~ hototube Oscilloscope

***,,~* i Y Detector circvit essure asurement +**,*'.~ L~ ~･~ ~L~ ~ X ;***･+ vUv x ;"~;' ~ P~imary part Secondary part ~ ste m ',*~ ~ Excimer lamp ~.~ Oscilloscope Potent'a! d~!ider

,~*=1:*_~*}i-･'*'*+;~i.~,~~,.**~*: *~~:~*;i. .~~~~;~?;~･*'~*~ ~ :~~~'~~~

Va ve l To pump (a) Configuration of piezoelectric transformer-based excimer lamp (b) Discharge chamber and VUV detecting system

Figure 1 Experlmenta] arrangement

be passed to the outer side of the lanrp. The quartz glass ioo ~ has an 80"/, transmittance at 170 nm. As shown in the _i~1 o figure, the PT and dielectric layer together with the back ~K 80 Q~ Ro(~) electrode are fixed by a spacer to be supported at the node h~: oc oe) s; 60 Rf> (~ )=Ro (~) Tf¥(~) l( of the mechanical vibration of the PT. Provided that the L~= =a' I oo PT is driven at (1/2)A-mode vibration, a high voltage is " " ~s induced on the secondary part of the PT. This results in E= 40 // Tf(X) H~! 20 ~' '~ Xe2' (17~ nm) the DBD plasma generation in the gap space as shown in " the figure . The ( 1/2)A-mode vibration is de alt with as ' o ~l~lo-' condition, where a half wavelength of the mechanical o 140 160 180 200 220 240 260 280 300 320 wavelength ~ (nm) vibration is equal to the length of the PT. However the Figure 2 Cathode tadiant sensitivities Ro(~) and R<~) and vibration is subj ect to the primary voltage and discharge transmittanoe of optical filter UV-250 T<~) current to a ~rtain amount of extent. Therefore frequency of the prirnary voltage is adjusted arbitral:ily between 26.3 Xe DBD. The cathode radiant sensitivity of the phototube and 26.6 kHz. The gap length is adjusted according to ~)Q~) is shown in Fig. 2. The phototube responds to 160- thickness of the spacer. A secondary voltage appearing at 320-nm wavelength light, where both VUV and uv the tip of the PT is measured by an oscilloscope and regions are included. Hence, an optional filter (uv-250), potential divider. Thus discharge power is estinated by a which is adopted to dis(ximinate the VUV and uv regions, Lissajous figure formed by the secondary voltage and is placed between the lamp and the phototube. voltage across the 2-nF capacitor counected between the The spectral transmittance of the uv-250 filter T}Q~) is back electrode and the ground. The experimental also shown in the figure. The transmittance of the uv-250 arrangement is illustrated in Fig. 1(b). The excimer larnp filter T}Q~:) of the filter is 50"/, at A=250 nm and Oo/~ for A < unit depicted in Fig. l(a) is placed in the chamber center. 200 mu. The cathode radiant sensitivity J~f(A) having the All electrical feedthroughs mounted on the chamber filter in place is considered by the product of I~) Q~) and flanges. In advance of experixnents, the chamber is T}Q~:) provided as shown below. evacuated to the full extent reaching 10-5 Pa to be filled with a He-Xe (10.8"/o) mixture in a range from 400 Torr Rf(~) R (~)･T/ (~) (53.3 kP~ to 1000 Torr (133.3 kPa). The radiated VUV is detected using a phototube, whereas the output phototube R~Q~:) exhibits a spectral response complying with the light voltage is recorded using an oscilloscope connected ranging from 200 to 320-nm brought about by the cutoff through a detector drcuit. characteristics of the filter. The light intensity of the 2.2 VUV detection method excirner lamp is measured by using and conculrently not Full expectation is made in this study that the VUV at by using the filter in an adequate place. Provided tbat 172 nm will be radiated from the Xe2* excimers in the He- radiation of the 172-nm VUV emitted fiom the lamp is

The llluminating Engineering Institute of Japan 6 J. Light & Vis. Env. Vol.31, No.1, 2007 7

available, the light intensity measured by means of the phototube used as a iuction of discharge power. The open filter in place will become remarkably smalLer than the and solid circles indicate types of intensity ~6 and Sf light intensity measured without utilizing the filter. measured by means of and without using the uv-250 Although it may also be possible to detect VUV emission filter in pla~, respectively. at 147 nm brought about by the Xe' resonance Ines by Both types of light intensity were multiplied as the means of the phototube, the sensitivity at this wavelength discharge power was increased, but the light intensity Sf almost corresponds to the 20th part of the one at 172 nm. measured by means of the ~ltcr in pla~ is 250 tixnes By considerlng, in addition to the above, that the present smaller than the light intensity S; measured without experiment is perforned under the gas pressure at more using the filter. By so doing, it is revealed that 99.6*/, of than 400 Torr (53.3kPa), most of the resonance emissions the radiation emitted by the excimer lamp is absorbed by are absorbed by the ground state atoms2G) 2D. Accordingly the filter. It is explained from the above that the VLrv in it can safely be said that the VUV light at 147 nm is the wavelength region ranging from 160 to 200 nm is believed to be negligible in this experirnent. radiated from the excimer lamp. Thus it is concluded that the detected ¥rUV can be regarded as 172 nm resulted 3. Experirnental results and discussions from radiation from Xe2'. Considering that most of the 3.1 Vlsible emission from exciner lamp radiation emitted from the lamp is absorbed by the filter, Figure 3(a)-(d) shows the images of emissions of visible there remains no radiation between 200 and 320 nm. It is light obtained at the gas pressure of 760 Torr (101.3 kPa). possible to infer from the results that light intensity These images were obtainable by using a single-reflex detected by means of the filter correspond to those of the camera with exposure time of O.5 and F-number of 3.5. 172-nm VUV The upper side of the images corresponds to the top of the 100 p = 760 Torr (101 .3 kPa) secondary part of the PT. The active emitting region, which is 30X15 mm, complies with the dischange space ~:) that is slightly larger than the area of the secondary part ~~i O~i Without filter : So of the PT. The expeximents were conducted by allowing > ~co the primary voltage Vp to be increased in a range from 20 c:(D i O~2 to 45 V. Micro-discharges together with homogeneous ~c: emissions are observed in a style offilaments between the ~:o)~ i 0~3 gaps. The DBD was first initiated at the primary voltage ~i With filter : Sf of 20 V. Secondly by increasing the primary voltage, the 10 o o.1 O.2 O 3 04 O 5 O 6 discharge regions were extended in a direction of the PT Discharg~ power (W) ' center (toward the bottom of the images). Figure 4 Light intensities with respect to discharge power measured with and without opticaf filter in pla~ ~~~>1i5 mm 3.3 VUV characteristics of excilner lamp The VUV intensity as a function of the discharge power measured in the gas pressure region ranging from 400 to 1000 Torr (53.3-133.3 kPa) is shown in Fig. 5(a). The gap length is maintained at 2 mm. Although the VUV intensity generally increases in proportion to heightening of the discharge power, the values come to be slightly (a)vp=20 V (b) Vp=30 V (c)vp=35 V (d) V,=45 V saturated at higher discharge powers for each region of (0.15 W) (o.35 W) (O.4i W) (0.55 W) the gas pressure. Figure 5(b) shows types of VUV Figure 3 Visible light emission tom tr]e exdmer lamp (Gas pressure: intensity plotted with respect to gas pressure for discharge 760 Torr (i O1 .3 kPa), Gap length: 2 mm) powers ranging from O.2 to 0.6 W. The VUV intensity in At the primary voltage of 45 V, the DBD appears on the this region is proportional to the gas pressure. These entire surface of the secondary part of the PT. From these results suggest that in the region ranging from 400 to results, it is understood that application of low voltages of 1000 Torr (53.3~133.3 kl)a), the concentration of the 15-45 V to the PT makes it possible to operate the excixner generated Xe2' excimers in the He-Xe DBD bringing about lamp in the discharge power region ranging from O.15 to the number of photons at 172 nm is directly proportional 0.55 W. Thus it becomes possible for the lanrp to work as a to the gas density. High-gas pressure conditions enhance compact surfa~ light source. e~icient excimer formations because the 3-body reaction 3.2 VUV detection using phototube and optical filter for the formation, Xe'+Xe+Xe ~> Xe2'+Xe rapidly arises Figure 4 shows the light intensity detected by the prior to the decay process for the Xe' atoms. Such being

7 The I!luminating Engineering Institute of Japan 8 J. Light & Vis. Env, Vol.3J, N0.1, 2007

Gas pressure p: increases linearly accompanied with increase of discharge A 400 Torr (53.3 kPa) ~1 500 Torr (66.7 kPa) power in the case of the gap length less than 2 mm. On a 1 c 600 Torr (80.0 kPa) 1 y 700 Torr (93.3kPa) (a) Discharge po~~r: (b) point of 3 mm in gap length, the redu~d intensity seems + 760 Torr (101 .3 kPa) e 0'2W to be saturated at discharge powers of more than 0.4 W. x 900 Torr (120.0 kPa) A 0'3W O 1000 Torr (133.3 kPa) ~ 0'4W Details of these results, which have not yet been made 0.8 o.8 + 0'5W ///;i/ ,,) (/? O 0'6W clear, might be explained quantitati'vely in the description :t:: c: C: s O shown below. Li o.6 ~~(U o.6 *(~; The present experiment was conducted not only in the llll gas pressure region ranging from 400 to 1000 Torr (53.3 to / ll >1 133.3 kPa) but also at a gap length ranging from 0.1 to 0.3 ,+- 0.4 ~ o.4 u) U) (Dc (DC cm. Accordingly the pd product, which is in the right side C // / c // / of the Paschen minimum, can be estimated in a region // / 0.2 o.2 // / ranging from 40 to 300 Torr ' cm. In this region, ////ll heightening of the pd product would lower ionization d=2 mm d::::2 mm coefficient a:i/p, and excitation coefiicient a~ /p because of O 200 400 600 800 1000 O 0.2Discharge 0.4 power 0.6 (W) Gas pressvre (Torr) decrease in the reduced electric intensity L7p corresponding to the starting voltage of discharge. In O 20 40 60 80 100i20140 Gas pressure (kPa) this experixnent, increasing discharge power makes it

Figure 5 VUV intensities for dffferent gas pressures possible to enhance the excitation and ionization, followed by the increase in light intensity as a result as depicted in the case, it is imagined in this experiment that the Fig. 6. However taking into account the fact that the mininum VUV intensity will be noti~d at the highest gas sustain voltage in the discharge gap is always constant pressure of 1000 Torr (133.3 kPa). Figure 6(a)-(d) show during the DBD generation even if the applied voltage types of VUV intensity complying with the discharge (primary voltage) of the PT increases, the reduced electric power obtained for gap lengths between 1.0 and 3.0 mm. field intensity Llp (= V 7(pd )) Iikewise remains constant. Measurement was made several times with different For this reason, the increase in the discharge power is types of pressure. As can be dedu~d from Fig. 5(b), the mainly due to the increase in the discharge curl'ent caused VUV intensity is proportional to the gas pressure. Thus it by development in the repetition of micro-discharge in the is preferred that the reduced VUV intensity I~,/p a.u.flbrr DBD. Thus it is hereby conclude d that the saturation should be indicated, as the intensity is redu~d to I Torr characteristics of the light intensity in Fig. 6(d) is derived (133.3 Pa) when obtained by dividing I~. by the gas from the temperature increase based on the high current pressure p. The redu~d VUV intensity I~./p, which is density. Furthermore a decrease in the excimer forrnation independent of the gas pressure for all gap lengths, is resulted from the above.

1 (a) d=1 mm (b) d=1 (c) d=2 mm (d) d=3 mm A 400 Torr (53.3 kPa) 500 Torr (66.7 kPa) 0.8 c 600 Torr (80.0 kPa) l~ 7 700 Torr (93.3 kPa) L + 760 Torr (10i .3 kPa) O 900 Torr (120.0 kPa) ~~., O iOOO Torr (1 33.3 kPa) :! O,6 CQ '-/ (1) O Y~x 0,4 CL *~>

0.2

oo o.2 0.4 0.6 0.8 O 0.2 0.4 0.6 08 O 02 04 06 08 O 02 0.4 0.6 08 DiSCharge poWer (W)

Figure 6 VUV intensrties reduced to I TorT (i33.3 Pa) for dmierent gap lengths

The I!/uminating Engineering Institute of Japan 8 」1Z｝なhご＆V該3．E1¢材 シわ∠31，ノ〉6．1，2007 9

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Aono，M：Develop士ng Mec賀anism of Emission in Xenon Dischεしrge L＆mps with，1薫mer and O魏ter Elec捻odes，」．Illum．Engng．1ns軌Jpn．，Vヒ）L87，No．2， pp．98404（2003）． （11）」㎞o，M．，Motomura，H．，Loo，K H．and Aono，M．： Em蛤sion Ch鯉acte蚕st丘（£of Xenon and Xe簸on－Rare Gas D圭elec頓c Bε翻er Discharge Fhorescent La搬ps， 」．Ligh』t＆Vis．Envl，VbL29，No．3，pp．13閣20（2005〉． （12）Itoh，H．，艶rε面shi，K．and Suzu短，S．：Observation of hghtEmissio鍛sayo撹ndaPiezoe墨ec面cT罫a雌brmer 痴V謡ousGases，IEEETrans．o簸PlasmaScience，Vb1． 30，No．1，PP．124－125（2002）． （13）馳r幽短，K．，S協噛，S．副lt・h，H．：Dy薮鐡ic Behavior of hght Em捻＄ions Generated by Piezoelec面c Trans鉋㎜e製s，IEEE Tra鵬．on Plasma Science，Vb1．30，No．1，PP．122－123（2002）． （14）Tbrε樋a圭shi，K，S舳，S．ε雛ユd Itoh．，H．：Lu斑taesceRce 丘om Fl膿oresce難t Materia㎏Exc｝ted by Piezoelec頓c 狂ε鵬fb㎜e鴎Jpn．J．AppL Phys．，VbL41，No．11，pp． 7160－7165（2002）． （15）馳rε癒shi，K．，S脇uki，S．and I紋）h，H．：‘A Novel Generぬo灘Method of Dielec頭c Ba麺er Discha灘ge ∈md Ozone P罫o伽c敏o鷺 Us血客 a Piezoelec伽c Tゼεmsfbme鶏Jp瓦J．Appl．Phys．，Vb1．43，No．9，pp． 6733・6739（2004）． （16）冗brε曲hi，K，S協ukl，S．ε滋1d ItDh，H．：Dielect巌c Ba血e齢 D捻Chεぼge Genera．ted by PieZOeleC樋C Transfbmer in At瓢osphe血c Pressure，IEEE T罫a聡． o鍛Plε芝sma Sci．，Vb1．33，No．2，pp．296－297（2005）． （i7）Itoh，H，Tbrεm沿hl，K．飢d Suzu垣，S．：Glow D捻chaヱge AroundPiezoelec磁cTransfb㎜erQPeでatedUnder HigherOr（ier Vibration Modes， 1EEE 贋ans． on Plas鵬Sα．，VbL33，N・．2，PP．352・353（2005）． （18）圧brε曲hi，K，S協ukl，S．and Itoh，H．：Absolute Measurement of Su幽ce Poten磁al a撮D蛤chε迂ge Power Dis㎞bα慧ons痴r Pie蹴1ec面c T『ans癒撒er B3sedPlas瓢aReacto蔦Jp甑」．App1．Phys．，Vb1．44，No． 9，PP．7083・7088（2005）． （19）ItDh，H．，硬br曲hi，K and Suz面，S．：Discharge Plasma8GeneratεdbyPiezoelec麺c野齪癒鞭erand The登Applications，Plasma So鞭ces Sci』fbc㎞oL，VbL 15，PP．S51・61（2006）． （20）Mikoshiba，S．and Murayama，S．：㎞itations of Lum血ance Chεぽacもe並sti（£ of Gεしs・（強scharge Colo翌 Display Pε㎜els，App1．Phys．Letむ，VbL37，pp．529・ 530（1981）． （21）Morozo努A．，Am．esen，A．，Hε曲，R，K尊10鴨B．ε瞭d Geras加o鷲G．：VUVEmおsionSpectra食o灘1B加町 InertGas醗x撫resneartheResonance㎞esofXeI and K翌1，J．Phlys．D：App1．Phys．，Vb1．36，No．9，pp． 1126－1134（2003）．

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