J. Light & Vis. Env. Vol.3J, No.J, 2007 5 Pa per A Compact Excimer 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 excimers 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 ozone generator was proposed and light or vacuum ultraviolet (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 noble gas/halogen 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 plasma 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.