The efficient coloration of LiF material, in the form of bulk and films, by EUV and soft X-rays emitted by a laser-plasma source is demonstrated. The short penetration depth of soft-X-rays is exploited to obtain high spatial resolution luminescent patterns while the high dynamic range of proportionality between X-ray dose and coloration is exploited for using LiF as image detector in micro-radiography and soft X-ray microscopy applications

Baldacchini G.

Bonfigli F.

Faenov A.

Flora F.

Limongi T.

Lisi N.

Montereali R. M.

Murra D.

Nichelatti E.

Palladino L.

Pikuz T.

Reale L.

English

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

16 July 2022POLITECNICO DI TORINORepository ISTITUZIONALEHigh resolution and high efficiency coloration of lithium fluoride by soft X-rays irradiation / Baldacchini, G.; Bonfigli, F.;Flora, F.; Montereali, R. M.; Murra, D.; Lisi, N.; Nichelatti, E.; Pikuz, T.; Faenov, A.; Limongi, T.; Palladino, L.; Reale, L.. -5131:(2002), pp. 300-304. ((Intervento presentato al convegno International Conference on New Laser Technologies andApplications nel San Diego, CA, United States [10.1117/12.513681].OriginalHigh resolution and high efficiency coloration of lithium fluoride by soft X-rays irradiationPublisher:PublishedDOI:10.1117/12.513681Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2859402 since: 2021-01-03T13:39:43ZSociety of Photo-Optical Instrumentation Engineers (SPIE)High resolution and high efficiency coloration of lithium fluoride by soft X-rays irradiationG. Baldacchini, F. Bonfigli, , F. Flora, R.M. Montereali, D. Murra,ENEA, C.R. Frascati, Via E. Fermi 45,00044, Frascati, Roma, ItalyN. Lisi, E. NichelattiENEA, C.R. Casaccia, Via Anguillarese 301, 00060 S. Maria di Galeria, Roma, ItalyT. Pikuz, A. FaenovMISDC ofVNTIFTRI Mendeleevo, Moscow region, 141570, Russia.T. Limongi, L. Palladino, L. RealeUniversità degli Studi di L'Aquila, Dip. di Fisica e INFM, INFN-LNGS, L'Aquila, Italy.AbstractThe efficient coloration of LiF material, in the form of bulk and films, by EUV and soft X-rays emitted by alaser-plasma source is demonstrated. The short penetration depth of soft-X-rays is exploited to obtain highspatial resolution luminescent patterns while the high dynamic range of proportionality between X-ray doseand coloration is exploited for using LiF as image detector in micro-radiography and soft X-ray microscopyapplications.1-IntroductionAs well known ionizing radiation like UV, EUV (Extreme Ultra-Violet), X-rays, electron beams,etc., can generate color centers (CCs) in alkali halide crystals [1]. In particular, in lithium-fluoride (LiF)several types of point defects can re-emit visible light when excited by blue light and, after being generated,they remain stable for a very long time (centuries) even at room temperature [2]. For this reason LiF, in theform of bulk and films, is interesting for many applications like optical memories [2], miniaturized opticaldevices like broad-band optical emitters [3] and active waveguides [4], microcavities [5] and point lightsources [6,7]. The increasing demand for low-dimensionality photonic devices imposes the utilization ofadvanced lithographic systems for producing structures with submicrometric spatial resolution. On thisregard, the emerging technologies presently investigated in the field of EUV lithography allow to push thespatial resolution to values below 100nm [8,9]. An essential aspect for low-dimensionality is the use of lowpenetrating radiation like EUV or low energy electron beams.In the present work we show how the EUV radiation produced by a laser plasma source canefficiently create high resolution luminescent patterns at a much higher rate compared with electron beamtreatment on large areas and how this combined use of laser plasma sources and LiF films as sensitiveradiation detectors can improve the performances of soft X-ray microscopy and micro-radiography.2-Coloration of LiF by EUVThe photon energy of EUV radiation (20<hv<200 eV) and soft X-rays (O.2<hv<8 keV) issufficient to generate CCs in alkali halide crystals and in particular in LiF [1]. The irradiated areas of thecrystal, where CCs are generated, become non-transparent to some specific wavelengths, mainly in the blueregion, where the F3 and F2 CCs posses almost overlapping broad absorption bands. They emit a strongluminescence in the green-red region when optically excited by blue light. The penetration depth of EUVradiation in LiF is so short that the colored layer of the crystal can be as thin as 20-100 urn, as shown infigure 1 . The CCs formation process can involve non-linear mechanisms so that the CCs density increaseswhen the EUV (or soft X-rays) energy deposition rate is larger. For this reason laser plasma sources,characterized by a high peak brightness, are extremely suitable to be used for the LiF material coloration.Third GR-I International Conference on New Laser Technologies and Applications,Alexis Carabelas, Giuseppe Baldacchini, Paolo Di Lazzaro, Dimitrios Zevgolis, Editors,Proceedings of SPIE Vol. 5131 (2003) © 2003 SPIE · 0277-786X/03/$15.00300Downloaded From: http://proceedings.spiedigitallibrary.org/ on 11/01/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspxDue to their high peak power and to the short penetration depth of EUV in the material, the dose depositionrate ,P, can be as high as some gigawatts per cubic centimeter. In our case we used a laser plasma sourcepumped by a large volume excimer laser having a pulse energy ranging between 1 and 7 J and a pulseduration ranging between 10 and 120 ns [10]. The efficiency of the laser plasma is as high as 20% in theEUV [10] so that at 10 cm from the source the dose rate in LiF is as high as 3 GW/cm3 and with just 1000shots we could reach in the LiF samples a dose of some tens kJ/cm3; after the irradiation, a CCs density inthe order of 1019 cm3 is reached in the the irradiated LiF samples and a strong luminescence is observed.According with the literature, when using hard X-rays the F-type CCs results to scaleapproximately as the root-square of the X-ray dose (D) and with the logarithm of the dose rate (P), asshown in figure 2 for a NaC1 crystal; in this case the experimental data are taken from page 221 of [1]... . . .. . T_15_The empirical best-fit function for the debns density in figure 2 is F =F0 .A(P) • j— , where D isthe deposited EUV dose, P is the dose rate, D0= 1 J/cm3 is the unity dose, A(P)=1+2log(1+PIP), P= 10mW/cm3 is the threshold dose-rate for non-linear effects and F0=7.7 1015 centers/cm3 is the centers densitydue to a unity dose at P<<P. From our preliminary experiments on LiF crystals, it seems that exactly thesame function is also followed by the F-type CCs in LiF when irradiated with EUV.3-Luminescent patterns formationSince the EUV penetration depth is very short, the LiF crystal can be substituted by a LiF filmobtained as a layer deposited on different kind of thermal substrates by evaporation. In this way, smoothsurfaces can be obtained and the active CCs formation efficiency is found (for the F2 and F3 CCs) to be oneProc. of SPIE Vol. 5131     301Downloaded From: http://proceedings.spiedigitallibrary.org/ on 11/01/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspxorder ofmagnitude larger than for a LiF crystal as shown in fig. 3, where photoluminescence spectra of CCs,pumped by the 458 nm line of our Ar laser are reported [1 1]. The reason for this result lies in the differentstructure ofthe film (polycrystalline) compared with the bulk crystal (monocrystalline).Under the above mentioned experimental conditions for the EUV irradiation (1000 shots at 10 cmfrom the plasma source, absorbed EUV dose 30 kJ/cm3) we irradiated a LiF crystal with a copper meshplaced in contact over the crystal [12]. In this way, colored patterns are obtained on the LiF surface since theareas under the mesh structure are not colored while those which received directly the EUV radiationpresents a strong luminescence (when excited by blue light), as shown in fig. 4. The resolution in theluminescent patterns is less than 1 pin, limited mainly by diffraction effects as explained in [12].3: Comparison between the photo-luminescent spectra re-emitted by a LWcrystal and by a 6 jtm thick LW filmafter being irradiated by EUV in thesame conditions (120 mJ/cm2). (Roomtemperature; excitation by argon laserat X=458 nm)Another technique for the generation of luminescent patterns consists on writing the patterns by e-beams [2], but salts like LiF are insulators and hence charge-space effects appear and low e-beam currentvalues must be used; the main advantage of the use of EUV radiation is that large areas (some squarecentimeters) can be treated at sub-micron resolution with exposure-times orders of magnitude lower than fore-beam direct writing: in our case by operating the laser at 10 Hz it is possible to reach the proper exposureof the LiF plate ('-' 100 mJ/cm2) over some centimeters area in less than two minutes.Considering the above described properties of LiF, this material can be used as soft X-ray imagingdetector. Compared with photographic films, typically used for ti-radiography of small animals, or withPMMA (PolyMethylMethAcrylate) photoresist, typically used as detector for soft X-ray contact microscopy[10, 13, 14, 15], the LiF film allows a much smaller resolution (the CCs size is less than 1 nm) and a muchwider contrast dynamic range, respectively. This second characteristic of LiE films is due to the quadraticresponse of the CCs density vs X-ray dose shown in figure 2. Consequently, images of 10-12 bits can beobtained and no development is needed after the exposure to X-rays, while by using PMMA the dynamicrange is limited by the development process so that the final dynamic in the images is just 5-6 bits [10,14].The problem of the contrast dynamic is much smaller when photographic films are used but, in this case, thec/DCCl)Cl)E1.6I .41.21.00.80.60.40.20600Wavelength (nm)Fig. Fig. 4: Image of a LiF crystal irradiated by EUVobserved by an optical microscope influorescence mode (through a yellow ifiter,objective 40X) while excited by an argonlaser at X458 nm. Clearly visible is theluminescent zones of the crystal, as well asthe shadow of the mesh placed over thesample during the EUV exposure.302     Proc. of SPIE Vol. 5131Downloaded From: http://proceedings.spiedigitallibrary.org/ on 11/01/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspxemulsion grains size limits the resolution to few microns. For comparison between LiF and PMMA, in figure5 we show the micro-radiography of a polypropylene phantom: the much larger dynamic of LiF is evidentallowing a clear imaging of both regions exposed at a low X-ray intensity (the regions where the phantom isthicker) as well as regions exposed to a large X-ray intensity (where the phantom thickness is zero). On thephantom image on PMMA, the low dose regions can't be distinguished each other nor at optical analysisneither at AFM analysis, as shown in the expanded details of figure 5.6- Conclusions and acknowledgmentsWe have demonstrated that laser-plasma sources can efficiently generate color centers in LiF andthat high resolution luminescent patterns can be obtained. The application of LiF films as image detectorsseems to be very promising for microradiography and soft X-ray contact microscopy. Authors thank Dr. A.Reale and his group of L'Aquila University (Italy) for their support in the experiments and Dr. A. Lai ofENEA C.R. Frascati for important discussions and for her contribution.served a. .....Fig. 5: Micro-radIography of a step-shaped polypropylene phantom on PMMA photoresist and on LiFfilm exposed in the same conditions (600 mJ/cm2). The phantom step-structure reduced the softX-ray fluence by a factor 0, 150, 300 and 600 for areas a,b,c and d, respectively. The PMMAhas been developed 0.5 minutes in MIBK developer. Its original thickness was 600 nm whileafter development resulted (from AFM analysis as shown in the details) 200 nm for zone-a,588 nm for zone-b and .-6OO nm for both zones c and d.Proc. of SPIE Vol. 5131     303Downloaded From: http://proceedings.spiedigitallibrary.org/ on 11/01/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx(L661) 'OT96 dd 'ZTd 'LST TOA 'Xdoosoioq, jo ç 'suoqinp osind ios pu SJ1UOW uoJoJJp TM O1flOS msjd isi jouxioxo u rnsn XdoosoJoTw rnuoo Ai- : p o ouiiqj 'dEc11 (o66T) '09 -1 c dd '6c 'T°A 'AdoosoJDTj,s ,JO 'f 'JOS1 -x ijos t q1TM Adoosoiojm oiuoj : ji o iuuq HJ [t'i] 66T 'IdOS 'LT-T dd'sisjrnxypui AdoooJoTy 'soouoios-jTj ioj onbrnqoo i4ou i :AdoosoJoTm ouoo i(iJ- jo :uolloD y UTqo).J [ ii (zooz) ' zT st'-oi dd 'Os 'bA '°i sAq jdd 'ooinos iusjd-iosij moij sXi- jos cq poonpoid sps(io opuonj wnq1 rn suio11d uosouTmnJo1oqd :j TURJOOPTIH o [IT] (966 I ) 'T Z d '9 1 1 H JDJtOSO)J 'SXJd isuj 'T°NI "11 o iunjoopjg [ I i] (866t) 'ToLT-c891 'dd 'j ot '!°A ' oiuauqj OtOflN/J ' suoqioTJddi iotpo pUt AooTqoTpiu 'Adoosoioim onssaid ouoqdsom1 ioj ooinos iuisiid A-x so :j uijoj s [oil (666T) )J11 'loojils uouTdmiuj 'SSOJd XITSJOATUfl op.iqwi3 'suoTiioTJddi pu sjdTouud :uoTirnp>.j i01O1AJTIfl ouiix put si- ijo,, :pooM pTAQ [61 (666 i) 6691"69 'dd '() 'pj uvnjj T ri 'AUOOMS wa pui uoins HèT [sI (9661) oc-cc 'dd '9 g 'S(CtIc[ icMv 'AOTp[OO SA PU1 1pTSi1)TOS ')I'S [LI '(ioo) 6c-c 'dd '6 o 'i ° "Pv d [91 (iooz) 'TSr d '681 1°A 'UflWWOJ 'ØQ '.T1 onooj y Ed (iooz) tso17-zsot' 'SL(9z) 'fl7 'YJ V '"'H E 'y 'qoiouoy j,c>j [171 (6661) 681 '6tI 8/71/Og fl)([ ff pv 'q aroiuoj, 'wa [El (8661) 9EEEZE '(9)91 U 'lPL PS VA 'f 'TT1V TS pU OUT3 'f' [ 'Z961 'SSOJd UOUflJOd 'Pii0S Ut SJOUJ JOJO3 uoiduioj prn umjntpg HI [il 304     Proc. of SPIE Vol. 5131Downloaded From: http://proceedings.spiedigitallibrary.org/ on 11/01/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx

High resolution and high efficiency coloration of lithium fluoride by soft X-rays irradiation

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High resolution and high efficiency coloration of lithium fluoride by soft X-rays irradiation

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