Erosion of PMMA Induced by Multiple X-ray Laser Shots Below the Single-shot Ablation Threshold

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1 WDS'12 Proceedigs of Cotributed Papers, Part II, 58 63, ISBN MATFYZPRESS Erosio of PMMA Iduced by Multiple X-ray Laser Shots Below the Sigle-shot Ablatio Threshold T. Buria, V. Hájková, J. Chalupský, L. Juha, M. Toufarová, V. Vorlíček Istitute of Physics, Academy of Scieces of the Czech Republic, Na Slovace 2, Prague 8, S. P. Hau-Riege Lawrece Livermore Natioal Lab, 7000 East Ave., Livermore, CA , USA. J. Krzywiski, J. D. Bozek, M. Messerschmidt, S. Moeller, C. Bostedt SLAC Natioal Accelerator Lab, 2575 Sad Hill Road, Melo Park, CA , USA. Abstract. We report results of ivestigatio of damage to a thi layer of a orgaic polymer (PMMA) iduced by irradiatio with multiple laser shots below the sigle-shot ablatio threshold. After exposure to a focused beam of a free-electro laser (LCLS, CA, USA) geeratig X-ray laser radiatio at 835 ev the sample has bee ispected with a atomic force microscopy (AFM) eablig aalysis of the eroded craters. As expected, depedece of the maximum depth of the damaged area exhibits a oliear behaviour with respect to the accumulated dose of eergy. A simple model describig this pheomeo i term of competitio of two radiatio drive processes, i.e. chai scissio ad cross-likig, has bee developed ad modified for X-ray laser sources. The first result of compariso of the experimetal data ad the simulatios of the developmet of the maximum crater depth calculated with the model metioed above is also reported i this cotributio. Itroductio Sice the eergetic X-ray photos carry eough eergy to break may bods iside macromolecules of orgaic polymers, surface of such materials ca be easily damaged or modified with short-wavelegth radiatio at very low level of fluece of radiatio impactig the sample surface. The irreversible chages iduced by irradiatio ca be observed eve i case the fluece of radiatio is sigificatly lower tha the sigle-shot ablatio threshold, i.e., desorptio-like damage of the material occurs. Durig such a process, the material removal is ot as massive as i case of laser ablatio, i.e. i case the fluece is well above the sigle-shot ablatio threshold [1, 2]. Nevertheless, the kowledge of behaviour of material irradiated uder these coditios is of the high importace i umerous braches of sciece ad idustry, e.g., optical elemets developmet ad protectio, EUV (Extreme UltraViolet) ad soft X-ray aopatterig, radiatio chemistry, pulsed laser depositio, etc. So far the method of ablative imprits ito the surface of the orgaic polymers has bee used also for characterizatio of trasversal ad logitudial itesity distributio i focused short-wavelegth laser beams [3, 4, 5]. However, the shorter wavelegth of the radiatio is used, the loger atteuatio legth of the material is observed. This implies the decrease i the local absorbed dose of eergy ad the desorptio starts beig more sigificat tha the effect of ablatio. Moreover to that, experimetal results metioed below show the etch rate is strogly dose-depedet i desorptio mode as well as it decreases with the umber of accumulated laser shots. This ca be explaied i terms of phase trasitios ad structural chages i the irradiated material. I this cotributio we report the results of compariso of the experimetal data obtaied from the orgaic polymer (Poly(Methyl MethAcrylate) PMMA) irradiated with the focused soft X-ray laser beam, with the umerical model of the desorptio of the PMMA based o two mai radiatio-drive processes of chai-scissio ad cross-likig. Experimetal The experimet was performed at the free-electro laser facility LCLS (Light Coheret Light Source), CA, USA [6]. This system is desiged to produce the radiatio with the wavelegth tuable from 1,5 m (825 ev) to 0,15 m (8,25 kev). For our experimet, the photo eergy of 835 ev was 58

2 chose. The PMMA sample was irradiated with laser pulses with time duratio of 100 fs. Average eergy of the uatteuated laser pulses fluctuated aroud 1 mj. For varyig the fluece impactig the sample surface a gas atteuator filled with krypto was used as well as a set of beryllium foils with various thickesses. The samples were mouted o a motorized X-Y-Z stage iside the vacuum chamber placed i a AMO experimetal statio [7] (Atomic, Molecular ad Optical Physics). Icomig laser beam was focused oto the sample surface by meas of two Kirkpatrick-Baez mirrors. Eergies of the laser pulses were measured by a photo-ioizatio detector GMD (Gas Moitor Detector). The sigle-shot ablatio threshold for PMMA at photo eergy of 835 ev was determied through aalysis of the craters made o the surface of bulk of PMMA. The craters were ispected with the Nomarski DIC microscope (Differetial Iterferece Cotrast). The value of threshold fluece was foud to be 93.4 ± 5.2 mj/cm 2 [8]. Therefore, three sets of craters were made at differet fluece levels (0,4 mj/cm 2, 4 mj/cm2 ad 40 mj/cm 2 ) ad with a differet umber of accumulated pulses (30 to 300) o the surface of 5μm thick layer of PMMA deposited o the silico slab. Morphology of these craters was ivestigated with a AFM (Atomic Force Microscope) workig i tappig mode (Veeco D3100 NaoScope Dimesio). Fially, a backscatterig Rama spectrometer with a spatial resolutio of aroud 4 μm (Reishaw Ramascope) was used to idetify ad ivestigate the chemical chages iduced i PMMA irradiated by the soft X-ray laser radiatio. Results ad discussio The mai goal of the experimet was to ivestigate the effects of chemical chages iside the irradiated material iduced by the X-ray radiatio to the efficiecy of material erosio. Sice the PMMA is well kow to udergo the process of chai scissio ad cross-likig whe exposed to the radiatio of various wavelegths [7, 8], these effects were expected to occur eve for the soft X-rays. Fig. 1 shows schematically these key processes takig place iside the irradiated PMMA. It is obvious the effect of chai scissio is essetially resposible for the producig of tiy volatile fragmets, such as methyl or ester groups, etc., which easily escape the material surface ad thus cotribute to the crater developmet. O the other had, whe free ed-liks are preset i the irradiated material, the effect of cross-likig ca take place. This effect is idicated by meas of a formatio of double C=C bods i the backboe chai of the PMMA which origially cotais oly sigle C C bods. Presece of both these effects was prove by the compariso of the Rama spectra take from the irradiated ad uirradiated part of the PMMA. Fig. 2 summarizes the differece i these spectra. A ew peak at 1650 cm 1 appears i the spectrum correspodig to the irradiated area. This peak belogs to the double C=C bods. The umber of double C=C bods formed iside the irradiated material was supposed to icrease with the deposited dose of eergy. Fig. 3 illustrates the progress i probability of PMMA cross-likig with icreasig dose of eergy. Figure 1. Formatio of a PMMA molecule with a double bod C=C; (a) uaffected PMMA molecule, (b) chai scissio caused by icidet radiatio, (c) ad (d) cross-likig. 59

3 Figure 2. Rama spectra of PMMA irradiated by 300 X-ray laser shots at a fluece level of 40 mj/cm 2. The differece betwee the irradiated (solid lie) ad the uirradiated (dotted lie) area is characterized by a ew peak at 1650 cm 1 represetig the presece of C=C bods formed due to the effect of cross-likig. Figure 3. Progress i ratios betwee areas of ew bad at 1650 cm 1 correspodig to formed C=C double bods ad bads represetig the desity of backboe chais (1125 cm 1 ) ad ester (800 cm 1 ) ad methyl (1450 cm 1 ) groups i depedece o accumulated dose of eergy. It is clearly visible the ratios betwee areas of the ew peak at 1650 cm 1 ad the peaks correspodig to aother bods i the PMMA molecule grows with the dose, eve steeper tha i the liear way. Actually, such a growth is caused both by the icrease i umber of formed double C=C bods ad by the decrease i desity of ester ad methyl groups which desorb out of the material due to the chai scissio effect. The data was take from craters made at fluece level of 4 mj/cm 2. Sice the bidig eergy of double C=C bod is higher tha of the sigle C C oe, the efficiecy of radiatio-iduced desorptio is supposed to decrease with the icreasig umber of accumulated pulses. This presumptio was prove by the aalysis of depedece of maximum crater depth o the umber of accumulated pulses. The results of this aalysis are summarized i Fig

4 Figure 4. Depedece of maximum crater depth o the deposited dose of eergy at three differet flueces: (black) 0.4 mj/cm 2, (red) 4.0 mj/cm 2, ad (blue) 40 mj/cm 2. Deposited eergy was determied cosiderig a eergy cotet of each particular laser pulse measured by photo-ioizatio detector. The oliearity of at least two kids ca be observed i the depedeces i Fig. 4. The first oe correspods to the decrease i the etch rate with the icreasig dose of eergy ad it is the result of the higher umber of more strogly coupled double C=C bods. The secod kid of oliearity ca be observed i discotiuities i maximum depths correspodig to the same dose of eergy but to the differet level of fluece of the icidet radiatio. This pheomeo ca be explaied i term of the producig of the volatile fragmets i the deeper layers of the PMMA where they caot escape out ito the free space. These fragmets remai iside the material, cause the additioal losses ad thus lower the effective yield of the erosio. It meas part of the icidet radiatio is absorbed by these trapped fragmets; however, it does ot cotribute to the yield of chai scissio effect. For the simulatio of the crater developmet uder irradiatio of PMMA, a simple model [9] based o evaluatio of the desity of sigle C C bods ( SB ), double C=C bods ( ) ad the free fragmets ( FRAG ) was used. Origially, this model was used for simulatio of a iteractio of PMMA with high order harmoics radiatio. Nevertheless, it was modified for the soft X-rays ad thus could be used eve for the experimet with LCLS radiatio. This model deals with a sequece of laser pulses with time duratio well separated i time. Variatios i desities SB, ad FRAG are to be described by kietics equatios. The fluece F [mj/cm 2 ] decreases iside the material with a depth z[m] i accordace with the Lambert-Beer law: df( z) = -ξ( z) F( z) ( z) ξ F( z) ( z) SB - FRAG FRAG (1) dz where ξ [cm 2 ], ξ FRAG [cm 2 ], SB [cm 3 ] ad FRAG [cm 3 ] deotes a cross-sectio for the effect of mai chai scissio, cross-sectio of absorptio i o-desorbed fragmets, desity of itact molecules (without a double bod i the backboe chai) ad desity of o-desorbed fragmets, respectively. Sice LCLS pulses are very short (100 fs), cross sectios ξ ad ξ FRAG are assumed to be costat over the pulse duratio period. Effects of cross-likig ad other relaxatios are eglected as well. O the other had, a pause betwee each two pulses (more tha 30 ms) is may orders of magitude loger i compariso with characteristic time of relaxatio ad cross-likig effect. Thece, chages i desities SB, ad FRAG ca be evaluated separately for each sigle pulse of the irradiatio sequece ad they are described by the set of sequetial equatios ad iitial coditios as follows: 61

5 ( ) ( -1) ( ) ( -1) ( -1 ) ( ) ( -1) = + FRAG = + αξ F SB 1 2 ( ) ( -1) ( -1 ) ( ) ( -1) SB = SB αξ F SB ( ) ( -1) ( -1 ) ( ) ( -1) FRAG = FRAG + 2αξ F SB ( 0 ) SB ( 0 ) FRAG ( 0 ) = = 0 = 0 0 (2) where the iteger i superscript deotes the umber of accumulated pulse ad α expresses the umber of bods broke by the oe X-ray photo. The oliearity of the process caused by the icreasig desity of double C=C bods ca be iclude ito the depedece of ξ o this desity: SB ( ) ( ) ~ - ξ = ξ SB + ξ = ξ ~ (3) SB where ~ is a wavelegth-depedet parameter of the model. The set of equatios (2) ca be solved with the fiite step i the depth. Sice the maximum crater depth is supposed to follow some isodesity of sigle C C bods [9], the experimetal data were iteratively fitted with the simulated curves. Fig. 5 shows the result of this procedure. It was foud out the experimetal data ca be fitted with the isodesity of of the iitial desity of sigle C C bods. Although the experimetal data ca be fitted with the mathematical model described above ad crater depth ca be predicted, some limitatios for this model were determied. First of all, for short sequeces of laser pulses a liear depedece of the depth of the eroded crater o the umber of accumulated pulse is presumed. Cotrary to that, oe of isodesities i the model ca match such a presumptio. Other limitatio is joied with the parameter ξ FRAG. Whe it is set to zero, it meas all fragmets desorb out of the material. O the other had, whe this parameter is o-zero ad costat, it implies all fragmets remais iside the material or very close to the surface, ad all of them cotribute to the losses i fluece. I our case, the fragmets produced i layers very close to the surface of the sample mostly desorb out, whilst the fragmets i deeper layers remai iside the material. Here they ca absorb icomig radiatio or recombie with the ed-liks ad thus lower the yield of formig double bods. SB Figure 5. Maximum depths of craters eroded with accumulated LCLS laser pulses at fluece level of 4mJ/cm 2 fitted with iteratively computed isodesity of SB / 0 =

6 Coclusio Efficiet erosio of orgaic polymer PMMA has bee observed eve for the fluece level may times lower tha that of the sigle-shot ablatio threshold. This sub-threshold, desorptio-like damage to the material is iitiated by the sigle-photo radiolytical effect kow as chai scissio. Depedece of the efficiecy of the material removal o the total dose of eergy exhibits oliear behaviour of at least two types. First of them is related to the decrease i etch rate with the icreasig umber of accumulated shots. The secod oe correspods to the lower efficiecy of erosio whe material is exposed to the radiatio at higher fluece. Both of these types of oliearity ca be described i term of chemical chages iside the irradiated material iduced by the icidet radiatio, especially the effect of formatio of double C=C bods (kow as cross-likig) ad creatio of free fragmets which remai i the material. The progress i maximum depth of the eroded area ca be simulated umerically as a fuctio of umber of accumulated shots. Although the experimetal data ca be fitted with the umerical model described above, some pheomea, such as ability of simulatio of short irradiatio sequeces with almost liear icrease i depth, are to be implemeted ito the model i future. Ackowledgmets. This work was partially supported by the Czech Miistry of Educatio (Projects LC510, LC528, LA08024, ad ME10046), the Czech Sciece Foudatio (P205/11/0571 ad P108/11/1312) ad the Academy of Scieces of the Czech Republic (Grats Z , IAAX , ad KAN ). Refereces [1] J. O. Choi et al., Degradatio of PMMA by deep ultraviolet, X-ray, electro beam, ad proto beam irradiatios, J. Vac. Sci. Techol. B, 6(6), pp , [2] C. K. Marcia et al., Photodecompositio of PMMA thi films by moochromatic soft X-rays, J. Vac. Sci. Techol. A, 13(4), pp , [3] J. Chalupský et al., No-thermal desorptio/ablatio of molecular solids iduced by ultra-short soft X-ray pulses, Opt. Express 17(1), pp , [4] J. Chalupský et al., Spot size characterizatio of focused o-gaussia X-ray laser beams, Optics Express, 18(26), pp , [5] J. Chalupský et al., Comparig differet approaches to characterizatio of focused X-ray laser beams, Nuclear Istrumets ad Methods i Physics Research A 631, pp , [6] P. Emma, et al., First lasig ad operatio of a ågstrom-wavelegth free-electro laser, Nature Photoics 4(9), pp , [7] J. D. Bozek, AMO istrumetatio for the LCLS X-ray FEL, Eur. Phys. J., Spec. Top. 169, pp , [8] T. Buria et al., Damage to Molecular Solids Irradiated by X-ray Laser Beam, i WDS'11 Proc. of Cotributed Papers: Part II Physics of Plasmas ad Ioized Media, Prague, Matfyzpress, pp , [9] M. de Grazia et al., Desorptio Mechaisms i PMMA Irradiated by High Order Harmoics, Proc. of SPIE Vol. 8077, 80770L, 1 10,