Morphology Analysis of Cu Film Fractures in Sandwiched Methylmethacrylate Plates

Transcription

1 ISSN MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 21, No Morphology Anlysis of Cu Film Frctures in Sndwiched Methylmethcrylte Pltes Cristino FIDANI 1, Murizio De CRESCENZI 2 1 Osservtorio Sismico A. Bin, Borgo XX Giugno, 74, 06121, Perugi, Itly 2 Diprtimento di Fisic, Università di Rom Tor Vergt, Vi dell Ricerc Scientific 1, 00133, Rom, Itly Received 22 Ferury 2014; ccepted 20 My 2014 Thin films of Cu were evported on solid pltes of polymethylmethcrylte (PMMA). A successive polymeriztion process ws mde to relize sndwiched structure to protect the Cu films. Frcturing of the metl film surfce ws oserved with severl morphologies showing two different frcture systems. Surfce film morphology ws nlysed in terms of the distriution re of the islnds nd contour frctl dimension. The islnd re showed mximum corresponding to 42 nm of the Cu thickness, which ws threshold to oserve two min frcture systems. The frctures pttern resulted to e scle invrint with frctl dimension etween 1.55 nd 1.7. The minimum frctl dimension lso depended from the film thickness nd corresponded to the mximum of the islnd res. The reported effects cn e understood on the sis of different therml expnsion coefficients of the two mterils, their thermlly induced dhesion nd the ction of the monomer. Keywords: copper thin films, e-em evportion, deposition thickness, polymethylmethcrylte, surfce nlysis. 1. INTRODUCTION Deposition of thin films of severl metllic mterils on polymer surfces is frequently used to otin mirrors with selective properties for reflectivity nd trnsmission of visile light [1]. Nowdys the thin metl films re widely used for industril products of mny ojects rnging from reflector for cr lights, compct discs nd electriclly shielded computer cses to foils for food pcking [2]. Finlly, the deposition of thin metllic films on polymers re of primry importnce oth for ctive nd pssive use, to increse the chemicl-physicl properties of the ojects [3]; while the sndwiched structure re uilt to increse ending stiffness nd ending strength [4]. Generlly, severl metllic-sed pints [5] give wide rnge of colours nd effects when mixed with the pre-polymer. When these pints re used s films, they re generlly deposited fter tht the polymer, used s sustrte, hs een modelled to otin the desired oject. These methods intrinsiclly suffer of the non uniformity of coverge induced y the non regulr shpe of the oject. For this reson, the use of sndwiched pltes with metl cores, cn constitute vlid solution to overcome the undesired effect of inhomogeneity. Severl methods of deposition re now ville y epitxil growth, nmely: electro-deposition, electro-less deposition, sol-gel deposition, nno-prticle growth nd crystlliztion, chemicl precipittion, chemicl vpour deposition nd vcuum therml evportion [6]. The possiilities offered y the vcuum technology re now well estlished nd wit for wider use in the industril processes [3, 7]. A fundmentl prolem to e solved still remins the protection of the metllic films from the oxidtion nd Corresponding uthor. Tel.: ; fx.: E-mil ddress: c.fidni@virgilio.it (C. Fidni) 220 mechnicl scrtches. A vlid solution cn e found in the sndwiched structures tht cn e relized y susequent polymeriztion. However, this process modify strongly the morphology of the film surfces depending from the chemicl-physicl condition of the polymeriztion nd metllic thickness. The morphology of metl film fter the second polymeriztion of PMMA ws investigted in this work. The thin metl film ppered frctured fter the tretment, showing n interesting complexity due to the presence of two frcture system. The form nd the re of the different islnds were nlysed with respect to the thickness of the metllic film. The physicl properties descriing the oserved morphology were quntified nd discussed. Cu ws chosen for depositions ecuse of its chep cost nd prcticl fcility of evportion, while PMMA for good sticking coefficient. The studied smples were squred of 250 mm of side nd 8 mm of complete thickness, eing of suitle dimension in the production of smll ojects. 2. EXPERIMENTAL Sustrtes of PMMA were supplied y Frtelli Guzzini s.p.. ( Recnti, Itly, to otin new composite mterils for illumintion systems. Sustrtes strted s ig pltes of ( ) mm 2 nd 3 mm of thickness. Then ech plte hs een cut in mny squres of ( ) mm 2. This dimension resulted to e very similr to tht of mny Guzzini s houses rticles nd, in ddition, very suitle for the vcuum chmer nd the crucile geometry. The vcuum chmer ws constituted y ell of 350 mm of dimeter nd 500 mm of height, the vcuum system ws constituted y rottive pump coupled to turo-moleculr. The finl vcuum of 10 6 Torr ws esily otined. The deposition system ws composed y Mo

2 crucile BD T filled to otin n emission re of (10 5) mm 2. PMMA temperture of smples ws incresed y the crucile irrdition during the metl deposition. The evlution of the thickness ws mde through the sensor crystl INFICON , which position is shown in Fig. 1 with the geometry of the experimentl deposition system. Strting from the mesurement of the thickness S 0 detected on the sensor position, the thickness of ny other point on the plte surfces cn e clculted [8]. Introducing n (x, y) system of coordintes, s shown in Fig. 2, the x = 0 is put on the centre of the plte surfces. If L 0 is the distnce etween the crucile nd sensor nd L(y) the distnce etween the crucile nd the xes of the plte with x = 0, one cn clculte the metl thickness in ny point. If AE nd AD re the section res of emission nd deposition perpendiculr to the tomic em, the effective res of emission is indicted y AE eff (x, y) perpendiculr to the em nd tht of deposition y AD eff (x, y). These res cn e relted to the slope of the em with respect to the plte nd crucile plnes. Then it is: 2 L A e ( xfd, yf) A S( x, y) = S 0 0 x + L ( y) A e ( xfe, yf) A (1) nd it cn e clculted pproximtely s: ( x, y) ( d + c αo) L ( ys) d ( 1+ d ) 2c ( yl) 2 2 ( x + c) L0 ( x c) L0 + ( x + c) + L ( y) ( x c) + L ( y) S S0 3, (2) where L(y) = [y y 2( + y)cosα] ½, nd d = ( + y)cosα. In the presented configurtion, 2c = 10 mm ws the length of the crucile, = 24.4 mm nd = mm were distnces defined from the geometry of the deposition system, finlly α = 55 ws the inclintion of the plte. The contour grphics of (2) with the ove mentioned constnts is shown in Fig. 2, where the grey colour is progressively clerer for thickness more nd more thicker. With Cu thickness of 30 nm mesured on the sensor, one cn clculte 10 nm 70 nm of Cu films thickness on the plte, distriuted s shown in Fig. 2. prepred y mixing the monomer, of chemicl formul CH 2 C(CH 3)(COOCH 3) [9], with the ctlyst zoisoutyrronitrile (AZDNM). The pltes with the films were fixed etween two crystl surfces seprted y spce of 2.5 mm. Then, the pre-polymer ws poured to fill the empty spces etween crystl-polymer nd crystl-film surfces to uild ( ) mm sndwiched structures. The polymeriztion ws otined y mintining constnt nd uniformly distriuted temperture of 60 C for four hours, then the temperture ws incresed up to 105 C for one hour to check the complete polymeriztion of the monomer. Fig. 2. The contour grphics of the deposited thickness S(x, y) computed ccording (2). It reflects the oserved symmetry of the metl deposition To study the frctured surfces severl photos were relized in trnsmission light, using the microscope OLYMPUS BX60 together with photo cmer OLYMPUS DP10. The formt of the res photos ws 20 mm 15 mm, with n imge definition of 640 pixels 480 pixels corresponding to 32 pixels/mm. This size hs een chosen to cover the smllest re contining the mximum numer of structures with ner constnt thickness. Specil ttention hs een devoted on focusing cmer imges, ecuse depth-of-field must cover the whole depth of secondry frcture system. Lrger field photos were mde with norml photo cmer with zoom ojectives, then the photos were converted to digitl imges y scnner. The photos were mnipulted nd nlysed y two progrms of free softwre: SCIONIMAGE ( to study the res of frcturing nd FDC2D ( /pckges/rchitec/frctls/fdc2d.se.hqx) to study the frctl dimension. All of imges were mnipulted to otin grey scle colours with etter contrst. Fig. 1. Schemtic experimentl system locted inside the vcuum ell, the section is shown for x = 0 The polymeriztion fter metllistion ws relised in the Guzzini s electricl oven. The pre-polymer ws MORPHOLOGY OBSERVATIONS Cu thermlly evported t room temperture on PMMA pltes evolves progressively from smll droplets up to the formtion of continuous thin film [10 12]. Then, the surfces ppered completely mirror in the centre of the smple while showing prtil reflection on the corners, where it ws oserved mrked trnsprency due to the reduced thickness of the film. After the second polymeriztion ws concluded, the Cu films resulted completely frctured. Although, the

3 frctures were not uniformly distriuted on the surfce nd forming complex structures. Strting from the surfce regions corresponding to Cu minimum thickness, on the right nd left corners on the top, nd moving towrd the centre of symmetry of Fig. 2, n increse of the islnd verge re ws oserved. However, ech islnd hs ecome frctured moving nerer the centre of symmetry, creting n dditionl complex frcturing visile in Fig. 3. The pttern of the frcture system hs shown symmetry very similr to tht reported in Fig. 2. Moving from the centre towrd the ottom of Fig. 2, the principl islnds decresed ecoming very smll, while the secondry frctures disppered. Fig. 3. Photos of two zones of the smple in trnsmitted light fter the second polymeriztion. The photos reported on the left covers n re of (120 90) mm 2 while tht on the right covers n re of (90 60) mm 2 The secondry frcture system ppered with different morphology hving contours rounded off, wheres the primry frcture presented with very shrp roken lines. It ws possile to oserve n irregulr film profile looking t section of the sndwich structure t some positions. This irregulrity ws oserved in correspondence of the secondry frcture system. Such system ws chrcterised y Cu islnd edges folded towrds the second polymeriztion, nd occupied the third dimension principlly round the centre of the Fig. 2. Folding ws responsile of pprent rounded off contours. Irregulrity thickness reched 2 mm round the centre of the Fig. 2. The width of the frctures of the principl frgmenttion incresed in this region ecuse the surfce re of the smple ws the sme while the re of the metl film occupied the third dimension. A shrp frgmenttion occurred t the ottom position of Fig. 2. This ppered difficult to explin following the Cu thickness only, ecuse it hs different symmetry. However, the shrp frgmenttion cn e correlted to the modulus of the grdient of the thickness, which is shown in the contour plot of Fig. 4,. The frcturing incresed where there were strong chnges of the thickness. The principl frctures were oserved to diverge from the centre of symmetry of smple, follows the vector field of the grdient plotted in Fig. 4,. The longer size of the islnds were directed long the sme direction. Fig. 4. Contour plot of the modulus of the grdient of the thickness () nd vector field plot of the sme grdient () The surfce of the film ppered not smooth when oserved in reflection light. Film surfce irregulrities were oserved on ll the region smples, nd they were very fine on the corners on the top of Fig. 2, while they were corser where size of the re of the islnds hd got mximum. The secondry frcture system prevented to oserve the irregulrities of the Cu islnds ner the centre of the smple. To otin more quntittive lw on the frgmenttion, some grphicl methods of nlysis such s the re nlysis nd the frctl nlysis were used. 4. ANALYSIS AND DISCUSSION A digitl nlysis of the frctured Cu film ws conducted. The file imges creted y the photo cmer were initilly converted in grey scle, then incresed the contrst nd converted in inry files. The results re shown in Fig. 5,, nd Fig. 6,. After this procedure, the islnd s re sizes were nlysed employing SCIONIMAGE routine. It ws set choosing the minim considerle size. This procedure ws very importnt to eliminte the imperfections of imge. In ddition, prticles touching edge imges were ignored while interior islnd holes were included. The nlysis gve the chrcteristic wideness of the islnds in the different regions y distriution, verge nd stndrd devition. Distriutions n(a) with respect to the numer of the islnds of re A, 222

4 reltive to the res reported in Fig. 5,, nd Fig. 6,, re shown respectively in Fig. 5,, nd Fig. 6,. They re Poissonin distriutions. islnds re delimited y the principl frcture system. The mximum islnd re ws out 7.6 mm 2 nd it occurred for Cu thickness greter thn 50 nm. Looking to Fig. 3, islnds pper to e divide y secondry frcture system where Cu thickness overcme 42 nm. Therefore, the true mximum re of islnds is oserved t 42 nm thickness. These true mximum re islnds follows the profile with the sme thickness of Fig. 2, s it is confirmed y Fig. 3. The stndrd devition hs lso mximum for Cu thickness corresponding to 42 nm. Finlly, monotone decresing of the islnd res creted y secondry frcture system is shown in Fig. 7 right down, it strts with islnd res of out 1 mm 2. Fig. 5. A photo, in trnsmitted light, of zone of the smple where the thickness ws 22 nm () nd the distriution of the res of the islnds with men vlue of 3.4 mm 2 () Fig. 7. Men re nd stndrd devition with respect to the thickness clculted from the nlysis of mny smple photos Interestingly, if the primry frcture system is compred with the modulus of the Cu thickness grdient shown in Fig. 4,, it is possile to oserve res to ecome smller where the plot is white. In other terms, the res size of the islnds lso depends from the Cu thickness vrition. The effect strts to e effective in the white re of Fig. 4,, which is equivlent to overcome 5 nm/mm s shown in Fig. 8. Decresing re is responsile for stndrd devition mximum s white region is crossed y 42 nm Cu thickness. Fig. 6. A photo, in trnsmitted light, of zone of the smple where the thickness ws 64 nm () nd the distriution of the res of the islnds with men vlue of 0.6 mm 2 () The plot of the re verge vlues s function of the thickness of the film is reported in Fig. 7. This shows monotone incresing re of the islnds up to the centre of the smple, where Cu thickness ws mximum. These 223 Fig. 8. Plot of the solute vlue of the thickness grdient long the direction x = 30 mm where ll the photos were tken A further digitl tretment ws necessry to nlyse the frcture system. Becuse the frctures re of finite thickness, we hve to modify the imges to otin skeletristion. The imges were converted in grey scle, enhnced in contrst, converted to inry file to otin the negtive photo nd then skeletrised with SCIONIMAGE options. The results of

5 these procedures re shown in Fig. 9. PAINT SHOP PRO resulted to e very useful to delete mny imperfections introduced y the conversion nd it ws suitle to convert the formt file of imge in PICT for Mc. FDC2D performed the log[1/s] nd log[n(s)] nlysis, eing s the dimension of the ox counting nd n(s) the numer of the oxes necessry to cover ll the grphics. The nlysis reveled the frcture system geometry of the metl film to e frctl. An exmple of the scle invrince of film frctures ws reported in Fig. 10, where the frctl dimension is defined s the ngulr coefficient of the stright line tht fits the vlues of the nlysis of Fig. 9,. Dimension ws scle invrint etween 2 mm nd 20 mm for the principl frcture system while it extended etween 0.8 mm nd 20 mm for the whole frcture primry plus secondry systems. The frctl dimension ws clculted etween 1.55 nd 1.63 for the principl frcture system, while it ws clculted etween 1.55 nd 1.7 for the whole structure, comprising the secondry system s well. Interestingly minimum in the frctl dimension ws oserved in close correspondence to the trnsition etween the two systems. Fig. 9. Grphicl preprtion to the frctl nlysis: ws reltive to the photo of Fig. 5, ; ws reltive to the photo of Fig. 6, The different therml dilttion coefficients of the two mterils must e considered to explin the origin of film frctures. In fct, the second polymeriztion ws otined y incresing the temperture of the smple up to 60 C. Dilttion coefficient of Cu films is 17.7 ppm/ C [13], equl to the ulk Cu dilttion coefficient, while for ulk PMMA the dilttion coefficient is 90 ppm/ C [14]. Being so, Cu film experienced frctures ecuse of the strin field pplied y PMMA mjor expnsion. The primry frcture system hving incresing size of the islnd res with the film thickness occurred ecuse the incresing cohesion of Cu [15]. Fig. 10. The mesurements of the frctl dimension of the frcture system of the photo in Fig. 6,, ws mde y ox counting of Fig. 9,, the geometry shown to e scle invrint etween 0.8 mm nd 20 mm To understnd the secondry frcture system, it cn e considered tht metl dhesion on PMMA improves with incresing temperture [16]. Chemicl rections t the metl/polymer interfce were usully discussed in terms of cron-oxygen ond-reking of the croxylic group giving rise to C O Metl groups [17] or the formtion of chrge trnsfer complex, s hs een climed in metllised polyimide smples [18]. However, the chemicl onding ws considered wek in Cu/PET smples nd dhesion strength correlted with the metl diffusion into the polymer lyer to form clusters [16, 19]. PMMA ws heted y the crucile high temperture due to the proximity of the smple for irrdition. Strting t room temperture, in the rnge etween 80 C 120 C, it ws oserved tht the metl dhesion incresed very rpidly from 240 g in 1 up to the upper limit vlue of 1300 g in 1 [16]. So, the in-plne film trction ws creted during the smple polymeriztion in the oven, nd it ws greter in the region with mjor dhesion. Incresed dhesion thus produced the secondry frcture system. Then, the ction of chemicl melting from the monomer on the polymer cn e consider. Becuse the specific weight of the monomer is 0.92 while tht of the polymer is 1.2 [14] n increse of sustrte volume should e expected where the contct re etween the prepolymer nd PMMA occurred, in the frcture spces. The sustrte volume incresing folded the Copper islnd edges towrds the pre-polymer, with consequent incresing size of the frctures. Scle invrince of two frcture systems cn indicte tht the sme process is frcturing t different dimensions, which is the stress due to different dilttion coefficients. The minimum frctl dimension ner to 42 nm of Cu thickness cn e interpreted s due to the opposite contriute to islnd frgmenttion of the incresing metl thickness nd the incresing dhesion to PMMA. 5. CONCLUSIONS Thin film frcturing morphologies of Cu in polymer composite sndwich structures were digitlly investigted. The thin film morphology, fter the polymeriztion, consisted of two frcture systems giving rise to two different islnd s re sizes. Surfce stress effects during the polymeriztion were considered to yield the frctures, 224

6 ecuse of different therml expnsion coefficients of Cu nd PMMA, depending from Cu thickness nd dhesion to the polymer surfce. A folding of Cu film islnd edges ws explined with the monomer ction on the PMMA in the frcture res. Cu thickness ws simulted with mthemticl model nd compred with the frcture pttern. Islnd res nd frctl dimension of the frcture systems were quntified y digitl nlysis, so to know the prmeter to reproduce the sme esthetic effect. The sndwiched structures were prepred with the ojective to crete new diffused light effects s lmpshdes produced y the Compny iguzzini for Indoor lighting nd outdoor lighting luminries ( Acknowledgments The finncil ssistnce given y Guzzini s.p.. is grtefully cknowledged. Acknowledgement to R. Bernrdini for the severl suggestions to the deposition of the films, G. Pisuri for the chievement of the polymeriztion nd the XAS group of the Cmerino University for the pprtus nd the ssistnce. REFERENCES 1. Wooten, F. Opticl Properties of Solids, Springer, Acdemic Press, Mittl, K. L. (Ed.) Metllized Plstics 1 5; Fundmentl nd Applied Aspects, Plenum New York, Tg, Y. Recent Progress of Nnotechnologies of Thin Films for Industril Applictions Mterils Sciences nd Engineering C : pp Goswmi, S., Becker, W. Anlysis of Deonding Frcture in Sndwich Plte with Hexgonl Core Composite Structures 49 (4) 2000: pp ENGELHARD CORPORATION in: Industril Grde Perlescent Pigments Color Crd Ctlougue, Pigments & Additives Group, Iseline, New Jersey, Fendler, J. H., Tin, Y. Nnoprticles nd Nnostructured Films. J. H. Fendler (ed.), WILEY-VCH, Mttox, D. M. Hndook of Physicl Vpor Deposition (PVD) Processing. 2nd edition, Elsevier, Scher, E., Kowlczyk, S. P., Pireux, J.-J., Metlliztion of Polymers. Oxford University Press, ATO, elf utochem NORSOCRILMMA. July, Lewis, B., Anderson, J. C. Nucletion nd Growth of Thin Films. Acdemic, New York, Yu, X., Duxury, P. M., Jeffers, G., Duson, M. A. Colescence nd Percoltion in Thin Metl Films Physicl Review B : pp Adms, J. B., Wng, Z., Li, Y. Predicting the Microstructure of Cu Thin Films Thin Solid Films : pp Zho, J.-H., Du, Y., Morge, M., Ho, P. S. Simultneous Mesurement of Young's Modulus, Poisson Rtio, nd Coefficient of Therml Expnsion of Thin Films on Sustrtes Applied Physics : pp Lide, D. R. Hndook of Chemistry nd Physics. 72 nd Edition, CRC Press, Streitz, F. H., Cmmrt, R. C., Sierdzki, K. Surfce- Stress Effects on Elstic Properties. I. Thin Metl Films Physicl Review B : pp Silvin, J. F., Ehrhrdt, J. J. An Overview on Metl/PET Adhesion Thin Solid Films : pp Atnsosk, Lj., Anderson, S. G., Meyer III, H. M., Lin, Z., Wever, J. H. Aluminum/Polyimide Interfce Formtion: An XPS Study of Selective Chemicl Bonding Journl of Vcuum Science nd Technology A 5 (6) 1987: pp Hight, R., White, R. C., Silvermn, B. D., Ho, P. S. Complex Formtion nd Growth t the Cr- nd Cu- Polyimide Interfce Journl of Vcuum Science nd Technology A 6 (4) 1988: pp LeGoues, F. K., Silvermn, B. D., Ho, P. S. The Microstructure of Metl-Polyimide Interfces Journl of Vcuum Science nd Technology A 6 (4) 1988: pp