physica status solidi applications and materials science

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www.-.com pplictions nd mterils science Stiffening nd hydrophilistion of SOG low-k mteril studied by ellipsometric porosimetry, UV ellipsometry nd lser-induced surfce coustic wves A. M. Urbnowicz1, B.

Bklnov1 1 2 3 IMEC, Leuven, Belgium IRSET, Sn Jose, USA Frunhofer Institute for Mterils nd Bem Technology, Dresden, Germny Received 7 June 2007, ccepted 20 November 2007 Published online 20 Mrch 2008

1 pplictions nd mterils science Stiffening nd hydrophilistion of SOG low-k mteril studied by ellipsometric porosimetry, UV ellipsometry nd lser-induced surfce coustic wves A. M. Urbnowicz1, B. Meshmn2, D. Schneider3, nd M. R. Bklnov IMEC, Leuven, Belgium IRSET, Sn Jose, USA Frunhofer Institute for Mterils nd Bem Technology, Dresden, Germny Received 7 June 2007, ccepted 20 November 2007 Published online 20 Mrch 2008 PACS Pt, B, Pv, f, Ci, Qn In this pper we evlute the correltion between UV light bsorption of SOG (spin-on glss) low dielectric constnt (low-k) films nd chemicl chnges induced by n excimer UV source t severl wvelengths: 172 nm, 222 nm, nd 308 nm. Low-k bsorption in the UV rnge ws mesured by nitrogen purged UV ellipsometry. It is shown tht im- provement of mechnicl properties is ccompnied by degrdtion (hydrophilistion) of low-k films. Chnges in hydrophobic properties were evluted by wter-source ellipsometric porosimetry (WEP). The Young s moduli of the films were mesured by lser-induced surfce coustic wves (LAwves). phys. stt. sol. () 205, No. 4, (2008) / DOI / T N I R P E R

2 phys. stt. sol. () 205, No. 4, (2008) / DOI / Stiffening nd hydrophilistion of SOG low-k mteril studied by ellipsometric porosimetry, UV ellipsometry nd lser-induced surfce coustic wves A. M. Urbnowicz *, 1, B. Meshmn 2, D. Schneider 3, nd M. R. Bklnov 1 1 IMEC, Leuven, Belgium 2 IRSET, Sn Jose, USA 3 Frunhofer Institute for Mterils nd Bem Technology, Dresden, Germny pplictions nd mterils science Received 7 June 2007, ccepted 20 November 2007 Published online 20 Mrch 2008 PACS Pt, B, Pv, f, Ci, Qn * Corresponding uthor: e-mil urbno@imec.be, Phone: , Fx: In this pper we evlute the correltion between UV light bsorption of SOG (spin-on glss) low dielectric constnt (low-k) films nd chemicl chnges induced by n excimer UV source t severl wvelengths: 172 nm, 222 nm, nd 308 nm. Low-k bsorption in the UV rnge ws mesured by nitrogen purged UV ellipsometry. It is shown tht im- provement of mechnicl properties is ccompnied by degrdtion (hydrophilistion) of low-k films. Chnges in hydrophobic properties were evluted by wter-source ellipsometric porosimetry (WEP). The Young s moduli of the films were mesured by lser-induced surfce coustic wves (LAwves) WILEY-VCH Verlg GmbH & Co. KGA, Weinheim 1 Introduction Integrtion of low-k mterils is continuing issue in the microelectronics industry. To reduce the RC dely nd improve the operting speed of multilevel Cu interconnects in modern ultr lrge scle integrted (ULSI) circuits, the k-vlue of dielectric lyers hs to be s low s possible. To decrese the k-vlue, porosity hs been implemented to new genertion of dielectrics [1]. However, porous low-k mterils hve lower mechnicl strength compred to conventionl dielectric films. Improving mechnicl properties (such s Young modulus) of new low-k mterils becomes chllenge. The low-k dielectrics UV curing processes could solve this issue. The combined ction of UV rdition nd therml ctivtion is shown to generte rerrngement in the bonding structure of the SiCOH bsed low-k [2]. As result, the mechnicl properties re improved, but the hydrophilicity increses. Subsequent moisture bsorption into the porous structure significntly increses the k-vlue of dielectric film becuse of high polrizbility of wter molecules. Most of results of low-k dielectrics UV curing reported so fr re relted to widebnd UV sources. It is expected tht nrrow bnd UV sources re more efficient for understnding of precise mechnisms of modifiction nd such dt re importnt for optimistion of technologicl processes. Opticl trnsmission mesurements of SiCOH bsed lyers showing trnsprency in the fr UV rnge with incresing bsorption in the VUV suggest the use of excimer light source. The min dvntge of n excimer light source is tht it cn emit light with very nrrow spectrl distribution. In this pper we nlyse the effect of UV nd VUV excimer light of different wvelengths (172 nm, 222 nm nd 308 nm), different power nd exposure time to study mechnism of photo induced chnges of SOG low-k. The originl (pristine) nd UV treted SOG low-k films were chrcterized by nitrogen-purged ultr-violet (PUV) ellipsometry for determintion of bsorption in the UV rnge, ellipsometric porosimetry (EP) for determintion of porosity nd pore size [3], nd ellipsometric porosimetry with wter source (WEP) for evlution of hydrophobic properties [4]. The mechnicl properties of the films were evluted by lser-induced coustic wve spectroscopy 2008 WILEY-VCH Verlg GmbH & Co. KGA, Weinheim

3 830 A. M. Urbnowicz et l.: Stiffening nd hydrophilistion of SOG low-k mteril (LAwve) [5]. Fourier trnsform infrred (FTIR) spectroscopy ws used for chrcteristion of the chemicl composition before nd fter UV irrdition. 2 Experimentl 2.1 Smples preprtion The smples used in this study were silic-bsed low-k films deposited on 200 mm silicon wfers by SOG technology. The films hd 10 12% crbon-contining-hydrophobic groups, 32% porosity, nd pore size close to 2 nm fter stndrd curing (therml nneling). The k-vlue of the films ws in the rnge of Such prepred smples were irrdited by plsm-bsed, plnr VUV (vcuum ultr-violet) source, which provides single or multiple bnds between 120 nm nd 350 nm (Fig. 1). For this evlution, 172 nm, 222 nm nd 308 nm were used. The plsm is induced in chmber with excimer gs mixture. The light from the plsm is emitted into processing chmber through specil qurtz window. The spectrl distribution of the emitted light depends on excimer gs composition s shown in Fig. 2. During the UV exposure, the smple chmber is flowing with N 2 (99.99% purity) nd continuously pumped down. N 2 pressure is bout 1 Torr with flow 0.1 liter per minute. 2.2 Metrology The refrctive index nd extinction coefficient of the film studied were mesured by Sopr PUV-EP ellipsometric system (λ = nm). Mesurements were done in nitrogen mbient t tmospheric pressure. Amount of H 2 O nd O 2 during mesurement ws below the level of 5 ppm. Porosity, pore size distribution nd bulk hydrophilicity were mesured by using ellipsometric porosimeter EP-10 equipped with SENTECH 801 spectroscopic ellipsometer (λ = nm). The ellipsometer is mounted on vcuum chmber tht cn be filled with solvent vpor (such s toluene or wter) in controllble wy [3, 4]. The elstic modules of the films studied were mesured by LAwve. This technique is bsed on high frequency surfce coustic wves, which re very sensitive to surfce film, even if their thickness is much thinner thn the penetrtion depth of the wves. The coustic wves re generted by short lser pulse in wide spectrl rnge. The phse velocity of the wve propgtion is mesured depending on frequency. Fitting the theoreticl curve to Figure 2 Power versus wvelength of emitted light for different excimer mixtures. the curve mesured yields the elstic modulus (Young s modulus) of the film [5]. The chemicl composition of low-k films before nd fter UV irrdition ws nlyzed using Biord QS2200 ME FTIR system. 3 Results nd discussion Figure 3 shows the refrctive index nd the extinction coefficient of the pristine low-k dielectric film (no UV tretment) in rnge from 150 nm to 650 nm. The pristine smple hs strong bsorption below 180 nm. It indictes tht the UV region below 180 nm is photochemiclly ctive ccording to fundmentl lw of photochemistry [6]. Figure 4 reflects chnges of Young moduli (YM) fter UV irrdition. The y-xis presents exposure. The exposure is product of UV power, time of tretment nd light frequency. The highest chnges in YM re observed for 172 nm. This improvement of mechnicl properties indictes tht wvelength of light below 180 nm cuses the highest efficiency of photochemicl rections. Dielectric films treted with 222 nm nd 308 nm UV revels smll improvement of YM. This correltes well with FTIR nd EP dt s presented in Figs. 5 nd 6. Figure 1 Scheme of the experimentl setup. Figure 3 Refrctive index nd extinction coefficient of the low-k (s mesured by PUV) versus wvelength WILEY-VCH Verlg GmbH & Co. KGA, Weinheim

4 Originl Pper phys. stt. sol. () 205, No. 4 (2008) 831 Figure 4 Young s moduli (s mesured by LAwve) versus UV exposure. The numbers on the grph correspond to the smple numbers in Tble 2. Figure 5 shows rtio Si O Si (1050 cm 1 ) cge bond to Si CH 3 (1276 cm 1 ) FTIR peks versus exposure. This rtio reflects well the compromise between hydrophilicity nd mechnicl properties. Incresing the number of Si O Si groups gives better mechnicl properties. While the number of Si O Si groups increses, reduction of the number of CH 3 (methyl) groups is observed [7]. The methyl groups re hydrophobic due to low polrizbility. Depletion of CH 3 groups leds to hydrophilistion of the low-k. The degree of hydrophilistion is shown in Fig. 6. The hydrophilisition of UV treted low-k s cn be relted to photo-dissocition of crbon contining groups nd cross-linking mechnism of the Si O Si network. Possible rections for UV nd VUV irrdition re presented in Tble 1. Another reson of hydrophilistion cn be relted to influence of residul oxygen in the UV vcuum chmber. Excited oxygen cn remove crbon-contining hydrophobic groups. Figure 6 Hydrophilicity (s mesured by EPW) versus exposure. The energies of the rections listed in Tble 1 were clculted by Ushio et l. who used moleculr models of the rectnts nd products nd first-principles moleculr orbitl progrm, Gussin 03 [8]. Since the rection energy corresponds to the driving force of rection in thermodynmic sense, rections 1 nd 2 re the most probble to occur. Rection 4 is insignificnt, becuse the rection energy is negligible. Considering the rection energies nd compring the chnges in mesured FTIR spectr, we cn conclude tht Si O Si cross-link genertion from CH 3 group nd OH group is the primry rection. Methne (CH 4 ) releses s rection product of this rection. Therefore, Si O Si cross-link genertion leds to CH 3 groups depletion nd, s result, strong hydrophilistion occurs. The results of our tests nd settings of the UV system re summrized in Tble 2. These dt show tht the highest modifiction of low-k films occurs fter 172 nm irrdition due to strong bsorption of our mteril in this region (Fig. 3). This correltes with reduction of porosity nd men pore rdius (s mesured by EP). Moreover the rtio Si O/Si CH 3 (s clculted from FTIR) reflects tht improvement of mechnicl properties leds to reduction of the number of hydrophobic CH 3 groups. This correltes with WEP dt. Mechnicl properties re improved but hydrophobic properties become significntly worse. In the region of nm the impct of photochemicl rec- Tble 1 Possible rections during UV nd VUV irrdition nd their rection energies (i.e., differences between energies of rectnts nd those of products) [2]. Figure 5 Si O Si/Si CH 3 rtios (s mesured by FTIR) versus exposure. rection energy [ev] 1 O 3 Si CH 3 + HO SiO 3 + CH O 3 Si H + HO SiO O 3 Si CH 3 Si H + CH O 3 Si OH + HO SiO 3 O WILEY-VCH Verlg GmbH & Co. KGA, Weinheim

5 832 A. M. Urbnowicz et l.: Stiffening nd hydrophilistion of SOG low-k mteril studied Tble 2 Summry of experimentl conditions nd results. smple wvelength (nm) time (min) power FTIR Si O/Si CH 3 EP pore rdii (nm) EP porosity (%) EPW wter (%) low med med high low high high low low high pristine tions is low becuse of low low-k bsorption. However, we still observed some chnges fter 222 nm nd 308 nm UV irrdition. We cn ssume tht it my be relted to hot plte nture. When smple is irrdited with UV wvelength from hot plte region, most of the light is trnsmitted through low-k nd wrms up the Si substrte due to strong bsorption of silicon for the wvelengths shorter thn 400 nm. This effect could be relted to multi-photon bsorption on the Si substrte ner interlyer with the low-k. Temperture of the Si substrte my ply role in l nd chemicl processes occurring during the UV tretment of the low-k. 4 Conclusions The lrgest improvement of the mechnicl properties of the low-k studied occurs during 172 nm UV irrdition due to the fct tht the bsorption edge of the SOG low-k mtrix lies below 180 nm. However, improving the mechnicl properties of low-k films by UV light leds to significnt hydrophilistion. This phenomenon is relted to removing of hydrophobic crbon-contining groups s result of UV induced photodissocition nd Si O Si cross-linking mechnism. The degree of hydrophilistion of 172 nm ws higher thn in the cse of 222 nm nd 308 nm. There re two possible resons. One reson could be relted to the smller degree of photo-chemicl rections due to lower UV light bsorption for 222 nm nd 308 nm wvelengths. Another reson might be relted to influence of residul oxygen in UV vcuum chmber which could be excited by 172 nm rdition. Therefore, n optiml blnce between improvement of mechnicl properties nd hydrophilistion of low-k must be estblished. Acknowledgement It is our plesure to thnk Dennis Shmiryn for fruitful discussions. References [1] K. Mex, M. R. Bklnov, D. Shmiryn, F. Icopi, S. H. Brogersm, nd Z. S. Ynovitsky, J. Appl. Phys. 93(11), 8793 (2003). [2] J. Ushio, T. Ohno, T. Hmd, S.-I. Nko, K. Yoned, M. Kto, nd N. Kobyshi, Jpn. J. Appl. Phys. 46, 17 (2007). [3] M. R. Bklnov nd K. P. Mogilnikov, Microelectron. Eng. 64, 335 (2002). [4] M. R. Bklnov, K. P. Mogilnikov, nd Q. T. Le, Microelectron. Eng. 83, 2287 (2006). [5] D. Schneider, P. Siemroth, T. Schülke, J. Berthold, B. Schultrich, H. H. Schneider, R. Ohr, B. Petereit, nd H. Hilgers, Surf. Cot. Technol. 153, 252 (2002). [6] J. G. Clvert nd J. N. Pits, Photochemistry (Wiley, New York, 1967), pp [7] K. Yoned, M. Kto, N. Kobysi, N. Mtsuki, K. Mtsushit, N. Ohr, A. Fukzw, nd T. Kiur, Proc Inerconnect Technology Conf., p [8] M. J. Frisch et l., Gussin 03 (Gussin, Inc., Wllingford, CT, 2004), Revision B WILEY-VCH Verlg GmbH & Co. KGA, Weinheim

Measuring Electron Work Function in Metal

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