Microelectronic Engineering

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1 Microelectronic ngineering 88 () Contents lists ville t ScienceDirect Microelectronic ngineering journl homepge: Mechnicl properties nd frcture mechnism of porous SiOC low-k dielectrics.l. Chng, C.T. Kuo, M.S. Ling Institution of xecutive Mster of Business Administrtion, Ntionl Chio Tung University nd Deprtment of Mteril Science nd ngineering, Ntionl Chio Tung University, Tiwn rticle info strct Article history: Received Septemer Received in revised form 6 Mrch Accepted 9 Mrch Aville online 2 Mrch Keywords: Indenttion Low-k Mechnicl properties A series of low-k dielectric films with vrious mechnicl properties were prepred for 32-nm ck-end-of-line technology node. Vrious precursors were used, nd the porogen removl tretment ws performed using the 2 or -em-ssisted method. This study presents novel pproch for determining the loss of mterils during tretment. This pproch includes determining yield strength, frcture toughness, onding structure, nd frcture mechnism of series of low-k silic films. The results show tht low-k film formed using the precursor trimethylsilne hs higher yield strength nd frcture toughness thn the low-k films formed using the precursor tetrmethylsilne or octmethylcyclotetrsiloxne. The residul gs nlysis ws conducted to determine the loss of mterils nd predict the onding structure; the results show tht the -em tretment rerrnges the structure more effectively thn the 2 tretment y using the rdil to decompose the methyl group. Finlly, the frcture mechnism of these low-k films ws determined y relting the crck ptterns of the indents on these films to their indenttion lod displcement curves. Ó lsevier B.V. All rights reserved.. Introduction Corresponding uthor. Tel.: mil ddress: gldies.chng@gmil.com (.L. Chng). As the feture sizes continue to shrink to improve their resistnce cpcitnce performnce, porous low-k mterils re used in their ck-end-of-line (BOL). Becuse low-k mterils re mechniclly wek, the post-process tretment is crucil for reconstructing the onding nd efficiently removing porogen. The fundmentl understnding of such tretment is importnt for controlling the tretment depth nd the film properties. Becuse low-k mterils re porous, determining their mechnicl properties is chllenging. In generl, mteril frctures elsticlly under stresses much lower thn its cohesive strength []. The discrepncy etween the oserved frcture strength nd the theoreticl cohesive strength ws explined y the pioneering work of Griffith, who proposed tht the propgtion of crcks from defects or the crcking in the mteril y mgnifiction of locl stress concentrtions cuses the finl frcturing [2]. Low-k mterils re normlly low in density, ecuse pores were dded to them to chieve low-k vlues. The degrdtion of mechnicl properties cused y these defect-like intrinsic pores is the min concern in the pplictions of these low-k films s semiconductor BOL interconnect technologies, ecuse such degrdtions my result in films peeling nd crcking during processing. Mechnicl properties such s yield strength nd frcture toughness of thin-film/thick-sustrte composites re difficult to mesure using conventionl techniques. This pper demonstrtes n pproch to mesure these properties using nnoindenttion technique lredy used to mesure the hrdness nd Young s modulus of thin films. This pproch descries systemtic reltionship etween thin film nd the thin film/sustrte system [3]. The results of ductile/rittle ehvior studies of vrious low-k films deposited on Si sustrtes suggest tht gret enefits cn e gined in low-k film evlution nd development y using this pproch. 2. xperiments Six low-k films with k = 3., 2.7, 2.7, 2.6, 2.4, nd 2.4, leled s Smples 6, respectively, were deposited on Si wfers y plsmenhnced chemicl vpor deposition. Films were prepred using different precursors nd post-tretments, s shown in Tle. Smple ws prepred using the trimethylsilne (Si:C:O = :3:) precursor nd Smple 4 ws prepred using the tetrmethylsilne (4MS) (Si:C:O = :4:) precursor. Smples 2 nd 3 were prepred using the octmethylcyclotetrsiloxne (Si:C:O = :2:) precursor, ut Smple 3 hd n dditionl e tretment pplied to it. Smples nd 6 were prepred using the 4MS precursor with dditionl tretment y -em nd 2, respectively. All the -nm-thick films were nnoindenttion-tested to otin their indenttion curves y using nnoindentor on CSM instrument with force resolution of ln nd displcement resolution of.3 nm. The /$ - see front mtter Ó lsevier B.V. All rights reserved. doi:.6/j.mee..3.8

2 624.L. Chng et l. / Microelectronic ngineering 88 () Tle Film chrcteriztion of Smples 6. Smple Precursor Tretment K 3MS 3. 2 OMCTS OMCTS e MS 2.6 4MS -em MS corresponding indent mrks were investigted y n tomic force microscope (AFM). rdness, MP () Smple rdness 8, GP 3. Results nd discussion Depth of indenttion / Film thickness Indenttion curves re well known to specify the reltionship etween lod P nd displcement h, which re continuously monitored nd recorded during indenttion. From the indenttion curve, hrdness is defined s the pek lod divided y the projected contct re, nd Young s modulus is defined s the initil slope of the unloding prt of the indenttion curve. Fig. () (d) shows the mesured hrdness nd Young s modulus s function of the indenttion depth/film thickness for Smples 4, s clculted from their indenttion curves with vrious test lods, with exmples shown in Fig. 2() (d). The elstic/plstic deformtion zone in thin film/sustrte composite produced y indenttion expnds to the sustrte s the indent goes deeper into the thin film. Beyond certin depth, hrdness mesurements re ffected y not only film properties ut lso sustrte properties. Thin film/sustrte composites hve een studied y finite-element method to chrcterize their elstic plstic response [4]. In the cse of soft film on hrder sustrte, similr to the cse considered in this study, the film s hrdness cn e descried s ¼ þ f s s exp ðr f =r s Þ ð f = s Þ ðh c=t f Þ 2 ; ðþ where t f is the film thickness, h c is the contct depth, is Young s modulus, r is the yield strength, is the hrdness, nd the suscripts f nd s represent the film nd the sustrte, respectively, wheres no suscript represents the composite system. From Fig. () (d) nd q. (), the yield strength vlues of Smples 4 re clculted to e 478, 2, 3, nd 6.2 MP, respectively. Under the sme pplied lod, film with higher yield strength hs smller deformtion zone compred to the one with lower yield strength. A soft low-k film exhiits low yield strength, indicting its tendency for rittle frcture under n pplied stress. This suggests tht for films with higher yield strength, the intrinsic film hrdness cn e mesured t greter depths in ulk films, s demonstrted in the mesured hrdness depth profiles of Smples 4 shown in Fig. () (d), with the lengths of the mrked A plteus corresponding to the intrinsic film hrdness, in proportion to the film yield strength just clculted. Consequently, the intrinsic hrdness vlues of Smples 4, which re independent of sustrte properties, re mesured t indenttion depths of less thn %, 3%, %, nd % of the film thickness; these hrdness vlues re found to e 2.37,.66,., nd.43 GP, respectively, s specified y the mrked A plteus in Fig. () (d). Nnoindenttion tests on Smple t pek indenttion lods of, 2., nd mn yielded the smooth indenttion curves in Fig. 2(). The yield strength vlue indictes the pplied stress t which the mteril egins to deform plsticlly; the yield strength vries for Smples 4. No crcking ws detected y inspection with n rdness, MP c rdness, MP d rdness, MP () Smple 2 rdness Depth of indenttion / Film thickness (c) Smple 3 rdness Depth of indenttion / Film thickness (d) Smple 4 rdness Depth of indenttion / Film thickness Fig.. () (d) rdness nd Young s modulus s the function of depth/film thickness of Smples 4., GP, GP, GP

3 .L. Chng et l. / Microelectronic ngineering 88 () () Smple mn c.4 (c) Smple 3 Lod, mn mn Lod, mn.3.2 Pop - in mn. Lod, mn Displcement, nm () Smple 2 Pop - in d Lod, mn Displcement, nm (d) Smple 4 Pop-in. 8 8 Displcement, nm. 8 Displcement, nm Fig. 2. () (d) Indenttion curves of Smples 4, with the first pop in for Smples 2 4 indicted in () (d). Fig. 3. () (d) Corresponding indent mrk AFM imges to the indenttion curves shown in Fig. 2() (d) with rrows indicting the rdil crcks t the edge of indent mrks. AFM for the mximum lod of mn; Fig. 3() revels tht Smple underwent plstic deformtion without crcking in the test lod rnge of mn. owever, the indenttion curves of Smples 2 4 under vrious test lods indicte sudden dvnces of the indenter tip into the mterils, which re clled pop-in kinks. Discontinuities in the indenttion curves of Smples 2 4 t the first pop-in kinks were locted t.3,.24, nd.9 mn, respectively, s shown in Fig. 2() (d). The AFM imges of Smples 2 nd 3 shown in Fig. 3() nd (c), respectively, revel tht these two films udged slightly upwrd nd tht rdil crcks ppered t the edge of the indent mrks. The AFM imge of Smple 4 shown in Fig. 3(d) revels tht the film moved significntly upwrd nd tht rdil crcks ppered t the edge of the indent mrk. Becuse the indenttion depths t which the pop-in kinks occurred re sustn-

4 626.L. Chng et l. / Microelectronic ngineering 88 () () Wfer in (2) -em on (3) Wfer out. Pressure (Log ) Cron Mss4 C3 C4 2O Ar Mss28 C2 Fig. 4. Schemtic digrm of rekthrough crck formtion. Time (sec) () Wfer in (3) Wfer out (2) 2 plsm (4) Wfer clen Pressure (Log ) Cron N2 C3 C4 2O Mss29 C2-6 Time (sec) Fig. 6. () RGA spectrum of the -em treted low-k. () RGA spectrum of the 2 treted low-k. Fig.. Schemtic digrm of the relesed strin energy. Tle 2 Mechnicl properties of Smples 6. Smple rdness, Gp Young s modulus, Gp Yield strength, MP Frcture toughness (MP m /2 ) tilly less thn the thickness nd hrdness of the films, nd the Young s modulus of the Si sustrte is much higher thn those of the films, the kinks oserved in Fig. 2() (d) must result from the crcks in the films themselves rther thn from crcks t the film/sustrte interfces or in the Si sustrtes. The presence of kink in the indenttion curve is ssocited with the formtion of rekthrough crck in the film, schemticlly depicted in Fig. 4. A rekthrough crck in the film llows the indenter to e displced frther without incresing the pplied lod. According to this rekthrough crck formtion model, the relesed strin energy tht is required to form new crck surfce cn e determined using the specific re projected y the indenttion curve, s first proposed y Li et l. []. The frcture of the films under lod-controlled indenttion cn e oserved in the lod displcement curve shown in Fig.. When rittle film yields, crck is generted to relese high strin, which is indicted y the shdowed region in Fig.. The ehvior of the pplied lod nd the displcement chnges when crck is generted; the AB segment (Fig. ) represents the drop in lod strength when n internl crck is generted. As mode I frcture is dominnt, the opening mode in plne strin, the criticl stress-intensity fctor or the frcture toughness, K IC, cn e written s G =2 U =2 K IC ¼ ð m 2 Þ ð m 2 Þ2pCR t nd du G ¼ 2pCR dc where is the elstic modulus, G is the strin energy relese rte, m is Poisson s rtio, C R is the crck length in the film plne, nd U is ssessed from the kink. Bsed on q. (2), the clculted frcture toughness vlues re.4 MP m /2,.26 MP m /2, nd.49 MP m /2 for Smples 2, 3, nd 4, respectively. The frcture toughness for Smple is not ville, ecuse it underwent plstic deformtion for the entire lod rnge. The mechnicl properties of the films were mesured y the nnoindenttion technique nd re summrized in Tle 2. Becuse Smple 4 hs the lowest hrdness nd yield strength, tretments to improve its mechnicl strength were studied. Smples nd 6 were formed using the sme precursor s Smple 4, ð2þ

5 .L. Chng et l. / Microelectronic ngineering 88 () , GPrdness, MP, GPrdness, MP () /=. GP/9.26 GP Depth of indenttion/film thickness ut then followed y the -em tretment for Smple nd the 2 tretment for Smple 6. Fig. 6 nd presents residul gs nlysis (RGA) spectr of Smples nd 6, respectively, following their post-tretments. Fig. 6 shows the three stges typicl of RGA spectr of films with -em post-tretment: () the wfer processing stge, which corresponds to the wfer eing moved in nd out of the chmer; (2) the -em ctivtion stge, which corresponds to the -em eing turned on fter the chmer pressure is stilized; nd (3) the methyl/methne group stge, which corresponds to when the methyl/methne group cn e detected once the film is under the -em tretment. Similr ehvior is oserved for the film under 2 tretment. To summrize, methyl/methne groups re the min yproducts of the -em tretment, nd 2 outgssing is the min yproduct of the 2 tretment. The -em tretment cuses the loss of times s mny methyl groups s the 2 tretment, indicting tht the -em tretment cuses greter structurl rerrngement. To further study the mechnicl properties of Smples nd 6, Fig. 7 nd presents their hrdnesses nd Young s modulus. The greter hrdness following the -em tretment (.72 GP) thn tht following the 2 tretment (. GP) is consistent with the nlysis of the RGA spectr of Fig. 6 nd, which suggests tht the -em tretment cuses greter structurl rerrngement thn the 2 tretment. FTIR onding identifiction revels tht new Si C 2 Si onds re formed nd Si C 3 onds re eliminted. Fig. 8 shows the FTIR spectr of Smple efore nd fter the -em tretment. Fig. 8 is the sutrction curve derived from (2) () /=.72 GP/.7 GP rdness Depth of indenttion/film thickness (2) rdness Fig. 7. () rdness nd Young s modulus of Smple.() rdness nd Young s modulus of Smple 6. (3) (3) Asornce Asornce Fig. 8 to study the chnge in the onding structure fter the tretment. The tretment mechnism is considered chin rection etween rdils nd methyl groups. The dissocition of terminl onds in Si C 3 y the formtion of C 3 (methne) cn chnge the onding structure y forming new Si C 2 Si onds. The Si C 2 Si network in the low k skeleton in Fig. 8 is considered to fvor onding tht strengthens low-k films. 4. Conclusions An pproch sed on indenttion is developed to mesure the mechnicl properties of low-k films. Yield strength nd frcture toughness, the most criticl mechnicl properties of low-k films in semiconductor processing were mesured nd compred. Chemicl rective tretments of low-k films tht fvor new ond formtion to improve mechnicl properties were lso purposed. Tremendous enefits cn e gined in low-k film evlution for etter film nture understnding nd key indices cn e identified nd mesured in film development for expedited process optimiztion, which gretly shorten the cycle time in delivering integrtion comptile low-k films. References After Before Wvenumer (cm-) Si- (+) formed Si-O (+) Si- (+) Si-C2-Si cross links(+) 2.22 Si-C (-) After trt Before Sustrction (After - Before) Wvenumer (cm-) Fig. 8. () FTIR spectrum of Smple efore nd fter tretment. () Bonding structure chnge fter tretment. [] K.M. Lin,.L. Chng, Mter. Sci. ng. A 87 (997) 39. [2] G.D. Dieter, Mechnicl Metllurgy, McGrw-ill, London, 988. pp [3].L. Chng, C.T. Kuo, Di. Relt. Mter. () 9. [4] B. Bhushn, ndook of Micro/Nno Triology, CRC, Bot Rton, 99. pp [] X. Li, D. Dio, B. Bhushn, Act Mter. 4 (997) 443.