CITARA.TERISTI.S
`
`oF Ta AS AN UNDERLAYBR FOR CU INTERCONNECTS
`
`Tpeparment of Materials Scienc_e anf Engineering, Slanford university, Stanford' cA 94305
`$#"g$.ffiPi[''Hoo-rEoNclret'cHANGsuPRYUtt'RoBERrsn'iclARi'and
`icenter fo, tnt.grul'Ji,3i';J,,i,' s""*il it"*"tffi' stunford' cA 94305
`
`ABSTRACT
`,-^+rar rFyrrlre. and interdiffusion of
`.
`oi'-
`rhe effects of the ra un{err11;1.on the 'dh*l:1,:;:f:[:l;#*T:#; ;;a
`"n"'gv
`overlying cu films iu"" u""-*"tf tl;u,.|1X?r:iltiil:il are obtained owing to the hete-
`ut +00 'c with ra to rorm
`p.,,ii," {p:.,,?ll.lt* ::* :ffil]'l!.]';;; r";"; .,i '"".i. '*aitv
`fr +Jffi**ini".r*iur,**,n.,n,ffi :*:'*:ix:Si*:,:*t;"ilIli:'i:+"1i":'t
`unarl.nTr.:::,j,*;i'JJ,.T:::'j:#Ht!:t'llil;J*.qo"n'rv, athickraunderra'",n,
`5 nm into ra, whire ve.?#il;;
`laver rema
`needed to .o*pr"Lrv ,oeng-rg arrr-G rhe extenr;;;;;#qr":r^]"erface
`Jinu'ion. fh: "*]:1.:|||:*?ffiffi;,^r,in r" und cu lay-
`unchanged.u"n uir., annealing ut ooo;c. n' laminate multilayer of aiternat'
`to'o6b;g' b"t Jgglomt'ut"s when annealed at 800 'C'
`ers is intact "p
`1 E^
`NTRODUCTION
`In very large scale integrated ('LSD devices,.the.signal a.lll *.:o interconnects lS no
`i Ji ; iil; r* it' r' i n l'o"i ^" ii i o: : f :'j:'.,i'tffi ;3it*:
`l on ger ne g r i gib r e .:;;;; i n ri
`H"Ti:J::*,1il":"J.fr l'liltrtT**f ;l*fi fi :l:ii1':';1:il;ffi "?;'ir''^0""'-1""
`of cu to dielectr;';;;';p'"y"n1'1" iii*t cu ions tnlu;n *'* *"1""'f::*'"1'i"""1""T'::3;
`,oryme,a,sandry;-;'i#'{**11frjilS;:#S*il"::::i1i:,?Iffi I
`:1ifi::Tff;rfii""ii or.e^-
`r.sistun..18l as welr as
`has t"'n it'o*n to provide enhanced elr
`Ji'i*#*trnffiiifi Tl",lffi :'$tt"#;'*gtransmissionerecff
`-.r[S.which
`int iur;;;.;" co uniiu i' unuryrro ro reveal crystallograpr
`prrr"*un"" or Ta as an underlayer'
`as weri as X-ray aitrru[tio", tt
`and reactio" t .**, ., rh. int rfuc. ffi ,. .".r,r*, tt
`
`{ ,
`
`i1
`
`.T
`
`In
`
`irI
`
`.lrj
`
`14i
`
`"i
`
`onmicroscopv(rEM)
`
`"
`
`"
`
`EXPERIMENT
`
`Cu struc$re-to
`
`Two types of specimens w::e.fabricated: (1).the'Si/Sio2i25-n.1'J1]-u*
`invesdgare the inteiace in questi"l #;'6i?:t;y fl:,},f,#'':fr or 13-nm ra and 18-
`il"ll'qJif fa;"fi :Ii::Iil:,il:t'll!?{::Tnx*4ilfJ,"3qil:::iruft "
`The cu grain size disiribution *u, i.ir*ined from pr^"*i,i* and cross-sectional TEM micro-
`?;:"1*;,::T"X',"',1J,',T{ffi ltl'd'fr"'111tr[:lih**fr :*ff##r:i.*ffi iii'i;
`srains u, r.*p.ru*ris-as row as,*.,a* b"rn ,.portllrlilirr rnr'1]**ntJorientations
`and Ta ar the interface were invesG"r.a uv ,t".t on'iitrru.rion' k-'uy iiffraction was used to
`.on 6/
`.t
`;.
`l
`Coniefenee Frocearlintre I ll Sl ylll @ t Oot ilalarialc gaaaarah Gar
`^/\
`
`,cu
`
`711
`
`,-reaaia*rr
`
`of
`
`li,:l
`,I i
`
`1..
`
`.!
`
`:1
`!,,l
`
`Iit.
`
`{I 1,
`
`{iII I 1I1Iit 1
`
`Page 1 of 6
`
`IP Bridge Exhibit 2033
`TSMC v. IP Bridge
`IPR2016-01249
`
`

`

`obtain the global texture of the deposited films'
`The synthesized Ta films exhibited a tetragonal structure, classified as beta-Ta (B-Ta)
`r+;i*"*'ivaaat.rconsequently, throughout this paper, Ta refers strictly to B-Ta'
`x,-_'-- .._*--:-'='
`
`with
`
`RESULTS AND DISCUSSION
`Since the most popular underlayer in semiconductor fabrication is Ti/TiN, the approach of the
`investigation starts with comparison of Cu films on Ti/TiN and Ta' Fig' 1 shows the bright field
`imagesofCufilmsdepositedonTiniNandTa.Itcanbeseenataglancethate*u-on*th-e&-under-
`laver has larser grains. The Cu film on Ti/TiN has been p,99lq4-gf at times during the TEM sam-
`il;ffihighresolutionelectronrrfcroscope_(HRE,M)imagesattheinterface
`between underlayer and Cu shown in Fig. 2 explain clearly the improv-ement of adhesion with the
`thickness. of
`Ta underlayer. iu-regsls-rgaddy--ulh-T;-to-fornq-an elLgab-o-qs-l$9&q9Jaygurylth
`about 3 nm while_TiN/Cuinterface-has-no-ev-idence of-interfacsreaerioileYerrafterbeingannealed- - -
`AZ-OO:-ff".. the adhesion of the thin film is determined by the comp$i$qn between the ther-
`*ar31r.SanJ the physical/chemical bonding, it would be enhanced by thb'ihemical bonding due
`to the interface reaction such as that between Cu and Ta. The difference in the grain size on the
`two types of underlayers, however, could not be explained from the HREM micrographs'
`The selected area electron diffraction patterni of cross-sectional and plan-view specimens
`shown in Fig. 3 reveal that Cu on Ta has certain crystallographic orientation relationships not only
`in-plane, bui also in out-of-plane directions. The cu ( 1 1 I ) and Ta (002) planes are aligned parallel
`k:=+---r-;-
`
`totheinterface,andbq]rhthg'C'-u-Q?ol_oirection_andjla_13-3-0]*djrep.tlon_are.-parallel,.onlh-o.1etO LIrtr llltctlaetr, otrt_!!l!!L_U5o=-.---o-
`1^^-.- ^^^.-^_- --^i^*:
`eleg$, rhat is at1ffiintl+a(oozlriirllt is interesting that cu and l1 Tl"-Tj:"Hl*ff:
`;i*tutil ""i ;r,bi'.ii5?i:e" iffilt-l singre cu grain covers numerous Ta grains. The median
`| ---\
`'r-7 /
`
`Fig. 1. Bright field TEM images of cu films grown on (a) tetragonal Ta and (b) TilTiN'
`
`Page 2 of 6
`
`

`

`Fig.2. Cross-sectional high resolution electron microscope images at the interfaces of (a) Ta/Cu
`and (b) TiN/Cu after anneal at 400 oC.
`
`grain sizes of Ta and Cu are 90 A and 1600 A respectively. The large grains of Cu on Ta evidently
`result from the heteroepitaxial growth of Cu on the Ta underlayer.
`An important feature of interface coherency is how the hexagonally symmetric Cu atoms fit
`onto the tetragonally symmetric Ta atoms. Fig. 4 (a) shows the projection view of atoms in a unit
`cell of a Ta crystal which has P42lmnm symmetry along the c-axisfi0,11]. Unexpectedly, the Ta
`atoms on the (002) plane at elevation of zlc = 0 or 0.5 have pseudohexagonal array, and superim-
`pose with eleveylf{fteen Cu atoms on its (111) plane as shown in Fig 4 (b).The misfit srrain at
`the interface i9?*p%o.])te heteroepitaxial growth of Cu also results in a strong texture in Cu which
`can improu" tFirtfiiUility of Cu interconnects. X-ray diffraction pattern in Fig. 5 verifies the
`strong (l 11) texure in Cu Layer.
`The formation of the amorphous interface layer observed in Fig. 2 between Cu and Ta is not
`
`Fig. 3. Selected area electron diffraction patterns of (a) cross-sectional specimen and (b) plan-
`view sample within a single Cu gran. Prefixes C and 'none' in indices stand for diffraction
`from Cu and Ta, respectively. A and B in (b) indicate two different sets of diffraction from
`Ta t3301 planes.
`
`Page 3 of 6
`
`

`

`se
`
`'fuzJc= l14,3l4
`
`Cu
`
`@o
`Ta, zlc = 0
`Tu zlc = ll2
`Fig. 4. Schematic projection view. of.atoms (a) in a unit cell of tetragonal Ta and (b) in mona-
`tomic layers of C; on Ta at the int.rf*..'fu lattice is strained U89^ 0) to illustrate
`itoic'i
`
`clearry understood yet. This is presumabty aue to. a solid state amorphization process which is
`common in other metal_metal interfaces such as the co-zr system[l2]. The composition across
`the interface was unuf,r.a before una uti* annealing at 400 oC by energy dispersive spectroscopy
`using an electron prob. of 1 nm in diameter. The as-deposited cu firm has a clear interface with ra
`and the composition changes abruptly at the interface as shown in Fig. 6. During the anneal' how-
`ever, cu diffuses at a distince into Ta while little of ra is detected in the cu layer' The composi-
`tionintheinterfacelayervariesg,uounrrv.rtisknown;!4-15@tetragonalT@y.
`toalignitsc-axisn9rqr-a'!g9-tt,'gs-urr-q.';a,andfh1ta@ec-axis[tl]ascanbe
`seen in Fig. 4 (a). Therefore, a rerativ;ry ii,tr. r. tuy.t=rtfl?nr.o.d to block rapid cu diffusion
`
`=O
`
`90
`
`100
`
`c!
`
`o (
`
`6Ft
`
`(g
`c)to
`20 30 40 50 60 70 80
`20 (degree)
`
`zc
`ri
`
`(d
`
`=aco
`
`c
`
`Fig. 5. X-ray diffraction pattern oJ Cu deposited on Ta'
`
`Page 4 of 6
`
`

`

`--l- Cu (annealed)
`---t- Ta (annealed)
`
`----o-- - Cu (as-deposited)
`---0-- - Ta (as-deposited)
`
`100
`
`80
`
`{d9
`
`60
`.F
`(6
`
`L5
`
`+ooc
`
`10
`
`0-
`
`10
`
`O
`
`Relative Distance (nm)
`Fig. 6. Composition profiles across the Cu/Ta interface before and after anneal at 400 oC. The
`electron probe in energy dispersive spectroscopy was I nm in diameter.
`
`at elevated temperatures.
`According to the thermodynamic study on Cu-bcc Ta system[l4],gtr-e5g1wo metals have little
`solubilltyjn-e*qch other,and*there is-no--in-qe.Ilg_d,gt_e_p!1se in between. The ffi
`face amorphous layer and high content of Cu in tiiaie-prob-atity Eatised by the fact that Cu is in
`contact with metastable Ta, not with bcc Ta. A high temperature anneal is essential to examine the
`stability of the interface between metastable Ta and Cu. A 20-ply alternating multilayer of l3-nm
`Ta and l8-nm Cu was constructed and subject to heat treatment. Fig. 7 shows a series of bright
`field TEM images of the composite structure annealed at 600 and 800 oC for t hour. The constitu-
`ent Cu and Ta layers are maintained with clear interfaces at 600 oC. The thickness of the interface
`layer remains unchanged at 3 nm up to 600 oC. When the multilayer was annealed at 800 oC, two
`or three Ta layers agglomerate, and the original layered structure is destroyed. This reaction tem-
`{Li^t u^ 7r}
`
`Fig. 7. Bright field TEM micrographs of Cu/Ta multilayers (a) as-deposited, (b) annealed at 600
`oC and (c) 800 oC. Dark and bright layers are Ta and Cu layers, respectively.
`
`Page 5 of 6
`
`

`

`';a-L'*
`perarure agrees well with the crystallization temperafure of an coevaopated amorphous Cu/Ia
`alloytl5l. th. th.r*al stalllrfrresulrSimply the CrlTa interface is quite stable under typical back
`end process temperatures(il90 "C)- '
`.-r'
`
`CONCLUSION
`
`The crystallography, adhesion, interdiffusion, and reaction at the Cu/Ta interface have been
`Ta as an underlayer for Cu interconnects. The interface reaction between
`investigatea to
`"uiluite
`the two film at 400 .C enhances the adhe_s.io_n_of.Cu. -However, the reaction and rapid diffusion of
`cufito Ta;t a A;d; oT?6oua5 nm requires a relatively thick Ta underlayer to ensure its diffu-
`sion barrier capability at elevated temperatures---Cri-grows-heteroepit4xially on the tetragonal Ta
`The-heteroepitaxial growth of
`with the cryst;llographic orientation ofldu111 Dlz1o1llTar002)t3301'
`Cu enhances the formation of large grains rfitnitron-g (ffD-tcxffi-re in Cu film. This is expected to
`improve the reliability of Cu interconnects. The multilayer of alternating 13-nm Ta and 18-nm Cu
`maintains its structure with clear interfaces up to 600 oC, but agglomerates at 800'C. The thermal
`stability of the Cu/Ta interface is confirmed at 600 "C.
`
`ACKNOWLEDGMENTS
`
`The authors wish to thank Mr. Alvin L.S. Loke at Stanford University for his fruitful discus-
`sion. This work is partially supported by the Spmiconductor Research Corporation under contract
`IJ-400.
`
`REFERENCE
`
`l. S.P. Jeng, R.H. Havemann, and M.-C. Chang, Mater. Res. Soc. Syrnp.Proc., 337,25 (1994)
`2. S. P. Murarka and S. W. Hymes, Critical Reviews in Solid State and Materials Sciences 20,8'7
`(1ee5)
`3. M.H. Tsai, S.C. Sun, C.E. Tsai, S.H. Chuang, and H.T. Chiu, J. Appl. Phys. 79,6932 (1996)
`4. H. Ono, T. Nakano, and T. Ohta, Appl. Phys. Lett- 64,1511 (1994)
`5. S.C. Sun, M.H. Tsai, H.T. Chiu, S.H. Chuang, and C.E. Tsai, in Proc.IEEE-IEDM,461 (1995)
`6. J.S. Reid, E. Kolawa, R.P. Ruiz, and M.-A. Nicolet, Thin Solid Films, 236,319 (1993)
`7. K.W. Kwon, C. Ryu, R. Sinclair, and S.S. Wong, Appl.Phys. Lett. (to be published)'
`8. C. Ryu, A.L.S. Loke, T. Nogami, and s.S. wong, in Proc. IEEE-IRPS,1997 201
`9. E.M. Zielinski, R.P. vnci, and J.c. Bravman, J. Appl. Phys. 76,4516 (1994)
`10. P.T. Moseley and C.J. Seabrook, Acta Cryst.B29,l 170 (1973)
`11. G.J. Dickins, Audrey M.B. Douglas, and W.H. Taylor, Acta Cryst. 9,297 (1956)
`12. R. Busch, F. Gaertner, S. Schneider, R. Bormann, P. Haasen, in Proc. Mater. Res. Soc., Spring,
`1994p.229
`13. R.A. Huggins, in Diffusion in Solids: Recent Developments, edited by A.S. Nowick (Aca-
`demic Press, 1975) ch. 9
`t4.L. Kaufman, CALPHAD, 15(3), 243 (1991)
`15. M. Nastasi, F.W. Saris, L.S. Hung, and J.W. Mayer, J. Appl. Phys. 58(8)' 3052 (1985)
`
`Page 6 of 6
`
`