Reliability of Cu Pillar Bumps for Flip-Chip Packages with Ultra low-k Dielectrics

Transcription

1 Reliability of Cu Pillar Bum for Fli-Chi Package with Ultra low-k Dielectric Yiwei Wang, Kuan H. Lu, Jay Im and Paul S. Ho Microelectronic Reearch Center, The Univerity of Texa at Autin, Autin, TX 88 Abtract The reliability of Cu/low k interconnect tructure uing Cu illar bum wa invetigated in thi aer. Firt the characteritic related to electromigration (EM) of Cu illar with Sn-Ag ti were tudied and comared with full Pb-free Sn-Ag older bum. The imulation reult revealed a ignificant reduction in the current crowding when Sn-Ag C older wa relaced by Cu illar tructure. A a reult, the current-induced Joule heating and local temerature gradient were reduced in the Cu illar tructure. Thi wa followed by a tudy of the imact of the Cu illar bum on the mechanical reliability of ultra low-k dielectric. The crack driving force induced by chi-ackage-interaction (CPI) for delamination in the ultra low-k interconnect tructure wa evaluated uing a D ub-modeling technique. The energy releae rate wa found to increae ignificantly for ackage with Cu illar bum comared with thoe with Pb-free older only. Finally, the characteritic of thermal fatigue life of Cu illar bum were invetigated baed on Darveaux train energy denity model. The reult howed that the fatigue life of the older ti adjacent to Cu illar could be imroved by reducing the Cu illar height in relation to the older ti height. Structural otimization of Cu illar bum to imrove the mechanical tability of ackage with ultra low-k dielectric in the chi wa dicued. Introduction The fli-chi ackaging technology i facing increaing demand in the fine itch alication, driven by high denity and roduct miniaturization requirement. The conventional older bum have eriou technology limitation due to the ace required for the herical geometry and reliability concern. Cu illar bum have recently been introduced to relace C older bum a a firt-level ackage interconnect [-]. The tructure conit of a non-reflow-able Cu illar bae with a reflow-able older ca for connection to the ubtrate a hown in Figure []. ackaging level [, ]. In addition, it i eaier to control the illar bum diameter and tand-off height, which are critical for fine-itch ackage. In ite of the inherent advantage, thermomechanical reliability roblem have been reorted for Cu illar bum, articularly for ackaging aembly of ultra low-k chi. Mechanical failure in low-k interlayer dielectric (ILD) beneath Cu illar and delamination between Cu bum and Al bond ad during aembly have been oberved [, ]. In thi aer, we invetigated firt the characteritic related to EM of Cu illar with Sn-Ag ti and comared with that of full Pb-free Sn-Ag bum. Thi wa followed by a modeling tudy of the imact of the Cu illar bum on the thermomechanical reliability of Cu/ultra low-k interconnect. The crack driving force for delamination in ultra low-k interconnect tructure due to chi-ackage-interaction (CPI) wa evaluated uing a D ub-modeling technique together with a modified virtual crack cloure method []. Finally, the thermal fatigue life of Cu illar tructure wa invetigated baed on Darveaux train energy denity model [8]. Baed on the reult, the tructural otimization of Cu illar bum to imrove the mechanical tability of ackage with ultra low-k chi wa dicued. EM Characteritic of Cu Pillar Structure Electromigration in older bum i one of the key reliability iue for microroceor uing the fli-chi technology. Imrovement of EM lifetime ha been reorted for Cu illar bum comared with conventional C bum []. To gain further undertanding, we analyzed current crowding and Joule heating under current treing for the Cu illar bum, which are two material characteritic having direct imact on EM erformance. For thi tudy, we comared thee two older tructure with an identical under bum metallization (UBM) a illutrated in Figure. Si Pb-free Pillar Figure. Cu illar bum with Pb-free Sn-.% Ag older ca [from ] EM imrovement ha been reorted for the Cu illar bum, roviding the otential for further caling of the dimenion to uort increaing interconnect denity at the PCB Figure. Schematic of a) Conventional C bum b) Cu illar Finite element method (FEM) wa emloyed to evaluate the current ditribution and Joule heating for both C bum and Cu illar bum a hown in Figure. The tructure i

2 chematically reented a two older bum with a connecting Cu trace. The Cu illar and trace are hown in yellow, and the older joint are in blue. The current flow direction i a hown by the red arrow. Both trace and older are urrounded with Si die, ubtrate, and underfill material. Simulation demontrated that the illar tructure ha the advantage of ditributing current flow more evenly through the bum. For conventional C bum, the current crowding occur at the entrance of the metal trace into the bum. Wherea for Cu illar, the current wa more evenly ditributed at the entrance region, reulting in a reduction of the current denity by about % a hown in Figure. The location of maximum current crowding (in red color) modeled for the right hand ide bum in Figure alo hifted from the older contact at the to trace to that at the bottom trace a illutrated in Figure (c). an excellent thermal conductor, thi facilitate the heat diiation and imrove the electrical erformance of the device. Reult in Figure how a reduction of about o C in the maximum temerature for the illar tructure. The reduced local temerature in combination with le current crowding make Cu illar more reitant to current treing than conventional C bum. A a reult, an imrovement of the EM lifetime i to be exected. Joule Heating (degree C) illar w/o illar Figure. Comarion of joule heating Changing the illar height while keeing the ame chito-board ga wa found to have little imact on current crowding and Joule heating, a hown in Figured and. The current crowding and Joule heating induced temerature rie remained nearly contant when the illar height wa reduced from µm to µm.. Current Denity (^ A/cm^) illar w/o illar Current Denity (^ A/cm^) / / Pillar Height/Solder Height (c) Figure. FEM model; Current crowding comarion between illar and C bum; (c) Current denity ditribution inide Pb-free older for the right hand ide bum in. Simulation of local Joule heating howed a reduction in the Joule heating for the Cu illar bum due to better electrical conductivity and le current crowding. Since Cu i Joule Heating (degree C) 8 / / Pillar Height/ Solder Height Figure. Comarion of current crowding; joule heating between different ratio of illar/older ca height in the fixed chi/board ga. Chi-Package Interaction (CPI)

3 In ite of the imrovement in EM erformance, imlementation of Cu illar bum ha had it own iue. The mechanical tability of the weak ultra low-k dielectric material in the Cu interconnect raie eriou reliability concern for ackage with Cu illar bum. Failure of low-k interlayer dielectric (ILD) beneath the bum and delamination between Cu bum and the Al ad have been oberved during ackaging aembly [, ]. Ultra low-k material are mechanically weak and ucetible to interfacial delamination or coheive failure during aembly []. In thi aer, we invetigated the CPI reliability iue for Cu illar tructure with the objective to develo otimum illar deign to imrove the CPI reliability. The tudy of thi roblem for ultra low-k dielectric wa analyzed uing a D finite element analyi baed on a multilevel ub-modeling aroach a hown in Figure. In the interconnect ub-model, crack with a fixed length were introduced into a -level tructure at everal interface of interet a hown in Figure. The crack driving force ERR for thee interface induced by fli-chi attach roce wa calculated uing a modified virtual crack cloure technique []. The material roertie ued in thi tudy can be found in Ref.. (c) (d) Figure. Illutration of four-level ub-modeling: ackage level; critical older level; (c) die-older interface level; (d) detailed interconnect level. with Pb-free older a hown for crack in Figure 8. Thi can be attributed to the mechanical trength of the Cu bum comared with Sn-Ag older, reulting in an increae in the thermal tree generated by thermal mimatch between the die and ubtrate during ackaging aembly. The tre can be directly couled into the Cu/ultra low-k interconnect to raie the crack driving force to induce delamination. Energy Releae Rate (J/m ) Pillar w/o Pillar Crack Crack Crack Crack Figure 8. Comarion of ERR between illar and Pb-free bum at for all crack in Figure. To enure the mechanical tability of low-k dielectric, the Cu illar bum tructure hould be otimized. Our tudy howed that the crack driving force for low-k dielectric failure can be reduced by changing the older height and the illar height and diameter. Figure 9 how that reducing the illar height can alleviate the ERR for low-k delamination, eecially for crack which ha the highet ERR. Here, reducing the older height while keeing the ame illar height can alo hel a demontrated in Figure 9. Shorter older height generate maller ERR for all four crack. Therefore the bet aroach would be to reduce both the illar and older height. However, ince a certain total tandoff of the illar and the older i required for the aembly roce, an effective aroach would be to otimize the illar to older height ratio while keeing the total tandoff height contant. Thi will yield a maller illar to older height ratio leading to a lower ERR for low-k delamination a hown in Figure 9(c). SiO SiO Figure. Cro-ection view of interconnect model The crack driving force for low-k delamination, induced by chi-ackage-interaction, wa found to increae by about % for ackage with Cu illar bum comared with thoe Energy Releae Rate (J/m ) um um um Crack Crack Crack Crack

4 Energy Releae Rate (J/m ) Energy Releae Rate (J/m ) um um um Crack Crack Crack Crack illar/older=um/um Pillar/Solder=um/um Crack Crack Crack Crack (c) Figure 9. Effect of illar height at fixed older height, µm; older height at fixed illar height, µm; (c) illar to older height ratio on ERR at fixed total tandoff height, µm Increaing the illar diameter can alo imrove the CPI reliability of low-k interconnect. Two illar diameter were comared in thi tudy, 8µm v. µm, while keeing the illar/older height contant at µm /µm. A lotted in Figure, the ERR for low-k dielectric delamination wa reduced by roughly three time by a % increae in the illar diameter. A larger diameter reduced the amount of tre in the Cu/low-k interconnect induced by CPI. Energy Releae Rate (J/m ) Small Pillar Diameter larger Pillar Diameter Crack Crack Crack Crack Figure. Effect of illar diameter on the crack driving force for fixed illar/older ratio of µm /µm. It ha alo been reorted that, for tructural otimization in the Cu UBM tructure, a thicker olyimide (PI) aivation layer i effective in maintaining the mechanical tability of Cu/low-k interconnect during aembly and i imortant for imlementation of ultra low-k dielectric and Pb-free older bum into fli-chi ackage for advanced technology node []. In addition to the geometrical otimization, we have invetigated other technique to mitigate the imact of CPI on the reliability of low-k interconnect. One of thee technique i called ad hift deign in which the thermal mimatch between the die and organic ubtrate i comenated for by adjuting the metal ad location on the ubtrate [8]. Another technique that i widely adoted i the reditribution layer (RDL) technology [8]. The idea behind thi method i to decoule the older bum away from the landing ad and weak low-k dielectric underneath. Reditribution line are ued to electrically connect the original ad to the relocated older bum. Significant reliability imrovement for low-k dielectric can be achieved by uing the RDL [8]. Solder Joint Fatigue The low-cycle fatigue life time of older bum i a concern for Pb-free older and ha been extenively tudied by thermal cycling tet at elevated temerature [9-]. Such accelerated fatigue tet are time-conuming and often require week or even month to collect a et of data. Beide, direct obervation of fatigue crack initiation and roagation during cyclic tet i a formidable tak. Therefore imulation method have been widely ued to tudy the older fatigue behavior. Several older joint fatigue life rediction model have been rooed baed on variou tre, train, and fracture criteria [9, ]. One oular aroach i an energybaed method rooed by Darveaux [9]. In thi model, the inelatic train in older joint during thermal cycling conit of two art, a time-deendent cree train and a timeindeendent latic train. ε in = ε c + ε () The time deendent cree train of older joint can be exreed a follow: ε c = t + ε T ( ex( B t)) () n Qa = C[inh( ασ )] ex( ) () kt where i the teady tate train rate, ε T i the tranient cree train, B i the tranient cree coefficient, k i Boltzmann contant, T i the abolute temerature, σ i the alied tre, Qa i the aarent activation energy, n i the tre comonent, α i the break-down tre level baed on a ower law relation, and C i a contant. The latic train comonent i decribed by the following train hardening relationhi: m ε = C ( σ ) () G where ε i the time-indeendent latic train, G i the hear modulu, C and m are contant.

5 To aly Darveaux model, a contitutive relationhi rereenting the elatic-latic behavior of the older joint i required. A commercial FEM ackage, Any, ha vicolatic element available which ue Anand contitutive model []. Anand model conit of a flow equation and three evolution equation are: Flow Equation / m Q = A[inh( ξσ / )] ex () kt Evolution Equation: d B α = { ho ( B ) } () B B = where * n ^ / * Q = S ex () A kt where Q i the activation energy, k i Boltzmann contant, A i the re-exonential factor, ξ i the multilier of tre, m i the train rate enitivity of tre, i the deformation ^ reitance, ho i the hardening contant, S i the coefficient for deformation reitance aturation value, n i the train rate enitivity of aturation (deformation reitance) value, α i the train rate enitivity of hardening. The contant for Anand model for lead-free older are lited in Table. []. Table : Contant for lead-free older in Anand model [] Crack growth: da K = K ΔW ave (9) dn ΔW * V ΔW = () ave V where K to K are contant deending on older material and modeling methodologie. Δ Wave i the average vicolatic train energy denity accumulated er cycle for the element at the die/older interface, Δ W i the train energy denity accumulated er cycle of each element and V i the volume of the lice element. The thermal fatigue life time of older joint wa found to decreae with increaing Δ Wave [, ]. Here we invetigated the effect of imlementing the Cu illar tructure on the older fatigue life by calculating the change in the accumulated train energy, Δ Wave, during thermal cycling.. Fli-Chi Package Die Subtrate Figure : A chematic lice model ued for older fatigue tudy To comare the Cu illar tructure with conventional bum, the total tandoff height wa fixed at 8µm. For the illar tructure, the height of Cu illar and older were µm and µm, reectively. The imulation reult demontrated a % increae of ΔWave when the Cu illar are imlemented a hown in Figure. Thi increae will negatively imact on the fatigue life time of Pb-free older ortion during thermal cycling.. S i the initial value of deformation reitance In our tudy, a tyical lice model wa emloyed a hown in Figure. Two thermal cycle were imulated in order to etablih a table tre-train hyterei loo. The train energy denity accumulation er thermal cycle within the critical older bum wa calculated. The train energy denity i related to the crack initiation and growth lifetime a follow: Crack initiation: N K o K ΔW ave = (8) Pb free Bum w/o Pillar Figure. Effect of Cu illar on the accumulated average vicolatic train energy denity er cycle, Δ W The effect of changing the illar and older bum height wa invetigated and the reult are hown in Figure Pillar ave

6 and. The Δ Wave can be reduced either by decreaing the illar height at fixed older height or increaing the older height at fixed illar height. Thi i reaonable becaue Cu illar i much tiffer than Pb-free older and thu induce more thermal deformation in the older during thermal cycling. The effect on fatigue life due to change in the illarto-older height ratio with a contant total tandoff height i hown in Figure (c). A maller illar to bum ratio yielded a longer older joint fatigue life. Thi trend track with the CPI reult for low-k dielectric delamination. Therefore, a maller illar to bum ratio i referred for Cu illar tructure for imroving both CPI and older joint fatigue life reliability um um um Pillar Height um um 8um 8um/um Solder Height um/um Pillar Height/Solder Height (c) Figure. Effect of illar height at fixed older height, µm; older height at fixed illar height, µm; (c) illar to older height ratio on Δ W ave Concluion and Dicuion In thi aer, the imact of imlementing Cu illar bum on the mechanical reliability of fli-chi ackage with ultra low-k dielectric wa invetigated. The EM characteritic, CPI reliability, and older fatigue life of the Cu illar tructure were analyzed and comared with conventional C bum. A ignificant reduction in the current crowding and local Joule heating wa oberved becaue of the uerior reitivity and thermal conductivity of Cu illar, which oint to a better EM erformance for Cu illar bum. The effect of Cu illar tructure on CPI and it imact on Cu/low-k interconnect reliability have alo been invetigated. The origin of the roblem wa traced to the weak thermomechanical roertie of the low-k dielectric material and the large thermal tre induced by ackage deformation during ackaging aembly which drive crack roagation. ERR reult demontrated that the imlementation of Cu illar bum can degrade the mechanical tability of the Cu/low k tructure due to the increae in the driving force for fracture. Structural otimization uch a reducing the illar height and increaing illar diameter wa found to be effective in imroving the mechanical reliability of Cu/ultra low-k interconnect under CPI. Other method to imrove low-k material integrity during ackage uch a increaing the thickne of PI aivation layer and uing RDL tructure have alo been dicued. Finally, the older joint fatigue life time wa tudied for ackage with Cu illar. The fatigue lifetime of Pb-free older, in contact with the Cu illar, wa found to be degraded comared with conventional C buming uing full Pb-free older. Thi i becaue of a larger thermal deformation accumulated in the older in the reence of the tiff Cu illar. Reducing the illar height or increaing older bum height can imrove the older fatigue erformance. Overall, a maller illar to bum ratio i referred for Cu illar tructure for better thermo-mechanical reliability with regard to both CPI and older joint fatigue life. In ummary, the Cu illar tructure i generally beneficial for older EM lifetime imrovement, but oe a challenge for CPI-induced ultra low-k dielectric reliability and the older fatigue life. Reference. A. Yeoh, M. Chang, et al., Coer Die Bum (Firt Level Interconnect) and Low-K Dielectric in nm High Volume Manufacturing, Proc. th ECTC,,. -. J. Yu, A. Anand, et al., Reliability Study on Coer Pillar Buming with Lead Free Solder, Proc. th ECTC,,. 8-. B. Eberberger and C. Lee, Cu Pillar Bum a a Lead-Free Dro-in Relacement for Solder-Bumed, Fli-Chi Interconnect, Proc. 8 th ECTC, 8,.9-

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