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Interconnect Al Slides.Pdf Interconnects Outline •Interconnect scaling issues •Aluminum technology •Copper technology araswat tanford University 1 EE311 / Al Interconnects Properties of Interconnect Materials Material Thin film Melting resistivity point (˚C) (µ! " cm) Cu 1.7-2.0 1084 Metals Al 2.7-3.0 660 W 8-15 3410 PtSi 28-35 1229 Silicides TiSi2 13-16 1540 WSi2 30-70 2165 CoSi2 15-20 1326 NiSi 14-20 992 TiN 50-150 ~2950 Barriers Ti3 0W7 0 75-200 ~2200 N+ polysilicon 500-1000 1410 araswat tanford University 2 EE311 / Al Interconnects 1 Interconnect Architecture SILICIDES - Short local interconnections which have to be exposed to high temperatures and oxidizing ambients, e.g., polycide and salicide structures. REFRACTORY METALS – Via plugs, future gate electrodes, local interconnections which need very high electromigration resistance. TiN, TiW – Barriers, glue layers, anti reflection coatings and short local interconnections. Al, Cu - for majority of the interconnects araswat tanford University 3 EE311 / Al Interconnects Why Aluminum? Al has been used widely in the past and is still used Low resistivity Ease of deposition, Dry etching Does not contaminate Si Ohmic contacts to Si but problem with shallow junctions Excellent adhesion to dielectrics Problems with Al • Electromigration ⇒ lower life time • Hillocks ⇒ shorts between levels • Higher resistivity araswat tanford University 4 EE311 / Al Interconnects 2 Electromigration Electromigration induced hillocks and voids Electromigration due to electron wind induced diffusion of Al through grain boundaries voiMdetal Void Dielectric Hillock Metal SEM of hillock and voids formation due to electromigration in an Al line araswat tanford University 5 EE311 / Al Interconnects Electromigration Theory E field FTOTAL = FSCAT + FFIELD Ffield + Fscat # r " qz E Where qz* is effective ion charge r q * ! r Using Einstein’s relation "D = µ # = " D " E kT r r E current density is related to E fie!ld by J = "E = # ! Diffusivity is given by "Ea D = Do e kT Ea qZ * $ Theref!or e " = J # D e kT D kT 0 Ea Meantime to failure is thus of the !for m of MTF " e kT A A phenomenological model for M!T F is MTF = e Ea γ is the duty factor m kT M, n are constants " J n araswat ! tanford University 6 EE311 / Al Interconnects ! 3 Thermal Behavior in ICs 9 175 n o i t a p 8 i ] s 2 s 150 i m D c [ 7 r a e ] e w r W o [ A 125 P p i 6 m h u C m i 100 x 5 a Source: ITRS 1999 M 4 75 50 70 90 110 130 150 170 190 Technology Node [nm] • Energy dissipated is increasing as performance improves • Average chip temperature is rising araswat tanford University 7 EE311 / Al Interconnects Parametric Dependencies of Electromigration Temperature Current Density A ! E $ Mean time to failure is MTF = exp# a & rmJ n " kT % r is duty cycle J is current density Ea is activation energy T is temperature A, m, and n are materials related constants araswat tanford University 8 EE311 / Al Interconnects 4 Electromigration-Induced Integration Limits A ! Ea $ MTF = exp# & r = duty cycle rmJ n " kT % J =current density EM Limited Interconnect Width EM Limited Device Integration Density Ref: Yi, et al., IEEE Trans. Electron Dev., April 1995 Better packing techniques will be needed in the future to araswat minimize the chip temperature rise tanford University 9 EE311 / Al Interconnects Electromigration: Material and Composition Materials Composition ) s r ) u o V e h ( ( a e E m i T e r u l i a F Temperature (K) Cumulative failure (%) Materials with higher activation Adding Cu to Al decreases its self energy have higher resistance to diffusivity and thus increases electromigration resistance to electromigration araswat tanford University 10 EE311 / Al Interconnects 5 (a) Electromigration: Grain Structure Top view (b) (c) Near bamboo structure Self Diffusion in Polycrystalline Materials DS DGB • For Al grain boundary diffusion DGB >> Ds or DL (Ds = surface diffusion, DL = lattice diffusion) • In a bamboo structure grain boundary diffusion is DL minimized araswat tanford University 11 EE311 / Al Interconnects Effect of Post Patterning Annealing on the Grain Structure of the Film. Increasing time Bamboo Structure and/or temperature . With sufficient grain boundary migration a "bamboo structure" may develop . No grain boundary diffusion in the bamboo structure araswat tanford University 12 EE311 / Al Interconnects 6 Microstructural Inhomogeneities and Stress Induced Electromigration On an average the flux is uniform in If the grain size is non uniform a uniform grain size textured film. the flux will vary accordingly. electric current, J e- spanning grain A B e- atomic flux polygranular cluster cluster backPolygranular flux from clusterstress cluster !ss SpanningTensile stress grain CompressiveSpanning grain stress t e ns i le metal atom depletion metal atom accumulation !(t1) t !o o ! (stress) t1 t" comp. build up of stress within polygranular cluster with time due to electromigration L araswat tanford University 13 EE311 / Al Interconnects e- Build up of stress within polygranular cluster with time due to electromigration. !ss t e ns i le !(t1) t !o o ! (stress) t1 t" comp. L •Eventually, a steady-state is approached where the forward flux is equal to the backward flux due to stress. •A criterion for failure of an interconnect under electromigration conditions could be if the steady-state stress is equal to or greater than some critical stress for plastic deformation, encapsulent rupture, large void formation, etc. •The maximum steady state stress would occur at x=0 or x=L araswat tanford University 14 EE311 / Al Interconnects 7 e- !ss !crit !crit !ss ! ! !crit !ss !ss !crit L 1 L 2 • For very large line width/grain size (w/d) values, no spanning grains are present. No points of flux divergence are present and the time to failure is relatively high. • As w/d decreases, some spanning grains occur. This results in divergences in the flux, causing stress to develop that can be greater than the critical stress for failure (with L > Lcrit for most or all of the polygranular clusters). The lines would fail and the median time to failure would decrease. • As w/d decreases more, more spanning grains are present, but the lengths of the polygranular cluster regions between them are shorter, many of them shorter than Lcrit. The stresses that develop, which create the back fluxes and which in turn eventually stop the electromigration flux, are in many cases smaller than the critical stress. • For small enough values of w/d bamboo or near-bamboo structures are present. The polygranular clusters are few, and for some lines, all the clusters are shorter than Lcrit and the lines will not fail by this mechanism. The MTTF thus increases dramatically. araswat tanford University 15 EE311 / Al Interconnects Electromigration: Grain Texture • Empirical relationship (for Al & Al alloys) µ e I(111) 3 S. Vaidya et al., Thin Solid Films, Vol. 75, 253, 1981 MTF ! 2 log[ ] " I(200) 7 263 238 213 188°C 10 ) c 6 e 10 s ( (111) CVD Cu e r E = 0.86 eV a u l i 5 a 10 F - o t - (200) CVD Cu e 104 m i E = 0.81 eV a T 103 1.8 1.9 2.0 2.1 2.2 2.3 1/T (10-3/K) araswat Ref: Ryu, Loke, Nogami and Wong, IEEE IRPS 1997. tanford University 16 EE311 / Al Interconnects 8 Layered Structures: Electromigration Layering with Ti reduces Electromigration e r u l Structure i a f e Al-Si v i t a l u m u C Layered Al-Si/Ti Time (arbitrary units) Improvements in layered films due to redundancy araswat tanford University 17 EE311 / Al Interconnects Mechanical properties of interconnect materials Material Thermal Elastic Hardness Melting expansion modulus, (kg/mm2) point coefficient Y/(1- ) (MPa) (˚C) (1/˚C) Al (111) 23.1 x 10-6 1.143 x 105 19-22 660 Ti 8.41 x 10-6 1.699 x 105 81-143 1660 -6 TiAl3 12.3 x 10 - 660-750 1340 Si (100) 2.6 x 10-6 1.805 x 105 - 1412 Si (111) 2.6 x 10-6 2.290 x 105 - 1412 -6 5 SiO2 0.55 x 10 0.83 x 10 - ~1700 araswat tanford University 18 EE311 / Al Interconnects 9 Layered Structures: Hillocks Pure Al Homogeneous Al/Ti Layered Al/Ti Surface profile Layering with Ti reduces hillock formation araswat tanford University 19 EE311 / Al Interconnects Stress due to Thermal Cycling Difference in the expansion coefficients of the film and substrate causes stress in the thin film. (a) Film as-deposited (stress free). (b) Expansion of the film and substrate with increasing temperature. (c) Biaxial forces compress the Al film to the substrate dimension araswat tanford University 20 EE311 / Al Interconnects 10 Process Induced Stress Expansion of Al with respect to Si Pure Al Aluminum 300 ! (in Al) 0.64 micron thick ! (in Si) 200 Cooling Silicon Elastic deformation 100 Stress (MPa) 0 Si pulls inward on Al at interface, Heating Plastic deformation inducing compressive stress on Al. -100 0 100 200 300 400 500 Temperature (˚C) araswat tanford University 21 EE311 / Al Interconnects Stress Measurement Biaxial stress in the film % E ( t 2 " = ' s * s & 1#$ s ) 6t f R f • Es is Young’s modulus of the substrate, • vs is Poisson’s ratio for the substrate, !• (Es/1-vs) is the biaxial modulus of the substrate, • ts is the substrate thickness, • tf is the film thickness, and • Rf is the radius of curvature induced by the film araswat tanford University 22 EE311 / Al Interconnects 11 Hillocks Al ILD Al ILD Al Hillock formation due to compressive stress and diffusion along grain boundaries in an Al film.
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