Adhesion and Electromigration in Cu Interconnect. Jim Lloyd, Michael Lane and Eric Liniger. Yorktown Heights, NY 10598

Adhesion and Electromigration in Cu Interconnect Jim Lloyd, Michael Lane and Eric Liniger Yorktown Heights, NY 10598

Adhesion and Electromigration Cu and Al act very differently with respect to electromigration performance The differences have been attributed to the difference in the nature of the interfaces Cu adheres poorly to most materials of interest as compared to Al Al forms a very adherent tough stable oxide that inhibits diffusion at that interface Cu does not

Ellingham Diagram Free Energy of a reaction vs temperature For oxide formation Elements lying below will reduce oxides lying above Aluminum to Al 2 O 3 Copper to Cu 2 O Silicon to SiO 2

Cu electromigration has been shown to depend strongly on the liner and the liner/cu interface. Good adhesion appeared to improve performance CMP and damascene process does not permit the top surface to be covered with liner. Liner covered with cap material

Adhesion of Different Metals to SiO 2 2000 Å Ta Metal 500 Å Metal Pt Cu W SiO 2 Si H f a (kcal/nonmetal atom) -10-39 -70 Interface Debond Energy, G (J/m 2 ) 10 8 6 4 2 0 Pt Cu W Ta Ta -98

Relationship Between Enthalpy of Formation of oxide and Adhesion to SiO 2 Interface Debond Energy, G c (J/m 2 ) 10 8 6 4 2 Expected bond density (slope) 1.4 1.9*10 19 atoms/m 2 Slope=1.2*10 19 atoms/m 2 0 0 2 4 6 8 Enthalpy of Formation/metal-oxygen bond (J*10 19 ) G O N(H M + H H M-O Plot of G o vs H M-O straight line with slope N. O Linear behavior and slope consistent with bond density and suppressed plasticity Indicates bulk enthalpy predictor of adhesion )

Interfacial Chemistry G G o = γ O + γ M γ M-O (Griffith) O = N(g M + g O (from Weiderhorn, Lawn, Cook) g M-O ) Metal a o Oxide H f (kcal/mol) G f (kcal/mol) SiO 2 Cu 2 O -40.3-34.9 MgO -143.8-136.1 SiO 2-217.3-204.4 Oxide H f (kcal/mol) H f a (kcal/nonmetal atom) G O G O g M-O ~ H M-O N(H M + H O = α N(H H M-O M-O ) ) NbO -97-97 NbO 2-189 -95 Nb 2 O 5-452 -90

Interfacial Adhesion Measurements Four Point Flexure Ψ ~ 43 0 h 1 =h 2 =h G = 21(1 ν 16Eb 2 2 )P h 3 2 L 2 Load Displacement P c Adhesion measurement only dependant on plateau load and sample geometry.

Electromigration in Interconnect Structures e - Cap/Cu interface fast diffusion path (Hu et al) Does better adhesion of cap/cu interface lead to improved EM performance?

In-situ Void Growth Measurements in SEM (a) Cu e - 5h v = αe H kt (b) (c) (d) 1µm 8h 10h 12h Void Growth Rate, v (µm/hr) 1 10-1 10-2 CoWP SiC NSiC SiN 10-3 16 17 18 19 1/Temperature*10 4, (K -1 )

Void Growth Rate and Interface Adhesion Cap Adhesion (J/m 2 ) Growth Rate (µm/hr) Activation Energy (ev) SiC 11.5 0.32 0.76 NSiC 20.0 0.013 0.89 SiN 18.5 0.017 0.90 CoWP >40 0.001 1.05 Void Growth Rate, v (µm/h) 1 10-1 10-2 10-3 SiN SiC NSiC CoWP, No delamination > 40 J/m 2 10-4 0 20 40 60 Interface Debond Energy, G (J/m 2 ) Growth rate, adhesion and activation energy changes are all consistent. Suggests, H kt v = αe v e βg kt

Purposely Contaminating Interfaces e - Intentionally contaminating interface should decrease adhesion and increase void growth rate

Clean vs Dirty Interfaces Void Growth Rate, v (µm/h) 1 10-1 10-2 10-3 NSiC dirty interface SiC SiN NSiC NSiC clean interface CoWP, No delamination > 40 J/m 2 10-4 0 20 40 60 Interface Debond Energy, G (J/m 2 ) Structures purposely made with dirty interfaces consistent with previous data.

Traditional EM Experiments Percent Failed 95 90 80 70 60 50 40 30 20 t H A kt 50 = e n 10 5 1 10 100 1000 Time to Failure (hours) j Ln(T 50 ), normalized 6 4 2 SiN NSiC SiC 0 16 17 18 19 20 21 1/T*10 4 (K -1 )

Work of adhesion and EM activation Energy x 1 x 2 x 2 a o k is the spring constant a k is the spring constant P W ad G λ x G~0.5a 2 k W ad =kδ 2 /π 2 if a ~ δ then, G ~ W ad

Electromigration Testing Normalized Time T( C) SiC T( C) BLoK T( C) SiN 326 1.0 317 8.5 327 23.2 297 2.5 297 16.2 ---- --- 283 3.2 276 25.3 276 84.2 244 8.4 244 113.9 239 350.5 H (ev) 0.69 ±0.06 0.93 ±0.08 0.83 ±0.07

Activation Energy Comparison EM Activation Energy 1.2 1 0.8 0.6 EM Void growth Consistent Results Correspondence between testing methods 0.4 0 10 20 30 40 Adhesion Energy, G (J/m 2 )

Interface Fracture Resistance σ τ # Material 1 Dissipation Zone plasticity Stress, σ σy S Strain, ε crack interactions asperity contact bridging ligaments Energy associated with the near tip fracture process zone (inc. chemical bonding) Fracture Mechanism Stress, σ Work of Adhesion, W a d σo Separation, δ δo Debond # Material 2 G mac =G o +G pl +G r G pl and G r are fn(g o ) Energy dissipation zone surrounding the debond arises from plasticity of the surrounding ductile layers and crack face interactions

Accounting for Plasticity in Adhesion Measurements 80.0 60.0 40.0 20.0 G o 0.0 10-2 10-1 1 10 20 Copper Layer Thickness (µm) )100.0 Interface Fracture Energy, G (J/m 2 c G max (Lane, Dauskardt, Vainchtein, Gao) Macroscopic Work of Fracture, G c (J/m 2 ) 100 80 60 40 20 0 m = 20 4 6 8 Work of Adhesion, G o (J/m 2 ) Interface fracture energy measured as a function Cu thickness Elastic-perfectly plastic model predicts linear relationship between plastic energy dissipation and work of adhesion Measured data shows similar trend and allows for estimate of plastic energy dissipated for a given Cu layer thickness

Accounting for Plasticity in Adhesion Measurements SiC NSiC/SiN % Change Macroscopic Adhesion (J/m 2 ) 11 20 45 Intrinsic Adhesion (J/m 2 ) 4.5 6.0 25 Adhesion energy/atom (ev) 1.5 2.0 25 Electromigration H (ev) 0.69 0.88 28 G O = α N(H ) kt M-O v = αe H t A 50 = e n j H kt Enthalpy and activation energy the same or closely related?

Comparison to Cu Grain Boundary Grain boundary is just another albeit special interface Grain boundary adhesion energy estimated as the Cu cohesive energy less the grain boundary energy 1.4 Estimate is 3.3 ev/atom (e.g. Kuar, Gust, Kozma) H = 0.36 + 0.26*G EM Activation Energy, H (ev) 1.2 1 0.8 SiC CoWP SiN/NSiC Cu gb 0.6 1 1.5 2 2.5 3 3.5 Adhesion Energy, G (ev/atom) H is non-zero when G = zero

Comparison with Surface Mobility The Surface is just another albeit special interface Adhesion energy vanishes STM measurements Cu adatom H m from 0.25 to 0.40eV 0.37 ± 0.06 ev along steps Varies according to surface EM H PVD Cu, 0.42 ev (Sematech 1996) H = 0.36 + 0.26*G

Consistent Throughout Range EM Activation Energy, H (ev) 1.2 0.8 0.4 SiC Cu, adatom, steps CoWP SiN/NSiC Cu gb 0 0 1 2 3 4 Adhesion Energy, G (ev/atom)

Summary Adhesion energy shown to be related to enthalpy of compound formation Adhesion energy and electromigration activation energy found to scale General relationship found between electromigration activation energy and work of adhesion H = 0.36 + 0.26*G At G = 0, H consistent with adatom diffusion

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