Reliability Concerns in Microelectronic Packages due to Interactions between Stress, Temperature and Electric Current

Transcription

1 Reliability Concerns in Microelectronic Packages due to Interactions between Stress, Temperature and Electric Current Praveen Kumar Assistant Professor Department of Materials Engineering Indian Institute of Science, Bangalore Workshop on Mechanical Behaviour of Systems at Small Length Scales 4 February 24 th to 28 th, Coorg

2 Overview Microelectronics package, interconnects: Why to worry about electric current besides stresses and temperature? Interfaces and interfacial sliding at metal/si hetero-interfaces Electromigration, electro-mechanical coupling; effect of substrate surface layer Electric current induced damages in solders (lead-free, Sn based solders) post damage microstructures; effect of electro-mechanical coupling on creep and fracture Electric current related unconventional phenomenon, such as electromigration in thin films of liquid metals Summary and outlook

3 Microelectronic Package & Interconnects Thermal Interface Interconnects Integrated Heat Spreader (Cu) Silicon Device HMP LMP Device Level Interconnects Organic Substrate/PCB Cu In 5 mm Package Level Interconnects e.g. In-based composites, polymer based thermal glues, etc. e.g. Cu (mostly) Sn-based lead free solders, e.g. Sn-3.Ag-.5Cu (SAC35), etc. Curtsey: K N Tu, UCLA

4 Miniaturization of Microelectronic Packages Maximum current density, j max (A/cm 2 ) Device Level Interconnects 9 nm ITRS - 2 IRTS nm Year j max-32nm > 1 j max-9nm Moore s Law is going to stay Actually Intel progresses faster than predictions! Rapid increase in operating temperature due to non-linear increase in resistivity with decreasing interconnect line-width Pad pitch, Pad diameter (m) Package Level Interconnects Pad Pitch Pad Diameter Line Width Source: ITRS Year Line width (m) Up to.4 A current through solders, j~ 1 4 A/cm 2 for 5 m diameter solder EM Temperature up to 1 o C (>.75 T m for Sn-Ag-Cu solders) EM, TM and other high temperature phenomenon [International Technology Roadmap for Semiconductors (ITRS), 27]

5 Electromigration: Mass Flow Driven by Electron Flow e - wind Momentum transfer during e - scattering E Strehle et al, Microelectron. Eng. 86(29) 2396 EM rate is given by : Deff * Deff * vem Z ee Z e j kt kt Deff f Dvol, Dgb, Di If width of line >> grain size, T T m D eff D gb

6 Quantifying Failure due to Electromigration Mean time to fail (MTTF) correspond to 5% increase in electrical resistance MTTF is predicted using Black s law: t MTTF Q n eff A j exp RT n ~2 At low temperature, a small temperature increase dramatically reduces MTTF Most likely scenario in modern MEMS devices MTTF/A (AU) Cu (Q gb = 14 kj/mol, n = 2) j = 1 5 A/cm 2 j = 1 6 A/cm 2 j = 1 7 A/cm Temperature ( o C) A further increase of 1 o C can reduce MTTF by a factor of 5!

7 Miniaturization: Explosion in Interfacial Volume Fraction INTEL Core 2 extreme Release date: Jul 26 > 3 million transistors with gate size of 65nm Thin film lines with interfacial area >1 8 m 2 /m 3 INTEL Core i7 extreme Release date: Jan 211 >1.2 billion transistors with gate size of 32nm Thin film lines with interfacial area >1 1 m 2 /m 3 INTEL Xeon Westmere-EX Release date: July 211 >2.6 billion transistors with gate size of 32nm Thin film lines with interfacial area >1 11 m 2 /m 3 wikipedia Interfaces & interface related phenomena become important, such as Interfacial Sliding

8 Diffusionally Accommodated Interfacial Sliding Mechanism : Applied stress sets up chemical potential gradient along interface Mass transport along interface sliding between phases (no interfacial fracture) i Film Film Substrate i Peterson et al, Acta Mater 51 (21) 2831 Sliding rate is given by : U i 4iDi kt h2 i Driven by interfacial shear stress Sliding rate increases rapidly with decreasing roughness

9 Interfacial Sliding due to Electromigration Before After (j ~ 2.5 x 1 4 A/cm 2, T = 18 o C for 72hrs) Kumar and Dutta, Acta Mater 59 (211) 296 Electromigration can also cause interfacial sliding, provided: grains are smaller than width of the film EM flux passes through I/F

10 Effect of Electro-Mechanical Coupling on Interfacial Sliding Model shear-lap sample: Pb foil (thickness ~ 1 μm, grain size 15 m was diffusion bonded between two Si strips One of the interfaces was locked by depositing 4 nm of Cr on Si Fixed Flexible Thin Film Heater Capacitance Gauge Assembly Very Thin Cu Wires for Current Solder Bumps Alignment Rods (Fixed) Si Strip Thermocouple Pb Foil Friction Fixture Assembly Sliding Plate Dead Weight Fixed Plate (Attached to Upper Plate via Alignment Rods)

11 Evidence of Interfacial Sliding at Only One Interface Si/Cr Pb Si 35 3 Si-Pb-Si j = 1.4 x 1 4 A/cm 2 =.17 MPa W/ +e - (Cr at one I/F) 25 T = 81.5/.5 o C Al grids Displacement (nm) x 1-3 nm/s W/O e - (Cr at one I/F) 3.3 x 1-3 nm/s Cr at both I/F's 1 μm x 1-4 nm/s 3.3 x 1-4 nm/s Time (s) W/ +e - W/O e - Kumar and Dutta, Acta Mater 59 (211) 296 Pb/Si interface predominantly slides Deposition of thin layer of Cr locks an interface Limited deformation of Pb-foil measured displacement is mainly due to interfacial sliding at the Pb-Si interface

12 Co-ordinate System and Sign Convention

13 Measurement of Interfacial Sliding 8 7 Si-Pb-Cr/Si j = 2.36 x 1 4 A/cm 2 =.5 MPa W/-j 6 5 Si-Pb-Cr/Si j = 1.4 x 1 4 A/cm 2 =.17 MPa 6 T = 81.5/.5 o C 4 T = 81.5/.5 o C Displacement (nm) x 1-2 nm/s 1.72 x 1-2 nm/s W/O j Displacement (nm) x 1-3 nm/s 3. x 1-3 nm/s W/-j W/O j 2 1 W/+j 6.44 x 1-3 nm/s Time (s) Time (s) -3.5 x 1-3 nm/s W/+j Electron flow parallel to shear stress Accelerated displacement rates Electron flow anti-parallel to shear stress Decelerated displacement rates Kumar and Dutta, Acta Mater 59 (211) 296

14 Interfacial Sliding Rate: Dependence on & j Displacement Rate (nm/s) Si-Cr/Pb/Si T = 81.5/.5 o C 5.56 x x 1-2 j = 1 4 A/cm 2 j = A/cm 2 j = -1 4 A/cm 2 Displacement Rate (nm/s) Si-Cr/Pb/Si T = 81.5/.5 o C 6.14 x x 1-7 =.5 MPa =.33 MPa =.17 MPa 4.7 x Shear Stress, (MPa) 4.32 x Current Density, j (A/cm 2 ) Linear dependence of displacement rate on shear stress and electric current density Linear superposition of effects of electromigration and shear stress on the interfacial sliding may give net effect: U C C j 1 2 C 1 = 4.93x1-17 m 2 s/kg, C 2 = 5.33x1-2 m 3 /As for the Pb-Si system at 81.5 ±.5 o C

15 Interaction between EM and Shear Stress : Modeling x h 2 cos y 2 2 sin 2 n y h 2 Vy E y sin y J D kt J stress D ( ) kt D JEM Z* ev kt

16 Step 1: Determination of Chemical Potential Find (x,y) by applying conservation of mass: 2 ( xy, ) where, * xy, n xy (, ) ZeV Boundary conditions: *, y, y ZeV(, y), y ZeV(, y) * n Solution 2 - * 2 * h x 2 ( xy, ) -ZeE- y ZeE e sin y h i 2

17 Continuity Condition Film s Unconstrained Motion Film Fixed Interface Film into substrate Open space Substrate Film Mass Transport Continuity Cond n Substrate Interfacial Sliding Film Substrate

18 Continuity Condition and Final Solution Continuity Condition: Relate to fluxes by applying Continuity across interface: U For ( ie.., h/ 1), this gives x y y U t.sin. s t J (, y) s J (, y y) J (, y) dx U dy J x f dx U Df i Di dy kt x x Final Solution: f (, y) i J y i (, y) y 2 2 x kt x i x i substrate y i y S J (,y) i X f U tsin J (,y y) y i i J (,y) U t film 4 * i eff 2iDi 2Z ee eff 2iDi 2 f f U D D kth kth Where eff D f D, D f vol gb If D f eff << D i U 8 D 4 D kth i kth i i i i 2 * Z ee E j U C C j 1 2 Kumar and Dutta, Acta Mater 59 (211) 296

19 EM Induced Interfacial Sliding: Effect of Substrate Surface Layer Displacement (nm) T = o C j = 1. x 1 4 A/cm 2 Si/Cr-Pb-Si Si/Cr-Pb-W/Si Si/Cr-Pb-TaN/Si 1.17 x 1-2 nm/s x 1-3 nm/s x 1-3 nm/s Displacement (nm) T = ~ o C j = 2. x 1 4 A/cm 2 Si/Cr-Pb-Si Si/Cr-Pb-W/Si Si/Cr-Pb-TaN/Si 1.62 x 1-1 nm/s x 1-2 nm/s x 1-2 nm/s Time (s) Time (s) Pb-W/Si interface is slowest and Pb-TaN/Si interface is fastest Substrate surface layer determines electromigration induced interfacial sliding Substrate surface layer determines structure and chemistry of interfacial layer Kumar and Dutta, J. Phys. D (In press)

20 An Implication Initial u d Unconstrained v d du dt d Constrained v d ' v d Interfacial sliding accommodates film motion due to electromigration

21 Even in Classical Experiments Using Blech Structure v (m/s) EM in Cu Ti x N TaN TaN/Ta/TaN Kumar and Dutta [unpub.], 1.m Hu et al, APL [1999],.7m Lee et al APL [1995],.5-1.m Strehle et al MicroEng [29] Frankovic et al IEEE TED [1996], 1.35m Chai et al IEEE TEDL[28] j..e (-Q/RT) /T (V/m K) TaN W/Ta (Ta-cap) W/Ti/Ta v (m/s) SiO 2 TiN Glass Schieber, SSE [1985] Proost et al, ScriptaMat [1998] Spolenak et al, MicroRel [1998] Blech, JAP [1976] Hummel et al, JPCS [1976] j..e (-Q/RT) /T (V/m K) EM in Al TiN Dominant mechanism for mass transfer is grain boundary diffusion However, substrate surface layer does affect electromigration induced atom flow!

22 Cu-filled Through Silicon Vias (TSVs) TSVs are now widely used in high density, 3-dimensional microelectronic packaging chips/devices TSVs Question: How do thermal cycling and electromigration affect interfacial sliding in Cu filled TSVs?

23 Effect of TC, Small T (Annealed) As-fabricated After Annealing at 425oC for 3 minutes 4 m Cu 4 m Cu Si 4 m After 2 cycles between -25oC and 135oC Si 4 m Cu 4 m Si Cu protrudes due to thermal cycling Stress relaxation due to inelastic strains, Si/Cu interface slides (& do not fracture) 4 m

24 Effect of Electric Current + Thermal Cycling As-fabricated Annealing + 3 cycles bet n 25 o C and 425 o C Additional 3 cycles + EM (e - ) Cu Cu e - 4m Si 4m Si 4m Interfacial sliding, and hence TSV protrusion, is enhanced by EM depends of direction of electron flow relative to direction of interfacial shear stress on the Cu-side As Fab. Differential Length, L Cu - L Si (m) Annealed Ann+3c Ann+6c Ann+3c+ 3c-w/EM Cu/TSV, large T T min = 25 o C, T max = 425 o C Heating and o C/s j = 5.2 x 1 4 A/cm 2 Kumar et al, J. Elec. Mater. 41 (212) 322

25 Effects of Creep, Plasticity, Electromigration and Interfacial Sliding: 1-D Model Model reported in: Kumar et al, J. Elec. Mater. 41 (212) 322 Summarily: 1. Continuity equation: l o ( Cu - Si ) = h i 2. Estimate initial stress in Cu 3. Calculate incremental stress in Cu due to T: Cu - 8h D - T - t o lo kth V Cu 1 Si 1 1 EV Si Si 1 Cu ECu RWH Cr i i cu Si Cu 3 i,z l / 2 o 3. Update Cu stress as: Cu, j+1 = Cu, j+1 + Cu 4. Calculate strains using: Si: Cu: T1 th el f Si Si Si dt E T2 f 1 T2 YS n1 th el pl Cr Cu Cu Cu Diff Cu Cu Cu Cu Cu dt ACu Cu E T Cu K1 1 : Thermo-elastic solid : Thermo-elastic-plastic-creeping solid

26 Interfacial sliding: Model vs. Experiment Differential Length, L Cu - L Si (m) Cu/TSV (j = ) T = 25K, T = 41K min max K/s Original Height = 4m Model ( i D,i = 1-5 gb D,gb, = 5MPa) Experiment Thermal Cycles Increment in differential strain (or, length) between Cu and Si saturates at higher number of thermal cycles 1-D model predicts experimentally observed behavior (qualitative & quantitative)

27 Effect of TC+ Electromigration (Model) Position of Topmost Point of Cu & Si, Z, Z (m) Top,Cu Top,Si Cu/Si - w/ i/f sliding T = 25K, T = 41K min max Heating and Cu, -j ( parallel to e - ) D = 1-5 D i,i gb,gb Temperature (K) Cu, no EM Cu, +j Si (+EM, no EM, -EM) j = 1x1 12 A/m 2 Differential Length, L Cu - L Si (m) Cu/TSV - w/ i/f sliding T min = 25K, T max = 41K Heating and D = 1-5 D i,i gb,gb j = 1x1 12 A/m Thermal Cycles EM: -j No EM EM: +j Protrusion (or intrusion) of Cu wrt Si can be increased, decreased or reversed with EM With EM, protrusion (or intrusion) of Cu wrt Si does not saturate with number of cycles at large number of cycles, protrusion (or intrusion) is determined principally by EM Stress-state does not seem to be affected by EM

28 Electric Current Induced Damages in Solders: Summary Crack nucleation & growth Differential growth of IMC layers Phase separation Sn-3.5Ag-1.Cu 1.5x1 4 A/cm o C Current crowding Sn-58Bi 5x1 3 A/cm o C Chan and Yang, Prog. Mater. Sci. 55 (21) 428 Tin whiskering Voltage (V).4 Sn-3.5Ag-1.Cu 1.5x1 4 A/cm o C A B C D E Time (h) Yang, PhD Thesis, University of Hong Kong (28) X 2 (m 2 ) 14 Sn-3.7Ag-.7Cu x 1 4 A/cm 12 o C 1 j = 2 x 1 4 A/cm 2 (Anode) 8 No electric current j = 2 x 1 4 A/cm 2 (Cathode) Time (h) Gan and Tu, J. Appl. Phys. 97 (25) hr e hrs Sn-3.7Ag-.7Cu 1.x1 4 A/cm RT hrs hrs Ouyang et al, Appl. Phys. Lett. 91(27)231919

29 Effect of Electric Current on Fracture Behavior Sn-3.7Ag-.7Cu No EM, 7 Mpa, 1 o C 3.3x1 3 A/cm 2, 7 MPa, 1 C 5x1 3 A/cm 2, 7 MPa, 1 C Fracture surface moves towards anode (thicker IMC) Ren et al, Appl. Phys. Lett. 89 (26)141914

30 Microstructural Coarsening of SAC Solders SAC 35: Sn-3.%Ag-.5%Cu: Isothermal Aging at 15 o C

31 Electric Current Induced Phase Coarsening Sn-3.5Ag-.5Cu Precipitate Size (m) SN-3.5Ag-.5Cu - 5 o C, W/EM 125 o C, No EM 125 o C, W/ EM Time (h) Wu et al, J. Elec. Mater. 37 (28)469 Is the following possible for Sn-based Pb-free solders? n p n o Q n d d Ae RT j t f j,,, T, t? If yes, is a microstructurally adaptive model for creep (or, other mechanical behavior) possible comprising of effects of current, strain (thermo-mechanical cycling) and temperature?

32 Effect of Electric Current on Creep Temperature (K) Sn Chamber Temperature I (A) = Impression depth (mm) Sn T = 398 K chamb. = 19 MPa imp. I (A) Impression velocity (m/s) Sn imp Time (s) T chamb. = 398 K Impression velocity (m/s) Time (s) I (A) Q app (kj/mol) I 2 (A 2 ) V V j EM o Q Q j EM 1/RT (mol/kj) Chen and Yang, J. Elec. Mater. 39 (21) ? Sn 2 Q j n p Ab RT b m 2 SA, C De j f j,,, T, t kt G d

33 Unconventional Effects of Electric Current: An Example Liquefaction and flow of exposed adhesion layer μm Real worry - worsens under AC loading!

34 Summary & Outlook: Device & Package Level Interconnects Stress, Fracture, fatigue, etc. whiskering Creep, enhanced phase coarsening, etc. Interaction with effects of, enhanced phase coarsening, etc. Temperature, T Diffusion, phase transformation, etc. Thermo-migration Current, I Electromigration EM fracture, magnetic saw effect, etc. Liquid electromigration, differential chemical reactions/phase growth Major concern: decoupling of Joule heating and electromigration effects during in-situ experiments

35 Acknowledgements Collaborator Professor I. Dutta (WSU) IISc Santanu Talukder Nalla Somaiah Piyush Jagtap