Zhen Zhang, Zhigang SuoDivision of Engineering and Applied Sciences

Harvard University

Jean H. PrévostDepartment Civil and Environmental Engineering

Princeton University

Cracking in Interconnectsdue to Thermal Ratcheting

MRSEC

Time

Temperature

1500C

1250C

-550C

Packaging temperature

Loadingrange

Cyclic loading test

Silicon

Lower level interconnects (10-15 m thick)

Al-Cu 2m thick

Underfill

Polyimide(4 m thick)

SiN (0.45m thick)

underfill

Flip-chip structure

Plan view of SiN

Silicon die

Organicsubstrate

What is the origin of high stress?

Ratcheting Plastic Deformation

Huang, Suo, Ma, Fujimoto, J. Mater. Res., 15, 1239 (2000)

Biased Shear Stress

Al or Cu

Silicon

10~100 µm

2 µm 0.5 µm SiN

Silica and low level interconnects (10~15µm thick)

Polymeric underfill

underfill Silicon die

Organic Substrate

Time

Temperature

1500C

1250C

-550C

Packaging temperature

Loadingrange

Packaging and loading

First cycle

m

SiN film

What is the crack behavior?

membrane stressdue to CTE mismatch

Metal yields every cycle !

Many cycles

mAl / Cu pad

Stress builds up in SiN

m

m biased shear stress

Al / Cu pad

Ratcheting Plastic Deformation

p

2D Shear Lag Modelstress

YE

strain

Two challenges for simulation• Crack growth• Plasticity

0

Elastic substrate

x

yzx

0

y0

Elastic film

Elastic-plastic sublayer

X-FEMLinear creep analogy

Gradual loss of constraintStress relaxes in crack wake, but intensifies at crack tip.

Extended Finite Element Method (X-FEM)

Nodal Enrichment functions:

Moës, Dolbow, Belytschko, Int. J. Num Math. Eng, 46, 131 (1999).

– Displacement jumps– Singular crack tip field

– Relative coarse mesh– No remeshing required for

crack growth simulations

Benefits:

Time-saving

Linear Ratcheting-Creep Analogy

Y3

/p N Strain per cycle

Uni-directional shear stress

metal filmcyclic membrane stress

substrateCycle

Temperature

125 °C

-55 °C

Cyclic loading

1 cycle

Y

strain

stress

E

p

R

d

dN

1

212(1 ) (1 )

m mR

m m

E E Twhere

v v Y

Linearapproximation

pd d

dN dt

Ratcheting-Creep analogy

Time-saving

Huang, Suo, Ma, Acta Materialia, 49, 3039-3049 (2001)

p

Semi-infinite Stationary Crack in Blanket Film

• Comparison of time cost:• Creep: 1hr 20min• Ratchet: 22 hr

1/ 40~K N

Creep

Ratchet

N

0

IK

Rl hHEN Length scale

• Both creep and ratcheting calculation show the same trend.

K

l(N)

Kl(N)

Finite Stationary Crack in Blanket Film

0

IK

a

Normalized cycles 2/NEHh a

Creep

Ratchet

0K a

Final stage l>>aGriffith crack limit

Early stage l<<aInfinite crack limit

1/ 4

01.05 /K NEHh

2a

Earlystage

l

2a

Finalstage

l>>a

2a

0

,K l

faa

Evolvingl ~ a

Crack Propagation in a Blanket Film

a

0

IK

a

/ c

V

N

Normalized cycles / cN N

Preparation

Initiation

TransientPropagation Steady-state

Length scale2

0

ssK

HhEN c

2Cycle scale

Kss

Simulation of Cracks Propagation in Interconnects

Initial state After 100 cycles

Time

Temperature

150 °C

125 °C

-55 °C

Packaging temperature

Loadingrange

Cyclic loading

Tensile stress

Compressive region

SummaryRatcheting

deformation

in metal layer

High stress

in SiN

passivation film

X-FEM + Linear creep analogy

Simulation of cracking in interconnects becomes feasible

High

temperature

packaging

Thermalcyclic

loading

Cracking in interconnects