Post on 05-Jan-2016
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