7 Fundamentals and Failures in Die Preparation for 3D Packaging
181
ablation is to remove most of metal stacks in the street to allow the saw blade to
singulate the remaining Si. This technique will ensure blade loading is reduced
during saw dicing. Typically the entire metal stack is ablated during the laser scribe
process which is then followed by saw singulation which can be a 2-step or 1-step
process [64]. The two-step process uses two blade widths to cut through remaining Si
while the 1-step process uses only one blade width to cut through the Si. The distance
between laser scribe edge and saw cut edge is dictated by street width, scribe width
and saw blade width. This distance is one of the key process parameters because
micro cracks that are generated during laser and saw process can propagate during
reliability stress and causes reliability issue.
There are multiple publications that discuss laser-solid interaction [67–69]. One
of the key parameters to focus on in understanding laser-material interactions is the
characteristics time for electrons and lattice to reach thermal equilibrium [70]. In
nano-second scale, the energy transport can be described by the Fourier conduction
model [71, 72]. In this case, the deposition of laser energy is instantaneous compared
to pico or femtosecond laser regime. In the latter case, the electrons and material
lattice do not attain thermal equilibrium [70].
A typical heat conduction equation is valid for any spatial and temporal scales as
in Eq. (7.16):
C
∂ T
∂t
= −∇ · q
+ S
(7.16)
where C is the heat capacity (J/m
3 K), q
is the heat flux vector, and S is the internal
heat source (W/m
3 ) that can be caused by volumetric heating or coupling between
electron. The heat capacity is C = ρc p and the heat flux is related to the temperature
gradient by the Fourier’s law in Eq. (7.17):
q
= −k∇T
(7.17)
Substituting Eqs. (7.17) into (7.16) results in the following heat conduction
equation:
ρc p
∂ T
∂t
= ∇ · (k∇T ) + S
(7.18)
Further information on ultrafast laser such as pico and femto second laser are well
explained by a few researchers particularly by Zhang [73].
When laser irradiance exceeds 10
9 W/cm
2 for a nanosecond laser pulse, the
temperature near the surface of a solid target can exceed the boiling point, thus
a superheated liquid layer is formed [74]. It was suggested that explosive boiling
takes place where homogeneous vapor bubble nucleation occurs when the target
material reaches ~ 0.9T tc (T tc is the thermodynamic critical temperature) [74]. As a
consequence, the target material makes an abrupt transformation from superheated
181
ablation is to remove most of metal stacks in the street to allow the saw blade to
singulate the remaining Si. This technique will ensure blade loading is reduced
during saw dicing. Typically the entire metal stack is ablated during the laser scribe
process which is then followed by saw singulation which can be a 2-step or 1-step
process [64]. The two-step process uses two blade widths to cut through remaining Si
while the 1-step process uses only one blade width to cut through the Si. The distance
between laser scribe edge and saw cut edge is dictated by street width, scribe width
and saw blade width. This distance is one of the key process parameters because
micro cracks that are generated during laser and saw process can propagate during
reliability stress and causes reliability issue.
There are multiple publications that discuss laser-solid interaction [67–69]. One
of the key parameters to focus on in understanding laser-material interactions is the
characteristics time for electrons and lattice to reach thermal equilibrium [70]. In
nano-second scale, the energy transport can be described by the Fourier conduction
model [71, 72]. In this case, the deposition of laser energy is instantaneous compared
to pico or femtosecond laser regime. In the latter case, the electrons and material
lattice do not attain thermal equilibrium [70].
A typical heat conduction equation is valid for any spatial and temporal scales as
in Eq. (7.16):
C
∂ T
∂t
= −∇ · q
+ S
(7.16)
where C is the heat capacity (J/m
3 K), q
is the heat flux vector, and S is the internal
heat source (W/m
3 ) that can be caused by volumetric heating or coupling between
electron. The heat capacity is C = ρc p and the heat flux is related to the temperature
gradient by the Fourier’s law in Eq. (7.17):
q
= −k∇T
(7.17)
Substituting Eqs. (7.17) into (7.16) results in the following heat conduction
equation:
ρc p
∂ T
∂t
= ∇ · (k∇T ) + S
(7.18)
Further information on ultrafast laser such as pico and femto second laser are well
explained by a few researchers particularly by Zhang [73].
When laser irradiance exceeds 10
9 W/cm
2 for a nanosecond laser pulse, the
temperature near the surface of a solid target can exceed the boiling point, thus
a superheated liquid layer is formed [74]. It was suggested that explosive boiling
takes place where homogeneous vapor bubble nucleation occurs when the target
material reaches ~ 0.9T tc (T tc is the thermodynamic critical temperature) [74]. As a
consequence, the target material makes an abrupt transformation from superheated