176
H. Ma et al.
In order to reduce the wafer bow (H), we need to reduce the residual stress (σ f )
from the two sides of wafers or increase wafer thickness (h S ).
Challenges in thermoslide debond as discussed above are driving the industry to
investigate more on room temperature debond options, such as mechanical debond
and laser debond. These processes are compatible with a larger range of thermoplastic and thermoset adhesives, even when the shear modulus is beyond which
thermal slide-off debond could process. Thanks to the room temperature process,
wafers can be mounted before mechanical or laser debond, eliminating the need
to directly contact the exposed device wafer surface, and handle a standalone thin
warped wafer. The dicing tape needs to be compatible to the chemical cleaning post
debond. The remaining challenge for dicing tape mounting is to mount on a wafer
with surface topography, survive through debond, clean, and singulation processes
without delamination and seepage, and finally can still release the die from the dicing
tape with high yield at die pick and place.
In case of mechanical debond [34, 55], the separation is initiated from one side
of the wafer pair, and gradually progress through the diameter of the wafer, with the
debond line perpendicular to the debond direction. The carrier wafer is either tilted
or wrapped up, and the device wafer is held flat by dicing tape on a vacuum chuck.
The bond strength needs to be lower than the dicing tape adhesion strength, to avoid
separation at the device wafer to dicing tape interface causing wafer loss. On the
other hand, the bond strength also needs to be high enough sustain the wafer through
post bond fabrication processes without delamination, wafer crack, or chipping. This
defines the narrow adherence energy process window of 0.4–1.2 J/m
2 for material
selection [56]. For device wafers with bumps encapsulated in the adhesive, the bond
strength is also a function of bump critical dimensions and density, as well as passivation layer surface property, which further increases the challenge to find a single
solution for all products. The process window is enlarged when a laser release layer
is added to either the adhesive to device wafer interface or to the adhesive to carrier
wafer interface.
In case of laser debond, a carrier wafer transparent to the laser is used and a release
layer with strong laser absorption is added between the carrier and adhesive layer [38,
57, 58]. The release layer needs to demonstrate good adhesion between the carrier
wafer surface and the bonding adhesive, thermally stable to sustain through bond and
post bond fabrication processes, and have high laser absorbance at the wavelength
of debond. After laser ablation, the adhesion to glass carrier is significantly reduced
to enable the subsequent mechanical separation with minimal force, thus minimal
stress on wafer.
Laser ablation is a combined photochemical effect and photothermal effect,
depending on the properties of the irradiated material and on the irradiation conditions, such as wavelength and pulse energy [59]. In the UV wavelength range, the
photochemical effect is predominant, which is referring to the direct bond breaking
by UV photons. UV laser, e.g. 308 or 355 nm, is more often selected for laser debond
applications to ablate the polymeric release layer, for its effective fragmentation of
the polymer chains, low carbonization of ablated material, and low thermal effect to
the surrounding adhesive layer and device wafer. The laser fluence must be above the
H. Ma et al.
In order to reduce the wafer bow (H), we need to reduce the residual stress (σ f )
from the two sides of wafers or increase wafer thickness (h S ).
Challenges in thermoslide debond as discussed above are driving the industry to
investigate more on room temperature debond options, such as mechanical debond
and laser debond. These processes are compatible with a larger range of thermoplastic and thermoset adhesives, even when the shear modulus is beyond which
thermal slide-off debond could process. Thanks to the room temperature process,
wafers can be mounted before mechanical or laser debond, eliminating the need
to directly contact the exposed device wafer surface, and handle a standalone thin
warped wafer. The dicing tape needs to be compatible to the chemical cleaning post
debond. The remaining challenge for dicing tape mounting is to mount on a wafer
with surface topography, survive through debond, clean, and singulation processes
without delamination and seepage, and finally can still release the die from the dicing
tape with high yield at die pick and place.
In case of mechanical debond [34, 55], the separation is initiated from one side
of the wafer pair, and gradually progress through the diameter of the wafer, with the
debond line perpendicular to the debond direction. The carrier wafer is either tilted
or wrapped up, and the device wafer is held flat by dicing tape on a vacuum chuck.
The bond strength needs to be lower than the dicing tape adhesion strength, to avoid
separation at the device wafer to dicing tape interface causing wafer loss. On the
other hand, the bond strength also needs to be high enough sustain the wafer through
post bond fabrication processes without delamination, wafer crack, or chipping. This
defines the narrow adherence energy process window of 0.4–1.2 J/m
2 for material
selection [56]. For device wafers with bumps encapsulated in the adhesive, the bond
strength is also a function of bump critical dimensions and density, as well as passivation layer surface property, which further increases the challenge to find a single
solution for all products. The process window is enlarged when a laser release layer
is added to either the adhesive to device wafer interface or to the adhesive to carrier
wafer interface.
In case of laser debond, a carrier wafer transparent to the laser is used and a release
layer with strong laser absorption is added between the carrier and adhesive layer [38,
57, 58]. The release layer needs to demonstrate good adhesion between the carrier
wafer surface and the bonding adhesive, thermally stable to sustain through bond and
post bond fabrication processes, and have high laser absorbance at the wavelength
of debond. After laser ablation, the adhesion to glass carrier is significantly reduced
to enable the subsequent mechanical separation with minimal force, thus minimal
stress on wafer.
Laser ablation is a combined photochemical effect and photothermal effect,
depending on the properties of the irradiated material and on the irradiation conditions, such as wavelength and pulse energy [59]. In the UV wavelength range, the
photochemical effect is predominant, which is referring to the direct bond breaking
by UV photons. UV laser, e.g. 308 or 355 nm, is more often selected for laser debond
applications to ablate the polymeric release layer, for its effective fragmentation of
the polymer chains, low carbonization of ablated material, and low thermal effect to
the surrounding adhesive layer and device wafer. The laser fluence must be above the