7 Fundamentals and Failures in Die Preparation for 3D Packaging
177
ablation threshold to provide sufficient energy for bond breaking. Under the photochemical model, ablation happens when the absorbed photon density in the irradiated
material volume is higher than the material atomic density, and the photon energy is
higher than the atomic bonding energy. The threshold fluence F thr.. can be written as
[59].
F thr. =
hvn at.
α
(7.14)
where h is the Planck constant, v is the photon frequency, n at. is the material atomic
density, and α is the material absorption coefficient at the laser wavelength.
In the IR and visible wavelength range, the photothermal effect is predominant,
which is a result of laser heating of material. IR laser debond provide a unique opportunity to use Si as the carrier wafer, as Si wafer demonstrates over 52% transmittance
to laser wavelength above 1.2 um [39]. Under the thermal model, the threshold fluence
represents the energy density necessary to evaporate in vacuum the irradiated mass
of the target, including the heating energy, fusion energy, and the evaporation energy
(sublimation) of the irradiated mass [59]. To minimize the thermal impact to device
wafer, it is critical to select a release layer material with lower laser release energy
density [39].
After debond, the adhesive material on device wafer needs to be cleaned. The
most common way to remove the remaining adhesive is through a wet cleaning
process. Mechanical peel of adhesive prior to solvent clean is needed in some two
layer systems, where the release layer shows poor dissolution in the same solvent
as adhesive layer. Besides, it could also be utilized as a way to reduce the solvent
consumption and cleaning time [60].
A typical solvent clean process would include 4 key steps: puddle creation,
puddling, spin rinse, and spin dry. In the first step, a solvent is dispensed onto
wafer while the wafer spins at low speed to create a puddle of solvent on wafer
fully covering the wafer surface. Then, the wafer would sit for a certain period of
time to let solvent interact with adhesive. After that, solvent is dispensed while the
wafer spins at a fast speed to rinse the wafer surface. Lastly, solvent dispense stop
and wafer continue to spin at high speed to dry. When a high boiling point solvent is
used in the cleaning, a secondary rinse using a low boiling point solvent is typically
required to allow quick evaporation in spin dry.
The dissolution of an adhesive into a solvent mainly happens in the puddling step.
For a non-crosslinked thermoplastic adhesive polymer, dissolution starts with the
diffusion of solvent molecules into the polymer matrix. After the induction time, the
solvent concentration in polymer exceeds a critical value, then true dissolution occurs
with chain disentanglement [61]. During dissolution, the surface layer structure at
the adhesive to solvent interface is shown in Fig. 7.16. The polymer-solvent system
gradually transition from pure polymer to a glassy state in infiltration layer and
solid swollen layer, further to a rubber like state in gel layer, and finally to a liquid
state in liquid layer, which connects to pure solvent. From pure polymer layer to
177
ablation threshold to provide sufficient energy for bond breaking. Under the photochemical model, ablation happens when the absorbed photon density in the irradiated
material volume is higher than the material atomic density, and the photon energy is
higher than the atomic bonding energy. The threshold fluence F thr.. can be written as
[59].
F thr. =
hvn at.
α
(7.14)
where h is the Planck constant, v is the photon frequency, n at. is the material atomic
density, and α is the material absorption coefficient at the laser wavelength.
In the IR and visible wavelength range, the photothermal effect is predominant,
which is a result of laser heating of material. IR laser debond provide a unique opportunity to use Si as the carrier wafer, as Si wafer demonstrates over 52% transmittance
to laser wavelength above 1.2 um [39]. Under the thermal model, the threshold fluence
represents the energy density necessary to evaporate in vacuum the irradiated mass
of the target, including the heating energy, fusion energy, and the evaporation energy
(sublimation) of the irradiated mass [59]. To minimize the thermal impact to device
wafer, it is critical to select a release layer material with lower laser release energy
density [39].
After debond, the adhesive material on device wafer needs to be cleaned. The
most common way to remove the remaining adhesive is through a wet cleaning
process. Mechanical peel of adhesive prior to solvent clean is needed in some two
layer systems, where the release layer shows poor dissolution in the same solvent
as adhesive layer. Besides, it could also be utilized as a way to reduce the solvent
consumption and cleaning time [60].
A typical solvent clean process would include 4 key steps: puddle creation,
puddling, spin rinse, and spin dry. In the first step, a solvent is dispensed onto
wafer while the wafer spins at low speed to create a puddle of solvent on wafer
fully covering the wafer surface. Then, the wafer would sit for a certain period of
time to let solvent interact with adhesive. After that, solvent is dispensed while the
wafer spins at a fast speed to rinse the wafer surface. Lastly, solvent dispense stop
and wafer continue to spin at high speed to dry. When a high boiling point solvent is
used in the cleaning, a secondary rinse using a low boiling point solvent is typically
required to allow quick evaporation in spin dry.
The dissolution of an adhesive into a solvent mainly happens in the puddling step.
For a non-crosslinked thermoplastic adhesive polymer, dissolution starts with the
diffusion of solvent molecules into the polymer matrix. After the induction time, the
solvent concentration in polymer exceeds a critical value, then true dissolution occurs
with chain disentanglement [61]. During dissolution, the surface layer structure at
the adhesive to solvent interface is shown in Fig. 7.16. The polymer-solvent system
gradually transition from pure polymer to a glassy state in infiltration layer and
solid swollen layer, further to a rubber like state in gel layer, and finally to a liquid
state in liquid layer, which connects to pure solvent. From pure polymer layer to