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Laser debonding in 2D and 3D heterogeneous applications

By Koen Kennes [imec] Alice Guerrero [Brewer Science]
T his article will discuss the

i mpact of la ser debond
processing for 2D and 3D
heterogeneous applications.
To this end, the fundamentals of laser
debonding will be introduced. The focus,
however, will be on highlighting its use
in several imec programs such as fan-out
wafer-level packaging (FOWLP), die-to- Figure 1: Laser debonding a thin wafer on tape frame.
wafer stacking, collective hybrid bonding, uses glass carrier substrates whenever laser using the following assembly flow. For
and 2D material transfer. debonding is required. The UV photons are more information on this project and flow
The semiconductor industry is readying transmitted through the glass and absorbed we refer to the literature [2]. In short,
itself for the transition from mechanical by the LRL (Figure 1). This layer will after through-package via (TPV) and
peel debond processing to laser debond photochemically degrade and break up logic placement on a temporary carrier
processing. Several aspects of the latter into smaller fragments, which will result system, the silicon bridges are stacked
process that make it very appealing include in adhesive strength loss between the LRL on top (Figure 2). Wafer-level molding
the improved throughput compared to a and the temporary adhesive. Next, the and planarization via grinding exposes
peel debond process and the selectivity glass can be lifted off with near zero force. the µ-bumps on the front side. Next, the
regarding debond interface. Where a In this specific example, the LRL is coated memory needs to be stacked on the TPVs.
mechanical peel debond process can take on the glass wafer before bonding with In order to do this, a first wafer flip is
from several minutes to up to tens of the TBM, but the LRL can also be coated introduced. After memory placement and
minutes, laser debonding typically takes on top of the thin wafer on the condition the second molding process, the system
less than a minute for a full 300mm wafer. that the TBM is transparent for the laser is placed on tape for die singulation via a
In more complex systems encountered in wavelength used. second wafer flip process.
2D-3D heterogeneous processing, multiple The design and selection of the LRL Several wafer flipping operations
wafer flip operations can be required in depend on several criteria. First, the using temporary carrier substrates are
order to enable both front-side and back- specific laser wavelength used will required in order to complete the whole
side processing. The adhesive strength, on dictate the optical properties of the assembly process flow. During chip
which a successful peel debond depends, LRL. The material will absorb the UV placement on the first temporary carrier,
can become a limiting factor in these photons that penetrate just a few hundred the chips are typically placed face down
cases, but may be overcome when peel nanometers of the LRL. In this way, the into the adhesive material. Re-accessing
debond is replaced by laser debonding primarily photochemical reaction does this original device front side after
[1]. Furthermore, the strength required to not contribute to a significant thermal bridge placement, molding and grinding
debond might damage fragile systems like hot spot near the device interface, and requires a first wafer flip. Hence, there is
ultra-thin Si layers or 2D monolayers. In therefore, the device is unaffected. a strong selectivity competition between
the laser debonding scheme, the adhesive Second, the LRL will be selected based the adhesive on carrier 1 (TBM 1) and
strength of the temporary bonding material on its compatibility with the stack (on the adhesive on carrier 2 (TBM 2). The
(TBM) is not an issue during debonding top or below the TBM) as required by latter needs to enable a high selectivity
because this function is replaced by the so- the application. In each of the following during the first carrier removal. However,
called laser release layer (LRL) [1]. This described applications, the choice of the it still needs to be able to release during
transition, however, does require a redesign used LRL and the stack order will be the second wafer flip, which is required
of existing temporary bonding flows and explained in detail. for package singulation. Furthermore, in
a material selection reassessment based this specific application, TBM 1 needs to
on the type of laser used for the ablation FOWLP meet a very specific set of requirements
process. In this review, we describe several The flip-chip on FOWLP concept aims regarding the high die placement accuracy
imec programs that clearly benefit from to enable a high chip-to-chip connection that is expected. These properties involve
laser debonding. Before describing these density within a package [2]. This density transparency for alignment pattern
benefits, we will briefly introduce laser is achieved via the use of a silicon recognition, low temperature die-TBM
debonding for a thin wafer flip on tape. bridge with back-end-of-line (BEOL) tackiness, and zero-reflow/–die shift
During the laser debond process, an interconnection layers and fine-pitch during the molding process. It is this set
LRL is ablated by the laser. Imec opted microbumps. The final target is built of requirements on TBM 1 that makes
for an ultraviolet (UV) laser and, as such, finding a suitable TBM 2 more complex.

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