Chip Scale Review May • June • 2018[ChipScaleReview.com]
8 ways to make RDLs for FOW/PLP
By John H. Lau
[ASM Pacific Technology Ltd.]
edistribution layers (RDLs)
[1-2] are the most integral
part of wafer-level packaging
(WLP). In this study, various methods
for the fabrication of RDLs for fan-
out wafer/panel-level packaging (FOW/
PLP) are presented. Emphasis is placed
on four different methods in making the
RDLs for FOWLP, and the other four for
FOPLP. Some recommendations are also
provided. Finally, the critical issues of
panel-level technology are presented.
FOWLP RDL methods
As noted in the introduction, there are
four methods for making RDLs used in
FOWLP. This section discusses them.
FOWLP RDLs by polymer and ECD
Cu + etching.
This is the oldest method
to make RDLs for fan-in WLP—for
examples, see [3,4]. The RDL consists of
two layers, the dielectric layer and the Cu
conducting layer. The dielectric layer is
made of a polymer, e.g., polyimide (PI),
benzocyclobutene (BCB), or polybenzo-
bisoxazole (PBO) and the conductor layer
is made by electrochemical deposition
(ECD) of Cu and etching. The key process
steps are described as follows: 1) First,
spin coat a polymer on the whole wafer;
2) That step is followed by spin coating
a photoresist; 3) Then the photoresist is
opened with a mask aligner or stepper.
4) The polymer is then etched, and the
resist is stripped off; 5) Next, the adhesive/
seed layer (Ti/Cu) is sputtered using
physical vapor deposition (PVD); 6) The
photoresist is then spin coated, and then
the photoresist is opened with a mask
aligner or stepper; and 7) Next comes
electroplating the Cu. After the resist is
stripped off and the TiCu is etched off,
we have the first RDL. If one repeats the
processes, you get the other RDLs. For
shows the schematic of
the chip and the RDL, the PCB assembly,
and the cross section of the fan-in WLP
. Today, most outsourced semiconductor
assembly and test suppliers (OSATS) use
this method to make RDLs for FOWLP
with chip-first and chip-last processing.
FOWLP RDLs by photosensitive
polymer and ECD Cu + etching.
FOWLP RDLs process flow using photosensitive polymer and ECD Cu + etching.