Chip Scale Review May • June • 2018[ChipScaleReview.com]
better and simpler process is shown in
. It can be seen that for the
PI (polyimide) development, the whole
reconst it ut ed wafe r is spi n- coat ed
with a photosensitive PI. It is followed
by applying a stepper and then using
photolithography techniques to align,
expose, and develop the vias of the PI.
Finally, the PI is cured at 200°C for
one hour—this will form a 5µm-thick
PI layer. It is followed by sputtering
Ti and Cu by PVD at 175°C over the
entire reconstituted wafer. Then, apply
a photoresist and a stepper and use
photolithography techniques to open
the redist r ibution t race’s locations.
Nex t , elect roplat e t he Cu by ECD
at room t empe r at u re on t he Ti /Cu
i n t he photoresist open i ngs. These
steps are followed by stripping off the
photoresist and etching off the Ti/Cu;
RDL1 is thereby obt ained. Finally,
repeat all the above steps to obtain
shows the cross
sect ion of a FOWLP with 2 RDLs.
Th is can be used for FOWLP wit h
chip-f irst and chip-last processing.
The RDLs made by the polymer (either
photosensitive or not) and ECD Cu +
etching are called organic RDLs.
FOWLP RDLs by PECVD and
Cu - Dama s c e n e + CMP.
T h i s i s
t he oldest back- end semiconductor
process. Th is process uses SiO
SiN for the dielectric layer and ECD
to deposit the Cu on the whole wafer.
That is followed by usi ng CMP to
remove the overburden Cu and seed
layer to make the Cu conductor layer
of the RDLs. The key process steps
a r e shown i n
F i gure 4
. Fi r s t , u s e
PECVD to form a thin layer of SiO
(o r Si N ) on a f u l l t h ick ne s s ba r e
si l icon wa fe r a nd t hen u s e a s pi n
coater to lami nate t he photoresist.
These steps a re followed by usi ng
a st eppe r t o open t he r e sist a nd a
react ive ion et ch (R I E) t o remove
t he SiO
. The n , a s t e pp e r i s u s e d
to open the resist wider and RIE to
et ch more of t he SiO
. Next , st r ip
off the resist, sputter the TiCu, and
ECD t h e Cu on t h e whol e wa f e r.
The se st eps a r e fol lowed by CMP
t o r emove t he ove r bu r den Cu a nd
the TiCu, and then we have the first
RDL1 and V01 (the via connecting
t h e S i a n d R DL1) a s s h ow n i n
. This is called the dual
Cu- damascene method . Finally,
repeat all t he processes to get t he
o t he r RDLs . T h i s me t ho d c a n b e
FOWLP RDLs process flow using PECVD and Cu-damascene + CMP.
Cross section of RDLs using PECVD and
Cu-damascene + CMP.
FOWLP RDLs process flow for hybrid-RDLs.
Cross section of FOWLP RDLs using
photosensitive polymer and ECD Cu + etching.