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Chip Scale Review May • June • 2018


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


Figure 1

shows the schematic of

the chip and the RDL, the PCB assembly,

and the cross section of the fan-in WLP

[3]. 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.



Figure 1:

Fan-in WLP.

Figure 2:

FOWLP RDLs process flow using photosensitive polymer and ECD Cu + etching.