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Chip Scale Review March • April • 2017

[ChipScaleReview.com]

systems, and the development

of TBDB materials for UV

laser debonding.

TBDB process flow and

system

T h e p r o c e s s f l ow o f

temporary bonding, including

the device wafer thinning

process followed by backside

processing for 3D packages,

is shown in

Figure 1a

. Wafer

thinning is carried out by

grinding until the thickness

reaches 100μm or thinner.

On the other hand, FOWLPs,

which comprise molded

compound embedding dies,

also require a wafer support

system. As a representative

f l o w o f F O W L P , a

redistribution layer (RDL)-first

process is shown in

Figure

1b

. In this process, a TBDB

material is coated on the

support wafer first, followed

by building up of RDL layers

using photoresist, dielectrics,

copper plating materials, and

epoxy molding compounds

(EMCs). It should be noted

that the bonded wafer must withstand the

high-temperature conditions, chemical

exposure and subsequent release without

damage to the device.

A laser release system has advantages in

terms of debonding temperature, mechanical

stress and throughput, compared with the

other conventional debonding systems such

as thermal slide, mechanical and solvent

release. For applications where glass carriers

are acceptable, a laser release system is a

promising procedure for debonding.

Laser release systems are classified by

the laser wavelengths as shown in

Table

1

. These are different in terms of the laser-

induced reaction mechanism and debonding

equipment cost, while all the systems listed in

the table require a glass carrier. Generally, the

UV laser debonding system has advantages

in terms of lower heat generation. The

ablation reaction induced by a high energy

density ultraviolet (UV) laser is speculated to

have photochemical characteristics including

a two-photon absorption mechanism.

This results in direct activation followed

by cleavage of the covalent bonds of the

TBDB material. In contrast, an infrared (IR)

laser is speculated to have a photothermal

mechanism that results only in the thermal

decomposition of molecules.

There are several sources for oscillating

UV lasers. In this study, we focused on UV

laser release systems with 308nm and 355nm

wavelengths because there are numerous

advanced packaging manufacturers that can

handle glass carriers and also because of the

trend to decrease operation temperatures to

minimize thermal damage to packages.

Temporary bonding material with high sensitivity for

laser release in advanced packaging processing

By Kenzo Ohkita, Yooichiroh Maruyama, Hikaru Mizuno, Takashi Mori, Hiroyuki Ishii, Koichi Hasegawa

[JSR Corporation]

r i v e n b y e x p o n e n t i a l l y

increasing demand in big

data for the Internet of Things

(IoT), powerful and multifunctional devices

with low energy consumption have been

developed. As Moore’s law is reaching its

limitations, innovative evolutions of advanced

electronic packages are required. Devices

that deploy 2.5D/3D integration and fan-

out wafer-level packages (FOWLPs) have

progressed to satisfy these requirements in the

recent decade [1-5]. The 2.5D/3D structures

consist of stacked chips with thinned silicon

wafers and through-silicon vias (TSVs),

while FOWLPs include device chips (dies)

embedded in a mold compound. Even though

their designs are different, the basic concept

for improving device performance tends to be

focused on two directions, one is to achieve

3D packaging by stacking thinned wafers,

and the other is to increase I/O density by

constructing fan-out packaging designs.

In order to handle thin and fragile

substrates, temporary bonding/debonding

(TBDB) technologies have been utilized.

In this technology, the device wafers are

rigidly fixed onto a carrier while processing

and then released. To release the substrate

from the carrier, four predominant

debonding systems – thermal slide,

mechanical release, solvent release, and

laser release – have been reported. Some

of the systems have already been put into

practical use. However, the requirements

for TBDB materials are broad and depend

on the target application. This is one of the

reasons that development and optimization

of each TBDB system is still underway.

Among the options listed above, the

laser release system has advantages in very

high-throughput debonding specifically

with respect to low mechanical and thermal

stresses. The temporary bonding layer is

designed to absorb the laser beam to be

decomposed. As a result, the TB layer loses

adhesion immediately and completely thereby

allowing the device a force-free release from

the carrier. In this paper, we will outline

TBDB technology, features of laser TBDB

D

Figure 1:

Schematic drawing of the TBDB process: a) Device wafer thinning

and backside processing; b) Mold wafer build-up for fan-out packages.

Table 1:

Laser release system comparison.