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Chip Scale Review July • August • 2018

[ChipScaleReview.com]

Temporary bonding for high-temperature processing of

thin glass

By Robert A. Bellman, Prantik Mazumder, Robert G. Manley, Kaveh Adib, Shiwen Liu, Leena Sahoo

[Corning Research and

Development Corporation]

hin glass substrates offer

low d iele c t r ic con s t a nt

a n d h i g h - t emp e r a t u r e

process capability for faster, thinner

packages, but handling is challenging

on account of the f lexibility of the

substrate. Polymeric and tape wafer

bonding solutions are available, but

only for low-temperature processes.

This paper describes Advanced Lift-

off Technology (ALOT), a temporary

wafer bonding method for thin glass

that permits processing up to 580°C.

F luo r oca r bon pl a sma s a r e shown

t o mod i f y t he su r f a ce ene r ge t ic s

of a glass continually f rom that of

a clean “glass-li ke” su r face ( h igh

surface energy) to that of a “Teflon™️-

like” surface (low surface energy)

permitting controllable van der Waals

bonding between a thin glass and a

glass car r ier at room temperat u re.

This modification can withstand the

va cuum, t he rma l , wet- proce s si ng

st eps of back- end- of-l i ne (BEOL)

processing. However, the bond energy

between the pair remains low enough

after the thermal processing steps that

render the pair fully detachable.

Introduction

Ultra-thin glass offers the promise

of cheape r dev ice s u si ng r ol l- t o -

rol l processi ng, and t he pot ent ial

t o make t h i n ne r, l ight e r,

more f lexible and durable

d i s p l a y s . Howe v e r , t h e

technology, equipment, and

processes required for roll-

to-roll processing of high

quality displays are not yet

f ully developed. Because

panel makers have already

heavily invested in toolsets to

process large sheets of glass,

laminating flexible thin glass

to a glass carrier and making

display devices by a sheet-to-sheet

process offers a shorter term solution to

develop the value proposition of thinner,

lighter, and more f lexible displays.

While displays have been demonstrated

on polymer sheets such as polyethylene

naphthalate (PEN) laminated to a glass-

glass carrier, the upper temperature

limit (usually less than 200°C) of the

PEN limits the process capability and

in turn, device quality, not to mention

the high permeability of the polymer

substrate leading to environmental

degradation of organic light-emitting

d iode (OLED) dev ic e s . The r e i s ,

therefore, much value in inventing

a material/process combination that

would allow temporary bonding of thin

glass to a rigid, transparent and high-

temperature glass carrier substrate,

such as glass, and de-bond the thin

glass from the glass carrier after the flat

panel display (FPD) processing steps

(

Figure 1

). This realization nucleated

ALOT technology, which could be

summarized as the following problem

statement: a) the thin glass must bond

spontaneously with a glass carrier at

room temperature with sufficient bond

or adhesion energy; b) the bonded pair

must withstand the thermal, chemical,

vacuum and wet processing steps of

FPD fabr ication; c) the outgassing

a nd /o r con t ami n a t ion du r i ng t he

thermal processing should be as low as

possible; and d) at the end of the high-

temperature steps, one should be able

to separate the thin glass from the glass

carrier with relative ease.

Van der Waals bonding of glass

It is wel l-k nown t hat clean and

hydroxylated surfaces of glass are replete

with silanol or hydroxyl groups (number

density anywhere between 2-5/nm

2

[1]).

These hydroxyl groups are highly polar

in nature – the glass surface is essentially

an array of permanent dipoles sticking

out of the surface – twisting, turning, and

vibrating on account of thermal energy.

When clean and smooth (Ra<0.6nm)

glass surfaces are brought into optical

contact, the surfaces spontaneously bond

(

Figure 2a

). The glass surfaces adhere

either by direct hyd rogen bonding

between the opposing silanol groups, or

by mediation through one or two layers

of molecular water absorbed on the

silanol groups [1]. Either way, these polar

interactions lead to a polar bonding energy

~150mJ/m

2

. The attractive force between

permanent dipoles is called Keesom force,

named after W.H. Keesom who worked

out the theoretical details and is one of the

three types of van der Waals forces [2].

In addition, all materials have fluctuating

instantaneous dipoles distributed in the

bulk due to the continuous motion of

T

Figure 1:

Schematic showing the process path of bonding thin

glass to glass-glass carrier, device fabrication, and de-bonding.

Figure 2:

a) Van der Waals bonding between a pair of

hydroxylated glass substrates, and b) Covalent bonding between

the pair due to condensation reaction at high temperature.