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26

Chip Scale Review January • February • 2019

[ChipScaleReview.com]

emporar y bonding is used

f o r m a n y a p p l i c a t i o n s

i n a d v a n c e d p a c k a g i n g

and microelectromechanical systems

(MEMS). Device wafers are bonded to a

carrier wafer with a temporary bonding

material in order to perform subsequent

backside processing without affecting

the device side. After completing the

fabrication steps, the wafer must be

removed from the temporary carrier

by a release mechanism appropriate to

the type of temporary bonding material

used. After debonding, the device wafer

must be sufficiently clean in order to

complete fabrication. If the device wafer

was thinned, it will be debonded onto a

film frame with ultraviolet (UV) tape.

The cleaning process and chemistry need

to be compatible with the tape and film

frame structure.

Optimized cleaning is a result of

selecting the most effective chemistry,

equipment and process. The chemistry

should dissolve the temporary bonding

material while leaving the device wafer

undamaged. The equipment needs to

mai nt ai n process pa rameters, such

as temperature and f low rates, while

recirculating the chemical for reduced

chemical consumption. The process

determines the optimized parameters and

sequence for the most effective cleaning

at the lowest cost of ownership.

This paper describes the methodology

and experimentation done to optimize

the cleaning process for BrewerBOND

®

305 bond i ng mat e r ia l. Pa r amet e r s

addressed included chemical purity,

bath life and consumption, process

temperature, dispense method, time,

and process sequence. A design of

experiment (DOE) methodology was

performed to determine the impact of

tool and chemical parameters that would

yield residue-free wafers. In addition,

the effect of adhesive film thickness

on process conditions was considered.

Processes will be presented for both

debonded wafers and wafers mounted

onto f ilm frames. The methodology

provided an optimized cleaning process

wit h lower cost usi ng reci rcu lated

chemistry at ambient temperature.

The growing importance of TBDB

Wa f e r- l e v e l p a c k a g i n g ( WL P)

technologies continue to evolve and

have been adopted for use in large-scale

manufacturing. As demand for smaller,

thinner packages has grown, temporary

bonding and debonding (TBDB) has

become an increasingly critical step in

the fabrication process [1]. Consumer

electronics, automotive and inter net

of things (IoT/5G)-based applications

and services have been driving growth

in the semiconductor industry. High

p e r fo r ma nc e , sma l l fo r m

factor, and dense integration

with low power consumption

are requirements for today’s

advanced devices.

A critical requirement for

advanced technology is the

use of a temporar y car rier

wafer to support the device

wafer that has been processed

with temporary bonding and

release material. Downstream

f a b r i c a t i o n p r o c e s s e s

i nclude h igh -t empe r at u r e

thermal cycles, high-vacuum

environments, and various

chemic a l t r e a t me nt s t h a t

can damage an unsupported

device wafer. Device damage

d u e t o b ow i n g / w a r p i n g

induced f rom downst ream

processes can be prevented

with the use of a temporary

bonded carrier substrate.

I n a s t a nd a r d TBDB p r o c e s s , a

thermoplastic bonding material is spin-

coated onto the front side of a device wafer

then baked to remove excess solvent from

the film. The carrier wafer, typically glass

or silicon, is coated with a thin layer of

release material used to facilitate debonding

at the appropriate interface (

Figure 1

). The

carrier and device wafers are then bonded

together, under vacuum, using a bond

temperature high enough for the bonding

material to soften and become tacky.

Once the device wafer is supported

by the carrier wafer, backside grinding

or thinning using chemical mechanical

planarization (CMP) can be performed.

During this process, the device wafer is

typically thinned to less than 100µm and

in some instances, as thin as 30µm. The

bonded pair then can undergo various

backside processes, including high-

temperature and vacuum exposure. Once

T

Temporary bonding and the challenge of cleaning

post-debond

By Phillip Tyler, Kenji Nulman

[Veeco Instruments - Precision Surface Processing]

Michelle Fowler, Seth Molenhour

[Brewer Science, Inc.]

Originally published at the International Wafer-Level Packaging Conference, San Jose, California; October 2018.

Figure 1:

Illustration of wafer processing and handling flow.