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Chip Scale Review November • December • 2016

[ChipScaleReview.com]

Photoresist strip using advanced single-wafer

process technology

By Laura Mauer, Scott Kroeger

[Veeco Precision Surface Processing]

, and Amy Lujan

[SavanSys Solutions]

hotoresist strip is a key lithography

step in many electronics packaging

process flows. There are a variety

of photoresists and processes

available depending on the lithography

requirements. In addition, there are different

methods for stripping the negative dry

film photoresist that is typically used for

advanced packaging applications.

The focus of this analysis is the Veeco

Precision Surface Processing WaferStorm®

tool featuring ImmJET™ high-performance

immersion and single-wafer spray process

technology. The tool and chemistry are

introduced, along with a cost analysis of the

key variables of the strip process. The cost

modeling portion, conducted by SavanSyS

Solutions LLC, includes a comparison of the

Veeco tool to other available processes, such

as batch immersion and spray tools.

The approach to understanding the cost

elements associated with each technology

is activity-based cost modeling. In this

methodology, a process flow is broken

down i n t o a c t i v i t i e s , and t he cos t

components of each activity–including

labor, material, capital, tooling, and yield–

are analyzed. The goal of this analysis is

to evaluate the ImmJET technology, and to

understand the key cost drivers associated

with photoresist strip.

Background

A thick photoresist pattern is required for

the formation of tall bump structures, shown

in

Figure 1

. Dry film resist provides the

advantage of uniform resist thickness across

the entire wafer, eliminating the need to do

multiple coatings and edge bead removal.

However, since these are negative working

polymers, they are highly cross-linked and

more challenging to remove. The complete

removal of the film and cleaning of the surface

is critical to the yield of the next step, which is

etching of the underlying metal seed layers.

A successful process results from the

proper choice of tool technology, solvent,

and process to completely remove the

dry film without any residuals that would

block subsequent etching. Wet benches are

commonly used for photoresist stripping

processes. A batch of wafers is loaded

into an immersion station with heated

chemistry for a long soak. Typical wet-bench

configurations include multiple soak tanks.

The first immersion station is the “dirty”

tank where the bulk of the resist saturates the

solvent. The second tank typically involves

a “cleaner” solvent process after which the

wafers are transferred to a final rinse tank

before drying. Single-wafer spray systems

have also been used as an alternative to

batch processes. However, the length of

time for the spray process is quite long for

these highly cross-linked dry film resists

and, though this process can be effective at

removing the resists, the cost-of-ownership

is too high to be feasible in a high-volume

manufacturing process. Here, we present a

new high-performance immersion and spray

process technology, called ImmJET, which

combines an optimized immersion step and

single-wafer spray process to produce high-

yield results at a low cost-of-ownership.

Process flow

The combination of two process

t e chn i que s ( s equenc ed imme r s i on

and single-wafer spray) in a single

system provides unique capabilities for

photoresist and dry film strip. Each wafer

is soaked under precisely controlled

conditions in a heated, recirculating,

solvent immersion bath with a nitrogen

environment. Sequencing is based on

the downstream process times, ensuring

each wafer is soaked an equal length

of time. With the appropriate selection

of chemistry for the composition and

thickness of the dry film, the soaking time

allows for swelling and dissolution of the

highly cross-linked resist. The process

sequence is shown in

Figure 2

.

Following the sequenced immersion step,

the surface of the wafer remains solvent-

wet during transfer to the single-wafer spin

process station. The use of a high-pressure

chemical fan spray enhances the removal

of residuals, ensuring a clean, resist-free

surface. The wafer is then transferred to a

P

Figure 2:

Dry film strip process sequence.

Figure 1:

Optical images a) Pre-dry film resist strip; b)

Post-DFR strip; c) SEM image post-DFR strip.