Wet chemical processing with megasonics assist for micro-bump resist stripping

IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ
Wet Chemical Processing with Megasonics Assist
for the Removal of Bumping Process
Photomasks
Hongseong Sohn and John Tracy
Akrion Systems
6330 Hedgewood Drive, Suite 150
Allentown, PA 18106, USA

Contents
 Background and goals
 Process Overview
 Real World Process Development
 Conclusions and Next Steps
 Acknowledgements
 References

Background – Akrion Systems
 Based in Allentown, PA with sales and service
offices worldwide
 Batch and Single-wafer wet process systems
 Customer base in Semiconductors, WLP, MEMS,
Solar
 Strategic acquisitions over time to build
technology portfolio

New and Thick Photoresist Adoption
Driven by Cu Pillar Applications
 A shift to new negative tone and chemically amplified
positive tone PR has occurred for Cu pillar with
micro-bump as bump densities increase
 Why the new photoresist products?
• PR must fully enclose the pillars prior to reflow
• At PR thickness 20 ~ 80µm acrylate or other negative
resists are much more transparent than Novolak resists
• Response to exposure is 10x-100x faster [1]
• Shorter litho cell times (exposure, PEB, etc.)
• Downside is that strip times can be 10x longer [1,2,3]
 Processing Goals
• Cost Effective PR Stripping by improving productivity
and reducing chemical consumption
• No damage or corrosion to bump metals, UBM
• No photoresist residue SEM image of 70 µm height pillars prior
to reflow (courtesy A*Star IME)

Thick Resist Strip Process Overview
 Based on processes developed to strip chemically amplified DUV resists [4], a two
step solvent process has emerged as most efficient for PR up to 60 µm thick
1. Solvent exposure at 60 – 80oC and low RPM wafer spin – begin the reaction
2. Continue solvent feed during input of 100+ Watts of megasonic energy
 Solvent spin-off steps, DI rinse and spin dry are adequate for removing reactants
from the wafer
Puddle solvent
Low RPM to
Dissolve PR
Solvent with
high-power
megasonics
Spin off solvent
DI water rinseSpin dry
Flow chart of the thick photoresist removal process

Thick Resist Strip Process Overview
 The megasonic energy imparted by
frontside or backside megasonics plays an
important role in driving the reaction
• Agitation helps drive the polymer chain
scission reaction throughout the layer of
photoresist
• Decreases the fluid boundary layer to nearer
the surface to help with removal of surface
residues
 This megasonics assisted process has
been applied successfully to a variety of
negative and chemically amplified
photoresists
Megasonics impact on fluid boundary layer
Source: Souvik Banerjee 2001, UC Berkeley Extension [5]

Relative sizes of Some Semi Structures
 90 nm width polysilicon
lines spaced at 180 nm
 Borderline of possible
damage for high power
0.85 MHz or 1.0 MHz
megasonics operation
 Leti 7 µm x 70 µm TSVs
 Structures are 7000 nm
wide x 70,000 nm deep!
 Leti 50 µm bump array
 50,000 nm sized
structures!

Thick Resist Strip Process Development
 The single-wafer thick photoresist stripping process was applied to a process
development effort with a Korea based bumping line customer
• 20 µm and 50 µm levels with JSR negative tone resists and Dynaloy solvents
• For a given solvent and resist type the contribution of megasonics was studied
 The same process methodology was applied to development work with a
Singapore customer
• 20 µm and 40 µm levels with a TOK PMER series chemically amplified positive tone
resist and solvent products by TOK, Dow Chemical, and Cheil Industries
• Use best known methods from previous work to optimize process times for this resist
 Thirdly, the typical process was modified to optimize removal of 100 µm thick TOK
PMER series resist

JSR Negative Tone 20 µm Process Results
 20 µm thick resist mask required 90 sec. process with megasonics to clear resist vs.
150 sec. without megasonics
 Process time and solvent usage were reduced by 40% per wafer (60 sec., 1 L solvent)
Parameter w/ Megs w/o Megs
Bump dia. 50 µm 50 µm
Photoresist JSR THB-126N,
20 µm thick
JSR THB-126N,
20 µm thick
Solvent DynaloyAP7700 DynaloyAP7700
Temp., Flow 60oC, 1.0 Lpm 60oC, 1.0 Lpm
Solvent feed Side nozzle w/megs Topside oscillating
Megasonics XT 1.0, 100 W none
Strip time 90 sec. to clear 150 sec. to clear
With megasonics and 90
second strip time
Without megasonics and 150
second strip time

JSR Negative Tone 50 µm Process Results
 50 µm thick resist mask required 135 sec. process with megasonics and 0.7 Lpm
 Without megasonics - resist remaining at 180 seconds
 Process time reduced at least 60 sec. and solvent reduced at least 1.5 L per wafer
(50% solvent reduction)
Parameter w/ Megs w/o Megs
Photoresist JSR THB-151N,
50 µm thick
JSR THB-151N,
50 µm thick
Solvent DynaloyAP7880-C DynaloyAP7880-C
Solvent feed Side nozzle w/megs Topside oscillating
Megasonics XT 1.0, 100 W None
Solvent strip
time 135 sec. to clear 180 sec.
residue remaining
With megasonics and 135
second strip time
Without megasonics and 180
second strip time

TOK PMER Series Resist Process Results
 The TOK chemically amplified resist required shorter process times for removal than
negative tone, but much longer than for traditional i-line positive tone resists
 70 seconds strip time for TOK resist at 20 µm vs. ~30 seconds for 20 µm of AZ P4620
i-line resist
Parameter 20 µm resist 40 µm resist
Bump size 20 µm H x 10 µm D 30 µm H x 20 µm D
Photoresist TOK PMER
P-CR4000, 20 µm thick
TOK PMER
P-CR4000 , 40 µm thick
Solvent Microposit 1165 Microposit 1165
Solvent feed Side nozzle w/megs Side nozzle w/megs
Megasonics XT 1.0, 100 W XT 1.0, 100 W
Strip time 70 sec. to clear 80 sec. to clear
20 µm bumps after 70
second strip time
30 µm pillars after 80
second strip time
Chemically amplified positive tone resist

Ongoing Process Development
TOK PMER at 100 µm Thickness
 The same chemically amplified TOK resist is used at 100 µm thickness to pattern large 70
µm high landing bumps
 Customer’s POR with TOK ST-120 (DMSO based) stripper resulted in >7 minute process time
– shortest process time achieved on-site was 140 sec. with 100 W megasonics, Microposit
1165 and max. achievable temp. 65oC – marginal results
 Decision was made to investigate higher process temp., greater chemical flow +
megasonics at the Akrion Systems lab
Pre-SEM of 100 µm mask
Clean results with final processMarginal results – resist remaining

TOK PMER at 100 µm Thickness
 The best process time and results achieved at the customer site were improved by >30%
with additional process development
• ~60 second decomposition step with megasonics (80oC, topside spray and side dispense, total 1.9 Lpm)
• ~30 second removal step using 100oC Hot N2 spray (1.0 Lpm chemical front and back)
• DMSO based StarStrip by Cheil Industries was used for most of the splits at Akrion
140
180
129
122
95
50
70
90
110
130
150
170
190
A B C D E
Process Time
Center nozzle scan
180 sec. 80oC
(no megs)
at Akrion lab
Center spray and
Side feed 105 sec.
80oC
Front megs 15 sec.
Center/Side feed
105 sec. 80oC
Back megs 10 sec.
Center/Side feed
w/ BS megs 62 sec.
100oC N2 spray 33 sec.
80oC chemical temp.
 For the thickest bumping photoresist masks, higher chemical flow and recirculation, combined with
high power megasonics and a powerful heat source are required to optimize results
Center nozzle scan
125 sec. 65oC
10 sec. Front megs
at customer site

Conclusions
 Single-wafer process with megasonics assist can provide 40-50% cost and
chemical consumption reduction for bumping resists 20 µm – 50 µm
 A new capability was developed and optimized for the efficient removal of
the very thickest bumping photomasks, adding heat to drive the chemical
action to breakdown the resist, together with megasonics
 The new process shortened removal time for 100 um TOK PMER series by
an additional 30% beyond the base case with megasonics and existing
spray technology
 Next Steps – continue this work on 20 um and 40 um PMER resist films to
determine if similar process gains may be achieved, and to compare CoO
of the two methods

Acknowledgements
 The authors wish to thank Steven (Lee Hou) Jang of A*Star IME for his
contributions to the work with three levels of TOK PMER series photoresist
patterned for bumping levels.

References
1. Flack, W., Nguyen, H., Neisser, M., Sison, E., Ping, H. L., Plass, B., Makii, T. & Murakami, Y., A
Comparison of new thick photoresists for solder bumping, SPIE 2005, #5753-104
2. Henderson, C., Introduction to chemically amplified photoresists, published online by the School of
Chemical and Biomolecular Engineering at the Georgia Institute of Technology, retrieved February
12, 2014 from https://sites.google.com/site/hendersonresearchgroup/helpful-primers-
introductions/introduction-to-chemically-amplified-photoresists
3. Flack, W., Nguyen, H. & Capsuto, E., Characterization of an ultra-thick positive photoresist for
electroplating applications, Future Fab Intl., 17, June 21, 2004
4. Franklin, C., Megasonic agitation allows removal of chemically amplified photo-resists, SPIE 2009,
#7273, Advances in Resist Materials and Processing Technology XXVI, April 01, 2009
5. Banerjee, S., course material from the online class, Cleaning technology for integrated circuit
manufacturing, UC Berkeley Extension, 2001

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