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 rinse Spin 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 Dynaloy AP7700 Dynaloy AP7700

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 Dynaloy AP7880-C Dynaloy AP7880-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 process Marginal 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

Science

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