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

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  • 1. 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
  • 2. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ Contents  Background and goals  Process Overview  Real World Process Development  Conclusions and Next Steps  Acknowledgements  References
  • 3. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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
  • 4. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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)
  • 5. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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
  • 6. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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]
  • 7. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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!
  • 8. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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
  • 9. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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
  • 10. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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 Temp., Flow 65oC, 0.7 Lpm 65oC, 1.0 Lpm 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
  • 11. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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 Temp., Flow 70oC, 1.0 Lpm 70oC, 1.0 Lpm 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
  • 12. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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
  • 13. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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
  • 14. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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
  • 15. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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.
  • 16. IMAPS 10TH INTERNATIONAL CONFERENCE & EXHIBITION ON DEVICE PACKAGING | MARCH 10-13 | FOUNTAIN HILLS, AZ 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