The document describes research into fabricating micro vias in LCP substrate using mechanical punching and subsequent cleaning. Chemical etching using sodium permanganate and sodium hydroxide was found to effectively remove LCP and copper burr leftovers from punching. However, carbonated LCP debris remained. 30 minutes of oxygen plasma cleaning at 125 watts power successfully removed the debris, yielding uniform via walls. The sequential process of wet etching and plasma cleaning was necessary for clean micro via fabrication using mechanical punching.

1. The 60 th ECTC Paris Las Vegas, Nevada, June 1 - 4, 2010 Investigation of Chemical De-burring and Subsequent Plasma Cleaning of Mechanically Punched Micro Via Array Fabricated in LCP Substrate Mohammad K. Chowdhury , 1 Li Sun, 2 Shawn Cunningham, 2 and Ajay P. Malshe 1* 1 Materials and Manufacturing Research Laboratories (MMRL) University of Arkansas, Fayetteville, AR 72701 2 WiSpry Inc., Irvine, CA 92618 * Contact: E-mail: apm2@uark.edu, Tel: (479) 575-6561, Fax: (479) 575-2669 Chowdhury , Li Sun, Shawn, and Ajay

2. Acknowledgements The National Science Foundation wi Spry Inc., Irvine, CA High Density Electronics Center (HiDEC) Staff Rogers Corporation Rohm and Hass ECTC 2010 Chowdhury , Li Sun, Shawn, and Ajay

3. Need for a Cost effective way of micro via fabrication High speed processing of through via fabrication with high yield throughput Compatible process with the conventional via fabrication tool E limination of thermo processing of the substrate during through via fabrication Fabrication of vias with uniform through via wall Motivations of the Research: Chowdhury , Li Sun, Shawn, and Ajay

4. How The Punching System Works? Chowdhury , Li Sun, Shawn, and Ajay µ -Via Fabrication by Mechanical Punching: Die Bushing Pin LCP Copper Copper LCP Copper Copper Before Punching Copper Copper LCP After Punching APS 8718 Automatic Punching System Pacific Trinetics Corporation 6” x 6” Sample Holder Punch Pin Holder & Die Bushing LCP Copper Copper LCP Copper Copper Copper Copper LCP Figure: Uniform and nice via formation

5. Driver Applications: Chowdhury , Li Sun, Shawn, and Ajay Ref:http://www.mpdigest.com/issue/Articles/2009/june/mmic/Default.asp In MMIC (Microwave Monolithic IC) Ref: http://www.pcdandf.com/cms/magazine/220-2009-issues In 3D Packaging & MCM Module Ref:http://www.cmst.be/projects/img9.jpg In Flexible Electronics Ref: http://www.techwear-weblog.com In Wearable Electronics www.smalltimes.com In RF-MEMS Devices in Cell Phone www.ec.europa.eu – ANASTASIA Project In Satellite and Aerospace Applications

6. Dealing with the Fabrication Issues Chowdhury , Li Sun, Shawn, and Ajay Z-axis expansion of the LCP Film Warpage of the LCP sample LCP Burr Copper Burr Expansion Warpage 50 µm Via, 75 µm Pitch, 10 x 10 Array LCP Burr Bottom Cu Film Bottom Cu Film Copper Burr

7. Addressing the Issues Chowdhury , Li Sun, Shawn, and Ajay By Wet Chemical Etching Anisotropic Isotropic (For LCP & Cu Burr) By Dry Etching Reactive Ion Etching ( O 2 Plasma Cleaning ) Deep Reactive Ion Etching Ref: http://www.emeraldinsight.com/fig/0870220101014.png Ref: http://knol.google.com/k/-/-/2ufdlj2yc019u/u4nyvf/deep-reactive-ion-etching%20(1).jpg

8. Chemical Etching Using Ethanolamine Chowdhury , Li Sun, Shawn, and Ajay Molecular Formula: C 2 H 7 NO Chemical Structure: HO CH 2 CH 2 NH 2 Physical State: Viscous Hygroscopic Liquid 50 µm Via, 75 µm Pitch, 10 x 10 Array Chemical Property of Ethanolamine Chemical Property of LCP Due to its Hygroscopic nature the Ethanolamine was not an effective etchant for low moisture absorbent ULTRALAM 3850 TM LCP 20 wt% EA + 40 wt% KOH + 40 wt% Water @ 85 0 C 3314 Neutralizer (3% H 2 SO 4 + 3% H 2 O 2 ) 1 Min 5 Min 5 Min 10 Min

9. Chemical Etching Using Promoters The LCP burr need to be functionalized before it can be etched out Permanganate solution is a strong oxidizer can help in this process Chowdhury , Li Sun, Shawn, and Ajay Chemical Property of LCP Promoter A: NaMnO 4 (Strong Oxidizer) Promoter B: NaOH (Strong Etchant) Oxidation of the LCP burr by 20 – 40% NaMnO4 Etching of the LCP burr by < 30% NaOH 5 Minutes Neuratilzation by (3% H 2 SO 4 + 3% H 2 O 2 )

10. Chemical Etching Using Promoters Chowdhury , Li Sun, Shawn, and Ajay 75 µm Via, 187.5 µm Pitch, 10 x 10 Array PROMOTER 3308A (20 – 40 % NaMnO 4 ) @ 85 0 C PROMOTER 3308B (NaOH < 30%) @ 85 0 C 3314 Neutralizer (3% H 2 SO 4 + 3% H 2 O 2 ) 5 Min 1 Min 5 Min 5 Min 10 Min 15 Min PROMOTER 3308A (20 – 40 % NaMnO 4 ) @ 85 0 C PROMOTER 3308B (NaOH < 30%) @ 85 0 C 3314 Neutralizer (3% H 2 SO 4 + 3% H 2 O 2 ) 1 Min 5 Min 5 Min 5 Min 10 Min 15 Min PROMOTER 3308A (20 – 40 % NaMnO 4 ) @ 85 0 C PROMOTER 3308B (NaOH < 30%) @ 85 0 C 3314 Neutralizer (3% H 2 SO 4 + 3% H 2 O 2 ) 5 Min 5 Min 1 Min 5 Min 10 Min 15 Min

11. Chemical Etching – Process Conditions 2.5 Liter of Promoter A & B were taken in a beaker to submerge the 5” LCP wafer sample The solutions were continuously stirred using a magnetic stirrer A solution temperature of 85 0 Cs was monitored continuously during the process for Promoter A & B Top view SEM images of the same spot of the samples were taken before and after The cross sectional analysis was done for the same sample after taking the SEM images Chowdhury , Li Sun, Shawn, and Ajay

12. Chemical Etching Using Promoters Chowdhury , Li Sun, Shawn, and Ajay PROMOTER 3308A (20 – 40 % NaMnO 4 ) @ 85 0 C - 5 Minutes PROMOTER 3308B (NaOH < 30%) @ 85 0 C – 5 Minutes 3314 Neutralizer (3% H 2 SO 4 + 3% H 2 O 2 ) – 5 Minutes Best recipe out of the matrices

13. Chemical Etching Using Promoters Chowdhury , Li Sun, Shawn, and Ajay PROMOTER 3308A (20 – 40 % NaMnO 4 ) @ 85 0 C - 5 Minutes PROMOTER 3308B (NaOH < 30%) @ 85 0 C – 5 Minutes 3314 Neutralizer (3% H 2 SO 4 + 3% H 2 O 2 ) – 5 Minutes Best recipe out of the matrices

14. Chemical Etching Using Promoters Chowdhury , Li Sun, Shawn, and Ajay Delamination due to longer etching time of 15 minutes Promoter A – 5 min , Promoter B – 15 min, Neutralization – 5 min 200 μ m 50 μ m

15. Chemical Etching Using Promoters Chowdhury , Li Sun, Shawn, and Ajay Delamination due to higher processing temperature (90 0 C) Promoter A – 1 min (90 0 C) , Promoter B – 5 min, Neutralization – 5 min After 200 μ m 50 μ m Delamination of the bottom Cu film

16. Issue to Overcome after Chemical Etching Chowdhury , Li Sun, Shawn, and Ajay Carbonated LCP debris leftover after chemical etching Carbonated LCP Debris

17. O 2 Plasma Cleaning of LCP Debris Micro vias were cleaned by using 5 Min Promoter A (NaMnO 4 ) – 5 Min Promoter B (NaOH), 5 Min Acid Neutralization (3% H 2 SO 4 + 3% H 2 O 2 ) 125 watt power at 320 miliTorr pressure with 160 sccm of oxygen flow was used Samples were treated for 5 min, 15 min, and 30 minutes SEM images were taken for the same spot before and after of plasma cleaning process Chowdhury , Li Sun, Shawn, and Ajay

18. 05 minute oxygen plasma cleaning Chowdhury , Li Sun, Shawn, and Ajay O 2 Plasma Cleaning after Chemical Etching Precipitation of LCP After After Before Before LCP precipitation

19. Chowdhury , Li Sun, Shawn, and Ajay 15 minute oxygen plasma cleaning O 2 Plasma Cleaning after Chemical Etching LCP debris did not get cleaned very well due to large volume of the debris After After Before Before

20. Chowdhury , Li Sun, Shawn, and Ajay O 2 Plasma Cleaning after Chemical Etching 30 minute oxygen plasma cleaning Small Precipitation of LCP debris also copper film get etched After After Before Before LCP Precipitation Copper Etching

21. Chowdhury , Li Sun, Shawn, and Ajay 30 minute oxygen plasma cleaning O 2 Plasma Cleaning after Chemical Etching Very nice uniform cleaned via fabrication after 30 minutes oxygen plasma cleaning Bottom Copper Layer

22. Chowdhury , Li Sun, Shawn, and Ajay O 2 Plasma Cleaning Without Chemical Etching No improvement on the LCP and copper burr removal process due to ignoring the chemical etching process before the plasma cleaning 5 min 5 min 15 min 15 min 30 min 30 min

23. Summary of µ- Via fabrication Micro via fabrication by mechanical punching 5 Minutes Oxidation of the LCP burr by 20 – 40% NaMnO4 Chowdhury , Li Sun, Shawn, and Ajay 5 Minutes Etching of the LCP burr by < 30% NaOH 5 Minutes Neuratilzation by (3% H 2 SO 4 + 3% H 2 O 2 ) 30 Minutes oxygen plasma cleaning with 125 watt power at 320 mili torr pressure with 160 sccm of oxygen flow

24. Conclusions Ethanolamine with KOH is not a good chemistry to address the burr elimination NaKMnO 4 for oxidation needed to functionalize the LCP before it can be etched by NaOH Acid neutralization is needed after strong caustic solution etching 5 minutes oxidation, 5 minutes etching, and 5 minutes neutralization gives the best cleaned micro vias Oxygen plasma cleaning is needed to remove carbonated LCP burr formation during the etching process Therefore, a sequential treatments of wet chemical etching and oxygen plasma etching is need to make uniform micro via fabrication by mechanical punching technique Chowdhury , Li Sun, Shawn, and Ajay

25. Thank You! Chowdhury , Li Sun, Shawn, and Ajay