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1、The Cutting Edge of Copper Wire Ball Bonding 2 Agenda Growth of copper wire bonding Challenges and solutions Pd Coated Copper Wire Summary 3 Copper Wire - The Journey Engineers had been experimenting with alternate wire to gold for more than 20 years Until recently, the only alternative bonding wire
2、 is aluminum which is usually wedge bonded Copper is chosen because of the excellent conductivity and a commonly used metal for electrical circuits Initial work with copper wire found that forming a FAB is relatively easier - K&S had patented an earlier version of the copper kit in 1999 Early Cu Wir
3、e Bond adoption cost saving for heavy wire in 1990s First used on discretes and Power ICs which have a robust pad structures Cu kits designed via trial and error Wire size usually thick than 2.0mil Renewed interest to convert 1mil wire to Cu 2006 Gold prices began to shot up significantly Effort was
4、 focus on understanding the fundamental knowledge for Cu wire bonding Engineering models and characterizations were initiated to study the fundamentals Today Gold prices continuing to rise dramatically Cu Process Knowledge increased dramatically and reliability had been proven Fine Pitch/High perfor
5、mance apps entering Cu Wire production at a rapid pace A few large IDMs are transitioning to Cu very aggressively Subcons Competing for Cu Business (aggressively adding capacity) 4 Copper Wire Penetration Estimated 15% automatic ball bonders worldwide are installed with a copper kits and running reg
6、ular production By end 2010, estimate about 1215% units are bonded with copper wire, compared to less than 2% before 2008 Prismark projects that copper wire penetration will grow from just over 5% in 2009 to almost 30% in 2014 IC applications now account for more than 70% of the copper wire volume.
7、Taiwan and China lead the world in transition to copper Copper kit installed by K&S 5 Challenges And Solutions 6 Controlled enviroment for EFO firing Design based on Computational Fluid Dynamics analysis and Gas Flow Visualization Copper Kit Example 7 Intensive use of gas flow modeling to determine
8、design to exclude oxygen in the EFO firing process Contour plots showing mass fraction of oxygen (levels corresponds to previously reported measured oxygen) Predicts trade-off between forming gas usage (flow rate) and enviroment robustness (size of usable low oxygen region) FAB Region Ceramic Tube E
9、FO Wand Range for good FAB formation Flow rate: 0.2 lpm EFO Wand FAB Region Ceramic Tube Range for good FAB formation Flow rate: 0.6 lpm Designing the Kit 8 Establishing the right level of gas flow Malformed and Oxidized Low forming gas flow Visible oxide layer Pointed Flow is too high Oxide layer n
10、ot visible but surface tension disrupted Round and clean Ideal forming gas flow Making The Right Free-Air-Ball 9 Copper is a Harder Material ANSYS 10.0 NODAL SOLUTION STEP=9 SUB =46 TIME=9 EPTOX (AVG) RSYS=1 PowerGraphics EFACET=1 AVRES=Mat DMX =.219E-03 SMN =-.459073 SMX =.48208 1 -.48 -.32 -.16 -.
11、555E-16 .16 .32 .48 ANSYS 10.0 NODAL SOLUTION STEP=9 SUB =46 TIME=9 EPTOX (AVG) RSYS=1 PowerGraphics EFACET=1 AVRES=Mat DMX =.192E-03 SMN =-.421537 SMX =.279271 1 -.48 -.32 -.16 -.555E-16 .16 .32 .48 Au Total Radial Strain Cu Bare copper wire is about 40% harder than gold 85Hv compared to 60Hv To ac
12、hieve the same ball height, 2025% higher force is needed on the copper wire compared to gold The higher bond force causes stress on the pad and the underlying layer Pad peel and crack are the common failures 10 Dealing With First Bond Issues Critical 1st bond issues are Pad crack Aluminum splash Mat
13、erial supplier had developed softer copper wire that is up to 1015% softer in term of Hv hardness Equipment maker develop processes to minimizes pad crack and aluminium splash, features like Various mode of table scrub Segmented bonding And lately, dedicated bonder for copper wire (IConn ProCu from
14、K&S) The next step is to develop a complete solution that involves Optimized material - wire and capillary Optimized copper bonder Pad structure design Pad SplashPad Crack 11 Ball Shear Requirement Higher USG power generally gives a better ball shear but also create higher level of pad crack and Al
15、splash Copper wire requires slightly more than 2x the energy to increase 1 unit of bonded ball size Need to review spec to limit upper limit, balance between lifted ball and Al splash, ex. Not more than 8 gm/mil2 The danger of too high USG power causes potential reliability failure as the Al under t
16、he ball is too thin - potential stress on underlying layer Ball Diameter vs. Current Factor for Au and Cu Wire 40.00 45.00 50.00 55.00 60.00 65.00 0.750.80.850.90.9511.051.11.151.21.25 Copper) Au 1 um / 20% 1 um / 8% Al splash vs SUA 48 50 52 54 56 58 60 67891011 Shear per Unit Area (g.) 12 Improve
17、Al Pad Splash Extensive research was accomplish to study how the weld was formed We observed the traditional Au wire processes with impact deformation followed by USG scrub and deformation do not work well They commonly result in an uneven Cu-Al interface, causes Al pad splash, pad damage, small pro
18、cess window between ball lift and pad lift. New Cu processes were developed to optimize a Cu to Al bonding by balancing bond force and bond power and their interaction Traditional process Concave Interface Pad Damage/Splash Small process window Slower UPH Al Depletion No Al depletion Special Cu Proc
19、esses Flat Interface Reduced Pad Damage/Splash Larger process window Faster UPH 13 New Copper Process for 1stBond Traditional Process Concave Interface Pad Damage/Splash Require table scrub Small process window New Cu Process Achieves Flat Interface Reduced Pad Damage/Splash Larger process window De
20、dicated Copper Bonder 14 Au ( baseline process) Au Cu Cu (baseline process with LOOP FACTOR changed from -2.8 to - 4.0) Cu (with baseline process) Cu (baseline process with SF5 changed from 35 to 17) Cu vs Au Looping Optimization Basic guidelines for copper wire looping optimization is similar to go
21、ld wire However, gold and copper looping parameters are not directly portable. Significant differences in material properties requires looping parameter optimization. Cu has 40-60% higher stiffness than Au Compared to gold, the stiffer copper wire results in less defined bends and thus extra shaping
22、 is sometimes needed to make the desired loop shapes. 15 Low Loop Height Application Example 16 Copper Wire Looping Capability Advanced Looping Capability Stack die looping Compressed loop shapes Ultra-Low loop height applications High loop height applications Long wire applications SSB type applica
23、tions 17 Copper Wire SSB Bond Example of a 1-mil die-to-die bonding application 18 In copper wire bonding obtaining a good 2ndbond is critical and a challenge As copper is harder, it a challenge to get a good 2ndbond shape, a bog variation between the 4 sides The balance between bond force and USG p
24、ower is critical Various features and material were developed to improve the 2ndbond performance Table scrub and segmented bond are some of the features developed reduces 2nd bond variations 2ndBond Issues 19 2ndBond Issues Lowering CV to avoid peeling and NSOLs will often result in wire sway As TS
25、deformation progresses, the wire twists between the cap face and surface causing sway (ultrasonic deformation replaces impact deformation as the CV is lowered) Reducing ultrasonic levels to reduce sway requires the use of scrubbing to complete the needed deformation (which hurts the UPH) Low impact
26、with high USG deformation resulting in wire sway Low impact with spiral scrubbing using ProStitch 20 Bonding Segments Special Processes have been developed for Cu wire bonding Developed multi segment bonding process to equalize stitch bonds Bonding segments opens a newly formatted parameter entry ta
27、ble, allowing adjustment of up to 10 segments 21 New Process = Well deformed stitch shape: 1.5xWD stitch width, symmetrical weld shape. Normal US process gives poor x vs. y control Special Stitch Process - S1 XY Dist move toward 1stbond - S2 XY Dist move away from 1stbond Seg-2 Seg-1 (Scrub) (Skid)
28、22 22 Unique granular morphology Minimize wire slippage during bonding Improves gripping between the wire and the capillary 2nd Bond Improvements Granular Surface Tools Matte Polish 23 Pd Coated Wire 24 Pd Coated Copper Wire Pd coated copper wire provides an oxidation resistant coating on the copper
29、 surface - coating is typically about 0.2 m thick It also prevents the formation of the lubricious copper oxide surface Improved corrosion resistance on the wire and may ease stringent requirements on molding compounds Possible to use nitrogen instead of forming gas for FAB formation - performance u
30、sing forming gas is still better 25 Pd Coated Copper Wire FAB Pd-Cu alloy formation in the FAB causes greater hardness than for FABs from bare Cu wire Higher EFO currents cause greater mixing of the Pd into the FAB The greater mixing at higher EFO currents likely cause greater hardness Due to these
31、effects, Pd-Cu wire generally causes more pad damage and peeling than bare Cu wire Higher EFO currents appear to aggravate this effect 120 mA 60 mA 30 mA Pd Coated CopperBare Cu wire 0 20 40 60 80 100 120 Courtesy Heraeus Vickers Hardness of FABs 60 mA EFO, 0.8 mil diameter 26 Metal Peel at Differen
32、t EFO Current Bonding test with bare Cu and Pd- Cu wires at EFO currents = 30, 60, 90mA 1stbond pull test (at TOL) revealed rate of metal peel Cu wire has far less metal peel than Pd-Cu wire Higher EFO current gives more metal peel Higher EFO current led to more Pd mixing into core of FAB correspond
33、ing to higher Vickers hardness Cu Pd-Cu Effect of EFO current on metal peel with pure Cu and Pd-Cu wires Typical Al pad peel after pull test 27 Al Splash Comparison using Bare Cu and Pd-Cu Wires Al Splash vs. Shear/Area 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 4.05.06.07.08.09.0 Shear/Area (g/mil2) S
34、plash - Cu Splash - Pd-Cu Linear (Splash - Cu ) Linear (Splash - Pd-Cu ) Al splash as a function of shear per area using pure Cu and Pd-Cu wires Typical Al splash after etching off the bonded ball More severe Al splash observed with Pd-Cu wire at low shear/area At higher shear/Area, the difference b
35、ecame subtle Trade-off between high shear and Al splash control is more challenging with Pd-Cu wire Confidential 28 First Bond DOE using Pd-Cu Wire TOL pull suffered a lower pull strength in some runs with N2 Higher ball lift occurrence in some runs with N2 Ball lift % and TOL pull response comparin
36、g different cover gases Ball lift % normalized against shear per area comparing different cover gases Under normalized Shear/Area, N2 still showed higher ball lift rate at low Shear/Area. At high enough Shear/Area, the difference is subtle. Confidential 29 2ndBond Improvements Bare Cu wire has a low
37、er average stitch pull strength Pd-Cu wire has a significantly higher stitch pull strength and better Cpk Larger 2ndbond process window from Pd-Cu provide better MTBA Relatively simpler 2ndbond process can be used with Pd-Cu wire so improving UPH as well Pd layer present at the wire and lead interfa
38、ce giving better adhesion. Stitch Pull results on silver-plated lead frame Each data point corresponds to a cell in a DOE Stitch Pull with Bare Cu and Pd-Cu wires 0 0.5 1 1.5 2 2.5 3 3.5 4 1234567891011121314 DOE cell 0 2 4 6 8 10 12 14 Cpk - Cu Cpk - Pd-Cu Stitch Pull - Cu Stitch Pull - Pd-Cu 30 Pd
39、-Cu Wire - Conclusion Advantages of Pd-Cu wire More robust 2ndbond process in many applications Allows the use of the nitrogen instead of forming gas for free air ball formation Possibly more resistance to corrosion in reliability tests Disadvantages of Pd-Cu wire Cost About 3 times more expensive t
40、han bare Cu wire but still about 70% less expensive than Au wire Harder free air ball and could cause higher level of bond pad peeling and dielectric damage A smaller 1stbond process window for pad sensitive devices 31 Summary 32 Bond Pad Structure Key Differences Al* 5kA TiN 500A Al* 10.5kA TiN 630
41、 A Cu 9.3kA SiO2 FSG *Bond pad Al is 99.5%Al 0.5%Cu for all 3 technologies WorkedWorked With OptimizationNever Worked Al* 10.5kA TiN 630 A Cu 16kA SiO2 Cu development baseline Thin Cu and softer FSG may increase stack flexing during bonding Al* 10.5kA TiN 630 A Cu 3.2kA FSG 33 Copper Wire Roadmap 80
42、um60um40um45um1 mil0.9mil0.7mil0.8mil0.6milWire Size Mostly Power And Discrete Started IC In QFP, QFN BGA Used in All pkgs - limited by device sensitivity 70% (Cu wire length) are IC pkg 25% of All wire bonded ICs? Packages 2 um11.2um0.91.0um1um?Al thickness Non low-K CUP, some Low-K Low-K, thin Al
43、Low-K Ultra Low-K, Cu friendly Pad Pad Structure 34 Summary The semiconductor industry had come a long way in copper wire bonding development In fine wire development, pitch capability is getting close to gold wire performance, seeing 40um BPP in production before 2012/13 Pd coated copper wire is an
44、 alternative for the short term and eventually will convert to pure copper A complete solution involving material, pad design and equipment capability with accelerate the use of copper wire The penetration rate to continue to increase rapidly and could reach 50% of all semiconductor packaging within 5 years, and 70% a couple of years later - excluding LEDs 35