The Adhesion Between Electroless Nickel Deposit and Si/Al ... · PDF fileThe Adhesion Between...

© International Microelectronics And Packaging Society

The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number3, Third Quarter 2000 (ISSN 1063-1674)

Intl. Journal of Microcircuits and Electronic Packaging

236

The Adhesion Between Electroless NickelDeposit and Si/Al/Cu Bump Pad

Kwang-Lung Lin and Chih-Li ChenDepartment of Materials Science and EngineeringNational Cheng Kung UniversityTainan, Taiwan 701, R. O. C.Phone: 886-6-275-7575 x 62929Fax: 886-6-234-6290e-mail: [email protected]

Abstract

The adhesion strength of Si/Al/Cu/EN(electroless nickel) multilayer structure has been investigated by pull test as well as by sheartest. Electroless nickel deposit lowers the pull strength of Al/Cu deposit on Si from 4.8 kpsi to 3.0 kpsi. The electroless nickel wasdeposited by a continuous and an intermittent deposition process. The intermittent deposition process will enhance the shear strengthof the electroless nickel bump, a shear strength of 60 g/bump was achieved for a bump diameter of 60 µm. Thermal cycling testdegrades the pull strength of the deposit from 3 kpsi to 2 kpsi, while it lowers the shear strengh of an electroless nickel bump from 60g/bump to around 30 g/bump. Fracture mechanism was investigated by analyzing the fracture surface of the structure. It was foundthat the fracture mechanisms are different for these two adhesion test methods.

Key words:

Adhesion Strength, Shear Strength, Electroless Nickel, Stress,and Fracture Mechanism.

1. Introduction

The Flip Chip bonding technology, developed in 1960s byIBM1, is an area array chip bonding technology. It becomes in-teresting and important to electronic industry due to the trend ofhigh I/O and low profile product. Flip Chip bonding technologyneeds to apply solder bump or gold bump2 as the interconnec-tion. Solder bump is preferable for most IC application due to itslow melting temperature. The IC structure, depending on cir-cuitry, may have Al or Cu terminal pad on which the solder bumpis produced. In fact, it is not appropriate to directly contact sol-der on Al or Cu, as both metals have good solderability, and thussolubility in molten solder3-5. The terminal film may dissolve

fastly during reflow of solder bump. Accordingly, a solder bumpconsists of suitable UBM (under bump metallurgy) which in-cludes a diffusion barrier layer between the solder and the termi-nal pad. Mechanism of diffusion barrier layer has been discussedpreviously6 The barrier layer should exhibit small solubility insolder, low reaction rate with solder and terminal pad. Bothsolder and terminal pad materials must also have low diffusivityin the barrier layer.

A number of barrier layers have been reported. These includeTi-W7, Cu8, Cr-Cu9, Ta10, and electroless nickel11. Ti-W, Cr-Cu,and Ta have slow reaction rate with solder and terminal pad,while Cu reacts rapidly with solder. Accordingly, a thick layer ofCu, around 10 µm or more7, is needed for barrier layer applica-tion while the others may be thin films of thousands angstroms.In addition to slow reaction with contacting materials, a materialhaving no diffusion path is also ideal for barrier application.Amorphous material6 is in lack of grain boundary and thus ex-hibits no diffusion path. It is of interest to produce amorphousdeposit for diffusion barrier application. Wet chemical deposi-tion process, such as electroless nickel deposition, can be appliedfor producing amorphous deposit12. Most of the amorphous de-posits produced by wet chemical process contain metalloid12 thatdestroys the lattice arrangement and thus yields amorphous struc-ture.

The Adhesion Between Electroless Nickel Deposit and Si/Al/Cu Bump Pad


© International Microelectronics And Packaging Society 237

Electroless nickel deposit is an amorphous deposit when itsphosphorus contents is greater12 than 7%. It is thus of interest totry to apply this deposit as diffusion barrier for Flip Chip solderbump. Nevertheless, the electroless nickel deposit may containporosity and thus it would need a thick layer of around 6 µm orabove to gurantee the barrier performance13. It has been attemptedin a series of study applying intermittent deposition to eliminatethe porosity of the electroless nickel deposit. Intermittent depo-sition is conducted by periodically interrupting the depositionreaction so as to refine the nodule size, removing the adsorbedhydrogen gas bubble, and thus eliminating porosity.

The Si/Al/Cu/EN (electroless nickel) is not in practical appli-cation yet. The Al layer is to simulate the actual Al conductorpad of the IC chip. It is thus desired to have a thickness of less 1µm. However, the EN process undergoing development in in-dustry generally needs zincating14 of the Al layer prior to ENdeposition due to the oxide formation and the lack in d-orbital ofthe Al layer. Zincating is generally conducted in alkaline ZnO-NaOH solution14 that will strictly corrode the Al layer. Thus, theAl layer needs to have more than 2 µm in order to incorporateEN deposition. In addition, the zincating process is not a stable,easy to manipulate process as the reaction of alkaline with Al isvery fast. Accordingly, it was tried to investigate the possibilityof applying a thin deposit of Cu on Al as a pre-required layer forEN. The present work studies the properties of this produced Si/Al/Cu/EN structure.

2. Experimental Procedure

Si wafer was degreased with trichloroethylene and acetonefollowed by ultrasonic clean in isopropanol. Thus, cleaned Siwafer was then etch cleaned in a 10% (volume percent) HF solu-tion to remove oxide. The Al and Cu films were produced withmagnetron sputtering deposition. The distance between the tar-get and the substrate is around 10 cm. Both targets were pre-sputtered at a DC power of 150 Watt for 15 minutes to removesurface contaminants. Sputtering deposition for both films wasoperated at a working pressure of 3.5 x 10-3 torr and a DC powerof 150 Watt for 30 minutes of deposition time. The electrolessnickel was deposited on top Cu film from a nickel sulfate solu-tion using sodium hypophosphite as the reducing agent14. TheCu film was contacted with iron to initiate the electroless nickelreaction. The deposition was conducted at a bath temperature of90 oC and pH of 4.6. Intermittent deposition of electroless nickelwas carried out by momentarily interrupting the deposition reac-tion moving the specimen out from the deposition bath after cer-tain deposition interval, the on-interval. The time of on-intervalincludes 1, 3, 5, and 10 minutes. Thermal cycling test was con-ducted between -45oC and 150oC at a ramp speed of 4.75oC/min.The specimen was held at the peak and lowest temperature for20 minutes during each cycle of test.

Adhesion strength of the deposit was investigated by pull test

as well as by shear test. Pull test was conducted attaching a studon to the deposit surface using epoxy glue. The epoxy was curedat 150 oC for one hour. The contact area is 2.7 mm in diameter.The stud was pulled with increasing load until the deposit delami-nates. The value of force applied when the deposit delaminateswas recorded. This value is termed as the “pull strength”.

For shear test, the electroless nickel was produced as a bumpwith the aid of photolithography. The bump has a diameter of 60µm. The pitch, center to center distance between bumps, is 140µm. Shear test was conducted with Dage 4000 shear tester. Themoving speed of the shear blade was 100 µm/s. The shear heightwas set at one third of the bump height from the substrate. Sheartest ceases when the bump was moved away from the bump pad.The force applied at this moment when the bump moves wastermed as the shear strength. The fractured surface was ana-lyzed with SEM (Scan Electron Microscopy) for surface appear-ance, while the compositional image was analyzed with EDS(energy dispersive spectroscopy) of the SEM at fixed electronbeam energy of 20 KeV.

3. Results and Discussion

The Al/Cu/EN bumps have been successfully produced onsilicon wafer. Figures 1a and 1b present the SEM morphology ofproduced bumps at different magnifications. These electrolessnickel bumps were produced at a deposition interval of 3 min-utes for a total deposition time of 90 minutes. It is seen that theround bumps have somewhat uniform dimension in diameterand shape. Figure 2 presents the pull strength of the Al/Cu (with-out electroless nickel deposit) and Al/Cu/EN deposits on Si. Thefilm thickness is 0.5 µm for Al and 0.3 µm for Cu. The totaldeposition time for the EN is thirty minutes for all experiments.The Al/Cu deposit has a pull strength of around 4.8 kpsi. Figure3a shows a fractured surface of the Si/Al/Cu specimen. Resultsof elemental analysis, Figures 3b to 3d, indicate that the fractureoccurs at both Si/Al and Al/Cu interfaces.

Figure 1a.




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Figure 1b.

Figure 1. The electroless nickel bumps, (a) array, (b) athigher magnification.

Figure 2. Pull strength of Al/Cu and Al.Cu/EN (electrolessnickel) deposits.

Figure 3a.

Figure 3b.

Figure 3c.

Figure 3d.

Figure 3. (a) Fracture surface of Si/Al/Cu deposit, andelemental analysis of (b) Si, (c) Al, (d) Cu.




The results of Figure 2 further show the pull strength of Si/Al/Cu/EN. It is seen that the pull strength decreases to 3.0 kpsionce the EN is deposited. In addition, both the continuous EN (0minute interval) and intermittent EN (with various depositioninterval) deposition lowers the pull strength to 3.0 kpsi. Figure4a presents the fracture surface of the Si/Al/Cu/EN specimen.Result of nickel analysis, Figure 4b, shows that the central Nilayer of the pull area dissapears, indicating that the pull test re-sults in delaminating of electroless nickel deposit. The results ofAl (Figure 4c) and Si analysis (Figure 4d) further show that Al isabsent in the pull area. It delineates that most of the fractureoccurs at the Si/Al interface. These observations indicate that“as deposited” electroless nickel and Cu have good adhesionstrength. On the other hand, the electroless nickel depositionlowers the adhesion strength of the Al deposit. The latter isrealizable as EN deposition is recognized to introduce stress tothe specimen.

Figure 4a.

Figure 4b.

Figure 4c.

Figure 4d.

Figure 4. (a) Fracture surface of Si/Sl/Cu/Ni deposit, andelemental analysis of (b) Ni, (c) Al, (d) Si.

The thermal cycling test of this study renders the specimenunder great periodical thermal expansion and contraction. Themultiple layers did not peel off or show any blister after 200thermal cycles. Nevertheless, as shown by Figure 5, increasingthe number of thermal cycle keeps decreasing the adhesionstrength of the deposits. Figure 6a shows the fracture surface ofa specimen after 200 cycles of thermal cycle test. Nickel analy-sis, Figure 6b, shows that the pull test not only pulled off thedeposit on contact region of the stud, but also caused peeling ofthe deposit around the contact region. The Al analysis, Figure6c, shows that a large portion of Al left behind after pull test. Cuwas not detected in an elemental analysis of this fracture surface.In other words, the fracture after thermal cycle test occurs at theAl/Cu interface. In theory, thermal cycle test will greatly affectthe interface between two layers where a great CTE (coefficientof thermal expansion) mismatch exists. The CTEs (in 10-6/oC)of the various materials of the Si/Al/Cu/EN combination are 2.5,24, 16.4, and 14, respectively, for Si, Al, Cu, and EN. It is seen




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that the Si/Al interface exhibits the largest mismatch, followedby the Al/Cu interface. It is clear that the fracture behavior afterthermal cycling did not correspond to CTE mismatch. This be-havior is ascribed to the interaction between Al and Si. It is acommon practice to enhance the adhesion between Al and Si byannealing. The thermal cycling test up to 200 cycles is expectedto result in the formation of silicide between Al and Si and thusimprove the adhesion strength. On the other hand, no such in-teraction is expected between Al and Si. On the other hand, nosuch interaction is expected between Al and Cu. Therefore, ther-mal cycling will accumulate thermal stress at the Al/Cu inter-face. The adhesion between Al and Cu is degraded continuouslyduring thermal cycling and thus the fracture occurs at the Al/Cuinterface.

Figure 5. Effect of thermal cycling on pull strength ofelectroless nickel deposit.

Figure 6a.

Figure 6b.

Figure 6c.

Figure 6. (a) Fracture surface of Si/Al/Cu/Ni deposit afterthermal cycle test, and elemental analysis of (b) Ni, (c) Al.

Solder bump is generally characterized by shear test. Accord-ingly, the shear strength of the electroless nickel is also of impor-tance for practical application. Figure 6 presents the shear strength(gram per bump) of the intermittently deposited electroless nickelbumps. The electroless nickel was deposited at various intervalsand different total deposition time. It is of interest to find that theshear strength shows the highest value at around 60 minutes ofdeposition time when the time interval is around 3 to 5 minutes.Further investigation is needed to better understand the variationbehavior of the shear strength with respect to deposition interval.A decrease in shear strength when deposition for longer than 60minutes may be resulted from the stress induced by thick deposit.On the basis of the results of Figure 7, it is seen that the approachof this work has the benefit of enhancing the shear strength ofthe electroless nickel bumps.




Figure 7. Shear strength of electroless nickel deposit withrespect to deposition interval and total deposition time.

Fracture surface of the shear test specimen is presented inFigure 8. The electroless nickel of this specimen was depositedat an interval of 5 minutes for a total deposition time of 60 min-utes. Figure 8a shows the photograph of the fractured surface.The Al and Cu films are continuous deposits while the electro-less nickel bump was produced on the continuous deposit. Theelectroless nickel was completely removed after shear test as in-dicated by Ni analysis, Figure 8b. The result of Al analysis,Figure 8c, and Cu analysis, Figure 8d, evidenced that the frac-ture occurs at the Al/Cu interface. This result further indicatesthat the Cu/EN has good adhesion strength.

Figure 8a.

Figure 8b.

Figure 8c.

Figure 8d.

Figure 8. (a) Shear test fracture surface of electroless nickeldeposited at 5 minute interval for total deposition time of 60minutes, and elemental analysis of (b) Ni, (c) Al, (d) Cu.




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The shear strength of the Si/Al/Cu/EN structure also degradesafter thermal cycle test. It is interesting to compare the behaviorof pull strength and shear strength curves. The shear strengthcurve, Figure 8, shows that the shear strength is around 30 gafter thermal cycling. The shear strength remains the same from50 to 200 cycles, Figure 9. In other words, the shear strengthreaches its lowest value very soon. On the other hand, the pullstrength, Figure 5, tends to decrease continuously with respect tothermal cycle test. Figure 10 shows the photograph of the frac-ture surface of a shear test specimen. It is seen that the fractureoccurs at the electroless nickel, instead of at the Al/Cu interfaceas seen in pull test. It is likely that the electroless nickel becomesweakened during thermal cycling although it is not realized atthis moment why EN layer was weakened.

Figure 9. Efffect of thermal cycle test on shear strength ofelectroless nickel deposit.

Figure 10. Fracture surface of Si/Al/Cu/Ni after shear test,showing fracture at the electroless nickel deposit.

4. Conclusion and Future Directions

Electroless nickel deposit lowers the pull strength of Si/Al/Cumultilayer structure from around 5 kpsi to around 3 kpsi. Ther-mal cycling further lowers the pull strength. Intermittent depo-sition results in the highest value of shear strength of the electro-less nickel bump on the Si/Al/Cu multilayer. The fracture mostlyoccurs at Al/Cu interface regardless of adhesion test method.However, shear fracture occurs at the electroless nickel depositafter thermal cycle test. Shear strength of Si/Al/Cu/electrolessnickel bump lowers from 60 g/bump to around 30 g/bump afterthermal cycle test. It is seen that the intermittent deposition hasthe possibility of producing the bump with high shear strength,60 g/bump. It is certainly necessary to further establish practicaloperation condition inn the future work to verify its suitabilityfor industrial practice. Furthermore, the application of Cu de-posit in this work is to simulate the Cu circuit of recently devel-oped Cu chip. It is still needed to actual work on more realisticCu chip structure to investigate the applicability of the intermit-tent electroless nickel deposition process.

Acknowledgment

The financial support of this work from the National ScienceCouncil of Republic of China under NSC87-2216-E-006-027 andNSC88-2216-E-006-025 is gratefully acknowledged.

References

1. A. C. Dixon and E. G. Myszka, “MCM Packaging Selection:A Materials and Manufacturing Perspective,” Multichip Mod-ule Technologies and Alternatives: The Basics, D. A. Doaneand P. D. Franzon, Editors, Van Nostrand Reinhold, New York,Chapter 2, 1993.

2. C. G. Woychik and R. C. Senger, “Joining Materials and Pro-cesses in Electronic Packaging,” in Principles of ElectronicPackaging, D. P. Seraphim, R. C. Lasky, and C. Y. Li, Edi-tors, McGraw-Hill, New York, Chapter 19, 1989.

3. W. R. Wrenner and J. D. Larnerd, “Mechanical Processes,”in Principles of Electronic Packaging, D. P. Seraphim, R. C.Lasky, and C. Y. Li, Editors, McGraw-Hill, New York, Chapter20, 1989.

4. H. H. Manko, “Solders and Soldering”, 2nd Edition, McGraw-Hill, Chapter 6, 1979.

5. M. A. Nicolet, “Diffusion Barriers in Thin Films,” Thin SolidFilms, Vol. 52, pp. 415-453, 1978.

6. M. Warrior, “Reliability Improvements in Solder Bump Pro-




cessing for Flip Chips,” Proceedings of Electronic Compo-nents Conference, ECC ‘90, Las Vegas, Nevada, Vol. 1, pp.460-469, May 20-23, 1990.

7. K. K. Yu and F. Tung, “Solder Bump Fabrication by Electro-plating for Flip Chip Applications,” Proceedings of 1993IEEE/CHMT International Electronic Manufacturing Tech-nology Symposium, pp. 277-288, 1993.

8. G. H. Adema, C. J. Berry, N. G. Koopman, G. A. Rinne, E.K. Yung, and E. Turlike, “Flip Chip Technology: a Methodfor providing Microcircuits and Electronic Packaging,” In-ternational Journal of Microcircuits and Electronic Pack-aging, IJMEP, Vol. 17, No.4, pp. 352-360, Fourth Quarter,1994.

9. K. L. Lin and Y. T Liu, “Manufacturing of Solder Bumpswith Cu/Ta/Cu as Under Bump Metallurgy,” IEEE Transac-tions on Advanced Packaging, Vol. 22, No. 4, pp. 580-585,1999.

10. M. Inaba, K.Yamakawa, and N.Iwase, “Solder Bump For-mation Using Electroless Plating and Ultrasonic Soldering,”IEEE Transactions on Components, Hybrids, and Manufac-turing Technology, CHMT, Vol. 13, No. 1, pp. 119-123, 1990.

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About the authors

Professor Kwang-Lung Lin receivedhis B.S. and M.S. Degrees in Chemis-try from Tunghai University (Taiwan)and The Catholic University of America,respectively. He accomplished his Ph.D.study in Metallurgy from PennsylvaniaState University in 1984. After doingone year of research at Ames Labora-tory of Iowa State University of Scienceand Technology, Professor Lin joined the

Department of Materials Science and Engineering at NationalCheng Kung University as a faculty. He has served as the De-

partment Chairman and is currently the Director of the Instru-ment Center of the University. His research work includes thematerials study and process development of surface modificationtechnology and electronic packaging. Most of his recent researchwork is focussed on Flip Chip bumping materials and process,lead-free solder materials, and wetting property of solders. Pro-fessor Lin is a member of IEEE-CPMT.

Mr. Chih-Li Chen received his M.S. Degree from the Depart-ment of Materials Science and Engineering, National ChengKung University in 1999. He is now serving in military.

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