Chemistry

Physical & Theoretical Chemistry fields

Okayama University Year 1997

Current evolution of electrodeposited

copper bumps with photoresist angle

Kazuo Kondo Keisuke FukuiOkayama University Himeji Institute of Technology

This paper is posted at eScholarship@OUDIR : Okayama University Digital InformationRepository.

http://escholarship.lib.okayama-u.ac.jp/physical and theoretical chemistry/9

840 J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.

n, moles of species iX cationic fraction of species i

REFERENCES1. G. H. Kesall and R. A. Williams, This Journal, 138,

931 (1991).2. W. B. Crow, J. B. Myers, and J. V. Jeffreys, Corrosion,

28, 77 (1972).3. A. T. Kuhn, B. B. Lartey, D. W. Wakeman, and N. El-

Nasr Br. Corros. J., 13, 123 (1978).4. Y. Omurtag and M. Doruk, Corros. Sci., 10, 225 (1970).

5. U. Wolff, Private communication.6. 5. Haupt, C. Calinski, U. Collisi, H. W. Hoppe, H. D.

Speckmann, and H-H. Strehblow, Surf. InterfaceAnal., 9, 357 (1986).

7. J. M. Sanz and S. Hofmann, Surf. Interface Anal., 5,210 (1983).

8. D. A. Shirley, Phys. Rev. B, 5, 4709 (1972).9. D. Schaepers and H-H. Strehblow, This Journal, 142,

2210 (1995).10. J. H. Scofield, J. Electron Spectrosc., 8, 129 (1976).11. G. C. Nelson, J. Vac. Sci. Technol. A, 4, 1567 (1986).

Current Evolution of Electrodeposited Copper Bumpswith Photoresist Angle

Kazuo Konclo*

Faculty of Engineering, University of Okayama, Okayama 700, Japan

Keisuke Fukui

Himeji Institute of Technology, Himeji 671-22, Japan

ABSTRACT

We report the current distribution of copper bumps with photoresist sidewall angles. The role of outer diffusion, vor-tices, and penetration flow within the cavity is discussed, with numerical fluid dynamics computed in order to preventside bumping. The current distributions were calculated at the diffusion controlled overpotential. The mass transfer-lim-ited current distribution showed that a zero or negative angle reduced diffusion from the outer surroundings andenhanced vortex formation at the cathode corners. Reduction in the diffusion and enhancement in vortices reduced cur-rent at the cathode corners and prevented side bumping.

( au ax ai' (a2u d"uplu—+Vl= +ax ay) ax x

InfroductionElectrodeposited bumps are the indispensable micro-

connectors for high-density interconnection in the latestmicroelectronics applications.' For the higher frequencycircuit, shorter interconnection length by bumps is re-quired in order to reduce the reflection noise. Ball gridarray (BGA) of solder bumps is necessary for high pin-count chips in order to reduce their packaging size. Bumpscan also be used as a new testing method for bare chips ofhigh pin-counts.2 Another application is the interconnec-tion between liquid crystal display (LCD) and driverchips, which are mostly interconnected with tape auto-mated bonding (TAB). The bumps act as microconnectorsof TAB technology and conduct digital signals from thechip to the LCD pixels.3

The bumps are electrodeposited onto the dot shapedcathode of 10 to 200 p.m in diam. The cathode, or photo-lithography patterns, are patterned by photomask andphotoresist. The control of electrodeposited bump shape,as well as the uniformity in their height, is important forobtaining proper interconnection reliability.'

The role of mass transport in etching of rectangular cav-ities formed by photoresist was extensively discussed byseveral authors. Alkire and Deligianni developed two-di-mensional numerical fluid dynamics computations to studythe electrolytic etching of copper.45 The flow patterns wereclassified into two modes for the rectangular cavity ofaspect ratios of 1.0 and 0.25, and their etch rate distributionnumbers were compared. Alkire and Georgiadou reportedthe experimental results on chemical etching of copper."They also developed numerical fluid dynamics computationto study the presence of sparingly soluble surface film andcompared the average etch rate with experimental data forcavity aspect ratios of 1.0 and 0.20.' Shenoy and Dattastudied the effect of mask wall angle on through-masketching,' solving the primary current distribution numeri-

* Electrochemical Society Active Member.

cally with boundary element method and discussing theetch factor with mask wall angle.

On the other hand, bumping is electrodeposition ontodots and rectangular cavities formed by photoresist. Du-kovic developed a two-dimensional numerical computa-tion of the tertiary current distribution. This distribution,however, does not include the convection. The movingboundary problem was discussed in regard to the shapeevolution of copper electrodeposit and the effect of resistwall angle and leveling agent was reported." The ex-perimental studies on shape evolution of gold bumps werediscussed by Kondo et al.1' They studied the effects of dotdiameter, electrolyte flow, and additive on bump shapes.The numerical fluid dynamics computation study on theinitial stage of gold and copper bumps at Peclet numbersless than 100 were reported.'2'13 The numerical fluiddynamics computation study of copper bumps at higherPeclet numbers more than hundred was also reported.'4

The present investigation discusses the shape evolutionof copper bumps with photoresist angle. The role of outerdiffusion, penetration flow, and vortex flows within thecavities is discussed in order to prevent side bumping,which is necessary for the high-density interconnection,Current distributions at diffusion controlled overpoten-tials were calculated by the numerical fluid dynamicscomputations.

Numerical AnalysisTwo-dimensional cross sections of dot patterns are num-

erically analyzed by solving the equation of continuity,Navier-Stokes equations, and mass transfer equation

3u dv—+—=0 [11dx dy

[21

j. iiecrrocnem. soc., VOl. 1 4b, NO. i, arcn 1 () I lie Electrocliemical Society, Inc. 41

( av av aP (a2v a2vp1U+V1 _+I__T+_TL ax ay) ax Lax ay

ac ac (a2c a2cu— + v— = Di +ax ay [x2 ay2

The computational area and the boundary conditions withresist angle 0 are illustrated in Fig. 1. The details withresist angle of 900 are given in the Ref. 13 and 14. Thenumerical method proposed by Patankar1 is adopted fordiscritization and pressure equation calculation. Smallergrid line spaces at the vicinity of both cathode and pho-toresist surface and also double precision variables calcu-lation are adopted in order to reduce computational error.The Peclet number is defined as

Pe = huzh/D [5]

The bump factor, a factor to represent the side bumping,is defined as

BF = (ieit + right)/(2center)

Results and DiscussionStream functions and isoconcentration contours with Pe

numbers .—Figure 2 illustrates the normalized streamfunctions for the Pe numbers of 1.31, 41.6, and 7311 in (a),(b), and (c), respectively. Cathodes 100 .m in width arelocated at the cavity bottoms and 10 tim height photore-sists are located on both sides of the cathode. The streamfunctions indicate the trajectory of fluid flow. The elec-trolyte flows from left to right as indicated by the arrows.

Vortices occur both at up and downstream corners.1214The stream functions indicated at 6 are the penetrationflows that start from the upstream side bulk solution to

C=C bulk and u=u,, at 50 p.m

Fig. 1. Schematic illustration of two-dimensional cross section ofphotoresist and cathode. Resist angle of 0, x and y coordinates,and boundary conditions are shown.

a

j'j11111111uui

[6]

the cathode centers at cavity bottoms and to the down-[3] stream side bulk solution. The vortices at upstream side

corners increase and grow toward the downstream sidewith the increase in the Pe numbers. The vortices at down-

[4] stream side corners, on the other hand, decrease with theincrease in the Pe numbers.

Figure 3 shows the isoconcentration contours of the Penumbers of 1.31, 41.6, and 7311 in Fig. 3a, b, and c, respec-tively. The isoconcentration contours illustrate how theelectrolyte is consumed on the cathodes at the cavity bot-toms and how the concentration boundary layers develop.The isoconcentration contours converge and come close tothe cathode with the increase in the Pe numbers.

Isoconcentration contours and stream functions withresist angle.—Figure 4 shows isoconcentration contoursfor the resist angle of —10 and 30° (Fig. 4a and b), respec-tively. The Pe numbers are 1.31 for both Fig. 4a and b. Themass-transport process is controlled mainly by diffusionfor this small Fe number. Mass transport at the cornersadds the diffusion from the outer surroundings, if com-pared at the center. This additional diffusion is shaded bythe resist sidewall with negative resist angles of 0 (Fig. 4a).On the other hand, this diffusion is enhanced with positiveresist angles (Fig. 4b). Shading and enhancement of theouter diffusion is illustrated with closeness of adjacentcontours. The adjacent contours are much closer at thecathode corner for the positive resist angle of Fig 4b thanfor that of Fig. 4a (arrows in Fig. 4). This closeness indi-cates larger current on the cathode corners.

Figure 5 shows the normalized stream functions for re-sist angles of —10 and 30° for Fig. 5a, b and c, d, respec-tively. The Fe numbers are 41.6 and 1407 for Fig. 5ä,c andb, d, respectively. Vortices occur both at up and down-stream side corners. The vortices increase for the cases of

Fig. 3. Isoconcentration contours for 100 p.m cavity width; (a)Pe = 1.31, (b) Pe = 41.6, and (c) Pe = 7311.

Fig. 2. Normalized stream functions for 100 p.m cavity width; (a) Fig. 4. Isoconcentration contours with different resist angles forPe = 1.31, (b) Pe = 41.6, and (c) Pe = 7311. Pc = 1.31; (a) 0 = —10° and (b) 0 = 30°.

— C.thodeJ, I I If

L- 100p.

XL+5011 m

842 J Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.

Fig. 5. Normalized stream functions with different resist anglesfor Pe = 41.6 and 1407; (a, b) 0 = —I0 and (c, d) 0 = 30°.

the resist angle of —10°, but on the other hand, decreasesfor 30°. These vortices circulate within themselves and in-troduce local mass-transfer resistance of electrolyte to thecathode. The stream function 6 is the penetration flow.

The normalized stream functions (Fig. 2) and isoconcen-tration contours (Fig. 3) are obtained by changing Pe num-bers. The isoconcentration contours are obtained for smallFe number of 1.31 by changing resist angles (Fig. 4).Furthermore, the normalized stream functions are ob-tained for Pe = 41.6 and 1407 by changing resist angles(Fig. 5). Based on these results, the current distributions atFe = 1.31, 41.6, and 1407.2 are discussed in the followingsections.

Current distributions at Pe = 1.31.—Figure 6 is the cur-rent distributions for the Fe number of 1.31. The mass-transport process is mainly controlled by diffusion for thissmall Fe number. For the resist angle of —40° of Fig. 6 (a),a decrease in current distribution is observed at upstreamcorner of x = 0 sm and gradually decrease toward down-

Fig. 6. Current distributions for 10 #sm cavity width of Pc = 1.31with different resist angles: (a) 0 = —10°, (b) 0 = 0°, and (c) 0 = 30°.

stream side. A small decrease is also observed at down-stream corner of x = 100 p.m. The diffusion from the outersurroundings is shaded by the resist sidewalls and causesthis decrease. The penetration flow causes a gradualdecrease toward downstream side.12-14

For the resist angle of 0° of Fig. 6 (b), the resist sidewallsdo not shade the outer surroundings diffusions. The cur-rent distributions at both corners are flattened. For theresist angle of 30° of Fig. 6 (c), the surroundings diffusionsare enhanced by the positive resist angle and sharp in-creases in current distribution exist at both corners.

Current distributions at Pe = 41.6.—Figure 7 is the cur-rent distributions for the Fe number of 41.6. The process iscontrolled both by diffusion and convection. For the resistangle of —10° of Fig. 7 (a), a large decrease at the upstreamcorner and a gradual decrease in current distributiontoward the downstream side is observed. A small decreasein current distribution also exists at the downstream cor-ner. The vortices occur at up and downstream corners(Fig. ba b). Not only these vortices, but also the shading ofsurrounding diffusion (Fig. 4a), causes these decreases.

For the resist angle of 0° of Fig. 7 (b), the resist side-walls do not shade the surroundings diffusions. The cur-rent distribution decreases because of the vortices at theup and downstream corners. For the resist angle of 30° ofFig. 7 (c), the vortices reduce (Fig. 5c, d). The positiveresist angle enhances diffusion from the surroundings(Fig. 4b) and current density shows sharp increases at bothcorners.

Current distributions at Pe = 1407.2.—Figure 8 is thecurrent distributions for the Fe number of 1407.2. Theprocess is mainly controlled by convection. For the resistangle of —10° of Fig. 8 (a), a large decrease at the upstreamcorner and the gradual decrease in current distributiontoward the downstream side is observed. This largedecrease is caused by vortices. The current distributionshows a small rise at downstream cornet This small rise isdue to the vortex reduction at downstream corner (Fig. 2c).This vortex reduction is due to the collision between thepenetration flows and the photoresist side walls at thedownstream cornet14

x, .tm100

Fig. 7. Current distributions of Pe = 41.6 with different resistangles: (a) 0 = —10°, (b) 0 = 0°, and (c) 0 = 30°.

01=)

I-'I=

0 50 100 0 50

x, m

J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc. 843

Fig. 8.Current distributions of Pe = 1407.2 with different resistangles: (a 0 = 10°, (b) 0 = 0°, and (c) 0 = 30°.

For the resist angle of 0° of Fig. 8 (b), the same currentdensity behavior is observed as Fig. 8 (a). The vortices,however, decrease for the resist angle of 30° of Fig. 8 (c)(Fig. 5c, d). At the upstream corner, the current distribu-tion shows sharp increase because of the diffusion fromthe surroundings (Fig. 4b). The current shows the mini-mum and a small rise toward downstream side because ofthe vortex formation. At the downstream corner, the vor-tex again causes a minimum and the surroundings diffu-sion causes a small rise.

Bump factor.—The amount of side bumping can be rep-resented by the bump factors. Figure 9 summarizes a rela-tion of bump factor (BF) and resist wall angle. The BFdecreases as the decrease in resist wall angles. For smallPe number of 1.31, whose mass transport is controlled bythe diffusion, the BF decreases monotonically as thedecrease in resist wall angle.

For larger Pe numbers of 41.6 and 1407, the BF decreas-es because of the formation of vortices. The zero or nega-tive resist angle enhances the vortices formation (Fig. 5a,b). These vortices are the local mass-transfer resistance tothe cathode. The BF especially show small values for thesezero or negative resist angles.

3.0

2.01.0

Pe=1.31Pe 41 6—

ci

-

147I i

—

'- I

0 500, deg

Fig. 9. Relation between BF and resist angle for Pe = 1.31,41.6,and 1407.

SummaryThe relation between resist wall angles and current dis-

tributions was discussed based on the numerical fluiddynamics computations. The role of outer diffusion, pene-tration flow, and vortex flows within the cavities was dis-cussed in order to prevent side bumping.

1. Additional diffusion from the outer surroundingsexists at the corners for the Pe number of 1.31. This outerdiffusion is shaded by the negative resist sidewall angle of—10' and is enhanced with a positive angle of 30°. Vorticesoccur both at up and downstream side corners for the Penumbers of 41.6 and 1407. The vortices increase for theangle of —10° and decrease for 30°. These vortices are thelocal mass-transfer resistance to the cathode.

We conclude that iii order to prevent side bumping, theresist wall angles should be zero or negative. These zero ornegative angles shade the outer diffusion and increase the vor-tices. These angles reduce the current at the cathode corner.

2. For a Pe number of 1.31, the process is mainly con-trolled by diffusion. A decrease in current distribution isobserved at upstream corner for the negative angle of—10°. A decrease is also observed at downstream corner.The diffusion from the outer surroundings is shaded bythis negative angle sidewall. The diffusion from the outersurroundings is shaded by this negative angle sidewall.For the positive angle of 30°, the surroundings diffusionsare enhanced by this angle and sharp increases in currentdistribution exist at both corners.

3. For a Pe number of 41.6, the process is controlled byboth diffusion and convection. A large decrease in currentdistribution at the upstream corner is observed, for theangle of —10°. A decrease in current distribution alsoexists at the downstream corner. Not only the vortices atup and downstream corners, but also the shading of thesurrounding diffusion, causes these decreases. For thesidewall with an angle of 30°, the vortices reduce. Thisangle enhances diffusion from the surroundings and thecurrent density shows a sharp increase at both corners.

4. For the Pe number of 1407.2, the process is mainlycontrolled by convection. A large decrease in current dis-tribution at the upstream corner, caused by vortices, is ob-served for the angle of —10°. The current distributionshows a small rise at downstream corner. The vortices, how-ever, decrease for the angle of 30°. At the upstream corner,the current distribution shows a sharp increase because ofthe diffusion from the surroundings. The current shows themmimum because of the vortex formation. At the down-stream corner, the vortex causes a minimum and the sur-roundings diffusion causes a rise in the current distribution.

AcknowledgmentsThe authors thank Professor R. C. Alkire and his re-

search group for the instructive discussions. Financialsupport from the Okayama Foundation for Science andTechnology is gratefully acknowledged.

Manuscript submitted July 14, 1997; revised manuscriptreceived November 12, 1997.

University of Okagama assisted in meeting the publica-tion costs of this article.

LIST OF SYMBOLSc concentration, mol m3D diffusion coefficient, m2/sh resist height, mi current density, mA/cm2L cathode length, mPe =

hu5_2h/D(—)u velocity in the x-direction, mv velocity in the y-direction, m sx coordinate in the streamwise direction, my coordinate normal to the cathode surface, m8 resist angle (°)

REFERENCES1. R. R. Tummala and E. J. Rymaszewski, Microelectron-

ics Packaging Handbook, p. 361, Van Nostrand Rein-hold, New York (1989).

I-I.

0 50 100

x, tm

844 J. Electrochem. Soc., Vol. 145, No. 3, March 1998 The Electrochemical Society, Inc.

2. Y. Yamamoto, M. Sugimoto, and K. Miyake, in Pro-ceedings of 1SHM '93, p. 370, Dallas, TX (1993).

3. K. Hatada, Introduction to TAB Technology, Kougiy-ou-chiyousakai, Tokyo (1989).

4. R. C. Alkire and H. Deligianni, This Journal, 135, 1093(1988).

5. 11. C. Alkire, and H. Deligianni, and J-B. Jo, ibid., 137,818 (1990).

6. M. Georgiadou and B. C. Alkire, ibid., 140, 1340 (1993).7. M. Georgiadou and B. C. Alkire, ibid., 140, 1348 (1993).8. M. Georgiadou and B. C. Alkire, ibid., 141, 679 (1994).

9. B. V. Shenoy and M. Datta, ibid., 143, 544 (1996).10. J. 0. Dukovic, IBM.!. Res. Develop., 37, 125 (1993).11. K. Kondo, T. Miyazaki, and Y. Tamura, This Journal,

141, 1644 (1994).12. K. Kondo and K. Fukui, Kagaku Kogaku Ronbunshu,

22, (1996).13. K. Kondo, K. Fukui, K. Uno, and K. Shinohara, This

Journal, 143, 466 (1997).14. K. Kondo, K. Fukui, M. Yokoyama, and K. Shonohara,

ibid., 144, 466 (1997).15. 5. V Patankar, Numerical Heat Transfer and fluid Flow,

Hemisphere Publishing Corp., New York (1980).

Self-Discharge of Sealed Nickel—Metal Hydride Batteries

Mechanisms and Improvements

Pah'ick Leblanc,° Philippe Blanchard,° and Stéphane Senyarichb

aSAF? Research Department, 91460 Marcoussis, France5SAFT Portable Batteries Technical Division, 16440 Roullet St-Estéphe, France

ABSTRACT

Mechanisms likely to accelerate self-discharge of a nickel—metal hydride (Ni—ME) battery have been investigated. Ahigher self-discharge rate of sealed Ni—ME batteries compared to Ni—Cd is explained by shuttle reactions between nitriteions and ammonia. Even if a polyolefin separator stable in the electrolyte is used, the positive electrode is an importantsource of nitrogen impurities. It was found that some grafted polyolefin separators have a special ability to trap ammo-nia. Using this specially designed separator and provided that the entire quantity of ammonia can be trapped, the self-discharge rate of Ni—ME batteries can be reduced to the self-decomposition of the charged active material in the positiveelectrode.

IntroductionNickel—metal hydride (Ni—ME) batteries using hydrogen

storage alloys yield a higher energy density than Ni—Cdbatteries and favorably replace Ni—Cd cells in portableappliances. However, higher self-discharge rates are oftenreported in Ni—ME batteries. Several papers have beenpublished on the subject, but conclusions are contradicto-ry. It has been reported that the self-discharge of sealedNi—ME batteries is accelerated by ammonia and amineions which participate in redox shuttle reactions.'2 Basedon this mechanism, the use of stable polyolefin separatorswhich do not produce ammonia during degradation havebeen recommended.3'4 The use of battery components de-pleted in nitrogen impurities,5 as well as cell-design mod-ifications,6 have also been proposed in order to improveself-discharge of ME batteries. On the other hand, otherworkers have concluded that the chemical reactionbetween H7 and the charged positive active mass is themain reason for the high self-discharge rate.7-9

In this work, mechanisms likely to accelerate self-dis-charge are clearly identified: (i) self-decomposition (ther-modynamic instability) of the positive electrode leading toa release of oxygen which ultimately reacts with the neg-ative electrode, (ii) reduction of the positive electrode byhydrogen gas coming from the ME electrode, and (iii)redox shuttles.

The contribution of each of these mechanisms to self-discharge bas been evaluated and the most dominant hasbeen identified. In light of these results, practical solutionsfor reducing the self-discharge rate to a level comparableto Ni—Cd cells are proposed.

ExperimentalNi—ME sealed cylindrical cells with a typical capacity of

1.2 Ah (AA size) were used for battery self-discharge meas-urements. These batteries were obtained by spirally wind-ing a nickel hydroxide positive electrode and a hydrogen-storage alloy negative electrode along with a polyamide ora polyolefin separator. The electrolyte used was a ternary

8.7 M KOE, 0.5 M NaOE, 0.5 M LiOE aqueous solutionwith 9.9 N total normality.

The composition of the hydride-forming alloy used in thenegative electrode was MmNi355Mn04AI53Co07, (Mm: mischmetal). Spherical nickel hydroxide containing 0.75% copre-cipitated Co and 3% coprecipitated Zn was used as a stan-dard positive active material.

For self-decomposition measurements of the positiveelectrodes, positive foam electrodes containing sphericalnickel hydroxide powders were initially fully charged atthe 0.1 C rate f or 16 h in half-cells using Cd counter elec-trodes and a polyolefin separator. The cell was disassem-bled and the positive electrode was partially immersed inthe electrolyte for 7 days at 40 °C in a container continu-ously purged by air or hydrogen or argon. In order to avoidelectrolyte carbonation or evaporation, the gas wasallowed to bubble in a separate flask containing the sameelectrolyte at 40 °C before introduction to the container.

Nitrate and nitrite ions concentrations were determinedby ionic chromatography using a Dionex 100 apparatuswith a AS4A-SC column. The ammonia content was meas-ured by Kjeldhal's technique using a Buchi distillation unit.

Self-discharge rates of positive plates or AA cells wereevaluated after a week's storage at 40 °C. The remainingdischarge capacity after storage was measured at the0.2—C rate and compared to the discharge capacity beforestorage.

Results and Discussion

Self-decomposition of the positive electrode—It isknown that the charged nickel hydroxide electrode is notthermodynamically stable in the electrolyte. A decrease inthe state of oxidation of the positive material is observed,and 02 gas evolution takes place according to eq 1

NiOOH + 'E20 -' Ni(OE)2 + '02 [1-I

Conway and Bourgault showed that the oxygen evolutionreaction was the rate-determining step in the self-dis-charge of nickel oxyhydroxide.1°