Lead-free Solder Bumping by the Electroplating Process for Electronic Packaging1

Slianv JoseDh", G. J. Phatak*", Tanay Setha, K. Gurunathd', D. P. Anialnerkaf' and T. R. N. Kuttyb I' Centre for Materials for Electronics Technology (C-MET), Panchawati, Off Pashan Road, Pune- 4 I 1 008, India

*Corresponding author. Phone: +91 020 5899273, e-mail: gjp@cnietp.ernet.in "Materials Research Centre, Indian Institute of Science, Bangalore - 560 012, India.

Abstract-Replacement of tin-lead solder bumps by a suitable lead-free material for flip chip attachment has become an urgent necessity due to the impending ban on the use of lead i n electronics. Alloy compositions containing Sn and Ag, with some additions of Cu and Bi are the most promising candidates for the replacement of Sn-Pb. These materials have shown better mechanical strength and wetting, while its fatigue resistance surpasses even that of Sn-Pb. Amongst various competing deposition techniques, electroplating is the most attractive technique due to low cost and capability of preparing small dimension bumps. However, the co-deposition of elements is seriously complicated by the large difference in the electrode potentials of each element. Though these problems are not intractable, suitable chemicals need to be selected as additives by way of chelating agents, surfactants, buffers etc. so as to deposit multiple elements simultaneously. In this work, we report preliminary results of a co-deposition bath for Sn-Bi-Cu. This bath was used to obtain SdAg-Bi- CLI alloy after reflow, with a targeted composition of Sn- 3.1 Ag-3.1 Bi-O.5Cu. For this, Ag was deposited separately. The plating bath chosen for tlie eo-deposition of Sn-Bi-Cu contains sulphate salts of these metals dissolved in sulphuric acid i n the targeted proportion, along with chelating agents and buffers for a pH of -4.5. Titanium was chosen as substrate i n view of its importance as barrier layer in Under BLIIII~ Metallurgy (UBM). The bath was found to be stable for more than a week and produced compact films having composition close to the target. The bath was characterized for the dependence of film composition on the storage time up to 72 hours and current density between 5 and 25 niA/cm2. I n both experiments, it was found that the film composition is more or less invariant. To obtain solder bumps, Cu and Ni were deposited on patterned Ti substrates, followed by Ag and Sn-Bi-Cu. The deposited films show slight deviation from the targeted composition, which could be due to higher duration deposition done to obtain large thickness. The reflown bunips show variation in bump size and composition close to that targeted. Further study and optimization seems necessary in order to obtain improved bumps.

I. INTRODUCTION

The rapid advancement in the complexity of silicon chips

has naturally brought flip chip process into the forefront of today's packaging technology. As compared to other chip interconnection processes, flip chip has many advantages like highest performance in terms of speed, reduced inductance, superior power and ground distribution, improved signal propagation and noise reduction (due to relaxed pitches), higher silicon efficiency with higher I/O count per die through increased use of the die surface [ 1,2]. This technology utilizes solder bumps formed over the wettable pads on the chip, which is then flipped over and placed on the matching pads. The connection is finned-up by reflowing them together. Suitable Under Bump Metalhrgy (UBM) layers are required for proper adhesion of the bunips to the chip. UBM usually consists of two or more metal layers deposited on the conducting pads of the chip. These layers also act as diffusion barrier for the pad layer (mostly AI), preventing it from dissolving into molten solder during reflow. The UBM also provides wettable surface for solder. Ti, W, Cr, Cu & Ni are some of the most commonly reported metals to be used for UBM [3].

Amongst the various acposition tcclinologies in circulation, electroplating is seen as the most commercially viable process for deposition of solder. Due to its commercially viable process for deposition of compatibility with lithography process, i t is possible to make much smallcr bunips with fine pitches (50-1 O O p ) using electroplating process [4].

With increasing cnvironmental concerns regarding the toxicity of lead [SI, substantial research efforts arc now being expended towards identification of the best " Lead- free" alloy that could suitably replace tin-lead. We have reported electroplating of binary lead-free alloys of Sn-Bi and Sn-Cu [6]. Amongst a host of binary, ternary and quaternary alloy option, the ternary tin-copper-silver family of lead-free alloys has denionstrated most promise for lcad free applications, albeit with slight disadvantage of high melting point [7]. However, i t has been shown that addition of a small amount of bismuth to this system can further lower the melting point froni 21 7 for tlie Sii-Ag-Cu eutectic to 209°C [ 8 ] . Bismuth in small quantities also helps in improving the mechanical properties of this alloy, which are demonstrably better than Sn- Pb [8]. However. a

TENCON 2003 / 1368

cu cleaning of the Deposition

disadvantage of this alloy is that presence of even trace amount of Pb causes formation of a low melting (96°C) ternary phase of Sn-Pb-Bi, which may give away during thermal cycling [9]. Nevertheless, the quaternary alloys of Sn-Bi-Ag-Cu are amongst the best lead-free options available today.

Ni -+Deposition

I n this work, we have attempted to make solder bumps with targeted composition of Sd3.1 Ag/3.1 Bi/OSCu using electroplating process. Electroplating of [our elements from a single bath is perceivably difficult due to the intrinsic difference (more than 2OOniV) in the deposition potentials of each element [IO]. In the present system, the deposition potentials vs SCE of individual metal ions in aqueous media are:

Si1: - 0.136V Bi: +0.317V Ag: + 0.790V Cu: + 0.340V

Ag being the most noble, it has most positive deposition potential of +0.79V whereas Sn being most electronegative, i t has a potential of -O.l36V[lO,I 11. CO-deposition in such cases can be made possible by suitably altering their potentials with the help of appropriate chelating agents and

' other additives [IO]. The situation is further complicated due to mutual interference of salts and additives affecting tlie stability of the bath. Clearly, it is crucial to choose appropriate chelating agents and additives to obtain a stable bath that produces films with composition in the targeted w i n dow .

Solder Reflow It Deposition -

Here we report deposition of Sn-Bi-Cu i n a single electroplating bath. The paper presents results of the bath stability and its first level characterization with respect to the dependence of composition on current density an'd time elapsed. This bath was used to deposit Sn-Bi-Ag-Cu lead- free bunips over a chosen UBM, wherein Ag was deposited separately. An initial characterization of bump in terms of composition is also reported here.

Silver Deposilioii '

2. EXPERIMENTAL Solder bumps can be obtained by depositing UBM on the conducting pad followed by deposition of solder. Reflow of solder would form bumps. In an usual process on silicon, the re-distributed Aluminum pads must have a layer of diffusion barrier, such as Ti to prevent AI dissolution in molten solder during reflow. In order to simplify the process, we used Ti substrates for subsequent depositions. The other two metal layers that constitute UBM in the present case are, Cu and Ni, deposited i n that order. Copper provides adhesion to the solder after reflow through the formation of intermetallic layers, while Ni prevents complete dissolution of copper into molten solder, or, in other words, i t controls the formation of Cu intermetallics. Due to the difficulty in obtaining a stable bath for four- element deposition, wherein it was found that silver salt could be a source of bath instability, silver was deposited separately (using commercial bath), while Sn, Bi and Cu were co-deposited in a single bath.

Two different sets of experiments were carried out. For the bath characterization experiments where the feasibility of the bath for the desired co-deposition was studied, the depositions were done directly on Ti substrates without tlie lithography step. For solder bumping, UBM layers were deposited on Ti substrates after the lithography, followed by deposition of a layer of Ag. The ternary solder was deposited on this Rg layer. Figure 1 shows the sequence steps for the solder bump formation.

Figure 1: Sequence steps for solder bump formation

chemically and niechanically treating Ti substrates to create sufficient porosity for good adhesion for subsequent deposited layers.. The substrate was first polished on the machine using silicon carbide paper after wlikh the substrate surface was coarsened by manual polishing using coarser silicon carbide powder dispersed on silicon carbide paper. After polishing, the substrate was cleaned with soap water in an ultrasonic bath, followed by similar cleaning i n distilled water and finally i n trichloroethylene. Thus prepared substrate was then ready for photolithography or solder deposition. Lithography was done using coinniercial (M/s Image Technologies, Pune) liquid photoresist (negative type). A mask containing circular test pattern of diameter varying from 100 to 900pin was used to obtain such circular openings on the otherwise photoresist-polymer covered Ti substrates. Subsequent electro depositions were done on such patterned substrate. After developing the patterns, copper (Rochelle CoppeGk', Grauer & Weil ( I ) Itd) was deposited at 8mA/cm2 at 50-60'C for 2 min to obtain - l p m thickness, Nickel (Bright Nickelw Grauer & Weil ( I ) Itd) was deposited at 40mAkm' at -55°C for 7.5minutes to obtain -5pni thickness and silver (Argolume Bright Silver'"', Grauer & Weil (1) Ltd) was deposited at 1 5niA/cni2 at rooin temperature for 1 minute to get a 1 pni thick deposition. For all depositions, a graphite strip was used as anode, while the bare or patterned Ti substrate formed cathode. After each deposition tlie Ti substrate was rinsed i n distilled water and dried.

Poster Papers / 1 369

The bath prepared for co-deposition of Sn-Cu-Bi coniprised of their sulphate salts dissolved in 0.5 M sulphuric acid. Specifically, the bath contained 30g/l of SnS04, 2g/l of Bi2(S04)% and 0.2 g/l of CuS04. Other additives in the bath were polynionocarboxylic acids for chelation, surface finishing agents and buffers to finally raise the pH to -4.5. All chemicals used were of analytical grade. As earlier, graphite, non-dissolving electrodes were used as anode. The above bath was characterized for the film composition at different current densities in the range 5-25 mA/cni*. The photoresist was stripped by immersing in methyl ethyl ketone. RMA type liquid flux was then applied on deposited solder and it was reflown at - 250°C using IR Reflow Soldering Station (IRRS, PRECISOLD, Switzerland, TYP PS 3000). The film composition was also studied for its dependence on bath storage time i.e. the time measured from the end of the bath preparation process. The deposition done for bump preparation reported here was done at room temperature and at 5niA/cm2 current density and for a duration of two hours to obtain solder films with an expected average thickness of -5Opm.

After deposition the Sn-Bi-Cu films deposited on bare Ti substrates were first observed under the microscope and the thickness was determined using metallurgical microscope (Nikon MM-40). The detailed microstructure and elemental composition of tlie films were studied using Scanning electron microscope attached with Energy Dispersive Spectroscopy (SEM-EDS, Phillips XL-30 with EDAX@). A portion of film was used for adhesion test using 'scotch tape' method. For this, a small piece of scotch tape was stuck on the film and pulled off. The film having poor adhesion would come off with the tape. The Sn-Bi-Cu films deposited on patterned, UBM coated Ti substrates too were Characterized for their thickness and composition as above. The post reflow studies included wetting studies, compositional and size uniformity of the bumps.

3. Results and Discussion Bath Characterization The electroplating bath for co-deposition of Sn-Bi-Cu was found to be clear and transparent for almost one week, indicating good bath stability over that period. Some white particles could be seen settled down after seven days. It was inferred therefore, that tlie chelating agents and additives used in this bath were compatible to each other. The films deposited using this bath had composition within the targeted range. In order to cross check the stability of bath in terms of film composition, co-deposition of the films was done after varying hours of bath preparation. These depositions were done immediately, and after 24, 48 & 72 hours of bath preparation at a current density of 5mA/cm2. Figure 2 presents the composition of the films with respect to the elapsed time.

As can be seen from Figure 2, the coniposition of tlie film ,

4 0 SI1

- - ' - - - - - - - " - . . . . - .~ . . -~ .~~~- - - - . "~~~~~.~~.~~~~*

10 30 50 70 'Time Elapwd after I3ath Pmparalioii (Iluun)

Figure 2: Effect of time elapsed after bath preparation on deposited film composition for Sn-Bi-Cu system

deposited from the fresh bath and the ones deposited LIP to after 72 hours of bath preparation are very close. The composition of Sn in these films varied between 92% and 94% while the content of Bi and Cu was found to be varying between 4.5 to 6 '% and I .6 to 1.72 o/u respectively. This variation in composition may be ignored because it is within or of the same order of the expected errors i n EDS technique. These results indicate that there is a right combination of chelating agents used for the reported electroplating bath for Sn-Bi-Cu deposition. Thus, this bath can be used even after 3-4 days of preparation and the deposited film would exhibit similar coinpositional properties.

10 * -

In order to check the effect of current density on the properties of the deposited film, tlie electro deposition of Sn-Bi-Cu system was carried out at various current densities i.e. at 5 , 10, 15 & 25iiiA/cm2. The deposited films were then characterized for the film composition, microstructure and adhesion. Figure 3 shows the effect of current density on the

'

- 0 SI1 2 70- 8 - m B t

60-

50-

f 4 0 - v -

- A CU - 4 - -

-e ---_ * - _ _ _ __--- ------. . 10-

0 - '

5 10 15 20 25 Curmiit Density (n~A/rtii 3

Figure3: Effect of Cur ren t Density on deposited film composition for Sn-Bi-Cu system

composition of the deposited .film for the co-deposition of I

Sn-Bi-Cu laycr ,c,,. ~i a l l r y C r s TENCON 2003 / 1370

Phqtorcsisl

Sn-Bi-Cu system i n the reported bath. Thesc depositions were done immediately after bath preparation.

As can be seen from Figure 3, the composition of Sn in the films varied between 93.1% and 94% as the current density was changed from 5 nL4/cm2 to 25mA/cm2. In the same films the composition of Bi and Cu were 4.7 to 5.5% and 1. I to 2.2% respectively. This variation in composition too is within EDS error limit. Thus. change in current density does not seem have much effect on the deposited film composition. The bath being insensitive to current density fluctuations is a very useful property especially in commercial applications. However, it was found that the films had increasingly porous microstructures and t!ie film had poor adhesion to the substrate at higher current densities. Such behavior is e expected beyond a critical current density, which happens due to higher over potential and hydrogen evolution at cathode [ I I]. The film had poor adhesion at both 15 and 25niA/cni2. The film was very loose and came off very easily.

Birinpirig E.rperiniei1t.s In view of acceptable microstructure with good adhesion at low current densities, the bumps were prepared at 5 mA/cn?. Figure 4 shows the SEM Micrographs of the as deposited film, the solder bumps obtained after reflow and a depiction of deposited solder in cross section.

b Figure 4: (a)- SEM micrograph of as deposited solder, (b)- SEM micrograph of Solder bumps obtained (continued)

As can be scen from this figure, the solder has followed the same contour as that of the pattern, though there i s an

Ti Substratc

E

Figure 4: (c) Depiction of Cross-section of the deposited bump

increasing dimension as scen in the plan. This is expected, as the electrodeposition tcnds to spread over the resist surface due to very low resist thickness as compared to that of solder. This results in an overhang, as depicted in Fig 4c. Small area or point deposition of solder is also seen at certain places over photoresist. This could be due to pinholes in the photo resist. The deposited solder had a composition of 97.7Sn/0.64Bi/l. 13Agi0.53Cu. The film had a compact niicrostructiire. The thickness of the deposited solder was just sufficient to form IOOpni bumps. Figure 4b shows the SEM micrograph of the solder bumps formcd after reflow. As can be seen from the picture, there is a good amount of variation ir. the size of the bumps. The composition of the bumps as obtained by EDAX analysis was 94.67Sn/3.42Ag/I .4GBi/0.45Cu, whereas the expected composition was 93.3Sn/3. I Ag/ 3.1 Bi/OSCu. The picture also shows an extra amount of solder (as confirmed by EDAX analysis) around the bumps, which has not contributed to the bump. This may have happened due to the melting and flow of extra overhung (Fig 4c) solder over decomposing photoresist, a thin film of which was found to have remained even after stripping. Roughened Ti substrate around the bump may have offcred a partially wetting surface to the solder material. These difficulties in forming solder bumps can be overcome if the substrate is non- wetting or by using thicker resist that is stripped completely. Optimization of reflow tcmperature would also improve the uniformity in bump size. The bumps -showed good compositional uniformity. Thc variatioii was less than 1 %.

4. Conclusions This study presents the feasibility of deposition of quaternary Sn-Bi-Ag-Cu solder by the electroplating process and helps in identifying important process parameters affecting the formation of solder bumps. Optimization of the entire process and selection of proper reflow condition would help in improving the size arid compositional uniformity of the bumps. Larger bump size would require thicker films that in turn would need the deposition to be carried out at higher current densities. The adherence of the film at higher current densities have to be improved by carrying out the deposition under stirring or other means of agitation. This can also be achieved by changing the composition of the bath, which raises the hydrogen evolution potential so that higher current density co-depositions arc possible. Finally it is concludcd that this

Poster Papers / 1371

work establishes feasibility of depositing ternary alloy of Sn-Bi-Cu in order to form Sn-Ag-Bi-Cu lead free bumps.

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[ I O ] Mordechay Schlesinger, and Milan Paunovic,

[ 1 I ] N. V. Partha Sarathy, Practical Electroplating Moderii electroplating, W iley Interscience, 2000.

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