Competitive Adsorption Between Suppressor and Accelerator in Copper Methanesulfonic Acid Bath tor Electrodeposition Dongfan Wang l , Xiaoying Miao l , Huiqin Ling l *, Ming Li l , IState Key Lab 0/ Metal Matrix Composites, Schoolo/ Materials Science and Engineering, Shanghai Jiao Tong University Shanghai 200240, China Email:
[email protected] Abstract-The rapid development of three dimensional packaging makes it necessary to develop smaller and more reliable microbumps. In the electrodeposition process of bump cylinder, the filling quality is largely determined by the combination of additives. In this work, the effect and competitive adsorption between suppressor polyethylene glycol (PEG) and accelerator Bis-(3-sodiumsulfopropyl disulfide) (SPS) were investigated in cop per methanesulfonic acid (MSA) plating bath. The results indicated that suppressor inhibited the Cu+/Cu reduction process in copper deposition by forming PEG-CI--Cu+ passivation film on the electrode surface. And the inhibiting effect was found to reach saturation when suppressor concentration reached above 9 mg/L, due to the coverage limitation of suppressor absorption. When accelerator was added into plating bath with PEG, SPS was supposed to disrupt or displace the adsorbed PEG and to form a bridge (RS-Cu 2+-Cn with the electrode metal, wh ich speed up the charge-transfer process. Thus the electrochemical process was converted to be diffusion controlled. Keywords-bump; copper electrodeposition; methanesulfonic acid plating bath; additives; competitive adsorption; I.
INTRODUCTlON
In the three-dimensional packaging, copper pillar bumps, due to excellent electrical and thermal properties [1], are promising to achieve ultra-fine pitch interconnection with layers. In Cu/Sn microbumps, the bottom copper pillar can prevent the short circuit and provide enough mechanical strength. And electroplating void-free and tlat Cu pillar is essential to ensure filling quality and reliability ofupper solder [2]. But with the rapid shrink down of packaging size, microbumps are required to scale down from 40-50 11m to several microns [3]. The high density preparation of fine-pitch bumps especially for Cu pillar has met with new issues in electroplating process. In Cu electroplating process, the filling quality is largely determined by the combination of additives [4, 5]. The linear polyethylene glycol (PEG, H(OCH2CH2)nOH) and Bis-(3sodiumsulfopropyl disulfide) (SPS, Na2[S(CH2)3S03]z) are commonly used as suppressor and accelerator in sulfate bath. As is known to all, PEG can be absorbed on the entrance of via and form an intermediate film (PEG- CI-- Cu+) on the copper
2017 18th International Conference on Electronic Packaging Technology 978-1-5386-2972-7/17/$31.00 ©2017 IEEE
Fengwei Dai2, Wenqi Zhani, Liqiang ea02
2Institute 0/Microelectronic 0/ Chinese Academy 0/ Sciences, Beijing 100029, China
surface to inhibit cop per deposition [6-8]. And SPS could be adsorbed on electrode surface by the coordination between sulfonate end group and CuCI adsorbed species on the surface [9]. Lee et al. [10] demonstrated that SPS adsorbed on cathodic surface was reduced to 3-mercapto-l-propanesulfonate (MPS) firstly. Then the combination of MPS and Cl- could obstruct the inner sphere electron transfer net and form the bridge (RSCu 2+-Cn, thus speed up the charge transfer. The competitive adsorption between suppressor PEG and accelerator SPS in sulfate bath in copper electroplating have been made some attentions. M.A. Pasquale et al [11] demonstrated that SPS and PEG had a competitive tendency to be absorbed on Cl- or CuCI. Ying Jin et al [12] found that PEG could accumulate evidently on copper surface without the existence of SPS. But with the addition of SPS, SPS would displaced most of PEG molecules to be adsorbed on Cu surface in short time. As discussed above, SPS could react with both Cu+ and CuCI to generate weakly-adsorbed intermediate, while PEG could be strongly chemisorbed on copper surface. However, in recent years, Methanesulfonic acid (MSA) system was taken as a new promoting plating bath because it could accommodate much more Cu 2+ and acid ions than sulfate bath to increase the plating rate [13]. And MSA was quite different from sulfate bath in many aspects such as plating bath properties, filling mechanism [14, 15]. The synergistic and competitive behavior of additives in MSA plating bath for Cu void-free electrodeposition remained unclear. Therefore, in this work, the effect of suppressor and the competitive adsorption between suppressor and accelerator were investigated in MSA plating bath by several electrochemical techniques. 11.
EXPERIMENT AL PROCEDURE
The MSA plating bath as weil as additives were a11 supplied by commercial company. And the MSA solutions mainly consisted of 110 g/L Cu 2+, 15 g/L CH2=C(CH3)COOH, and 50 ppm cr. And 0 to 13 mg/L UPT3360S, mainly composed by PEG, was added in sequence as suppressor to investigate its adsorption behavior in MSA during Cu electrodeposition. Besides, to examine the competitive adsorption, 0 to 4 mg/L UPT3360A mainly composited by SPS was subsequently added as accelerator while the concentration ofUPT3360S was fixed on 9 mg/L.
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The electrochemical measurements were performed on an electrochemical workstation at room temperature. It was a three-electrode cell which included a pure copper working electrode (area = 0.5 cm 2 , purity = 99.9%), a Pt counter electrode and a saturated calomel reference electrode (SCE). In the preparation stage, the working electrode surface was polished and etched by acid to be c1eaned. Electrochemical impedance spectroscopy (EIS) measurements were performed at -0.05 V with 5 mV AC signal amplitude and frequency ranging from 0.01 to 10 6 Hz. Chronopotentiometry tests were performed at 1 A/dm 2 cathode current. And the whole experiment process was completed in a shielding box to prevent electromagnetic interference.
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40
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o
The coverage of additives on electrode surface could be calculated according to the reaction equilibrium potential given by the result of chronopotentiometry. The surface coverage 8",v might be expressed by equation [16]: (Jav
= 1-
0.39F
exp[--- (~A,n O
RT
- ~A)]
Where F was the Faraday constant, T was the absolute temperature, R was the gas constant,
