Electrochemical investigations for copper electrodeposition of through-silicon via

Microelectronic Engineering 88 (2011) 195–199

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Electrochemical investigations for copper electrodeposition of through-silicon via Tzu-Hsuan Tsai ⇑, Jui-Hsiung Huang Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei 10608 Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 28 June 2010 Received in revised form 8 September 2010 Accepted 12 October 2010 Available online 21 October 2010 Keywords: Copper Electrodeposition Through-silicon via Impedance Polarization

a b s t r a c t This study uses electrochemical techniques with a rotating disk electrode (RDE) to investigate the effects of Cu2+ concentration and additives on electrodeposition of through-silicon vias (TSV). The plating bath with both PEG and SPS has an obvious suppression effect on Cu-RDE with a thin boundary layer from 350 to 634 mV (vs. Hg/Hg2SO4) and a wide potential range for the via-filling operation. The impedance and potentiodynamic scans show the adsorption of small molecule SPS is more stable than PEG, and the effect of PEG or SPS depends on the thickness of boundary layer obviously only in Tafel region. This study obtained high filling powers in both deep and shallow vias using the plating bath of 50 g/L Cu2+, and TSV filling in wafer-segment scale, with 20 lm via diameter and 100 lm via depth, verifies the performance predicted by the electrochemical techniques. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

In recent years, through-silicon via (TSV) allows the shortest chip-to-chip vertical interconnections, which provide 3D siliconbased technology with a number of significant advantages, such as high performance, low power consumption, multi-functionality, and the small form factor required for advanced and portable electronic products [1–4]. For most TSV developments, Cu electrodeposition is the core and critical technology for the implementation of 3D interconnection [5,6]. Electrodeposition is a well-known technology used for Cu damascene [7]. The largest difference between Cu damascene and TSV is their filling dimensions. TSV is larger in diameter (tens of microns) and deeper in depth (hundreds of microns) than the damascene via. Therefore, it is necessary to develop a new Cu-via electrodeposition approach or chemistries due to its extremely high-aspect ratio and depth [8,9]. For Cu electrodeposition in TSV, the depth of the via is comparable to the thickness of a diffusion boundary layer [10]. It is very challenging and important to improve mass transport and the concentration gradient of reactants within via to avoid via void or seam [7]. This paper investigates the effects of Cu2+ concentration and additives by using electrochemical techniques with a rotating disk electrode (RDE) to evaluate Cu electrodeposition in TSV. This study further examines wafer-segment plating using the real TSV structure.

The basic plating-bath consisted of Cu2+ from CuSO4-5H2O, 100 g/L H2SO4 and 50 ppm Cl from HCl. The Cu2+ concentrations were examined in the ranges given 25 g/L, 50 g/L and 75 g/L. The additives included 10 ppm bis(3-sulfopropyl) disulfide (SPS) and 10 ppm polyethylene glycol (PEG, Mw = 6000). Potentiodynamic scans at a beaker scale are measured by a potentiostat (Autolab PGSTAT30) in a three electrode-system setup. This setup includes a Cu-RDE working electrode (area = 0.197 cm2, purity = 99.9%), a Cu wire counter electrode, and a saturated Hg/Hg2SO4 reference electrode. This study scans the potential from 1000 mV to 200 mV (vs. Hg/Hg2SO4) at a rate of 5 mV/s. In addition, electrochemical impedance measurements were performed at 200 mV vs. open circuit potential (OCP) by superimposing a 10 mV sinusoidal potential over the frequency range from 100 kHz to 0.01 Hz with Autolab FRA2 system. All electrochemical scans were controlled at 25 °C. The RDE rotation speeds are 1000, 100, and 10 rpm to simulate the mass-transfer environment on the wafer surface, at the bottom of shallow vias, and the deep vias, respectively [11]. This study defines the filling power as a ratio of the corresponding current densities at 10 or 100, with that at 1000 rpm. A high filling power is necessary for superfilling in a high-aspect ratio TSV. Based on characteristics of the plating-bath using electrochemical scans, this experiment performs the filling using commercial tool platforms with fountain-flow systems (Semitool Raider-M) at a wafer-segment scale to examine the effect on Cu filling of plating baths with various Cu2+ concentrations. The experiments use a wafer with

⇑ Corresponding author. Tel.: +886 2 2771 2171x2775; fax: +886 2 2778 7579. E-mail address: [email protected] (T.-H. Tsai). 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.10.018

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blind vias that etched by using deep reactive ion etch (DRIE). The blind via diameter and depth is 20 and 100 lm, respectively. After via formation, SiO2 insulator was deposited by plasma enhanced chemical vapor deposition (PECVD). Then, TiN barrier and Cu seed deposition were applied by physical vapor deposition (PVD) to provide good conductivity on the wafer surface and interior blind via. Before Cu via-filling, the samples were pre-wetted by 5 min using high-pressure DI water to ensure no air bubble inside the blind via. After the Cu via-filling process, the cross sections of vias were observed using optical microscopy (Nikon/MM-60) to determine the filling qualities. 3. Results and discussion Fig. 1 presents the effects of PEG, SPS and rotation speeds on the cathodic polarization curves of Cu-RDE in the plating bath of 50 g/L Cu2+. Due to different mass-transfer environments, the current density at 10 rpm reaches the limiting current density at approximately 570 mV (vs. Hg/Hg2SO4). This potential achieving masstransfer controlled regime is less negative than at 100 or 1000 rpm. The different polarization behaviors for RDE at 1000, 100, and 10 rpm make it possible to simulate the mass-transfer

20

i (ASD)

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(a) 10 ppm SPS 10 rpm 100 rpm 1000 rpm Without PEG/SPS 10 rpm 100 rpm 1000 rpm

0 20

i (ASD)

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10

5

(b) 10 ppm PEG 10 ppm SPS 10 rpm 100 rpm 1000 rpm Without PEG/SPS 10 rpm 100 rpm 1000 rpm

0 20

i (ASD)

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10

(c) 10 ppm PEG 10 rpm 100 rpm 1000 rpm Without PEG/SPS 10 rpm 100 rpm 1000 rpm

5

0

-400

-500

-600

-700

E (mV vs Hg/Hg SO ) 2

4

Fig. 1. Cathodic polarization curves of Cu-RDE in plating baths of 50 g/L Cu2+, 100 g/ L H2SO4 and 50 ppm Cl and in that with (a) 10 ppm PEG; (b) 10 ppm PEG and SPS; (c) 10 ppm PEG.

environment on the wafer surface, at the bottom of shallow vias, and the deep vias, respectively. Fig. 1 shows that the current density of the plating bath, without PEG and SPS, is higher at 1000 rpm (wafer surface) than at 100 or 10 rpm (via bottom). This creates an undesirable electrodepositing situation that naturally results in a void formation within the via. In Fig. 1a, the curves with 10 ppm SPS show a minor growth in current density for each rotation speed before reaching the limiting current density, indicating the overall acceleration effect of SPS on Cu electrodeposition with the different thicknesses of masstransfer boundary layer. However, after reaching the limiting current density, the mass-transfer of copper ion gradually controls the reductive reaction, and the acceleration effect of SPS becomes weak. With 10 ppm PEG and SPS, the curves indicate an obvious reduction in current density for each rotation speed, especially from 350 to 634 mV (vs. Hg/Hg2SO4), as Fig. 1b illustrates. From 350 to 634 mV, the current density is lower at 1000 than at 100 or 10 rpm, indicating a more obvious suppression effect of PEG on the wafer surface than at via bottom. Because the molecular weight of PEG is larger than SPS, the mass-transfer of PEG would be more difficult than SPS, resulting in a weak suppression effect, especially in a thick boundary layer. This polarization characteristic is helpful to fill TSV from the bottom upwards. Similarly, after reaching the limiting current density, the mass-transfer of copper ion gradually controls the reductive reaction, instead of by the effect of the PEG or SPS. Therefore, the current density is higher at 1000 than at 100 or 10 rpm. In Fig. 1c, the curves with 10 ppm PEG show an obvious polarization, showing the plating reaction occurred at more negative potential than that without PEG. In addition, the polarization effect of PEG is higher at 1000 than at 100 or 10 rpm, indicating Cu electrodeposition could be inhibited on wafer surface. This should be useful for TSV-filling. As indicated by comparison with the curves in Fig. 1b, however, the plating bath with only PEG shows a narrow feasible potential range (568 to 648 mV), where the current density is lower at 1000 than at 100 or 10 rpm. By considering the operational window, the plating bath with PEG and SPS has a wide feasible potential range and is recommended for TSV-filling. In order to analyze the suppression and acceleration effects of PEG and SPS, the impedance of Cu-RDE was measured in various plating baths as shown in Fig. 2. The plating bath without PEG and SPS (Fig. 1a) shows larger polarization resistance from 100 kHz to 436 Hz at 10 or 100 than at 1000 rpm, due to a thicker boundary layer at 10 or 100 than at 1000 rpm. Below 1.74 Hz, a near 45° line appears at 10 rpm instead of a semicircular arc, indicating the diffusion control occurred in this thick boundary layer. When adding 10 ppm SPS (Fig. 2b), the polarization resistance from 100 kHz to 436 Hz decreases more obviously at 100 than at 10 or 1000 rpm, showing the acceleration effect of SPS is stronger for filling the bottom of shallow vias than that of deep vias or on the wafer surface. In the low frequency region of Fig. 2b, a line appears below 5.24 Hz at 10 rpm, indicating the diffusion control occurred in this thick boundary layer, while an inductive loop appears below the Z0 axis at 100 or 1000 rpm, showing the adsorption of SPS. Moreover, a larger adsorption resistance below 436 Hz at 100 rpm than at 1000 rpm indicates more SPS molecules adsorbed on RDE at 100 than at 1000 rpm. In Fig. 2c, with 10 ppm PEG, the impedance curves from 100 kHz to 436 Hz indicate a smaller polarization resistance at 10 or 100 than at 1000 rpm, due to the weak suppression effect of PEG in a thick boundary layer. Below the Z0 axis, an inductive loop due to the adsorption of PEG could also been found as that due to SPS in Fig. 2b. However, SPS molecules are smaller than PEG, and the inductive loop due to SPS in Fig. 2b is more obvious than that due to PEG in Fig. 2c. In addition, Fig. 2c shows the inductive loop transfers to a capacitive loop at 10 rpm near 0.03 Hz,

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(a)

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Cu2+=50 g/L 10 rpm 100 rpm 1000 rpm

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Cu2+=75 g/L 10 rpm 100 rpm 1000 rpm

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i (ASD) Fig. 3. Polarization curves of Cu-RDE in plating baths of Cu2+, 100 g/L H2SO4, 50 ppm Cl, 10 ppm PEG, and 10 ppm SPS: (a) 25 g/L Cu2+; (b) 50 g/L Cu2+; (c) 75 g/L Cu2+.

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Z' (Ω) Fig. 2. Impedance plots of Cu-RDE in plating baths of 50 g/L Cu2+, 100 g/L H2SO4 and 50 ppm Cl: (a) without PEG/SPS; (b) with 10 ppm SPS; (c) with 10 ppm PEG.

charactering the unstable adsorption of PEG in a thick boundary layer. In Fig. 2a and b, the impedance cures at 10 rpm in the low frequency region appears a linear region. This reveals that the mass-transfer of Cu2+ is insufficient within the via, especially at the bottom of deep vias. Therefore, to improve mass transport

and the concentration gradient of Cu2+ within via are important for Cu electrodeposition of TSV. Fig. 3 shows the effect of rotation speeds on polarization curves in the plating bath of 100 g/L H2SO4, 50 ppm Cl, 10 ppm PEG, 10 ppm SPS and Cu2+ with different concentrations. The corresponding electrode kinetic parameters are listed in Table 1. In Fig. 3 from 350 to 600 mV (vs. Hg/Hg2SO4), the plating baths with different Cu2+ concentrations all show higher cathodic current density at 10 or 100 than at 1000 rpm. In further comparison with the electrode kinetic parameters in Table 1, at the same rotation speed, the exchange current density (i0) and mixed potential (Emix) increase with Cu2+ concentration. This shows the increased Cu2+ as a reactant would enhance the reduction potential and reaction rate. It is known the current density increases with potential linearly near Emix. In this linear-polarization region, the Emix raises with decreasing rotation speeds, due to the increased masstransport resistance, but the i0 decreased, showing an undesired filling distribution. When the applied potential reaches the Tafel region, where the current density increases with potential exponentially, the cathodic current density is higher at 10 or 100 rpm than at 1000 rpm as shown in Fig. 3. The difference between the linear-polarization region and Tafel region is the reaction was controlled from an ohmic resistance to an activation resistance. The suppression effect of

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Table 1 Electrode kinetic parameters for Cu-RDE in plating baths.

25 g/L Cu

2+

50 g/L Cu2+

75 g/L Cu2+

X (rpm)

Emix (mV)

i0 (ASD)

ba (mV/dec)

bC (mV/dec)

1000 100 10 1000 100 10 1000 100 10

373 362 332 367 355 341 361 345 326

2.19  102 1.20  102 5.35  103 5.91  102 1.26  102 6.50  103 6.32  102 2.02  102 7.15  103

78 71 61 131 80 71 101 68 46

138 71 72 486 76 54 133 64 45

Without PEG/SPS Cu 2+=50 g/l

4

Filling Power

Solutions

5

With PEG/SPS Cu 2+=25 g/l Cu 2+=50 g/l Cu 2+=75 g/l

3

2

1

0

5

Without PEG/SPS Cu 2+ =50 g/l

Filling Power

4

1

3

4

5

isurface (ASD)

With PEG/SPS Cu 2+ =25 g/l Cu 2+ =50 g/l Cu 2+ =75 g/l

3

2

Fig. 5. Filling power in shallow vias predicted by RDE techniques.

2

1

0 1

2

3

4

5

isurface (ASD) Fig. 4. Filling power in deep vias predicted by RDE techniques.

PEG is obvious in Tafel region especially for RDE with a thin boundary layer. In addition, Table 1 shows the cathodic Tafel slope (bC) is larger at 1000 than at 10 or 100 rpm, indicating the current density decreases significantly for RDE with a thin boundary layer (1000 rpm), where the adsorption of PEG is easy. The adsorption of PEG created a higher energy barrier for Cu electrodeposition on the surface than at via bottom. Furthermore, the plating bath of 50 g/L Cu2+ shows a largest bC, 486 V/dec, at 1000 rpm (Table 1), showing the higher activation polarization occurred in the plating bath of 50 g/L Cu2+ than in the bath of 25 or 75 g/L Cu2+. When Cu2+ concentration was insufficient, the mass-transfer effect of Cu2+ weakens the polarization of the additives at 1000 rpm, resulting in a smaller bC in the bath of 25 than in 50 g/L Cu2+. However, when Cu2+ concentration was very high, the suppression of PEG was not enough to produce a high energy barrier at 1000 rpm for all Cu2+. Therefore, bC in the bath of 75 g/L is also smaller than in 50 g/L Cu2+. After reaching mass-transfer region (more negative than 600 mV), where the current density is unchanged with potential, Cu electrodeposition is controlled by the mass-transfer of Cu2+, instead of by the effect of the additives. As shown in Fig. 3, Cu-RDE at 10 rpm reaches the mass-transfer control at 634 mV (vs. Hg/ Hg2SO4), less negative than that at 1000 rpm. When the potential is more negative than 634 mV, the current density increases more significantly at 1000 than at 10 or 100 rpm, resulting in the higher current density at 1000 rpm. The filling power is plotted in Figs. 4 and 5 for predicting Cu electrodeposition in deep vias and shallow vias, respectively. To ensure a perfect filling, the filling power must be much higher than one in a wide range of surface current density (isurface), i.e. the cur-

Fig. 6. Cu electrodeposition in 100  20 lm TSV: (a) 25 g/L Cu2+; (b) 75 g/L Cu2+; (c) 50 g/L Cu2+.

rent density of RDE at 1000 rpm. When the filling power is less than one or close to one, the result is the formation of undesirable seams and voids. In Figs. 4 and 5, the filling power is almost higher in the baths with PEG and SPS than that in the bath without PEG and SPS, with the exception of that in the bath of 25 g/L Cu2+. When the isurface shifts over 2.5 ASD, the filling power is less than that in the bath without PEG and SPS in shallow or deep vias. In Fig. 4, the highest filling power is 4.94 and 5.21 in the plating baths of 25 and 50 g/L Cu2+, respectively, while the filling power is lower than 2.5 in the plating bath of 75 g/L Cu2+. This predicts a better filling in deep vias could be obtained in the plating baths of 25 and 50 g/L Cu2+ than in 75 g/L Cu2+. However, due to the extremely large via depth and diameter of TSV, the higher deposition rate at via bottom than on the wafer surface through the whole electrodeposition process is important. Therefore, the high filling power in shallow vias is necessary. In Fig. 5, the filling power, higher than 2, could only be obtained in the plating bath of 50 g/L Cu2+. This predicts a better TSV filling could be done by using the plating bath of 50 g/L Cu2+ than the plating baths of 25 and 75 g/L Cu2+. Fig. 6 shows the cross sections of TSV-filling using wafersegment scale. The plating parameters, such as waveform, current

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density, plating time, etc are all identical, with the exception of Cu2+ concentrations. The filling results agree with the polarization characteristics of RDE. With the plating baths of 25 and 75 g/L Cu2+, Fig. 6a and b show that voids and seams remain within the via. The top of the via closes easily for low Cu2+ concentrations that result in an insufficient Cu2+ supplement. In the same way, the diameter of the via narrows easily for high Cu2+ concentrations that cause conformal sidewall growth. This study observes a clear V-shape within the via and a defect-free filling when using an optimum Cu2+ concentration, 50 g/L, as Fig. 6c shows. 4. Conclusion Electrochemical techniques can evaluate Cu electrodeposition of TSV. The obvious suppression effect of PEG in a thin boundary layer (1000 rpm) assists in filling the TSV from the bottom upwards. The plating bath with both PEG and SPS has a wide feasible potential range and is recommended for TSV-filling. The impedance results show the adsorption of the large molecule PEG is more unstable than SPS, and mass transport and the concentration gradient of Cu2+ within via are important for Cu electrodeposition of TSV. Polarization curves show the effect of PEG or SPS depends on the thickness of a mass-transfer boundary layer obviously only in Tafel region. In addition, the plating bath of 50 g/L Cu2+ presents the highest filling power in deep and shallow vias, predicting a

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superfilling in TSV. The real deposition in TSV, with a 20 lm via diameter and a via depth of 100 lm, verifies the prediction. Acknowledgement The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Contract No. NSC 96-2221-E-032-060. References [1] S. Lassig, Solid State Technol. 50 (2007) 48–56. [2] Y. Xie, in: Proceedings of the 2010 23rd International Conference on VLSI Design, 2010, pp.446-451, doi:10.1109/VLSI.Design.2010.60. [3] C.W. Lin, H.A. Yang, W.C. Wang, W.L. Fang, J. Micromech. Microeng. 17 (2007) 1200–1205. [4] J.U. Knickerbocker, P.S. Andry, B. Dang, R.R. Horton, M.J. Interrante, C.S. Patel, et al., IBM J. Res. Dev. 52 (2008) 553–569. [5] X. Gagnard, T. Mourier, Microelectron. Eng. 87 (2010) 470–476. [6] G. Druais, G. Dilliway, P. Fischer, E. Guidotti, O. Lühn, A. Radisic, et al., Microelectron. Eng. 85 (2008) 1957–1961. [7] K. Kondo, T. Yonezawa, D. Mikami, T. Okubo, Y. Taguchi, K. Takahashi, et al., J. Electrochem. Soc. 152 (2005) H173–H177. [8] C. Song, Z. Wang, L. Liu, Microelectron. Eng. 87 (2010) 510–513. [9] L. Hofmann, R. Ecke, S.E. Schulz, T. Gessner, Microelectron. Eng. (2010). doi:10.1016/j.mee.2010.06.040. [10] D.P. Barkey, J. Callahan, A. Keigler, Z. Liu, A. Ruff, J. Trezza, et al., Proc. IEEE Electron. Comp. Tech. Conf. (2007) 638–642. [11] W.P. Dow, C.W. Liu, J. Electrochem. Soc. 153 (2006) C190–C194.