Volume 3 Issue 8 - March 21, 2008
Effect of Oxidizer on the Galvanic Behavior of Cu/Ta Coupling during Chemical-Mechanical Polishing
Szu-Jung Pan, Jui-Chin Chen and Wen-Ta Tsai*

Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan
*Corresponding author: Email: wttsai@mail.ncku.edu.tw

(published in J. Electrochemical Society, Vol. 153(6), pp. B193-B198, 2006. (SCI))

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Introduction

To meet the requirement of global planarization, chemical-mechanical polishing (CMP) has emerged as a critical technique in Cu metallization process. Although Cu CMP exhibits some advantages over the conventional techniques, such as dry-etching, etc., there still exist some unfavorable factors affecting the CMP efficiency. One of them is the galvanic effect between materials in direct contact within a microelectronic component. If these materials in intimate contact have substantial difference in electrochemical properties in CMP slurry, the galvanic effect will become important in affecting the CMP efficiency. In this investigation, the effects of different oxidizers (H2O2, KIO3 and Fe(NO3)3) on the galvanic behavior of Cu/Ta couples were explored and compared. The polarity change due to area ratio was also examined.

Experimental

The electrochemical tests were conducted in the base slurries consisting of 0.01 M Na2SO4 supporting electrolyte and with 1 wt% Al2O3 abrasive particles added. Three different oxidizers, namely 10 vol% H2O2, 0.1 M KIO3 and 0.1 M Fe(NO3)3, were added in the base slurries. Electrochemical tests were conducted either in static or under CMP conditions, using a self-designed and manufactured polisher equipped with an electrochemical cell (Fig. 1). The galvanic current (Ig) flowing from Cu to Ta in each slurry during and after CMP was measured using a zero resistance ammeter (ZRA). The sign of Ig was taken as positive if Cu was the cathode and vice versa.
Figure 2 Potentiodynamic polarization curves of Ta in 0.01 M Na2SO4 + 1 wt% Al2O3 base slurries with various oxidizers, under static condition.
Figure 1 Schematic diagram of CMP apparatus for open circuit potential and galvanic current measurements of the Cu/Ta coupling.


Effect of oxidizer on the open circuit potentials of uncoupled Cu and Ta

Potentiodynamic polarization curves of Ta determined in 0.01 M Na2SO4 + 1 wt% Al2O3 base slurry and those with different oxidizer additions, under static condition, are demonstrated in Fig. 2. It was noted that the corrosion potential of Ta was significantly increased when Fe(NO3)3 was added into the base slurry. Clearly, Fe(NO3)3 was even more effective in passivating Ta in the base slurry considered.
Figure 3 Effects of (a) 10 vol% H2O2, (b) 0.1 M KIO3, and (c) 0.1 M Fe(NO3)3 on the open circuit potentials for uncoupled Cu and Ta in 0.01 M Na2SO4 + 1 wt% Al2O3 base slurry under CMP condition.

Under CMP, the effect of oxidizer on the OCP of uncoupled Cu and Ta is shown in Fig. 3. As shown in Figs. 3(a) and (b), the values of OCP for Cu measured in H2O2 and KIO3-containing slurries were higher than those of Ta. Figure 3(c) shows the potentials measured in the Fe(NO3)3-containing slurry. The potential of Cu remained almost constant at about +85 mV, which was slightly higher than that in static condition. For uncoupled Ta, the initial potential in Fe(NO3)3-containing slurry was -50 mV, more negative than Cu. However, as shown in Fig. 3(c), the potential of Ta increased and eventually became higher than that of Cu as CMP continued.
Figure 4 Effects of (1) 10 vol% H2O2, (2) 0.1 M KIO3, and (3) 0.1 M Fe(NO3)3 on the galvanic currents of the Cu/Ta coupling in 0.01 M Na2SO4 + 1 wt% Al2O3 base slurry during CMP and in static condition.

Effect of oxidizer on the galvanic current of coupled Cu/Ta

The galvanic currents for the Cu/Ta couples measured in the base slurry with different oxidizer additions are depicted in Fig. 4. Curves 1 and 2 in Fig. 4 demonstrated the galvanic currents for Cu/Ta couple (both with an exposed surface area of 4.9 cm2) measured under CMP in the slurries containing 10 vol% H2O2 and 0.1 M KIO3, respectively. The negative currents measured indicated that Ta was the anode with respect to Cu cathode in these two different slurries. Curve 3 in Fig. 4 represents the galvanic current measured in Fe(NO3)3-containing slurry. A switch of polarity was seen due to the passivation of Ta in such slurry.


Effect of Cu/Ta area ratio on galvanic current

Figures 5 and 6 show the effect of Cu/Ta area ratio on the galvanic currents measured under CMP in the slurries with 10 vol% H2O2 and 0.1 M KIO3 additions, respectively. In both cases, Ta was the anode regardless of the Cu/Ta area ratio. It was noted that the galvanic current density with a Cu/Ta area ratio of 5:1 was higher than that of 1:1. The results indicated that accelerated dissolution on anode occurred if the area of cathode was increased.
Figure 6 Effect of Cu/Ta area ratio on the galvanic current density of the Cu/Ta coupling in 0.1 M KIO3 + M Na2SO4 + 1 wt% Al2O3 slurry during CMP and in static condition.
Figure 5 Effect of Cu/Ta area ratio on the galvanic current density of the Cu/Ta coupling in 10 vol% H2O2 + M Na2SO4 + 1 wt% Al2O3 slurry during CMP and in static condition.


Conclusions

The presence of Fe(NO3)3 promoted the passivation of Ta. A change of polarity of Ta with respect to Cu was observed in the Fe(NO3)3-containing slurry. The magnitude and polarity of the galvanic current also depended on the Cu/Ta area ratio. CMP is a common and required practice employed in the processing streams in semiconductor, electronic and photoelctronic industries. Knowing the effects of additives and area ratio on the electrochemical behavior of metals in CMP slurries may be helpful for quality assurance.

Acknowledgement
The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 91-2216-E-006-036.
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