Investigation of Galvanic Corrosion Between TaNx Barriers and ...

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Chi-Cheng Hung,a Wen-Hsi Lee,a Yu-Sheng Wang,a Shih-Chieh Chang,b and ... aDepartment of Electrical Engineering, National Cheng Kung University, ...
Electrochemical and Solid-State Letters, 10 共10兲 D100-D103 共2007兲

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1099-0062/2007/10共10兲/D100/4/$20.00 © The Electrochemical Society

Investigation of Galvanic Corrosion Between TaNx Barriers and Copper Seed by Electrochemical Impedance Spectroscopy Chi-Cheng Hung,a Wen-Hsi Lee,a Yu-Sheng Wang,a Shih-Chieh Chang,b and Ying-Lang Wangb,c,z a

Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan Department of Applied Physics, National Chia-yi University, Chia-yi, Taiwan c Department of Materials Science, National University of Tainan, Tainan, Taiwan b

In this article, electrochemical impedance spectroscopy is used to characterize the mechanism of galvanic corrosion between copper 共Cu兲 seeds and tantalum nitride 共TaNx兲 barriers deposited with different N2 flow rates. By way of software simulating with EIS data, an equivalent circuit is built up to explain the corrosion behavior of the TaNx films’ relation to the Cu seeds in an acidic chemical-mechanical-polishing slurry. The equivalent circuit can respond to changes in resistance and capacitance elements of the Cu–TaNx electrochemical system. It is found that the charge-transfer resistance of the TaNx galvanic corrosion increases with the N2 flow rate, whereas the resistance of a tantalum-oxide layer is opposite because increasing the N content of the TaNx films inhibits corrosion and oxidation of the Ta metals. The result is consistent with our previous investigation that the galvanic corrosion of the TaNx films to the Cu seeds is retarded by the N element 关C. C. Hung, Y. S. Wang, W. H. Lee, S. C. Chang, and Y. L. Wang, Electrochem. Solid-State Lett., 10, H127 共2007兲兴. © 2007 The Electrochemical Society. 关DOI: 10.1149/1.2756339兴 All rights reserved. Manuscript submitted April 16, 2007; revised manuscript received May 29, 2007. Available electronically July 23, 2007.

In the manufacturing of semiconductor devices, copper 共Cu兲 has been used as the interconnection metal because of its higher electromigration resistance and lower resistivity.1,2 Different from the metal-etching patternization in the aluminum metallization process, the Cu-metal lines are produced by the damascene process that includes the deposition of barrier/Cu metals and the removal of overburden metals by chemical mechanical polishing 共CMP兲.3-15 For the Cu metallization process, a barrier layer is necessary to prevent Cu metal from diffusing into a dielectric layer. Tantalum and tantalum nitride 共Ta/TaNx兲 films have been widely used as the diffusion barriers for Cu metallization due to their excellent chemical and thermal stability. The Ta/TaNx should not only be thin enough to reduce metal resistance but also have compatible adhesion between the Cu metals and the dielectric layers.16-24 The physical properties of the TaNx films deposited with different nitrogen 共N2兲 gas flow rates have been widely investigated in prior literature.25-28 Except for the physical properties, the chemical behaviors of the TaNx films in the CMP process should be considered as well to get a robust production. Our previous study has demonstrated that the TaNx films had self-corrosion and the corrosion current decreased when the N2 flow rate increased from 2 to 30 sccm.29 In this article, electrochemical impedance spectroscopy 共EIS兲 is used to characterize galvanic corrosion between the TaNx barrier and the Cu seed. EIS has been widely recognized as a powerful tool for the investigation of electrochemical behaviors.30-35 It allows for fast 共time-resolved兲 detection of impedance spectra and, hence, is particularly useful for studying transient corrosion effects. Baranski et al. have developed high-frequency impedance measurements 共up to 5 MHz兲, which enabled the studies of very fast surface processes.36,37 Zou et al. have used a pair of microelectrodes for the measurements of local ac currents which allowed a local impedance to be determined.38 Through the EIS study, the galvanic behavior between the Cu seeds and the barrier metals can be clarified in more detail that benefits the defect reduction after the Cu-CMP process, especially for sub90 nm technologies. Experimental In this study, blanket wafers were deposited with 50 nm thick TaNx films and 200 nm thick Cu seed films using the ionized metal plasma 共IMP兲 process on 200 nm SiO2 /Si共100兲 substrates. The IMP

z

E-mail: [email protected]

deposition used inductively coupled Ar/N2 plasma to ionize sputtered atoms, which were highly directed by the potential difference between plasma and substrate. The TaNx barriers were deposited by rf 共13.56 MHz兲 magnetron sputtering at 1 kW forward power from a Ta target 共99.995%兲. While the Ar flow was kept at 50 sccm, the nitrogen flow was varied from 0 to 30 sccm resulting in the process pressure to ⬃0.2 Torr. The gas purity was 99.9999% and the gas flow was controlled within ±0.1 sccm by mass flow controllers, which guaranteed reproducible deposition conditions. After the deposition of the TaNx film, a 200 nm thick Cu seed was subsequently deposited without vacuum break. In the corrosion analyses, the TaNx films deposited with different N2 flow rates were used as working electrodes 共4 cm2兲 and the Cu seed films were used as counter electrodes 共4 cm2兲 and Ag/AgCl used as a reference electrode in a Cu-CMP slurry, which contained aluminum oxide abrasive, surfactant, and hydrogen peroxide 共⬃5% H2O2兲 with pH of 6–7. The Nyquist plot was measured from Princeton Applied Research PARSTAT 2273 and the ac impedance behavior of the capacitor cells was analyzed. The impedance measurements were carried out at different potential values with dc potential amplitude of 0 mV associated with open circuit potential and a frequency range of 10 mHz to 1000 kHz. The equivalent circuit was built up and simulated from the software of ZSimpWin version 3.1 with EIS data.

Results and Discussion The Bode plots of Fig. 1a show the effect of applied frequency on the corrosion impedance between the Cu seeds and the TaNx films deposited by various N2 gas flow rates from 2 to 30 sccm. In this case, the TaNx films deposited with different N2 flow rates were used as working electrodes, the Cu films were used as counter electrodes and all potentials were reported relative to the Ag/AgCl reference electrode. In the low-frequency region 共0.01–0.1 Hz兲, the corrosion impedance apparently increases with increasing the N2 gas flow rate. On the contrary, the corrosion impedance slightly decreases with increasing the N2 gas flow rate while the measurement is in the high-frequency region 共⬎100 kHz兲, as shown in Fig. 1b. Figure 2a shows Nyquist plots of the Cu–TaNx electrochemical system in the CMP slurry. The Nyquist plots also reveal that the value of Zim /Zre ratio increases with increasing the N2 flow rate in the low-frequency region indicating that the TaNx films with higher N doping are less corrosive. However, Fig. 2b shows that when the Cu–TaNx system is measured in the high-frequency region, the

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Electrochemical and Solid-State Letters, 10 共10兲 D100-D103 共2007兲

Figure 1. 共a兲 Bode plots of the Cu seeds and the TaNx films deposited with various N2 gas flow rates from 2 to 30 sccm and 共b兲 is a magnification of the high-frequency region.

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Figure 2. 共a兲 Nyquist plots of the Cu seeds and the TaNx films deposited by various N2 gas flow rates from 2 to 30 sccm and 共b兲 is a magnification of impedance close to zero at the high-frequency region.

Zim /Zre ratio decreases as the N2 flow rate of the TaNx deposition increases. The smaller corrosion resistance indicates that the TaNx film is easier to be corroded. Figure 3 displays the simulated equivalent circuit of the Cu–TaNx electrochemical system. Chart 共a兲 inserted in Fig. 3 is the simulation result from the software 共ZSimpWin兲 and the exactitude of the equivalent circuit has been clarified. The equivalent circuit shows that the Cu–TaNx electrochemical system consists of three resistances and two capacitances. In this circuit, the series and parallel combination of resistance or capacitance elements indicate whether their representative reactions occur sequentially 共in series兲 or simultaneously 共in parallel兲.34 The chemical reaction of the TaNx films in the Cu–TaNx electrochemical system occurs through the following intermediate steps35 Ta ↔ Ta5+ + 5e−

关1兴

Ta5+ + 5 O2− ↔ Ta2O5

关2兴

Hence, in this equivalent circuit, Rs is the bulk-solution resistance, CEDL is the double-layer capacitance of the electrogenerated from surface corrosion of the TaNx films, Cox is the capacitance of the TaNx oxidization layer for Reaction 2, Rcorr is the charge-transfer resistance 共associated with double layer兲, and Rox is the resistance of the TaNx oxidation layer for Reaction 2. Furthermore, Rox and Rcorr are in parallel because surface oxidation and corrosion of the TaNx films always occur at the same time in the Cu–TaNx electrochemical system. On the other hand, two semicircles 共as seen in Fig. 3兲 re-

Figure 3. Proposed equivalent circuit diagram shows the electrochemical characteristics of TaNx galvanic corrosion in the CMP slurry solution, where Rs was the bulk solution resistance, CEDL was the double-layer capacitance, Cox was the oxidization-layer capacitance, Rcorr was the charge-transfer resistance 共associated with double layer兲 and Rox was the oxidization resistance of TaNx and chart 共a兲 is a simulation map.

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Electrochemical and Solid-State Letters, 10 共10兲 D100-D103 共2007兲

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Table I. Element values of equivalent circuit in Fig. 3 required for the best fitting of impedance spectra in Fig. 2a. CPEox TaNx–Cu

Rs 共⍀兲

Rox 共⍀兲

Y ox 共␮⍀−1兲

␣ox

Cox 共␮F兲

Rcon 共k⍀兲

CEDL 共␮F兲

N N N N N

62.54 63.82 63.53 63.23 62.77

171.5 169.1 149.5 143.2 133.7

71.88 71.49 80.69 79.06 85.27

0.9241 0.9317 0.9231 0.9325 0.9212

50.1 51.7 55.8 57.2 58.2

38.64 46.42 58.38 91.25 112.01

0.008958 0.00986 0.01929 0.03113 0.05123

= = = = =

2 10 20 28 30

spectively express transient surface oxidation of the TaNx films. The first semicircle 共high-frequency region兲 indicates the Ta metal oxidizes in peroxide-based solutions and the second semicircle 共middlelow frequency region兲 means the continuously corrosion of the TaNx films. The values of circuit’s components in Fig. 3 are obtained by fitting the Nyquist spectra of Fig. 2a and are summarized in Table I. CPEox is the impedance in the constant phase element 共CPE兲 of the TaNx oxidation layer. The values of system’s capacitances can be transferred from CPE using Eq. 3 C=

共Y ox ⫻ Rox兲共1/␣ox兲 Rox

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Consequently, the Cox values of oxidation layers increase from 48.6 to 56.8 ␮F as the N2 flow of the TaNx deposition increases from 2 to 30 sccm. For a chemistry perspective, the Ta metal oxidizes rapidly in aqueous environments to form the tantalum oxide 共Ta2O5兲, especially for hydrogen peroxide existing in the solutions. Equation 4 defines the overall oxidation process for the Ta metal in the presence of hydrogen peroxide39 2Ta + 5H2O2 ↔ Ta2O5 + 5H2O

关4兴

Equation 5 depicts the relationship between the tantalum-oxide capacitance, Cox, and tantalum-oxide thickness, d Cox = ␧0␧

s d

关5兴

where ␧0 is the permittivity of free space 共8.85 ⫻ 10−12 F/m兲, ␧ is the dielectric constant of the Ta2O5 film 共27.6兲, S is the tested Ta wafer surface area 共4 cm2兲, and d is the film thickness. The Ta2O5 layer is formed on the surface of the TaNx films by Reaction 4. The N element is the factor to reduce the oxidation rate of the TaNx films. Hence, the dielectric constant of the Ta2O5 layer should be slightly dependent on the composition of the TaNx films. Figure 4

Figure 4. Capacitance and thickness of Ta2O5 for TaNx film deposited with various N2 gas flow rates.

displays that the thickness of the oxidation layer decreases and the capacitance of the oxidation layer increases as the N2 flow rate of the TaNx deposition increases. The film thickness of Ta2O5 calculating from Cox value 共48.6–56.8 ␮F兲 is 17.2–20.1 Å. The value is comparable to ⬃20 Å that has been reported on the tantalum surface.40 On the other hand, Rcorr increases with increasing the N2 gas flow rate while Rox decreases with an increase in the N2 gas flow rate, as shown in Table I. This is because the TaNx films with higher N doping have difficulty oxidizing and are less corrosive. Figure 5 reveals that the reaction time constant 共RC兲 meaning corrosion degree is Rcorr multiplied by capacitance of CEDL. Consequently, RC increases from 0.346 to 5.738 ms when the TaNx films are deposited from 2 to 30 sccm. The phenomenon reveals that the galvanic corrosion of the TaNx films is suppressed by N element. Similarly, the value of CEDL increases with the N2 gas flow rate because TaNx films’ corrosion is retarded to accumulate many electrons. Furthermore, according to Stern and Geary, developing the following equation shows the mathematical relationship between corrosion resistance and corrosion current for polarization data in Eq. 641,42 Icorr =

␤a ⫻ ␤b 2.303Rcorr共␤a + ␤b兲

关6兴

in which Icorr and Rcorr are corrosion current and corrosion 共chargetransfer兲 resistance, respectively. ␤a and ␤b are anodic Tafel slope and cathodic Tafel slope, respectively. According to our previous studies, the TaNx films have self-corrosion and the self-corrosion current declines when the N2 flow rate increases from 2 to 30 sccm.29 Through the results of impedance measurements and potential difference between the Cu seeds and the TaNx films measured by an electrometer system, the corrosion currents reduce from 14.32 to 3.75 ␮A when the N2 flow rate of the TaNx deposition increases from 2 to 30 sccm, as shown in Fig. 5. The result is consistent with our previous investigation that the galvanic corro-

Figure 5. Relationship of TaNx film deposited with various N2 gas flow rates between corrosion current and RC time delay in Cu–TaNx system.

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Electrochemical and Solid-State Letters, 10 共10兲 D100-D103 共2007兲 sion current is inhibited by the N element of the TaNx films. In addition, above-mentioned elements of the circuit in this electrochemical system have been defined by their physical meanings. By way of the parameters of the impedance spectroscopy, larger Rcorr and smaller Rox mean the TaNx metal is hard to be corroded and to form the Ta2O5 layer. On the other hand, Cox is relative to the thickness of the Ta2O5 films while CEDL correlates with the generation of electrons. Conclusions The mechanism of galvanic corrosion in the Cu–TaNx electrochemical system is investigated. By way of computer software simulating with EIS data, an equivalent circuit of the surface reaction within the TaNx films’ relation to Cu seeds is built up. In addition, the charge-transfer resistance 共Rcorr兲 associated with double-layer capacitance 共CEDL兲 increases with the N2 flow rate of the TaNx deposition, however, the tantalum-oxide resistance 共Rox兲 associated with tantalum-oxide capacitance 共Cox兲 is opposite. It demonstrates that the N element content is effective to inhibit tantalum-metal oxidation and to reduce galvanic corrosion between the Cu seed and the TaNx films. Acknowledgment This work was supported by the National Science Council of Taiwan 共grant no. NSC 95-2221-E-006-086兲. The authors thank National Cheng Kung University, Tainan, Taiwan, for technical support. National Cheng Kung University assisted in meeting the publication costs of this article.

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