CHEMICAL AND ELECTROCHEMICAL CHARACTERIZATION OF PEROXIDE- INDUCED PASSIVATION OF COPPER IN AQUEOUS GLYCINE SOLUTIONS

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1 Proceedings VMIC 23 (Twentieth Int. VLSI Multilevel Interconnection Conf), Marina Del Rey, CA, Sept. 23, pp CHEMICAL AND ELECTROCHEMICAL CHARACTERIZATION OF PEROXIDE- INDUCED PASSIVATION OF COPPER IN AQUEOUS GLYCINE SOLUTIONS Fiona M. Doyle and Ling Wang University of California Department of Materials Science and Engineering 21 Hearst Mining Building Berkeley, CA Abstract Under many circumstances, copper passivates in CMP slurries when hydrogen peroxide is present as an oxidizing agent. This behavior is observed under acidic conditions, where no solid oxidized phases appear on the potential-ph diagram, and hence where passivation would not be expected. This passivation behavior is highly desirable for effective CMP. However, passivation is not seen in slurries of similar ph and complexing agent concentration, where oxidizing potentials are controlled electrochemically. As part of an effort to model the effects of chemistry on CMP rates, we are endeavoring to better characterize the oxidation rates observed in slurries containing hydrogen peroxide, especially in the passive region where the rates are highly non-linear. Here we report the behavior in slurries containing glycine, a typical complexing agent, and the influence of buffering agents. Introduction As copper replaces aluminum as the interconnect metal in integrated circuits, it is becoming increasingly important to be able to predict the behavior during chemical mechanical planarization (CMP). CMP is key in the dual damascene method for producing devices with multilevel metallization [1,2], yet involves intrinsically different kinetics from those of most fabrication processes. Furthermore, copper CMP is still poorly understood compared to CMP for silicon dioxide, because of electrochemical interactions between the slurry and the metal film during polishing, and the coupled effect of these on the mechanical properties of the surface. Early studies on copper CMP have demonstrated the importance of oxidizers and complexing agents in successful slurries [3]. Complexing agents have included ammonia [4,5,6], ethylenediamine-tetraacetic acid (EDTA) [7], glycine [8,9,1,11] and other amino acids. Mixtures of glycine and hydrogen peroxide have shown promising CMP behavior in both alkaline [8] and neutral [9] slurries. However, neutral ph s are more attractive, because the copper polish rate selectivity with respect to silica is unfavorable under alkaline conditions, because of erosion of the interlayer dielectric.

2 As part of an effort to model the effects of chemistry on CMP rates, incorporating both mechanical and chemical factors, we are endeavoring to better characterize the oxidation rates observed in slurries containing hydrogen peroxide, especially in the passive region where the rates are highly non-linear. Electrochemical studies are particularly convenient for CMP [12,13,14]. However, such studies are only intrinsically useful if the behavior at a given potential is comparable with the behavior that would be exhibited if a chemical oxidizer were used to control the potential, because chemical oxidants are used commercially. Aksu and Doyle recently reported that when copper is oxidized by hydrogen peroxide in the presence of glycine, the behavior is not consistent with the electrochemical behavior [15]. Figure 1 shows their polarization curves for copper at ph 4, 9, and 12 in.1 M glycine [12]. There is clearly active dissolution at ph 4 and 9, and passivation at moderately oxidizing potentials at ph 12. This behavior is consistent with the potential-ph diagram for the copperwater-glycine system (Figure 2 [12]), which shows that at ph 4 and 9, copper oxidizes to form Cu(H 2 NCH 2 COO) +, and Cu(H 2 NCH 2 COO) 2, respectively. These are soluble species that would not be expected to provide passivation. However, CuO starts forming at about ph 12, and may be responsible for the passivation observed at this ph. Polishing, either with a pad or with a pad and abrasives, resulted in marginal increases in current density at all potentials for ph 4 and 9, attributed to increased mass transport, whereas polishing eliminated the passivation seen at ph 12 in Figure 1 [15]. In contrast, at both ph 4 and 9, both copper dissolution rates and polishing rates (measured in weight loss experiments using copper coupons) showed a maximum when plotted as a function of hydrogen peroxide concentration [15]. Figure 3 shows the behavior for ph 4; that at ph 9 was very similar. To compare directly the dissolution and polishing rates observed in the presence of H 2 O 2 with the electrochemical behavior measured in the absence of H 2 O 2, the open circuit potential of copper was measured at each H 2 O 2 concentration, at both ph 4 and 9. These potential data were then fitted to a Nernst-type behavior, and the rates for these concentrations were converted to equivalent current densities using Faraday s Law. Figures 4 and 5 compare the equivalent polarization curves obtained in this way for copper in the presence of hydrogen peroxide with the electrochemical polarization curves obtained in the absence of hydrogen peroxide, at ph 4 and 9, respectively. Whereas active oxidative dissolution occurred in the absence of H 2 O 2, chemical oxidation with H 2 O 2 clearly induced passivation at both ph 4 and 9. It is important to note that this passivation behavior appeared to be rapid in onset; it was observed during polishing, as well as during dissolution. This is different from the passivation observed during electrochemical polarization, which disappeared during polishing [15]. Clearly, then, while electrochemical polarization tests are extremely convenient for characterizing the rate of copper dissolution under different chemical conditions, they have limitations for providing the information needed for realistic models of CMP. Accordingly, more detailed work is reported here, aimed at examining the passivation behavior exhibited by copper in solutions containing both glycine and hydrogen peroxide.

3 18 E mv vs. SHE ph 4 ph 9 ph i, A/m 2 Figure 1. Effect of ph on the polarization behavior of copper in.1 M glycine at a scan rate of 2 mv/sec and 2 rpm [12]. E, V vs. SHE Cu 2+ CuL + Cu CuL 2 CuO Cu 2 O CuO ph Figure 2. Potential-pH diagram for the copper-water-glycine system, {Cu T } = 1-5, {glycine(l) T } =.1 [12]. Removal Rate, nm/min Dissolution Rate Polish Rate H 2 O 2, wt% Figure 3. Effect of H 2 O 2 concentration on copper dissolution and polish rates (at 27.6 kpa, 2 rpm) in aqueous.1 glycine at ph 4 [14].

4 Polarization in the absence of H 2 O 2 Dissolution in the presence of H 2 O 2 Polishing in the presence of H 2 O Polarization in the absence of H 2 O 2 Dissolution in the presence of H 2 O 2 Polishing in the presence of H 2 O E, mv vs. SHE E, mv vs. SHE i, A/m 2 i, A/m 2 Figure 4. Equivalent polarization curves for copper dissolution and polishing in aqueous.1 M glycine solutions containing different amounts of H 2 O 2 at ph 4 [15]. Figure 5. Equivalent polarization curves for copper dissolution and polishing in aqueous.1 M glycine solutions containing different amounts of H 2 O 2 at ph 9 [15]. Experimental Procedure The oxidation behavior of copper was studied in glass beakers containing 2 ml of.1 M aqueous glycine solutions, at various ph s, containing a wide range of H 2 O 2 concentrations. The solutions at ph 3 and 4 were buffered with acetic acid/sodium acetate, and the solution at ph 9 was buffered with carbonate. An unbuffered.1 M glycine solution (ph ~4.5) was also used. Copper coupons (23 x 23 x 1 mm, Aldrich Chemical Co., > %), with a single 5 mm circular hole, were first mechanically ground on 6 grit pads, and polished to a.25 µm finish. The coupons were then washed with double distilled water, then acetone. The coupons were weighed, suspended by a platinum wire through the hole, and immersed in the test solution for a predetermined time interval, while stirring magnetically. Small solution samples were withdrawn periodically during each dissolution test, and analyzed for copper using either flame atomic absorption spectrophotometry, or by UV-visible spectroscopy. In addition, at the end of each test, the copper coupons were repeatedly washed with double distilled water and reweighed. An equivalent dissolution rate was then calculated from the weight loss and the duration of each test. It should be noted that weight losses would not correlate exactly with the solubilization rate of copper if thick films of oxide were forming on the copper surface. Results and Discussion Figure 6 (a) shows the weight loss of copper coupons exposed to 2 ml of aqueous glycine solutions at different ph s for 5 minutes, as a function of hydrogen peroxide concentrations. Figure 6 (b) shows the copper dissolved into solution during the same tests. For

5 comparison, each figure shows the data converted to a copper removal rate in nm/min, assuming no uptake of mass. The weight loss, copper solubilization and removal rates were clearly significantly faster at ph 3 and 4 than in the unbuffered solution, or at ph 9. The discrepancy between the data obtained in the acetate-buffered ph 4 solution and the unbuffered solution is particularly marked, given that the ph of the unbuffered solution, around 4.5, was exceedingly close. This suggests that the acetate buffer used at ph 3 and 4 contributed actively to the dissolution of copper. This may be through catalyzing the decomposition of peroxide to form hydroxyl radicals. Closer inspection reveals that while the two figures are fairly consistent at ph 9 and for the unbuffered solution, at the lower ph s the weight loss measurements suggested a significantly lower copper removal rate than did the copper solubilization measurements. This suggests that surface films formed at low ph contain more oxygen/water/glycine than films formed at the higher ph s. In view of the likelihood of film formation, it is possible that the kinetics of oxidation would change as such films formed. This was investigated, because accurate knowledge of the kinetic behavior is essential for any modeling effort. To maximize accuracy of weight loss measurements, the experiments were of longer duration than is realistic for CMP. Figures 7 through 1 show the time dependence for ph 3, 4, 4.5 (unbuffered) and 9, respectively. Each figure shows the dissolution at different sampling times, as a function of H 2 O 2 concentration, and the dissolution for different H 2 O 2 concentrations, as a function of sampling time. The general trend in all these figures is for the sensitivity of the dissolution to the hydrogen peroxide concentration to increase with increasing sampling times. In the acetate-buffered solutions at ph 3 and 4, there was a maximum in the amount of dissolution at hydrogen peroxide concentrations slightly below 1% (Figures 7 (a) and 8 (a)). In contrast, the unbuffered solutions and the ph 9 solutions showed a steady decline in dissolution with increasing hydrogen peroxide concentrations, over the range studied (Figures 9 (a) and 1 (a)). It is particularly elucidating to examine the same data plotted as a function of time. At ph 3, 4, and 4.5 (unbuffered) (Figures 7 (b), 8 (b) and 9(b)), the dissolution rates are fairly constant at low concentrations of hydrogen peroxide. At higher peroxide concentrations, however, the rates clearly decrease with increasing contact time. The non-linear kinetics are characteristic of the formation of a protective film, and its progressive thickening. Hence it appears that there is little impediment to dissolution at low hydrogen peroxide concentrations, but that protective films form with increasing peroxide concentrations. At ph 9, the kinetics were strongly non-linear at all hydrogen peroxide concentrations used, suggesting that protective films form more readily at the higher ph. Although the difference between weight loss and copper dissolution at ph 9 is significantly lower than that at ph 3 and 4, indicating a relatively low uptake of oxygen, water or glycine, it is evident that the film that does form is much more protective. Further evidence for a difference between the films formed in the presence of acetate or carbonate buffers comes from the fact that although the potentials generated by different hydrogen peroxide concentrations were strongly consistent with Nernstian behavior at both ph s, the gradients of the Nernst plots differed significantly (59 mv/decade [H 2 O 2 ] at ph 4, and 166 mv/decade [H 2 O 2 ] at ph 9). These different gradients are presumed to reflect differences in the polarization curves of the dominant anodic and cathodic reactions at ph 4 and 9. These differences have been discussed in detail elsewhere [16]. In summary, in the presence of hydrogen peroxide, the dominant cathodic

6 ph 3, acetate buffered ph 4, acetate buffered no buffer ph 9,carbonate buffered ph3, acetate buffered ph4, acetate buffered no buffer ph9, carbonate buffered concentration of copper ion, mg/l Weight loss copper ion concentration. mg/l H 2 O 2 Concentration, wt% 5 Removal rate, copper ion concentration, mg/l H2O2 concentration, wt% Figure 6. (a) Weight loss and corrosion rate; (b) Copper solubilization and corrosion rate of copper coupons exposed to 2 ml of.1 M aqueous glycine solutions for 5 minutes H2O2 concentration, wt% 5 minutes 4 minutes 3 minutes 2 minutes 1 minutes Figure 7. Dissolution from copper coupons exposed to 2 ml of.1 M aqueous glycine solutions at ph 3. (a) Effect of [H 2 O 2 ], different sampling times. (b) Effect of sampling times, different [H 2 O 2 ] H2O2 concentration, wt% 5 minutes 4 minutes 3 minutes 2 minutes 1 minutes Figure 8. Dissolution from copper coupons exposed to 2 ml of.1 M aqueous glycine solutions at ph 4. (a) Effect of [H 2 O 2 ], different sampling times. (b) Effect of sampling times, different [H 2 O 2 ]. copper ion concentration, mg/l copper ion concentration, mg/l wt% H2O2.75 wt% H2O2 1 wt% H2O2 3 wt% H2O2 5 wt% H2O sampling time, minutes.5 wt% H2O2.75 wt% H2O2 1 wt% H2O2 3 wt% H2O2 5 wt% H2O2 sampling time, minutes 1 Removal Rate, nm/min

7 minutes 4 minutes 3 minutes 2 minutes 1 minutes wt% H2O2.75 wt% H 2 O 2 1 wt% H 2 O 2 3 wt% H 2 O 2 5 wt%h2o concent H 2 O 2 concentration, wt% 5 Dissolved Cop copper ion concentration, mg/l sampling time, minutes Figure 9. Dissolution from copper coupons exposed to 2 ml of unbuffered.1 M aqueous glycine solutions (ph about 4.5). (a) Effect of [H 2 O 2 ], different sampling times. (b) Effect of sampling times, different [H 2 O 2 ] minutes 4 minutes 3 minutes 2 minutes 1 minutes wt% H 2 O 2.75 wt% H 2 O 2 1 wt% H 2 O 2 3 wt% H 2 O 2 5 wt% H 2 O Copper ion concentration, mg/l H 2 O 2 concentration, wt% 5 copper ion concentration, mg/l sampling time, minutes Figure 1. Dissolution from copper coupons exposed to 2 ml of.1 M aqueous glycine solutions at ph 9. (a) Effect of [H 2 O 2 ], different sampling times. (b) Effect of sampling times, different [H 2 O 2 ]. reaction would be reduction of hydrogen peroxide. There would be competing anodic reactions, namely the oxidation of hydrogen peroxide to oxygen and water, and the anodic oxidation of copper. The open circuit potential is a mixed potential, dependent upon the exchange current density and polarization behavior of both the anodic and cathodic reactions, and cannot be predicted a priori. The kinetics of the anodic oxidation of hydrogen peroxide, which evolve oxygen, are likely to be sensitive to the substrate, because of the need to nucleate gas bubbles. The resulting differences in the Nernst gradient, then, provide further evidence of fundamental differences in the nature of the surface films formed at ph 4 and ph 9. Several other researchers have also reported decreases in the dissolution and polish rates of copper above a threshold H 2 O 2 concentration in slurries containing complexing agents [9,11,16]. Hernandez et al. interpreted the XPS spectra of copper surfaces exposed to 7.5 % 2 5

8 H 2 O 2 and phthalic acid at ph 3 to 5 as demonstrating the presence of Cu 2 O, which initially increased steadily in quantity with increasing H 2 O 2 concentration, then leveled off, suggesting passivation of the surface [16]. If Cu 2 O is, indeed, responsible for the passivation seen in the presence of glycine, consideration of Figure 2, where Cu 2 O appears between ph 11 and 15, immediately above the stability region of metallic copper, demonstrates that the local potential and ph at the copper-solution interface must be significantly different from those recorded in the bulk solution. In solutions containing glycine, it is also possible that glycine is adsorbing onto copper surfaces, thereby inhibiting oxidative dissolution of the copper. Although glycine could conceivably undergo condensation polymerization upon the copper surface, forming peptide links, the strong propensity of glycine to complex copper makes such a process improbably. Moreover, the kinetics of copper dissolution are more consistent with the formation of a film that gradually thickens, rather with blockage of active sites by an inhibiting agent. From a practical viewpoint, regardless of the mechanism for passivation of copper in acidic solutions containing hydrogen peroxide and glycine, this phenomenon is desirable, and hence extremely fortuitous. Passivation, or at least inhibition of active etching, is essential if a device surface is to be planarized. When polarizing electrochemically, passivation is only seen in glycine-based slurries at ph 12, where significant dissolution and erosion of SiO 2 would be expected, giving poor selectivity. However, if H 2 O 2 is used as an oxidizing agent, passivation will occur at lower ph, where the polish selectivity for copper over SiO 2 is much more attractive. Consequently, it appears that one can simultaneously achieve good selectivity and good planarization, using a glycine slurry with H 2 O 2 at mildly acidic and neutral ph s. Glycine-based slurries seem to offer a significant advantage over the phthalic acid-based slurries studied by Hernandez et al. in that passivation was more dramatic, and occurred at lower H 2 O 2 concentrations. At ph 3.89, the latter authors observed a maximum etch rate at 2 vol% H 2 O 2, then a drop in etch rate at 5 vol% to about 2% of its maximum value [16]. The polish rates were much higher, and showed a maximum at about 5 vol% H 2 O 2, with only a modest (about 5%) decrease in removal rate at 7.5 vol%, for a slurry at ph 3.9 [16]. In contrast, glycinecontaining slurries had maxima somewhat below 1 wt % H 2 O 2. The reason for passivation being more readily attainable in slurries containing glycine than in those containing phthalic acid is thought to be due to the strong catalytic activity of Cu(II)-glycine complexes in decomposing H 2 O 2 to yield hydroxyl radicals. Various metal ions catalyze the decomposition of H 2 O 2 to HO radicals [17]. However, Hariharaputhiran et al. demonstrated the rate of formation of HO was significantly faster in the presence of both Cu(II) and glycine than in the presence of either Cu(II) or glycine alone [18]. The standard reduction potential of HO, 2.8 V, is considerably higher than that of H 2 O 2, 1.76 V [17]. Hence, one could expect that lower concentrations of H 2 O 2 induce passivation in the copper-glycine-water system than in the copper-phthalic acidwater system, because of the comparatively higher concentration of HO radicals. However, it should be noted that this appears to be inconsistent with passivation by Cu 2 O, since this forms at low potentials, and does not contain Cu(II). It might be noted that, using 1 wt% glycine, Hariharaputhiran et al. observed significantly faster polishing rates with 5 wt % H 2 O 2 than were observed at the same H 2 O 2 concentration, using.1 M glycine in this work. This suggests that modest concentrations of glycine are also more effective in achieving passivation.

9 Conclusions Copper undergoes passivation in aqueous solutions containing glycine, when hydrogen peroxide is used as an oxidant, even under acidic conditions where no solid oxidized phases appear on the potential-ph diagram. This passivation behavior is desirable for effective CMP. In contrast, passivation is not seen in slurries of similar ph and complexing agent concentration, where the potential is increased electrochemically. As part of an effort to model the effects of chemistry on CMP rates, detailed kinetic studies of passivation were made. The acetate buffer that has been used to maintain low ph s appears to greatly accelerate dissolution of copper, but does not appear to be involved in passivation. The dissolution kinetics are consistent with the formation of a protective copper film that gradually thickens, rather than adsorption of an inhibiting species from the glycine solutions containing hydrogen peroxide. It is recognized that the experiments used to measure the kinetics of film formation were of longer duration than typical CMP operations. In order to measure these kinetics more accurately at time-scales more representative of commercial CMP operations, experiments are now starting, using an electrochemical quartz crystal microbalance, capable of detecting extremely small weight changes. It is anticipated that the results of this work will assist us in developing models relevant to the industry. Acknowledgements This work was funded by the University of California Discovery Grant through the Small Feature Reproducibility Project. References 1. P. Singer, Semiconductor International, 6, 91 (1998). 2. C. Lingk and M. E. Gross, J. Appl. Physics, 84, 5547 (1998). 3. R. Carpio, J. Farkas and R. Jairath, Thin Solid Films, 266, 238 (1995). 4. K. Osseo-Asare and K. K. Mishra, J. Electronic Materials, 25, 1599 (1996). 5. Q. Luo, D. R. Campbell, and S. V. Babu Thin Solid Films, 311, 177 (1997). 6. J. M. Steigerwald, S. P. Murarka, and R. J. Gutmann, Chemical Mechanical Planarization of Microelectronic Materials, (New York, NY: John Wiley & Sons 1997), J. W. Carr, L. D. David, W. L. Guthrie, L. William, F. B. Kaufmann, W. J. Patrick, K. P. Rodbell, R. W. Pasco, and A. Nenadic, US Patent No. 4,954,142 (199). 8. H. Hirabayashi, M. Huguchi, M. Kinoshita, H. Kaneko, N. Hayasaska, K. Mase, and J. Oshima, US Patent No. 5,575,885 (1996). 9. H. Hirabayashi, M. Kinoshita, H. Kaneko, N. Hayasaska, M. Huguchiet, K. Mase, and J. Oshima, in Proceedings of 1 st International CMP for VLSI/ULSI Multilevel

10 Interconnection Conference (CMP-MIC), Institute for Microoelectronics Interconnection (IMIC), p. 119 (1996). 1. S. V. Babu, Y. Li, M. Hariharaputhiran, S. Ramarajan, J. Zhang, Y-S. Her, and J. E. Prendergast, in Proceedings of 3 rd International CMP for ULSI Multilevel Interconnection Conference (CMP-MIC), Institute for Microoelectronics Interconnection (IMIC), p. 443 (1998). 11. M. Hariharaputhiran, S. Ramarajan, S. V. Babu, in Chemical-Mechanical Polishing Fundamentals and Challenges, S. V. Babu, S. Danyluk, M. Krishnan, and M. Tsujimura, Editors, PV 566, p. 129, The MRS Symposium Proceedings, Warrendale, PA (1999). 12. S. Aksu and F.M. Doyle, Electrochemistry of copper in aqueous glycine solutions, Journal of the Electrochemical Society, 148 (1) (21) pp. B51-B S. Aksu and F.M. Doyle, The role of glycine in the chemical mechanical planarization (CMP) of copper, Journal of the Electrochemical Society, 149, (6) (22) G352-G S. Aksu and F.M. Doyle, Electrochemistry of Copper in Aqueous Ethylenediamine Solutions, Journal of the Electrochemical Society, 149 (7) (22) B34-B S. Aksu and F.M. Doyle, Electrochemistry of Copper in the Chemical Mechanical Planarization (CMP) Slurries Containing Glycine and Hydrogen Peroxide, in Chemical Mechanical Polishing V, Ed. S. Seal, The Electrochemical Society, Pennington, New Jersey, PV-22-1 (22) pp J. Hernandez, P. Wrschka, and G. S. Oehlein, J. Electrochem. Soc., 148, G389 (21).