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Chem. Mater. 2005, 17, 2076-2080
Amino Acids as Complexing Agents in Chemical-Mechanical Planarization of Copper Venkata R. K. Gorantla,†,§ Egon Matijevic´,*,‡,§ and S. V. Babu†,§ Center for AdVanced Materials Processing, Department of Chemical Engineering, and Department of Chemistry, Clarkson UniVersity, Potsdam, New York 13699 ReceiVed September 6, 2004. ReVised Manuscript ReceiVed January 3, 2005
This study explains the relative importance of the *OH radical in the presence of other additives in hydrogen peroxide based slurries during chemical-mechanical planarization of copper. For this purpose, we investigated the effects of three different amino acids, that is, glycine, serine, and cysteine, in the presence of H2O2 on the dissolution of copper coupons and on the polishing of copper disks with welldispersed slurries of fumed silica particles over the pH range 2.0 and 8.0. While these amino acids produced different amounts of *OH, the polish rates did not depend on the concentration of this radical alone but were influenced by other parameters such as the pH of the slurries and the specific interactions of the amino acids with other components of the slurry.
Introduction Copper has emerged as the metal of choice for interconnect wiring at 0.13 µm and lower technology nodes, because of its low electrical resistivity and better electromigration resistance as compared to aluminum.1 Chemical-mechanical planarization (CMP) has become an essential enabling process step that makes the integration of copper as interconnect metal feasible.2 This process not only removes the excess copper deposited over the interconnect lines but also generates a “globally” planarized surface at the die and wafer level, which is critical for multilevel metallization. Copper CMP is a complex process involving both chemical and mechanical components3-5 and is usually performed in two or more steps. The bulk of the excess copper is removed first by a slurry at a fast removal rate and at a high copper to barrier material (Ta/TaN) removal rate selectivity. The residual copper and the barrier layer are later planarized by a nonselective slurry. The slurries used in the CMP of Cu consist of submicrometer or nanosized abrasive particles, such as alumina,6,7 silica,8,9 mixed abrasives,10 coated polymer * To whom correspondence should be addressed. E-mail: matiegon@ clarkson.edu. † Department of Chemical Engineering. ‡ Department of Chemistry. § Center for Advanced Materials Processing.
(1) Peter, S. Semicond. Int. 1998, June, 91. (2) Hu, C. K.; Luther, B.; Kaufman, F. B.; Humnel, J.; Uzoh, C.; Pearson, D. J. Thin Solid Films 1995, 262, 84. (3) In Chemical Mechanical Polishing: Fundamentals and Challenges, Materials Research Society Symposium Proceedings, Warrendale, PA, 1999; Babu, S. V., Danyluk, S., Krishnan, M., Tsujimura, M., Eds.; Elsevier: New York, 1999; Vol. 566. (4) In Chemical-Mechanical Polishing 2000: Fundamentals and Materials Issues, Materials Research Society Symposium Proceedings, Warrendale, PA, 2000; Singh, R. K., Bajaj, R., Moinpour, M., Meuris, M., Eds.; Elsevier: New York, 2000; Vol. 613. (5) In Chemical-Mechanical Planarization, Materials Research Society Symposium Proceedings, Warrendale, PA, 2002; Babu, S. V., Singh, R. K., Hayasaka, N., Oliver, M., Eds.; Elsevier: New York, 2002; Vol. 732E. (6) Luo, Q.; Campbell, D. R.; Babu, S. V. Langmuir 1996, 12, 3563. (7) Hernandez, J.; Wrschka, P.; Oehrlein, G. S. J. Electrochem. Soc. 2001, 148, G389.
particles,11 and so forth, dispersed in deionized water containing additives, including oxidizers, complexing and buffering agents,12 corrosion inhibitors,13 and surfactants.14 Some of these chemicals modify the copper surface layer, which is then abraded by the particles in the slurry during polishing. For example, amino acids,15 particularly glycine,16 interact with copper in aqueous solutions and affect the dissolution rate of the metal. More recently, a rather extensive electrochemical study has been carried out on copper/glycine complexes.17 Thus, the chemical reactions (passivation and etching of copper metal) and the mechanical action (abrasion) of the slurry act synergistically in enhancing and controlling the material removal rates. Ferric nitrate13 and ammonium compounds,18,19 which dissolve copper, and several different oxidizing agents, such as KIO320,21 or H2O2,22 have been investigated in the CMP (8) Kondo, S.; Sakuma, N.; Homma, Y.; Ohashi, N. In Electrochemical Society Proceedings, 1998; Vol. 98-6, 195. (9) Li, Y.; Hariharaputhiran, M.; Babu, S. V. J. Mater. Res. 2001, 16, 1066. (10) Jindal, A.; Hegde, S.; Babu, S. V. Electrochem. Solid-State Lett. 2002, 5, G48. (11) Kawahasi, N.; Hattori, M. In Chemical-Mechanical Polishing- AdVances and Future Challenges, Materials Research Society Symposium Proceedings, Warrendale, PA, 2001; Babu, S. V., Cadien, K. C., Yano, H., Eds.; Elsevier: New York, 2001; Vol. 671, M2.2.1. (12) Caprio, R.; Farkas, J.; Jairath, R. Thin Solid Films 1995, 266, 238. (13) Luo, Q.; Campbell, D. R.; Babu, S. V. Thin Solid Films 1997, 311, 177. (14) Basim, G. B.; Vakarelski, I. U.; Moudgil, B. M. J. Colloid Interface Sci. 2003, 263, 506. (15) Halpern, J.; Milant, H.; Wiles, D. R. J. Electrochem. Soc. 1959, 106, 647. (16) Halpern, J. J. Electrochem. Soc. 1953, 100, 421. (17) Aksu, S.; Doyle, F. M. J. Electrochem. Soc. 2001, 148, 1354. (18) Steigerwald, J. M.; Murarka, S. P.; Gutmann, R. J. Chemical Mechanical Planarization of Microelectronic Materials; John Wiley & Sons: New York, 1996. (19) Luo, Q.; Mackay, R. A.; Babu, S. V. Chem. Mater. 1997, 9, 2101. (20) Li, Y.; Babu, S. V. Electrochem. Solid-State Lett. 2001, 4, G20. (21) Lee, S.-M.; Mahajan, U.; Chen, Z.; Singh R. K. In ChemicalMechanical Polishing 2000- Fundamentals and Material Issues, Materials Research Society Symposium Proceedings, Warrendale, PA, 2000; Singh, R. K., Bajaj, R., Meuris, M., Moinpour, M., Maury, A., Eds.; Elsevier: New York, 2000; Vol. 613, E7.8.1.
10.1021/cm048478f CCC: $30.25 © 2005 American Chemical Society Published on Web 03/25/2005
Amino Acids as Complexing Agents
of copper. The addition of complexing agents, including glycine,23 citric acid,8 and so forth, to the H2O2 based slurry enhances the copper removal rates. Although significant information is available on the effects of oxidizing agents,20-22,24 the understanding of the role of these complexing agents needs further investigation to optimize the efficiency of the slurry. Hariharaputhiran et al.25 proposed that the decomposition of H2O2 to hydroxyl radicals (*OH), which are more powerful oxidizing agents than H2O2, is catalyzed by the Cu2+-amino acid complexes. Thus, the increase in the copper polish rates with the addition of Cu2+ ions to the glycineH2O2 based slurry has been attributed to the increased generation of these hydroxyl radicals. However, addition of Cu2+ ions to glycine-peroxide systems caused a decrease in the pH of the system,25 which also has a strong influence on the copper removal rates.26,27 Hence, it will be useful to identify, if only one or both parameters (pH drop and higher *OH concentration) cause increased Cu dissolution rates, as well as if the additional processes in the system affect the efficiency of planarization. Hydroxyl radical trapping experiments, performed with different amino acids as complexing agents in aqueous H2O2 solutions at pH ∼8.0 (Table 1), showed that serine and cysteine generated higher concentrations of the OH* radical than glycine. According to the suggested mechanism, one would expect that these amino acids should yield higher copper removal rates than glycine, which did not occur. Obviously, other parameters must have significant influence on the Cu removal rates in these amino acid-H2O2 based slurries, as discussed in this study. Experimental Section Materials. All chemicals were of reagent grade and were not further purified. The structures of the amino acids used in this study are given below.
(22) Zeidler, D.; Stavreva, Z.; Plotner, M.; Drescher, K. Microelectron. Eng. 1997, 33, 259. (23) Hirabayashi, H.; Higuchi, M.; Kinoshita, M.; Kaneko, H.; Hayasaka, N.; Mase, K.; Oshima, J. In Proceedings of CMP-MIC Conference, Santa Clara, CA, 1996; p 119. (24) Du, T.; Tamboli, D.; Desai, V.; Seal, S. J. Electrochem. Soc. 2004, 151, G230. (25) Hariharaputhiran, M., Zhang, J.; Ramarajan, S.; Keleher, J. J.; Li, Y.; Babu, S. V. J. Electrochem. Soc. 2000, 147, 3820. (26) Jindal, A.; Li, Y.; Babu, S. V. In Chemical-Mechanical PolishingAdVances and Future Challenges, Materials Research Society Symposium Proceedings, Warrendale, PA, 2001; Babu, S. V., Cadien, K. C., Yano, H., Eds.; Elsevier: New York, 2001; Vol. 671, M6.8.1. (27) Jindal, A.; Babu, S. V. J. Electrochem. Soc. 2004, 151, in press.
Chem. Mater., Vol. 17, No. 8, 2005 2077 Table 1. Concentration of Hydroxyl Radicals (*OH) in Aqueous Solutions Containing Cu2+ Ions and H2O2 at pH ∼8.0a amino acid present in solutionb
[*OH] × 1014 (mol dm-3)
no amino acid glycine serine cysteine
0.32 1.84 2.00 3.40
a Para-nitroso dimethyl aniline (PNDA) was used as the *OH trapping agent. (Data from ref 22). b 2 wt % H2O2 + 0.05 mmol dm-3 copper acetate + PNDA + 0.013 mol dm-3 amino acid at 21 °C.
Figure 1. Weight loss per unit area of copper coupon in aqueous solutions of 1 wt % glycine and 5 wt % H2O2 at pH 4.0 (circles) and pH 8.0 (squares), as a function of time.
Copper Dissolution Rates. Copper dissolution experiments were carried out by immersing a square (23 × 23 × 1 mm) copper coupon (99.99% pure) in 500 cm3 of aqueous solutions of various chemical compositions, contained in 600 cm3 glass beakers. Before each run, the coupon was rubbed with 1500 grit sandpaper, washed with dilute HCl, followed by DI water, and finally air-dried and weighed. The coupon was then dipped into the solution, stirred mechanically at 1000 rpm for desired periods of time, after which it was washed with DI water, dried in an air stream, and reweighed. The dissolved amounts of copper were determined from the weight loss of the coupon after given reaction times. Blank experiments showed that no change in the pH was detected under the same conditions with coupons kept in water only. Figure 1 displays the results using aqueous solutions containing 1 wt % glycine + 5 wt % H2O2 at pH 4.0 and 8.0, respectively. The pH was adjusted either by HClO4 or KOH, as needed. Each point is the average of four repeated experiments, always with fresh solutions. It is clear that the weight loss was approximately linear with time up to 5 min, a relation which may not hold over longer durations.28 However, the usual polish time in the planarization process is typically ∼3 min and, therefore, all results to follow will be compared for reactions at 3 min. Chemical-Mechanical Polishing of Copper Disks. Copper disks (99.99% pure), 3-mm thick with a cross-sectional area of 8.0 cm2, were polished on a Struers DAP-V benchtop polisher. The disk holder was held stationary with a downforce of 6.3 psi (41.4 kN/m2) and the platen was rotated at 90 rpm during polishing. An IC1400 k-groove pad was hand conditioned with a 220 grit sandpaper prior to each experiment. The slurry was made of fumed silica (Aerosil-130, Degussa Corp.) and was pumped onto the pad at 60 mL/min, while continuously stirred throughout the experiment. The polish rates were determined from the weight loss of the disk (28) Wang, L.; Doyle, F. M. In Chemical-Mechanical Planarization, Materials Research Society Symposium Proceedings, Warrendale, PA, 2003; Boning, D. S., Devriendt, K., Oliver, M. R., Stein, D. J., Vos, I., Eds.; Elsevier: New York, 2003; Vol. 767, F6.5.1.
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Table 2. Copper Dissolution Rates in Glycine-Peroxide and Serine-Peroxide Systems with Admixed Cu2+ Ions (Column 3) and without Cu2+ Ions (Column 4) at the Same pH Values (as in Column 2)a solution composition
the pH of solutions in column 1
Cu dissolution rates in solutions of column 1
Cu dissolution rates in 1 wt % glycine + 5 wt % H2O2 solutions without Cu(NO3)2
1 wt % glycine + 5 wt % H2O2 + 0.06 wt % Cu(NO3)2 1 wt % glycine + 5 wt % H2O2 + 0.125 wt % Cu(NO3)2 1 wt % glycine + 5 wt % H2O2 + 1.0 wt % Cu(NO3)2 1 wt % serine + 5 wt % H2O2 + 0.06 wt % Cu(NO3)2 1 wt % serine + 5 wt % H2O2 + 0.125 wt % Cu(NO3)2
3.7 3.4 2.6 3.4 3.1
224 ( 3 mm/min 243 ( 7 365 ( 9 290 ( 10 360 ( 5
203 ( 3 (mm/min) 247 ( 3 340 ( 42 280 ( 15 330 ( 10
a
All data are in nm/min. Table 3. Copper Dissolution and Polish Rates in Glycine, Serine, and Cysteine Systems Containing H2O2 at pH ) 8.0
slurry composition 0.13 mol dm-3 glycine + 5 wt % H2O2 0.13 mol dm-3 serine + 5 wt % H2O2 0.13 mol dm-3 cysteine + 5 wt % H2O2
Figure 2. Copper dissolution rates in aqueous solutions containing 1 wt % glycine, 5 wt % H2O2, and different concentrations of Cu(NO3)2 at pH 4.0 and 8.0.
polished for 3 min, and the reported data were averages of four experiments.
Results Effects of Cu2+ Ions and pH on Copper Dissolution Rates. Chemical-mechanical planarization of copper can be viewed as a synergistic process involving mechanical abrasion and chemical dissolution. At higher dissolution rates, copper may be removed from both the protruding as well as the recessed regions, thereby resulting in poor planarization. The slurries are typically acidic and the understanding of the effects of various chemical additives on copper dissolution rates at these conditions is essential if their planarization efficiency is to be improved. This section describes the results of copper dissolution experiments in glycine- and serineH2O2 systems, carried out to establish if only one or both parameters (decrease in the pH and increase in the *OH concentration) cause higher Cu dissolution rates. Table 2 compares the copper dissolution data in glycineand serine-peroxide systems with and without admixed Cu2+ ions, but at the same pH at three different values. These results suggest that the pH is the dominant factor under acidic conditions, while the addition of Cu2+ had no significant effect. Figure 2 displays dissolution data for copper in the glycine-H2O2 system containing different concentrations of Cu2+ ions at pH values of 4.0 and 8.0. The result at the lower pH 4.0 is consistent with the above argument and was also confirmed in the serine-H2O2 system (data not presented here). In contrast, the addition of Cu2+ ions to 1 wt % glycine + 5 wt % H2O2 solution at pH 8.0 causes an enhancement in the dissolution of copper (Figure 2), which could be due to the increased *OH generation, according to Hariharaputhiran et al.25 Effect of Complexing Agents on the Copper Dissolution and Polish Rates. Since *OH radical concentrations could
dissolution polish rates rates (nm/min) (nm/min) (with 3 wt % (no abrasives) fumed silica abrasives) 280 ( 20 110 ( 10 ∼0
375 ( 15 240 ( 10 50 ( 10
be measured only at pH ∼ 8.0,22 the dissolution and polish rates of copper with glycine, serine, and cysteine at the same molar concentrations in H2O2 based systems were determined at this pH only. Table 3 shows that the order of the copper removal rates is opposite to the expectation, if *OH were the dominant factor (Table 1). The addition of H2O2 to cysteine resulted in the formation of a white precipitate, which also affected the stability of the slurry. It was found29,30 that in the presence of hydrogen peroxide, cysteine (HS-R, where R f - CH2CH(NH2)COOH) undergoes oxidation to yield cystine (RSSR), which is sparingly soluble in water (solubility ∼0.11 g/kg31), according to (H2O2)
2HS - R 98 RSSR + 2H + + 2e-
(1)
In this study, it was observed that the solids were formed over the pH range 2.6-8.0 and that the rate of precipitation increased with higher pH. During this reaction, H2O2 should reduce to H2O at acidic or to OH- ions at alkaline conditions. The dissolution and polish rates of copper with the three amino acids at different concentrations (0, 1, and 5 wt %) of H2O2 are shown in Figure 3, at the “natural” pH (not adjusted) values given in Table 4. Under these conditions, all systems are acidic. In serine- and glycine-peroxide systems, both processes follow a similar trend, which differs from that of the cysteine-peroxide system. The decrease in the removal rates of copper, observed when the H2O2 concentration was increased to 5 wt % in the first two systems, can be attributed to the higher passivity and hardness of the oxide layer formed on the copper surface7,22,24,27 (for simplicity, the term “removal rate” will be used to refer to both “dissolution rate” and “polish rate”, unless differently specified). In the cysteine-peroxide system, copper removal rates increased at the higher H2O2 concentration, suggesting a different mechanism. Under the same conditions, the decrease (29) Hill, J. W.; Coy, R. B.; Lewandowski, P. E. J. Chem. Ed. 1990, 67, 172. (30) Neville, R. G. J. Am. Chem. Soc. 1957, 79, 2456. (31) Lide, D. R. CRC Handbook of chemistry and physics, 81st ed.; CRC press: Boca Raton, FL, 2001.
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Chem. Mater., Vol. 17, No. 8, 2005 2079
However, a similar reaction may also take place at acidic conditions, as shown in eq 3: 4RSH + 2Cu2+ S 2RSCu(I) + RSSR + 4H+
(3)
The addition of a clear blue solution of 0.125 wt % Cu(NO3)2 to 1 wt % cysteine solution resulted in a white precipitate suspended in a pale green solution, while the pH of the system rapidly decreased from 4.7 to 2.5. Introducing a 1 wt % H2O2 solution to this dispersion caused a change in the color from pale green to blue with the white colored precipitate still dispersed in it and no significant shift in the pH. Hydrogen peroxide could have oxidized RSCu(I) complex to Cu2+ in a Fenton type reaction35 to produce *OH radicals and cystine, according to 2RSCu(I) + 4H2O2 f 2Cu2+ + RSSR + 4*OH + 4OH- (4)
Figure 3. (a) Copper coupon dissolution rates in different amino acidH2O2 systems with no abrasives, and (b) copper disk polish rates in the presence of 3 wt % fumed silica (Aerosil-130). Table 4. Measured “Natural” pH (Not Adjusted) of Solutions in Figure 3a amino acid (AA) glycine serine cysteine
1 wt % AA + 1 wt % 1 wt % AA + 1 wt % AA + 1 wt % H2O2 + AA in 1 wt % H2O2 5 wt % H2O2 0.125 wt % Cu(NO3)2 DI water in DI water in DI water in DI water 6.0 5.5 4.7
5.2 4.9 4.2
5.0 4.6 2.6
3.9 3.3 2.5
a
The addition of fumed silica abrasives to the systems did not change the pH significantly.
in the pH was much larger than with the other amino acids (Table 4). H2O2 oxidizes cystine (the white precipitate) to cysteic acid (RSO3H)32,33 in acidic conditions, according to RSSR + 3H2O2 f 2RSO3H + 4H+ + 4e-
(2)
Thus, an increase in the concentration of H2O2 oxidizes more of cysteine to cystine and then to cysteic acid, causing a decrease in the pH. Cysteic acid could be forming soluble complexes with oxidized Cu, resulting in higher Cu dissolution rates. Also, it was shown27 that a decrease in the pH at 5 wt % H2O2 enhanced copper dissolution. Thus, the observed effects at higher H2O2 levels in cysteine-H2O2 system (Figure 3) may be caused by both these changes in the slurry. Kolthoff and Stricks34 suggested that, in aqueous alkaline solutions containing copper and cysteine, Cu2+ reacts with cysteine to yield cystine and cuprous cysteinate (RSCu(I)). (32) Savige, W. E.; Maclaren, J. A. In Organic Sulphur Compounds; Karash, N., Meyers, C. Y., Eds.; Pergamon Press: Oxford, U.K., 1966; Vol. 2. (33) Katritzky, A. R.; Akhmedov, N. G.; Denisko, O. V. Magn. Reson. Chem. 2003, 41, 37. (34) Kolthoff, I. M.; Stricks, W. J. Am. Chem. Soc. 1951, 73, 1723.
Furthermore, H2O2 could have further oxidized cystine to cysteic acid (eq 2), because of the catalytic effect of Cu2+.33 The finding that the pH was essentially constant on addition of H2O2 could be expected, if both reactions 2 and 4 were taking place. The addition of Cu2+ ions has an insignificant effect on the copper polish rates in the cysteine-H2O2 system (Figure 3b), even though the dissolution rate increased somewhat (Figure 3a). Hence, in the cysteine-peroxide system, the removal rates of copper are influenced more by the pH and the complexation of oxidized Cu with cysteic acid than by the *OH radicals alone. Consequently, cysteine lacks the potential to be a complexing agent in Cu CMP slurries. Discussion The above results suggest that the copper dissolution rates are dependent not only on the concentration of *OH radicals but also on other factors, such as the chemical interactions between the additives in the slurry and on its pH. The order of the copper removal rates in glycine-, serine-, and cystine-H2O2 systems (Table 3) is in contrast to the expectation that the increase should be related to the concentration of *OH radicals (Table 1). When comparing the results with the three amino acids, one finds the concentration of *OH radicals generated in the Cu2+cysteine-peroxide system to be the highest, but the rate was the lowest. This effect is explained by the reactions of cysteine with Cu2+ and H2O2 (eqs 2-4), which resulted in the formation of chemical byproducts, like cystine and cysteic acid, which adversely affected the Cu removal. Furthermore, copper dissolution rates in the glycine-H2O2 and serineH2O2 systems at pH 8 (Table 4) correlated well with the magnitude of the respective critical stability constants with Cu2+ ions (Table 5). An increase in the dissolution and polish rates (Figure 3) by the addition of Cu2+ ions to the glycine-H2O2 systems was also reported by Hariharaputhiran et al.24 They attributed this increase to higher concentration of [*OH] radicals, (35) Eberhardt, K.; Ramirez, G.; Ayala, E. J. Org. Chem. 1989, 54, 5922.
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Table 5. Critical Stability Constants of Cu2+-Amino Acid Complexes34 amino acid
-log K for “Cu2+ + L- ) (CuL)+”
-log K for “Cu2+ + 2L- ) (CuL2)”
glycine serine cysteine
8.15 7.89 reduction of metal ion by ligand
15.03 14.48
generated by the addition of Cu2+ ions. However, this study has shown that this enhancement in acidic conditions is mainly due to the change of the pH of the slurry. The increase in the dissolution rates by the addition of Cu2+ in the glycine-H2O2 system at pH 8.0 (Figure 2), on the other hand, appears to be in agreement with the mechanism proposed by Hriharaputhiran et al.25 However, there could be another explanation of the observed effect as offered by Robbins and Drago,36 who suggest that the activation of peroxide involves the formation of copper(II)hydroperoxide complexes, which are stronger oxidizing agents than H2O2. They observed that the rate of oxidation by these complexes increases with the pH and is also dependent on the complexation. Hence, in addition to the (36) Robbins, M. H.; Drago, R. S. J. Catal. 1997, 170, 295. (37) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum: New York, 1974; Vol. 1.
generation of *OH, the addition of Cu2+ to glycine-peroxide system at pH 8.0 may affect the dissolution rates by the formation of Cu(II)-hydroperoxide complexes. Conclusions A comparison of the dissolution and polish rates of copper in glycine, serine, and cysteine systems containing H2O2 indicated that an increase in the generation of *OH radicals alone may not be responsible for the enhancement of copper dissolution rates. The specific interactions between Cu2+ and the complexing agents present in the slurry, as well as the pH, affect the removal rates of copper. For example, the chemical interactions of cysteine with H2O2 and Cu2+, which resulted in the formation of different byproducts, yielded lower copper removal rates despite the generation of highest concentration of *OH radicals among the three systems. Acknowledgment. The authors acknowledge the invaluable advice of Dr. Dan Goia and Mr. Zhenyu Lu. This work was partially supported by the Intel Corporation through the Semiconductor Research Corporation (SRC). The authors also acknowledge Degussa Corporation and Rodel Inc. for providing fumed silica abrasives and polishing pads, respectively. CM048478F
