Copper Dissolution Behavior in Acidic Iodate Solutions - Langmuir

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Copper Dissolution Behavior in Acidic Iodate Solutions Qiuliang Luo Rodel, Inc., 451 Bellevue Road, Newark, Delaware Received December 13, 1999 Copper dissolution was investigated using electrochemical techniques including potentiodynamic scans and electrochemical quartz crystal analysis. The adsorption of iodate on a copper surface depends on the solution pH and the concentration of iodate. IO3- behaves both as an oxidizer and a passivating agent for copper dissolution. At low IO3- concentrations, IO3- acts more like an oxidizer while it passivates the copper surface at higher concentrations. The copper dissolution rate decreases with increasing solution pH and increases with increasing concentration of potassium iodate up to 1 wt %. The dissolution rate does not change significantly when the potassium iodate concentration is >1 wt %. A strong adsorption of IO3- ions on a copper surface has been observed at higher solution pH’s and higher iodate concentrations. This may be ascribed to the formation of a thin film of Cu(IO3)2 due to the high surface Cu2+ concentration and higher concentration of IO3- adjacent to the copper surface.

Introduction Copper is becoming the interconnect material of choice in semiconductor manufacturing because of its low resistivity and high electromigration resistance.1 The current strategy for copper implementation is to use the dual-damascene technique, in which chemical-mechanical polishing (CMP) is used to remove the overburden materials.2 This overburden material comes from copper deposition used to fill the trenches and vias to fabricate conductive lines and plugs for wiring the devices. Chemical-mechanical polishing of copper becomes an enabling technology for copper implementation because a reliable dry etch technology is not available to date. In addition, the significant advantage of global planarization after CMP compared with traditional planarization techniques makes CMP the most suitable choice for planarization, as the feature size is shrinking to sub-quarter-micron with the future trend of shrinking more and more.3 Copper CMP has been intensively investigated in recent years both by academic institutions and by industrial technology leaders.3-10 It has been accepted that copper CMP can be facilitated by using a two-step process: the first CMP step is to remove the overburden copper and stop on the barrier layer such as tantalum or tantalum nitride while the second step is used to remove the barrier layer.3 The first-step copper CMP generally uses a slurry (1) Pai, P. L.; Ting, C. H. IEEE Electron Device Lett. 1989, 10, 423. (2) Lakshminarayanan, S.; Steigerwald, J. M.; Price, D. T.; Bourgeois, M.; Chow, T. P.; Gutmamn, R. J.; Murarka, S. P. IEEE Electron Device Lett. 1994, 15, 307. (3) Hu, C. K.; Luther, B.; Kaufman, F. B.; Humnel, J.; Uzoh, C.; Pearson, D. J. Thin Solid Films 1995, 262, 84. (4) Steigerwald, J. M.; Zirpoli; Murarka, S. P.; Price, D. T.; Gutmamn, R. J.; J. Electrochem. Soc. 1994, 141, 3512. (5) Steigerwald, J. M.; Duquette, D. J.; Murarka, S. P.; Gutmamn, R. J. J. Electrochem. Soc. 1995, 142, 2379. (6) Luo, Q.; Campbell, D. R.; Babu, S. V. Thin Solid Films 1997, 311, 177. (7) Luo, Q.; Ramarajan, S.; Babu, S. V. Thin Solid Films 1998, 335, 160. (8) Luo, Q.; Campbell, D. R.; Babu, S. V. Proceedings of the 1st International VMIC Specialty Conference on CMP Planarization, Santa Clara, CA, Feb, 1996; p 145. (9) Matsumoto, M.; Suzuki, K.; Sakamoto, T.; Kamisaka, A. Proceedings of the 4th International VMIC Specialty Conference on CMP Planarization, Santa Clara, CA, Feb, 1999; p 176. (10) Mahulikar, D.; Pasqualoni, A. Proceedings of the 4th International VMIC Specialty Conference on CMP Planarization, Santa Clara, CA, Feb, 1999; p 221.

pH in the neutral or acidic region to remove the overburden copper, and the second-step CMP is used to remove the barrier materials, most commonly tantalum and tantalum nitride. The slurry for the second step is usually at neutral or higher pH with colloidal silica as the abrasive because of its excellent surface finish on both copper and silicon dioxide dielectric layers. The first-step copper CMP slurry consists of abrasives, oxidizer(s), copper complexing agent(s), and passivator(s) (inhibitors). The abrasives are used to apply mechanical abrasion on the copper surface, the oxidizer is used to oxidize the copper surface to copper oxides, the complexing agent is used to enhance the copper removal rate during CMP, and the passivators are used to protect the copper from dishing.11 The oxidizers used may be hydrogen peroxide, potassium iodate, and so forth. The understanding of the copper dissolution behavior in the solutions containing hydrogen peroxide and potassium iodate is essential to reduce copper dishing during polishing and enhance the slurry polishing performance. Copper dissolution behavior has been reported in various fields ranging from copper corrosion protection to silicon wafer cleaning in the semiconductor industry; especially in silicon wafer cleaning, hydrogen peroxide and/or ozone has been used to oxidize the organics and metals to facilitate their removal.12-17 The copper dissolution behavior in the solutions containing hydrogen peroxide and ozone is well studied. However, the copper dissolution behavior in iodate solution has not been reported in the literature. In this paper, both potentiodynamic scans and electrochemical quartz crystal analyzer measurements were used to investigate the copper dissolution behavior in potassium iodate solutions. Solutions with pH 2.0 and pH 4.0 are investigated using various concentrations of potassium iodate to understand the (11) Luo, Q. Ph.D. Thesis, Clarkson University, July 1997. (12) Jope, D.; Sell, J.; Pickering, H. W.; Weil, K. G. J. Electrochem. Soc. 1995, 142, 2170. (13) Luo, Q.; Mackay, R. A.; Babu, S. V. Chem. Mater. 1997, 9, 2101. (14) Halper, J. J. Electrochem. Soc. 1953, 100, 421. (15) Fisher, J.; Halpern, J. J. Electrochem. Soc. 1956, 103, 282. (16) Morita, H.; Joo, A. D.; Messoussi, D.; Kawada, K.; Kim, J. S.; Ohmi, T. Proceedings of the 5th International Symposium on Cleaning Technology in Semiconductor Device Manufacturing, 1998; p 143. (17) Hattori, T. Proceedings of the 5th International Symposium on Cleaning Technology in Semiconductor Device Manufacturing, 1998; p 3.

10.1021/la991626+ CCC: $19.00 © 2000 American Chemical Society Published on Web 04/29/2000

Copper Dissolution Behavior

Figure 1. Copper electroplating on the platinum electrode surface. The slope of the deposition curve is the deposition rate on the platinum surface. The deposition rate is about 2000 Å/min.

copper dissolution and the adsorption of IO3- on the copper surface. Experimental Section 1. Preparation of Copper Film for Electrochemical Quartz Crystal Analyzer Measurements. The copper film was electroplated on the platinum electrode using an electroplating solution that contains 25 g of 95% ethanol, 25 g of concentrated sulfuric acid (98%), and 40 g of copper sulfate pentahydrate dissolved in 500 g of distilled (DI) water. All those chemicals were purchased from Aldrich Chemicals. The plating solution was transferred to the quartz crystal analyzer (QCA) cell using a 5-mL pipet. A constant current density of 0.01A/cm2 was used, and the electroplating time was 20 s in the QCA cell. The frequency shift response of copper deposition from the electroplating solution in the QCA is shown in Figure 1. The linear decrease of the frequency indicates the deposition of copper on the platinum electrode at a constant rate. The slope of the linear line indicates the copper deposition rate. The deposition rate was approximately 2000 Å/min on the basis of the current density. The thickness of deposited copper on the electrode for the electrochemical quartz crystal analyzer was about 700 Å for all the experiments. 2. Electrochemical Measurements. The iodate test solutions were prepared using the ACS grade potassium iodate purchased from Aldrich Chemicals. A 0.1 M K2SO4 solution was used for the electrochemical test in the absence of KIO3 to increase the conductivity of the solution. The solution pH was adjusted using diluted nitric acid and/or KOH solutions. The potentiodynamic scan was performed using an EG&G Princeton Applied Research Potentio/Galvanostat model 373. A corrosion software SoftCorr 352 III (also from EG&G Princeton Applied Research) was used to control the potential scan. The scan rate was 2 mV/s for all the experiments. The electrochemical cell was made in a 250 mL glass beaker, and a rotating disk electrode EG&G model 616 from EG&G Princeton Applied Research was used as the working electrode. The copper disk was purchased from EG&G Princeton Applied Research, and the purity of the copper was high (99.999%). A saturated calomel electrode was used as the reference electrode. The counter electrode was platinum and was placed far away from the working electrode to minimize the interference with the working electrode. The electrochemical quartz crystal analyzer model QCA 917 was provided by EG&G Princeton Applied Research associated with Seiko Corporation. The response measured was the frequency shift relative to a standard or reference frequency. This frequency shift reduces with deposition or adsorption and increases with dissolution or desorption. A QCA cell with a platinum electrode deposited with copper as the substrate (also from EG&G Princeton Applied Research) was used to test the dissolution of copper in this cell. The exposing area for the solution

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Figure 2. Effect of KIO3 concentration at pH 2.0 under static conditions. The scan rate used for these potentiodynamic scans is 2 mV/s. was 1 cm2 with a rubber seal to protect the test solution from leaking. A constant potential was applied to the platinum electrode covered with a copper film, electroplated as described above. The counter electrode was a platinum wire inserted in the cell (away from the working electrode as much as possible to avoid interference). The potential was varied from 0.6 to 1 V versus the Ag/AgCl reference electrode. An EG&G Princeton Applied Research Potenio/Galvanostat model 373 was used to control the applied potential on the electrode using the software SoftCorr 352 III. After each test, the platinum electrode was cleaned using electrodissolution using 0.01 A/cm2 for 60 s and washed with plenty of DI water and then dried with compressed lab air. After cleaning, this electrode was used for the next electroplating deposition for the next test.

Results and Discussion 1. Potentiodynamic Measurement. Figure 2 shows the effect of KIO3 concentration on the electrochemical behavior under static conditions. The solution pH was fixed at pH 2.0 for the tests conducted here. As shown in this figure, the current density increases significantly when the concentration of KIO3 is increased from 0% to 1 wt % both on anodic and cathodic branches. This is because IO3- is an oxidizer and easier to reduce because of its much higher concentration than that of dissolved oxygen. The current density does not increase further when the concentration of KIO3 increases from 1 to 2 wt %. The sharp increase in potential on the cathodic branch implies the reaction is controlled by the diffusion of the oxidizer IO3- from solution to the electrode surface. Since the solution pH is low (pH 2.0), copper oxides are not stable thermodynamically according to the Pourbaix diagram.19 Thus, the copper surface is either pure copper or copper with adsorbed IO3-, more likely the latter. When the IO3- concentration is low, the copper surface may be pure copper or the copper surface may be only partially covered with IO3-, the copper dissolution rate is controlled by the availability of IO3- to oxidize copper to copper oxides, and then these copper oxides dissolve in the pH 2.0 solution. When the IO3- concentration is high, the copper surface is fully covered with IO3- and a protection film of Cu(IO3)2 is formed; thus, the copper dissolution may be controlled by the diffusion of IO3- through the Cu(IO3)2 film to the surface, and the concentration effect may not (18) Carpio, R.; Farkas, J.; Jairath, R. Thin Solid Films 1995, 266, 238. (19) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solution; NACE: Houston, TX, 1974.

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Figure 3. Effect of KIO3 concentrations at pH 4.0 under both static and dynamic (5000 rpm of rotational rotating disk electrode) conditions.

be significant because of the slow diffusion of IO3- through the Cu(IO3)2 film. The effect of KIO3 concentration at pH 4.0 is revealed in Figure 3 under both static and dynamic conditions. It was found that the current density increases with the rotational speed of the disk electrode up to 5000 rpm, indicating the effect of mass transfer (including 0% KIO3, which is not shown in this figure). The current density does not increase further when the rotational speed of the disk electrode is further increased above 5000 rpm, implying that the electrochemical reaction on the electrode surface is controlled by the slow diffusion of IO3- to the copper electrode surface or of the reaction products through the Cu(IO3)2 film back to the solution. There is not a significant difference, as mentioned earlier, when the concentration of KIO3 is increased from 1 to 2 wt % at pH 4.0. This is because the electrode surface may be covered with Cu(IO3)2 and the reaction is controlled by the diffusion of IO3- from solution to the electrode surface or of the reaction products back to the solution, or both through the Cu(IO3)2 film. Comparing Figure 2 with Figure 3, the anodic branch is very different at pH 2.0 and pH 4.0. There is a peak observed at -70 mV versus SCE under static conditions at pH 4.0 but not at pH 2.0. This peak may be explained by the transition of Cu+ to Cu2+.18 At pH 2.0, Cu+ may not be stable according to the Pourbaix diagram19 while it may be stable at pH 4.0. Thus, the transition peak from Cu+ to Cu2+ does not exist at pH 2.0. However, as shown in Figure 3, this transition peak disappears at pH 4.0 when the rotational speed of the disk electrode is increased to 5000 rpm, indicating that this transition peak is due to the diffusion of Cu+ reaction products back to the solution.11 This may be due to the thinning of the Cu(IO3)2 film by the shear stress from electrode rotation (hydrodynamics) and facilitate the diffusion of Cu+ reaction products back to the solution. It is clear from Figure 3 that the overall copper dissolution in KIO3 solution at pH 4.0 is still controlled by the diffusion of IO3- from the solution to the electrode surface through the Cu(IO3)2 film even at high disk electrode rotational speed. The formation of a Cu(IO3)2 film on the copper electrode surface due to the strong adsorption of IO3- will be verified by the electrochemical quartz balance data later. Figure 4 demonstrates the copper dissolution behavior at different pH values under static and dynamic conditions. The concentration of KIO3 was fixed at 2 wt %. As shown in this figure, pH affects the current density significantly. When solution pH is increased from 2.0 to 4.0, the current

Luo

Figure 4. Effect of solution pH at a concentration of 2% KIO3 under static and dynamic (5000 rpm of rotational disk electrode) conditions.

Figure 5. Copper dissolution behavior at applied external potentials at pH 4.0 measured using electrochemical quartz crystal analysis.

density decreases dramatically under both static and dynamic conditions, and more so on the cathodic side than the anodic side. As discussed previously, the copper surface is covered with a Cu(IO3)2 film at 2% KIO3 by weight for both pH 2.0 and pH 4.0; this significant difference may be due to the copper oxide formed at pH 4.0 on the basis of the Pourbaix diagram,19 resulting in the synergetic effect of Cu(IO3)2 and copper oxides, which may form a denser Cu(IO3)2 film and thus provide better protection. The more dramatic decrease of the cathodic current than that of the anodic current from pH 2.0 to pH 4.0 under static conditions may be ascribed to the lower diffusion coefficient of IO3- than that of copper ions, since the IO3- ion is larger than copper ions, implying that IO3- is more difficult to diffuse through the Cu(IO3)2 film from the solution to the electrode surface than the copper ions from the electrode surface to the solution. 2. Electrochemical Quartz Crystal Analyzer Measurements. The copper dissolution behavior in the solutions at pH 2.0 and pH 4.0 in the absence of KIO3 is shown in Figures 5 and 6. The pH was adjusted using a diluted nitric acid. The potential used here for the copper dissolution was 0.6, 0.8, and 1.0 V, respectively, versus the Ag/AgCl reference electrode. At pH 4.0, as shown in Figure 5, the copper dissolution was inhibited at 0.6 V versus Ag/AgCl, probably due to the passivation of copper by forming stable copper oxides. The electrode gained some

Copper Dissolution Behavior

Figure 6. Copper dissolution behavior at applied external potentials at pH 2.0 measured using electrochemical quartz crystal analysis.

Figure 7. Copper dissolution behavior at applied external potentials at pH 4.0 in the presence of 1% KIO3 measured using electrochemical quartz crystal analysis.

weight that is consistent with the formation of a thin copper oxide. When the applied potential is >0.6 V, the copper starts dissolving at pH 4.0 (the passivation film may be broken down by the applied potential). Since the slope of the curves implies the dissolution rate at that potential, it is not surprising that the dissolution rate is higher at 1.0 V than that at 0.8 V. At pH 2.0, as shown in Figure 6, the dissolution rate is constant (uniform dissolution); again, the higher the applied potential, the higher the dissolution rate; and the passivation behavior has not been observed at 0.6 V versus Ag/AgCl at pH 2.0 due to the unstable oxides in this medium on the basis of the Pourbaix diagram.19 Figure 7 shows the copper dissolution behavior in the presence of KIO3 at pH 4.0. The addition of 1% KIO3 increases the copper dissolution rate compared with that in the absence of KIO3 (Figure 5) that is consistent with the results from potentiodynamic scans discussed previously. The dissolution at 0.6 V occurs in the presence of KIO3 while for the solution in the absence of KIO3 (passivation) it does not (Figure 5). Not surprisingly, the copper dissolution rate increases with the applied potentials. When the KIO3 concentration is further increased to 2%, as shown in Figure 8, at lower applied potentials such as 0.6 V, the copper dissolution rate is not increased compared with that at the 1% KIO3 concentration. At 0.8 V, the copper starts dissolving at a higher rate up to 4 s,

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Figure 8. Copper dissolution behavior at applied external potentials at pH 4.0 in the presence of 2% KIO3 measured using electrochemical quartz crystal analysis.

and a significant weight gain was detected on the electrode. This weight gain may be due to the strong adsorption of IO3- on the copper surface, leading to the formation of a Cu(IO3)2 thin film that is not soluble in the solution. The formation of Cu(IO3)2 on the electrode reaches equilibrium at about 10 s, and the copper dissolution rate is reduced by the formation of a Cu(IO3)2 film on the copper surface (the slope is smaller after 10 s than that at the beginning). During the time period from 4 to 10 s, even though the dissolution is still continuing, the response observed here is the overall effect of dissolution of copper (lose weight) and deposition of Cu(IO3)2 (gain weight). It is obvious that the overall dissolution process is dominated by the formation of a Cu(IO3)2 film on the copper surface. The Cu(IO3)2 film grows up to 10 s, and then the copper dissolution dominates the process again but at a significantly lower rate because of the resistance from the Cu(IO3)2 film. The further weight loss response is most probably due to the dissolution of copper, and the rate is limited by the diffusion of copper reaction products through the Cu(IO3)2 film that is the rate-determining step. Since the copper dissolution rate is relatively low, the growth of the Cu(IO3)2 film is relatively slower and thus the time to reach steady state is relatively longer. At 1.0 V, however, the copper dissolution rate is higher, and the formation of the Cu(IO3)2 film starts after applying the potential for only about 2 s. The time to reach steady state in this case is also shorter. After reaching the steady state, again, the copper dissolution rate is significantly reduced because of the resistance of the Cu(IO3)2 film on the copper surface relative to the beginning. The dissolution rate at 0.8 V is very close to that at 1.0 V after reaching steady state, implying that the copper dissolution is controlled by the diffusion of copper reaction products through the Cu(IO3)2 film on the copper surface. The significant copper dissolution rate difference after 20 s could be due to the better film formation by the reorientation or relaxation of the film stress. As discussed before, the copper dissolution rate increases with decreasing solution pH in the absence of KIO3. This seems to be true from the experimental results even in the presence of 1% KIO3, as shown in Figure 9. The copper dissolution behavior is different from what was observed previously when the applied potential is at 0.8 V versus Ag/AgCl. At 0.8 V, the copper dissolution rate is higher than that at 1.0 V up to 15 s, and then the copper dissolution rate at 1.0 V exceeds that at 0.8 V. The reason is not clear at present. The formation of a Cu(IO3)2 film

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Luo

Figure 9. Copper dissolution behavior at applied external potentials at pH 2.0 in the presence of 1% KIO3 measured using electrochemical quartz crystal analysis.

Figure 10. Copper dissolution behavior at applied external potentials at pH 2.0 in the presence of 2% KIO3 measured using electrochemical quartz crystal analysis.

has not been observed, and this may be due to the lower copper dissolution rate and lower IO3- concentration. The copper ion concentration is not high enough on the electrode surface because of the slow formation of Cu2+ and the diffusion of Cu2+ ions back to the solution. The adsorption of IO3- is not dominant at this time even though the adsorption may be still existent. When the concentration of KIO3 is increased from 1% to 2%, similar behavior to that at pH 4.0 with 2% KIO3 by weight has been observed, as shown in Figure 10. The strong adsorption of IO3- to form Cu(IO3)2 may be due to the dominant effect of the higher IO3- concentration. Again, at 0.6 V, the dissolution in the presence of a 2% KIO3 concentration is increased compared with that at 1% KIO3 at the same solution pH; however, the copper dissolution rate is not high enough to form a Cu(IO3)2 film even though the IO3concentration is increased. When the applied potential is increased to 0.8 V, a faster formation of Cu(IO3)2 has been observed. The accumulation time for the Cu(IO3)2 film formation is much shorter than that with the same KIO3 concentration at pH 4.0. Since the copper dissolution rate increases with decreasing solution pH, this can be explained simply because the copper dissolution rate is much higher and the required concentration of Cu2+ ions on the surface can be achieved in a short period of time.

The adsorption of IO3- on the copper electrode surface at 0.8 V is very similar to that at 1.0 V even though the copper dissolution rate is higher at 1.0 V than that at 0.8 V. Conclusions Copper dissolution behavior in iodate solutions has been investigated using potentiodynamic scans and electrochemical quartz crystal analysis. Copper dissolution increases with decreasing solution pH in the presence of IO3-. IO3- behaves both as an oxidizer and as a passivating agent for copper dissolution. At low IO3- concentration, IO3- acts more like an oxidizer while it passivates the copper surface at a higher concentration. The copper dissolution increases with KIO3 concentration up to 1%, then a strong adsorption of IO3- on the copper surface has been observed at high concentration (2 wt %), and then the copper dissolution rate does not increase further because of the adsorption of IO3- to form a Cu(IO3)2 film on the copper surface. The IO3- adsorption strongly depends on the surface Cu2+ concentration: the higher the surface Cu2+ concentration, the more the IO3adsorption. LA991626+