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Low-Temperature Deposition of Conformal Copper Films in Supercritical CO2 by Catalytic Hydrogen Reduction of Copper Hexafluoroacetylacetonate Hiroyuki Ohde,† Steve Kramer,‡ Scott Moore,‡ and Chien M. Wai*,† Department of Chemistry, University of Idaho, Moscow, Idaho 83844, and Micron Technology, Inc., Boise, Idaho 83707 Received March 17, 2004. Revised Manuscript Received June 24, 2004
Conformal copper films were deposited onto various copper diffusion barrier layers with catalytic hydrogen reduction of copper(II) hexafluoroacetylacetonate, Cu(hfa)2, in supercritical CO2. In the presence of 2-5 at. % of Pd(hfa)2 (relative to Cu(hfa)2), device quality copper films (resistivity 2.1 × 10-6 Ω-cm) could be obtained at temperatures as low as 70 °C. The amounts of Pd in the Cu films were found to be very low (∼0.2 at. %) throughout the bulk of the Cu films. Adhesion of Cu films onto barrier layers was strong despite no Cu seed layer being used. The bottom-up supercritical fluid deposition mechanism allowed Cu films to fill up small features patterned on Si wafers.
Introduction The performance of microelectronic devices has become increasingly dependent on the properties of materials used as interconnects, insulators, and semiconductors. Recently, copper is used as an interconnecting material for microelectronic devices because of its lower resistivity and higher reliability in performance compared with the conventional aluminum interconnect. Since copper interconnect is difficult to fabricate using a subtractive etch process, which is widely used for aluminum interconnect, due to difficulty in plasma etch, the copper patterning would be achieved by the so-called “damascene process”.1 In the damascene process, copper is deposited in holes and trenches patterned in dielectric films such as silicon dioxide followed by removal of the superfluous copper film with a chemical mechanical polishing process. One big challenge associated with this process is to fill small holes and narrow trenches (200 °C) is another drawback of this technique. Supercritical fluid carbon dioxide has been a solvent of much interest to the semiconductor industry because of its low environmental impact and chemical properties such as tunable solubility, high diffusivity, and little surface tension. Utilization of supercritical CO2 in (6) Cohen, S. L.; Liehr, M.; Kasi, S. Appl. Phys. Lett. 1992, 60 (1), 50. (7) Chen, Y. D.; Reisman, A.; Turlik, I.; Temple, D. J. Electrochem. Soc. 1995, 142 (11), 3903. (8) Lecohier, B.; Calpini, B.; Philippoz, J.-M.; van den Bergh, H. J. Appl. Phys. 1992, 72 (5), 2022.
10.1021/cm049542w CCC: $27.50 © 2004 American Chemical Society Published on Web 09/18/2004
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Figure 2. XPS analysis of Cu films formed on TaN, Ta, W2N, W, and Cu/Ta barrier layers. (a) XPS survey spectra of the surfaces of the 5 Cu films; (b) XPS depth profiles of various elements in Cu film on W barrier layer.
microelectronic fabrication has been focused mainly on cleaning of semiconductor devices since the late 1990s.9-12 Recently, new applications such as supercritical resist drying (SRD),13,14 supercritical CO2-based chemical and mechanical planarization (CMP) process,15,16 and supercritical fluid metal deposition17-22 have been reported as potential candidates of fabrication processes for next generation microelectronic devices. The deposition of conformal copper film in supercritical CO2 was reported by Watkins et al.19 They used organometallic copper(I) and copper(II) compounds as starting materials for copper deposition onto the native oxide of Si and TiN. Conformal copper films were obtained at relatively high temperatures (225-250 °C). When catalytic clusters or film such as Pd and Ni were seeded on Si wafer, the deposition temperature was lowered to 200 °C or less. Recently, we have developed a new supercritical fluid method for depositing conformal copper films onto copper diffusion barrier layers (TaN, Ta, W2N, W).23 The method employs hydrogen reduction of copper(II) hexaflu(9) Douglas, M. A.; Templeton, A. C. Method of removing inorganic contamination by chemical derivitization and extraction. U.S. Patent 5,868,856, 1999. (10) McCullough, K. J.; Purtell, R. J.; Rothman, L. B.; Wu, J. Residue removal by supercritical fluids. U.S. Patent 5,908,510, 1999. (11) Wallace, R. M.; Douglas, M. A. Method of cleaning and treating a semiconductor device including a micromechanical device. U.S. Patent 6,024,801, 2000. (12) Chandra, M.; Mount, D. J.; Costantini, M. A.; Moritz, H. D.; Jafri, I.; Boyd, J.; Heathwaite, R. M. Supercritical fluid cleaning process for precision surfaces. U.S. Patent 6,602,349, 2003. (13) Namatsu, H.; Kurihara, K.; Nagase, M.; Iwadate, K.; Murase, K. Appl. Phys. Lett. 1995, 66, 2655. (14) Namatsu, H.; Yamazaki, K.; Kurihara, K. Microelectron. Eng. 1999, 46, 129. (15) Bessel, C. A.; Denison, G. M.; DeSimone, J. M.; DeYoung, J.; Gross, S.; Schauer, C. K.; Visintin, P. M. J. Am. Chem. Soc. 2003, 125, 4980. (16) McClain, J. B.; DeSimone, J. M. Methods, apparatus and slurries for chemical mechanical planarization. U.S. Patent 6,623,355, 2003. (17) Watkins, J. J.; Blackburn, J. M.; McCarthy, T. J. Chem. Mater. 1999, 11, 213. (18) Blackburn, J. M.; Long, D. P.; Watkins, J. J. Chem. Mater. 2000, 12, 2625. (19) Blackburn, J. M.; Long, D. P.; Cabanˇas, A.; Watkins, J. J. Science 2001, 294, 141. (20) Cabanˇas, A.; Shan, X.; Watkins, J. J. Chem. Mater. 2003, 15, 2910. (21) Cabanˇas, A.; Blackburn, J. M.; Watkins, J. J. Microelectron. Eng. 2002, 64, 53. (22) Kondoh, E.; Kato, H. Microelectron. Eng. 2002, 64, 495. (23) Wai, C. M.; Ohde, H.; Kramer, S. Methods of forming metalcontaining films over surfaces of semiconductor substrate; and semiconductor constructions. U.S. Patent 6,653,236, 2003.
oroacetylacetonate [Cu(hfa)2] catalyzed by 2-5 at. % (relative to the Cu precursor) of palladium hexafluoroacetylacetonate [Pd(hfa)2] dissolved in supercritical CO2. Using this catalytic deposition method, device quality copper films (resistivity 2.1 × 10-6 Ω-cm) were formed at temperatures as low as 70 °C. The copper films showed strong adhesion to the barrier layers. Also the high diffusivity and extremely low surface tension of supercritical CO2 allowed the Cu precursor to penetrate into small trenches (∼100 nm) patterned on an Si wafer and to fill the trenches with Cu. Experimental Section The copper deposition experiments were performed using a stainless steel reactor system illustrated in Figure 1. A 180 mL volume, stainless steel reactor was equipped with a 5 cm × 5 cm square quartz heating table on the upper interior wall and thermal blocks on both upper and lower portions of the exterior wall. The heating elements were heated and controlled independently using three temperature controllers (Barnant Company, Barrington, IL). A 5 cm × 5 cm square piece of Si wafer seeded with barrier layer was held on the quartz heating table by the spring force of a thermocouple for localized heating of the Si wafer. In this upside-down arrangement, heating efficiency on the Si wafer was better than the right-side-up arrangement. Vessel 2 was a 20 mL volume, tubular stainless steel cell for dissolution and injection of the precursor. Carbon dioxide (Oxarc, Spokane, WA) was metered and pressurized using an ISCO syringe pump, model 260D. Hydrogen gas was purchased from Oxarc. Cu(hfa)2‚H2O (Aldrich) and Pd(hfa)2 (Aldrich) were used as received. Six types of silicon wafers were used for this work, 5 plane Si wafers with TaN(300 A), Ta (300 A), W2N (300 A), W (300 A), or Cu/Ta(300 A) barrier layer and 1 patterned Si wafer with Ta barrier layer (300 A). The Si wafers were provided by Micron Technology Inc. Hydrogen gas was flowed through the 180 mL reactor chamber to purge ambient air and then pressurized to 10 atm (0.075 mol). Thermal blocks on the exterior wall of the reactor were turned on and preheated the reactor to 50 °C. After the temperature reached 50 °C, CO2 was introduced into the reactor chamber to mix with the hydrogen at a total pressure of 80 atm. Then 1.5 g of Cu(hfa)2 (3 mmol) and 50 mg of Pd(hfa)2 (0.1 mmol) were put into the injection vessel, and CO2 was flowed through the injection vessel chamber to purge of ambient air and pressurized to 150 atm (35 °C) to dissolve the Cu and Pd compounds in the supercritical CO2. After a dissolution time of 1 h, the quartz heating table in the reactor chamber was turned on and heated to a desired deposition temperature (70-120 °C) (the thermal blocks used for preheating were turned off). Immediately after reaching the deposition temperature, the Cu and Pd precursors were injected into the reactor with the aid of the CO2 pressure
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Figure 3. SIMS depth profiles of palladium in Cu films formed on TaN, Ta, W2N, and W barrier layers. difference by opening an interconnecting valve between the reactor (80 atm) and the injection vessel (150 atm). The reactor wall temperature was much lower than the quartz heating plate throughout the deposition time, allowing selective deposition of Cu to occur on the Si wafer surface. After a deposition time of 30 min, the quartz heating plate was turned off, and the CO2 was vented from both the reactor and the injection vessel. Morphology and gap-fill of Cu films were studied by scanning electron microscopy (SEM) using a JEOL SEM JSM-6301 and a JEOL JSM-6600. X-ray photoelectron spectroscopy (XPS) analyses of sample surfaces and depth profiles were carried out using a Perkin-Elmer PHI 5600 ESCA system. Also low levels of Pd in Cu films were analyzed by secondary ion mass spectroscopy (SIMS) using a Cameca magnetic sector SIMS model IMS-4f and a Physical Electronics Quadrupole SIMS model 6650.
Results and Discussion Copper films were deposited on 5 plane Si wafers with different barrier layers (TaN, Ta, W2N, W, and Cu/Ta) at a deposition temperature of 120 °C. All Cu films formed on these barrier layers were uniform and highly reflective. The Cu films showed strong adhesion to all barrier layers. The Cu films could not be rubbed off from the barrier surfaces. XPS survey spectra of the Cu films shown in Figure 2a identified the presence of carbon, oxygen, fluorine, silicon, and copper on the surfaces. However, the carbon, oxygen, fluorine, and silicon were
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not detected in the bulk of the Cu film as shown in the depth profile of the Cu film (Figure 2b). Since palladium was not detected both on the surface and in the bulk by XPS analysis, SIMS was also employed for the Cu film analysis. Figure 3 shows the depth profiles of Pd in the Cu films. Although the amount of Pd(hfa)2 added in Cu(hfa)2 was 3 at. %, very low levels of Pd (∼0.2 at. %) were actually detected throughout the bulk of the Cu films. It was also confirmed that Pd was not segregated at the interfaces between the Cu films and the barrier layers. Based on the above analyses, the Pd-catalyzed Cu film formation mechanism is proposed as follows. First, hydrogen reduction of Pd(hfa)2 takes place in supercritical CO2 to produce Pd catalytic clusters. These Pd clusters form an incipient catalytic layer on the barrier layer. The Pd clusters catalyze hydrogen reduction of Cu(hfa)2 at the catalytic surface to form Cu film on the barrier layer. The Pd clusters have continuously floated out to the surface of the growing Cu film during the deposition process to catalyze hydrogen reduction of Cu(hfa)2 at the surface. Finally, the Pd clusters segregated out to the Cu film surface agglomerate and fall off the surface. This deposition mechanism may be similar to the catalyst-enhanced CVD using iodine as a catalytic surfactant2 that allows Cu film to fill up small trenches in a bottom-up fashion. X-section SEM images of the above 5 samples were obtained to measure thicknesses of the Cu films. The x-section image of Cu film deposited on Ta barrier is shown in Figure 4a. The Cu film was flat throughout the entire surface, and no delamination was observed at the interface between the Cu film and the barrier layer. The average thicknesses of these Cu films were estimated to be between 100 and 150 nm. Also, the resistances of a Cu film (deposited on Ta barrier) were measured randomly at several areas using 4-point probe method.24 Based on the thickness measurements and the resistance measurements, the average resistivity of the Cu film was determined to be 2.1 × 10-6 Ω-cm,
Figure 4. X-section SEM images of Cu films deposited in supercritical CO2 (a) on plane Si wafer with a Ta barrier layer at 120 °C, (b)-(d) on patterned Si wafers [trench widths: (b) 220 nm, (c) 320 nm, (d) 430 nm] with a Ta barrier layer at 100 °C, and (e) on a patterned Si wafer (trench width: 220 nm) with a Ta barrier layer at 70 °C. Scale ) 100 nm.
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which is close to the pure bulk Cu (1.7 × 10-6 Ω-cm). The gap-fill ability of Cu film deposited in supercritical CO2 was also studied using the x-section SEM. In this experiment, patterned Si wafers with Ta barrier layer were used. X-section SEM images of Cu films deposited on the test wafers at 100 °C were shown in Figures 4b-4d. The test wafer had three types of small trenches with widths 220, 320, and 430 nm. As seen in these figures, the three types of trenches were filled completely with Cu after the deposition process. The thickness of the Cu film exceeded 500 nm occasionally. No delamination of the Cu films was observed at the bottom and side surfaces of trenches and at the top surface. The gap-fill ability was tested at an even lower temperature as 70 °C. The Cu film deposited at 70 °C (Figure 4e) appeared to be thinner than those at 100 °C so that a seam was observed in one of the trenches. Conclusion Low-temperature supercritical fluid deposition of conformal copper with palladium-catalyzed hydrogen reduction of Cu(hfa)2 was demonstrated in this study. (24) McCormick, J. R.; Kitchin, J. R.; Barteau, M. A.; Chen, J. G. Surf. Sci. 2003, 545, L741.
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With use of this method, device quality copper films having reasonable resistivity (2.1 × 10-6 Ω-cm) and negligible impurities in the bulk film could be deposited on silicon wafers at temperatures as low as 70 °C. The good gap-filling ability inherent in supercritical CO2 deposition was also demonstrated. This article shows that the supercritical fluid deposition process can be a potential candidate for fabrication of next generation microelectronic devices. Since supercritical fluid is considered a potential medium for fabrication processes such as wafer cleaning, etching, and CMP processes, introduction of supercritical fluid deposition technologies may create an efficient dry process that could dramatically simplify the microelectronic device fabrication process. Current fabrication processes usually require shuttling wafers between wet processes using aqueous and organic solvents (e.g., cleaning and CMP processes) and dry processes (e.g., vapor depositions such as CVD and PVD). Acknowledgment. This work was supported by Micron Technologies, Inc., and by the University of Idaho. CM049542W
