Dissolution of Precious Metals in Supercritical Carbon Dioxide

Journal of the American Chemical Society 2008 130 (49), 16659-16668 ... Development of a homogeneous/heterogeneous metal recovery system based on ...
0 downloads 0 Views 194KB Size
Download PDF
922

Ind. Eng. Chem. Res. 2005, 44, 922-926

SEPARATIONS Dissolution of Precious Metals in Supercritical Carbon Dioxide Joanna Shaofen Wang and Chien M. Wai* Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343

Precious metals including copper, gold, and palladium can be dissolved in supercritical CO2 by oxidation with HNO3 and subsequent chelation with hexafluoroacetylacetone to form CO2-soluble metal β-diketonate complexes. The oxidizing agent HNO3 is carried into the supercritical fluid phase by the CO2-soluble Lewis base tri-n-butyl phosphate (TBP) as a complex of the general form TBP(HNO3)x(H2O)y. For the dissolution of Pd metal, a Lewis acid-base complex of the form TBP(HNO3)1.0(H2O)0.4 is required. Au and Cu can be oxidized with a lesser amount of HNO3 in the complex. This supercritical fluid dissolution technique provides a “dry” method for recovering precious metals from abandoned electronics and spent catalysts with minimum waste generation. Introduction Dissolving metal species using supercritical fluid carbon dioxide (SC CO2) as a solvent has many potential chemical, environmental, and materials related applications. The in situ chelation method for extracting metal ions in SC CO2 was first reported in the early 1990s.1-3 In this method, a CO2-soluble chelating agent is used to convert insoluble metal ions into CO2-soluble metal chelates. Fluorinated chelating agents and phosphoruscontaining complexing agents are highly effective for this in situ chelation/supercritical fluid extraction (SFE) method because of their high solubilities in SC CO2.1,3 Recent reports show that oxides of the f-block elements can also be dissolved in SC CO2 using proper complexing agents.4-6 The dissolution of uranium dioxide (UO2) in SC CO2 is particularly interesting because it is related to a potential green technology for treatment of uranium contaminated wastes and for recycling spent nuclear fuels. UO2 in the 4+ oxidation state does not form stable complexes with commonly known complexing agents. An oxidizing agent is required to convert UO2 to the 6+ oxidation state in the form of the uranyl ion (UO22+), which forms stable complexes with a number of complexing agents that become soluble in SC CO2. In the reported SC CO2 processes for the dissolution of UO2, one method uses hydrogen peroxide to oxidize UO2 to UO22+ ions, followed by complexation with a fluorinated β-diketone to form a uranyl-β-diketonate complex that is soluble in SC CO2.7 The solubility of the uranyl-βdiketonate complex in SC CO2 can be further increased by adding the CO2-soluble Lewis base tri-n-butyl phosphate (TBP), which replaces coordinated water molecules, making the complex more soluble in CO2.8 Another method uses a CO2-soluble TBP-HNO3 complex to dissolve UO2 in SC CO2.4-6 Nitric acid is conventionally used for metal dissolution and extraction, but it is not soluble in SC CO2. However, when HNO3 * To whom correspondence should be addressed. Tel.: (208)885-6787. Fax: (208)885-6173. E-mail: [email protected].

is bound to a CO2-philic Lewis base such as TBP, the resulting Lewis acid-base complex is highly soluble in SC CO2.9 TBP in this case serves as a carrier for introducing the acid into the SC CO2 phase for chemical reactions. In this approach, the HNO3 carried by TBP oxidizes UO2 to the UO22+ ion, which is followed by formation of a CO2-soluble (UO2)(NO3)2‚(TBP)2 complex. The same principle can be used to dissolve precious metals in SC CO2. Precious metals in their elemental states do not dissolve in SC CO2; they must be oxidized first and then subjected to complex formation with a proper chelating agent to make them soluble in CO2. In a recent communication, Bessel et al. used an organic oxidizing agent to convert zerovalent copper Cu(0) into Cu(II), which was then complexed with a β-diketone to form a CO2-soluble Cu(II)-β-diketonate.10 The reaction is significant for potential development of a dry process to replace the wet chemical mechanical planarization (CMP) process that is currently used by the microelectronics industry. Copper is considered a better interconnect metal for semiconductor devices because of its superior electrical conductivity relative to the current tungsten- or aluminum-based materials. In the previous report by Bessel et al., the organic oxidizing agent ethyl peroxydicarbonate (EPDC) was introduced into the SC CO2 phase in a hexane solution. The results for Cu dissolution were obtained after 20 h in SC CO2, indicating low reaction rates. Because EPDC can be explosive, safety precautions must be considered, as noted by the authors. The CO2-soluble TBP-HNO3 complex described recently in the literature appears to be a better oxidizing system for dissolving zerovalent metals in SC CO2.9 Traditional methods for dissolving gold (Au) usually involve aqua regia, alkali cyanides, or thiocyanate media and use of organic solvents for extracting the Au complexes.11,12 Larger quantities of acid wastes are also generated for the recovery of copper from the electronic and galvanic industries13 and for dissolution of metallic palladium (Pd).14 If supercritical CO2 could be used as a solvent for the dissolution of precious metals, it would

10.1021/ie040198m CCC: $30.25 © 2005 American Chemical Society Published on Web 01/21/2005

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 923

Figure 1. Apparatus of the high-pressure system for the dissolution of metals.

significantly reduce the generation of liquid acid and organic solvent wastes for their recovery and processing. In this report, we present our results of dissolving metallic copper, gold, and palladium in SC CO2 using a TBP-HNO3 complex as an oxidizing agent and a β-diketone as a chelating agent. Not only are methods for dissolving zerovalent metals in SC CO2 significant for potential applications in semiconductor device processing, but they can also be used in other chemical processes such as recovery of precious metals from abandoned electronics and used catalysts.

with the view cell, a stainless steel extraction vessel (14 mL), or a stainless steel extraction cell containing a porous internal cell with a total volume of 6.2 mL (5 mL of the internal cell and 1 mL of a conduit underneath the internal cell connecting to the outlet valve), which was placed in an oven to maintain a desired temperature. The flow rate of the SC CO2 fluid was controlled by the Isco pump. At the oven exit, stainless steel tubing (316 SS, 1/16-in. o.d. and 0.030-in. i.d.) with a length of 20 cm was used as the pressure restrictor for the exit CO2. All SFE experiments were performed at a temperature of 40 °C and a pressure of 150 or 200 atm. The SFE apparatus and general procedures are similar to those given in the literature.15 Surface morphology of the metal films was examined with an AMRAY 1830 scanning electron microscope (SEM). An energy-dispersive X-ray spectrometer was used to measure the metal film composition. A Bruker 300 MHz NMR spectrometer was used to identify metal complexes dissolved in the CO2 phase. Nondestructive instrumental neutron activation analysis (NAA) was used to determine Au in solid and liquid samples. The gold samples and standard were irradiated for 3 min in a 1-MW TRIGA nuclear reactor at a steady neutron flux of 6 × 1012 N cm-2 s-1.

Experimental Section

Results and Discussion

Reagents and Materials. Instrument-grade carbon dioxide (purity 99.99%, Oxarc, Spokane, WA) was used in the experiments. TBP was purchased from Avocado (ordered through Alfa Aesar, Ward Hill, MA). Nitric acid [69.5% (w/w)] was obtained from Fisher Chemical (Fair Lawn, NJ). The preparation procedure for the complexes TBP(HNO3)0.7(H2O)0.7 and TBP(HNO3)1.0(H2O)0.4 is described in the literature.4,9 All other reagents and solvents used in this study were of analytical grade. Silicon wafers (∼1 mm thick) coated with thin Cu layers or with thin Pd layers were obtained from Micron Technology Inc. (Boise, Idaho). Obsolete CPU processors with gold connectors and copper strips (0.8 × 2 cm2) cut from a circuit board were taken from old computers including IBM and Intel Pentium models for this study. The size of gold pin connectors from the CPU processors was 0.4 cm (length) × 480 µm (diameter), with an average thickness of 34 µm of pure gold coating (SEM image). The connectors of CPU processors are made of Co, Ni, and Fe alloy and coated with gold. Energydispersive X-ray spectrometry (EDS) data show that the surface layer of the copper strip from the computer circuit board contains a small amount of gold and the layer below it is copper. Palladium shot (1-mm diameter), Cu shot (3-mm diameter), hexafluoroacetylacetone (Hhfa), palladium hexafluoroacetylacetonate [Pd(hfa)2], and copper hexafluoroacetylacetonate [Cu(hfa)2] were purchased from Aldrich. Deionized water (Millipore Milli-Q system, Bedford, MA) was used for the preparation of all aqueous solutions. Instrumentation. The apparatus for the SFE experiments (Figure 1) includes the following: a liquid CO2 tank, a high-pressure syringe pump, a highpressure view cell with quartz windows (20-mL volume and 5-cm path length), a stirring and heating plate, a thermocouple, a pressure transducer, and a collection vial. Detailed descriptions of the view cell and highpressure system are given in the literature.15 SFC-grade CO2 was supplied with a syringe pump (Isco, model 260D, Lincoln, NB). The experiments were performed

Copper Dissolution. A TBP-HNO3 complex with the composition TBP(HNO3)0.7(H2O)0.7 was prepared by mixing 5 mL of TBP with 0.82 mL of concentrated HNO3 (15.5 M) following the procedure reported in the literature.4 The complex had a solubility of about 1.7 mol % in SC CO2 at 40 °C and 110 atm.10 Hexafluoroacetylacetone was used as a chelating agent to convert the oxidized metal species into CO2-soluble metal chelates. For the Cu dissolution experiments, a piece of silicon wafer (1.5 × 1.5 cm2) coated with Cu was placed in a small glass beaker (0.7-cm diameter), and Hhfa (0.20 mL) was placed in another beaker (2-cm diameter). Both beakers were then placed in the stainless steel reactor (14-mL volume, preheated at 40 °C) with 1.5 mL of the TBP(HNO3)0.7(H2O)0.7 complex in it. The reaction cell was pressurized to 200 atm, and the Hhfa in the beaker and the TBP complex in the reaction cell were each agitated with a miniature stirring bar. The collection vessel from the outlet was a 2-L plastic container with multiple holes on the lid to provide enough room for CO2 expansion after the dissolution reactions. Reaction time was set at 0.25, 0.5, 1, 2, and 4 min individually. After the reaction, the stainless steel cell was depressurized by fully opening the outlet valve, and the silicon wafer was then removed from the system, rinsed with hexane, and dried with a nitrogen gas stream. SEM images of the wafer before and after the SC CO2 dissolution are shown in Figure 2. The SEM images and EDS data indicated that all metallic Cu on the silicon wafer surface was removed in 4 min. The thickness of the Cu film in the original silicon wafer was about 128 nm (Figure 2a). After the SC CO2 treatment, the thickness of the Cu film was reduced to 49 nm (30 s), as shown in Figure 2b. The dissolution of Cu by the TBP(HNO3)0.7(H2O)0.7 complex and Hhfa in SC CO2 is obviously very fast according to our experiments. When only pure TBP and Hhfa were used, no Cu removal was observed from the silicon wafer under the same experimental conditions. The HNO3 carried by the TBP into the SC CO2 phase is apparently important for the oxidation of Cu

924

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005

Figure 4. NAA γ spectra of the irradiated trap solution.

Figure 2. SFE cross section of Cu/Si wafer (a) before and (b) after SFE at 40 °C and 200 atm (scale ) 100 nm).

Figure 3. (a) Cu and (b) Au dissolved in SC CO2 at 40 °C and 150 atm.

leading to subsequent complexation and dissolution in the SC CO2 phase. The color of the SC CO2 solution was deep green when the copper strip (2 pieces) or copper shot (3 grains) dissolution experiments were conducted in the high-pressure view cell with quartz windows (Figure 3a). Because the layer of Cu on the silicon wafer is so thin, a distinctive green color of Cu(hfa)2 could not be detected by the naked eye. The extraction speed of SC CO2 at 40 °C and 150 atm was much higher than the extraction speed of liquid CO2 at room temperature (25 °C) and 150 atm, by about 1 order of magnitude. UV-vis spectra of the supercritical fluid phase are consistent with a Cu(hfa)2 standard in TBP complex solution. Because of its paramagnetic property, Cu2+ complexed with Hhfa, i.e., Cu(hfa)2, does not show a measurable NMR spectrum. Gold Dissolution. The Au dissolution experiments were carried out using connector pins cut from a CPU processor. The gold-coated pins (6 pieces with a total weight of ∼20 mg) were placed in the high-pressure view

cell preheated to 40 °C. The TBP(HNO3)0.7(H2O)0.7 complex (6 mL) and Hhfa (0.5 mL) were added to the reactor, and CO2 was introduced into the reactor to a pressure of 150 atm. The SC CO2 solution was agitated with a magnetic stirrer. After a couple of minutes, the solution turned to yellow and then became orange red after 35 min (Figure 3b). The observed color change indicated that Au coated on the surface of the pins was dissolved first, followed by the exposed iron and other metals from the interior of the pins. The color of Fe(hfa)3 in SC CO2 is red. After investigation of the conditions of dissolution of gold through the high-pressure view cell, a specially designed stainless steel vessel was used for gold extraction experiments. This extraction vessel was fitted with a porous stainless steel cup with a conduit underneath the porous cup on the bottom of the vessel. The porous sintered cup served as a filter to remove particulates greater than 20 µm in size. It was designed for dissolving metals from solid materials in SC CO2. A total of 80 pins from a computer processor was loaded into the cup, and then 6 mL of TBP(HNO3)0.7(H2O)0.7 complex and 0.5 mL of Hhfa were added. The system was preheated to 40 °C and then pressurized to 200 atm. A magnetic stirring bar was used for agitation of the SC CO2 solution in the cup, and the system was left for static extraction for 2 h. After that, the fluid phase was released slowly through the conduit underneath the porous cup and collected in a small container. A known amount of the trap solution was placed in a polyethylene vial (2/5 dram) and heat-sealed for neutron activation analysis (NAA) of gold. The samples and the standards were irradiated for 3 min in a 1-MW TRIGA nuclear reactor at a steady neutron flux of 6 × 1012 N cm-2 s-1. The irradiated samples and standards were counted individually 3 h after irradiation on a Ge(Li) detector and counted again after 18 h. The γ spectrum obtained 3 h after the irradiation exhibited the major 198Au (t1/2 ) 2.69 days) peak at 411.8 keV and a satellite 198Au peak at 675.9 keV, as shown in Figure 4. The 846.8 keV peak was assigned to 56Mn, which has a half-life of 2.58 h. When the sample was counted 18 h after irradiation, the 846.8 keV peak was significantly reduced, and the decrease was consistent with the decay of 56Mn. The Mn most likely came from the stainless steel extraction cell system.16 A 511 keV positron annihilation peak was also observed that could be from the decay of other highenergy γ rays from 56Mn (1810 and 2113 keV). The NAA indicated that 619 µg of gold was extracted with a counting error of 0.69%. Before SFE, the original gold connectors showed only Au peaks in the EDS spectrum (Figure 5a), indicating that the exterior of the connector was composed mainly of pure Au. After SFE, the Au peaks became smaller, and other peaks, including those of Fe, Ni and Co, were observed (Figure 5b). The interior of the connector pins

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 925

Figure 5. EDS analyses of Au pin surface (a) before and (b) after SFE.

Figure 6. 19F NMR spectra of Pd(hfa)2, Hhfa, and trap solution after SFE of Pd shot.

was made of other metals that are visible from the cross section of the cut end. The results presented in this study clearly indicate that Au in the CPU connectors could be rapidly dissolved in SC CO2 using a mixture of Hhfa and a TBP(HNO3)0.7(H2O)0.7 complex, with the former serving as a chelating agent and the latter as the oxidizing agent. When the same dissolution process was conducted using liquid CO2 (at 24 °C and 150 atm), the speed of dissolution of Au was much lower, by a factor of 5-10. When dissolution of Au-coated connector pins was carried out at room temperature and at ambient pressure in TBP(HNO3)0.7(H2O)0.7 phase with Hhfa directly, the dissolution rate was 1-2 orders of magnitude lower than when the same Hhfa and TBPHNO3 complex as extractants were used in SC CO2. The high diffusivity of SC CO2 is probably an important factor facilitating oxidation and transport of metal species into the SC CO2 fluid phase. Palladium Dissolution. Dissolution of palladium shot in SC CO2 was performed using the TBP(HNO3)1.0(H2O)0.4 complex and Hhfa at 40 °C and 150 atm. 19F NMR spectra of Pd(hfa)2, Hhfa, and a trap solution obtained from a typical Pd dissolution experiment are shown in Figure 6. Palladium dissolved in SC CO2 using Hhfa and TBP/HNO3 exhibits a light yellow color. Our experiments indicate that TBP(HNO3)0.7(H2O)0.7 is not strong enough for rapid oxidation of Pd metal; only a slight amount of Pd was converted to Pd(hfa)2 complex (-73.59 ppm), as shown in the 19F NMR spectra of Figures 6c and 7a. Using TBP(HNO3)1.0(H2O)0.4 as an oxidizing agent, a much higher dissolution rate of Pd

Figure 7. Dissolution of Pd shot using the two different Lewis acid-base complexes TBP(HNO3)0.7(H2O)0.7 and TBP(HNO3)1.0(H2O)0.4.

shot in SC CO2 could be achieved. Spectra 7b and 7c show the Pd(hfa)2 peak at -73.68 ppm increases with decreasing Hhfa peak (-77.11 ppm) from 2 to 4 h of SC CO2 extraction. Dissolution of Pd films (100 nm thick) on Si wafers was also tested using this technique. SFE conditions were set at 40 °C and 200 atm, and extraction times at 0.5, 1, 2, and 4 min. EDS results showed that only 1% of palladium metal remained on the Si wafer surface in 30 s of extraction, and no palladium was found on the Si wafer surface in 4 min of dissolution, indicating that the dissolution reaction was perhaps completed within seconds. Because TBP has a UV absorbance around 230 nm with an absorption shoulder (280 nm), Hhfa chelating agent has an absorption band centered at 280 nm,17 and Pd(hfa)2 standard solution in TBP matrix has a broad absorption band around 230-450 nm, it is difficult to identify individual peaks and to quantify the Pd(hfa)2 complex in the trap solution mixture. The 19F NMR spectrum of the trapped solution confirms that the extracted palladium is in the chemical form of Pd(hfa)2 with the chemical valence of 2+, consistent with the Pd(hfa)2 standard. The results presented in Figure 7 also suggest that TBP(HNO3)1.0(H2O)0.4 is more effective for the oxidation of Pd in SC CO2 than TBP(HNO3)0.7(H2O)0.7. Recovery of Precious Metals. Several approaches have been reported recently in the literature regarding

926

Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005

the reduction of metal β-diketonate complexes to their elemental state in SC CO2. For example, Ye et al. reported that, by using a supercritical fluid immersion deposition (SFID) technique, β-diketonate complexes of Pd, Cu, Ag, and other metals dissolved in CO2 could be reduced to their elemental state, forming thin films on silicon surfaces in the presence of hydrogen fluoride.18 The reduction of the hfa complexes of these metals was attributed to the following reaction

4HF + Si0 + 2M(hfa)2 f SiF4 + 4Hhfa + 2M0 Another method of reducing metal β-diketone complexes in SC CO2 reported in the literature is by adding a reducing agent such as H2 or NaBH3CN to the fluid phase.19,20 Copper and palladium films can be formed by hydrogen reduction of Cu(hfa)2 or Pd(hfa)2 at moderate temperatures in SC CO2. This kind of in situ metal reduction approach can be used to recover precious metals extracted in the SC CO2 phase. Recovery of precious metals can also be done in a trap solution at ambient pressure after their removal from the supercritical fluid phase by pressure reduction. Investigation of several possible methods for precious metal recovery after their extraction by the SC CO2 method described in this paper is currently in progress. Conclusion This study has demonstrated that metallic copper, gold, and palladium in different forms (film, coating, strip, and solid granule) can be dissolved in SC CO2 using HNO3 as an oxidizing agent and a fluorinated β-diketone as a chelating agent. The introduction of HNO3 via a Lewis acid-base complex utilizing a CO2philic Lewis base such as TBP implies that other CO2insoluble acids can also be introduced into SC CO2 by the same principle. This supercritical fluid dissolution process can, in principle, be applied to the dissolution of other metals in SC CO2 for various chemical applications. Acknowledgment This work was supported by the Idaho NSF-EPSCoR Program. Literature Cited (1) Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. Solubility of Fluorinated Metal Dithiocarbamates in Supercritical Carbon Dioxide. J. Supercrit. Fluids 1991, 4, 194. (2) Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. Extraction of Metal Ions from Liquid and Solid Materials by Supercritical Carbon Dioxide. Anal. Chem. 1992, 64, 2875.

(3) Wai, C. M.; Wang, S. SFE: Metals as Complexes. J. Chromatogr. A 1997, 785, 369. (4) Enokida, Y.; El-Fatah, S. A.; Wai, C. M. Ultrasound Enhanced Dissolution of UO2 in Supercritical CO2 Containing a CO2-Philic TBP-HNO3 Complexant. Ind. Eng. Chem. Res. 2002, 41, 2282. (5) Tomioka, O.; Meguro, Y.; Yoshida, Z.; Enokida, Y.; Yamamoto, I.; Yoshida, Z. Dissolution Behavior of Uranium Oxides with Supercritical CO2 Using HNO3-TBP Complex as a Reactant. J. Nucl. Sci. Technol. 2001, 38, 1097. (6) Samsonov, M. D.; Wai, C. M.; Lee, S. C.; Kulyako, Y.; Smart, N. G. Dissolution of Uranium Dioxide in Supercritical Fluid Carbon Dioxide. Chem. Commun. 2001, 1868. (7) Trofimov, T. I.; Samsonov, M. D.; Lee, S. C.; Smart, N. G.; Wai, C. M. Ultrasound Enhancement of Dissolution Kinetics of Uranium Oxides in Supercritical Fluid Carbon Dioxide. J. Chem. Technol. Biotechnol. 2001, 76, 1223. (8) Lin, Y.; Wu, H.; Smart, N. G.; Wai, C. M. Investigation of Adducts of Lanthanide and Uranium β-diketonates with Organophosphorus Lewis Bases by Supercritical Fluid Chromatography. J. Chromatogr. A 1998, 793, 107. (9) Enokida, Y.; Tomika, O.; Lee, S. C.; Rustenholtz, A.; Wai; C. M. Characterization of a Tri-n-butyl Phosphate-Nitric Acid Complex: A CO2-soluble Extractant for Dissolution of Uranium Dioxide. Ind. Eng. Chem. Res. 2003, 42, 5037. (10) Bessel, C. A.; Denison, G. M.; Desimone, J. M.; DeYoung, J.; Gross, S.; Schauer C. K.; Visintin, P. M. Etchant Solutions for the Removal of Cu(0) in Supercritical CO2-Based “Dry” Chemical Mechanical Planarization Process for Device Fabrication. J. Am. Chem. Soc. 2003, 125 (17), 4980. (11) Das, N. R.; Bhattacharyya, S. N. Solvent Extraction of Gold. Talanta 1976, 23, 535. (12) Budavari, S.; O’Neil, M. J.; Smith, A.; Heckelman, P. E. the Merck Index, Encyclopedia of Chemicals, Drugs, and Biologicals, 11th ed.; Merck & Co., Inc.: Rahway, NJ, 1989. (13) Veglio, F.; Quaresima, R.; Fornari, P.; Ubaldini, S. Recovery of Valuable Metals from Electronic and Galvanic Industrial Wastes by Leaching and Electrowinning. Waste Manage. 2003, 23 (3), 245. (14) Takao, H. Recovery of Platinum-Group Metals from Nitric Acid Solutions. Japan Patent 63307123, 1987. (15) Wang, S.; Koh, M.; Wai, C. M. Nuclear Laundry Using Supercritical Fluid Solutions. Ind. Chem. Eng. Res. 2004, 43 (7), 1580. (16) See: http://www.alliedtrading.com/shop_misjewel316ins.htm. (17) Cheng, K. L.; Ueno, K.; Imamura, T. CRC Handbook of Organic Analytical Reagents; CRC Press: Boca Raton, FL; pp 85108. (18) Ye, X. R.; Wai, C. M.; Zhang, D. Q.; Kranov, Y.; McIlroy, D. N.; Lin, Y.; Engelhard, M. Immersion Deposition of Metal Films on Silicon and Germanium Substrates in Supercritical Carbon Dioxide. Chem. Mater. 2003, 15, 83. (19) Blackburn, J. M.; Long, D. P.; Cabanas, A.; Watkins, J. J. Deposition of Conformal Copper and Nickel Films from Supercritical Carbon Dioxide. Science 2001, 294 (5), 141. (20) Blackburn, J. M.; Long, D. P.; Watkins, J. J. Reactive Deposition of Conformal Palladium Films from Supercritical Carbon Dioxide Solution. Chem. Mater. 2000, 12, 2625.

Received for review July 9, 2004 Revised manuscript received November 5, 2004 Accepted November 12, 2004 IE040198M