ethylenediimine and Dialkyldithiocarbamate Ligands for Copper

Nov 23, 2006 - Andrew Dunbar,† Donna M. Omiatek,† Susan D. Thai,† Christopher E. ... Laurel L. Grotzinger,‡ Walter J. Boyko,† Randy D. Weins...
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Ind. Eng. Chem. Res. 2006, 45, 8779-8787

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Use of Substituted Bis(acetylacetone)ethylenediimine and Dialkyldithiocarbamate Ligands for Copper Chelation in Supercritical Carbon Dioxide Andrew Dunbar,† Donna M. Omiatek,† Susan D. Thai,† Christopher E. Kendrex,† Laurel L. Grotzinger,‡ Walter J. Boyko,† Randy D. Weinstein,‡ Dorothy W. Skaf,‡ Carol A. Bessel,*,† Ginger M. Denison,§ and Joseph M. DeSimone| Departments of Chemistry and Chemical Engineering, VillanoVa UniVersity, 800 Lancaster AVenue, VillanoVa, PennsylVania 19085; Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599; and Department of Chemical Engineering, North Carolina State UniVersity, Raleigh, North Carolina 27695

Chemical-mechanical planarization (CMP) is a process of oxidizing and chelating the copper overburden present in an interconnect device while mechanically polishing the surface of the wafer. Because the use of condensed CO2 as the solvent for CMP would be environmentally and technically advantageous, several substituted bis(acetylacetonate)ethylenediimine (R4BAE, where R ) CH3 or CF3) and lithium or sodium dialkyldithiocarbamate (M+(R2DTC-), where M+ ) Li+ or Na+ and R ) ethyl, n-propyl, n-butyl, or 1,1,1trifluoroethyl) ligands were used with t-butylperacetate (t-BuPA, as oxidant) for the oxidative dissolution of copper(0) in supercritical (sc) CO2 at 40 °C and 170-210 bar or in hexanes at 40 °C and atmospheric pressure. The reaction products from the copper etching were determined to be Cu(R4BAE) or Cu(R2DTC)2, respectively. The R2DTC- ligands had higher etch rates than the R4BAE ligands with comparable substituents, and the lithium dialkyldithiocarbamate salts gave higher copper etching rates than their sodium counterparts. The highest average etch rates were observed for Li((CF3CH2)2DTC): 16.7 nm/min in sc CO2 and 11.2 nm/min in hexanes. While hexanes have similar physical properties when compared to sc CO2, the rates of copper(0) removal with the R2DTC- ligands were 17-49% higher in sc CO2 than in hexanes at comparable temperatures and solvent densities. Scanning electron microscopy (SEM) images of the postreaction copper surfaces using the various ligands showed significant variations in surface roughness. X-ray photoelectron spectroscopy (XPS) measurements indicated that the lower R4BAE etch rates may be due to surface passivation by the R4BAE ligands and/or the Cu(R4BAE) complexes. Introduction Copper has emerged as the preferred interconnect metal for the miniaturization of computer microchips because of its superior electrical conductivity, migration, and processing properties when compared to aluminum, tungsten, gold, and silver.1-3 Unfortunately, because of difficulties in patterning copper by conventional dry-etch techniques, chemical-mechanical planarization (CMP) is required for its processing in microprocessor devices.4,5 CMP is a process of oxidizing and chelating the copper overburden while mechanically polishing the surface of the wafer.6 Often CMP is accomplished in aqueous media where the slurry composition includes a combination of oxidants, chelants, buffers, abrasives, and/or cosolvents. As the current aqueous and organic slurries lead to both technical and environmental challenges,1-3,7,8 we reported the first model of a “dry” copper CMP process based on supercritical (sc) CO2 using β-diketone chelating agents.9,10 This model examined only the chemical aspects of the CMP process, i.e., the oxidation and chelation of copper, without mechanical polishing. Condensed CO2 is particularly amenable for use in the CMP process because CO2 can be removed from the device by simple * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (610) 519-4876. Fax: (610) 5197167. † Department of Chemistry, Villanova University. ‡ Department of Chemical Engineering, Villanova University. § University of North Carolina at Chapel Hill. | North Carolina State University.

depressurization. In contrast, aqueous slurries require water removal by high-temperature processing or the residual moisture may cause undesirably high dielectric constants in the insulating layers of the device. Unlike aqueous solvents, condensed CO2 has an extremely low viscosity and surface tension.11,12 These properties are also desirable for CMP processing as they allow the entire wafer surface to be efficiently and effectively covered by the slurry media, preventing unreacted regions on the wafer surface and avoiding image collapse of the patterned interconnect regions. Condensed CO2 also circumvents many of the problems associated with traditional organic solvents. Unlike halogenated organic solvents, condensed CO2 has low toxicity, is not flammable, and is not susceptible to solvent oxidation.13 Finally, CO2 has “tunable” solvent properties (based on changes in temperature and pressure) such that the solubility of the reactants and/or products may be varied to provide a recyclable process.12 Hexanes have also been examined as a possible solvent. Hexanes are more environmentally friendly, inexpensive, and nontoxic than chlorinated organic solvents and may offer some of the recycling capabilities not possible with aqueous CMP slurries. Notably, hexanes have properties similar to those of sc CO2 but do not require the high-pressure equipment. Table 1 highlights the solvent properties for water, the commonly used CMP solvent, and compares it with sc CO2 and hexanes (used in this study).12,14-17 Notably, hexanes provide similar solubility properties to sc CO2, yet the lower viscosity and surface tension of sc CO2 provides more advantageous transport properties. In order to better understand the copper chelation and dissolution processes in sc CO2, substituted bis(acetylacetone)-

10.1021/ie060947v CCC: $33.50 © 2006 American Chemical Society Published on Web 11/23/2006

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Table 1. Comparison of Solvent Properties for sc CO2, Hexanes, and Water material

dielectric constant

density (g/cm3)

viscosity (mPa‚s)

surface tension (dyn/cm)

CO2 hexanes water

1.51a,b 1.86e,f 78.4e,h

0.852c,d 0.64e,d 0.997e,h

0.08c,d 0.25e,d 0.89e,h

∼0 16.32d,g 69.60e,h

At 45 °C and 214 bar. b Reference 14. c At 40 °C and 214 bar. d Reference 15. e At 40 °C and 1 bar. f Reference 16. g At 40 °C and 0.37 bar. h Reference 17. a

Figure 1. Structures of the R4BAE and R2DTC- families.

ethylenediimine (R4BAE) ligands and lithium or sodium dialkyldithiocarbamates (Li(R2DTC) or Na(R2DTC)) salts have been explored. Figure 1 shows the structure of these two families. The R4BAE compounds were first synthesized by Combes and Combes in 1889 and later by Schwarzenbach and Lutz.18 Martell and co-workers investigated the infrared19,20 and UVvisible spectra21 and dipole moments22-24 of the R4BAE ligands and their metal complexes. The extraction of copper by R4BAE ligands in traditional solvents has been studied by spectroscopic25-27 and electrochemical28 methods. The R4BAE ligands offer significant improvements over the β-diketone ligands for chelating and solubilizing copper(0). Notably, the coordination chemistry of the R4BAE ligands with copper is fairly simple: only the Cu(II) complexes have been isolated and the R4BAE ligands strongly prefer coordination in a tetradentate, square planar geometry. These properties can be compared to the more complex coordination chemistry of bis(β-diketone)copper compounds, where, depending on the substituent of the (β-diketone) ligand, both copper(I) and copper(II) species may be isolatable and the structures of the preferred copper products can vary considerably (i.e., Cu2(acac)4 is dimeric, Cu(tfac)2 is square planar, and Cu(hfac)2·H2O is square pyramidal). The solubilities of R4BAE complexes in CO2 have not been previously reported, and thus, we present the phasebehavior measurements here in the form of cloud-point determinations. Dialkyldithiocarbamates have also been synthesized29 and characterized previously.30-34 These ligands have been particularly useful as analytical reagents for metal analyses using gas chromatography.35-37 They form CO2-philic compounds with over 40 metals and nonmetals.38-45 The R2DTC- ligands were expected to be a better choice than the previously examined β-diketone ligands for copper dissolution because their experimental stability constants (β2) are significantly higher: the β2 value for Cu(Et2DTC)2 is 18.42 (dissolution method),45 while β2 for Cu(hfac)2 is only 3.84.46 Their stability to heat treatment (especially as copper complexes) and their excellent solubility in CO2 make the R2DTC- ligands excellent candidates for microelectronics processes such as cleaning (e.g., chemical vapor deposition (CVD) reactors and heated sections of processing equipment or the back-side of the wafers) or copper CMP. While the H+(R2DTC-) salts are preferable for semiconductor process-

ing, they are not stable in sc CO2 and quickly decompose. The solubilities of the M+(R2DTC-) salts, where M+ ) Li+ or Na+, have been previously reported in condensed CO2.47 Experimental Section Materials. Carbon dioxide (99.99% or higher) was obtained from BOC gases or National Specialty Gases. Bis(2,2,2trifluoroethyl)amine was obtained from Synquest. tert-Butyl peracetate (75 wt % in aliphatic hydrocarbons) and all other reagents were obtained from Sigma-Aldrich and used as received. Caution! Although we haVe experienced no difficulty with t-BuPA, this solution should be treated as potentially explosiVe and handled with care. Etching studies in CO2 were performed on copper coupons (99.99%) obtained from Alfa Aesar or on copper films supported on semiconductor-grade silicon wafers obtained from International Sematech. The copper coupons were 0.25 mm thick and cut to 3 mm × 3 mm squares. Copper on silicon wafer samples were composed of silicon, a 250 Å TaN barrier layer, a 1 kÅ copper seed layer, and a 15 kÅ layer of electrodeposited copper. The wafers were cut to 10 × 15 mm pieces. Prior to reaction, the copper on silica wafer samples were soaked in 5 vol % H2SO4 for 3 min, rinsed with distilled water for 5 s, and air-dried. The critical temperature and pressure of supercritical CO2 are 31.1 °C and 73 atm, respectively. Our reaction conditions are slightly above this temperature and pressure yet represent a minimal energy investment in the etching process (while still working under sc conditions) to make this process as environmentally friendly as possible. Significantly higher temperatures would negate a comparison study with hexanes (which boils at 65-70 °C) and/or may cause ligand and/or complex decomposition (e.g., 109-178 °C for the R4BAE ligands and 220350 °C for the Cu(R2DTC)2 complexes).31,33 This range was also used for published studies using β-diketone ligands, which we used for comparison.9,10 Notably, Yazdi and Beckman studied the effect of pressure on the extraction of arsenic into condensed CO2 by fluoroether dithiocarbamates and found that, once a minimum pressure for solubilizing the chelating agent was reached, increasing the CO2 pressure had little or no effect on the efficiency of the extraction.48 Similarly, Murphy and Erkey investigated the extraction efficiencies of copper ions from aqueous solution by chelation in sc CO2 using tfacH and 2,2-dimethyl-6,6,7,7,8,8,8heptafluoro-3,5-octanedione (fodH) and found that no significant changes were observed with changes in temperature and pressure at a fixed initial amount of chelating agent.49 While our process differs from transition metal ion extractions because copper(0) etching also includes an oxidation step, we do not anticipate pressure or temperature effects on the reaction rates. Studies are underway to test these hypotheses and will be reported separately. Ligand and Complex Syntheses and Characterization. The synthesis of the R4BAE ligands followed the procedures of Schwarzenbach and Lutz18 and Martell and co-workers.19-24 The synthesis of the lithium and sodium R2DTC salts followed the procedures developed by Sucre and Jennings29 and Sakai et al.50 The characterization of the R4BAE and R2DTC- ligands and their respective copper(II) complexes were accomplished by elemental analysis (Atlantic Microlabs, Inc., Norcross, GA), melting points, cyclic voltammetry, and 1H, 13C, and 19F NMR, infrared, and UV-visible spectroscopies, as appropriate. Notably, the only copper species isolated from the etching reactions were Cu(R4BAE) or Cu(R2DTC)2.

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(a) Characterization of Cu(Me4BAE). IR (KBr): 3362(w) ν(H2O), 2921(m) ν(CH3), 1590(s) ν(C...O), 1516(s) ν(C...N), 1460(m) ν(C...C), 1418(s) δd(CH3), 1354(m) δs(CH3), 1283(m) ν(C_CH3) + ν(C...C), 1225(m), 1114(m) βacac(CH), 1040(m), 1016(m) Fr(CH3), 946(m) ν(C...C) + ν(C...O), 752(s) γacac(CH), 649(m), 457(s) cm-1. UV-vis (CH2Cl2, λ ((nm)  (M-1 cm-1))): 546(240), 373(sh), 335(sh), 310(21 400), 276(8 600). Cyclic voltammetry: E1/2 (V vs SSCE in 0.1 M TBAB/CH3CN): +0.825 V (∆Ep ) 0.133 V, ip,c/ip,a ) 0.2), Ep,c ) -0.51 V. Anal. Calcd for C12H18O2N2Cu: C, 50.42; H, 6.34. Found: C, 50.38; H, 6.40. (b) Characterization of Cu(Me2(CF3)2BAE). IR (KBr): 2922(w) ν(CH3), 2851(w), 1623(s) ν(C...O), 1545(s) ν(C...N), 1468(m) ν(C...C), 1375(m) ν(C...C) + ν(CF3), 1253(s) ν(CF3), 1224(m), 1181(s) βtfac(CH), 1148(s), 1129(s), 896(m) γtfac(CH) + ν(C-CF3), 857(s) ν(CF3), 791(m) and 789 ν(C-CF3), 732(s) cm-1. UV-vis (CH2Cl2, λ ((nm)  (M-1 cm-1))): 557(170), 370(sh), 331(sh), 304(15 200), 270(7300). Cyclic voltammetry: E1/2 (V vs SSCE in 0.1 M TBAB/CH3CN): +1.34 V (∆Ep ) 0.127 V, ip,c/ip,a ) 0.3), Ep,c ) -1.05 V. Anal. Calcd for C12H12O2N2F6Cu: C, 36.61; H, 3.05. Found: C, 34.43; H, 3.11. (c) Characterization of Cu((CF3)4BAE). IR (KBr): 3261(m) ν(H2O), 2923(w), 1662(m) (C...O), 1634(m) ν(C...N), 1525 and 1475(m) ν(C...C) + δhfac(CH), 1525(m), 1475(s) δ(CH) + ν(C...C), 1325(s) ν(C...C) + ν(CF3), 1271 and 1247(s) ν(CF3), 1199(s) βhfac(CH), 1140(s), 1065(m), 1017(w), 951(m) ν(C...C) + ν(C...O), 918(m), 910(s) ν(CF3), 796(m) γhfac(CH) + ν(CCF3), 731(s) ν(C-CF3), 672(w), 670(m), 614(m), 578(m), 538(w) cm-1. UV-vis (CH2Cl2, λ ((nm)  (M-1 cm-1))): 619(200), 388(sh), 345(sh), 327(10 300), 267(5 200). Cyclic voltammetry: E1/2 (V vs SSCE in 0.1 M TBAB/CH3CN): +1.46 V (∆Ep ) 0.88 V, ip,c/ip,a ) 0.2), Ep,c ) -0.53 V. Anal. Calcd for C12H6O2N2F12Cu·H2O: C, 226.78; H, 1.49. Found: C, 27.48; H, 1.94. (d) Characterization of Cu(Et2DTC)2. IR (ATR): 3674(w), 3573(w), 2952(s), 2930(s), 2870(s), 2730(w), 2362(w), 2157(w), 1502(vs) ν(C_N), 1464(m), 1456(m), 1436(m), 1426(s), 1367(m), 1299(m), 1263(w), 1248(m), 1223(m), 1188(w), 1144(m), 1109(w), 1091(m), 1052(w), 1012(w), 995(s) ν(C...S), 968(m), 951(w), 932(m), 911(w), 838(w), 801(w), 739(w), 727(w). UV-vis (EtOH, λ ((nm)  (M-1 cm-1))): 217(sh), 269(11 900), 289(6400), 433(5100), 458(sh). E1/2 (V vs SSCE in 0.1 M TBAB/acetone): +0.574 V, ∆Ep ) 0.061 V, ip,c/ip,a ) 0.65; E1/2: -0.512 V, ∆Ep ) 0.147 V, ip,c/ip,a ) 0.39. Anal. Calcd for C10H20S4N2Cu: C, 33.36; H, 5.60%. Found: C, 33.20; H, 5.58%. 13C NMR (CP-MAS) δ 32.26 (c), 74.59, 98.73 (b), ∼500 (a). (e) Characterization of Cu(Pr2DTC)2. IR (ATR): 2961(s), 2932(s), 2872(m), 2318(w), 1482(vs) ν(C_N), 1461(m), 1420(s), 1371(m), 1353(m), 1302(m), 1285(w), 1263(w), 1240(m), 1196(m), 1150(m), 1092(m), 1075(w), 1041(w), 980(s) ν(C...S), 893(w), 871(w), 834(w), 802(w), 773(w), 745(w). UV-vis (EtOH, λ ((nm)  (M-1 cm-1))): 218(sh), 271(15 400), 290(8700), 434(6300), 458(sh). E1/2 (V vs SSCE in 0.1 M TBAB/ acetone): +0.577 V, ∆Ep ) 0.092 V, ip,c/ip,a ) 0.56; E1/2: -0.521 V, ∆Ep ) 0.325 V, ip,c/ip,a ) 0.35. Anal. Calcd for C14H28S4N2Cu: C, 40.40; H, 6.78%. Found: C, 40.06; H, 6.79%. (f) Characterization of Cu(Bu2DTC)2. IR (ATR): 2953(s), 2870(s), 2729(w), 2261(w), 1464(s) ν(C_N), 1465(m), 1456(m), 1425(s), 1365(m), 1297(m), 1263(w), 1247(m), 1222(s), 1187(m), 1144(m), 1109(m), 1090(m), 1052(w), 1011(w), 968(s) ν(C...S), 951(m), 932(m), 911(w), 837(w), 823(w), 739(w), 727(w). UV-vis (EtOH, λ ((nm)  (M-1 cm-1))): 221(sh), 270-

Figure 2. Cloud-point diagram for R4BAE ligands as a function of (a) pressure and (b) density: ([) Me4BAE, (9) Me2(CF3)4BAE, and (2) (CF3)4BAE. Reaction conditions: 1.82 × 10-3 (mol of ligand)/(mol of CO2); constant CO2 density contours for the various solutions are given as solid, dashed, and dotted lines.

(20 400), 289(9900), 434(6400), 465(sh). E1/2 (V vs SSCE in 0.1 M TBAB/acetone): +0.568 V, ∆Ep ) 0.074 V, ip,c/ip,a ) 0.54; E1/2: -0.509 V, ∆Ep ) 0.214 V, ip,c/ip,a ) 0.33. Anal. Calcd for C18H36S4N2Cu: C, 45.77; H, 7.68%. Found: C, 45.49; H, 7.86%. (g) Characterization of Cu((CF3CH2)2DTC)2. IR (ATR): 2964(w), 1458(s) ν(C_N), 1408(m), 1386(m), 1319(m), 1293(w), 1259(m), 1133(s), 1103(vs), 1007(s) ν(C...S), 977(m), 921(w), 834(m), 688(m). UV-vis (EtOH, λ ((nm)  (M-1 cm-1))): 220(sh), 265(42 000), 293(21 000), 434(8900), 458(sh). Cyclic voltammetry: E1/2 (V vs SSCE in 0.1 M TBAB/acetone): +0.856 V, ∆Ep ) 0.077 V, ip,c/ip,a ) 0.34; E1/2: -0.059 V, ∆Ep ) 0.345 V, ip,c/ip,a ) 0.55. Anal. Calcd for C10H8S4N2F12Cu: C, 20.85; H, 1.40%. Found: C, 21.22; H, 1.61%. Copper Characterization Methods. Scanning electron microscopy (SEM) of the copper coupons before and after etching was performed on a Hitachi S-570 scanning electron microscope at an accelerating voltage of 15 keV and a working distance of 25 mm. A field emission scanning electron microscope (FESEM; model 6400F, JEOL) was used to observe the surface morphology of the copper films on silicon wafers. A scanning probe microscope (Nanoscope IIIa, Digital Instruments) was operated in atomic force microscopy (AFM) mode to observe the topography of the copper surfaces and to calculate rootmean-squared (rms) roughnesses. Scans were taken at room temperature under ambient conditions. A profilometer (AS200, Tencor) was also used to calculate the surface roughness of the copper on silicon wafer samples. The chemical composition of the copper on silicon wafers was analyzed by X-ray photoelectron spectroscopy (XPS; model 5400, PHI) with a monochromatized Al KR source at a power of 350 W. Shifts were corrected with a binding energy scale fixed at the C(1s) binding energy of 284.6 eV. Phase-Behavior Measurements of the R4BAE Ligands. The cloud points of the R4BAE ligands in condensed CO2 were measured in a stainless steel variable-volume cell, which has been described previously.51 Cloud points were confirmed visually through a sapphire viewing window; see Figure 2. The R4BAE ligand (1.3 × 10-4 mol) and a stir bar were placed in the high-pressure cell, which was then heated to 60 °C and pressurized with CO2 to 275 bar at a fixed starting volume. A ligand mole fraction of 1.8 × 10-4 in CO2 was used for all measurements. The pressure was varied by changing the volume of the cell until the system became turbid upon visual inspection. The temperature was then varied to obtain the cloud-point

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pressures at different temperatures. Each point was observed and repeated 2-4 times, with an average uncertainty of (10 bar. No trend in the errors was observed with increasing temperature or pressure. The accuracy of the cloud-point measurements reflects the accuracy of the pressure transducer. Notably, no color changes in the ligand were observed at the end of the cloud-point measurements, and the maximum temperatures of the solubility determinations were kept wellbelow the melting points of the BAE ligands (BAE ) 109110 °C; Me2(CF3)2BAE ) 152-154 °C; and (CF3)4BAE ) 176-178 °C) so that ligand decomposition is not anticipated. Removal-Rate Studies with Copper Coupons. Etchant solutions were prepared in a 10 mL high-pressure stainless steel cell loaded with a stir bar, the lithium or sodium (R2DTC)ligand (4.72 × 10-6 moles), t-BuPA (9.5 × 10-5 moles), and a copper(0) coupon (∼0.002 g). The reaction vessel was quickly pressurized to, and held at, 40 °C and 172 bar (CO2 density ) 0.81029 g/mL15). After 4 h of reaction with stirring, the cells were slowly vented while flushing with CO2 to minimize deposition of byproducts from solution onto the copper surface. Once the copper coupon was removed from the cell, it was rinsed with acetone and ethanol to further remove loosely trapped byproducts. The reaction duration was selected to obtain accurate weight differences between the unreacted and reacted copper coupons. The copper removal rate, in nm/min, was calculated from the copper coupon weight loss data, the area of the coupon × 2 (for the two-sided coupon), and the density of copper (8.92 g/cm3). The reported dissolution rates for the copper(0) coupons were calculated using the total reaction time and represent an average removal rate. The reported % Cu(0) removed is based on the maximum percentage of copper that could be removed given that the ligand is the limiting reagent (a copper/ligand mole ratio of 0.15 was maintained for all copper coupon reactions). Similar reactions were performed in 10 mL of hexanes under 1 bar of nitrogen for comparison. Note: While ethanol has been shown to remove copper oxides from a copper surface in the vapor phase,52 our mild and brief rinsing step, coupled with the relatively long chemical reaction times, allowed us to assume that the rinsing step did not significantly change the postreaction surface. Removal-Rate Studies with Copper on Silicon Wafer Surfaces. The removal rates using the R4BAE ligands were significantly slower than those for the (R2DTC)- ligands, and so, to obtain more accurate measurements of copper removal rates, the R4BAE reactions were performed using samples of copper on silicon wafers and studied using AFM and profilometer measurements. For these studies, the 10 mL cell was preheated to 40 °C and 7.0 × 10-4 mol of BAE ligand, 3.5 × 10-4 mol of t-BuPA, a stir bar, and a 10 × 15 mm piece of wafer were added. These conditions correspond to a ligand and oxidant-to-copper ratio of 10. The cell was then pressurized to 207 bar (CO2 density ) 0.846 g/mL15), and the reaction was allowed to proceed for 18 h with stirring. The pressure and temperature were sufficient in all cases to fully dissolve the desired chelating agent (at least initially); this was confirmed by visual inspection. The contents of the cell were rinsed twice with pure CO2, and each rinse was helium displaced before the wafer was removed from the cell and stored under N2 (g). The copper thickness was determined by measuring the film height over several areas of the wafer. The copper removal rates (nm/ min) were calculated by subtracting the thickness of copper remaining after reaction from the thickness of the copper before

Table 2. Reaction of Cu(0) on Silicon Wafer with Various R4BAE Ligands and t-BuPA in sc CO2

ligand none Me4BAE Me2(CF3)2BAE (CF3)4BAE

thickness (microns)

copper removed (%)a

1.729 ( 0.050 N/A 1.249 ( 0.098 27.8 ( 5.7 1.344 ( 0.047 22.2 ( 3.7 0.963 ( 0.106 44.3 ( 6.1

average rate of removal (nm/min)

rms roughness (nm)

N/A 0.44 0.35 0.71

0.44 ( 0.15 68.4 ( 18.3 41.0 ( 33.4 20.5 ( 7.8b

a Reaction conditions: 40 °C, 207 bar (CO density ) 0.846 g/mL), 2 and 18 h. b AFM measurements were taken on a recessed area.

reaction and dividing this by the total reaction time to give an average removal rate. Results and Discussion Phase Behavior of the R4BAE Ligands in sc CO2. The cloud points of the various R4BAE ligands were measured in sc CO2 as a function of temperature and pressure. The cloud point reflects the condition at which a solute starts to precipitate from solution (i.e., the solution becomes turbid upon visual inspection). Single-phase systems are formed above each curve. The cloud-point diagram for the R4BAE ligands is shown in Figure 2 along with constant-density contours for the sc CO2. The R4BAE ligands showed the same general solubility trends as were recently reported for Li(R2DTC) ligands.47 In general, the cloud-point pressure increased (signifying a lower solubility) with increasing temperaturesmost likely because of a decrease in CO2 density.15 As temperature is increased at a fixed density, the solubility of the ligand increases. This has been observed previously.53,54 The solubilities of the R4BAE ligands were also observed to increase with increasing fluorine content. It has been noted that, for relatively nonvolatile molecules, fluorocarbons tend to be more soluble than hydrocarbons of similar molecular weight.43,47,55-58 Because solubility depends upon solute-solute, solvent-solvent, and solute-solvent interactions, the differences in solute-solvent interactions account for the variation in solubility within a ligand family. The R4BAE compounds are all at least an order of magnitude higher in solubility than the Li(R2DTC) salts.47 These higher solubilities of the R4BAE compounds were expected because the ionic nature of the Li(R2DTC) salts make them more difficult to dissolve in sc CO2 than the neutral species (i.e., R4BAE). The Na(R2DTC) salts were much less soluble than their lithium counterparts. Removal Rate Studies Using the R4BAE Ligands and Copper on Si Wafer Surfaces. The results of the copper etching studies by t-BuPA and the R4BAE ligands are given in Table 2. Because the weight losses are small, profilometry measurements were conducted for accurate measurements. The first entry of Table 2 gives the measured thickness of the unreacted copper on silicon wafer as 1.729 ( 0.050 µm. A 1.6 µm copper film thickness was specified by the manufacturer. The Me4BAE and Me2(CF3)2BAE ligands removed about 27.8 and 22.2% of the available copper surface, respectively. Assuming this copper loss over the entire time period gives average overall rates of copper removal at 0.44 and 0.35 nm/ min, respectively. The (CF3)4BAE ligand was more effective, removing ∼44.3% of the initial copper surface at an average rate of 0.71 nm/minsa 37% improvement. This improvement is attributed, in part, to the greater solubility of the (CF3)4BAE ligand and the Cu((CF3)4BAE)2 complex in sc CO2. Notably, these copper(0) dissolution rates represent the net copper oxidation, chelation, and dissolution processes but should not

Ind. Eng. Chem. Res., Vol. 45, No. 26, 2006 8783 Table 3. Literature Values for Copper Binding Energies copper species

binding energy (eV)

Cu metal Cu2O CuO Cu(OH)2 Cu(acac)2

932.5 ( 0.15a,b 932.5 ( 0.2a,b 933.8 ( 0.2a,b 934.4 ( 0.2a,b 934.7c,d

a All references refer to Cu(2p3/2) unless noted. b Reference 59. c Refers to Cu(3d5/2). d Reference 60.

Figure 4. Cu 2p3/2 region for copper species treated with slurries containing (a) (CF3)4BAE, (b) Me2(CF3)2BAE, (c) Me4BAE, (d) a “cleaned” copper surface, and (e) the “as-received” copper surface. The intensities of the ligand peaks were multiplied by a factor of 2, and the intensity of the cleaned copper surface was reduced by a factor of 5 for scaling purposes. Binding energies were measured in eV.

Figure 3. SEM micrographs of Cu(0) films on Si surface after reaction with R4BAE ligand-containing CO2 slurries: (a) Me4BAE, (b) Me2(CF3)2BAE, and (c) (CF3)4BAE.

be compared directly with CMP removal rates because no mechanical action was performed on the surfaces. Surface Analysis of Copper on Si Wafer Surfaces Etched Using the R4BAE Ligands in Supercritical CO2. Table 2 also gives the AFM rms surface roughness measurements for the unreacted and reacted copper on silicon wafers. The “asreceived” surface was fairly uniform with a measured surface roughness of 0.44 ( 0.15 nm. Figure 3 shows the SEM images of the postreaction copper surfaces. A comparison of parts a-c of Figure 3 shows that the overall surface roughnesses decreased in the order: Me4BAE (rms roughness ) 68.4 nm) > Me2(CF3)2BAE (41.0 nm) > (CF3)4BAE (20.5 nm). Notably, while the Me4BAE and Me2(CF3)2BAE were more rough, the surfaces were fairly homogeneous in appearance. In contrast, the post (CF3)4BAE-treated copper surface was smoother overall but had a much more heterogeneous morphology as the surface was interrupted by infrequent pits and eruptions. The increased solubility of the (CF3)4BAE (and its subsequent copper products)

decreases the overall roughening of the copper surface, while the Me4BAE and Me2(CF3)2BAE ligands cause more extensive, deeper surface corrosion over most of the exposed region. The copper surface roughness such as that observed here is often observed in concentration cells when studying metal corrosion. In such instances, areas that are in contact with different concentrations of solution can create a galvanic cell. This can happen when the deposit settles on the copper surface and reduces the access of the surface to the solution for the small region under the deposit. To better understand the etch behavior, the copper on silica samples were analyzed using X-ray photoelectron spectroscopy (XPS). Literature values of the copper binding energies for Cu(0), CuO, Cu2O, and the structurally similar copper(II) complex, Cu(acac)2, are given in Table 3.59,60 Partial XPS spectra (Cu 2p3/2 region only) of the copper on silica wafers before and after reaction with the R4BAE/t-BuPA/CO2 slurry are shown in Figure 4. The Cu(2p3/2) region shows that the as-received copper on the silica surface (Figure 4e) is composed of Cu(0) and/or Cu2O species (these are indistinguishable in terms of binding energy at 932.7 eV) as well as CuO and/or Cu(OH)2 as determined by the prominent satellite structures on the high-binding-energy side of the copper core lines (at 940.7 and 944.3 eV).59,60 Additional very broad peaks on the as-received surface were not specifically identified. Treatment with H2SO4 produced a “cleaned” unreacted copper surface (Figure 4d) that did not show the satellite peaks attributed to the Cu(II) species (CuO or Cu(OH)2) nor the presence of unidentified peaks. The surface of Figure 4d is assumed to be only Cu(0) and/or Cu2O. The XPS data from the Me4BAE slurry (Figure 4c) reveal a strong peak centered at 932.7 eV, which has been assigned to

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Table 4. Reaction of Cu(0) Coupons with t-BuPA and Li(R2DTC) in sc CO2 or Hexanes % Cu(0) removeda

average rate of removal (nm/min)

ligand

hexanes

CO2

hexanes

CO2

Li(Et2DTC) Li(Pr2DTC) Li(Bu2DTC) Li((CF3CH2)2DTC)

19.8 20.2 20.0 27.2

24.3 26.7 27.7 36.4

8.39 8.40 8.40 11.2

9.78 11.2 11.2 16.7

Reactions were run at 40 °C and 172 bar (CO2 density ) 0.81 g/mL) for 4 h. a

the Cu(0)/Cu2O, as well as a peak at 935.4 eV, which has been assigned to the Cu(Me4BAE) complex. Although Cu(OH)2 has a similar binding energy to that expected for the Cu(Me4BAE) complex, we have assigned this peak to the complex only. Notably, Cu(OH)2 is well-known to partially decompose in the X-ray beam unless deposited in a very thin film on the metal.60 In practice, this decomposition appears as a broadened peak or as high-energy satellites. Because these are not observed, the peak at 934.5 eV has been assigned to the Cu(Me4BAE) complex and not to Cu(OH)2. This assignment was confirmed by the O(1s) XPS spectrum, which showed two peakssone of which was assigned to the uncoordinated Me4BAE ligand and the other of which was assigned to the Cu(Me4BAE) complex. These results imply that the etch rate may be lower than expected because of surface passivation by both the ligand and the copper complex. The Cu(0) and/or Cu2O peak increased dramatically relative to the peak assigned to the Cu(Me2(CF3)2BAE) or Cu((CF3)4BAE) complexes in parts b and a of Figure 4, respectively. Only a small amount of the Me2(CF3)2BAE ligand was observed on the copper wafer surface, as indicated by small peaks in the O(1s) and N(1s) spectra. Inspection of the (CF3)4BAE-reacted surface showed no peaks in the N(1s) region and small oxide peaks in the O(1s) region. The higher observed etch rates coupled with the lack of surface passivation indicates that increased solubility of the Me2(CF3)2BAE) and (CF3)4BAE ligands and their corresponding metal complexes is critical to their increased reactivities. Removal-Rate Studies Using Lithium and Sodium R2DTC Salts and Copper(0) Coupons. The R2DTC- ligands reacted much more quickly with copper(0) than the R4BAE ligands, and thus, reliable dissolution rates were obtained based on massloss measurements from copper coupons rather than profilometry measurements on copper(0) on silicon surfaces. The results of the reaction of various R2DTC- ligands with t-BuPA and copper(0) coupons are given in Table 4. The results for sc CO2 indicate that the percent Cu(0) removed using the lithium salts increases in the following order: Et2DTC- < Pr2DTC- ≈ Bu2DTC- < (CF3CH2)2DTC-. The percentage of copper(0) removed generally increases as the alkyl chain length of the R2DTC- ligand increases or as fluorinated substituents are introduced. Upon the basis of the average percent copper removal for the entire etching period, the most effective ligand (Li(CF3CH2)2DTC) had a dissolution rate of 16.7 nm/min of Cu(0). Although the copper on silicon wafer used for the R4BAE ligands represents a seeded, vapor-deposited copper surface while the copper foil used for the R2DTC- ligands resulted from a rolling process (and, thus, is considerably more disordered), the increase in dissolution rate for the (CF3CH2)2DTC- ligand over the (CF3)4BAE ligand is nearly 24×. Hexanes are often used as a model for sc CO2 since it displays similar solvent behavior. Both solvents are nonpolarspossessing similar dielectric constants, densities, and often ligand and

Table 5. Effect of Varying Cation of R2DTC- Ligands on Cu(0) Removal in sc CO2 % Cu(0) removeda DTC-

R2

anion

Et2DTCPr2DTCBu2DTC(CF3CH2)2DTC-

Li+

Na+

24.3 26.7 27.7 36.4

19.8 19.9 23.4 not tested

a Reactions were run at 40 °C and 172 bar (CO density ) 0.81 g/mL) 2 for 4 h.

complex solubilities. Table 4 also shows that, at similar solvent densities, the percentage of copper(0) removed in sc CO2 is higher than that in hexanes (18-28%) for all of the R2DTCligands. The strong quadrupole moment of CO2 may be responsible for the increased solubilities and subsequent increased reactivities in the condensed media relative to hexanes. Another factor which may be responsible for the increased solubilites are van der Waals forces, which may favor decomplexation for the hexanes as these interactions strengthen. This type of behavior has been studied quantitatively for systems in which the hydrogen bonding was compared to the cohesive energy density of the solvent but, to the best of our knowledge, has not yet been studied with transition metal complexes.61 Table 5 shows the effect of the cation (M+) of the M+(R2DTC-) compounds on the copper removal rates in sc CO2. The Li+(R2DTC-) salts were 16-26% more effective in removingcopper(0)fromthecouponsurfacethantheNa+(R2DTC-) salts. This is most likely due to the fact that the lithium salts are more soluble than sodium salts in sc CO2. As stated previously, Na+ and Li+ (R2DTC-) salts are used in this study because they are easily synthesized and inexpensive ligands for comparison; neither of these alkali metal cations is appropriate for use in actual semiconductor manufacture because they have high mobilities in the oxide films, creating local fields that cause device failure. The H+(R2DTC-) salts are unstable in CO2. SEM images of the copper coupons before and after reaction with Li(R2DTC) and t-BuPA in sc CO2 are given in Figure 5. Notably, the blank copper coupon (a) had a nonuniform surface consistent with the “rolling” of the copper foil. The copper surface after reaction with Li(Et2DTC) (b), Li(Pr2DTC) (c), and Li(Bu2DTC) (d) shows increasing amounts of complex and/or ligand precipitation on the surface, which reduced the effective area for etching. After treatment with Li((CF3CH2)2DTC), the copper coupon showed the smoothest surfaceswithout the grooves associated with the blank coupon and virtually free of any extraneous ligand or complex precipitation or corrosion pitting. Notably, the smooth surface of the copper after Li((CF3CH2)2DTC) treatment was obtained without the addition of mechanical polishing. Factors Effecting Average Etching Rates. It has been demonstrated that the solubilities of both the R2DTC- and R4BAE ligand families increase with increasing fluorination in sc CO2 (see Figure 2 and ref 47); however, the solubilities of the alkylated and fluorinated ligands in hexanes contradict this trend. In hexanes, the ligand solubilities are as follows: LiEt2DTC ) 0.31 g/mL; Li(CF3CH2)2DTC ) 0.091 g/mL; Me4BAE ) 0.17 g/mL; and (CF3)4BAE ) 0.032 g/mL. Notably, the average copper etching rates increase with increased fluorination for both ligands, in both sc CO2 and hexanes. These trends indicate that, while ligand solubilities in the pure solvent may play an important role in the etching reactivity, it is not the only determining parameter. Reactivity may also be strongly influenced by cosolvent-induced interactions (here, with t-BuPA),

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Figure 5. SEM micrographs (at 1000× magnification) of copper coupons after reaction with R2DTC- ligand-containing CO2 slurries: (a) unreacted Cu(0) coupon, (b) Li(Et2DTC), (c) Li(Pr2DTC), (d) Li(Bu2DTC), and (e) Li((CF3CH2)2DTC).

hydrogen bonding, ion-pair formation, solvent clustering, and the volume changes of the pure liquid components on mixing.62 Lagalante et al. studied the solubility of copper and chromium β-diketonate complexes in sc CO2, demonstrating that solubility is strongly dictated by the substituents on the ligands surrounding the central metal atom.62 They used the method of Fedors (a group-contribution approach)63,64 to calculate the solubility parameters of uncomplexed β-diketonate ligands and correlated these parameters to experimentally measured extraction solubilities. For the β-diketonate ligands, the solubility constants (δ, reported in cal1/2 cm-3/2) were reported as follows: acac) 10.65, tfac- ) 9.54, and hfac- ) 8.71.62 Their data showed that a difference in solubility constants (∆δ) of 1.94 correlated to a ∼250-fold increase in mole fraction solubilities of the Cu(β-diketonate)2 complexes at 40 °C and a sc CO2 density of 0.760 g/mL.62 They attributed these solubilities to the character of the hydrocarbon or fluorocarbon shell surrounding the metal center. Using Fedors’ method, we have calculated the δ values for the ligands used in this study as follows: Et2DTC- ) 10.53; (CF3)2DTC- ) 8.31; Me4BAE ) 10.03; and (CF3)4BAE ) 5.59.63-65 If the trends for the β-diketonates hold, we would expect the Cu((CF3)2DTC)2 complex to be 290× more soluble than the Cu(Et2DTC)2 complex; however, Erkey reports the Cu((CF3)2DTC)2 complex to be 360× more soluble under similar conditions.66 We have reported a 60% decrease in the mole fraction solubility of the Li(CF3)2DTC ligand relative to the

LiEt2DTC ligand, under similar conditions.47 These comparisons indicate that (1) Fedors’ solubility values (δ) may predict the magnitude of copper complex solubilities based on ligand substituent changes, but greater resolution may not be possible without more advanced calculation techniques; and (2) Fedors’ solubility values are better at predicting copper complex solubility than ligand solubility in sc CO2. This may be due to geometric changes caused by metal coordination. Given these limitations, we have further compared Fedors’ solubility parameters with the reported average etching rates for the β-diketone ligands, tfacH (0.3 nm/min) and hfacH (3.9 nm/min) ligands in sc CO2 at 40 °C and 214 bar.10 The average copper etching rate increases 13×, while the solubility parameter for the β-diketones decreases 0.83×. Using this ratio, we would anticipate that the observed decrease in δ for the corresponding alkylated and fluorinated BAE ligands would cause an improvement in the BAE etching rate of 700% and, for the alkylated and fluorinated DTC ligands, an increase in the etching rate of ∼350%. As stated previously, an etching rate improvement of only 37% was observed for the (CF3)4BAE ligand over the Me4BAE ligand, and an etch rate increase of 70% was observed for the DTC family. Clearly, δ values, while relatively effective for predicting complex solubilities, are not effective for predicting copper etching reactivities, at least when used in a linear correlation.

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Conclusions Several R4BAE and Li+ or Na+(R2DTC-) ligands were successfully compared for the oxidative dissolution of copper(0) (with t-BuPA) in sc CO2 or hexanes. Copper etch rates increased with increasing fluorine substitution for both the R4BAE and R2DTC- ligands, but the R2DTC- ligands outperformed the R4BAE ligands by a ∼20-25-fold increase in dissolution rate (ignoring changes in the copper surface). The lower dissolution rates for the BAE ligands may be attributed to the lower solubility of their respective copper complexes causing increased passivation (as confirmed by XPS) and a corrosion-like pitting morphology (as confirmed by AFM and SEM) of the copper surface. The Li(R2DTC) salts gave 1626% higher average copper etching rates than their sodium counterparts, and the rates of copper(0) removal with the R2DTC- ligands were 18-28% higher in sc CO2 than in hexanes. Calculated solubility parameters could not linearly account for experimentally determined etching rates, and further correlation studies are needed. Acknowledgment We thank Professor Norman R. Dollahon of Villanova University for his assistance with the SEM and Christopher Gallo for his assistance with copper complex characterization. We also acknowledge helpful comments by the reviewers. Semetech International kindly provided the copper on silicon wafer samples. This material is based upon work supported by the National Science Foundation under Agreement No. CHE9876674 (STC Program) and Grant No. CHE-0416040. We also received financial support from the Kenan Center for the Utilization of CO2 in Manufacturing and the Harry S. Truman Scholarship Foundation (G.M.D.). This material is based upon work supported while serving at the National Science Foundation (C.A.B.). Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Nomenclature 2,4-pentanedione ) acacH 1,1,1-trifluoro-2,4-pentanedione ) tfacH 1,1,1,5,5,5-hexafluoro-2,4-pentanedione ) hfacH 1-phenylpentane-1,3-dione ) bzacH 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione ) fodH bis(acetylacetone)ethylenediimine ) Me4BAE bis(1,1,1-trifluoroacetylacetone)ethylenediimine ) Me2(CF3)2BAE bis(1,1,1,5,5,5-hexafluoroacetylacetone)ethylenediimine ) (CF3)4BAE lithium bis((diethyl)dithiocarbamate) ) Li(Et2DTC) lithium bis((dipropyl)dithiocarbamate) ) Li(Pr2DTC) lithium bis(di(n-butyl)dithiocarbamate) ) Li(Bu2DTC) lithium bis(di(1,1,1-trifluoroethyl)dithiocarbamate) ) Li((CF3CH2)2DTC) Literature Cited (1) Steigerwald, J. M.; Murarka, S. P.; Gumann, R. J. Chemical Mechanical Planarization of Microelectrode Materials; John Wiley & Sons: New York, 1997. (2) Singh, R. K.; Bajaj, R. Advances in Chemical-Mechanical Planarization. MRS Bull. 2002, 27, 743. (3) Hanazono, J.; Amanokura, J.; Kamigata, Y. Development and Application of an Abrasive-Free Polishing Solution for Copper. MRS Bull. 2002, 27, 772.

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ReceiVed for reView July 20, 2006 ReVised manuscript receiVed October 16, 2006 Accepted October 17, 2006 IE060947V