Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Full Paper Substrate-Independent Palladium Atomic Layer Deposition By Jay J. Senkevich,* Fu Tang, Diana Rogers, Jason T. Drotar, Christopher Jezewski, William A. Lanford, Gwo-Ching Wang, and Toh-Ming Lu A novel method is presented for the atomic layer deposition (ALD) of palladium on a tetrasulfide self-assembled monolayer functionalized SiO2 surface. Additionally, a novel reducing agent (glyoxylic acid) was used to remove the organic ligands from the chemisorbed palladium(II) hexafluoroacetylacetonate metallorganic. Glyoxylic acid is an effective reducing agent above 200 C, which is not optimal for palladium but it is effective enough to show proof-of-concept and deposit a Pd ªseedº layer. Palladium was also deposited on iridium at 80 C and 130 C via a hydrogen process or on the Pd seed layer at 80 C. The 60 Pd film grown on the tetrasulfide self-assembled monolayer (SAM) showed nearly random texture and higher carbon and fluorine contamination levels compared to the one grown on Ir. The 55 film grown on Ir at 80 C is highly (111) textured with a grain size of ~60 , as shown by reflection high energy electron diffraction (RHEED). The higher contamination levels of the Pd film deposited on the tetrasulfide SAM, as measured by X-ray photoelectron spectroscopy (XPS), is attributed to the high temperatures needed to deposit the Pd seed layer. The higher deposition temperatures cause more dissociation of the hfac ligand and a higher metallorganic desorption rate. These equate to less Pd being deposited and with higher contamination levels. 1. Introduction The use of self-limiting chemical reactions to sequentially grow monolayers of transition metals will positively impact many diverse applications. Among these applications for metal ALD are noble metal catalysts on rough electrode and mesoporous bulk materials,[1,2] Cu seed layers for the electrochemical deposition and CVD of Cu,[3] conformal adhesion layers to Cu metallic overlayers,[4] and alkylthiolate self-assembled monolayers.[5] In particular, palladium is useful as a catalyst in fuel cells[6] and for hydrogenation reactions, gas sensors[7], and hydrogen permselective membranes.[8] Palladium, like the other noble metals, is rather costly. Therefore, a driving force exists to reduce the quantity used as a function of its activity. ALD is ideal to extract the highest performance out of noble metal catalysts with least cost, since the growth it produces is highly conformal and well-controlled. As a result, monolayer-by-monolayer control of growth can take place over complicated geometries and bulk mesoporous supports. To date, metal ALD has had limited success. This is primarily due to the lack of metallorganic chemisorption on oxide-terminated surfaces and the lack of appropriate re± [*] Prof. J. J. Senkevich, Prof. F. Tang, Dr. J. T. Drotar, Dr. G.-C. Wang, Prof. T.-M. Lu Dept. of Physics, Rensselaer Polytechnic Institute 110 8th St., Troy, NY 12180-3590 (USA) E-mail: [email protected] Dr. D. Rogers Dept. of Chemistry, Rensselaer Polytechnic Institute 110 8th St., Troy, NY 12180-3590 (USA) Dr. C. Jezewski, Prof. W. A. Lanford Dept. of Physics, University at Albany 1400 Washington Ave., Albany, NY 12222 (USA) 258 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ducing agents. The lack of chemisorption with palladium(II) hexafluoroacetylacetonate [PdII(hfac)2] on oxidized Ta, Cu, and P surfaces has been shown previously.[9] The inefficency of hydrogen as a reducing agent on non-metallic surfaces is well established, and is attributed to the lack of polarizability and high bond strength (435.99 kJ mol±1) of molecular hydrogen.[10] Alcohols and aldehydes have been investigated as possible reducing agents for Cu ALD with CuII(hfac)2 as a precursor. Unfortunately, Solanki and Pathangey[3] undertook their depositions above the decomposition temperature of CuII(hfac)2, which is 230 C.[11] Coincidentally, 230 C is also the decomposition temperature of PdII(hfac)2 used here.[9,12] Blackburn et al. did not show any growth of Pd via chemical fluid deposition on oxideterminated surfaces below 230 C.[13] Metal ALD has been successful in limited cases on noble metal surfaces or with halogenated precursors that interact preferentially on oxide-terminated surfaces. Martensson and Carlsson used CuII(tmhd)2 with H2 on Pt/Pd seeded glass substrates.[14] As has been established,[9] the noble metals serve two purposes. These surfaces preferential chemisorb CuII(tmhd)2 or PdII(hfac)2, and are known to dissociate molecular hydrogen.[15] H2 ® 2 H (on noble metals) (1) The dissociated hydrogen then serves two purposes. It reduces the Cu2+ or Pd2+ metal ions to the elemental metal and it cleaves the tmhd or hfac ligand, allowing a second pulse of the metallorganic to chemisorb onto the newly created surface. Another example where a metal has successfully been deposited via ALD is tungsten (via WF6 and Si2H6).[16] In this chemical system, WF6 does not have much difficulty inDOI: 10.1002/cvde.200306246 Chem. Vap. Deposition 2003, 9, No. 5 Full Paper teracting with the hydroxylated SiO2 surface and Si2H6 acts here as a reducing agent. However, two drawbacks of this method exist; HF is generated as a side reaction and Si2H6 is only effective reducing metal fluorides. As a result, there is still great interest in developing an ALD technique where metallorganics can be used and deposited on nonnoble metal surfaces, for example, polymers, oxidized metals and oxidized barrier layers. The work presented here explores the ALD of Pd with PdII(hfac)2 and H2, on an Ir surface, and with PdII(hfac)2 and glyoxylic acid (both shown in Fig. 1) on a tetrasulfide self-assembled monolayer. The technique is substrate-indeCF3 sulfide tetrasulfide SAM (13 Å) alkyl SiO2 (14 Å) Si(100) Tetrasulfide SAM C OO - CH - Pd2+ O O HC C O O H-C-C-OH C glyoxylic acid CF3 palladium (II) hexafluoroacetylacetonate Fig. 1. The chemical structures of the Pd ALD precursor and novel reducing agent used in this study. pendent since any surface can be sulfide- or thiol-terminated via self-assembled chemistry, plasma-enhanced surface modification, or sulfur-functionalized CVD oligomers. The ALD thin films (< 60 ,) are analyzed via Rutherford backscattering (RBS) spectrometry, XPS, and RHEED. 2. Results and Discussion 2.1. Palladium ALD on Ir The use of Ir as an ALD substrate accomplishes two things. First, it has been established that PdII(hfac)2 preferentially chemisorbs on Ir.[9] Second, Ir is known to dissociate molecular hydrogen,[15] which acts as a reducing agent and removes the hfac ligands from the PdII(hfac)2 metallorganic. Further, it is important to establish two experimental findings to make the ALD process viable. First, the precursor (or metallorganic) should be able to be sublimed without decomposing. This can be undertaken with thermogravimetric analysis (TGA). Fortunately, PdII(hfac)2 sublimes without decomposing, as has been shown by Poston and Reisman.[17] Second, the metallorganic should not decompose at the surface of the substrate until temperatures are reached that exceed the deposition temperature of the metal. This has been shown previously with PdII(hfac)2 to be 230 C on Ir[9] and on an unspecified surface.[18] As a result of the above work, the processing window for the ALD of Pd is between 60 C and 230 C. The lower limit is due to the condensation of the precursor and the upper limit is due to its thermal decomposition (non-self-limiting growth). The substrate structure of Ir and the tetrasulfide SAM is shown in Figure 2, and explained in more detail in the experimental section. Figure 3 shows the thickness of Chem. Vap. Deposition 2003, 9, No. 5 http://www.cvd-journal.de Noble Metal Fig. 2. The structure of the tetrasulfide SAM and Ir substrates used in this study. Thickness (Å) O O PdII(hfac)2 + H2 a. at 80 °C b. at 130 °C 150 Å Ir Si-O-C linkage CF3 C CF3 Step 1: PdII(hfac)2 + glyoxylic acid at 210 °C, 120 cycles Step 2: PdII(hfac)2 + H2 at 80 °C, 120 cycles 65 60 55 50 45 40 35 30 25 20 15 10 5 0 130 °C 80 °C 0 40 80 120 160 200 240 # of Cycles Fig. 3. Deposition of Pd vs. number of cycles. The lower rate at 130 C is attributed to desorption of PdII(hfac)2 from the Pd surface. the ALD Pd thin films as a function of the number of cycles. The areal density of the Pd films was obtained by RBS, and then converted to a thickness value by: 1 = 6.80 1014 atoms cm±2 obtained from the density and molar mass of palladium. As can be seen from Figure 3, the Pd films grown at 130 C do not exhibit a linear relationship between film thickness and the number of cycles. This paralinear growth is attributed to the difference in bond energies between Ir± PdII(hfac)2 and Pd±PdII(hfac)2. The atomic polarizabilities of Ir and Pd are (7.6 ± 1.9) 3 and (4.8 ± 1.2) 3 respectively.[19,20] The larger polarizability helps to stabilize the van der Waals forces between the Ir surface and the PdII(hfac)2 metallorganic. The change between the initial growth (Ir± PdII(hfac)2) and the subsequent growth (Pd±PdII(hfac)2) does not occur until nearly ~10 , due to the growth mechanism of ALD of Pd via PdII(hfac)2. It should be realized that monolayer-by-monolayer deposition of metallic atoms does not occur, but instead the monolayer deposition of, for example, PdII(hfac)2 occurs assuming no desorption. The primary reason for the lack of monolayer-by-monolayer growth is due to the large size of the organic ligands associated with the metallorganic precursor. At 80 C the vapor pressure of PdII(hfac)2 is much lower, and the Pd±PdII(hfac)2 bond is strong enough that little desorption occurs. However, when Pd is grown on Ir via PdII(hfac)2/H2, an incubation period is consistently observed. It is known that H2 has a very high reactivity with Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 259 Full Paper 2.2. Palladium ALD on Ir and the Tetrasulfide SAM (XPS) Ex-situ XPS was used to investigate the chemical quality of the Pd ALD films. In particular, comparisons were made for Pd deposited on Ir versus the tetrasulfide SAM. The primary difference between the two types of depositions is that a Pd seed layer is deposited via ALD on the tetrasulfide SAM at temperatures > 200 C, where glyoxylic acid is active, and then Pd is deposited by the PdII(hfac)2/H2 process at 80 C used with Ir. Glyoxylic acid was chosen as an effective metallorganic reducing agent due to its use as an electroless Cu reducing agent,[22] and its thermal dissociation to form CO2 and H2CO at temperatures greater than 200 C.[23] The positions of both the Pd 3d5/2 peaks in Figure 4 at 335.7 eV are close to what has been previously established in the literature for elemental Pd. Elemental Pd has been shown to have a Pd 3d5/2 binding energy of 334.8 eV,[27] 335.3 eV,[28] and 335.4 eV.[29] The slight shift to larger binding energies may be attributed to charge referencing or slight contamination from the dissociation of the hfac ligand.[30] However, the relationship between the chemical contamination and the binding energy shift may be less important since the Pd deposited on the tetrasulfide SAM has higher contamination levels (C and F) in contrast to the Pd deposited on Ir but still has the same Pd 3d binding energy. The C 1s and F 1s XPS spectra of Pd deposited on the tetrasulfide SAM and Ir surfaces are shown in Figures 5 and 6. The Pd deposited on the tetrasulfide SAM has significantly higher levels of carbon and fluorine compared to the Ir substrate. This is likely caused by the high temperatures (>200 C) needed to deposit the Pd seed layer with C-C 285.0 eV Pd on tetrasulfide ML C/Pd ratio 500 0.16 C-O 286.5 eV C=O, CFX 287.7-288.3 eV 400 χ 2 = 4.2 300 Pd 3d5/2 335.3 eV Pd on tetrasulfide SAM Pd 3d3/2 340.6 eV C-H 284.6 eV 200 3000 intensity (cps) PdII(hfac)2, and therefore this observation is likely attributed to carbon residing at the Ir surface.[21] After the prerequisite number of cycles of H2, the carbon can be adequately cleaned and Pd ALD can commence. The adventitious carbon on the Ir surface was observed with XPS (not shown) and was significantly greater than on, for example, a SiO2 surface. Further, as the temperature of the substrate is raised, the ability of hydrogen to clean the Ir surface should increase. Pd on Ir 335.7 eV 2000 341.0 eV shake-up shake-up intensity (cps) 0.09 1000 346 344 342 340 338 336 334 0 332 100 Pd on Ir 290 χ = 5.2 2 288 286 284 282 0 280 Binding Energy (eV) Fig. 5. The C 1s XPS spectra for the Pd ALD films deposited on the tetrasulfide SAM and Ir surfaces. The high binding energy shoulder on the peak associated with the C±C or adventitious bonding is associated with ±CFx, C=O, or C±O bonding due to the dissociation of the hfac ligand. Binding Energy (eV) Figure 4 shows the Pd 3d XPS spectra for Pd deposited on Ir and on the tetrasulfide SAM. The Pd 3d5/2 peak for the tetrasulfide SAM surface is referenced to carbon (C 1s peak) at 285.0 eV from previous work.[24,25] The Pd 3d5/2 peak for the Ir surface is referenced to adventitious carbon at 284.6 eV for the C 1s peak. The value for adventitious carbon on Ir was determined with a separate experiment. This measurement must be done with care since the tail of the Ir 4d5/2 peak at 297 eV can interfere with the C 1s peak at 284.6 eV.[26] As a result, quantitative results are hard to come by, though the peak position is easily obtainable. 260 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 250 Pd on tetrasulfide ML 688.9 eV 200 χ 2 = 3.4 150 F/Pd ratio 0.12 100 intensity (cps) Fig. 4. The Pd 3d XPS spectra for 60 of Pd deposited on the tetrasulfide SAM and for 55 of Pd deposited on Ir. The peak position of the Pd 3d5/2 peak is slightly shifted from elemental Pd, which might be attributed to charge referencing or slight chemical contamination from the dissociation of the hfac ligand. 689.5 eV 0.02 Pd on Ir 50 χ 2 = 4.6 0 694 692 690 688 686 684 Binding Energy (eV) Fig. 6. The F 1s XPS spectra for the Pd ALD films deposited on the tetrasulfide SAM and Ir surfaces. The peak position of the spectra is correlated with the fluorine being associated with carbon. http://www.cvd-journal.de Chem. Vap. Deposition 2003, 9, No. 5 Full Paper Pd-S 162.7 eV 120 90 S2p1/2 60 intensity (cps) S2p3/2 30 0 168 166 164 162 160 Binding Energy (eV) Fig. 7. S 2p XPS spectrum showing the Pd±S interfacial reaction between PdII(hfac)2 and the tetrasulfide SAM. 63.70eV 240 cycles Ir-Pd 60.72eV 1600 120 cycles 63.89eV Ir4f5/2 60.91eV Ir4f7/2 1200 800 intensity (cps) glyoxylic acid. Higher temperatures favor a greater degree of ligand dissociation, which is problematic with the b-diketonate metallorganics.[30] Both C 1s spectra show evidence of CFx, C=O, or C±O bonding due to dissociation from the original hfac ligand. Much of the carbon evident from Figure 6 is attributed to adventitious sources existing at the top of the Pd deposit due to ex-situ analysis. Proper chemical analysis of the Pd ALD films would be better undertaken in-situ; however, experimental difficulties preclude this set up at this time. Golub et al. have shown that the position of the F 1s peak is correlated to atomic percent of fluorine that exists in polymer repeat units.[31] Poly(tetrafluoro ethylene) (PTFE) has 67 at.-% of fluorine, and a corresponding binding energy of 689.2 eV.[31] The same ±CF2± group has a binding energy in the C 1s spectra of 290.0±291.4 eV. Lin et al. showed that physisorbed PdII(hfac)2 on a clean Cu surface at 120 K had a F 1s binding energy of 688.2 eV.[30] After annealing the physisorbed PdII(hfac)2 at 473 K, they saw the formation of a new peak at 682.9 eV. However, the fluorine peaks associated with the Pd ALD films in Figure 6 are at 688.9 eV (on tetrasulfide SAM) and 689.5 eV (on Ir), possibly indicating a loss of carbonyl groups from the hfac ligand. Also, it should be noted that carbon-bonded fluorine typically has larger F 1s binding energies compared to metal fluorides, and the binding energies of the F 1s spectra in Figure 6 are consistent with the former. 400 15 cycles 0 II 2.3. Interfacial Interactions between Pd (hfac)2 and the Tetrasulfide SAM and Ir Surfaces It has been established previously that PdII(hfac)2 interacts weakly, or not at all, with hydroxyl- or oxygen-terminated surfaces.[9] Further, weak van der Waals interactions are not strong enough to make ALD possible. As a result, chemisorption needs to exist between PdII(hfac)2 and the tetrasulfide SAM and Ir surfaces. That chemisorption exists means that interfacial Pd±S and Pd±Ir bonding should be evident. In order to see such bonding in the XPS spectra, for example, the Pd±S signal should be large relative to the C±S or S±S bonding, if the S 2p spectrum is used for analysis. Figure 7 shows the S 2p spectrum for a monolayer of PdII(hfac)2 deposited on the tetrasulfide SAM surface (Fig. 2). The tetrasulfide SAM has a peak at 164.1 eV in the S 2p XPS spectrum (previously established).[25] After pulsing PdII(hfac)2 over the surface, chemisorption is evident and a significant binding energy shift occurred in the S 2p spectrum, as shown in Figure 7. The composite S 2p peak exists at 163.1 eV, whereas the deconvoluted spectra has two peaks, one at 162.7 eV associated with the S 2p3/2 peak, and one at 163.9 eV associated with the S 2p1/2 peak. The spin orbital splitting is 1.18 eV between the S 2p peaks. The shift in the Pd 3d spectrum for one monolayer versus a 60 film is not significant. Chem. Vap. Deposition 2003, 9, No. 5 http://www.cvd-journal.de 70 68 66 64 62 60 58 Binding Energy (eV) Fig. 8. The Ir 4f XPS spectra showing the Pd±Ir interfacial reaction between PdII(hfac)2 and the Ir surface. The binding energy shift is only obvious for the 240 cycle Pd film because only the interfacial chemistry contributes to the Ir signal. The Pd ALD films deposited on Ir are slightly different than the ones deposited on the tetrasulfide SAM because the Ir film is ~150 versus just one monolayer for the sulfur-terminated SAM. As a result, interfacial bonding is only evident in Figure 8 for the 31 Pd film that was deposited with 240 cycles at 130 C. Before that point, the Ir± Ir bonding overwhelms the Pd±Ir bonding, and this peak is convoluted in the Ir 4f spectra. The Ir 4f7/2 peak is centered at 60.91 eV, the literature value.[24] The Pd±Ir interfacial bond is centered at 60.72 eV. No literature value exists for this bonding, but Ir±Si bonding has been reported at 60.70 eV, which may be similar to Pd±Ir.[32] 2.4. Reflection High-Energy Electron Diffraction (RHEED) The structure of the Pd ALD films is most easily obtained by using a RHEED camera under ultra high vacuum (UHV) conditions. RHEED is sensitive to the structure of angstrom thick metallic films, whereas conventional X-ray Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 261 Full Paper ferred (111) texture. The lattice constant is (3.8 ± 0.1) , based on the (111) d-spacing. The grain size is ~ 60 , which is very similar to the thickness of the Pd film. Also, it has a texture spread of ~ 70 at the surface and ~ 40 in the bulk of the film. A high-quality Pd ALD film would be expected for the deposition of a metal on a higher surface free energy metal that is relatively smooth, ~ 10 , root mean square (rms) (from atomic force microscope (AFM) measurements). The Pd ALD film grown on the tetrasulfide SAM possesses very small grains and has nearly random texture. This is uncharacteristic of a metal film that exhibits monolayer wetting, as previously shown.[25] However, the difficulty that arises with the use of PdII(hfac)2 is the high volatility it (111) Pd on Ir possesses at low temperatures (e.g., lattice constant=3.8 Å ± 0.1 Å 1 torr at 60 C).[12] Further, the hfac ligand shows some instabilities even (200) at room temperature,[30] and as the (220) (222) dosing or substrate temperature is raised the dissociation of the hfac li(331) gand becomes worse. As previously explained, glyoxylic acid is only effective as a reducing agent at temperatures greater than 200 C. Therefore, 2 4 6 8 10 the processing method developed -1 Ring Position (Å ) here is more suitable for lower vapor Pd on tetrasulfide SAM pressure metallorganics that do not (111) dissociate at the deposition temperatures needed to undertake ALD for (220) their respective metal. diffraction (XRD) is not appropriate due to the small X-ray cross-section. Figure 9 shows the RHEED images and the corresponding spectra of Pd on Ir, Pd on the tetrasulfide SAM, and the Ir substrate itself obtained from digitizing the images. The ~150 Ir thin film thermally evaporated on hydrogen-terminated Si(100) shows some slight (111) orientation but exhibits nearly random texture. Based on a d-spacing of (2.86 ± 0.04) for the (111) plane, the calculated lattice constant is (3.81 ± 0.06) , close to the literature value of 3.839 .[33] Despite the nearly random texture of the Ir substrate, the 55 Pd ALD film deposited on it has the much pre- Intensity (arb.) 8000 6000 4000 2000 0 0 Intensity (arb.) 6000 (222) 4000 2.5. Chemical Analysis of Glyoxylic Acid 2000 0 0 2 4 6 8 10 -1 Ring Position (Å ) Ir substrate (111) 12000 -1 (200) 10000 Intensity (arb.) -1 (111) postion = 2.856 Å ± 0.042 Å lattice constant = 3.81 Å ± 0.06 Å 8000 (220) (113) (222) 6000 4000 2000 0 0 2 4 6 8 10 -1 Ring Position (Å ) Fig. 9. RHEED images and spectra for 55 of Pd on Ir (top), 60 of Pd on tetrasulfide SAM (middle), and the 150 Ir substrate (bottom). The Pd on Ir exhibits a higher quality texture than the Ir substrate itself. However, the Pd on the tetrasulfide SAM is nearly random, which is attributed to the higher temperatures needed to achieve the Pd seed layer. 262 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cvd-journal.de Glyoxylic acid is used here to deposit a Pd seed layer via ALD on top of the tetrasulfide SAM. After the seed layer is deposited, Pd can be deposited via H2 at low temperatures where the growth rate per cycle is higher. The preliminary experiments showed that glyoxylic acid is only effective as a reducing agent above 200 C, the temperature where it thermally decomposes (according to previous work by Back and Yamamoto).[23] They showed that, at lower temperatures (~ 200 C), a 5:1 ratio exists between CO2 and formaldehyde for the decomposition products. As the temperature is raised, a lower ratio is evident but CO2 is always the dominant species. Nevertheless, glyoxylic acid is an effective reducing Chem. Vap. Deposition 2003, 9, No. 5 Full Paper agent since it decomposes cleanly to yield oxidation products and electrons available for reduction reactions with the metallorganic. The as-received glyoxylic acid is a hydrate, and water is known to dissociate b-diketonate metallorganics.[34] Proton nuclear magnetic resonance (1H NMR) spectroscopy was used to analyze the glyoxylic acid as-received and after dehydration to ensure that this glyoxylic acid was utilized as a reducing agent rather than effects caused by the presence of water. Figure 10 shows the 1H NMR spectra for the glyoxylic acid before and after dehydration under vacuum heating to 60 C for 60 min. There are four peaks in the glyoxylic acid TMS acetone intensity (arb. units) dehydrated acid-H2O proton exchange as-received (hydrated) 9 8 7 6 5 4 3 2 1 0 ppm Fig. 10. 1H NMR spectroscopy of the glyoxylic acid reducing agent before (bottom spectrum) and after (top spectrum) dehydration in vacuum (10 sccm Ar carrier and 10±4 torr base pressure) at 60 C for 60 min. The broad peaks at 7.2 ppm and 8.5 ppm are attributed to a proton exchange mechanism between glyoxylic acid and water. When the peak is at higher chemical shifts it is more associated with the acid than free water. proton spectra for glyoxylic acid dissolved in d6-acetoneÐ the tetramethylsilane reference peak at 0 ppm, 2.1 ppm acetone (impurity in the d6-acetone), 5.3±6.1 ppm glyoxylic acid, and a broad peak attributed to proton exchange between the glyoxylic acid and water.[35] Two items are apparent from the 1H NMR spectra. First, the integrated area between the broad peak at 7.2 ppm for the as-received sample and the group of peaks at 5.3±6.1 ppm associated with glyoxylic acid is 2.9:1. In contrast, the ratio for the dehydrated sample is 0.8:1. Second, the peak associated with proton exchange shifts from 7.2 ppm (full width at half maximum (FWHM) = 17.1 Hz) to 8.5 ppm (FWHM = 113 Hz). The conclusions to be made from these observations are that much less water exists in a glyoxylic acid sample without taking into consideration the water in the d6-acetone and the water adsorbed after the sample was dehydrated. Further, the protons are much more associated with the acid than free' water after dehydration. This last conclusion comes from the larger chemical shift for the dehydrated sample. As a result, glyoxylic acid is effective as a reducing agent, there is little associated water, and therefore water plays a minor role in the removal of the hfac ligands from the PdII(hfac)2 metallorganic. Chem. Vap. Deposition 2003, 9, No. 5 http://www.cvd-journal.de 3. Conclusions To date there has been very little success with the ALD of metals via metallorganics. The method presented here is a novel one, which allows ALD metals to be deposited onto dielectric and barrier layer surfaces. The method is achieved by activating the dielectric surface with sulfide terminated silanes, then depositing a Pd seed layer by the sequential pulsing of PdII(hfac)2 and glyoxylic acid above 200 C. Once the seed layer is deposited then Pd ALD is achieved by the sequential pulsing of PdII(hfac)2 with H2 at low temperature (80 C), where PdII(hfac)2 has a long transient time on the newly created Pd surface. The method is appropriate to all metals where the metallorganics can sublime without decomposing and chemisorb on an appropriate substrate without dissociating. Since glyoxylic acid is only effective above 200 C, precursors that have a low vapor pressure at those temperatures are required, or the development of reducing agents that are effective at lower temperatures is needed. The thicknesses of the Pd ALD films were measured as a function of the number of ALD cycles via RBS. At a deposition temperature of 130 C, the relationship was not linear due to the high volatility of PdII(hfac)2, or in other words, its high desorption rate from the freshly created Pd surface. However, at lower temperatures (80 C) the desorption rate is not significant and linear growth is observed. The Pd ALD films were also characterized by XPS and RHEED. The films grown on the tetrasulfide SAM had a higher levels of carbon and fluorine contamination attributed to the high temperature need to deposit the Pd seed layer. This was related to the dissociation of the hfac ligand. When the films were grown on Ir the contamination levels were lower due to the temperature dependence of the hfac dissociation. Further, the films grown on Ir exhibited (111) texture with a grain size of ~ 60 for a 55 thick film. In contrast, the Pd films grown on the tetrasulfide SAM showed very small grains with a nearly random texture. This relatively poor structure, as compared to the Pd film grown on Ir, was attributed to the dissociation of the hfac ligand, thus allowing fluorine and carbon contamination at the Pd seeded surface. 4. Experimental Two surfaces were used for the ALD of palladium, as shown in Figure 2. 150 of Ir (99.95 %, Alfa Aesar, Ward Hill, MA) was electron beam-deposited onto a Si(100) wafer. The other surface was a tetrasulfide-terminated self-assembled monolayer solution grown on the native oxide of Si(100) via bis[3-(triethoxysilyl)propyl] tetrasulfide (>97 %, Gelest Inc., Morrisville, PA). The details of the growth and characterization of this monolayer has been published previously [17]. A custom-built vacuum chamber with computer controlled gas flow was used for the ALD of palladium via PdII(hfac)2. The base pressure of the vacuum system was 5.0 10±4 torr with the use of a roots blower/direct drive roughing pump (Leybold RUVAC WS/WSU 151 and TRIVAC D25 BCS (hydrocarbon prepped)) and a 3.7 L foreline trap (bronze gauze) to prevent oil backstreaming. The vacuum chamber was warm walled to prevent con- Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 263 Full Paper densation of the PdII(hfac)2 precursor. Further, pneumatic valves (that turn off or turn on the gas flow and mass flow meters) can control the flow of gases into the system under steady state conditions. Two types of deposition were undertaken, one on a noble metal surface (Ir) and one on a tetrasulfide SAM. The depositions (outlined in Fig. 2) on Ir were undertaken at a deposition temperature of (80 ± 4) C or (130 ± 4) C, and a PdII(hfac)2 sublimation temperature of (49.8 ± 0.2) C. The deposition chamber walls were kept at 60±65 C, and the lines between the sublimation tube and the deposition chamber at 90±95 C to ensure no cold spots existed to condense PdII(hfac)2. During each experiment, 30 sccm Ar (99.999 %, Air Products) was flowed as a purge gas, 10 sccm Ar as a carrier gas for PdII(hfac)2, and 8.0 sccm H2 (99.999 %, Air Products) as a reducing gas. The pulse sequence was 5 s of PdII(hfac)2 followed by 10 s of dead time' (with just 30 sccm Ar flowing as a purge gas), and then 20 s of H2. The number of cycles per deposition varied from 8 to 240 for Pd ALD on Ir. The depositions on the tetrasulfide SAM were slightly different due to the inefficacy of glyoxylic acid (Alfa Aesar, MA) as a reducing agent below 200 C [31], and the high rate of desorption of PdII(hfac)2 on Pd at temperatures above 200 C due to its high vapor pressure and low interaction energy with Pd. This behavior may be followed in terms of Langmuir kinetics [36]. As a result, these depositions consisted of two parts. The first part was the deposition of a seed layer of Pd, with PdII(hfac)2 pulsed for 5 s alternatively with 60 s pulses of glyoxylic acid (sublimation temperature (60 ± 2) C and 15 sccm Ar carrier). As with all of the depositions, a 10 s dead time was used. It took 120 cycles to deposit the seed layer of Pd, after that point the PdII(hfac)2/H2 process was used at 80 C for a further 120 cycles. The RHEED images were taken with a 15 keV electron beam source that produces a 100 lm diameter spot size. The pressure in the stainless steel chamber was ~1 10±9 torr. The high-energy electron beam was incident at 1 from the sample plane. The diffracted electrons pass through two metal retarding grids before striking the phosphor-coated screen, imaged using a 16-bit Princeton Instruments CCD camera. The NMR work on the analysis of the water in the glyoxylic acid was undertaken on a Varian Unity INOVA 300 (300 MHz). 1H NMR spectra were undertaken on dehydrated and as-received glyoxylic acid with eight scans with a pulse width of 8 ls at 90 in d6acetone with 1 v/v-% tetramethylsilane as a proton standard. Received: September 27, 2002 Final version: February 25, 2003 ± [1] H. P. Hsieh, Inorganic Membranes for Separation and Reaction, Elsevier, New York 1996. [2] G. Xomeritakis, Y.-S. Lin, in Chemical Vapor Deposition±Proc. 14th Int. CVD Conf. and EUROCVD-11 (Eds: M. Allendorf, C. Bernard), Vols. 97±25, The Electrochemical Soc., Pennington, NJ 1997, p. 1604. [3] R. Solanki, B. Pathangey, Electrochem. Solid-State Lett. 2000, 3, 479. [4] J. Chae, H.-S. Park, S.-W. Kang, Electrochem. Solid-State Lett. 2002, 5, C64. [5] S. Busse, J. Kashammer, S. Kramer, S. Mittler, Sens. Actuators B 1999, 60, 148. [6] A. Wieckowski, H. Kim, C. Rice, P. Waszczuk, Proc. Electrochem. Soc. 2001, 2000-20, 91. [7] J. R. Pitts, P. Liu, S.-H. Lee, C. E. Tracey, R. D. Smith, C. Salter, Proc. 2000 DOE Hydrogen Program Rev. National Renewable Energy Laboratory, US Department of Energy, Golden, CO 2000, Vol. 1; http://www.eere.energy.gov/hydrogenanafuelcells/hydrogen/annual-review2000.html [8] R. C. Hughes, W. K. Schubert, R. J. Buss, J. Electrochem. Soc. 1995, 142, 249. [9] J. J. Senkevich, F. Tang, G.-C. Wang, T.-M. Lu, T. S. Cale, C. Jezewski, W. A. Lanford, Chem. Vap. Deposition 2002, 8, 189. [10] K. P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure Constants of Diatomic Molecules, Van Nostrand, New York 1979. [11] C. Jezewski, J. J. Senkevich, unpublished. [12] Gmelin Handbook of Inorganic Chemistry, Pt Suppl Vol. A1 (Eds: G. J. K. Acres, K. Swars), Springer-Verlag, Berlin 1986. [13] J. M. Blackburn, D. P. Long, J. J. Watkins, Chem. Mater. 2000, 12, 2625. [14] P. Martensson, J.-O. Carlsson, J. Electrochem. Soc. 1998, 145, 2926. [15] C. J. Hagedorn, M. J. Weiss, W. H. Weinberg, Phys. Rev. B 1999, 60, R14016. [16] J. W. Klaus, S. J. Ferro, S. M. George, Thin Solid Films 2000, 360, 145. [17] S. Poston, A. Reisman, J. Electron. Mater. 1989, 18, 553. [18] Gmelin Handbook of Inorganic Chemistry, Pt Suppl. Vol. A1 (Eds: G. J. K. Acres, K. Swars), Springer-Verlag, Berlin 1986, p. 45. [19] G. D. Doolen, Los Alamos National Laboratory, unpublished. [20] A. Zangwill, P. Soven, Phys. Rev. A 1980, 21, 1561. [21] V. Bhaskaran, M. J. Hampden-Smith, T. T. Kodas, Chem. Vap. Deposition 1997, 3, 85. [22] Y. Y. Shacham-Diamand, Electrochem. Solid-State Lett. 2000, 3, 279. [23] R. A. Back, S. Yamamoto, Can. J. Chem. 1985, 63, 542. [24] R. Lenigk, M. Carles, N. Y. Ip, N. J. Sucher, Langmuir 2001, 17, 2497. [25] J. J. Senkevich, B. Li, G.-R. Yang, G.-C. Wang, T.-M. Lu, Electrochem. Solid-State Lett. 2002, 5, C94. [26] J. F. Moulder, W. F. Stickle, P. E. Sobol, K. D. Bomben, Handbook of X-ray Photoelectron Spectroscopy (Eds: J. Chastain, R. C. King Jr.), Physical Electronics, Eden Prairie, MN 1995. [27] M. Schildenberger, R. Prins, Y. C. Bonetti, J. Phys. Chem. B 2000, 104, 3250. [28] M. Peuckert, J. Phys. Chem. 1985, 89, 2481. [29] M. Brun, A. Berthet, J. C. Bertolini, J. Electron Spectrosc. Relat. Phenom. 1999, 104, 55. [30] W. Lin, B. C. Wiegand, R. G. Nuzzo, G. S. Girolami, J. Am. Chem. Soc. 1996, 118, 5977. [31] M. A. Golub, T. Wydeven, R. D. Cormia, J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 2683. [32] M. Wittmer, P. Oelhafen, K. N. Tu, Phys. Rev. B 1986, 33, 5391. [33] B. D. Cullity, Elements of X-ray Diffraction, 2nd ed., Addison-Wesley, Reading, MA 1978. [34] E. L. Hahn, D. E. Maxwell, Phys. Rev. 1952, 88, 1070. [35] D. Yang, J. Hong, D. F. Richards, T. S. Cale, J. Vac. Sci. Technol. B 2002, 20, 495. [36] A. W. Adamson, Physical Chemistry of Surfaces, 5th ed., John Wiley & Sons, New York 1990. ______________________ 264 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cvd-journal.de Chem. Vap. Deposition 2003, 9, No. 5