Anal. Chem. 1994,66, 522-530
Theoretical and Experimental Investigation of Internal Reflection at Thin Copper Films Exposed to Aqueous Solutions Kenneth P. Ishida and Peter I?.Grifflths' Department of Chemistty, University of Idaho, Moscow, Idaho 83843
Research directed toward the adsorption of biopolymersat an aqueous/solid interface is of particular interest due to the prevalence of corrosion influenced by biopolymers and/or microorganisms adsorbed at the interface. A method for depositing continuous copper films (10-12 nm) on a germanium internal reflectionelement (IRE) by physical vapor deposition is described. The Maxwell and Fresnel equations were used to model reflectance and mean square electric field intensity as functionof distance from the Cu/H20 interfacefor stratified media consisting of Ge/Cu/air and Ge/Cu/HzO. Calculation of the absorptionof the H20 bending mode allows an estimate of the feasibilityof measuring biofilmsat the Cu surface.Results of theoretical calculations were verified by exposing the Cucoated Ge three-reflection IRE to flowing saline. Initially the intensity and location of the 1640-cm-l band was consistent with the theoretical calculations. After -1 min, surfaceenhanced infrared absorption was observed leading to anomalous band shifts to 1650 cm-l and dispersive band shapes due to the removal of Cu from the IRE surface and the formation of Cu islands that were originally formed during the deposition. On-going research, both in our and those of others3-$has been directed toward studying the adsorption of biological macromolecules at aqueous/metal interfaces. Adsorption phenomena at metal surfaces are of particular interest due to the prevalence of microbially influenced corrosion (MIC) of surfaces submerged in aquatic environments. Since corrosion-induced failure of submerged materials is of great economic concern, a detailed understanding of the processes occurring during MIC is needed if methods of combating its effects are to be developed. When most surfaces, including clean metals, are submerged in natural waters (fresh or salt), they rapidly become fouled with an organic film composed primarily of protein or gly~oprotein.~,~ Only after this conditioning film has formed are bacteria and other microorganisms observed at the surface. Under certain conditions that are believed to be enhanced by chemical and biological ( 1 ) Geesey. G.G.;Iwaoka, T.; Griffiths, P. R. J. Colloid ZnferfaceSci.1987,120,
370-376. (2) Ishida, K. P.; Griffiths, P. R. In FT-IR in Colloid and Interface Science; Scheuing, D. R., Ed.; ACS Symposium Series No. 447; American Chemical Society; Washington, DC, 1990; pp 208-224. (3) Jolley, J. G.; Geesey, G. G.; Hankins, M. R.; Wright, R. B.; Wichlacz, P. L. Appl. Specfrosc. 1989, 43, 1062-1067. (4) Bremer, P. J.; Geesey, G. G.Appl. Enuiron. Microbiol. 1991,57, 1956-1962. ( 5 ) Geesey, G.G.;Bremer. P. J. Mar. Technol. SOC.J . 1990, 24, 36-43. (6) Baier, R.; Meyer, A. E.; DePalma, V.A,; King, R. W.; Fornalik, M. S . J . Heat Transfer 1983, 105, 618-624. (7) Loeb, G. I.; Neihof, R. A. In Applied Chemistry ut Protein Znferfaces;Baier, R. E., Ed.; Advances in Chemistry Series No. 145; American Chemical Society: Washington, DC, 1975; pp 319-335.
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processes associated with the adherent bacteria, MIC is observed.8-10 The adsorption of polysaccharideslJ and bacterial cells4,5on thin metallic films has been studied in the past by attenuated total reflection infrared (ATR-IR) spectrometry. In typical experiments, a thin copper film (nominal thickness, 2-7 nm) is deposited on a germanium cylindrical internal reflection element (IRE) such that sufficient energy is transmitted through the crystal that a spectrum with reasonable signal-to-noise ratio (SNR) can be obtained. The high number of reflections commonly associated with commercially available cylindrical IRES (typically 10) has severely limited the thickness of metal that can be deposited due to the high absorptive and reflective losses associated with metal films. Thus, either fewer reflections or thinner films are preferable. One recent study of thin metal deposits by atomic force microscopy (AFM) has revealed that when Cu layers that are 7 nm or less in nominal thickness are produced by physical vapor deposition (PVD) using a conventional basket source, the metal film is not continuous.ll A film thickness of 9-10 nm is required to produce a continuous Cu film by PVD using a basket source. Although continuous films as thin as 5-6 nm have been deposited on a Ge prism when a point source was utilized,12 even a continuous 5-nm metal film deposited on a standard 10-reflectioncylindrical Ge IRE attenuates the beam to such an extent that a useful ATR spectrum cannot usually be measured with adequate SNR. By reducing the number of internal reflections significantly (e.g., from 10 to 3), the absorption by a continuous 10-nmCu film is still weak enough that enough energy is transmitted to the detector for an acceptable ATR spectrum of a liquid in contact with the outer surface of the metal to be measurable. In this paper, we will show both theoretically and experimentally that an adequate SNR can be attained when a Fourier transform infrared (FTIR) spectrometer equipped with a mercury cadmium telluride (MCT) detector is used for the spectral measurement, despite the weak absorption of the evanescent wave by an analyte in contact with the outer surface of the metal layer and the fact that the beam is strongly attenuated because of absorption by the metal film.
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(8) Characklis, W. G.;Marshall, K. C. In Biofilms; Charaklis, W. G., Marshall, K. C., Us.; John Wiley and Sons: New York, 1990; pp 3-15. (9) (9) Microbial Aspects ofMefullurgy;Miller, J. D. A,, Ed.; American Elsevier: New York, 1970. (10) Iverson, W. P. Adv. Appl. Microbiol. 1987, 32, 1-36. (1 1) Bremer, P. J.; Geesey, G.G.;Drake, B.; Jollcy, J. G.;Hankins, M. R. Surf: Interface Anal. 1991, 17, 767-772. (12) Ishida, K. P.; Griffiths, P. R., unpublished data.
0003-2700/94/036&0522$04.50/0
@ 1994 Amerlcan Chemical Society
To determine the extent of absorption by the metal film, the Maxwell and Fresnel equations were first applied to calculate the theoretical reflectance for systems of parallelboundary stratified media. A program originally described by Dluhyl3 permitted the reflectance and mean square electric field intensity (MSEFI) at any distance from the surface of the IRE to be calculated. In our study, the three-layer system Ge/Cu/HzO was characterized in terms of its reflectance and MSEFI as a function of Cu thickness. Calculation of the intensity of one or more absorption bands of water together with the overall reflectance of the system allowed an estimate of the feasibility of detecting the presence of a thin film at the Cu/H20 interface to be made. To verify the results of these calculations experimentally, continuous films of Cu were deposited on a three-reflection Ge IRE by PVD. The stability of these metal films upon exposure to flowing saline solutions was then assessed. Comparisons of the results of experimental measurements and theoretical calculations were made.
EXPERIMENTAL SECTION Deposition of Thin Copper Films. Copper (99.999%) shot (4-6 mm) was purchased from Johnson Matthey (Ward Hill, MA). Thin metallic films were deposited by evaporation onto a 45O, three-reflection (20 X 36 X 6 mm) Ge trapezoidal IRE (Harrick Scientific, Ossining, NY). The Cu shot was placed in a conical tungsten basket (R. D. Mathis Co., Long Beach, CA) which was mounted 18.3 cm from the top surface of the prism such that the deposited Cu film had a thickness that varied by less than 2% across the entire surface. The Ge substrate was cleaned by glow discharge at a pressure of 250 mTorr (20-25 mA at 30 kV) for 3 min prior to the deposition. When a pressure of 5 X 10" Torr was achieved, sufficient current was passed through the tungsten basket to melt the Cu. The Ge prism was initially masked with a shutter. After 0.5-1 nm of Cu was evaporated, the shutter was opened and Cu was allowed to deposit on the IRE at -0.1 nm/s as measured with a quartz crystal microbalance (Phelps Electronics, Santa Barbara, CA). The pressure within the bell jar Torr. during the deposition was -5 X Atomic Force Microscopy. Surface integrity and morphology were investigated by AFM. Cu films were deposited on Ge disks (0.125 X 0.25 in. diameter) polished with 6-pm diamond paste and 0.05-pm alumina (Buehler, Lake Bluff, IL). The Ge surface was washed with detergent and rinsed thoroughly with tap water and then with Milli-Q water (Millipore Corp., Bedford, MA). Cu deposits of 7.8-, 10.1-, and 12.3-nm nominal thickness were deposited as described above. The AFM was performed by Imaging Services, Santa Barbara, CA. The surfaces were imaged in air using a Nanoscope I1 contact mode AFM (Digital Instruments, Santa Barbara, CA), equipped with a 100-pm silicon carbide cantilever (Digital Instruments) with a spring constant of 0.58 N/m. The applied force while imaging was lo-* N.
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Theoretical Reflectance Calculations. The theoretical reflectance (R) of plane-polarized radiation from three-phase stratified media with parallel boundaries was calculated by (13) Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373-1379.
useof the Maxwell and Fresnel equations"l6 using a computer program written in FORTRAN 77 that was provided to us by Richard Dluhy of the University of Georgia. A detailed description of the basis of these calculations has been reported by him 13. The optical constants, Le., the real refractive index (n) and attenuation index (k), were obtained from the literature.17J8 The reflectance for the three-phase system Ge/ Cu/air served as the reference (Ro). Individual calculations of reflectance for parallel (p) and perpendicular (s) polarized radiation were made and the weighted average of the total reflectance was determined, accounting for our instrument characteristics (i-e.,60% s-polarization and 40% p-polarization in the region around 1640 cm-l). The ratio of reflectance, R , for a Ge/Cu/H20 system of varying Cu thickness to the air reference was converted to absorbance (-log(R/Ro)). ASCII files generated by the program were imported into Lotus 1-2-3 software (Lotus DevelopmentCorp., Cambridge, MA), where simple calculations were carried out, and then exported into SpectraCalc software (Galactic Industries, Salem, NH) for display and further spectral manipulations. The center of gravity of both theoretical and experimental bands due to the bending mode of water absorbing near 1640 cm-l was determined between 1590 and 1670 cm-l before and after baseline correction at 1800 and 1500 cm-l. Flow-Through Experiments. A rectangular stainless steel plate was mounted on top of the Cu-coated Ge prism with an O-ring seal. A background spectrum (512 scans) of the dry IRE was collected and then a saline (0.15 F NaCl) solution at pH 7.0 or 4.8 was pumped through the flow cell at a rate of 84 pL/min and was not recirculated. Spectra were measured at 4-cm-l resolution with a Nicolet 740 FT-IR spectrometer equipped with a medium-range MCT detector. A series of 175-scan spectra (1-min acquisition time) was collected every 5 min for the first hour and then every 10 min for the next 3 h. At the end of the 4-h period, the data acquisition program was restarted. After 8 h, spectra were collected every hour for 8 h or more for a total run time of 16 h. The ratios of single-beam spectra to a background spectrum that was obtained with the dry metal-coated IRE installed were calculated and converted to absorbance. Both single-beam and absorbance spectra were transferred to a network of personal computers. Each absorbance spectrum was baseline-corrected to zero at 2000,1900,1800,1500, and 1100 cm-l by fitting to a fourth-order polynomial utilizing SpectraCalc software.
RESULTS AND DISCUSSION Atomic Force Microscopy of Thin Copper Films. Photomicrographs measured by AFM illustrating the surface morphology of the Cu deposits are shown in Figure 1. The surface of the bare Ge was determined to be largely smooth but some scratches, probably caused by the 6-km diamond abrasive used to polish the surface, are noticeable (Figure 1A). The Cu deposited at a nominal thickness of 7.8 nm was not continuous. Relatively flat islands as large as 260 nm in (14) Hansen, W. N. J . Opr. SOC.Am. 1968, 58, 38C-390. (15) Born, M.; Wolf, E. Principles of Optics, 4th ed.;Pergamon h a s : New York,
1973.
D.E. In Optical Properties of Solids: New Developments; Seraphin, B. 0.. Ed.; American Elsever: New York, 1976. (17) Downing, H. D.; Williams, D. J. Geophys. Res. 1975, 80, 1656-1661. (18) Ordal, M. A.; Long, L. L.; Bell, R. J.; Bell, S. E.; Alexander, R. W. Jr.; Ward, C. A. Appl. Opt. 1983, 22, 1099-1119. (16) McIntyre, J.
Analytical Chemistry. Vol. 60, No. 4, February 15, 1994
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Ge / Cu /H,O 1200 cm-1
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I
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coppERFItMmcKNEss(nm) Flgure 2. Theoretical reflectance of parallel@) and perpendicular (s) polarized radiation at 1200 cm-1 for three-phase stratified medium (Ge/Cu/H20) as a function of Cu film thickness. One reflection: (+) R(s) and (X) Np). Three reflections: (A)4 s ) and (V) Np).
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COPPER FILM THICKNESS (nm) Flgure 3. Theoretical (weighted) total reflectance at 1200 cm-1 for three reflections from three-phase stratified medium (Ge/Cu/H20) as a function of Cu film thickness.
30 1755 1
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D Figure 1. AFM images of (A) polished Ge disk and copper deposits evaporated on Ge disks by physical vapor deposkkm at (8)7.8, (C) 10.1, and (0)12.3-nm nominal thickness.
diameter, but with a height of only about 1&15 nm, arevisible (Figure 1B). These islands are separated by gaps as large as 100 nm, leaving the underlying Ge substrate exposed. At a nominal thickness of 10.1 nm, the Cu islands grew in diameter and coalesced (Figure 1C); in this photograph, the tops of Cu islands are visible, but the film appears to be continuous. A smoother Cu film resulted when the thicknessof the Cu deposit was increased to 12.3 nm (Figure 1D). The tops of a few islands are just barely visible but the surface itself is quite smooth. Similar results have been reported by Bremer and Geesey." These results indicate that it is necessary to deposit a film greater than 8 nm thick to achieve a continuous film under the conditions described above. Metal films deposited 524 AnalyticalChemMry, Vol. 66, No. 4, February 15, 1994
by PVD that are 8 nm or less in nominal thickness are not continuous and thus are not suitable for studies of adsorption phenomena at the aqueous/metal interface. Optical Properties of Stratified Media. The calculated reflectance of p- and s-polarized radiation for a three-phase system that occurs at the Cu/HzO interface is displayed as a function of increasing Cu film thickness in Figure 2 for both a one- and three-reflection IRE. As the Cu film increases in thickness, the reflectance drops rapidly due to the high absorption index of the metal. The reflectance reaches a minimum at a Cu thickness between 3 and 5 nm depending on the polarization of the light and then gradually increases as the film thickness increases. For thick films, most of the reflection occurs at the Ge/Cu interface and absorption of the evanescent wave by the analyte is weak. For a 3-nm Cu layer on a 10-reflection Ge IRE, only (0.4)1°,or 0.01% of the incident ppolarized radiation, and (0.7)1°,or 2.8%, of the s-polarized radiation is transmitted through the IRE. If a weak ATR spectrum is to be measured under this condition, the noise level on an ATR spectrum measured in 1 min or less will be excessively high. Although the reflectance increases when the thickness of the Cu film is increased to 10 nm (thickness at which a continuous film can be deposited by PVD), the absorption of the evanescent wave by an analyte will be much smaller than for the uncoated IRE (see later, Figure 6), so that the SNR of the measured ATR spectrum will still be excessively low. The optimum number of reflections for a Cu-coated IRE is -3. The calculated transmission of 1200-cm-I radiation (where kwater =0) by a three-reflection IRE for the three-phasesystem consisting of Ge/Cu/HzO is shown as a function of the
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.
.
.
.
.
.
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.
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thickness of the Cu layer in Figure 3 for radiation polarized at 0.6 perpendicular and 0.4 parallel (corresponding to the characteristics of the Nicolet 740 FT-IR spectrometer). It can be seen that the minimum in the theoretical reflectance curve occurs at a Cu film thickness of 3 nm, well below the thickness at which a coherent Cu film can be deposited by PVD. As for the Ge/Cu/air system, the reflectance increases when the film thickness is increased beyond 3 nm as the Cu film becomes increasingly more reflective. Because the intensity of the radiation that is transmitted to the IR detector is directly proportional to the value of RN (where N is the number of reflections), the intensity of the single-beam spectrum can be used to monitor the integrity of the Cu film when exposed to the aqueous solutions. Magnitudes of the MSEFI ((EZ2))as a function of distance from the Ge/Cu interface are shown in Figure 4. The z-direction is defined as being parallel to the plane of incidence and normal to the substrate. The MSEFI at the Cu/H20 interface drops exponentially with increasing distance from the interface and with increasing Cu film thickness. Defining dpafterHarricklg as that distance at which the energy intensity has dropped to l / e of its value at the Cu/H20 interface, the value of dp(-200 nm) is independent of the Cu film thickness. More significant is the effect of the Cu film on the electric field intensity, E,, at the Cu/H20 interface and the effective thickness, de, which represents the actual thickness of a film required to obtain the same absorption in a transmission measurement as that obtained in the reflection measurement using a semiinfinite bulk sample. The low absorption (
