Infrared Spectroscopic Ellipsometry of Self-Assembled Monolayers

Published 2000 American Chemical Society ... By matching measurement and prediction, the thicknesses and molecular structures of a series of alkanethi...
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Langmuir 2000, 16, 9483-9487

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Infrared Spectroscopic Ellipsometry of Self-Assembled Monolayers Curtis W. Meuse* Biotechnology Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8313 Received February 2, 2000. In Final Form: July 19, 2000

Ellipsometry measures the relative intensity of and the phase difference between the parallel and perpendicular components of a polarized electric field vector interacting with a sample. In this paper, a technique using polarized Fourier transform infrared spectroscopy for measuring this information, as the complex optical density function, is presented. The advantage of the complex optical density function is that it relates the properties of the polarized electric field vector to the supporting surface instead of the plane of incidence. In this configuration, the precise positioning of the sample surface compared to the reference surface and the reproducibility of the analyzer movement are the most important contributions to the errors in the complex optical density function. We demonstrate that these errors are small compared to the complex optical density function measured from the presence of an organic monolayer on a metal surface. The measured complex optical density function provides a signature that is related to both the thickness and molecular structure of the sample layers. Electromagnetic wave theory can be used to predict the signature for any set of sample properties. By matching measurement and prediction, the thicknesses and molecular structures of a series of alkanethiol monolayers are determined.

Introduction Infrared absorption spectroscopy is used to study the composition and molecular structure of thin films on metal surfaces. Visible spectroscopic ellipsometry is used to characterize the thickness and morphology of similar thin films. The benefit of infrared spectroscopic ellipsometry (IRSE) is that it has the potential to describe the composition, structure, thickness, and morphology in the same measurement of the same sampling area. In addition, IRSE is useful for samples that need to be measured in situ because it can discriminate between contributions from surface and ambient species, such as water vapor or solvent. That is to say, because IRSE measures changes in the polarization and not the intensity of the electric field vector, the isotropic contributions of the ambient species are minimized. IRSE has been used to characterize thin inorganic and organic films such as (SiO2/Si),1-5 formic acid, and Langmuir-Blodgett layers.6-9 The advantage of spectroscopic ellipsometry is that ellipsometry at a single wavelength cannot generally describe a thin film, as only two independent quantities (the relative amplitude and phase of the polarized electric field vectors) are measured to describe three unknowns: the film thickness, the refractive index, n, and the absorption coefficient, k. This problem can be overcome either by making assumptions about the optical constants * E-mail: [email protected]. (1) Ro¨seler, A. Infrared Phys. 1981, 21, 349. (2) Ro¨seler, A.; Molgedey, W. Infrared Phys. 1984, 24, 1. (3) Ro¨seler, A. Thin Solid Films 1993, 234, 307. (4) Drevillon, B. Thin Solid Films 1988, 163, 157. (5) Canillas, A.; Pascual, E.; Drevillon, B. Rev. Sci. Instrum. 1993, 64, 2153. (6) Dignam, M. J.; Moskovits, M.; Stobie, R. W. Can. J. Chem. 1971, 49, 1115. (7) Stobie, R. W.; Rao, B.; Dignam, M. J. Appl. Opt. 1975, 14, 999. (8) Ro¨seler, A.; Dietel, R.; Korte, E. H. Mikrochim. Acta [Suppl.] 1997, 14, 657. (9) Benferhat, R.; Drevillon, B.; Robin, P. Thin Solid Films 1988, 156, 295.

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of the film or by making spectroscopic measurements. Spectroscopic ellipsometry circumvents the problem of two equations and three independent variables, because, for each wavelength, the film thickness is the same. If at any wavelength the film is nonabsorbing (k ≈ 0), then the thickness can be obtained at that wavelength, and n and k can be obtained for the other wavelengths. In the infrared region, descriptions of n and k are useful in determining structural properties such as molecular conformation. If descriptions of the optical properties of components of the sample are available, IRSE can also be used to characterize the molecular orientation and composition of the sample. Specifically, film properties are determined from ellipsometric data using electromagnetic wave theory and spectroscopic values of n and k in an iterative process. The theory is used to calculate the complex optical density function for a proposed model for the structure of the sample. The calculated complex optical density function is then compared to the measured value, and the model is revised. The process continues until changes in the model do not produce a better fit to the experimental results. Because IRSE allows the evaluation of the complex optical density function for thousands of frequencies, many different properties of the sample can be determined. The continued development of IRSE could provide a description of the composition, morphology (film thickness), and structure (molecular orientation) of thin films in a single measurement. In this paper, we use IRSE to study self-assembled alkanethiol monolayers on gold surfaces. Several previous groups have made IRSE measurements of organic monolayers on different substrates but did not report film thickness.6-9 Generally, these studies focus on measuring the complex optical density function. For reflection, the complex optical density function, D, can be defined as6

D ) ln(F0/F)

This article not subject to U.S. Copyright. Published 2000 by the American Chemical Society Published on Web 10/31/2000

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where F and F0 refer to the ellipsometric properties of the interface with and without the organic monolayer, respectively. F is defined as

F ) tan(Ψ) ei∆ where Ψ and ∆ refer, respectively, to the relative intensity of and the phase difference between the parallel and perpendicular components of the polarized electric field vector interacting with the sample. By determining F with and without the sample layer, we perform a differential measurement, so the calibration of the positions of the polarizer and analyzer in relation to the plane of incidence are unimportant. The absolute positions of the polarizer and analyzer only change the total amount of radiation viewed by the detector. Because we have the gold surface without the film to calibrate the amount of radiation present, only the reproducible movement of the analyzer and the reproducible positioning of the samples are required to determine D. Electromagnetic wave theory can then be used to relate D to the thicknesses of our self-assembled monolayers. Other groups have measured the thickness of selfassembled alkanethiol monolayers using a variety of techniques including X-ray reflectivity,10 visible ellipsometry,11 and visible spectroscopic ellipsometry.12 X-ray measurements are difficult and not widely available, and they may damage the sample. Visible ellipsometry and visible spectroscopic ellipsometry are widely available and do not damage the samples. However, the interpretation of the results for self-assembled alkanethiol monolayers has proven difficult. First, to determine the thickness of a self-assembled alkanethiol monolayer, the optical constants of the gold surface and the self-assembled alkanethiol monolayer must be known. The optical constants of the gold surface are not difficult to obtain but do require a measurement of the surface without the self-assembled alkanethiol monolayer, just as in IRSE. The optical constants of the self-assembled alkanethiol monolayers are more difficult. Early single-wavelength visible ellipsometry measurements to determine self-assembled alkanethiol monolayer thicknesses were performed using a variety of values similar to the reported values for bulk alkanes for the real part of the refractive index, n.13 More recently, spectroscopic ellipsometry measurements have revealed that there may also be an imaginary component, k, in the optical constants of self-assembled alkanethiol monolayers in the visible region.12 This study accounted for the value of k using a thin layer, ∼0.2 nm thick, with fitted optical constants to describe the sulfur-gold region of the selfassembled alkanethiol monolayers.12 It has also been reported that the surface of the gold rearranges during the formation of a self-assembled alkanethiol monolayer.14 This rearrangement could change the optical constants of the gold in the sample compared to those measured for the bare surface. These changes in the optical constants can also be accounted for by considering an extra layer to describe the sulfur-gold region of the optical model. In either case, using an extra layer to describe the sulfur(10) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (11) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (12) Collins, R. W.; Allara, D. L.; Kim, Y.-K.; Lu, Y.; Shi, J. Characterization of Organic Thin Films; Ulman, A., Ed.; ButterworthHeinemann: Boston, MA, 1995; pp 48, 49. (13) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (14) Poirier, G. E. Chem. Rev. 1997, 97, 1117.

Figure 1. Schematic diagram of the optical components of the infrared spectroscopic ellipsometer (IRSE). The angles P, A, and θ describe the position of the pass axis of the polarizer, the position of the pass axis of the analyzer, and the angle of incidence, respectively.

gold region of the film revealed slightly thinner monolayers than the single-wavelength measurements and values consistent with film thicknesses determined using X-ray reflectivity.12 In contrast, the infrared spectra of self-assembled alkanethiol monolayers have been simulated using electromagnetic wave theory and bulk optical constants quite successfully.15-17 In the infrared region, the bulk optical constants can be used directly to simulate the spectra and do not require the inclusion of a thin unknown layer in the sulfur-gold region. Optical analysis in the infrared region is simpler because the bands are narrower and so new contributions to k after monolayer formation do not alter the analysis regions of our spectra. In addition, the longer infrared wavelengths are less sensitive to changes in surface roughness due to the rearrangement of the gold surface upon layer formation. Here, we use infrared spectroscopic ellipsometry to measure thickness and structural information about a series of self-assembled alkanethiol monolayers. We measure the spectral changes observed with the repositioning of the surface and the rotation of the analyzer and use these results to describe the error in the complex optical density function. We match simulated and measured complex optical density functions to show that the changes observed with the addition of the film are much larger than the measurement errors. Finally, we present the film thicknesses, obtained from our simulations, of a series of self-assembled alkanethiol monolayers. Materials and Methods18 For all measurements, gold substrates were prepared on silicon(100) wafers (Virginia Semiconductor, Fredricksburg, VA) previously coated with a layer of chromium by magnetron sputtering at a base pressure of ∼1.3 × 10-6 mbar to a nominal thickness of 2000 Å, as described previously.19 Alkanethiols were obtained from Aldrich and used as received. The monolayers were prepared by immersing the gold substrates in ∼1 × 10-3 mol/L (mM) thiol solutions in 200 proof ethanol (Warner Graham Co., Cockeysville, MD) for a minimum of 12 h. The experimental setup (Figure 1) is based on a Bruker Equinox 55 spectrometer (Billerica, MA) with an MCT detector. The polarizer is a germanium double-diamond polarizer from Harrick Scientific Corp. (Ossining, NY). The pass axis of the polarizer was oriented at ∼45° to the plane of incidence of the light and ∼45° to the gold surface. The VeeMax reflection optics, analyzer, and analyzer rotation accessory were from Pike Instrument Co. (Madi(15) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (16) Meuse, C. W.; Krueger, S.; Majkrzak, C. F.; Dura, J. A.; Fu, J.; Conner, J. T.; Plant, A. L. Biophys. J. 1998, 74, 1388. (17) Meuse, C. W.; Conner, J. T.; Richter, L. J.; Plant, A. L. Appl. Spectrosc., manuscript to be submitted. (18) The specification of commercial products is for clarity only and does not constitute endorsement by the NIST. (19) Vanderah, D. J.; Meuse, C. W.; Silin, V.; Plant, A. L. Langmuir 1998, 14, 6916.

Ellipsometry of Self-Assembled Monolayers

son, WI). The VeeMax system was set and aligned to provide a 70° angle of incidence. Although our measurements are consistent with a 70° angle of incidence from the normal, our electromagnetic wave theory calculations reveal that small changes to the absolute value of the angle of incidence, say 69° instead of 70°, only alter the observed thickness value by ∼0.1 nm per degree. As suggested in recent work,20 we use face-down sampling and a 70° angle of incidence to minimize differences caused by changes in the angles of incidence and beam focus between the sample and reference measurements. In this configuration, the throughput of the instrument was ∼90% of maximum for the detector. The entire setup was sealed into the sample compartment of the FTIR spectrometer using plastic so that only the 18-mm sampling hole of the VeeMax system was open to the atmosphere. To collect data at three analyzer positions, collection was automated using the Opus Macro language of the Bruker Equinox spectrometer. The collection macro was written to pause for 4 min to let the instrument purge using dry air (FTIR purge gas generator, Whatman, Haverhill, MA). This is important because, although IRSE data is less sensitive to contributions from the ambient, it is sensitive to changes in the amount of water vapor present between the acquisitions of each of the singlebeam spectra that are used to create the IRSE data. The macro scanned the single-beam spectra, at 4 cm-1 resolution, accumulating for 2.5 min at each of the three analyzer positions. By repeating the measurements, contributions due to water vapor changes as a function of time were further reduced. The shorter the accumulation time and the more revolutions used, the smaller the water vapor contributions observed. The data workup was performed in MathCad 7.0 (MathSoft, Cambridge, MA). The fitting of electromagnetic wave theory models to the measured data was performed using WVASE 32 software from J. A. Woollam Inc. (Lincoln, NE) or software developed in our laboratory17 based on the methods of Parikh and Allara.15 Results and Discussion Using the apparatus in Figure 1, we can determine the influence of the pass angle of the analyzer on the interaction of the light with a sample layer. When the angle of incidence is 70° and the pass axis of the polarizer is nominally 45° from the plane of incidence, Figure 2 shows that changing the analyzer angle from 0° to 45° to 90° decreases the amount of interaction the light has with a hexadecanethiol monolayer on gold. For illustrative purposes, each of the three single-beam reflection spectra is plotted using the infrared reflection convention of the base 10 logarithm, by plotting -1 × log(SBRx/SBRx0) to show the amount of interaction with the film relative to the amount of light passed without a film present. In the above expression, SBRx and SBRx0 are the single-beam reflection spectra of the sample and reference, respectively, taken with the pass axis of the analyzer at an angle of x, where x is approximately 0°, 45°, or 90° to the plane of the incidence. Because light polarized perpendicular to the plane of incidence (analyzer at 90°) undergoes an approximately 180° phase change at a metal surface, the incoming and outgoing waves destructively interfere with each other, leaving no infrared energy at the gold surface to interact with the monolayer. This is illustrated in the spectra of Figure 2 by the decrease in the intensity of the spectra as the analyzer position approaches 90°. The influence of the pass angle of the analyzer can be completely described in terms of Ψ and ∆, the relative (20) Kurth, D. G. Langmuir 1998, 14, 6987.

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Figure 2. Spectra of a hexadecanethiol self-assembled monolayer on gold. In all cases, the pass axis of the polarizer is at approximately 45°, and the pass axis of the analyzer is at approximately 0°, 45°, and 90° with respect to the plane of incidence of the light.

intensity of and the phase difference between the parallel and perpendicular components of the polarized electric field vector, respectively. The three SBRx sample spectra are not normalized to the amount of light passed and can be combined to determine values for Ψ and ∆ by rearranging the equations of Azzam and Bashara21 for our three analyzer positions. In this manner, we obtain

Ψ ) 1/2 arccos[(SBR90 - SBR0)/(SBR90 + SBR0)] and

∆ ) arccos{(2 SBR45 - SBR0 - SBR90)/ [sin(2Ψ)(SBR90 + SBR0)]} where SBR0, SBR45, and SBR90 are the single-beam spectra with the analyzer at approximately 0°, 45°, and 90°, respectively. The process is repeated using the SBRx0 spectra of the gold reference surface to obtain Ψ0 and ∆0. Because the polarizer and analyzer positions are not calibrated compared to the plane of incidence of the light, the absolute values of Ψ, ∆, Ψ0, and ∆0 have little meaning. These values could be calibrated to the plane of incidence; however, this procedure does not reveal much additional information. Even in the visible wavelength region where these values are regularly calibrated, the most reliable film thickness measurements are obtained by comparing to an uncoated reference sample. Ψ, ∆, Ψ0, and ∆0 are related to F and F0 according to

F ) tan(Ψ) exp(i∆) and F0 ) tan(Ψ0) exp(i∆0)22 where i ) (-1)1/2, which can then be used to calculate the (21) Parikh, A. N.; Allara, D. L. Physics of Thin Films; Francombe, M., Vossen, J. L., Eds.; Academic Press: New York, 1994; Vol. 19, pp 279-323.

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complex optical density function, D. The real part of the complex optical density function, Re(D), refers to the relative intensity of the p-polarized spectrum compared to the s-polarized spectrum. The imaginary part, Im(D), refers to the phase spectrum, which describes the difference in the phase shifts between the p- and s-polarized light at the interface. To interpret D, we utilize electromagnetic wave theory to simulate the reflectivity coefficients of a multilayer model designed to describe our self-assembled monolayer samples. Simulated reflectivity coefficients are mathematically related to Ψ and ∆ using the equations21

Ψ ) tan-1{[(Re(rp/rs))2 + (Im(rp/rs))2]0.5} and

∆ ) tan-1{[Re(rp/rs)]/[Im(rp/rs)]} where rp0, rs0, rp, and rs refer to the reflectivity coefficients of the reference and sample for light polarized parallel (p) and perpendicular (s) to the plane of incidence. These predictions for Ψ and ∆ can be converted to predictions for the real and imaginary parts of the complex optical density function in a manner similar to the conversion of the experimental measurements. The predicted phase changes can be compared to the measured phase changes, as the Im(D), to determine the optical thickness of the film. Similarly, the orientations of the absorbing functional group constituents of the monolayers can be determined using Re(D). The ability of IRSE to determine both optical thicknesses and specific structural information about the same sampling area is its greatest strength. Using the methods described above, the components of the complex optical density function, Re(D) and Im(D), were determined for a self-assembled hexadecanethiol monolayer on gold and are shown in Figure 3a and b, respectively. The solid lines depict experimental measurements with especially straight baselines, and the dashed lines depict data simulated using electromagnetic wave theory. The real part of the complex optical density, Re(D) (Figure 3a), shows a spectrum similar to an infrared reflection-absorption spectrum. This is because, for a high angle of incidence on a metal surface, the phase shift of s-polarized light is nearly 180°, so the s-polarized light makes very little contribution to the spectrum. With the cancellation of the s-polarized light intensity, components of the film interacting with the p-polarized light are preferentially observed. For our experimental conditions, the equivalence of the rs and rs0 coefficients is illustrated in Figure 2, where the lack of intensity in the 90° spectrum shows that rs2 ≈ rs02. This equivalence is sufficient to assume that rs ≈ rs0, which can be used to simplify

Figure 3. (a) Illustrates two spectra of Re(D), the real component of the complex optical density function, of a hexadecanethiol self-assembled monolayer on gold. The solid line is experimental data, and the dashed line is simulated using electromagnetic wave theory as described in the text. (b) Illustrates two spectra of Im(D), the imaginary component of the complex optical density function, of a hexadecanethiol selfassembled monolayer on gold. The solid line is experimental data, and the dashed line is simulated using electromagnetic wave theory as described in the text.

which is to say that Re(D) only differs from the standard infrared reflection spectrum in that it uses a natural logarithm. The imaginary part of the complex optical density, Im(D) (Figure 3b), describes the change in the phase between the p- and s-polarized light induced by the monolayer sample compared to the gold reference.

Figure 3 shows experimental (solid) and simulated (dashed) spectra of a self-assembled hexadecanethiol monolayer on gold. The shift in the baseline of the simulated Im(D), Figure 3b, matches the measured Im(D), indicating a film thickness of 2.0 nm. The peak intensities in the simulation of Re(D), Figure 3a, also match the peak intensities of the measured Re(D), which indicates that the orientations of the alkane chains in the monolayer are similar to that of the hexadecane chains in the electromagnetic model. We used a single-chain model in which all the chains have the same twist angle and a sign convention similar to those used in ref 23. The best fit was obtained when the hexadecane chains contained 27% end guache conformations, were tilted at an angle of -26° with respect to the surface normal, and were twisted by

(22) Azzam, R. M.; Bashara, N. M. Ellipsometry and Polarized Light, North-Holland; Amsterdam, 1977.

(23) Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. Langmuir 1998, 14, 845.

D ) ln[(rp0/rs0)/(rp/rs)]6 to

D ≈ ln(rp0/rp)

Ellipsometry of Self-Assembled Monolayers

Figure 4. During each measurement, the polarizer undergoes two complete revolutions, and spectra are acquired at three polarizer positions for each revolution. These spectra can be used to determine two measurements of Im(D). Examples of these two different experimental Im(D) spectra without moving the sample are shown to illustrate the error of our measurements related to analyzer repositioning.

48° around their long axis. Similar orientations have been previously observed using infrared spectroscopy.15,17,23 In contrast, the baseline of the measured Re(D) is not exactly the same as the simulated Re(D). Similar previous small discrepancies have been attributed to mixing of Im(D) into Re(D) because of the imperfect discrimination between s- and p-polarized light by a wire grid polarizer such as our analyzer.24 We evaluated two sources of error in our measurements. During each measurement, the analyzer undergoes two complete revolutions, and six spectra are obtained, two at each of the three analyzer positions. From these six spectra, two measurements of Im(D) can be calculated. These can be averaged, as in Figure 3b, or we can compare the two measurements to characterize the effect of the reproducibility of the analyzer positions on the value of Im(D), as illustrated in Figure 4. If a simulated Im(D) is matched to each curve, then thicknesses that are 0.05 nm different from each other are revealed. We also made a series of four measurements of the thickness of a heptanethiol monolayer to evaluate the error caused by differences in the angle of incidence between the sample and the reference. Analysis revealed a thickness of 1.0 ( 0.2 nm, very similar to the value and standard deviation previously measured using visible spectroscopic ellipsometry.12,19 These results indicate that the error due to the analyzer position is smaller than the error due to differences in the angles of incidence, which, in turn, is no larger than the error in visible spectroscopic ellipsometry using a split detector to correct the angle of incidence.19 Figure 5 shows Im(D) measurements for a series of selfassembled monolayers on gold. In the three spectra, the band intensities decrease from top to bottom, that is from hexadecanethiol to dodecanethiol to hexanethiol monolayers. In the top two spectra, the most prominent deviation is centered around 2917 cm-1. The 2917 cm-1 band can be assigned to the CH2 asymmetric stretching vibration.15 Because the number of CH2 groups decreases (24) Dignam, M. J.; Fedyk, J. Appl. Spectrosc. Rev. 1978, 14, 249.

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Figure 5. Im(D) spectra for (a) hexadecanethiol, (b) dodecanethiol, and (c) hexanethiol self-assembled monolayers on gold illustrate that the error contributions from changes in the angle of incidence related to changing from sample to reference are small compared to film thickness contributions.

with chain length, our peak intensities also decrease until they are too small to be discriminated from the noise in the hexanethiol monolayer spectrum. These measurements can be compared to electromagnetic wave theory simulations of Im(D), as in Figure 3b, to reveal the thickness of each monolayer. For these three self-assembled monolayers, the simulations reveal thicknesses of 0.8, 1.5, and 2.0 nm for hexanethiol, dodecanethiol, and hexadecanethiol monolayers, respectively. These values are similar to those obtained using visible ellipsometry.12 However, they did not require the inclusion of an extra unknown layer to describe the interface between the gold and the alkanethiol. The error in the reproducibility of these measurements, (0.2 nm, is equivalent to (0.0005 Im(D) units. Therefore, Figure 5 illustrates that IRSE can be utilized to discriminate between the thicknesses of hexanethiol, dodecanethiol, and hexadecanethiol monolayers. This type of sensitivity is possible only if the error contributions from changes in the angle of incidence related to changing from sample to reference are small compared to film thickness contributions. Conclusion We have shown that IRSE can be used to determine the complex optical density function for the study of monolayers on metal surfaces. By measuring the complex optical density function compared to a metal reference sample, the calibration of the measurements, and the appropriate optical model, are simplified. Specifically, on a metal surface, there are only three unknowns in the optical model, the film thickness and the components of the optical constants, n and k, normal to the surface at each frequency. Because IRSE measures two independent parameters, Re(D) and Im(D), and because the film thickness is the same at each frequency, IRSE can be used to describe these three unknowns if k is anywhere known to equal zero. By matching IRSE measurements to simulations of IRSE data for different possible models of the film, we can determine both molecular structure, such as orientation, and film morphology, such as film thickness, for precisely the same sampling area. LA000146B