Chapter 2
Determining the Structure of Polyimide—Metal Interfaces Using Fourier Transform IR and Raman Spectroscopy
Downloaded by MONASH UNIV on March 2, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch002
F. J . Boerio, J . T. Young, and W. W. Zhao Department of Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012 Reflection-absorption infrared spectroscopy (RAIR) and surface-enhanced Raman scattering (SERS) were used to determine the structure of interphases formed by polyamic acids which were spin-coated onto silver and gold substrates and then heated at elevated temperatures to complete the imidization reaction. Polyimides having relatively rigid backbones, such as those preparedfrompyromellitic dianhydride (PMDA) and oxydianiline (ODA), formed films in which the long axes of the molecules were oriented mostly parallel to the substrate surface. Polyimides having relatively flexible backbones, such as those prepared from 4,4'-hexafluoroisopropylidene dianhydride (6FDA) and ODA, mostly formed films in which there was little preferred orientation of the polymer molecules. However, for 6FDA/ODA films having a thickness of only a few tens of angstroms, a preferred orientation was observed in which the C H rings of the ODA moieties were oriented parallel to the substrate surface. Regardless of the flexibility of the backbone, the polymers interacted with silver substrates by formation of carboxylate species but there was little evidence of carboxylate formation on gold substrates. Thiol-terminated oligomers such as 46
4
mercaptophenylphthalimide (4-MPP) formed monolayers when adsorbed onto goldfromdilute solutions. However, the orientation of the molecular axes was much different than for PMDA/ODA. Whereas PMDA/ODA was oriented with the long axes of the molecules almost parallel to the gold surface, 4-MPP was oriented with the two-fold symmetry axes of the molecules almost perpendicular to the surface.
The purpose of this paper is to describe the use of reflection-absorption infrared spectroscopy (RAIR) and surface-enhanced Raman spectroscopy (SERS) to 0097-^156/95/0598-0008$15.25A) © 1995 American Chemical Society In Multidimensional Spectroscopy of Polymers; Urban, Marek W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by MONASH UNIV on March 2, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch002
2.
BOERIO ET AL.
Determining the Structure of Polyimide—Metal Interfaces 9
determine the molecular structure and orientation in thin films of polymers and model compounds formed on metal substrates. RAIR has been extensively discussed in the literature (7-5). In order to obtain an infrared spectrum of a thin film on a reflecting metal substrate, radiation polarized parallel to the plane of incidence and large, grazing angles of incidence must be used. Under most conditions, the incident and reflected waves combine to form a standing wave that has a node at the metal surface. However, for parallel polarized radiation and grazing angles of incidence, the incident and reflected waves combine to form a wave that has significant electric field amplitude at the metal surface. The dependence of RAIR absorbance on the angle of incidence and polarization is simulated in Figure 1. In these calculations, it was assumed that the refractive indices of the air, film, and substrate were 1.0, 1.3 - O.li, and 3.0 - 30.0i, respectively (2). The ratio of film thickness dl to the wavelength of light was 0.0003. From the figure, it can be observed that absorbance is negligible for radiation polarized perpendicular to the plane of incidence. Absorbance is also small for radiation polarized parallel to the plane of incidence when the angle of incidence is small. However, for parallel polarized radiation, absorbance increases rapidly for angles of incidence greater than about 45° and reaches a maximum at approximately 88°. The absorbance in a properly designed experiment is such that spectra of monolayers of polymers are obtained. RAIR spectroscopy has several important characteristics. One is that the resultant electric field vector is perpendicular to the metal surface. Therefore, if molecules are adsorbed onto the substrate with a preferred orientation, vibrational modes having transition moments perpendicular to the surface will appear with greater intensity than modes having transition moments parallel to the surface. As a result, RAIR is a powerful technique for determining the orientation of adsorbed molecular species. Another characteristic of RAIR is that spectra will depend on both the real and complex parts of the refractive index of the absorbing species whereas spectra obtained in transmission depend almost entirely on the complex part. As a result, certain types of "distortions" may appear in RAIR spectra when compared to transmission spectra of the same compound. An example is shown in Figure 2 where optical constants given by Allara et. al. (5) for the region of the carbonyl stretching mode of polymethylmethacrylate (PMMA) adsorbed onto a gold substrate were used to simulate the RAIR spectrum corresponding to a transmission spectrum consisting of a single band at 1700 cm" 1. It can be seen that for a film having a thickness of 200 À, the band in RAIR is shifted to higherfrequenciesby several wavenumbers. When the film thickness increased to 5,500 Â, the band shift increased to about 10 cm"l. However, when the film thickness increased to 14,500 Â, a second component appeared at about 1696 cm"l. One final comment regarding RAIR spectra is that the absorbance increases almost linearly with thickness for films having thickness less than about 4,000 Â. However, for thickerfilmsthe absorbance is a non-linear function of thickness due to interference effects.
In Multidimensional Spectroscopy of Polymers; Urban, Marek W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by MONASH UNIV on March 2, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch002
10
MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS
./_..
n1 «= 1.3-0.li; n2 = (.3 - 30.0 ; dl/λ = ).0003
Pa rallel Polai ization
·
Per Dendicular Polarizati( η _
10
20
30
_ \
40
50
60
70
80
90
Angle of Incidence (degrees)
Figure 1. The dependence of RAIR band size on the angle of incidence and polarization. The refractive indices of the ambient, film, and substrate were 1.0, 1.3 - 0.1 i, and 3.0 - 30.0i, respectively (2). The ratio of film thickness to the wavelength of light was 0.0003.
In Multidimensional Spectroscopy of Polymers; Urban, Marek W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by MONASH UNIV on March 2, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch002
BOERIO ET AL.
1600
1620
Determining the Structure of Polyimide—Metal Interfaces 11
1640
1660
1680
1700
1720
1740
1780
1760
1800
Wavenumber
Figure 2. RAIR spectra corresponding to a transmission spectrum consisting of a single band at 1700 cm"l. In this simulation, it was assumed that k = 0.38, γ = 25 cm" , v = 1700 cm" , η*) = 1.38, φ = 82°, and n2 = 9.5 - 30i. k and γ were the maximum and full width at half height of the absorption band in k-space while v was the position of the band, rioo was the refractive index of the film away from the absorption band, φ was the angle of incidence, and n2 was the refractive index of the substrate. m a x
1
1
0
0
m a x
0
0
In Multidimensional Spectroscopy of Polymers; Urban, Marek W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
Downloaded by MONASH UNIV on March 2, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0598.ch002
12
MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS
SERS has also been discussed extensively in the literature (4,5). The Raman scattering by a monomolecular layer of molecules is usually too weak to be detected by conventional techniques. However, when the monolayer is placed on the roughened surface of certain metals, such as copper, gold, or silver, the intensity of the Raman scattering is enhanced by several orders of magnitude. Two mechanisms seem to be responsible for the enhancement. The first is electromagnetic in origin and is associated with the large electric fields that can be obtained when the roughened surface of a metal is illuminated with electromagnetic radiation. The other is chemical in nature and is associated with a resonance effect involving charge transfer states that are formed when an organic compound is chemisorbed onto the surface of a metal. Electromagnetic enhancement can be as great as 10^ while chemical enhancement can be as great as 10^. Both mechanisms are very short-range. The chemical enhancement is only obtained for molecules that are chemisorbed and is therefore restricted to molecules in the "first layer." Electromagnetic enhancement is usually considered to have a form such as Enhancement ~ (r/r + d)12 where r is the radius of an asperity on the metal surface and d is the distance of the scattering molecule from the surface of the asperity (5). As a result, molecules adjacent to the surface experience a large enhancement but there is little enhancement for molecules only a few molecular layers awayfromthe surface. Therefore, SERS is surface-selective and can be used for non-destructive characterization of polymer/metal interfaces. Polyimides are widely used as dielectrics in multilevel interconnects. As a result, there has been a great deal of interest in determining the structure of polyimide films on metal substrates. Interest in determining the effect of the substrates on the molecular structure of polyimides and the polyamic acidsfromwhich they are usually derived has been especially strong. Much of the work has involved the use of vacuum techniques such as X-ray photoelectron spectroscopy (XPS) to determine the structure of bulk polyimides and their interfaces with metal substrates. However, several applications of RAIR and SERS to the study of polyamic acid and polyimidefilmson metal substrates have been reported. Burrell et. al. (
