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Thin Polyimide Films Prepared by Ionic Self-Assembly Mark R. Anderson,*,† Richey M. Davis,*,‡ C. Douglas Taylor,† Michaiah Parker,† Spencer Clark,† Daniela Marciu,§ and Michael Miller§ Departments of Chemistry and Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212, and Luna Innovations, Incorporated, Blacksburg, Virginia 24060 Received July 9, 2001. In Final Form: October 22, 2001 Layer-by-layer electrostatic deposition of polyelectrolytes was used to modify a gold substrate with polydiallyl dimethylammonium chloride and an Ultem-type poly(amic acid) salt. Following deposition, heat treatment converted the poly(amic acid) salt into a polyimide. A decrease in the thin film thickness and a substantial increase in the interfacial contact angle accompanied thermal treatment. Results from cyclic voltammetry measurements also show that the imidized surface had a decreased dielectric constant relative to an unmodified or a poly(amic acid)-modified interface. Each of these results is consistent with formation of the polyimide. Infrared spectra of the thin films formed by the electrostatic deposition were virtually identical with spectra obtained when the polyimide was formed by the normal spin casting procedure. These results suggest that the electrostatic method for depositing precursors of polyimides followed by heat treatment produces thin films that have structural and physical properties consistent with those of spin cast polyimide thin films.
Molecular design of interfaces has been an active area of research for many years.1 Using the Langmuir-Blodgett method with amphiphiles or the spontaneous adsorption of mercaptans onto metal substrates, many research groups have demonstrated that the presence of different chemical overlayers can alter interfacial properties in a controlled fashion.2-4 Structural investigations with these overlayers show that small changes in interfacial molecular structure might result in large changes in the surface energy.2,3 Consequently, much effort has gone toward understanding how interfacial molecular structure can be used in a variety of applications such as chemical sensing, adhesion, lubrication, and corrosion protection.5-9 Many of the techniques used to control interfacial molecular structure/composition, however, are labor intensive and are not amenable to large-scale application. In 1991, Decher et al. demonstrated that interfaces could be modified with polyelectrolytes using the electrostatic attraction of alternating polycations and polyanions to build up, in a layer-by-layer fashion, a multilayer thin film modified interface.10 This relatively simple procedure * To whom correspondence should be addressed. E-mail:
[email protected] or
[email protected]. † Department of Chemistry, Virginia Polytechnic Institute and State University. ‡ Department of Chemical Engineering, Virginia Polytechnic Institute and State University. § Luna Innovations, Inc. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932-950. (2) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350-4358. (3) Walczak, M. M.; Chung, C. K.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370-2378. (4) Slowinski, K.; Bilewicz, R.; Kublik, Z. Electrochem. Commun. 1999, 1, 437-440. (5) Napier, M. E.; Thorp, H. H. Langmuir 1997, 13, 6342-6344. (6) Young, J. T.; Boerio, F. J.; Zhang, Z.; Beck, T. L. Langmuir 1996, 12, 1219-1226. (7) Zhang, Z.; Beck, T. L.; Young, J. T.; Boerio, F. J. Langmuir 1996, 12, 1227-1234. (8) Casero, E.; Darder, M.; Takada, K.; Abruna, H. D.; Pariente, F.; Lorenzo, E. Langmuir 1999, 15, 127-134. (9) Haneda, R.; Aramaki, K. J. Electrochem. Soc. 1998, 145, 18561861.
requires no special equipment, and stable interfacial structures with an arbitrary number of alternating polyelectrolyte monolayers can be assembled. The simplicity of the deposition offers flexibility in the preparation of multilayer thin films and suggests that the method could easily be applied to a variety of substrate sizes and shapes. Since Decher’s report, several groups have demonstrated the utility of the ionic self-assembly (ISAM) method to a diverse range of applications.11-21 Many of these ISAM studies and applications were recently reviewed.22 While the ISAM method relies on the electrostatic attraction of alternating polycationic and polyanionic polymers to build up the thin film, subsequent chemical manipulation of the polyelectrolyte films can alter the chemical and/or physical properties of these multilayers. Rubner et al. demonstrated this by assembling the charged precursor of poly(phenylene vinylene) using the electrostatic layer-by-layer deposition procedure and subsequently thermally treating the sample to convert the interface to the neutral polymer.12,13 Baur et al. used both thermal and chemical methods to prepare a pyromellitic (10) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1991, 95, 1430-1434. (11) Heflin, J. R.; Figura, C.; Marciu, D.; Liu, Y.; Claus, R. O. Appl. Phys. Lett. 1999, 74, 495-497. (12) Onitsuka, O.; Fou, A. C.; Ferreira, M.; Hsieh, B. R.; Rubner, M. F. J. Appl. Phys. 1996, 80, 4067-4071. (13) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501-7509. (14) Liu, Y.; Claus, R. O. J. Appl. Phys. 1999, 85, 419-424. (15) Lenahan, K. M.; Wang, Y. X.; Liu, Y.; Claus, R. O.; Heflin, J. R.; Marciu, D.; Figura, C. Adv. Mater. 1998, 10, 853. (16) Arregui, F. J.; Liu, Y. J.; Matias, I. R.; Claus, R. O. Sens. Actuators, B 1999, 59, 54-59. (17) Liu, Y. J.; Wang, Y. X.; Claus, R. O. Chem. Phys. Lett. 1998, 298, 315-319. (18) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115-7120. (19) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 71077114. (20) Cheung, J. H.; Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2712-2716. (21) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 27172725. (22) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348.
10.1021/la011042j CCC: $20.00 © 2001 American Chemical Society Published on Web 11/30/2001
Thin Polyimide Films through Ionic Self-Assembly
dianhydride oxydianiline (PMDA-ODA) polyimide film that was deposited by the ionic self-assembly of the poly(amic acid) salt precursor.23 Thermal imidization of these films resulted in a substantial increase in the contact angle of the film. Importantly, Baur shows that the imide films formed following thermal or chemical imidization are stable despite the loss of the electrostatic attraction between alternate layers. Likewise, Bruening et al. form passivating multilayers on Al substrates by cross-linking alternating polyanionic and polycationic layers with either amide or imide bonds formed by thermal treatment of ISAM multilayers.24,25 Here, we investigate the properties of Ultem-type polyimide films prepared by the electrostatic deposition method. Polyimides are materials frequently used by the microelectronics industry for interfacial modification because they are tough, have high thermal stabilities, and have favorable dielectric properties for use as insulating layers.26 Normally, polyimides are deposited onto substrates in a two-step process by spin casting a thin film from a solution of the poly(amic acid) form of the polymer from an organic solvent, such as 1-methyl-2pyrrolidinone, followed by thermal curing to convert the poly(amic acid) into the imide. The viscosity of the polymer solution and the rate of rotation limit thin film thickness obtainable by spin casting, with a typical minimum thickness on the order of micrometers. Use of organic solvents in this procedure is also a limitation as they require costly disposal. Baur et al. show that poly(amic acid)s can be used to prepare interfacial films by electrostatic assembly.23 Treatment of the poly(amic acid) with a base will ionize the poly(amic acid), making the polymer water soluble and suitable for deposition by the ISAM method. Formation of the poly(amic acid) salt in this fashion, however, can decrease the molecular weight of the subsequent polyimide, possibly altering the thin film physical properties. Several groups use the ISAM approach for preparing thin polyimide films, although no detailed structural and only limited physical properties of the films have been reported.23,24,27,28 We report the preparation of thin films of an Ultem-type polyimide by the electrostatic layer-bylayer deposition method of the corresponding poly(amic acid) salt from aqueous solutions followed by thermal imidization using procedures simlar to those of Baur.23 Following formation of the ISAM films, we investigate the dielectric and structural properties of the ISAM film both before and after imidization and compare these parameters to those of polyimide thin films prepared by spin casting. Experimental Section ISAM films consisting of the tripropylammonium salt of an Ultem-type poly(amic acid), denoted as TPA-PAA (Figure 1A), and polydiallyl dimethylammonium chloride, denoted as PDDA (Figure 1C), were made on gold-coated glass slides. The TPAPAA salt was prepared using the procedure of Facinelli et al.29 The salts were collected, dried, and stored at 0 °C until being (23) Baur, J. W.; Besson, P.; O’Connor, S. A.; Rubner, M. F. Mater. Res. Soc. Symp. Proc. 1996, 413, 583-588. (24) Dai, J.; Sullivan, D. M.; Bruening, M. L. Ind. Eng. Chem. Res. 2000, 39, 3528-3535. (25) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006-2013. (26) Young, J. T.; Boerio, F. J. Surf. Interface Anal. 1993, 20, 341351. (27) Liu, Y. J.; Wang, A. B.; Claus, R. O. Appl. Phys. Lett. 1997, 71, 2265-2267. (28) Moriguchi, I.; Teraoka, Y.; Kagawa, S.; Fendler, J. H. Chem. Mater. 1999, 11, 1603-1608. (29) Facinelli, J. V.; Gardner, S. L.; Dong, L.; Sensenich, C. L.; Davis, R. M.; Riffle, J. S. Macromolecules 1996, 29, 7342-7350.
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Figure 1. Repeat unit structures of (A) the Ultem-type poly(amic acid) salt, (B) the Ultem-type polyimide after thermal treatment of the TPA-PAA salt, and (C) the polydiallyl dimethylammonium chloride, PDDA. used. Upon thermal imidization, the weight-average molecular weight of the Ultem-type polyimide (Figure 1B) was 19 700 g/mol as measured by gel permeation chromatography. PDDA was obtained from Aldrich and used as received. The weight-average molecular weight of the PDDA was reported by the manufacturer to be in the range of 400 000-500 000 g/mol. Aqueous solutions of these polyelectrolytes were prepared using Nanopure water (17.5 MΩ resistivity, Barnstead). The pH of the TPA-PAA and the PDDA solutions was controlled using tripropylamine (98%, Aldrich). It was important to maintain the pH of the TPA-PAA solution above 8 to suppress chain hydrolysis in solution. ISAM films consisting of 1, 2, 5, and 10 bilayers of TPA-PAA/PDDA were made by a stepwise dipping procedure. Substrates were 1 in. × 1 in. glass microscope slides with a 50 Å vapor-deposited layer of Cr followed by a 1000 Å layer of Au vapor deposited on top of the Cr (EMF, Inc., Ithaca, NY). Prior to polymer deposition, the Au surfaces were cleaned with piranha solution (3:1 concentrated H2SO4/30% H2O2) for 1 min followed by rinsing with water. Extreme care must be taken when using piranha solution as it is highly oxidative; it should not be stored in a closed container, and it should be disposed of properly after use. This procedure produced hydrophilic surfaces suitable for adsorption of the polyelectrolytes. Solutions of both polymers were prepared at concentrations of 1 mM of repeat unit. The pH for both the PDDA solution and the TPA-PAA solution was adjusted to 10 with tripropylamine. The dipping time for the first PDDA/TPA-PAA bilayer was 10 min;23 subsequent bilayers were deposited using dipping times of 3 min. Lvov et al. showed that ∼90% surface coverage occurred within less than 1 min when ISAM films were made using PDDA solutions 50 times less concentrated than the solutions used in this study.30 After each dipping step, the slides were rinsed in Nanopure water for approximately 1 min. Once the final layer was deposited, drying was done immediately after the rinsing step using purified nitrogen. No drying was done between the deposition of bilayers. The slides were then imidized at 275 °C for 10 min in a programmable convection oven. A temperature of 275 °C was chosen for the imidization for several reasons. This (30) Lvov, Y. M.; Rusling, J. F.; Thomsen, D. L.; Papadimitrakopoulos, F.; Kawakami, T.; Kunitake, T. Chem. Commun. 1998, 1229-1230.
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heating schedule was found to result in complete imidization of TPA-PAA, resulting in a well-defined Ultem-type polyimide.31 In addition, thermal gravimetry measurements of spin cast PDDA films show that these materials are stable up to 300 °C, having only a 6% weight loss at approximately 280 °C. At higher temperatures, between 300 and 320 °C, the PDDA experiences a 70% weight loss, suggestive of thermal decomposition of the polymer. Film characterization tests were conducted both before and after imidization. Reference thin films of pure TPA-PAA and PDDA were spin-coated onto the Au surfaces using a custommade spin coater operated at 3000 rpm. Ellipsometry measurements were made with a Gaertner ellipsometer using a 70° reflection angle. A minimum of five points were measured for each modified substrate. The optical constants for the gold substrates were measured prior to deposition of the thin film. In addition, the optical constants of an unmodified gold substrate were also measured before and after the thermal treatment and found to not significantly change as a result of heating at 275 °C for 10 min. The real part of the refractive index of the thin film samples was set to 1.45 for the thickness calculations. Because the actual values of the refractive indices for the asdeposited ionic and the imidized thin films were not known, the thickness values calculated by the ellipsometry measurements are approximate and are used to indicate trends in thickness. Contact angle measurements were conducted using the sessile, static drop method with a Rame-Hart NRL contact angle goniometer. Drops (5 µL) of Nanopure water were used as the probe liquid, and values reported are the average of a minimum of 3 drops per sample. Infrared spectra were measured using a Nicolet model 710 FTIR with an HgCdTe detector. All spectra were acquired using 512 interferometer scans, 4 cm-1 resolution, and boxcar apodization. Reflection spectra were obtained using a Spectra-Tech grazing-incidence sampling accessory. For reflection spectra, the source radiation was oriented so that the electric vector was polarized normal to the substrate surface using a Cambridge Physical Sciences IR polarizer (IGP228). Electrochemical measurements were made using a CH Instruments model 600A potentiostat (Austin TX) and a drop electrochemical cell described previously.32 With this electrochemical cell, the contact area that a drop of electrolyte solution makes with the modified substrate (for these measurements, 0.13 cm2) defines the surface area of the working electrode. Solution resistance was compensated 80% using positive feedback IR compensation for all measurements. The capacitive current was determined using cyclic voltammetry measurements with an aqueous 0.10 M KCl solution and a 0.10 V/s rate of potential scan. Electrochemical measurements are made on both modified and unmodified substrates that had been subject to similar conditions prior to measurements. Potentials are measured relative to an aqueous Ag/AgCl reference electrode, and the reported capacitance was determined at the open circuit potential of the cell.
Results and Discussion The poly(amic acid) salt form of the polyimide was deposited onto the Au substrates with alternating layers of the polycation PDDA using the ISAM technique. Samples with 1, 2, 5, and 10 bilayers were prepared and investigated in both the as-deposited poly(amic acid) salt and thermally treated forms. Results of thin film thickness measurements by ellipsometry are given in Table 1. The average thickness per bilayer prior to the thermal treatment lay in the range of 0.5-0.7 nm/bilayer, a typical value for ISAM films.23 As expected for the layer-by-layer deposition, the thickness of the films increased in an approximately linear fashion with increasing number of bilayers (polycation/polyanion pairs). Films typically (31) Gardner, S. H. Effect of Controlled Interphase Structure on Performance of Thermoplastic Composites. Ph.D. Dissertation, Virginia Polytechnic Institute and State University, Blacksburg, VA, 1998. (32) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691.
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Figure 2. Reflection infrared spectra of the thin film (A) before and (B) after thermal treatment of a 10-bilayer thin film prepared from TPA-PAA/PDDA. Spectra are the result of 512 interferometer scans collected at 4 cm-1 resolution. Table 1. Measured Thickness of TPA-PAA/PDDA Multilayer Films by Ellipsometry number of thickness of poly(amic acid) thickness of polyimide bilayers form, nm form, nm 1 2 5 10
could not determine 1.28 ( 0.94 3.70 ( 0.72 5.42 ( 1.97
could not determine could not determine 1.40 ( 0.51 2.45 ( 0.67
showed a large decrease in thickness after imidization. Decreased thickness of the thin film was not unexpected since the imidization process releases two molecules of water for each repeat unit of TPA-PAA and ISAM films are known to swell in response to changes in ambient humidity;33 however, this alone could not account for the magnitude of the thickness change. It is likely that the optical properties of the as-deposited film and the heattreated films are different, and using of the same approximate refractive index value for the thickness calculation likely introduces error in the measured film thicknesses. For this reason, the magnitude of the thickness change on heat treatment is an approximation and is taken as an indication of a trend toward decreased film thickness on heat treatment. Reproducible thickness measurements for the 1- and 2-bilayer samples could not be obtained by ellipsometry following thermal treatment. For that reason, most of the subsequent analysis was conducted with the 5- and 10-bilayer samples. Reflection infrared spectra of these thin films also showed that thermal treatment resulted in the imidization of the poly(amic acid) salt (Figure 2), consistent with Baur’s results with PMDA-ODA.23 Prior to thermal treatment, the dominant features in the IR spectrum were found at 1660 and 1600 cm-1 and are characteristic of an amide and a carboxylate salt, respectively. This is consistent with the electrostatic interaction between the poly(amic (33) Kleinfeld, E. R.; Ferguson, G. S. Chem. Mater. 1995, 7, 23272331.
Thin Polyimide Films through Ionic Self-Assembly
Figure 3. Comparison of the reflection infrared spectra of a spin cast film of PDDA from a 5% solution (A) prior to thermal treatment and (B) after thermal treatment for 10 min at 275 °C.
acid) salt and the cationic sites of PDDA being responsible for the assembly of the thin films. After thermal treatment, the amide and carboxylate vibrational features were replaced in the reflection spectrum by vibrational features characteristic of an imide. Specifically, the asymmetric carbonyl stretching mode at 1730 cm-1 and the axial imide C-N-C stretching mode at 1370 cm-1 are diagnostic of an imide.7 It was anticipated that imidization would not only change the structure of the film but would also alter the interfacial physical properties. Contact angle measurements conducted before and after imidization confirmed this expectation. Before imidization, the 10-bilayer PDDA/ TPA-PAA-modified surface had a contact angle with water of 59 ( 2°. After imidization, the contact angle of this 10-bilayer modified interface increased to 87 ( 2°, consistent with the formation of a more hydrophobic interface. These contact angle results are also consistent with values reported by Baur before and after their PMDAODA poly(amic acid) ISAM films are converted to the polyimide.23 Thermal treatment at 275 °C also may affect the structure and properties of the PDDA polycationic layer. Figure 3 shows the infrared spectrum of a spin cast PDDA thin film (cast from a 5% solution at 2000 rpm, with a thickness of approximately 5 µm) before and after heating at 275 °C for 10 min. Prior to heating, the spectrum is dominated by a feature at 3400 cm-1 characteristic of the quaternary ammonium group. After thermal treatment, this feature decreases in intensity and is replaced by a new feature at 1710 cm-1. This new feature is assigned to a carbonyl mode and is attributed to oxidation of the PDDA polymer that occurs during thermal treatment. Both the heat-treated PDDA and the Ultem-type polyimide have features characteristic of carbonyl functionality that may contribute to the spectrum of the ISAMprepared multilayer films. Figure 4 shows a comparison of the heat-treated, spin cast PDDA thin film with a thin film of the polyimide (also prepared by heat treating a film of the poly(amic acid) prepared by spin casting from
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Figure 4. Comparison of the reflection infrared spectra of spin cast thin films of (A) PDDA after thermal treatment at 275 °C for 10 min and (B) the Ultem-type polyimide thin film prepared by spin casting a 5% poly(amic acid) solution followed by imidizing with heat treatment at 275 °C.
a 5% 1-methyl-2-pyrrolidinone solution). Each of these spin cast films has approximately the same thickness (5 µm); however, the intensity of the carbonyl feature of the oxidized PDDA is substantially smaller than that of the polyimide. While it is clear that the heat-treated PDDA may contribute to the total absorbance found in the carbonyl region for the ISAM-prepared multilayer, the intensity differences observed in the comparison of the spectra from spin cast films suggest that the absorbance by the vibrational features in the PDDA layers will be significantly less than that of the polyimide layer. This result suggests that the infrared spectrum of the ISAM multilayer films will be dominated by the polyimide contribution. The infrared spectra of the polyimide films formed by ISAM deposition are nearly identical to the spectrum of the spin cast film (Figure 5). This result suggests that polyimide films prepared by ISAM formation with the poly(amic acid) salt followed by thermal imidization are structurally similar to bulk films prepared by spin casting despite the presence of the PDDA in the electrostatically assembled layers. In the reflection spectra of thin films, the absorbances of individual vibrational modes that have weak to moderate absorptivities are a function of the orientation of the transition dipole moment relative to the surface normal.34,35 The relative intensities of different vibrational modes, therefore, may be used to qualitatively describe interfacial molecular structure. The interfacial structure of the polyimide thin films may be qualitatively described by the surface selection rules for grazing-incidence infrared reflection spectroscopy and the method of Miller et al. using these infrared spectra.34-37 In this analysis, the relative peak intensities (34) Greenler, R. G. J. Chem. Phys. 1969, 50, 1963-1969. (35) Greenler, R. G. J. Chem. Phys. 1966, 44, 310-315. (36) Cammarata, V.; Atanasoska, L.; Miller, L. L.; Kolaskie, C. J.; Stallman, B. J. Langmuir 1992, 8, 876-886. (37) Kwan, W. S. V.; Atanasoska, L.; Miller, L. L. Langmuir 1991, 7, 1419-1425.
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Figure 6. Cyclic voltammograms of an aqueous 0.10 M KCl solution using a potential scan rate of 100 mV/s with a gold electrode modified with 10 bilayers of TPA-PAA/PDDA before and after thermal treatment. The i vs E behavior of an unmodified gold electrode is also shown for comparison. For these experiments, the potential was continuously cycled until a steady-state current response was obtained. Figure 5. Comparison of the reflection infrared spectra of (A) a spin cast film of TPA-PAA which has been thermally imidized, (B) a 10-bilayer thin film of electrostatically deposited TPAPAA/PDDA which has been thermally treated, and (C) a 5-bilayer thin film of electrostatically deposited TPA-PAA/ PDDA which has been thermally treated. Spectra were collected using the parameters described previously.
of two or more vibrational modes whose transition dipole moments are orthogonal to each other provides qualitative information regarding the net orientation of the molecule at the interface. For the Ultem-type polyimides, the axial imide C-N-C vibration and the asymmetric carbonyl vibration are orthogonal to each other within the molecule and are suitable for the qualitative structural analysis. The ratio of the peak areas for the axial imide C-N-C absorbance to the asymmetric carbonyl absorbance for the spin cast film and the ISAM-prepared film are 0.10 ( 0.03 and 0.09 ( 0.02, respectively. The closeness of these values implies that these vibrational modes are similarly oriented in both the spin cast film and the film prepared by the ISAM method. In addition, the strength of the asymmetric carbonyl modes relative to the axial imide C-N-C modes in the polyimide spectra suggests that the aromatic imides are aligned so that a significant portion of the transition dipole moment of the asymmetric carbonyl stretch is perpendicular to the substrate. This interpretation places the polymer with the aromatic imide largely parallel to the substrate. A structural arrangement with the long axis of the polymer parallel to the substrate is consistent with the expected arrangement of the poly(amic acid) salt following electrostatic deposition and agrees with the quantitative polyimide structural analysis by Young.6
Cyclic voltammetry measurements provide a qualitative measure of the changing dielectric properties of the interface that occur with modification. In the absence of a faradaic charge-transfer reaction, the measured current in an electrochemical experiment is representative of the capacitance of the interface. By cycling the potential of the gold substrate between +0.15 and -0.15 V versus Ag-AgCl in the presence of aqueous 0.1 M KCl, the capacitive current, ic, can be measured. The capacitive current in this cyclic voltammetry experiment is related to the interfacial capacitance Cdl by
ic ) AνCdl
(1)
where A is the area of the electrode and ν is the rate of potential scanning. The capacitance of the thin film is related to the dielectric constant of the interface, , as well as the distance of closest approach of the doublelayer ions to the metallic substrate, d0, as shown by
Cdl )
0 d0
(2)
Example cyclic voltammograms for a 10-bilayer sample are shown in Figure 6, and capacitance values calculated from the cyclic voltammetry data for the different bilayermodified interfaces are given in Table 2. Interestingly, the capacitance of the as-deposited films is approximately 15 µF/cm2 regardless of the number of bilayers deposited. Given that the capacitance of the interface is related to both the thickness of the film (e.g., the barrier properties) as well as the dielectric of the film, this result suggests that the as-deposited films are not efficient barriers to
Thin Polyimide Films through Ionic Self-Assembly Table 2. Measured Capacitance of the TPA-PAA/PDDA Bilayer Films
bilayers
capacitance, µF cm-2 unimidized film
capacitance, µF cm-2 imidized film
calculated dielectric of the imidized filma
1 2 5 10
15.5 ( 0.8 18.3 ( 0.9 15.0 ( 0.2 16.2 ( 0.2
6.5 ( 1.3 7.1 ( 0.5 4.5 ( 0.9 3.4 ( 0.7
could not determine could not determine 7.13 9.65
a The calculated dielectric uses the thickness of the heat-treated film as determined by ellipsometry.
electrolyte permeation, consistent with previous measurements.25,38,39 When the thin films are imidized by heat treatment, however, the capacitance of the interface decreases considerably. In addition, the capacitance of the heat-treated 2-, 5-, and 10-bilayer films systematically decreases as the number of bilayers increases. This result is consistent with the trend in the thickness of the polyimide films, and it suggests that the imidized films are more efficient barriers to electrolyte permeation than the as-deposited poly(amic acid) films. After conversion of the interface to the polyimide by thermal treatment, there was a substantial decrease in the capacitance for each of the bilayer films (relative to the corresponding as-deposited film). The observed differences when comparing the capacitance of the asdeposited and the imidized films are taken to represent a lowering of the interfacial dielectric constant and an increase in the barrier properties of the interface to aqueous electrolyte, consistent with the conversion of the film to a polyimide. This can be qualitatively demonstrated by calculating an approximate dielectric of the thin films using the film thicknesses determined by ellipsometry. The dielectric values calculated (Table 2), 7.1 and 9.6 for the 5- and 10-bilayer films, respectively, are approximations at best; however, the values obtained are not unreasonable for polyimide films. For comparison, polyimide films prepared by spin casting typically have dielectric values on the order of 3.5.40 The electrochemical and infrared spectroscopy results are consistent with the films deposited by electrostatic deposition having the desired dielectric properties of polyimide thin films required by the microelectronics industry. (38) Losche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893-8906. (39) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626-7634. (40) General Electric Corp. Ultem Profile. 20. 1998.
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Summary Electrostatic deposition of alternating layers of a polycation (PDDA) and a polyanion (the poly(amic acid) salt precursor of a polyimide) was used to build up a polymer thin film on a gold substrate. Thermal treatment of the deposited ISAM film induced imidization of the poly(amic acid) salt and was accompanied by a decrease in the thin film thickness. Reflection infrared spectra of the polyimide thin films prepared by the ISAM method were nearly identical to spectra of pure polyimide films prepared by spin casting. These results suggest that polyimide films prepared by these two methods are structurally similar. Electrochemical measurements with the electrostatically deposited films (before and after imidization) show that the imidized films have altered dielectric properties and improved resistance to aqueous electrolyte permeation relative to the as-deposited poly(amic acid) salt. Estimates of the dielectric for the 5- and 10-bilayer films from the capacitance measurements are consistent with values typical of spin cast films, suggesting that the reduced molecular weight of the ionized poly(amic acid) does not dramatically alter the physical properties of the ISAM-deposited polyimide films relative to the much thicker spin cast films. The spectral and electrochemical results suggest that the structural and physical properties of thin polyimide films prepared by the ISAM deposition of the poly(amic acid) salt precursor are similar to those of films prepared by the normal spin casting method. These results indicate that the ISAM method for polyimide deposition could be used in place of spin casting films with little difference in thin film behavior. The ISAM method, however, offers several advantages over spin casting that suggest the power of this method. ISAM deposition may be applied to substrates that are not planar and are not amenable to spin casting. Also, thin films prepared by the ISAM method can be made significantly thinner than is possible by spin casting, a significant advantage for miniaturization. These attributes when combined with subsequent chemical manipulation of the thin layers indicate that the ISAM method of surface modification is a powerful technique for interfacial modification. Acknowledgment. Funding for this work was provided through United States Air Force SBIR Phase I Contract F33615-98-C-5127. The authors thank A. Bounds and J. S. Riffle for the synthesis and characterization of the Ultem-type polyamic acid and M. Guzy for help with the film preparation. LA011042J
