Langmuir 1998, 14, 6743-6748
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Influence of Relative Humidity during Film Formation Processes on the Structure of Ultrathin Polymeric Films U. Hecht,* C. M. Schilz, and M. Stratmann Lehrstuhl fu¨ r Korrosion und Oberfla¨ chentechnik, Universita¨ t Erlangen-Nu¨ rnberg, Martensstrasse 7, D-91058 Erlangen, Germany Received April 29, 1998. In Final Form: August 13, 1998 Film formation processes of ultrathin films of a curable model polymer system in the presence of a metallic substrate were studied. The structure of prepolymerized films strongly depends on the relative humidity during the spin-coating process. With increasing atmospheric humidity, typical defect structures were observed which never healed during full curing. A mathematical simulation shows that condensation of water forming liquid drops can be excluded as source of holes in the films. We propose that these defects are caused by physisorbed water layers on the substrate surface resulting in a dewetting of the polymer.
Introduction In the past, adhesion of a broad variety of polymers to different metals was extensively studied. For its widespread use in industry, adhesion to aluminum is of special interest. A lot of work has been done concerning the pretreatment of aluminum and its alloys in order to improve the strength and lifetime of adhesive bonds as recently thoroughly reviewed by Critchlow and Brewis.1 In most cases the roughness of the aluminum surfaces is quite high in order to achieve mechanical interlocking in combination with chemical or physical bonding. The aim of our work is to gain a deeper understanding of the different influencesschemical or physical in natures which are critical for the strength and lifetime of adhesive bonds excluding the influences of mechanical interlocking. The orientation and structure of the polymer at the interface established during bond formation will later dominate the effectiveness of the bond. In this paper, we emphasize the effect of the relative humidity on phase boundary formation. It has been reported that, for instance, the influence of the relative humidity on bond strength can be significant during bonding.2 As one possible approach, we try to simulate the phase boundary of adhesive joints by means of ultrathin films, an attempt also performed by other groups.3 We chose aluminum as a model substrate for its importance, e.g., in airospace, transport, packaging, and automobile industries. A thermoset polycyanurate network served as adhesive, for it exhibits interesting properties such as high glass transition temperature, high toughness, and good thermal stability.4,5 Experimental Details To obtain a reproducible and smooth surface we evaporated thin aluminum films (around 200 nm) on commercially available silicon (100) wafers (Wacker Siltronic AG, Burghausen; evaporation pressure around 5 × 10-5 mbar, Al pellets, 99.995%, Johnson (1) Critchlow, G. W.; Brewis, D. M. Int. J. Adhes. Adhes. 1996, 16, 255. (2) Thorne, N. A.; Thue´ry, P.; Frichet, A.; Gimenez, P.; Sartre, A. Surf. Interface Anal. 1990, 16, 236. (3) Gesang, T.; Possart, W.; Hennemann, O.-D.; Petermann, J. Langmuir 1996, 12, 3341. (4) Bauer, M.; Bauer, J. In Chemistry and Technology of Cyanate Ester Resins; Hamerton, I., Ed.; Blackie Academic & Professional: London, 1994; p 57. (5) Georjon, O.; Galy, J. Polymer 1998, 39, 339.
Figure 1. The monomer and trimer of DCBA. Matthey Alfa Products, Karlsruhe). The morphology of the substrates was measured with AFM (description of type and scan parameters are given below) and showed typical small crystallites (50-250 nm base) well-known for evaporated metal films. The calculated root-mean-square roughness (RMS) of these films ranged from 4 to 6.5 nm. As a model system for curable polymers we used a dicyanate of bisphenol A (DCBA) with a prepolymerized conversion of 44 mol % (Figure 1). A polymer of the same kind has already been taken as a model system for high-temperature adhesives by other groups.3,5 The prepolymer shows a broad molar mass distribution consisting of monomers, trimers, and higher oligomers (Figure 2). The oligomers are linked via triazine rings.4 For preparation of ultrathin films, the substrates were spin coated with a prepolymer solution (5 mg/mL in tetrahydrofuran, 99.9+% HPLC grade, Sigma Aldrich Chemie GmbH, Steinheim) resulting in an integral overall film thickness of around 10 nm as determined for some samples with spectroscopic ellipsometry. Before and during spin coating the relative humidity (RH) was
10.1021/la9804987 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/14/1998
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Hecht et al. Table 1. Influence of the Relative Humidity during the Spin-Coating Process on the Surface Coverage by Prepolymer RH 80% surface 100% 100% 96 ( 3% 84 ( 3% 81 ( 3% 80 ( 3% coverage
Results
Figure 2. Distribution of oligomers. adjusted by an argon flux of fixed humidity through a chamber covering the spin-coating machine. The sample was stored in that humidity for about 5 min prior to wetting with the prepolymer solution. The chamber has a small opening in order to drop the solution on the horizontal substrates with a Pasteur pipette. Immediately after substrate wetting the chamber is closed again and after a delay of 3 s the substrate is spinned for 20 s at a speed of 5000 r/min. For full curing the films must be heated for 2 h at around 200-220 °C. In this process the end-standing cyanate groups form triazine rings with a conversion of around 99 mol % (controlled with FTIR spectroscopy). A PicoAFM (Molecular Imaging, Tempe) was used to analyze the resulting structures of the prepolymerized as well as the cured films. All measurements were performed in contact mode at very low contact forces (controlled via force-distance plots). We used pyramidal silicon nitride cantilevers (Digital Instruments, St. Barbara) with a typical spring constant of 0.12 N/m and scanning frequencies between 1.5 and 3 Hz. X-ray photoelectron spectra (PHI 5600, Physical Electronics, Eden Prairie, XPS) revealed the chemical composition of the various surfaces and Auger-spectra, line scans and mappings (Scanning Auger Microscope PHI 670, Physical Electronics, Eden Prairie, SAM) were taken to gain chemical information at high lateral resolution. Surface plasmon resonance (SPR) spectra provided thickness information of the adsorbed water layers on substrates covered with aluminum oxide. The setup is described in detail elsewhere.6 SPR uses the high sensibility of an evanescent electromagnetic wave to little variations in thickness and/or dielectric properties of thin layers. Surface plasmons (SP), two-dimensional vibrations of the free electron gas, are excited along a metal/dielectric interface. The dielectric can be vacuum or gas phase or any solid phase with a positive real part of the dielectric function. For isotropic materials it provides direct, nondestructive, and integral thickness information. For our SPR experiments we chose LaSFN9 glass (J. D. Mo¨ller Optische Werke GmbH, Wedel) for both substrate and prism material. Thin film samples for the SPR have been produced in a vacuum chamber at pressures of approximately 5 × 10-5 mbar. First, 48 nm Au (pellets, 99.99%, C. Hafner GmbH & Co., Pforzheim) was deposited thermally and then covered with approximately 18 nm Al2O3 (99.6%, Balzers Materials, Liechtenstein) using electron beam sputtering. Pure nitrogen served as carrier gas for changing moisture in the cell. RH in the cell was measured with a little sensor (MiniCap2, Panametrics GmbH, Hofheim) mounted beneath the exposed surface. SPR spectra were recorded at T ) 28 °C. SPR samples were investigated with XPS to determine surface composition. (6) Schilz, C. M.; Unger, M.; Knoll, W.; Stratmann, M. Joint ISE/ ECS; Paris, Fall Meeting 1997; Abstract No. 906.
Depending on the relative humidity before and during the spin-coating process, the resulting ultrathin prepolymeric films show different typical structures. Below 25% RH the films are always homogeneously spread over the whole sample and very smooth (typical RMS values between 1.7 and 2.7 nm). At 30% RH the films start to show very small uniformly distributed holes of around 100 nm in diameter. At higher humidities some larger holes appear additionally with diameters between 1.4 and 2.2 µm (60% RH) and approximately 200 nm sized drops of prepolymer (SAM measurements) are forming inside the larger holes (Figure 3). The film coverage evaluated by analyzing the AFM data is shown in Table 1. One can see that polymer films with defects appear at relative humidities larger than 20%, the surface coverage decreasing to 81% with increasing humidity up to 60% RH. Although they have the same degree of coverage, the typical morphology of films coated at 60% RH and films coated at >80% RH is quite different (Figure 4). Films coated at 60% RH exhibit a lot of small (around 100 nm) and medium-sized (around 2 µm) holes, very rarely showing drops of prepolymer inside mediumsized holes. Films coated at 80% RH have significantly less small and medium-sized holes; large ones (3-15 µm) with quite a number of prepolymer drops inside of them predominate. SPR experiments determine the water layer thickness on aluminum oxide correlated to the varying RH. Figure 5 shows the change of the water layer thickness versus RH value. The adsorption isotherm seems to be of type III of the Brunauer et al. classification.7,8 This result is in pretty good agreement with various other works on aluminum oxide.9,10 When the humidity is changed to 25% the layer thickness raises by 5 Å, near 96% RH (detection limit of the sensor) 27 Å are adsorbed. If we assume a cubic closest packing of spheric water molecules (2.75 Å diameter) the measured layer thickness of 27 Å equals 17 monolayers (ML) of water (layer thickness of 1.587 Å). Warren et al.10 found a slightly higher value, 22 ML, on R-alumina and also evidence for hysteresis while cycling. Furthermore, calculations we made from quartz micro balance measurements on evaporated Al films confirm our thickness values. Warren et al.10 also mention the strong influence of contaminants on the amount of adsorbed water. This may cause the difference in reported thicknesses. XP spectra from the samples used for SPR in this work show only poor contamination. Despite this the assumption of a looser packing of water molecules can also lead to a slight increase of ML number. Morimoto et al.11 proposed models for understanding the growth mode of water on aluminum oxide. As adsorption starts the first physisorbed monolayer is bound (7) Burwell, R. L.; Smudski, P. A.; May, T. P. J. Am. Chem. Soc. 1947, 69, 1525. (8) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723. (9) Khanna, V. K.; Nahar, R. K. Appl. Surf. Sci. 1987, 28, 247-264. (10) Warren, G. W.; Chatterjee, I. Proceedings of the 1st International Symposium on corrosion of electronic materials and devices; ECS: Vols. 91-2, p 185. (11) Morimoto, T.; Nagao M.; Tokuda, F. J. Phys. Chem. 1969, 73, 243.
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Figure 3. Typical film morphologies for films coated at (a) 20% RH, (b) 30% RH, (c) 40% RH, and (d) 60% RH. Holes are clearly visible, arroys point to drops of prepolymer.
Figure 4. Typical film morphologies for films coated at (a) 60% RH and (b) >80% RH.
through H-bonding to two OH- groups on the oxidic surface. This layer is held quite firmly. The following layers are more loosely held and mobile. They are subject to cyclic humidity changes. This model could explain the offset value of a few Å in our SPR experiments close to zero humidity and, moreover, is consistent with XP-O1s spectra. They show a high OH-/O2- ratio of 1/1.9, the residue of the initially built surface hydroxyls. On Al bulk material we observe an increase of the hydroxyl
content of the native oxide to 48% during exposure to laboratory air over a period of approximately 30 days. Similar and more precise results were reported by Thorne et al.2 and are explained by the formation of Al(OH)3 islands on the native aluminum oxide. When the thin prepolymer films are cured for maximum conversion the mean thickness of the films is reduced by 30-40% primarily because of evaporation of monomers. Despite this loss of mass and the drying of the interface
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T(x) )
F Fcl
t+
[
F 3x2 - l2 l K 6l2 n 2 ∞ (-1) π2
Figure 5. Correlation between the relative humidity and the layer thickness of adsorbed water on bare Al2O3.
during the curing process, we always find that the structure established during the coating process is not changing significantly. Uniformly distributed and closed films remain closed, while the typical defect pattern which exists after coating at higher RH is found again on the cured samples (Figure 6). For a detailed chemical analysis of these holes, Auger spectra were taken at a lateral resolution of 40 nm. For this investigation, only cured films were used to avoid artifacts resulting from the degassing of monomers into vacuum. The measurements show that the holes are free of polymer. Figure 7 presents a 12 µm line scan of Al and N taken with 256 points of a film spin coated at 60% RH. Discussion Basically three different possible explanations exist for the described structural behavior of the ultrathin polymeric films, showing an influence of the RH during the film formation process: 1. Adsorbed water with varying thickness covering the substrate surface before and during coating could result in a dewetting of the prepolymer film. 2. Tiny drops of water could condensate from the gas phase because of an undercooling of the substrate during the evaporation of the solvent. These could displace the polymer resulting in the observed holes in the film. 3. Water contained in the solvent remains after evaporation of the solvent on the surface and could also result in water drops. (The last two possibilities were also proposed by others.12) This last explanation was excluded by experiments with dried tetrahydrofuran which gave the same results. The second explanation predicts the formation of tiny drops of water, which could be the source of the typical round holes. These holes broaden with increasing humidity. (This however fails to explain the existence of drops of prepolymer inside holes at 60% and especially at 80% RH.) For a mathematical evaluation of substrate cooling we made the following assumptions: 1. The sample is a slab bounded by two parallel planes, 2. the surface suffers a constant negative flux of heat for a fixed time (evaporation time of the solvent), and 3. the sample is thermally isolated. This problem was solved and described in detail by Carslaw and Jaeger13 (12) Partridge, A.; Toussaint, S. L. G.; Flipse C. F. J. Appl. Surf. Sci. 1996, 103, 127. (13) Carslaw, H. S.; Jaeger, J. C. In Conduction of Heat in Solids, 2nd ed.; Oxford University Press: London, 1959; p 112.
Hecht et al.
[∑[ n)1
n2
(
) ( )]]]
nπ2 κn2π2 x exp t cos l l2
where T(x) is the temperature of the sample at thickness x, F is the flux of heat, F is the density of the sample, c is the heat capacity, l is the sample thickness, t is the time of flux, K is the thermal conductivity, and κ ) K/Fc. For our estimation we took the material constants of silicon (F ) 2.33 g/cm3, c ) 0.704 J/(g K), K ) 1.49 J/(s cm K)) neglecting the thin aluminum evaporation layer (typically of some 100 nm) compared to the silicon wafer with a thickness of 525 µm. Moreover the material constants of silicon are in the same order of magnitude as the ones of aluminum (F ) 2.70 g/cm3, c ) 0.901 J/(g K), K ) 2.37 J/(s cm K)). The evaporation rate of tetrahydrofuran was measured by weighing the loss of mass to be 1.84 × 10-2 mg/(s cm2), the flux of heat during evaporation being therefore 7.57 × 10-3 J/(s cm2). After the rotation of the spin coater has started an equilibrium is established between the centrifugal and the liquid shear forces. The excess solution is removed immediately. As the resulting film thickness of the prepolymer is known (around 10 nm, ellipsometry measurement) the thickness of the equilibrium solution film in the spin-coating process before drying is calculated to be 1.78 µm. (For the given concentration and the density of the prepolymer of 1.22 g/cm3.) At the measured evaporation rate it takes 11.8 s for a film of the pure solvent of such thickness to evaporate; this is the maximum time for the flux of heat to cool the sample. The cooling of the sample surface is therefore less than 1.1 °C. Considering the humidity diagram at 25 °C, even at 80% RH 3.5 °C cooling of the sample would be required for condensation to take place. As pointed out above the typical holes in the polymer films can neither be caused by water contained in the solvent nor by condensation of water drops. The determining factor for the polymer structure therefore has to be the amount of adsorbed water on the substrate surface. The investigated polymer model system is slightly hydrophobic with contact angles of water on polymer between 71° and 76°. The relationship between the degree of prepolymer coverage and the thickness of the water layer on the bare substrate (values taken from the fit of the adsorption isotherm) at given relative humidities is shown in Figure 8. With a growth of the water layer of some 5 Å (corresponding to about 3 ML of water) between 20% and 50% RH, the degree of surface coverage by polymer film drops from a closed film (100%) to almost the minimum value of around 82%. There are two possible explanations for such behavior: 1. Any water on the substrate under dry conditions (one or two monolayers bound to the hydroxylated surface) is chemisorbed and strongly oriented.9 Investigations on mica with scanning polarization force microscopy indicate that the first monolayer of water molecules is solid indeed.14 When the humidity increases the next adsorbed layers are only physisorbed and the orientation induced by the alumina surface is loststhe water molecules are unoriented as in the liquid state (Figure 9). When the polymer, initially spread into an uniform film by the (14) Hu, J.; Xiao, X.-d.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 344, 221.
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Figure 6. Film morphologies (a) before and (b) after curing for 2 h at 200 °C (spin coating at 60% RH).
Figure 7. Auger linescan of N and Al of a (cured) film coated at 60% RH.
Figure 9. Model for the effect of adsorbed water on the polymer structure; for details see the text.
Figure 10. Line profile of a film coated at 60% RH. Rims around holes are always found, smooth film parts, and substrate crystallites are clearly visible. Figure 8. Correlation between surface coverage by prepolymer and adsorbed water layer thickness.
centrifugal forces in the spin-coating process, senses a surface covered by oriented water molecules, it will wet the surface and build up a homogeneous film. Sensing unoriented “liquid” water the polymer dewets the surface, resulting in the typical defect structures with high rims around round holes (Figure 10) and the formation of drops inside these holes at high relative humidities. (Theoretical details of dewetting processes were published by Sharma and Reiter.15) 2. A second possibility is that the attractive effect of aluminum on the polymer is simply more and more (15) Sharma, A.; Reiter, G. J. Colloid Interface Sci. 1996, 178, 383.
shielded by the rising amount of water. Being hydrophobic the polymer will then dewet with increasing thickness of the water layer. Additional water adsorbed at humidities above 50% RH no longer has any significant effect on the surface coverage as most of it will probably just be dissolved in the THF solution. (But note that there are still some structural changes in the dewetting pattern.) The degree of dewetting described heresand therefore the resulting structure of the polymer filmsdepends presumably on the mobility and time for retraction during the evaporation process. The thickness of the water film adsorbed on the substrate surface is the determining parameter. As the relative humidity changes significantly
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throughout the year (around 50-70% in summer and only 15-30% in the winter in our laboratory), the quality of such a coating will vary a great deal if the humidity is not controlled during the coating process. Outlook A proper examination of the environmental conditions during formation of phase boundaries in adhesive joints is of significant importance for basic adhesion loss investigations. We believe that one possible reason for such behavior could be a different structure of the polymer at the interface depending on the water coverage of the substrate, developing, for example, nanopores at the interface. The adhesive strength in not only dry atmo-
Hecht et al.
spheres but especially in humid or electrolytic environments would be dominated by effects such as this. Acknowledgment. We thank the Bundesministerium fu¨r Bildung und Forschung (BMBF) for the funding of this project (FKZ: 03 D 0038 C3), Wacker Siltronic AG for the silicon wafers, and our partners at the Fraunhofer Institut fu¨r Zuverla¨ssigkeit und Mikrointegration (IZM) in Berlin-Teltow for the synthesis of the polymers, all information about the polymer system, the spin-coating parameters, and the ellipsometric thickness determination of the polymeric films. LA9804987
