Article pubs.acs.org/ac
Hollow Interior Structure of Spin-Coated Polymer Thin Films Revealed by ToF-SIMS Three-Dimensional Imaging Xianwen Ren,† Lu-Tao Weng,‡ Chi-Ming Chan,*,†,§ and Kai-Mo Ng∥ †
Department of Chemical and Biomolecular Engineering, ‡Materials Characterization and Preparation Facility, §Division of Environment, and ∥Advanced Engineering Materials Facility, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong S Supporting Information *
ABSTRACT: Surface patterns were observed on spin-coated poly(bisphenol A decane ether) (BA-C10) films prepared with chloroform and tetrahydrofuran as the solvents. The interior structure of these surface patterns were analyzed using a timeof-flight secondary ion mass spectrometry (ToF-SIMS) equipped with a bismuth cluster source for ion imaging and a C60+ cluster source for depth profiling. For the first time, the surface patterns have been shown to be hollow rather than solid using ToF-SIMS three-dimensional (3D) analysis and optical techniques. Moreover, the microarea depth profiling analysis indicated that the hollow structure was sandwiched between two polymer layers rather than sitting on the substrate. The height of the hollow structure and the thicknesses of the polymer layers above and below the hollow structure were also estimated from the depth profiling results. olymer thin films are widely used in various practical applications such as coatings, adhesives, and biomedical devices. Among the many techniques used to prepare these films, spin coating is probably the most popular one because it can yield highly reproducible thin polymer films over large areas with excellent structural uniformity even on nonwettable substrates.1 However, these spin-coated thin films are not always without surface defects. For instance, cellular patterns and radial striations have often been detected at the center and the periphery of the spinning.2−9 Much research effort has been devoted to the study of the surface morphology and the stability of spin-coated thin films in order to understand why structures are formed on the surface during spin coating. Up to now, several explanations have been put forward to explain the formation of surface structures,2,10,11 but none of them could prevail over others due to the absence of conclusive evidence. At present, most of the research is focused on elucidating the surface morphologies of spin-coated films using analytical tools such as optical microscopy, surface profilometry, and to a lesser extent atomic force microscopy (AFM). The interior structure, however, has never been reported in detail. Obtaining a detailed knowledge of the exterior and interior structures of spin-coated films is of great importance if we are to advance our understanding of the mechanisms of their formation. The chemical characterization of polymer thin films has been difficult, if not impossible, in analytical science. Such characterization in practice requires analytical techniques that are capable of revealing not only the lateral distribution but also the depth profile with resolution in the nanometer range. For inorganic materials, the in-depth chemical characterization of thin films has been usually realized using an imaging surface analytical
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© 2012 American Chemical Society
technique such as Auger electron spectroscopy (AES) or secondary ion mass spectrometry (SIMS) equipped with an atomic sputter ion gun (e.g., Ar+) for sputtering. A threedimensional (3D) characterization of the elements inside an inorganic sample with a lateral resolution and a depth resolution down to the scale of 10 nm and the nanometer range, respectively, can be obtained. Unfortunately, these approaches cannot be applied to organic materials as it is well known that organic materials would be severely damaged upon increased exposure to atomic ion bombardment. In other words, molecular information will be lost when the ion fluence exceeds a threshold value (static limit: ∼1012 ion cm−2). Therefore, the depth profiling analysis of organic materials using atomic ion sources will basically provide elemental information only. This is the reason why SIMS has been mainly operated in the static mode for organic/polymeric materials, for which it has indeed shown a very wide range of applications.12 In order to obtain molecular information, static SIMS will usually have to be combined with a microsectioning technique such as cryo-microtomy. For spin-coated thin polymer films, which are only a few hundred nanometers thick at maximum, however, microsectioning is hardly possible. The above situation has been drastically improved since the recent discovery of cluster ion sources which offer many advantages for the SIMS analysis of organic materials.13 Not only do these sources enhance the ion yields of molecular fragments by up to 3 orders of magnitude,14 more importantly, they offer the capability Received: March 27, 2012 Accepted: September 10, 2012 Published: September 10, 2012 8497
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Characterization of Thin Films. Optical micrographs showing the surface morphologies of BA-C10 films were obtained using a Leica DMLP microscope equipped with a CCD camera operated in the reflection mode. The surface profiles of BA-C10 films were recorded with an optical profilometer (Veeco Wyko NT 2000). The instrument was calibrated before measurement to ensure accuracy. A vertical scanning interferometer (VSI), with a vertical resolution of 1 nm and a maximum height limitation of 2 mm, was used to obtain the 3D exterior profiles of the surface structures of the spin-coated films. ToF-SIMS 3D microanalysis were facilitated by a ToF-SIMS V instrument (Ion-ToF GmbH, Münster, Germany) equipped with a bismuth liquid-metal ion source for analysis and a C60+ ion source for sputtering. A focused 10 keV C60+ beam scanning over an area of typically 800 μm × 800 μm at a current of 0.5 nA and an angle of incidence of 45° was used to sputter through the polymer films. Sample imaging was performed over an area of 100 μm × 100 μm at the center of the sputter region using a 25 keV Bi3+ cluster beam. The target pulsed current of Bi3+ beam was 0.1 pA. The primary beam was optimized for best spatial resolution (∼300 nm) by sacrificing mass resolution, so only nominal mass resolution was obtained. Each imaging scan contained 128 × 128 pixels. The whole profiling analysis was run in the interlaced mode, consisting of cycles of short pulses of Bi3+ primary beam for analysis followed by a longer period of C60+ sputtering. The analyzer extraction potential was switched off during sputtering.29 Data were stored in the raw data stream mode so that the images or profiles could be reconstructed after the analysis. Charge compensation was realized using a low energy flood gun. The vacuum during the analysis was about 1.5 × 10−9 Torr. Calibration of the Film Thickness. In order to take into account the difference in sputter rates between the polymer and the silicon substrate, we used the procedure proposed by Wucher et al.30 to interpolate the sputter rate, S:
of molecular depth profiling for organic materials, as summarized in recent reviews.15,16 The success of cluster ion sources for molecular depth profiling is believed to be due to a combination of two beneficial effects: (1) Cluster ion beams tend to deposit their energy on the very near surface, causing only surface-localized damage. (2) The erosion of organic materials is so fast that the surface damage created by the cluster ion beam is not accumulative. Among the cluster ion sources (SF5+, Aun+, Bin+, C60+, Arn+, etc.) developed so far, Aun+ and Bin+ are mainly used for analysis while SF5+, C60+, and Arn+ are mainly used for sputtering. Nowadays, C60+ is probably the most popular sputter source for organic materials and has been adopted in many commercial time-of-flight SIMS (ToF-SIMS) instruments. The combination of C60+ molecular depth profiling and ToF-SIMS imaging produces a unique analytical technique for the 3D imaging of organic materials. In recent years, ToF-SIMS 3D imaging has been extensively applied in the characterization of biological systems,17,18 drug delivery systems,19,20 optoelectronic materials,21 etc. As for polymeric materials, most of the research work has been focused on the fundmental study of molecular depth profiles, such as the attempts to understand why molecular depth profiling works or to optimize the experimental conditions for depth profiling, using either standard homopolymers or multilayered polymer films of well-defined thicknesses as models.16,22−24 A few studies have also been carried out to examine the macroscopic phase separation in immiscible polymer blends by utilizing ToFSIMS 3D imaging.25,26 However, this technique, to the best of our knowledge, has never been applied to analyze the interior structures of dewetted thin films. In this work, we applied ToF-SIMS 3D imaging to determine the detailed interior stucture of spin-coated polymer thin films. Our recent work27 has shown that, depending on the solvents used, the surface morphologies of poly(bisphenol A decane ether) (BA-C10) films spin-coated on silicon wafers differed. For example, a uniformly flat film was obtained when 1,2dichlorobenzene (ODCB) was used as the solvent, while various surface patterns were observed when tetrahydrofuran (THF) and chloroform (CHCl3) were used as the solvents. These surface patterns were tens of micrometers in lateral size while the films were a few hundred nanometers thick. ToF-SIMS 3D imaging is ideally suited to analyzing the surface structures in such thin films.
S = C PSP + CSiSSi
(1)
where SP and SSi are, respectively, the sputter rates of the polymer and the silicon substrate, while CP and CSi are the weighting factors calculated with eq 2:
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Ci =
EXPERIMENTAL SECTION Materials and Preparation of Thin Films. BA-C10 was synthesized by condensation polymerization of bisphenol A with 1,10-dibromodecane.28 An excess of dibromodecane was used to ensure that the polymer chains were terminated by bromine. The molecular characteristics of BA-C10 were determined by gel permeation chromatogramphy (GPC) at room temperature with THF as the eluent. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) Mw/Mn of the polymers are 42 000, 60 700 and 1.45, respectively. THF and CHCl3 were used to dissolve the polymer to prepare 30 mg mL−1 solutions. The polymer films were prepared by spin coating the polymer solutions on silicon wafers at a rate of 2000 rpm for 90 s. Silicon wafers were cleaned in a hot piranha solution (98 wt % H2SO4:30 wt % H2O2 = 3:1, v/v) at 120 °C for 30 min. After cooling to room temperature, the cleaned silicon wafers were washed several times with deionized water and dried in a stream of nitrogen.
Ii Iimax IP IPmax
+
ISi ISimax
(2)
where Ci and Ii are the weight factor and signal intensity for the i max species, respectively, Imax P and ISi are the signal maxima of the polymer and substrate peaks, respectively. SP was calibrated with a flat and uniform BA-C10 film (∼100 nm thick) prepared using ODCB as the solvent. The thickness of the film was determined with a single-wavelength (He−Ne laser; λ = 633 nm), null-type ellisopmeter (Gaertner Scientific, Model L116C). The film thickness was also estimated by measuring the crater depth after depth profiling analysis using a surface profiler (Tencor AlphaStep 200). The consistency between the two measurements was better than 10%. SSi was determined by measuring the sputter depth of a piece of bare silicon wafer under the same sputter conditions. As Gillen et al.31 demonstrated that 10-keV C60+ beam cannot sputter silicon well after the first few tens of nanometers due to carbon deposition, the sputter depth of the silicon wafer was thus limited to only about 50 nm. The sputter depth was measured using the surface profiler. 8498
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Figure 1. Spin-coated BA-C10 films with THF as the solvent: (a) optical micrograph; (b) optical profilometry image; (c) ToF-SIMS total ion image of all sputter cycles stacked together; (d) ToF-SIMS images of the polymer and substrate ions at different sputter depths; and (e) cross-section views of the polymer ion. The structure parameters of surface patterns defined in Figure 3b are labeled. ToF-SIMS images were normalized to the intensity of the brightest pixel. The brightest pixel (mc = maximum counts per pixel) of each image is shown.
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RESULTS AND DISCUSSION Thin films Prepared Using THF as Solvent. As reported in our paper27 and elsewhere,32,33 the surface morphology of spincoated films depends greatly on the solvents used. With THF as the solvent, the BA-C10 films on a silicon substrate exhibit caplike droplets and necklace-like striations at the center and the periphery of the spinning, respectively.27 Parts a and b of Figure 1
It should be pointed out that the above calibration method has a very minimal effect on the depth scale of the polymer film because (ISi)/(Imax Si ) is negligible before the substrate is reached. However, it does have a significant influence on the depth scale at the interface region. Such a calibration would provide a more accurate presentation of the interface width, although some authors argued that such treatment might underestimate the interface width.16 8499
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Figure 2. Optical observations of fractured BA-C10 films prepared with THF as the solvent: (a) optical micrographs of droplets; (b) 3D surface profiles of droplets obtained with an optical profilometer; (c) and (d) magnification of the intact and fractured areas in part b, respectively.
taller than the surrounding valleys, the only explanation to these unexpected results is that the droplets are hollow. In fact, if the droplets were solid, the substrate ion should appear first at the valleys. It is also interesting to note that the substrate ion has a tendency to be left-of-center rather than concentric with respect to the lower polymer signal areas, as indicated by the green circles in Figure 1d, probably due to the fact that the C60+ sputter ions hit the film with an incident angle of 45°. The hollow structure of the droplets can also be deduced from the cross-section analysis. As presented in Figure 1e, the left shows the C9H11O+ ion image that stacking all sputter cycles (XY view), while the middle and the right images are, respectively, the XZ and YZ cross-section views of the ion along the green lines marked in the XY view. Note that the scale of the Z axis is different from that of the X and Y axes. The surface of the film in the XZ and YZ views appears to be flat due to the fact that the ions do not carry any depth information. The ions from the topmost layer of a wavy surface are treated like ions emitted from a flat surface. As a result, a wavy surface appears to be flat while a flat substrate becomes wavy instead. Nevertheless, the crosssection views do provide the effective film thicknesses along the green lines shown in the XY view. It can be seen that the effective thicknesses of the droplets are much smaller than those of the surrounding valleys. Given that the droplets are about 300 nm taller than the surrounding valleys, these results strongly suggest again that the droplets are hollow. Optical observations were performed in order to provide further proof on the hollow interior structures of the surface patterns. In brief, the polymer films were frozen in liquid nitrogen and then immediately detached using an adhesive tape. In this way, the polymers were detached at most areas of the films but occasionally some polymer residual patches may remain on the
present, respectively, a typical optical micrograph and an optical profilometry image obtained at the center of such a thin film. Cap-like droplets can be seen clearly. The 2D surface height profile showed that the amplitude of the droplets (peak-to-valley distance) was about 300 nm on average. In addition, the isotropic power spectral density (PSD) analysis indicated that the characteristic wavelength λ of the surface patterns was about 30 μm. In other words, the separation between the droplets was about 30 μm on average. Positive ion ToF-SIMS depth profiling was carried out at the centers of the films where the droplets were present. Within an area of 100 μm × 100 μm, at least 2 to 3 droplets could be imaged. Figure 1c shows the total ion image of all sputter cycles. It is surprising to observe that the total ion intensity at the droplet areas is lower. Figure 1d presents ToF-SIMS images of the representative polymer and substrate ions at different sputter depths. Two polymer ions were selected. C3H5+ is the most intense fragment while C9H11O+ is the most characteristic ion of the BA-C10 polymer.34 SiOH+ was selected to represent the substrate.29 Each image in Figure 1d is produced by integrating 10 consecutive sputter cycles, which is about 5 nm in thickness, so that it is bright just for discerning. At the surface (0 nm), the polymer ions are homogenously distributed and the substrate signal is absent. In the middle depth (255 nm), the intensities of the polymer ions become lower at the centers of the droplets but, surprisingly, the substrate ion signal is still not detected. Close to the interface (300 nm), unexpectedly, the substrate ion appears first at the centers of the droplets where the intensities of the polymer ions are lower. It is reasonable to assume that the sputter rate was the same at the valley and droplet areas. Obviously, the effective sputter depth of the droplets is much smaller than that of the valleys. Given the fact that the droplets are about 300 nm 8500
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Figure 3. A schematic illustration of two possible locations of a gap inside a thin film: (a) on the substrate and (b) between the top and bottom polymer layers. It should be pointed out that the vertical direction of the schematic has been “exaggerated”. Typical scale: only a few hundreds of nanometers for the vertical direction and tens of micrometers for the horizontal direction.
Figure 4. Depth profiles reconstructed retrospectively from the areas indicated in Figure 1c: (a) valley area (a blue rectangle in Figure 1c) and (b) droplet area (a red square in Figure 1c). Black dashed line indicates the interface; red dot-dashed line indicates the depth at which the polymer ion intensities start to decrease; blue dot-dashed line indicates the depth at which the substrate ion starts to increase; purple dot-dashed line is used to represent both the red- and blue-dotted lines because they are merged at the same depth.
substrate. At the edge of the residual patches, fractured areas could be expected, providing a chance to investigate the interior structure of the surface patterns using optical techniques. Parts a and b of Figure 2 present an optical micrograph and an optical profilometry image, respectively, on the residual patches at the spinning center. Parts c and d of Figure 2 are, respectively, the magnification of the intact (marked with c) and fractured areas (marked with d) shown in Figure 2b. In contrast to the bumps in the intact areas, the depressions in the fracture areas after the removal of the polymer surface layers can be clearly identified. Thus, in agreement with ToF-SIMS 3D microanalysis, optical observations confirmed that the droplets are hollow. Although the above ToF-SIMS and optical results clearly show that there are gaps inside the droplets, it is still unclear whether these gaps are positioned near the substrate (c.f. Figure 3a) or sandwiched between a top polymer layer and a bottom polymer layer on the substrate (c.f. Figure 3b). In order to examine whether ToF-SIMS has the ability to determine the exact location of the gaps, depth profiles were retrospectively reconstructed from the droplet and valley areas. A pair of depth profiles, which was reconstructed from the two areas indicated by a red square and a blue rectangle in Figure 1c, are displayed in Figure 4. At the valley area (c.f. Figure 4a), the polymer ion intensities remain constant until the substrate is reached. The substrate ion, on the contrary, is absent until the polymer ion intensities start to decrease. The decrease in polymer ion intensities and the increase in substrate ion intensity occur simultaneously, which is the normal observation in the depth profiling of continuous polymer films.35 From the depth profile, the film thickness at this valley area was estimated to be about 460 nm. At the droplet area
Table 1. Structural Parameters of the Surface Patterns as Defined in Figure 3ba droplet solvent
valley (L)/nm
ls/nm
li/nm
THF CHCl3
400−475 250−300
190−270 90−140
90−120 120−200
a The surface patterns were from the five pairs of reconstructed depth profiles on spin-coated BA-C10 films, prepared using THF and CHCl3 as the solvents.
(c.f. Figure 4b), the depth profiles of the polymer ions, however, are distinct from those in Figure 4a. The polymer ion intensities start to decrease at about 190 nm while the substrate ion appears at about 260 nm. The appearance of the substrate ion is about 90 nm deeper than the decrease in the polymer ion intensities. The special distribution of the polymer ion intensity in Figure 4b provides a strong indication to verify the two models shown in Figure 3. Indeed, if the gap were to sit on the substrate (c.f. Figure 3a), there would only be one polymer layer on the top of the gap and, as a consequence, depth profiles of the polymer ions similar to those of the valley area (c.f. Figure 4a), in which the decrease in the polymer ion intensity is correlated with the appearance of the substrate ion, would be expected. But it was not the case. However, the depth profiles in Figure 4b can be explained with the sandwich model displayed in Figure 3b. As illustrated in Figure 4b, the depth profiles of the polymer ions can be divided into two regions. The first region of the depth profiles corresponds to the top polymer layer above the gap (ls in Figure 3b), while the 8501
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Figure 5. Spin-coated BA-C10 films with CHCl3 as the solvent: (a) optical micrograph; (b) optical profilometry image; (c) ToF-SIMS total ion image of all sputter cycles stacked together; (d) ToF-SIMS images of the polymer and substrate ions at different sputter depths; and (e) cross-section views of the polymer ion. The structure parameters of surface patterns defined in Figure 3b are labeled. ToF-SIMS images were normalized to the intensity of the brightest pixel. The brightest pixel (mc = maximum counts per pixel) of each image is shown.
droplets #1 to 5 vary significantly, indicating that the structural parameters are highly dependent on which droplet is selected. It must be pointed out that the depth profiles as shown in Figure 4b are often observed in polymer depth profiling15,16 and should be interpreted carefully. In general, several possibilities are often considered regarding the ion intensity changes during depth profiling, just to name a few: (1) interlayer mixing in multilayer
second region of the depth profiles is attributed to the bottom polymer layer underneath the gap (li in Figure 3b). From the depth profiles, ls and li were estimated to be about 190 and 110 nm, respectively. Given the peak-to-valley distance D = 300 nm and the thickness of the valley areas L = 460 nm, the gap thickness in the droplet (#2, Figure 1c) was estimated to be lb = (D + L) − (ls + li)= 460 nm. In addition, it is seen from Figure 1e that the heights of 8502
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Figure 6. Depth profiles reconstructed retrospectively from the areas indicated in Figure 5c: (a) valley area (a blue rectangle in Figure 5c) and (b) droplet area (a red square in Figure 5c). Black dashed line indicates the interface; red dot-dashed line indicates the depth at which the polymer ion intensity starts to decrease; blue dot-dashed line indicates the depth at which the substrate ion intensity starts to increase; purple dot-dashed line is used to represent both the red- and blue-dotted lines because they are merged at the same location.
films;23 (2) yield-quenching effect due to the decrease of sputter rate during depth profiling;15,16,36 and (3) topographical effects.37,38 In our case, the interlayer mixing is not relevant because only a single component BA-C10 was used. The yieldquenching effect has been observed in the depth profiling of pure polymers.16 However, it should be equally effective on both the valley and the droplet areas if it existed. The fact that the polymer ion intensity was constant in the valley areas until the substrate was reached strongly suggests that the yield-quenching effect can be excluded in our system. Therefore, we believe that topographical effects are the main factor behind the depth profiles in Figure 4b. Indeed, the topographical effects can cause the distortion of the extraction field in ToF analyzer, leading to the loss of ion intensity.37,38 In order to examine whether Figure 4 is a common observation, depth profiles were reconstructed from five different areas of the valleys and droplets which were chosen from different samples. Similar to the depth profiles in Figure 4, the results (not shown) showed that in the valley areas, the decrease in polymer ion intensity and the increase in substrate ion intensity occur simultaneously, while in the droplet areas, the appearance of substrate ion is much later than the decrease in polymer ion intensity. Except for the ion intensity which depends on the size of the selected area, the main difference among the different valley areas or droplet areas is in the sputter depth. With the same methodology described earlier, the structural parameters of all the droplet and valley areas were determined and are summarized in Table 1. It can be seen that L varies from 400 to 475 nm while ls and li are in range of 190−270 nm and 90−120 nm, respectively. Thin Films Prepared Using CHCl3 as Solvent. Similarly, the structure of the spin-coated BA-C10 films prepared with CHCl3 as the solvent was analyzed by ToF-SIMS 3D imaging. The optical micrograph (c.f. Figure 5a) and the optical profilometry analysis (c.f. Figure 5b) show that D was about 100 nm and λ was about 42 μm. The hollow structure of the droplets can be deduced from both the ion images at different sputter depths (c.f. Figure 5d) and the cross-section analysis (c.f. Figure 5e). Figure 5d shows that at the droplet areas, the polymer ion intensities start to decrease at a depth of 135 nm but without substrate signal detected. Near the interface (about 275 nm), the substrate ion appears and the polymer ion intensities are very low
at the center of the droplets. These results suggest that the effective sputter depth of the droplet areas is smaller than that of the valley areas. The cross-section analysis in Figure 5e also shows that the effective film thickness of droplet areas was slightly thinner than those of their surrounding valleys. Given that the droplets were about 100 nm taller than the valleys, these results strongly suggest that the droplets were hollow inside. Moreover, the hollow structure of the droplets was also confirmed by optical observations (see Supporting Information). The depth profiles were also retrospectively reconstructed from the droplet and valley areas indicated by a red square and a blue rectangle, respectively, in Figure 5c and the results are presented in Figure 6. The depth profiles from the valley area (c.f. Figure 6a) correspond to what is expected for a single polymer layer on the substrate, which is characterized by the observation that the decrease in the polymer ion intensities and the increase in the substrate ion intensities occur simultaneously. From Figure 6a, L was estimated to be 300 nm. At the droplet area, the depth profiles of the polymer ions (c.f. Figure 6b) can be clearly divided into two regions. In the first region, the distribution is very similar to that of Figure 6a, i.e. after a surface transient region, the polymer ion intensities remain constant before a depth of 90 nm. In the second region after the depth of 90 nm, the polymer ion intensities drop suddenly by about one-third and then remain relatively constant until the substrate is reached at a much greater sputter depth of about 270 nm. The results once again suggest that a gap was sandwiched between two polymer layers in each droplet. From Figure 6b, ls and li were estimated to be 90 and 200 nm, respectively. With the same method described earlier, lb was estimated to be 110 nm. Apparently, the gaps in this film are much smaller than those in the film prepared using THF as the solvent. Depth profiles were also reconstructed from five different valleys and droplets areas. The structural parameters are summarized in Table 1. It can be seen that although the films prepared using THF were on average thicker than those prepared using CHCl3, the bottom polymer layer of the former was thinner than those of the later. It is also worth mentioning that the depth profiles of the polymer ions on the bottom layer behaved quite differently in the two cases. For the film prepared using CHCl3 as the solvent, the polymer ion intensity is almost constant before the substrate is reached (c.f. Figure 6b), while for 8503
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the film prepared using THF as the solvent, the polymer ion intensity decreased continuously (c.f. Figure 4b). This difference is believed to be caused by the variation in the surface roughness of the bottom layers between the two films (see Supporting Information). In summary, the droplets observed on the spin-coated BA-C10 films prepared using two different solvents were found to have a hollow interior structure. The location of the hollow structure inside the thin films and its size can be determined from ToFSIMS 3D microanalysis. For the two solvents used in this work, the hollow structure in the thin films prepared using THF was much bigger than in those prepared using CHCl3. Furthermore, the bottom polymer layer was thinner and rougher in the film prepared using THF than in that prepared using CHCl3. The exact mechanism that led to this difference is still unclear and is out of the scope of this paper. It is nevertheless worth pointing out that these findings are consistent with the previous conclusion on the stability of spin-coated BA-C10 films on the silicon substrate: the films prepared using CHCl3 were partially dewetted while those prepared using THF were completely dewetted.27
(6) Muller-Buschbaum, P.; Gutmann, J. S.; Kraus, J.; Walter, H.; Stamm, M. Macromolecules 2000, 33, 569. (7) Rehg, T. J.; Higgins, B. G. AIChE J. 1992, 38, 489. (8) Strawhecker, K. E.; Kumar, S. K.; Douglas, J. F.; Karim, A. Macromolecules 2001, 34, 4669. (9) Birnie, D. P.; Kaz, D. M.; Taylor, D. J. J. Sol-Gel Sci. Technol. 2009, 49, 233. (10) Du, X. M.; Orignac, X.; Almeida, R. M. J. Am. Ceram. Soc. 1995, 78, 2254. (11) Spangler, L. L.; Torkelsson, J. M.; Royal, J. S. Polym. Eng. Sci. 1990, 30, 644. (12) Weng, L. T.; Chan, C. M. Appl. Surf. Sci. 2006, 252, 6570. (13) Winograd, N. Anal. Chem. 2005, 77, 142A. (14) Wucher, A. Appl. Surf. Sci. 2006, 252, 6482. (15) Wucher, A.; Winograd, N. Anal. Bioanal. Chem. 2010, 396, 105. (16) Mahoney, C. M. Mass Spectrom. Rev. 2010, 29, 247. (17) Fletcher, J. S.; Lockyer, N. P.; Vickerman, J. C. Mass Spectrom. Rev. 2011, 30, 142. (18) Fletcher, J. S.; Vickerman, J. C. Anal. Bioanal. Chem. 2010, 396, 85. (19) Gillen, G.; Fahey, A.; Wagner, M.; Mahoney, C. Appl. Surf. Sci. 2006, 252, 6537. (20) Mahoney, C. M.; Fahey, A. J. Anal. Chem. 2008, 80, 624. (21) Ninomiya, S.; Ichiki, K.; Yamada, H.; Nakata, Y.; Seki, T.; Aoki, T.; Matsuo, J. Surf. Interface Anal. 2011, 43, 95. (22) Shard, A. G.; Green, F. M.; Gilmore, I. S. Appl. Surf. Sci. 2008, 255, 962. (23) Wagner, M. S. Anal. Chem. 2005, 77, 911. (24) Mouhib, T.; Delcorte, A.; Poleunis, C.; Bertrand, P. Surf. Interface Anal. 2011, 43, 175. (25) Mayerhofer, K. E.; Heier, J.; Maniglio, Y.; Keller, B. A. Thin Solid Films 2011, 519, 6183. (26) Mahoney, C. M.; Yu, J.; Fahey, A.; Gardella, J. A., Jr Appl. Surf. Sci. 2006, 252, 6609. (27) Ren, X. W.; Weng, L. T.; Chan, C. M.; Ng, K. M., In preparation. (28) Li, L.; Chan, C. M.; Yeung, K. L.; Li, J. X.; Ng, K. M.; Lei, Y. G. Macromolecules 2001, 34, 316. (29) Shard, A. G.; Green, F. M.; Brewer, P. J.; Seah, M. P.; Gilmore, I. S. J. Phys. Chem. B 2008, 112, 2596. (30) Wucher, A.; Cheng, J.; Winograd, N. Appl. Surf. Sci. 2008, 255, 959. (31) Gillen, G.; Batteas, J.; Michaels, C. A.; Chi, P.; Small, J.; Windsor, E.; Fahey, A.; Verkouteren, J.; Kim, K. J. Appl. Surf. Sci. 2006, 252, 6521. (32) Petri, D. F. S. J. Braz. Chem. Soc. 2002, 13, 695. (33) Birnie, D. P. J. Mater. Res. 2001, 16, 1145. (34) Li, L.; Chan, C. M.; Ng, K. M.; Lei, Y. G.; Weng, L. T. Polymer 2001, 42, 6841. (35) Shard, A. G.; Brewer, P. J.; Green, F. M.; Gilmore, I. S. Surf. Interface Anal. 2007, 39, 294. (36) Shard, A. G.; Ray, S.; Seah, M. P.; Yang, L. Surf. Interface Anal. 2011, 43, 1240. (37) Lee, J. L. S.; Gilmore, I. S.; Seah, M. P.; Fletcher, I. W. J. Am. Soc. Mass Spectrom. 2011, 22, 1718. (38) Lee, J. L. S.; Gilmore, I. S.; Seah, M. P.; Levick, A. P.; Shard, A. G. Surf. Interface Anal. 2012, 44, 238.
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CONCLUSIONS ToF-SIMS 3D imaging has been successfully applied to determine the interior structure of the surface patterns of spincoated thin films prepared using two different solvents. For the first time, the structure of the surface patterns has been revealed by ToF-SIMS analysis to be hollow, which has been further verified using optical techniques. Moreover, the location and the size of the hollow structure, as well as the thickness of polymer layers around the hollow structure were estimated from the ToFSIMS microarea depth profiling.
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ASSOCIATED CONTENT
S Supporting Information *
Details of the investigation of the hollow interior structure by optical techniques and the surface roughness of the bottom layers by AFM observation. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work described in this paper was fully supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Grant No. 600210, SEG_HKUST04).
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REFERENCES
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dx.doi.org/10.1021/ac3014466 | Anal. Chem. 2012, 84, 8497−8504
