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Unstable Polymer Bilayers. 1. Morphology of Dewetting Alessandro Faldi,? Russell J. Composto,” and Karen I. Winey* Department of Materials Science and Engineering and The Laboratory for Research on the Structure of Matter, The University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272 Received March 31, 1995. I n Final Form: August 29, 1995@ We present a detailed study of the morphology of dewetting of a polycarbonate (PC) melt from a poly(styrene-co-acrylonitrile) ( S A N ) melt supported on a rigid substrate. PC and SAN, which are immiscible, were spin coated to form 200 nm films for these studies. Optical microscopy is used to monitor the dewetting
process. Cross-sectional scanning electron microscopy, atomic force microscopy, and Auger electron spectroscopy are used to study the detailed morphology of an isolated hole having a diameter of -30 pm. The rim surrounding the hole in the PC layer is steeper on the inside than the outside (i.e., asymmetric) and its near-surface composition is characteristic of PC. The maximum height of this rim is more than twice that of the original bilayer thickness. The floor of the hole uncovered by the PC is a S A N layer which is thinner than the original SAN layer. This thinning of the underlying S A N layer is much greater than the equilibrium PCISAN interfacial width, 5 nm. During the initial stages of dewetting, a depression or “dimple” in the SAN layer was observed in the center of each hole which was -10 pm wide and 50 nm deep. These dimples persist until the late stages of dewetting when the PC broke up into droplets on the SAN.
Introduction The spreading and dewetting of polymeric films is of considerable importance in many industrial applications involving, for example, the application and stability of protective coatings, lubricants, and adhesives on avariety of surfaces. A contiguous film deposited on a substrate can reduce its interfacial free energy by rupturing to uncover the underlying substrate if the spreading coefficient, S, is negative. The spreadingcoefficient is defined as
where ysois the surface energy of the substrate, yi is the fildsubstrate interfacial energy, and y is the surface energy of the film. In addition to the surface and interfacial energies, film rupture will depend on other forces acting on the film. For relatively thick liquid films (’100 nm) the van der Waals forces are weak and film stability will be determined by the balance of the interfacial and hydrostatic (gravitational) f0rces.l Such films will be metastable if the spreading coefficient is negative and their film thickness is smaller than a critical thickness of -1 pm.lS2 Thin films ( < l o 0 nm) subject to attractive van der Waals forces are unstable and rupture spontaneo~sly,l,~-~ although the instabilities can be retarded in viscoelasticmaterials and suppressed in elastic ones.6 Upon film rupture, a dry patch or hole is formed and will grow driven by the interfacial and hydrodynamic forces acting at the three-phase contact line.7 We refer to the hole growth process as dewetting. +
Present address: Exxon Chemical Co.,P.O.Box 5200, Baytown,
Texas 77522-5200. Abstract published in Advance ACS Abstracts, November 15, 1995. (1) Brochard-Wyart, F.;Daillant, J. Can. J.Phys. 1990,68, 1084. (2)Redon, C.;Brochard-Wyart,F.;Rondelez,F.Phys.Rev.Lett. 1991, 66,715. (3)Vrij, A.Discuss. Faraday SOC.1967,42, 23. (4)Ruckenstein, E.;Jain, R. K. J . Chem. SOC.,Faraday Trans. 2. 1974,70,132. (5) Williams, M. B.; Davis, S . H. J. Colloid Interface Sci. 1982,90, 220. (6)Safran, S.A.;Klein, J. J. Phys. II 1993,3,749. (7)Brochard-Wyart,F.; deGennes, P. G. Adu.Colloid Interface Sci. 1992,39,1.
The morphology ofnonvolatile liquids and polymer melts spreading on, or dewetting from, solid substrates has received considerable theoretical and experimental attention in recent year^.^-^ For example, deGennes described a spreading polymer droplet on a rigid surface as having a spherical cap that crosses over to a macroscopic foot with a thin precursor film.8 In the case of a nonwetting liquid on a solid, Brochard-Wyart and Daillantl suggested a drying process involving formation of a rim that collects the dewetted liquid. The rim profile was assumed to be an arc of a circle that made a finite dynamic contact angle with the substrate. This assumption is common to most theoretical treatments of the velocity of hole growth during d e ~ e t t i n g ,with ~ , ~anotable recent ex~eption.~ Knowledge of the rim shape is critical for predicting the velocity of the rim, and our study will show that the rim in polymer dewetting can be asymmetric suggesting a nonuniform pressure in the rim. ReiterlOJ’ used optical microscopy to observe the dewettingofthin(< 100 nm) polystyrene (PSIfilms on silicon wafers. Upon annealing above the glass transition temperature, the PS film dewetted the silicon forming holes with rims. At later stages, the rims coalesced and formed cellular patterns. Similar morphological features of dewetting were observed by Shull and Karis12while investigating poly(ethy1ene-co-propylene)on glassy polystyrene or poly(methy1 methacrylate). In this study, an aliphatic alcohol was used to induce the dewetting of the copolymer film. Zhao et al.13 studied the dewetting of poly(ethy1ene-co-propylene) from silicon wafers using X-ray reflectivity and atomic force microscopy. They found that films thinner than the bulk radius of gyration of the polymer chain dewetted the rigid substrate and formed irregularly shaped droplets with curved surfaces. Liu et al.14 deposited thin films of polystyrene on microphase-
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(8) deGennes, P. G. Rev. Mod. Phys. 1986,57, 827.
(g)Brochard-Wyart,F.;deGennes, P. G.; Hervert, H.; Redon, C. Langmuir 1994,10,1566. (10)Reiter, G.Phys. Rev.Lett. 1992,68, 75. (11)Reiter, G.Langmuir 1993,9, 1344. (12)Shull, K.R.; Karis, T. E. Langmuir 1994,10,334. (13)Zhao, W.; Rafailovich, M. H.; Sokolov, J.;Fetters, L. J.;Plano, R.; Sanyal, M. K.; Sinha, S. K.; Sauer, B. B. Phys. Rev. Lett. 1993,70, 1453. (14)Liu, Y.;Rafailovich, M. H.; Sokolov, J.; Schwarz, S. A.; Zhong, X.; Eisenberg, A.; Kramer, E. J.; Sauer, B. B.; Satija, S . Phys. Rev. Lett. 1994,73,440.
0 1995 American Chemical Society
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Faldi et al.
Table 1. Polymer Characteristics Mw M J M , AN(wt %) T,"("C) source
polymer SAN27 242000 PC 19 100
2.98 2.94
27
115 153
Monsanto General Electric
Measured via differential scanning calorimetry at 20 "C/min.
separated polybtyrene-eo-vinylpyridine) copolymers supported on a silicon wafer. The ratio of homopolymer to copolymer molecular weights was an important parameter in this study. Dewetting of oligostyrene from a silicon substrate was found to be greatly retarded by the addition of surface-attached chains and long free chains.15 The mechanism of stabilization is still under investigation. l5 The dewetting of a liquid layer by another liquid has received considerably less attention than its liquidsolid counterpart. Martin et al. have studied the hole velocity of thick (50 pm to 1mm) poly(dimethylsi1oxane) (PDMS) films deposited on a fluorinated PDMS substrate.16 The hole velocity was found to be constant in time and decreased as a function of PDMS thickness. Data analysis required the assumption of a hemispherical rim. Brochard-Wyart et al.17 have described the morphology of 1iquidAiquiddewetting for the case of thin and thick liquid substrates. In contrast to the liquidsolid interface, the liquiuiquid interface is not constrained to a plane and its macroscopic contact angle will be determined by the Neuman triangle, rather than the Young equation. In general, the cross-sectional shape of the rim is expected to be that of a liquid lens floating on the lower liquid. Fully characterizing the shape of the hole and rim produced by polymer melt/polymer melt dewetting is the primary objective of this work. We consider polymer/polymer dewetting for a film (-200 nm) of polycarbonate (PC) deposited over a layer (-200 nm) of poly(styrene-co-acrylonitrile)(SAN27).18 Using optical microscopy, we find that the PC film spontaneously dewets the SAN27 layer upon annealing the bilayer at 190 "C, a temperature above the glass transition temperatures of both PC and SAN27. Upon arresting the dewetting process by quenching the sample, the morphology and topography of the rim and hole are investigated using scanning electron and atomic force microscopies. The profile of the rim is found to be noncircular and the exposed SAN27 film thinner than the original film thickness. Auger electron spectroscopy studies show that the near-surface composition of the rim and hole floor correspond to PC and SAN27, respectively. In a subsequent paper the dynamics of polymer meltlpolymer melt dewetting and the effect of acrylonitrile (AN)content on the hole velocity will be presented.lsJ9
Experimental Section The polymers were bisphenol-A polycarbonate (PC)-more 1,4formally poly(oxycarbony1oxy)-1,4-phenyleneisopropylidenephenylene)-and poly(styrene-co-acrylonitrile)(SAN27)with an acrylonitrile (AN)weight fraction of 0.27. PC and SAN27 were dissolved in dichloromethane and chloroform, respectively, and precipitated with methanol. This process was carried out twice to remove impurities and oligomers. The characteristics of the polymers are reported in Table 1, where AN indicates the (15) Yerushalmi-Rozen, R.; Klein, J.; Fetters, L. J. Science 1994, 263, 793. (16)Martin, P.;Buguin,A,; Brochard-Wyart,F. Europhys. Lett. 1994, 28, 421. (17) Brochard-Wyart, F.; Martin, P.; Redon, C. Langmuir 1993, 9, 3682. .._
(18)Faldi, A.; Winey, K. I.; Composto, R. J. In Materials Research Society Symposium Proceedings; Materials Research Society: Pittsburgh, 1995; Vol. 366, p 71. (19) Faldi, A.; Composto, R. J. Manuscript in preparation.
acrylonitrile content of SAN27. Earlier investigations of PC/ SAN blends have found these polymers to be immiscible.20 The width of a PClSAN22 interface was determined to be 4.5 nm using neutron reflectivity.21 Although the AN content was 0.22 by weight in the latter study, we expect a similar interfacial thickness for the PC/SAN27 system used in this work. For optical microscopy (OM), Auger electron spectroscopy (AES),and atomic force microscopy (AFM) experiments, bilayer films were prepared on silicon wafers using a spin coating technique. The silicon substrates, n-type (Sb dopant) having a (100) orientation, were obtained from Recticon Co. and used as received. A layer of SAN27 was deposited on a wafer by spin casting a solution of SAN27 in 4-methyl-2-pentanone. The SAN27 film was then dried at 80 "C under vacuum for 8-12 h. A solution of PC in tetrachloroethane was spin coated onto a glass slide to produce a PC film. The film was scored a t the edges, floated onto a bath of deionized water, and picked up with the SAN27-coated wafer. The bilayer sample was then dried at 80 "C under vacuum for 8-12 h. For scanning electron microscopy (SEM), the sample preparation was modified so that cross sections of the bilayer could be microtomed. A polyimide (ULTEM)substrate about 2 mm thick served as a rigid substrate that could be microtomed. Agold layer (-75 nm) was deposited on the polyimide sheet to serve as a marker. The polymer bilayer was then prepared on the gold-coated polyimide sheet as described above. For samples deposited on silicon substrates, the thicknesses of the SAN27 and PC layers were measured by ellipsometryusing indices of refraction of 1.576 and 1.585, respectively. The measured values were 200 f 5 n m and 240 =! 5 nm for SAN27 and PC, respectively. The layer thicknesses of selected samples were further measured by forward recoil spectrometry (FRES),22 and values of -200 nm were obtained for both layers. The FRESmeasured PC thickness is in better agreement with SEM measurements; see results below. To measure the film thicknesses of samples deposited on polyimide substrates, the surfaces were coated with gold and Rutherford backscattering spectrometry (RBW3 was used to determine the distance between the surface and buried gold layers. For both the SAN27 and PC layers, a thickness of -200 n m was measured, in excellent agreement with the values for samples prepared on silicon. Thus, the OM, AES, AFM, and SEM measurements were taken on samples having identical film thicknesses. Both types of samples, on silicon and on polyimide substrates, were annealed in a Mettler FP90 hot stage a t 190 "C in air and then quenched to room temperature before further analysis. The dewetting process occurs upon heating without deliberate external intervention such as pricking the film. The hot stage was calibrated using standards with known melting points, specifically benzophenone and benzoic acid (Mettler ME-18870 and ME-18555). No further sample preparation was required for OM, AES, or AFM, which probe the bilayer from the top. Control experiments with SAN27 on silicon and gold-coated polyimide showed no evidence of dewetting after extended annealing at 190 "C. Similarly, no dewetting was observed for a layer of PC (-400 nm) on silicon after almost 24 h at 190 "C. Cross sections ofthe bilayers were prepared to investigate the morphology of dewetting using electron microscopy. Crosssectional microtomy was performed at room temperature using a Reichert Ultracut S equipped with a diamond knife (35"angle, Diatome). The sections, -100 nm thick, were transferred from the water in the knife trough to pieces of gold-coated mica (-3 x 3 mm), which provided a flat and conductive support. To simplify the interpretation of the SEM images, we developed a procedure to ensure that the cross sections studied were cut across the diameter of a hole. Optical microscopy inspection during microtoming was used to locate an isolated hole and determine when the center had been reached. In addition, the sequence of the sections was preserved so that, when the sections were later (20) Callaghan, T. A.; Takakuwa, K.; Paul, D. R.; Padwa, A. R. Polymer 1993,34,3796. (21) Mansfield,T. L. Ph.D.Thesis,UniversityofMassachusetts,1993. (22) Genzer, J.; Rothman, J. B.; Composto, R. J. Nucl. Instrum. Methods Phys. Res., Sect. B 1994, 86, 3453. (23)Chu. W. K.: Maver. J. W.: Nicolet. M. A. Backscatterine Spectrometry; Academic Press: New York, 1978.
-
Unstable Polymer Bilayers
imaged with SEM, the hole width could be seen to increase and decrease as the center of the hole was first approached and then passed. Optical micrographs were obtained in reflection using a Leitz Laborlw microscope equipped with phase-contrast objectives. Images were recorded periodically during the dewetting process using a 35 mm camera. Scanning electron micrographs were obtained in a JEOL 6300FV equipped with a field-emission gun. An accelerating voltage of 3 kV at 8 pA of current was determined to be a good compromise between charging, contrast, and resolution. The images were recorded on Polaroid 55 film using the slowest scan rate. The AES spectra were obtained with a Perkin-Elmer Phi 600 ScanningAuger Multiprobe system. The sample was irradiated by a 3 keV electron beam at a current of 0.1 mA. The electron beam was defocused to -1 pm diameter to reduce sample charging. The relative yield was determined by subtracting the small AES signal from the large background produced from the secondary electrons. To improve the yield statistics,the Auger signals for oxygen, nitrogen, and carbon were averaged over 20, 20, and 3 scans, respectively. The AFM images were acquired in air using a Digital Instrument Nanoscope I11 equipped with a silicon tip. The scan range was 100pm. The AFM was operated in the tapping mode to minimize tip-induced sample degradation. Zhong et al. demonstrated that scanning in the contact mode produced tipinduced damage on polymer surfaceswhereas tapping mode AFM generated reproducible images.24In our study, image reproducibility was checked by scanning across the hole from left to right and then from right to left. These two images produced indistinguishable topographic features. Both full images and individual traces were useful in accessing the dewetting morphology.
Results and Discussion To calculate the spreading coefficient S , the surface tensions of PC and SAN27 as well as the interfacial tension of the PC/SAN27 interface are needed at 190 "C. The interfacial tension between PC and S A N has been measured as a function of AN content at a temperature of 200 "C.25 For PC/SAN27, the value is 2.8 ( f 0 . 5 dyn/ cm). The surface tension of PC at 190 "C is 33 dyn/cm.26 Although the surface tension for SAN melts has not been measured, the critical surface tensions of SAN solids at 20 "C are known.26 We can estimate the SAN27 surface tension by extrapolating the critical surface tension values of 27%AN content. Using the temperature dependence of the polystyrene surface tension,26the SAN27 surface tension was calculated as 36 dyn/cm at 190 "C. Using these values, the spreading coefficient is -0 dyn/cm. Given the uncertainties in the values of the surface and interfacial energies, the stability of PC films deposited on SAN27 cannot be predicted reliably. Nevertheless, we observe that a 200 nm layer of PC dewets a 200 nm underlayer of SAN27, suggesting that the spreading coefficient for PC/SAN27 is negative. The series of optical micrographs in Figure 1illustrates qualitatively the many stages of the PC/SAN27 dewetting process. Initially, the surface of the sample appears smooth, Figure 1A. After 0.5 h of annealing a t 190 "C, surface defects have grown sufficiently to be detected by optical microscopy, although they are not yet so large as to be resolved in detail. These defects form upon heating without deliberate external intervention. Defects occur preferentially at irregularities in the sample, such as dust particles and scratches, though it is not known whether an irregularity is necessary for nucleation. With further (24)Zhong, Q.;Inniss, D.; Kjoller, K.; Elings, V. B. Surf. Sci. Lett. 1993,290,L688. (25)Watkins, V. H.; Hobbs, S.Y. Polymer 1993,34, 3955. (26)Brandrup, J.;Immergut, E. H. In Polymer Handbook,3rd ed.; Wu, S., Ed.; John Wiley & Sons: New York, 1989;p V-411.
Langmuir, Vol. 11, No. 12, 1995 4857
annealing to 1.24 h, the defects have grown to -40 pm and are clearly identifiable in the images as circular holes each of which was surrounded by a rim, Figure 1B. A "black dot" is also evident at the center of each hole; its nature will be discussed later. Growth of the holes continues with further annealing, and eventually the holes impinge on one another, Figure 1C. We have monitored the rate of growth of isolated defects as a function of temperature and AN composition; these results will be reported in a subsequent paper.18J9 The coalescence of neighboring holes resulta in a cellular pattern of connected polygons formed by the material in the rims, Figure 1D. This stage has also been described as a two-dimensional This observation is consistent with previous results published by Reiter'OJ' and Shull and Karis12for dewetting of polymer films on rigid substrates. If annealing continues, the contiguous cellular structure ruptures and individual droplets are formed, Figure lE, again in accordance with previous studies.1°-12 A n interesting phenomenon in the PC/SAN27 system is illustrated in Figure 1F. After annealing for 3 days at 190 "C,bright, poorly-resolved phases appear inside the droplets. These bright regions disappear quickly upon raising the temperature to 240 "C, which is above the melting point ofPC (T,= 225 "C). Thus, we attribute the features in Figure 1F to crystalline PC, the result of slow crystallization within the droplets at an undercooling of 35 "C. It should also be noted in Figure 1Fthat the "black dot" has disappeared after extensive annealing. The above description encompasses the entire evolution of dewetting from a PC/SAN27 bilayer to droplets of PC on SAN27. Qualitatively, the optical microscopy results suggest that melt/solid and melvmelt dewetting follow a similar scheme. However, in the remainder of this paper important differences between meltlsolid and melumelt dewetting will be identified. We will now focus on the morphology of an isolated hole, that is away from any apparent irregularities in the sample, at an intermediate stage of PC/SAN27 dewetting, such as the center hole shown in Figure 1B. The scanning electron micrograph in Figure 2A shows a cross-sectional view of a hole formed when the PC layer dewets the SAN27 layer. From the bottom, the image displays the polyimide substrate (dark gray), the gold marker layer (white),the PC/SAN27 bilayer (dark gray), and the gold-coated mica support (medium gray). The edge of the microtomed section appears bright due to the common edge effect in SEM images and does not indicate contrast between the PC and SAN27 layers. Diffuse, semicircular light regions are also observed along the edge of the microtomed section and arise from sample charging. These features are ripples at the edge of the microtomed section that have poor contact with the conductive support and, consequently, no conductive path for the incident electrons. For the sample shown, the cutting direction of the microtome is right to left. As shown in Figure 2A, and other images, the rims are mirror images of each other indicating that the hole profile has not been significantly perturbed by microtoming. Note that the PC and SAN27 layers are indistinguishable in Figure 2. Therefore, the total thickness of the bilayer sample will be given by the distance between the gold marker layer and the edge of the microtomed section. We next describe the hole morphology shown in Figure 2A in terms of the regions far outside the hole, the rim of the hole, and the floor of the hole. Far from the hole the thickness of the polymer layers is 390 f20 nm, as measured on SEM images. This value is in excellent agreement with the initial thickness of the bilayer, -400 nm, measured by FRES and RBS. The
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Figure 1. Optical micrographs showing the progression of dewetting of a PC film from SAN27 on silicon upon annealing at 190 "C: (A) no anneal, (B)1.24 h, (C) 1.81 h, (D)3.51 h, (E) 6.84 h, (F)70.93 h, that is, 3 days.
surface composition of the bilayer far outside the' hole was determined using AES, which probes the outer 1.5 nm of the surface. Figure 3 shows the relative intensity of the KLL transitions of oxygen and nitrogen as the electron beam was scanned across a hole. The intensity fluctuations for 0 and N both outside and inside the hole are typical ofAES intensities that are derived from a small Auger signal superimposed on a large background of secondary electrons and do not represent real compositional fluctuations. For comparison, an AES scan of a PC layer on silicon was taken and showed the expected 0
signal as well as a background N signal comparable to the one obtained from outside the holes. An AES scan of a SAN27 layer showed the expected N signal and a background 0 signal, the latter comparable to the signal obtained from inside the holes. Furthermore, to check for beam damage, a profile was also recorded from the same hole in a direction perpendicular to the first scan. Within experimental accuracy, the same concentration profiles as shown in Figure 3 were measured. Thus, away from the hole the AES spectra in Figure 3 clearly show that PC predominately covers the surface.
Langmuir, Vol. 11,No. 12, 1995 4859
Unstable Polymer Bilayers
Figure 2. (A) Field emission scanning electron micrograph of a microtomed cross section from the middle of a hole. From the bottom, the image displays the polyimide substrate (dark), the gold marker layer (light), the PC/SAN27 bilayer (dark), and the gold-coated mica support (medium). (B) Higher magnification of the left rim shown in Figure 2A.
0
20
40
60
80
100
Lateral Profile (pm)
Figure 3. Auger electron spectroscopy line scan across a hole showing the relative intensities of oxygen (PC) and nitrogen (SAN27) as a function of lateral distance. AES spectra were taken every micrometer.
We conclude that at this intermediate stage of dewetting and away from the holes, the PC/SAN27 bilayer is unperturbed in both thickness (-400 nm) and surface composition (PC). This is further confirmed by finding
that the estimated volume displaced from the hole equals the estimated volume in the rim. The rim of the hole shown in Figure 2A is enlarged in Figure 2B. For this particular hole the maximum rim height, 920 f4 nm, is more than twice the original bilayer thickness of -400 nm. The distance between the peaks of the rims is -28 pm. These images also clearly show that the rim is asymmetric, rising gradually as the hole is approached and then decreasing more sharply into the hole. The rim is -6 pm wide and reaches its maximum height at a distance of -2 pm from the hole perimeter. If we approximate the inner and outer portions of the rim by arcs, the radius of curvature on the steeper side of the rim is approximately twice that of the outer more gradual side. As mentioned earlier, many theoretical descriptions of dewetting approximate the rim profile as an arc of a circle. A notable exception is the prediction for polymer melts dewetting a semi-ideal substrate which is a solid substrate with a low concentration of adsorbed polymer^.^ In the presence of strong slip an asymmetric rim profile was predicted. In our case of polymer meltlpolymer melt dewetting we do not believe that an asymmetric rim necessarily implies slip at the PC/SAN27 interface.
4860 Langmuir, Vol. 11, No. 12, 1995
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a f
.A-
0
IC I
\
Expanded Height Scale 1
I
0.30 n
5
0.10
v
E
.P -0.10 Q) -r -L
-0.30
-0.50
I
0
10
#
I
I
20
30
40
1
I
60
50
70
Across Hole Diameter (pm)
n
= - 2 18
I
I
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Eaual Height and Lateral Scales I
1
I
12
1
14
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16
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Figure4. (a)Atomicforce microscopy(AFM)perspectiveimage of a hole grown under the same conditions as the one shown in Figure 2. (b) Height profile across the center of the hole shown with an expanded vertical scale. (c) Height profile of the lett rim shown with equally scaled axes.
Rather, an asymmetric rim could be the result of a finite dynamic contact angle inside the hole where three phases meet and a smaller dynamic contact angle outside the hole where only two phases exist. Our direct observation of the actual rim shape, although only a snapshot a t a particular time during dewetting,will hopefully motivate more theoretical investigations of polymer/polymer dewetting which include asymmetric rim profiles. Figure 4a shows an AFM image of the surface of the PC/SAN27 bilayer after annealing a t the same temperature as the sample investigated by SEM. Subsequent AFM images of this same hole were indistinguishablefrom that in Figure 4a, implying that tapping mode AFM can provide reproducible images of the surface topography of polymers and does not significantlymodify the hole width, rim shape, or rim height. To show the topographic features, the AFM image is shown in the perspective view using isoheight contours to reveal the height variation of the rim, Figure 4a. The AFM images confirm the asymmetry of the rim previously discussed in conjunction with the SEM cross-sectional image. The protrusions on the hole floor are due to either dust or the polymer sticking
to the probe tip. The rim and hole were quantitatively analyzed by evaluating several line scans across the nearcenter of the hole. Using an expanded height scale, Figure 4b shows a trace across the rim, floor, and opposite rim. For comparison with the SEM images, Figure 4c shows an AFM trace of the left rim using equal height and length scales. The diameter of this hole, as measured from the rim maxima, is -35pm for this particular hole. Although Figures 2B and 4c represent entirely different holes, the rim profiles are very similar.27Relative to the unperturbed bilayer outside the hole, the AFM images give a maximum rim height of 400 nm. Given that the unperturbed bilayer is -400 nm thick, the total rim height is -800 nm, which again indicates a rim height twice that of the unperturbed bilayer in this case. Finally, we discuss the floor of the hole. The crosssectional image in Figure 2 indicates that the thickness of the floor inside the hole is 85-115 nm. The floor thickness varies systematically showing a gradual decrease toward the center of the hole. It should be noted a t this point that the initial thickness of the SAN27 layer on the gold-coated polyimide sheet was determined to be 200 nm using both ellipsometry and RBS. Therefore, the thickness of the floor is only about 50% of the original SAN27 layer thickness. This difference between the initial thickness and that of the floor has been observed in all other holes that were imaged using cross-sectionalSEM. Unlike meltlsolid or liquidlsoliddewetting, the dewetting of a melt from a melt perturbs the underlying material because the hole opening induces flow in the underlying melt. Figure 3 shows the nitrogen (SAN)and oxygen (PC) AES signals accumulated during a line scan across a hole located on the same sample that was studied by AFM. Note that the hole diameter is -30 pm, in reasonable agreement with AFM results. The profile indicates that the near-surface composition of the floor is SAN27 with no detectablePC. Therefore, we can surmise that the PC fully uncovers (dewets)the underlying SAN27. Furthermore, the SAN27 removed from the hole is not detected on the surface of the rim, which indicates that the SAN27 must be incorporated into the bulk of the rim. Presently, experiments are in progress to determine if the PC and SAN27 layers can be distinguishedby electron microscopy. The slight depression on the floor of the hole is readily detected by AFM as shown by the light area a t the center of Figure 4a and then accentuated in Figure 4b due to the. enhanced height scale. The depression is -10 pm wide and -50 nm deep. This “dimple”correspondsto the “black dot” in the optical micrographs shown in Figure 1B-D. A larger hole, 80 pm, was measured by AFM, and the width and depth of the dimple were approximately the same as measured for the smaller (-35 pm) hole. Note that this dimple is not evident in the AES spectra indicating that the near-surfacecompositionof the dimple is SAN27, as for the rest of the floor. Furthermore, the AFM image of the center of the dimple does not contain a protrusion, which implies that contaminationmight not be required for initiation of the dewetting process.
Concluding Remarks This study has identified a number of important characteristicsof the morphologyof polymer meltlpolymer melt dewetting for the PC/SAN27 system. The initiation of dewetting produces a shallow depression or dimple on (27) While the dewetting morphology appears comparable for the two substratesstudied,note that the dynamicswere significantlyslower on the gold-coatedpolyimide relative to the silicon substrate. Experimental modifications are underway in our laboratories to perform all experiments on identical substrates.
Langmuir, Vol. 11, No. 12, 1995 4861
Unstable Polymer Bilayers the floor of the hole, which persists with time until very long annealing times. The profile of the rim of the hole is definitely not semicircular in nature and is asymmetric. The PC layer dewets the SAN27 underlayer by pulling it into the rim, thereby reducing the SAN27 thickness. Consequently, the growth ofholes in a PC melt on a SAN27 melt substrate differs profoundly from the dewetting behavior on a solid substrate.
Acknowledgment. This work was supported by NSF Grants DMR91-58462 (A.F. and R.J.C.) and DMR9457997(K.I.W.). We acknowledge the use of the Ion Scattering, Electron Microscopy, and Scanning Probe
Microscopy central facilities of the Laboratory for Research on the Structure of Matter supported by NSF Grant DMR91-20668. Acknowledgment is made for the partial support from the donors of the Petroleum Research Fund (A.F., R.J.C., and K.I.W.), administered by the ACS, and Monsanto Chemical Company (A.F. and R.J.C.). Mr. Bruce Rothman and Dr. David Carroll provided valuable assistance with the AES and AFM studies, respectively. We thank Dr. Paul D. Garrett (Monsanto) for supplying materials and advice and Ms. Qi Pan for measuring the glass transition temperatures. LA9502570
