A Criterion for Dewetting Initiation from Surface Disturbances on

9194

Langmuir 2005, 21, 9194-9198

A Criterion for Dewetting Initiation from Surface Disturbances on Ultrathin Polymer Films Ioannis Karapanagiotis*,† and William W. Gerberich‡ ORMYLIA Art Diagnosis Centre, Ormylia, Chalkidiki, 63071, Greece, and Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received April 14, 2005. In Final Form: July 19, 2005 Nanoindentation-induced defects on ultrathin (h ) 17 nm) polystyrene (PS) films that are spin cast on silicon (Si) substrates, with residual depths of penetration lower than the film thickness ( ∆Fγ,crit and levels when ∆Fγ < ∆Fγ,crit. This conclusion is in agreement with previous reports, which used ∆Fγ to distinguish the two (dewetting/leveling) opposing processes (1) in the case of indents deeper than the film thickness and (2) in the case of built-in ordered surface disturbances by capillary force lithography.

Introduction The stability of thin polymer films placed on rigid, hard substrates is of great importance because of the numerous and diverse applications of the organic coatings in the microelectronics industry, printing technology, and their utilization as membranes, adhesives, lubricants, protective layers, and decorative layers. A thin polymer film on top of a solid nonwettable surface (the equilibrium contact angle, θe, between the film and the substrate is θe * 0) can rupture, either spontaneously or because of surface or interfacial defects, upon heating above the glass transition temperature (Tg).1-11 Film rupturing leads to the onset of dewetting, that is, the uncovering of the substrate.7-14 Dewetting is usually an undesirable process because it results in a discontinuous film; polymer mass is accumulated in droplets, and the exposed substrate area is therefore maximized. Within this context, several researchers have proposed strategies to minimize or even suppress dewetting, including structural alterations of the polymer molecules,15-17 substrate surface modifica* Corresponding author. Address: Ormylia Art Diagnosis Centre, Sacred Convent of Annunciation, 63071 Ormylia, Chalkidiki, Greece. Phone: +30 23710 98400. Fax: +30 23710 98402. E-mail: [email protected]. † ORMYLIA Art Diagnosis Centre. ‡ University of Minnesota. (1) Ruckenstein, E.; Jain, R. K. Chem. Soc., Faraday Trans. 1974, 70, 132. (2) de Gennes, P.-G. Rev. Mod. Phys. 1985, 57, 827. (3) Sharma, A.; Ruckenstein, E. J. Colloid Interface Sci. 1986, 113, 456. (4) Sharma, A.; Ruckenstein, E. Langmuir 1986, 2, 480. (5) Brochard-Wyart, F.; Daillant, J. Can. J. Phys. 1990, 68, 1084. (6) Kheshgi, H. S.; Scriven, L. E. Chem. Eng. Sci. 1991, 46, 519. (7) Reiter, G. Phys. Rev. Lett. 1992, 68, 75. (8) Reiter, G. Langmuir 1993, 9, 1344. (9) Stange, T. G.; Hendrickson, W. A.; Evans, D. F. Langmuir 1997, 13, 4459. (10) Kim, H. I.; Mate, C. M.; Hannibal, K. A.; Perry, S. S. Phys. Rev. Lett. 1999, 82, 3496. (11) Mu¨ller-Buschbaum, P. J. Phys.: Condens. Matter 2003, 15, 1549. (12) Geoghegan, M.; Krausch, G. Prog. Polym. Sci. 2003, 28, 261. (13) Mu¨ller-Buschbaum, P.; Stamm, M. Physica B 1998, 248, 229. (14) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Phys. Rev. Lett. 1998, 81, 1251. (15) Henn, G.; Bucknall, D. G.; Stamm, M.; Vanhoorne, P.; Je´roˆme, R. Macromolecules 1996, 29, 4305.

tions,18 and the utilization of additives.19-24 On the other hand, controlled dewetting has been used as a surfacepattern process to fabricate ordered polymer and biomolecule arrays at the micrometer scale.25-31 Therefore, depending on the application, dewetting can be considered either an undesirable process that needs to be prevented or a process that can be exploited for the fabrication of ordered microstructures, which are extremely useful in microelectronics. Consequently, developing criteria that could predict the conditions in which film rupturing and therefore dewetting occurs is of great importance. Furthermore, controlling these conditions will allow us to initiate or prevent the dewetting of organic films from solid substrates, depending on the desired application. Within this framework, several efforts, aimed to determine a critical parameter for the onset of dewetting, have been reported. In particular, energetic criteria based on the Young-Laplace equation of capillarity for the breakup of thin films, have been investigated,32,33 and (16) Feng, Y.; Karim, A.; Weiss, R. A.; Douglas, J. F.; Han, C. C. Macromolecules 1998, 31, 484. (17) Mounir, E. S.; Takahara, A.; Kajiyama, T. Polym. J. 1999, 31, 89. (18) Reiter, G.; Schultz, J.; Auroy, P.; Auvray, L. Europhys. Lett. 1996, 33, 29. (19) Barnes, K. A.; Karim, A.; Douglas, J. F.; Nakatani, A. I.; Gruell, H.; Amis, E. J. Macromolecules 2000, 33, 4177. (20) Barnes, K. A.; Douglas, J. F.; Liu, D.-W.; Karim, A. Adv. Colloid Interface Sci. 2001, 94, 83. (21) Sharma, S.; Rafailovich, M. H.; Peiffer, D.; Sokolov, J. Nano Lett. 2001, 1, 511. (22) Mackay, M. E.; Hong, Y.; Jeong, M.; Hong, S.; Russell, T. P.; Hawker, C. J.; Vestberg, R.; Douglas, J. F. Langmuir 2002, 18, 1877. (23) Yurelki, K.; Karim, A.; Amis, E. J.; Krishnamoorti, R. Macromolecules 2003, 36, 7256. (24) Li, X.; Han, Y.; An, L. Polymer 2003, 44, 5833. (25) Karthaus, O.; Gråsjo¨, L.; Maruyama, N.; Shimomura, M. Thin Solid Films 1998, 327-329, 829. (26) Zhang, G.; Yan, X.; Hou, X.; Lu, G.; Yang, B.; Wu, L.; Shen, J. Langmuir 2003, 19, 9850. (27) Zhang, Z.; Wang, Z.; Xing, R.; Han, Y. Surf. Sci. 2003, 539, 129. (28) Zhang, Z.; Wang, Z.; Xing, R.; Han, Y. Polymer 2003, 44, 3737. (29) Kim, Y. S.; Lee, H. H. Adv. Mater. 2003, 15, 332. (30) Luo, C.; Xing, R.; Han, Y. Surf. Sci. 2004, 552, 139. (31) Lee, L.-T.; Leite, C. A. P.; Galembeck, F. Langmuir 2004, 20, 4430. (32) Sharma, A.; Ruckenstein, E. J. Colloid Interface Sci. 1990, 137, 433. (33) Sharma, A. J. Colloid Interface Sci. 1993, 156, 96.

10.1021/la050998h CCC: $30.25 © 2005 American Chemical Society Published on Web 08/25/2005

Dewetting Initiation on Ultrathin Polymer Films

critical nucleus sizes were determined for initiating dewetting.34-36 Similarly, in a previous study, we proposed an energetic criterion to predict the evolution of nanoindentation-induced defects on thin polystyrene (PS) films on silicon (Si) substrates upon heating above the Tg.37 Indents with residual depths of penetration (zo) that are slightly higher than the thicknesses (h) of the tested films (zo > h ) 50 and 100 nm) either grew laterally, resulting in dry patches (dewetting onset), or healed, resulting in a flat surface (leveling). To distinguish the growing from the healing indents, the excess surface energy (∆Fγ) of the system, because of the presence of indents, was measured prior to heating. For a particular film thickness, a critical value, ∆Fγ,crit, was then determined, and the following conclusion was drawn: if ∆Fγ < ∆Fγ,crit, then the indent heals upon heating above the Tg, and dewetting is prevented, whereas, if ∆Fγ > ∆Fγ,crit, the indent grows, and dewetting is initiated.37 The same concept of the ∆Fγ,crit was successfully applied in another study to determine the evolution (dewetting or leveling) of 34-nm-thick PS films with built-in ordered disturbances by capillary force lithography.30 In this investigation we test whether ∆Fγ can be used as a criterion to predict the evolution of nanoindents that are imposed on ultrathin PS film upon heating the film above the Tg. The system in this study has two major differences from the system examined in our previous report:37 (1) film thickness (h ) 17 nm) is lower than the thicknesses of 50 and 100 nm tested before. (2) In contrast to our previous study in which zo > h, here, indent residual depths are lower than the film thickness (zo < h ) 17 nm), resulting in a modified formula for the excess surface energy, ∆Fγ. The 17-nm-thick films spontaneously rupture when heated above the Tg. Rupturing is initiated at the free polymer surface on which surface disturbances are formed, which grow toward the substrate and initiate dewetting.9,38 These disturbances resemble the intentionally induced nanoindents.38 The latter, however, either grow or level. The dry patches that formed because of the presence of indents were distinguished from the spontaneously formed holes, which are developed randomly, by inducing the indents in well-defined positions, forming ordered arrays. The evolution of the ordered indentation area was monitored with annealing time. In the following section, we first describe the materials, sample preparation procedures, and instruments utilized in this investigation. Then, the two opposing processes (growing/leveling) are described, clarifying the goal of the study. Parameters that affect indent topography, and therefore ∆Fγ,crit, are discussed in detail, along with the interaction driving forces of indent evolution. Finally, we compare the healing versus the growing indents, on the basis of (1) indent depth, zo and (2) excess surface energy, ∆Fγ, measurements. Experimental Section Standardized, low molecular weight PS (Mr ) 10900 g/mol, Mr/Mn ) 1.02; Polymer Labs, U.K.) was dissolved in spectroscopic grade toluene to prepare stock solutions of 1.0%. Solutions were (34) Sykes, C.; Andrieu, C.; De´tappe, V.; Deniau, S. J. Phys. III 1994, 4, 775. (35) Bausch, R.; Blossey, R.; Burschka, M. A. J. Phys. A: Math. Gen. 1994, 27, 1405. (36) Liu, H.; Bhattacharya, A.; Chakrabarti, A. J. Chem. Phys. 1998, 109, 8607. (37) Karapanagiotis, I.; Gerberich, W. W.; Evans, D. F. Langmuir 2001, 17, 2375. (38) Karapanagiotis, I.; Evans, D. F.; Gerberich, W. W. Langmuir 2001, 17, 3266.

Langmuir, Vol. 21, No. 20, 2005 9195

Figure 1. Micrograph of indentation area (a) before and (b) after heating at 140 °C for 20 min. (a) Big indents, which served as markers (M), are shown. The relative positions of 12 nanoindents are indicated. (b) Upon heating, nanoindents 1, 2, 3, 4, and 5 evolve to circular dry patches, i.e., act as nucleation sites for dewetting initiation. Nanoindents 6, 7, 8, 9, 10, 11, and 12 level. All big indents (markers) grow laterally and initiate dewetting. filtered with 0.2-µm Teflon filters to minimize particle contamination. Polished Si wafers (Virginia Semiconductor, Fredericksburg, VA), with a diameter of 50 mm, were used as substrates for the deposition of the PS films. Ellipsometric measurements on uncoated wafers revealed the presence of a 1.8-nm-thick native oxide film. The 〈100〉-oriented wafers were used as they were received without any further treatment. PS solutions were spin coated onto Si wafers in a class 10 clean room. The coatings were then annealed at 60 °C overnight in a vacuum to remove residual solvent. This annealing temperature is well below the Tg of PS (for Mr ) 10900 g/mol, Tg ) 91 °C),39 and therefore spontaneous dewetting was not initiated. On the contrary, the free film surface appeared to be atomically smooth, according to atomic force microscopy (AFM) images, with the exception of some scarce airborne particles.9,38,40 The thickness of the polymer coatings was measured by ellipsometry and found to be 17 nm. This result was verified later by AFM images of the dry patches that developed upon heating the samples above the Tg. A nanoindentation apparatus was then used to impose a series of nanoindents on the free undisturbed PS surface at room temperature. Nanoindents were induced at discrete positions, according to Figure 1a. Indent depths, zo, were lower than the film thickness, h (zo < h). Big indents, which are shown in Figure 1a, were induced around the nanoindents. These served as markers to locate the area of interest with an optical microscope and position the AFM tip to scan the nanoindents. The indented (39) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley: New York, 1989. (40) Hamley, L. W.; Hiscutt, E. L.; Yang, Y. W.; Booth, C. J. Colloid Interface Sci. 1999, 209, 255.

9196

Langmuir, Vol. 21, No. 20, 2005

coatings were then heated at 140 °C (>Tg), and the nanoindents either grew or leveled, which was recorded with an optical microscope (Figure 1b) and AFM. A Nanoscope III SPM (Digital Instruments, Santa Barbara, CA), operated in the contact mode, was utilized to record the topography of the nanoindents. Forces in the range of 1-10 nN were applied from the 200-µm cantilever to the polymer surface. Repeated scanning of the same surface area showed no alteration in the recorded topography due to the applied scanning force. Controlling the interaction force within a relatively short range (1-10 nN) ensures that, even if the applied scanning force affected the indent shape to some extent, it would introduce a repeated error to all AFM scans. In this worst scenario, the measured values of the topographical features might contain a minor, but repeated, error. All of the images in this study were obtained using the same AFM tip. Indents were induced using a Hysitron Triboscope (Hysitron Inc., Minneapolis, MN) mounted with a standard three-sided pyramid (Berkovich) diamond tip. The instrument adapts to an AFM that utilizes a Hysitron microsensor system, instead of the standard AFM head component, for applying loads electrostatically. The apparatus can serve two purposes: (1) performing standard nanoindentation measurements and (2) scanning the indented surface. Neither of these operations was performed in this study. Inducing indents on a soft polymeric surface with a residual depth of penetration, zo, less than 17 nm (zo < 17 nm) required the development of a careful experimental procedure. Nanoindents were induced by bringing the Berkovich tip just in contact with the PS surface. Contact forces were in the range of 2-5.5 µN. Immediately after contact, the tip was forced to withdraw from the polymer to avoid any surface damage that could be caused by scanning the indented surface with the Berkovich tip or by scanner drift. Afterward, the indent topography was recorded with an AFM and not with the Hysitron Triboscope. Big marks were imposed under high forces (11 mN) following the standard indentation procedure recommended by Hysitron Inc. Apparently, these big indents penetrated the film and damaged the substrate; that is, in this case, zo > h.

Results and Discussion Figure 1a shows the indentation pattern induced on a PS film, and Figure 1b shows the resulting micrograph obtained after heating the sample at 140 °C for 20 min. In Figure 1a, 10 big indents, serving as markers (M) to locate the indentation area, are shown. Within the area surrounded by the markers, 12 nanoindents that were not visible with the optical microscope, were induced. Their relative positions are indicated in the micrograph. Figure 1b shows that five of the nanoindents acted as nucleation sites for dewetting initiation upon heating the film above the Tg; that is, they grew and resulted in dry patches, similar to the spontaneously formed holes (not shown). Seven of them, however, did not initiate dewetting. On the contrary, they leveled, resulting in a flat polymer surface. The leveling process of these indents was verified by AFM, which was used to scan the indentation area before and after thermal treatment above the Tg. As expected, all of the big marks grew laterally and initiate dewetting upon heating. Several nanoindents, like those in Figure 1, were imaged with AFM. Figure 2 shows an AFM image of a nanoindent and the corresponding cross section prior to any thermal treatment above the Tg. As described later, the characteristics of the recorded topography are used to distinguish the leveling from the growing nanoindents. Below, we provide a short discussion regarding the parameters that determine and affect the shape of a nanoindent. Although the indent of Figure 2 was induced using a three-sided pyramid (Berkovich) tip, the top view shape is not triangular because of the very small indent depth (zo ) 15.6 nm). At this small length scale, the precise indenter tip shape cannot be controlled by the manufacturer. At larger depths

Karapanagiotis and Gerberich

Figure 2. AFM image and corresponding cross section of a nanoindent induced by a Berkovich tip. The film thickness, h, is 17 nm and the indent depth, zo, is 15.6 nm (zo < h).

of penetration (>50 nm) the same indenter tip resulted in “triangular” indents, as expected, because of the shape of a Berkovich tip. Although the topography of a residual indent is mainly determined by the shape of the indenter tip, three more parameters might have considerable effects on the indent shape at the scale of penetration depth ( h.37 Recently, eq 2 was used by Han et al. to distinguish the leveling from the growing of surface disturbances induced on PS films by capillary force lithography.30 The extra PS area created with the formation of an indent, APS, can be measured by image analysis of the acquired AFM data as follows: Each AFM image, like the one shown in Figure 2, is divided into an N number of cross sections, each of which has a width W. In a particular cross section, which is illustrated in Figure 4, the extra length LPS, which is created upon indentation, is calculated as

LPS ) 2(w1 + w2 + zo)

(3)

in which w1 and w2 are the rim heights. The latter and the indent depth, zo, can be easily measured by AFM, and therefore LPS can be calculated from eq 3. We note here that the indent cross section illustrated in Figure 4 has been simplified, compared to the real situation shown in Figure 2, to make the derivation of eq 3 easier. However, eq 3 is independent of the indent shape, and it is valid in any case. APS can be then calculated as follows:

APS ) W(LPS(1) + LPS(2) + ........ + LPS(N))

(4)

The nanoindents in this study were analyzed using W ) 35 nm. On average, each image was divided into 15 () N) cross sections. The above methodology, used to measure APS, does not include any assumption associated with the indent shape because it is based on a pixel by pixel image analysis. The resulting APS measurements are presented in Figure 5, which suggests that a critical value, APS,crit ) 1.92 × 104 nm2, exists that determines the final evolution of an indent (growing/leveling) upon heating above the Tg. Considering that the surface tension of PS (Mr ) 10900 g/mol) at 140 °C is γPS ) 31.7 mN/m,39 we calculated the critical excess surface energy from eq 2 to be ∆Fγ,crit ) 6.1 × 10-16 J. If ∆Fγ > ∆Fγ,crit, then the indent grows, and if ∆Fγ < ∆Fγ,crit, the indent heals upon heating above the Tg. The value of the measured ∆Fγ,crit is valid only for the conditions of the present study, that is, for indents with zo < h that were induced on PS (Mr ) 10900 g/mol) films with h ) 17 nm, placed on the Si wafers of this investigation (no cleaning method was applied), and heated at 140 °C. The annealing temperature, however, is expected to have

Figure 4. Schematic illustration of the system film/substrate prior to (a) and after (b) indentation. Comparison of the two schemes results in the conclusion that the extra length created in the PS film, LPS, upon indentation, refers to the perpendicular (z) direction. Therefore, LPS ) 2(w1 + w2 + zo).

9198

Langmuir, Vol. 21, No. 20, 2005

Karapanagiotis and Gerberich

a result of the new surfaces that were created upon indenting the system. Conclusion

Figure 5. Extra area of PS induced in the system PS/Si upon nanoindentation, APS. Indents are separated with respect to their resulting evolution process (leveling or growing) after being heated above the Tg. A critical value, APS,crit ) 1.92 × 104 nm2, exists, which determines that, if APS > APS,crit, the indent grows (dewetting), whereas, if APS < APS,crit, the indent heals (leveling).

a minor impact on the measured ∆Fγ,crit because it has a minor effect on γPS. No distinguishable difference was reported in the ∆Fγ,crit measurements that were performed within a temperature range of 110-140 °C.37 As expected, the measured ∆Fγ,crit is lower than the values reported for indents deeper than the film thickness (zo > h) and thicker films:37 ∆Fγ,crit ) 2.5 × 10-15 J for h ) 50 nm, and ∆Fγ,crit ) 1.2 × 10-14 J for h ) 100 nm. The argument that the growing and leveling of artificially induced surface disturbances on thin polymer films can be determined by the excess surface energy seems to be well-established, when one considers the results of this investigation and the data reported recently in the literature.30,37 ∆Fγ,crit can be considered an energetic barrier (activation energy) for the dewetting process that is to be initiated by preexisting defects. Only indents with ∆Fγ > ∆Fγ,crit possess sufficient energy to overcome this barrier to growing. It must be clear, however, that ∆Fγ does not correspond to the total energy induced in the system upon indent formation. It includes only the energy induced as

The effect of nanoindentation-induced defects on ultrathin (h ) 17 nm) PS films placed on Si substrates was studied. Indents with residual depths of penetration lower than the film thickness were induced. Upon heating at 140 °C (>Tg), some of the indents grew and exposed substrate to the air, thus initiating a dewetting process, whereas others healed and resulted in a flat polymer surface. Indent depth measurements, which were performed by AFM, appeared to be insufficient to distinguish the two opposing processes (leveling/growing). Nanoindents with comparable residual depths can either grow or heal. Detailed topographical data of the nanoindents were then analyzed to measure the extra area of PS, APS, that was created upon nanoindentation. Measurements were performed for several healing and growing indents, and a critical value, APS,crit ) 1.92 × 104 nm2, was determined as a “cutoff” border for the two opposing processes. The corresponding critical excess surface energy (∆Fγ,crit) was found to be ∆Fγ,crit ) 6.1 × 10-16 J. A nanoindent grows when ∆Fγ > ∆Fγ,crit and heals when ∆Fγ < ∆Fγ,crit. The same conclusion has been reported in other studies for different film thicknesses and artificially induced surface disturbances. Consequently, the excess surface energy can be a very useful concept in the processing of thin organic films because it can be used to predict the evolution of surface defects upon heating. Similarly, controlling the topographical characteristics of a manmade surface disturbance can allow manufacturers to initiate or prevent dewetting at specific sites, according to their will. Acknowledgment. Prior support by the Center for Interfacial Engineering (CIE), a National Science Foundation Engineering Research Center at the University of Minnesota, is gratefully acknowledged. LA050998H