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Langmuir 2007, 23, 12984-12989
Fabrication and Characterization of Plasma Processed Surfaces with Tuned Wettability A. Ruiz, A. Valsesia, G. Ceccone, D. Gilliland, P. Colpo,* and F. Rossi Institute for Health and Consumer Protection, Joint Research CentresEuropean Commission, TP 203, 21020 Ispra (VA) Italy ReceiVed August 7, 2007. In Final Form: September 12, 2007 Engineered surfaces with controlled hydrophilic/ hydrophobic character have been fabricated by tailoring the substrate topography and chemistry. In this method, the substrate to be treated was first coated by a photoresist, which was then surface-roughened using SF6 plasma etching. The resulting rough texture was then transferred to the underlying silicon surface by over-etching of the photoresist. At this point, the topographically modified surface was modified chemically by controlled deposition of a thin polymer layer using plasma processing. In this way, both the surface texture and the surface chemistry could be varied independently, producing surfaces with variable wetting character, including super-hydrophilicity and super-hydrophobicity, depending on the choice of plasma polymer deposited. Chemical characterization demonstrates a correlation between the surface chemistry and the wettability of the samples after etching. The surface elementary composition contained more C-F groups as the measured contact angle increased, indicating that the change of wettability is due to both the roughness and the surface energy of the deposited photoresist. In the case of materials deposited on the plasma-treated rough surfaces, the strengthening of the wetting character is only due to the created surface roughness, as XPS analyses showed no significant chemical difference as compared to the flat polymer.
Introduction Controlling the wettability of surfaces is an important issue attracting increasing interest from both fundamental and practical perspectives. A variety of applications in medical devices or biosensing can take advantage of surfaces with selected wettability.1,2 For instance, the possibility of patterning hydrophobic and hydrophilic polymers on the same substrate is of particular interest for bio-analytical applications and microfluidics. Moreover, the use of patterned surfaces containing superhydrophobic areas permits the development of surfaces with unique properties of floatability, flow acceleration, inhibition of contamination or self-cleaning,3-6 and consequently much research in recent years has been focused on the study of such surfaces.7,8 In terms of wettability, a surface is hydrophilic when the water contact angle is lower than 90°, it is considered superhydrophilic when it goes below 5°. When the water contact angle is larger than 90°, the surface is hydrophobic, and the surface becomes superhydrophobic for contact angles above 150°. Besides the high contact angle, another criterion to distinguish superhydrophobicity is that water drops can easily glide along the surface at low tilting angles.9,10 It is known that the wettability of polymers is determined by a combination of the topography, i.e., the surface roughness, and by the chemical properties of the surface, which * To whom correspondence should be addressed. E-mail: Pascal.colpo@ jrc.it. Tel: +39 0332 789979 Fax: +39 0332 785787. (1) Morra, M. Water in Biomaterial Surface Science; John Wiley & Sons, Ltd.: Chichester, U.K., 2001. (2) McHale, G.; Shirtcliffe, N. J.; Newton, M. I. Analyst 2004, 284-287. (3) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1-8. (4) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644-652. (5) Delamarche, E.; Bernard, A.; Schmid, H.; Michel, B.; Biebuyck, H. Science 1997, 276, 779-781. (6) Sun, T.; Tan, H.; Han, D.; Fu, Q.; Jiang, L. Small 2005, 10, 959-963. (7) Tserepi, A. D.; Vlachopoulou, M. E.; Gogolides, E. Nanotechnology 2006, 17, 3977-3983. (8) Tsougeni, K.; Tserepi, A.; Boulousis, G.; Constantoudis, V.; Gogolides, E. Plasma Process. Polym. 2007, 4, 398-405. (9) Gao, L.; McCarthy, T. J. Langmuir 2006, 22, 5998-6000. (10) Marmur, A. Langmuir 2004, 20, 3517-3519.
regulate the surface energy.7,8,11,12 There are two models explaining the effects of roughness on the contact angle. The Wenzel’s model assumes that the liquid is in contact with all the parts of the irregular surface, whereas the Cassie’s model considers that the contact surface is smaller because it supposes that the drop does not wet the bottom part of the pillars.13,14 In nature there are several examples of superhydrophobic surfaces, among which the most popular is the lotus leaf.15,16 Its main characteristics are considerable roughness and low surface energy. The topography of the lotus leaf has been reproduced artificially by nanocasting, a process based on soft lithography that Sun et al.17 used to replicate the macro and nanostructures of the leaf in PDMS. Despite the different chemistry, the artificial polymeric surface exhibited the same hydrophobic properties as the natural one. Plasma treatments have been extensively applied to polymers, metals, or elastomers for fabrication of engineered surfaces,18-20 including processes for altering the wetting properties to create either hydrophilic or hydrophobic surfaces. As previously stated, the wettability behavior is determined by several parameters, among which the surface energy and the surface roughness are the most important ones. It has been demonstrated that plasma etching is an excellent tool for inducing surface roughness and (11) Guo, Z. G.; Fang, J.; Hao, J. C.; Liang, Y. M.; Liu, W. M. Chem. Phys. Chem. 2006, 7, 1674-1677. (12) Song, X.; Zhai, J.; Wang, Y.; Jiang, L. J. Phys. Chem. B 2005, 109, 4048-4052. (13) Herberston, D. L.; Evans, C. R.; Shirtcliffe, N. J.; McHale, G.; Newton, M. I. Sens. Actuators A 2006, 130, 189-193. (14) Zhu, L.; Feng, Y.; Ye, X.; Zhou, Z. Sens. Actuators A 2006, 130, 595600. (15) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. AdV. Mater. 2002, 15, 1857-1860. (16) Gao, X.; Jiang, L. Nature 2004, 432, 36-36. (17) Sun, M.; Luo, C.; Xu, L.; Ji, H.; Ouyang, Q.; Yu, D.; Chen, Y. Langmuir 2005, 21, 8978-8981. (18) Bre´tagnol, F.; Valsesia, A.; Ceccone, G.; Colpo, P.; Gilliland, D.; Ceriotti, L.; Hasiwa, M.; Rossi, F. Plasma Process. Polym. 2006, 3, 443-455. (19) Favia, P.; Stendardo, M. V.; d’Agostino, R. Plasma Polym. 1996, 1, 91. (20) Satyaprasad, A.; Jain, V.; Nema, S. K. Appl. Surf. Sci. 2007, 253, 54625466.
10.1021/la702424r CCC: $37.00 © 2007 American Chemical Society Published on Web 11/20/2007
Plasma Processed Surfaces with Tuned Wettability
surface nanostructuring on different types of materials.21-23 Tailoring the roughness of surfaces has been proven to be an effective method for enhancing its hydrophilic or hydrophobic character.24,25 Although several methods to prepare superhydrophobic materials have been proposed, there are few references to the control of the other extreme of wettability, i.e., hydrophilicity.26,27 In a previous work, we have reported how plasma treatments can be usefully employed to create significant roughness in a photoresist.28 The plasma etching yielded nanotopography in the photoresist, thus enhancing the hydrophobic or hydrophilic character of polymer surfaces. However, in order to design and fabricate nanostructured surfaces with desired and controllable wettability, a clear relationship between nanostructure, surface chemistry, and wetting behavior needs to be established. With this objective in mind, we have used a combination of plasma processes including plasma etching and polymerization to prepare rough surfaces with different hydrophilic/hydrophobic character. The first step in this process, producing surfaces with different defined roughness properties, was done using fluorocontaining gases to plasma etch a layer of photoresist deposited on silicon. Using this process and varying the treatment duration, it was found possible to obtain surfaces with a range of different surface roughness values. The surfaces produced were then analyzed in terms of chemistry, roughness, and wettability. The etched surfaces were analyzed by XPS, ToF-SIMS, and SEM, and the results were compared with the wettability and roughness obtained from contact angle and surface profile measurements, respectively. The results obtained show a correlation between the contact angle, i.e., the hydrophilic character, the roughness, and the chemical composition of the surface. In particular, XPS and SIMS both show that the etching of the photoresist creates a large amount of C-F bonding at the surfaces, which corresponded to an increase of its contact angle. Teflon-like, acrylic acid, and silicon oxide layers were deposited on the rough plasma-treated surface by plasma polymerization. The hydrophilic or hydrophobic character of the deposited layers was more pronounced due to the previously created roughness. The use of a photoresist as substrate in this process is particularly interesting because it allows the nanoscale modification by plasma technology to be combined with micro-patterning capabilities of photolithography. Applications such as tailored surfaces for droplet deposition for microarrays, nanostructuring PDMS for microfluidics, or modifying the cell adhesion behavior are envisaged. Experimental Methods The process used in this work is shown schematically in Figure 1. The smooth starting surfaces were prepared by spin-coating a 2.5-µm layer of a photosensitive resist (Shipley s1813) onto a silicon substrate. After deposition, the photoresist was soft baked at 110 °C for 2 min and exposed to natural light for 24 h. The resist was then etched in an inductively coupled plasma reactor using SF6 as an etching reagent. Treatments were done at 10 mTorr of pressure, 400 (21) Vlachopoulou, M.-E.; Tserepi, A.; Beltsios, K.; Boulousis, G.; Gogolides, E. Microelec. Eng. 2007, 84, 1476-1479. (22) Gogolides, E.; Constantoudis, V.; Patsis, G. P.; Tserepi, A. Microelec. Eng. 2006, 83, 1067-1072. (23) Gogolides, E.; Boukouras, C.; Kokkoris, G.; Brani, O.; Tserepi, A.; Constantoudis, V. Microelec. Eng. 2004, 73-74, 312-318. (24) Favia, P.; Cicala, G.; Milella, A.; Palumbo, F.; Rossini, P.; D’Agostino, R. Surf. Coat. Technol. 2003, 169-170, 609-312. (25) He, B.; Lee, J.; Patankar, N. A. Coll. Surf. A 2004, 248, 101-104. (26) Wang, S.; Song, Y.; Jiang, L. J. Photochem. Photobiol. C 2007, 8, 18-29 (27) Xie, Q.; Xu, J.; Feng, L.; Jiang, L.; Tang, W.; Luo, X.; Han, C. C. AdV. Mater. 2004, 16, 302-305. (28) Lejeune, M.; Lacroix, L. M.; Bretagnol, F.; Valsesia, A.; Colpo, P.; Rossi, F. Langmuir 2006, 22, 3057-3061.
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Figure 1. Fabrication process of the superhydrophobic and the superhydrophilic surfaces. W inductive power, and -60 V of DC bias. In order to produce samples with different values of surface roughness, the plasma treatment time was varied across the range 30-600 s. Full details of the plasma etching process used can be found in ref 29. After this plasma treatment, the roughened photoresist surfaces were coated with thin hydrophobic (CFx) or hydrophilic (Poly Acrylic Acid) layers using Plasma Enhanced Chemical Vapor Deposition (PECVD). The deposition of Poly Acrylic Acid (PAA) was done using a capacitive plasma source working in pulsed mode.30 The mixture introduced was PAA/Ar ) 5/5 sccm at a pressure of 50 mTorr. The CFx layer deposition was done in a homemade capacitively coupled plasma reactor described in ref 31, using C4F8 as a gas precursor. The continuous flow of gas was adjusted to reach a pressure of 50 mTorr in the chamber, and then an RF power of 10 W was applied for 5 min. Three series of samples were fabricated by the same method and characterized. Sample surface chemistry was studied in a surface analysis workstation equipped with separate XPS and ToF-SIMS spectrometers. In this system, the two spectrometers are connected together via a UHV sample transfer chamber which permits samples to be stored and moved between both instruments under constant UHV ( 150°) when deposited on rough surfaces. Similarly, for the hydrophilic coating, the roughness makes the substrate more hydrophilic. PAA exhibits a contact angle of 38°, which decreased below 5° when deposited on a rough surface. The superhydrophobicity and superhydrophilicity were attained on substrates etched for just 1.5 min. After that time, the surface has been roughened enough and the polymer deposited remains superhydrophobic or superhydrophilic with little variation of the contact angle. To explore the application to other substrates, we performed the same experiments on photoresist coated on glass. The results are presented in Figure 7b. Superhydrophilicity and superhydrophobicity were also reached, but in this case, larger preetching times were needed (5 min). The results of the plasma deposition on rough and flat silicon are summarized in Table 1. Pictures of drops on the fabricated superhydrophobic and superhydrophilic surfaces are shown in Figure 8. XPS analyses were carried out in order to check if the increase of the wetting character could be due to a chemical effect instead of roughness. Table 2 reports the elemental composition of the different films for the CFx and PAA layers deposited on flat and on rough silicon (etching time 5 min). In the case of the Teflonlike layer, no appreciable difference in composition is observed and the F/C ratio is similar in both cases (F/C ≈ 0.54).
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Figure 7. Contact angles of the SF6 etched photoresist-coated silicon (a) and photoresist-coated glass (b) before and after the deposition of CFx and PAA. Table 1. Contact Angle and Roughness for the Different Materials Deposited on Flat and Rough Silicon substrate flat silicon flat silicon rough silicon flat silicon rough silicon
roughness polymer etching contact thickness deposited time angle Ra (nm) Rq (nm) (nm) none PAA PAA CFx CFx
0 0 5 0 5
34a 38 150
1.33 0.8 233 0.8 214
1.66 0.9 300 1 269
100 100 90 90
a After washing ultrasonicating 5 min in trichloroethylene, acetone, ethanol; and dried at 110 °C 1h.
Figure 6. ToF-SIMS ion mappings of CF and Silicon at different etching times, and the relative intensities. Field of view 150 × 150 µm.
Furthermore, XPS showed that the C1s core level spectra are also very similar with the same relative amount of CFx bonds (data not shown), indicating that the change from hydrophobic to superhydrophobic can be attributed only to the changing roughness of the substrate. In the same way, XPS showed that the chemistry of the hydrophilic coating was independent of whether it was deposited on flat or rough silicon, further supporting the idea that the surface texture was the critical factor in converting a hydrophilic surface into a superhydrophilic one.
Conclusions Engineered surfaces with selected hydrophilic/hydrophobic character have been obtained by using simple plasma-based techniques to independently tailor the substrate topography and the surface chemistry. It has been shown that controlled surface
Figure 8. Drop of water on superhydrophobic and superhydrophilic surfaces prepared after etching and CFx (a) or PAA(b) deposition.
roughness can be achieved by appropriate plasma etching of a photoresist and that this roughness can be transferred to an underlying silicon wafer by extended plasma etching. A clear link between the chemistry of the etched photoresist surface and the hydrophobicity has been found. C-F groups detected on the surfaces were thought to be responsible for the increase in hydrophobicity after the photoresist was etched. In order to controllably separate the influence of topography and surface chemistry, thin films of plasma-deposited polymers of known wetting behavior were produced on both smooth and
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Table 2. Elemental Composition of the Different Materials Deposited on Flat and Rough Silicon as Measured by XPS substrate flat silicon rough silicon flat silicon rough silicon
polymer etching deposited time CFx CFx PAA PAA
0 5 0 5
elemental concentration (atom %) C
F
O
37.9 ( 0.27 59.48 ( 0.22 1.36 ( 0.17 34.55 ( 0.9 63.42 ( 1.6 1.12 ( 0.07 75.56 ( 1.3 23.03 ( 1.4 77.42 ( 1.04 21.48 ( 0.6
variably roughened silicon substrates. XPS analyses of deposited films have shown the resultant surface chemistry to be independent of the underlying surface topography. The characterization of thin layers deposited on the pretreated surfaces showed that layers with the same chemistry exhibited an enhancement of their wetting characteristics with a normally hydrophobic or hydrophilic surface exhibiting, respectively, superhydrophobic or superhydrophilic behavior as a result of the increased roughness. These results
show the critical influence played by surface roughness in producing extreme wetting behavior (superhydrophobic or supehydrophilic) while using surface chemistry with intermediate wetting characteristics. The fabrication technique discussed in this work is a fast, easy to implement method which has the advantage of being applicable to different substrates as well as permitting large area processing and good repeatability. These factors mean that this process offers not only potential for use in fundamental studies, but also could be effectively adapted to many practical applications where surfaces with extreme wetting properties could be exploited. Acknowledgment. This project has been financed by the JRC Action “NanoBiotechnology for Health”. LA702424R
