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Topography, Crystallinity and Wettability of Photoablated PET Surfaces J. S. Rossier, P. Bercier, A. Schwarz, S. Loridant, and H. H. Girault* Laboratoire d’E Ä lectrochimie, E Ä cole Polytechnique Fe´ de´ rale de Lausanne, CH-1015 Lausanne, Switzerland Received August 5, 1998. In Final Form: April 26, 1999 Surface topography, crystallinity, and wettability of photoablated poly(ethylene terephthalate) (PET) resulting from various ablation conditions have been characterized by atomic force microscopy (AFM), microconfocal Raman spectroscopy, and wettability measurements. Two ablation modes have been considered here: (i) static ablation, where the samples are immobilized in front of the pulsed laser beam and (ii) dynamic ablation, where the samples are moved in order to write three-dimensional structures in the polymer. Laser fluence, repetition rate, and speed of the substrate motion during the ablation process have been varied. The laser fluence has been observed to strongly affect the resulting surface roughness, which increased to a maximum value at fluences between 70 and 600 mJ‚cm-2. For all fluences in the range of 1000-3000 mJ‚cm-2, the roughness was found to be similar. No remarkable effects could be attributed to the pulse frequency of the 23 ns laser pulses. Raman spectroscopy studies demonstrated that the polymer surface exhibits a high degree of crystallinity when ablated in the static mode. Raman imaging of the surface indicated that these conditions also led to a more homogeneous surface state than when the polymer is ablated in the dynamic mode. Experiments measuring channel filling velocities by capillary action showed that the surfaces of structures fabricated in static photoablation mode were much more hydrophobic than those fabricated under dynamic photoablation.
Introduction The aim of this work is to examine the microscopic characteristics resulting from the photoablation of polymers. This study uses atomic force microscopy (AFM), microconfocal Raman spectroscopy (µ-confocal Raman), and wettability measurements to point out some important parameters of these materials, which can be used in a variety of applications including analytical microsystems. Most of these parameters cannot be predicted in advance and, therefore, empirical information is required prior to use in applications. Since the first developments in the 1980s, the photoablation mechanism of polymers has been a rather controversial issue.1,2 Some authors proposed that photoablation is mainly the result of photochemically initiated electron excitation, followed by the decomposition of the polymer substrate into monomers and gases. Other authors have attributed thermal degradation as having greater importance in polymer photoablation. Although the relative contribution of the proposed mechanisms remains unknown,3 many authors now suggest that the phenomena can be described as a photochemical process with subsequent thermal degradation of fragmented particles and molecules.4,5 It is reasonable to think that these thermal effects will grow with increasing laser fluence and pulse duration, thereby affecting the surface properties of the polymer substrate. Despite the relatively poor understanding of the physical mechanisms, photoablation of polymers has already found a broad field of applications, from eye surgery to integrated (1) Ba¨uerle, D.; Himmelbauer, M.; Arenholz, E. J. Photochem. Photobiol. AsChem. 1997, 106, 27-30. (2) Ba¨uerle, D. Appl. Phys. B 1988, 46, 261-270. (3) Krajnovich, D. J. J. Appl. Phys. 1997, 82, 427-435. (4) Zhigilei, L. V.; Kodali, P. B. S.; Garrison, B. J. J. Phys. Chem. B 1998, 102, 2845-2853. (5) Watanabe, H.; Yamamoto, M. J. Appl. Polym. Sci. 1997, 64, 12031209.
circuits.6,7 This technique has proven to be an interesting method to pattern dielectric materials at the micrometer scale. 8-10 More recently, photoablation has been employed in the fabrication of components for microanalytical systems. The successful use of such devices depends not only on the dimensions and topography of photoablated structures but also on the resulting surface chemistry. For instance, in the development of microfluidic devices, it has been shown that photoablated microchannels exhibit increased surface charges and hydrophilic properties. These features allow the generation of capillary flow or even electroosmotic flow when filled with a buffer and placed in high electrical fields.11 Studies of protein micropatterning on polymer surfaces have revealed that physisorption of hydrophobic proteins is larger on surfaces ablated in the static mode than on the original polymers.12 Laser ablation of dielectric materials or carbon polymer inks13 also provide highly active surfaces which are ideal for electrochemical detection.14 In order to enlighten the ablative phenomena, comprehensive studies of the surface properties have been undertaken by a number of groups. Numerous works have highlighted the increase of the surface roughness induced by the photoablation processes.15,16 These changes are (6) Srinivasan, R.; Bodil, B. Chem. Rev. 1989, 89, 1303-1316. (7) Srinivasan, R. Science 1986, 234, 559-565. (8) Marella, P. F.; Tuckerman, D. B.; Pease, F. R. Appl. Phys. Lett. 1990, 56 (26), 2625-2627. (9) Hohman, J. L.; Keating, K. B.; Kelley, M. J. Mater. Res. Soc. Symp. Proc. 1995, 354, 571-577. (10) Niino, H.; Yabe, A. Appl. Surf. Sci. 1993, 69, 1-6. (11) Roberts, M. A.; Rossier, J. S.; Bercier, P.; Girault, H. H. Anal. Chem. 1997, 69, 2035-2042. (12) Schwarz, A.; Rossier, J. S.; Roberts, M. A.; Girault, H. H.; Roulet, E.; Mermod, H. Langmuir 1998, 14, 5526-5531. (13) Seddon, B. J.; Shao, Y.; Fost, J.; Girault, H. H. Electrochim. Acta 1993, 39 (6), 783-791. (14) Osborne, M. D.; Seddon, B. J.; Dryfe, R. A. W.; Lagger, G.; Loyall, U.; Schafer, H.; Girault, H. H. J. Electroanal. Chem. 1996, 417, 5-15. (15) Bolle, M.; Lazare, S. Appl. Surf. Sci. 1993, 69, 31-37. (16) Novis, Y.; Pireaux, J. J.; Brezini, A.; Petit, E.; R., C.; Lutgen, P.; Feyder, G.; Lazare, S. J. Appl. Phys. 1988, 64 (1), 365-369.
10.1021/la9809877 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/25/1999
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Figure 1. Schematic representation of dynamic ablation. The substrate is translated at a velocity of 200 µm/s and the ablated fragments are partially redeposited on the µ-channel surface.
dependent on the laser fluence. The formation of small carbon islands has been linked to roughening processes. These islands shield the original polymer from the incoming laser beam generating conical structures on the photoablated surfaces.3 It has also been shown that the crystallinity of the original polymer is an important factor for the resulting roughness. More crystalline polymers provide more regular cones distribution.16 Additionally, mechanical stress before the photoablation can orientate the polymer roughness.16,17 The relationship between surface roughness and crystallinity is a rather important point for a practical process. For instance, adhesion and wettability of a polymer can be dramatically affected by slight changes in its surface crystallinity.18 The surface chemistry of polymers can change enormously when modified by the photoablation processes. Grazing angle XPS studies of PET have shown that the oxygen/carbon ratio decreases dramatically after ablation.19 Nevertheless, contact angle measurements indicate that the polymer surface becomes more hydrophilic due to redeposition of charged materials.20 Other chemical changes such as new functional groups have been investigated by some authors. Mainly alcohol, carboxyl, and olefin groups were identified.21 Discrepancies in the surface net charge of photoablated PET surfaces have been reported.10,11,21 These contradictions may appear because of various experimental conditions. Most of the physical characterization studies of photoablated polymers have been performed by a standard technique where the laser beam is focused on the surface of the polymer in the absence of translational motion of either the beam or the substrate, i.e., static mode. Nevertheless, interesting applications such as the fabrication of microfluidic devices can be performed in the dynamic mode, which implies motion of the polymer target relative to the pulsed UV laser. It is important to note that the dynamic photoablation opens the ability to fabricate structures in the micrometer range but translated over much longer distances (mm to cm range). Furthermore, as the target movement is under computer (17) Bahners, T.; Kesting, W.; Shollmeyer, E. Appl. Surf. Sci. 1993, 69, 12-15. (18) Hayes, N. W.; Beamson, G.; Clark, D. T.; Law, D. S.-L.; Raval, R. Surf. Interface Anal. 1996, 24, 723-728. (19) Lazare, S.; Hoh, P. D.; Baker, J. M.; Srinivasan, R. J. Am. Chem. Soc. 1985, 106, 4288-4290. (20) Chan, C. M.; Ko, T. M.; Hiraoka, H. Surf. Sci. Rep. 1996, 24, 3-54. (21) Lazare, S.; Srinivasan, R. J. Phys. Chem. 1986, 90, 2124-2131.
control, it becomes relatively easy to make complex patterns with a simple photomask. Differences in the surface states can be expected between these two techniques and, therefore, the present work describes depth profile, roughness, and surface crystallinity of poly(ethylene terephthalate) (PET) after photoablation in the two different fabrication modes, static and dynamic. The aim of this study is to highlight the microscopic characteristics of both ablation modes by atomic force microscopy (AFM) and by microconfocal Raman spectroscopy (µ-confocal Raman). The underlying morphology of the surface and its homogeneity are analyzed. The measurement of the velocity of fluid flow by capillary action is known to be dependent on the surface wettability and has been used here to study the effect of laser photoablation. These experiments provide information on the resulting surface hydrophilicity as would contact angle measurements. Material and Methods Laser Ablation Procedure. All the experiments were achieved with a 100 µm thick Melinex s Grade poly(ethylene terephthalate) substrate (PET) obtained from Melinex (ICI, UK). The static laser ablation procedure has already been described in detail.11 Briefly, to drill a pattern, a polymer substrate is rinsed with distilled water and ethanol and mounted on an X,Y stage (Microcontrol, National Instrument, US). Then UV laser (Lambda Physik LPX 205 i, D) pulses (193 nm, 23 ns pulse length) are fired and focused on the target through a beam homogenizer (Excitech, Oxford, UK) and a photomask. The pattern of the laser ablation on the surface is 200 × 1000 µm2. The beam homogenizer creates a flat-top fluence distribution at the sample plane that enhances the distribution of energy on the substrate. In order to control the stability of the pulsed laser, the energy at the target is measured with an energy monitor (Gentec, Canada) before and after each sample irradiation. The repetition rate of the laser pulses is varied from 1 to 50 Hz, as well as the fluence per pulse between 70 and 3000 mJ‚cm-2. Depth and Roughness Measurements. Each sample is exposed to 20 laser pulses at various repetition rates and fluences. The depth is measured with a profilometer (Tencor, USA). The surface roughness defined as the mean amplitude is studied by AFM (Burleigh, USA). Static and Dynamic Ablation. For the studies of the surface chemical modifications, laser fluence and repetition rate are fixed at 900 mJ‚cm-2 and 50 Hz, respectively. During the static ablation, the substrate is not moved and 250 pulses are shot. In the dynamic ablation, the substrate is moved horizontally with the X,Y stage at a speed of 0.2 mm/s in order to generate ablated lines as shown in Figure 1. In both ablation modes, the laser
Photoablated PET Surfaces
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Figure 2. Ablation depth dependence on the repetition rate and the fluence of the laser pulses. A logarithmic relationship between depth and fluence is observed up to 1000 mJ‚cm-2. Above 1000 mJ‚cm-2, the ejected fragments are thought to mask the substrate and the ablation rate is therefore lower than the logarithmic expression. fires 250 pulses in 5 s on a given area. The dynamic ablation allows the fabrication of 5 mm × 200 µm lines with a depth of 40 µm. This depth is too large for the AFM cantilever to reach the surface from the top of the channel. In order to circumvent this difficulty, the structures are drilled at the border of the polymer sheet allowing the cantilever to reach the microchannels from the side. Crystallinity Measurement with Microconfocal Raman Spectroscopy. The micro Raman spectroscopy is suitable for characterizing ablation patterns. The confocal geometry provides information with a penetration depth of about 1 micron in the case of transparent compounds. Furthermore, it is possible to map the surface and take the spectrum on a larger surface area (20 × 50 microns) with a micrometer resolution and therefore analyze the homogeneity of the polymer. The homogeneity of the polymer surface is estimated by imaging the half-width of the peak at about 1756 cm-1. The Raman spectra are obtained with a XY Dilor spectrometer. The 514.53 nm exciting line of an argon laser is focused on the samples through an objective (G ) ×50) of a microscope such that the diameter of the analyzed area is about 1 µm. The backscattered light is recollected with this objective and analyzed with a CCD detector. The laser beam power is limited to 5 mW in order to prevent heating of the sample. Capillary Fill Experiments. The µ-channels are fabricated following previously described procedures.11 Briefly 1 mm long, 200 µm wide and 40 µm deep µ-channels were photoablated either dynamically or statically and closed with a 35 µm thick PET/PE lamination (Morane, UK). 20 µL of water or 10 mM Eosin B (Fluka, Switzerland) aqueous solution for a better visualization are then placed on one reservoir and the capillary flow was observed under a microscope, imaged with a CCD camera and recorded on a videotape.
Results and Discussion Dependence of the Ablation Depth on Fluence and Pulse Repetition in Static Ablation. The depth of the cavities obtained by laser photoablation, as a function of the fluence, is shown in Figure 2. Twenty pulses were employed at various repetition rates. The relationship between depth and fluence in the range from 70 to 1000 mJ‚cm-2 is clearly logarithmic. This behavior is in agreement with the general phenomenological model of laser ablation:22
D ∝ log(F/Fth)
(1)
Figure 3. Ablation roughness and depth as a function of the repetition rate and the fluence for 20 laser pulses. From 70 mJ‚cm-2, the roughness increases up to a maximum value. The decrease of the roughness at higher fluence suggests a melting process.
where D, F, and Fth are the ablated depth, the ablation fluence, and the threshold fluence, respectively. The laser pulse duration (23 ns) is rather long compared with the time scale of the ablation processes and plume formation (1000 mJ‚cm-2), it seems that part of the incoming photons may be masked by the ejected fragments. An ablation threshold of about 32 mJ‚cm-2 is extrapolated from the behavior in Figure 2. This value is in fairly good agreement with previous reports. Indeed, many authors have measured or extrapolated threshold fluences from 21 to approximately 30 mJ‚cm-2.3,5 Krajnovich et al.3 mentioned that the ablation fluence threshold is lower for the first pulses than for the following ones due to a carbon enrichment of the sample surface during the first few pulses. In our experiments, all measurements were performed after 20 pulses, when the top layer of the polymer might be affected resulting in an overestimation of the threshold fluence. Furthermore, the commercially available Melinex samples used for these experiments contain some UV stabilizers that can make the ablation more difficult. At higher fluences, parts of the photons may only contribute to increase both plume and surface temperature, leading to melting of the cones in a so-called fusion layer.5,24 As expected, the variation of the repetition rate of the laser pulses from 1 to 50 Hz does not significantly change the ablation depth at any fluence. It should be considered that the laser fluence cannot be controlled with an accuracy of more than 10%, leading to scattering of the results. It is also worth noting that the effect of the repetition rate on the ablation rate is small for strongly absorbing polymers.25,26 Pulse Repetition Rate and Fluence Dependence of the Surface Roughness in the Static Ablation. The surface roughening of PET by the photoablation is a well(22) Pettit, G. H.; Sauerbrey, R. Appl. Phys. A 1993, 56, 51-63. (23) Ku¨per, S.; Stuke, M. Appl. Phys. B 1987, 44, 199-204. (24) Kesting, W.; Bahners, T.; Knittel, D.; Schollmeyer, E. Angew. Makromol. Chem. 1993, 212, 129. (25) Reyna, L. G.; Sobehart, J. R. J. Appl. Phys. 1995, 78, 34233427. (26) Burns, F. C.; Cain, S. R. J. Phys. DsAppl. Phys. 1996, 29, 13491355.
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Figure 4. Comparison between (a) static and (b) dynamic ablated polymer by AFM measurement. No significant difference can be pointed out between static and dynamic ablated polymer surfaces.
known phenomenon. In order to better understand this effect in our experimental conditions, an AFM analysis of the surface is carried out on laser-treated samples, taking the mean amplitude as the roughness parameter. Figure 3 displays the dependence of the roughness on the fluence and the repetition rate of the laser. In the fluence range of 70-600 mJ‚cm-2, the roughness increases dramatically with the ablation fluence. Ablation depth as measured by a profilometer is also shown in Figure 3. Both measurements indicate that the surface roughness is comparable to the depth of photoablation in the low fluence range. At higher fluences, parts of the cones may be molten in a fused layer,5,24 thereby leading to a decrease in the roughness. These observations seem to be consistent with the model of carbon islands enrichment of the surface, leading to the formation of cones.3 Nevertheless, other models explaining the origin of the roughness with surface scattered waves,27 mechanical stress effects,16,17 or product redeposition5 cannot be completely discarded. Finally, the
roughness cannot be related to the repetition rate of the laser as already discussed for the ablation depth. Surface State Comparison between Static and Dynamic Ablation. Parts a and b of Figure 4 contrast a 70 × 70 µm2 surface topography as measured by AFM after the static and the dynamic ablation. No significant differences are observed between the two surfaces; they both reveal cones with a base of approximately 3 µm diameter. The amplitude of the roughness is comparable to those measured at a fluence between 600 and 1000 mJ‚cm-2. The apparent similarity between dynamic and static ablation may arise from the fact that the coverage of the laser beam on the treated surface during the dynamic ablation is 96%, even though the sample is displaced in dynamic mode. It should be noted that the surface in both ablation modes received the same number of laser pulses per unit area. (27) Zhang, J. Y.; Boyd, I. W.; Esrom, H. Surf. Interface Anal. 1996, 24, 718-722.
Photoablated PET Surfaces
Figure 5. Comparison between the microconfocal Raman spectra of the original polymer (a, upper) and the polymer after 250 pulses and (b, lower) in the frequency range between 1100 and 1900 cm-1. The nonablated polymer exhibits a large fluorescence band that disappears after photoablation.
Figure 5 shows two µ-confocal Raman spectra of the PET surface before and after the laser treatment in static mode. The Raman-active modes of PET observed at 1292 cm-1 have been attributed to the ring and the CdO stretch, whereas the 1616 and 1727 cm-1 modes were attributed to CdC ring and carbonyl stretch, respectively.28 The Raman spectrum of the non ablated polymer reveals a strong fluorescence band on which the PET Raman bands are superimposed. This large fluorescence band in commercially available polymers is thought to be induced by UV stabilizers adsorbed at the surface. These molecules are often functionalized benzenes, which might partially be responsible for the detected fluorescence.29,30 The Raman spectra of the static ablated PET does not show such a fluorescence band, which is probably due to the removal of this adsorbed layer, during photoablation. A Raman imaging study is performed to compare the static and dynamic ablated areas. Figure 6 a shows the superposition of 192 spectra taken in different places in a 20 × 50 µm2 static ablated pattern in the frequency range from 1050 to 1800 cm-1. All spectra are similar, containing some characteristic PET bands, and reveal a high mean homogeneity of the surface structure. These measurements indicate that the cones shown with AFM are composed of PET. The presence of the peak at 1092 cm-1 attributed to carbonyl stretching31 has been previously used to characterize high crystallinity in PET.28 The crystallinity of the static ablated surface may be due to the fact that the layer exposed to the laser melts reaches the glass transition temperature (already observed at 125 (28) Grasselli, J. G.; Bulkin, B. J.; Wiley, J. Anal. Raman Spectrosc. 1991. (29) Wright, S. J.; Dale, M. J.; Langridgesmith, P. R. R.; Zhan, Q.; Zenobi, R. Anal. Chem. 1996, 68, 3585-3594. (30) Koenig, J. L. Spectroscopy of Polymers; American Chemical Society: Washington, DC, 1992. (31) Szabo, N. J.; Winefordner, J. D. Anal. Chem. 1997, 69, 24182425.
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°C 18) and cools after the laser treatment. It is worth noting that ablation is performed under high power conditions (about 40 MW). Figure 6 b shows the superposition of 192 spectra taken in the dynamic ablation mode. The spectra show the PET bands in the frequency range from 1050 to 1800 cm-1 with a fluorescence band superimposed. It is obvious from Figure 6 b that the spectra taken in separate locations differ and, therefore, the homogeneity of the dynamically ablated surface is lower than in the static ablation pattern. An analysis of the peak half width of the carbonyl stretch band at 1727 cm-1 also gives information upon the crystallinity degree of the polymer.28 Parts c and d of Figure 6 show two images of the peak half-width at 1727 cm-1 as a function of the location where the spectra have been measured for static and dynamic ablations, respectively. The intensity distribution may show that the PET crystallinity in the static mode is better distributed on the overall pattern than in the case of dynamic ablation. This interpretation must be considered with caution due to the large fluorescence background which may artificially change the peak half-width. This fluorescence band in the dynamic ablation is thought to be related to the contribution of the ejected fragments. During photoablation, parts of the ejected fragments are redeposited on the surface around the ablated pattern. In the dynamic mode, the number of these fragments may increase by the fact that the laser-treated surface remains exposed to the plume and the redeposition of the debris during certain periods of time. The size of these redeposited debris must be much smaller than the cones or be incorporated on the surface, since they cannot be measured by AFM in our experiment. In the static ablation, the number of the redeposited fragments is thought to be smaller due to the fact that every pulse, but the last one, cleans the surface of these debris. Raman studies did not evidence new spectral bands in connection to the surface debris. This observation reflects that the redeposited debris may lie in a rather thin layer beyond the sensitivity of the surface Raman analysis (50 nm). Nevertheless, on the basis of other studies, we can consider that the surface in the dynamic ablated area may contain polymer fragments that are mainly monomers and other lower molecular weight species down to carbon.3,6,32 Wettability and Capillary Fill. Capillary fill experiments show that µ-channels drilled in static or dynamic ablation exhibit very different wetting properties. Indeed, the velocity of the liquid front observed in the static ablated µ-channel is between 0 and 10 µm‚s-1 whereas in the dynamic ablated one, between 1 and 10 mm‚s-1. This large change in velocity (at least 3 orders of magnitude) reveals a dramatic difference in the ablated surface hydrophobicity/hydrophilicity resulting from the two fabrication methods. The hydrophobicity of polymers ablated in the static mode is probably a result of the decrease in the surface O/C ratio previously observed after the static laser ablation.19 This effect may be related to an enhanced surface crystallinity after photoablation. Indeed, a 2-5% increase of the aromatic component in comparison to clean PET has been observed after thermal annealing experiments.18 The resulting surface may therefore be more hydrophobic. In the case of the dynamic ablation, the fast velocity of the flow front reveals a more hydrophilic surface. This wettability may result from the redeposited fragments which have already been characterized as hydrophilic.20 (32) Feldmann, D.; Kutzner, J.; Laukemper, J.; MacRobert, S.; Welge, K. H. Appl. Phys. B 1987, 44, 81-85.
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Figure 6. Comparison between Raman spectra of laser treated surface: (a and b) superposition of 192 scans taken at different places on static and dynamic ablated surfaces, respectively; (c and d) imaging of half-width of the peak at 1727 cm-1 (carbonyl stretch26) on the static and dynamic ablated substrate, respectively.
Conclusion The photoablation of commercially available PET has been characterized under different conditions. The surface roughness increases with the laser fluence until 600 mJ‚cm-2. At higher fluences, where stronger thermal effects are thought to melt the surface, the roughness is more homogeneous. The surface characterization by Raman spectroscopy reveals a homogeneous surface with a high degree of crystallinity for polymers ablated in the static mode whereas dynamic ablation provides an inhomogeneous surface. The effects of redeposited fragments are thought to be partially responsible of the observed difference. The redeposited fragments also affect the surface wettability. PET ablated in the static mode is found to be hydrophobic with a poor wettability whereas
dynamically ablated surfaces are found to be hydrophilic and highly wettable. Acknowledgment. This work has been achieved in collaboration with the Laboratoire d’Optique Applique´e (EPFL). F. Wagner and P. Hofmann as well as D. Fermin and M. A. Roberts are gratefully acknowledged for helpful discussions. The authors thank Prof. Lucazeau of the Laboratoire d’E Ä lectrochimie et de Physico-Chimie des Mate´riaux et des Interfaces de Grenoble for access to the Raman spectrometer. The authors acknowledge the financial support given by the Priority Progam SPP Biotechnology from the Swiss National Foundation (Fonds National pour la Recherche Scientifique Suisse and by the Office Fe´de´rale de l’Education et de la Science). LA9809877
