- Email: [email protected]
Studies of KrF laser-induced long periodic structures on polyimide
ARTICLE IN PRESS Optics and Lasers in Engineering 47 (2009) 180– 185
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Studies of KrF laser-induced long periodic structures on polyimide H.Y. Zheng a,, T.T. Tan b, W. Zhou b a
Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore Precision Engineering and Nanotechnology Centre, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b
a r t i c l e in f o
a b s t r a c t
Article history: Received 8 April 2008 Received in revised form 13 June 2008 Accepted 13 June 2008 Available online 21 July 2008
KrF excimer laser-induced long periodic structures on polyimide were investigated. At a low laser fluence slightly above the material ablation threshold, traces of micron-scale irregular ripples were observed, whereas at a fluence much higher than the ablation threshold, long-period (10-mm) and regular ripple structures were clearly visible. By examining the transition region between the peaks and valleys using a transmission electron microscope (TEM), we observed two phases within the region: amorphous structure in the vicinity of the peak and crystalline structure in the vicinity of the valley. The amorphous region was formed probably due to rapid cooling as a result of polymer melting and re-solidification, which did not allow sufficient time for the molecular chains to become completely aligned. The two distinctive regions of different degrees of crystallinity within the periodic structure can provide opportunities for industrial applications as polymer crystallinity affects mechanical and optical properties, as well as surface wettability. & 2008 Published by Elsevier Ltd.
PACS: 61.80.Ba 68.37.Lp 61.80. x 68.37.Ps Keywords: Polyimide Amorphization Transmission electron microscope Laser-induced periodic structures Laser-induced surface ripples
1. Introduction The use of pulsed ultraviolet lasers as tools for micromachining polymers has generated great interest in both engineering and scientific community [1–13] ever since Srinivasan first demonstrated the use of UV excimer lasers for ablating polymeric materials [1,2]. Due to the short UV wavelength of the KrF excimer laser (248 nm), the photon energy of the KrF laser is sufficiently high to break some molecular bonds such as C–N bonds. Therefore, the laser-ablated surface contains residues of carbonized molecules and carbon clusters. It is also well understood that stronger molecular bonds such as CQO are mainly broken through thermal–chemical reactions, where partial polymer melting is present [1–6]. Polyimide possesses excellent mechanical, electrical properties as well as good thermal stability and therefore has been an important flexible substrate for microelectronics applications. Significant amount of research has been carried out with regard to laser polarization [3], wavelength [4], fluence and pulse duration [3,5] to study changes in both physical and chemical properties of
Corresponding author.
E-mail address: [email protected] (H.Y. Zheng). 0143-8166/$ - see front matter & 2008 Published by Elsevier Ltd. doi:10.1016/j.optlaseng.2008.06.015
polyimide after laser irradiation. Formation of micron and submicron periodic structures or ripples has been observed after pulsed UV laser irradiation on polyimide [14–17] and other polymer materials [18,19]. It is more commonly believed that the laser-induced periodic structures are due to the interference between the laser-induced surface electromagnetic waves and the incident laser beams [20] or the Fresnel diffraction in the laser beam causing intensity modulation of the transverse intensity profile [21]. More recently, the period was found to increase with increasing laser fluence but to be independent of the number of laser pulses [22,23]. The literatures have provided some in-depth understanding of the mechanisms to form periodic structures. However, detailed studies of the microstructure of the laserinduced periodic structures, in particular the changes in the polymer crystallinity, have not been well reported. As the degree of polymer crystallinity affects mechanical properties, optical transparency and surface wettability [24], an improved understanding of the changes in polymer crystallinity as a result of the laser irradiation is important. The objective of this study is to generate and characterize the KrF laser-induced periodic structures on polyimide in terms of geometry, surface morphology and crystallinity. It is hoped that an improved understanding of the laser-induced periodic surface structures can be gained.
ARTICLE IN PRESS H.Y. Zheng et al. / Optics and Lasers in Engineering 47 (2009) 180–185
181
2. Experimental
3. Results and discussion
A KrF excimer laser (Lambda Physik’s Novaline 100) which emits 25 ns (FWHM) laser pulses at a wavelength of 248 nm was used to irradiate a Kapton polyimide film (thickness of 50 mm) in air. Kapton has semi-crystalline structures, i.e. it consists of both amorphous and crystalline structures [25,26]. The laser beam was homogenized by a pair of lens arrays to produce a near top-flat beam profile (Fig. 1), which was measured using the Exitech Beam Diagnostic System. The laser beam was then projected through a rectangular mask and a 4 reduction imaging lens onto the polyimide surface. Laser fluence was set at 58 mJ/cm2 slightly above the ablation threshold of about 45 [27] and 772 mJ/cm2, respectively, to assess the fluence effect on the formation of the periodic structures. Surface morphology of the laser-irradiated pockets was examined using an optical microscope (CamScan S360), a scanning electron microscope (Joel 5600 SEM) and an atomic force microscope (AFM) in tapping mode (Digital Instruments Nano Scope IIIa). The sample was then thinned by ion milling (Gatan 691 Precision Ion Polishing System) from the reversed side for the analyses of the crystallinity using a transmission electron microscope (JOEL 2010 TEM).
The major processing parameters are listed in Table 1 for easy reference. The polyimide surface was irradiated with the KrF excimer laser for a fixed number of pulses at a fixed repetition rate. But the laser fluence was set at just above the ablation threshold and at a much higher fluence. The corresponding optical images of the irradiated areas are shown in Figs. 2 and 3. At the low fluence, the surface appears with traces of irregular grooves. Granular particles in the range of a few microns are also observed on the surface as shown in Fig. 2b, which is believed to be due to the surface roughening under the weak laser energy irradiation on the surface [28]. At the high fluence, however, the surface appears with clearer periodic grooves without visible particles (Fig. 3). AFM was used to further examine the surface morphology as well as the period and depth of the grooves (Figs. 4 and 5). It can be seen from the 2D AFM image and the sectional analysis that there are traces of irregular periodic structures for the surface irradiated at the laser fluence of 58 mJ/cm2 and apparent surface roughening. It is further seen from Fig. 2b that there existed ripples with varying periods between the major peaks and valleys. For instance, in the circled region, the ripple period was calculated to be around 1.5 mm and depth of a few nanometers. For the entire measured region, the average period was around 2.3 mm. Such micron-scale ripples are in agreement with the reported results in the literature [4]. Clearer and longer periodic grooves can be observed in Fig. 5 for the surface irradiated at the laser fluence of 772 mJ/cm2.
Fig. 1. A homogenized excimer laser beam intensity profile.
Table 1 Major processing parameters Energy fluence (mJ/cm2) No. of pulses Repetition rate (Hz)
58 30 100
772 30 100
Fig. 3. Optical image of clear periodic grooves on polyimide surface irradiated at a laser fluence of 772 mJ/cm2.
Fig. 2. Optical images of (a) traces of grooves on polyimide surface irradiated at a laser fluence of 58 mJ/cm2 and (b) SEM image of irregular granular particles on the surface.
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Fig. 4. AFM image of (a) surface structures produced by KrF excimer laser at the laser fluence of 58 mJ/cm2 and (b) sectional analyses of the surface structures.
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Fig. 5. AFM images of (a) periodic structures produced by KrF excimer laser at the laser fluence of 772 mJ/cm2 and (b) sectional analysis of the periodic structures with a period of about 10 mm and depth about 400 nm.
The period and depth were averaged at about 10 mm and 400 nm, respectively. The dimensional scale of the laser-induced periodic structures is one order of magnitude higher than the micron and sub-micron structures reported in literatures [24,27]. One of the likely reasons is the much-higher-than-threshold fluence used in our study as it has been reported that the period increased with increasing laser fluence [22]. To look into the details of Fig. 5a and b, it was further observed that there existed smaller ripples in the individual major peaks such as the one circled. The ripple period was calculated to be around 3 mm. The smaller ripples within the individual major peaks may be explained by the light interference between the laser-induced surface electromagnetic waves and the incident laser beams [20] or the Fresnel diffraction in the laser beam causing intensity modulation of the transverse intensity profile [21]. There is no common understanding for the formation of the long periodic structures, although self-organization of the polymer molecules and clusters under UV laser irradiation at
the fluence above the ablation threshold was cited as a reason for the formation of the long periodic structure [29–31]. Our reasoning is as follows: At the low laser fluence around the ablation threshold, the strong molecular bonds such as C–O and C–C (existing in polyimide) with bond energies greater than 5 eV (KrF laser photon energy) will not be broken. Only weak bonds such as N–H and C–H are ablated photochemically, which would lead to a small amount of material removal (smaller and shallower ripples). At the high fluence level, the strong C–O and C–C bonds are broken through a thermal process, which would lead to a large amount of material removal (larger and deeper ripples) and involve material melting and re-solidification. The difference in the material removal mechanisms and removal rates could be conjectured as one of the reasons for the formation of the peaks and valleys of the long periodic structures. However, a thorough understanding of the long periodic structures will require much further research.
ARTICLE IN PRESS H.Y. Zheng et al. / Optics and Lasers in Engineering 47 (2009) 180–185
In order to study the crystallinity of the KrF laser-irradiated surfaces, the polyimide samples of both non-laser-irradiated and laser-irradiated areas were thinned using ion milling from the reverse side for TEM analyses. A TEM image is formed due to the electron beam being diffracted by crystal lattices. The variation in the electron intensity (the bright rings) of the diffracted beam reveals information on the crystal structure. Therefore, the diffraction patterns (the bright rings) in the TEM images indicate the existence of crystalline structures. The TEM image of the surface structure and the diffraction pattern of the non-irradiated polyimide are shown in Fig. 6. The surface appears spongy with dark pockets that are networked together. Judging from the bright diffraction ring around the centre, there exist some crystalline structures in the original polyimide. It implies that the original polyimide is a mixture of crystalline and amorphous structures. This agrees with the literature [24,25]. The existence of the crystalline structure within the original polymer is important as it is a critical factor for the resultant surface roughness upon UV laser irradiation. Higher degree of polymer crystallinity within the polymer has been shown to lead to more regular cone structures at the laser fluence just above the ablation threshold [15]. For the area irradiated at the laser fluence of 58 mJ/cm2, the TEM image of the surface structure and the diffraction pattern are shown in Fig. 7. The spongy structure observed in Fig. 6a disappeared. The laser-irradiated area is characterized with closely packed granular particles in the range of 20–30 nm. Compared to Fig. 6b, there are more bright rings in the diffraction pattern as shown in Fig. 7b. This indicates that the laser-treated area becomes more crystalline. The degree of the polymer
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crystallinity increased. This is not surprising as excimer laser irradiation was reported to increase the crystalinity in polyethylene terephthalate (PET) [32]. Adhesion and wettability of a polymer surface can be dramatically affected by slight changes in its surface crystallinity [27]. On examining the transition region between the peak and valley of the periodic structure irradiated at the fluence of 772 mJ/cm2, two distinctive regions are observed (Fig. 8a and b). For easy identification, the lighter region was named as region A and the darker region as region B. From Fig. 8a, it appears that region A is in the vicinity of a peak region and region B is in the vicinity of a valley. However, for the present study, we are unable to relate accurately regions A and B to the peaks and valleys of the longperiod periodic structures. Further studies will be needed to obtain an accurate correlation. The close-up images of the two regions are shown in Fig. 9a and b, respectively. Region A is covered by 4 nm particles, which are much smaller than those observed on the surface irradiated at the fluence of 58 mJ/cm2 (Fig. 2b). The diffraction pattern of region A is shown in Fig. 9c. There are no strong bright diffraction rings detectable, which indicate that region A is mostly amorphous in structure. Region B is characterized with closely packed granular particles similar to those shown in Fig. 7a. The diffraction rings can be observed in Fig. 9d, indicating that region B contains crystalline structures. However, it is further observed that the diffraction rings are weaker than those shown in Figs. 6b and 7b, indicating a reduction in the degree of the polymer crystallinity. The reduction in the degree of polymer crystallinity can be interpreted as a result of the fast heating and fast cooling during
Fig. 6. TEM images of (a) non-irradiated polyimide structure and (b) diffraction pattern.
Fig. 7. TEM image of (a) the polyimide surface irradiated at the fluence of 58 mJ/cm2 and (b) its diffraction pattern.
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Fig. 8. TEM images of the polyimide structure irradiated at the fluence of 772 mJ/cm2 (a) at a lower magnification and (b) at a high magnification showing two distinct different regions.
Fig. 9. TEM images of the polyimide structure irradiated at the fluence of 772 mJ/cm2. (a) Close-up of region A showing nano particles of about 4 nm; (b) close-up of region B showing closely packed granular particles of about 15 nm; (c) the diffraction pattern of region A showing a dull amorphous ring; (d) the diffraction pattern of region B showing traces of crystalline rings.
polymer melting and re-solidification under the UV laser interactions with the polyimide. The fast cooling process allows insufficient time for the polymer molecules to get completely aligned. With the increase in laser energy fluence, a larger amount of material becomes molten during the laser beam irradiation and a prolonged period of time is needed for the polymer molecules to get completely aligned. Therefore, increasing laser fluence would lead to reduction in the degree of polymer crystallinity. The two distinctive regions of amorphous and crystalline phases may provide opportunities for potential applications as polymer crystallinity affects the polymer transparency, surface wettability and mechanical properties.
4. Conclusions KrF excimer laser was used to treat polyimide surfaces at both low and high laser fluences. Different surface morphologies were observed.
At the high fluence of 772 mJ/cm2, periodic structures with an average period of 10 mm and depth of 400 nm were produced. The larger-than-wavelength periodic structures are one order of magnitude higher than those reported in the literatures. TEM analyses further suggested that an amorphous region in
ARTICLE IN PRESS H.Y. Zheng et al. / Optics and Lasers in Engineering 47 (2009) 180–185
the vicinity of the peak and a crystalline region in the vicinity of at the valley were formed. The degree of crystallinity is found to be reduced at the higher laser fluence. At the low fluence of 58 mJ/cm2 just above the ablation threshold, traces of irregular ripples were observed and with apparent surface roughening. TEM analyses revealed the existence of closely packed granular particulates in the range from 20 to 30 nm on the laser-irradiated area.
Acknowledgement One of the authors would like to thank NTU for the financial support and SIMTech for access to its equipment. References [1] Srinivasan R, Mayne-Banton V. Self-developing photoetching of poly(ethylene terephthalate) films by far-ultraviolet excimer radiation. Appl Phys Lett 1982;41:576–8. [2] Srinivasan R, Leigh WJ. Ablative photodecomposition: action for far-ultraviolet (193 nm) laser irradiation on poly (ethylene terephthalate) film. J Am Chem Soc 1982;104:6784–6. [3] Baudach S, Bonse J, Kautek W. Ablation experiments on polyimide with femtosecond laser pulses. Appl Phys A: Mater Sci Process 1999;69:S695–8. [4] Lu QH, Li M, Yin J, Zhu ZK, Wang ZG. Polyimide surface modification by pulsed ultraviolet laser irradiation with low fluence. J Appl Poly Sci 2001;82: 2739–43. [5] Schmidt H, Ihlemann J, Wolff-Rottke B, Luther K, Troe J. Ultraviolet laser ablation of polymers: spot size, pulse duration, and plume attenuation effects explained. J Appl Phys 1998;83:5458–68. [6] Pettit GH, Ediger MN, Hahn DW, Brinson BE, Sauerbrey R. Transmission of polyimide during pulsed ultraviolet laser irradiation. Appl Phys A: Mater Sci Proc 1994;58:573–9. [7] Brannon JH, Lankard JR, Baise AI, Burns F, Kaufman J. Excimer laser etching of polyimide. J Appl Phys 1985;58:2036–44. [8] Adhi KP, Owings RL, Railkar TA, Brown WD, Malshe AP. Chemical modifications in femtosecond ultraviolet (248 nm) excimer laser radiation-processed polyimide. Appl Surf Sci 2004;225:324–31. [9] Gotok K, Kikuchi S. Improvement of wettability and detergency of polymeric materials by excimer UV treatment. Colloid Polym Sci 2005;283:1356–60. [10] Weichenhain R, Wesner DA, Pfleging W, Horn H, Kreutz EW. KrF-excimer laser pretreatment and metallization of polymers. Appl Surf Sci 1997;109/ 110:264–9. [11] Qin ZY, Du BY, Zhang J, He TB, Qin L, Zhang YS. Thermal influence on excimerlaser-induced electrical conductivity on polyimide film surfaces. Appl Phys A: Mater Sci Proc 2001;72:711–5.
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[12] Ball Z, Sauerbrey R. Lowering of the conduction threshold by carbon fiber formation in KrF excimer laser irradiated polyimide. Appl Phys Lett 1994;65: 391–3. [13] Ball Z, Phillips HM, Callahan DL, Sauerbrey R. Precolative metal–insulator transition in excimer laser irradiated polyimide. Phys Rev Lett 1994;73: 2013–99. [14] Bolle M, Lazare S. Large scale excimer laser production of submicron periodic structures on polymer surfaces. Appl Surf Sci 1993;69:31–7. [15] Hiraoka H, Sendova M. Laser-induced sub-half-micrometer periodic structure on polymer surfaces. Appl Phys Lett 1994;64:563–5. [16] Mei L, Hua LQ, Jie Y, Yu LS, Wang ZG. Investigation of the periodic microstructure induced by a 355 nm UV polarized laser on a polyimide surface. Chin Phys Lett 2002;19:255–8. [17] Sendova M, Hiraoka H. Sub-half micron periodic structures on polymer surfaces with polarized laser irradiation. Jpn J Appl Phys 1993;32:6182–4. [18] Csete M, Marti O, Bor Zs. Laser-induced periodic surface structures on different poly-carbonate films. Appl Phys A: Mater Sci Process 2001;73: 521–6. [19] Cooper J, Julian B, Morrison H, Song P, Albin S, Zheng J. Surface characterization of pulsed UV-laser modified polyamide films. Thin Sol Films 1997;303:180–90. [20] Bench-Bmevich AM, Libenson MN, Makin VS, Trubaev V. Surface electromagnetic waves in optics. Opt Eng 1992;31:718–30. [21] Sipe E, Young JF, Preston JS, Driel HM. Laser-induced periodic surface structure. I. Theory. Phys Rev B 1983;27:1141–54. [22] Wang XC, Lim GC, Zheng HY, Ng FL, Liu W, Chua SJ. Femtosecond pulse laser ablation of sapphire in ambient air. Appl Surf Sci 2004;228(1–4):221–6. [23] Qian HX, Zhou W, Zheng HY, Lim GC. Morphological and chemical evolution on InP(1 0 0) surface irradiated with femtosecond laser. Surf Sci 2005;595: 49–55. [24] Hayes NW, Beamson G, Clark DT, Law DS-L, Raval R. Crystallisation of PET from the amorphous state: observation of different rates for surface and bulk using XPS and FTIR. Surf Interface Anal 1996;24:723–8. [25] Caroline A, O’Mahoney, Willams DJ. Single-crystal X-ray studies of aromatic oligomers. Macromolecules 1991;24:6527–30. [26] Liu J, Kim D, Harris FW, Cheng ZD. Crystal structure, morphology, and phase transitions in aromatic polyimide oligomers. J. Polym Sci B 1994;32:2705–13. [27] Revesz A, Ochin P, Donnadieu P, Simon JP, Guyot P. Nanocrystallization in a Zr57Ti5Cu20Al10Ni8 bulk metallic glass. Philos Mag Lett 2001;81:767–75. [28] Krajnovich DJ. Near-threshold photoablation characteristics of polyimide and poly(ethylene terephthalate). J Appl Phys 1997;82:427–35. [29] Murthy NS, Prabhu RD, Martin JJ, Zhou L, Headrick RL. Self-assembled and etched cones on laser ablated polymer surface. J Appl Phys 2006;100:2035381–203538-12. [30] Niino H, Nakano M, Nagano S, Yabe A, Miki T. Periodic morphological modification developed on the surface of polyethersulfone by XeCl excimer laser photoablation. Appl Phys Lett 1989;55:510–3. [31] Dyer PE, Fareley RJ. Periodic surface-structures in the excimer laser ablative etching of polymers. Appl Phys Lett 1990;57:765–7. [32] Rossier JS, Bercier P, Schwarz A, Loridant S, Gurault HH. Topography, crystallinity and wettability of photoablated PET surfaces. Langmuir 1999;15: 5173–8.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Studies of KrF laser-induced long periodic structures on polyimide H.Y. Zheng a,, T.T. Tan b, W. Zhou b a
Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore Precision Engineering and Nanotechnology Centre, School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b
a r t i c l e in f o
a b s t r a c t
Article history: Received 8 April 2008 Received in revised form 13 June 2008 Accepted 13 June 2008 Available online 21 July 2008
KrF excimer laser-induced long periodic structures on polyimide were investigated. At a low laser fluence slightly above the material ablation threshold, traces of micron-scale irregular ripples were observed, whereas at a fluence much higher than the ablation threshold, long-period (10-mm) and regular ripple structures were clearly visible. By examining the transition region between the peaks and valleys using a transmission electron microscope (TEM), we observed two phases within the region: amorphous structure in the vicinity of the peak and crystalline structure in the vicinity of the valley. The amorphous region was formed probably due to rapid cooling as a result of polymer melting and re-solidification, which did not allow sufficient time for the molecular chains to become completely aligned. The two distinctive regions of different degrees of crystallinity within the periodic structure can provide opportunities for industrial applications as polymer crystallinity affects mechanical and optical properties, as well as surface wettability. & 2008 Published by Elsevier Ltd.
PACS: 61.80.Ba 68.37.Lp 61.80. x 68.37.Ps Keywords: Polyimide Amorphization Transmission electron microscope Laser-induced periodic structures Laser-induced surface ripples
1. Introduction The use of pulsed ultraviolet lasers as tools for micromachining polymers has generated great interest in both engineering and scientific community [1–13] ever since Srinivasan first demonstrated the use of UV excimer lasers for ablating polymeric materials [1,2]. Due to the short UV wavelength of the KrF excimer laser (248 nm), the photon energy of the KrF laser is sufficiently high to break some molecular bonds such as C–N bonds. Therefore, the laser-ablated surface contains residues of carbonized molecules and carbon clusters. It is also well understood that stronger molecular bonds such as CQO are mainly broken through thermal–chemical reactions, where partial polymer melting is present [1–6]. Polyimide possesses excellent mechanical, electrical properties as well as good thermal stability and therefore has been an important flexible substrate for microelectronics applications. Significant amount of research has been carried out with regard to laser polarization [3], wavelength [4], fluence and pulse duration [3,5] to study changes in both physical and chemical properties of
Corresponding author.
E-mail address: [email protected] (H.Y. Zheng). 0143-8166/$ - see front matter & 2008 Published by Elsevier Ltd. doi:10.1016/j.optlaseng.2008.06.015
polyimide after laser irradiation. Formation of micron and submicron periodic structures or ripples has been observed after pulsed UV laser irradiation on polyimide [14–17] and other polymer materials [18,19]. It is more commonly believed that the laser-induced periodic structures are due to the interference between the laser-induced surface electromagnetic waves and the incident laser beams [20] or the Fresnel diffraction in the laser beam causing intensity modulation of the transverse intensity profile [21]. More recently, the period was found to increase with increasing laser fluence but to be independent of the number of laser pulses [22,23]. The literatures have provided some in-depth understanding of the mechanisms to form periodic structures. However, detailed studies of the microstructure of the laserinduced periodic structures, in particular the changes in the polymer crystallinity, have not been well reported. As the degree of polymer crystallinity affects mechanical properties, optical transparency and surface wettability [24], an improved understanding of the changes in polymer crystallinity as a result of the laser irradiation is important. The objective of this study is to generate and characterize the KrF laser-induced periodic structures on polyimide in terms of geometry, surface morphology and crystallinity. It is hoped that an improved understanding of the laser-induced periodic surface structures can be gained.
ARTICLE IN PRESS H.Y. Zheng et al. / Optics and Lasers in Engineering 47 (2009) 180–185
181
2. Experimental
3. Results and discussion
A KrF excimer laser (Lambda Physik’s Novaline 100) which emits 25 ns (FWHM) laser pulses at a wavelength of 248 nm was used to irradiate a Kapton polyimide film (thickness of 50 mm) in air. Kapton has semi-crystalline structures, i.e. it consists of both amorphous and crystalline structures [25,26]. The laser beam was homogenized by a pair of lens arrays to produce a near top-flat beam profile (Fig. 1), which was measured using the Exitech Beam Diagnostic System. The laser beam was then projected through a rectangular mask and a 4 reduction imaging lens onto the polyimide surface. Laser fluence was set at 58 mJ/cm2 slightly above the ablation threshold of about 45 [27] and 772 mJ/cm2, respectively, to assess the fluence effect on the formation of the periodic structures. Surface morphology of the laser-irradiated pockets was examined using an optical microscope (CamScan S360), a scanning electron microscope (Joel 5600 SEM) and an atomic force microscope (AFM) in tapping mode (Digital Instruments Nano Scope IIIa). The sample was then thinned by ion milling (Gatan 691 Precision Ion Polishing System) from the reversed side for the analyses of the crystallinity using a transmission electron microscope (JOEL 2010 TEM).
The major processing parameters are listed in Table 1 for easy reference. The polyimide surface was irradiated with the KrF excimer laser for a fixed number of pulses at a fixed repetition rate. But the laser fluence was set at just above the ablation threshold and at a much higher fluence. The corresponding optical images of the irradiated areas are shown in Figs. 2 and 3. At the low fluence, the surface appears with traces of irregular grooves. Granular particles in the range of a few microns are also observed on the surface as shown in Fig. 2b, which is believed to be due to the surface roughening under the weak laser energy irradiation on the surface [28]. At the high fluence, however, the surface appears with clearer periodic grooves without visible particles (Fig. 3). AFM was used to further examine the surface morphology as well as the period and depth of the grooves (Figs. 4 and 5). It can be seen from the 2D AFM image and the sectional analysis that there are traces of irregular periodic structures for the surface irradiated at the laser fluence of 58 mJ/cm2 and apparent surface roughening. It is further seen from Fig. 2b that there existed ripples with varying periods between the major peaks and valleys. For instance, in the circled region, the ripple period was calculated to be around 1.5 mm and depth of a few nanometers. For the entire measured region, the average period was around 2.3 mm. Such micron-scale ripples are in agreement with the reported results in the literature [4]. Clearer and longer periodic grooves can be observed in Fig. 5 for the surface irradiated at the laser fluence of 772 mJ/cm2.
Fig. 1. A homogenized excimer laser beam intensity profile.
Table 1 Major processing parameters Energy fluence (mJ/cm2) No. of pulses Repetition rate (Hz)
58 30 100
772 30 100
Fig. 3. Optical image of clear periodic grooves on polyimide surface irradiated at a laser fluence of 772 mJ/cm2.
Fig. 2. Optical images of (a) traces of grooves on polyimide surface irradiated at a laser fluence of 58 mJ/cm2 and (b) SEM image of irregular granular particles on the surface.
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50.0
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500.0 nm Section Analysis 450
nm
25.0
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−450
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Fig. 4. AFM image of (a) surface structures produced by KrF excimer laser at the laser fluence of 58 mJ/cm2 and (b) sectional analyses of the surface structures.
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Fig. 5. AFM images of (a) periodic structures produced by KrF excimer laser at the laser fluence of 772 mJ/cm2 and (b) sectional analysis of the periodic structures with a period of about 10 mm and depth about 400 nm.
The period and depth were averaged at about 10 mm and 400 nm, respectively. The dimensional scale of the laser-induced periodic structures is one order of magnitude higher than the micron and sub-micron structures reported in literatures [24,27]. One of the likely reasons is the much-higher-than-threshold fluence used in our study as it has been reported that the period increased with increasing laser fluence [22]. To look into the details of Fig. 5a and b, it was further observed that there existed smaller ripples in the individual major peaks such as the one circled. The ripple period was calculated to be around 3 mm. The smaller ripples within the individual major peaks may be explained by the light interference between the laser-induced surface electromagnetic waves and the incident laser beams [20] or the Fresnel diffraction in the laser beam causing intensity modulation of the transverse intensity profile [21]. There is no common understanding for the formation of the long periodic structures, although self-organization of the polymer molecules and clusters under UV laser irradiation at
the fluence above the ablation threshold was cited as a reason for the formation of the long periodic structure [29–31]. Our reasoning is as follows: At the low laser fluence around the ablation threshold, the strong molecular bonds such as C–O and C–C (existing in polyimide) with bond energies greater than 5 eV (KrF laser photon energy) will not be broken. Only weak bonds such as N–H and C–H are ablated photochemically, which would lead to a small amount of material removal (smaller and shallower ripples). At the high fluence level, the strong C–O and C–C bonds are broken through a thermal process, which would lead to a large amount of material removal (larger and deeper ripples) and involve material melting and re-solidification. The difference in the material removal mechanisms and removal rates could be conjectured as one of the reasons for the formation of the peaks and valleys of the long periodic structures. However, a thorough understanding of the long periodic structures will require much further research.
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In order to study the crystallinity of the KrF laser-irradiated surfaces, the polyimide samples of both non-laser-irradiated and laser-irradiated areas were thinned using ion milling from the reverse side for TEM analyses. A TEM image is formed due to the electron beam being diffracted by crystal lattices. The variation in the electron intensity (the bright rings) of the diffracted beam reveals information on the crystal structure. Therefore, the diffraction patterns (the bright rings) in the TEM images indicate the existence of crystalline structures. The TEM image of the surface structure and the diffraction pattern of the non-irradiated polyimide are shown in Fig. 6. The surface appears spongy with dark pockets that are networked together. Judging from the bright diffraction ring around the centre, there exist some crystalline structures in the original polyimide. It implies that the original polyimide is a mixture of crystalline and amorphous structures. This agrees with the literature [24,25]. The existence of the crystalline structure within the original polymer is important as it is a critical factor for the resultant surface roughness upon UV laser irradiation. Higher degree of polymer crystallinity within the polymer has been shown to lead to more regular cone structures at the laser fluence just above the ablation threshold [15]. For the area irradiated at the laser fluence of 58 mJ/cm2, the TEM image of the surface structure and the diffraction pattern are shown in Fig. 7. The spongy structure observed in Fig. 6a disappeared. The laser-irradiated area is characterized with closely packed granular particles in the range of 20–30 nm. Compared to Fig. 6b, there are more bright rings in the diffraction pattern as shown in Fig. 7b. This indicates that the laser-treated area becomes more crystalline. The degree of the polymer
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crystallinity increased. This is not surprising as excimer laser irradiation was reported to increase the crystalinity in polyethylene terephthalate (PET) [32]. Adhesion and wettability of a polymer surface can be dramatically affected by slight changes in its surface crystallinity [27]. On examining the transition region between the peak and valley of the periodic structure irradiated at the fluence of 772 mJ/cm2, two distinctive regions are observed (Fig. 8a and b). For easy identification, the lighter region was named as region A and the darker region as region B. From Fig. 8a, it appears that region A is in the vicinity of a peak region and region B is in the vicinity of a valley. However, for the present study, we are unable to relate accurately regions A and B to the peaks and valleys of the longperiod periodic structures. Further studies will be needed to obtain an accurate correlation. The close-up images of the two regions are shown in Fig. 9a and b, respectively. Region A is covered by 4 nm particles, which are much smaller than those observed on the surface irradiated at the fluence of 58 mJ/cm2 (Fig. 2b). The diffraction pattern of region A is shown in Fig. 9c. There are no strong bright diffraction rings detectable, which indicate that region A is mostly amorphous in structure. Region B is characterized with closely packed granular particles similar to those shown in Fig. 7a. The diffraction rings can be observed in Fig. 9d, indicating that region B contains crystalline structures. However, it is further observed that the diffraction rings are weaker than those shown in Figs. 6b and 7b, indicating a reduction in the degree of the polymer crystallinity. The reduction in the degree of polymer crystallinity can be interpreted as a result of the fast heating and fast cooling during
Fig. 6. TEM images of (a) non-irradiated polyimide structure and (b) diffraction pattern.
Fig. 7. TEM image of (a) the polyimide surface irradiated at the fluence of 58 mJ/cm2 and (b) its diffraction pattern.
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Fig. 8. TEM images of the polyimide structure irradiated at the fluence of 772 mJ/cm2 (a) at a lower magnification and (b) at a high magnification showing two distinct different regions.
Fig. 9. TEM images of the polyimide structure irradiated at the fluence of 772 mJ/cm2. (a) Close-up of region A showing nano particles of about 4 nm; (b) close-up of region B showing closely packed granular particles of about 15 nm; (c) the diffraction pattern of region A showing a dull amorphous ring; (d) the diffraction pattern of region B showing traces of crystalline rings.
polymer melting and re-solidification under the UV laser interactions with the polyimide. The fast cooling process allows insufficient time for the polymer molecules to get completely aligned. With the increase in laser energy fluence, a larger amount of material becomes molten during the laser beam irradiation and a prolonged period of time is needed for the polymer molecules to get completely aligned. Therefore, increasing laser fluence would lead to reduction in the degree of polymer crystallinity. The two distinctive regions of amorphous and crystalline phases may provide opportunities for potential applications as polymer crystallinity affects the polymer transparency, surface wettability and mechanical properties.
4. Conclusions KrF excimer laser was used to treat polyimide surfaces at both low and high laser fluences. Different surface morphologies were observed.
At the high fluence of 772 mJ/cm2, periodic structures with an average period of 10 mm and depth of 400 nm were produced. The larger-than-wavelength periodic structures are one order of magnitude higher than those reported in the literatures. TEM analyses further suggested that an amorphous region in
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the vicinity of the peak and a crystalline region in the vicinity of at the valley were formed. The degree of crystallinity is found to be reduced at the higher laser fluence. At the low fluence of 58 mJ/cm2 just above the ablation threshold, traces of irregular ripples were observed and with apparent surface roughening. TEM analyses revealed the existence of closely packed granular particulates in the range from 20 to 30 nm on the laser-irradiated area.
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