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Ultra-short pulsed laser ablation of polymers
Applied Surface Science 180 (2001) 42±56
Ultra-short pulsed laser ablation of polymers A.A. Serafetinidesa,1, M.I. Makropouloua,*, C.D. Skordoulisb,2, A.K. Karc,3 a
Physics Department, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece b Atomic and Molecular Physics Laboratory, Physics Department, University of Ioannina, Greece c Physics Department, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK Received 15 November 2000; accepted 12 April 2001
Abstract We report the experimental results of the ablation rate per pulse as a function of the laser ¯uence and images of the surface morphology, as examined by atomic force microscopy, for a number of organic polymer materials of special interest in microelectronics and biomedical applications. The ablation parameters and the surface modi®cations are examined under various irradiation conditions using laser wavelengths ranging from the ultraviolet through the visible to the infrared and pulse widths ranging from nanoseconds to femtoseconds. Our results are discussed in the view of interplay between the material properties and the radiation dependent parameters governing the ablation process. Visible and infrared ultra-short pulsed laser ablation of the polymer samples was performed with a very low threshold ¯uence of approximately 0.2 mJ/mm2. The irradiated polymers exhibit different optical transmission properties in the corresponding spectral regions. The quantitative results on ablation rate versus laser energy ¯uence show that the picosecond laser ablation is more ef®cient than the subpicosecond and nanosecond ablation, i.e., it exhibits higher etch rates before the onset of any saturation. # 2001 Elsevier Science B.V. All rights reserved. PACS: 42.62.-b; 79.20.D Keywords: Polymer ablation; Picosecond laser ablation; Ultra-short laser ablation
1. Introduction Pulsed laser ablation is well established as a universal tool for surface processing of organic polymer materials. The dynamics of the removal process are in¯uenced by a variety of parameters originating from * Corresponding author. Tel.: 30-1-772-2934; fax: 30-1-772-2928. E-mail addresses: [email protected] (A.A. Serafetinides), [email protected] (M.I. Makropoulou), [email protected] (C.D. Skordoulis), [email protected] (A.K. Kar). 1 Tel.: 30-1-7722931; fax: 30-1-772928. 2 Tel.: 30-651-98542; fax: 30-651-4531. 3 Tel.: 44-131-3049; fax: 44-131-451-3136.
the material properties, the parameters of the laser pulse and the laser irradiation environment. The parameters of the laser radiation and the properties of the material govern the possible physical processes such as optical absorption, heat conduction, phase transitions, evaporation kinetics and plasma dynamics, which altogether result in the ®nal properties of the processed material [1]. Ef®cient photoetching of synthetic polymers (laser ablation) was reported by exposure of polymers to intense excimer laser light [2], by exposure of doped polymers to low UV laser light [3] and by using infrared nanosecond laser pulses [4±6]. Under these conditions, the polymeric material is photochemically and photothermally degraded and,
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 3 2 4 - 5
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in some cases, rather disrupted than etched by ablation, especially, in the case of shock wave generation by excimer laser pulses [7]. Compared to nanosecond and microsecond laser pulses, it was reported that the picosecond laser pulses change the physical conditions during material processing, offering advantages to the treatment of the polymer surfaces. The main advantage of using very short laser pulses (picosecond and femtosecond laser pulses) for the ablation of polymers is that the heat diffusion into the polymer material is negligible and the energy loss into the sample is minimised. As a result, high precision patterning of the sample without thermal damage of the surroundings becomes possible and the ablation threshold can be lowered [8]. Such ultra-short pulses are of potential value in areas of thin-®lm deposition, micromachining, and surgical procedures. For optically transparent materials, in the case of ultra-short pulse visible laser wavelengths, the mechanism of ultra-short pulse laser ablation is complex involving a combination of photochemically induced bond dissociation and a photothermal process [9]. In the present work, we report experimental results of the ablation rate per pulse as a function of the laser ¯uence, measurements being done with pulse duration ranging from femtoseconds to nanoseconds. These experiments include wavelengths from the ultraviolet through the visible into the infrared, while the laser radiation targets are various organic polymers such as polyetheretherketone (PEEK), polytetra¯uoroethylene (PTFE), and polycarbonate (PC). These polymers present special interest in various microelectronics and biomedical applications and the microarchitectural preparation of their surface is very important. PEEK is an engineering thermoplastic material and PTFE has several applications in microelectronic and biomedical technology due to its high thermal stability, chemical stability, low dielectric constant and low friction coef®cient. PC is used in the fabrication of advanced arti®cial organs. The relative signi®cance of the duration and the photon density of the laser pulses have been considered, and their consequences for the occurrence of various processes during the laserinduced material removal have been outlined. The surface topology of the polymers was investigated by atomic force microscopy in the continuous contact mode.
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2. Materials and methods Commercially available PTFE bulk samples of thickness 0.55 mm (DuPont), PEEK ®lms of thickness 0.10 mm (Goodfellow), and PC ®lms of thickness 0.10 mm (Goodfellow) were irradiated in air with different laser sources as follows: 1. A regenerative ampli®er system (RGA) provided laser pulses at 1064 nm wavelength (Nd:YAG laser ampli®cation). The pulse duration was 100 ps, as observed on the 7633 Tektronix main frame oscilloscope which incorporates a 7512/ TDR sampler. The laser energy, measured with the Rj 7200 energy ratiometer, varied between 0 and 70 mJ. The repetition rate was ®xed at 10 Hz. A BK7 glass lens was used to focus the laser beam. This lens had a 10 cm focal length. 2. The second harmonic from the RGA laser output provided laser pulses at 532 nm wavelength with pulse duration of 100 ps. The repetition rate was ®xed at 10 Hz. 3. A sub-picosecond tuneable dye ampli®er emitting at 595 nm wavelength with a pulse width of 800 fs, as recorded through the autocorrelator on a Tektronix 2201 digital storage oscilloscope. The dye was kiton red 620 (sulphorodamine B) in a water solvent. The pulse repetition rate was 10 Hz. 4. A XeCl excimer laser (Lumonics 500) provided laser pulses at 308 nm wavelength and 10 ns pulse duration. The laser energy, measured with the Rj 7200 energy ratiometer, varied between 0 and 4.4 mJ. The repetition rate was ®xed at 10 Hz. A BaF2 lens was used to focus the laser beam. This lens had a 10 cm focal length. 5. A dye laser (HyperDYE 300, with Cumarin 480 dye) provided laser pulses at 473 nm wavelength and 2 ns pulse duration. For the ablation experiments, either a long focal length lens (30 cm) made from BK7 or a shorter focal length lens (10 cm) made from BaF2 was used to focus the laser beam. The repetition rate was ®xed at 10 Hz. Prior to use, the transmission spectra of all samples were obtained with a Shimatzu UV-3100 spectrophotometer in the wavelength range 0.4±2.0 mm. This was a double check of the absorption characteristics
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available from the manufacturers or the literature for the polymers under investigation and it was very valuable, at least for the non-diffusing samples. Etch depths corresponding to a particular laser energy ¯uence were measured at room temperature in open air, by counting the number of pulses required to perforate the sample. To avoid ablation debris redeposition, the laser pulses were ®red horizontally onto the polymer surface, while the sample was placed in a Perspex box equipped with a fume evacuator. The ablation end point was recorded by a Rj 7200 joulemeter connected to a joulemeter monitor, placed immediately behind the sample and connected directly to a Tektronix 7623 storage oscilloscope. The size of the ablated spots, produced by the focused laser beam, was varied by changing the position of the micrometer driven sample holder. The minimum spot size determined by the burn pattern at the focus of the laser beam was 0:018 0:018 mm2 . The laser energy output was reduced by using special ®lters and the transmission ®gures of the ®lters were used to calibrate the output energy values used in the perforation experiments. The ¯uence F at each setting was calculated as the ratio of laser energy per laser spot area. For all the experiments, ¯uence measurements are within an uncertainty of 10%.
The simpli®ed experimental set-up, for both picosecond and femtosecond laser ablation experiments, has been published elsewhere [9]. The surface topology of the polymer sample craters was investigated by means of an atomic force microscope (Quesant AFM). The polymer samples were cleaned with compressed air to remove loose dust particles. The samples were traced in the continuous contact mode, with a piezoelectric ceramic at 2 Hz scan rate, soft zoom mode. For the optical alignment, the sample surface was monitored by an 80 optical microscope and the probe was directed to the sample via the automatic approach mechanism. The scan probe was directed perpendicularly into the crater walls, at zero scan angle, and any tilting was removed from the AFMs. The image mode used was Z height. This is a constant force mode between tip and surface and the true Z height features of the surface are revealed. 3. Results The experimental results are presented quantitatively (plots of polymer etch rate as a function of the applied laser energy ¯uence) in Figs. 1±5 and
Fig. 1. Etch depth versus laser energy ¯uence at 308 nm, 10 ns pulse duration, for PC, PEEK and PTFE samples.
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Fig. 2. Etch depth versus laser energy ¯uence at 473 nm, 2 ns pulse duration, for PC and PEEK samples.
Fig. 3. Etch depth versus laser energy ¯uence at 532 nm, 100 ps pulse duration, for PC, PEEK and PTFE samples.
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Fig. 4. Etch depth versus laser energy ¯uence at 1064 nm, 100 ps pulse duration, for PC, PEEK and PTFE samples.
Fig. 5. Etch depth versus laser energy ¯uence at 595 nm, 800 fs pulse duration, for PC, PEEK and PTFE polymer samples.
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Fig. 6. Optical microscopy image of PEEK sample irradiated by a dye laser at 473 nm, with a 2 ns pulse duration. Magni®cation of 100 and focusing on the top surface of the polymer reveal any effects around the edge of the crater and give only a very general impression of the crater walls.
qualitatively (optical microscopy and AFM images) in Figs. 6±13. The data points illustrated in Figs. 1±5 are mean values of a number of data collected from measurements performed during the multiple experiments.
3.1. Etch rate measurements Fig. 1 shows the etch depth versus laser energy ¯uence at 308 nm, 10 ns pulse duration and 10 Hz pulse repetition rate, for PEEK, PC and PTFE
Fig. 7. Optical microscopy image of PEEK sample irradiated by a Nd:YAG laser at 1064 nm, with a 100 ps pulse duration. Magni®cation of 40 and focusing on the bottom of the crater reveal a circular top edge and a rather symmetric bottom edge of the crater and symmetrically conical crater walls.
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Fig. 8. Contact mode AFM image of PEEK sample irradiated with 10 ns laser pulses at l 308 nm. Three-dimensional view of crater wall at zero scan angle.
samples. It is clear from Fig. 1 that the ablation rates of PEEK, PC and PTFE are similar in the ¯uence range 4±19 mJ/mm2, while the PTFE polymer demonstrates a tendency for higher ablation rates at ¯uences 20 mJ/mm2. The corresponding threshold ¯uence is 1.3 mJ/mm2 for PEEK, 0.55 mJ/mm2 for PC and 3.5 mJ/mm2 for PTFE. In a previous work [6], we performed XeCl laser ablation of PTFE with 17 ns laser pulse duration and approximately 1 Hz pulse repetition rate and the corresponding threshold ¯uence was equal to 11 mJ/mm2. It seems, therefore, that a decrease in the pulse repetition rate results in an increase in the ¯uence threshold, which is in good agreement with a photothermally driven ablation process in the case of polymer XeCl laser ablation [6,10]. Fig. 2 shows the etch depth versus laser energy ¯uence at 473 nm, 2 ns pulse duration, for the PC and the PEEK polymers. The corresponding threshold
¯uence is 0.35 mJ/mm2 for both PEEK and PC polymer samples. For PTFE polymer samples, perforation (the ablation end point) was not observed after 5±10 min or 3000±6000 pulses and ¯uences ranging between 1 and 4 mJ/mm2. Fig. 3 shows the etch depth versus laser energy ¯uence at 532 nm, 100 ps pulse duration, for PC, PEEK and the PTFE polymer samples. PEEK demonstrates the lower ablation rates than PC and PTFE for the same energy ¯uence range with a strong saturation behaviour. The ablation rate increases rapidly above the threshold ¯uence and reaches the 80% saturation point at 10 mJ/mm2, while the corresponding ablation rate is 5.3 mm/pulse. The polymer PTFE at the same laser settings shows higher ablation rates than those of PEEK and PC as well as a strong saturation effect. The ablation rate increases very rapidly above the threshold ¯uence and reaches the 80% saturation point at 12.0 mJ/mm2, while the corresponding
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Fig. 9. Contact mode AFM image of PTFE sample irradiated with 10 ns laser pulses at l 308 nm. Three-dimensional view of crater wall at zero scan angle.
ablation rate is 17.5 mm/pulse. The corresponding threshold ¯uence is 0.03 mJ/mm2 for PEEK, 0.14 mJ/ mm2 for PC and 0.18 mJ/mm2, respectively, for PTFE. The threshold ¯uence for PTFE is approximately 20 times lower than the respective value obtained previously with the second harmonic Nd:YAG laser l 532 nm ablation of PTFE at 6 ns pulse duration and 10 Hz pulse repetition rate [11]. Fig. 4 shows the etch depth versus laser energy ¯uence at 1064 nm, 100 ps pulse duration, for PC, PEEK and the PTFE polymer samples. It is clear from Fig. 4 that the polymer PEEK has lower ablation rates than PC and PTFE for the same energy ¯uence range with a tendency for saturation. The ablation rate reaches the 80% saturation point at 10 mJ/mm2, while the corresponding ablation rate is 4.0 mm/ pulse. The threshold ¯uence is approximately 0.05 mJ/mm2 for both PEEK and PC polymers. The polymer PTFE at the same laser settings shows higher ablation rates and a strong saturation effect, while the
threshold ¯uence is lower than 0.015 mJ/mm2. The ablation rate increases very rapidly above the threshold ¯uence and reaches the higher value at 20.0 mJ/ mm2, while for higher ¯uences, the corresponding ablation rates are decreasing in a nonlinear mode. The threshold ¯uence estimated in the present work, for Nd:YAG picosecond laser ablation of PTFE polymer, is two or three orders of magnitude lower than the value of 6.0 J/cm2 reported in a previous work for the PTFE laser ablation with a Q-switched Nd:YAG laser of 25 ns pulse duration [6]. Fig. 5 shows the etch depth versus laser energy ¯uence at 595 nm, 800 fs pulse duration, for PEEK, PC and the PTFE polymer samples. The ablation rate of the PTFE polymer sample demonstrates a nonlinear behaviour. At relatively high laser ¯uences (12±20 mJ/ mm2), the etch rate measured was higher than those of PEEK and PC, but at ¯uences ranged between 1 and 3 mJ/mm2, no perforation was observed for more than 1800 pulses.
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Fig. 10. Contact mode AFM image of PEEK sample irradiated with 800 fs laser pulses at l 595 nm. Three-dimensional view of crater wall at zero scan angle.
3.2. Surface topology In addition to the etch rate measurements, the crater surface pro®le was investigated by optical microscopy and AFM and thus the physical changes, related to multiple-pulse irradiation of the polymers, were further evaluated. Although optical microscopy pictures and AFM images of the polymers were obtained for all the laser wavelengths and pulse duration used in this work, we have selected and present here only some of them. As far as the more conventional optical micrographs is concerned, the images of PEEK polymer irradiated by a dye laser l 473 nm and a Nd:YAG laser l 1064 nm are shown in Figs. 6 and 7, respectively. In addition, AFM images from two representative polymers in three laser conditions are illustrated in Figs. 8±13. An optically transparent polymer (PEEK) and an opaque polymer (PTFE) was irradiated
with a UV l 308 nm laser of nanosecond pulse duration, a visible l 595 nm laser of femtosecond pulse duration and an infrared l 1064 nm laser of picosecond pulse duration. The representative AFM images, for the relevant irradiation conditions, are indicated in detail in the ®gure captions. In all AFM images, obtained from the inner crater lateral surface of the polymer samples, changes in brightness indicate height differences, i.e., the brightest regions have the maximum height and the darkest regions have the lowest. As the AFM can accurately measure in three dimensions with sub-micrometer scale resolution, the AFM images in this work represent polymer areas as large as a few micrometer square, smaller than the laser spot area. Therefore, it is very dif®cult to conclude if the hump formations in the PTFE polymer, irradiated at 1064 nm (Fig. 13), are due to lower energy laser pulse regions in the periphery of the near-Gaussian spot
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Fig. 11. Contact mode AFM image of PTFE sample irradiated with 800 fs laser pulses at l 595 nm. Three-dimensional view of crater wall at zero scan angle.
pro®le, resulting in a thermally induced volume increase, or can be related to a non-thermal effect. For example, humps or volume increase can be the result of either amorphisation of crystalline domains, or fragmentation of polymer chains, as Himmelbauer et al. [12] reported for polyimide ®lm ablated by long UV laser pulses. Hump formations were also observed previously in Nylon-6,6 and PMMA polymer samples, irradiated at 1064 nm [9]. The AFM image of PEEK irradiated with 10 ns laser pulses at 308 nm (Fig. 8) reveals an approximately perfect smooth lateral crater surface, except a rippled hump in the left lower part of the image, probably corresponding to the etched polymer surface at the border of the laser spot, since the height of the hump is 3.5 mm, very close to the etch depth ablated by 3±4 pulses. The same aspect, as far as the smoothness of the crater surface is concerned, but with a smooth surface elevation is observed for PEEK irradiated with
100 ps laser pulses at 1064 nm (Fig. 12). In contrast, the AFM image of PEEK, irradiated with 800 fs laser pulses at 595 nm, shows a ripple structured crater surface with a sub-micron periodicity and a weak hump formation (Fig. 10). This ripple morphology could not be considered as a result of coherent light interference patterns, because of the size, compared to the laser wavelength. Most likely, the parallel ripples observed are the result of the relaxation of unidirectional laser induced stresses on the polymer surface. KruÈger and Kautek [13] reported a similar mechanism for the ripple formations, observed by scanning electron microscopy (SEM Ð a more conclusive technique when stress release and interference phenomena are of prime importance) on fused silica, irradiated with a femtosecond pulse visible laser. The AFM image of PTFE, irradiated with 10 ns laser pulses at 308 nm, reveals a ripple structured crater surface with a sub-micron periodicity (Fig. 9). The ripple aspect of
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Fig. 12. Contact mode AFM image of PEEK sample irradiated with 100 ps laser pulses at l 1064 nm. Three-dimensional view of crater wall at zero scan angle.
the crater wall is probably due to an ablation pattern involving cone formation at the bottom of the bore oriented into the direction of the incoming beam, as reported in [14]. PTFE sample, irradiated with 800 fs laser pulses at 595 nm, shows also a ripple structured crater surface with a sub-micron periodicity and a weak hump formation (Fig. 11). AFM images are a valuable tool for quantifying the polymer surface microstructure and for predicting their in¯uence on any light scatter losses that in¯uence the laser ¯uence deposition of subsequent pulses. However, the AFM images offer the ®ne evaluation of the ablated holes, while the more conventional optical microscopy or SEM images show the general shape and the edge quality of the ablated crater, giving the overall and thus very important information for an application oriented study. In addition, the detailed understanding of the imaging of polymer materials by using a very sophisticated technique, such as AFM,
requires a more careful investigation of the imaging process in relation with the physicochemical characteristics of the sample and precise removal of any image artefacts produced by the method. 4. Discussion Plots of the etch rate (depth of the material ablated per pulse) as a function of the incident laser ¯uence (laser energy per spot area), in a regime in which Beer's law is applicable, are useful in determining the threshold ¯uence for signi®cant ablation and the effective absorption coef®cient which characterise the process, since not all polymers exhibit the same ablation characteristics with a given set of irradiance conditions and a given polymer can exhibit a variety of ablation characteristics depending on the irradiance parameters applied.
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Fig. 13. Contact mode AFM image of PTFE sample irradiated with 100 ps laser pulses at l 1064 nm. Three-dimensional view of crater wall at zero scan angle.
Regarding the threshold ¯uence for the onset of ablation, Figs. 3±5 demonstrate that the threshold ¯uence is very low, <0.2 mJ/mm2, for visible and infrared ultra-short pulsed laser ablation of PEEK, PC and PTFE polymer samples. We used multiple pulses to determine threshold values for the damage while, according to [15], incubation effects may serve to reduce this value. For minimising this effect, we irradiated relatively thick samples, as some preliminary measurements with bulk polymers showed a strong dependence of the incubation effect on the sample thickness. It seems that the incubation pulses produce new chromophores within the polymer by photochemical modi®cations, resulting in an increase of the absorption coef®cient, while saturation effects may also in¯uence the absorption depth. Further uncertainties in the energy deposition arise from the plasma developing above the irradiated area. Due to its strong absorption, it may shield the surface from
further irradiation. The high-density ablation induced plasma behaves as a mirror and, therefore, changes the surface re¯ectivity [16]. Plasma formation was also observed macroscopically, during the experimental procedure, in case of hard dental tissue ablation with similar laser parameters, giving evidence for a ``plasma mediated ablation'' mechanism [17]. As shown in Figs. 1±5, the ablation rates at low ¯uence levels (near the Fth) for the same irradiation conditions are very similar for the polymers under investigation. Another interesting parameter, in¯uencing the overall ablation effect, is the relationship between the ¯uence threshold and the pulse duration. It has been reported that, for the same irradiation wavelength, the ¯uence threshold for signi®cant damage is tp 1=2 dependent (where tp is the pulse width) [18], while a deviation from the tp 1=2 scaling of threshold ¯uences, for pulses shorter than 20 ps, was also reported by the
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Fig. 14. The photon density for threshold damage versus pulse duration for the polymers PEEK and PC.
same group [18], for fused silica and calcium ¯uoride irradiated at 1053 and 526 nm, with pulses ranging from 270 fs to 1 ns. In this work, we used various laser wavelengths and pulse widths for polymer ablation. As the comparison of different pulse duration at different laser wavelengths could lead to limited value results, it is more appropriate to introduce the photon density parameter for threshold damage and therefore increase the value of the comparative studies. The photon density for threshold damage, F, is de®ned as F Fth =hn, where hn is the photon energy. Fig. 14 illustrates the relationship between the photon density for threshold damage and the pulse duration for two polymer samples and namely an optically transparent polymer (PEEK) and a less transparent one (PC). As shown in Fig. 14, the dependence for PEEK is approximately linear and for PC is of a second order. This is most probably attributed to the different optical properties of the two materials. PEEK samples of 0.10 mm thickness transmit 80± 90% of the incident light in the visible and nearinfrared region of the spectrum, while PC samples of the same thickness transmit only 10%. PTFE samples of higher thickness d 0:55 mm transmit even smaller percentage of light, but as this is due to
the intense light diffusion, hiding the relatively poor absorption of the material, it is more dif®cult to reach any coherent conclusion relating the PTFE polymer absorption and the photon density for threshold damage. Nevertheless, the coupling mechanism of the ultrashort laser pulses to the polymer samples is very complex and only the relative contribution of their optical properties in the overall ablation mechanism can be considered. When the irradiance is very high such as that produced by the ultra-short pulsed lasers, the absorption of laser radiation becomes a nonlinear process, involving a multiphoton absorption caused by a high photon density rather than simple resonant effects, i.e., the absorption due to the coincidence of the laser radiation wavelength with characteristic absorption peak of the target. Our quantitative results on the ablation rate versus laser energy ¯uence show that the picosecond laser ablation is more ef®cient than the sub-picosecond and nanosecond ablation, i.e., it exhibits higher etch rates before the onset of any saturation. Similar results, as far as the ef®ciency of picosecond and sub-picosecond laser ablation is concerned, were reported in a previous work with optically transparent polymers
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(Nylon and PMMA) [9]. As far as the observed differences in PTFE is concerned, it seems that this polymer exhibits a non-predictable ablational behaviour. PTFE ®lm has been known as one of the most dif®cult materials for laser ablation because it consists of C±F and C±C bonds, whose binding energies are relatively high, resulting in an absorption edge near 160 nm [19]. For this reason, contradictory results on PTFE laser ablation are published. For example, Davis et al. [20] have reported an unsuccessful attempt to ablate PTFE thin ®lms with a 25 ns XeCl laser, although they have used energy ¯uences as high as 120 mJ/mm2, until a small quantity of polyimide was incorporated. Other researchers [21] reported that PTFE ablation with pulsed excimer radiation has been met only by using 300 fs long pulses at a ¯uence of 1 J/cm2. In a previous work [6], we reported ablation rates of PTFE, irradiated with XeCl laser pulses of 25 ns pulse duration, with threshold ¯uence equal to 1.1 J/cm2 and we assumed that PTFE ultra-violet laser ablation involves the simultaneous absorption of two photons by the material chromophores. Kumagai et al. [19] reported that high intensity IR (at 798 nm) femtosecond laser pulses give rise to simultaneous absorption of ®ve photons for PTFE. 5. Summary and conclusions In this paper, we have reported experimental data of the ablation rate per pulse as a function of the laser ¯uence, and the phenomenological evidence of the surface morphology for a number of organic polymers, selected on the basis of their technological signi®cance, as well as on the basis of their different optical properties. Pulsed laser ablation measurements have been performed from the femtosecond to the nanosecond time scale and at wavelengths ranging from the ultraviolet through the visible into the infrared. We have also investigated the relative signi®cance of the duration and the photon density of the laser pulse on the occurrence of the various photochemical and photothermal processes during the laser-induced material removal. An interesting conclusion of this work is the dependence of the threshold ¯uence, required for ablation, primarily on the pulse duration and not on the external
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optical transmission of the material, since the use of shorter pulses lowers the ablation threshold by nearly two orders of magnitude. It has been demonstrated that the threshold ¯uence is of the same very low value (<0.2 mJ/mm2) for visible and infrared ultra-short pulsed laser ablation of PEEK, PC and PTFE polymer samples, while these three polymers exhibit different optical transmission properties in the corresponding spectral regions, derived from the transmission spectra of the samples. The relationship between the photon density for ablation threshold and the pulse duration seems to be well correlated to the individual polymer optical properties, as for the optically transparent polymer PEEK, the dependence is approximately linear and for the less transparent PC is of a second order. Finally, our quantitative results on ablation rate versus laser energy ¯uence, in agreement with similar results of other researchers, show that picosecond laser ablation is more ef®cient than the sub-picosecond and nanosecond ablation, i.e., it exhibits higher etch rates before the onset of any saturation. It has to be noted, however, that a complete investigation of the phenomena involved cannot be based on pure experimental evidence solely. The ®nal conclusion for this and any other research can only be reached after the comparison of the experimental data with the appropriate theoretical model which will be the subject of our future work. Acknowledgements The authors would like to thank the members of the Non Linear Optics Group of the Heriot-Watt University for their help in carrying out the experiments. References [1] J. Jandeleit, G. Urbasch, H.D. Hoffmann, H.-G. Treusch, E.W. Kreutz, Appl. Phys. 63 (1996) 117. [2] R. Srinivasan, V. Mayne-Banton, Appl. Phys. Lett. 41 (1982) 576. [3] C.E. Kosmidis, C.D. Skordoulis, Appl. Phys. 56 (1993) 64. [4] C.D. Skordoulis, M.I. Makropoulou, A.A. Serafetinides, Opt. Laser Technol. 27 (1995) 185. [5] C.D. Skordoulis, M.I. Makropoulou, A.A. Serafetinides, Appl. Surf. Sci. 86 (1995) 239. [6] M.I. Makropoulou, A.A. Serafetinides, C.D. Skordoulis, Lasers Med. Sci. 10 (1995) 201.
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Ultra-short pulsed laser ablation of polymers A.A. Serafetinidesa,1, M.I. Makropouloua,*, C.D. Skordoulisb,2, A.K. Karc,3 a
Physics Department, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece b Atomic and Molecular Physics Laboratory, Physics Department, University of Ioannina, Greece c Physics Department, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK Received 15 November 2000; accepted 12 April 2001
Abstract We report the experimental results of the ablation rate per pulse as a function of the laser ¯uence and images of the surface morphology, as examined by atomic force microscopy, for a number of organic polymer materials of special interest in microelectronics and biomedical applications. The ablation parameters and the surface modi®cations are examined under various irradiation conditions using laser wavelengths ranging from the ultraviolet through the visible to the infrared and pulse widths ranging from nanoseconds to femtoseconds. Our results are discussed in the view of interplay between the material properties and the radiation dependent parameters governing the ablation process. Visible and infrared ultra-short pulsed laser ablation of the polymer samples was performed with a very low threshold ¯uence of approximately 0.2 mJ/mm2. The irradiated polymers exhibit different optical transmission properties in the corresponding spectral regions. The quantitative results on ablation rate versus laser energy ¯uence show that the picosecond laser ablation is more ef®cient than the subpicosecond and nanosecond ablation, i.e., it exhibits higher etch rates before the onset of any saturation. # 2001 Elsevier Science B.V. All rights reserved. PACS: 42.62.-b; 79.20.D Keywords: Polymer ablation; Picosecond laser ablation; Ultra-short laser ablation
1. Introduction Pulsed laser ablation is well established as a universal tool for surface processing of organic polymer materials. The dynamics of the removal process are in¯uenced by a variety of parameters originating from * Corresponding author. Tel.: 30-1-772-2934; fax: 30-1-772-2928. E-mail addresses: [email protected] (A.A. Serafetinides), [email protected] (M.I. Makropoulou), [email protected] (C.D. Skordoulis), [email protected] (A.K. Kar). 1 Tel.: 30-1-7722931; fax: 30-1-772928. 2 Tel.: 30-651-98542; fax: 30-651-4531. 3 Tel.: 44-131-3049; fax: 44-131-451-3136.
the material properties, the parameters of the laser pulse and the laser irradiation environment. The parameters of the laser radiation and the properties of the material govern the possible physical processes such as optical absorption, heat conduction, phase transitions, evaporation kinetics and plasma dynamics, which altogether result in the ®nal properties of the processed material [1]. Ef®cient photoetching of synthetic polymers (laser ablation) was reported by exposure of polymers to intense excimer laser light [2], by exposure of doped polymers to low UV laser light [3] and by using infrared nanosecond laser pulses [4±6]. Under these conditions, the polymeric material is photochemically and photothermally degraded and,
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 3 2 4 - 5
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in some cases, rather disrupted than etched by ablation, especially, in the case of shock wave generation by excimer laser pulses [7]. Compared to nanosecond and microsecond laser pulses, it was reported that the picosecond laser pulses change the physical conditions during material processing, offering advantages to the treatment of the polymer surfaces. The main advantage of using very short laser pulses (picosecond and femtosecond laser pulses) for the ablation of polymers is that the heat diffusion into the polymer material is negligible and the energy loss into the sample is minimised. As a result, high precision patterning of the sample without thermal damage of the surroundings becomes possible and the ablation threshold can be lowered [8]. Such ultra-short pulses are of potential value in areas of thin-®lm deposition, micromachining, and surgical procedures. For optically transparent materials, in the case of ultra-short pulse visible laser wavelengths, the mechanism of ultra-short pulse laser ablation is complex involving a combination of photochemically induced bond dissociation and a photothermal process [9]. In the present work, we report experimental results of the ablation rate per pulse as a function of the laser ¯uence, measurements being done with pulse duration ranging from femtoseconds to nanoseconds. These experiments include wavelengths from the ultraviolet through the visible into the infrared, while the laser radiation targets are various organic polymers such as polyetheretherketone (PEEK), polytetra¯uoroethylene (PTFE), and polycarbonate (PC). These polymers present special interest in various microelectronics and biomedical applications and the microarchitectural preparation of their surface is very important. PEEK is an engineering thermoplastic material and PTFE has several applications in microelectronic and biomedical technology due to its high thermal stability, chemical stability, low dielectric constant and low friction coef®cient. PC is used in the fabrication of advanced arti®cial organs. The relative signi®cance of the duration and the photon density of the laser pulses have been considered, and their consequences for the occurrence of various processes during the laserinduced material removal have been outlined. The surface topology of the polymers was investigated by atomic force microscopy in the continuous contact mode.
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2. Materials and methods Commercially available PTFE bulk samples of thickness 0.55 mm (DuPont), PEEK ®lms of thickness 0.10 mm (Goodfellow), and PC ®lms of thickness 0.10 mm (Goodfellow) were irradiated in air with different laser sources as follows: 1. A regenerative ampli®er system (RGA) provided laser pulses at 1064 nm wavelength (Nd:YAG laser ampli®cation). The pulse duration was 100 ps, as observed on the 7633 Tektronix main frame oscilloscope which incorporates a 7512/ TDR sampler. The laser energy, measured with the Rj 7200 energy ratiometer, varied between 0 and 70 mJ. The repetition rate was ®xed at 10 Hz. A BK7 glass lens was used to focus the laser beam. This lens had a 10 cm focal length. 2. The second harmonic from the RGA laser output provided laser pulses at 532 nm wavelength with pulse duration of 100 ps. The repetition rate was ®xed at 10 Hz. 3. A sub-picosecond tuneable dye ampli®er emitting at 595 nm wavelength with a pulse width of 800 fs, as recorded through the autocorrelator on a Tektronix 2201 digital storage oscilloscope. The dye was kiton red 620 (sulphorodamine B) in a water solvent. The pulse repetition rate was 10 Hz. 4. A XeCl excimer laser (Lumonics 500) provided laser pulses at 308 nm wavelength and 10 ns pulse duration. The laser energy, measured with the Rj 7200 energy ratiometer, varied between 0 and 4.4 mJ. The repetition rate was ®xed at 10 Hz. A BaF2 lens was used to focus the laser beam. This lens had a 10 cm focal length. 5. A dye laser (HyperDYE 300, with Cumarin 480 dye) provided laser pulses at 473 nm wavelength and 2 ns pulse duration. For the ablation experiments, either a long focal length lens (30 cm) made from BK7 or a shorter focal length lens (10 cm) made from BaF2 was used to focus the laser beam. The repetition rate was ®xed at 10 Hz. Prior to use, the transmission spectra of all samples were obtained with a Shimatzu UV-3100 spectrophotometer in the wavelength range 0.4±2.0 mm. This was a double check of the absorption characteristics
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available from the manufacturers or the literature for the polymers under investigation and it was very valuable, at least for the non-diffusing samples. Etch depths corresponding to a particular laser energy ¯uence were measured at room temperature in open air, by counting the number of pulses required to perforate the sample. To avoid ablation debris redeposition, the laser pulses were ®red horizontally onto the polymer surface, while the sample was placed in a Perspex box equipped with a fume evacuator. The ablation end point was recorded by a Rj 7200 joulemeter connected to a joulemeter monitor, placed immediately behind the sample and connected directly to a Tektronix 7623 storage oscilloscope. The size of the ablated spots, produced by the focused laser beam, was varied by changing the position of the micrometer driven sample holder. The minimum spot size determined by the burn pattern at the focus of the laser beam was 0:018 0:018 mm2 . The laser energy output was reduced by using special ®lters and the transmission ®gures of the ®lters were used to calibrate the output energy values used in the perforation experiments. The ¯uence F at each setting was calculated as the ratio of laser energy per laser spot area. For all the experiments, ¯uence measurements are within an uncertainty of 10%.
The simpli®ed experimental set-up, for both picosecond and femtosecond laser ablation experiments, has been published elsewhere [9]. The surface topology of the polymer sample craters was investigated by means of an atomic force microscope (Quesant AFM). The polymer samples were cleaned with compressed air to remove loose dust particles. The samples were traced in the continuous contact mode, with a piezoelectric ceramic at 2 Hz scan rate, soft zoom mode. For the optical alignment, the sample surface was monitored by an 80 optical microscope and the probe was directed to the sample via the automatic approach mechanism. The scan probe was directed perpendicularly into the crater walls, at zero scan angle, and any tilting was removed from the AFMs. The image mode used was Z height. This is a constant force mode between tip and surface and the true Z height features of the surface are revealed. 3. Results The experimental results are presented quantitatively (plots of polymer etch rate as a function of the applied laser energy ¯uence) in Figs. 1±5 and
Fig. 1. Etch depth versus laser energy ¯uence at 308 nm, 10 ns pulse duration, for PC, PEEK and PTFE samples.
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Fig. 2. Etch depth versus laser energy ¯uence at 473 nm, 2 ns pulse duration, for PC and PEEK samples.
Fig. 3. Etch depth versus laser energy ¯uence at 532 nm, 100 ps pulse duration, for PC, PEEK and PTFE samples.
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Fig. 4. Etch depth versus laser energy ¯uence at 1064 nm, 100 ps pulse duration, for PC, PEEK and PTFE samples.
Fig. 5. Etch depth versus laser energy ¯uence at 595 nm, 800 fs pulse duration, for PC, PEEK and PTFE polymer samples.
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Fig. 6. Optical microscopy image of PEEK sample irradiated by a dye laser at 473 nm, with a 2 ns pulse duration. Magni®cation of 100 and focusing on the top surface of the polymer reveal any effects around the edge of the crater and give only a very general impression of the crater walls.
qualitatively (optical microscopy and AFM images) in Figs. 6±13. The data points illustrated in Figs. 1±5 are mean values of a number of data collected from measurements performed during the multiple experiments.
3.1. Etch rate measurements Fig. 1 shows the etch depth versus laser energy ¯uence at 308 nm, 10 ns pulse duration and 10 Hz pulse repetition rate, for PEEK, PC and PTFE
Fig. 7. Optical microscopy image of PEEK sample irradiated by a Nd:YAG laser at 1064 nm, with a 100 ps pulse duration. Magni®cation of 40 and focusing on the bottom of the crater reveal a circular top edge and a rather symmetric bottom edge of the crater and symmetrically conical crater walls.
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Fig. 8. Contact mode AFM image of PEEK sample irradiated with 10 ns laser pulses at l 308 nm. Three-dimensional view of crater wall at zero scan angle.
samples. It is clear from Fig. 1 that the ablation rates of PEEK, PC and PTFE are similar in the ¯uence range 4±19 mJ/mm2, while the PTFE polymer demonstrates a tendency for higher ablation rates at ¯uences 20 mJ/mm2. The corresponding threshold ¯uence is 1.3 mJ/mm2 for PEEK, 0.55 mJ/mm2 for PC and 3.5 mJ/mm2 for PTFE. In a previous work [6], we performed XeCl laser ablation of PTFE with 17 ns laser pulse duration and approximately 1 Hz pulse repetition rate and the corresponding threshold ¯uence was equal to 11 mJ/mm2. It seems, therefore, that a decrease in the pulse repetition rate results in an increase in the ¯uence threshold, which is in good agreement with a photothermally driven ablation process in the case of polymer XeCl laser ablation [6,10]. Fig. 2 shows the etch depth versus laser energy ¯uence at 473 nm, 2 ns pulse duration, for the PC and the PEEK polymers. The corresponding threshold
¯uence is 0.35 mJ/mm2 for both PEEK and PC polymer samples. For PTFE polymer samples, perforation (the ablation end point) was not observed after 5±10 min or 3000±6000 pulses and ¯uences ranging between 1 and 4 mJ/mm2. Fig. 3 shows the etch depth versus laser energy ¯uence at 532 nm, 100 ps pulse duration, for PC, PEEK and the PTFE polymer samples. PEEK demonstrates the lower ablation rates than PC and PTFE for the same energy ¯uence range with a strong saturation behaviour. The ablation rate increases rapidly above the threshold ¯uence and reaches the 80% saturation point at 10 mJ/mm2, while the corresponding ablation rate is 5.3 mm/pulse. The polymer PTFE at the same laser settings shows higher ablation rates than those of PEEK and PC as well as a strong saturation effect. The ablation rate increases very rapidly above the threshold ¯uence and reaches the 80% saturation point at 12.0 mJ/mm2, while the corresponding
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Fig. 9. Contact mode AFM image of PTFE sample irradiated with 10 ns laser pulses at l 308 nm. Three-dimensional view of crater wall at zero scan angle.
ablation rate is 17.5 mm/pulse. The corresponding threshold ¯uence is 0.03 mJ/mm2 for PEEK, 0.14 mJ/ mm2 for PC and 0.18 mJ/mm2, respectively, for PTFE. The threshold ¯uence for PTFE is approximately 20 times lower than the respective value obtained previously with the second harmonic Nd:YAG laser l 532 nm ablation of PTFE at 6 ns pulse duration and 10 Hz pulse repetition rate [11]. Fig. 4 shows the etch depth versus laser energy ¯uence at 1064 nm, 100 ps pulse duration, for PC, PEEK and the PTFE polymer samples. It is clear from Fig. 4 that the polymer PEEK has lower ablation rates than PC and PTFE for the same energy ¯uence range with a tendency for saturation. The ablation rate reaches the 80% saturation point at 10 mJ/mm2, while the corresponding ablation rate is 4.0 mm/ pulse. The threshold ¯uence is approximately 0.05 mJ/mm2 for both PEEK and PC polymers. The polymer PTFE at the same laser settings shows higher ablation rates and a strong saturation effect, while the
threshold ¯uence is lower than 0.015 mJ/mm2. The ablation rate increases very rapidly above the threshold ¯uence and reaches the higher value at 20.0 mJ/ mm2, while for higher ¯uences, the corresponding ablation rates are decreasing in a nonlinear mode. The threshold ¯uence estimated in the present work, for Nd:YAG picosecond laser ablation of PTFE polymer, is two or three orders of magnitude lower than the value of 6.0 J/cm2 reported in a previous work for the PTFE laser ablation with a Q-switched Nd:YAG laser of 25 ns pulse duration [6]. Fig. 5 shows the etch depth versus laser energy ¯uence at 595 nm, 800 fs pulse duration, for PEEK, PC and the PTFE polymer samples. The ablation rate of the PTFE polymer sample demonstrates a nonlinear behaviour. At relatively high laser ¯uences (12±20 mJ/ mm2), the etch rate measured was higher than those of PEEK and PC, but at ¯uences ranged between 1 and 3 mJ/mm2, no perforation was observed for more than 1800 pulses.
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Fig. 10. Contact mode AFM image of PEEK sample irradiated with 800 fs laser pulses at l 595 nm. Three-dimensional view of crater wall at zero scan angle.
3.2. Surface topology In addition to the etch rate measurements, the crater surface pro®le was investigated by optical microscopy and AFM and thus the physical changes, related to multiple-pulse irradiation of the polymers, were further evaluated. Although optical microscopy pictures and AFM images of the polymers were obtained for all the laser wavelengths and pulse duration used in this work, we have selected and present here only some of them. As far as the more conventional optical micrographs is concerned, the images of PEEK polymer irradiated by a dye laser l 473 nm and a Nd:YAG laser l 1064 nm are shown in Figs. 6 and 7, respectively. In addition, AFM images from two representative polymers in three laser conditions are illustrated in Figs. 8±13. An optically transparent polymer (PEEK) and an opaque polymer (PTFE) was irradiated
with a UV l 308 nm laser of nanosecond pulse duration, a visible l 595 nm laser of femtosecond pulse duration and an infrared l 1064 nm laser of picosecond pulse duration. The representative AFM images, for the relevant irradiation conditions, are indicated in detail in the ®gure captions. In all AFM images, obtained from the inner crater lateral surface of the polymer samples, changes in brightness indicate height differences, i.e., the brightest regions have the maximum height and the darkest regions have the lowest. As the AFM can accurately measure in three dimensions with sub-micrometer scale resolution, the AFM images in this work represent polymer areas as large as a few micrometer square, smaller than the laser spot area. Therefore, it is very dif®cult to conclude if the hump formations in the PTFE polymer, irradiated at 1064 nm (Fig. 13), are due to lower energy laser pulse regions in the periphery of the near-Gaussian spot
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Fig. 11. Contact mode AFM image of PTFE sample irradiated with 800 fs laser pulses at l 595 nm. Three-dimensional view of crater wall at zero scan angle.
pro®le, resulting in a thermally induced volume increase, or can be related to a non-thermal effect. For example, humps or volume increase can be the result of either amorphisation of crystalline domains, or fragmentation of polymer chains, as Himmelbauer et al. [12] reported for polyimide ®lm ablated by long UV laser pulses. Hump formations were also observed previously in Nylon-6,6 and PMMA polymer samples, irradiated at 1064 nm [9]. The AFM image of PEEK irradiated with 10 ns laser pulses at 308 nm (Fig. 8) reveals an approximately perfect smooth lateral crater surface, except a rippled hump in the left lower part of the image, probably corresponding to the etched polymer surface at the border of the laser spot, since the height of the hump is 3.5 mm, very close to the etch depth ablated by 3±4 pulses. The same aspect, as far as the smoothness of the crater surface is concerned, but with a smooth surface elevation is observed for PEEK irradiated with
100 ps laser pulses at 1064 nm (Fig. 12). In contrast, the AFM image of PEEK, irradiated with 800 fs laser pulses at 595 nm, shows a ripple structured crater surface with a sub-micron periodicity and a weak hump formation (Fig. 10). This ripple morphology could not be considered as a result of coherent light interference patterns, because of the size, compared to the laser wavelength. Most likely, the parallel ripples observed are the result of the relaxation of unidirectional laser induced stresses on the polymer surface. KruÈger and Kautek [13] reported a similar mechanism for the ripple formations, observed by scanning electron microscopy (SEM Ð a more conclusive technique when stress release and interference phenomena are of prime importance) on fused silica, irradiated with a femtosecond pulse visible laser. The AFM image of PTFE, irradiated with 10 ns laser pulses at 308 nm, reveals a ripple structured crater surface with a sub-micron periodicity (Fig. 9). The ripple aspect of
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Fig. 12. Contact mode AFM image of PEEK sample irradiated with 100 ps laser pulses at l 1064 nm. Three-dimensional view of crater wall at zero scan angle.
the crater wall is probably due to an ablation pattern involving cone formation at the bottom of the bore oriented into the direction of the incoming beam, as reported in [14]. PTFE sample, irradiated with 800 fs laser pulses at 595 nm, shows also a ripple structured crater surface with a sub-micron periodicity and a weak hump formation (Fig. 11). AFM images are a valuable tool for quantifying the polymer surface microstructure and for predicting their in¯uence on any light scatter losses that in¯uence the laser ¯uence deposition of subsequent pulses. However, the AFM images offer the ®ne evaluation of the ablated holes, while the more conventional optical microscopy or SEM images show the general shape and the edge quality of the ablated crater, giving the overall and thus very important information for an application oriented study. In addition, the detailed understanding of the imaging of polymer materials by using a very sophisticated technique, such as AFM,
requires a more careful investigation of the imaging process in relation with the physicochemical characteristics of the sample and precise removal of any image artefacts produced by the method. 4. Discussion Plots of the etch rate (depth of the material ablated per pulse) as a function of the incident laser ¯uence (laser energy per spot area), in a regime in which Beer's law is applicable, are useful in determining the threshold ¯uence for signi®cant ablation and the effective absorption coef®cient which characterise the process, since not all polymers exhibit the same ablation characteristics with a given set of irradiance conditions and a given polymer can exhibit a variety of ablation characteristics depending on the irradiance parameters applied.
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Fig. 13. Contact mode AFM image of PTFE sample irradiated with 100 ps laser pulses at l 1064 nm. Three-dimensional view of crater wall at zero scan angle.
Regarding the threshold ¯uence for the onset of ablation, Figs. 3±5 demonstrate that the threshold ¯uence is very low, <0.2 mJ/mm2, for visible and infrared ultra-short pulsed laser ablation of PEEK, PC and PTFE polymer samples. We used multiple pulses to determine threshold values for the damage while, according to [15], incubation effects may serve to reduce this value. For minimising this effect, we irradiated relatively thick samples, as some preliminary measurements with bulk polymers showed a strong dependence of the incubation effect on the sample thickness. It seems that the incubation pulses produce new chromophores within the polymer by photochemical modi®cations, resulting in an increase of the absorption coef®cient, while saturation effects may also in¯uence the absorption depth. Further uncertainties in the energy deposition arise from the plasma developing above the irradiated area. Due to its strong absorption, it may shield the surface from
further irradiation. The high-density ablation induced plasma behaves as a mirror and, therefore, changes the surface re¯ectivity [16]. Plasma formation was also observed macroscopically, during the experimental procedure, in case of hard dental tissue ablation with similar laser parameters, giving evidence for a ``plasma mediated ablation'' mechanism [17]. As shown in Figs. 1±5, the ablation rates at low ¯uence levels (near the Fth) for the same irradiation conditions are very similar for the polymers under investigation. Another interesting parameter, in¯uencing the overall ablation effect, is the relationship between the ¯uence threshold and the pulse duration. It has been reported that, for the same irradiation wavelength, the ¯uence threshold for signi®cant damage is tp 1=2 dependent (where tp is the pulse width) [18], while a deviation from the tp 1=2 scaling of threshold ¯uences, for pulses shorter than 20 ps, was also reported by the
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Fig. 14. The photon density for threshold damage versus pulse duration for the polymers PEEK and PC.
same group [18], for fused silica and calcium ¯uoride irradiated at 1053 and 526 nm, with pulses ranging from 270 fs to 1 ns. In this work, we used various laser wavelengths and pulse widths for polymer ablation. As the comparison of different pulse duration at different laser wavelengths could lead to limited value results, it is more appropriate to introduce the photon density parameter for threshold damage and therefore increase the value of the comparative studies. The photon density for threshold damage, F, is de®ned as F Fth =hn, where hn is the photon energy. Fig. 14 illustrates the relationship between the photon density for threshold damage and the pulse duration for two polymer samples and namely an optically transparent polymer (PEEK) and a less transparent one (PC). As shown in Fig. 14, the dependence for PEEK is approximately linear and for PC is of a second order. This is most probably attributed to the different optical properties of the two materials. PEEK samples of 0.10 mm thickness transmit 80± 90% of the incident light in the visible and nearinfrared region of the spectrum, while PC samples of the same thickness transmit only 10%. PTFE samples of higher thickness d 0:55 mm transmit even smaller percentage of light, but as this is due to
the intense light diffusion, hiding the relatively poor absorption of the material, it is more dif®cult to reach any coherent conclusion relating the PTFE polymer absorption and the photon density for threshold damage. Nevertheless, the coupling mechanism of the ultrashort laser pulses to the polymer samples is very complex and only the relative contribution of their optical properties in the overall ablation mechanism can be considered. When the irradiance is very high such as that produced by the ultra-short pulsed lasers, the absorption of laser radiation becomes a nonlinear process, involving a multiphoton absorption caused by a high photon density rather than simple resonant effects, i.e., the absorption due to the coincidence of the laser radiation wavelength with characteristic absorption peak of the target. Our quantitative results on the ablation rate versus laser energy ¯uence show that the picosecond laser ablation is more ef®cient than the sub-picosecond and nanosecond ablation, i.e., it exhibits higher etch rates before the onset of any saturation. Similar results, as far as the ef®ciency of picosecond and sub-picosecond laser ablation is concerned, were reported in a previous work with optically transparent polymers
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(Nylon and PMMA) [9]. As far as the observed differences in PTFE is concerned, it seems that this polymer exhibits a non-predictable ablational behaviour. PTFE ®lm has been known as one of the most dif®cult materials for laser ablation because it consists of C±F and C±C bonds, whose binding energies are relatively high, resulting in an absorption edge near 160 nm [19]. For this reason, contradictory results on PTFE laser ablation are published. For example, Davis et al. [20] have reported an unsuccessful attempt to ablate PTFE thin ®lms with a 25 ns XeCl laser, although they have used energy ¯uences as high as 120 mJ/mm2, until a small quantity of polyimide was incorporated. Other researchers [21] reported that PTFE ablation with pulsed excimer radiation has been met only by using 300 fs long pulses at a ¯uence of 1 J/cm2. In a previous work [6], we reported ablation rates of PTFE, irradiated with XeCl laser pulses of 25 ns pulse duration, with threshold ¯uence equal to 1.1 J/cm2 and we assumed that PTFE ultra-violet laser ablation involves the simultaneous absorption of two photons by the material chromophores. Kumagai et al. [19] reported that high intensity IR (at 798 nm) femtosecond laser pulses give rise to simultaneous absorption of ®ve photons for PTFE. 5. Summary and conclusions In this paper, we have reported experimental data of the ablation rate per pulse as a function of the laser ¯uence, and the phenomenological evidence of the surface morphology for a number of organic polymers, selected on the basis of their technological signi®cance, as well as on the basis of their different optical properties. Pulsed laser ablation measurements have been performed from the femtosecond to the nanosecond time scale and at wavelengths ranging from the ultraviolet through the visible into the infrared. We have also investigated the relative signi®cance of the duration and the photon density of the laser pulse on the occurrence of the various photochemical and photothermal processes during the laser-induced material removal. An interesting conclusion of this work is the dependence of the threshold ¯uence, required for ablation, primarily on the pulse duration and not on the external
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optical transmission of the material, since the use of shorter pulses lowers the ablation threshold by nearly two orders of magnitude. It has been demonstrated that the threshold ¯uence is of the same very low value (<0.2 mJ/mm2) for visible and infrared ultra-short pulsed laser ablation of PEEK, PC and PTFE polymer samples, while these three polymers exhibit different optical transmission properties in the corresponding spectral regions, derived from the transmission spectra of the samples. The relationship between the photon density for ablation threshold and the pulse duration seems to be well correlated to the individual polymer optical properties, as for the optically transparent polymer PEEK, the dependence is approximately linear and for the less transparent PC is of a second order. Finally, our quantitative results on ablation rate versus laser energy ¯uence, in agreement with similar results of other researchers, show that picosecond laser ablation is more ef®cient than the sub-picosecond and nanosecond ablation, i.e., it exhibits higher etch rates before the onset of any saturation. It has to be noted, however, that a complete investigation of the phenomena involved cannot be based on pure experimental evidence solely. The ®nal conclusion for this and any other research can only be reached after the comparison of the experimental data with the appropriate theoretical model which will be the subject of our future work. Acknowledgements The authors would like to thank the members of the Non Linear Optics Group of the Heriot-Watt University for their help in carrying out the experiments. References [1] J. Jandeleit, G. Urbasch, H.D. Hoffmann, H.-G. Treusch, E.W. Kreutz, Appl. Phys. 63 (1996) 117. [2] R. Srinivasan, V. Mayne-Banton, Appl. Phys. Lett. 41 (1982) 576. [3] C.E. Kosmidis, C.D. Skordoulis, Appl. Phys. 56 (1993) 64. [4] C.D. Skordoulis, M.I. Makropoulou, A.A. Serafetinides, Opt. Laser Technol. 27 (1995) 185. [5] C.D. Skordoulis, M.I. Makropoulou, A.A. Serafetinides, Appl. Surf. Sci. 86 (1995) 239. [6] M.I. Makropoulou, A.A. Serafetinides, C.D. Skordoulis, Lasers Med. Sci. 10 (1995) 201.
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