Efficient micromachining of poly(vinylidene fluoride) using a laser-plasma EUV source
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In this paper an efficient micromachining of poly(vinylidene fluoride) (PVDF) by direct photo-etching with a laser-plasma EUV (extreme ultraviolet) source was demonstrated for the first time. Mass spectroscopy was employed to investigate the ablation products and revealed emission of numerous molecular species of C-containing fragments of the polymer chain. Chemical surface changes after irradiation were investigated using X-ray photoelectron spectroscopy (XPS). The XPS spectra obtained for PVDF samples, irradiated with low and high EUV fluence, indicate significant differences between chemical structures in near-surface layers. It was shown that irradiation with low fluence results in defluorination and thus carbon enrichment of the polymer in near-surface layer. In contrary, irradiation with high fluence leads to intense material ablation and hardly modifies the chemical structure of the remaining material.
KeywordsVinylidene Fluoride Quadrupole Mass Spectrometry Carbon Enrichment PVDF Sample Pristine PVDF
Poly(vinylidene fluoride) (PVDF) is an important fluoropolymer because of its piezoelectric, pyroelectric and ferroelectric properties. It is also known to have an extremely high chemical stability and electrical resistivity. Micro- or even nanopatterning of PVDF is highly desirable for applications in multifunctional and integrated devices. Many works have been performed on surface processing of PVDF using ion beams [1, 2] synchrotron X-ray [3, 4] and UV laser radiation [5, 6]. Irradiation of PVDF with these sources resulted in strong modification of the molecular structure in a near-surface layer of the polymer. The chemical changes are associated with dehydrogenation, defluorination [2, 4, 7, 8] and cross-linking between the polymer chains [4, 9]. The changes, especially the defluorination, strongly influence the ablation process. In case of utilizing the synchrotron radiation for photomicromachining the maximum achievable etch depth was ∼10 μm due to a saturation behavior . QMS (quadrupole mass spectrometry) showed that only H2, F and HF gaseous species were emitted from the surface during irradiation [3, 4]. The removal of the fluorine atoms resulted in decrease of the irradiated region volume, hence, was responsible for the photomachining mechanism. Simultaneously the F fraction in PVDF decreased gradually as the etching progressed and finally the process stopped. Similar behavior was reported for PVDF irradiated with ion beams [1, 2] and excimer lasers [5, 6, 7].
In this work micromachining of PVDF by using a laser-plasma EUV (extreme ultraviolet) source was demonstrated for the first time. It was shown that C-containing molecules were emitted from the polymer during irradiation, which is contrary to the experiments mentioned above. The ablation rate in this case was comparable to EUV ablation rate of polytetrafluoroethylene (PTFE) and fluorinated ethylene-propylene (FEP) .
In our experiments, a 10-Hz laser-plasma EUV source, based on a double-stream gas-puff target, irradiated with the 3-ns/0.8 J Nd:YAG laser pulse, was used. The target was created by pulsed injection of a krypton–xenon (90/10%) mixture gas into a hollow stream of helium by employing electromagnetic valve system equipped with a double nozzle set-up. The focusing conditions and plasma parameters were adjusted to obtain maximum intensity in the EUV spectral region. The radiation was focused using a gold-plated grazing incidence ellipsoidal collector, manufactured in Reflex s.r.o., (at present: Rigaku Innovative Technologies Europe s.r.o.), Czech Republic. The collector allowed for effective focusing of radiation emitted from Kr/Xe plasma in the wavelength range λ=9–70 nm. The most intense emission was in the relatively narrow spectral region centered at λ=11±1 nm. The spectral intensity at longer wavelength range was much smaller, however, the spectrally integrated intensities in both ranges were comparable. The EUV fluence in the focal plane of the collector exceeded 60 mJ/cm2 in the center of the focal spot. FWHM diameter of the intensity distribution in the focal spot was 1.4 mm. Detailed description of the source and parameters of the focused EUV radiation can be found elsewhere .
PVDF foils of 50-μm thickness from Goodfellow, utilized in this experiment, were used without any treatment. The samples were mounted on the XYZ motorized translation stage, placed in the EUV collector focal plane and irradiated through a contact metallic mask with square orifices 60×60 μm2 in size. Irradiation was performed at 10-Hz repetition rate with different exposure times up to 2 min.
Similar results of XPS measurements were obtained using a KrF excimer laser for irradiation of PVDF . In this case the fluorine concentration on the polymer surface decreased after irradiation to about 40% according to the initial value. In our experiment decrease of the fluorine concentration reached 36, 25, 19% for 150, 450 and 600 EUV pulses, respectively. Irradiation with low fluence EUV pulses gives then similar effect as KrF laser. There is, however, significant difference concerning irradiation fluence: surface carbonization using KrF laser requires at least 140 mJ/cm2 while in case of EUV the fluence should not be higher than 10 mJ/cm2. For PVDF samples irradiated with X-rays UPS measurements of the polymer valence band were performed. Also in this case strong decrease of peak corresponding to fluorine content in the near-surface layer was reported [3, 4].
Different behavior of PVDF irradiated with EUV pulses against the response of the polymer to X-ray and UV radiation is likely associated with the unique properties of the EUV radiation in respect to the other radiation sources. The EUV photons are strongly absorbed in any polymer (absorption depth of 10–100 nm in PVDF). Energy of a single EUV photon is sufficient to excite or release any electron from the valence band. Each of these photoelectrons has enough energy to excite several electronic states. The relative probabilities of triplet states formation are greatly enhanced with respect to excitation with electromagnetic radiation. It means that radiationless de-excitation processes, including bond breaking, are predominant without preferences regarding the type of bond (C–C, C–F, C–H). Similar mechanism is responsible for interaction of X-rays with PVDF but the absorption depth in this case is much larger (10 μm–1 mm) and the exposure time is much longer—up to several hours, which results in the power density several order of magnitude lower compared to the EUV irradiation. Apart from that, C-containing fragments of the polymer chain, produced deep inside the polymer, can be too large to diffuse to the surface. Thus mainly H2, and HF molecules are emitted changing the chemical structure and the composition of the polymer chains. In case of irradiation of PVDF with the UV lasers the power density in a single pulse is comparable to that with the EUV irradiation. The mechanism of interaction, however, is different. Energy of the UV photon is sufficient to break a single C–F bond and, in consequence, allows formation of the HF molecule. The HF molecule is being released from the polymer surface and a C–C bond is converted to the C=C bond . Further cross-linking leads to the formation of benzene rings and finally graphite crystals on the polymer surface.
The laser-plasma EUV source was used for micromachining of PVDF by direct photo-etching for the first time. Efficient material ablation was obtained in case of EUV irradiation with the high fluence (60 mJ/cm2). Investigation of the ablation products with QMS demonstrated emission of C-containing fragments of the polymer chain. It was shown that the C:H:F ratio obtained from the spectrum, is in a good agreement with the stoichiometric composition of PVDF molecules. XPS spectra acquired for the polymer after ablation are almost identical to the spectrum of pristine PVDF, indicating preservation of the chemical structure of the remaining material. This way ablation process remains undisturbed during irradiation with consecutive EUV pulses. XPS measurements applied to the polymer irradiated with low fluence (<10 mJ/cm2) indicate strong chemical modification in the near-surface layer. In this case defluorination and thus carbon enrichment in the surface material was revealed.
This work was supported by the grant No. N N202 174939 of the Ministry of Science and Higher Education of Poland and was partially performed under COST Action MP0601.
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