Novel structure formation in poly(ethylene therephthalate) by scanning excimer laser ablation
Introduction
A large number of articles about polymer ablation with excimer lasers have appeared since the first results obtained by Srinivasan and Mayne-Banton in 1982 [1]. The major topic in this area is the ablation rate of different polymers with various fluence ranges, wavelengths, and pulse lengths. Another topic of interest is the surface structure and the chemical composition that results from ablation [2].
Several types of polymer ablated surface structures are reported in the literature. For poly(ethylene therephthalate) (PET), three classes of structures are known. First, at fluences below the ablation threshold, a periodic pattern with a period close to the irradiation wavelength develops within a small fluence range (3–5 mJ/cm2 for irradiation at 193 nm). These kind of structures are often referred to as light-induced periodic surface structure (LIPSS) [3]. Second, at fluences around the ablation threshold, dendrite-like structures develop when the irradiation is carried out in vacuum [4], [5].
Finally, for fluences well above the ablation threshold, a nap or wall type structure develops in stretched PET foils [1], [6], [7]. This type of structure gets more pronounced with an increase in fluence and number of pulses. Our experiments were all carried out at fluences well above the ablation threshold. In order to distinguish this known type of structure from the ones that we observed only in scanning ablation experiments, we named it the “static structure”.
A detailed investigation of the static structure was given by Hopp et al. [8]. Among other effects, they examined the structure changes occurring by variation of the angle of incidence of the light. They found that the polygon borders separating the naps became longer in the direction of the incident light when the angle of light incidence exceeded 70°. For angles below 70°, only minor structural changes were observed.
In scanning ablation, we also irradiate a surface under an angle. The difference is that in our case, we observed an important structural change already at angles bigger than 11°. We repeated the experiment of Hopp et al. [8] at an angle of 45° and could not observe a structural change, i.e., we confirmed the results of Hopp et al. [8]. As consequence, we need to study the microscopic differences between these two experiments.
In this article, we show that scanning ablation leads to two new types of surface structures — “scanning structure” and “smooth structure” — in a certain parameter range. Furthermore, we present the results that we obtained up to now on the structural change between static and scanning structures.
Section snippets
Experimental
The samples were 100 μm thick, and composed of commercial PET [Melinex, type S, ICI]. The samples were usually exposed to 200 pulses of about 20 ns duration at 193 nm [LPX 205 Excimer Laser, Lambda Physik]. The repetition rate of the pulses varied between 1 and 50 Hz and the investigated fluence range covered 75–1200 mJ/cm2, i.e., well above the measured ablation threshold of 36 mJ/cm2. The experimental setup is a standard ablation setup consisting of an excimer laser, a variable attenuator
Channel geometry and parameters of scanning ablation
In multiple pulse scanning ablation, theoretically, the irradiated area has the shape of a stairway. Fresh material is fed to the beam at the top of the stairway and used material leaves the irradiated zone at the bottom. The total length of the stairway equals the effective mask length, a. The total depth of the stairway corresponds to the depth of the channel. The step width of the stairway equals the distance by which the substrate is moved between two subsequent laser pulses and the step
Discussion
When we compared the data presented in this article with the data of Hopp et al. [8], we noticed that we address the fundamental question: In which system can we describe structure formation during ablation? We ask this question because the system usually used, which consists of light, the material properties of the irradiated surface, and a gas environment, is the same in both experiments. We suggest that the neighborhood of the irradiated surface play an important role in the formation of the
Summary and conclusion
We presented new surface structures, called “scanning structures”, which were observed after scanning ablation of stretched PET foils. Further more, we investigated the influence of the mask length, a, fluence, Φ, pulse repetition rate, f, and scanning velocity, v, on the structure change for the static and scanning structure. We showed that the structural change that occurred on the ramps was the origin of the structural change on the channel floor. Because all ramps with “static structure”
Acknowledgements
We thank James Derose for the critical revision reading of this article.
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