Abstract. Structure formation upon 500 fs 248 nm KrFlaser irradiation of PolyEthylene Terephthalate (PET) and Polylmide (PI) has been investigated. The results obtained with fs pulses have been compared to those with ns pulses. 42.55, 81.40, 82.65 Irradiation of polymers with pulsed UV-laser light and pulse lengths of, typically, 1-50 ns frequently results in surface-structure formation [1][2][3]. These structures are often periodic or quasi-periodic in the sense that either the one-or two-dimensional lateral Fourier transform of the structure height h (x, y) corresponds to a single frequency or a relative narrow group of frequencies, respectively. The structures can be classified according to two groups: coherent and non-coherent structures. PACS:Coherent laser-induced surface structures such as ripples and dots are due to interference between the incident laser light and surface-scattered waves [4,5]. The periodicity of such structures is of the order of the wavelength of the light and depends on the angle of incidence of the laserbeam. Ripple formation has been observed on various polymers, including Polylmide (PI) and PolyEthylene Terephthalate (PET), at laser fluences below [6-8] and above [9 11] the ablation threshold ~th" On PI, coherent structures with linewidths below 100 nm have also been obtained by means of an excimerlaser interference technique [11][12][13].Non-coherent structures with a periodicity in the micrometer range are frequently observed in polymers. These structures are either regular spaced naps, or joining or branching walls with a preferred direction (similar to Fig. 1). For uniaxially and biaxially stretched foils wall-and nap-type structures, respectively, have been observed [14][15][16][17]. The internal stress-fields are related to either the production process of the polymer foil itself or/and to post-fabrication stretching.Non-periodic microstructures on excimer-laser irradiated PET have been reported in [18]. These structures consist of branched dendrites that have a lateral dimension in the micrometer range and a height of 5-30 nm.Femtosecond UV excimer-laser ablation was investigated for polymers [19][20][21], LiNbO3 [22], superconductors [23], and fused silica [24]. Ultra-short pulses require lower threshold fluences for ablation than ns pulses and yield an improved surface quality within the ablated areas.In this paper we study the formation of structures induced by femtosecond excimer-laser pulses on polymer surfaces.

Periodic line structures with a period of 167 nm and linewidths varying from 35 to 100 nm have been produced on polyimide by direct ablation with a KrF laser using an interferometric technique. Since ablation is a nonlinear process, the resolution can exceed that expected from the wavelength and numerical aperture of the system and the linewidth can be controlled by varying the laser fluence. This externally generated period of 167 nm prevents the spontaneous growth of periodic surface structures due to radiation remnants.

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An array of permanently electrically conducting lines with a width of 0.5 μm and a period of 0.9 μm has been produced in polyimide with a KrF laser using a holographic technique. The electrical resistivity of these submicron structures shows an ohmic characteristic and has a value of less than 0.5 Ω cm, while the resistivity perpendicular to the structures exceeds this value by more than a factor of 107. The temperature dependence of the conductivity in these wires is found to be similar to that found for macroscopic regions of polyimide which show laser induced conductivity.

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