Direct observation of excimer-laser photoablation products from polymers by picosecond-uv-laser mass spectroscopy

Organic nanostructures on silicon, created with semitransparent polystyrene spheres and 248 nm laser pulses

Arrays of nanostructures are made starting with a template of close-packed, polystyrene spheres on a silicon surface. The spheres are either 1.091 or 2.99 µm in diameter (d) and are of polystyrene (PS). They are irradiated with a pulse of either 308 or 248 nm light to which they are transparent and semitransparent, respectively. A transparent sphere with d = 1.091 µm diameter concentrates incident light onto a small substrate area. As has been previously reported, that creates silicon nanobumps that rise from circular craters. At 248 nm and d = 2.99 µm, the light energy is mainly absorbed, destroys the sphere, and leaves a shrunken mass (typically about 500 nm wide and 100 nm high) of organic material that is probably polystyrene and its thermal degradation products. At 248 nm and d = 1.091 µm, the residual organic structures are on the order of 300 nm wide and 100 nm high. A distinctive feature is that these organic structures are connected by filaments that are on the order of 50 nm wide and 10 nm high. Filaments form because the close-packed PS spheres expand into each other during the early part of the laser pulse, and then, as the main structures shrink, their viscoelasticity leads to threads between them. Our results with 248 nm and d = 1.091 µm differ from those described by Huang et al with 248 nm and d = 1.0 µm. Future studies might include the further effect of wavelength and fluence upon the process as well the use of other materials and the replacement of nanospheres by other focusing shapes, such as ellipsoids or rods.

Influence of Polymer Molecular Weight on the Chemical Modifications Induced by UV Laser Ablation

This paper investigates the influence of polymer molecular weight (M(W)) on the chemical modifications of poly(methyl methacrylate), PMMA, and polystyrene, PS, films doped with iodonaphthalene (NapI) and iodophenanthrene (PhenI), following irradiation at 248 nm (KrF excimer laser, 20 ns fwhm and hybrid excimer-dye laser, 500 fs fwhm) and at 308 nm (XeCl excimer laser, 30 ns fwhm). The changes of intensity and position of the polymer Raman bands upon irradiation provide information on cleavage of the polymer bonds. Degradation of PMMA, which is a weak absorbing system at 248 nm, occurs to a higher extent in the case of a larger M(W), giving rise to the creation of unsaturation centers and to degradation products. For highly absorbing PS, no degradation is observed upon irradiation with a KrF laser. Consistently irradiating doped PS at 308 nm, where the absorption is low, induces degradation of the polymer. Results provide direct support for the bulk photothermal model, according to which ejection requires a critical number of broken bonds. In the case of irradiation of doped PMMA with pulses of 248 nm and 500 fs, neither degradation nor dependence with polymer M(W) are observed, indicating that mechanisms involved in the femtosecond laser ablation differ from those operating in the case of nanosecond laser ablation. Participation of multiphoton/avalanche processes is proposed.

Molecular analysis by ionization of laser-desorbed neutral species

A powerful molecular surface analysis technique for the analysis of complex materials, such as polymer/additive systems, consists of laser desorption of surface molecules and subsequent ionization of these gas-phase molecules with resonant or nonresonant laser ionization. These molecular ions are subsequently detected by Fourier-transform mass spectrometry or time-of-flight mass spectrometry. We show that different wavelengths for the postionization step permit selectivity that provides important additional information on the chemical makeup of these complex materials. Near-UV wavelengthsselectively ionize aromatic polymer additives, far-UV wavelengths photoionize other nonaromatic species; and vacuum-UV wavelengths provide access to all the desorbed species. In addition to these applied results, we study many fundamental issues of laser desorption, such as desorption thresholds, velocity distributions, postionization wavelength selectivity, etc. The Fourier-transform mass spectrometer and time-of-flight mass spectrometer are discussed in detail.