Bodil Braren

Ultraviolet laser ablation of polyimide films

Pulsed laser radiation at 193, 248, or 308 nm can etch films of polyimide (DuPont KaptonTM). The mechanism of this process has been examined by the chemical analysis of the condensible products, by laser‐induced fluorescence analysis of the diatomic products, and by the measurement of the etch depth per pulse over a range of fluences of the laser pulse. The most important product as well as the only one condensible at room temperature is carbon. Laser‐induced fluorescence analysis showed that C2 and CN were present in the ablation plume. At 248 nm, even well below the fluence threshold of 0.08 J/cm2 for significant ablation, these diatomic species are readily detected and are measured to leave the polymer surface with translational energy of ∼5 eV. These results, when combined with the photoacoustic studies of Dyer and Srinivasan [Appl. Phys. Lett. 48, 445 (1986)], show that a simple photochemical mechanism in which one photon or less (on average) is absorbed per monomer is inadequate. The ablation proces...

Ablation and etching of polymethylmethacrylate by very short (160 fs) ultraviolet (308 nm) laser pulses

The effect of pulse width on the ablation of polymers has been extended to ultrashort pulses (160 fs) of 308 nm wavelength. Polymethylmethacrylate has negligible absorption at this wavelength for one‐photon excitation. With the ultrashort pulse clean etching without any sign of thermal damage can be achieved at fluences as little as 0.2–0.3 J/cm2. This is the first demonstration that the high power of ultrashort pulses of ultraviolet light can produce photochemical etching by taking advantage of multiphoton excitation to dissociative states.

Laser sputtering: Part I. On the existence of rapid laser sputtering at 193 nm☆

Abstract Irradiation, i.e.bombardment, with 193 nm laser pulses having an energy fluence of 2.5 J cm 2 and a duration of ~12 ns leads to rapid sputtering with Au, Al2O3, MgO, MgO. Al2O3, SiO2, glass and LaB6, relatively slow sputtering with MgF2 and diamond, and mainly thermal-stress cracking with W. Scanning electron microscopy (SEM) suggests that the mechanism for the sputtering of Au in either vacuum or air is one based on the hydrodynamics of molten Au, while an SEM-derived surface temperature estimate confirms that thermal sputtering (which might have been expected) is not possible. SEM with W shows that the near total lack of material removal is due to the thermal-stress cracking not leading to completed exfoliation, together with the surface temperature being too low for either hydrodynamical or thermal processes. Corresponding SEM with Al2O3 shows, in the case of specimens bombarded in vacuum, topography of such a type that all mechanisms except electronic ones can be ruled out. The topography of Al2O3 or other oxides bombarded in air through a mask is somewhat different, showing craters as for vacuum bombardments but ones which have a cone-like pattern on the bottom.

Direct imaging of the fragments produced during excimer laser ablation of YBa2Cu3O7−δ

The evolution dynamics of the fragments produced during KrF excimer laser (248 nm, 25 ns) ablation of YBa2Cu3O7−δ has been observed by ultrafast photography using a synchronized dye laser beam (∼1 ns) to probe above the target surface. The images show that fragment removal is initiated near the beginning of the laser pulse (≳10 ns), continues for a maximum of a few hundred nanoseconds, and has an expansion front velocity suggesting a target temperature varying from about 1500 to 4000 K or greater. The shock wave formed due to interaction of the ablated fragments with background oxygen gas has also been photographed, and its temporal evolution as a function of oxygen pressure has been shown to agree better with a planar than spherical model. The overall picture is that of an ablation process showing pronounced but understandable gas‐dynamic effects.

Nature of ‘‘incubation pulses’’ in the ultraviolet laser ablation of polymethyl methacrylate

The term ‘‘incubation pulses’’ is applied to the pulses of ultraviolet laser radiation which are directed initially at a polymer surface and which etch less depth per pulse than subsequent pulses which remove identical depths of material at each pulse. This incubation effect which is particularly strong when the polymer absorbs moderately at the laser wavelength has been studied in polymethyl methacrylate at 193 and 248 nm. The transformation of the surface during incubation pulses has been followed by 1) photographing the blast wave that is produced by the products that emerge from the surface, using a fast (<1 ns) dye laser that is electronically triggered to light the ablation plume after a set delay, and 2) by treating the exposed surface with a solvent to dissolve out any photolyzed material that is left in the exposed area. The results show that ablation, as shown by the ejection of the products at high velocity, occurs even during the incubation period but the surface is not etched at all or only p...

Ultrafast imaging of ultraviolet laser ablation and etching of polymethylmethacrylate

Ablation and etching of the surface of polymethylmethacrylate (PMMA) by pulses of 248 nm laser radiation [∼20 ns full width at half maximum (FWHM)] have been probed by pulses of visible laser radiation (596 nm; <1 ns FWHM). The results were recorded photographically. Modification of the surface structure of the polymer is first visible at 12 ns and appears to be complete in about 60 ns. Emerging solid material, preceded by a shock wave which first becomes visible at 60 ns, reaches a maximum in intensity at 6 μs and continues until about 20 μs. The average velocity of the solid material, which is probably a low molecular weight polymer of PMMA, is 1.5×104 cm/s.

Primary and secondary mechanisms in laser-pulse sputtering☆

Abstract Primary sputtering mechanisms are conventionally grouped in terms of the categories collisional, thermal , and electronic . With pulsed photons one must. in addition, consider the emission of droplets and fragments in thermomechanical processes. Pulsed photons also lead to the situation that the density of emitted particles is sufficiently high for gas-dynamic effects to enter and for the system therefore to lose memory of the primary mechanism. One then distinguishes secondary mechanisms which include outflow, as when a finite reservoir expands, effusion , effusive release from the outer surface without recondensation, and recondensation . effusive release with recondensation. If the photon pulse interacts with the emitted particles then still further secondary mechanisms are relevant due to energy deposition in the plume of emitted particles as well as due to ionization. Finally. the laser-pulse sputtering of the polymer PMMA (polymethylmethacrylate) and the superconductor YBCO ( YBa 2 Cu 3 O 7− x ) is discussed on the basis of explicit photographs of the sputtered particles. In the case of PMMA there are two groups of particles. the first group having primary and secondary mechanisms which are presently unestablishable but the second group being reasonably attributed to thermal primary release and to secondary behavior of the effusion (or recondensation) type. In the case of YBCO there is only one group of particles having a primary mechanism which is almost certainly electronic and a secondary mechanism which is tentatively identified with outflow .

Magnetron sputtering and laser patterning of high transition temperature Cu oxide films

High Tc Y‐Ba‐Cu‐O films have been prepared by dc magnetron sputtering of metal alloy targets. To circumvent the negative ion effect, two alloy targets, YCu and BaCu, are sputtered in an argon atmosphere with an oxygen spray near the substrate. Films deposited on sapphire with onsets at 92 K and a 6° transition width (10–90%) have been achieved using this technique. These films have been successfully patterned with the technique of laser ablation.

Photothermal description of polymer ablation: Absorption behavior and degradation time scales

A photothermal model of ablation is used to investigate the time scales for polymer degradation by UV laser light. In the absence of a significant incubation effect, strong absorbers (e.g., polyimide) are expected to decompose roughly three orders of magnitude faster than weak absorbers (e.g., polymethylmethacrylate), owing to the higher surface temperature attained during the absorption. This difference in the time scales reflects processes taking place at significantly different temperatures and should extrapolate to the overall ablation process. The very short calculated time scales (femtoseconds for polyimide and picoseconds for polymethylmethacrylate) indicate that polymer decomposition occurs rapidly compared to the actual ejection of material. For multipulsed ablation experiments, significant incubation modification tends to increase the absorbance of weak absorbers, making the effect less marked than in the ideal case. Incubation effects are attributed to nonablative decomposition that occurs at c...

The significance of a fluence threshold for ultraviolet laser ablation and etching of polymers

The minimum fluence at which a laser pulse (FWHM 20 ns; 248 nm) causes a change in the surface of a film of polyethylene terephthalate (=PET) or polyimide (KaptonTM) in air has been measured by probing the surfaces with visible laser pulses of <1 ns at delay times of 10–10 000 ns. At fluences at which a single pulse left a permanent etch pit, the probe pulse showed an intense and rapid (<60 ns) darkening of the surface which may be attributed to the scattering of the beam by gas bubbles at the polymer surface. In 100 ns, a blast wave was visible which could be tracked for over 1000 ns. At the end (∞ ns), the polymer surface was not darkened but an etch pit was apparent. Progressively decreasing the fluence showed that even at fluences <0.025 J/cm2 for PET and<0.050 J/cm2 for Kapton, a single pulse transiently blackened the surface but did not leave an etch pit. The threshold for the ablative photodecomposition of these polymers appears to lie at a value of the fluence that is well below the threshold for ...