Sylvain Lazare

ULTRAVIOLET LASER PHOTOABLATION OF POLYMERS: A REVIEW AND RECENT RESULTS

The evolution since 1982, of far-UV laser photoablation of polymers is described. The experimental data can be fitted by using a dynamic model which states that the irradiated interface moves at a rate proportional to the difference between, the intensity reaching it, and the ablation threshold intensity It. The screening effect of the ablated gaseous products is taken into account. The experimental etch depth versus fluence, obtained with our new quartz crystal microbalance technique, can be fitted by adjusting two parameters of this model; the mean absorption coefficient of the products β and the so-called ablation rate constant k, which is the etch rate for I=It

Characterization of submicrometer periodic structures produced on polymer surfaces with low‐fluence ultraviolet laser radiation

Periodic ripples and dot patterns with spacing smaller than 200 nm have been produced on various highly absorbing polymers [poly(ethylene terephthalate), poly(butylene terephthalate), polystyrene, polycarbonate, etc.] using a polarized beam of an ArF and KrF excimer laser. The period of the structures increases with the wavelength used and with the angle of incidence of the beam, whereas the ripple direction is parallel to the polarization direction. The fluence interval in which the pattern can be produced is far below the ablation threshold of the polymer but probably allows a local melting or photolysis of the surface. For polystyrene, a strong influence of the presence of oxygen was observed, which leads to the conclusion that photooxidative processes play a major role in the structure growing mechanisms. Due to the small size (0.1 μm) of the structure scattering techniques using visible light (0.4–0.8 μm) could not be used in this study and scanning electron microscopy and transmission electron micro...

Fabrication of refractive microlens arrays by excimer laser ablation of amorphous Teflon

The fabrication of refractive microlens arrays by the technique of excimer laser ablation of doped amorphous Teflon combined with the subsequent annealing and melting of the produced polymer islands is described. The microlenses obtained are optically clear from the far UV (190 m) to the near IR (2000 nm) and are of good optical quality. They vary in size from 50 to 385 µm in diameter with numerical apertures between 0.2 and 0.3. Utilization of these microlenses for material processingby excimer lasers at 193 nm is demonstrated, and possible applications are discussed.

Submicron periodic structures produced on polymer surfaces with polarized excimer laser ultraviolet radiation

For the first time, pure laser induced periodic structures (without any ablation or any larger structure) of submicron size (spacing and amplitude of 0.2 μm) are developed on polymer surfaces [poly(ethylene terephthalate), poly(butylene terephthalate) and polystyrene] by irradiation with one thousand pulses of the polarized beam of the excimer laser (193 and 248 nm). Fluence is below the ablation threshold and must be chosen in a narrow window which depends on the polymer and the wavelength. The obtained relief is observed by SEM, TEM and characterized by ellipsometry. Structures are obtained also by irradiation of thin films (2000 A) coated on silicon wafers.

Excimer laser light induced ablation and reactions at polymer surfaces as measured with a quartz‐crystal microbalance

We have developed a new technique to measure the etch rate of polymer films under the ablative photodecomposition condition obtained by absorption of the far‐UV radiation of the excimer laser. The technique is based on the use of a quartz‐crystal microbalance interfaced with a microcomputer. It has high accuracy (17 ng/cm2). Etch rates of poly(ethylene terephthalate), polycarbonate, and polystyrene were measured as a function of the laser intensity, at 193 and 248 nm. The etch rate in the region of the ablation threshold intensity is obtained precisely. A slow ablation regime between the threshold and the linear regime is evidenced. For polycarbonate and polystyrene, a reaction of the irradiated surface with molecular oxygen accelerates the ablation process at low fluence of the 248 nm radiation. The degradation reaction of poly(ethylene therephthalate) is monitored as a function of fluence by selective dissolution in acetone. A sudden increase of the degraded depth is seen at the transition between the t...

Structural origin of surface morphological modifications developed on poly(ethylene terephthalate) by excimer laser photoablation

The ablation of poly(ethylene terephthalate) (PET) by Ar‐F laser radiation has been studied at different fluences for amorphous and semicrystalline samples by scanning electron microscopy (SEM) and by etch‐depth measurements. The surface structure created on PET has been interpreted as resulting mainly from a difference in the etch rate—and not the ablation threshold—between amorphous and semicrystalline PET. SEM pictures clearly show the melting of the PET surface at high fluence (≥80 to 100 mJ/cm2) and not below that value. UV laser etching is proposed as a quick and easy method for observing the crystalline subsurface structure of aromatic semicrystalline polymers, as long as low fluence and a low pulse repetition rate are used.

Large scale excimer laser production of submicron periodic structures on polymer surfaces

Abstract By absorption of a polarized excimer laser beam of low fluence a new class of submicron laser induced structures is produced on various polymer surfaces. Parallel ripples of less than 0.2 μm spacing in the direction of the electric field are obtained with the ArF or KrF radiation when it is strongly absorbed by the polymer. The amplitude of the roughening increases with the number of pulses used, with a maximum obtained after one thousand. Several polymers in thin or thick films of commercial or laboratory origin can be textured in air or in vacuum or both depending on the chemical structure. Fluence is below the ablation threshold and must be chosen in a narrow window, which depends on the polymer and the wavelength used. The obtained relief is observed by SEM, TEM and characterized by ellipsometry. Structures are obtained also by irradiation of thin film (2000 A) coated on silicon wafers. In many applications, a controlled submicron level roughening of surfaces is desired. Owing to the simplicity of this irradiation procedure large scale surface treatments are possible.

Surface modifications of Kapton and cured polyimide films by ArF excimer laser: applications to imagewise wetting and metallization

Abstract Polymer surfaces of certain kinds, particularly polyimide, become highly hydrophilic upon irradiation by deep UV or UV laser in air. Unlike UV lamp light, laser exposure in air is extremely efficient in making polymer surfaces hydrophilic and wettable with aqueous solutions or similar liquids. The change in chemical nature of the polymer surfaces upon laser and deep UV irradiation is studied by contact-angle measurements in both advancing and receding modes. The results are compared with those of PPQ, PPES, polystyrene and PMMA. Under UV irradiation in air, polymer surfaces become highly hydrophilic, resulting in a decrease of the contact angles in both modes. Laser exposure in air makes the surfaces highly hydrophilic and wettable by both photo-induced reactions and debris/roughness formation. Water spray reduces the hysteresis by removing the debris, but the polymers still keep certain roughness and hydrophilicity on the remaining surfaces. Laser irradiation makes imagewise wetting possible with polyimide films at a low fluence. Direct imagewise metallization is also observed with polyimides and PPQ films. The laser-exposed areas cannot have electrically conducting metal films deposited on them while on unexposed surfaces uniform metal film deposition took place. Although the mechanism is still not clear, oxygen RIE is found to be possible in the exposed, non-conducting part, while galvanic plating was carried out on unexposed, conducting surfaces with a Au-Pd metallization.

Surface amorphization of Mylar(R) films with the excimer laser radiation above and below ablation threshold: Ellipsometric measurements

Ablation is the main phenomenon which occurs on a polymer surface when a pulse of ultraviolet radiation is absorbed. The front edge of the radiation pulse is absorbed by a volume which eventually vaporizes and the rest is partially blocked by the ablation plume of products and partially transmitted to the new surface left behind after ablation. Most of this transmitted energy is transformed into heat and raises the temperature of the surface sometimes above the melting point of the polymer. Due to the transient character of the irradiation which lasts 25 ns, a rapid cooling occurs. For semicrystalline polymers like poly(ethylene terephthalate) the surface is left in an amorphous state by this cooling. In this work the amorphous depth was measured by monochromatic ellipsometry as a function of the pulse energy. It is shown that amorphization has a threshold fluence (7 mJ/cm2 at 193 nm) which is lower than the ablation threshold (17 mJ/cm2). The ablation depth reaches a maximum of 850 A at 193 nm when the ablation threshold is reached. Similarly the ablation depth measured at 248 nm is 1600 A. The amorphization depths are proportional to the radiation penetration depths in the materials.

Direct and accurate measurement of etch rate of polymer films under far‐UV irradiation

We have developed a new technique for measuring the etch rate of polymer films under ablative photodecomposition condition obtained by absorption of the far‐UV radiation of the excimer laser. The technique is based on the use of a quartz crystal microbalance interfaced with a microcomputer. It has good accuracy and thus provides precise evidence of a new feature of the etch curve for laser intensity right above the ablation threshold value. Further applications are proposed.