RO127505A2 - Process for structuring surfaces by means of laser radiation by near-field optical intensification effect - Google Patents

Abstract

The invention relates to a process for structuring surfaces on extended areas by direct printing by using the laser ablation effect induced by the near-field intensification of the optical radiation. According to the invention, the process consists in manufacturing a micro-structural mask (1) by photopolymerization, then positioning the mask (1) on the surface of a material (2) to be structured and in irradiating it with a laser beam (3), by irradiation there being obtained micro-structures (4); the mask (1) comprises a support (5) and some focusing micro-optical elements (6), the structure of said elements (6) being formed of elements in the shape of cylinder or other structures with a geometry different from the spherical geometry, with micrometric and sub-micrometric dimensions, obtained from a transparent polymer material at the wavelength of the laser beam (3) used for the ablation; the material (2) to be processed may be the surface of a solid material or a thin film of a material deposited on a sub-layer (7), the mask (1) and material (2) to be processed are irradiated on a large surface with a single laser pulse or a laser beam (3) and, by the near-field intensification effect, the laser beam (3) is focused at the interface between the micro-optical elements (6) and the surface of the material (2), locally the optical irradiance exceeds the threshold of optical damage causing the laser ablation of the material (2), only at the surface, without affecting the mask (1); as a consequence of focusing the laser beam (3), the cylindrical micro-optical elements (6) generate line-shaped micro-structures (4) by the laser ablation, the layout thereof respecting the layout of the cylinders within the microstructured masks (1).

Description

PROCESS FOR STRUCTURING LASER RADIATION SURFACES BY OPTICAL INTENSIFICATION EFFECT IN APPROPRIATE FIELD

The invention relates to a process for structuring surfaces on extended areas by direct printing using the laser ablation effect induced by intensifying optical radiation in the near field, with the help of micro-optical focusing elements with geometry different from the spherical one, which allow the generation of micro and nanostructures with different geometries.

Laser ablation material processing is a versatile method, applicable to a wide class of materials: metals, ceramic materials, semiconductors, polymers, biological materials, etc. For the purpose of processing, classical laser microfabrication techniques consist of focusing laser radiation using optical components, such as lenses or microscope lenses, whose characteristics directly depend on the resolution of structures created by laser ablation. The geometry of a particular structure is formed either by the fixed holding of the sample and the displacement of the laser beam, or by the translation or rotation of the sample and the fixed holding of the laser beam. In either case, the processing method is a sequential one, scanning the sample requiring a long processing time. The resolution of the realized structure depends in this case not only on the focusing optics, but also on the accuracy of the mechanical scanning systems.

A recently developed method of parallel processing of materials over large areas consists of using the effect of intensifying in the near field optical radiation at the interface of microspheres and nanospheres of transparent material at laser radiation [1], or even metal nanoparticles deposited by self-organization on surface of the material to be processed [2]. Following the irradiation of the surface deposited with micro particles, each micro-sphere acts as a micro-lens. The disadvantage of using microspheres as micro-optical focusing elements was to obtain geometries with periodic structure limited to hexagonal symmetry, given the placement of self-organized microspheres on the surface.

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The invention consists in the manufacture of microstructured masks of the polymer and their use as micro-optical elements for focusing the laser radiation on the surface of a solid material (metal, semiconductor, dielectric). The process according to the invention removes the limitation on the hexagonal symmetry obtained in the near field processing with microspheres, as well as the disadvantage of a long processing time in the case of sequential laser scanning processors.

An example of the invention is given below, with reference to Figures 1, 2, 3 and 4, which represent:

FIG. 1, the processing stages with intensified optical radiation in the near field with microstructured masks.

FIG. 2, the microscopic image of the atomic force of a mask made of polymer SU8 on the quartz substrate.

FIG. 3, numerical simulations of light propagation through the mask with micro-optical focusing elements from SU-8.

FIG. 4, microstructures on a copper film, obtained by optical intensification in the field near the interface between the focusing elements of a transparent mask and the copper film.

The steps of laser processing in the near field, according to the invention, are:

a) Manufacture of a microstructured mask 1.

b) Positioning the mask on the surface of a material 2 to be structured and irradiation with a laser beam 3.

c) Following laser irradiation 3 microstructures 4 are obtained simultaneously.

Masks are manufactured using a photopolymerization process: either by lithographic methods or by direct laser writing, respecting a certain predetermined geometry. Mask 1 is composed of a support 5 and micro-optical focusing elements 6 made on that support. The support 5 is made of glass, quartz, or other transparent material at the wavelength of the laser radiation 3. The structure of micro-optical focusing elements 6 is composed of elements based on cylinders, or other regular or irregular, periodic or non-periodic structures, with predefined geometry, other than spherical, with micrometric and submicrometric dimensions. The micro-optical focusing elements 6 are obtained from a polymer material, transparent at the wavelength of the laser 3 used for ablation. The criterion of choice of the polymer material for the manufacture of the micro-optical focusing elements 6 is given by the optical destruction threshold of the polymer. The threshold of optical destruction of this material must be higher than the ablation threshold of the material to be processed 2.

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The material to be processed 2 may be the surface of a massive material, or a thin film of material deposited on a substrate 7. The mask 1 is placed on the surface of the material to be processed 2, so that the micro-optical focusing elements 6 are in contact or in proximity. surface of solid material. The mask 1 and the material to be processed 2 are irradiated on a large area, either with a single laser pulse or with a train of laser pulses 3, with the fluence below the optical destruction threshold of the polymer material from which the optical focusing elements are manufactured. 6. Fluence The average of the laser beam 3 is also lower than the destruction threshold of the material to be processed 7. By the optical intensification effect in the near field, the laser radiation 3 is focused at the interface between the micro-optical elements 6 and the surface of the material 2. Local, optical irradiation exceeds the optical destruction threshold, causing laser ablation of the material only at the surface, without affecting the mask.

The micro-optical focusing elements 6 with the cylindrical shape generate by laser ablation, following the focus of the laser radiation 3, the microstructures 4 of the line type. Combinations of cylindrical structures arranged according to a certain predetermined arrangement produce on the surface of the material 2, following laser irradiation, microstructures 4 with the shape corresponding to the arrangement of the cylindrical structures. The arrangement on the surface of the structures 4 obtained by laser ablation respected the arrangement of the cylinders in the component of the microstructured masks.

The following are given in connection with the invention, examples of masks and microstructures obtained by the process of micro and laser nanostructuring.

Figure 2 shows the microscopic image of the atomic force of a SU-8 polymer mask on the quartz substrate and the semi-elliptical transverse profile of the polymerized structure. The structures were obtained using the method of direct writing by photopolymerization induced by the effect of biphotonic absorption of the laser radiation pulsed from a Ti: Sapphire laser, with the pulse duration of the order of femtoseconds. To generate the optical microstructures in the polymer, a microscope objective with a numerical aperture of 0.5 was used. Due to the confocal parameter of the focused laser beam larger than the minimum spot diameter in the focal point, and because the laser beam was focused at the interface between the polymer and the substrate, the shape of the photopolymerized structure has a semi-elliptical profile. By translating in the XY direction of the computer controlled sample, a certain arrangement of the focusing elements is obtained in the plane, respecting a predetermined design.

Figure 3 shows the numerical simulations of light propagation through the microelement mask of SU-8 with semi-elliptical section and the optical intensity distribution, demonstrating the laser radiation at the interface between the mask by an order of magnitude.

^ 2010-01174-2 5 -11- 2010 transparency and surface to be processed, in this case the surface being a 50 nm thick copper film deposited on glass substrate.

Figure 4 shows microstructures on a Cu film, obtained on a large area with a single laser pulse, by ablation following the optical intensification effect in the field near the interface between the focus elements of the transparent mask and the copper film.

The process of micro and laser nanoprocessing has the following advantages according to the invention:

- allows parallel processing of micro and nanostructures over an extended area.

- allows processing with a single laser pulse with expanded beam.

- reduces the laser processing time of the materials.

- allows the integration into the mass production of microstructures using the same mask.

REFERENCES [1] Y. Zhou, Μ. H. Hong, J. Y. Fuh, L. Lu, B. S. Luk'yanchuk, Z. B. Wang, L. P. Shi, and T. C. Chong; Direct femtosecond laser nanopatterning of glass substrate by particleassisted near-field enhancement; Appl. Phys. Lett. 88, 023110 (2006).

[2] Alex Heltzel, Senthil Theppakuttai, S C Chen and John R Howell; Surface plasmonbased nanopatterning assisted by gold nanospheres; Nanotechnology, Vol. 19, page 025305 (2008).

Claims (2)

1. Process for structuring the surfaces with laser radiation through optical intensification effect in the near field, characterized in that, for the purpose of parallel processing of the materials and obtaining structures on extended surfaces, transparent microstructured masks are used as micro-optical elements for laser radiation focus. 2. Process for structuring the surfaces with laser radiation, according to claim 1, characterized in that microstructured masks made of transparent materials with predetermined geometry other than spherical geometry are used in order to obtain structures with some predefined geometry.