Silicon bump arrays by near-field enhanced femtosecond laser irradiation in fluorine liquid precursors
Graphical abstract
Highlights
► A simple approach to fabricate periodic array of conical bumps on silicon (Si) substrate is reported. ► The influence of the medium on the near-field interactions for both sizes silica particles layer is investigated. ► By varying the sizes for the colloidal particles, i.e. 700 nm and 390 nm, we were able to tune, in the near field, the surface density of the Si bumps. ► The developed Si nanobump pattern has potential applications for sensitive detectors, efficient photovoltaic cells, field emitter arrays and displays.
Introduction
Nanoprocessing technology plays an important role for emerging nanotechnology. Most widely used industrial technology for ultralarge scale integration is photolithography using vacuum-UV ArF (193 nm) and F2 (157 nm) excimer lasers, which requires highly precise mask, expensive advanced dispersion-free optical system, complex multilayer resists, and accurate optical alignment system. Many research groups have demonstrated patterning of surfaces at resolution below the diffraction limit using optical near-field effects through the use of light-coupling masks [1], an evanescent near-field optical lithography method [2], or embedded-amplitude mask [3]. A nanofabrication method induced by the optical near-field around transparent particles, without using a scanning probe, enables to realize high throughput nanohole patterning with a simple apparatus [4]. Since the localized spot is governed mainly by the particle size, the method permits the fabrication of nanoscale pits or dents on a solid surface [5], [6], [7], [8], [9]. Theoretical and experimental results on the tuning effect of a liquid (water) media on the focusing properties of self-assembled particle-lens array and a single particle on a glass substrate were also presented in the literature [10]. Due to the presence of water medium, the multiple focusing spots of the microsphere array illuminated by femtosecond (fs) laser were tuned to positions beneath the surface, and focal length is greatly extended. Depending on the laser fluence, it has been demonstrated that different micro/nano-structures such as nano-height ring-bumps and convex bumps can be fabricated on glass surface in large area without cracks and debris. In this article, we report on the fabrication of silicon conical shape bumps on an n-doped Si (1 0 0) wafer (Crystal GmbH). The method employs a regular two-dimensional (2D) lattice of 700 nm and 390 nm silica microspheres (Bangs Laboratories, Inc). Such lattices were formed by well known self-assembly processes from colloidal suspensions [11]. In contrast to previous reports published in the literature, we have placed the lattice of silica microspheres (deposited on the Si wafer) at the bottom of a glass container filled with liquid precursors, such as 1,1,2 trichlor-trifluorethan (C2Cl3F3). This liquid precursor with refractive index of has been chosen due to the silicon etching activity of the molecules containing chlorine and fluorine [12]. The effects of femtosecond laser processing of Si in a fluorine liquid precursor have been investigated by our group in the far field. In this paper, the ablation and photochemical/photothermal processes induced by the optical near-field around the particles are investigated. The created nanofeatures under the particles and its morphology, both on 700 nm and 390 nm layered system, are studied.
Section snippets
Materials and methods
In the experiments, silica colloidal particles with diameter of 700 nm and 390 nm were used. The monodisperse silica spheres were applied to an n-doped Si (1 0 0) substrate after the suspension had been diluted with deionized water. The Si substrate, previously kept for some minutes in an oxygen plasma environment in order to transform them from hydrophobic to hydrophilic behavior, was kept still until all of the water had been evaporated. As a result, a silica bead, mono layer array, within a
Results and discussion
It is known that there is an optical field enhancement by micro/nanospheres in the near-field region, which can be explained by the Rayleigh and Mie scattering [13]. Rayleigh scattering takes place when the diameter of the sphere is less than the wavelength of the light. In this case, the sphere is treated as dipole radiator. When the diameter of the sphere is larger than the wavelength, light is scattered elastically according to the Mie scattering law. For the sphere diameter larger than the
Conclusion
In summary, we presented a simple and efficient technique to fabricate periodical array (hexagonal) of Si bumps onto silicon substrate surface. Due to the presence of liquids, like trichlor-trifluorethan, the multiple focusing spots of the silica spheres array illuminated by fs laser were tuned to positions beneath the surface and the induced structures have the shape of bumps, with a silicon composition. By varying the sizes for the colloidal particles, i.e. 700 nm and 390 nm, we were able to
Acknowledgements
This work is supported through the project Nucleu 37N/2012.
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