Abstract
Periodic structures of alternating amorphous-crystalline fringes have been fabricated in silicon using repetitive femtosecond laser exposure (800 nm wavelength and 120 fs duration). The method is based on the interference of the incident laser light with far- and near-field scattered light, leading to local melting at the interference maxima, as demonstrated by femtosecond microscopy. Exploiting this strategy, lines of highly regular amorphous fringes can be written. The fringes have been characterized in detail using optical microscopy combined modelling, which enables a determination of the three-dimensional shape of individual fringes. 2D micro-Raman spectroscopy reveals that the space between amorphous fringes remains crystalline. We demonstrate that the fringe period can be tuned over a range of 410 nm – 13 µm by changing the angle of incidence and inverting the beam scan direction. Fine control over the lateral dimensions, thickness, surface depression and optical contrast of the fringes is obtained via adjustment of pulse number, fluence and spot size. Large-area, highly homogeneous gratings composed of amorphous fringes with micrometer width and millimeter length can readily be fabricated. The here presented fabrication technique is expected to have applications in the fields of optics, nanoelectronics, and mechatronics and should be applicable to other materials.
Highlights
Periodic structures of alternating amorphous-crystalline fringes have been fabricated in silicon using repetitive femtosecond laser exposure (800 nm wavelength and 120 fs duration)
We provide a complete characterization of the structures, using optical microscopy, femtosecond microscopy, micro-Raman spectroscopy and scanning electron microscopy, which allows a 3D reconstruction of individual fringes and yields important insights on their formation process
The reflectivity increase within the annular region is consistent with surface amorphization as reported by Bonse[17, 18], since the absorption coefficient of the amorphous phase of Si at the illumination wavelength is known to be much higher than that of the crystalline phase[19], leading to a reflectivity increase. Superimposed to this relatively large structure, parallel bright fringes can be appreciated, which are oriented vertical and perpendicular to the direction of the laser polarization. This is a characteristic of the so-called low spatial frequency (LSF) laser-induced periodic surface structures (LIPSS)
Summary
Periodic structures of alternating amorphous-crystalline fringes have been fabricated in silicon using repetitive femtosecond laser exposure (800 nm wavelength and 120 fs duration). The method is based on the interference of the incident laser light with far- and near-field scattered light, leading to local melting at the interference maxima, as demonstrated by femtosecond microscopy Exploiting this strategy, lines of highly regular amorphous fringes can be written. The use of shorter wavelengths (i.e. UV) and high numerical aperture optics becomes essential for modifying the material with nanometric spatial resolution Under such conditions of tight focusing for writing each individual feature sequentially, processing times become prohibitively long when large areas need to be fabricated. We exploit the full potential of this fabrication technique by controlling systematically the laser irradiation conditions, demonstrating full control over their final properties such as fringe period, size, thickness, surface topography, lateral extension and optical contrast. We provide a complete characterization of the structures, using optical microscopy, femtosecond microscopy, micro-Raman spectroscopy and scanning electron microscopy, which allows a 3D reconstruction of individual fringes and yields important insights on their formation process
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