In this paper, we proposed a tapered hollow tube which can produce a near diffraction-limit spot and focus the incident
light in far-field region. From previous researches, the sub-wavelength annular aperture (SAA) made on metallic film
generates a Bessel beam in far-field region. Also, the traditional tapered fiber has been widely used in near-field
scanning optical microscope (NSOM) to achieve super-resolution in near-field. Combining these two concepts, tapered
hollow tube was shown to have great potential in creating a small sub-micron spot size and long depth of focus (DOF)
emitted light beam. By using the commercially available capillary and fiber heat-pulling method, it was found that tube
processed per design to be disclosed in this paper can achieve Bessel beam as well. It will be shown that the SAA-like
structure was actually implemented by the geometry of the tube tip. From FDTD simulation and experiment, the emitted
beam was identified to have more than 10 μm DOF and 250-300 nm focal spot excited by using the 408 nm laser source.
These results not only can help us pursue lithography applied to create through silicon via (TSV) process in far-field
region while maintaining near diffraction-limit spot size. The high throughput and side lobe became a serious problem
when continuous incident light was used. To circumvent this problem, the incident light from was changed from
continuous to pulse type and a suitable lithography experimental system designed by using three-axis displacement
platform was developed. All results will be detailed in this paper.
We experimentally examined the effect of laser energy fluence on the ablation of a silicon wafer using a Ti:sapphire
femtosecond laser system. A femtosecond laser was focused through an oxide-metal-oxide (Al2O3/Al/Al2O3) film
engraved with a subwavelength annular aperture (SAA) structure, i.e., a Bessel beam composed of a femtosecond laser
created using a SAA. The optical performance, such as depth-of-focus (DOF) and focal spot of the SAA structure, was
simulated using finite-difference time domain (FDTD) calculations. We found that a far-field laser beam propagating
through the SAA structure possesses a sub-micron focal spot as well as high focus intensity. The experimental results
demonstrated that silicon can be ablated using an input ablation threshold of an order of 0.05 J/cm2 with a pulse duration
at around 120fs. We found the obtained surface hole to have a diameter smaller than 1μm. Different surface ablation
results obtained by using different threshold fluences of input laser energy are shown. Possible applications of this
technique includes executing high aspect ratio laser drilling for thin film microfabrication, undertaking thru silicon via
(TSV) for 3DIC, etc.
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