Focused Laser Lithographic System Based On Vortex Laser Induced Opacity

In this thesis, based on the technology of far fields modulating near fields, a focused laser lithographic system combines with vortex laser induced opacity of photochromic layer to write patterns with linewidth below wavelength. Experimentally, using a 20 x lens (NA= 0.4), this system condenses the linewidth of written patterns from 6614 to 350 nm. This system realizes laser direct writing with sub-wavelength resolution, which is extremely beneficial for mass production of nano-components such as waveguides, gratings, and nano-circuits.The laser with 532 nm wavelength is transformed to a vortex laser after passing through a beam expander, a polarizer, a 1/4 wave plate, and a vortex phase plate; then its center coincides with the center of an expanded laser with a 405 nm wavelength. The polarizer and the 1/4 wave plate are used to provided matched polarization for the rotation of the vortex phase plate. Two lasers are focused by anachromatie objective lens. The substrate, composed of a piece of silicon wafer, a photoresist layer, and a photo-chromic layer, is illuminated by a focused spot at a 405 nm wavelength and an annular spot at a 532 nm wavelength. The photochromic layer contains homogeneous-dispersed metanil ellow. The size of the two spots in the focal plane is the same because the effective numerical aperture of the two lasers is equivalent. In the photochromic layer, the light intensity of the focused spot presents Gaussian distribution and the light intensity of the annular spot presents hollow distribution. Because the absorbance at 405 nm wavelength decreases upon exposure to light with a 405 nm wavelength, and increases upon exposure to light with a 532 nm wavelength, a transparent aperture for a lithographic laser with a 405 nm wavelength is created. After passing through the aperture, the lithographic laser exposes the underlying photoresist which is sensitive to light with a 405 nm wavelength, but insensitive to light with a 532 nm wavelength. The size of the transparent aperture within the photochromic layer could be tuned by changing the ratio of the I532/1405, and it decreases as the ratio increases. In the experiment, desired patterns could be written by the movement of motorized stage. Furthermore, because the light intensity presents Gaussian distribution, the relationship between the diameter of the transparent aperture and the vortex laser power is nonlinear.The principle of the above-mentioned absorbance modulation is based on the photoisomerization of metanil yellow. Upon exposure to light with a 405 nm wavelength, the trans-isomer is transformed to the cis-isomer. This trans-formation leads to a variation of absorption spectrum which is measured by a UV-VIS spectroscopy. Similarly, the reverse reaction is favored upon exposure to light with a 532 nm wavelength. Besides, to build film, metanil yellow was dissolved in an 8% aqueous PVA solution to form a saturated solution; then the solution was spun onto the photoresist layer to form a photochromic layer with a 400 nm thickness. After exposure, samples were rinsed in an aqueous NaOH solution for 60 s and the photochromic layer was removed because metanil yellow is a water-soluble material. After that, the photoresist was developed. Remarkably, metanil yellow, a kind of water-soluble photochromic material, presents several advantages when it is used in photolithography. First, only one procedure is needed to remove the photochromic layer and develop the photoresist. Second, it is suitable for the green industry for it is ecofriendly.In summary, combining the traditional focused laser lithographic system with a vortex laser induced opacity of photochromic material, we discovered that written lines with a sub-wavelength resolution could be obtained. Furthermore, desired patterns could be written by the movement of a motorized stage. This technique is beneficial for the development of massive production of nano-components which are the cores of nano-sensors, nano-probes, etc.