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Femtosecond Laser Micro-nanofabrication of Semiconductors

Author: GuoLi
Tutor: SongJunFeng
School: Jilin University
Course: Microelectronics and Solid State Electronics
Keywords: Femtosecond laser micro-nanofabrication Reduction of graphene oxide Antimony doped tin dioxide Patterning
CLC: TN304.05
Type: Master's thesis
Year: 2010
Downloads: 370
Quote: 1
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Abstract

In recent years, micro-nanofabrication of conductive semiconductors has been paid much attention due to the requirements of miniaturization and integration of microelectronic devices. Conventional micro-nanofabricative methods such as photoresist-wet chemical etching (Wet etching), reactive ion etching (RIE), focused ion beam etching (FIB), e-beam directwritten (EBDW), nano-imprint lithography (NIL) and sol-gel direc patterning have been successfully used for reducing the size of micro-devices. However, it is still very difficult to create complex microstructures arbitrarily for highly integrated micro-nanodevices .Femtosecond(FS) laser micro-nanofabrication exhibits a series of unique advantages. For example, the focus area of femtosecond laser is very small, which give rise to a much higher resolution compared with the traditional micro-processing technologies. Additionally, the low thermal effects of femtosecond laser is of benefit to good topography. FS laser induced multi-photon absorption could be used for fabrication of three-dimensional microstructures. Therefore, FS laser would be a powerful technique for micro-nanofabrication of desire microstructures towards miniaturization and integration of microelectronic devices.In this work, as two representative examples, we demonstrate the FS laser micro-nanofabrication of conventional semiconductor of SnO2: Sb and novel graphene-based materials for microelectronic devices use.In the fabrication of microstructured SnO2: Sb semiconductor, we prepared a photosensitive SnO2: Sb sol as precusor. Typically, Acetylacetone(AcAc) was introduced into metal ions solution to forme chelate rings in the solvent of 2-methoxyethanol. The as-formed sol-like precursor shows long-term stability at room temperature. From UV-vis absorption spectrum, the obvious absorption could be identified at around 302 nm. After UV irradiation for 30 min, the absorption peak decreases, and the gel film behaves insoluble in acetone. In this work, the light source is FS laser with 790nm wavelength, while the gel has no absorption of 790nm wavelength. Therefore, it is believable that two-photon or multi-photon absorption occurs in processing of the fabrication.During the laser processing, FS laser directly wrote on the SnO2: Sb gel films. The regions irradiated by laser become insoluble in acetone. So after development in acetone, desired microstructures were successfully obtained. 2D and 3D microstructures could be fabricated according to preprogrammed patterns.Crystallized microstructures were finally obtained after high temperature calcination at 500℃for two hours, XRD study shows that the micronanostructures was highly crystallized. The whole set of diffraction peaks were attributed to the tetragonal rutile structure. From the SEM images, before sintering, the gel structure showed the highest resolution of 400nm. After high temperature calcinations in air, the structure shrank, but remained intact, and the resolution is as high as 150nm.Finally, we also fabricated a SnO2: Sb micro-wire between two gold electrodes as a humidity microsensor. Resistance of the wire was calculated to be 5500 ?·cm . When changing the humidity from 11% to 95%, the range of the variational resistance is as wide as 5 orders of magnitude, exhibiting excellent sensing property.In the second case, we present a direct imprinting of graphene microcircuits on graphene oxide (GO) film by FS laser induced deoxidation. Graphene oxides used for processing were prepared by Hummers’ methode. Then graphene oxides aqueous solution was spin-coated on glass substrate at 1000 rpm, giving a transparent film. The thickness of the film was estimated to be about 55nm from AFM characterization. The as-obtained graphene oxide film was used for subsequent processing. After FS laser direct writing on this film according to preprogrammed patterns, various microcircuits were created on the GO film. We carefully characterized the surface height of patterned part and compared it with primary GO film. The microcircuits surface is lower than that of unpatterned part. Possible reason for this phenomenon would be the laser induced deoxidation of GO. XRD, XPS and Raman spectra confirm this explanation. After exposure to FS laser, most of oxygen-containing groups were removed. The reduced GO show conductivity and could be used for electrical devices.We have also reduced and patterned GO film with different output laser power (0.5-3.0mW). The results show that the conductivity of reduced GO has strong dependence on laser power. With the increase of laser power, the conductivity of reduced and patterned GO also increased. The highest conductivity was about 2.56×104S/m, which was obtained with 3.0mW of FS laser. Further increase of laser power could result in a broken GO film.In conclusion, by using FS laser micro-nanoprocessing, we have successfully fabricated microstructured semiconductors of SnO2:Sb and graphene microcircuits towards microelectronic devices. It is believable that the FS laser micro-nanofabrication would be a powerful technique for miniaturization and integration of various microdevices.

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