Neeraj Shukla

Increasing lifetime of the plasma channel formed in air using picosecond and nanosecond laser pulses

We report experiments on a pump-probe configuration to elucidate the formation of a plasma channel by the hydrodynamic evolution of air breakdown in laser focus. A stable air breakdown was produced by focusing a picosecond laser pulse to create a shock driven plasma channel in the laser focus for propagating a nanosecond pulse. A four fold increase in the lifetime of the channel estimated by monitoring the temporal evolution of the fluorescence of a spectral line at 504.5nm of N+ transition 3pS3−3sP03 is reported. Assuming plasma in local thermal equilibrium plasma temperature of ∼8.2eV and an electron density of ∼1.4×1018cm−3 were determined using a Stark broadening of 649.2nm line of NII transition 3dD03−4pD3 in the channel. An enhancement in the electron density of the plasma channel was observed at the 7ns delay of the nanosecond laser pulse relative to the picosecond laser pulse.

The out of beam sight effects in focused ion beam processing

We report that during focused ion beam chemical vapor deposition (FIB-CVD) the effect of deposition is not limited to the area where the ion beam scanning takes place but occurs on regions which are out of sight of the incident beam and extends up to several micrometers away from the specified site. This phenomenon has deleterious effects, especially when the nanocontacts are fabricated for the electrical characterization of the nanodevices. The deposition occurs into the gap between the contact pads and acts as a resistance in parallel with the resistance of the nanostructure to be measured. The extended deposition has been explained on the basis of molecular cracking of the precursor gas molecules induced by forward scattered Ga ions, and appropriate measures to remove the effect have been suggested.

Exploring a new strategy for nanofabrication : deposition by scattered Ga ions using focused ion beam

We report a new strategy of nanofabrication using the focused ion beam (FIB)-based chemical vapor deposition method. It utilizes scattered Ga ions to decompose organometallic molecules of the precursor gas for depositing the metallic element on a surface with the advantage of producing uniform metallic coats on those surfaces of nanostructures which are not directly accessible to the primary beam. The method can be used to provide electrical contacts on inaccessible regions of prototype nanodevices, such as ion batteries, electrophoresis cells, cantilevers, etc, which have been demonstrated and explained by depositing Pt and W on different surfaces of 3D nanostructures.

ELECTRICAL TRANSPORT CHARACTERISTICS OF FOCUSED ION BEAM FABRICATED Au, Cu NANOWIRES

Nanowires of Au and Cu were fabricated using a top–down method in which focused ion beam (FIB) milling process has been used. The width of the fabricated nanowires has been kept in the range of 45 to 300 nm and the length in the range of 2–10 μm. In situ electrical measurements of the nanowires were carried out. The resistivities of these wires are found to be higher (five to nine times larger than their bulk values). These results have been understood on the basis of increase in the electron surface scattering due to one-dimensional confinement of the electrons. Also, other effects such as nanogap formation in the range of 40 nm to few hundreds of nanometers, structural changes of the wires, increase of current density with time at constant applied voltage, etc. have been observed during measurements.

Proton beam writing for minimum step lithography in multilayer patterning

We report the use of proton beam lithography as a convenient procedure consisting of minimum steps for micropatterning in polymer multilayers comprising of a metallic intermediate layer – a structure which can facilitate many application including vertical organic field effect transistors. We have used focused micro-beam of 2 MeV protons to pattern array of micro-holes in a 1 μm thick tri-layer film. A tri-layer film was prepared by spin coating 400 nm poly(methyl methacrylate) (PMMA) film on a glass substrate followed by thermal evaporation of 200 nm thick aluminium film and spin coating of a second PMMA layer of thickness 400 nm on the top. MeV protons suffer very little scattering inside the tri-layer film and pass through the tri-layer following almost straight path. Thus MeV proton beam causes molecular chain scissions in the top and the bottom PMMA layer in desired patterns in a single exposure which is not possible with other techniques such as e-beam lithography (EBL) or photolithography. By following a step by step development/etching of the three layers, we achieved an array of through micro-holes in the tri-layer film on the glass substrate. Potential application of such micro-patterns is discussed. We also report optimized parameters of proton beam writing for arrayed micro-structures fabrication in single layer of two different common polymer resists, SU-8 and PMMA. The surface morphology and side wall broadening of the fabricated structures were characterized using SEM and profilometer study respectively.