Nitul S. Rajput

Electron- and ion-beam-induced maneuvering of nanostructures: phenomenon and applications

Electron-and ion-induced bending (EIB/IIB) phenomena have been studied in self-supported polycrystalline metallic and metal-amorphous bilayered nanocantilevers. The experiments reveal many interesting facts regarding electron/ion-matter interaction, which builds a proper foundation for the understanding of the phenomenon. The mechanism for bending of metallic cantilevers has been proposed to be primarily due to void-induced stress generation during ion beam irradiation. On the other hand, thermal effects have been found to play the dominant role in the case of bending of bilayer (amorphous-metal) nanocantilevers. The instantaneous, reversible, highly controllable and permanent nature of the process has been exploited to fabricate several complicated nanostructures in three dimensions. IIB of the fabricated cantilevers is shown to have a high precession mass sensing aptitude, capable of detecting a change in mass of the order of femtograms.

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.

Role of the substrate in the electrical transport characteristics of focused ion beam fabricated nanogap electrode

Precise metallic nanogap structure is fabricated on a glass substrate by using a 30 keV focused Ga ion beam. While investigating the I-V behavior of the nanogap structure, tunneling through the substrate has been found to play a vital role in the electrical transportation process. Substrate breakdown occurs at a certain applied voltage and a metal vapor state is initiated through intense heat generation at the nanogap region. The experimental observation confirms the role of the substrate in the explosion process. Metallic spherical particles are formed during cooling/condensation of the metal vapors or splashing of the liquid droplets showing a wide distribution of size from few tens of nanometers to few microns.

Unusual dimensional dependence of resonance frequencies of Au nanocantilevers fabricated with self-organized microstructure

Metallic nanocantilevers of gold are fabricated from self-supporting polycrystalline thin film (100 nm) by focused ion beam assisted milling and ion induced manipulation processes. The surfactant assisted growth of the thin film leads to self-organized dendrite like morphology. This self-organized dendrite like morphology of the gold film imposes a new characteristic length scale corresponding to the mean size of gold grains present within the branches of the dendrite pattern in the film. The resonance characteristic investigated on cantilevers having different widths shows a significant drop in energy dissipation and hence an enhancement in the resonance amplitude at a characteristic width. At this width the resonance frequency of a vibrating cantilever approaches the theoretically expected value anticipated from an ideal cantilever treated like an elastic continuum.

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.

Beam scanning effect on focused-ion-beam-induced processing

In order to fabricate a nanostructure precisely, the effect of different parameters involved in the fabrication processes should be extensively studied. We have studied the effect of beam parameters such as beam dwell time, mode of deposition as well as pattern size, on focused-ion-beam-induced deposition of nanopillars. While fabricating the nanopillars in the circular and serpentine mode deposition, long tails of the deposited material are observed. The amount deposited in the tails is found to be a function of the beam dwell time as well as the diameter of the pillar. The tail disappears at higher beam dual time and pillar diameter. The elemental composition of the tail is found to be same as that of the pillar.

P1–18: I-V Characteristics of nanogap electrodes formed by thermally assisted electromigration

Electrodes with nano-scale gaps have been fabricated using metallic nano-wires, derived via milling of thin films with focused ion beam (FIB) and passing current ~10¹²A/m². Their I-V characteristics measured at a pressure ~10⁻⁶ mbar are shown to follow Child-Langmuir law or Fowler-Nordheim field emission depending upon the gap.

Nanoimprint lithography - the past, the present and the future

Background: Nanoimprinting lithography technique uses a very simple concept of transferring pattern of nanoscale features from a mold to a target substrate. In the past two decades, this technique has successfully broken through the barrier of laboratory scale production and become an industrial scale production technique. The aim of this paper is to introduce to readers to the basic working principle, applications, analysis the technological limitations. It will also point out future research direction of this useful nanofabrication technique. Methods: We adopted a systematic approach to give a comprehensive review of the work principle, hardware and analysis of advantaged and disadvantages of major nanoimprint lithography techniques. Moreover, a technical comparison of these methods is carried out to provide future research direction. Results: 87 papers were reviewed. Four techniques including thermal NIL, ultraviolet light NIL, laser-assisted direct imprint and nanoelectrode lithography have been identified as main stream of NIL techniques. These techniques possess certain advantages and disadvantages in terms of cost, throughput, attainable resolution. Lack of flexibility is the common limitation of current NIL techniques. NIL has gained wide applications in the fabrication of optoelectronics devices, solar cells, memory devices, nanoscale cells, hydrophobic surfaces and bio-sensors. The potential applications of NIL in biochips, artificial organs, diagnostic system, and fundamental research in cell biology will demand large scale 3D fabrication capability with resolution towards 10nm or less. Conclusions: The findings of this review confirm that NIL is one of the most employed commercial platforms for nanofabrication which offers high throughput and cost-effectiveness. One of the disadvantages of NIL over other nanofabrication techniques is the flexibility of patterning. Integrating NIL with other existing nanofabrication techniques can be helpful to overcome such issue. The potential applications of NIL in biochips, artificial organs, diagnostic system, and fundamental research in cell biology will attract researchers to push nanoimprint lithography forward at a resolution of 10 nm or less in the future.

FIB Micro-/Nano-fabrication

Focused Ion Beam (FIB) is one of the state of the art micro/nano fabrication tools. It adopts both bottom-up and top-down approaches to generate micro/nano structures and the biggest advantage of FIB lies in its maskless fabrication capability. The efficacy of the machine is now well proven among the scientific community. Modern FIB systems offer highly focused and precised beam of ions, which are effectively used for fabricating precised, high aspect ratio micro/nano structures.