Recent developments in nanofabrication using scanning near-field optical microscope lithography
Introduction
Nanofabrication aims at building structures with nanoscale features, which can be used as components, devices, or systems in large quantities and at potentially low costs. These nanostructures should allow their integration into complex hierarchical products. Generally, nanoscale features refer to any characteristic that is between 0.1 and 100 nm in size. Most current technologies used in the nanofabrication industry have evolved from conventional lithographic processes, which have been developed for the semiconductor industry for making microelectronic circuits and components. However, there are many challenges to conventional lithographic techniques as they are approaching their fundamental size limits. New fabrication strategies are therefore required to secure cost-effective and technically feasible options. Currently, scanning near-field optical microscopy (SNOM)-based technology has become increasingly popular in the fabrication of nanoscale structures due to its low cost and great technical potential. In this paper, an overview of the applications of the SNOM technology for nanofabrication is presented with an emphasis on its ability to fabricate a wide variety of nanostructures.
SNOM, also known as near-field scanning optical microscopy (NSOM), is one of the major proximal probe technologies that allows optical excitation of materials at spatial resolutions well below the diffraction limit and has become a powerful emerging tool for nanofabrication. SNOM was independently demonstrated by Pohl et al. [1] and Lewis et al. [2] in 1983, shortly after the invention of the first proximal probe technology, the scanning tunneling microscope (STM), by Binnig and Rohrer [3] in 1981. The basic principle of SNOM is similar to its predecessor. STM is based on the detection of electron tunneling currents, whereas SNOM senses the tunneling of photons. In fact, this difference is what gives SNOM an advantage over STM, i.e., the SNOM sample can be non-conductive.
Proximal probe technologies are revolutionizing the physical, chemical, and biological sciences, owing to their ability to explore and modify matter at the nanoscale, and to operate in various environments (including liquids and air). Proximal probe techniques rely on the use of nanoscale probes that are positioned and scanned in the immediate vicinity of the material surface. The technologies continue to open new doors, while serving as our eyes and hands in the exploration and manipulation of material surfaces at nanometer scales. In addition to STM and SNOM, atomic force microscopy (AFM) has also become one of the most popular proximal probes in the nanotechnology arena. Generally, STM is little more than a pointed electrode scanned closely over a conductive surface, where AFM uses a cantilever probe to detect the attractive and repulsive forces between the tip and sample in close proximity. A recent review of these two technologies for nanofabrication can be found in Tseng et al. [4].
In SNOM, an optical probe with an aperture diameter well below the optical wavelength is typically used to replace the STM or AFM probe. The aperture is often made by tapering an optic fiber to a confined tip and by coating all but the tip with metal. As shown in Fig. 1a, by illuminating the sample with near-field light, the probe tip is held very close to the sample. The evanescent light through the aperture is incident on the sample and excites the atoms in the surface to re-radiate propagating waves. The same probe can then pick up the propagating light and transmit it to an appropriate detector. SNOM has been used for studying molecular exciton, optical properties, the properties of organic and inorganic films, and microelectronic devices. The fundamentals and applications of SNOM with aperture probes for analysis and measurement can be found in a recent review paper by Hecht et al. [5].
In aperture-based SNOM, the nanometer-scale aperture limits the light throughput and resolution. To overcome these limitations, Zenhausern et al. [6], [7] developed an apertureless SNOM that uses an apertureless probe, a sharp tip without any apertures similar to an STM or AFM tip, to scan within the near field of the sample. The sample is first irradiated from above by far-field light in order to produce the evanescent field on the surface of the sample as shown in Fig. 1b. The evanescent photons then excite the atoms in the probe tip, which re-radiate the propagating photons. Far-field optics is used for both illumination and collection of light scattered from the vibrating scanning sharp tip. Unlike in aperture SNOM, where the contrast results from a weak source (aperture) dipole interacting with the polarizability of the sample, imaging by aperatureless SNOM relies on a different contrast mechanism: sensing the dipole–dipole coupling of two externally driven dipoles (the tip and sample dipoles) as their spacing is modulated. In this paper, techniques using both aperture and apertureless probes for nanofabrication will be evaluated and their respective pros and cons will also be discussed.
In this paper, the nanolithographic applications of SNOM or SNOM lithography (SNOML) will be studied. The principles and approaches of different techniques developed for SNOML are first evaluated. The nanostructures made by these techniques are specifically presented in order to illustrate the versatility and advancement of these SNOM-based techniques. The major variances as well as the associated strengths and weaknesses for the different techniques considered are examined. Finally, prospective developments and research focuses for SNOM-based techniques are presented. The organization of this paper will follow the taxonomy of fabrication; the techniques examined will be clustered into three categories: material modification (including resist exposure), material removal (including etching) and material addition (mainly induced deposition). A wide variety of induced energy sources, including electrostatic, optical, optochemical, optoelectrical, optomagnetic, and thermal have been adopted by the SNOM-based techniques, which enable the SNOML to be a versatile manufacturing tool in a nanoscale world.
Section snippets
Material modification
In applications for material modification, the SNOM probe is used as an energy source for modifying a variety of materials at the nanoscale level, including conventional photoresists and non-conventional electro-magneto-optical (MO) materials as well as ferroelectric and self-assembled monolayers (SAM). Patterns are generated by scanning the probe over these target surfaces. The effects of direct photoexposure, hydrogen passivation, photoisomerization, optical magnetization, and thermal phase
Material removal or etching
SNOM can be utilized to perform two types of material removal functions. One is employing photon energy to induce or assist a chemical etching process, while the other is exercising photon or thermal energy to physically ablate or melt materials. Possible applications of the SNOM ablation technique for nanostructuring, repair, and production of lithographic masks will be discussed in this section.
Wysocki et al. [24] performed laser-induced chemical etching of Si(100) in a 300 mbar Cl2
Material addition or deposition
Diesinger et al. [32] have applied an SNOM with an Al-coated optical fiber tip to electrochemically deposit Ni dots on Si substrates. The optical tip, which has an aperture of 100 nm in diameter, acts as a local near-field light source to control a photocurrent on a nanoscale to create Ni structures by locally triggering the electrochemical reduction of nickel ions. The photocurrent that is generated by a He–Ne gas laser (λ=633 nm) chopped at 400 Hz and that flows between the tip and substrate
Apertureless SNOM
To have a lithographic resolution at nanoscales, the tip aperture has to be well below 100 nm. However, the amount of light that can be transmitted by a small aperture poses a limit on how small it can be made. For typical wavelengths adopted, if the aperture is 100 nm, less than third orders of magnitude of light can pass through, while when it reaches 50 nm, only one part in 107 makes it through [37]. The input power cannot be increased too high because about one third of the power can be
Concluding remarks
Nanofabrication is an essential tool for the sustained evolution of electronic, photonic, biomedical, and nanosystem technologies. This paper has reviewed the current developments of the SNOM-based techniques for nanofabrication. The principles, procedures, achievements, and potential for each technique have been presented. In particular, a wide variety of nanostructures made by these techniques have been examined to specifically illustrate their respective feasibilities and limitations.
Acknowledgements
The author gratefully acknowledges the financial support for this study by the US National Science Foundation under Grant Nos. DMI-0002466, CMS-0115828, and DMI-0423457 and by Pacific Technology of Arizona. The encouragement and help from Dr. Andrea Notargiacomo of Roma TRE University (Italy) should be specifically acknowledged. Thanks are also due to Mr. Timothy M. Russell and Jeremiah J. Gutierrez-Jensen of Arizona State University for their help in preparing this manuscript.
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