Near-field Lithography Research Articles

Femtosecond-Laser-Coupled Near-Field Scanning Optical Microscopy Patterning Using Self-Assembled Monolayers

Investigations into the fabrication of nanoscale patterns on gold substrates via direct scanning near-field lithography were carried out. A laser beam is scanned over the workpiece where the pattern formation is desired by near-field scanning optical microscopy (NSOM). A pipette-type nanoprobe having a 100 nm aperture at its apex is used with the NSOM and a frequency-doubled Ti:sapphire femtosecond laser at a wavelength of 395 nm as the illumination source to prevent pulse dispersion of the femtosecond laser pulse. Self-assembled monolayers (SAMs) formed by the adsorption of alkanethiols onto the gold substrate are employed as very thin photoresists. The process underlying photopatterning of SAMs on gold is well-known at the phenomenological level. Alkanethiolates formed by the adsorption of alkanethiols are oxidized to alkylsulfonates upon exposure to UV light in the presence of air. Specifically, it is known that deep-UV light of wavelength less than 200 nm is necessary for oxidation to occur. The results show that an ultrafast laser can replace a deep-UV laser source for the photopatterning of thin organic films. Femtosecond-laser photolithography may be applied to create the patterning of a surface chemical structure or a three-dimensional nanostructure by combination with suitable etching methods.

Super-resolution near-field lithography using planar silver lenses: A review of recent developments

The near-field imaging properties of planar silver layers are reviewed here, with a view to identifying their potential and pitfalls for sub-100 nm photolithography. Initial studies have shown that half-pitch features of 60–70 nm can be achieved using simple contact exposure with low-cost mercury lamp sources, which is less than 1/5th of the exposure wavelength. Sub-100 nm isolated features and line pairs with sub-100 nm spacing have also been resolved, and conventional resolution limits are exceeded in these cases too. Multi-layer silver lenses have also been studied, as it has been predicted that these will improve image resolution for a fixed mask-resist separation. Initial experimental results are inconclusive, as roughness on the additional interfaces tends to mitigate the benefits of going to the laminated structure. Simulations have been used to support the findings of the silver-lens experiments, and it is predicted that sub-50 nm half-pitch resolution should be possible.

Generating continuous variable quantum codewords in the near-field atomic lithography

Recently, Gottesman et al (2001 Phys. Rev. A 64, 012310) showed how to encode a qubit into a continuous variable quantum system. This encoding was realized by using non-normalizable quantum codewords, which therefore can only be approximated in any real physical set-up. Here we show how a neutral atom, falling through an optical cavity and interacting with a single mode of the intracavity electromagnetic field, can be used to safely encode a qubit into its external degrees of freedom. In fact, the localization induced by a homodyne detection of the cavity field is able to project the near-field atomic motional state into an approximate quantum codeword. The performance of this encoding process is then analysed by evaluating the intrinsic errors induced in the recovery process by the approximated form of the generated codeword.

Recent developments in nanofabrication using scanning near-field optical microscope lithography

In addition to its well-known capabilities in imaging and spectroscopy, scanning near-field optical microscopy (SNOM) has recently shown its great potentials for fabricating various structures at the nanoscale. A variety of SNOM-based fabrication techniques have been developed for different applications. In this paper, the SNOM-based techniques involving three major functions: material modification, addition, and removal, are examined with emphasis on their abilities and reliability to make structures with resolutions at the nanometer level. The principles and procedures underlying each technique are presented, and the differences and uniqueness among them are subsequently discussed. Finally, concluding remarks are provided to summarize the major techniques studied and to recommend the scopes for technology improvement and future research.

A comparison of near-field lithography and planar lens lithography

Abstract The resolution of a near-field lithography technique that uses a planar silver lens to form a near-field image has been investigated and compared with hard-contact lithography. Sub-diffraction-limited imaging has been observed through a 50 nm silver film, confirming a recent superlensing proposal [J.B. Pendry, Phys. Rev. Lett. 85 (2000) 3966], with grating periods down to 170 nm pitch being patterned into resist using simple broadband UV-illumination.

Fabrication of Biological Nanostructures by Scanning Near-Field Photolithography of Chloromethylphenylsiloxane Monolayers

We demonstrate the fabrication of sub-100-nm DNA surface patterns by scanning near-field optical lithography using a near-field scanning optical microscope coupled to a UV laser and a chloromethylphenylsiloxane (CMPS) self-assembled monolayer (SAM). The process involves 244-nm exposure of the CMPS SAM to create nanoscale patterns of surface carboxylic acid functional groups, followed by their conversion to the N-hydroxysuccinimidyl ester and reaction of the active ester with DNA to spatially control DNA grafting with high selectivity.

Deep subwavelength nanolithography using localized surface plasmon modes on planar silver mask

The development of a near-field optical lithography is presented in this paper. By accessing short modal wavelengths of localized surface plasmon modes on a planar metallic mask, the resolution can be significantly increased while using conventional UV light source. Taking into account the real material properties, numerical studies indicate that the ultimate lithographic resolution at 20nm is achievable through a silver mask by using 365nm wavelength light. The surface quality of the silver mask is improved by adding an adhesion layer of titanium during the mask fabrication. Using a two-dimensional hole array silver mask, we experimentally demonstrated nanolithography with half-pitch resolution down to 60nm, far beyond the resolution limit of conventional lithography using I-line (365nm) wavelength.

Enhancement of near-field phase-shifting contact lithography by immersion technique

Near-field phase-shifting contact lithography is theoretically modeled incorporating the immersion technique for improvement of photoresist features. The absorption patterns in the photoresist layer, which correspond to the resolution of features after development, are found to be localized in a more compact and uniform fashion with the immersed lithographic system than with the dry system. Therefore, the resolution and profiles of the high-aspect-ratio features can be notably enhanced by immersion lithography.

Optical near-field lithography in chalcogenide films

Abstract Chalcogenide glasses have attracted much interest for use in many fields of optics: microoptics, diffractive optics, optical communication, non-linear optics, etc. This paper focuses on the fabrication of subwavelength gratings based on nano-photolithography using chalcogenide films and scanning atomic force microscope (AFM) equipped with an optical near field source. The nano-lithography system is described and the results obtained using As 2 S 3 chalcogenide films are presented. Variable period gratings with 100 nm line width are presented.

Near-field optical lithography method for fabrication of the nanodimensional objects

A new method of contact scanning near-field optical lithography has been developed to enable fabrication of elements with characteristic dimensions of 30–50 nm. The method involves the deposition of a thin-layer polymer-metal coating, the thermal destructive deformation of a top metal layer with a probe of scanning near-field optical microscope (SNOM), the transfer of the pattern through the polymer by using dry etching and the formation of various nanoelements through this prepared mask. The method is applicable to any material, e.g. metal, dielectric, light/heavy doped semiconductors for the formation of nanometre objects.

A two-dimensional photonic structure made from a conjugated, fluorescent polymer

We report the fabrication of a two-dimensional periodic structure made from the conjugated, fluorescent polymer poly(p-phenylene vinylene), PPV, by direct-write near-field lithography of a photosensitive precursor of the conjugated polymer. The precursor film is illuminated with ultraviolet radiation using the apertured fibre of a home-built scanning near-field optical microscope (SNOM), and unexposed areas are subsequently dissolved in methanol. The fully conjugated polymer is obtained by thermal conversion under vacuum. The structure is approximately 10 mu m x 10 mu m, and consists of 30 nm tall pillars on a 330 nm periodicity lattice. We present a calculation of the photonic properties and propose that structures made with the method investigated here are promising candidates for photonic crystal lasers. We also report the results of preliminary investigations aimed at increasing the pillar height.

Electromagnetic modeling of near-field phase-shifting contact lithography with broadband ultraviolet illumination

Near-field phase-shifting contact lithography is modeled to characterize electromagnetic absorption in a photoresist layer with one face in contact with a quartz binary phase-shift mask. The broadband ultraviolet illumination is represented as a frequency-spectrum of normally incident plane waves. A rigorous coupled-wave analysis is carried out to determine the absorption spectrum of the photoresist layer. The specific absorption rate in the photoresist layer is calculated and examined in relation to the geometric parameters. Columnar features in the photoresist layer are of higher quality on broadband illumination in contrast to monochromatic illumination, in conformity with some recent experimental results. Feature resolution and profile are noticeably affected by the depth of the grooves in the phase-shift mask. Ideally, the feature linewidth can be less than about 100 nm for broadband illumination in the transverse-magnetic mode. These conclusions are subject to modification by the photochemistry-wavelength characteristics of the photoresist.

Fabrication of sub-100nm Patterns using Near-field Mask Lithography with Ultra-thin Resist Process

A resist pattern of half-pitch (hp) 50 nm, 120 nm deep was fabricated with near-field lithography (NFL) of i-line (l = 365 nm) using a positive-tone chemically amplified resist and a tri-layer resist process developed for NFL. The experimental results were in close agreement with the numerical results of electro-magnetic analysis using finite-difference time domain (FDTD) method. The possibility of fabricating sub-50 nm patterns was discussed over the numerical results.

Near-field optical lithography using a planar silver lens

A variation of proximity lithography has been proposed using a planar metallic film in a near-field configuration, where metal acts as a near-field “superlens.” We report here on experimental evidence of such optical lithography with a planar silver lens. Silver layers of varying thickness (85–120nm) placed at specific distances (40–60nm) below a patterned mask were able to image the mask’s features onto a photoresist located after a gap (26–60nm) below the silver. The entire structure was exposed from above with a mercury lamp. Feature sizes as small as 250nm (at a 500nm period) were imaged, demonstrating the lensing ability of the planar silver slab.

Investigation of heating effects in near-field experiments with luminescent organic semiconductors

We present a study of heating effects in apertured near-field gold-coated probes which makes use of the temperature dependence of the photoluminescence spectra of the polymer semiconductor poly(p-phenylene vinylene) (PPV). The small throughput of metal-coated probes used for aperture-scanning near-field optical microscopy (aperture-SNOM) can induce significant probe heating, even when operated with laser powers of only a few mW. According to recent literature the probe apex can reach several hundred degrees celsius, possibly producing local heating of the specimen. Such a possibility needs to be investigated carefully, since it could affect the results of both near-field spectroscopy or lithography, especially where the photosensitive material happens to be sensitive to high temperatures, as for the PPV precursor. Interestingly, the sensitivity of conjugated polymers optical properties to changes in temperature (blue-shift of either photoluminescence or absorption upon heating), can be used as a convenient tool for investigation of the sample heating. Here, we report a comparison between photoluminescence spectra collected in the far-field but with excitation in either the far- or near-field, which demonstrate the absence of any significant difference, and thus indicate a negligible heating of the polymers during SNOM experiments. In addition, we demonstrate that near-field illumination of PPV precursor films with red light (i.e. with a wavelength longer than the absorption edge of PPV precursor), does not result in any lithographic effect, confirming that thermolithography of the PPV precursor is not significant in our lithography experiments.

Submicron imaging with a planar silver lens

Optical imaging through a thin planar silver layer has been achieved by utilizing near-field lithography techniques. A 120 nm thick silver lens that was placed 60 nm below a patterned mask, imaged the mask’s features onto a photosensitive material located 60 nm below the silver. The entire structure was exposed from above with a mercury lamp. Features sizes as small as 350 nm (at a 700 nm period) were imaged onto the photosensitive material, demonstrating the lensing ability of the planar silver slab.

Writing subwavelength-sized structures into aluminium films by thermo-chemical aperture-less near-field optical microscopy

An optical near-field at the tip of an atomic force microscope probe is utilised to pattern aluminium thin films on glass substrates by photo-thermally induced corrosion in water. Aluminium forms a thin passivating oxide layer when immersed into neutral water at room temperature. Owing to the high energy density of the near-field, the metal below the probe tip can be heated to 100°C due to absorption of the light, which then provokes breakdown of the passivation and metal corrosion. The localised near-field is generated by tip-induced enhancement of an evanescent field originating from a laser beam, that is totally internally reflected at the glass–aluminium–water interface. The process is governed by surface plasmons excited in the aluminium film by the evanescent waves and the field enhancement of the probe tip. Holes of 40 nm diameter and lines below 100 nm width have been written into a 20-nm-thick aluminium film. Applications of the scanning probe lithography process may include the one-step fabrication of point contacts or contact masks for near-field optical lithography and reactive ion etching.

Fabrication of conjugated polymers nanostructures via direct near-field optical lithography

We report our investigations into the fabrication of nanostructures of poly( p-phenylene vinylene) via direct scanning near-field lithography of its soluble precursor. Our technique is based on the spatially selective inhibition of the precursor solubility by exposure to the ultraviolet optical field present at the apex of commercially available, Au-coated near-field probes with aperture diameters between 40 and 80 nm (±5 nm). After development in methanol and thermal conversion under vacuum we obtain features with a minimum dimension of 160 nm. We analyse our results via tapping-mode atomic force microscopy, and find a clear phase contrast between the core and the centre of the lithographed features, corroborating the hypothesis that hard, fully insolubilised regions are surrounded by a gel-like phase, which we estimate of the order of 110–130 nm for the smallest features, by comparing our experiments with simulations carried out using a Bethe–Bouwkamp model. Use of such model also allows us to discuss the influence of probe size, tip–sample distance, and film thickness on the resolution of the lithographic process. We demonstrate the use of the technique for the direct writing of two-dimensional periodic structures with intentional defects and a periodicity relevant to applications in the visible range.

Optimizing the near field around silver tips.

Owing to their promise of obtaining optical as well as topographic information in nanometer scale, apertureless near-field scanning optical microscopy (NSOM) and apertureless near-field scanning optical spectroscopy have drawn much attention recently. However, NSOM is still not a mature technique. A proper understanding of and the ability to tune the near field around the tip end is critically important in NSOM instrumentation and in NSOM image interpretation. On the basis of reflection geometry, we systematically studied the effects of a number of parameters pertinent in the application of apertureless NSOM, e.g., polarization, incident angle, wavelength of the incident laser, tip material, and tip length, by using the generalized field propagator technique. Our results show that all the above parameters have a significant influence on near-field enhancement and that care must be taken in the design of the experiment in order to maximize the near field. In addition to apertureless NSOM and spectroscopy, apertureless near-field lithography can benefit from these simulation results.

Rigorous electromagnetic modeling of near-field phase-shifting contact lithography

Electromagnetic absorption in a photoresist layer sandwiched between a silicon substrate and a quartz binary phase-shifting mask (employed in near-field phase shift contact lithography) is theoretically analyzed. A rigorous coupled-wave analysis is used to compute the distribution of specific absorption rate (SAR) in the photoresist layer on either monochromatic and polychromatic illumination. The nonuniform distribution of the SAR in the photoresist layer for the printing of high-aspect-ratio features is systematically examined in relation to the geometric dimensions of the binary phase mask, the photoresist layer thickness, and the polarization state of the incident plane wave. Two methods for the improvement of feature resolution and profile are also discussed.