Werayut Srituravanich

Flying plasmonic lens in the near field for high-speed nanolithography

The commercialization of nanoscale devices requires the development of high-throughput nanofabrication technologies that allow frequent design changes. Maskless nanolithography, including electron-beam and scanning-probe lithography, offers the desired flexibility but is limited by low throughput. Here, we report a new low-cost, high-throughput approach to maskless nanolithography that uses an array of plasmonic lenses that flies above the surface to be patterned, concentrating short-wavelength surface plasmons into sub-100 nm spots. However, these nanoscale spots are only formed in the near field, which makes it very difficult to scan the array above the surface at high speed. To overcome this problem we have designed a self-spacing air bearing that can fly the array just 20 nm above a disk that is spinning at speeds of between 4 and 12 m s(-1), and have experimentally demonstrated patterning with a linewidth of 80 nm. This low-cost nanofabrication scheme has the potential to achieve throughputs that are two to five orders of magnitude higher than other maskless techniques.

Plasmonic Nearfield Scanning Probe with High Transmission

Nearfield scanning optical microscopy (NSOM) offers a practical means of optical imaging, optical sensing, and nanolithography at a resolution below the diffraction limit of the light. However, its applications are limited due to the strong attenuation of the light transmitted through the subwavelength aperture. To solve this problem, we report the development of plasmonic nearfield scanning optical microscope with an efficient nearfield focusing. By exciting surface plasmons, plasmonic NSOM probes are capable of confining light into a 100 nm spot. We show by nearfield lithography experiments that the intensity at the near field is at least one order stronger than the intensity obtained from the conventional NSOM probes under the same illumination condition. Such a high efficiency can enable plasmonic NSOM as a practical tool for nearfield lithography, data storage, cellular visualization, and many other applications requiring efficient transmission with high resolution.

Realization of optical superlens imaging below the diffraction limit

Recently, the concept of superlensing has received considerable attention for its unique ability to produce images below the diffraction limit. The theoretical study has predicted a superlens made of materials with negative permittivity and/or permeability, is capable of resolving features much smaller than the working wavelength and a near-perfect image can be obtained through the restoration of lost evanescent waves (Pendry 2000 Phys. Rev. Lett. 85 3966–9). We have already demonstrated that a 60 nm half-pitch object can indeed be resolved with λ0/6 resolution with the implementation of a silver superlens with λ0 = 365 nm illumination wavelength, which is well below the diffraction limit (Fang et al 2005 Science 308 534–7). In order to further support the imaging ability of our silver superlens, a two-dimensional arbitrary object with 40 nm line width was also imaged (Fang et al 2005 Science 308 534–7). In this paper, we present experimental and theoretical investigations of optical superlensing through a thin silver slab. Experimental design and procedures as well as theoretical studies are presented in detail. In addition, a new superlens imaging result is presented which shows the image of a 50 nm half-pitch object at λ0/7 resolution.

Sub-100 nm lithography using ultrashort wavelength of surface plasmons

The development of a nanolithography technique utilizing ultrashort wavelength of surface plasmons (SPs) is presented in this article. The mask consists of silver thin film perforated with two-dimensional hole arrays exhibiting superior confinement due to SPs with a wavelength equal to 14 of that of the illuminating light (365 nm). This short wavelength of SPs can confine the field on an area much smaller compared to the excitation light wavelength, leading to the higher resolution lithography than conventional photolithography methods. Finite-difference time-domain simulations show significantly enhanced electric field and tight confinement of the near-field profile obtained from silver plasmonic masks, where features as small as 30 nm can be resolved. Furthermore, the lithography experiments have been performed with demonstration of sub-100 nm spatial resolution.

Broad Band Two-Dimensional Manipulation of Surface Plasmons

A plasmonic interference pattern can be formed when multiple surface plasmon waves overlap coherently. Utilizing a sharp edge coupling mechanism, we experimentally demonstrate plasmonic interference patterns that can be designed at will by shaping the edges in a metallic film. The patterns can also be dynamically tailored by adjusting the wavelength, the polarization, and the incident angle of the excitation light beam. Possessing the subdiffraction limited feature resolution, this dynamical manipulation method of surface plasmon patterns will have profound potentials in nanolithography, particle manipulation, and other related fields.

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.

Fabrication and characterization of novel microneedles made of a polystyrene solution

Nowadays, microneedles are attracting a lot of attention because microneedles can deliver drugs, vaccines and hormones into the body without pain unlike conventional hypodermic needles. Furthermore, microneedles are safe for self-injection and disposal. This work aims to develop novel microneedles made of a solution of polystyrene (PS) in toluene. The mechanical properties of the fabricated PS microneedles were characterized in failure strength test and skin penetration test. According to the experimental results, a PS microneedle could withstand a large force up to 1.0 N without fracturing. Owing to the superior mechanical strength, the PS microneedles could penetrate the skin without any deterioration making them a promising alternative for commercial applications.

Investigation of Optimal Silk Film Thickness in Silk Microneedle Fabrication

Injection is one of the most commonly used methods for delivering drugs or vaccines into human bodies. It is rapid, low-cost and compatible with almost any drugs. However, the major drawbacks of the injection by hypodermic needles are the pain associated with the injection and the disposal of used needles. Microneedles have then received wide attention since they can overcome such drawbacks, especially dissolving microneedles. Recently, silk fibroin has been used to fabricate dissolving silk microneedles for transdermal drug delivery at low temperature. In the fabrication process, the quality of the silk microneedles relies on the solidification of silk fibroin solution. This research aims to study the role of silk fibroin concentration (silk film thickness) in the formation of silk microneedles. In the experiment, silk microneedles were fabricated using various concentrations of silk fibroin solution from 3 to 7% while the volume of the silk fibroin solution was fixed. According to the experimental resuls, it was found that the concentrations of 4-5% were suitable for producing silk microneedles (silk film thickness of 470 μm) while the concentrations of 6-7% caused wrinkles on microneedle patch due to mismatch of upper and lower layers of microneedles. Furthermore, the concentration of 3% had a problem with the demolding step of microneedles since it caused mold damage due to strong adhesion force between microneedles and mold.

Plasmonic nearfield scanning optical microscopy

Nearfield scanning optical microscopy (NSOM) offers a practical means of optical imaging at a resolution well beyond the diffraction limit of the light. However, its applications are limited due to the strong attenuation of the light transmitted through the sub-wavelength aperture. To solve this problem we report the development of plasmonic nearfield scanning optical microscope with a high optical coupling efficiency. By exciting surface plasmons, plasmonic NSOM probes are capable of focusing light into a 100 nm spot. Both numerical simulation and nearfield exposure experiments have demonstrated that the intensity at the focal point is at least 10 times stronger than can obtained from the conventional NSOM probes under the same illumination condition. By providing a strong nano-scale light source, plasmonic NSOM can be used as a high speed nano-scale imaging tool for cellular visualization, molecule detection, and many other applications requiring high temporal resolution.

Subwavelength nanolithography using surface plasmons

We have investigated the novel plasmonic nanolithography by exposing a photoresist layer through a plasmonic mask, which is an opaque metal film with subwavelength hole arrays in it. The hole arrays of various diameters are fabricated by using focused ion beam (FIB). Through the lithography, the hole array patterns are transferred to negative photoresists. As a result, high contrast dot arrays with the smallest diameter of 120 nm, equivalent to /spl sim//spl lambda//3, are observed by atomic force microscope (AFM).