Patterning via Optical Saturable Transitions - Fabrication and Characterization
Summary
We report that the diffraction limit of conventional optical lithography can be overcome by exploiting the transitions of organic photochromic derivatives induced by their photoisomerization at low light intensities.1-3 This paper outlines our fabrication technique and two locking mechanisms, namely: dissolution of one photoisomer and electrochemical oxidation.
Abstract
This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical Saturable Transitions (POST). In this technique the chemical properties of organic photochromic molecules that undergo single-photon reactions are exploited, enabling rapid top-down nanopatterning over large areas at low light intensities, thereby, allowing for the circumvention of the far-field diffraction barrier.4 Simple, cost-effective, high throughput and resolution alternatives to nanopatterning are being explored, such as, two-photon polymerization5,6, beam pen lithography (BPL)7, scanning electron beam lithography (SEBL), and focused ion beam (FIB) patterning. However, multi-photon approaches require high light intensities, which limit their potential for high throughput and offer low image contrast. Although, electron and ion beam lithographic processes offer increased resolution, the serial nature of the process is limited to slow writing speeds, which also prevents patterning of features over large areas. Beam-pen lithography is an approach towards parallel near-field optical lithography. However, the gap between the source of the beam and the surface of the photoresist needs to be controlled extremely precisely for good pattern uniformity and this is very challenging to accomplish for large arrays of beams. Patterning via Optical Saturable Transitions (POST) is an alternative optical nanopatterning technique for patterning sub-wavelength features1-3. Since this technique uses single photons instead of electrons, it is extremely fast and does not require high light intensities1-3, opening the door to massive parallelization.
Introduction
Optical lithography is of key importance in the fabrication of nanoscale structures and devices. Increased advancements in novel lithography techniques has the ability to enable new generations of novel devices.8-11 In this article, a review is presented of a class of optical lithographic techniques that achieve deep sub-wavelength resolution using novel photoswitchable molecules. This approach is called Patterning via Optical-Saturable Transitions (POST).1-3
POST is a novel nanofabrication technique that uniquely combines the ideas of saturating optical transitions of photochromic molecules, specifically (1,2-bis(5,5’-dimethyl-2,2’-bithiophen-yl))perfluorocyclopent-1-ene. Colloquially, this compound is referred to as BTE, Figure 1, such as those used in stimulated emission-depletion (STED) microscopy12, with interference lithography, which makes it a powerful tool for large-area parallel nanopatterning of deep subwavelength features onto a variety of surfaces with potential extension to 2- and 3-dimensions.
The photochromic layer is originally in one homogeneous state. When this layer is exposed to a uniform illumination of λ1, it converts into the second isomeric state (1c), Figure 2. Then the sample is exposed to a focused node at λ2, which converts the sample into the first isomeric state (1o) everywhere except in the near vicinity of the node. By controlling the exposure dose, the size of the unconverted region may be made arbitrarily small. A subsequent fixing step of one of the isomers may be selectively and irreversibly converted (locked) into a 3rd state (in black) to lock the pattern. Next, the layer is exposed uniformly to λ1, which converts everything except the locked region back to the original state. The sequence of steps may be repeated with a displacement of the sample relative to the optics, resulting in two locked regions whose spacing is smaller than the far-field diffraction limit. Therefore, any arbitrary geometry may be patterned in a “dot-matrix” fashion.1-3
Protocol
Representative Results
Discussion
The fabrication, experimental setup and related operational procedures of Patterning via Optical Saturable Transitions (POST) have been described. By exploiting the linear switching properties of thermally stable photochromic molecules, POST offers new perspectives on circumventing the far-field diffraction limit.1-2,4
Previously long-term storage requirement of the samples was solved by storing the samples under N2, directly after the initial evaporation.2 How…
Disclosures
The authors have nothing to disclose.
Acknowledgements
Thanks to Michael Knutson, Paul Hamric, Greg Scott, and Chris Landes for helpful discussions and assistance related to the custom inert atmosphere sample holder and assistance in the University of Utah student machine shop. P.C. acknowledges the NSF GRFP under Grant No. 0750758. P.C. acknowledges the University of Utah Nanotechnology Training Fellowship. R.M. acknowledges a NSF CAREER Award No. 1054899 and funding from the USTAR Initiative.
Materials
| Name of Material/ Equipment | Company | Catalog Number | Comments/Description |
| Isopropanol | Fisher Scientific | P/7500/15 | CAUTION: flammable, use good ventilation and avoid all ignition sources. |
| Buffered Oxide Etch | |||
| Methanol | Ricca Chemical | 48-293-2 | CAUTION: flammable, use good ventilation and avoid all ignition sources. |
| Ethylene Glycol | Sigma-Aldrich | 324558 | CAUTION: Harmful if swallowed |
| Silicon wafer | |||
| Diamond Scribe | |||
| Glass Beakers | |||
| Tweezers | Ted Pella | 5226 | |
| Reactive Ion Etching System | Oxford | Plasma Lab 80 Plus | |
| Inert Atmosphere Sample Holder | Proprietary In-house Designed | ||
| Polarizing beamsplitter cube | Thorlabs | PBS052 | |
| HeNe Laser | Melles Griot | 25-LHP-171 | CAUTION: Wear safety glasses |
| Half-wave plates | Thorlabs | WPH05M-633 | |
| Thermal Evaporator | Proprietary In-house Designed | ||
| TMV Super | TM Vacuum Products | TMV Super | |
| Voltammograph | Bioanalytical Systems | CV-37 | |
| Shortwave UV lamp 365nm | UVP Analytik Jena Company | UVGL-25 | CAUTION: Wear UV safety glasses |
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