Name: Patterning via Optical Saturable Transitions - Fabrication and Characterization (Video) | JoVE Journal
Uploaded: 2014-12-11T14:00:00.000+00:00
Duration: 8 min 19 s
Description: Learn optical lithography for nanostructure fabrication using photochromic molecules on silicon substrates. This method overcomes diffraction limits to create sub-wavelength features efficiently.

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5 minutes

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.

NOTE: perform all the following steps under cleanroom class 100 conditions or better.
1. Sample Preparation
<ol>
	<li>Clean a 2&#8221; diameter silicon wafer with Buffered Oxide Etch (BOE) solution (6 parts 40% NH4F and 1 part 49% HF) for 2 min (Caution: Hazardous chemicals). Choose this etch time to remove any organics or contaminants on the surface. Rinse with deionized (DI) water for approximately 5 min. Dry wafer with dry N2. 
	NOTE: Never work alone when using HF. Always wears eye protection with face shield and personal protective equipment (PPE) in case of spills. Post guidelines for the use and handling of HF waste in the lab where the etching is performed. 
	NOTE: Steps 1.2 to 1.7 are for electrochemical locking only. If performing locking via dissolution proceed to Step 2.</li>
	<li>To lay down the working electrode, sputter 100 nm of Platinum (Pt) onto the clean 2&#8221; diameter silicon wafer.</li>
	<li>Before etching the platinum thin film, clean the RIE chamber of any impurities or leftover photoresist from previous dry etches.</li>
	<li>Pump down the chamber until a base pressure of 1 &#215; 10-5 Torr is achieved. Make sure that the RF power is set to 200 W and the flow rates for the oxygen and argon are set to 50 sccm and 10 sccm respectively. Ignite the Ar/O2 plasma and run for at least 1 hr.</li>
	<li>Turn off the Ar/O2 plasma and allow the chamber vent for approximately 10 min.</li>
	<li>To etch the platinum thin film surface, load the sample into the RIE chamber and pump the chamber down to a base pressure of 1 &#215; 10-5 Torr. This time set the argon flow rate to 0 sccm. Ignite the O2 plasma and let this process run for 30 min.</li>
	<li>Turn off the O2 plasma and let the chamber vent for 10 min.</li>
</ol>
2. Thermal Evaporation of Photochromic Molecule Using Custom Low Temperature Evaporator (LTE)
<ol>
	<li>Fill AlO2 boat with 30 mg of BTE and load into custom LTE source (Figure 6).</li>
	<li>Load silicon wafer into sample mount.</li>
	<li>Seal chamber ports and pump chamber down to a base pressure of 1 x 10-6 Torr.</li>
	<li>Evaporate the BTE at a setpoint temperature of 100 &#176;C, with a film thickness of 30 nm.</li>
	<li>Immediately after evaporation, flood illuminate the sample to 5 min of UV to transform the BTE material to the closed form, 1c.</li>
	<li>In order to define the sample size, cleave a small piece of the wafer using a diamond scribe to scratch a line from the edge of the silicon surface. Grab the wafer on both sides of the scratch line and bend the wafer downwards until it breaks along the crystal plane.</li>
	<li>Perform profilometer measurements to validate BTE thin film thickness. To do this, scratch the sample using a fine edge tweezers. Measure the step height from this scratch, which is the difference in height between the right and left cursor position. 
	NOTE: Inaccuracies in film thickness will result in discrepancies in exposure dose.</li>
	<li>Store remaining sample in N2 filled glovebox.</li>
</ol>
3. Exposures
NOTE: Perform all exposures under inert atmosphere conditions to prevent degradation of sample.
<ol>
	<li>Cleave the sample by following the same procedure as outlined in step 2.6.</li>
	<li>Load sample in inert atmosphere sample holder.</li>
	<li>Mount inert sample holder on stage. Purge sample with N2.</li>
	<li>Expose the sample to the desired exposure time using an interferometer, such as the one shown in Figure 8.</li>
</ol>
4. Electrochemical Oxidation Using Three Electrode Cell
NOTE: Perform electrochemistry under inert atmosphere conditions to prevent degradation of sample.
<ol>
	<li>Clamp a clean glass vial on top of the hot plate. Place a clean stirring bar in the vial. Turn on the stirrer.</li>
	<li>Clean a new copper clip with methanol. Clean the platinum counter electrode with methanol.</li>
	<li>Using a clean copper clip, clip the sample through one of the holes in the Teflon vial cap. Make sure to clip onto the exposed platinum only.</li>
	<li>Place the Teflon vial cap onto the vial. Clip the red lead onto the platinum counter electrode and the black lead onto the copper clip holding the sample.</li>
	<li>Using a clean syringe, fill the vial with filtered deionized (DI) water through the second hole in the Teflon vial cap. Fill as high without immersing any of the bare platinum on the sample.</li>
	<li>Bubble nitrogen through the water for 3-5 min. Turn off the nitrogen.</li>
	<li>Place the reference electrode in the second hole in the Teflon vial cap. Clip the white lead onto the reference electrode. Check to make sure none of the bare platinum on the sample is immersed.</li>
	<li>Using a voltammograph, set the oxidation voltage to 0.5 V/sec.</li>
	<li>After the desired oxidation time has elapsed, turn the power to the voltammograph off.</li>
	<li>Remove the red, black, and white clips from the platinum counter electrode, copper clip, and reference electrode.</li>
	<li>Expose the sample to UV for 5 min.</li>
</ol>
5. Sample Development &#8212; Electrochemical Locking
NOTE: Perform development under inert atmosphere conditions to prevent degradation of sample.
<ol>
	<li>Develop the sample in filtered 5 (wt%) isopropanol, 95 (wt%) ethylene glycol for the desired amount of time. Note: Typically 50 nm samples are developed for 30-60 sec while 80 nm samples are developed for 60-180 sec.</li>
	<li>Dry sample with dry N2.</li>
	<li>Immediately expose sample to 5 min of UV.</li>
</ol>
6. Sample Development &#8212; Dissolution Locking
NOTE: Perform development under inert atmosphere conditions to prevent degradation of sample.
<ol>
	<li>Using 100 ml of ethylene glycol in a clean glass beaker, develop the exposed sample for the desired development time.</li>
	<li>Dry sample with dry N2. Immediately expose sample to 5 min of UV.</li>
</ol>
7. Multiple Exposures
<ol>
	<li>If performing multiple exposures repeat steps 3-6 with a translation of the sample relative to the optics.</li>
</ol>

<ol><li>Brimhall, N., Andrew, T. L., Manthena, R. V., Menon, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Breaking+the+far-field+diffraction+limit+in+optical+nanopatterning+via+repeated+photochemical+and+electrochemical+transitions+in+photochromic+molecules%5BTitle%5D)+AND+%22Physical+Review+Letters%22%5BJournal%5D)">Breaking the far-field diffraction limit in optical nanopatterning via repeated photochemical and electrochemical transitions in photochromic molecules.</a> Physical Review Letters. 107 (20), 205501(2011).</li>
<li>Cantu, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Subwavelength+nanopatterning+of+photochromic+diaryethene+films%5BTitle%5D)+AND+%22Applied+Physics+Letters%22%5BJournal%5D)">Subwavelength nanopatterning of photochromic diaryethene films.</a> Applied Physics Letters. 100 (18), 183103(2012).</li>
<li>Cantu, P., Andrew, T. L., Menon, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Nanopatterning+of+diarylethene+films+via+selective+dissolution+of+one+photoisomer%5BTitle%5D)+AND+%22Applied+Physics+Letters%22%5BJournal%5D)">Nanopatterning of diarylethene films via selective dissolution of one photoisomer.</a> Applied Physics Letters. 103 (17), 173112(2013).</li>
<li>Abbe, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Beitr%C3%A4ge+zur+Theorie+des+Mikroskops+und+der+mikroskopischen+Wahrnehmung%5BTitle%5D)+AND+%22Archiv+f%C3%BCr+mikroskopische+Anatomie%22%5BJournal%5D)">Beiträge zur Theorie des Mikroskops und der mikroskopischen Wahrnehmung.</a> Archiv für mikroskopische Anatomie. 9 (1), 413-418 (1873).</li>
<li>Li, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Achieving+%CE%BB%2F20+resolution+by+one-color+initiation+and+deactivation+of+polymerization%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Achieving λ/20 resolution by one-color initiation and deactivation of polymerization.</a> Science. 324 (5929), 910-913 (2009).</li>
<li>Fischer, J., von Freymann, G., Wegener, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+materials+challenge+in+diffraction-unlimited+direct-laser-writing+optical+lithography%5BTitle%5D)+AND+%22Advanced+Materials%22%5BJournal%5D)">The materials challenge in diffraction-unlimited direct-laser-writing optical lithography.</a> Advanced Materials. 22 (32), 3578-3582 (2010).</li>
<li>Mirkin, C. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Beam+pen+lithography%5BTitle%5D)+AND+%22Nature+Nanotechnology%22%5BJournal%5D)">Beam pen lithography.</a> Nature Nanotechnology. 5, 637-640 (2010).</li>
<li>Xie, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Manipulating+spatial+light+fields+for+micro-+and+nano-photonics%5BTitle%5D)+AND+%22Physica+E%3A+Low-dimensional+Systems+and+Nanostructures%22%5BJournal%5D)">Manipulating spatial light fields for micro- and nano-photonics.</a> Physica E: Low-dimensional Systems and Nanostructures. 44, 1109-1126 (2012).</li>
<li>Leroy, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((High-speed+metal-insulator+transition+in+vanadium+dioxide+films+induced+by+an+electrical+pulsed+voltage+over+nano-gap+electrodes%5BTitle%5D)+AND+%22Applied+Physics+Letters%22%5BJournal%5D)">High-speed metal-insulator transition in vanadium dioxide films induced by an electrical pulsed voltage over nano-gap electrodes.</a> Applied Physics Letters. 100 (21), 213507(2012).</li>
<li>Carr, D., Sekaric, L., Craighead, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Measurement+of+nanomechanical+resonant+structures+in+single-crystal+silicon%5BTitle%5D)+AND+%22Journal+of+Vacuum+Science+%26+Technology+B%22%5BJournal%5D)">Measurement of nanomechanical resonant structures in single-crystal silicon.</a> Journal of Vacuum Science & Technology B. 16 (6), 3821-3824 (1998).</li>
<li>Wilhelmi, O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Rapid+prototyping+of+nanostructured+materials+with+a+focused+ion+beam%5BTitle%5D)+AND+%22Japanese+Journal+of+Applied+Physics%22%5BJournal%5D)">Rapid prototyping of nanostructured materials with a focused ion beam.</a> Japanese Journal of Applied Physics. 47 (6), 2010-5014 (2008).</li>
<li>Hell, S. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Far-field+optical+nanoscopy%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">Far-field optical nanoscopy.</a> Science. 316 (5828), 1153-1158 (2007).</li>
<li>Chou, S. Y., Krauss, P. R., Renstrom, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Nanoimprint+lithography%5BTitle%5D)+AND+%22Journal+of+Vacuum+Science+%26+Technology+B%22%5BJournal%5D)">Nanoimprint lithography.</a> Journal of Vacuum Science & Technology B. 14, 4129(1996).</li>
<li>Guillemette, M. D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Surface+topography+induces+3D+self-orientation+of+cells+and+extracellular+matrix+resulting+in+improved+tissue+function%5BTitle%5D)+AND+%22Integrative+Biology%22%5BJournal%5D)">Surface topography induces 3D self-orientation of cells and extracellular matrix resulting in improved tissue function.</a> Integrative Biology. 1 (2), 196-204 (2009).</li>
</ol>

The authors have nothing to disclose.

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...

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&#8217;-dimethyl-2,2&#8217;-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 &#955;1, it converts into the second isomeric state (1c), Figure 2. Then the sample is exposed to a focused node at &#955;2, 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 &#955;1, 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 &#8220;dot-matrix&#8221; fashion.1-3

<table><tbody><tr><td>Isopropanol</td><td>Fisher Scientific</td><td>P/7500/15</td><td>CAUTION: flammable, use good ventilation and avoid all ignition sources.</td></tr><tr><td>Buffered Oxide Etch</td><td></td><td></td><td></td></tr><tr><td>Methanol</td><td>Ricca Chemical</td><td>48-293-2&amp;nbsp;</td><td>CAUTION: flammable, use good ventilation and avoid all ignition sources.</td></tr><tr><td>Ethylene Glycol</td><td>Sigma-Aldrich</td><td>324558</td><td>CAUTION: Harmful if swallowed</td></tr><tr><td>Silicon wafer</td><td></td><td></td><td></td></tr><tr><td>Diamond Scribe</td><td></td><td></td><td></td></tr><tr><td>Glass Beakers</td><td></td><td></td><td></td></tr><tr><td>Tweezers</td><td>Ted Pella</td><td>5226</td><td></td></tr><tr><td>Reactive Ion Etching System</td><td>Oxford</td><td>Plasma Lab 80 Plus</td><td></td></tr><tr><td>Inert Atmosphere Sample Holder</td><td>Proprietary In-house Designed</td><td></td><td></td></tr><tr><td>Polarizing beamsplitter cube</td><td>Thorlabs</td><td>PBS052</td><td></td></tr><tr><td>HeNe Laser</td><td>Melles Griot</td><td>25-LHP-171</td><td>CAUTION: Wear safety glasses</td></tr><tr><td>Half-wave plates</td><td>Thorlabs</td><td>WPH05M-633</td><td></td></tr><tr><td>Thermal Evaporator</td><td>Proprietary In-house Designed</td><td></td><td></td></tr><tr><td>TMV Super</td><td>TM Vacuum Products</td><td>TMV Super</td><td></td></tr><tr><td>Voltammograph</td><td>Bioanalytical Systems</td><td>CV-37</td><td></td></tr><tr><td>Shortwave UV lamp 365&amp;nbsp;nm</td><td>UVP Analytik Jena Company</td><td>UVGL-25</td><td>CAUTION: Wear UV safety glasses</td></tr></tbody></table>

patterning via optical saturable transitions - fabrication and characterization

Patterning via Optical Saturable Transitions - Fabrication and Characterization

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.

Fabricated samples:
Different oxidation times were characterized as illustrated by the atomic-force micrographs in Figure 3 at an oxidation voltage of 0.85 V determined from cyclic voltammetry. The 50 nm-thick films were exposed to a standing wave at λ = 647 nm of period 400 nm for 60 sec at a power density of 0.95 mW/cm2. As the oxidation time is increased from 10 min to 25 min, one can clearly see a loss of contrast as some of the regions comprise...

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.

This protocol describes the fabrication and characterization of nanostructures using a novel nanolithographic technique called Patterning via Optical ...

The overall goal of the following experiment is to fabricate sub wavelength nanostructures using a novel single photon optical lithographic technique. This is achieved by first applying a layer of a photochromic molecule on a silicon substrate. As a second step, the sample is irradiated with short wavelength ultraviolet light, which converts the open ring isomers to closed ring isomers.Next, the sample is illuminated with a standing wave to convert the molecules back to the open ring form, except in the near vicinity of the nodes. By optically saturating this transition, the molecules in the closed ring isomer remain in a region that is far smaller than the far field diffraction limit. Finally, the sample is placed in a polar solvent where the closed ring isomer dissolves away at a faster rate than the open ring isomer leaving behind nanoscale topography results are obtained, which show that patterning via optical saturable transitions is an alternative optical nano patterning technique for obtaining sub wavelength features.The main advantages of this technique over existing methods like scanning electron beam lithography are that patterning via optical ible transitions can be orders of magnitude faster, provide more precise patterning, and be simpler and cheaper to implement. Demonstrating the procedure will be precious Cantu, a graduate student from my laboratory. This protocol requires that all steps be performed in a class 100 or better clean room.To prepare the sample for dissolution locking, begin with a two inch silicon wafer that has been cleaned with buffered oxide etch solution and dried with dry nitrogen with wafer in hand, proceed to the thermal evaporation step. This lab uses a custom low temperature evaporator, load the wafer into the evaporator sample holder, then mount the holder on the evaporator. For this protocol, an aluminum oxide boat has been filled with 30 milligrams of BTE.Load this boat into the source. Well of the evaporator proceed by sealing the chamber ports and pumping the chamber down to a base pressure of 10 to the negative six. Tor then evaporate the BTE at a set point temperature of 100 degrees Celsius with a film thickness of approximately 30 nanometers.Immediately after the evaporation process is complete and the system can be opened unmount the sample. Then take it to be loaded into a glove box with a nitrogen environment and a UV lamp. Here the sample has been loaded into the glove box.In the glove box. The sample should be positioned under the UV light for flood illumination flood. Illuminate the sample with UV for five minutes to transform BTE to the closed form.After the UV illumination, take a portion of the sample to characterize. Do this by using a diamond scribe to define a small region by scratching a line from the edge of the silicon surface. Then grab the wafer on both sides of the line and bend it downwards until it breaks along the crystal plane.The smaller portion will be used for characterization after it has been removed from the glove box. Take the small piece of sample to a profilometer to validate the BTE film thickness. To make the measurement use fine edge tweezers to make a scratch to the wafer surface on the sample.Then measure the step height from this scratch. After measuring BTE film thickness, return to the sample in the glove box to prepare it for exposure. Working in the inert atmosphere of the glove box.Use a diamond scribe to cleave the wafer and break off a piece for exposure. In this case, the piece is about one square centimeter. Put the larger piece away and get an inert atmosphere sample holder.Load the exposure sample in the sample holder. Prepare to remove the sample holder from the glove box. When ready, remove the sample holder from the glove box in order to take it to the interferometer.At the interferometer mount, the inert atmosphere sample holder on the sample stage. After it is in position, attach a nitrogen line to the sample holder. Open the port on the sample holder to purge the sample with nitrogen.Then close the port. Keeping the nitrogen line in place. Use the interferometer to expose the sample for the desired exposure.Time After exposure with the interferometer, remove the inert atmosphere, sample holder and its sample. Transport them back to the glove box and place them inside. With the sample in the glove box, get a clean glass beaker containing 100 milliliters of ethylene glycol.Remove the exposed sample from the holder and place it in the beaker for the desired development time. When the development time has elapsed, remove the sample from the beag. It will quickly dry in the nitrogen atmosphere.Then as soon as possible, mount the sample for illumination with UV light and expose it for five minutes. This is a plot of the thickness of molecular layers versus development time for the two forms of BTE molecules. Data for the open form is connected with a blue line.Data for the closed form is connected by a red line. The plot demonstrates the higher solubility of the closed form molecule in polar solvents. To collect this data, half of a sample was converted to closed form and the other half to open form.The sample was developed in ethylene glycol for several different development times, and then the thickness of the remaining layer was measured with a profilometer. Here are scanning electron microscopy images of pattern samples from single development exposures performed in the inert atmosphere Sample holder. The feature sizes are 196 nanometers, 160 nanometers and 85 nanometers.The last corresponding to a line width of approximately lambda. Over 7.4. Data from several exposures are shown with crosses in a plot of line width versus exposure time.For comparison, the blue curve represents the prediction of a simple model, assuming incident sinusoidal illumination with a period of 457 nanometers, and using the exposure threshold as the sole fitting parameter. After watching this video, you should have a good understanding of how to implement super resolution patterning via optical ible transitions.

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Patterning via Optical Saturable Transitions - Fabrication and Characterization

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