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 polymerization^5,6, 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 features^1-3. Since this technique uses single photons instead of electrons, it is extremely fast and does not require high light intensities^1-3, 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 3^rd 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

NOTE: perform all the following steps under cleanroom class 100 conditions or better.

1. Sample Preparation

1. Clean a 2” 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.
2. To lay down the working electrode, sputter 100 nm of Platinum (Pt) onto the clean 2” diameter silicon wafer.
3. Before etching the platinum thin film, clean the RIE chamber of any impurities or leftover photoresist from previous dry etches.
4. Pump down the chamber until a base pressure of 1 × 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.
5. Turn off the Ar/O₂ plasma and allow the chamber vent for approximately 10 min.
6. 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 × 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.
7. Turn off the O₂ plasma and let the chamber vent for 10 min.

2. Thermal Evaporation of Photochromic Molecule Using Custom Low Temperature Evaporator (LTE)

1. Fill AlO₂ boat with 30 mg of BTE and load into custom LTE source (Figure 6).
2. Load silicon wafer into sample mount.
3. Seal chamber ports and pump chamber down to a base pressure of 1 x 10^-6 Torr.
4. Evaporate the BTE at a setpoint temperature of 100 °C, with a film thickness of 30 nm.
5. Immediately after evaporation, flood illuminate the sample to 5 min of UV to transform the BTE material to the closed form, 1c.
6. 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.
7. 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.
8. Store remaining sample in N₂ filled glovebox.

3. Exposures

NOTE: Perform all exposures under inert atmosphere conditions to prevent degradation of sample.

1. Cleave the sample by following the same procedure as outlined in step 2.6.
2. Load sample in inert atmosphere sample holder.
3. Mount inert sample holder on stage. Purge sample with N₂.
4. Expose the sample to the desired exposure time using an interferometer, such as the one shown in Figure 8.

4. Electrochemical Oxidation Using Three Electrode Cell

NOTE: Perform electrochemistry under inert atmosphere conditions to prevent degradation of sample.

1. Clamp a clean glass vial on top of the hot plate. Place a clean stirring bar in the vial. Turn on the stirrer.
2. Clean a new copper clip with methanol. Clean the platinum counter electrode with methanol.
3. 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.
4. 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.
5. 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.
6. Bubble nitrogen through the water for 3-5 min. Turn off the nitrogen.
7. 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.
8. Using a voltammograph, set the oxidation voltage to 0.5 V/sec.
9. After the desired oxidation time has elapsed, turn the power to the voltammograph off.
10. Remove the red, black, and white clips from the platinum counter electrode, copper clip, and reference electrode.
11. Expose the sample to UV for 5 min.

5. Sample Development — Electrochemical Locking

NOTE: Perform development under inert atmosphere conditions to prevent degradation of sample.

1. 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.
2. Dry sample with dry N₂.
3. Immediately expose sample to 5 min of UV.

6. Sample Development — Dissolution Locking

NOTE: Perform development under inert atmosphere conditions to prevent degradation of sample.

1. Using 100 ml of ethylene glycol in a clean glass beaker, develop the exposed sample for the desired development time.
2. Dry sample with dry N₂. Immediately expose sample to 5 min of UV.

7. Multiple Exposures

1. If performing multiple exposures repeat steps 3-6 with a translation of the sample relative to the optics.

Results

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...

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...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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 365 nm	UVP Analytik Jena Company	UVGL-25	CAUTION: Wear UV safety glasses