Patterning via Optical Saturable Transitions - Fabrication and Characterization

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.