The overall goal of this procedure is to fabricate ultra-thin color films by using oblique angle deposition to control the porosity of the highly absorbent deposited film. This method can help answer key questions about color purity and tunability in ultra-thin film coloration research. The main advantage of this technique is that the porosity of the ultra-thin color films can be easily controlled.

The implications of this technique extend to semiconductors and flex electrodes. Because, this nanofabrication process yields or transcends structures. Though this method can provide insight into thin film coloration, it can also be applied to optical resonance applications in other wavelength ranges, such as UV and IR.To begin the procedure, first cut silicon wafers into two centimeter by two centimeter squares.

Use a PTFE dipper to immerse the silicon substrates in a six to one buffered oxide etchant for three seconds. Load the etched substrates into a PTFE cleaning jig. Sonicate the substrates in acetone for three minutes.

Then, rinse the substrates in isopropanol and deionized water in sequence. Dry the clean substrate with a nitrogen blowgun. Next, use forceps and carbon tape to fix the clean, dry substrates on a flat sample holder.

Load the samples into an electron beam evaporator equipped with titanium and gold sources. Evacuate the sample chamber for one hour, and then deposit 10 nanometers of titanium as an adhesion layer at one angstrom per second. Then, deposit 100 nanometers of gold, as a reflection layer, at two angstroms per second.

Vent the chamber, and remove the samples. Use carbon tape to mount four samples on a custom-built inclined sample holder at angles of zero, 30, 45 and 70 degrees. Load the samples into an electron beam evaporator with a germanium source, and evacuate the chamber for one hour.

Once the chamber is at high vacuum, deposit a layer of germanium at one angstrom per second to the desired thicknesses. Following germanium deposition, vent the chamber and remove the samples. First, clean a two-inch silicon wafer as previously described.

Deposit on the wafer a 10 nanometer adhesion layer of titanium, and a 100 nanometer reflection layer of gold. Fix the wafer to a 45 degree inclination sample holder. Load the sample into the electron beam evaporator for germanium deposition, and evacuate the chamber for one hour.

Then, deposit germanium to half of the target thickness at a rate of one angstrom per second. For convenience, it is recommended that the sample holder is loaded facing the center of the chamber. Vent the chamber and remove the sample holder.

Reposition the sample so that it is upside-down with respect to its former position. Place the sample holder in the chamber facing the same direction as in the previous deposition. Evacuate the chamber for one hour, and deposit the remaining germanium at the same electron beam power and deposition rate as previously used.

Once germanium deposition is complete, vent the chamber and remove the sample. Oblique angle deposition of germanium films yielded inclined arrays of germanium nanocolumns. As the deposition angle increased, the color change with increasing thickness became less pronounced.

Reflectance measurements showed that higher deposition angles correspond to smaller shifts of the dip in reflectance with increasing thickness. The reflectance measurements were converted to chromatic values, which showed that samples with high deposition angles had wider ranges of color expressions, and higher color purity. There was little change in color at different viewing angles, which was attributed to the thinness of the coatings.

After experiment, this technique paved the way for researchers in thin-film colorations to expand controllability of thin-film parameters by tuning optical arrangements with the film porosity. After watching this video, you should have a good understanding of how to control porosity using oblique angle deposition. Once mastered, this technique can also be used in a variety of nanofabrication processes to produce other patterns and structures.

While attempting this procedure, it is important to stabilize the deposition rate of the film.