Name: fabrication of ultra-thin color films with highly absorbing media using oblique angle deposition
Uploaded: 2017-08-29T15:00:00
Description: Watch this Scientific Journal Video about Fabrication of Ultra-thin Color Films with Highly Absorbing Media Using Oblique Angle Deposition at JoVE.com

This research was supported by Unmanned Vehicles Advanced Core Technology Research and Development Program through the Unmanned Vehicle Advanced Research Center (UVARC) funded by the Ministry of Science, ICT and Future Planning, the Republic of Korea (2016M1B3A1A01937575)

Caution: Some chemicals (i.e., buffered oxide etchant, isopropyl alcohol, etc.) used in this protocol can be hazardous to health. Please consult all relevant material safety data sheets before any sample preparation takes place. Utilize appropriate personal protective equipment (e.g., lab coats, safety glasses, gloves, etc.) and engineering controls (e.g., wet station, fume hood, etc.) when handling etchants and solvents.
1. Preparation of the Si Substrate
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In conventional thin film coatings for coloration3,4,5,6, the color can be controlled by altering different materials and adjusting the thickness. The choice of materials with different refractive indices is limited for tuning various colors. To relax this limitation, we exploited the oblique angle deposition to thin-film color coating. Depending on the deposition angle, the porosity of the Ge ...

In general, the performance of thin-film optical coatings is based on the type of optical interference they produce, such as high reflection or transmission. In dielectric thin-films, optical interference can be obtained simply by satisfying conditions such as quarter wave thickness (&#955;/4n). Interference principles have long been used in various optical applications such as Fabry-Perot interferometers and distributed Bragg reflectors1,2. In recent years, thin film structures using highly absorbent materials such as metals and semiconductors have been widely studied3,4,5,6. Strong optical interference can be obtained by thin-film coating an absorbent semiconductor material on a metal film, which produces non-trivial phase changes in the reflected waves. This type of structure allows ultra-thin coatings which are considerably thinner than dielectric thin-film coatings.
Recently, we studied ways of improving the color tunability and color purity of highly absorbent thin-films using porosity7. By controlling the porosity of the deposited film, the effective refractive index of the thin-film medium can be changed8. This change in the effective refractive index allows the optical characteristics to be improved. Based on this effect, we designed ultra-thin color films with different thicknesses and porosities by calculations using rigorous coupled wave analysis (RCWA)9. Our design presents colors with different film thicknesses at each porosity7.
We employed a simple method, oblique angle deposition, to control the porosity of highly absorbent thin-film coatings. The oblique angle deposition technique basically combines a typical deposition system, such as an electron beam evaporator or thermal evaporator, with a tilted substrate10. The oblique angle of incident flux creates atomic shadowing, which produces areas that the vapor flux cannot reach directly11. The oblique angle deposition technique has been widely used in various thin-film coating applications12,13,14.
In this work, we detail the processes for fabricating ultra-thin color films by oblique deposition using an electron beam evaporator. Also, additional methods for large-area processing are presented separately. In addition to the process steps, some notes that should be taken into consideration during the fabrication process are explained in detail.
We also review processes for measuring the reflectance of the fabricated samples and converting them into color information for analysis, so that they can be expressed in CIE color coordinates and RGB values15. Furthermore, some issues to consider in the fabrication process of ultra-thin color films are discussed.

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			<td height="23" style="height:23px;width:216px;">&#160;KVE-2004L</td>
			<td style="width:185px;">Korea Vacuum Tech. Ltd.</td>
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			<td style="width:227px;">E-beam evaporator system</td>
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			<td height="21" style="height:21px;width:216px;">Cary 500</td>
			<td style="width:185px;">Varian, USA</td>
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			<td>UV-Vis-NIR spectrophotometer</td>
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			<td height="21" style="height:21px;width:216px;">T1-H-10</td>
			<td style="width:185px;">Elma</td>
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			<td height="21" style="height:21px;width:216px;">HSD150-03P</td>
			<td style="width:185px;">Misung Scientific Co., Ltd</td>
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			<td height="21" style="height:21px;width:216px;">Isopropyl Alcohol (IPA)</td>
			<td style="width:185px;">OCI Company Ltd.</td>
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			<td style="width:227px;">Isopropyl Alcohol (IPA)</td>
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			<td height="21" style="height:21px;width:216px;">Buffered Oxide Etch 6:1</td>
			<td style="width:185px;">Avantor</td>
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			<td style="width:227px;">Buffered Oxide Etch 6:1</td>
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			<td height="21" style="height:21px;width:216px;">Acetone</td>
			<td style="width:185px;">OCI Company Ltd.</td>
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			<td style="width:227px;">Acetone</td>
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			<td height="63" style="height:63px;width:216px;">4inch Silicon Wafer</td>
			<td style="width:185px;">Hi-Solar Co., Ltd.</td>
			<td style="width:129px;"></td>
			<td style="width:227px;">4inch Silicon Wafer (P-100, 1-20 ohm.cm, Single side polished, Thickness: 440&#177;20&#956;m)</td>
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			<td height="63" style="height:63px;width:216px;">2inch Silicon Wafer</td>
			<td style="width:185px;">Hi-Solar Co., Ltd.</td>
			<td style="width:129px;"></td>
			<td style="width:227px;">2inch Silicon Wafer (P-100, 1-20 ohm.cm, Single side polished, Thickness: 440&#177;20&#956;m)</td>
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fabrication of ultra-thin color films with highly absorbing media using oblique angle deposition

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present an advanced method for fabricating ultra-thin color films with improved characteristics. The proposed process addresses several fabrication issues, including large area processing. Specifically, the protocol describes a process for fabricating ultra-thin color films using an electron beam evaporator for oblique angle deposition of germanium (Ge) and gold (Au) on silicon (Si) substrates.&#160; Film porosity produced by the oblique angle deposition induces color changes in the ultra-thin film. The degree of color change depends on factors such as deposition angle and film thickness. Fabricated samples of the ultra-thin color films showed improved color tunability and color purity. In addition, the measured reflectance of the fabricated samples was converted into chromatic values and analyzed in terms of color. Our ultra-thin film fabricating method is expected to be used for various ultra-thin film applications such as flexible color electrodes, thin film solar cells, and optical filters. Also, the process developed here for analyzing the color of the fabricated samples is broadly useful for studying various color structures.

Figure 2a shows images of the 2 cm x 2 cm fabricated samples. The samples were fabricated so that the films had different thicknesses (i.e., 10 nm, 15 nm, 20 nm, and 25 nm) and were deposited at different angles (i.e., 0&#176;, 30&#176;, 45&#176;, and 70&#176;). The color of the deposited films changes depending on the combination of both the thickness of the samples and the deposition angle. The changes in color result from changes in the porosity of the film. Depending on...

Watch this Scientific Journal Video about Fabrication of Ultra-thin Color Films with Highly Absorbing Media Using Oblique Angle Deposition at JoVE.com

Fabrication of Ultra-thin Color Films with Highly Absorbing Media Using Oblique Angle Deposition

We present a detailed method for fabricating ultra-thin color films with improved characteristics for optical coatings. The oblique angle deposition technique using an electron beam evaporator allows improved color tunability and purity. Fabricated films of Ge and Au on Si substrates were analyzed by reflectance measurements and color information conversion.

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present ...

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.

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