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

Summary

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.

Abstract

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.  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.  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.

Introduction

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 (λ/4n). Interference principles have long been used in various optical applications such as Fabry-Perot interferometers and distributed Bragg reflectors^1,2. In recent years, thin film structures using highly absorbent materials such as metals and semiconductors have been widely studied^3,4,5,6. 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 applications^12,13,14.

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.

Protocol

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

1. Using a diamond cutter, cut a 4 inch silicon (Si) wafer into 2 cm x 2 cm sized squares. To make colored samples, the substrate is typically cut 2 cm x 2 cm, but can be larger, depending on the size of the sample holder used for oblique angle deposition.
2. To remove native oxide using Polytetrafluoroethylene (PTFE) dipper, dip the cleaved Si substrates in buffered oxide etchant (BOE) for 3 s. Caution: Please wear appropriate protection for safety.
3. Clean the cleaved Si substrates sequentially in acetone, isopropyl alcohol (IPA), and deionized (DI) water for 3 s each.
   1. Using PTFE cleaning jig, sonicate the cleaved Si substrates with acetone in an ultrasonic bath for 3 min at a frequency of 35 kHz.
   2. To remove the acetone, rinse the cleaved Si substrates with IPA.
   3. As the last step of cleaning, rinse the cleaved Si substrates with DI water.
4. To remove moisture, dry the clean substrate with a nitrogen blow gun while holding it with forceps. 

2. Deposition of the Au Reflector

1. Using forceps and carbon tape, fix the cleaned Si substrates onto a flat sample holder and place the holder into the chamber of the electron beam evaporator with Ti and Au sources.
2. Evacuate the chamber for 1 h to reach high vacuum. The base pressure of the vacuum chamber should be 4 x 10^-6 Torr.
3. Deposit the Ti layer as an adhesion layer to a thickness of 10 nm with 5 - 7% of electron beam power controlled in manual mode at a DC voltage of 7.5 kV, which gives a deposition rate of 1 Å/sec.
   Note: A Cr layer of the same thickness, instead of a Ti layer, can be deposited as the adhesion layer.
4. Deposit the Au layer as a reflection layer to a thickness of 100 nm with 13-15% of electron beam power controlled in manual mode at a DC voltage of 7.5 kV, which gives a deposition rate of 2 Å/sec.
   Note: The thickness of the Au reflection layer can be greater than 100 nm. A thickness of 100 nm is deposited here to make the reflection layer as thin as possible while maintaining the optical properties of the Au.
5. After the Au layer deposition, vent the chamber and take out the samples. They will need to be reloaded with the inclined sample holder for the oblique angle deposition.

3. Preparation of the Inclined Sample Holder for Oblique Angle Deposition

Note: There are several methods that can be used for oblique deposition, such as the z-axis rotating chuck¹⁶, but this requires equipment modification and films can only be deposited at one angle at a time. To efficiently observe the changes in color produced by different deposition angles, we used sample holders that inclined the samples at different angles. For precision, the inclined sample holder can be made using metal processing equipment. However, in this paper, we introduce a simple method that can be easily followed.

1. Prepare a metal plate made of an easily bendable metal such as aluminum.
2. Cut the metal plate into three 2 cm x 5 cm pieces.
3. Fix the metal piece to the floor alongside a protractor, hold the short side and bend the metal to the desired deposition angle (i.e., 30°, 45°, and 70°).
4. Attach the bent metal pieces to the 4 inch sample holder using carbon tape.

4. Oblique Angle Deposition of Ge Layer

Note: In this section, refer to the schematic diagrams in Figure 1 of the samples deposited on the inclined sample holders, and porous Ge films, following oblique angle deposition.

1. Fix the four Au deposited samples with carbon tape to an inclined sample holder at angles of 0°, 30°, 45°, and 70°, respectively.
2. Load the Au-deposited samples on the inclined sample holder into the electron beam evaporator with a Ge source for oblique angle deposition.
3. Evacuate the chamber for 1 h to reach high vacuum. The base pressure of the vacuum chamber should be 4 x 10^-6 Torr.
4. Deposit the Ge layer as a coloring layer with 6 - 8% of electron beam power controlled in manual mode at a DC voltage of 7.5 kV, which gives a deposition rate of 1 Å/sec. The deposition thicknesses of the Ge layer on the four samples are 10 nm, 15 nm, 20 nm, and 25 nm, respectively.
   Note: The deposition thicknesses of 10 nm, 15 nm, 20 nm, and 25 nm were selected to facilitate comparison of the color changes for each deposition angle. A different angle and thickness (5 - 60 nm) can be chosen to achieve a particular color.
5. After the Ge layer deposition, vent the chamber and take out the samples.

5. Oblique Angle Deposition Process for Large Areas

Note: If the size of the sample used for oblique angle deposition is small, it can be fabricated by the process detailed in step 4. However, if the size of the sample to be fabricated is large, it becomes difficult to maintain film uniformity due to variation in the evaporation flux along the z-axis¹⁶. Therefore, a separate additional process, step 5, is required to fabricate larger samples and achieve a uniform color.

1. For a 2 inch wafer, after depositing the Au layer on the large sample in step 2, fix the Au-deposited large sample to the 45° inclined sample holder.
   Note: Since our inclined sample holder is designed to fit small samples, loading large samples at all the angles (i.e., 0°, 30°, 45°, and 70°) will create interference between samples. Therefore, to obliquely deposit large-sized samples at various angles in one process, it is necessary to have an inclined sample holder suitable for large-sized samples.
2. Load the Au-deposited large sample on the inclined sample holder into the electron beam evaporator with a Ge source for oblique angle deposition.
   Note: When loading the sample, the second deposition layer must be deposited in the same direction as the first deposition, so note the direction of the loaded sample. For convenience, it is recommended that the sample holder is loaded facing the front of the chamber.
3. Evacuate the chamber for 1 h to reach high vacuum. The base pressure of the vacuum chamber should be 4 x 10^-6 Torr.
4. Deposit the Ge layer as a coloring layer to a deposition thickness of 10 nm, which is half of the target thickness of 20 nm, with 6 - 8% of electron beam power controlled in manual mode at a DC voltage of 7.5 kV, which gives a deposition rate of 1 Å/sec.
5. After the deposition of the first Ge layer is finished, vent the chamber and take out the sample, because the sample needs to be repositioned and reloaded.
6. Fix the sample to the inclined sample holder in a position which is upside down with respect to the position of the first deposition.
7. Load the sample on the inclined sample holder with the Ge source so that the holder faces in the same direction as the first deposition.
8. Evacuate the chamber for 1 h to reach high vacuum. The base pressure of the vacuum chamber should be 4 x 10^-6 Torr.
9. Deposit the Ge layer as a coloring layer to a deposition thickness of 10 nm, which is half of the target thickness of 20 nm, with 6 - 8% of electron beam power controlled in manual mode at a DC voltage of 7.5 kV, which gives a deposition rate of 1 Å/sec.
10. After the Ge layer deposition, vent the chamber and take out the sample.

Results

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°, 30°, 45°, and 70°). 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 the deposition angl...

Discussion

In conventional thin film coatings for coloration^3,4,5,6, 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 ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
KVE-2004L	Korea Vacuum Tech. Ltd.		E-beam evaporator system
Cary 500	Varian, USA		UV-Vis-NIR spectrophotometer
T1-H-10	Elma		Ultrasonic bath
HSD150-03P	Misung Scientific Co., Ltd		Hot plate
Isopropyl Alcohol (IPA)	OCI Company Ltd.		Isopropyl Alcohol (IPA)
Buffered Oxide Etch 6:1	Avantor		Buffered Oxide Etch 6:1
Acetone	OCI Company Ltd.		Acetone
4 inch Silicon Wafer	Hi-Solar Co., Ltd.		4 inch Silicon Wafer (P-100, 1 - 20 ohm.cm, Single side polished, Thickness: 440 ± 20 μm)
2 inch Silicon Wafer	Hi-Solar Co., Ltd.		2 inch Silicon Wafer (P-100, 1 - 20 ohm.cm, Single side polished, Thickness: 440 ± 20 μm)