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,

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 ....

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)