Two-layer widebandantireflection coatings with an absorbing layer

Yanfei Zheng, Kazuo Kikuchi, Masafumi Yamasaki, Kenichirou Sonoi,and Kazuhiro Uehara

A method for designing antireflection coatings by the use of an absorbing layer and a dielectric layer isproposed. Theoretical and experimental results of three two-layer wideband antireflection coatings withAu, Cu, and TiOxNy are given. The antireflection coatings have low reflectance, good conductivity, and asimple structure. They are especially suitable for display applications. © 1997 Optical Society of America

Key words: Antireflection, electromagnetic radiation shielding, antistatic electricity.

1. Introduction

Antireflection ~AR! coatings have been widely used invarious optical devices for increasing transmission orreducing reflection. In the past, a number of paperson various aspects of AR coatings have beenpublished.1–6 AR coatings are usually designed andfabricated with nonabsorbing materials, such asthose AR coatings used in eyeglass and cameralenses. The purpose of these applications is to in-crease transmission. It is also possible to reduce thereflectance with the use of absorbing films in thedesign.7 The absorption reduces the transmittedlight, but in some applications reducing the reflectedlight is more important than keeping the transmittedlight at a high level.8

For video display terminals, such as computermonitors and television sets, thin-film coatings arerequired to reduce reflection and increase contrast~AR coatings! or reduce static charge and shieldagainst electromagnetic radiation ~conductive coat-ings!. For the second purpose, gold ~Au!, silver ~Ag!,and indium tin oxide films have been used in displaydevices. Conducting AR coatings that are nonab-sorbing in the visible spectral region have been re-ported.9,10 If indium tin oxide is used as a high-index material in AR coatings, then these coatingshave both lower reflectance and good conductivity.

The authors are with Shincron Co., Ltd., Research & Develop-ment Division, 2-6 Minami-ohi 3 Chome, Shinagawa-ku, Tokyo140, Japan.

Received 1 November 1996; revised manuscript received 7 April1997.

0003-6935y97y256335-04$10.00y0© 1997 Optical Society of America

In contrast, in order to improve the image contrast ofcathode ray tubes and flat panel displays, one or moreabsorbing layers are used in AR coatings.11–13

Narrow-band AR coatings can be easily designedwith an absorbing layer and a dielectric layer.7However, because the performance of the coating ismainly dependent on the complex refractive index ofthe absorbing material used, it is difficult to design awideband AR coating with two such layers even ifoptimization methods are used. In contrast, com-plex refractive indices of some materials, such asTiOxNy, vary sensitively with process conditions. Asa result, for designing a two-layer wideband AR coat-ing, it is important to investigate the complex refrac-tive index required.

In this paper we propose a method to design two-layer wideband AR coatings by using absorbing anddielectric materials. The theoretical and experi-mental results of three two-layer wideband AR coat-ings that use Au, Cu, and TiOxNy are given. TheseAR coatings have fewer layers and are thinner thanconventional AR coatings.

2. Design Method

The dispersions of the complex refractive indices ofabsorbing materials ~N 5 n 2 ik! are usually largerthan those of dielectric materials. This property canbe used for designing a wideband antireflection coat-ing in the visible region. Figure 1 shows a two-layerAR coating that consists of an absorbing layer next toa glass substrate and a dielectric layer on the outside.The incident medium is air. The refractive indexand thickness of the dielectric layer are n1 and d1,respectively. The refractive index of the substrate isns. The complex refractive index of the absorbinglayer is N2 5 n 2 ik.

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In order to obtain a wideband AR coating in thevisible region, the requirements on the dispersion ofan absorbing material are analyzed by using the ad-mittance diagram technique.7 Because the endingadmittance of the dielectric layer has to be 1.0 inorder to ensure zero reflectance at a given wave-length, the starting admittance of the dielectric layerwith a given thickness can be determined to obtainthe ending admittance of 1.0. According to thisstarting admittance, as well as the refractive index ofthe substrate and the thickness of the absorbinglayer, we can solve the complex refractive index of theabsorbing layer. The admittance loci of a two-layerAR coating having zero reflectance at the wave-lengths of 400, 500, and 700 nm are shown in Fig. 2.

If the starting and ending admittances of the di-electric layer are assumed to be x0 1 iy0 and 1.0,respectively, then x0 and y0 can be given by7

x0 51

cos2 d1 1 sin2 d1yn12 , (1)

y0 5~1yn1 2 n1!sin d1 cos d1

cos2 d1 1 sin2 d1yn12 , (2)

where d1 5 2pn1d1yl. Because the ending admit-tance of the absorbing layer is equal to the startingone of the dielectric layer,

x0 1 iy0 5ns cos d2 1 iN2 sin d2

cos d2 1 ins sin d2yN2, (3)

where d2 5 2pN2d2yl. In order to design a two-layer wideband AR coating, we first assume that therefractive index and the thickness of the dielectriclayer and the thickness of the absorbing layer areknown, for example, ns 5 1.536, n1 5 1.46, d1 5 85nm, and d2 5 15 nm. Next, we utilize an optimiza-tion algorithm to solve the complex refractive index ofthe absorbing layer for each wavelength in the visibleregion by using Eqs. ~1!–~3!. The result is shown inFig. 3~a!. A wideband AR coating can be obtained ifthe complex refractive index dispersion of the absorb-ing layer has a form like that shown in Fig. 3~a!.The reflectance and transmittance of such layer sys-tem are shown in Fig. 3~b!. For different combina-tions of thicknesses d1 and d2, the dispersion of theabsorbing layer desired is also changed. Therefore,

Fig. 1. Two-layer AR coating with an absorbing layer.

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to determine what material should be used, we mustcalculate desired dispersions for different thicknesscombinations of the two layers. Figure 4 gives someof the results. It is clear that the form needed is onein which the refractive index is decreasing and theextinction coefficient is increasing with wavelength.

3. Experimental Results

A. GlassyAu or CuySiO2yAir

Fortunately, the most commonly used metal materials,Au and Cu, have dispersion properties similar to therequired dispersion forms.14 As a way to demon-strate the proposed method, two wideband AR coatingsusing SiO2 and an Au or Cu layer were designed. Thecalculated reflectances and transmittances for thesetwo AR coatings are shown in Figs. 5~a! and 5~b!, re-spectively. AR coatings were deposited in a BMC-850RD box coater. Film thickness was controlled bythe quartz crystal monitor. The substrate was notheated. The measured spectral reflectances andtransmittances are shown in Figs. 5~c! and 5~d!. It isevident that the theoretical and experimental resultsare rather close. The small difference between thecalculated and measured performances may resultfrom the difference in the complex refractive indexbetween a thick metal layer and a thin metal layer.

B. GlassyTiOxNyySiO2yAir

A different AR coating using TiOxNy film as an ab-sorbing layer has been also designed and produced.TiOxNy film has been widely used in various applica-tions because of its high hardness, good wear prop-erty, and corrosion resistance. The optical andmechanical properties of TiOxNy films usually varywith stoichiometry, crystallinity, microstructure,morphology, and density.15 For different x and y,the optical constants of the films are also different.In our case, TiOxNy films were deposited by magne-tron sputtering. The material of the target is Ti andthe substrate is BK-7 glass without heating. Thelayer termination is based strictly on time alone.This requires a stable process with closely controlledtarget powers. TiOxNy films were deposited in dif-ferent partial pressures of O2 and N2, and their com-plex refractive indices were determined by using

Fig. 2. Admittance diagram of a two-layer wideband AR coating.Points S and A are the admittances of the substrate and incidentmedium. Curves SB, SC, and SD are the admittance loci of theabsorbing layer and curves BA, CA, and DA are those of the di-electric layer at wavelengths of 400, 500, and 700 nm, respectively.

Fig. 3. ~a! Desired dispersion of thecomplex refractive index of an absorb-ing film on the assumption that ns 51.536, n1 5 1.46, d1 5 85 nm, and d2 515 nm. ~b! Calculated spectral reflec-tance and transmittance of the abovetwo-layer AR coating.

measured reflectance and transmittance. The ex-perimental results showed that for an N2 flow of 10SCCM and an O2 flow of 0 SCCM ~where SCCMdenotes cubic centimeter per minute at STP!, thedispersion of the complex refractive index of theTiOxNy film @shown in Fig. 6~a!# is closest to the val-ues required. The measured spectral reflectanceand transmittance of a two-layer AR coating are

shown in Fig. 6~b!. The photopically weighted re-flectance is near 0.1%, the average transmittance isapproximately 70%, and the sheet resistance is ap-proximately 200 Vyh.

4. Conclusion

We have shown that two-layer wideband antireflec-tion coatings can be obtained by using absorbing ma-

Fig. 4. Desired dispersions of complex re-fractive indices of absorbing films on theassumption that ns 5 1.52 and n1 5 1.46,and different thickness combinations oftwo layers: thin curve, d1 5 90 nm; thickcurve, d1 5 80 nm; dotted curve, d1 5 70nm.

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Fig. 5. Spectral performances ofdesigns and experiments of two-layer wideband AR coatings: ~a!glassyCuySiO2yAir ~the thick-nesses of Cu and SiO2 are 5 and 62nm, respectively!, ~b! glassyAuySiO2yAir ~the thicknesses of Au andSiO2 are 5.4 and 63 nm, respective-ly!, ~c! glassyCuySiO2yAir, ~d!glassyAuySiO2yAir.

Fig. 6. ~a! Dispersion of the complexrefractive index ~n 2 ik! of TiOxNy film.~b! Measured spectral reflectance andtransmittance of a two-layer AR coatingconsisting of an SiO2 layer and aTiOxNy layer with thicknesses of 85 nmand 14 nm, respectively.

terials such as Au, Cu, or TiOxNy combined withSiO2. We believe that any absorbing material witha dispersion like the one shown in Fig. 4 can be usedto design a two-layer wideband antireflection coating.This kind of AR coating is very suitable for displayapplications, in which low reflectance and good anti-static properties are important.

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