Antireflection coating design for plastic optics

Ulrike Schulz, Uwe B. Schallenberg, and Norbert Kaiser

The coating of plastics for optical applications is intended to improve the mechanical durability of softpolymers and to serve an antireflection function. Usually a classic four-layer antireflection system isadded on top of a single-layer hard coating. With needle optimization, an alternative coating design hasbeen developed. The design is characterized by thin high-refractive-index layers that are almost evenlydistributed over the whole stack. Plasma ion-assisted deposition was used to deposit coatings uponpoly�methyl methacrylate�, polycarbonate, and cyclo-olefin copolymer. Uniform antireflection and highscratch resistance have been achieved. © 2002 Optical Society of America

OCIS codes: 310.1210, 310.1860.

1. Introduction

Transparent plastics are widely used as optical com-ponents. Coating of soft materials is intendedmainly to improve the materials mechanical durabil-ity. Additionally, an antireflection coating is neces-sary for many optical applications. Most experiencewith abrasion-resistant antireflection coatings hasbeen with ophthalmic coatings. In general, coatedeye-glasses have to pass severe environmental andabrasion tests to demonstrate their quality. Usuallyophthalmic hard coatings are lacquers based on sil-anols or organically modified silica layers depositedby physical vapor deposition or chemical vapor dep-osition.1,2 An antireflection coating, which consist’sof four to six single layers as known from classicaltheory,3 is arranged typically on top of the hard coat-ing.

For injection molding of optical parts, thermoset-ting materials such as poly�methyl methacrylate��PMMA� and polycarbonate �PC� are used. Some-times only moderate abrasion resistance is requiredfor sensitive handling and cleaning procedures. Aswe know from our tests, an oxide layer with a thick-ness of �1 �m ensures that conditions for the abra-sion test with rubber �International Organization for

U. Schulz �ulrike.schulz@iof.fhg.de� and N. Kaiser are with theFraunhofer Institute for Applied Optics and Precision Engineering,Schillerstrasse 1, 07745 Jena, Germany. U. B. Schallenberg is withMSO, Jena Mikroschichtoptik GmbH, Carl-Zeiss-Promenade 10,07745 Jena, Germany.

Received 27 September 2001; revised manuscript received 3 Jan-uary 2002.

0003-6935�02�163107-04$15.00�0© 2002 Optical Society of America

Standardization standard ISO9211-02-03� requiredor optical instruments will be met. Nevertheless,the application of thermoplastics as transparent win-dows for displays will require thicker coatings to ful-fill higher demands.

The aim of performing this study has been to de-velop for plastic substrates a broadband antireflec-tion coating with an average residual reflectance of�0.4% in the visible spectral range and with a highabrasion resistance. The physical thickness of thecoating was to range from 1 to 3.5 �m. It is wellknown that the thicker the overall coating, thegreater the design possibilities for a desired opticalfunction.4 Therefore we used a needle optimizationprocedure to design an antireflection coating by dis-tributing the high-index material in one thick low-index layer instead of the usual combination of athick hard coating with a thin antireflection coatingon top of it. Plasma ion-assisted deposition �IAD�was been applied for deposition of coatings of the newtype onto several kinds of plastic. The optical andmechanical properties of coatings have been investi-gated and compared with those of a four-layer anti-reflective coating combined with a single silica hardcoating.

2. Experiment

We prepared flat samples with optical grade surfacesby injection molding, using a polymer granulate thatcomprises PMMA 7N �Rhoem�, the �PC� Makrolon�Bayer AG, Germany�, and the cyclo-olefin copolymer�COC� Topas 5030 �Ticona, Germany�.

Uncoated substrates were handled carefully to pre-vent contamination. We cleaned the substrates be-fore coating them by blowing the samples withionized nitrogen.

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A Leybold APS-904 box coater was used for plasmatreatment of substrates and plasma IAD of coatings.During film growth, substrates are bombarded withargon ions emitted by the advanced plasma source,which ensures the formation of dielectric films withhigh density and stable optical properties.6

As a first step in the vacuum process, a differentinitial treatment for each kind of plastic substrate isuseful for good adhesion of evaporated dielectricfilms. For PC an argon-ion treatment with an ad-vanced plasma source �120 s; ion energy, �80 eV� wasapplied. PMMA requires a special glow dischargetreatment with a dc cathode and an Ar�H2O gas mix-ture to improve adhesion properties.7 COC can beactivated promptly if an ion-assisted layer depositionprocess is started, and no further treatment is nec-essary. Initial treatment was followed immediatelyby electron-beam evaporation of coating materials.Low ion energy ��80 eV� for ion assistance was usedfor SiO2 layers to produce coatings with minimal re-sidual stress. The high-index material Ta2O5 wasdeposited by Ar-ion assistance with an energy of�120 eV. Deposition rates were 0.8 and 0.2 nm�sfor SiO2 and Ta2O5, respectively.

The maximum temperature of a substrate duringcoating was controlled by use of Thermostrip heat-sensor tapes. Spectral transmittance and reflec-tance of coated samples were measured with aPerkin-Elmer �19 spectrophotometer. Environ-mental tests in accordance with the standardISO9022 and a qualitative steel-wool test were per-formed.

3. Results and Discussion

A. Design of Hard Antireflection Coatings

For simplification, all calculations presented in thispaper were performed with constant refractive indi-ces and normal light incidence with the followingvalues: nL � 1.46 for SiO2 �low-index material, L�,nH � 2.1 for Ta2O5 �high-index material, H�, and ns �1.49 for PMMA �plastic substrate�.

One common antireflective �AR� layer system isderived from the quarter-half-quarter design

sub�M2HL�air,

where material M with an intermediate refractiveindex was replaced by parts of high- and low-refraction materials. Such a design �AR-4� has thefollowing structure with respect to the physical layerthickness:

Design AR-4 sub�15H24L108H83L�air,

which means, starting from the substrate, 15-nmhigh-index material, 24-nm low-index material, andso on, with 123-nm Ta2O5 and 107-nm SiO2 com-bined. Usually design AR-4 or similar AR designsare adapted to a single hard coat layer for ophthalmicapplications.

Our first step in looking for a new design was toreduce the amount of high-index material to make

the process for PMMA as cold as possible. Addition-ally, the low-index part was increased to improvescratch resistance. The needle optimization tech-nique was applied, starting simply with a 1000-nm-thick SiO2 layer and a target value for residualreflection of 0.4% defined for the spectral range 420–670 nm. The optimization procedure was inter-rupted after incorporation of four high-index layers.After refinement, a nine-layer-stack �design AR-hard-9; see below for a definition� with a total phys-ical thickness of 1114 nm was achieved:

Design AR-hard-9

sub�215L5H252L9H247L13H235L22H116L�air.

In Fig. 1 the index profiles and the performance ofAR-4 and AR-hard-9 are shown. The high-index-layer thickness of AR-hard-9 is only 47 nm �com-bined�. We found that design AR-hard-9 can bemodified to comprise more or fewer layers. HL layerpairs, where H is �10 nm and L is �245 nm, have tobe added or removed, followed by design refinement.Uneven numbers of layers, from 7 to 35 are at leastpossible. Typically, the high-index layers are thin-ner than the low-index layers. We call the antire-flective coating AR-hard. Figure 2 shows some moreindex profiles and the resultant reflectance. An ex-cellent uniform antireflection effect combined withhigh scratch resistance can be expected for thethicker AR-hard coatings.

It is obvious that coatings of the type of AR-hardmay be regarded as periodic structures. Each pe-riod consists of approximately 5–15-nm Ta2O5 and�240-nm SiO2. The optical thickness of the periodsis �3 ��4 for a wavelength of nearly 516 nm, which isthe reference wavelength for all coatings.

Sometimes the results of needle optimization arenot suitable for practical use because of the sensitiv-ity to thickness of very thin needle layers. It hasbeen estimated that for design type AR-hard the sen-

Fig. 1. Index profiles and optical performance of the novel designAR-hard-9 and of the classic four-layer design AR-4. Reflectancewas calculated for substrate index n � 1.49 �back side disabled�.

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sitivity of performance to thickness errors in eachsingle H layer is low. If systematic thickness errorsof as much as 20% occur, the whole system is shiftedslightly to lower or higher wavelength only. Thegeneral mechanism of periods of this design type asdescribed is expected to work well even when thecalculated thickness of the H layers cannot be accu-rately achieved in a real manufacturing process.The coating as an arrangement of symmetric three-layer periods LHL will be considered in upcomingresearch.

B. Antireflective-Hard-9 upon Poly�Methyl Methacrylate�

PMMA is one of the materials for which the coatingtemperature is critical, because of its low glass-transition temperature of 107 °C. No controlledsubstrate heating was applied during the plasmaIAD process, but heat radiation emitted by electron

beam guns and an ion source increased the temper-ature. The thermal load on the substrates dependedmainly on the thickness of the Ta2O5 layers becausethe evaporation temperature of Ta2O5 is higher thanthat of SiO2. During the deposition of the AR coat-ing AR-4 �which has a high content of Ta2O5� thetemperature on the substrate was increased to�90 °C. As a consequence of high thermal stress,film cracking was seen to occur.8

Alternatively an AR-hard-9 coating �47 nm Ta2O5only� was deposited. The maximum temperature onthe substrate was raised to �50 °C, and no film crack-ing was observed. Figure 3 shows transmissionmeasurements of PMMA samples from differentbatches. The average transmission achieved at440–670 nm was �95.5%. The deviations of differ-ent batches from this value were low, as expected.

C. Antireflective-Hard-27 upon Polycarbonate andCyclo-olefin Copolymer

To demonstrate the applicability of the design AR-hard as a hard coating, we deposited a system con-sisting of 27 layers with a total physical thickness of�3 �m upon PC and COC. It had been assumed

Fig. 2. Index profiles and optical performance of designs AR-hardconsisting of 7 �AR-hard-7�, 13 �AR-hard-13�, 19 A�R-hard-19�, and27 �AR-hard-27� layers Substrate index n � 1.49, back side dis-abled�.

Fig. 3. Transmission of PMMA coated with AR-hard-9; compar-ison of samples from five batches �No backside coating�.

Fig. 4. Light microscope image of a PC surface, half of which hadbeen coated with AR-hard-27, after the entire surface was rubbedwith steel wool. The coated area �left� is not damaged.

Fig. 5. Transmission of COC and PC after coating with AR-hard-27 on both sides.

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that the mechanical properties of a coating are dom-inated by the properties of silica, because of the lowcontent of high-index material. The mechanicalstress of �250 MPa compressive and the scratch re-sistance of coating AR-hard-27 were indeed the sameas for a single silica layer of 3-�m thickness. Figure4 demonstrates the result of a steel-wool test of a PCsubstrate, half of which was coated with AR-hard-27.No damage occurred to the coated area. In contrast,a conventional four-layer antireflection coating�AR-4� upon a plastic substrate was easily damagedby steel wool.

The desired antireflective performance wasachieved again without manufacturing problems.Only a quartz-crystal monitoring system was usedfor thickness control of single layers during electron-beam evaporation. Figure 5 shows measuredtransmission curves of COC and PC samples withAR-hard-27 coating on both sides. The averagetransmission achieved from 440 to 670 nm was�98.5%.

Environmental stability can be an essential crite-rion of coatings on plastics. In particular, tempera-ture changes are critical for inorganic coatings onplastics because the two have different thermal ex-pansion coefficients. Results of climatic testingshow a surprising difference in the behavior of a sin-gle silica layer and an AR-hard-27 coating that havethe same thickness �Table 1�. Thermal stress crack-ing was observed only for silica coating but not forAR-hard coatings. A relaxation effect of needle lay-ers on the elasticity of the whole coating is thereforeassumed.

4. Conclusion

A new type of antireflection coating has been dis-cussed. In contrast to common antireflection coat-ings, it consists of a periodic arrangement of thinhigh-index layers and thicker low-index layers. Theperiods may have an optical thickness of �3 ��4 at

the reference wavelength to achieve a broadband an-tireflective effect. Coatings of the new design typewere deposited upon PMMA, COC, and PC by plasmaIAD. The hardness of AR-hard coatings correspondsto that of single SiO2 layers of the same thickness.With coatings on both sides of thermoplastic materi-als, transmission was increased uniformly to morethan 98% in the visible spectral range. Low sensi-tivity of the AR-hard design type to systematic thick-ness errors of the high-index layers during depositionwas observed. The low volume of high-index mate-rial inside coatings of the AR-hard type could be in-teresting also for use in other spectral regions.

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7. U. Schulz, P. Munzert, and N. Kaiser, “Surface modification ofPMMA by DC glow discharge and microwave plasma treatmentfor the improvement of coating adhesion,” Surf. Coat. Technol.142–144, 507–511 �2001�.

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Table 1. Results of Environmental �ISO 9022� and Adhesion �ISO 9211� Tests of AR-Hard-27 Coating and Single-Layer SiO2 upon COC and PCa

Test Method AR-Hard-27 Coating Single-Layer SiO2

Tape test snap No peeling No peelingHeating to 55 °C for 16 h No defect No defectHeating to 55 °C for 16 h, 90% R.H. No defect Film crackingHeating to 70 °C for 16 h No defect Film crackingHeating to 100 °C for 16 h No defect Film crackingHeating to �25° C for 16 h No defect Film cracking

a Both coatings have a physical thickness of �3 �m.

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