Microelectronic Engineering 61–62 (2002) 435–440www.elsevier.com/ locate /mee

Nano-structured anti-reflective surfaces replicated by hotembossing

a , a b b b* ¨ ¨C. David , P. Haberling , M. Schnieper , J. Sochtig , C. ZschokkeaLaboratory for Micro- and Nanotechnology, Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland

bCentre Suisse d’Electronique et de Microtechnique SA, CH-8048 Zurich, Switzerland

Abstract

We developed a fabrication technique of anti-reflective structures for applications in the visible spectral range.The depth and duty cycle of the grating structures optimised for wavelengths around 500 nm were calculatedusing rigorous diffraction theory. We used electron-beam lithography for the manufacturing of linear and crossedgrating patterns with pitches of 200 nm and various duty cycles over areas of several square millimeters. Thestructures are transferred to a quartz master, which is then replicated by Ni electroforming into a stamper for thehot embossing into polycarbonate (PC) sheets. The optical properties of the replicated PC gratings weremeasured over a wide wavelength range. The general shapes of the curves are in good agreement with thecalculations, and for both polarizations, the reflectivity has been reduced significantly. 2002 ElsevierScience B.V. All rights reserved.

Keywords: Anti-reflective surfaces; Hot embossing; Electron-beam lithography; Nanoreplication

1. Introduction

Light reflection from surfaces such as lenses, displays, or computer monitors, can significantlydeteriorate the optical performance of a device by causing stray light or by degrading the transmissionof optical components. Anti-reflective surfaces are therefore widely used. They mostly consist ofvacuum-deposited coatings of dielectric material layers. Depending on the required optical per-formance, the necessary number and precision of the dielectric layers makes this method too costly formany applications.

An alternative method for reducing the reflectivity is to pattern the reflecting surface with a periodicgrating structure. When the period of the structure is smaller than the wavelength, the incoming lightis not diffracted in a classical sense, since all other than the transmitted and the reflected zeroth order

*Corresponding author. Tel.: 1 41-56-3103-753; fax: 1 41-56-3102-646.E-mail address: christian.david@psi.ch (C. David).

0167-9317/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0167-9317( 02 )00425-2

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are evanescent. The grating can, nevertheless, significantly modify the transmitted and the reflectedlight with respect to its intensity, phase, and state of polarization [1]. Compared to dielectric coatings,anti-reflective (AR) gratings have a number of potential advantages, depending on the specificapplication:

(i) AR gratings can cover a wide wavelength range with a single surface pattern.(ii) The surface relief can be replicated into polymers by fast, low-cost mass production techniques

such as injection moulding or hot embossing.(iii)The principle of AR gratings also works for UV light, where the choice of dielectric coating

materials with low absorption becomes very limited. Especially for intense, pulsed UV sources,AR gratings can withstand substantially higher fluences.

In this work, the second issue was addressed within a collaboration with the industrial partner LeicaGeosystems, Switzerland. We investigated the optical properties of electron-beam generated testpatterns replicated into polycarbonate sheets by hot embossing. The comparison with calculatedoptical properties will serve for the optimization of a larger area origination of AR structures, e.g., byholographic exposures.

2. Optical design calculations

According to scalar diffraction theory, the reflectivity of a substrate covered with a thin dielectriclayer is zero, when the two interfaces (substrate-coating and coating-surrounding medium) cause tworeflected waves of (i) equal amplitude and (ii) half-wavelength phase shift. According to thewell-known Fresnel formulas, the reflectivity of each interface is determined by the ratio refractiveindices on either side the interface. To meet the first condition, the ratios n /n and n /n of the0 1 1 2

refractive indices on either side the interfaces (see Fig. 1a) have to be equal, which leads to the1 / 2antireflective criterion n 5 (n n ) [2]. In the case of normal incidence, the second condition is1 0 2

fulfilled for a layer thickness d equal to a quarter of the light wavelength inside the dielectric coatinglayer l /n , i.e., d 5 l /4n .1 1

Fig. 1b illustrates the situation of a binary dielectric grating with a pitch of L, a grating depth d,and a duty cycle (i.e., the line width divided by L) DC. As an approximation, a sub-wavelengthgrating can be regarded as a layer with an effective refractive index n , with (n 2 1) 5 (n 2 1) DC.eff eff 1

Fig. 1. Schematic view to illustrate the geometry of a dielectric AR coating (a) and an AR grating (b).

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Fig. 2. Calculated spectral reflectivities of polycarbonate gratings with 200 nm pitch obtained by rigorous diffraction theory.The left plot shows curves for TE and TM polarization at a constant duty cycle of 0.5 for 60, 80, and 100 nm grating depth.The right plot shows curves of a 100 nm deep grating at various duty cycles.

This simple model already gives us an indication for the design of a AR-grating. In our case, thesurrounding medium is air (n ¯ 1) and the grating structures consist of the same material as the0

substrate (n 5 n ¯ 1.5), which leads us to n 5 1.22, DC 5 0.45 and an optimum grating depth of1 2 eff

about d 5 100 nm at l 5 500 nm.More accurate results can be obtained using rigorous diffraction theory, i.e., by solving Maxwell’s

equations under the boundary conditions of the grating structure. These calculations also reveal thepolarization dependent properties of sub-wavelength AR gratings. We used the commercial programGSOLVER (by Grating Solver Development) to calculate a large variety of polycarbonate (PC)grating geometries. Some examples are shown in Fig. 2. In the left graph, the reflectivity of a lineargrating with L 5 200 nm and DC 5 0.5 for different grating depths is shown for polarization parallel(TE) and orthogonal (TM) to the grating lines. As anticipated, the reflectivity for l ¯ 500 nm, andd 5 100 nm is significant lower than that of an unpatterned PC sheet, but a clear polarizationdependence is observed. The right graph shows the spectral reflectivity for d 5 100 nm and varyingDC for TM polarization.

3. Fabrication technology

To verify the theoretical predictions described in the previous section, we used a LION-LV1electron-beam lithography system (Leica Microsystems Jena) for the manufacturing of linear andcrossed grating patterns with pitches of 200 nm and various duty cycles. Each test grating had an areaof 2 mm 3 2 mm. The grating lines were each generated by a single sweep of the electron beam, theline width and thus the duty cycle were adjusted by controlling the line dose and defocus settings ofthe electron beam [3]. The pattern transfer processes are depicted in Fig. 3. A 20 nm thick Cr layerunderneath the PMMA resist was applied to avoid electrostatic charging. Reactive ion etching in aCl /CO plasma was used to pattern the Cr layer. The Cr patterns were transferred into a quartz2 2

substrate by reactive ion etching in a CF /O plasma. The quartz master was replicated by Ni4 2

electroforming into a stamper for hot embossing at 150 8C into 1 mm thick PC sheets. This two-step

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Fig. 3. Processing steps used for fabricating of polycarbonate (PC) AR-grating structures.

replication technique allows for a generation of several nickel stampers from the same quartz master.Fig. 4 shows scanning electron microscopy (SEM) images of two linear gratings with different dutycycle and a dot array generated by two orthogonal grating exposures.

4. Optical measurements

The optical characteristics of the gratings were determined using a commercial spectrometer fromPerkin-Elmer (Lambda 9, UV/Vis /NIR spectrometer) with a reflection cell in a dual beam (sampleand reference beam) configuration. In order to separate the reflected beam, the incidence angle waschosen to be 68, which introduces only a negligible effect on the measurements compared to normalincidence. An example for such a measurement is shown in Fig. 5. For illustration of the AR

Fig. 4. SEM images of linear and crossed gratings with 200 nm pitch generated by electron-beam lithography and etchedinto the quartz masters substrate. The changing contrast in horizontal direction of the right image is due to electrostaticcharging.

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Fig. 5. Measured spectral reflectivities for TE and TM polarization of a replicated polycarbonate grating and of anunpatterned PC sheet. The grating has a pitch of 200 nm and a depth of 100 nm. The duty cycle was determined to be0.560.05 by SEM inspection.

properties of the gratings the reflectivity curve of a bare PC-foil (single foil facet, backside reflectionsuppressed by black coating) is included in the graph. Although the performance around the designwavelength is not quite as good as anticipated in the theoretical calculations, the general shape of thecurves are in good agreement, and for both polarizations, the reflectivity has been reducedsignificantly.

5. Conclusion and outlook

Our measurements indicate that hot embossing is a viable method for the generation of AR-gratings. The good agreement of the optical performance with the theoretical predictions show that wewere able to replicate the electron-beam generated masters with high fidelity.

These results are of special interest in context with the mass fabrication of optical components byinjection molding. It has been shown, that this technique is capable of replicating surface structureswith the required dimensions [4]. Although the patterning of mold surfaces with the required smallgrating lines appears challenging, the method could provide means for the fabrication polymer opticalelements with AR properties without any additional processing steps.

Acknowledgements

¨We are grateful to D. Bachle for his technical support. This work was funded by the Commissionfor Technology and Innovation (KTI-SEGOEM) and the TOP NANO21 initiative (NODE 1).

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References

[1] J. Turunen, in: H.P. Herzig (Ed.), Micro-Optics, Taylor and Francis, London, 1998.[2] M. Bass (Ed.), Handbook of Optics. Fundamentals, Techniques and Design, McGraw-Hill, New York, 1995.[3] C. David, D. Hambach, Microelectron. Eng. 46 (1999) 219–222.[4] H. Schift, C. David, J. Gobrecht, A. D’Amore, D. Simoneta, W. Kaiser, M. Gabriel, J. Vac. Sci. Technol. B 18 (2000)

3564–3568.