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Disposable master technology is a promising method for creating large-area sub- ... Keywords: nanoimprint technology, structural characterization, diffraction ...
Structural and optical characterization of photonics structures prepared by nanoimprint technology Daniel Haško*a, Jaroslav Kováča,b, Alexander Šatkaa,b, Milan Držíka, František Uhereka, Graham Hubbardc and Duncan W.E. Allsoppd a International Laser Center, Ilkovičova 3, Bratislava 841 04, Slovak Republic b Dept. of Microelectronics, Slovak University of Technology, Ilkovičova 3, Bratislava 812 19, Slovak Republic c MacDermid Autotype Ltd, Grove Road, Wantage OX12 7BZ, England d Faculty of Engineering and Design, University of Bath BA2 7AY, England ABSTRACT Nanoimprinting provides an alternative approach for production of highly ordered arrays of nanostructures on a wafer scale cheaply and rapidly. Disposable master technology is a promising method for creating large-area sub-wavelength photonic elements, solar cells and PhC structures. The geometrical parameters of the surface profile of disposable master samples and its replicas in the nanometer scale can be determined by applying standard methodology of the surface morphology measurement by AFM or SEM. Systematic studies will be focused on the process control for pattern transfer into different types of resist and its homogeneity on Si wafers. In particular, using a single spent polymer mold, imprint results shows, that the conditions for spin coating and curing the resist determine the homogeneity and replication fidelity that can be achieved. To analyze structures over large areas the above techniques can be used for statistical sampling. In addition, the general uniformity of the materials will be assessed using large-scale optical techniques. For the visualization and testing of structure pattern homogeneity as well as the pattern defects identification a large field diffraction-based diagnostic method has been utilized. The results indicate that choice of processing conditions is, in addition to materials selections, extremely important in achieving high-fidelity nanostructures. Keywords: nanoimprint technology, structural characterization, diffraction diagnostic

1. INTRODUCTION The continued improvement in capabilities for patterning structures with nanometer dimensions is essential for the semiconductor technology. The minimum feature sizes that can be produced by conventional photolithographic techniques are limited by the wavelengths of visible light. Beyond this level of resolution are the fabrication techniques for nanosized patterns, such as deep-ultraviolet lithography [1], X-ray lithography [2], e-beam lithography [3] and Atomic Force Microscopy lithography [4]. These techniques are very powerful but expensive, and they have low throughput. On the other hand nanoimprint lithography [5] is a new manufacturing technology to realize fine structures over large areas substrates (glass, silicon, SiO2 coated silicon, aluminium coated silicon, etc.) with high throughput at low cost [6]. The aim of the nanoimprint lithography is to develop tools that can modify surfaces with nanoscale resolution in a massively parallel fashion. In this procedure the mold (made from either a hard or soft material) creates patterns by mechanical deformation of imprint photoresist and subsequent processes. The imprint photoresist is typically a monomer or polymer that is cured by heat or UV light during the imprinting. The mold matters as well as the photoresist influence the resolution. Both mold and photoresist must be able to come into perfect conformal contact with the interface to establish an atomically accurate replica of the relief structures. The nanoreplication based on mold is extremely sensitive to any particle existing between the spin-coated wafer and the mold during the imprinting process. Since the feature size of the nanostructures is typically at < 50–100 nm, and the thickness of the resist layer is around the same scale (~100–200 nm), any soft or hard particles with size larger than ~50 nm existing on either the mold or the wafer could potentially generate a defect area on the imprinted wafer.

*[email protected]; phone +421 2 65421575; fax +421 2 65423244; www.ilc.sk Photonics, Devices, and Systems IV, edited by Pavel Tománek, Dagmar Senderáková, Miroslav Hrabovský, Proc. of SPIE Vol. 7138, 713824 · © 2008 SPIE · CCC code: 0277-786X/08/$18 · doi: 10.1117/12.818074

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In this paper we demonstrate flexible wafer-scale nanoimprint lithography process - disposable master technology (DMT), which allows imprinting 3D nano-scale features into a UV curable lacquer spin-coated on to the target substrate, using a single spent soft master. General interests of the investigations were connected with detailed geometric measurement of used disposable masters and its replicas to photoresist. Analysis results were used for improving the imprinting process. Atomic Force Microscope (AFM) and Field Emission Gun Scanning Electron Microscope (FE SEM) were employed for characterization of DMT applicable on 4” Si wafers. For the structure pattern homogeneity visualization as well as the pattern defects identification a diffraction-based diagnostic method has been proposed and utilized. In this paper the structural and large-scale optical characterization of 300 nm moth eye pattern replication made in UV sensitive photoresist is described by using novel nanoimprint lithography process.

2. PHOTONICS STRUCTURES PREPARATION The DMT utilize a soft master structure based on polyethylene terephthalate (PET) foil for nanoimprinting. This “disposable master” is prepared using a roll-to-roll UV replication process from the nickel master. The nickel pattern was molded by the standard electroforming technique across photoresist with moth eye structure made by two-beam laser interference photolithography [7]. Wafer scale nickel replicas can be routinely produced in PET thin films (100’s of meters). The resulting features of pseudo-hexagonal structure array can be used as an antireflection film for displays [8]. For the pattern transfer on 4” Si wafers two types of special UV sensitive photoresist were used. One is based on acrylate, second on oxetanyl silsesquioxane (OXSQ). Imprinting procedure consist of substrate cleaning, single-layer spin coating of photoresist, short low temperature pre-bake, hand-roll disposable master with light force, UV exposure for few seconds, thermal cure to complete the cross-linking of the imprint and release disposable master by peeling it from substrate. After imprinting 100 % successful coverage of pseudohexagonal array in the OXSQ photoresist on 4” Si wafer by using the above technique was achieved (see Fig. 1).

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3. STRUCTURAL CHARACTERIZATION Morphology of disposable masters and replicas to photoresist has been examined because of its impact on final array of nanoscale surface structures. The geometrical parameters of the pattern profiles in the nanometer scale were measured by high-resolution AFM using ultra-sharp tips (tip radius is ~ 2 nm). The images were collected utilizing semicontact mode with scan velocity 11 µm/s and scan range of 5×5 µm2. The AFM measurements were realized at 9 areas of the imprints marked in Fig. 1 and at approximately the same areas on the used masters, to show the depth of structure and to determine cross-section profiles. Fig. 2 shows the corresponding AFM images of used disposable master at position 7.

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This analysis gives the information about the change of the depth of the structure between the master and its replica. The examples of AFM pictures of the imprinted moth eye structure in OXSQ photoresist, spun at 3000 rpm on SiO2/Si substrate with period of ~ 300 nm and height of ~ 140 nm is shown in Fig. 3. The imprinted pattern with pseudohexagonal symmetry is regular and shows good filling of the relief on the master.

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Special SEM characterization was directed to structure depth and residual layer determination. It was found that OXSQ imprinted with disposable master gives ~150 nm structure depth with residual layer less than 30 nm while UV sensitive acrylate imprinted with the same moth eye master gives ~165 nm structure depth with residual layer less than 20 nm. Fig. 4 shows SEM photograph of moth eye on SiO2/Si wafer nanoimprinted by DMT replication technology. The nanopattern replication process has a good fidelity except some technological errors. The peaks and depressions in the pattern have a near elliptical rather than circular cross-section.

Fig. 4. SEM top view of imprinted structure (a), and angle view of the moth eye grating (b)

Moth eye disposable masters with 450 nm and 230 nm period and linear grating patterns have been also imprinted successfully, when the thickness of the photoresist film is optimized. More experiments have been done support that the DMT nanoreplication process does not cause feature size shrinkage on the lateral dimension. However, on the vertical dimension, there is about 7 % - 10 % shrinkage in feature size.

4. HOMOGENITY TESTING Analysis of geometrical parameters of the disposable master and its replicas to photoresist can be done by applying standard methods of the surface morphology measurements. However, this approach gives effect only to very small surface extent with an area of several squared micrometers. In order to overcome such limitation and thus, to obtain the opportunity of the regular surface inspection, the whole field surface pattern diffraction observation can be realized. This

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possibility follows from the fact that moth eye patterns have a periodic surface structure with the dimensions comparable to those of visible light wavelengths. Hence, after illumination of the structure the diffraction angles by a digital CCD camera can be identified. Therefore, the diffraction-based principle of measurement consists in the illumination of the surface tested at proper angle of incidence (Fig. 5a) with a subsequent focusing of the diffracted light in the Fourier filtering arrangement. In the scheme, besides of the diffracted structure inhomogeneities, also any kind of periodical structure small-scale defects are visualized throughout the whole field of view (Fig. 5b).

ψ φ d (sin φ + sinψ ) = λR Diffraction angles: φ = 66°, ψ = 0° at λ = 473 nm Diffraction efficiency: η = 2.78 % Periodicity of the grating is 299 nm Fig. 5. Grazing angle incidence and beam diffraction on the surface grating (a), visualization of the whole-field white light diffraction efficiency with surface defects identification (b)

The realization of the assembly needs only monochromatic or white light source, large diameter lens and appropriate digital CCD camera. Diffraction efficiency of the structure can be determined from the recorded photographic pictures by scanning the intensity distribution and its comparison with the intensity of the light reflected from the surface at the angle of mirror-like reflection. Precise quantitative measurement of the diffraction efficiency was carried out using the Nd:YAG (λ= 473 nm/1 mW) blue light laser. The amount of the light reflected at perpendicular direction was compared with the intensity of the beam diffracted at the angle of first order diffraction. Only in such configuration, the ratio of diffracted vs. reflected intensities was correct to evaluate the diffraction efficiency. The diffraction-based measurements of the imprints on 4” SiO2/Si wafer using OXSQ photoresist (Fig. 5b) shows the quality of DMT and are in good correspondence with the 300 nm period of the grating structure.

5. CONCLUSIONS Disposable master technology (DMT), by which photoresist patterns are fabricated by embossing with a soft master mold is a very useful nanostructure replication technique. This has been demonstrated by making 300 nm moth eye grating nanostructure patterns. On the same samples AFM, SEM and large-scale diffraction-based diagnostic method were employed to determine the morphology, depth profiles and homogeneity of the disposable masters and its replicas to photoresist. All used methods revealed a good quality of nanostructure arrays on 4” SiO2/Si wafers with reproducible periodicity of the moth eye grating. The results demonstrate the applicability of DMT for large area photonic nanostructures as an attractive alternative approach, offering a much simpler technique, higher throughput, high-speed and lower cost. From among the various factors affecting DMT quality, the process is a promising method for low-cost formation of sub-micron size photonic structures in one-step and is potentially suitable for high volume production. With many advantages, such a technique can be used in the fabrication of a wide range of nanoscale electronic, photonic and biological devices where patterns of various sizes and densities are needed.

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ACKNOWLEDGEMENT This work is supported by European Union under Framework 6 contract number 017481, STREP “N2T2”, by Slovak Research and Development Agency grant APVV-RPEU-0005-6 and by Ministry of Education of the Slovak Republic project AV4/0022/05.

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