THIN METALLIC LAYERS STRUCTURED BY E-BEAM LITHOGRAPHY. Miroslav HORÁČEK, Vladimír KOLAŘÍK, Michal URBÁNEK, František MATĚJKA, Milan MATĚJKA

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1 THIN METALLIC LAYERS STRUCTURED BY E-BEAM LITHOGRAPHY Miroslav HORÁČEK, Vladimír KOLAŘÍK, Michal URBÁNEK, František MATĚJKA, Milan MATĚJKA Ústav přístrojové techniky AV ČR, v. v. i., Královopolská 147, CZ Brno, Abstract The group of electron beam lithography runs the laboratory equipped with a shaped beam electron writer (BS600) and the basic technology for the lithographic process. The group is able to prepare micro and nano structures in thin layers of metals and other materials; including the characterization of the realized structures (using AFM, SEM, and CLSM). Within a few months (in the frame of the 'ALISI' project) a new e-beam writer with a better resolution will be installed; it will enable the realization of the actual structures in a better quality and the development of new structures with a very high innovation potential. Key words: E-beam lithography, shaped electron beam, thin metallic layers, micro structures. 1. INTRODUCTION This contribution is aimed at the preparation of microstructures in thin metallic layers. Data preparation for microstructures, e-beam exposure, resist development, metal etching and sputtering issues are discussed as well as characterization and evaluation methods. Next, an overview of practical applications is presented e. g. special photo masks for optical lithography, direct written structures, calibration specimens for microscopic distance and orthogonality check, and metallic grating for optical applications. Finally, the potential using the new high-resolution equipment is summarized. 2. E BEAM TECHNOLOGY 2.1 E Beam Pattern Generator E beam writer (pattern generator) with a rectangular-shaped electron beam (Tesla BS 600, see Fig. 1) allows for a fast exposure of high resolution image information into a thin layer of electron resist spun on Silicon (or glass) substrates. The basic step of the writing system is 50 nm; the maximum grating density is about lines per mm. The permanent improvement of the pattern generator technical parameters allows for the increase of the writing speed. Besides that, the recent installation of the magnetic field cancellation system improved also its precision noticeably [1]. 2.2 Data preparation The first technological step is similar for all processes. Required patterns are prepared in the machine readable format and the data are checked for the consistency. Thin metallic working layers however imply a necessity to provide a special care and handling of electron scattering during exposures. Also, the isotropic etching process parameters (widening of delineated patterns) is to be considered. 2.3 E Beam Exposure and Resist Development The required pattern is transferred into the resist layer during the exposure. Generally, the shaped beam system has a higher writing speed then the Gaussian one; larger area could be patterned. Both positive and

2 Fig. 1 E beam pattern generator BS600. negative tone resists are available. The resist layer mask is created by the development of the exposed patterns. The areas exposed into the positive resist (PMMA is a commonly used) are dissolved in an appropriate developer. 2.4 Metal Etching and Metal Sputtering The substrate surface (or the working layer previously deposited on the substrate either a metallic or a dielectric one) is modified through the resist openings. Alternatively a thin metallic layer can be sputtered on the relief created in the resist. Final structures are checked using different microscopic techniques (profile meter, optical microscope, atomic force microscope AFM, confocal laser scanning microscope CLSM, scanning electron microscope). 3. PRACTICAL APPLICATIONS 3.1 Photo Masks for Optical Lithography The main industrial application of the e-beam lithography is the preparation of the masks for other (mainly optical) lithographic processes. An example of the 4 by 4 inches photo mask is depicted in Fig. 2. This mask includes a matrix of testing structure chips used in imaging analysis of implanted regions (Boron, Phosphorus) with different concentration for scanning electron microscopy. The photo masks are sometimes used directly as a standalone product for special purposes. We can mention the application for Earth movement monitoring (results of the monitoring provided by P. Kalenda are accessible on-line, cf detail of the mask is shown in Fig. 3). A photo mask used for the calibration of measurements apparatus for stone assortment mesh screen could be another example.

3 Fig. 2 Lithography mask, Chromium on 4 glass substrate. Fig. 3 Detail of a mask used for Earth movement monitoring, lettering 50 microns.

4 Fig. 4 Comb structure made in the Aluminum layer on a glass substrate. 3.2 Direct Written Structures Thin layer structures are usually prepared by the etching of working (metallic) layer or by a lift-off technique. The etching process (both isotropic and anisotropic) is performed through the resist mask openings. Silicon, glass or ceramic substrates can be used see Fig. 4. A large variety of sizes and patterns were alienated with the resolution down to sub submicron resolution. Application field covers micro sensors applications (temperature sensors, pressure sensors, illumination sensors, surface acoustic wave devices). Structures were also used as masters for nano imprint lithography process [2]. Special cases calibration specimens and various types of diffractive gratings are discussed in the following sections. 3.3 Calibration Specimens The metrics of the pattern generator is derived from the laser interferometer module and the prepared structures can be delineated with a remarkable precision. A natural application is then calibration specimens used for metrology check of various types of microscopes. Over a hundred samples were prepared during last few years for optical and scanning electron microscopy that are used for dimensional and orthogonality check. The precision of the specimens is guaranteed with the certificate of calibration issued by the Czech Metrology Institute. The specimens are basically composed of various patterns as linear and cross gratings, scales, geometric shapes and description. They can be easily customized. A cross grating with the period of 462 nm prepared by an anisotropically etch of the Silicon (100) covered by a thin metallic layer (Platinum) is shown in the Fig. 5.

5 Fig. 5 Detail of a calibration specimen: cross grating with a period of 463 nm. 3.4 Metallic Gratings Metallic gratings in the micron and submicron resolution can be prepared either in the resist layer and consequently covered by a selected metal layer (Silver, Gold, and Platinum) either by etching the metallic layer through the resist mask openings. Regular diffraction grating as well as irregular structures (Fresnel lenses, computer generated holograms) can be prepared in a very flexible way examples shown in Fig. 6 and Fig TUNGSTEN TIP FORMING A little bit apart from the thin film technology there is a metallic tip forming technology. Using a precisely controlled anodic etching process (bath temperature, hydroxide concentration, etching current, process Fig. 6 Part of a Fresnel lens, resist layer covered by Silver, optical microscope. Fig. 7 Multilevel computer generated hologram, basic pixel is 2.5 microns, AFM.

6 Fig. 8 Tungsten tip, diameter 800 nm. timing) we can achieve the predefined shaping of metallic wires. A Tungsten wire of 100 microns in diameter can be shaped in the sub micron region in a way that is required for electron beam emitters [3] see Fig CONCLUSIONS A potential of the high resolution equipment. So far we have presented the results that were achieved with the actual pattern generator (resolution of several hundred nanometers). Soon, a new pattern generator with a considerably better resolution will be installed. This system, in combination with the reactive ion etching equipment, will push the capacities as the resolution is concerned but also a much higher aspect ratio (ratio of the deepness and the width of the structures) will be achievable. Such structures, with a substantially larger surface area given the volume of the metallic material remaining constant, can change optical, mechanical, frictional, electrical or thermal properties of the metal surfaces in a very interesting way. ACKNOWLEDGEMENT This work was partially supported by European Commission and Ministry of Education, Youth and Sports of the Czech Republic (project No. CZ.1.05/2.1.00/ ALISI). REFERENCES [1] Kolařík, V. et al.: Nanolithography and Magnetic Field Cancellation in the Industrial Area. Jemná mechanika a optika. Vol. 56, (2011), pp ISSN [2] Kettle, J. et al.: Fabrication of poly(3-hexylthiophene) self-switching diodes using thermal nanoimprint lithography and argon milling. Journal of Vacuum Science & Technology B. Vol. 27, No. 6 (2009), pp ISSN [3] Matějka F. et al.: Modification of the Schottky FE ZrO/W electron emitter. Proceedings of the 17th IFSM International Microscopy Congress pp. I1.12: 1-2. ISBN