Iridium wire grid polarizer fabricated using atomic layer deposition
© Weber et al; licensee Springer. 2011
Received: 18 July 2011
Accepted: 25 October 2011
Published: 25 October 2011
In this work, an effective multistep process toward fabrication of an iridium wire grid polarizer for UV applications involving a frequency doubling process based on ultrafast electron beam lithography and atomic layer deposition is presented. The choice of iridium as grating material is based on its good optical properties and a superior oxidation resistance. Furthermore, atomic layer deposition of iridium allows a precise adjustment of the structural parameters of the grating much better than other deposition techniques like sputtering for example. At the target wavelength of 250 nm, a transmission of about 45% and an extinction ratio of 87 are achieved.
Keywordsoptics nanostructure fabrication polarizing devices
Wire grid polarizers offer a large spectral working range, small feature size, and good integrability and are of utmost importance for various applications such as microscopy or imaging systems. Generally, a wire grid polarizer consists of a periodical arrangement of conductive (metallic) wires on a transparent substrate. Upon illumination, a wire grid polarizer shows a higher transmission for light with the electrical field vector perpendicular to the wires (transverse magnetic (TM) polarization) than for the parallel counterpart (transverse electric (TE) polarization). In addition to the transmission of TM-polarized light, the extinction ratio which is defined by the ratio of TM- and TE-polarized light is another characteristic optical property of a wire grid polarizer. The spectral working range and the optical properties of a wire grid polarizer are determined by the grating material and the structural parameters of the grating such as period, grating height, or ridge width. First wire grid polarizers for the IR spectral region were demonstrated by Bird and Parrish  in 1960. Subsequent work in the past years showed polarizers for the terahertz , IR , visible , and UV  spectral ranges. Decreasing the operation wavelength requires smaller grating periods. For a wire grid polarizer, it is necessary that only the zeroth diffraction order is propagating. For this purpose it is known that the grating period must be much smaller than the incident wavelength. Consequently for a target wavelength of 250 nm a period of about 100 nm is required. The fabrication of such a high frequency metallic grating with high aspect ratios is technologically very challenging and demands a sophisticated lithography and metal structuring process. Possible lithographical processes include nanoimprint lithography , interference lithography , or electron beam lithography . Metal structuring can be accomplished by means of a lift off process , dry etching , or a spatial frequency doubling technique .
In this Letter, we present an iridium wire grid polarizer for broadband applications down to a wavelength of 250 nm fabricated by a spatial frequency doubling technique based on ultrafast electron beam lithography and atomic layer deposition (ALD). Moreover, we compare the suitability of ALD and sputtering as deposition technique for the frequency doubling process. In our previous work, we already demonstrated that iridium is suitable as an alternative grating material to the frequently used aluminum . The refractive index of iridium and aluminum is shown in . Compared with aluminum, iridium shows a superior corrosion resistance and the optical properties of an iridium wire grid polarizer generally comply with the requirements for broadband applications with wavelength of less than 300 nm. To realize these aimed optical properties, a grating with a period of 100 nm, a grating height of about 150 nm, and a ridge width of approximately 35 nm was prepared. ALD was the method-of-choice for the frequency doubling process  as it is superior in terms of step coverage and uniformity of the coating of high aspect ratio structures [14, 15] compared to conventional sputtering techniques, especially since a straight forward ALD process for iridium is available [16, 17]. It provides the possibility to accurately adjust the ridge width of the metal grating simply by controlling of the metal layer thickness through the number of ALD cycles. Furthermore, atomic layer deposition is a non-line-of-sight deposition technique and thus not affected by shadowing effects as it is observed in physical deposition processes under oblique incidence. Hence, the value of the ridge width of the fabricated iridium grating is not limited compared to our previous work  where sputtering was used as deposition technique.
2 Results and discussion
In conclusion, we have implemented the ALD technology in a frequency doubling process for the fabrication of a high aspect ratio iridium wire grid polarizer. Besides that, we showed that ALD technology is a more suitable deposition technique for the frequency doubling process than sputter deposition. The broadband optical function of the polarizer was shown from the IR down to a wavelength of 230 nm. Furthermore, the results were compared with a polarizer fabricated using a sputter technique and showed a notably higher extinction ratio in the whole investigated spectral range. A decrease of the target wavelength below 200 nm requires smaller grating periods and is the aim of further investigations.
atomic layer deposition
inductively coupled plasma
scanning electron microscope
Financial support from the Federal Ministry of Education and Research (BMBF) (project FKZ 13N9712 and FKZ 13N9711) is acknowledged.
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