Meta-foils are all-metal free-standing electromagnetic metamaterials based on interconnected S-string architecture. They
provide a versatile applications' platform. Lacking any substrate or embedding matrix, they feature arrays of parallel
upright S-strings with each string longitudinally shifted by half an S compared to its neighbour to form capacitance-inductance
loops. Geometric parameters include length a, width b, thickness t, and height h of an S, the gap between
adjacent S-strings d, and the periodicity p of the interconnecting lines. Equidistant strings at p=1 form a 1SE meta-foil.
Grouped in pairs of gap d, exhibiting a gap d<sub>p</sub> between pairs, they are named 2SP. Geometric parameters a, b, t, h, d, d<sub>p</sub>,
pS(E or P) and materials' properties like electric conductivity, Young's modulus, thermal expansion coefficient, and heat
capacity determine the electromagnetic, mechanical, and thermal properties of meta-foils including the spectral
dependence of resonance frequencies, refractive index, transmission, reflection, and bending. We show how the
frequency and transmission of left-handed pass-bands depend on a, p, and d<sub>p</sub>, the pSP geometry exhibiting higher
resonance frequency and transmission. Equivalent circuit considerations serve to explain physical reasons. We also
demonstrate mechanical behavior versus p and d<sub>p</sub> justifying the design of a cylindrical hyperlens depending on bent
Ideal metamaterials would consist of metal conductors only that are necessary for negative ε and μ. However, most of
present-day metamaterials include dielectrics for various support functions. Overcoming dielectrics, we manufactured
free-standing THz metamaterials as bi-layer chips of S-string arrays suspended by window-frames at a small gap that
controls the resonance frequency. Remaining problems concerning their useful range of incidence angles and the
possibility of stacking have been solved by manufacturing the first self-supported free-standing all-metal metamaterials
featuring upright S-strings interconnected by metal rods. Large-area slabs show maximum magnetic coupling at normal
incidence with left-handed resonances between 3.2 - 4.0 THz. Such metamaterials which we dub the meta-foil represent
an ideal platform for including index-gradient optics to achieve optical functionalities like beam deflection and imaging.
Micro/nanomanufactured electromagnetic metamaterials in the THz spectral range may help extending the use of
metamaterials in transportation. S-string based THz metamaterials as manufactured by SSLS, in particular, the meta-foil,
provide a promising platform for applications. Special emphasis may be given to antennas being conformal or quickly
steerable or tunable for inter-traffic communication. Achievements by SSLS in co-operation with MIT and Zhejiang
University are discussed and potential applications outlined.
X-ray phase-contrast tomographic microimaging is a powerful tool to reveal the internal structure of opaque soft-matter objects that are not easily seen in standard absorption contrast. In such low Z materials, the phase shift of X-rays transmitted can be important as compared to the absorption. An easy experimental set up that exploits refractive contrast formation can deliver images that are providing detailed structural information. Applications are abundant in fields
including polymer science and engineering, biology, biomedical engineering, life sciences, zoology, water treatment and filtration, membrane science, and micro/nanomanufacturing. However, available software for absorptive contrast tomography cannot be simply used for structure retrieval as the contrast forming effect is different. In response, CSIRO has developed a reconstruction code for phase-contrast imaging. Here, we present a quantitative comparison of a micro phantom manufactured at SSLS with the object reconstructed by the code using X-ray images taken at SSLS. The phantom is a 500 μm thick 800 μm diameter cylindrical disk of SU-8 resist having various eccentric cylindrical bores with diameters ranging from 350 μm to 40 μm. Comparison of these parameters that are well known from design and post-manufacturing measurements with reconstructed ones gives encouraging results.
Up to date, electromagnetic metamaterials (EM<sub>3</sub>) have been mostly fabricated by primary pattern generation via electron beam or laser writer. Such an approach is time-consuming and may have limitations of the area filled with structures.
Especially, electron beam written structures are typically confined to areas of a few 100×100 μm<sup>2</sup>. However, for meaningful technological applications, larger quantities of good quality materials are needed. Lithography, in particular X-ray deep lithography, is well suited to accomplish this task. Singapore Synchrotron Light Source (SSLS) has been applying its LIGA process that includes primary pattern generation via electron beam or laser writer, X-ray deep
lithography and electroplating to the micro/nano-manufacturing of high-aspect ratio structures to produce a variety of EM<sup>3</sup> structures. Starting with Pendry's split ring resonators, we have pursued structure designs suitable for planar lithography since 2002 covering a range of resonance frequencies from 1 to 216 THz. More recently, string-like structures have also been included. Latest progress made in the manufacturing and characterization of quasi 3D
metamaterials having either split ring or string structures over areas of about ≈1 cm<sup>2</sup> extension will be described.
The production of X-ray masks is one of the key techniques for X-ray lithography and the LIGA process. Different ways for the fabrication of X-ray masks has been established. Very sophisticated, difficult and expensive procedures are required to produce high precision and high quality X-ray masks. In order to minimize the cost of an X-ray mask, the mask blank must be inexpensive and readily available. The steps involved in the fabrication process must also be minimal. In the past, thin membranes made of titanium, silicon carbide, silicon nitride (2-5μm) or thick beryllium substrates (500μm) have been used as mask blanks. Thin titanium and silicon compounds have very high transparency for X-rays; therefore, these materials are predestined for use as mask membrane material. However, the handling and fabrication of thin membranes is very difficult, thus expensive. Beryllium is highly transparent to X-rays, but the processing and use of beryllium is risky due to potential toxicity. During the past few years graphite based X-ray masks have been in use at various research centers, but the sidewall quality of the generated resist patterns is in the range of 200-300 nm Ra. We used polished graphite to improve the sidewall roughness, but polished graphite causes other problems in the fabrication of X-ray masks. This paper describes the advantages associated with the use of polished graphite as mask blank as well as the fabrication process for this low cost X-ray mask. Alternative membrane materials will also be discussed.
During the past few years, graphite based X-ray masks have been in use at CAMD and BESSY to build a variety of high aspect ratio microstructures and devices where low side wall surface roughness is not needed In order to obtain lower sidewall surface roughness while maintaining the ease of fabrication of the graphite based X-ray masks, the use of borosilicate glass was explored. A borosilicate glass manufactured by Schott Glas (Mainz, Germany) was selected due to its high purity and availability in ultra-thin sheets (30 μm). The fabrication process of the X-ray masks involves the mounting of a 30 μm glass sheet to either a stainless steel ring at room temperature or an invar ring at an elevated temperature followed by resist application, lithography, and gold electroplating. A stress free membrane is obtained by mounting the thin glass sheet to a stainless steel ring, while mounting on an invar ring at an elevated temperature produces a pre-stressed membrane ensuring that the membrane will remain taut during X-ray exposure. X-ray masks have been produced by using both thick negative- and positive-tone photoresists. The membrane mounting, resist application, lithography, and gold electroplating processes have been optimized to yield X-ray masks with absorber thicknesses ranging from 10 μm to 25 μm. Poly(methyl methacrylate) layers of 100 μm to 400 μm have been successfully patterned using the glass membrane masks.
Poly-methylmethacrylate (PMMA), a positive resist, is the most commonly used resist for deep X-ray lithography (DXRL)/LIGA technology. Although PMMA offers superior quality with respect to accuracy and sidewall roughness but it is also extremely insensitive. In this paper, we present our research results on SU-8 as negative resist for deep X-ray lithography. The results show that SU-8 is over two order of magnitude more sensitive to X-ray radiation than PMMA and the accuracy of the SU-8 microstructures fabricated by deep X-ray lithography is superior to UV-lithography and comparable to PMMA structures. The good pattern quality together with the high sensitivity offers rapid prototyping and direct LIGA capability. Moreover, the combinational use of UV and X-ray lithography as well as the use of positive and negative resists made it possible to fabricate complex multi-level 3D microstructures. The new process can be used to fabricate complex multi-level 3D structures for MEMS, MOEMS, Bio-MEMS or other micro-devices.
For patterning thick photoresist films, x-ray lithography is superior to optical lithography because of the use of a shorter wavelength and a very large depth of focus. SU-8 negative resist is well suited to pattern tall, high-aspect ratio microstructures in UV optical and x-ray lithography with rapid prototyping capability due to its high sensitivity. The negative tone of the SU-8 resist offers advantages in fabricating multi-level and non-planar microstructures using x-ray lithography or a combination of x-ray and UV optical lithography. In this paper, we present a fabrication process for multi-level metallic mold insert by a combination of multi-layer SU-8 patterning, poly-dimethylsiloxane (PDMS) molding, and nickel electroplating to make final nickel mold inserts that are suitable for injection molding and hot embossing of plastics and ceramics.
SU-8 has great potential in low cost ultra-thick high aspect ratio MEMS applications. Although a broad range of thickness (from micrometer to mm) can be obtained by spin coating, the works about the sidewall profile and dimension control of SU-8 microstructures have not been published in detail. This paper describes the detailed investigations on the effects of processing parameters such as UV wavelength and exposure dose on dimensional change and sidewall profile of SU-8 microstructures. The optimized processing parameters for SU-8 structures with the thickness from 10 to 360 micrometer are presented.