Deep X-ray Lithography Research Articles

Note: Gratings on low absorbing substrates for x-ray phase contrast imaging.

Grating based X-ray phase contrast imaging is on the verge of being applied in clinical settings. To achieve this goal, compact setups with high sensitivity and dose efficiency are necessary. Both can be increased by eliminating unwanted absorption in the beam path, which is mainly due to the grating substrates. Fabrication of gratings via deep X-ray lithography can address this issue by replacing the commonly used silicon substrate with materials with lower X-ray absorption that fulfill certain boundary conditions. Gratings were produced on both graphite and polymer substrates without compromising on structure quality. These gratings were tested in a three-grating setup with a source operated at 40 kVp and lead to an increase in the detector photon count rate of almost a factor of 4 compared to a set of gratings on silicon substrates. As the visibility was hardly affected, this corresponds to a significant increase in sensitivity and therefore dose efficiency.

방사광 LIGA 공정을 이용한 플라스틱 성형용 마이크로 금형 제작

ABSTRACT In the present study, copper micromolds with a microhole array were precisely manufactured by a synchrotron LIGA process. Like in the traditional LIGA process, a deep X-ray lithography based on a synchrotron radiation was employed as the first manufacturing step. Due to the excellent optical performance of the synchrotron X-ray used, cylindrical micropillar arrays with high aspect ratio could be efficiently obtained. The fabricated microfeatures were then used as a master of the subsequent copper electroforming process, thereby resulting in copper micromolds with a microhole array. Thermoplastic hot embossing experiments with the copper micromolds were carried out for imprinting cylindrical microfeatures onto a polystyrene sheet. Through the hot embossing, the effect of embossing temperature and usefulness of the present manufacturing method could be verified.KeyWords:LIGA Process(LIGA 공정), Micromold(마이크로 금형), Synchrotron Radiation(방사광), Electroforming(전주도금), Hot Embossing(핫 엠보싱)

Development and characterization of ultra high aspect ratio microstructures made by ultra deep X-ray lithography

A technique of casting of thick pre-polymer layers was developed based on the proposed model of diluent evaporation. For the first time, microstructures with 7mm height and up to 18 microns wide (an aspect ratio of up to 389) were manufactured by means of ultra deep X-ray lithography in a cured pre-polymer that can be functionalized. The microstructures were characterized. The difference in the size of patterned microstructures and their initial size on the X-ray mask decreases to 400nm with the lessening lateral dimension of the microstructures from 1500 to 18μm. The sidewall roughness also decreases to 20nm.

Sinusoidal wavy surfaces for curvature-guided migration of T lymphocytes

Micro/nanofabricated surfaces have been widely used for the study of topography-guided migration of cells. While the current studies mostly utilized micro/nanostructures containing sharp edges, internal tissues guiding migration of cells such as blood and lymphatic vessels, bone cavities, perivascular tracks have smooth microscale topographical structures. To overcome these limitations, we fabricated sinusoidal wavy surfaces with various wavelengths by deep X-ray lithography enabling precise and simultaneous control of amplitudes and wavelengths. Using these surfaces, we systematically studied curvature-guided migration of T cells. The majority of T cells migrated along the concave surfaces of sinusoidal wavy structures and as wavelength increased (or curvature decreased), preference to concave surfaces decreased. Integrin-mediated adhesion augmented the tendency of T cells crawling along grooves of highly curved wavy surfaces. To understand mechanisms of curvature-guided migration of T cells, T cells were treated with small molecule drugs such as blebbistatin and CK636, inhibiting myosin II activity and lamellipodia formation, respectively. While lamellipodia-inhibited T cells frequently crossed ridges, myosin II-inhibited T cells were mostly confined within concave surfaces. These results suggest that lamellipodia regulate local actin polymerization in response to surface curvature to maintain T cells within concave surfaces while myosin II-mediated contractile forces push T cells out of concave surfaces to make T cells less sensitive to surface curvature.

Performance of SU-8 Membrane Suitable for Deep X-Ray Grayscale Lithography

In combination with tapered-trench-etching of Si and SU-8 photoresist, a grayscale mask for deep X-ray lithography was fabricated and passed a 10-times-exposure test. The performance of the X-ray grayscale mask was evaluated using the TERAS synchrotron radiation facility at the National Institute of Advanced Industrial Science and Technology (AIST). Although the SU-8 before photo-curing has been evaluated as a negative-tone photoresist for ultraviolet (UV) and X-ray lithographies, the characteristic of the SU-8 after photo-curing has not been investigated. A polymethyl methacrylate (PMMA) sheet was irradiated by a synchrotron radiation through an X-ray mask, and relationships between the dose energy and exposure depth, and between the dose energy and dimensional transition, were investigated. Using such a technique, the shape of a 26-μm-high Si absorber was transformed into the shape of a PMMA microneedle with a height of 76 μm, and done with a high contrast. Although during the fabrication process of the X-ray mask a 100-μm-pattern-pitch (by design) was enlarged to 120 μm. However, with an increase in an integrated dose energy this number decreased to 99 μm. These results show that the X-ray grayscale mask has many practical applications. In this paper, the author reports on the evaluation results of SU-8 when used as a membrane material for an X-ray mask.

A simple approach for an ultra-precise patterning using deep x-ray lithography with a micron-patterned x-ray mask

We present a novel method of fabricating ultra-precise patterns using multiple x-ray irradiations and precision stage movement. As the typical deep x-ray mask by ultraviolet (UV) lithography can have a minimum several-microns-scale pattern, fabrication of smaller patterns using general deep x-ray lithography with such a UV-process-based x-ray mask has limitations. In the present study, a substrate was loaded onto a precision stage allowing independent motion in the horizontal and vertical directions. The vertical stage, during x-ray irradiation, moves only up and down; after the initial x-ray irradiation, the horizontal stage moves the substrate in the horizontal direction in preparation for the next x-ray irradiation, which subsequently is carried out. The horizontal movement distance, crucially, can be adjusted to control the fabricated pattern size. By these combinations of precision stage movements and multiple x-ray irradiations, a pattern smaller than the original can be fabricated. The experimental results show in fact that this simple technique can be easily utilized for sub-micron-scale pattern fabrication using the typical UV-based x-ray mask.

On the direct X-ray lithographic formation of deep microstructures

Apparatus and technological protocols developed at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences (BINP SB RAS), for the method of the deep X-ray lithography of high-aspect-ratio microstructures with an arbitrarily specified topological pattern are presented. The novelty of the approach described is the use of the direct formation of deep LIGA structures in a SU-8 negative photoresist with a diaphragmed SR (synchrotron radiation) microbeam with the possibility of changing the diaphragm during exposure.

Recent Micro X-ray Focusing Optics and High-Pressure Applications at SPring-8

We have developed unique x-ray focusing optics by using compound x-ray refractive lens (CRL) designated for the high-pressure XRD beamline BL10XU [1] of SPring-8. The CRL is a set of planar cross-lenses with a quasi-parabolic profile for focusing in two directions, and are fabricated from the SU-8 polymer by deep x-ray lithography at the IMT in Germany [2]. The focusing optics consisted by two sets of CRL and a pinhole in tandem. The set of the upstream CRL and the pinhole as a virtual source are used for generation of homogeneously divergent beam. The downstream CRL, which is placed between a pinhole and a focal point (sample position), is utilized for focusing the x-ray towards the micron size. The focused beam size is defined by the relationship between a pinhole diameter and the ratio of source-lens versus lens-focal point distances. We have succeeded in focusing the x-ray beam down to 2.4 microns in vertical and 2.1 microns in horizontal, as we had designed, under the following conditions: 30 keV x-ray, 10 micron pinhole in diameter and 11:1 ratio of source-lens:lens-sample distances. The beam flux, which was converted to flux density as about 1015 cps/mm2, was enough strong to obtain XRD profiles in sub-second exposure, even at the sample condition under few hundred GPa. Using this system, we have obtained clearly districted XRD profiles from sub-structures of microns size in DAC, such as electrodes for electrical resistance measurement, micro-anvils settled in the sample chamber, and so on. Micro-beam technique will bring us new applications for high-pressure experiments, and become more important in order to enhance precise analysis, such as grains/elements distribution, partial melting, solution and/or chemical reactions in a sample chamber of DAC, as well as in order to collect high-quality XRD under above 500 GPa. Furtehrmore, the sub-micron beam focusing is now achievable and will be a key-technique for a probe of a scanning XRD microscope.

3D Spatially Controlled Chemical Functionalization on Alumina Membranes

Among the myriad microfabrication approaches, Deep X-ray Lithography (DXRL) takes advantage of the high penetration depth of hard X-rays. For the first time, this feature has been exploited for the precise control of surface chemical functionalities on a thick porous ceramic material. As a proof of concept, porous alumina membranes with controlled thickness (50 µ m) have been chosen to test the potential of DXRL. The Al 2 O 3 membranes were decorated with fluoro- and amino-silanes. These functionalized ceramic membranes were exposed to hard X-rays in a synchrotron facility, which allowed for the selective decomposition of the chemical functionalities in controlled areas. The water contact angle of hydrophobic-functionalized samples was measured to confirm the decomposition of the fluoro-silane in the exposed area, and water diffusion through the 200 nm pores of the alumina membranes was observed to occur only in the exposed area. The patterned amino-functionalized Al 2 O 3 samples were tested with an alcoholic solution containing Au cations, where it was found that gold nanoparticles only formed in the unexposed areas, whereas the amino functionality survived the radiation damage induced by the X-rays.

Microfocussing of synchrotron X-rays using X-ray refractive lens developed at Indus-2 deep X-ray lithography beamline

X-ray lenses are fabricated in polymethyl methacrylate using deep X-ray lithography beamline of Indus-2. The focussing performance of these lenses is evaluated using Indus-2 and Diamond Light Source Ltd. The process steps for the fabrication of X-ray lenses and microfocussing at 10 keV at moderate and low emittance sources are compared.

Fabrication of high aspect ratio comb-drive actuator using deep X-ray lithography at Indus-2

We report microfabrication of high aspect ratio comb-drive using deep X-ray lithography at Indus-2 synchrotron radiation source. Analysis shows that the comb-drive actuator of aspect ratio 32 will produce nearly 2.5 μm displacement when 100 V DC is applied. The displacement increases as the gap between the comb finger decreases. For fabrication of comb-drive, polyimide–gold X-ray mask using UV lithography is made for the first time in India. To pattern on an 800 μm thick X-ray photoresist (PMMA) exposures are performed using our deep X-ray lithography beamline (BL-07) at Indus-2. Metallization on the selective regions of the developed X-ray photoresist with comb-drive pattern was carried out by RF sputtering. Following this the comb-drive actuator of PMMA was fabricated by one-step X-ray lithography. The comb-drive can also be used as a sensor, energy harvester, resonator and filter.

X-ray Full Field Microscopy at 30 keV

In our X-ray full field microscopy experiments, we demonstrated a resolution better than 260 nm over the entire field of view of 80 μm × 80 μm at 30 keV. Our experimental setup at PETRA III, P05, had a length of about 5 m consisting of an illumination optics, an imaging lens and a detector. For imaging, we used a compound refractive lens (CLR) consisting of mr-L negative photo resist, which was fabricated by deep X-ray lithography. As illumination optics, we choose a refractive rolled X-ray prism lens, which was adapted to the numerical aperture of the imaging lens.

마이크로 밀링과 X-선 리소그래피 공정을 이용한 다층 마이크로 구조물 제작 공정 개발

Conventional machining technologies such as a milling process have limitations in accuracy to fabricate microstructures. Deep X-ray lithography using the synchrotron radiation is a promising micromachining process with an excellent accuracy, whereas there are difficulties in the fabrication of multi-layered structures. Therefore, it is mainly used for fabricating simple monolayered microstructures with a high aspect ratio. In this study, a novel technology for fabricating multi-layered microstructures is proposed by combining two processes. In advance, an X-ray resist material is cut and machined into various shapes and heights by the micro milling process. Subsequent X-ray irradiation process facilitates the fabrication of multi-layered microstructures. The proposed technology can overcome the limitation of the pattern accuracy in conventional milling process and the difficulty of the multi-layered machining in x-ray process. The usefulness of the proposed technology is demonstrated in this study by applying the technique in the realization of various multi-layered microstructures.

Development of high aspect ratio X-ray parabolic compound refractive lens at Indus-2 using X-ray lithography

A synchrotron beamline dedicated to soft and deep X-ray lithography is operational on Indus-2 synchrotron source and is being used for high aspect ratio microfabrication. This X-ray lithography facility provides the access to X-ray mask fabrication, X-ray exposures and development of micro-nano structures. We report the development of planar parabolic refractive X-ray lenses in SU-8 for energy range 8---20 keV using this facility. The focussing properties of X-ray lenses were studied with synchrotron radiation in the X-ray energy range 8---20 keV on the moderate emittance machine Indus-2. A focal spot of 11 μm at 15.9 keV is obtained with a gain of 18.

Characterization method for new resist formulations for HAR patterns made by X-ray lithography

At the Karlsruhe Institute of Technology (KIT), Institute of Micro Structure Technology (IMT) high aspect ratio (HAR) micro structures are manufactured by means of deep X-ray lithography and gold electroplating (LIGA technology). The technology is used to fabricate grating structures for differential phase contrast X-ray imaging (DPCI). Using an epoxy based negative resist material; electroplated grating structures are fabricated having absorber lamellas with heights up to 100 μm and a period down to 2.4 μm. However, in DPCI there is an increasing demand for improved quality gratings with periods down to 1 μm, areas larger than 50 mm × 50 mm with a high homogeneity in terms of the lamella height distribution and defect-free grating patterns. Pattern deformations are due to limited mechanical stability of the resist during the development process as well as to resist shrinkage during crosslinking, affecting mostly gratings with small periods and HARs. The purpose of this contribution is to present a methodology for the characterization of different epoxy based negative resist formulations, aiming to increase the quality of the HAR free standing grating lamellas by increased mechanical stability of the resist.

Exfoliated Graphene into Highly Ordered Mesoporous Titania Films: Highly Performing Nanocomposites from Integrated Processing

To fully exploit the potential of self-assembly in a single step, we have designed an integrated process to obtain mesoporous graphene nanocomposite films. The synthesis allows incorporating graphene sheets with a small number of defects into highly ordered and transparent mesoporous titania films. The careful design of the porous matrix at the mesoscale ensures the highest diffusivity in the films. These exhibit an enhanced photocatalytic efficiency, while the high order of the mesoporosity is not affected by the insertion of the graphene sheets and is well-preserved after a controlled thermal treatment. In addition, we have proven that the nanocomposite films can be easily processed by deep X-ray lithography to produce functional arrays.

Large tuning ratio high aspect ratio variable capacitors using leveraged bending

Variable capacitors are a key component in Radio Frequency Micro Electro-Mechanical Systems (RF MEMS). They comprise fixed and flexible electrodes. Deformation, or actuation, of the flexible electrode changes the capacitance of the capacitor. This way, electrical properties of high frequency circuits can be modified. Traditionally, variable capacitors are based on a planar layout architecture, while a newer, vertical-wall, quasi three-dimensional approach theoretically enables increased device performance. Such devices depend on high aspect ratios, i.e. relatively high micro structures with very thin walls and gaps. A few vertical-wall variable capacitors made of nickel or gold have been fabricated to date, using deep X-ray lithography and subsequent electroplating (Achenbach et al. 2006; Klymyshyn et al. 2007, 2010) as the fabrication approach. They feature, amongst others, excellent quality factors of Q ≤ 95 at 5.6 GHz with 50 Ω reactance, but suffer from a very limited tuning range of the capacitance value (tuning ratio of, e.g., 1.38:1). The devices presented here exploit the same architecture and materials selection, resulting in similar, excellent Q-factors, but feature a different electrode layout approach, referred to as leveraged-bending. This layout is based on pulling a flexible electrode sideways, towards a fixed electrode, increasing the capacitance when actuating the variable capacitor. The leveraged bending approach theoretically enables infinitely high tuning ratios for components with perfect structure accuracy. To date, a significantly increased tuning ratio of 1.9:1 has been demonstrated. Limiting factors are an electrically non-ideal layout geometry chosen as a compromise to increase the fabrication yield, and structure deviations of ~1.6 μm from CAD layout to the electroplated component. Electrostatic actuation requires voltages between 0 and 72 V for capacitance values on the order of C = 0.3 pF at device dimensions of about 1.5 mm overall length, 5---10 μm gap and wall widths, and 100 μm metal height. Device performance measured with a vector network analyzer is in 97 % agreement with simulation results based on two-dimensional electrostatic-structural coupling (ANSYS Multiphysics) and three-dimensional electromagnetic field simulations (Ansoft HFSS). These simulations also indicate that an optimized gap geometry will allow to reduce the actuation voltage required by up to 40 %.

A Monte Carlo study of the primary absorbed energy redistribution in X-ray lithography

The minimum feature size producible by LIGA X-ray lithography is fundamentally limited by the redistribution of primary doses via photoelectrons and the influence of the resulting dose distribution on resist development. Secondary radiation from mask and substrate are well known as source for pattern distortion in deep X-ray lithography. Numerical simulations by means of Monte Carlo simulations using PENELOPE (Salvat et al. in PENELOPE-2008: a code system for Monte Carlo simulation of electron and photon transport. http://www.nea.fr/html/dbprog/penelope-2008.pdf , 2008) are applied to quantify these additional dose values in the resist/substrate interface and the irradiated/shadowed interface. A significant reduction of the additional dose by secondary radiation from the plating base is not observed for Au and Ti layers thicker than 10 nm. The influence of polarized or unpolarized X-rays might be neglected for structure dimensions larger than a few 10 nm. As an example of critical dimension, simulations were used to predict the structure quality of grating structures with a period of 2.4 μm and duty cycle 0.5 in a resist layer of 300 μm.

Microfabrication of sharp blazed gratings by a two-step height amplification process based on soft and deep X-ray lithography

We describe a scheme for the fabrication of sharp blazed gratings involving two cascaded steps of X-ray lithography (XRL) with electroplating (EP) steps interposed. The adopted iterative scheme enables the amplification by two orders of magnitude of the height of high-resolution structures of an original X-ray mask produced by Electron Beam Lithography (EBL), while minimizing the loss of lateral resolution. Nickel structures of up to 200μm in height with blazed grating profile were obtained with <500nm resolution details.The described approach is suitable for the fabrication of nickel structures, to be used in particular as master tools in hot embossing processes, in a broad range of optical applications requiring non-binary microstructures.

Mosaic-like micropillar array for hard x-ray focusing—one-dimensional version

An array of microstructures resembling a Fresnel version of the classical parabolic x-ray refractive lens shape shows a significant improvement towards focal spot size and intensity. The proposed polymer microstructures with the acting refractive surface have a pillar-like form with a triangular cross-section. Their arrangement and the parameters of the triangles change along the lenses in accordance with the x-ray propagation law, yielding a focusing effect in the hard x-ray range. The great number of micropillars arranged in one row conduct the x-ray beam like a waveguide, with its transparency increasing with decreasing pillar size. A multi-row lens can collect x-rays into a focus down to several nanometres. The specific regular arrangement of the pillars resembles a mosaic ornament, which led to the chosen name. Fabricating the lenses by means of deep x-ray lithography is essential for avoiding various potential manufacturing difficulties. The one-dimensional focusing lenses overcome the diffraction limit defined for the classical parabolic lenses due to their large aperture that results from the waveguide-like rows formed by the pillar arrangement.