Fabrication of X-ray imaging zone plates by e-beam and X-ray lithography

X-ray imaging and microscopy techniques have been developed in worldwide due to their capabilities of large penetration power and high spatial resolution. Fresnel zone plates is considered to be one of the most convenient optic devices for X-ray imaging and microscopy system. The zone plates with aspect ratio of 7 and 13 have been fabricated by e-beam lithography combined with X-ray lithography in this paper. Firstly, the X-ray lithography mask of zone plates with outermost zone width of 100 nm was fabricated by e-beam lithography and gold electroplating techniques. Secondly, the zone plates with gold profile thickness of 700 and 1,300 nm were replicated by X-ray lithography and gold electroplating techniques. X-ray imaging and microscopy techniques were introduced to characterize the high-aspect-ratio zone plates’ inner structures. At the X-ray energy of 7.5 keV, the first-order focusing efficiency of zone plates with gold profile thickness of 700 nm is about 8.63%.     

X-ray zone plates with 25 aspect ratio using a 2-μm-thick ultrananocrystalline diamond mold

Hard X-ray phase zone plates are focusing optics used for X-ray microscopes at synchrotron radiation facilities. The resolution is determined by the outer-most zone width (OZW) and modern lithographic techniques are capable of patterning OZW less than 100 nm. Efficiency of a phase zone plate will peak when the zones have a thickness that provides a π-phase shift to the X-rays. Thus, a hard X-ray zone plate with ideal efficiency and sub-100-nm resolution requires fabricating high-aspect-ratio, dense-packed structures in materials suitable for exposure to synchrotron radiation. The fabrication method implemented involves an electroforming mold process where a top resist layer is lithographically patterned and used for pattern transfer into a bottom layer which acts as the electroform mold. The resulting mold is filled with Au by electroplating, and afterwards the mold is not removed but remains in place for mechanical support. Ultrananocrystalline diamond (UNCD) was used as the mold layer. UNCD is deposited by hot-filament chemical vapor deposition with well-controlled stress and thickness up to 2 μm. The top resist layer is hydrogen silsesquioxane, which is a high-contrast electron beam lithography resist and resistant to the oxygen reactive ion etching required for UNCD pattern transfer. Using this fabrication method, we successfully produced zone plates with OZW down to 80 nm and an aspect ratio up to 25 for a thickness of 2 μm. The efficiency of several fabricated zone plates were measured, demonstrating their functionality.     

Fabrication of intermediate mask for deep x-ray lithography

 This paper presents the fabrication of intermediate x-ray mask for deep x-ray lithography. In order to have working mask with absorbers thickness larger than 10 μm, the intermediate mask should have absorbers of 0.7 μm in thickness. To demonstrate intermediate mask fabrication, x-ray zone plates are fabricated on the 1.2 μm low-stress silicon-rich silicon nitride (SiN_x) membrane with the tri-layer Chromium-Tungsten-Chromium (Cr–W–Cr) as the x-ray absorbers. The chromium layers both 200 angstroms are used as adhesion and for stress relief. The SiN_x film is deposited with low pressure chemical vapor deposition (LPCVD) and the free standing membrane are formed by KOH silicon backside etching. With the e-beam lithography and reactive ion etching, width of 0.8 μm of outmost zone of the x-ray zone plates has been achieved on the membrane. The scanning electron microscopy (SEM) images of the x-ray zone plates and pictures of intermediate masks are demonstrated. Received: 25 August 1997/Accepted: 3 September 1997     

Direct imprinting of organic–inorganic hybrid materials into high aspect ratio sub-100 nm structures

The challenging fabrication of sub-100-nm structures with high aspect ratio by UV-nanoimprint lithography (NIL) is addressed in this work. Thermal shrinkage is induced by cooling the structures below room temperature to avoid the issues commonly arising during the release of the polymeric nanostructures from the master. The UV-NIL has been performed to obtain OrmoComp^® nanostructures using OrmoStamp^® working stamps copied from Si masters. Nanoridges and nanopillars with 45 nm width and 380 nm thickness have been fabricated with a corresponding aspect ratio of 8.5. This is, to the best of our knowledge, the highest aspect ratio achieved using organic–inorganic hybrid materials at the sub-100-nm scale.     

Experimental considerations and fabrication analysis of surface micromachined RF NEMS switch

This paper presents fabrication and issues related to fabrication of RF NEMS switch using surface micromachining approach. The switch has been fabricated using electron beam lithography. Lift off process has been used in achieving patterned metal layers. Fabrication process has been optimized by overcoming different issues like proximity effect, field stitching. A high resolution 20 nm gap has been achieved during fabrication. Dose-shape correction method has also been used to achieve desired patterns. A four metal layer fabrication process has been developed for fabrication of RF NEMS switch. Simulation and measured results are also presented.     

Fabrication of micro sloping structures of SU-8 by substrate penetration lithography

In this paper, three-dimensional (3D) micro sloping structures were fabricated by ordinary mask pattern and diffraction phenomenon. Especially, we fabricated the structures with SU-8 negative photoresist and substrate penetration lithography. In this method, exposure is performed arranging in order of a mask, a substrate and the SU-8 resist. There is a gap that is equal to the thickness of the substrate between resist and mask. In narrow slit of mask, resist is less exposed than usual because of Fraunhofer diffraction. The amount of exposure depends on slit width so that the height of SU-8 resist can be controlled. A 173 μm height of structure was obtained in the case of 27 μm width slit and 24.2 μm height of structure was obtained in the case of 7.4 μm width slit. By using this method, high aspect ratio 3D SU-8 structures with smooth sloping were fabricated in the length of 100–300 μm and in the height of 50–200 μm with rectangular triangle mask pattern. In the same way, there is influence of Fresnel diffraction on edge of aperture so that micro taper structures were fabricated. A lot of taper structures were fabricated by the method to make the surface repellency. The contact angle was achieved more than 160° in this study.     

Resist evaluation for proton beam writing,Ni mold fabrication and nano-replication

Proton beam writing (PBW) is a new direct-write technique which has shown great potential to fabricate structures down to 20 nm level in resist material. Protons can be accelerated up to a high energy (3.5 MeV) at Centre for Ion Beam Applications. Because the mass of a proton is much larger than the mass of an electron (m_p:m_e = 1,800:1), the energy of the secondary electrons is very small compared with secondary electrons generated by electron beam lithography. Therefore, a proton will travel along a straight path into resist and secondary electrons will only expose the resist within several nanometers around the path of the proton. PBW is capable of fabricating structures with very straight, vertical and smooth sidewalls without proximity effect. This is very important when combining PBW with Ni electroplating and nanoimprinting as well as injection molding. High quality Ni molds with smooth and vertical side walls are critical in nanoimprint lithography and injection molding. In our experiments, several new resists including AR-P 3250, a mixture of AR-P 3250 and AR 300-12, and ma-N 2401 are tested with PBW for the production of high aspect ratio Ni molds and thermoplastic replication with these molds. High aspect ratio structures (up to 7) are fabricated at a width of 500 nm in Ni molds. The structures are transferred to plastic via nanoimprinting and injection molding.     

Fabrication of ceramic microcomponents and microreactor for the steam reforming of ethanol

Ceramics have several advantages over other materials in MEMS, such as heat resistance, hardness, corrosion resistivity even in harsh environments, chemical inertness for biological applications and catalytic activity of surfaces and so on. For these advantages ceramic microstructures will be very potentially useful in microsystem, especially in microreactor. A novel method for the high aspect ratio micro ceramic structures fabrication based on deep X-ray lithography and lost-mold technique is developed. By using this method, ceramic microreactors have been successfully fabricated. The ceramic microreactor consists of 14 identical microchannels in parallel, each with typical dimensions of approximately 300 μm in width, 400 μm in height and 20 mm in length. The Ni film catalyst with thickness of 300 nm is uniformly coated through sputtering process. The steam reforming of Ethanol into hydrogen on the ceramics microreactor was studied at temperatures between 500 and 700°C. The microreactors have been characterized by studying of C₂H₆O conversion, H₂ selectivity, and product stream composition.     

Soft X-ray lithography of high aspect ratio SU8 submicron structures

SU8 submicron structures with an aspect ratio of more than 50 are made by soft X-ray lithography using modified spectra of the synchrotron radiation at the ANKA LITHO-1 beamline, which includes a chromium mirror. The X-ray spectrum is additional shaped by a beam stop and a filter to a narrow band in order to reduce the influence of diffraction and photoelectrons. The exposure determination is based on the measured threshold doses for used SU-8 resist layers as well as on the calculated diffractive distribution of an absorbed power. Post-exposure bake of the resist is performed at low temperature and low pressure to avoid changes of the structural size because of shrinkage due to temperature changes and to eliminate a “skin” layer at the top of the resist. SU8 structures with lateral dimensions of 1 μm and heights from 50 to 80 μm have been fabricated defect free with the optimized process.     

Fabrication of high precision X-ray mask for X-ray grating of X-ray Talbot interferometer

X-ray imaging is used in many applications such as medical diagnosis and non-destructive inspection, and has become an essential technologies in these areas. In one image technique, X-ray phase information is obtained using X-ray Talbot interferometer, for which X-ray diffraction gratings are required; however, the manufacture of fine, highly accurate, and high aspect ratio gratings is very difficult. X-ray lithography could be used to fabricate structures with high precision since it uses highly directive syncrotron radiation. Therefore, we decided to fabricate X-ray gratings using X-ray lithography technique. The accuracy of the fabricated structure depends largely on the accuracy of the X-ray mask used. In our research, we combined deep silicon dry etching technology with ultraviolet lithography in order to fabricate untapered and high precision X-ray masks containing rectangular patterns. We succeeded in fabricating an X-ray mask with a pitch of 5.3 μm. The thickness of the Au absorber was about 5 μm, and the effective area was 60  × 60 mm², which is a sufficient size for phase tomography imaging. We demonstrated the utility of the Si dry etching process for making high precision X-ray masks.     

Orthogonal and fine lithographic structures attained from the next generation proton beam writing facility

A second generation proton beam writing (PBW) system has been built at the Centre for Ion Beam Applications at the National University of Singapore for fabrication of high aspect ratio 3D nano lithographic structures. System improvements and a few lithographic structures obtained with this facility are presented in this paper. Through accurate alignment of the magnetic quadrupole lenses and the electrostatic scanning system, orthogonal beam scanning has been achieved. The earlier constrain of limited beam scan area has been overcome by adopting a combination of beam and stage scanning as well as stitching. With these improvements smallest ever Ni structure of 65 nm in width has been fabricated using nickel electroplating on a proton beam written PMMA sample in the second generation PBW facility. Using this improved PBW facility, we have also demonstrated the fabrication of fine lithographic patterns with 19 nm line width and 60 nm spacing in 100 nm thick negative high resolution hydrogen silsesquioxane resist. Future possible system improvements leading to finer resolution will be discussed briefly.     

A sub-micron metallic electrothermal gripper

We report the design, fabrication, and characterization of a multiple bent beam, sub-micron metallic electrothermal gripper. A bottom electroplating mold for electrodes was patterned using electron beam lithography in an SU-8, followed by nickel electroplating. A top electroplating mold for a sub-micron metallic gripper with high aspect ratio bent beams (thickness of 1 μm, width of 350 nm) was prepared using electron beam lithography in a polymethyl methacrylate (PMMA), followed by nickel electroplating and dry release of the top and bottom molds. The sub-micron gripper was characterized using a nanomanipulator system installed in a dual column scanning electron microscopy/focused ion beam system. The ability of the jaw to close up to 1.39 μm displacement with high precision and reliability has been reproducibly observed at an applied current of 28 mA, corresponding to the maximum power consumption of 11.2 mW. Finite element modeling displacement results performed using ANSYS for effective bent beam widths of 370 nm showed a good agreement with the measured displacement results. The sub-micron gripper demonstrated herein will enable the reproducible manipulations with nano-scale resolution displacement and could provide an effective means of interface between nano-scale objects and the micro/macro scale robotic systems.     

Structure quality in deep X-ray lithography applying commercial polyimide-based masks

The structure quality of deep X-ray lithography components strongly depends on the quality of the applied X-ray mask. In this article we compare the results obtained with two different mask types. Sophisticated working masks generated by e-beam lithography, soft X-ray lithography and electroplating of gold absorbers on a titanium mask membrane have been fabricated at the Institute for Microstructure Technology, Research Center, Karlsruhe (FZK/IMT), Germany. Prototype masks generated by e-beam lithography, optical lithography and electroplating of gold absorbers on a polyimide mask membrane have been fabricated by Optnics Precision, Japan, with the aim to offer commercially available low cost masks. Both mask types were applied to pattern PMMA resist layers of 300–750 μm thickness at the 2.5 GeV electron storage ring ANKA, Germany, using comparable process parameters. FZK/IMT masks provide microstructures with significantly better structure quality. The layout area, however, is currently limited to 12 cm², and the Ti mask membrane tends to lead to a slight resist surface attack, such as rounding of the resist edges. Optnics masks provide microstructures with reduced structure quality due to sidewall striations (sidewall roughness up to 2 μm) and thermal distortions (of up to 3–5 μm) which limit the potential scope of applications. They could nevertheless potentially be applied as low quality, low cost X-ray masks. High resolution and high accuracy applications, however, require more sophisticated but also more expensive masks, like the Ti-masks from FZK/IMT.     

Submicron-scale surface acoustic wave resonators fabricated by high aspect ratio X-ray lithography and aluminum lift-off

A submicron-scale surface acoustic wave (SAW) resonator fabricated by high-aspect-ratio X-ray lithography (XRL) and metal lift-off that operates at microwave frequencies is presented. We demonstrate that XRL is especially well suited for SAW device templating, as long submicron-scale interdigitated transducer structures can be batch patterned with excellent structure quality. 0.4–2.0 μm thick PMMA layers were structured by X-ray lithography shadow projection using silicon nitride-based X-ray masks. Structures with a critical lateral feature size of down to 200–700 nm were processed. The polymer structures served as templates in a subsequent aluminum lift-off process. The metal electrodes were successfully tested as SAW resonators for high frequency applications, e.g. around 1.3 GHz, using calibrated 1-port RF wafer probing measurements. Compared with standard fabrication techniques, the high structure quality of submicron-scale polymer templates made of unusually thick PMMA layers offers additional possibilities to fabricate thicker metal transducers.     

Movable microstructures made by a sub-micron LIGA process

Movable microstructures with high aspect ratios were made with lateral dimensions down to the sub-micron domain by the LIGA process and a sacrificial layer technique. Compared to microstructures usually made by LIGA, all dimensions were reduced approximately by a factor of 10, while the aspect ratio was kept constant. The smaller resist thickness in the range of some ten micrometers allowed much lower X-ray doses and energies to be used for exposure and the absorbers with a thickness of only 3 μm to be employed. As the lateral dimensions are smaller, a much larger number of devices can be fabricated in one batch. Therefore, the production costs of deep etch X-ray lithography are reduced dramatically. Electrostatic linear actuators with lateral dimensions as small as 500 nm were manufactured to demonstrate the advantages of LIGA in sub-micron dimensions. An X-ray mask with absorbers 2.8 μm high was produced by a three-level technique. The linear actuators consisted of several arrays of capacitor plates combined into an electrostatic driving unit with an active area, typically, 0.3 mm² and less. The driving unit was supported by folded beam flexures to avoid frictional forces. They also guaranteed parallel movement of the capacitor plates. The functionality of these devices was demonstrated by measuring displacement as a function of the voltage applied.     

Design and microfabrication of a polymer membrane-based submicron scale electrophoretic flow detector for biomedical applications

This study demonstrates the design and microfabrication of single cylindrical submicron-sized pores in 1 μm-thick PMMA membranes, and their integration and assembly into all-polymeric electrophoretic detectors. Pore sizes vary from 200 to 600 nm. Fabrication includes electron beam lithography of the pore and mechanical microfabrication and assembly of the remaining detector system, using UV-curing glues and a silicon sacrificial substrate wafer. Initial electrophoretic translocation experiments have been performed for various potassium chloride (KCl) electrolyte concentrations between 0.1 and 1 M. Experiments prove that the detector system is hermetically sealed, that the pores are capable of sustaining an open pore current, and that they respond with a steady and low-noise signal. The same experiments have also been applied to analyze the pore metrology, and revealed that submicron pore sizes have been underestimated by roughly 150 nm.     

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.     

A novel method to overcome photoresist collapse with high aspect ratio structures

A novel method based on electromagnetic wave (EW) was firstly proposed to dry photoresist with high aspect ratio after electron beam lithography. This method used EW to penetrate photoresist and heat the water stored between resist patterns directly, then the water evaporated with absorbing energy of EW. An array of 15,625 pillars and lines with 14.9 nm width of aspect ratio 13 and 17 respectively were dried successfully. By analyzing the heating mechanism, we demonstrated that the EW can decrease surface tension of water effectively and may be applicable for cleaning via hole and the inner wall of carbon nanotubes.     

Electropolishing as a method for deburring high aspect ratio nickel RF MEMS

High aspect ratio nickel radio frequency microelectromechanical systems (RF MEMS) were fabricated by X-ray lithography and electroplating. Control of growth during electroforming of micro components is in general a problem in terms of achieving homogeneous thickness due to a non-uniform current distribution across a layout. It is necessary to level the deposited layer by means of a lapping process which results in burr formation. To ensure the functional properties of the devices these burrs have to be removed. Electropolishing with a current density of 80 A/dm² is used for burr removal from nickel micro components containing small width (~10 μm) structural elements. The investigated metal removal rates range from 0.2 to 1.8 μm/s depending on burr formation, presence or absence of resist and device position in the layout during electropolishing. Furthermore, edge rounding, a common electropolishing effect, is only observed when electropolishing in the absence of resist.     

Fabrication of silicon-based two-dimensional photonic crystals

Both triangular lattice and square lattice photonic crystals with lattice constant 400 nm and the radius 200 nm of the air pores in the silicon substrate have been designed, fabricated and realized. The processes of the fabrication of the two-dimensional photonic crystals, based on silicon, including mask making, electron-beam lithography, and inductively coupled plasma etching are introduced.