Fabrication of Fresnel zone plates with high aspect ratio by soft X-ray lithography

High focusing efficiency Fresnel zone plates for hard X-ray imaging is fabricated by electron beam lithography, soft X-ray lithography, and gold electroplating techniques. Using the electron beam lithography, Fresnel zone plates which has an outermost zone width of 100 nm and thickness of 250 nm has been fabricated. Fresnel zone plates with outermost zone width of 150 nm and thickness of 660 nm has been fabricated by using soft X-ray lithography.     

Fabrication of antiscatter grids and collimators for X-ray and gamma-ray imaging by lithography and electroforming

Creatv MicroTech has developed unique fabrication techniques to make high precision, high-aspect-ratio metal microstructures to custom specifications. A lithography based fabrication method permits precise fabrication of various microstructures. Collimators and antiscatter grids with continuous, smooth, thin, parallel or focused septa have been fabricated using deep X-ray and optical lithography, combined with metal electroforming. Microfabrication of high-aspect-ratio structures, especially of relatively large areas, presents many challenges: specialized mask design and X-ray mask fabrication; resist preparation, optimal exposure parameters, post-exposure processing, electroforming, polishing, and final assembly. Here, we present microstructures of various designs that we fabricated and describe the challenges that had to be overcome.     

Fabrication of ceramic microcomponents using deep X-ray lithography

In the last years the fabrication of micro components made from ceramic materials became more and more evident with respect to the pronounced chemical stability and the outstanding thermomechanical properties in comparison to plastics and metals. The aim of this work is the lithographic generation of ceramic microstructures avoiding an intermediate molding step using SU8 as pronounced sensitive resist matrix filled with fine ceramic powder in the submicron range. Focus of the research was to investigate the composite formation, patterning by x-ray lithography, developing, debinding and sintering to form stable ceramic parts. The addition of fine ceramic particles to low viscous liquids like SU8-10 leads to an increase of the viscosity. For a successful debinding and sintering a volume content of at least 40% ceramic is required resulting in a change of the viscosity from around 2 Pas up to a value of 1000 Pas at 25 °C and low shear rates. A modified casting procedure was developed for the formation of uniform resist films with a thickness around 300 m. Optimized exposure and development parameters allow the fabrication of good quality resist structures that can be further transformed into ceramic structures by sintering. Details of the work and results will be presented and discussed in this paper.At this point the authors would like to thank all people who supported this work.     

Fabrication of RF MEMS variable capacitors by deep X-ray lithography and electroplating

Radio frequency micro electro-mechanical systems (RF MEMS) vertical cantilever variable capacitors fabricated using deep X-ray lithography and electroplating are presented. Polymethylmethacrylate (PMMA) layers of 100 μm and 150 μm have been patterned and electroplated with 70 μm and 100 μm thick nickel. A 3 μm thick titanium layer was used as plating base as well as etch time-controlled sacrificial layer for the release of the cantilever beam. The parallel plate layout includes narrow gaps and cantilever beams with an aspect ratio in nickel of up to 60 for 1 mm long features. Auxiliary structures support the beams and gaps during the processing. Room temperature electroplating significantly reduces the risk of deformations compared to the standard process temperature of 52°C. The capacitors operate in the 1–5 GHz range, and demonstrate good RF performance, with quality factors on the order of 170 at 1 GHz for a 1 pF capacitance.     

Fabrication of sub-micron structures for MEMS using deep X-ray lithography

Fabrication techniques of microstructures with high resolution and high aspect ratio are necessary for practical microelectromechanical systems (MEMS) that have high performance and integration. In order to fabricate microstructures with sub-micron resolution and high aspect ratio, deep X-ray lithography has been investigated using the compact synchrotron radiation (SR) light source called “AURORA”. An X-ray mask for sub-micron deep X-ray lithography, which is composed of 1 μm thick Au as absorbers, 2 μm thick SiC as a membrane and 625 μm thick Si as a frame, was designed. In preliminary experiments, the following results were achieved: EB resist microstructures with an aspect ratio of 22 corresponding with 0.07 μm width and 1.3 μm height were formed; a 10 μm thick PMMA resist containing no warp was formed by direct polymerization, enabling more precise gap control.     

High-aspect-ratio nanoporous membranes made by reactive ion etching and e-beam and interference lithography

Nanoporous membranes engineered to mimic natural filtration systems can be used in “smart” implantable drug delivery systems, hemodialysis membranes, bio-artificial organs, and other novel nano-enabled medical devices. Conventional membranes exhibit several limitations, including broad pore size distributions and low pore densities. To overcome these problems, lithographic approaches were used to develop porous silicon, silicon nitride, ultrananocrystalline diamond (UNCD), and polymer film membranes. Here we report processing of high porosity, high-aspect-ratio membranes by two techniques: UNCD fabricated by reactive ion etching after e-beam lithography and epoxy fabricated by interference lithography.     

Fabrication of micro pore optics with smooth sidewall using X-ray lithography

Micro pore optics (MPO) as an X-ray imaging system is perfectly suited for the applications in space telescopes due to its light-weight and high-resolution properties. We report the fabrication of MPO samples by LIGA process focusing on its sidewall surface used as mirrors for X-ray reflection. An intermediate mask is fabricated and used to obtain the working mask in order to avoid the UV exposure to a very thick photo resist layer. Around 400 μm-thick nickel MPO plate is obtained with the aspect ratio of the square pore and sidewall of 8 and 32, respectively. The root mean square roughness of the sidewall surface is below 10 nm in a 5 × 5 μm² region. Some striations are found on the sidewall surface originating from the jagged edge of the chromium coating on the UV mask.     

Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.     

Fabrication of microgratings on PMMA plate and curved surface by using copper mesh as X-ray lithography mask

We demonstrate experimentally the X-ray lithography technique to fabricate microgratings on a PMMA plate and on curved surfaces such as PMMA cylinder lens surfaces with X-ray lithography by copper mesh as mask. Some gratings with 12.7 μm pitches on the plate and on PMMA curved surface with large area (10 mm × 10 mm) by vertically moving or rotating the resist stage exposure are realized.     

Fabrication method of two-level polymeric microstructure with the help of deep X-ray lithography

Two- or multi-level microstructures are getting more important in several applications such as multi-component micro optical elements and various microfluidic systems. In the present study, a simple and efficient method is newly proposed for a fabrication of the two-level polymeric microstructures. Making a mother two-level microstructure consists of two processes: (1) the hot embossing process for a fabrication of microstructures on a PMMA substrate, and (2) the deep X-ray lithography using the hot embossed substrate for a high aspect ratio microstructure fabrication, resulting in a high aspect ratio microstructure containing smaller microstructures on its surface. Making use of so fabricated two-level microstructures as a mother structure, one could achieve a mass replication of the same microstructures via injection molding process with a metallic mold insert obtained by a nickel electroforming onto the mother microstructure. In order to demonstrate the proposed method, a polymeric high aspect ratio microstructure having smaller square microstructures on its top surface was fabricated. The fabricated two-level microstructure shows fine vertical sidewalls, which is a characteristic feature of the deep X-ray lithography. In addition, a metallic mold insert for a mass replication was fabricated by a nickel electroforming process.     

Fabrication of glassy carbon microstructures by soft lithography

This paper describes fabrication techniques for the fabrication of glassy carbon microstructures. Molding of a resin of poly(furfuryl alcohol) using elastomeric molds yields polymeric microstructures, which are converted to free-standing glassy carbon microstructures by heating (T ∼500–1100°C) under argon. This approach allows the preparation of macroscopic structures (several mm²) with microscopic features (∼2 μm). Deformation of the molds during molding of the resin allows preparation of curved microstructures. The paper also presents a method of incorporating these structures on a chip by masking and electroplating.     

Validation of X-ray lithography and development simulation system for moving mask deep X-ray lithography

This paper presents a newly developed 3-Dimensional (3-D) simulation system for Moving Mask Deep X-ray Lithography (M/sup 2/DXL) technique, and its validation. The simulation system named X-ray Lithography Simulation System for 3-Dimensional Fabrication (X3D) is tailored to simulate a fabrication process of 3-D microstructures by M/sup 2/DXL. X3D consists of three modules: mask generation, exposure and resist development (hereafter development). The exposure module calculates a dose distribution in resist using an X-ray mask pattern and its movement trajectory. The dose is then converted to a resist dissolution rate. The development module adopted the "Fast Marching Method" technique to calculate the 3-D dissolution process and resultant 3-D microstructures. This technique takes into account resist dissolution direction that is required by 3-D X-ray lithography simulation. The comparison between simulation results and measurements of "stairs-like" dose deposition pattern by M/sup 2/DXL showed that X3D correctly predicts the 3-D dissolution process of exposed PMMA.     

Fabrication of sealed nanofluidic channels using site-selective direct write (maskless) X-ray lithography

Sealed nanofluidic channels with cross-sections as small as 60 nm × 60 nm were created in polymer bilayers using the focused X-rays of a scanning transmission X-ray microscope. These structures were then characterized by near-edge X-ray absorption fine structure spectromicroscopy, atomic force microscopy and scanning electron microscopy. The cross-sectional area of the nanochannels could be tuned by adjusting the area patterned in x and y and/or manipulating the bottom layer thickness. The maximum length was found to be limited by the efficiency of excavation of patterned material out of the channel, and the stability of the polymer overlayer which seals the channel. Schemes toward interfacing these nanochannels with conventional microfluidics are discussed.     

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.     

Micro gas bearings fabricated by deep X-ray lithography

Micro bearing systems for Micro Electromechanical Systems (MEMS) have drawn attention for several decades as critical components for micro rotating machinery. Ideally, frictionless bearings are needed, and in practice, micro gas bearings approach the ideal. Typically, bearings function as a separate component, assembled onto sliding counterparts. However, in micro scale devices, assembly procedures are known to be very tedious and time consuming. This leads to the pursuit of single material monolithic structures. Critical issues arising from these approaches include: limitation of materials, friction, and reliability, among others. In this paper, new approaches have been pursued. Micro gas bearings were fabricated as a single component through X-ray lithography. A stainless steel gauge pin, machined to ultra precision, was used as a journal shaft. Simple and very easy assembly processes using self-aligning concepts were developed as an alternative method to conventional assembly. This article presents the design, fabrication, assembly, and testing of micro gas bearings.This project was funded from National Science Foundation (DMI-0115527) and Atoz CompuNet Ltd. The authors also acknowledge partial support from Center for Nano and Molecular Science and Technology and Welch foundation in The University of Texas at Austin.     

Reflectivity test of X-ray mirrors for deep X-ray lithography

The reflectivity of grazing angle X-ray mirrors, used for X-ray deep lithography, is tested by means of a calorimetric method. A deviation in the reflectivity of a used mirror compared with the reflectivity of a clean surface is observed. This deviation is caused by an oxide layer on the mirrors surfaces. The density, thickness and roughness of the assumed oxide layers are determined experimentally.     

Fabrication of HARM structures by deep-X-ray lithography using graphite mask technology

The cost-effective fabrication process for high-aspect-ratio microstructures using X-rays depends largely on the availability and quality of X-ray masks. The fabrication of X-ray masks using commercially available graphite sheet stock, as a mask membrane is one approach that is designed to reduce cost and turnaround time. Rigid graphite offers unique properties, such as moderate X-ray transmission, fairly low cost, electrical conductivity, and the ability to be used with either subtractive or additive processes [1, 2]. This paper will demonstrate the potential of a cost-effective, rapid prototyping of high-aspect-ratio microstructures (HARMs) using graphite masks. The graphite wafer accommodates both the intermediate mask and the working mask. In order to allow a direct comparison of the graphite mask quality with other X-ray masks, the primary pattern was derived from a Ti X-ray mask using soft X-ray lithography (XRL). Received: 7 July 1999 / Accepted: 29 September 1999     

Geometrical strengthening and tip-sharpening of a microneedle array fabricated by X-ray lithography

A novel fabrication method for LIGA (from the German “Lithographie”, “Galvanik”, and “Abformung”) microneedles with through holes is presented. Such microneedles are in demand by most bio-medical MEMS applications and in some fluidic MEMS applications. We propose a technique that combines conventional deep X-ray lithography, plane-pattern to cross-section transfer (PCT) process, and alignment X-ray lithography. The technique provides precise hole alignment with ± 3 μm tolerance. Finite-element simulations on various hole locations were performed to determine the optimum position. We previously fabricated a microneedle with a 100-μm base and a 300-μm height by a right-triangular mask. The resultant microneedle had a very sharp tip but was excessively steep, and thus resulted in a very low strength. Improved strength and tip sharpness was consequently achieved by changing the mask-pattern from a triangular pattern to a polygonal mask and changing the dimensions of the microneedle to have a 300-μm base with various heights between 350 and 800 μm. Using the proposed technique, we could produce a total of 100 hollow microneedles on a 5 × 5 mm² chip. Moreover, we successfully fabricated sharpened microneedles that were stronger than that we have fabricated so far. The molding process or electroplating and the cost list of the LIGA microneedle will also be included.     

Fluid filter fabricated by deep X-ray lithography for micro fluidics

A new method and fluid filter with micro through capillary array for high-throughput micro fluidics were proposed and fabricated. The method, utilizing liquid surface tension and directing fluid flow in vertical direction, was achieved by using the fluid filter we originally proposed. The computational fluid dynamics analysis was conducted to examine the feasibility of vertical fluid flow operation using the fluid filter. And the results indicated that the vertical fluid flow operation is useful and the good properties of the fluid filter. Fabrication of fluid filter was successfully conducted by using deep X-ray lithography. And vertical fluid flow operation and its high throughput properties were successfully demonstrated.     

Surface roughness control by energy shift in deep X-ray lithography

 Previous studies show that the surface roughness of the sidewall generated by deep X-ray lithography (DXL) is a function of the photon energy of X-rays. Present study demonstrates and reveals the ideas of controlling surface roughness by tuning irradiation photon energy on the sidewall using the developing temperature. The ideas are resulted from two observations (1) X-rays of higher energy induce photoelectrons of higher energy in the resist and the corresponding scattering distance is nearly a square function of electron energy [1], and (2) high-energy X-rays are expected to induce more surface roughness and this effect has been observed by different laboratory [2]. Received: 10 August 2001/Accepted: 24 September 2001 Major part of this work was carried out in Hsinchu. This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.