X-Ray masks for very deep X-Ray lithography

 The high aspect ratio, deep x-ray lithography and electrodeposition process [Becker et al. (1986)] can be expensive unless throughput is high enough. The use of a very high energy synchrotron has allowed the cost of exposure to be significantly reduced through simultaneous exposure of stacked photoresist [Guckel et al (1994)]. Synchrotron radiation at high photon energies has resulted the use of a large area x-ray mask. Both stacked exposures and a large area x-ray masks have significantly increased the throughput of the deep x-ray lithography and electrodeposition process. Received: 25 August 1997/Accepted: 3 September     

Microfabrication of thick tungsten films for use as absorbers of deep X-ray lithography masks

 We fabricated thick (5 μm) tungsten (W) film patterns by sputtering and dry etching, and realized a new deep X-ray lithography mask. The X-ray mask with 5-μm-thick W absorbers could expose about 1-mm-thick resist structures. In the deposition process of W films, the column structure of about 0.2 μm grain size, from which pattern edge roughness originates, disappeared by adding nitrogen into the sputtering gas. W film etching was carried out by reducing gas pressure and cooling the substrate (−40 °C), and a side etch width of below 0.2 μm was obtained. From the results of the pattern edge roughness and the side etch width, a pattern fabrication accuracy below ±0.5 μm was achieved. Furthermore, film stress, which induces pattern distortion, was reduced to below 50 MPa by controlling the sputtering gas pressure. The obtained mask achieved a pattern distortion below ±0.3 μm. Received: 7 July 1999/Accepted: 29 May 2000     

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.     

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.     

High speed hydraulic scanner for deep X-ray lithography

 From our research and development in hard x-ray lithography, we have found that our conventional leadscrew driven scanner stages do not provide adequate scan speed or travel. These considerations have led us to develop a scanning system based on a long stroke hydraulic drive with 635 mm of travel and closed loop feedback to position the stage to better than 100 micrometers. The control of the device is through a PC with a custom LabView interface coupled to simple x-ray beam diagnostics. This configuration allows us to set a variety of scan parameters, including target dose, scan range, scan rates, and dose rate. Results from our prototype system at beamline X-27B are described as well as progress on a production version for the X-14B beamline. Received: 25 August 1997/Accepted: 3 September 1997     

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.     

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.     

Numerical simulation of thermal distortions in deep and ultra deep X-ray lithography

 One major process step in deep X-ray lithography is the exposure of the resist with synchrotron radiation. High energy photons are absorbed in mask, resist and substrate. About 95% of this energy is deposited as thermal heat [Schweizer (1997)]. This may lead to a temperature rise in the system and result in thermal distortions during the patterning process. A sample layout is used to determine the distortions during irradiation. Typical radiation parameters of the ELSA storage ring at Bonn University (2.7 GeV, 35 mA) and material properties are applied to simulate the heat effects. Mask membranes made of titanium or beryllium are modeled to irradiate PMMA layers of 200 and 2500 μm thickness. Copper is used as substrate material. Mask support and the bottom of the substrate are cooled to 21 °C as the system is scanned through the synchrotron beam. In the case of 200 μm PMMA and titanium mask membranes, mask temperatures increase to 40.1 °C, whereas only 22.3 °C are reached if beryllium masks are simulated. Maximum distortions are 0.74 μm for Ti-masks and 0.03 μm for Be-masks. With increasing resist thickness, the incident synchrotron radiation power as well as the temperature rise are reduced. In the case of 2500 μm thick PMMA, temperatures of 21.45 °C are simulated. Received: 10 August 2001/Accepted: 24 September 2001 This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

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.     

SU-8 as resist material for deep X-ray lithography

 A new negative tone resist for deep X-ray lithography is presented. This resist is a nine parts to one mixture of the EPON SU-8 resin with 2,2-bis-(3,5-dichloro-4-hydroxyphenyl)propane (Tetrachlorobisphenol A, TCBA), the latter acting as the photoinitiator. The resist was irradiated at the synchrotron source of DCI at LURE. It was dried for 7 to 20 days beforehand over silica gel while under a light vacuum (20 mbar). Best results for a 150 μm high resist were obtained with a X-ray bottom dose of 3 kJ cm⁻³ and a post exposure bake at 33 °C. Differential Scanning Calorimetry measurements (DSC) determined the glass transition temperature of the resist. The glass transition for the undried, loose resist was 34.7 °C, and it was 28.7 °C when the resist was pressed on a silicon substrate. For a sample of the dried resist, the glass transition was 33.4 °C for the loose resist and 29.8 °C when it was pressed on a Silicon substrate. CD measurements were made on top surface of a set of 100 μm long columns structures, which were produced in 150 μm of this resist. These structures have a constant 100 μm pitch, and the structures themselves varied in width from 20 to 17 μm. For these structures, the CD was calculated to be 0.15 ± 0.03 μm. Received: 8 February 2000/Accepted: 3 March 2000     

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.     

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.     

Optimisation of SU-8 processing parameters for deep X-ray lithography

The negative photoresist SU-8 has been recognised as an unique resist, equally useful for conventional UV lithography as well as deep X-ray lithography (DXRL) applications [2, 7, 12, 17, 18]. One of the major limitations in the use of SU-8 in lithographic processes is the occurrence of internal stress [15]. The processing parameters investigated for DXRL of SU-8 included resist thickness (450–850 m), soft bake time (7–11 h), exposure dose (30–70 J/cm³), post exposure bake time (20, 40, 60 min) and development time. The effect of these parameters on stress was evaluated using wafer curvature measurements. Taguchi optimisation techniques have been used to asses the contribution of these parameters on the stress of the developed structures. This study shows that softbake time contributes the most to stress in the SU-8 film at 50%, followed by the exposure dose and post exposure bake with 30% and 15% respectively. Stress varied somewhat linearly with thickness. At higher thickness, the deposition process needs to be changed for very high aspect ratio structures. The main objective of this work has been to optimise the processing conditions of thick SU-8 films for DXRL.This paper was first presented at the High Aspect Ratio Microstructurres (HARMST) conference in Montery California, June 2003.This work was supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, under Contract No. W-31-109-Eng-38. Support received from CRC for microTechnology (Australia) is also gratefully acknowledged. We also thank Dr. Brett Sexton and Fiona Smith from CSIRO (Australia), Dr. Francesco DeCarlo , Dr. Chian Liu, and Judy Yaeger from APS, and Dr. Jason Hayes and Dr. Matthew Solomon from Swinburne University for useful discussions and their help during some experimental work.     

Development of a freestanding copper antiscatter grid using deep X-ray lithography

A prototype freestanding copper antiscatter grid with parallel lamellar walls has been fabricated using deep X-ray lithography and electroforming. The freestanding copper grid has square shaped lamellar walls that are 25 m thick and 1 mm tall with a 550 m period. For mammography applications, the lamellar walls have to be aligned to a point X-ray source of the X-ray system. To achieve this goal, we investigated a dynamic double-exposure technique. Our progress in using stacked exposures and use of the photoresist SU-8, both to reduce fabrication cost, is also presented.We would like to thank Francesco De Carlo, Judith Yaeger, Joseph Arko and Shenglan Xu for their assistance. Use of the APS was supported by US Department of Energy, Office of Sciences, under Contract No. W-31-109-ENG-38. We thank SRI CAT for the support of this research. The work is supported by NIH SBIR Phase II Grants: 2 R44 CA76752-02 and 5 R44 CA76752-03.This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Low cost transparent SU-8 membrane mask for deep X-ray lithography

Deep X-ray lithography masks require good transparency and mechanical resistance to the intense synchrotron X-ray beam, large active areas (cm)² and compatibility with the standard fabrication processes (optical lithography and gold electroforming). Moreover higher resolution can be achieved with low roughness flat membrane. Furthermore multiple aligned exposures require an optically transparent material. Diamond like Carbon membranes fulfill those requirements but have a prohibitive cost. Our approach consists in using an SU-8 epoxy resin layer as membrane material. In this communication the different steps of the fabrication process will be presented, as well as the results obtained using the mask for particular applications.     

Influence of mask substrate materials on resist sidewall roughness in deep X-ray lithography

Absorption of X-rays in deep X-ray lithography masks can significantly increase exposure time, harden the X-ray spectrum and lead to heating and distortion of the mask. To reduce the impact of such absorption, we have evaluated low atomic mass (low-z) materials that allow the utilization of thick substrates (>100 μm) for reliable mask fabrication. Various forms of graphite, vitreous carbon (VC), boron nitride and beryllium were chosen for testing. Transmission tests were conducted to evaluate resulting surface roughness in the X-ray resist sidewalls. We found that VC, beryllium and pyrolytic graphite all have minimal effect on the resist sidewall surface roughness; however, graphite and boron nitride both significantly increase the roughness to about 300 nm RMS. We could show that this increase in surface roughness is directly related to the crystal structure of these materials. From the tests conducted, VC proves a promising mask substrate, superior to the more expensive and hazardous beryllium that is commonly used for thick high precision masks. VC has been successfully employed as a mask substrate and corresponding resist structures are introduced.     

3D deep X-ray lithography for producing a multi-channel Fourier transform interferometer

We introduce a modified LIGA process architecture to manufacture a static lamellar grating Fourier-transform spectrometer invented by Moser and Möller (European patent EP 0 765 488 B1, 1994). Such spectrometers hold unique advantages over common Michelson-type FTIRs including high time resolution, speed, compactness, and robustness. To cope with the spectrometer’s demand for precise high-aspect-ratio micro-fabrication, we present a modified LIGA process which enhances the X-ray lithography by means of a moving mask technique (Heussler and Moser Lithography method and apparatus PCT/SG2011/000376, 2011). The technique relies on independently moving multiple masks stacked on top of each other during the lithographic step and thus allows to locally vary the deposited dose in a positive tone photoresist. First manufacturing results as well as a performance test of a prototype spectrometer are reported.     

Improvement of capillary electrophoresis property for microchannels fabricated by deep X-ray lithography

We fabricated the electrophoresis microchips using the UV polymerization technique. We employed plastic substrates that were suitable for rapid prototyping instead of glass and quartz. A thick UV negative photo resist was used to form molds and poly-dimethylsilozane (PDMS) was polymerized by a thermal curing process on the mold to obtain replica microchips. Electroosmotic flow (EOF) was measured to evaluate the surface. Characteristic differences between UV-fabricated and SR-fabricated microchips were evaluated by electro osmotic flow (EOF) measurement. It was observed that microchannels fabricated by SR lithography show constant peak heights and FWHMs. We also investigated the effect of the change of the channel width along the EOF direction. It is demonstrated that broadening width channel significantly restricts the sample diffusion towards the EOF direction and leads to the high resolusion separation on the PDMS microchips. Thus the advantage of the application of SR lithography to the mold fabrication is also demonstrated.     

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.