The copolymer of methyl methacrylate and methacrylic acid as a sensitive resist for deep-etch X-ray lithography

The copolymer of methyl methacrylate and methacrylic acid was developed as a new sensitive resist for the LIGA (Lithographie, Galvanoformung, Abformtechnik) process. The resist exposures were carried out at the LIGA beamline of a superconducting compact light source NIJI-III. The absorbed energy density required to remove the entire exposed resist and produce a defect-free microstructure was between 0.4 and 7 kJ/cm³ (4 and 20 kJ/cm³ for PMMA). The sensitivity and patterning depth of the copolymer were 10 times and 3.5 times, respectively, those of poly methyl methacrylate (PMMA) assuming that both resists were exposed to synchrotron radiation (SR) of the same wavelength. Moreover, the copolymer showed high contrast and process stability. This work was performed under the management of the Micromachine Center as part of the Research and Development of Micromachine Technology supported by NEDO (New Energy and Industrial Technology Development Organization).     

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     

Characterisation of defects in very high deep-etch X-ray lithography microstructures

 In deep X-ray lithography synchrotron radiation is applied to pattern several hundred micrometer thick resist layers. This technique has been used to obtain micro structures with an aspect ratio up to 100 and dimensions in the micrometer range. The structures are characterised by straight walls and a typical sidewall roughness of approximately 50 nm. To be able to fabricate n-coherent structures with any lateral shape and to have the possibility to use these resist microstructures in an additional electroforming process the resist is usually mounted on a ceramic or metallic substrate. Due to the different thermal expansion coefficients of the resist material and the substrate a developing temperature of 37 °C produces cracks in the resist structures depending on the microstructure design. These defects are not observed if the developing temperature is reduced to 20 °C. Better structure quality is obtained using the GG-developer instead of MIBK/IPA, but the developing rate is decreased. Measurements of the developing rate of PMMA in GG-developer at different temperatures show that the contrast of the developer-resist system is increased at 20 °C compared to 37 °C. Received: 25 August 1997/Accepted: 3 September 1997     

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.     

A new UV sensitive positive resist for X-ray masks manufacture

LIGA is a well-established process to fabricate metallic micro parts with high resolution, high precision and very low sidewall roughness by means of X-ray lithography and electroplating. The availability of a precise X-ray mask is a precondition for the final precision of the manufactured micro parts. Typical mask substrate materials, e.g. beryllium, carbon based foils, Si₃N₄ or SiC show different disadvantages such as low X-ray transparency or high toxicity or high prices or low conductivity or high thermal expansion or surface porosity causing X-ray scattering. For the fabrication of X-ray masks, PMMA with its unique features such as high aspect ratio patterns with high precision, exhibits low sensitivity and the layers preparation is not easy. SU-8, an epoxy-based UV and X-ray sensitive, chemically amplified, negative tone photoresist exhibits high aspect ratio patterns with vertical sidewalls. The difficult remove of the resist after the electroplating process significantly hinders the inspection of the fabricated X-ray mask. We present the use and suitability of an UV sensitive, chemically amplified, viscous, aqueous-alkaline developable, and easy removable positive tone photoresist, XP mr-P 15 AV, exhibiting high aspect ratio patterns with vertical sidewalls for the fabrication of X-ray masks by means of UV lithography on vitreous carbon substrates.     

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.     

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.     

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.     

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     

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%.     

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.     

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.     

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.     

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.     

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.     

Fast and accurate X-ray lithography simulation enabled by using Monte Carlo method. New version of DoseSim: a software dedicated to deep X-ray lithography (LIGA)

This paper presents the recent development of a simulation tool for deep X-ray lithography. The simulation tool named DoseSim (Meyer et al. in Rev Sci Instrum 74(2):1113–1119, 2003) is a graphical user interface, working under Windows, specially dedicated to the necessary requirements of X-ray lithography setting at a synchrotron. The previous version included the computation of synchrotron radiation from bending magnets, the effects of the optical properties of materials, single mirror and the necessary parameters for the resist exposure. New functionalities, including among others, the exposure time calculation, the insertion of a double mirror, secondary effects (Fresnel diffraction, dose deposited under the absorber) have been added. Also, DoseSim includes traceability concerning the database and calculations used, and de facto the results obtained. Furthermore, Monte Carlo calculations using the PENetration and Energy LOss of Positrons and Electrons (PENELOPE) (Salvat et al.in OECD/NEA Data Bank, France, NEA N°6416, http://www.nea.fr/lists/penelope.html, 2008) code of the spatial distribution of the dose deposited by an X-ray beam in a resist are used. The PENELOPE results (simulations were done mono-energetically for a large range of energy) are the basis of the DoseSim routines for the calculations of the absorbed dose behind the absorber, and at the interface resist/seed layer/substrate. Example of calculations will be discussed along with the effects on dose from different seed layers and substrates.     

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 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 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.