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     

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.     

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.     

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).     

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).     

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.     

An intermediate-layer lithography method for generating multiple microstructures made of different conducting polymers

An intermediate-layer lithography (ILL) method has been developed in this work to generate multiple microstructures of different conducting polymers on the same substrate. Previous and current efforts in developing conducting polymer microsystems mainly focus on generating a device of a single function. When multiple micropatterns of different conducting polymers are produced on the same substrate, many microsystems of multiple functions can be envisioned. However, existing techniques present significant technical challenges of degradation, low throughput, low resolution, depth of field, and/or residual layer in producing conducting polymer microstructures. To circumvent these challenges, the ILL method has been explored to generate multiple micropatterns of different conducting polymers in a parallel manner. In this method, conducting polymer materials and a non-conducting polymer intermediate layer are first coated on a substrate, and are then patterned through a mold insertion at a raised temperature. In this work, the ILL has been used to successfully pattern three types of commonly used conducting polymers on the same substrate under a single mold insertion, and simulation has been conducted to gain a good understanding of the molding process. Due to distinctive advantages of simplicity, low cost and high throughput, the ILL has promising applications in fabricating micropatterns for polymer-based microsystems.     

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.     

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.     

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%.     

Improvements in graphite-based X-ray mask fabrication for ultradeep X-ray lithography

Hard x-ray masks for ultradeep x-ray lithography (UDXRL) at synchrotron radiation sources, such as the Advanced Photon Source, require a gold absorber thickness of 20–100 m on a low-Z substrate, such as silicon, graphite, or beryllium. Graphite sheets of 0.5 mm were used for the fabrication of x-ray masks by standard optical lithography using a SU-8 photoresist. The conductivity of graphite is sufficient to perform electroplating directly without the need for a metal-plating base layer. Gold electroforming was used to deposit a 85-m-thick patterned absorber layer. The masks were used for UDXRL using hard x-rays at the Advanced Photon Source.This work has been supported by the U.S. Department of Energy, Office of Science, under Contract No. W-31–109-ENG-38, by AFOSR Grant 49620–00–1-0088, and by DUSD (S&T) under the Innovative Microwave Vacuum Electronics MURI program managed by the AFOSR under grant F49620–99–1-0297.     

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.     

Mask design compensation for sloped sidewall structures fabricated by X-ray lithography

We have investigated and report in this paper the factors influencing the deformation caused by the dependence between the absorbed X-ray energy on the resist and the shape of the absorber on the X-ray mask. Based on the measurement of errors that occurred during the transferring process between the 2-D shape of mask pattern and the resulting wall of the fabricated 3-D structure, we have developed newly useful graphical data on the absorbed X-ray energy, dosage, and shape of a microstructure. As a result, it is being reported as a method for compensation for the deformed shape after the fabrication of a quadruplets-microneedle. We have considered a number of factors affecting the deformation and finally realized that the effect of a dose–depth nonlinear curve is the most possible cause. Without the compensation of the mask design, we could observe the deformed shapes of the sloped sidewall on the exposed structures. Polymethylmethacrylate microneedle structures fabricated by X-ray lithography with an additional plane-pattern to cross-section transfers technique are directly influenced by the absorber on the X-ray mask pattern. The sidewall of the microneedle was improved by changing the mask pattern from a double right-triangular pattern to a double semi-circular pattern, modeled by comparing the results from a mask-pattern and the actual structure.     

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     

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.     

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.     

Simulation of deep UV lithography with SU-8 resist by using 365 nm light source

The deep lithography of thick resist layer is the primary step of LIGA technology. UV Proximity lithography, which is used to fabricate high aspect ratio MEMS normally, is investigated in this paper. The light intensity distribution in the thick photoresist layer mainly depends on the diffraction produced by the gap between mask and wafer in proximity lithography. Fresnel diffraction model is used to simulate lithography process normally, which the thin photoresist layer is used in the lithography process. But it is not accurate in deep lithography process. A correction to the Fresnel diffraction theory is used to simulate the lithography process depending on the scalar diffraction theory of light propagation in this paper. The difference of this two models is given in this paper. The simulation results show that the corrected model can obtain more accurate simulation results than the Fresnel diffraction model. The experiment results and the theory analysis both indicate that: the structure contrast decreases with the increase of the film thickness. The smaller the structure linewidth is, the faster the structure contrast decreases. The linewidth of microstructure is not equivalent from the top to the bottom, broaden in the middle part normally. The theory simulation gives the quantitative analysis.This work was supported by of China 973 Program (No. 1999033109).     

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.     

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.