Equations of exposure time and X-ray mask absorber thickness in the LIGA process

 The LIGA X-ray exposure step was modeled into three inequalities from exposure requirements. From these inequalities, equations for the minimum and maximum exposure times required for a good quality microstructure were obtained. An equation for the thickness of an X-ray mask absorber was also obtained from the exposure requirement of threshold dose deposition. A power function of photon energy, approximating the attenuation length of the representative LIGA resist, PMMA, and the mean photon energy of the X-rays incident upon an X-ray mask absorber were applied to the above mentioned equations. Consequently, the trends of the minimum and maximum exposure times with respect to mean photon energy of X-rays and thickness of PMMA were examined and an equation for the maximum exposable thickness of PMMA was obtained. The trends of the necessary thickness of a gold X-ray mask absorber with respect to photon energy of the X-rays and PMMA thickness ratio were also examined. The simplicity of the derived equations has clarified the X-ray exposure phenomenon and the interplay of exposure times, the attenuation coefficient and the thickness of an X-ray mask absorber, the attenuation coefficient and the thickness of a resist, and synchrotron radiation power density. Received: 22 November 1999/Accepted: 27 January 2000     

LIGA fabrication of X-ray Nickel lenses

Nickel refractive planar lenses generating line and point X-ray spots were fabricated by LIGA technology. The optimum parameters of the lenses were simulated for a kinoform lens profile and an X-ray energy range from 100 keV to 1 MeV. The focusing elements of the lens are characterised by an aspect ratio of about 200 and a side-wall roughness of less than 5 nm r.m.s. The optical characteristics of the lenses were tested at the ESRF using a photon energy of 212 keV. A focal spot of 10 m size was measured using a X-ray sensitive film as a width at a half maximum of the darkening of this detector. A ratio of appr. 7 of the detector responses of the focused spot to the unfocused radiation was measured. The correspondent characteristics of the focal spot of the lens for the revealed photon flux are FWHM = 6 m and ~20 for the gain.     

Manufacturing pinhole arrays for X-ray source diagnostics using LIGA

 The manufacturing of pinhole arrays with diameters 10 μm up to 60 μm in 100–300 μm gold foils is described using LIGA. These structures are used to characterize the electron beam in a storage ring by means of the emitted synchrotron radiation. The source inside the bending magnet 13 at BESSY I (single bunch mode, 70 mA ring current, pinholes: ∅60 μm) is (448±40) μm in height and (768±40)μm in horizontal width. For BESSY II pinholes with ∅ 10–20 μm will be used utilizing bending magnet radiation (resolution ∼10 μm), single pinholes with 1–5 μm in diameter are dedicated to insertion device characterization (coherence). Received: 25 August 1997/Accepted: 23 October 1997     

Ultra light-weight and high-resolution X-ray mirrors using DRIE and X-ray LIGA techniques for space X-ray telescopes

We are developing novel ultra light-weight and high-resolution X-ray micro pore optics for space X-ray telescopes. In our method, curvilinear micro pore structures are firstly fabricated by silicon deep reactive ion etching (DRIE) or X-ray LIGA processes. Secondly, side walls of the micro structures are smoothed by magnetic field assisted finishing and/or hydrogen annealing techniques for high reflectivity mirrors. Thirdly, to focus parallel X-ray lights from astronomical objects, these structures are elastically or plastically bent into a spherical shape. Fourthly, the bent structures are stacked to form a multi-stage X-ray telescope. In this paper, we report on fabrication and X-ray reflection tests of silicon and nickel X-ray mirrors using the DRIE and LIGA processes, respectively. For the first time, X-ray reflections were confirmed on both of the mirrors. Estimated rms roughnesses were 5 nm and 3 nm for the silicon and nickel mirrors, respectively.     

The fabrication of LIGA mask using photolithography and synchrotron radiation lithography

 The fabrication of LIGA mask is a very important step in LIGA process. Usually an intermediate mask with gold absorber pattern of 2 μm thickness is fabricated firstly using gold electroplating for absorber pattern in the resist structure written by Electron Beam (e-Beam), then the LIGA mask can be copied from the intermediate mask using synchrotron radiation lithography and gold electroplating. Recently, we use photolithography (instead of e-beam) to make the primary structure, and produce the intermediate mask with gold absorber pattern of 1.5 μm thickness produced by etching gold film with 1.5 μm thickness under the photoresist structure using Ar⁺. The LIGA mask with absorber pattern of 13 μm thickness is copied from the intermediate mask using synchrotron radiation lithography and gold electroplating. Received: 30 October 1995/Accepted: 9 December 1995     

The influence of X-ray fluorescence on LIGA sidewall tolerances

 Fluorescence radiation during LIGA X-ray exposure and the impact of the resulting secondary dose on feature sidewall tolerances are examined using numerical methods. The study addresses fluorescence emitted by the substrate of the PMMA resist and focuses on the tolerances obtained for resists patterned with a range of feature sizes. New models of the secondary dose and subsequent feature development are presented and discussed. These models are coupled such that the evolving feature geometry during development depends on the local total dose, as well as the development temperature and the transport of PMMA fragments away from the dissolution front. We find that sidewall tolerances for typical exposure and development conditions are less than 0.1 micron for a metallized silicon substrate, but exceed several microns for thick titanium, copper and nickel substrates. We also find that these tolerances are very sensitive to the feature geometry, development temperature and the magnitude of the primary dose. Received: 10 August 2001/Accepted: 24 September 2001     

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.     

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.     

Linewidth design rules for microstructures manufactured in PMMA resist using X-ray LIGA

During the past decade, individual simulation modules for the LIGA process have been developed, such as the power spectrum of the X-rays available from a given synchrotron, the effect of various components in the beam line, the image structure in the resist and image development. This has lead to an understanding of the parameters affecting the basic dimensional relationship between the mask and the 3D image in the resist itself. The commercialization of X-ray LIGA now requires knowledge of the parameters affecting the production engineering of micro devices. Foremost among these parameters are the manufacturing tolerances arising from variations in key microfabrication steps. This paper seeks to integrate the various physical simulation models into a single interactive program, whereby the design engineer can see the effect of different synchrotron output spectra, different beamline constructions and different resist parameters on the dimensions of the planned 3D microstructure. A novel approach to estimating the dimensional tolerances to be expected with large scale production is developed and comparison made with data available from the literature. Predicted results from using a particular synchrotron with a beamline designed to approximate to a “standard” exposure spectra at the rest surface are calculated, showing the microstructure shape and tolerance expected in large scale production. From these data a basic set of equations are established from which “Linewidth Design rules for LIGA” can be established. The steps following definition of the resist structure, for example electrodeposition, injection moulding or hot embossing, are not studied here.     

The influence of mask substrate thickness on exposure and development times for the LIGA process

Optimizing mask substrate thickness is an important practical concern in the X-ray exposure of PMMA resists for LIGA. An overly thick substrate necessitates long exposure times due to excessive beam filtering, while a substrate too thin leads to long development times due to low absorbed doses at the PMMA bottom surface. To assist in this optimization, we have developed numerical models describing both the exposure and development of a PMMA resist. These exposure and development models are coupled in a single interactive code, permitting automated adjustment of mask substrate thickness to yield the minimum of a prescribed cost object function that depends on both the exposure and development times. Results are presented for several synchrotron sources and over a wide range of the PMMA thickness. Received: 15 July 1999/Accepted: 23: August 1999     

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.     

Microcomposite electroforming for LIGA technology

 Effective methods to improve hardness and thickness uniformity in electroforming for the LIGA process to produce metallic microstructures are described. The result shows the internal stress of Ni-Co alloy deposit can be well tuned between 2 to −2 kg/mm² by adding a stress release agent into the electroplating bath. A secondary cathode is applied to improve the thickness uniformity in electroforming without loss plating rate at the center of the features. Received: 7 July 1999/Accepted: 25 November 1999     

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.     

Microfabrication of massive parallel micromirror-lenses by X-ray LIGA technique: recent advances and prospects

Modern microfabrication techniques enable to obtain novel 2D lens systems which are the marriage of warped microchannel plates and Fresnel lenses. They rely on a multitude of precisely inclined micromirrors, acting together as very efficient macroscopic lenses for all electromagnetic wavelengths from the infrared, up to X-rays and even for particles like neutrons. Especially interesting is the fabrication of some-cm apertured lenses in the VUV and EUV region. X-ray LIGA is a favorite candidate for the fabrication of such lenses due to its inherent technological precision and capability to produce high aspect ratio structures. Theoretical consideration and limits for the design of such lens systems are presented, together with first investigations concerning surface-smoothing by spin-on glass in order to make such lens systems to work in the X-ray region.This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Photoresist application for the LIGA process

 Fabrication of high aspect ratio structures requires the use of a photoresist able to form a mold with vertical sidewalls. Thus the photoresist should have a high selectivity between the exposed and the unexposed area in the developer. It should be relatively free from stress when applied in thick layers necessary to make high aspect ratio structures. PMMA (Poly Methyl Methacrylate) is the photoresist of choice in the LIGA process, mainly for its ability to hold vertical sidewalls for tall structures. It is applied to the substrate by a glue-down process in which a pre-cast, high molecular weight, sheet of PMMA is attached to the plating base on a substrate. The applied photoresist is then milled down to the precise height by a fly-cutter prior to pattern transfer by x-ray exposure. The requirement that the applied layer be relatively free from stress dictates the choice of glue-down over casting. The substrate preparation steps, as well as the conditioning of the PMMA sheet prior to the glue-down, are done, in part, to reduce the stress in the glued down sheet of photoresist. The cutting of the PMMA sheet in the fly-cutter requires specific operating conditions as well as particular cutting tools to avoid introducing any stress and the resultant crazing of the photoresist. Received: 25 August 1997/Accepted: 3 September 1997     

Studies of the adhesion properties of LIGA microstructures by X-ray spectroscopy and mechanical measurements

In the first step of the LIGA process microstructures with high aspect ratios are fabricated by patterning a resist layer with deep etch X-ray lithography. As resist typically PMMA (polymethylmethacrylate) is used. For the following electroforming process and to achieve perfect microstructures the adhesion of the PMMA on the substrates is of great importance. For a better understanding the adhesion mechanism, particular induced through the adhesion promoter MEMO (methacryloxypropyl-trimethoxysilane) was investigated on various substrates. Two methods, namely XANES/EXAFS spectroscopy under grazing incidence and shear stress measurements were modified for the specific needs of microstructures. Our studies proved that the adhesion strength is determined by two factors. A rough surface, which allows mechanical interlocking, increases the adhesion strength by about 10%. The larger part of the adhesion strength is determined by formation of chemical bonds when adding an adhesion promoter. E.g. addition of 5% adhesion promoter increases the adhesion strength by 90%. These results were, as far as possible, confirmed by the X-ray spectroscopical measurements.     

Deep X-ray lithography using mask with integrated electrothermal actuator

We present a deep X-ray mask with integrated bent-beam electrothermal actuator for the fabrication of 3D microstructures with curved surface. The mask absorber is electroplated on the shuttle mass, which is supported by a pair of 20-m-thick single crystal silicon bent-beam electrothermal actuators and oscillated in a rectilinear direction due to the thermal expansion of the bent-beams. The width of each bent-beam is 10 m or 20 m and the length and bending angle are 1 mm and 0.1 rad, respectively, and the shuttle mass size is 1 mm × 1 mm. For 10-m-wide bent-beams, the shuttle mass displacement is around 15 m at 180 mW (3.6 V) dc input power. For 20-m-wide bent-beams, the shuttle mass displacement is around 19 m at 336 mW (4.2 V) dc input power. Sinusoidal cross-sectional PMMA microstructures with a pitch of 40 m and a height of 20 m are fabricated by 0.5 Hz, 20-m-amplitude sinusoidal shuttle mass oscillation.This research, under the contract project code MS-02-338-01, has been supported by the Intelligent Microsystem Center, which carries out one of the 21st centurys Frontier R & D Projects sponsored by the Korea Ministry of Science & Technology. Experiments at PLS were supported in part by MOST and POSCO.     

Studies of the adhesion properties of LIGA microstructures by X-ray spectroscopy and mechanical measurements

In the first step of the LIGA process microstructures with high aspect ratios are fabricated by patterning a resist layer with deep etch X-ray lithography. As resist typically PMMA (polymethylmethacrylate) is used. For the following electroforming process and to achieve perfect microstructures the adhesion of the PMMA on the substrates is of great importance. For a better understanding the adhesion mechanism, particular induced through the adhesion promoter MEMO (methacryloxypropyl-trimethoxysilane) was investigated on various substrates. Two methods, namely XANES/EXAFS spectroscopy under grazing incidence and shear stress measuremsents were modified for the specific needs of microstructures. Our studies proved that the adhesion strength is determined by two factors. A rough surface, which allows mechanical interlocking, increases the adhesion strength by about 10%. The larger part of the adhesion strength is determined by formation of chemical bonds when adding an adhesion promoter. E.g. addition of 5% adhesion promoter increases the adhesion strnegth by 90%. These results were, as far as possible, confirmed by the X-ray spectroscopical measurements. The financial support of this project by the Federal Ministry of Science and Technology (contract No. 05 5P DAX18) is gratefully acknowledged.