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     

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     

Materials of LIGA technology

 The progress in microfabrication technologies is currently characterized by an increasing number of microproducts and corresponding efforts for cost effective mass fabrication. In this context, the development and utilization of new application specific materials has become one of the key challenges for the commercial production of miniaturized functional units. This applies, in particular, to the LIGA technique, a sequence of process steps combining deep lithography, microelectroforming and micromoulding, which offers an extremely broad spectrum of materials for the generation of ultraprecise three-dimensional microstructures. A wide variety of polymers, metals, metal alloys and ceramic materials are by now accessible for fabricating microdevices. There are practically no major limitations in selecting the desired material properties, e.g. in respect of mechanical stiffness or biocompatibility, optical transparency or magnetic properties, resistance against corrosion or whatever is required by the specific function of a microdevice.     

Fabrication of LIGA mold inserts

 The present paper describes the fabrication sequence of a LIGA mold insert by electroforming after the patterning steps of the overall process. These tools are applied for large scale fabrication of microcomponents made by molding and embossing processes. The application of an intermediate layer system leads to optimized process performance and to a better surface quality of the mold insert. The plating processes are described and the materials properties, e.g. hardness, are used for the characterization of the recrystallization behavior of the electroformed nickel which yields the high temperature application limit of the tool. Received: 25 August 1997 /Accepted: 22 September 1997     

LIGA activity in Taiwan

 This paper describes activities and development of the LIGA technology in Taiwan. The third-generation synchrotron at SRRC, providing the radiation covering an energy range from VUV to soft X-ray, is fully optimized for the wide-range applications in science and technology. In its five-year plan started from 1995, SRRC has identified deep X-ray lithography and related LIGA technology as one of the main focuses of the applied research programs. To integrate a multiple discipline research, SRRC also seeks collaboration with the following organizations in Taiwan: Industrial Technology Research Institute, National Nano Device Laboratory, Chung-Shan Institute of Science and Technology, Universities and Textile Industry. In the near future, LIGA technology will be used to fabricate the microstructures for the scientific and industrial applications, such as zone plate, nozzle, sensor and heat transfer layer for VLSI. Received: 30 October 1995/Accepted: 25 January 1996     

LIGA technology in BSRF

Beijing Synchrotron Radiation Facility is a partly dedicated Synchrotron Radiation Source which is operated since the end of 1990. Some main specifications of the source are briefly illustrated. Beam line 3B1 is a soft X-ray beam line extracted from bending magnet exploited for lithography. In the end of 3B1 there is a simple exposure chamber, LIGA patterns were exposured in the chamber. The LIGA technologies in BSRF are in the basic research stage, only a few of demonstrated patterns of deep etching structure were obtained. A sample of electroplating apparatus has been set up for getting metal structure. Further plan of the next phase of LIGA technology are presented. A new exposure scanner is being designed and would be set up on the another beam line 4W1 which is extracted from wiggle source with critical wavelength 2.2 A which seems quite suitable for deep etching exposure.     

LIGA activity in Taiwan

This paper describes activities and development of the LIGA technology in Taiwan. The third-generation synchrotron at SRRC, providing the radiation covering an energy range from VUV to soft X-ray, is fully optimized for the wide-range applications in science and technology. In its five-year plan started from 1995, SRRC has identified deep X-ray lithography and related LIGA technology as one of the main focuses of the applied research programs. To integrate a multiple discipline research, SRRC also seeks collaboration with the following organizations in Taiwan: Industrial Technology Research Institute, National Nano Device Laboratory, Chung-Shan Institute of Science and Technology, Universities and Textile Industry. In the near future, LIGA technology will be used to fabricate the microstructures for the scientific and industrial applications, such as zone plate, nozzle, sensor and heat transfer layer for VLSI. During the past year, I had many valuable discussions with professor W. Menz, Dr. H. Lehr and Dr. J. Mohr who inspired and assisted in shaping the present LIGA program. I would like to thank all of our German friends. I would like to extend special thanks to professor A. C.-M. Yang at Tsing-Hua university for the preparation of the thick film photo resist. He developed a brand new method to produce the thick resist without polymerization process. My appreciation also extends to professor C.-L. Kuo at Yunlin institute of technology for the preparation of the Ta stencil mask by EDM machining. For the multiple exposure-development process, I want to thank Dr. M.-C. Jiang, the only full time staff in the LIGA program at present. In writing this paper, I have received a lot of support from the SRRC staff for providing me with the most current data and drawings of TLS. For that, I thank Dr. Ch. Wang, Dr. C.-C. Kuo, Dr. K.-K. Lin, Mr. Z.-I. Cheng and Mr. Z.-C. Chen.     

New electroforming technology pressure aid for LIGA process

A new microelectroforming technique using air-pressure assistance in the electrolyte is introduced. The related theories and experiments are reported in this paper. The pressurized electrolyte reduces hydrogen bubble formation and defect on the substrate surface. The high pressure electrolyte limits bubble formation in electroforming. It also produces good throwing power from the experimental sample observations. The edges of the electroformed workpiece using the pressurized method were smoother than those produced using conventional electroforming. The metallurgical grains were finer when higher pressures were applied in the electrolyte based on SEM micrographs. Pressurized electroforming can increase the allowable current density, which can shorten plating time in the LIGA process. The high growth rate due to large applied current density results in large electroforming microstructure grains. The surface morphology of pressurized electroformed samples were improved compared with the conventional method.This work was supported by the National Science Council (series no. NSC91-2623-7-005–002) of Taiwan. The National Synchrotron Radiation Research Center (NSRRC) provided synchrotron radiation for fabricating PMMA molds is acknowledged.     

LIGA technology in BSRF

 Beijing Synchrotron Radiation Facility is a partly dedicated Synchrotron Radiation Source which is operated since the end of 1990. Some main specifications of the source are briefly illustrated. Beam line 3B1 is a soft X-ray beam line extracted from bending magnet exploited for lithography. In the end of 3B1 there is a simple exposure chamber, LIGA patterns were exposured in the chamber. The LIGA technologies in BSRF are in the basic research stage, only a few of demonstrated patterns of deep etching structure were obtained. A sample of electroplating apparatus has been set up for getting metal structure. Further plan of the next phase of LIGA technology are presented. A new exposure scanner is being designed and would be set up on the another beam line 4W1 which is extracted from wiggle source with critical wavelength 2.2 A which seems quite suitable for deep etching exposure. Received: 30 October 1995 / Accepted: 26 January 1996     

Direct LIGA service for prototyping: status report

Since the early 1990s a variety of applications ranging from precision metallic, ceramic, or polymeric parts to complex devices for micro-fluidic and micro-optic applications have been discussed with the assumption that LIGA can either make a significant contribution or is the sole technology able to make these products. The question today is, have individual LIGA groups or even the entire LIGA community achieved this goal? Despite major investments, a lot of hard work from many students and researchers, and some promising “prototypes”, a true mass product made by direct LIGA, i.e., using X-ray lithography and electroplating, has still not entered the market. This paper will briefly present the current status of direct-LIGA technology and discuss some examples of commercialization efforts as well as prospects for high-volume production.     

Electrochemical post-processing of LIGA structures

The Louisiana State University (LSU) Microsystems Engineering Team (μSET) is developing techniques for producing microstructures with predictable 3-D geometry. One approach is based on controlled anodic dissolution of electroplated structures to obtain desired shapes. A finite element model was developed to predict the resulting shapes. As a demonstration of the shaping process, development of LIGA-based scanning probe microscope tips was investigated. An array of micro-posts 1.8 micrometers in diameter and 12 micrometers high was produced using x-ray lithography and subsequent electrodeposition. The posts were sharpened electrochemically. Other geometries are being investigated.     

Electrochemical post-processing of LIGA structures

 The Louisiana State University (LSU) Microsystems Engineering Team (μSET) is developing techniques for producing microstructures with predictable 3-D geometry. One approach is based on controlled anodic dissolution of electroplated structures to obtain desired shapes. A finite element model was developed to predict the resulting shapes. As a demonstration of the shaping process, development of LIGA-based scanning probe microscope tips was investigated. An array of micro-posts 1.8 micrometers in diameter and 12 micrometers high was produced using x-ray lithography and subsequent electrodeposition. The posts were sharpened electrochemically. Other geometries are being investigated. Received: 30 October 1995/Accepted: 18 December 1995     

Inexpensive, quickly producable X-ray mask for LIGA

 The capability to produce X-ray masks inexpensively and rapidly is expected to greatly enhance the commercial appeal of the LIGA process. This paper presents a process to fabricate X-ray masks both inexpensively (under $1000) and rapidly (within a few days). The process involves one UV lithography step and eliminates the need for an intermediate X-ray mask. The X-ray mask produced by this process consists of a 125 μm thick graphite membrane that supports a gold-on-nickel absorber pattern. The thickness of the absorber structures is great enough to supply sufficient contrast even when radiation sources with high characteristic photon energies up to 40 keV are utilized and/or when deep exposures are desired. The mask fabrication process is initiated by spin coating 30–50 μm of SU-8 directly on a graphite membrane. The SU-8 is then patterned using a UV mask. Gold-on-nickel absorber structures are electroplated directly onto the SU-8 covered graphite. Once the remaining SU-8 is removed, attaching the graphite membrane to a frame completes the mask. To test the performance of the mask, a nickel mold insert was fabricated. A sheet of PMMA 500 μm in thickness was bonded to a nickel substrate, then exposed to X-rays through the mask, and developed. Electroplating nickel into the patterned PMMA sheet produced a mold insert. SEM pictures taken of the SU-8, the X-ray mask, and the mold insert are shown. This method of rapidly producing an inexpensive X-ray mask for LIGA resulted in a mold insert with smooth, vertical sidewalls whose dimensions were within two micrometers of the UV mask dimensions. Received: 12 December 1998/Accepted: 2 February 1999     

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     

Processing-microstructure-resulting materials properties of LIGA Ni

Electroplated nickel is the main material used in LIGA technique for fabricating MicroElectroMechanical System (MEMS) components. This paper presents the recent results regarding LIGA Ni material property enhancement at Center for Advanced Microstructures and Devices at Louisiana State University. The methodology of this study is based on the fundamental materials science paradigm that processing and manufacturing determine the microstructure of materials, which results in specific properties, and governs the performance of the structures that are made from these materials. This paper will focus on results in three aspects of our current research efforts, which are: (1) manipulating the plating parameters to tailor deposited Ni microstructures for different grain size distributions that are in the range of tens nanometer in diameters; (2) analyzing the mechanical properties, such as nano-hardness, micro-hardness, and wearing property as the function of LIGA Ni metallurgical microstructures; (3) estimating the corrosion properties of nanocrystalline LIGA Ni.     

LIGA相关技术及应用

微电子机械系统(MEMS)技术的兴起及其在现代信息社会中的广泛应用,推动了能实现高深宽比三维微细加工的LIGA及准LIGA技术的迅速发展。介绍了LIGA相关技术的发展状况并举例说明了它们在射频、光学等方面上的一些应用。     

Alternative resist adhesion and electroplating layers for LIGA process

 The fabrication of high aspect ratio microstructures using the X-ray lithography and electroplating step of the LIGA process requires, that the metal layer on the substrate (plating base) enables a flawless resist adhesion and provides a high conductivity for the following electroplating process. In addition, a flawless metal adhesion after electroplating has to be ensured. Typically metal layers of titanium, but also of copper and gold are used. In case of higher structures, fluorescence radiation generated in the metal layer during exposure reduces the bond strength between the resist and the adhesion layer [7]. Since the fluorescence probability decreases with a decreasing atomic number, the suitability of conductive carbon as the plating base was investigated and compared to results achieved with titanium. The resist adhesion was increased tremendously on the carbon layer but the metal adhesion is insufficient. Therefore, platinum islands were deposited on the conductive carbon to facilitate good metal adhesion. The resulting resist adhesion was still superior compared to titanium. In addition, metal adhesion was achieved on these layers.     

Replication of LIGA structures using microcasting

Microcasting with a gold base alloy and an Al-bronze as a replication technique for LIGA structures is shown. A LIGA mold insert for injection molding is used to produce PMMA patterns as lost wax models in the microcasting process. Although there are several steps of replication the accuracy of the process is high and allows the fast manufacturing of middle size batches of microstructured parts. The mechanical properties can be varied within a wide range by choosing different casting alloys or cooling rates. Additionally the influence of the investment on the surface roughness is presented as well as results of flowlength tests showing maximal achievable aspect ratios with the gold base alloy and the Al-bronze.The funding of this work within Collaborative Research Center SFB 499 by Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged. Thanks also to the colleagues in the SFB 499 for good collaboration, especially to S. Rath for providing special investments for microcasting.