LIGA Process Research Articles

The Reliability of Micro-Spring under Dynamic Circumstances

This article mainly focuses on the reliability of micro-spring, which as a typical component of MEMS device, applies electrodeposited nickel material and is fabricated by LIGA process, being used under the circumstances like transporting and restoring which may induce shock and crash. Griffith theory of brittle fracture and Abaqus FEA simulation software are applied for analyzing the probable failure modes of the micro-spring as overloaded. And the principle of elastic deformation, energy-time and displacement-time variation curves are given by three groups of simulation experiments. It proves to be one novel method to evaluate the reliability and accelerate exposing the failure mode of these MEMS components under dynamic circumstances in short period. After comparison, this method is proved reliable for analyzing and providing evidence for enhancing the reliability of the micro-spring, also improving the performance in micron size.

Fabrication of micro spur gears by step powder extrusion of Zn-22wt%Al powder without sintering process

Micro spur gears were fabricated using the LIGA process and step powder extrusion without sintering. It is important to manufacture micro dies with high aspect ratios and determine the appropriate extrusion conditions for the micro-forming process. Micro-extrusion dies with close tolerance and long bearing length were produced by the LIGA process. Superplastic Zn-22wt%Al powders can deform under low stresses and exhibit a good micro-formability (average strain rate: 10−3s−1; constant temperature: 250°C). These powders were compacted to a cylinder (diameter: 3 mm; height: 10 mm) under 10kN compressive force, and sintered at 350°C for two hours. Micro spur gear shafts were not produced upon extrusion because of the high working pressure on the die surface. For the reduction of the forming load, step powder extrusion was carried out on the compacted powder without a sintering process, but controlling the temperature dwelling period. This process has succeeded in fabricating micro-gear shafts.

Collection efficiency and interstage loss of nanoparticles in micro-orifice-based cascade impactors

In this study, two micro-orifice-based cascade impactors, including the micro-orifice uniform deposit impactor (MOUDI, MSP Model 110) and the NCTU micro-orifice cascade impactor (NMCI), were tested for the collection efficiency and interstage loss of nanoparticles. In the NMCI, new nozzle plates with smooth nozzle shape made by the LIGA (Lithography, Electroplating, and Molding) process were used to replace the 7th–10th stages in one of the MOUDI. Test results show that after adjusting proper S/W ratios (S: jet to plate distance, W: nozzle diameter) to 2.52, 3.01, 13.44, and 24.75 for the 7th, 8th, 9th and 10th stage of the NMCI, respectively, and 5.56, 11.18, 9.3, and 10.9 for the 7th, 8th, 9th and 10th stage of the MOUDI, respectively, the cutoff aerodynamic diameters (dpa50) are close to the nominal values given in Marple et al. (1991). Different S/W ratios are needed due to differences in the nozzle shape and nozzle diameter between two cascade impactors. Total interstage loss of nanoparticles from the inlet to the 6th–10th stage of the MOUDI exists due to the convection-diffusion mechanism, which increases with decreasing dpa. For the MOUDI, total loss is 2.9–15.3 % (dpa: 105.8 to 15.4 nm) for the inlet to the 6th stage and it increases to 20.1–26.1 % (dpa: 23 to 15.4 nm) for the inlet to the 10th stage, respectively. Similar but slightly lower loss also exists in the NMCI. Field comparison tests in the ambient air show that mass size distributions measured by the MOUDI agree well with those of the NMCI. Finally, nozzle clogging tests using high concentration incense smokes indicate that the NMCI has a much less tendency for particles to clog in the nozzles than the MODUI.

Numerical simulation of micro hot embossing of polymer substrate

Micro hot embossing, one of the important steps in the LIGA process is aimed at increasing the scale of production. The process involves: molding-in which metallic mold is pressed into the polymer, cooling-in which mold and the polymer are held together and demolding — where in the mold is separated from the polymer. In the past, these steps in a micro hot embossing process are modelled independently. As the parameters in each of these stages influence the final product quality, the simulations were performed using DEFORM 3D and by transferring the strain history from one stage to another. The final product quality thus is a function of the preceding quality in each stage. The parametric analysis shows that, at lower level of molding speed (0.01 mm/min–0.1 mm/min) and lower level of molding temperature (105°C–115°C), complete mold filling was evident. In the cooling stage, the difference between the molding and demolding temperature governs contact stresses and thermal shrinkage. At the same time, in the demolding stage, lower rate of demolding ensures lower residual stresses in the micro components formed. Experimental validation of the results shows that the predicted results agree fairly with the simulated values.

Fabrication of Micro-Capacitive Inclination Sensor Using the LIGA Process

The resource saving of various electronic devices and advancement are demanded. Research and development of the MEMS device are briskly done from viewpoints of a miniaturization and highly-efficient. However, tip-shaped device manufacture is easy by the MEMS, but is hard to manufacture the tertiary structure. Therefore, as nanofabrication technology aiming at the mass production of the tertiary structure, LIGA process to make a microstructure body by copying it attracts attention of resin. In this research, the electric capacity type inclination sensor which used the LIGA process is produced. By using resin molding for a manufacturing step, a production cost can be low held down compared with the existing sensor. Former, the silicon mold which has a pattern of the size of 7×7 mm and structure height 100µm using ICP dry etching was produced. In addition, we performed resin molding using the silicon mold and succeeded in the manufacture and evaluation of the angle of inclination sensor. We expect that if techniques using resin-molded parts are introduced to the low-cost mass-production of MEMS devices (including sensors), the range of applications will further expand to new areas of technology and industry.

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 Moller (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.

LIGA 금형몰드를 이용한 Fe-Ni계 나노분말의 초미세 가스베어링 제조

This paper describes the manufacturing process of tilting pad gas bearing with a diameter of 5 mm and a length of 0.5-1 mm for power MEMS (Micro Electomechanical Systems) applications. The bearing compacts with nanopowder feedstock were prepared by Ni-metal mold with 2-mold system using LIGA process. The effect of the manufacturing conditions on sintering properties of nanopowder gas bearing was investigated. In this work, Fe-45 wt%Ni nanopowder with an average diameter of 30-50 nm size was used as starting material. After mixing the nanopowder and the wax-based binders, the amount of powder was controlled to obtain the certain mixing ratio. The nanopowder bearing compacts were sintered with 1-2 hr holding time under hydrogen atmospheres and under temperatures of <TEX>$600^{\circ}C$</TEX> to <TEX>$1,000^{\circ}C$</TEX>. Finally, the critical batch of mixed powder system was found to be 70% particle fraction in total volume. The maximum density of the sintered bearing specimen was about 94% of theoretical density.

Application of photo-etching of polytetrafluoroethylene induced by high energy synchrotron radiation to LIGA

Polytetrafluoroethylene (PTFE) microstructures’ processing characteristics using X-ray photo decomposition and desorption are studied in the highest energy region (2–12 keV). While the exposed surface states are seen melting and boiling from the remaining bubble structure of the irradiated surface, basic photochemistry of PTFE is also same as previous reports. Surface modification of PTFE from hydrophobic to hydrophilic was investigated in order to bond PTFE sheets to a brass substrate. Then, we have a successfully fabricated Ni microstructure by LIGA process using PTFE photo-etching using high energy X-ray. Proposed LIGA using SR-photo-etching of PTFE can simplify the total process, and it can ensure that dimensional errors remain small due to swelling in developer and electroplating bath.

Production of uniform droplets using membrane, microchannel and microfluidic emulsification devices

This review provides an overview of major microengineering emulsification techniques for production of monodispersed droplets. The main emphasis has been put on membrane emulsification using Shirasu Porous Glass and microsieve membrane, microchannel emulsification using grooved-type and straight-through microchannel plates, microfluidic junctions and flow focusing microfluidic devices. Microfabrication methods for production of planar and 3D poly(dimethylsiloxane) devices, glass capillary microfluidic devices and single-crystal silicon microchannel array devices have been described including soft lithography, glass capillary pulling and microforging, hot embossing, anisotropic wet etching and deep reactive ion etching. In addition, fabrication methods for SPG and microseive membranes have been outlined, such as spinodal decomposition, reactive ion etching and ultraviolet LIGA (Lithography, Electroplating, and Moulding) process. The most widespread application of micromachined emulsification devices is in the synthesis of monodispersed particles and vesicles, such as polymeric particles, microgels, solid lipid particles, Janus particles, and functional vesicles (liposomes, polymersomes and colloidosomes). Glass capillary microfluidic devices are very suitable for production of core/shell drops of controllable shell thickness and multiple emulsions containing a controlled number of inner droplets and/or inner droplets of two or more distinct phases. Microchannel emulsification is a very promising technique for production of monodispersed droplets with droplet throughputs of up to 100 l h−1.

Fabrication of Metal Micro Needle Array by LIGA Process

Polymer materials, such as PMMA and PDMS, have been of great interest in the research and development of MEMS recently due to their relatively low cost and easy process. In this research, we are trying to fabricate Ni Micro needle array from PMMA master by through PDMS mold based on multi-mold transformation technology. The fabricated micro nickel needles array is 900 microneedles in 1 cm2 chip area and the height of each needle is about 150um.

Simulation Research of the Effect of Thermal Reflow Processing Parameters on the Profile of Micro-Spherical Structure

In this paper reflow process is integrated into LIGA process in order to realize three dimensional fabrication. The melting and deformation process of photoresist, which has an initial cylindrical shape of D500μm and h66μm, and the influence of various processing parameters on the height of formed microlens, is simulated with Marc. The optimum processing parameters combination is obtained by orthogonal experiment method and the influence of different processing parameters on the height of micro-spherical structure is studied with single factor experiment method. The results showed that the optimum processing parameters combination was 1°C /s for heating rate, 110°C for heating temperature and 45min for holding time; the significance of processing parameters on the micro-spherical structure’s height can be ordered in holding time＞heating rate＞heating temperature.

사출/압축 및 RHCM 기술이 7인치 도광판 마이크로 패턴 전사성에 미치는 영향에 대한 실험적 연구

Recently, according to the rapid development of display, many display applications, such as, cellular phone, navigation, monitor and LCD TV have been changed from CRT type to LCD type. BLU(Back Light Unit) is one of main parts in LCD unit and generally, it consists of a light source, a reflective sheet, a LGP(Light Guide Plate), a diffuser sheet, and two prism sheets. The most important component of BLU is a light guide plate, which diffuses the input light to the TFT-LCD module uniformly. The LGP is usually made by injection molding process, and it has numerous optical micro patterns on the surface. In the present study the micro-patterned stamper which has cylindrical shape was fabricated by using the UV- LiGA process. And the replication characteristics have been compared among three different kinds of injection molding process; general injection molding, injection/compression molding and RHCM(Rapid Heating and Cooling Molding). Average replication ratios of CIM and ICM were 19.1% and 64.6%, respectively. On the other hand, the average replication ratio of RHCM process showed the higher value of 98.4% among these. It show that maintaining the mold surface above Tg could increase the replication ratio of micro patterns substantially.

Mechanical testing of micro samples produced by a novel LIGA related process chain

In this paper, a new LIGA related process chain, named MSG process [German acronym for multicomponent injection molding (MS) and electroplating (G)] is presented as a promising production cycle for the replication of micro-testing samples. Structures manufactured by the LIGA process but also e.g. micro-milled structures can be reproduced. The focus of this work was two fold: First, the accuracy of different replicated structures was investigated, and second, the influence of the surface quality on mechanical performance of electrodeposited nanocrystalline materials was tested. Therefore, micro samples were manufactured by the use of different methods resulting in a variation of the measured surface quality. The surface of LIGA and micro-milled samples can be accurately replicated using this process. Furthermore, it can be shown which dimensional variations within the process occur and how this is influenced by the surface of the original structure. Finally, preliminary mechanical tests are presented which show the successful application of the MSG process.

Cu MEMS

Many types of MEMS devices have been fabricated through electro- or electroless-plating of metals, such as Ni. Typically, this is accomplished using LIGA or LIGA-like processes where the metal is plated into a mold fabricated through patterning a photoimageable polymeric layer. After plating, the polymeric mold is removed though chemical etching or ashing, leaving the freestanding metal structure. Through the advent of TSV technology, techniques have been developed for plating Cu into DRIE etched vias in Si wafers. This technology has matured to the point where arrays of defect free plated Cu vias can be realized across the surface of the Si wafer. TSVs generally require small diameter Cu posts extending through the depth of the Si wafer between a bond pad on each surface. However, this technology can be modified to filling other types of DRIE etched features with plated Cu to realize complex 3D Cu microstructures in and on the Si wafer. Furthermore, the Si die containing these Cu structures can be eutectically bonded to other surfaces, such as pads on printed circuit boards. Then the Cu structure can be released from the bulk Si though exposing it to XeF2 gas in a low vacuum. This gas chemically converts solid Si to gaseous SiF2, but is nonreactive with most other materials, including Cu, SiO2 and E-glass. After the bulk Si has been removed through this process, the free standing Cu microstructure remains, attached to the substrate to which it was eutectically bonded. A variety of MEMS devices can be realized with this technique, such as antennas, inductors, air gap capacitors, sensors and electrostatic actuators.

Study of Micropart Fabrication via 17-4 PH Stainless Nanopowder Injection Molding

Micropart fabrication via 17-4 PH stainless nanopowder injection molding was investigated. The nanopowder was mixed with a binder that was based on wax to produce a feedstock composed of 45% powder and binder (the powder load). Initially, the fit and proper test was done before the micropart was made by making some bars of green samples, which the properties were examined after the sintering process. The examination involved the mechanical properties such as the porosity, hardness, and some of metallurgical aspects, such as the second-phase formation and the final compound after the sintering. The results showed that utilizing 17-4 PH stainless nanopowder is promising for micropart fabrication since it can form a nearly full-density sintered sample with a low porosity and good toughness, and can provide a smooth surface finish. After this, the investigations followed with the injection of the feedstock into the PDMS micromold that was formed by the nickel pattern from the X-Ray LIGA process. The green samples successfully produced a high-aspect-ratio sample with a thickness of up to 1 mm and an aspect ratio of 15 in the microchannel part. Then the green samples were sintered at 1,300 degrees C for 2 h, since from the initial test, they showed optimum parameters with nearly full density, low porosity, and a high degree of hardness. The research shows the excellent results of the application of the 17-4 PH stainless nanopowder to micropart fabrication.

LIGA 공정을 이용한 초소형 스퍼기어 금형 제작 및 하이브리드 분말 압출성형

This paper was designed to fabricate the micro-spur gear by the LIGA and hybrid powder extrusion process. It is important to manufacture a micro-die with a high aspect ratio and determine appropriate extrusion conditions for a microforming. Ni has been used to fabricate micro-dies. LIGA process was capable to produce micro-extrusion dies with close tolerance, longer bearing length and adequate surface quality. Superplastic Al-78Zn powders have the great advantage in achieving deformation under low stresses and exhibiting good micro-formability with average strain rate raging from <TEX>$10^{-3}$</TEX> to <TEX>$10^{-2} s^{-1}$</TEX> and constant temperature ranging from 503 to 563K. Al-78Zn powders were compacted into a cylindrical shape (<TEX>$\Phi3\times$</TEX>h10mm) under compressive force of 10kN and, subsequently, the compacted powders were extruded by the hybrid powder extrusion process controlling of the temperature holing time for a improvement on formability of Al-22Zn powder. Micro-extrusion has succeeded in forming micro-gear shafts.

Electrodeposition Assisted X-ray Lithography: Single Step Approach

This paper reports on the development of a novel single-step approach for the formation of patterned metal deposits using X-ray lithography. It is based on simultaneously exposing the substrate to masked X-rays within an electric field, utilizing the effects of radiolysis and potentiostatic electrodeposition. In the LIGA process a thick photo resist is exposed to a masked off X-ray beam and the processed resist is used as an electroplating template. Minimization of as many processing steps as possible would make the process a much more efficient and cost effective technique. The traditional molding and electroplating steps were marked for deletion in our proof of concept experiment, because of the beam time and effort that is required in manufacturing a LIGA mold. The method that is presented forgoes 4 traditional steps involved in fabricating high aspect ratio structures. We provide a proof of principle for single step patterning with x-ray lithography.

Fabrication of high hardness Ni mold with electroless nickel–boron thin layer

The nickel electroforming method using a high-concentration nickel sulfamate bath is commonly used to fabricate micro metal molds in the LIGA process; however, this method does not produce micro me...

Fabrication of High Aspect Ratio SU-8 Structures Using UV Lithography and Megasonic-Enhanced Development

In recent years SU-8 resist attracted a high interest for fabrication of structures with high topography or high aspect ratio structures. The reasons for SU-8 popularity can be found in its unique properties - high chemical and mechanical stability, biological compatibility, optical transparency, high aspect ratio capability and low cost of fabrication. SU-8 can be used as an alternative molding material to LIGA process; in comparison to the standard LIGA process, relatively thick (1 mm range) SU-8 layers can be processed with UV lithography and do not require expensive X-ray light source. Nevertheless, processing of thick layers of SU-8 is not without challenges - coating high viscosity material, critical soft bakes in order of several hours, exposure requiring good contact, critical post exposure bake and extremely long development times. One aspect of the SU-8 processing, which is explored in this paper, is improvement of the development process time - by using single wafer megasonic-enhanced development. In this paper, the experimental results from manufacturing of SU-8 structures with aspect ratio 1:23 by using UV lithography and megasonic-enhanced development will be presented. Significant development time reduction from 240 min down to 10 min was achieved.

An investigation of microstructure and mechanical properties of UV-LIGA nickel thin films electroplated in different electrolytes

Different electrolytes are used to fabricate nickel structures in MEMS devices by the LIGA or UV-LIGA process to meet different requirements. In order to investigate the microstructure and mechanical properties of nickel thin films electroplated in different electrolytes, four sets of nickel specimens were fabricated in different electrolytes: sulfamate bath with both saccharine and butynediol added (type A-I); sulfamate bath with saccharine added (type A-II); watts bath with saccharine added (type B); and chloride bath with saccharine added (type C). The function of these additives was to obtain the stress-free nickel films. The specimens were measured in our uniaxial tensile test system; their surface morphology and fractography, microstructure and texture were studied by SEM, TEM and XRD, respectively. The results show that the four sets of specimens have different mechanical properties and microstructures. The tensile strength of type A-II, type C and type A-I specimens increases with decreasing grain size, which is in accordance with the Hall–Petch law. In contrast, type B specimens have the highest value of ultimate tensile strength and elongation, but with the largest grain size among all the specimens. The XRD results show that there is no preferred orientation in type B while others have a preferred orientation in (2 0 0) along the growth direction. This might be the reason for the difference between type B and other types.