LIGA Process Research Articles

Speculation of Mechanical Properties of Ni-W Micro-gears Fabricated by LIGA Process

Speculation of Mechanical Properties of Ni-W Micro-gears Fabricated by LIGA Process

Novel Electroformed Ni-Au Alloy for High Temperature Semiconductor Test Structures

The ongoing reduction in semiconductor and package pitch leads to challenges for engineering suitable miniaturized test and measurement contact vehicles. Traditional probe card approaches for miniaturization may reach fundamental limits for reduction in scale and pitch. Direct electroforming of conductive electrical contacts at pitch using LIGA technology can allow for direct registration to the test points, the simultaneous formation of all contacts and an opportunity to extend the roadmap for miniaturization. We report here on a new and novel nickel-gold alloy (one to three percent gold by weight) that can be produced by electroforming. This new material was computationally designed to optimize the thermodynamics of the two alloy components such that the alloy has a temperature stable nanocrystalline grain structure which produces highly desirable mechanical and physical properties. Specifically, the alloy is highly electrically and thermally conductive with a resistivity of 10 micro-ohm-cm, while also having a yield strength in excess of 1.5 GPa and an elastic modulus of 130 GPa. This combination of high strength and moderate elastic modulus creates and optimum for mechanical elastic stored energy in a formed contact beam. That is to say, contacts engineered from this material are flexible and strong enough to remain in the elastic regime and have sufficient normal force to maintain low and stable contact resistance, which boosts their long term service life. Further, the alloy shows a fixed cantilever beam bending fatigue limit of more than 1 GPa, further enhancing its viability for long term service. For applications requiring even more strength a 2 GPa yield strength version has also been produced, however this version leads to minor trade-offs in terms of ductility and electrical resistivity and cost. Since these contacts will be used at elevated temperature, the alloy has been designed to be nearly as electrically conductive as plated unalloyed nickel and thermodynamically stable under long term high temperature storage conditions of 400 C. Characterization of the microstructure shows that it is a supersaturated solid solution of Au in a Ni host matrix with evidence of preferential Au segregation to the Ni grain boundaries. Xray diffraction analysis confirms a nanocrystalline grain structure whose size varies as a function of the alloying content. By segregating the Au to the grain boundaries, we extend the electron mean free path and boost electrical conductivity. We also reduce the grain size of the metal leading to improvements in mechanical properties such as yield and fatigue. The electroplating solution is environmentally friendly with no significantly hazardous materials and is REACH compliant. We have shown chemical compatibility with a limited test set of standard photo-resists and masks used for the LIGA process. The plating process is reasonably fast and has shown the ability to plate layers as thick as 400 um without cracks, defects or voids. For some applications it may be desirable to add more plated layers to engineer the contact surface. Gold and other precious metals can be plated on top of the Ni-Au layer with good adhesion and lead to improvements in long term low level contact resistance.

Experiments of Sub-THz Wave Folded Waveguide Traveling-Wave Tube Amplifier

This study showed the possibility of using a sub-terahertz (THz) traveling-wave tube (TWT) via measuring the transmission characteristics and TWT performance of the circuit by applying X-ray LIGA, a micro-fabrication process, to the interaction circuit. The applied circuit type, an E-bend folded waveguide, is a simple structure most suitable for lithography. A total of three applied frequencies were used the W-band, G-band, and 850 GHz. Among the manufactured circuits, the W-band circuit was applied to the TWT, one of the vacuum electronics devices (VEDs). This was done to prove the manufacturing accuracy of the circuit by comparing the nonlinear characteristics of the circuit with the prediction results. Through such testing, the small signal gain was measured as 13 ± 2 dB under the conditions of 13.96-kV and 24.2-mA electron beam energy. The frequency bandwidth was extremely wide, about 9 GHz, and showed similar characteristics to the simulation predictions. The maximum output of the device was obtained up to 1 W or more at 87.12 GHz by slightly increasing the beam current. These characteristic achievements showed the suitability of the TWT for very small circuits fabricated using the X-ray LIGA process, further suggesting the applicability of other sub-THz bands.

Investigation of the influence of material and sheet thickness on a three-stage process chain for cold forming of micro gears with a module of 0.1 mm

In many industries, there is a trend towards miniaturization of technical components, such as drive systems, which enable the solution of high-precision positioning tasks because of their low volume and high dynamics. Geared micro parts are currently mainly produced by cutting and LIGA processes, but in mass production cold forming offers ecological and economic benefits. However, the cold forming of micro gears is restricted to a lower module limit of m = 0.2 mm due to high tool stresses, size effects and handling difficulties. To solve these challenges, in this contribution a three-stage forming process chain for the manufacturing micro gears (m = 0.1 mm) is investigated. In the first stage, a pin as wheel blank is extruded from sheet metal, which is geared subsequently by lateral extrusion. Finally, the micro gear is separated from sheet by shear cutting. The aim of this study is to demonstrate the applicability of the process chain for the materials Cu-OFE and CuZn30. In addition, the influence of the sheet thickness, which has a major impact on the forming process and the material efficiency, is analyzed. The geometrical and mechanical component properties as well as the machine data are evaluated to assess the variables.

Investigation on the mechanical interface stability of curved high aspect ratio x-ray gratings made by deep x-ray lithography

We describe the rationale for selecting graphite as a substrate material suitable for manufacturing curved high-aspect ratio metallic x-ray gratings and experimentally validate that its properties satisfy requirements relevant for clinical phase-contrast and dark-field x-ray imaging. Selection criteria applied to two candidate materials graphite and polyimide were compliance to bending, mechanical tenacity of the attachment of the lamellar grating structure to the substrate, the substrate material’s x-ray robustness, and the compatibility with the x-ray LIGA process used to manufacture the grating structures. In contrast to other standard materials such as silicon wafers with titanium layer, graphite wafers could be bent to smaller radii and are natively electrically conductive. While polyimide wafers allowed for even smaller bending radii, we found their high risk of grating structure detachment to be a strong detractor. Minimum achievable bending radii were 55 and 70 mm for pure graphite wafers and graphite wafers with mounted grating structure, respectively. Electron microscopy of graphite surface and cross-sections revealed a fine porous structure conducting to a very stable metal–wafer interface. Grating structures with heights of more than 200 μm were bonded to graphite wafers and their integrity confirmed in flat as well as in bent state using microfocus x-ray imaging. We conclude that graphite is a very well-suited substrate material for manufacturing curved x-ray gratings.

X-Ray Lithography for Nanofabrication: Is There a Future?

X-ray lithography has been first proposed almost 50 years ago, and the related LIGA process around 25 years ago. It is therefore a good time to make an analysis of the technique, with its pros and cons. In this perspective article, we describe X-ray lithography’s latest advancements. First, we report the improvement in the fabrication of the high aspect ratio and high-resolution micro/nanostructures. Then, we present the radiation-assisted synthesis and processing of novel materials for the next generation of functional devices. We finally draw our conclusion on the future prospects of the technique.

The property research of the transmission sinusoidal grating with fabrication of LIGA process

The transmission sinusoidal grating (TSG) shows a different diffraction pattern with conventional black-white transmission grating (BWTG), which can focus the diffraction energy on a higher order and has been widely used in the spectrum instruments. According to the description of the scalar diffraction theory, the numerical simulation for diffraction properties of the TSG has been studied for years. However, the fabrication of the TSG is difficult and seldom reported. In this article, a TSG is designed and an approach of the X-ray LIGA process is used to fabricate the TSG and the device can obtain the brightest diffraction peaks at the ±1 order with the zero-order diffraction peak unobvious on the background for a certain wavelength of the incident light.

Development of oriented metallic patterns using LIGA process for focused anti-scatter grid application

X-ray projection imaging is troubled by beam scattering. Anti-scatter grids selectively remove such scattered X-rays and are classified as focused or unfocused, depending on the lamellar angle. Focused grids afford better transmittance than unfocused grids, but it is difficult to fabricate inclined microstructures with different angles. Here, we present a novel method for the fabrication of anti-scatter grids on bendable substrates. We utilized a synchrotron X-ray lithography process to obtain unique microstructures and subsequent copper-based electroforming process for metallization. Finally, lamellar micro-patterns were successfully realized with different angles according to each location of the focused anti-scatter grid application.

Fabrication of micro piezoelectric rod array using metallic mold system for mass production

LIGA process was carried out to fabricate the copper mold insert for a micro piezoelectric array. X-ray mask was fabricated using UV lithography and PMMA which is the X-ray photoresist was bonded to the titanium substrate done lapping. The bonded PMMA was exposed to X-ray exposure with the total amount of 4.0 kJ/cm3. Copper was then electroplated above the PMMA. After the post-processing, the copper mold insert with the pattern size 400 μm, the height 924 μm, was fabricated. To replicate the patterned micro rod array, PIM process was used. The entire optimized PIM process corresponding to the previous research was applied. The fabricated copper mold insert was used in injection molding process. The ejection pins were also fabricated using the same LIGA process for metal mold and used to extract the injection-molded specimens. After injection molding, two-step debinding and sintering processes were conducted and finally, micro piezoelectric rod array with a pattern size of 322 μm was fabricated without any defects such as collapse and break. The dimension accuracy was ensured through the SEM image analysis and dilatometry results. The physical and piezoelectric properties of sintered part were measured to confirm the part performance made by PIM process. Results indicated the possibility of using the metal mold for mass production system to produce the micro piezoelectric rod array.

Honeycomb Support Structures to Stabilize PMMA Mold during LIGA Processing of High Aspect Ratio Gratings for XPCI Systems

X-ray phase contrast imaging (XPCI) allows for medical and non-destructive testing of soft tissue [1]. High aspect ratio (HAR) gratings (100:1) are difficult to achieve and are a key element of XPCI gold gratings PMMA lithography, molding, and electroforming (LIGA) process [2]. Development of uniform, continuous HAR gold gratings over a large area is difficult as they easily tilt and delaminate. One technique for stabilizing the mold during development is to incorporate relief structures into mold, providing more surface area at the bottom, which results in segmented gratings [2]. In the current work, honeycomb supports are used to stabilize the PMMA mold during development and plating to realize continuous gratings centimeters in lateral length. Our goal is to fabricate micron size HAR gratings ~10 cm in lateral length that are ideal for XPCI applications enabling larger field of view for the XPCI imaging. Honeycomb structures were patterned on a 100 mm substrate. A sheet of PMMA is then bonded and fly cut to desired height. An x-ray exposure then patterns the PMMA mold prior to the fabrication of the top honeycomb structure. The gratings are then developed and electroplated having structural support on both sides. The use of bottom and top honeycomb arrays provide unique mechanical support to the fragile PMMA mold during development and electroplating of the HAR grating. Momose, A. and W. Yashiro, X-ray Phase Measurements with Talbot Interferometry and Its Applications. International Conference on Advanced Phase Measurement Methods in Optics an Imaging, 2010. 1236: p. 195-199. Mohr, J., et al., High Aspect Ratio Gratings for X-Ray Phase Contrast Imaging. International Workshop on X-Ray and Neutron Phase Imaging with Gratings, 2012. 1466: p. 41-50. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Figure 1: Schematic of mold stabilization design (left), honeycomb pattern (right) Figure 1

On the application of micro hot embossing for mass fabrication of template-based dielectric resonator antenna arrays

In this article, application of micro-replication of polymers in the field of next generation mm-wave antennas is presented for the first time. The concept is applied to recently proposed template-based dielectric resonator antenna arrays fabricated by deep X-ray lithography. In this work, the array templates are replicated by hot embossing of polymethyl methacrylate (PMMA) layers using a nickel mold insert fabricated from the direct X-ray exposed PMMA master (LIGA process). Transfer quality of the master pattern is examined through measurements by optical microscope. Under controlled conditions, a number of templates are replicated demonstrating repeatable quality and consistent performance among the devices. A four element dielectric resonator antenna array is fabricated and measured for the impedance bandwidth response. This proposed methodology has the potential to enable low cost and mass utilization of dielectric resonator antennas and arrays in future millimeter-wave applications.

Mechanical Properties of Ni-Pd Thin Films Fabricated by LIGA Process

Palmtop tensile and fatigue testing machines were developed in-house for small and thin specimens that were fabricated by the LIGA process. The dimensions of the specimens were selected in accordance with the international standard IEC62047-3, and were 10 micrometers in thickness, 20 micrometers in width, and 500 micrometers in parallel length. Since the loading mechanism and methods of measuring the load and displacement were specially designed, the stiffness of the specimen is quite low compared to that of the conventional large-size specimens. The mechanical properties of electroformed Ni2Pd8 and Ni4Pd6 alloys were studied with these developed testing machines and excellent values of tensile and fatigue strengths of 2.7 GPa and 1.8 GPa, respectively were obtained in the case of the Ni4Pd6 alloy.

Ball driven type MEMS SAD for artillery fuse

The SAD (safety and arming device) is an indispensable fuse component that ensures safe and reliable performance during the use of ammunition. Because the application of electronic devices for smart munitions is increasing, miniaturization of the SAD has become one of the key issues for next-generation artillery fuses. Based on MEMS technology, various types of miniaturized SADs have been proposed and fabricated. However, none of them have been reported to have been used in actual munitions due to their lack of high impact endurance and complicated explosive train arrangements. In this research, a new MEMS SAD using a ball driven mechanism, is successfully demonstrated based on a UV LIGA (lithography, electroplating and molding) process. Unlike other MEMS SADs, both high impact endurance and simple structure were achieved by using a ball driven mechanism. The simple structural design also simplified the fabrication process and increased the processing yield. The ball driven type MEMS SAD performed successfully under the desired safe and arming conditions of a spin test and showed fine agreement with the FEM simulation result, conducted prior to its fabrication. A field test was also performed with a grenade launcher to evaluate the SAD performance in the firing environment. All 30 of the grenade samples equipped with the proposed MEMS SAD operated successfully under the high-G setback condition.

방사광 LIGA 공정을 이용한 플라스틱 성형용 마이크로 금형 제작

ABSTRACT In the present study, copper micromolds with a microhole array were precisely manufactured by a synchrotron LIGA process. Like in the traditional LIGA process, a deep X-ray lithography based on a synchrotron radiation was employed as the first manufacturing step. Due to the excellent optical performance of the synchrotron X-ray used, cylindrical micropillar arrays with high aspect ratio could be efficiently obtained. The fabricated microfeatures were then used as a master of the subsequent copper electroforming process, thereby resulting in copper micromolds with a microhole array. Thermoplastic hot embossing experiments with the copper micromolds were carried out for imprinting cylindrical microfeatures onto a polystyrene sheet. Through the hot embossing, the effect of embossing temperature and usefulness of the present manufacturing method could be verified.KeyWords:LIGA Process(LIGA 공정), Micromold(마이크로 금형), Synchrotron Radiation(방사광), Electroforming(전주도금), Hot Embossing(핫 엠보싱)

DESIGN AND PROCESS PLANNING OF MICRO GEAR MANUFACTURING

Today a large no of microstructures are already employed as separate components or as constituents of large modules in a broad spectrum of production in medical technology, optics, biotech, mechatronics, fluidics, micro forming and tool constitution. Current research activities are directed towards the downscaling of manufacturing procedures or the formation of complex process chains for the manufacturing of the micro workpieces in this paper we are going to design a micro gears and a suitable manufacturing process for making of micro gears. We are also going to study in detail various processing parameters and control technology of manufacturing micro gear by LIGA process. We are also going to design the micro gears or gear train using CATIA software.

Nanocrystalline Stainless Steel Electrodeposition

FeCrNi stainless steel is widely used for industrial applications because of its excellent corrosion resistance and preferable mechanical properties. Electrodeposition of such FeCrNi alloy has attracted attention for years because of its potential application for fabrication of microcomponents by means of LIGA process. However, the technique has not been established so far mainly because of the complexity of bath chemistry, especially related to the instability of trivalent Cr ions as well as the difficulty to control the composition due to anomalous deposition. In order to overcome these technical issues, we have investigated the electrodeposition of FeCrNi stainless steel aiming at achieving the composition and properties close to those of SUS304 (18%Cr and 8%Ni), which is known to exhibit a superior corrosion resistance. In order to stabilize trivalent Cr ions, we have studied the effects of complexing agents for Cr ions on the quality of FeCrNi electrodeposits, and developed the chloride-based FeCrNi plating bath containing glycine as a complexing agent[1]. Further optimization of plating parameters enables us to fill FeCrNi electrodeposit in a UV-LIGA mold with depth of 300µm. Nevertheless, the deposits are brittle and highly stressed, which should be further optimized for practical applications. The crystallographic analysis by X-ray diffraction (XRD) and transmission electron microscope (TEM) showed that the deposit is predominantly amorphous, which may explain brittleness of the deposits. In terms of mechanics, nanocrystalline FeCrNi (nc-FeCrNi) is preferred because of its excellent mechanical strength.We report here the successful preparation of nanocrystalline FeCrNi electrodeposits by employing a double-compartment electrochemical cell where the anode electrode is separated from the cathode electrode compartment (hereafter referred to as the “double-cells”) instead of a conventional single compartment cell (single-cell), which yields amorphous FeCrNi (a-FeCrNi). Firstly, the microstructure and recrystallization behavior of FeCrNi electrodeposits from the single-cell and the double-cells are characterized and compared. The grazing-incidence XRD (GI-XRD) revealed that as-deposited FeCrNi films prepared in the double-cell is nanocrystalline consisting of the metastable ferrite phase. The analysis of TEM shows that the size of grain is about 7 nm. Further investigation of the microstructure was performed by the in-situ GI- XRD during thermal treatment up to 800°C. The experiments demonstrate that nc-FeCrNi recrystallizes at 550°C along with its phase transformation from the ferrite to the austenite. On the other hand, a-FeCrNi was shown to undergo recrystallization to form primarily the ferrite phase, which is subsequently transformed to the austenite phase. Chemical analysis of deposits revealed that the carbon content of nc-FeCrNi is significantly lower than that of a-FeCrNi, suggesting that the carbon inclusion is responsible for the formation of amorphous phase. The major source of carbon is attributed to reaction compounds at the anode electrode. In order to identify the anodic reactions in our FeCrNi electroplating bath, a series of cyclic voltammetry was carried out. The electrochemical study shows the evidence of the anodic oxidation of glycine as well as the oxidation of ferrous and trivalent chromium ions to ferric and hexavalent chromium ions on a Pt anode. Therefore, the oxidation products of glycine or the compounds generated by their further reactions are thought to be responsible for the inclusion of carbon. In this presentation, we will also discuss the corrosion properties of nc- and a-FeCrNi deposits along with the micromechanical properties of these FeCrNi electrodeposits.1. Philippe, L., C. Heiss, and J. Michler, Electroplating of stainless steel. Chemistry of Materials, 2008. 20(10): p. 3377-3384.

Fabrication of flexible microlens array through vapor-induced room temperature dewetting on plasma treated Parylene-C.

Microlens arrays have been widely used as an essential component in many optical systems. The conventional fabrication techniques, such as ink-jet printing, photoresist thermal reflow, grayscale photolithography, and LIGA process have some drawbacks in aspects of process complexity and high fabrication cost. This paper presents a low-temperature, rapid and cost-effective method to successfully generate SU-8 microlens arrays on biocompatible Parylene-C surfaces. Vapor-induced SU-8 dewetting has been achieved on chemically modified Parylene-C surfaces that were selectively pre-treated with O(2) or SF(6) plasma. By this technique, the SU-8 droplets are self-organized onto desired positions to form a microlens array. This method is also suitable for production of microlens array on curved surfaces. The progress of dewetting has been studied by both experiment and simulation.

Fabrication of micro pore optics with smooth sidewall using X-ray lithography

Micro pore optics (MPO) as an X-ray imaging system is perfectly suited for the applications in space telescopes due to its light-weight and high-resolution properties. We report the fabrication of MPO samples by LIGA process focusing on its sidewall surface used as mirrors for X-ray reflection. An intermediate mask is fabricated and used to obtain the working mask in order to avoid the UV exposure to a very thick photo resist layer. Around 400 μm-thick nickel MPO plate is obtained with the aspect ratio of the square pore and sidewall of 8 and 32, respectively. The root mean square roughness of the sidewall surface is below 10 nm in a 5 × 5 μm2 region. Some striations are found on the sidewall surface originating from the jagged edge of the chromium coating on the UV mask.

“LIGA2.X” process for mass production of single polymeric LIGA micro parts

The new modification of the LIGA process "LIGA2.X" is a promising process chain to replicate high accuracy single polymeric LIGA micro parts in large scale to feasible costs. The advantages of this new approach compared to the standard LIGA process are the reduction of fabrication costs, the elimination of any rework after replication, industry suitable mold technology for injection molding using a new developed demolding concept, optimized injection molding parameters for every single cavity, shot volumes of the parts below 0.5 mm3 and a new freedom in the arrangement of structure cavities in multicavity molds. Aspects covered in this paper are the introduction of the complete process chain of the new "LIGA2.X" process and the used mold concept for the micro-man 50 micro injection molding machine. Furthermore replicated single polymeric LIGA microparts fabricated using the new process chain are shown. To conclude, potential process improvements and future work will be outlined.

Effects of adding injection–compression to rapid heat cycle molding on the structure of a light guide plate

This study investigates the effects of adding injection–compression to rapid heat cycle molding (RHCM) (rapid heat cycle injection–compression molding (RICM)) on the physical quality and optical anisotropy of a molded light guide plate (LGP). Transcription ratio of microstructure, uniformity of part thickness and birefringence were experimentally evaluated on a 7 inch LGP of nominal thickness of 1.12 mm (including a microstructure array of 30 µm diameter and 14 µm height). The designed mold was equipped with rapid heating and compressing facilities and a microstructured nickel stamper was fabricated by UV LIGA process. In addition, to investigate the efficacy of RICM, experiments involving conventional injection molding (CIM), ICM, and RHCM were conducted in parallel with RICM using the same mold. RHCM and RICM yielded excellent transcription ratios for the microstructure, while CIM and RICM provided high thickness uniformity and low birefringence. Thus, RICM obtains high transcription ratio of microstructure, uniform thickness and low birefringence.