Modified LIGA Process Research Articles

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.

Replications and analysis of microlens array fabricated by a modified LIGA process

AbstractMicrolens arrays were fabricated using a modified LIGA process based on the exposure of a PMMA (polymethylmethacrylate) sheet to deep X‐rays from the synchrotron of PAL (Pohang Accelerator Laboratory) and a subsequent thermal treatment. A successful physical modeling and analyses for microlens formation were reviewed according to the experimental procedure of the modified LIGA process based on our previous study. A metallic nickel mold insert for the replication of microlens arrays was successfully fabricated by the nickel electroforming process on the PMMA microlens arrays fabricated by the modified LIGA process. For the replication of microlens arrays having various diameters with different foci on the same surface, the hot embossing and the microinjection molding processes have been successfully utilized with the nickel mold insert. The scanning electron microscope and atomic force microscope measurements showed that replicated microlenses have good surface quality with the surface roughness of the order of 1 nm. The injection‐molded microlens arrays were also found to have very small birefringence. Finally, the focal lengths of the injection‐molded microlenses were successfully measured experimentally and also estimated theoretically. The replicated microlens arrays with the good surface quality could be applied to various optical applications that require microlens arrays having the different foci on the same surface. POLYM. ENG. SCI. 46:416–425, 2006. © 2006 Society of Plastics Engineers

변형 LIGA 공정을 통해 제작된 마이크로 렌즈 어레이의 모델링 및 성형

Microlens arrays were fabricated by a modified LIGA process composed of the exposure of a PMMA (Polymethylmethacrylate) sheet to deep x-rays and subsequent thermal treatment. A successful modeling and analyses for microlens formation were presented according to the experimental procedure. A nickel mold insert was fabricated by the nickel electroforming process on the PMMA microlens arrays fabricated by the modified LIGA process. For the replication of microlens arrays having various diameters with different foci on the same substrate, both hot embossing and microinjection molding processes have been successfully utilized with the fabricated mold insert. Replicated microlenses showed very good surface roughness with the order of 1 nm. The focal lengths of the injection molded microlenses were successfully estimated theoretically and also measured experimentally.

Fabrication of integrated nozzle plates for inkjet print head using microinjection process

This paper presents a novel concept to fabricate nozzle plates for an inkjet print head using a modified LIGA process. This fabrication technique can reduce the production cost of current nozzle plate. It comprises of a multi-exposure and a single development (MESD) processes, the extra-hard Ni–Co (nickel–cobalt) electroforming and thin-wall plastic microinjection molding. The template of the nozzle plates is patterned using a dry film and MESD processes. Later, the template is transferred into metal Ni–Co mold by electroplating techniques. In this study, the technique of extra-hard Ni–Co alloy electroplating process with Hardness of Vickers over (HV) 550 is developed. Then with this stiffness of Ni–Co mold, it can withstand a high injection speed without deformation. Thin-wall microinjection molding process with a short cycle time to fabricate nozzle plate can be finished. Liquid crystal polymer (LCP) is used for the thin-wall microinjection molding process. Besides, this study presents a novel design of nozzle plates. It integrates of ink channels, cavities and nozzles all in one nozzle plate to reduce the assembly process. The dimensions of nozzle plate are 4.16 mm in width and 7.3 mm in length, respectively. Total thickness of this integrated nozzle plate is 100 μm, whereas the thinnest thickness around a nozzle hole is 40 μm. Nozzle holes with 50 μm in diameter and 169 μm in pitch are suitable for 1200 dpi inkjet print head.

An efficient method to improve the proximity effect for electron beam optical discmastering

For the next-generation optical disc, electron beam mastering has been considered as ahigh-potential technique to fabricate a high-density optical disc. However, for electronbeam mastering, the proximity effect caused by electron backscattering is an importantproblem. In this study, the influence of the proximity effect on the linewidth (full width athalf magnitude, FWHM) and thickness of the residual resist is discussed. Some methodsare presented to solve the proximity effect for optical disc mastering, i.e., by raisingthe electron beam voltage and depositing thin film material with low atomicnumber on a silicon substrate. In the study, thin film materials such as Al, Ni,SiO2,and Si3N4 are deposited on a silicon wafer to explore the proximity effect. The preliminaryexperimental results show that raising the electron beam voltage and depositingSiO2 orSi3N4 thin film on a silicon substrate can efficiently solve this problem. Later, theresist with a nano-pattern is transferred into a metal Ni–Co (nickel–cobalt)mould by electroplating. The technique of the Ni–Co electroplating processwith hardness at least Vicker hardness (Hv) 650 and residual stress below1.5 kg mm−2 is developed. Then, with the Ni–Co mould, a modified LIGA process is applied to producea high-density optical disc. The Ni–Co mould serves as the master for the hot embossingprocess to replicate the nano-pattern onto the PMMA sheet. Since the feature size is downto the nanometre range, the study presents an innovative demoulding mechanism todemould the master from the PMMA sheet without damaging the nanometre patterns.

Replication of microlens arrays by injection molding

Injection molding could be used as a mass production technology for microlens arrays. It is of importance, and thus of our concern in the present study, to understand the injection molding processing condition effects on the replicability of microlens array profile. Extensive experiments were performed by varying processing conditions such as flow rate, packing pressure and packing time for three different polymeric materials (PS, PMMA and PC). The nickel mold insert of microlens arrays was made by electroplating a microstructure master fabricated by a modified LIGA process. Effects of processing conditions on the replicability were investigated with the help of the surface profile measurements. Experimental results showed that a packing pressure and a flow rate significantly affects a final surface profile of the injection molded product. Atomic force microscope measurement indicated that the averaged surface roughness value of injection molded microlens arrays is smaller than that of mold insert and is comparable with that of fine optical components in practical use.

Deterministic Micro Asperities on Bearings and Seals Using a Modified LIGA Process

Deterministic micro asperities show potential for enhancement of lubrication in conformal contacts as found in many bearing and seal designs. Several manufacturing methods have been proposed for deterministic micro asperities. Of these, laser texturing has emerged as the most viable option. This paper proposes the LIGA MEMs manufacturing method as an alternative. Using LIGA, surfaces with patterned micron sized surface features of arbitrary cross section (cylindrical, hex, triangular, etc.) can be fabricated from electroplated nickel, gel-cast silicon nitride, or plastic. The resulting asperities can be positive (protuberances) or negative (recesses) and can have heights (depths) from 1–1000 microns and be patterned over surface areas up to about 150mm×150mm. In this paper, the LIGA method is used to fabricate a sample thrust bearing surface with a hexagonal array of positive asperities. The resulting asperities are 550 μm in average diameter, 165 μm in edge-to-edge spacing and have heights of 3–100 μm. Surface metrology indicates submicron accuracy of form and 13 nm Ra roughness on the asperity tops (land). Tribology testing in a nonpressurized oil bath indicates full film conditions and shows a 14–22% reduction in friction coefficient for a thrust surface covered with the micro asperities. A model confirms the experimental trends and indicates the potential to further reduce the friction coefficient by about 60% through optimization of the asperity geometry and layout.

Physical modeling and analysis of microlens formation fabricated by a modified LIGA process

In this paper, we present the physical modeling and analysis method of microlens formation using deep x-ray lithography followed by the thermal treatment of a polymethylmethacrylate (PMMA) sheet. According to the modeling, x-ray irradiation causes a decrease of the molecular weight of PMMA, which in turn decreases the glass transition temperature and consequently causes a net volume increase during the thermal cycle, resulting in a swollen microlens. In this modeling, the free volume theory including volume relaxation phenomena was considered during the thermodynamically non-equilibrium cooling process. Based on this modeling, an analysis method has been proposed to predict the shape of the microlens with the surface tension effect taken into account. The analysis results are favorably compared with experimental data. The present analysis method enables us to predict the fabricated microlens shapes and the variation pattern of the maximum heights of the microlens, which depend on the conditions of the thermal treatment. The prediction model could eventually be used in determining the detailed thermal treatment conditions for a desired microlens shape fabricated by a modified LIGA process.

A simple method for microlens fabrication by the modified LIGA process

Microlenses and microlens arrays were fabricated using a novel fabrication technology based on the exposure of a resist (usually PMMA) to deep x-rays and subsequent thermal treatment. The fabrication technology is very simple and produces microlenses and microlens arrays with good surface roughness (less than 1 nm). The molecular weight and glass transition temperature of PMMA is reduced when it is irradiated with deep x-rays. The microlenses were produced through the effects of volume change, surface tension, and reflow during thermal treatment of irradiated PMMA. The geometry of the microlens was determined by parameters such as the x-ray dose applied to the PMMA, the diameter of the microlens, along with the heating temperature, heating time and cooling rate in the thermal treatment. Microlenses were produced with diameters ranging from 30 to 1500 μm. The modified LIGA process was used to construct not only hemispherical microlenses, but also structures that were rectangular-shaped, star-shaped, etc.

Tapered microvalves fabricated by off-axis X-ray exposures

A method for creating angled structures for use in microvalve devices applicable to control of liquid flow is presented. This technique utilizes a modified LIGA process with successive angled and rotated exposures into free standing acrylic sheets to form a tapered valve seat structure. These valve seats are integrated with bulk micromachined silicon diaphragms and tapered PMMA valve bosses to complete the microvalve. The long term goal of this research effort is to develop a normally-closed, low power, microfabricated valve for use in an implantable drug delivery system. This paper reports on the design and fabrication of microvalves using off-axis LIGA exposures. Flow testing and fluid handling characterization results are also presented.

Fabrication and characterization of microlenses realized by a modified LIGA process

We present a fabrication technology for the creation of high-aperture polymer microlenses and their optical characterization both as single lenses and mounted in a micro-objective. The microlenses are made by the LIGA process (x-ray lithography, electroforming and moulding techniques), which is modified by an additional x-ray irradiation step and a melting process. The spherical lenses are realized by melting of PMMA micro-cylinders, which have been flood exposed in the additional synchrotron irradiation step. For the cost-effective replication by hot embossing a moulding tool is made out of the primary PMMA lenses.

Fabrication of high-aspect-ratio microstructures on planar and nonplanar surfaces using a modified LIGA process

Large surface areas (tens of square centimeters to square meters) covered with high-aspect-ratio microstructures (HARMs) have potential applications in a wide range of fields including heat transfer, adaptive aerodynamics, acoustics, catalysts, seal and bearing design, and composite materials. HARMs are typically hundreds of micrometers in height, with widths ranging from a few micrometers to tens of micrometers, and they can be manufactured from a variety of materials such as metals, polymers, and ceramics. Three of the barriers to extensive use of large HARM-covered surfaces are cost, nonplanarity of typical surfaces, and adhesion of the microstructures to the surface. A starting point for inexpensive reproduction of large arrays of HARMs is the plastic molding step of the LIGA micromanufacturing process. In order to address the latter two problems, the standard LIGA process was modified/extended. Free-standing polymer sheets, perforated with a pattern of high-aspect-ratio throughholes, were clamped to conductive substrates. The sheets provide a template for electrodeposition of nickel microstructures onto the target surface. This process makes it economically feasible to electroform metal microstructures directly onto large planar and nonplanar metal surfaces (cylinders).

Fabrication of stepped microoptical benches for fibre and free space applications

A modified LIGA process is well suited to fabricate hybrid mounted low-cost microoptical benches for fibre and free-space applications. The benches enable optical components to be assembled with lithographic precision in the LIGA plane without any active alignment. Due to the different sizes of the optical components, stepped substrates for X-ray lithography are required in order to guarantee a vertical alignment on the optical axis. Silicon and high-precision mechanical micromachining have been combined with the LIGA process to fabricate these benches. In addition, two alignment strategies were tested.

Fabrication and testing of the planar magnetic micromotor

Planar micromotors have the potential for high-speed control applications. These systems require closed loop control and therefore involve not only the motor but also associated high-speed control circuitry. Electrostatic devices with submicron gaps and CMOS controllers offer one possible construction technique. Magnetic motors with bipolar microelectronics are an attractive alternative. Magnetic micromotors have been constructed by using deep X-ray lithography and metal plating with a modified LIGA process. The devices are designed as reluctance motors and consist of a stator that is rigidly attached to the substrate and a rotor that is fabricated as a fully released, free part. Both pieces are formed from nickel and are typically 100 mu m in thickness. The rotor is assembled onto the stator shaft with submicron tolerances. The structure is driven by an external rotating magnet or a fixed electromagnet. Rotational speeds of up to 8000 rpm have been obtained and maintained for several days.