New development strategies for high aspect ratio microstructures

 In the first step of the LIGA process a resist layer, typically PMMA (polymethylmethacrylate), is pattered by deep X-ray lithography. The exposed parts are subsequently dissolved by an organic developer. The quality and the achievable height of the microstructure is decisively determined by the development process. In order to increase the aspect ratio and maintain the quality of the microstructures the parameters influencing the development process were investigated. In the case of dip development and ultrasound development a strong dependency of the development rate on the temperature, dose value and depth of deposition has been noticed. The development rate increases with increasing dose value and temperature and decreases with increasing depth of deposition. In case of dip development the development course can be described by a phenomenological equation which considers the three mentioned parameters. In the case of ultrasound further parameters have to be taken into account: the geometry and the dimensions of the strucutres. Received: 25 August 1997/Accepted: 3 September 1997     

The micromilling process for high aspect ratio microstructures

 High aspect ratio microstructures are currently created by several processes which include lithography (X-ray, deep ultraviolet, etc.) and mechanical machining (diamond machining, microdrilling, etc.). The lithographic processes require more extensive processing equipment such as an energy source, mask/mask holder/mask aligner, photoresist and substrate, and chemical development capacity. In addition, these processes are serial in nature and each adds to the tolerances of the finished structure. The current mechanical processes provide for the direct removal of the substrate material in a single step but are more limited in the geometric patterns which can be created. In conventional machining, the process which provides the most versatility in geometric patterns is milling. The micromilling process has two basic components. The first is the fabrication of small milling cutters with very sharp cutting edges. The second is the actual removal of the workpiece material with a very precise and repeatable machine tool. Several basic cutter designs have been fabricated using focused ion beam micromachining and are undergoing testing. The cutter diameters are nominally 100 micrometers and 22 micrometers. Results have been obtained which show that this process can be very effective for the rapid fabrication of molds and mask structures. Received: 30 October 1995 / Accepted: 4 March 1996     

The micromilling process for high aspect ratio microstructures

High aspect ratio microstructures are currently created by several processes which include lithography (X-ray, deep ultraviolet, etc.) and mechanical machining (diamond machining, microdrilling, etc.) The lithographic processes require more extensive processing equipment such as an energy source, mask/mask holder/mask aligner, photoresist and substrate, and chemical development capacity. In addition, these processes are serial in nature and each adds to the tolerances of the finished structure. The current mechanical processes provide for the direct removal of the substrate material in a single step but are more limited in the geometric patterns which can be created. In conventional machining, the process which provides the most versatility in geometric patterns is milling. The micromilling process has two basic components. The first is the fabrication of small milling cutters with very sharp cutting edges. The second is the actual removal of the workpiece material with a very precise and repeatable machine tool. Several basic cutter designs have been fabricated using focused ion beam micromachining and are undergoing testing. The cutter diameters are nominally 100 micrometers and 22 micrometers. Results have been obtained which show that this process can be very effective for the rapid fabrication of molds and mask structures.     

Manufacturing of microstructures with high aspect ratio by micromachining

The development of micromachining plays an important role in miniaturization of microsystems. Micromachining is a very flexible and compared to EDM or ECM a very fast machining process with a high material removal rate. A wide range of materials like polymers, metals and alloys as well as some sorts of ceramics can be machined. Also 3D-structures can be easily manufactured. Additionally, big advances have been made concerning the realization of high aspect ratios for small diameter end mills. Whereas end mills below 100 μm diameter are limited to an aspect ratio of 1.5, end mills of 100 μm diameter are available up to an aspect ration of ten now. A few years ago, end mills in this diameter range were uncoated. Nowadays, there was a big progress in the coating technology so that these end mills can be coated with layers as thin as 0.5 μm.     

Ultraviolet embossing for patterning high aspect ratio polymeric microstructures

In ultraviolet (UV) embossing, a substrate with a coating of liquid or semi-solid UV curable resin mix is pressed against a patterned embossing mold. The resin mix is irradiated with UV before demolding of the hardened microstructures. UV embossing can be done at room temperature and low pressure. However, demolding of UV embossed high aspect ratio microstructures from a metallic mold is typically difficult since there is no differential thermal contraction between the mold and the embossing. Several factors have been identified to influence demolding of UV embossed microstructures: (1) Roughness of mold, (2) Taper angle of microstructures of mold, (3) Chemical interaction between mold and embossing, (4) Tensile and crosslinking shrinkage properties of the irradiated resin and (5) Uniformity of crosslinking process through the thickness of the molded microstructures. By controlling these five parameters, a microarray with an aspect ratio of 5 was demonstrated using a Formulation containing epoxy acrylate, Irgacure® 651, silicone acrylate and other acrylates. The embossed microstructures replicated the features of the mold very well. It was also shown that by controlling the amount of irradiation, the tensile modulus of the UV formulation increased whilst the elongation decreased. An optimum irradiation is needed for clean demolding from the mold without microcracking.This research was supported by a Strategic Development Scheme fund (SDS 15/2001) from the Nanyang Technological University. The authors also acknowledge the kind contributions of chemicals by UCB Chemicals, Sartomer, Henkel (Singapore), Dupont (Singapore) and Ciba Chemicals and a microstructured mold by Dr R. C. Liang of SiPix Imaging (CA, USA). The second author acknowledges the financial support of Nanyang Technological University through a Research Scholarship.     

Micro-electroforming high aspect ratio microstructures under magnetic field

Microsystem Technologies - High-aspect-ratio (HAR) micro-electroforming is a significantly challengeable implementation to manufacture metallic microstructures and microparts mainly due to mass...     

Measurement of side walls of high aspect ratio microstructures

In this paper, the new method of measurements is presented for the technological control of the lithographic fabrication of high aspect ratio microstructures with complex side-wall profiles. The sampling procedure includes pouring polymer glue on a substrate with the microstructures and its hardening, sectioning the polymer matrix in the depth with planes parallel to the substrate and analysis of digital optical microscope images of the planes. The measured deviations the micro structure profiles in comparison with the designed geometry are used for the further optimization of the technological design of planar X-ray refractive lenses.     

Production of metallic high aspect ratio microstructures by microcasting

Microcasting is a competitive process for the production of metallic microparts. The remarkable suitability of this technique especially for extreme aspect ratio microsamples made of metal alloys will be shown. Based on the well known process of investment casting it is possible to produce microstructures by a gold basis alloy called Stabilor G^® with an aspect ratio of up to 90. Basic investigations on the ability of form filling and flow length show that microcasting can be improved by a increasing of the molding pressure to about 25 bar. Basic results will be presented as well as a testing structure for estimating the quality of the casting process in regard of its suitability for microcasting of high aspect ratio parts.     

国外高深宽比微细结构制造技术的发展

介绍了国外近年来高深宽比微细结构制造技术的最新发展,其中包括LIGA、准LIGA、深层反应离子刻蚀等主要高深宽比微细结构技术的具体研究现状和研究进展,还介绍了电化学沉积加工、质子束光刻、准LIGA与Si等离子刻蚀复合加工、局部电化学沉积、立体光固化微加工、微细电解加工等一些新发展的高深宽比微细结构技术。     

Investigations on possibilities of inline inspection of high aspect ratio microstructures

LIGA is the basic idea of promising developments for the manufacturing of microelectromechanical system parts containing high aspect ratio microstructures. Aim of the work is a brief discussion of the starting-points for inline process inspection within a direct LIGA technology using deep X-ray lithography for the production of micromechanical gear wheels with critical dimensions of ∼35 μm width at ∼1 mm height as well as to show methodic and technical measuring possibilities. Firstly, results of the determination of residual solvent content distribution within ultra-thick SU-8 films are shown obtained from refracted near field optical measurements. Furthermore, the capability of X-ray computer tomographic imaging is discussed and measurements for the determination of the three-dimensional shape of high aspect ratio microstructures are practically demonstrated with microscopic and interferometric optical methods. Finally, first results demonstrate the potential of the optical coherence tomography for several further important measurement tasks, among others, e.g. for the imaging of the distribution of mechanical stress at the resist–substrate interface. The results show that much information which is essential in the LIGA process can be achieved with recently available measurement methods. However, further development of non-destructive measurement techniques would be desirable for an effective inline process control of mass production of micromechanical parts. This work is a summary of the poster “Residual Solvent Content Distribution in Ultra-Thick SU-8 Films and Its Influence on the Imaging Quality” and of the presentation “Possibilities of Inline Process Inspection of High Aspect Ratio LIGA Micro Structures” to the High Aspect Ratio Micro Structure Technology workshop HARMST 2005 held in Gyeongju (Republic of Korea), June 10–13, 2005.     

A new micromachined sensor system for tactile measurements of high aspect ratio microstructures

A new tactile sensor with piezoresistive read-out is presented. The sensor is designed for measurements of high aspect ratio structures with a resolution of some 10 nm and a measuring range of hundreds of micrometer. Possible applications of the sensor are suggested. The silicon micromachining fabrication process is shown in detail next to the finite element simulations we performed. First measurements and a calibration process are described and the results are shown. The implementation into a measuring system is indicated.     

Development of separated micromold system for an efficient replication of high aspect ratio microstructures

In the present study, a separated micromold system (SMS) is newly proposed and developed for an efficient replication of high aspect ratio microstructures. The present SMS basically consists of micromold modules, each having a split structure of the complete microstructure to be replicated. So fabricated micromold modules are assembled for a replication of the microstructures and subsequently separated in a demolding stage. In this manner, serious problems commonly encountered in a conventional fabrication process for high aspect ratio structures can be effectively overcome. For a precise fabrication of the micromold modules, a deep X-ray lithography and nickel electroforming processes were carried out, resulting in nickel SMS including various half circular microstructures. By utilizing the obtained SMS modules, high aspect ratio micro-scale cilium structure and its array were successfully replicated by a hot embossing process.     

Arrays of high aspect ratio magnetic microstructures for large trapping throughput in lab-on-chip systems

Here we report a novel technology to obtain arrays of highly efficient magnetic micro-traps that relies on simple fabrication process. Developed micro-traps consist in chains of iron particles diluted in polydimethylsiloxane (PDMS). We analyzed the microstructure of the composite membrane by X-ray tomography. It revealed the predominance of aligned chain-like agglomerates. Largest traps, with diameter ranging from 4 to 11 µm, are found to be the most efficient. The trap arrays were characterized by a density of 1300 magnetic micro-traps/mm², an average nearest neighbor distance of 21 µm. Implemented in a microfluidic channel operating at a relatively high flow rate of 0.97 µL/s—a flow velocity of 8.3 mm/s—we measured a trapping efficiency of more than 99.7%, with a throughput of up to 7100 trapped beads/min. These performances are competitive with other approaches like hydrodynamic trapping. The strengths of this technology are its simple fabrication and easy handling.     

Characterisation of high aspect ratio non-conductive ceramic microstructures made by spark erosion

The spark erosion process is widely used for micro structuring. Its possibility to structure materials independent of their material properties like high hardness or melting temperature enables to address a large material diversity. However the process requires a minimal electrical conductivity of 0.1 Scm⁻¹. Nevertheless recent research has shown that the usage of an assisting electrode makes a processing of non-conductive materials possible. Thus even ceramics like Al₂O₃ or ZrO₂ can be processed. These materials are becoming more and more interesting for industrial application and the field of miniaturisation due to their outstanding material characteristics like high hardness, bending strength, melting temperature and chemical inertness. In this study a new lacquer based assisting electrode is used to erode bars in zirconia samples. For this purpose a modular tool concept is established. Bars with aspect ratios of more than 80 are generated. The achieved bar heights are 1.5 mm and the smallest bar width is 8 μm. Furthermore a characterisation of the sidewall angles showed mean values between 0.4° and 2.2° depending on the bar height and width.     

Experimental study of NiFe and CoFe throughmask electrodeposition of high aspect ratio features

The electrodeposition of NiFe and CoFe onto rotating disk electrodes and into high aspect ratio features in 300 μm-thick photoresist was studied using electrolytes previously considered in the literature. The through-thickness uniformity of the plated features was characterized with reference to the rotating disk electrode results. Current efficiencies for various deposition conditions were also characterized. The composition of NiFe was more sensitive to mixing than CoFe on a rotating disk electrode. This sensitivity was reflected in the higher nonuniformity in general of the plated high aspect ratio NiFe features; a feature with an aspect ratio of three showed decreasing Fe content towards the feature bottom, with the Fe content at the bottom less than half that at the top. CoFe showed only slightly decreasing Fe content for the same aspect ratios. The composition of both alloys near the feature opening (where more mixing occurs) was consistent with that of deposits obtained from rotating disk electrodes at high rotation rates.     

Investigations of SU-8 removal from metallic high aspect ratio microstructures with a novel plasma technique

First promising investigations of SU-8 removal experiments with a novel plasma etching technique are presented. The basic idea of this technique is to separate the highly effective generation of chemical radicals (e.g. oxygen radicals) using a traveling wave reactor (TWR) microwave source with water cooled plasma zone from the chemical reaction with the resist polymer. The etching tool operates in a remote and downstream mode with very high radical density allowing precise thermal management of the substrates on the chuck giving controlled process conditions without deviation in temperature, and generally preventing ion bombardment, at least resulting in gentle processing without jeopardizing the integrity of the metal structures. Very good removal of SU-8 with very few residues and very high etching rates up to 10 μm per minute are observed in first experiments which are offering chances to get even more than 20 μm per minute. The etching process is isotropic, and the rate stays stable during the whole removing process even for very thick films of 1 mm and more. First application examples of SU-8 removal are demonstrating the great potential of the presented microwave plasma based technique not only for the cleaning of metallic microparts but also for other more sensitive materials which is demonstrated by SU-8 removal from graphite X-ray mask substrates.     

Challenges with high aspect ratio nanoimprint

When nanoimprint is not used for lithography purposes (NIL), but for the direct patterning of polymeric layers, high aspect ratio patterns may be of interest for a number of applications. The definition of such patterns in a nanoimprint process deals with two aspects, a successful filling of the high aspect ratio cavities of the stamp used, followed by a successful separation of the high aspect ratio structures defined in the polymeric layer on the substrate, from the stamp. These two aspects are addressed by shedding light to the impact of capillary effects during the filling of high aspect ratio cavities, and to the deformation processes involved in the separation of the stamp from the polymeric structures, where adhesional energies have to be overcome without cohesional failure. Both aspects are discussed in terms of the geometries involved, the stamp geometries as well as the polymeric layer thickness, and correlations with thermally-assisted (T-NIL) and UV-assisted (UV-NIL) processing are deduced. The aspects discussed are typical of a nanoimprint situation with thin polymeric layers on hard substrates.     

Electrodeposition of copper into high aspect ratio PCB micro-via using megasonic agitation

This paper presents the use of micro-particle imaging velocimetry (micro-PIV) to analyse fluid flow and hence ion replenishment in PCB micro-via during the electroplating process. The cross section of a PCB via is fabricated in PMMA to allow optical access to the sample. Fluid flow within two 1:1 aspect ratio blind micro-vias, one with straight side walls and the other with tapered side walls were compared. Flow is also analysed in a 1:1 aspect ratio through via. Flow rates measured using micro-PIV are used to validate simulated flow models. The results show that there are increased flow rates within the blind via with tapered side walls. This goes some way to explaining the improved electroplating results obtained in industry when tapered vias are used. Initial experimental results using megasonic streaming to remove bubbles from blind micro-via and promote ion transportation within high aspect ratio PCB micro-via to enhance electrodeposition are also reported.     

A comprehensive study of ion track enabled high aspect ratio microstructures in flexible circuit boards

A process to form deep, vertical and high aspect ratio microstructures of solid as well as porous nature is presented. The process is capable of producing regions with perpendicular sub-micron metal wire connections, with a regulated effective metal density at numerous, arbitrarily specified locations. The structures are created in a two-metallic-layer polyimide laminate, i.e. a flexible printed circuit board. The high aspect ratio of the process is indebted to ion track technology. The laminate is irradiated with heavy ions creating a vertical damage anisotropy (individual ion tracks) in the polymer layer. Apertures in the front metallic layer define the geometry and the positions of the vertically projected structures. The tracks are selectively developed forming nanometer wide pores, which after prolonged etching grow in diameter and eventually merge creating fully opened cavities. Metallic structures have been replicated in these pores/cavities by electrodeposition of nickel and copper. We have fabricated open and dense clusters of separated micron or sub-micron sized wires as well as solid structures. Highly vertical, through hole microvias in average 39 m wide, with a pitch of 100 m have been fabricated. The smallest structures obtained were 25 × 25 m square columns. The process appears promising for ultra-high density via batch production and has a strong potential of further miniaturising via dimensions.The work was carried out within the Centre for Advanced Micro Engineering (AME), financed by the Swedish Foundation of Strategic Research (SSF). Part of the work has also been supported by the Commission of the European Communities under the Research and Training Network EuNITT, Contract No. HPRN-CT-2000-00047.     

Rapid replication of polymeric and metallic high aspect ratio microstructures using PDMS and LIGA technology

This paper present a method of rapid replication of polymeric high aspect ratio microstructures (HARMs) and a method of rapid reproduction of metallic micromold inserts for HARMs using polydimethylsiloxane (PDMS) casting and standard LIGA processes. A high aspect ratio (HAR) metallic micromold insert, featuring a variety of test microstructures made of electroplated nickel with 15:1 height-to-width ratio for 300 μm microstructures, was fabricated by the standard LIGA process using deep X-ray lithography (DXRL). A 10:1 mixture of pre-polymer PDMS and a curing agent were cast onto the HAR metallic micromold insert, cured and peeled off to create reverse images of the HAR metallic micromold insert in PDMS. In addition to the replication of polymeric HARMs, replicated PDMS HARMS were coated with a metallic sacrificial layer and electroplated in nickel to reproduce another metallic micromold insert. This method can be used to rapidly and massively reproduce HAR metallic micromold inserts in low cost mass production manner without further using DXRL.