High aspect ratio multi-level mold inserts fabricated by mechanical micro machining and deep etch X-ray lithography

 Complex microstructures can be fabricated in large quantities by thermoplastic molding processes. The shape of the microstructures is determined mainly by the mold insert. Until now, multi-level mold inserts have been fabricated either by deep etch X-ray lithography and electroforming, Harmening et al. (1992), or by milling of a brass substrate, Schaller et al. (1995). In both cases there are limitations on structuring either by the fabrication effort or by the sizes of the smallest available milling heads. To avoid these limitations on structuring, a new process for manufacturing multi-level mold inserts has been developed at Forschungszentrum Karlsruhe. Milling, drilling, deep etch X-ray lithography and electroforming have been combined to manufacture a mold insert which is characterized by high aspect ratios with small lateral dimensions and various level heights. Samples with two levels and an aspect ratio of 15 have been manufactured. Much higher aspect ratios seem to be achievable. This paper covers the fabrication process, first tests, and experimental results of manufacturing a multi-level mold insert for molding three-dimensional components of a microvalve system. Received: 30 October 1995 / Accepted: 17 January 1996     

Fabrication method of two-level polymeric microstructure with the help of deep X-ray lithography

Two- or multi-level microstructures are getting more important in several applications such as multi-component micro optical elements and various microfluidic systems. In the present study, a simple and efficient method is newly proposed for a fabrication of the two-level polymeric microstructures. Making a mother two-level microstructure consists of two processes: (1) the hot embossing process for a fabrication of microstructures on a PMMA substrate, and (2) the deep X-ray lithography using the hot embossed substrate for a high aspect ratio microstructure fabrication, resulting in a high aspect ratio microstructure containing smaller microstructures on its surface. Making use of so fabricated two-level microstructures as a mother structure, one could achieve a mass replication of the same microstructures via injection molding process with a metallic mold insert obtained by a nickel electroforming onto the mother microstructure. In order to demonstrate the proposed method, a polymeric high aspect ratio microstructure having smaller square microstructures on its top surface was fabricated. The fabricated two-level microstructure shows fine vertical sidewalls, which is a characteristic feature of the deep X-ray lithography. In addition, a metallic mold insert for a mass replication was fabricated by a nickel electroforming process.     

Manufacture of a micro-sized piezoelectric ceramic structure using a sacrificial polymer mold insert

There have been technical limitations to manufacture microstructures due to difficulty of demolding during replication process of high aspect ratio microstructure in mass production technologies. In the present study, the fabrication of a novel sacrificial micro mold insert and powder injection molding process using such a micro mold insert is proposed and developed. It utilizes a synchrotron radiation to fabricate the shape of polymer based sacrificial mold inserts and then these mold inserts were exposed at X-ray once more to adjust its solubility. This second X-ray exposure facilitates dissolving of mold inserts instead of demolding process which have difficulties like pattern collapses or defects in case of precise replication process. In this manner, severe problems of demolding process in conventional mass production technologies can be efficiently overcome. To verify the usefulness of the proposed technique, polymer based micro mold inserts with several tens of micrometer sized structure for piezoelectric sensor applications were fabricated using X-ray micromachining process radiated synchrotron. The solubility of mold inserts were optimized by the second X-ray exposure without an X-ray mask and then subsequent powder injection molding process was utilized with a piezoelectric based material. Finally, piezoelectric ceramics with micrometer-scale and high aspect ratio of 5 were successfully fabricated, verifying that the present sacrificial mold system is useful for the precise replication process such as the fabrication of microstructure with high aspect ratio or complicated structure.     

Study of Hot Embossing Using Nickel and Ni–PTFE LIGA Mold Inserts

Hot embossing is one of the main process techniques for polymer microfabrication, which helps X-ray lithography, electroplating, and molding (LIGA) to achieve low-cost mass production. Most problems in polymer micromolding are caused by demolding, especially for hot embossing of high-aspect-ratio microstructures. The demolding forces are related to the sidewall roughness of the mold insert, the interfacial adhesion, and the thermal shrinkage stress between the mold insert and the polymer. The incorporation of polytetrafluoroethylene (PTFE) particles into a nickel matrix can have the properties such as antiadhesiveness, low friction, good wear, etc. To minimize the demolding forces and to obtain high-quality polymer replicas, a Ni-PTFE composite microelectroforming has been developed, and the hot embossing process using Ni and Ni-PTFE LIGA mold inserts has been well studied in this paper. The morphologies, sidewall roughness, and friction coefficient have been explored in the fabricated Ni-PTFE LIGA mold insert. Finally, the comparison of embossed microstructures with various aspect ratios and the comparison of the embossing lifetimes of mold inserts have been carried out between Ni and Ni-PTFE mold inserts, which show a better performance of the Ni-PTFE mold and its potential applications.     

Numerical simulation and fabrication of microscale, multilevel, tapered mold inserts using UV-Lithographie, Galvanoformung, Abformung (LIGA) technology

Techniques for economic fabrication of high-aspect-ratio microscale structures (HARMS) are being investigated intensely. Microdevices employing metal-based HARMS are of particular interest for mechanical, electro-mechanical, and chemical applications. In many applications, HARMS with two or more heights are needed. Fabrication of these multi-level HARMS by compression molding requires two-level or multi-level mold inserts. In addition, tapered mold inserts would help achieving easy insert-part separation. This paper reports a process for fabricating two-level tapered mold inserts by combining UV-lithography of SU-8 resist, one-step metal electrodeposition, polish and level, followed by SU-8 resist removal. Without tilting and rotation during the lithography step, tapered plating molds are obtained by employing characteristics of UV-lithography and resist development. The SU-8 removal method used does not reduce the strength of the electrodeposited mold insert. Efficacy of our approach is demonstrated with a two-level mold insert prototype.     

Various replication techniques for manufacturing three-dimensional metal microstructures

 In microsystem technology a large range of different 6materials will be available only after the necessary micromanufacturing techniques have been developed or adapted. Existing manufacturing techniques are structuring or shaping techniques producing three-dimensional microstructures out of silicon (silicon etching, silicon surface micromechanics), mostly unfilled plastics (lithographic techniques, injection molding, hot embossing, reaction molding) or a few pure metals or binary alloys (electroforming). The choice of materials for microcomponents is determined by the function and conditions of use of microsystems. Especially the range of metals is still restricted considerably because the only processes available are electroforming and thin-layer techniques. It is for these reasons that we are developing various processes for manufacturing three-dimensional metal microstructures. In addition to direct electroforming of injection molding lost plastic micromolds, these are a new microcasting process and Micro Metal Injection Molding (Micro MIM). Microstructures have already been molded from mold inserts made by micromechanical cutting or by the LIGA technique. The results achieved, and future prospects, are outlined below. Received: 25 August 1997/Accepted: 3 September 1997     

Multi-functional valve components fabricated by combination of LIGA processes and high precision mechanical engineering

 The advantage of thermoplastic molding as a convenient method for fabricating large quantities of microstructures is restricted by the possibilities and the necessary efforts for structuring the required molding tools. In order to increase the complexity of mold inserts without significantly increasing the fabrication expenditure a new process combining LIGA techniques and precision mechanics had been suggested by Research Center Karlsruhe. Recent work on the optimization of this process made it possible to manufacture multi-leveled mold inserts with which different three-dimensional microcomponents have successfully been molded. The two- and three-level structures feature among other details integrated alignment aids which worked very well during the assembly of the valve system the components were designed for. This paper deals with the process optimization, the manufacturing of the mold inserts, the fabrication of the three-dimensional microstructures by hot embossing in PMMA as well as in polymers with high thermal resistance and the application in a microvalve system. Received: 25 August 1997/Accepted: 22 September 1997     

Fabrication of antiscatter grids and collimators for X-ray and gamma-ray imaging by lithography and electroforming

Creatv MicroTech has developed unique fabrication techniques to make high precision, high-aspect-ratio metal microstructures to custom specifications. A lithography based fabrication method permits precise fabrication of various microstructures. Collimators and antiscatter grids with continuous, smooth, thin, parallel or focused septa have been fabricated using deep X-ray and optical lithography, combined with metal electroforming. Microfabrication of high-aspect-ratio structures, especially of relatively large areas, presents many challenges: specialized mask design and X-ray mask fabrication; resist preparation, optimal exposure parameters, post-exposure processing, electroforming, polishing, and final assembly. Here, we present microstructures of various designs that we fabricated and describe the challenges that had to be overcome.     

Fabrication of a polymeric tapered HARMs array utilizing a low-cost nickel electroplated mold insert

A simple low-cost technique has been developed to fabricate a mold insert for replicating polymeric tapered high aspect ratio microstructures. A backside exposure technique is used to first obtain a tapered sidewall structure as an electroplating mold in SU-8 photoresist on a glass wafer. Nickel electroplating is utilized to form the mold insert. The lowest average surface roughness of the nickel mold insert on the side that interfaces with the glass wafer during electroplating is measured to be 7.02 nm. A novel technique involving use of titanium putty is introduced here to reduce cost and effort required to fabricate the mold insert. Replication of tapered microstructures in polymeric materials utilizing the fabricated mold insert is demonstrated here in polydimethylsiloxane by a direct molding process and in polymethyl methacrylate by hot embossing. The fabrication details for the mold insert are described. Advantages and disadvantages of the use of titanium putty for achieving superior metal surface finish are given.     

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.     

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.     

Studies of the adhesion properties of LIGA microstructures by X-ray spectroscopy and mechanical measurements

In the first step of the LIGA process microstructures with high aspect ratios are fabricated by patterning a resist layer with deep etch X-ray lithography. As resist typically PMMA (polymethylmethacrylate) is used. For the following electroforming process and to achieve perfect microstructures the adhesion of the PMMA on the substrates is of great importance. For a better understanding the adhesion mechanism, particular induced through the adhesion promoter MEMO (methacryloxypropyl-trimethoxysilane) was investigated on various substrates. Two methods, namely XANES/EXAFS spectroscopy under grazing incidence and shear stress measurements were modified for the specific needs of microstructures. Our studies proved that the adhesion strength is determined by two factors. A rough surface, which allows mechanical interlocking, increases the adhesion strength by about 10%. The larger part of the adhesion strength is determined by formation of chemical bonds when adding an adhesion promoter. E.g. addition of 5% adhesion promoter increases the adhesion strength by 90%. These results were, as far as possible, confirmed by the X-ray spectroscopical measurements.     

Micro injection molding for mass production using LIGA mold inserts

Micro molding is one of key technologies for mass production of polymer micro parts and structures with high aspect ratios. The authors developed a commercially available micro injection molding technology for high aspect ratio microstructures (HARMs) with LIGA-made mold inserts and pressurized CO₂ gasses. The test inserts made of nickel with the smallest surface details of 5 μm with structural height of 15 μm were fabricated by using LIGA technology. High surface quality in terms of low surface roughness of the mold inserts allowed using for injection molding. Compared to standard inserts no draft, which is required to provide a proper demolding, was formed in the inserts. To meet higher economic efficiency and cost reduction, a fully electrical injection molding machine of higher accuracy has been applied with dissolving CO₂ gasses into molten resin. The gasses acts as plasticizer and improves the flowability of the resin. Simultaneously, pressurizing the cavity with the gasses allows high replication to be obtained. Micro injection molding, using polycarbonate as polymer resins, with the aspect ratio of two was achieved in the area of 28 × 55 mm² at the cycle time of 40 s with CO₂ gasses, in contrast to the case of the aspect ratio of 0.1 without the gasses.     

Rapid fabrication of microcomponents – UV-laser assisted prototyping, laser micro-machining of mold inserts and replication via photomolding

 The rapid fabrication of microcomponents made from polymers or composites will be presented. The whole fabrication process is divided into three main steps: first, direct patterning of polymers with excimer laser radiation enables the fabrication of first prototypes. Second, laser assisted micromachining using Nd:YAG or KrF-Excimer laser radiation allows a rapid manufacturing of microstructured mold inserts made of steel or polymer. Third, the application of UV-light-induced reaction injection molding (UV-RIM, photomolding) process using reactive monomer/polymer resins gives the access to the replication of the previously fabricated mold insert. For mold insert fabrication, the development of different process strategies is in progress in order to meet the best fit for three dimensional shapes (e.g., steep walls or slowly inclined walls). The development of mold inserts made of polymers is performed via UV laser ablation. This is a new promising method for the rapid manufacturing of microcomponents. Received: 10 August 2001/Accepted: 24 September 2001 We are grateful to our colleagues A. Sporrer and P. Severloh for her technical assistance in SEM/EDX. We also thank M. Blumhofer for his support in laser profilometry and H. Besser and W. Hoffmann for helpful discussions. This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Studies of the adhesion properties of LIGA microstructures by X-ray spectroscopy and mechanical measurements

In the first step of the LIGA process microstructures with high aspect ratios are fabricated by patterning a resist layer with deep etch X-ray lithography. As resist typically PMMA (polymethylmethacrylate) is used. For the following electroforming process and to achieve perfect microstructures the adhesion of the PMMA on the substrates is of great importance. For a better understanding the adhesion mechanism, particular induced through the adhesion promoter MEMO (methacryloxypropyl-trimethoxysilane) was investigated on various substrates. Two methods, namely XANES/EXAFS spectroscopy under grazing incidence and shear stress measuremsents were modified for the specific needs of microstructures. Our studies proved that the adhesion strength is determined by two factors. A rough surface, which allows mechanical interlocking, increases the adhesion strength by about 10%. The larger part of the adhesion strength is determined by formation of chemical bonds when adding an adhesion promoter. E.g. addition of 5% adhesion promoter increases the adhesion strnegth by 90%. These results were, as far as possible, confirmed by the X-ray spectroscopical measurements. The financial support of this project by the Federal Ministry of Science and Technology (contract No. 05 5P DAX18) is gratefully acknowledged.     

Injection molding of polymeric LIGA HARMs

The primary goal of an ongoing research effort at LSU is to develop the three-step LIGA process to inexpensively manufacture high aspect ratio microstructures (HARMs). The first two steps of the process (lithography and electroplating) produce a metallic mold insert that can be used as a template for molding microstructures. The final step of LIGA is molding. This paper focuses on injection molding of thermoplastics to produce surfaces covered with HARMs. The resulting microstructures are hundreds of micrometers in height, tens of micrometers in width, and separated by gaps on the order of tens of micrometers. Injection molding experiments using high density polyethylene were performed using a commercially available injection molding machine. Experimental variables included injection speed, the tool temperature, and air pressure in the mold cavity. Elevating the tool temperature above the melting point ensured that the polymer completely filled the mold, producing microstructures with the desired geometry. As the temperature of the mold was reduced, higher injection speeds did not necessarily ensure filling of the mold cavity. The cycle time is shorter than the values previously reported in the literature [Madou (1996)]. Received: 30 March 1999 / Accepted: 12 April 1999     

Study on fabrication of high aspect ratio MEMS microparts using a compact SR beamline

 A compact beamline dedicated to hard x-ray deep lithography for fabrication of high aspect ratio MEMS microparts has been developed. The exposure stage was only 3 meters away from the synchrotron radiation (SR) source so that a relatively high photon flux could be achieved with a compact super-conducting SR source. The deep lithography using PMMA resist could be as deep as 1000 μm and the maximum aspect ratio achieved was about 50. The throughout for the 200 μm-deep lithography was found to be on the order of 5 cm²/h using the membrane-free mask under the routine SR conditions. Templates with the high aspect ratio microstructures have been made of the PMMA resist based on conducting substrates and applied further to electroforming to create metallic microstructures. In order to fabricate microparts for the MEMS applications, we have concentrated on development of masks for the hard x-ray deep lithography. The masks now can be made to have the 8 μm-thick gold absorber on the 2 μm-thick SiC membrane. Received: 25 August 1997/Accepted: 3 September 1997     

Micro molding for double-sided micro structuring of SU-8 resist

Advances in micro and nano fabrication technologies for MEMS require high-level measurement techniques with regard to sampling and sensitivity. For this purpose at the Institute of Microtechnology (IMT) highly sensitive piezoresistive 3D force sensors based on SU-8 polymer have been developed. In this paper we present an improved micro fabrication process for a double-sided micro structured design. The sensors are produced by multilayer processing techniques such as UV lithography and coating methods. The double-sided micro structured design demands a photoresist application method which simultaneously features a top side structuring and a casting from a mold. We use a new micro molding process to meet the demands. The micro fabrication technology is described, focusing on the development of the molding structure for shaping of the bottom side and a capable release process for the detachment of the molded structures. The fabrication process of the SU-8 mold layer is optimized to fabricate molding structures with heights from a few μm up to 350 μm. Therefore different SU-8 formulations, namely with classification numbers 5, 25, 50, and 100, have been used. The fundamental limitations for the mold design result from the lithography process, which defines the smallest lateral resolution, and from the characteristics of a molding process, e.g. the impossibility to realize an undercut. To allow for reliable release, the molding structures have to be coated with a sacrificial layer. Silicon nitride is deposited onto the substrate with accompanying monitoring of the deposition temperature during the PECVD process.     

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     

Feedstock development for micro powder injection molding

Powder injection molding of microstructured parts with high aspect ratios requires feedstocks, which have a high mechanical stability for demolding. The binders of the feedstocks have to allow pressure free and complete debinding and sintering without deformation in the submillimeter range. For complete molding of especially small and complex detailed microstructures, powders with a small particle size have to be used. Additionally the microstructured mold inserts themselves must have an appropriate design, which allows complete filling of the cavity and an easy removal of the molded microstructures. By the development of new binder compositions, adapted micro mold inserts and optimized processing parameters it was possible to manufacture specimens for micromechanical investigations without substrate plates. Thus many machining and finishing worksteps, which have great influence on the mechanical properties of the microstructures, can be omitted. Received: 10 August 2001/Accepted: 24 September 2001