Production issues for high aspect ratio Lobster-eye optics using LIGA

We have previously considered some of the parameters for, and production issues involved in making, a useful Lobster-eye prototype using X-ray lithography. Due to the high aspect ratio and dense packing of the array features, we were unable, in our initial attempts to produce optimal Lobster-eye optics using X-ray lithography and a PMMA resist, to consistently overcome adhesion issues between the substrate and the resist. In recent work we have overcome this problem by using SU-8 as the resist instead. In the light of this advance we reconsider the relevant parameters and production issues when using SU-8 and lower-energy exposures.     

The lifetime comparison of Ni and Ni-PTFE moulding inserts with high aspect-ratio structure

Micro molding is the key process of LIGA (UV-LIGA) technique for low-cost mass production. It is difficult to demold for high aspect ratio of the structure fabricated by LIGA technique due to adhesion and friction force between the side-walls of the molding inserts and the thermoplastic material (PMMA). In this paper, the life times of the molding inserts made of Nickel (the material usually used for LIGA) and Ni-Teflon (PTFE) compound material are studied. The experiment shows the lifetime of Ni-PTFE molding insert is longer than the one of Ni for across pattern.This work was supported by the national High technology Research and Development Program of China (863 Program, No. 2002 AA404130), and National Natural Science Fundation of China (No. 10375058)     

Modeling acoustic agitation for enhanced development of PMMA resists

  Although acoustic agitation is known to enhance the development rate of LIGA resists, the operative physical mechanism is not well understood. Analytical and numerical models presented here suggest that the observed enhancement results from steady acoustic streaming and associated polymer fragment transport induced by the high frequency sound waves. The computed steady flow within the feature is torroidal with downflow along the feature walls and upflow in the center. The numerically calculated enhancement of fragment transport is shown to be well approximated by a closed-form expression based on a pair of asymptotic formulas for weak and strong agitation. These transport models are combined with a model of PMMA dissolution kinetics and used to predict development rates for a range of LIGA feature sizes. The results are in good agreement with experimental data from Forschungszentrum Karlsruhe. Received: 10 August 2001/Accepted: 24 September 2001 This work was supported by the Sandia Materials Science Research Foundation and by the Accelerated Strategic Computing Initiative. Sandia is a multiprogram laboratory operated by Sandia Corporation for the United States Department of Energy under contract DE-AC04-94AL85000. This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Overcoming SU-8 stiction in high aspect ratio structures

Deep X-ray lithography is a well known technique used for making high aspect ratio structures (HARS). However, well known difficulties arise in the creation of some HARS; adhesion of the developed resist structures to the plating base commonly fails, and when resist structures are densely packed features can clump together. This in turn can exacerbate adhesion failure. The problem of clumping is sometimes known as stiction. In this paper we describe an idea involving a low dose UV second exposure that can help overcome stiction in certain designs and thus promote adhesion and the retention of the designed structure.AGP acknowledges receipt of an Australian Research Council QEII fellowship. This work was supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program.     

Microfabrication by hot embossing and injection molding at LASTI

LIGA process includes three processes as X-ray lithography, electroforming to fabricate metalic molds and replication, and can be fabricated nano and micro parts for various devices that it is difficult to product by conventional machining methods. A key technology which gathers mass-production efficiency in the LIGA process is micro-replication technology. We choiced hot embossing and injection molding methods for replication. For a demonstration, two kinds of Ni molds, a mesh pattern within a line width of 100 m, and an aspect ratio of 1.0 and a mesh pattern within a line width of 40 m, and an aspect ratio of 2.5, were prepared. These were produced with X-ray lithography and nickel electrofoming technique. In hot embossing, an experiment of micro-replication using polymethyl methacrylate (PMMA) and polycarbonate (PC) sheets succeeded. At injection molding, it could not transfer well with PMMA and PC, but injection temperature was set up highly, and it succeeded by cycloolefin polymer. Furthermore, we measured sidewalls surface roughness of microstructures produced at each steppes of the LIGA process, and it checked that the LIGA process had processing accuracy higher than a conventional machining method.We would like to thank Ms. A. Kitajima and Dr. R. Maeda at National Institute of Advanced Industrial Science and Technology (AIST), Mr. M. Ohtomo at Ikegami Mold Engineering Co., Ltd., and Mr. Noriaki Sato at Juken Kogyo Co., Ltd. for their valuable collaboration and contributions. This research was the contract research from the New Industry Research Organization (NIRO) supported financially by the New Energy and Industrial Technology Development Organization (NEDO).     

Reasoning about equality

This note contains a set of six theorems that can be used to assess the ability of a theorem-proving system to reason about equality. The six theorems are graduated in terms of difficulty: they range from fairly trivial to quite difficult. They do not cover all aspects of equality reasoning, but they have proved useful to us in developing our system.This work was supported in part by National Science Foundation grant MCS82-07496 and in part by the Applied Mathematical Sciences Research Program (KC-04-02) of the Office of Energy Research of the U.S. Department of Energy under Contract W-31-109-Eng-38.     

Movable microstructures made by a sub-micron LIGA process

 Movable microstructures with high aspect ratios were made with lateral dimensions down to the sub-micron domain by the LIGA process and a sacrificial layer technique. Compared to microstructures usually made by LIGA, all dimensions were reduced approximately by a factor of 10, while the aspect ratio was kept constant. The smaller resist thickness in the range of some ten micrometers allowed much lower X-ray doses and energies to be used for exposure and the absorbers with a thickness of only 3 μm to be employed. As the lateral dimensions are smaller, a much larger number of devices can be fabricated in one batch. Therefore, the production costs of deep etch X-ray lithography are reduced dramatically. Electrostatic linear actuators with lateral dimensions as small as 500 nm were manufactured to demonstrate the advantages of LIGA in sub-micron dimensions. An X-ray mask with absorbers 2.8 μm high was produced by a three-level technique. The linear actuators consisted of several arrays of capacitor plates combined into an electrostatic driving unit with an active area, typically, 0.3 mm² and less. The driving unit was supported by folded beam flexures to avoid frictional forces. They also guaranteed parallel movement of the capacitor plates. The functionality of these devices was demonstrated by measuring displacement as a function of the voltage applied. Received: 30 October 1995 / Accepted: 17 January 1996     

Naive asymptotics for hitting time bounds in Markov chains

A set of sufficient conditions is obtained for Markov chains to yield upper and lower passage time bounds. While obtaining expected passage times is strictly a numerical procedure for general Markov chains, the results presented here outline a simple approach to bound expected passage times provided the chains satisfy certain easy to check criteria. The results may be useful in modelling situations, such as in the analysis of algorithms, where simple ways of obtaining average complexity estimates are required.Research performed at the Mathematical Sciences Section of Oak Ridge National Laboratory under the auspices of the Faculty Research Participation Program of Oak Ridge Associated Universities, and supported by the Applied Mathematical Sciences subprogram of the Office of Energy Research, U.S. DOE, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.Also supported in part by the National Science Foundation, under award ASC-9002225 and NATO award CRG900108     

Movable microstructures made by a sub-micron LIGA process

Movable microstructures with high aspect ratios were made with lateral dimensions down to the sub-micron domain by the LIGA process and a sacrificial layer technique. Compared to microstructures usually made by LIGA, all dimensions were reduced approximately by a factor of 10, while the aspect ratio was kept constant. The smaller resist thickness in the range of some ten micrometers allowed much lower X-ray doses and energies to be used for exposure and the absorbers with a thickness of only 3 μm to be employed. As the lateral dimensions are smaller, a much larger number of devices can be fabricated in one batch. Therefore, the production costs of deep etch X-ray lithography are reduced dramatically. Electrostatic linear actuators with lateral dimensions as small as 500 nm were manufactured to demonstrate the advantages of LIGA in sub-micron dimensions. An X-ray mask with absorbers 2.8 μm high was produced by a three-level technique. The linear actuators consisted of several arrays of capacitor plates combined into an electrostatic driving unit with an active area, typically, 0.3 mm² and less. The driving unit was supported by folded beam flexures to avoid frictional forces. They also guaranteed parallel movement of the capacitor plates. The functionality of these devices was demonstrated by measuring displacement as a function of the voltage applied.     

Performance and simulation of thermoplastic micro injection molding

 Originally developed for the replication of high aspect ratio LIGA structures, micro injection molding is presently on its way to become an established manufacturing process. Enhanced technological products like micro optical devices are entering the market. New developments like the different kinds of injection molding with several components open up opportunities for increasing economic efficiency as well as for new fields of applications. Software tools for the simulation of the thermal household of the molding tool and/or the moldfilling process itself can provide useful but not wholly sufficient assistance for the optimization of micro injection molding. Received: 10 August 2001/Accepted: 24 September 2001     

Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.     

Solution of sparse positive definite systems on a shared-memory multiprocessor

Algorithms and software for solving sparse symmetric positive definite systems on serial computers have reached a high state of development. In this paper, we present algorithms for performing sparse Cholesky factorization and sparse triangular solutions on a shared-memory multiprocessor computer, along with some numerical experiments demonstrating their performance on a Sequent Balance 8000 system.Research was supported by the Applied Mathematical Sciences Research Program of the Office of Energy Research, U.S. Department of Energy under contract DE-AC05-84OR21400 with Martin Marieta Energy Systems, Inc., by the U.S. Air Force Office of Scientific Research under contract AFOSR-ISSA-85-00083 and by the Canadian Natural Sciences and Engineering Research Council under grants A8111 and A5509.     

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.     

Task scheduling for parallel sparse Cholesky factorization

This paper presents a solution to the problem of partitioning the work for sparse matrix factorization to individual processors on a multiprocessor system. The proposed task assignment strategy is based on the structure of the elimination tree associated with the given sparse matrix. The goal of the task scheduling strategy is to achieve load balancing and a high degree of concurrency among the processors while reduçing the amount of processor-to-processor data comnication, even for arbitrarily unbalanced elimination trees. This is important because popular fill-reducing ordering methods, such as the minimum degree algorithm, often produce unbalanced elimination trees. Results from the Intel iPSC/2 are presented for various finite-element problems using both nested dissection and minimum degree orderings.Research supported by the Applied Mathematical Sciences Research Program, Office of Energy Research, U.S. Department of Energy under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems Inc.     

The copolymer of methyl methacrylate and methacrylic acid as a sensitive resist for deep-etch X-ray lithography

The copolymer of methyl methacrylate and methacrylic acid was developed as a new sensitive resist for the LIGA (Lithographie, Galvanoformung, Abformtechnik) process. The resist exposures were carried out at the LIGA beamline of a superconducting compact light source NIJI-III. The absorbed energy density required to remove the entire exposed resist and produce a defect-free microstructure was between 0.4 and 7 kJ/cm³ (4 and 20 kJ/cm³ for PMMA). The sensitivity and patterning depth of the copolymer were 10 times and 3.5 times, respectively, those of poly methyl methacrylate (PMMA) assuming that both resists were exposed to synchrotron radiation (SR) of the same wavelength. Moreover, the copolymer showed high contrast and process stability. This work was performed under the management of the Micromachine Center as part of the Research and Development of Micromachine Technology supported by NEDO (New Energy and Industrial Technology Development Organization).     

The copolymer of methyl methacrylate and methacrylic acid as a sensitive resist for deep-etch X-ray lithography

The copolymer of methyl methacrylate and methacrylic acid was developed as a new sensitive resist for the LIGA (Lithographie, Galvanoformung, Abformtechnik) process. The resist exposures were carried out at the LIGA beamline of a superconducting compact light source NIJI-III. The absorbed energy density required to remove the entire exposed resist and produce a defect-free microstructure was between 0.4 and 7 kJ/cm³ (4 and 20 kJ/cm³ for PMMA). The sensitivity and patterning depth of the copolymer were 10 times and 3.5 times, respectively, those of poly methyl methacrylate (PMMA) assuming that both resists were exposed to synchrotron radiation (SR) of the same wavelength. Moreover, the copolymer showed high contrast and process stability. This work was performed under the management of the Micromachine Center as part of the Research and Development of Micromachine Technology supported by NEDO (New Energy and Industrial Technology Development Organization).     

Mixer slit plates fabricated by direct-LIGA

Direct-LIGA services combining X-ray based micro-fabrication and electroplating are routinely provided to customers worldwide on a “best effort” basis by several LIGA centers. In a publicly founded project in collaboration between BESSY and Ehrfeld Mikrotechnik BTS GmbH the direct-LIGA process was adapted and optimised for fabricating mixer slit plates for micro-reactors. The application of SU-8 as a resist in deep X-ray lithography opens new opportunities for a direct and cost-effective mass fabrication of LIGA metal micro-parts. This joint paper will report on efforts from BESSY, Micro Resist Technology, and Ehrfeld BTS in fabrication of mixer slit plates by direct-LIGA for use in modular micro-reaction technology. The high demands regarding precision and yield of the micro-slit plates need certain improvements in the LIGA processing chain. Our approach consists of X-ray patterned SU-8 resist molds with a layer thickness of up to 350 μm and an aspect ratio of up to 30:1. Furthermore, the pattern-transfer into the SU-8 resist was done by a low-cost X-ray mask based on a graphite membrane. Other processing steps like NiFe electroplating and SU-8 removal will be discussed in this paper.     

Microfabrication of massive parallel micromirror-lenses by X-ray LIGA technique: recent advances and prospects

Modern microfabrication techniques enable to obtain novel 2D lens systems which are the marriage of warped microchannel plates and Fresnel lenses. They rely on a multitude of precisely inclined micromirrors, acting together as very efficient macroscopic lenses for all electromagnetic wavelengths from the infrared, up to X-rays and even for particles like neutrons. Especially interesting is the fabrication of some-cm apertured lenses in the VUV and EUV region. X-ray LIGA is a favorite candidate for the fabrication of such lenses due to its inherent technological precision and capability to produce high aspect ratio structures. Theoretical consideration and limits for the design of such lens systems are presented, together with first investigations concerning surface-smoothing by spin-on glass in order to make such lens systems to work in the X-ray region.This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Optimisation of SU-8 processing parameters for deep X-ray lithography

The negative photoresist SU-8 has been recognised as an unique resist, equally useful for conventional UV lithography as well as deep X-ray lithography (DXRL) applications [2, 7, 12, 17, 18]. One of the major limitations in the use of SU-8 in lithographic processes is the occurrence of internal stress [15]. The processing parameters investigated for DXRL of SU-8 included resist thickness (450–850 m), soft bake time (7–11 h), exposure dose (30–70 J/cm³), post exposure bake time (20, 40, 60 min) and development time. The effect of these parameters on stress was evaluated using wafer curvature measurements. Taguchi optimisation techniques have been used to asses the contribution of these parameters on the stress of the developed structures. This study shows that softbake time contributes the most to stress in the SU-8 film at 50%, followed by the exposure dose and post exposure bake with 30% and 15% respectively. Stress varied somewhat linearly with thickness. At higher thickness, the deposition process needs to be changed for very high aspect ratio structures. The main objective of this work has been to optimise the processing conditions of thick SU-8 films for DXRL.This paper was first presented at the High Aspect Ratio Microstructurres (HARMST) conference in Montery California, June 2003.This work was supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities Program. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, under Contract No. W-31-109-Eng-38. Support received from CRC for microTechnology (Australia) is also gratefully acknowledged. We also thank Dr. Brett Sexton and Fiona Smith from CSIRO (Australia), Dr. Francesco DeCarlo , Dr. Chian Liu, and Judy Yaeger from APS, and Dr. Jason Hayes and Dr. Matthew Solomon from Swinburne University for useful discussions and their help during some experimental work.     

Design,fabrication and characterization of micro flow sensor inspired by biological hair cell

Design, fabrication and characterization of micro flow sensor were investigated based on the inspiration of biological hair cell in a nature. The micro scale artificial hair cell sensor was designed as considering two parts; first the high aspect ratio cilium structure which works as a hair cell of fish and second the mechanoreceptor structure where the drag force by flow are actually measured. Parameters of cilium structure were designed based on static modelling as follow: 300 μm diameter and 2 mm height. The high aspect ratio cilium structure was precisely fabricated using a hot embossing process with the developed separated micro mold system prepared by LIGA (from the German Lithographi, Galvanoformung, Abformung) process. The mechanoreceptor was formulated with a force sensitive resistor with four symmetric electrodes to analyze the direction and the magnitude of target flow. Performance of assembled sensor was characterized using the prepared water channel. Flow velocity was sensed with the magnitude of signal and the direction of flow was distinguished by analyzing the signals from four mechanoreceptors.