High aspect ratio micromolds for the electroplating of micro electro discharge machining tools

A new approach using micro systems technology for the fabrication of micro electro discharge machining tool electrodes using a combination of positive and negative photoresist fabricated with near UV-lithography as high aspect ratio micromolds was developed. Micromolds with heights varying from 200 to 650 μm and an aspect ratio of up to 26:1 were fabricated using SU-8?, a negative photoresist, as a micromold for the electrodeposition of the micro tool electrodes. Besides serving as a micromold, SU-8? is also used as an insulator to protect the sidewalls of the tool electrodes from corrosions. The fabrication of the SU-8? micromold was optimized to avoid cracks and delaminations. In the experiments, electrodes made of Cu, WCu, and CoFe were deposited into these High Aspect Ratio Micro Structure Technology (HARMST) micromolds. Furthermore, the process technique for electroplating in deep micromolds is discussed in this paper.     

Aqueous base developable: easy stripping, high aspect ratio negative photoresist for optical and proton beam lithography

A variety of different photo resists are used for the fabrication of polymer and metal high aspect ratio structures. Among them SU-8, a chemically amplified negative tone photoresist is the mostly used. However, after processing the finished resist pattern (SU-8) is hardly removed from the substrate. In the present work the formulation and process optimization of a negative tone chemically amplified photoresist (TADEP) is presented. TADEP resist owns two advantages: the dissolution of the uncrosslinked areas in IC standard aqueous developers and the easy stripping in acetone by the assistance of ultrasonic bath. The TADEP resist is successfully applied for the fabrication of polymer and metal structures, after electroplating and stripping and also in the case of Proton Beam Writing.     

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.     

Electroplated compliant metal microactuators with small feature sizes using a removable SU-8 mould

In the work presented in this article a fabrication process for high aspect ratio metal microstructures has been developed using photoresist EPON SU-8. A smallest feature size of 2 μm with near vertical sidewalls has been achieved on a routine basis. The SU-8 photoresist has been used as mould for electroplating high aspect ratio metal micro actuators. A removal process has also been developed. The removal of the SU-8 mould after plating is done in a plasma asher using an oxygen or oxygen/fluorine plasma with plasma power as low as 200 W. To demonstrate the fabrication process a 10 μm thick nickel, electro-thermal micro actuator has been fabricated.     

Reduction of diffraction effect of UV exposure on SU-8 negative thick photoresist by air gap elimination

 This paper reports a novel way to compensate the air gap between photomask and photoresist for eliminating UV light diffraction on photoresist, which greatly increases the sidewall straightness of high-aspect-ratio resist structures. In this research, SU-8 negative tone photoresist was used for experiments, and glycerol was employed as an index match material for bridging air gap between photomask and photoresist during exposure. Results showed that a high aspect ratio wall structure of 156 μm thick and 25 μm wide had a 45% pattern width error when exposed under 100 μm air gap, while glycerol compensated process accomplished a straight resist wall without appreciable error. This method is simple and cheap to employ, compared to the usage of costly thick-photoresist-film spinner for resist planarization. Numerical simulation on the diffraction effect upon the structure wall has also been conducted. The calculated and experiment wall profiles showed similarity in trend. Received: 10 August 2001/Accepted: 24 September 2001     

Improving the process capability of SU-8

SU-8 negative resists designed to produce uniform thick films in a single spin-coating step are widely used in the fabrication of MEMS devices. A potential limitation in using resists of this type in large-scale production is the relatively long processing cycle time, which is determined by the choice of solvent and bake conditions. A well-known disadvantage of SU-8 is its inability to wet low surface energy substrates which can result in non-uniform coating. In this paper we present the results of the first part of a study aimed at improving SU-8 process capability in which the focus is on solvent replacement. Some of the important trade-offs that should be considered in the selection of a practical solvent are discussed. The results of an investigation that led to the selection of cyclopentanone to replace gamma-butyrolactone are presented, the relative drying rates are compared and the drying mechanism is discussed. This study led to the commercialization of a new family of cyclopentanone based resists, SU-8 2000 (referred to as SU-8 CP in this paper) which shows significantly improved wetting, faster drying for film thicknesses up to about 50 microns and clean edge bead removal without the need for an intermediate bake step.This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Fabrication of photoplastic high-aspect ratio microparts and micromolds using SU-8 UV resist

 An innovative method for fabrication and rapid prototyping of high-aspect ratio micromechanical components in photoresist is discussed. The photoresist is an epoxy-negative-tone resist, called SU-8, which can be structured to more than 2 mm in thickness by UV exposure. Small gears of 530 μm in diameter and 200 μm in thickness have been realized in this photoplastic and their functionality has been demonstrated. In addition a process called MIMOTEC^TM (MIcroMOlds TEChnology) has been established for the fabrication of metallic micromolds. MIMOTEC^TM is based on the use of the SU-8 spun on high thicknesses and electrodeposition of nickel. Thermoplastic microcomponents have been injected and mounted in watches. Received: 25 August 1997/Accepted: 23 October 1997     

Two steps micromoulding and photopolymer high-aspect ratio structuring for applications in piezoelectric motor components

 We report on recent advances in micro fabrication technology using micromoulding and high-aspect ratio structuring of photopolymer. The direct application is the realization of components for millimeter-size, ultrasonic piezoelectric motors. A new fabrication process using a thick epoxy-based material (SU-8) and the electroplating of nickel is demonstrated. Photopolymer structures have also been realized and released using a positive tone resist as sacrificial layer. The main advantages over past fabrication methods are better design flexibility, simplicity of the fabrication process, the capability to combine metallic materials (Ni) with polymeric materials (SU-8), and the use of positive tone resist as a sacrificial layer. Received: 25 August 1997/Accepted: 23 October 1997     

Development of a freestanding copper antiscatter grid using deep X-ray lithography

A prototype freestanding copper antiscatter grid with parallel lamellar walls has been fabricated using deep X-ray lithography and electroforming. The freestanding copper grid has square shaped lamellar walls that are 25 m thick and 1 mm tall with a 550 m period. For mammography applications, the lamellar walls have to be aligned to a point X-ray source of the X-ray system. To achieve this goal, we investigated a dynamic double-exposure technique. Our progress in using stacked exposures and use of the photoresist SU-8, both to reduce fabrication cost, is also presented.We would like to thank Francesco De Carlo, Judith Yaeger, Joseph Arko and Shenglan Xu for their assistance. Use of the APS was supported by US Department of Energy, Office of Sciences, under Contract No. W-31-109-ENG-38. We thank SRI CAT for the support of this research. The work is supported by NIH SBIR Phase II Grants: 2 R44 CA76752-02 and 5 R44 CA76752-03.This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

One-step maskless grayscale lithography for the fabrication of 3-dimensional structures in SU-8

We propose a novel and simplified method to fabricate complex 3-dimensional structures in SU-8 photoresist using maskless grayscale lithography. The proposed method uses a Digital Micro-mirror Device (DMD^®) to modulate the light intensity across a single SU-8 photoresist layer. Top and back-side exposure are implemented in the fabrication of original structures such as cantilevers, covered channels with embedded features and arrays of microneedles. The fabrication of similar structures in SU-8 with other techniques often requires complex physical masks or the patterning of several stacked layers. The effects of critical process parameters such as software mask design, exposure and developing conditions on the quality of 3-D structures are discussed. A number of applications using bridges, cantilevers and micromixers fabricated using this methodology are explored.     

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.     

Fabrication of SU-8 photoresist micro–nanofluidic chips by thermal imprinting and thermal bonding

Micro–nanofluidic chips have been widely applied in biological and medical fields. In this paper, a simple and low-cost fabrication method for micro–nano fluidic chips is proposed. The nano-channels are fabricated by thermal nano-imprinting on an SU-8 photoresist layer followed by thermal bonding with a second SU-8 photoresist layer. The micro-channels are produced on the second layer by UV exposure and then thermal bonded by a third layer of SU-8 photoresist. The final micro–nano fluidic chip consists of micro-channels (width of 200.0 ± 0.1 μm and, depth of 8.0 ± 0.1 μm) connected by nano-channels (width of 533 ± 6 nm and, depth of 372 ± 6 nm), which has great potential in molecular filtering and detection.

     

Design and fabrication of an electrostatically suspended microgyroscope using UV-LIGA technology

A micromachined electrostatically suspended gyroscope, with a wheel-like rotor housed by top stator and bottom stator, using UV-LIGA microfabrication technology, was presented. The designed structure and basic operating principle of the gyroscope are described. The key steps in the fabrication process, such as wet etching of Pyrex glass pits for soldering, and integration of thick nickel structures by removal of SU-8 mold, were considered in detail and well solved. Cr/Pt/photoresist was used as etching mask and the etched pits, in depth of near 30 μm, with aspect ratio (depth to undercutting) of 0.75, were obtained. With metal foundations constructed for consolidation, successful integration of the nickel structures, in thickness of 200 μm, was achieved by successful removal of the SU-8 mold using oleum. After the two stators and the rotor were fabricated separately, they were assembled and soldering bonded to form axial and radial small gaps, hence, the initial prototype of the microgyroscope was realized. The key techniques described in this paper can be applied to fabrication of other micro devices. The metal foundation method, associated with removal of SU-8 mold by oleum, is expected to make SU-8 wider applications in making integrated microstructures with fabricated circuitry on the same chip.     

Proton micromachining: a new technique for the production of three-dimensional microstructures

A novel technique for the fabrication of high aspect ratio three-dimensional (3D) microstructures is presented. A suitable resist (e.g. PMMA or SU-8) is exposed using focused MeV (million electron volt) protons in a direct write process to produce 3D microstructures with sub-micrometer feature sizes. By adjusting the energy of the proton beam, the depth of the microstructures can be controlled very accurately (e.g. between 5 and 160 μm). Single layer SU-8, a newly developed, chemically accelerated, negative tone, near UV, photo-resist, has been used in multiple exposures using different proton energies to produce intricate 3D microstructures. The combination of a well controlled exposure depth coupled with the ability to tilt the sample with respect to the beam increases the manufacturing capability, and allows the production of complex microstructures with well defined edges in single layers of resist. Received: 15 July 1999/Accepted: 30 July 1999     

Sandwich structure based on back-side etching silicon (100) wafers for flexible electronic technology

An SU-8-silicon(100)-SU-8 flexible composite sandwich structure is studied. Besides preventing corrosion to the thin silicon membrane, SU-8 photoresist coated on the silicon membrane improves its flexibility as shown by an ANSYS finite element simulation. Using a plasma enhanced chemical vapor deposited SiO₂/Si₃N₄ composite film as an etch mask on the polished side, a 4″ (100) silicon wafer was thinned to 26 µm without rupture in a 30 wt% KOH solution. The wafer was coated on both sides with 20 µm of SU-8 photoresist and then bent over cylinders with various diameters, to flex the sandwich in a controlled manner. The determined minimal bending radius of the fabricated thin silicon-based sandwich structure is approximately 3.5 mm. The fabrication of this sandwich structure is compatible with conventional microelectronic fabrication processing. Thus, it allows one to thin fully fabricated devices in a post-fabrication process to make high-performance flexible electronics.

     

RF MEMS switches fabrication by using SU-8 technology

In this paper we present a novel process based on SU-8 technology for the fabrication of double clamped radio frequency (RF) micro-electro-mechanical system (MEMS) capacitive shunt switches in coplanar configuration. The key element of the exploited process is the MicroChem SU-8 2002 negative photoresist. The polymeric material is widely used in MEMS device processes because of its excellent thermal and chemical stability. In this paper, SU-8 polymer has been utilized in a double way to get suspended structures as double clamped beams: (1) SU-8 for the lateral supports, and (2) as a sacrificial layer for the release of the suspended membrane. Preliminary RF tests on the manufactured switches have been done, and the measured electrical performances are in good agreement with the performed simulations.     

Inexpensive, quickly producable X-ray mask for LIGA

 The capability to produce X-ray masks inexpensively and rapidly is expected to greatly enhance the commercial appeal of the LIGA process. This paper presents a process to fabricate X-ray masks both inexpensively (under $1000) and rapidly (within a few days). The process involves one UV lithography step and eliminates the need for an intermediate X-ray mask. The X-ray mask produced by this process consists of a 125 μm thick graphite membrane that supports a gold-on-nickel absorber pattern. The thickness of the absorber structures is great enough to supply sufficient contrast even when radiation sources with high characteristic photon energies up to 40 keV are utilized and/or when deep exposures are desired. The mask fabrication process is initiated by spin coating 30–50 μm of SU-8 directly on a graphite membrane. The SU-8 is then patterned using a UV mask. Gold-on-nickel absorber structures are electroplated directly onto the SU-8 covered graphite. Once the remaining SU-8 is removed, attaching the graphite membrane to a frame completes the mask. To test the performance of the mask, a nickel mold insert was fabricated. A sheet of PMMA 500 μm in thickness was bonded to a nickel substrate, then exposed to X-rays through the mask, and developed. Electroplating nickel into the patterned PMMA sheet produced a mold insert. SEM pictures taken of the SU-8, the X-ray mask, and the mold insert are shown. This method of rapidly producing an inexpensive X-ray mask for LIGA resulted in a mold insert with smooth, vertical sidewalls whose dimensions were within two micrometers of the UV mask dimensions. Received: 12 December 1998/Accepted: 2 February 1999     

Quasi-3D microstructure fabrication technique utilizing hard X-ray lithography of synchrotron radiation

 Quasi-three-dimensional (3D) microstructure fabrication technique utilizing hard X-ray lithography (HXL) has been developed. In this technique, as the intensity distribution of the X-rays is controlled by a newly developed bending mirror, the exposure residual depth of polymethyl methacrylate (PMMA) resist is controlled over the exposed area. The maximum difference of depths was approximately 50 μm over the large area more than 60 mm (horizontal) × 5 mm (vertical). We also investigated the effects of controlling the beam intensity distribution for exposure changing X-ray mask absorber shapes and angle on the obtained quasi-3D resist pattern shapes. As the results, Quasi-3D PMMA patterns with inclined shape sidewall and graded depths were successfully fabricated. We believe this technique greatly expands applications of LIGA process. Received: 10 August 2001/Accepted: 24 September 2001 This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Suspended SU-8 structures for monolithic microfluidic channels

SU-8 photoresist is commonly used in the field of microfabrication as structural material or for molding of microfluidic devices. One major limitation, however, is the difficulty to process partially freestanding SU-8 structures or monolithic closed cavities and channels on-chip. We propose here a simple method for the fabrication of suspended structures, in particular of monolithic SU-8 microchannels. The method is based on the processing of a SU-8 double-layer. Appropriate modification of the optical properties of the upper layer allows for selective crosslinking in the layer sandwich. This process is suitable for versatile layouts comprising open and hollow SU-8 structures on the same chip.     

In-situ fabrication of polymer microsieves for ��TAS by slanted angle holography

We present a simple, versatile method for the in-situ fabrication of membranes inside a microfluidic channel during a chip manufacturing process using only two extra slanted angle holographic exposure steps. This method combines the strengths of both inclined UV exposure and holographic lithography to produce micrometer-sized three-dimensional sieving structures. Using a common chip material, the photoresist material SU-8, together with this method, a leak-free membrane-channel connection is obtained. The resulting membranes are monodisperse, with a very well-defined pore geometry (i.e., microsieves with a pore diameter between 500 nm and 10 μm) that is easily controllable with the holographic set-up. The selectivity of in-situ fabricated microsieves with a pore diameter of 2 μm will be demonstrated using polystyrene beads of 1 and 3 μm.