Fabrication of a freestanding micro mechanical structure using electroplated thick metal with a HAR SU-8 mold

In this thesis, fabrication technology of a freestanding micro mechanical structure using electroplated thick metal with a high-aspect-ratio SU-8 mold was studied. A cost-effective fabrication process using electroplating with the SU-8 mold was developed without expensive equipment and materials such as deep reactive-ion etching (DRIE) or a silicon-on-insulator (SOI) wafer. The process factors and methods for the removal of SU-8 were studied as a key technique of the thick metal micro mechanical structure. A novel method that removes cross-linked SU-8 completely without leaving remnants of the resist or altering the electroplated microstructure was utilized. The experimental data pertaining to the relationship between the geometric features and the parameters of the removal process are summarized. Based on the established SU-8 removal process, an electroplated nickel comb structure with high-aspect-ratio SU-8 mold was fabricated in a cost-effective manner. In addition, a freestanding micro mechanical structure without a sacrificial layer was successfully realized. The in-plane free movements of the released freestanding structure are demonstrated by electromagnetic actuation. This research implies that various types of MEMS devices can be developed at a low-cost with design flexibility.     

In situ fabrication of SU-8 movable parts by using PAG-diluted SU-8 as the sacrificial layer

In this paper we proposed a novel technology to make SU-8 movable parts in situ by using PAG-diluted SU-8 or SU-8R, a SU-8 solution with no photoacid generator (PAG) content, as the sacrificial layer. Since they are not or less sensitive to UV light the unexposed bottom layer in UV lithography will be dissolved to release the movable components during the final develop in PGMEA (Propylene Glycol Monomethyl Ether Acetate). The use of PAG-diluted SU-8 or SU-8R as a sacrificial layer offers several advantages including significant reduction in processing steps. It becomes the simplest technique currently available for in situ fabrication of SU-8 movable parts. In this paper the lithographic sensitivities of SU-8 as a function of the PAG concentration, detailed fabrication process and examples of fabricated SU-8 movable parts will be presented.     

Releasing high aspect ratio SU-8 microstructures using AZ photoresist as a sacrificial layer on metallized Si substrates

This paper presents a successful method for releasing high aspect ratio SU-8 micro-structures by the use of positive photoresist (AZ 4620) as sacrificial layer. The AZ 4620 photoresist sacrificial layer was dissolved by the SU-8 developer (propylene glycol monomethyl ether acetate). Thus, this process reduces the need for complex microfabrication steps and equipments which are otherwise required in traditional methods using metal sacrificial layers. The current method is both cost-effective and time-effective because no additional releasing method or material is needed to remove the fabricated SU-8 structures. Further, the influence of surface energy on the adhesion between Si and SU-8 was demonstrated and metallic thin layer coating on Si was employed to further reduce the lift-off duration. The results obtained showed that the duration for lift-off of SU-8 structures from metal (Al) coated Si substrate is much lower (approximately 90 % time saving) and the surface morphology of the released structures has lesser micropore concentration compared to the process employing bare Si as the substrate. In both processes AZ 4620 was the sacrificial layer whereas the metalized Si substrate could be re-used.     

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.     

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     

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.     

Nickel and copper electroplating of proton beam micromachined SU-8 resist

 Proton beam micromachining (PBM) has been shown to be a powerful technique to produce three-dimensional (3D) high-aspect-ratio microstructures (Watt et al., 2000). Potential commercial applications of PBM, which is a fast direct write technique, will become feasible if the fabrication of metallic molds or stamps is realised. Metallic components can be produced by electroplating a master from a microstructure produced in resist. The production of high-aspect-ratio metallic stamps and molds requires a lithographic technique capable of producing smooth and near 90° sidewalls and a one to one conversion of a resist structure to a metallic microstructure. PBM is the only technique capable of producing high-aspect-ratio microstructures with sub-micron details via a direct write process. In PBM, SU-8 (Lorenz et al., 1997) resist structures are produced by exposing the SU-8 resist with a focused MeV proton beam followed by chemical development and a subsequent electroplating step using Ni or Cu. The data presented shows that PBM can successfully produce high-aspect-ratio, sub-micron sized smooth metallic structures with near 90° sidewall profiles. Received: 10 August 2001/Accepted: 24 September 2001     

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.     

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     

A novel method to fabricate complex three-dimensional microstructures

Conventional three-dimensional (3D) microstructures such as arcs or spiralities are generally fabricated using some complicated methods like LIGA or two-photon lithography. In this paper, a new approach of fabricating 3D microstructures is provided. The process is based on UV-LIGA technology yet including a novel reformation method in the post bake procedure. The fabrication process started with coating SU-8 as thick as 500 microns on the silicon substrate, and then it was followed by an exposure with patterned mask under UV light. Subsequently, a force on the exposed SU-8 photo resist was applied in the post-bake process. By adjusting the amount of force, the way in which the force was placed and the exposure dose, we directly fabricated some complicated three-dimensional structures on the SU-8 photo resist after development of the SU-8. We call this microfabrication method as Force-LIGA (F-LIGA). Firstly, orthogonal experiment method conducted to optimize the hot-press process is presented, and then we give some experiment examples using F-LIGA approach and discuss the relationships among the exposure time, pressure and the profile of microstructures. The fabrication process can be used widely in making useful three-dimensional devices.     

A quantitative study on the adhesion property of cured SU-8 on various metallic surfaces

SU-8 has received wide attention in recent years because of its application in the fabrication of high aspect ratio microstructures and devices. This negative resist is known for its excellent lithography properties using ultraviolet light source. As the microfabrication technology based on UV-lithography of SU-8 finds wide applications, a good understanding and characterization of cured SU-8 polymer on various substrate materials are therefore very important. A good adhesion on various substrate materials is essential to both the fabrication process and to the functionality of any final products that have cured SU-8 as part of the structural material. There are very limited studies reported in this important area in the literature. This paper presents a theoretical and experimental work to quantitatively study the adhesion properties of cured SU-8 on some of the most commonly used metallic surface materials. The adhesion strengths of cured SU-8 samples on Au, Ti, Cu, Cr, and Ni coated glass substrates were measured following ASTM-C633 standard. A detailed analysis of the experimental results was also provided based on the atomic structures and electron configurations of the respective metals.     

Comparison of PMMA and SU-8 resist moulds for embossing of PZT to produce high-aspect-ratio microstructures using LIGA process

 In this paper results are presented from a range of experiments to explore the feasibility of inserting a ceramic material PZT (lead zirconium titanate) into different kinds of high-aspect-ratio resist moulds. Polymethylmethacrylate (PMMA) and SU-8 on silicon substrates and free-standing SU-8 membranes with micro-cavities or through-holes (defined by X-ray lithography) have been used as moulding medium. Processing conditions for the resist materials including pre-bake, exposure, post-bake, development and stripping have been compared. The advantages of different types of resist mould for the LIGA process has been evaluated. Additionally a comparison of photosensitivity of PMMA and SU-8 has been carried out. Using a range of load pressures (5 to 60 MPa), appropriate conditions for PZT embossing into resist moulds have been determined (ensuring minimum void formation in the final PZT structures). In the final form, SU-8 moulds have been removed by laser ablation. This is the first reporting of high-aspect-ratio ceramic microstructures fabricated using a combination of SU-8 moulds and PZT embossing. Received: 10 August 2001/Accepted: 24 September 2001     

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.     

Deep photo-lithography characterization of SU-8 resist layers

SU-8 layers of different thickness that were formed by spinning and soft baking or a casting process with a solvent content of not more than 4% were used for experimental investigation of deep UV lithography using modified radiation of a mercury lamp. The specific absorbance of the SU-8 layers have been measured in dependence on the wavelength to calculate the power of the absorbed radiation as a function of depth in the resist layer. The resist layers which were formed on a photo-mask have been exposed with a variation of exposure dose to study the residual thickness of the resist. There are threshold exposures for the formation of insoluble SU-8 resist layer and for the disappearance of shrinkage of the resist layer, which depends on the size of the irradiated area, on microstructure topology and on the resist thickness for fixed parameters of pre- and post-exposure baking. It has been shown that exact filtration of a low-intensity band of exposure radiation at 334 nm allows to reduce strong diffraction distortion in the upper layer of the resist. Microstructures with an aspect ratio of 25 in a SU-8 resist layer of 1 mm thickness have been obtained using a 100 m thick SU-8 resist layer as a filter for the UV radiation and with optimal dose of exposure.     

SU-8: promising resist for advanced direct LIGA applications for high aspect ratio mechanical microparts

A case study of use of negative type SU-8 X-ray sensitive resist for fabrication of advanced, highly precise, ultra tall direct LIGA mechanical microparts is presented in this paper. Using direct LIGA technique, ～1 mm tall highly precise metallic gear wheels are being fabricated, previously using PMMA based process. Starting from a non-optimized non-satisfying SU-8 process, significant process parameters for process optimization were identified using statistical design of experiment. By varying the significant process parameters, SU-8 process was further optimized with respect to critical aspect of sidewall bow and tilt of metallic structures. After the optimization, metallic parts fabricated using SU-8 process showed comparable quality as those fabricated using PMMA based process.     

A new removable resist for high aspect ratio applications

A variety of different photo resists are used for fabrication of MEMS. Presently good results were reported for SU-8, a chemically amplified negative tone photoresist. But SU-8 has a disadvantage for some applications in LIGA technique, especially in the X-ray mask fabrication. After processing the finished resist pattern are hardly soluble from the substrate. This paper will briefly describe the current status of the development of the new negative tone photoresist CAR 44 whose big advantage is the easy removableness of the cross linked pattern. This work widely uses the contents of the presentation “A New Removable Resist for High Aspect Ratio Applications” to the High Aspect Ratio Micro Structure Technology workshop HARMST 2005 held in Gyeongyu (Republic of Korea), June 10–13, 2005.     

A new UV sensitive positive resist for X-ray masks manufacture

LIGA is a well-established process to fabricate metallic micro parts with high resolution, high precision and very low sidewall roughness by means of X-ray lithography and electroplating. The availability of a precise X-ray mask is a precondition for the final precision of the manufactured micro parts. Typical mask substrate materials, e.g. beryllium, carbon based foils, Si₃N₄ or SiC show different disadvantages such as low X-ray transparency or high toxicity or high prices or low conductivity or high thermal expansion or surface porosity causing X-ray scattering. For the fabrication of X-ray masks, PMMA with its unique features such as high aspect ratio patterns with high precision, exhibits low sensitivity and the layers preparation is not easy. SU-8, an epoxy-based UV and X-ray sensitive, chemically amplified, negative tone photoresist exhibits high aspect ratio patterns with vertical sidewalls. The difficult remove of the resist after the electroplating process significantly hinders the inspection of the fabricated X-ray mask. We present the use and suitability of an UV sensitive, chemically amplified, viscous, aqueous-alkaline developable, and easy removable positive tone photoresist, XP mr-P 15 AV, exhibiting high aspect ratio patterns with vertical sidewalls for the fabrication of X-ray masks by means of UV lithography on vitreous carbon substrates.     

2-DOF vibratory gyroscope fabricated by SU-8 based UV-LIGA process

This paper reports fabrication of 2-DOF vibratory gyroscope using SU-8 based UV-LIGA process. The device structure is designed to be symmetrical in order to match the resonance frequencies of drive and sense mode oscillators and also to minimize their relative temperature dependent drift. The overall arrangement is such that the two vibration modes do not affect each other and therefore, mechanical decoupling is achieved which helps in minimizing bias drift. The design is optimized to be compatible with the UV-LIGA process having 10 μm thick electroformed nickel as structural layer. Photolithography to create 11 μm thick SU-8 molds for electroforming sacrificial copper and structural nickel layer is optimized using multiple exposure technique that ensures near vertical side walls. Since the highly cross-linked SU-8 remaining after development is difficult to remove reliably from high aspect ratio structures without damage or alteration to the electroformed metals, a 2.45 GHz MW plasma etching process is developed with CF₄/O₂ mixes. The fabricated device is checked for off-plane misalignment between the stationary and movable comb fingers using white light interferometry and it is found to be almost negligible. Also, the prototype device is characterized for amplitude and phase spectral responses using Polytec MSA-500 Micro System Analyzer. The drive and sense mode resonance frequencies are observed at 7.3 and 7.1 kHz respectively against the mode matched designed frequency of 7.5 kHz.     

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.     

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.