Depth Lithography Research Articles

Electromagnetic Micromotors—Design, Fabrication and Applications

Microactuators have become essential elements of microelectromechanical systems, for example, for positioning purposes and for fluid-handling tasks in microfluidic systems. UV depth lithography and other new micromachining technologies, which have been developed since the 1990s, have initiated extensive investigations of electromagnetic microactuators, which are characterized by high forces, large deflections, low driving voltages resulting from low input impedances and robustness under harsh environments. This paper reviews the comprehensive research on the design, fabrication and application of electromagnetic micromotors performed in our laboratory over the past years.

Implementation of Synchronous Micromotor in Developing Integrated Microfluidic Systems

This paper introduces the synchronous micromotor concept and presents new investigations on its application as an integrated driving mechanism in microfluidic systems. A spiral channel viscous micropump and a microstirrer are considered and tested as examples to verify the concept. The fabrication technology of such integrated systems is based on UV depth lithography, electroplating and soft lithography. The synchronous micromotor consists of a stator including double layer coils, and a rotor disk containing alternate permanent magnets. The coils are distributed evenly around the stator and arranged in three phases. The phases are excited by sinusoidal currents with a corresponding phase shift resulting in a rotating magnetic field. Regarding the spiral channel viscous micropump, a spiral disk was fixed onto the rotor disk and run at different rotational speeds. Tests showed very promising results, with a flow rate up to 1023 µL·min−1 at a motor rotational speed of 4500 rpm. Furthermore, for the application of a microstirred-tank bioreactor, the rotor disk design was modified to work as a stirrer. The performance of the developed microbioreactor was tested over a time period of approximately 10 h under constant stirring. Tests demonstrated the successful cultivation of S. cerevisiae through the integration of the microstirrer in a microbioreactor system. These systems prove that synchronous micromotors are well suited to serve as integrated driving mechanisms of active microfluidic components.

Fabrication and test of multilayer microcoils with a high packaging density

In magnetic microactuation, the energy converted by a magnetic microactuator is proportional to its volume (Kohlmeier et al., Microsystem Technol 8(4–5):304–307, 2002a). For that reason, it is desirable not only to choose an actuator’s active part with a sufficient footprint size, but also with a substantial building height. By applying UV depth lithography, such a requirement may be fulfilled. This paper investigates the application of high aspect ratio systems technology to fabricating a compact microcoil as field generating element. The coil has four layers. To take optimal advantage of the building height, the insulation film between the coil layers was kept small by applying a thin Si3N4 layer. The coils are intended for the integration in a magnetic microlevitation system serving as an electromagnetic guide for a linear microhybrid step motor (Budde et al., In: Proceedings of the 9th int. conf. on new actuators, pp 665–668, 2004).

Application of UV depth lithography in micro system technology

This letter reports on new types of photo resists allowing UV-light exposure of thick layers. Optimized processes Cthat for the positive resist AZ 9260 and the negative resist Epon SU-8 have been developed enabling the fabrication of high aspect ratio metallic and polymer micro components. The high potential of this technology will be demonstrated on the basis of two examples, a variable reluctance micro motor with compensated attractive force and an optical microphone developed for application in electro-magnetic interference environments.

Linear Variable Reluctance (VR) Micro Motors With Compensated Attraction Force: Concept, Simulation, Fabrication and Test

This paper reports on optimized fabrication technologies for electromagnetic actuator systems and their characterization. These actuators consist of three-dimensional microstructures of increasing complexity like electroplated micro coils, flux guidances, insulation and planarization layers. The microstructures are fabricated by UV depth lithography using AZ9260, electrodepositable photo resist and Epon SU-8. Electroplated nickel iron and copper is used for the generation of flux guidance and coils. The magnetic flux is generated and guided in plane by a three dimensional double meander coil wound around laterally toothed stator poles. The traveller's comb shaped poles reach in between the stator poles and, therefore, are attracted from both sides. Thus, the attraction forces exceeding thrust, common to any VR-motor concept, are eliminated. The micro actuator was simulated, fabricated and successfully tested.

Complete 3D UV microfabrication technology on strongly sloping topography substrates using epoxy photoresist SU-8

A complete process of 3D UV depth lithography with electrodeposition was integrated into a complex technology for the fabrication of copper high-aspect-ratio micro coils. Three effects complicated the lithographic processing. A satisfactory adhesion of the resist structures on the copper seed layer could be achieved by application of a MCC Omnicoat TM adhesion layer. Locally uncontrolled crosslinking reactions within the SU-8 were caused by light reflections at strongly sloping topography. They could be avoided either by local removal of the reflecting seed layer or by redesign of the photomask. The SU-8 removal was accomplished by plasma etching followed by lifting off the resist residues in a wet etch step necessary for the removal of the seed layer. Resist structures of a width both of lines and of spaces of 4–4.5 μm were formed at 20 μm thickness, resulting in corresponding copper coils electroplated to a height of 15 μm.

Complete 3D UV microfabrication technology on strongly sloping topography substrates using epoxy photoresist SU-8

A complete process of 3D UV depth lithography with electrodeposition was integrated into a complex technology for the fabrication of copper high-aspect-ratio micro coils. Three effects complicated the lithographic processing. A satisfactory adhesion of the resist structures on the copper seed layer could be achieved by application of a MCC Omnicoat TM adhesion layer. Locally uncontrolled crosslinking reactions within the SU-8 were caused by light reflections at strongly sloping topography. They could be avoided either by local removal of the reflecting seed layer or by redesign of the photomask. The SU-8 removal was accomplished by plasma etching followed by lifting off the resist residues in a wet etch step necessary for the removal of the seed layer. Resist structures of a width both of lines and of spaces of 4–4.5 μm were formed at 20 μm thickness, resulting in corresponding copper coils electroplated to a height of 15 μm.

An investigation on technologies to fabricate microcoils for miniaturized actuator systems

Within the framework of the Collaborate Research Center “Design and Fabrication of Active Microsystems” (Sonderforschungsbereich 516), one aspect is the development of fabrication technology for driving coil systems to be applied to in electromagnetic microactuators. The two key focus areas are the fabrication of high aspect ratio conductors and insulating them by applying planarizing polymer material. The research activities included the development of technologies like for UV depth lithography, electroplating through lithographic masks (galvanoforming), as well as embedding in and planarization by insulation material. Combining these technologies allows to create high aspect ratio integrated multi-layer coils with integrated magnetic flux guiding structures. The materials applied were copper as a conductor, NiFe for magnetic cores and the negative tone resists SU-8 (MicroChem Corp.) as well as BCB (DOW Chemical) for insulation and embedding purposes. For electroplating molds, the DNQ/Novolak type photoresists AZ4562 and AZ9260 (Clariant) were utilized. The established technologies were integrated into process flows for fabricating thick and high aspect ratio planar and helical coils for electromagnetic linear actuators.

Application of UV depth lithography and 3D-microforming for high aspect ratio electromagnetic microactuator components

This paper was presented at the Fourth International Workshop on High Aspect Ratio Micro-Structure Technology (HARMST) 2001 in Baden-Baden, Germany. For building high aspect ratio parts, using a combination of UV lithography and electroplating is a very promising approach. This paper describes investigations regarding the application of UV lithography and electroplating to fabricate high aspect ratio component parts of microactuators, such as coils and pole pieces. As resist materials, both novolac-based resists and photosensitive epoxy were investigated. The high aspect ratio photomasks were used as micromolds for electroplating coils and magnetic structures in micromotors.

Microcoils and microrelays — an optimized multilayer fabrication process

In this paper a complete process technology for the integrated fabrication of inductive micro electromechanical systems (MEMS) is presented. The introduced technology comprises the technologies of 3D-UV–Depth Lithography, electroplating of various metals and alloys, insulation and planarization of magnetic or electrical microstructures using benzocyclobutene (BCB) and anisotropic trench etching of passivation layers to incorporate metallic structures into multilayer systems. By utilizing this technology inductive micro components and systems for use in sensing, actuation and microelectronics can easily be fabricated. To demonstrate the applicability of the introduced technology to complex micro systems microcoils with and without magnetic cores and a microrelay for high current switching have been built by means of this process.

Microcoils fabricated by UV depth lithography and galvanoplating

Three-dimensional (3D) UV microforming delivers promising results for advanced surface micromachining. This low-cost technology, a combination of UV patterning of very thick photoresist layers and moulding of the resulting patterns by electrodeposition, opens a wide range of applications. It allows the use of materials with interesting properties that could not be provided by standard processes. Furthermore, this method can be carried out directly on pre-processed chips. The practical application of the technology is demonstrated by planar and 3D microcoils.

Ultraviolet Depth Lithography and Galvanoforming for Micromachining

For advanced surface micromachining 3D UV‐microforming delivers excellent results. This low‐cost technology, a combination of UV patterning of very thick photoresist layers and molding of the resulting patterns by galvanoplating, opens a wide range of applications. An advanced spin coating and drying process was developed, which allows the homogeneous deposition of photoresist layers up to more than 60 μm in a single step. Simply by exposure to standard UV aligners and following immersion development, thick photoresist layers up to 100 μm could be patterned. Repeated exposures and developments were successfully used for structuring resist layers of more than 100 μm thickness. High aspect ratios of more then 10 and steep edges of more than 88° were achieved. The resist patterns were molded by electroplating. The widely used gold and nickel were completed by copper, zinc, and magnetic alloys like NiFe. If combined with sacrificial layers, this new technique offers a wide field for smart processing from the micron to the millimeter range. 3D UV‐microforming allows the use of materials with interesting properties which could not be provided by standard processes. It opens a way for faster acceptance of microfabrication in practice.

Application of ultraviolet depth lithography for surface micromachining

Very thick photoresist layers were patterned by contact ultraviolet (UV) lithography. In a following microelectrodeposition step the generated resist patterns were molded and three-dimensional (3D) microstructures were fabricated directly onto system surfaces. The new technology, called 3D UV-microforming, consists of an advanced resist preparation process, an UV lithographic step, resist development, a molding procedure by electrodeposition, and finally stripping and cleaning for finishing the structures. It enables the low-cost fabrication of a wide variety of microcomponents for many different uses. During resist preparation, layers up to 200 μm thickness were obtained. By using a standard UV mask aligner as an exposure tool followed by immersion development, thick resist layers up to 100 μm could be patterned in a single step on preprocessed silicon wafers. Repeated exposure and development were successfully used for structuring resist layers of up to 200 μm thickness. Using AZ 4000 series photoresist, the resolution is also limited by mechanical stability. For lines and spaces in 15-μm-thick resist a minimum width of 3 μm for the resist was found to be necessary to overcome the fabrication process. For thicker layers high aspect ratios of more than 10 as well as steep edges of more than 88° could be fabricated. The resist patterns used were molded by pulse or by direct current electroplating. For microsystem applications some metals and alloys were deposited. Three-dimensional microcomponents were fabricated as samples for demonstrating the new technique. The technique allows the use of materials with interesting properties, ones that could not be provided by standard processes.

Selectively released microstructures electroplated into thick positive photoresist

A fabrication process based on sputtering of a thin film plating base, on conventional UV depth lithography, and on electrochemical deposition of gold, makes microstructures of arbitrary shape in the third dimension and of considerable height (up to 80 mu m) and resolution (aspect ratio: about 10) possible. Applications for the fabrication of microstructures are discussed.

Compact synchrotron radiation depth lithography facility

X-ray depth lithography allows the fabrication of plastic microstructures with heights of up to 1 mm but with the smallest possible lateral dimensions of about 1 μm. A resist is irradiated with ‘‘white’’ synchrotron radiation through a mask that is partially covered with x-ray absorbing microstructures. The plastic microstructure is then obtained by a subsequent chemical development of the irradiated resist. In order to irradiate a reasonably large resist area, the mask and the resist have to be ‘‘scanned’’ across the vertically thin beam of the synchrotron radiation. A flexible, nonexpensive and compact scanner apparatus has been built for x-ray depth lithography at the beamline BN1 at ELSA (the 3.5 GeV Electron Stretcher and Accelerator at the Physikalisches Institut of Bonn University). Measurements with an electronic water level showed that the apparatus limits the scanner-induced structure precision to not more than 0.02 μm. The whole apparatus is installed in a vacuum chamber thus allowing lithography under different process gases and pressures.