Metrology of high aspect ratio MEMS

Lack of metrology tools for inspecting high aspect ratio MEMS severely limits the degree to which tolerances of a given part can be examined. Tools such as SEMs, AFMs, vision-based systems, and profilometers are good at examining two-dimensional entities of a part or for calculating surface roughness characteristics. None of these tools, however, can extract full three-dimensional data sets of high aspect ratio MEMS for part inspection. The hardware is either limited by the steep sidewalls of the part or by the simple fact that the acquisition method only collects two-dimensional data. This research proposes a method for extracting three-dimensional information from a part using multiple two-dimensional pointclouds. A fiducial setup is proposed which would allow for the registration of multiple pointclouds. A computer-aided inspection (CAI) software platform has been developed to handle the multiple data sets. The software platform implements a least-squares localization routine to compute the best-fit deviations from the nominal CAD geometry, as well as algorithms to determine the correct alignment between two pointclouds. With this software platform, both form errors of a single pointcloud and geometric errors with respect to multiple pointclouds can be calculated.This work was partially funded by Sandia National Laboratories and the National Science Foundation under Grant Number DMI-9988664. The government has certain rights in this material. Any opinions, findings and conclusions or recommendations are those of the authors and do not necessarily reflect the views of the Sandia National Laboratories or National Science Foundation.This paper was presented at HARMST 2003 in June 2003.     

Enhanced adhesion of PMMA to copper with black oxide for electrodeposition of high aspect ratio nickel-iron microstructures

 Copper is widely used as a plating base for soft magnetic alloy electrodeposition in sensors and actuators. PMMA, the X-ray resist used in the LIGA process, typically has poor adhesion with copper. The use of black oxide of copper to enhance PMMA-copper adhesion was investigated. In this work, peel strength as a function of treatment time and the method of bonding was evaluated using an ASTM standard T-peel test. Peel strength increased with increasing treatment time. The feasibility of producing microstructures with predictable 3-D geometry for use in resonating sensors was investigated using the process developed. Nickel-iron structures of 100-1000 micrometers wide and 500 micrometers tall were successfully electrodeposited. Growth of other representative microstructures is being investigated. Received: 25 August 1997/Accepted: 3 September 1977     

Challenges with high aspect ratio nanoimprint

When nanoimprint is not used for lithography purposes (NIL), but for the direct patterning of polymeric layers, high aspect ratio patterns may be of interest for a number of applications. The definition of such patterns in a nanoimprint process deals with two aspects, a successful filling of the high aspect ratio cavities of the stamp used, followed by a successful separation of the high aspect ratio structures defined in the polymeric layer on the substrate, from the stamp. These two aspects are addressed by shedding light to the impact of capillary effects during the filling of high aspect ratio cavities, and to the deformation processes involved in the separation of the stamp from the polymeric structures, where adhesional energies have to be overcome without cohesional failure. Both aspects are discussed in terms of the geometries involved, the stamp geometries as well as the polymeric layer thickness, and correlations with thermally-assisted (T-NIL) and UV-assisted (UV-NIL) processing are deduced. The aspects discussed are typical of a nanoimprint situation with thin polymeric layers on hard substrates.     

Micromechanical machining of high aspect ratio prototypes

 Micromechanical machining uses physical cutting tools in high precision machines to fabricate parts with micrometers features and sub-micrometer tolerances. An advantage of this process is the ability to use any machinable material, quick process planning and material removal, and three-dimensional geometry only limited by the machine and tools used. Disadvantages are that forces are placed on micro cutting tools causing deflection and possible breaking. Deflection reduces process precision and tool breakage results in repeated set up, slower production, and poorer tolerances. Nevertheless, these processes have created many diverse prototypes ranging from biomedical to space applications. Received: 10 August 2001/Accepted: 24 September 2001     

Pulsed electrodeposition of high aspect-ratio NiFe assemblies and its influence on spatial alloy composition

Direct- and pulse-reverse current electrodeposition processes and their influence on the spatial nickel-iron alloy composition are described. High-aspect ratio gear wheel structures are electroplated from a nickel sulphate/iron sulphate bath. Subsequent determination of the iron content at the gear-wheels cross section using scanning electron microscopy demonstrates that pulse reverse plating results in an uniform alloy composition across different structure dimensions. By contrast, local variations in iron content were observed in case of direct current plating.     

Design and simulation of high aspect ratio motion sensors

Microsystem Technologies -     

Large tuning ratio high aspect ratio variable capacitors using leveraged bending

Variable capacitors are a key component in Radio Frequency Micro Electro-Mechanical Systems (RF MEMS). They comprise fixed and flexible electrodes. Deformation, or actuation, of the flexible electrode changes the capacitance of the capacitor. This way, electrical properties of high frequency circuits can be modified. Traditionally, variable capacitors are based on a planar layout architecture, while a newer, vertical-wall, quasi three-dimensional approach theoretically enables increased device performance. Such devices depend on high aspect ratios, i.e. relatively high micro structures with very thin walls and gaps. A few vertical-wall variable capacitors made of nickel or gold have been fabricated to date, using deep X-ray lithography and subsequent electroplating (Achenbach et al. 2006; Klymyshyn et al. 2007, 2010) as the fabrication approach. They feature, amongst others, excellent quality factors of Q ≤ 95 at 5.6 GHz with 50 Ω reactance, but suffer from a very limited tuning range of the capacitance value (tuning ratio of, e.g., 1.38:1). The devices presented here exploit the same architecture and materials selection, resulting in similar, excellent Q-factors, but feature a different electrode layout approach, referred to as leveraged-bending. This layout is based on pulling a flexible electrode sideways, towards a fixed electrode, increasing the capacitance when actuating the variable capacitor. The leveraged bending approach theoretically enables infinitely high tuning ratios for components with perfect structure accuracy. To date, a significantly increased tuning ratio of 1.9:1 has been demonstrated. Limiting factors are an electrically non-ideal layout geometry chosen as a compromise to increase the fabrication yield, and structure deviations of ~1.6 μm from CAD layout to the electroplated component. Electrostatic actuation requires voltages between 0 and 72 V for capacitance values on the order of C = 0.3 pF at device dimensions of about 1.5 mm overall length, 5–10 μm gap and wall widths, and 100 μm metal height. Device performance measured with a vector network analyzer is in 97 % agreement with simulation results based on two-dimensional electrostatic-structural coupling (ANSYS Multiphysics) and three-dimensional electromagnetic field simulations (Ansoft HFSS). These simulations also indicate that an optimized gap geometry will allow to reduce the actuation voltage required by up to 40 %.     

The micromilling process for high aspect ratio microstructures

 High aspect ratio microstructures are currently created by several processes which include lithography (X-ray, deep ultraviolet, etc.) and mechanical machining (diamond machining, microdrilling, etc.). The lithographic processes require more extensive processing equipment such as an energy source, mask/mask holder/mask aligner, photoresist and substrate, and chemical development capacity. In addition, these processes are serial in nature and each adds to the tolerances of the finished structure. The current mechanical processes provide for the direct removal of the substrate material in a single step but are more limited in the geometric patterns which can be created. In conventional machining, the process which provides the most versatility in geometric patterns is milling. The micromilling process has two basic components. The first is the fabrication of small milling cutters with very sharp cutting edges. The second is the actual removal of the workpiece material with a very precise and repeatable machine tool. Several basic cutter designs have been fabricated using focused ion beam micromachining and are undergoing testing. The cutter diameters are nominally 100 micrometers and 22 micrometers. Results have been obtained which show that this process can be very effective for the rapid fabrication of molds and mask structures. Received: 30 October 1995 / Accepted: 4 March 1996     

The micromilling process for high aspect ratio microstructures

High aspect ratio microstructures are currently created by several processes which include lithography (X-ray, deep ultraviolet, etc.) and mechanical machining (diamond machining, microdrilling, etc.) The lithographic processes require more extensive processing equipment such as an energy source, mask/mask holder/mask aligner, photoresist and substrate, and chemical development capacity. In addition, these processes are serial in nature and each adds to the tolerances of the finished structure. The current mechanical processes provide for the direct removal of the substrate material in a single step but are more limited in the geometric patterns which can be created. In conventional machining, the process which provides the most versatility in geometric patterns is milling. The micromilling process has two basic components. The first is the fabrication of small milling cutters with very sharp cutting edges. The second is the actual removal of the workpiece material with a very precise and repeatable machine tool. Several basic cutter designs have been fabricated using focused ion beam micromachining and are undergoing testing. The cutter diameters are nominally 100 micrometers and 22 micrometers. Results have been obtained which show that this process can be very effective for the rapid fabrication of molds and mask structures.     

Micro-electroforming high aspect ratio microstructures under magnetic field

Microsystem Technologies - High-aspect-ratio (HAR) micro-electroforming is a significantly challengeable implementation to manufacture metallic microstructures and microparts mainly due to mass...     

New development strategies for high aspect ratio microstructures

 In the first step of the LIGA process a resist layer, typically PMMA (polymethylmethacrylate), is pattered by deep X-ray lithography. The exposed parts are subsequently dissolved by an organic developer. The quality and the achievable height of the microstructure is decisively determined by the development process. In order to increase the aspect ratio and maintain the quality of the microstructures the parameters influencing the development process were investigated. In the case of dip development and ultrasound development a strong dependency of the development rate on the temperature, dose value and depth of deposition has been noticed. The development rate increases with increasing dose value and temperature and decreases with increasing depth of deposition. In case of dip development the development course can be described by a phenomenological equation which considers the three mentioned parameters. In the case of ultrasound further parameters have to be taken into account: the geometry and the dimensions of the strucutres. Received: 25 August 1997/Accepted: 3 September 1997     

Measurement of side walls of high aspect ratio microstructures

In this paper, the new method of measurements is presented for the technological control of the lithographic fabrication of high aspect ratio microstructures with complex side-wall profiles. The sampling procedure includes pouring polymer glue on a substrate with the microstructures and its hardening, sectioning the polymer matrix in the depth with planes parallel to the substrate and analysis of digital optical microscope images of the planes. The measured deviations the micro structure profiles in comparison with the designed geometry are used for the further optimization of the technological design of planar X-ray refractive lenses.     

Eutectic bonding of Al-based high aspect ratio microscale structures

Metal-based microelectromechanical systems (MEMS) have important advantages over Si-based MEMS. To form any functional metal-based microdevice from metallic high-aspect-ratio microscale structures (HARMS), proper assembly and packaging are required. In this paper, we report successful eutectic bonding of Al-based HARMS using Al–Ge intermediate layers. A series of Al–Ge composite thin films were vapor deposited. Their composition and micro-/nano-scale structure were characterized. The morphology of bonded Al structures was examined, including the morphology of separated surfaces.     

Manufacturing of microstructures with high aspect ratio by micromachining

The development of micromachining plays an important role in miniaturization of microsystems. Micromachining is a very flexible and compared to EDM or ECM a very fast machining process with a high material removal rate. A wide range of materials like polymers, metals and alloys as well as some sorts of ceramics can be machined. Also 3D-structures can be easily manufactured. Additionally, big advances have been made concerning the realization of high aspect ratios for small diameter end mills. Whereas end mills below 100 μm diameter are limited to an aspect ratio of 1.5, end mills of 100 μm diameter are available up to an aspect ration of ten now. A few years ago, end mills in this diameter range were uncoated. Nowadays, there was a big progress in the coating technology so that these end mills can be coated with layers as thin as 0.5 μm.     

Hot embossing of transparent high aspect ratio micro parts

Accurate replications of complex, high aspect ratio nano and micro structured parts are still challenging due to the comparatively high surface-volume ratio. The critical process step of automated, undistorted demoulding in high precision replication techniques like hot embossing or thermal nano imprint require good process control at elevated temperatures just below the solidification of the thermoplastic material and higher adhesive forces of the polymer part to the substrate plate than to the structured tool insert. The required increase in interfacial surface to the substrate plate is typically done by a rough substrate plate which results in milky, in-transparent residual layer. We demonstrate a process modification which keeps the advantage of precise automated demoulding and allows for replication of micro structured parts with optically transparent residual layer even for high aspect ratio structures resulting in high demoulding forces.     

Production of metallic high aspect ratio microstructures by microcasting

Microcasting is a competitive process for the production of metallic microparts. The remarkable suitability of this technique especially for extreme aspect ratio microsamples made of metal alloys will be shown. Based on the well known process of investment casting it is possible to produce microstructures by a gold basis alloy called Stabilor G^® with an aspect ratio of up to 90. Basic investigations on the ability of form filling and flow length show that microcasting can be improved by a increasing of the molding pressure to about 25 bar. Basic results will be presented as well as a testing structure for estimating the quality of the casting process in regard of its suitability for microcasting of high aspect ratio parts.     

国外高深宽比微细结构制造技术的发展

介绍了国外近年来高深宽比微细结构制造技术的最新发展,其中包括LIGA、准LIGA、深层反应离子刻蚀等主要高深宽比微细结构技术的具体研究现状和研究进展,还介绍了电化学沉积加工、质子束光刻、准LIGA与Si等离子刻蚀复合加工、局部电化学沉积、立体光固化微加工、微细电解加工等一些新发展的高深宽比微细结构技术。     

Developing new markets for high aspect ratio micro-machined devices

 This paper reports on the commercialization strategy being developed by MEMStek Products, LLC (formerly CONCIS) to startup a component business using sacrificial LIGA (S-LIGA) as the initial manufacturing technology. S-LIGA provides the ability to fabricate fluidic, mechanical, and optical components and modules in the mesoscopic scale that exists between conventional silicon micro-machining and precision machining tools such as molding, stamping, and embossing. S-LIGA technology commercialization has some unique challenges. The S-LIGA technology must be developed from its current state into a viable manufacturing process. Since nearly every device fabricated sees x-ray exposure at least once, this creates the need for a production oriented synchrotron facility with the capacity for handling substantial wafer volume. Reliably electro-plating into 0.01 mm feature size becomes a major technical challenge to overcome. Finally, targeting the initial applications for the market requires a careful selection of initial customers and partners. Received: 25 August 1997/23 November 1997     

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.