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.     

Fabrication of compliant high aspect ratio silicon microelectrode arrays using micro-wire electrical discharge machining

This paper reports on the fabrication of high aspect ratio silicon microelectrode arrays by micro-wire electrical discharge machining (μ-WEDM). Arrays with 144 electrodes on a 400 μm pitch were machined on 6 and 10 mm thick p-type silicon wafers to a length of 5 and 9 mm, respectively. Machining parameters such as voltage and capacitance were varied for different wire types to maximize the machining rate and to obtain uniform electrodes. Finite element analysis was performed to investigate electrode shapes with reduced lateral rigidity. These compliant geometries were machined using μ-WEDM followed by a two step chemical etching process to remove the recast layer and to reduce the cross sections of the electrodes.     

Differential C<Superscript>4</Superscript>D sensor for conductive and non-conductive fluidic channel

This paper presents a novel design of a differential C⁴D (DC⁴D) sensor based on three electrodes for both conductive and non-conductive fluidic channel. This structure consists of two single C⁴D with an applied carrier sinusoidal signal to the center electrode as the excitation electrode. The electrodes are directly bonded on the PCB with built-in differential amplifier and signal processing circuit in order to reduce the parasitic component and common noise. In the non-conductive fluidic channel, the output voltage and capacitance changes 214.39 mV and 14 fF, respectively when a 3.83 μl tin particle crosses an oil channel. In conductive fluidic channel, the output voltage and admittance change up to 300 mV and 0.07 μS for the movement of a 4.88 μl plastic particle through channel. Moreover, the voltage change of this sensor is linear relation with the volume of investigated particle. This sensor also allows measuring velocity of particle inside fluidic channel and resistivity of the conductive fluidic.     

Prediction and optimization of performance measures in electrical discharge machining using rapid prototyping tool electrodes

In this work, the performance of rapid prototyping (RP) based rapid tool is investigated during electrical discharge machining (EDM) of titanium as work piece using EDM 30 oil as dielectric medium. Selective laser sintering, a RP technique, is used to produce the tool electrode made of AlSi10Mg. The performance of rapid tool is compared with conventional solid copper and graphite tool electrodes. The machining performance measures considered in this study are material removal rate, tool wear rate and surface integrity of the machined surface measured in terms of average surface roughness (Ra), white layer thickness, surface crack density and micro-hardness on white layer. Since the machining process is a complex one, potentiality of application of a predictive tool such as least square support vector machine has been explored to provide guidelines for the practitioners to predict various machining performance measures before actual machining. The predictive model is said to be robust one as root mean square error in the range of 0.11–0.34 is obtained for various performance measures. A hybrid optimization technique known as desirability based grey relational analysis in combination with firefly algorithm is adopted for simultaneously optimizing the performance measures. It is observed that peak current and tool type are the significant parameters influencing all the performance measures.

     

High aspect ratio meso-scale parts enabled by wire micro-EDM

Micro-electro discharge machining (EDM) is a subtractive meso-scale machining process. The Agie Excellence 2F wire micro EDM is capable of machining with a 25 micron diameter wire electrode and positioning the work piece to within ±1.5 microns. The over-burn gap can be controlled to within 3 microns to obtain a minimum feature radius of about 16 microns while achieving submicron surface finish and an imperceptible recast layer. For example, meso-scale gears that require vertical sidewalls and contour tolerances to within 3 microns can be wire EDMed into a variety of conductive materials. Material instabilities can affect the dimensional precision of machined meso-scale parts by material relaxation during the machining process. A study is done to investigate the machining performance of the wire micro EDM process by machining a high aspect ratio meso-scale part into a variety of metals (e.g. 304L stainless steel, Nitronic 60 Austentic Stainless, Beryllium Copper, and Titanium). Machining performance parameters such as, profile tolerance, perpendicularity, and repeatability are compared for the different materials. Pertinent inspection methods desirable for meso-scale quality assurance tasks are also evaluated. Sandia National Laboratories is developing meso-scale electro-mechanical components and has an interest in the assembly implications of piece parts fabricated by various meso-scale manufacturing processes. Although the wire EDM process is typically used to fabricate 2½ dimensional features, these features can be machined into a 3 dimensional part having other features such as hubs and chamfers to facilitate assembly.     

Characterisation of high aspect ratio non-conductive ceramic microstructures made by spark erosion

The spark erosion process is widely used for micro structuring. Its possibility to structure materials independent of their material properties like high hardness or melting temperature enables to address a large material diversity. However the process requires a minimal electrical conductivity of 0.1 Scm⁻¹. Nevertheless recent research has shown that the usage of an assisting electrode makes a processing of non-conductive materials possible. Thus even ceramics like Al₂O₃ or ZrO₂ can be processed. These materials are becoming more and more interesting for industrial application and the field of miniaturisation due to their outstanding material characteristics like high hardness, bending strength, melting temperature and chemical inertness. In this study a new lacquer based assisting electrode is used to erode bars in zirconia samples. For this purpose a modular tool concept is established. Bars with aspect ratios of more than 80 are generated. The achieved bar heights are 1.5 mm and the smallest bar width is 8 μm. Furthermore a characterisation of the sidewall angles showed mean values between 0.4° and 2.2° depending on the bar height and width.     

Design,fabrication and testing of a piezoelectric energy microgenerator

This article presents the design, fabrication and characterization of a micromachined energy harvester utilizing aluminium nitride (AlN) as a piezoelectric thin film material for energy conversion of random vibrational excitations. The harvester was designed and fabricated using silicon micromachining technology where AlN is sandwiched between two electrodes on top of a silicon cantilever beam which is terminated by a silicon seismic mass. The harvester generates electric power when subjected to mechanical vibrations. The generated electrical response of the device was experimentally evaluated at various acceleration levels. A maximum power of 34.78 μW was obtained for the device with a seismic mass of 5.6 × 5.6 mm² at an acceleration value of 2 g. Various fabricated devices were tested and evaluated in terms of the generated electrical power as well as the resonant frequency.     

A particle swarm approach for multi-objective optimization of electrical discharge machining process

This paper proposes an experimental investigation and optimization of various machining parameters for the die-sinking electrical discharge machining (EDM) process using a multi-objective particle swarm (MOPSO) algorithm. A Box–Behnken design of response surface methodology has been adopted to estimate the effect of machining parameters on the responses. The responses used in the analysis are material removal rate, electrode wear ratio, surface roughness and radial overcut. The machining parameters considered in the study are open circuit voltage, discharge current, pulse-on-time, duty factor, flushing pressure and tool material. Fifty four experimental runs are conducted using Inconel 718 super alloy as work piece material and the influence of parameters on each response is analysed. It is observed that tool material, discharge current and pulse-on-time have significant effect on machinability characteristics of Inconel 718. Finally, a novel MOPSO algorithm has been proposed for simultaneous optimization of multiple responses. Mutation operator, predominantly used in genetic algorithm, has been introduced in the MOPSO algorithm to avoid premature convergence. The Pareto-optimal solutions obtained through MOPSO have been ranked by the composite scores obtained through maximum deviation theory to avoid subjectiveness and impreciseness in the decision making. The analysis offers useful information for controlling the machining parameters to improve the accuracy of the EDMed components.     

Simple fabrication of an uncooled Al/SiO2 microcantilever IR detector based on bulk micromachining

A simple microfabrication process to make an uncooled aluminum/silicon dioxide bi-material microcantilever infrared (IR) detector using silicon bulk micromachining technology is presented in this work. This detector is based on high banding of the microcantilever due to the large dissimilar in thermal expansion coefficients between the two materials. It consists of a 1 μm SiO₂ layer deposited by 200 nm thin Al layer. Since no sacrificial layer is used in this process, complexity related to releasing sacrificial layer is avoided. Moreover Al is protected in Si etchant using dual-doped tetramethyl ammonium hydroxide. The other advantage of this process is that only three masks are used with four photolithography process. Thermal and thermal mechanical behaviors of this structure are obtained using finite element analysis, and the maximum temperature and displacement at the end of cantilever at 100 pW/μm² absorbed IR power density on top surface are 7.82°K and 1.924 μm, respectively.     

Thermally actuated buckling beam memory: a non-volatile memory configuration for extreme space exploration environments

A new nano-thermo-mechanical data storage memory is presented which combines two technologies of thermal actuation and buckling beam memory. The memory design is resistant in high radiation environments, making it a reliable memory for spacecraft computer systems. This memory has a data storage density, write/erase speed, and power consumption comparable with current memories. An integrated thermal–mechanical simulation of buckling in nano-mechanical memory is performed to optimize the design parameters. The preliminary system is a bridge with lengths of 20–40 μm, a width of 1 μm, and a thickness of 0.3 μm, in air with a pressure of 5 kPa. The simulation of high energy particle collisions shows radiation does not cause undesired buckling for silicon and silicon carbide bits, which makes the memory applicable for Jovian exploration. Optimization simulations are performed for silicon, silicon carbide, and kapton with various dimensions and actuation heating rates. The current work suggests the length of 20 μm for the bridge to balance the write time and the storage density. Among the beams with the fixed dimensions, kapton shows the fastest write time, with the lowest energy cost. However, high energy electron collision causes buckling in kapton, limiting its use in high radiation applications. The results show that silicon and silicon carbide based systems are viable for use in the extreme radiation environments that will be encountered in future space exploration missions.     

Ultra high aspect ratio penetrating metal microelectrodes for biomedical applications

Studying the functioning of the brain through the use of penetrating microelectrodes has revolutionized our understanding of the brain and has the potential to treat physical conditions such as the aftermath of a stroke, disease or other neural problems. Cochlear implant electrodes have transformed the lives of people who were suffering from cochlear auditory disorders. However, limitations of manufacturing procedures restrict the choice of work materials to mostly silicon based materials, and biocompatibility issues have constrained the extensive use of these devices. Metal microelectrodes can absolve this limitation and enable extensive study of the neural centers. In this paper we report the fabrication of tungsten penetrating microelectrodes using electrochemical machining. Ultra high aspect ratio penetrating metal microelectrodes with diameters 10 μm and below, with surface roughness (Ra) values in the range of 300–500 nm, have been fabricated by electrochemical machining process. Details regarding the fabrication process and a mathematical model developed for the electrochemical machining process are discussed in this paper.     

Application of micro structured photosensitive glass for the gravure printing process

Photosensitive glasses are well known as materials which are micro structurable with a high aspect ratio of 20:1. Typical applications are micro mechanical, fluidic and optical components due to the very good chemical, thermal and mechanical stability of this material. Currently the aim of the work is the development of micro structured clichés made from photosensitive glass for the gravure printing of electrically functionalised inks on flexible substrates. Glass clichés with a geometrical variation of recessed cells were fabricated and tested regarding the chemical and mechanical stability using process comparable conditions. Printing tests using particle less and particle loaded electrically functionalised inks were carried out to investigate the influence of cell geometry on printed ink layers. It was found that precise dot structures will be printable if cell openings are <80 μm and cell depths are <30 μm. Resulting from this ITO was printed on flexible glass substrates with a thickness of 30 μm for high temperature treatments.     

Development and characterization of a microthermoelectric generator with plated copper/constantan thermocouples

This work reports the development and the characterization of a microthermoelectric generator (μTEG) based on planar technology using electrochemically deposited constantan and copper thermocouples on a micro machined silicon substrate with a SiO₂/Si₃N₄/SiO₂ thermally insulating membrane to create a thermal gradient. The μTEG has been designed and optimized by finite element simulation in order to exploit the different thermal conductivity of silicon and membrane in order to obtain the maximum temperature difference on the planar surface between the hot and cold junctions of the thermocouples. The temperature difference was dependent on the nitrogen (N₂) flow velocity applied to the upper part of the device. The fabricated thermoelectric generator presented maximum output voltage and power of 118 mV/cm² and of 1.1 μW/cm², respectively, for a device with 180 thermocouples, 3 kΩ of internal resistance, and under a N₂ flow velocity of 6 m/s. The maximum efficiency (performance) was 2 × 10^?3 μW/cm² K².     

Real-time data acquisition and discharge pulse analysis in controlled RC circuit based Micro-EDM

Microsystem Technologies - Owing to the non-isoenergetic discharge pulses in an RC-based micro-electrical discharge machining (µEDM) process, the unit material removal analysis is difficult....     

Design and modeling of a novel microsensor to detect magnetic fields in two orthogonal directions

In this paper, we present the design and modeling of the electrical–mechanical behavior of a novel microsensor to detect magnetic fields in two orthogonal directions (2D). This microsensor uses a simple silicon resonant structure and a Wheatstone bridge with small p-type piezoresistors (10 × 4 × 1 μm) to improve the microsensor resolution. The resonant structure has two double-clamped silicon beams (1000 × 28 × 5 μm) and an aluminum loop (1 μm thickness). The microsensor design allows important advantages such as small size, compact structure, easy operation and signal processing, and high resolution. In addition, the microsensor design is suitable to fabricate using silicon on insulator (SOI) wafers in a standard bulk micromachining process. An analytical model is developed to predict the first bending resonant frequency of the microsensor structure using Macaulay and Rayleigh methods, as well as the Euler–Bernoulli beam theory. Air and intrinsic damping sources of the microsensor structure are considered for its electrical–mechanical response. The mechanical behavior of the microsensor is studied using finite element models (FEMs). For 10 mA of root mean square (RMS) excitation current and 10 Pa air pressure, this microsensor has a linear electrical response, a fundamental bending resonant frequency of 52,163 Hz, and a high theoretical resolution of 160 pT.     

A dual-axis MEMS capacitive inertial sensor with high-density proof mass

This paper reports a novel dual-axis microelectromechanical systems (MEMS) capacitive inertial sensor that utilizes multi-layered electroplated gold. All the MEMS structures are made by gold electroplating that is used as a post complementary metal-oxide semiconductor (CMOS) process. Due to the high density of gold, the Brownian noise on the proof mass becomes lower than those made of other materials such as silicon in the same size. The single gold proof mass works as a dual-axis sensing electrode by utilizing both out-of-plane (Z axis) and in-plane (X axis) motions; the proof mass has been designed to be 660 μm × 660 μm in area with the thickness of 12 μm, and the actual Brownian noise in the proof mass has been measured to be 1.2 \({\upmu}{\text{G/}}\sqrt {\text{Hz}}\) (in Z axis) and 0.29 \({\upmu}{\text{G/}}\sqrt {\text{Hz}}\) (in X axis) at room temperature, where 1 G = 9.8 m/s². The miniaturized dual-axis MEMS accelerometer can be implemented in integrated CMOS-MEMS accelerometers to detect a broad range of acceleration with sub-1G resolution on a single sensor chip.     

Machining of micro-systems

An increasing trend towards miniaturization of mechanical components necessitates the further development of micro-machining techniques in order to produce components within given tolerances. For the dressing of multilayered, metallically bonded, fine-grained grinding wheels that are used for micro-grinding the technique of electro contact discharge dressing was developed. In the following, the determined interrelations between the variables dressing voltage, limitation of dressing current, infeed and feed of electrode, and the volume flow rate in conditioning as well as the quality factor in conditioning are presented and illustrated. Thanks to the development of new geometries of CVD-diamond micro-grinding-pencils, a reduction of edge disruptions as well as a reduction of tool diameters to a minimum of 250 μm for the machining of brittle materials could be achieved. For the micro-drilling of ductile materials the maximal drilling depth could be increased by means of an adjusted process control. The smallest examined drill diameter was 50 μm. The utilizability of the developed processes and tools was proven in the production of aerostatic micro-bearings in the scope of the collaborative research center (SFB) 516.     

Micromachined ultrasonic transducers based on lead zirconate titanate (PZT) films

The design, fabrication and measuring of piezoelectric micromachined ultrasonic transducers (pMUTs), including the deposition and patterning of PZT films, was investigated. The (100) preferential orientation of PZT film have been deposited on Pt/Ti/SiO₂/Si (100) substrates by modified sol–gel method. PZT film and Pt/Ti electrode were patterned by novel lift-off using ZnO as a sacrificial layer avoiding shortcomings of dry and wet etching methods. pMUT elements have been fabricated by an improved silicon micromachining process and their properties were also characterized. As measured results, the pMUT tends to operate in a standard plate-mode. The receive sensitivity and transmit sensitivity of pMUT element whose active area only has 0.25 mm² are ?218 dB (ref. 1 V/μPa) and 139 dB (ref. 1 μPa/V), respectively.     

High aspect ratio micro tool manufacturing for polymer replication using μEDM of silicon, selective etching and electroforming

Mass fabrication of polymer micro components with high aspect ratio micro-structures requires high performance micro tools allowing the use of low cost replication processes such as micro injection moulding. In this regard an innovative process chain, based on a combination of micro electrical discharge machining (μEDM) of a silicon substrate, electroforming and selective etching was used for the manufacturing of a micro tool. The micro tool was employed for polymer replication by means of the injection moulding process.     

An experimental study on an electro-osmotic flow-based silicon heat spreader

An experimental study has been carried out to estimate the heat transfer improvement offered by a novel electro-osmotic (EO) heat spreader for microprocessor cooling. The proposed design of the elliptical silicon heat spreader can be fabricated at the back surface of the microprocessor as an integral part. Thus, no extra space may be required. The EO heat spreader developed is 0.6 mm³ in volume and it contains a pair of thin gold film electrodes of approximately 1 μm thickness for applying an external electric field that induces electro-osmotic flow. The inner channel surfaces of the heat spreader are electrically insulated with a thin SiO₂ layer to minimize current leakage into the wafer. The EO heat spreader constructed is able to generate a flow rate of 0.2028 μl/min at 2 V/mm. With this heat spreader, the temperature of a heat source may be reduced by up to 4°C without the aid of any external mechanical devices. The heat spreader has the potential to make the temperature uniform, if the heat source is non-uniform in nature.