Through-mask Electrochemical Micromachining Research Articles

Improved uniformity and anisotropy of through-mask electrochemical micromachining by localized etching and homogeneous flow

This contribution aims to point out the potential of through-mask electrochemical micromachining (TMEMM) as an environmentally friendly alternative for thin metal layer patterning for applications such as printed circuit boards (PCBs). Two process chamber concepts with shearing and impinging electrolyte flow, based on the concept of localized etching, are introduced. The etching results, obtained with simple mask designs, were investigated regarding homogeneity in etch depth and anisotropy. Compared to a simple setup with unspecific electrolyte flow and full area etching, a significant improvement in microscopic and macroscopic homogeneity (deviations reduced from >100 to <10%) and anisotropy (etch factors increased from ~4 to >7) could be shown. A comparison to an industrial chemical etching process using a simple mask design revealed equal macroscopic homogeneity and higher etch factors in TMEMM. Only the microscopic homogeneity leaves still room for improvement. It is presented for the first time that island formation can be prevented completely over a large area (5 × 20 mm2) with TMEMM. These results demonstrate the advantages and the potential that can be achieved with TMEMM by designing appropriate tools.

Investigation of micro pit array on titanium alloy with hydrophobic surface by through-mask electrochemical micromachining

In order to obtain higher contact angles and improve the hydrophobicity of titanium alloy, the micro pit arrays were fabricated by the through-mask electrochemical micromachining (TMEMM). The theoretical model of surface hydrophobicity between the contact angle and the geometry size of micro pit arrays was developed. Moreover, the multi physical field coupling simulation of TMEMM was carried out. Thus, the direct mapping relationship between the contact angle and the process parameters was obtained by combining the theoretical model with the simulation results. The effect of process prameters, such as electrolyte mass fraction, mask size and processing voltage, was investigated. The optimal combination of process parameters was predicted and verified by experiments. The results show that the errors of the measured values of diameter, spacing, depth and surface contact angle of the micro pit arrays are 2.49%, 6.87%, 7.40% and 6.01% respectively, which indicates that the hydrophobic textured surface with a contact angle of about 141° is successfully fabricated without the modification of low surface energy materials.

Study of Mass Transfer Enhancement of Electrolyte Flow Field by Rotating Cathode in Through-Mask Electrochemical Micromachining.

To solve the problem of the nonuniform distribution of temperature and electrolytic products in the electrolyte flow field during through-mask electrochemical micromachining, the use of a rotating cathode with surface structures is proposed. The rotation of the cathode increases the efficiency of heat and mass transfer by the electrolyte flow. Simulations are performed to analyze the influence of the type of surface structure, the number of surface structures, and the rotational speed of the cathode on the electrolyte flow field. The results show that the use of a rotating cathode with surface structures significantly improves the mass transfer efficiency of the electrolyte flow field in comparison with a conventional cathode structure, and, in particular, a grooved rotating cathode can increase the outlet flow velocity by about 23%. An experimental demonstration of micropit array processing shows that the use of a grooved rotating cathode increases the mass transfer efficiency by 34% and the processing efficiency by nearly 40% compared with a smooth-surfaced rotating cathode. The grooved rotating cathode also gives the highest machining accuracy. Using this cathode, a uniform micropit array with an average micropit diameter of 201.83 μm, a diameter standard deviation of 3.49 μm, and a depth standard deviation of 0.87 μm is processed.

Study on the uniformity of microgrooves in through-mask electrochemical micromachining with moving cathode

Study on the uniformity of microgrooves in through-mask electrochemical micromachining with moving cathode

Towards understanding uniformity of megasonic-assisted through-mask electrochemical micromachining based on bubble dynamics

In through-mask electrochemical micromachining (TMEMM), bubbles on the anode surface can lead to ohmic voltage drop and non-uniform conductivity distribution, resulting in lower etching accuracy. With the advantages of low cavitation effect, strong acoustic streaming and high acoustic intensity, megasonic facilitates the reduction of bubble accumulation on the anode surface, thus improving etching accuracy. In this paper, a multiphysics field simulation model is developed to predict the two-phase flow evolution and etching profile at the anode interface. The simulation results show that the gas content at the anode interface in the megasonic field is low, and the gas distribution is relatively uniform. The etching depth and etching uniformity of TMEMM are improved. Experiments demonstrate the accuracy of the proposed simulation model. In addition, micro pits array with CVdiameter and CVdepth of 2.28 % and 5.70 %, respectively, were prepared using the MA-TMEMM method. Visualization experiments systematically study the machining mechanism of MA-TMEMM. The underlying dynamic force balance on the gas bubble is analyzed to illustrate the mechanism and experimental observations. Finally, the advantage of megasonic in removing bubbles from the anode surface to improve etching uniformity was demonstrated by counting the detached bubble diameter and effective coverage.

Effect of the mask geometry on the microstructure forming in through-mask electrochemical micromachining

Effect of the mask geometry on the microstructure forming in through-mask electrochemical micromachining

Experimental and Numerical Investigations on Fabrication of Surface Microstructures Using Mask Electrolyte Jet Machining and Duckbill Nozzle

Abstract The consistency in the fabrication of microsurface structures on large workpieces remains a challenge for existing production techniques. Mask electrolyte jet machining (MEJM) is a hybrid mask-based electrochemical machining (ECM) process that combines the flexibility of Jet-ECM to flush the electrochemical by-products and the high throughput processing feature of through-mask electrochemical micromachining (TMEMM). In the present study, a duckbill-shaped nozzle is employed in the MEJM for the batch fabrication of microsurface structures which facilitates more uniform current density distribution over the entire machining area, resulting in better consistency. With a larger slit length, the duckbill nozzle will not only cover more processing area but also facilitates more uniform current density distribution over the entire machining area, resulting in a better consistency for batch fabrication. Thresholds of the ratio of slit length to the machining area were derived from a quantitative analysis, namely the efficient threshold and the performance threshold. The slit length of the duckbill nozzle should be at least twice as large as the machining area to wipe out any observable deviation on current density distribution in the machining area. An efficient and high-performance numerical simulation framework with a virtual gap concept is developed for the mask-based ECM processes to simulate microcavity profiles and associated current density distribution. The concept of a virtual gap is proposed to solve the simulation dilemma of elements being consumed in the mask-based ECM process. Quantitative analyses were carried out to study how the virtual gap influences the electric current density distribution in the interelectrode gap. A virtual gap smaller than 100 nm is recommended. Guidelines on how to ensure a smooth electric field transition across the coarse and fine-meshed zones are presented by conducting a quantitative analysis. As an example, this work has successfully fabricated several cavity arrays with different processing parameters. Both the experimental results and the numerical simulation frameworks are easy-to-implement and easy-to-extend for all the mask-based ECM processes.

Investigation on Bidirectional Pulse Electrochemical Micromachining of Micro Dimples.

Through-mask electrochemical micromachining (TMEMM) is a promising method to prepare micro dimples on the surface of metallic parts. However, the workpiece is machined one by one in traditional TMEMM. This paper introduced bidirectional pulse to TMEMM to improve the machining efficiency. Two masked workpieces were placed face to face, and connected to the ends of the bidirectional pulse power supply. Along with the change of the pulse direction, the polarities of the two workpieces were interchanged periodically, and micro dimples could be prepared on both workpieces at one time. The simulation and experiment results indicated that with bidirectional pulse mode, micro dimples with same the profile can be prepared on two workpieces at one time, and the dimension of micro dimple was smaller than that with unidirectional pulse mode. In bidirectional pulse current, the pulse frequency and pulse duty cycle played an important role on the preparation of micro dimple. With high pulse frequency and low pulse duty cycle, it is useful to reduce the undercut of micro dimple and improve the machining localization. With the pulse duty cycle of 20% and pulse frequency of 10 kHz, micro dimples with etch factor (EF) of 3 were well prepared on both workpieces surface.

Electrodissolution Processes and Their Application in Manufacturing for Shaping and Finishing of Advanced Materials

Controlled material removal by anodic dissolution at high rates forms the basis of non-conventional metal shaping and finishing processes such as Electrochemical Machining (ECM), Electrochemical Micromachining (EMM), and Electropolishing (EP). These processes are employed in the manufacturing and processing industries for a variety of applications ranging from the machining of complex shaped components for heavy industries to the fabrication of microstructures in thin films and foils and microfinishing of superconducting radiofrequency cavities (1-3). Application of these processes for the production of reliable, reproducible, and high-yielding components require an understanding of the parameters that influence the anodic dissolution processes.ECM, EMM, and EP can be understood and theoretically modeled based on the same principles provided the scaling effects related to each process are considered (2). In the first part of this presentation, the high rate anodic dissolution behavior of different materials including refractory materials are described which helps in establishing the basic concepts that govern ECM/EMM/EP processes. Electrochemical reactions taking place at high anodic potentials depend on the metal-electrolyte combination and the operating conditions. These reactions include active metal dissolution, passivity, oxygen evolution, and the phenomena of passive film breakdown leading to the transpassive metal dissolution. In passivating electrolytes (containing oxidizing anions), oxygen evolution and metal dissolution take place simultaneously. The current density dependence of meat dissolution efficiency in such electrolytes provides minimized stray cutting in ECM. Mass-transport, current distribution, and surface film properties play a significant role in the metal removal rate and surface finish. Whereas the formation of a salt film on the dissolving anode in ECM electrolytes is well established, the identification of the rate-controlling species during electropolishing of metals in concentrated acid solutions remains a controversial issue.The second part of the presentation deals with the applications of these processes in the manufacturing of advanced products for the aerospace, biomedical, electronics, and photovoltaics industries. Different ECM techniques such as die-sinking, combined tool machining, EC drilling, and EC deburring are described that are used in the manufacturing of complicated shaped parts such as turbine blades. Maskless EMM methods such as Jet EMM, micro-drilling, and wire EMM are applied to fabricate precision microstructures. For process enhancement, assisted techniques such as laser, abrasive, vibration, and ultrasonic are incorporated with maskless EMM. For through mask electrochemical micromachining (TMEMM), an understanding of mass transport in a cavity and shape evolution modeling is critical. The use of laser patterned oxide film mask for Ti patterning is presented as an interesting application of TMEMM (4). A few case-studies on electropolishing are described which include the fabrication of STM probes and Nitinol stents, and microfinishing of print bands. In the microprocessor industry, a significant effort was invested in the development of EP and electrochemical mechanical planarization (ECMP) approaches as a no-contact or low-contact planarization processes for the integration of ULK/copper interconnects. The development work, including processing details and planarization efficiency of EP and ECMP, are reviewed. A critical evaluation of the current status of EP/ECMP vis a vis the chemical mechanical planarization (CMP) process is presented.References J. McGeough, Principles of Electrochemical Machining, Chapman and Hall, Wiley, London, 1974.M. Datta, Electrodissolution Processes: Fundamentals and Applications, CRC Press/Taylor and Francis, Boca Raton, 2020.E. J. Taylor and M. Inman, Electrochemical Surface Finishing, The Electrochemical Society Interface, Fall 2014, p47.P.-F. Chauvy, P. Hoffmann, D. Landolt, Electrochemical Micromachining of Titanium Through a Laser Patterned Oxide Film, Electrochem. Solid-State Lett., 4(5), C31 (2001).

Through-mask electrochemical micromachining of micro pillar arrays on aluminum

Surface textures, such as micro pillar arrays, greatly promote the developments of many advanced fields. As many methods have been applied to fabricate the micro pillar arrays on silicon and polymer substrates, it is a still challenge to fabricate micro pillar arrays on metals. In this paper, through-mask electrochemical micromachining (TMEMM) was employed to fabricate micro pillar arrays on aluminum (Al) substrate. The effects of the direct and pulsed currents on surface morphology of the micro pillar arrays were investigated, and the ordered micro pillar arrays with different top diameters were obtained at the pulsed currents. Then, the influences of the current densities on the morphology of the micro pillar arrays were studied, and the largest etching anisotropy factor (EF) was obtained at a current density of 1 A/cm2. Finally, the wettability of the as-prepared micro pillar arrays with low surface energy modification was investigated. When the top diameters were 50– 90 μm and the heights were larger than 35 μm, all the micro pillar arrays obtained at different current densities showed superhydrophobicity with water contact angle larger than 150°.

Experimental Investigation Into Micro-Textured Surface Generation by Through-Mask Electrochemical Micromachining

Micro texturing with dimples is one of the most established methods for the enhancement of lubrication and tribological performance. In this work, a novel approach of through-mask electrochemical micromachining is instigated to fabricate high-aspect-ratio circular micro-dimples utilizing a low-cost photoresist (AZ-4903) as mask. Experiments were performed to understand the influence of the applied voltage along with the electrolyte type and concentration on the machining precision and surface characteristics of the generated micro array. The influence of three electrolytes has been studied and it has been experimentally established that 10% NaCl + 10% NaNO3 is favorable during maximization of the depth of the generated micro-dimples to 39.03 µm whereas undercut is minimized to 15.15 µm by utilizing 20% NaNO3. The surface quality of the generated micro-dimples has been found to be the finest, i.e. 0.062 µm, for the samples machined with 10% NaNO3. Patterned arrays containing micro-dimples having maximum depths of 30 μm, 35 μm, and 40 μm were successfully fabricated over a stainless steel (SS304) substrate by using these electrolytes with considerable accuracy and repeatability. The experiments confirm possibility to obtain high-aspect-ratio micro-dimples with the least degree of undercut using this technique. Further, friction test results were analyzed to figure out the alterations in the frictional coefficient of the micro-dimples with different depths as well as with non-dimpled samples.

Fabrication of micro-texture on cylindrical inner surface and its effect on the stability of hybrid bearing

In this paper, a removable through-mask electrochemical micromachining (EMM) is proposed for fabricating the microstructures with different sizes and structures on the inner surface of the hybrid bearing. By optimizing the machining parameters of EMM, micro-textures with considerable area consistency and excellent surface quality are obtained on the inner surface of the hybrid bearing. Subsequently, the stability tests of the non-textured bearing and other three textured bearings with various area ratios are carried out on the setup of hybrid rotor-bearing system to study the influence of micro-texture on the vibration characteristics and stability of the bearing rotor. The time domain, frequency domain, and axis trajectory of the rotor are compared and analyzed. The experiment results show that at low speed (2400 rpm), the micro-texture has little effect on the amplitude of bearing-rotor system. As the rotational speed increases, the amplitude of the rotor-bearing system gradually increases. It should be noted that the amplitude of specific micro-texture bearings is smaller than the non-textured bearing, but not all types of micro-texture perform like that. At 9000 rpm, the relative amplitude of one sample decreases by 22.6% compared with the non-textured bearing rotor system. Therefore, the micro-texture with reasonable size and structure could significantly reduce the vibration of the rotor-bearing system under the water film condition, which is of considerable significance to improve the stability of liquid film and provides a potential application background for high-speed bearing lubrication.

Three-dimensional simulation of performance in through-mask electrochemical micromachining

Through-mask electrochemical micromachining (TMEMM) is a kind of microfabrication. The major feature of through-mask electrochemical micromachining is that the tool is not limited by the shape, and can be processed in various shapes according to the pattern on the mask. This paper uses the finite element method to create a three-dimensional model to discuss the influence of flow direction, voltage, and electrolyte flow rate on the shape of the through-mask electrochemical micromachining. The simulation results show that processing shape is deeply affected by processing parameters, electrolyte flow rate affects processing temperature distribution, and temperature distribution at the bottom of aperture affects processing depth and flatness during processing, which in turn causes the difference in orifice size and roundness. At the same flow rate, the larger voltage, the larger average radius, flatness, and roundness. At the same voltage, the faster flow rate, the lower average depth, flatness, and average radius. Overall, it has best-processed aperture profile at a flow rate of 0.5 m/s. As a whole, the through-mask electrochemical micromachining is indeed suitable for array microfabrication of various shapes. In this study, experiments were added to verify the simulation results. At a fixed electrolyte flow rate, when the voltage, concentration, and mask aperture are changed, it will have an effect on the overall experimental results. From the experimental results, we know that the electrolyte flow rate and temperature distribution will indeed affect the overall processability.

Jet Electrochemical Micromachining of Micro-Grooves with Conductive-Masked Porous Cathode.

Surface structures with micro-grooves have been reported to be an effective way for improving the performance of metallic components. Through-mask electrochemical micromachining (TMEMM) is a promising process for fabricating micro-grooves. Due to the isotropic nature of metal dissolution, the dissolution of a workpiece occurs both along the width and depth. Overcut is generated inevitably with increasing depth, which makes it difficult to enhance machining localization. In this paper, a method of electrochemical machining using a conductive masked porous cathode and jet electrolyte supply is proposed to generate micro-grooves with high machining localization. In this configuration, the conductive mask is directly attached to the workpiece, thereby replacing the traditional insulated mask. This helps in achieving a reduction in overcut and an improvement in machining localization. Moreover, a metallic nozzle is introduced to supply a jetted electrolyte in the machining region with enhanced mass transfer via a porous cathode. The simulation and experimental results indicate that as compared with an insulated mask, the use of a conductive mask weakens the electric field intensity on both sides of machining region, which is helpful to reduce overcut and enhance machining localization. The effect of electrolyte pressure is investigated for this process configuration, and it has been observed that high electrolyte pressure enhances the mass transfer and improves the machining quality. In addition, as the pulse duty cycle is decreased, the dimensional standard deviation and roughness of the fabricated micro-groove are improved. The results suggest the feasibility and reliability of the proposed method.

Through-Mask Electrochemical Micromachining with Reciprocating Foamed Cathode.

A through-mask electrochemical micromachining process with a foamed cathode (foamed-cathode through-mask electrochemical micromachining (TMEMM)) has recently been proposed involving micro-scale surface microstructures with a high geometric consistency that are fabricated on the curved-surface workpiece. In this paper, to make the foamed-cathode TMEMM process more cost-efficient in the applications, significant modifications are made to this process and an upgraded version of the foamed-cathode TMEMM process is developed. In this modified process, the sandwich-like unit (including the foamed cathode, mask, and workpiece) is closely assembled by the magnetic field force instead of the conventionally-used mechanical force and is kept moving up-and-down inside the electrolyte, avoiding the use of the traditional pump-driven circulation for the electrode process. Experiments are carried out to evaluate the machining effect of this modified TMEMM for fabricating micro-dimples. The research results verify that this modified TMEMM process can produce highly uniform micro-dimples whose minimum CV (coefficient of variation) values in depth and in diameter are 5.4% and 1.9%, respectively, with smooth surfaces of the minimum Ra being 0.21–0.35 µm. These values are smaller than those previously reported. This results in the positive effects on the mass transfer driven by magnetohydrodynamic convection induced by the magnetic field within the interelectrode and the foamed electrode.

Micro-texturing on free-form surfaces using flexible-electrode through-mask electrochemical micromachining

Abstract Surface micro-texturing deals with the issues pertaining to various fields of engineering for enhancing essential functions such as tribological, wetting, biocompatibility, sustainability, cleanliness, etc. Among all the aforementioned sectors, micro-texturing of curved/free-form large areas is getting huge attention, e.g. micro-textures on artificial biomedical implants enhance sustainability and life cycle by better implant-tissue interface, cell-adhesion, and cell proliferation. Due to growing awareness towards the significances of micro-texturing, presently a lot of thrusts are being given on innovations employing surface textures on free-form/curved surfaces. The paper proposes a porous and flexible electrode based through-mask electrochemical micromachining (TMECMM) in which surface of any curvature can be textured by adapting the shape of the surface. A 3-D numerical simulation of the TMECMM texturing executed in COMSOL to analyze the effect of current density and topology of micro-textures. The minimum pitch of the micro-features in mask is determined through analysis of overlap of current density with the help of developed model. High resolution-reusable masks with micro-features arrays are fabricated using a two-step laser beam machining process. These masks are employed in-between the flexible electrode and target metal surfaces for localized anodic dissolution for texturing. The comparison of response surface methodology (RSM) based experimental and simulated results shows a close fit (and same trends) with respect to diameter and depth of micro-dimples array. The capabilities of the developed device are showcased by machining various types of texture arrays of micro-dimples, straight micro-channels, and turbulated micro-channels on flat, cylindrical, spherical, and free-form surfaces of SS304.

Fabrication of surface microstructures by mask electrolyte jet machining

This paper presents a novel electrochemical processing technique, mask electrolyte jet machining (MEJM), for the fabrication of surface microstructures. MEJM combines jet electrochemical micromachining (Jet-ECM) and through-mask electrochemical micromachining (TMEMM), combining the advantages of TMEMM, which is a high-throughput process, and of Jet-ECM, with its adjustable flow field. The effects of a mobile nozzle on electrolyte flow are investigated, and a new modeling approach for large translational movements is proposed. An analysis of the accuracy and reliability of the proposed method is presented. Microprotrusions and microdimples are produced to high precision and with excellent consistency of dimensional variation (maximum standard deviation 2.171 μm). The results suggest the possibility of an affordable technique for batch fabrication of surface microstructures with high efficiency and precision.

Fabrication of micro pits based on megasonic assisted through-mask electrochemical micromachining

Through-mask electrochemical micromachining (TMEMM) is the primary method to fabricate micro pits with controlled size, location, and density. In order to improve the machining localization and deep etching capability in TMEMM process, a novel method which combined megasonic vibration to TMEMM process is presented in this paper. Firstly, the coupling relationship between sound field, gas-liquid two-phase flow field and electrolytic process was theoretically analyzed. Theoretical analysis results indicate that acoustic wave agitation can promote the electrolytic process by increasing the conductivity of the electrolyte. Based on this theory, a numerical simulation method was used to predict anodic profiles under different megasonic intensity. The simulation results show that the addition of megasonic agitation can obviously improve the machining localization and deep etching capability in TMEMM process. Etching depth of the micro pit increased from 48.22 μm to 77.98 μm with megasonic agitation compared to the without megasonic one. Depth-diameter ratio of the micro pit increased from 0.30 to 0.45. Meanwhile, the etching factor (EF) increased from 1.55 to 2.10. Then, a megasonic electrolyser at 1 MHz was set up, micro pits were etched under different megasionc intensity. The experiment results show that megasonic assisted through-mask electrochemical micromachining (MA-TMEMM) had best process performance when it worked with the increase of megasionc intensity. When the megasonic intensity was 8 W/cm2, micro pits with average diameter of 167.77 μm and 79.62 μm in depth were successfully fabricated. The average depth-diameter ratio of the micro pits was as high as 0.47, and the EF was as high as 2.35. The working mechanism of megasonic in MA-TMEMM process was analyzed too.

Fabrication of Different Micro Patterned Arrays by Through Mask Electrochemical Micromachining

Patterned microstructures are among the basic micro features that are indispensable in improving the tribological performance and dependability of various mechanical components especially Micro-Electro-Mechanical Systems (MEMS). Through Mask Electrochemical Micromachining (TMEMM) is a feasible process for fabricating micro-patterned arrays with controlled dimension, location and density by maintaining suitable surface texture. In this paper, appropriate machining parameters along with optimal electrolyte combination have been utilized for achieving patterned arrays in the form of micro-dimples and micro-square features. Low aspect ratio mask made of a negative PR (AZ 4903) is being introduced during TMEMM due to its low cost, availability and chemical resistance. Pattern of micro-holes with an average diameter of 65μm as well as micro-squares with a side length of 65μm imprinted on the mask are being replicated over SS304 substrate with substantial repeatability. The influence of two major Electrochemical Micromachining (EMM) parameters viz. duty cycle and machining time are investigated on the machining accuracy and surface properties of the micro-dimples generated with mask of lean thicknesses. Circular micro-dimples with an undercut of 29.15μm, depth 44.10μm and Ra 0.091μm and square micro-dimples possessing 32.63μm undercut, 40.24μm depth and 0.081μm Ra have been successfully fabricated on SS304 by this process of TMEMM.

Electrochemical micromachining of micro square pattern using reusable masked tool

ABSTRACTMicro square pattern is the basic microfeature that can be applied to various products in different fields. The product duration and performance of textured surfaces are directly influenced by shape, size and surface quality. Maskless electrochemical micromachining (EMM) is a prominent method for producing numerous high-quality micro square patterns compared to photolithography and through-mask EMM, which are expensive and time consuming in mass production. In this paper, SU-8 2150 negative photoresist is used as a patterned tool in maskless EMM. Compared with other processes, this mask can be reused for generating many uniform micro square patterns. The maskless EMM setup consisting of EMM cell, electrode fixtures, electrical connections and vertical cross flow electrolyte system has been designed and developed indigenously for carrying out the experiments. Effects of major EMM parameters like interelectrode gap, flow rate and machining time on textured characteristics, i.e., current efficiency, length overcut, machining depth and surface roughness (Ra) are investigated during generation of micro square patterns. Arrays of 8000 micro square impressions have been fabricated during experiments. An attempt has also been made to analyze the textured characteristics on the basis of micrograph of micro square pattern for getting the best parametric combination.