Electrochemical Microfabrication Research Articles

From 2D to 3D electrochemical microfabrication of nickel architectures at room temperature: Synthesis and characterization of microstructure and mechanical properties

Recent development in direct electrochemical 3D microprinting allows the fabrication of 3D metallic micro-components of predominantly noble metals with the positive standard reduction potentials (e.g., Cu, Au, or Pt). These pure metals (e.g., Cu, Au, or Pt) have a small shear modulus up to 60 GPa, which limits one of the important strengthening mechanisms, Taylor hardening. Hence, their mechanical strength is relatively low and limits the application in a structural component. 3D printing of mechanically high-strength metals (e.g., Ni), specifically, printing of microscale parts by direct electrodeposition using a voxel-by-voxel printing technique, is so far very challenging due to the negative standard reduction potential, which leads to hydrogen evolution and unstable printing. In this paper, we demonstrate the stable and successful room temperature (RT) electrochemical printing of Ni micro-architectures by using a newly formulated Ni ink and supporting electrolyte, which were developed through 2D voltammetry and deposition analysis. The micro manufacturing of Ni succeeds at a high deposition speed up to ∼61 nm/s, which is the fastest speed reported so far for microprinting of Ni. 3D printed micropillars feature a heterogeneous nanostructure from ∼13 nm at the center to ∼27 nm at the perimeter. By performing micropillar compression tests, the printed pillars exhibit a high compressive yield-strength of 2.3 ± 0.1 GPa, only ∼15 % lower than 2D processed samples (2.7 ± 0.1 GPa) featuring a homogeneous nanostructure of 27 ± 16 nm. Thus, by deposition of stronger, but less noble metals we show a new pathway to enhance the mechanical robustness of 3D-printed structures, opening up new applications where mechanical rigidity is essential.

Development of nanosecond spike pulse power supply for electrochemical micromachining

The use of pulse voltage can greatly improve the precision of electrochemical microfabrication, and the narrower the pulse width of the applied pulse voltage signal, the higher the machining precision. However, the commonly used chopper circuit topology of pulse power supplies is limited by the maximum switching frequency of the field-effect transistor. To address this problem, this paper proposes a nanosecond pulse electrochemical micromachining power supply based on a differential circuit. The power supply uses the STM32F103C8T6 microcontroller as the control core to output high-performance rectangular waves through a DDS device. After differential, rectification, filtering, and power amplification processing, stable, frequency, amplitude, and pulse width adjustable spike pulse voltage signals are obtained. By establishing a system mathematical model and optimizing the time constant of the differential circuit, theoretically, the sub-nanosecond pulse width can be obtained. Prototype performance tests show that the power supply has a maximum frequency of 20 MHz, a minimum pulse width of 1.8 ns, and a maximum peak voltage of 10 V. By using this power supply for microhole electrochemical machining experiments, nanometer-level machining precision has been achieved.

Micromachining of nickel and nickel-based alloy surfaces using composite signal

Electrochemical micromachining refers an unconventional technology in the field of machining. With this technology, the ultrashort pulse power supplies are extensively used to address the issue of excessive machining of non-processing areas. However, the reduction of pulse duration is the only effective strategy to enhance the processing accuracy in ultra-short pulse electrochemical microfabrication. Nonetheless, the high cost of equipment and unsuitability in practical production has limited its progress. To resolve this issue, this paper proposes the use of a composite signal in electrochemical micromachining instead of ultrashort pulses. By changing the signal waveform during machining, the energy required for processing can be reduced with the same electromotive force input, thereby reducing the current used to decompose the anode in the circuit and effectively improving machining accuracy. This approach was employed to manufacture micro-structures on a pure nickel sheet, achieving micron-scale accuracy. Moreover, the same level of superior machining accuracy can be achieved when machining micro-structures on hard-to-cut super alloy plates.

Electrochemical micromachining based on sinusoidal signals with variable resistance

ABSTRACT Reducing pulse duration is the sole effective strategy to improve the processing accuracy in ultra-short pulse electrochemical microfabrication. High cost and unsuitability in practical production also restrain its development. This study aims to propose a method based on sinusoidal signals with variable resistance. The bilayer capacitor cannot be fully charged to achieve a transient machining state. An equivalent circuit differential equation is established, and the resistance becomes an effective parameter to control the machining accuracy. The experimental result shows that the machining accuracy increases significantly with the increase in resistance. The approach is applied to machine microstructures and achieves sub-micron scale accuracy on a pure nickel sheet. The same superior machining accuracy can be obtained by machining microstructures on hard-to-cut superalloy plates.

Electrochemical buckling microfabrication.

Can isotropic wet chemical etching be controlled with a spatial resolution at the nanometer scale, especially, for the repetitive microfabrication of hierarchical 3D μ-nanostructures on the continuously curved surface of functional materials? We present an innovative wet chemical etching method called "electrochemical buckling microfabrication": first, a constant contact force is applied to generate a hierarchical 3D μ-nanostructure on a mold electrode surface through a buckling effect; then, the etchant is electrogenerated on-site and confined close to the mold electrode surface; finally, the buckled hierarchical 3D μ-nanostructures are transferred onto the surface of a Ga x In1-x P coated GaAs wafer through WCE. The concave microlens, with a Fresnel structure, has an enhanced photoluminescence at 630 nm. Comparing with energy beam direct writing techniques and nanoimprint lithography, this method provides an electrochemical microfabrication pathway for the semiconductor industry, with low cost and high throughput.

Kinetic Investigation on the Confined Etching System of n-Type Gallium Arsenide by Scanning Electrochemical Microscopy

Confined etchant layer technique (CELT) has been proved an effective electrochemical microfabrication method for both 3D microstructures and a supersmooth surface. From a physical chemistry viewpoint, the confined etching system of n-GaAs includes an etchant generation reaction from Br– to Br2 (E) followed by two parallel reactions: the confining reaction between Br2 and l-cystine (C1), and the etching reaction between Br2 and n-GaAs (C2). In this paper, the homogeneous EC1 process is investigated first through the tip generation/substrate collection (TG/SC) mode of scanning electrochemical microscopy (SECM), and the reaction rate of C1 is determined as (8.0 ± 1.0) × 103 dm3 mol–1 s–1; second, the heterogeneous EC2 process is investigated through the feedback mode of SECM, and the reaction rate of C2 is determined as (3.2 ± 0.5) × 10–2 cm s–1; third, a deformed geometry finite element model is established to simulate the etching topography coupling E(C1∥C2) processes by using the obtained data. The theoretical profiles of pits etched at different concentrations of scavenger, l-cystine, are analyzed and compared with experimental results. This model allows the prediction of spatial resolution of CELT as a function of reaction rates of C1 and C2 but also of the concentration of scavenger.

Electrochemical mechanical micromachining based on confined etchant layer technique

The confined etchant layertechnique (CELT) has been proved an effective electrochemical microfabrication method since its first publication at Faraday Discussions in 1992. Recently, we have developed CELT as an electrochemical mechanical micromachining (ECMM) method by replacing the cutting tool used in conventional mechanical machining with an electrode, which can perform lathing, planing and polishing. Through the coupling between the electrochemically induced chemical etching processes and mechanical motion, ECMM can also obtain a regular surface in one step. Taking advantage of CELT, machining tolerance and surface roughness can reach micro- or nano-meter scale.

SECM Soft Contact Mode: An Approach to Electrochemical Microfabrication

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Design of experiments in electrochemical microfabrication

Electrochemical material etching techniques have attracted a significant amount of attention in the “wet” metal etching arena, as the process typically involves neutral salt electrolytes and is relatively safe to operate. There are also economical and environmental advantages associated with these techniques compared with competing etching methods. A new concept of electrochemical microfabrication on substrates has been developed. In the technique the workpiece, which is the anode in the electrochemical reactor, is placed closely to a tool, which is the cathode containing the micro-pattern. Selective pattern transfer results in a higher etching rate on the areas opposing “exposed” regions of the cathode, and lower etching rates in the areas directly opposite to the areas, on the cathode, covered by an insulator. In this investigation the electrochemical micro-patterning process has been evaluated and characterised in a vertical flow system described previously in literature. The experiments were carried out using copper disk anodes and patterned cathodes in a 0.1 M copper sulphate electrolyte. A 2 4 factorial experimental design procedure was adopted to determine the influence of process parameters on the electrochemical microfabrication process in terms of variability in pattern transfer over the electrode's surface area.

Electrochemical Process for Micropattern Transfer Without Photolithography: A Modeling Analysis

A concept of maskless electrochemical microfabrication has been presented. In this method, a substrate, which is the anode, is placed in an electrochemical reactor in close proximity to a tool carrying the micropattern, which is the cathode. Although the anode is etched over its entire surface, it etches at a higher rate on the areas opposing exposed regions of the cathode, hence, reproducing a pattern on the anode. This paper describes the electrochemical process that enables the transfer of micropatterns from the cathode to the anode. This investigation involved the development of a mathematical model for anodic dissolution from a substrate, which was tested and verified against experimental data. The model was solved using commercial software that allows the simultaneous solution of reaction distribution at two different length scales, millimeter and micrometer. It has been applied to copper dissolution in acidified and nonacidified media. The results show that selective etching in acidified media produces a surface topography that is sinusoidal, whereas a rectangular topography is obtained in nonacidified media due to the formation of an oxide at the surface.

Electrochemical Factors Controlling the Patterning of Metals on SAM-Coated Substrates

Alkanethiol self-assembled monolayers (SAMs) have been used in electrochemical microfabrication processes. The reductive desorption potential of alkanethiol SAMs, Edes, can be comparable to, greater than, or less than the metal reduction potential during electrodeposition, Emet. As a result, the SAM layer can passivate the surface or desorb simultaneously with metal deposition. We show that these electrochemical traits can be combined with a rastering microjet electrode to pattern SAMs directly and create patterned metal films without lithography steps. For the case of copper deposition on 1-octanethiol (OT)- and 1-dodecanethiol (DT)-coated substrates, Edes is significantly negative of Emet, resulting in high-resolution metal patterns with poor nucleation and poor adhesion to the substrate. However, nickel patterns deposited on 1-butanethiol (BT), OT, and DT have traits similar to bare gold (excellent nucleation and adhesion) because Edes is positive of Emet. Substrates with SAMs also suppress adventitious chemistries that occur distant from the rastering microjet electrode, such as oxygen reduction, making samples more corrosion resistant and improving the overall patterning process that we call electrochemical printing.

Monitoring of Instant-Mask Plating for Fabrication of Patterned Copper Microstructures

EFAB is a multilayer electrochemical microfabrication technology for the manufacture of truly 3-demensional (3-D) metal micro-devices. It includes a unique process, instant-mask (IM) plating, for the electrodeposition of patterned metal microstructures. As IM plating occurs in a completely closed micro plating cell and cannot be observed visually, this paper presents a method for monitoring the plating process for deposit quality control. In this method, the cell voltage of IM plating is recorded and presented in real-time after a plating current is applied. The recorded cell voltage reveals the real- time information of IM plating, including 1) if the plating process is normal; 2) if the anode and the cathode of a micro plating cell are shorted; and/or 3) if the plating process produces flash deposits.

Three-dimensional electrochemical microfabrication of n-GaAs using l-cystine as a scavenger

The confined etchant layer technique (CELT) was used to fabricate complex three-dimensional (3D) microstructures on gallium arsenide ( n-GaAs). The design of an appropriate chemical etching system is needed in order to realize successful microfabrication. In this study, Br 2 was electro-generated at the mold surface and used as an efficient etchant for n-GaAs. The use of l-cystine as a scavenger to replace the toxic scavenger H 3A SO 3 was explored. The resolution of the fabricated microstructure depended strongly on the composition of the electrolyte, and especially on the concentration ratio between l-cystine and KBr. A well-defined and polished Pt micro-cylindrical electrode with a diameter of ∼50 μm was employed as one kind of mold for CELT. By inspecting the deviation of the sizes of the etching spots from the real diameter of the microelectrode, the thickness of confined etchant layer (CEL) can be estimated and thus the composition of electrolyte can be optimized for better etching precision. By choosing an appropriate concentration ratio between l-cystine and KBr, complex microstructures were fabricated successfully on n-GaAs. The etched patterns on n-GaAs were approximately negative copies of the mold, and the precision of duplication could easily reach the submicrometer scale, which was better than that achieved with H 3A SO 3. The experimental results indicated that l-cystine is a good scavenger for microfabrication on n-GaAs by CELT. This technique avoids severe pollution of the environment, which will help to extend its future application in industry.

Applications of Metal Surface Modification in Microdevice Fabrication

EFAB{trade mark, serif} is a multilayer electrochemical microfabrication technology for the manufacture of truly 3-demensional (3-D) metal microdevices. Metal surface modification techniques can be applied to not only the EFAB process but also EFAB products. This paper first briefly introduces the applications of surface modification techniques in EFAB and then reports and discusses a surface treatment method for extending the use life of copper instant-masks (Cu-IM) used for instant-mask plating in EFAB. In this surface treatment process a copper substrate is immersed in a benzotriazole (BTA) solution to form a protective film on the copper substrate prior to the application of a patterned insulator layer. This protective film increases the delamination resistance of insulator layers on Cu-IMs in copper plating baths. Therefore the use life of Cu-IMs can be extended.

Electrochemical Microfabrication of Metallic Nanowires—A Revival of Electrochemical Methods for Modern Device Technologies?

An old dog taught new tricks is how electrochemical methods relate to modern device technologies. Penner et al. use electrochemical deposition, rather than chemical vapor deposition methods, to form molybdenum wires of monodisperse and controllable diameters in the range 20-900 nm; direct application to contempory nanotechnology and minaturization is envisioned.

Electrochemical Microfabrication of Metallic Nanowires—A Revival of Electrochemical Methods for Modern Device Technologies?

An old dog taught new tricks is how electrochemical methods relate to modern device technologies. Penner et al. use electrochemical deposition, rather than chemical vapor deposition methods, to form molybdenum wires of monodisperse and controllable diameters in the range 20–900 nm; direct application to contempory nanotechnology and minaturization is envisioned.

Fundamental aspects and applications of electrochemical microfabrication

The theory and applications of electrochemical microfabrication technology are reviewed focusing on electrodeposition and dissolution processes. Electrochemical microfabrication offers some unique advantages over competing vapor phase technologies and therefore finds increasing use in the electronics and microsystems industries. The present paper discusses the underlying principles of electrochemical microfabrication processes. The important role of mass transport and current distribution is stressed and it is shown how numerical modeling contributes to the present understanding of critical process parameters. The application of electrochemical microfabrication technology in the electronics industry is illustrated with selected examples.

Electrochemical microfabrication by laser-enhanced photothermal processes

Laser-enhanced processes have found an increasingly important role as the interest in microfabrication continues to grow. An especially attractive feature of laser processing in liquids is the ability to obtain maskless patterning in the form of material deposition or dissolution, dictated by the path of the focused laser on the sample. In the present discussion we review theoretical and experimental aspects of laser-enhanced plating and etching for a variety of materials. Some of the substrates readily etched with the help of the laser are extremely difficult to process by conventional methods. Even though the laser processes can handle materials only in a serial manner, the plating and etching rates in certain instances are sufficiently rapid to make them competitive with batch processing methods. This is particularly interesting as it pertains to laser-enhanced jet plating of copper and gold, where rates as high as 50 and 20 µm/s have respectively been obtained.