Microfabrication

Electrochemical microfabrication offers some unique advantages over competing technologies and therefore finds increasing applications in the electronics and microsystem industries (Datta and Landolt 2000). In the mid of the nineteenth century, electroforming was applied to precision manufacturing of printing plates for bank notes. Electrochemical technology entered the electronics industry some 50 years ago as a manufacturing process for low-end printed circuit boards. Today, electrochemical technology is employed widely for the processing of advanced microelectronic components, including high-end packages and interconnects, thin-film magnetic heads, and microelectromechanical systems (MEMS).
Electrochemical microfabrication technology includes cathodic, anodic, and open-circuit processes. The latter processes are often referred to as chemical processes but they involve cathodic and anodic partial reactions which follow electrochemical laws. An overview of different processes used in electrochemical microfabrication is presented in Table 1.

Electrochemical microfabrication is a relatively new technology which is rapidly evolving as a technology of choice in the electronics and microsystem industries. The successful implementation of electrochemical processes in the manufacturing industry has become possible thanks to progress achieved through fundamental research in electrochemistry, electrochemical materials science, and electrochemical engineering.
An improvement of the acoustic liquid manipulation (ALM)-assisted electroplating process by a potential tool-less fabrication (TLF) technique for microfabrication is reported by Gadkari and Nayfeh (2008). A plated dot size of 0.031 in., using highly backed ultrasonic transducers, was successfully achieved. At the start of electroplating process, an ion-depleted layer was formed at the cathode which acts as a natural mask. A fine stream of fresh electrolyte formed by acoustic streaming by an ultrasonic transducer was also directed to refresh the ion-depleted layer at the cathode which caused the plating process to continue at a localized point.
In their work, several key factors that can affect the quality of plating process were identified as (Gadkari and Nayfeh 2008):

1. Transducer type: It was determined that the minimum dot size in the focal plane was a function of the size of the transducer element, the size of the focal point, and the position of the target relative to the focal plane. To this end, a smaller element size transducer was used (0.25 in.) with a focal distance of 0.5 in. in the electrolyte fluid which resulted in a smaller-diameter stream along with reducing the stream spread.
2. Streaming power level: It was determined that the streaming power is critical to the process; higher power causes turbulence and stirring in the tank and results in a less-coherent geometry of the plated area. To this end, a highly damped transducer was used instead of the free ringing ones used in the previous work. This resulted in a reduced forward acoustic power and reduced the harmonics which both served to make the stream more laminar and reduced the beam spread.
3. Plating current and ion concentration levels: The relation between concentration of the ions in the electrolyte and the plating current is an important factor affecting the plating size and quality. Low ion concentration coupled with higher plating currents yielded the best plating results. The ideal situation would be where the rate of deposition of the copper ions on the target plate equaled the rate of replenishment of the fresh ions to the target plate.
4. Time of plating: As the time of plating increases, the plating thickness increases. This factor is important for building plating structures with thickness. The aspect ratio of structures by varying the time of plating could not be controlled.

Finally, although this work demonstrated plating copper on steel, however, the same process can be duplicated for plating other materials.
Three-dimensional (3D) micromachined objects are difficult to make by using current techniques; there are few alternatives to using a large number of process steps and masks. A facile approach to generate 3D microfabricated structures using very few steps and a single photolithographic mask was presented by Lavan and co-workers (2003). This new approach relies on a conductive template, which can be produced using conventional lift-off microfabrication, or by other means such as self-assembly or printing and then an electrodeposition step to produce the full structure. At first, gaps between regions of the template were intentionally introduced, and then the design of the conductive template determined the full 3D structure. As material is deposited, it expands both vertically and horizontally; the horizontal expansion bridges the spaces between the conductive regions. Once that space is bridged, the electrodeposited material forms an electrical connection with the new region and deposition continues on the existing structure, as well as initiating at the newly connected region. Figure 16 provides a schematic illustration of this process. If a small difference in height were desired between adjacent structures (regions), the gap would be small; a large difference in height is created with a larger gap.

Two different materials, polypyrrole (PPy) and Ni with a noticeable difference in growth rates and growth behavior, were prepared. The rate of electropolymerization for PPy is 780 nm min-1 vertically and 1,000 nm min-1 horizontally, while nickel had a deposition rate of 105 nm min-1, both vertically and horizontally. The 4:3 lateral-to-vertical growth ratio for PPy creates a well-defined faceted profile with the polymer growing slightly faster outward than upward (Fig. 17a). The nickel grows uniformly in all directions, which results in the edges of the pattern having a radius equal to the film thickness (Fig. 17b). Depending on the application, one or the other of the materials may be more desirable. The PPy surface appears smoother than the nickel surface, and it deposits more rapidly. The sidewalls of the nickel structures, however, are closer to vertical than the corresponding PPy patterns. Vertical sidewalls, or a limit on horizontal expansion, could be obtained by introducing a sacrificial boundary, such as one made from photoresist, around the conductive patterns.

The abovementioned technique opened the possibility of new methods to fabricate multilevel structures such as electrodes, interconnects, gratings, and photonic lattices. The next step could be to develop approaches to augment these structures with additional films patterned over the first layer. An additional area to explore is the ability of this method to bridge gaps to form low-impedance connections between devices and substrates produced by self-assembly or fluidic self-assembly. Finally, this approach could be used to modify the LIGA process (electroforming and molding process (referred to by its German acronym, LIthography, Galvanoformung, Abformung, or LIGA) of microelectromechanical systems (MEMS)) where regions of a microdevice are deposited to regulate the effect of current densities or to create composite or gradient materials from a single mold or pattern.
Chip-Interconnect Metallization: A paradigm shift in interconnect technology was announced by IBM in 1997 when the vacuum-deposited Al was changed to electroplated Cu. Since then, most of the leading chip manufacturers have converted to electroplated Cu technology. Relative to comparable Al interconnect, Cu interconnect has advantages of significantly low resistance, higher current carrying capability, and increased scalability (Datta 2003). For similar dimensions, electromigration lifetime of Cu interconnects is more than 100 times longer than for Al lines. Cu metallization, therefore, supports much higher current density specifications and makes it extendible to finer dimensions and pitches. Figure 18 shows a typical fabrication process of Cu interconnects.

Further miniaturization trends of the microelectronics and microsystem industries provide new opportunities and challenges which include issues related to nanoscale structuring, fabrication of high aspect ratio structures, new functional alloys, multidimensional interconnects, and automated large-scale processes including additive control and recycling of electrolytes. To successfully overcome such challenges, fundamental as well as technology-oriented research will be needed and interactions between the two should be stimulated.
Several techniques have been developed to fabricate MEMS-based through-wafer interconnections on silicon or silicon-on-insulator (SOI) substrates. In general, cavities formed by wet anisotropic etching or vertical through-wafer vias formed using the Bosch process, or the combination of the two, are used in the fabrication of through-wafer interconnects. The electrical connection from the front side of the substrate to the backside is typically made by metal or polycrystalline silicon layers formed on the cavity surface or through metal-filled vias.
One of the common techniques is to fabricate a separate wafer with through-wafer interconnects and bond the die with circuitry or interconnections on the front and backside of the wafer with interconnects. Typically, as the perforations for the via holes are formed on the substrate, a hole sealing process or separate substrate with seed layer is required to have a void-free fully metal-filled through-wafer interconnect. The vertical through-wafer via whose height equals the substrate thickness can be fabricated with deep silicon etch using the Bosch process and subsequent conductive material deposition inside via holes, but the aspect ratio is limited by the inherent characteristics of the Bosch process itself. For vertical trenches with a closed bottom surface, a void-free bottom-up fill with electroplated metal can be obtained with the aid of additive control, fountain plating, reverse pulse plating, and dissolve oxygen enrichment.
Magnetic heads: The electrodeposition of soft, high magnetic moment alloys has become the critical fabrication step in manufacturing of magnetic recording heads. The need for ultimately high magnetic moment alloys and the electrodeposition process capable of delivering magnetic structures with dimension in the range of several tens of nanometers are the future trends for next-generation magnetic recording devices. Films of CoNiFe ternary alloy with high saturation magnetic flux density BS and low coercivity, HC were successfully produced by electrodeposition. Osaka reported a typical film, designed as “HB-CoNiFe,” had the composition of Co65Ni12Fe23 (at.%) with BS = 2.0–2.1 T and H < 2 Oe (Osaka 2000). The key to the success in obtaining low HC with high BS was to form film with very fine crystals. The inclusion of small amount of sulfur was found to be essential for producing such a film with the desired magnetic properties. The film has been applied to the construction of a new type of merged-GMR head, which is considered as a breakthrough for materializing ultrahigh-density magnetic recording. The schematic diagram and morphology of GMR is presented in Fig. 19.

Microelectromechanical systems (MEMS): Electrodeposited metal films may play an important role in MEMS technology, as it provides an easy, simple, and low-cost technology for MEMS device development with comparable mechanical properties to the silicon-based material set, while offering an enhanced set of properties for transducers. Sensors and actuators based on plated metals have been fabricated and studied, including thermal actuators, microcoils, micromotors, and pneumatic actuators.
Ni and NiFe are the electroplated metals most commonly used for MEMS devices. Usually, electroplated nickel film can be used to vacuum seal a MEMS structure at the wafer level. The package is fabricated in a low-temperature (<250o C) 3-mask process by electroplating a 40-μm-thick nickel film over an 8-μm sacrificial photoresist that is removed prior to package sealing. A large fluidic access port enables an 800 x 800-μm package to be released in less than 3 h. MEMS device release is performed after the formation of the first-level package. The maximum fabrication temperature of 250o C represents the lowest temperature ever reported for thin-film packages (previous low ~400o C). Implementation of electrical feedthroughs in this process requires no planarization. 

Energy Conversion and Storage

Electrodeposition of metals, alloys, metal oxides, and conductive polymers plays a pivotal role in fabrication processes of some recently developed energy devices, such as fuel cells, supercapacitors, solar cells, and batteries.
Fuel cells: Porous platinum, bringing together high catalytic activity and a high catalyst surface area, is of high interest for fuel cells, as electrodes for electrooxidation of hydrogen, methanol, ethanol, formic acid, and glucose as well as for reduction of oxygen consist of platinum or platinum-alloy catalysts. Several strategies exist for the fabrication of porous platinum electrodes ranging from the use of nanoparticles, over electrodeposition, to dealloying.
Electrodeposition enables a facile fabrication of strongly adherent electrocatalysts on a conductive substrate within only one step. Techniques such as pulsed galvanostatic deposition (alternation of deposition and concentration relaxation) enable the generation of dendritic structures. Sacrificial templates such as microspheres, liquid crystals, polymers, or mesoporous structures (e.g., silica or alumina membranes) can be used to additionally control/design the pore structure on the corresponding scale (50 nm to 10 μm) (Kloke et al. 2012).
Supercapacitors: Electrochemical supercapacitors play an important role in power source applications such as hybrid electrical vehicles, computers, and short-term power sources for mobile electronic devices. They can be classified into two types based on their charge storage mechanisms: (i) electrical double-layer capacitors (EDLCs) and (ii) redox supercapacitors. Compared to the EDLC-based capacitors, redox capacitors based on transition-metal oxides or conducting polymers such as RuO2, MnO2, NiO, Co3O4, V2O5, and polyaniline may provide much higher specific capacitances up to 1,000 Fg-1 of the active material. For example, Co3O4 nanosheets/carbon foam with excellent supercapacitor characteristics was successfully fabricated without using metal substrates. The experimental results demonstrate that the as-deposited Co3O4 nanosheets (Figs. 20, 21) exhibited an ideal capacitive behavior with a maximum specific capacitance of 106 F/g in 1 M NaOH solution at a scan rate of 0.1 V/s.

Among the existing synthetic approaches to the transition-metal oxides electrodes for supercapacitors, electrochemical techniques are of great interest due to their unique principles and flexibility in the control of the structure and morphology of the film materials. The main advantage of the electrodeposition technique is its relatively easy and accurate control of the surface microstructure of metal oxides with high surface area by changing deposition variables, such as electrolyte, deposition potential, and bathing temperature.
Recently, graphene–metal oxide hybrid supercapacitors have attracted much research interest. As graphene is an ideal building block in composite materials combined with a variety of inorganic compounds, it is expected that fabrication of graphene/metal oxide hybrid nanomaterials will be an effective and practical method to overcome the abovementioned challenge. In such hybrid materials, the graphene acts as 2-dimensional conductive template or 3-dimensional conductive porous network for improving the poor electrical properties and charge transfer pathways of pure oxides. In addition, graphene provides 2D support for uniformly anchoring or dispersing metal oxides with well-defined size, shapes, and crystallinity; these metal oxides not only provide a high capacity but also suppress the agglomeration and re-stacking of graphene and thus to increase the effective surface area of the graphene, resulting in high electrochemical activity.
In situ electrodeposition can be applied to prepare the metal oxide nanostructures on graphene. The good conductive graphene will be used for the direct electrodeposition of nanostructural metal oxides without using other binders or conductive substrates. In addition, the electrodeposition is a simple approach to fabricate well-defined metal oxide nanostructures with excellent adhesion on the substrates in ambient temperature and atmosphere. In particular, the metal oxide nanostructural arrays, such as nanorod and nanotube arrays, offer several important advantages for the electrodes: as they possess abundant active sites and large surface areas, each nanorod/nanotube acts as a direct 1D pathway for the electron transport, more favorable for the enhancement of the specific capacitance and rate performance of the supercapacitors.
Solar cells: Electrodeposition has been shown to provide improved benefits in several solar cell technologies. It is one of the most selective processes because deposition only occurs at positions on a substrate where the substrate conductivity is highest. To date, significant progress has been made toward the scaling up of electrodeposition processes (both electrolytic and electroless) in microelectronics for printed circuit boards and for semiconductor wafers. The scale of substrates for the solar industry varies between 156 mm2 to m2 and within the range of the microelectronics processing. This scaling has been done on both rigid and metallic flexible substrates for different solar cell applications.
In a study, perylene bis(phenethylimide) (PPEI) was deposited on nonporous TiO2 electrode (Zaban and Diamant 2000). PPEI layer has a wide spectrum throughout the visible range resembling the polycrystalline form of this organic semiconductor, therefore the high optical density solar cell can be fabricated by only a thin layer of the deposited resistive PPEI film. It was found that liquid electrolyte is a quencher; the new electrode could operate as a front electrode in a high surface area bilayer OSC solar cell. The schematic view of solar cell is shown.
Wei et al. exhibited three kinds of Cu2O/ZnO heterostructure solar cell synthesized by electrochemical deposition (Wei et al. 2011). In their synthesis process, ZnO film, nanowires, and nanotubes were grown on the FTO first and the Cu2O was further coated on the ZnO. The absorption spectra of FTO glass, ZnO film, and Cu2O film are shown in the Fig. 22. The absorption spectrum of the Cu2O/ZnO bilayer exhibits two absorption edges at 380 nm and 600 nm, which correspond to the band gap absorption of ZnO and Cu2O films, respectively.

Another example of optimizing electrochemical conditions for fabricating Cu2O with a designed surface structure was explored to improve the photovoltaic performance of solar cells (Wei et al. 2012). The enhanced conversion efficiency of Cu2O film solar cell was carried out by tuning the homojunction interface crystal orientation and forming a pyramid-like textured surface. By tuning the n-Cu2O film growth orientation from [100] to [111] to match the p-Cu2O orientation, the formation of interface states during the homojunction epitaxial growth can be restrained, which can effectively prevent recombination of electrons in n-Cu2O with holes in p-Cu2O at the interface region and enhance the built-in potential.
Li-ion battery: For electrochemical energy storage device, one effective strategy to achieve high reversible capacities is to prepare nanostructured electrodes with sufficient buffering space, e.g., porous, 1D structures to alleviate the volume swings. Electrochemical process has been widely applied to fabricate nanostructured electrodes. For example, interconnected nanoflakes of ZnSb for Li-ion battery anode applications were prepared by a template-free electrochemical deposition process under high overpotential conditions (Saadat et al. 2011). In the synthesis part, copper substrate was immersed into the solution containing ethylene glycol, ZnCl2, and SbCl3 under the applied potential in the range of 3–9 V at room temperature without any stirring or bubbling inert gas.

Drug Delivery

Electrochemical methods, including electroplating and electroless plating methods, can be used to create micrometer-scale surface morphologies to induce specific types of cellular differentiation or to attract and/or repel a specific type of cell that can be used to increase the radiopacity of the device, to produce nanocomposite coatings on a device, to enhance or control drug delivery from a device, or to accommodate a nonmetallic coating. They can even be used to create a complete medical device. Normally, the electrochemical corrosive processes tend to destabilize surfaces and can undermine the mechanical stability of a device.
Micro- and nanofabrication technologies provide the possibility of designing small-scale drug carriers capable of delivering precise doses of drugs as near the target as possible. Ideally, drug delivery systems should be sufficiently miniaturized to reach the disease site but with enough volume to accommodate a relevant dose of drug (tens to hundreds of micrograms to be comparable with the current doses of potent) systemic drugs (LaVan 2003). In addition, they should satisfy biocompatibility and nontoxicity standards throughout their desired time of action. Finally, the device should be traceable and able to release drugs on demand based on external commands or local biochemical changes.
The intelligence of these devices can be defined during fabrication by choosing the appropriate combination of materials and methods. Chitosan, a polycationic saccharide derived from naturally available chitin, is an ideal candidate for medical and pharmaceutical applications due to its intrinsic properties. It is abundant, biocompatible, biodegradable (being metabolized by certain human enzymes), and bioadhesive and appears to act as a penetration enhancer by opening tight epithelial junctions. The pKa value of its primary amine groups (around 6.5) allows for the formation of pH-responsive and functionalizable hydrogel films for a large variety of applications, such as drug delivery systems, scaffolds for tissue engineering, and targeted radiotherapy. These functions can be incorporated into lab-on-a-chip devices by different fabrication methods, including casting, printing, and self-assembly. However, only electrodeposition provides the unique ability to create three-dimensional structures of various size and shape at a low cost and can conformally coat structures using local pH gradients on negatively charged electrodes.
In a recent study, Fusco and co-workers (2013) presented co-electrodeposition of drug-loaded chitosan hydrogels to functionalize steerable magnetic microdevices. The characteristics of the polymer matrix have been investigated in terms of drug loading, and release responses are shown in Table 2 and Fig. 23.

The effect of the environmental conditions on the drug release from as-deposited chitosan films was also investigated and shown in Fig. 22. When immersed at pH6.0, the hydrogels release almost the entire loaded drugs in only 72 h. This amount was five times higher than the release over the same period at pH 7.4, shown in Fig. 24.
This work provides a method to incorporate functional smart materials onto wirelessly controlled magnetic microdevices using electrodeposition. In the final design, magnetic microcylinders form the core of the device, while chitosan hydrogels form the pH-sensitive drug delivery device. Chemically mild and straightforward dip coating processes tailor the release profiles of BG, an ionic model drug used as a topical antiseptic, from the chitosan gels. The platform and the study proposed here represented new insight into creation of complex microdevices able to sense, act, and react to a complex environment such as the human body.
There is no doubt that the field of electrochemistry and its continual progress will have substantial potentials for the future of medical devices and would be viewed as an economical, simple, yet powerful technique to modify and create biomimetic surfaces and medical devices.