- Email: [email protected]
MATERIALS | Etching
Etching JA Kenney and GS Hwang, The University of Texas at Austin, Austin, TX, USA & 2009 Elsevier B.V. All rights reserved.
Introduction Electrochemical techniques can offer significant advantages over other surface modification technologies. They are capable of machining hard, brittle materials to high aspect ratio, unlike traditional techniques, which require mechanical interaction. In addition, electrochemical dissolution tends to leave surface properties such as composition and crystal orientation unmodified. This is in contrast to thermal methods, such as electrical discharge techniques, which leave heat-affected layers. Thus, the electrochemical approach is a preferred choice for applications in which surface properties are important and is often used as a finishing step following other means of modification. Recently, a novel electrochemical technique has been developed for modifying the surface morphology of conductive or semiconductive materials on submicron length scales through the application of ultrashort voltage pulses to a tool electrode. This technique allows the direct reproduction of tool shape on the substrate without applying a mask, by localizing surface reactions (dissolution or deposition). This may in turn make it possible to produce complex three-dimensional (3-D) structures of semiconductors and metals by controlling the tool shape as well as the pulsing and electrolyte conditions. Such surface modification of materials with submicron precision has emerged as a key future technology for manufacturing miniaturized devices for a variety of applications in electronics, optics, sensors, biology, and medicine. Herein, the following are described: the use of conventional electrochemical techniques in macroscopic machining processes; the application of voltage-pulsing techniques to electrochemical surface modification of conductive or semiconductive materials on micrometer and submicrometer scales; and modeling and simulation of such electrochemical micromachining with ultrashort voltage pulses.
Electrochemical Machining Techniques and Their Applications Conventional Macroscopic Electrochemical Machining Electrochemical machining (ECM) is the name given to a variety of processes that use electrochemical means to remove a substrate material by anodic dissolution. An electrolytic cell in ECM typically consists of a cathode
tool and an anode workpiece, where the electrolyte is pumped through the interelectrode gap to remove dissolution products that emanate from the workpiece. In the 1950s, ECM was initially developed as a method for shaping high-strength metal alloys, which were hard to be carved by conventional contact methods such as milling and grinding. Compared to conventional machining processes, ECM allows machining of complex features with high-quality surface finish, and may also offer economic savings through reduced processing time, little tool wear, and low metal scrap. Hence, today, ECM is widely employed in aerospace, automotive, and biomedical engineering industries. Some examples are as follows. Electrochemical drilling (ECD) is perhaps the most • common method by which holes are formed through
•
•
electrochemical means. In this technique, as illustrated in Figure 1, a metal tube serves as the cathode tool, with a concentrated salt electrolyte exiting the tube at high pressure. As dissolution occurs, the tool is lowered into the workpiece, with electrolyte exiting through the small gap between electrodes. This flow of electrolyte reduces heat buildup and serves to remove etch products, which otherwise may form precipitates. A schematic of the process is given in Figure 1. Shaped tube electrochemical machining (STEM) is essentially a modified ECD process, originally developed to drill high aspect ratio holes for which ECD processing proved inadequate. Its primary distinguishing characteristic is the use of a strong acid electrolyte rather than a concentrated salt. This keeps machined workpiece material in solution rather than forming precipitates, which may hinder electrolyte flow or cause short circuits between tool and workpiece. Electrochemical jet machining (ECJM) is a blanket term describing processes that employ a pressurized electrolyte jet for machining of holes and grooves. These techniques typically use capillary tubes or small nozzles to confine the flow of electrolyte, but operating pressures, applied voltages, and cathode materials vary greatly between methods. Different technologies here include the following: – Capillary drilling (CD), also known as electrochemical fine drilling (ECF), is a technique for forming holes with diameters ranging from 0.2 to 0.5 mm with aspect ratios as high as 100. It employs a glass capillary surrounding a wire electrode, with electrolyte forced through the annular region at moderate
625
626
Materials | Etching Tool (−)
Electrolyte Insulation
Workpiece (+)
Figure 1 Schematic of electrochemical drilling process. Electrolyte is fed through the cylindrical tool electrode at high pressure, with the tool lowered into the workpiece at a rate matching that of the workpiece dissolution. The side of the tool electrode is insulated to prevent additional etching of the sidewalls of the hole formed.
pressure (0.3–2 MPa) onto a workpiece. The wire electrode is typically 1mm or more from the tube outlet so as not to affect electrolyte flow, necessitating large applied potentials (100–200 V) due to the increased separation between electrodes. Holes are typically etched at the rates of 1–4 mm min1, with the tube steadily lowered into the workpiece. – Electrostream drilling (ESD), also known as electrojet drilling (EJD), is similar to CD but uses glass tubes drawn down to nozzles leading to fine capillaries. Wire electrodes are again employed, but in this case remain within the tube region rather than entering the capillaries. The longer, narrower path length between electrodes relative to that of CD means substantially larger applied potentials (150– 850 V) are required, and thus additional precautions must be taken when insulating the system. Operating pressures and etch rates are comparable to those of CD. In both CD and ESD, strong acid electrolytes (10–25 wt%) are used to provide a highly conductive medium for current flow and to ensure that dissolved metals do not form precipitates. Despite this, etch rates are typically low for machining of single holes, and thus these techniques are primarily used for machining of arrays of holes in parallel, with several capillaries/tubes fed from a manifold. When used in this arrangement, care must be taken to keep a similar arrangement for the wire electrode location among different elements of the array to prevent differing etch characteristics, particularly in the case of CD. – Jet electrolytic drilling (JED) also uses jets of pressurized electrolyte, with the nozzle feeding the
•
electrolyte remaining above the substrate approximately 2–4 mm rather than being lowered into the substrate during etching. Owing to this constant separation, an insulating material is no longer needed to feed the electrolyte, and thus metal nozzles are typically used, doubling as electrodes. As with ESD, the large separation between electrodes requires high applied voltages in the range of 400–800 V. Higher pressures (10–60 bar) than those used in CD and ESD are required for JED to keep the electrolyte jet tightly focused. Other characteristics are similar to those of CD and ESD, including the use of strong acid electrolytes, low etch rates (0.5–2.0 mm min1) for single holes, and the use of manifolds with multiple nozzles to create multiple holes simultaneously. Likewise, limitations are largely the same, with filtration of the feed electrolyte often required and special measures taken to ensure proper insulation and with waste handling. – Laser jet electrochemical machining employs laser beams to reduce undercutting in the etching of holes. This method requires the fabrication of a small chamber containing the cathode, an inlet for the electrolyte, and a nozzle outlet with laser aligned along its axis. The localized heating provided by the laser confines the location of the electrochemical reactions beyond that of the jet alone, further reducing stray cutting. Careful consideration of the electrolyte and workpiece materials must be taken, however, as photoelectrochemical and thermal effects may result in unwanted anodic reactions. Electrochemical polishing , sometimes referred to as electropolishing or reverse electroplating, is the controlled anodic leveling and/or brightening of a conductive surface, often resulting in improved surface properties and increased reflectivity. In this process, the surface to be modified is immersed in electrolyte and a bias applied, with the surface as anode. The nonuniform current distribution, resulting from the protrusions and recessions of the surface, gives rise to different rates of dissolution, with surface peaks preferentially removed.
Electrochemical Micro- and Nanostructuring with Ultrashort Voltage Pulses Conventional electrochemical techniques are generally ill-suited for machining materials on micrometric and submicrometric scales due to their inability to confine electrochemical reactions, whereas the use of ultrashort voltage pulses allows the 3-D structuring of conductive or semiconductive materials with such precision. Lithography and plasma etching techniques have been widely
Materials | Etching
used to construct well-ordered surface structures on these length scales, but the structures formed have largely been 2-D. For some specific materials such as silicon, micromachined 3-D structures can be crafted using a combination of wet and dry etch processes. However, this approach requires additional complex processing steps such as masking and wafer bonding, with the shape of the etched structure often determined by the reactivity of exposed surfaces to wet chemical etching solutions. Furthermore, the 3-D structuring of metals cannot be achieved using such etching methods. Thus, in recent years, the pulsed electrochemical approach has received much attention for high aspect ratio etching and controlled deposition of conductive or semiconductive materials due to well-established chemical procedures and ease of supplying materials dissolved in an electrolyte. As depicted in Figure 2(a), a pulse electrochemical technique employs a tool electrode held in close proximity (B1 mm) to a reactive workpiece electrode in the presence of an electrolyte and utilizes ultrashort (o100 ns) voltage pulses. Initially, a scanning tunneling microscope (STM) tip was chosen as the tool electrode but this has since evolved to employ a variety of tool shapes and compositions. The negative bias applied to the tool electrode for a short duration results in the localized charging of the electrochemical double layers in regions where the tool and workpiece electrodes are in close proximity. The ensuing localized zeta potential variation leads to selective dissolution of material at the workpiece, with resolutions on the submicron level possible. The bias is then removed for a duration sufficient to allow the double layers to discharge before a new pulse is initiated. The processes rely on the similarity of electrochemical systems to resistor–capacitor (RC) circuits during the transient phase. The electrolyte represents the resistance, which varies based on the differing current path lengths between regions of the tool
and workpiece. The electrochemical double layers at both the tool and workpiece may be considered as capacitors, with different regions charging at different rates due to the variation in the local current during the pulse. Thus, a particular pulse duration can be compared with the time constants of these RC circuits with varying electrolyte resistance to approximate the range at which electrochemical modification will occur. A variety of substrate and tool materials, electrolytes, and operating conditions have been used in pulsed ECM systems. Copper and stainless steel have been the most commonly etched workpieces, but doped semiconductors have also been shown to be capable of modification. Tool electrodes have been fashioned from platinum or titanium wires and may be simple cylinders or in the form of a complex template to be communicated to the substrate. Although pulse durations are sometimes as large as 5 ms, values are more typically under 100 ns, with current state-of-the-art systems capable of generating pulses on the order of 1 ns. The length of the pause following the pulse is often given as a ratio, with a 1:10 pulse/pause ratio the most commonly used value. Applied potentials fall in the range of 1–10 V; thus, no additional precautions need to be taken when insulating the system. A wide range of electrolytes and concentrations have been demonstrated to allow dissolution of the workpiece to occur. These include hydrochloric acid, copper sulfate þ sulfuric acid, copper sulfate þ hydrofluoric acid, and hydrofluoric acid þ sulfuric acid, among others, with acid concentrations ranging from 0.01 to 5 mol L1 depending on the workpiece material.
Modeling of Pulsed Electrochemical Micromachining Over recent years, extensive numerical studies have been undertaken to examine the underlying mechanisms of the
Tool electrode
ΔV Tool
627
RHigh
RLow
Substrate (a)
(b)
Substrate electrode
Figure 2 (a) Schematic of electrochemical machining (ECM) with ultrashort voltage pulses. Different resistances of current pathways through the electrolyte are indicated by RLow and RHigh. (b) Model of the pulsed ECM system. Parallel capacitors represent electrochemical double layers. Resistors represent the resistance of the electrolyte. Reflective boundary conditions are used as appropriate.
Materials | Etching
pulsed electrochemical micromachining and also to predict the performance of the electrochemical system at various processing conditions. The authors have developed a 3-D computational model that enables the study of complex electrochemical etching of sufficiently low aspect ratio microstructures in a moderate-to-strong electrolyte system, where no appreciable ion depletion occurs in the electrolyte during double layer polarization and no significant effect of dissolution products exists on electrolyte resistivity locally. Their simulation framework consists of two main modules: (1) transient charging simulation to describe the charge and discharge of electrochemical double layers during the pulse/pause cycles and (2) feature profile evolution simulation to delineate changes in the surface morphology of workpiece during the pulsed electrochemical micromachining. Transient Charging of Electrochemical Double Layers Electrochemical reactions at the workpiece are governed by the potential drop within the thin electrochemical double layer. Thus, an accurate description of double layer charging dynamics is essential for quantifying electrochemical dissolution. The double layer is analogous to an electrical capacitor, with some time required to charge or discharge upon a shift in the electrode potential. Electrolyte solutions commonly obey Ohm’s law and Kirchoff ’s voltage law like metallic materials. On an electrode surface with high charge density, the layer of counterions that effectively balance the surface charge will be a few angstroms thick from the surface. In addition, for its typical capacity of 10 mF cm2, the double layer polarization by 1 V requires the accumulation of 1014 ions cm2 (equivalent to about 0.1 ML). Thus, it could safely be assumed that the electrochemical double layers are negligible in size relative to the gap spacing between the tool and workpiece electrodes, unless the gap is only a few nanometers wide, and also ion concentrations in the gap region outside the double layers are unchanging. In addition, the double layer is assumed to be purely capacitive, as the polarization resistance could be ignored. Based on these assumptions, the modeling of the electrochemical system is simplified using equivalent circuit elements to describe the charging process rather tracking ion migration and diffusion and other complex interactions of the ions with the electrode surfaces. Figure 2(b) shows a simulation domain, where the double layer capacitances are considered as capacitors connected to their respective surfaces in parallel and the electrolyte resistance is likewise analogous to interconnected resistors, terminating at either one of the aforementioned capacitors or a reflective boundary condition. Nodes in the electrolyte region are connected to each other by resistors, which represent the resistance of
the electrolyte. Connections are made to the tool and workpiece surfaces through capacitors in parallel, representing the capacitances of the electrochemical double layers at the respective surfaces. Completing the model, the tool and workpiece are considered as equipotential surfaces, and reflective boundary conditions are used as appropriate. To find the initial potential of each node in the electrolyte region upon the application of a voltage pulse to the tool, Kirchoff ’s law can be applied, generating a system of linear equations. The induced current in the electrolyte in turn charges the capacitors. If for a given capacitor one considers only the equipotential surface and nearest-neighbor mesh point, the double layers reduce to a series of RC circuits with time-dependent, spatially varying applied potentials. The charging of the capacitors in these circuits is solved concurrently with the application of Kirchoff ’s law to the resistor network to evolve the double layers in time. To evaluate the RC model, the transient current response during a single pulse:pause period has been calculated for different tool–substrate separations using a 3-D computational model, as shown in Figure 3. Values of the pulse magnitude and duration, electrolyte resistivity, and double layer capacity were chosen to match experimental data. The results were qualitatively similar to that of the experimental data, with some discrepancies in the initial currents expected due to the different natures of the shapes of the voltage pulses – the experimental voltage pulse ramped upward to the full potential over a period of several nanoseconds whereas the simulation pulse reached the maximum value instantaneously. Most importantly, the simulation captured the peaking and rapid decline of the current at small separations between tool and substrate, indicative of the charging of the electrochemical double layers for these cases. The discharge behavior is similar, with a large initial
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1 μm 2 μm 10 μm
0.5 i DC (mA)
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Figure 3 Transient current, iDC, response calculated for a system with the following parameters: 1.6 V, 50 ns pulse, 1:10 pulse/pause ratio, 30 O cm resistivity, 10 mF cm2 capacitance, and 10 mm diameter tool electrode at the indicated separations between tool and substrate.
Materials | Etching 15
discharge of the double layers for the cases with small separations.
Prediction of surface morphology was accomplished through use of the feature profile evolution simulation. Here, the level set method was used for interface tracking, as this technique allows for robust handling of complicated etch and deposition features as well as simple manipulation of surface properties. This was particularly important for accurately estimating the resistances of connections to the workpiece from nodes in the electrolyte. In addition, the ability to resolve the properties of features at sizes below the length of the mesh allows for the use of less dense meshes, significantly increasing computational speed and allowing for the extension of this model into three dimensions. The implementation of the level set method used here implemented narrow banding to further increase computational efficiency. All forms of the profile evolution tool follow the same general algorithm. The tool location and level set description of the surface were used to form the simulation domain for the charging simulation (as the discharge during the pause period does not contribute to the modification of the tool or workpiece, it was ignored). Following the charging simulation, the transient dissolution current information was integrated for all capacitors at the workpiece for the duration of the pulse and divided by the length of the pulse and pause cycle to obtain average dissolution currents at each capacitor. These currents were then compared to a dissolution current for which the etch rate was known, obtained through an iterative process to match an experimental resolution. This generated the necessary surface velocities for the level set method, which were extended to the rest of the narrow band. The surface is then evolved for a set duration, and the tool is moved as appropriate. This overall process is repeated until a desired etch depth is reached (vertical etching) or a steady-state resolution downstream of the tool is found (lateral etch). To evaluate the etch model, simulations of shallow trench formation have been performed for varying pulse durations and the resolutions compared with those found experimentally. Experimental data for these features, formed by a short vertical etch followed by a lateral etch, showed a linear increase in etch resolution with pulse duration over a wide range of electrolyte resistivity. Again, experimental parameters were matched in the simulation. Owing to the uncertainty in tool diameter, two different sizes are examined, 10 and 30 mm. As summarized in Figure 4, greater resolutions were found for the larger diameter tool as expected because of the longer exposure of the trench sidewalls to etching
30 μm 10 μm Resolution (μm)
Surface Morphology Evolution of Electrodes
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5
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Pulse duration (ns)
Figure 4 Calculated resolution for a 0.1 mol L1 CuSO4/0.075 H2SO4 electrolyte using 10 and 30 mm diameter tools.
conditions, and these resolutions were roughly linear with respect to the pulse duration, matching the experimental trend. For the smaller diameter tool, however, the increase in resolution was sublinear with increasing pulse duration. The difference in the behavior of the resolution with tool diameter led to a study of the relationship among the tool diameter, pulse duration, and resolution, given in Figure 5. For the systems where the tool diameter is large relative to the resolution, the system was essentially 1-D. The linear increase in resolution with pulse duration seen was consistent with an analytical derivation of a 1-D model based on circuit elements. As the resolution grew larger relative to tool diameter, its value was no longer dependent simply on the pulse duration. The relationship among tool diameter, pulse duration, and resolution was found for a 2-D system consisting of a stationary cylindrical tool electrode concentrically arranged with a hollow cylindrical substrate, with electrolyte in the annular region. Subsequent 2-D computer simulations of a tool moving through a substrate closely matched the values predicted by this derivation, making it a useful method of estimating resolution for given system parameters. The variation in surface morphology with electrolyte concentration has also been considered through computational model, as shown in Figure 6. Profiles of a shallow lateral etch are given, demonstrating the decrease in the size of the trench formed with decreasing acid composition in a mixed electrolyte. In general, this trend has been found to hold down to electrolyte concentrations of roughly 0.05 mol L1, at which point dissolution of the substrate no longer occurs. The use of complex tool electrodes (templates) was also investigated, considering the creation of two deep holes in close proximity, as illustrated in Figure 7. Through a systematic study of profile evolution varying pulse durations and separations between features, the degree to
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Normalized resolution ratio
Materials | Etching
Resolution (μm)
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1 0.8 0.6 0.4 100
Tool radius (μm)
80
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Tool radius (μm)
0 100
(b)
80
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Pulse duration (ns)
Figure 5 (a) Predicted resolutions from a two-dimensional (2-D) stationary model with pulse duration and tool radius. (b) Normalized resolution ratios with pulse duration and tool radius, illustrating 1-D and 2-D behavior. Reproduced with permission from Kenney JA and Hwang GS (2006) Etch trends in electrochemical machining with ultrashot voltage pulses: Predictions from theory and simulation. Electrochemical and Solid-State Letters 9(1): D1–D4, Figure 3.
(a)
(b)
Figure 7 Profiles of the resulting holes formed by etching with (a) a simple cylindrical tool in two passes and (b) two properly spaced cylindrical tools in a single pass. Additional etching in the midpoint region occurs in the latter case due to the exponential nature of the dissolution current with respect to overpotential.
Figure 6 Surface morphologies following a shallow lateral etch using the same pulse characteristics and a mixed electrolyte of 0.1 mol L1 CuSO4 and (a) 0.025 mol L1 H2SO4, (b) 0.050 mol L1 H2SO4, and (c) 0.075 mol L1 H2SO4.
which the performance of these templates is decreased relative to that of a simple tool electrode was quantified. Both the discrepancies and the separation at which the maxima occur were found to be reduced with decreasing pulse duration. These discrepancies are explained by examining for the first time the effects of intra-tool electrode interactions on the overpotential and dissolution current at the substrate electrode. These studies have revealed that overpotential is largely additive among the
individual components of a tool electrode above a critical separation between components, whereas the exponential nature of the dissolution current with respect to overpotential leads to increased dissolution when using a complex tool electrode. Therefore, care must be taken when designing tool templates to account for the pulse duration to be used, ensuring that features in close proximity to each other do not merge.
Nomenclature Symbols and Units iDC RLow
RHigh
transient current lower resistance of shorter current path through electrolyte in pulsed ECM systems higher resistance of longer current path through electrolyte in pulsed ECM systems
Materials | Etching
Abbreviations and Acronyms CD ECD ECF ECJM ECM EJD ESD JED RC STEM STM
capillary drilling electrochemical drilling electrochemical fine drilling electrochemical jet machining electrochemical machining electrojet drilling electrostream drilling jet electrolytic drilling resistor–capacitor shaped tube electrochemical machining scanning tunneling microscope
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See also: Electrochemical Theory: Double Layer; Electrokinetics; Electrolytes: Overview.
Further Reading ASM Handbook Committee (1994) ASM Handbook, Vol. 5: Surface Engineering. Metals Park, OH: ASM International. ASM Handbook Committee (1995) ASM Handbook, Vol. 16: Machining. Metals Park, OH: ASM International. Kenney JA and Hwang GS (2006) Etch trends in electrochemical machining with ultrashot voltage pulses: Predictions from theory and simulation. Electrochemical and Solid-State Letters 9(1): D1--D4. Schuster R (2007) Electrochemical microstructuring with short voltage pulses. ChemPhysChem 8: 34--39. Schuster R, Kirchner V, Allongue P, and Ertl G (2000) Electrochemical micromachining. Science 289: 98--101.
Introduction Electrochemical techniques can offer significant advantages over other surface modification technologies. They are capable of machining hard, brittle materials to high aspect ratio, unlike traditional techniques, which require mechanical interaction. In addition, electrochemical dissolution tends to leave surface properties such as composition and crystal orientation unmodified. This is in contrast to thermal methods, such as electrical discharge techniques, which leave heat-affected layers. Thus, the electrochemical approach is a preferred choice for applications in which surface properties are important and is often used as a finishing step following other means of modification. Recently, a novel electrochemical technique has been developed for modifying the surface morphology of conductive or semiconductive materials on submicron length scales through the application of ultrashort voltage pulses to a tool electrode. This technique allows the direct reproduction of tool shape on the substrate without applying a mask, by localizing surface reactions (dissolution or deposition). This may in turn make it possible to produce complex three-dimensional (3-D) structures of semiconductors and metals by controlling the tool shape as well as the pulsing and electrolyte conditions. Such surface modification of materials with submicron precision has emerged as a key future technology for manufacturing miniaturized devices for a variety of applications in electronics, optics, sensors, biology, and medicine. Herein, the following are described: the use of conventional electrochemical techniques in macroscopic machining processes; the application of voltage-pulsing techniques to electrochemical surface modification of conductive or semiconductive materials on micrometer and submicrometer scales; and modeling and simulation of such electrochemical micromachining with ultrashort voltage pulses.
Electrochemical Machining Techniques and Their Applications Conventional Macroscopic Electrochemical Machining Electrochemical machining (ECM) is the name given to a variety of processes that use electrochemical means to remove a substrate material by anodic dissolution. An electrolytic cell in ECM typically consists of a cathode
tool and an anode workpiece, where the electrolyte is pumped through the interelectrode gap to remove dissolution products that emanate from the workpiece. In the 1950s, ECM was initially developed as a method for shaping high-strength metal alloys, which were hard to be carved by conventional contact methods such as milling and grinding. Compared to conventional machining processes, ECM allows machining of complex features with high-quality surface finish, and may also offer economic savings through reduced processing time, little tool wear, and low metal scrap. Hence, today, ECM is widely employed in aerospace, automotive, and biomedical engineering industries. Some examples are as follows. Electrochemical drilling (ECD) is perhaps the most • common method by which holes are formed through
•
•
electrochemical means. In this technique, as illustrated in Figure 1, a metal tube serves as the cathode tool, with a concentrated salt electrolyte exiting the tube at high pressure. As dissolution occurs, the tool is lowered into the workpiece, with electrolyte exiting through the small gap between electrodes. This flow of electrolyte reduces heat buildup and serves to remove etch products, which otherwise may form precipitates. A schematic of the process is given in Figure 1. Shaped tube electrochemical machining (STEM) is essentially a modified ECD process, originally developed to drill high aspect ratio holes for which ECD processing proved inadequate. Its primary distinguishing characteristic is the use of a strong acid electrolyte rather than a concentrated salt. This keeps machined workpiece material in solution rather than forming precipitates, which may hinder electrolyte flow or cause short circuits between tool and workpiece. Electrochemical jet machining (ECJM) is a blanket term describing processes that employ a pressurized electrolyte jet for machining of holes and grooves. These techniques typically use capillary tubes or small nozzles to confine the flow of electrolyte, but operating pressures, applied voltages, and cathode materials vary greatly between methods. Different technologies here include the following: – Capillary drilling (CD), also known as electrochemical fine drilling (ECF), is a technique for forming holes with diameters ranging from 0.2 to 0.5 mm with aspect ratios as high as 100. It employs a glass capillary surrounding a wire electrode, with electrolyte forced through the annular region at moderate
625
626
Materials | Etching Tool (−)
Electrolyte Insulation
Workpiece (+)
Figure 1 Schematic of electrochemical drilling process. Electrolyte is fed through the cylindrical tool electrode at high pressure, with the tool lowered into the workpiece at a rate matching that of the workpiece dissolution. The side of the tool electrode is insulated to prevent additional etching of the sidewalls of the hole formed.
pressure (0.3–2 MPa) onto a workpiece. The wire electrode is typically 1mm or more from the tube outlet so as not to affect electrolyte flow, necessitating large applied potentials (100–200 V) due to the increased separation between electrodes. Holes are typically etched at the rates of 1–4 mm min1, with the tube steadily lowered into the workpiece. – Electrostream drilling (ESD), also known as electrojet drilling (EJD), is similar to CD but uses glass tubes drawn down to nozzles leading to fine capillaries. Wire electrodes are again employed, but in this case remain within the tube region rather than entering the capillaries. The longer, narrower path length between electrodes relative to that of CD means substantially larger applied potentials (150– 850 V) are required, and thus additional precautions must be taken when insulating the system. Operating pressures and etch rates are comparable to those of CD. In both CD and ESD, strong acid electrolytes (10–25 wt%) are used to provide a highly conductive medium for current flow and to ensure that dissolved metals do not form precipitates. Despite this, etch rates are typically low for machining of single holes, and thus these techniques are primarily used for machining of arrays of holes in parallel, with several capillaries/tubes fed from a manifold. When used in this arrangement, care must be taken to keep a similar arrangement for the wire electrode location among different elements of the array to prevent differing etch characteristics, particularly in the case of CD. – Jet electrolytic drilling (JED) also uses jets of pressurized electrolyte, with the nozzle feeding the
•
electrolyte remaining above the substrate approximately 2–4 mm rather than being lowered into the substrate during etching. Owing to this constant separation, an insulating material is no longer needed to feed the electrolyte, and thus metal nozzles are typically used, doubling as electrodes. As with ESD, the large separation between electrodes requires high applied voltages in the range of 400–800 V. Higher pressures (10–60 bar) than those used in CD and ESD are required for JED to keep the electrolyte jet tightly focused. Other characteristics are similar to those of CD and ESD, including the use of strong acid electrolytes, low etch rates (0.5–2.0 mm min1) for single holes, and the use of manifolds with multiple nozzles to create multiple holes simultaneously. Likewise, limitations are largely the same, with filtration of the feed electrolyte often required and special measures taken to ensure proper insulation and with waste handling. – Laser jet electrochemical machining employs laser beams to reduce undercutting in the etching of holes. This method requires the fabrication of a small chamber containing the cathode, an inlet for the electrolyte, and a nozzle outlet with laser aligned along its axis. The localized heating provided by the laser confines the location of the electrochemical reactions beyond that of the jet alone, further reducing stray cutting. Careful consideration of the electrolyte and workpiece materials must be taken, however, as photoelectrochemical and thermal effects may result in unwanted anodic reactions. Electrochemical polishing , sometimes referred to as electropolishing or reverse electroplating, is the controlled anodic leveling and/or brightening of a conductive surface, often resulting in improved surface properties and increased reflectivity. In this process, the surface to be modified is immersed in electrolyte and a bias applied, with the surface as anode. The nonuniform current distribution, resulting from the protrusions and recessions of the surface, gives rise to different rates of dissolution, with surface peaks preferentially removed.
Electrochemical Micro- and Nanostructuring with Ultrashort Voltage Pulses Conventional electrochemical techniques are generally ill-suited for machining materials on micrometric and submicrometric scales due to their inability to confine electrochemical reactions, whereas the use of ultrashort voltage pulses allows the 3-D structuring of conductive or semiconductive materials with such precision. Lithography and plasma etching techniques have been widely
Materials | Etching
used to construct well-ordered surface structures on these length scales, but the structures formed have largely been 2-D. For some specific materials such as silicon, micromachined 3-D structures can be crafted using a combination of wet and dry etch processes. However, this approach requires additional complex processing steps such as masking and wafer bonding, with the shape of the etched structure often determined by the reactivity of exposed surfaces to wet chemical etching solutions. Furthermore, the 3-D structuring of metals cannot be achieved using such etching methods. Thus, in recent years, the pulsed electrochemical approach has received much attention for high aspect ratio etching and controlled deposition of conductive or semiconductive materials due to well-established chemical procedures and ease of supplying materials dissolved in an electrolyte. As depicted in Figure 2(a), a pulse electrochemical technique employs a tool electrode held in close proximity (B1 mm) to a reactive workpiece electrode in the presence of an electrolyte and utilizes ultrashort (o100 ns) voltage pulses. Initially, a scanning tunneling microscope (STM) tip was chosen as the tool electrode but this has since evolved to employ a variety of tool shapes and compositions. The negative bias applied to the tool electrode for a short duration results in the localized charging of the electrochemical double layers in regions where the tool and workpiece electrodes are in close proximity. The ensuing localized zeta potential variation leads to selective dissolution of material at the workpiece, with resolutions on the submicron level possible. The bias is then removed for a duration sufficient to allow the double layers to discharge before a new pulse is initiated. The processes rely on the similarity of electrochemical systems to resistor–capacitor (RC) circuits during the transient phase. The electrolyte represents the resistance, which varies based on the differing current path lengths between regions of the tool
and workpiece. The electrochemical double layers at both the tool and workpiece may be considered as capacitors, with different regions charging at different rates due to the variation in the local current during the pulse. Thus, a particular pulse duration can be compared with the time constants of these RC circuits with varying electrolyte resistance to approximate the range at which electrochemical modification will occur. A variety of substrate and tool materials, electrolytes, and operating conditions have been used in pulsed ECM systems. Copper and stainless steel have been the most commonly etched workpieces, but doped semiconductors have also been shown to be capable of modification. Tool electrodes have been fashioned from platinum or titanium wires and may be simple cylinders or in the form of a complex template to be communicated to the substrate. Although pulse durations are sometimes as large as 5 ms, values are more typically under 100 ns, with current state-of-the-art systems capable of generating pulses on the order of 1 ns. The length of the pause following the pulse is often given as a ratio, with a 1:10 pulse/pause ratio the most commonly used value. Applied potentials fall in the range of 1–10 V; thus, no additional precautions need to be taken when insulating the system. A wide range of electrolytes and concentrations have been demonstrated to allow dissolution of the workpiece to occur. These include hydrochloric acid, copper sulfate þ sulfuric acid, copper sulfate þ hydrofluoric acid, and hydrofluoric acid þ sulfuric acid, among others, with acid concentrations ranging from 0.01 to 5 mol L1 depending on the workpiece material.
Modeling of Pulsed Electrochemical Micromachining Over recent years, extensive numerical studies have been undertaken to examine the underlying mechanisms of the
Tool electrode
ΔV Tool
627
RHigh
RLow
Substrate (a)
(b)
Substrate electrode
Figure 2 (a) Schematic of electrochemical machining (ECM) with ultrashort voltage pulses. Different resistances of current pathways through the electrolyte are indicated by RLow and RHigh. (b) Model of the pulsed ECM system. Parallel capacitors represent electrochemical double layers. Resistors represent the resistance of the electrolyte. Reflective boundary conditions are used as appropriate.
Materials | Etching
pulsed electrochemical micromachining and also to predict the performance of the electrochemical system at various processing conditions. The authors have developed a 3-D computational model that enables the study of complex electrochemical etching of sufficiently low aspect ratio microstructures in a moderate-to-strong electrolyte system, where no appreciable ion depletion occurs in the electrolyte during double layer polarization and no significant effect of dissolution products exists on electrolyte resistivity locally. Their simulation framework consists of two main modules: (1) transient charging simulation to describe the charge and discharge of electrochemical double layers during the pulse/pause cycles and (2) feature profile evolution simulation to delineate changes in the surface morphology of workpiece during the pulsed electrochemical micromachining. Transient Charging of Electrochemical Double Layers Electrochemical reactions at the workpiece are governed by the potential drop within the thin electrochemical double layer. Thus, an accurate description of double layer charging dynamics is essential for quantifying electrochemical dissolution. The double layer is analogous to an electrical capacitor, with some time required to charge or discharge upon a shift in the electrode potential. Electrolyte solutions commonly obey Ohm’s law and Kirchoff ’s voltage law like metallic materials. On an electrode surface with high charge density, the layer of counterions that effectively balance the surface charge will be a few angstroms thick from the surface. In addition, for its typical capacity of 10 mF cm2, the double layer polarization by 1 V requires the accumulation of 1014 ions cm2 (equivalent to about 0.1 ML). Thus, it could safely be assumed that the electrochemical double layers are negligible in size relative to the gap spacing between the tool and workpiece electrodes, unless the gap is only a few nanometers wide, and also ion concentrations in the gap region outside the double layers are unchanging. In addition, the double layer is assumed to be purely capacitive, as the polarization resistance could be ignored. Based on these assumptions, the modeling of the electrochemical system is simplified using equivalent circuit elements to describe the charging process rather tracking ion migration and diffusion and other complex interactions of the ions with the electrode surfaces. Figure 2(b) shows a simulation domain, where the double layer capacitances are considered as capacitors connected to their respective surfaces in parallel and the electrolyte resistance is likewise analogous to interconnected resistors, terminating at either one of the aforementioned capacitors or a reflective boundary condition. Nodes in the electrolyte region are connected to each other by resistors, which represent the resistance of
the electrolyte. Connections are made to the tool and workpiece surfaces through capacitors in parallel, representing the capacitances of the electrochemical double layers at the respective surfaces. Completing the model, the tool and workpiece are considered as equipotential surfaces, and reflective boundary conditions are used as appropriate. To find the initial potential of each node in the electrolyte region upon the application of a voltage pulse to the tool, Kirchoff ’s law can be applied, generating a system of linear equations. The induced current in the electrolyte in turn charges the capacitors. If for a given capacitor one considers only the equipotential surface and nearest-neighbor mesh point, the double layers reduce to a series of RC circuits with time-dependent, spatially varying applied potentials. The charging of the capacitors in these circuits is solved concurrently with the application of Kirchoff ’s law to the resistor network to evolve the double layers in time. To evaluate the RC model, the transient current response during a single pulse:pause period has been calculated for different tool–substrate separations using a 3-D computational model, as shown in Figure 3. Values of the pulse magnitude and duration, electrolyte resistivity, and double layer capacity were chosen to match experimental data. The results were qualitatively similar to that of the experimental data, with some discrepancies in the initial currents expected due to the different natures of the shapes of the voltage pulses – the experimental voltage pulse ramped upward to the full potential over a period of several nanoseconds whereas the simulation pulse reached the maximum value instantaneously. Most importantly, the simulation captured the peaking and rapid decline of the current at small separations between tool and substrate, indicative of the charging of the electrochemical double layers for these cases. The discharge behavior is similar, with a large initial
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Figure 3 Transient current, iDC, response calculated for a system with the following parameters: 1.6 V, 50 ns pulse, 1:10 pulse/pause ratio, 30 O cm resistivity, 10 mF cm2 capacitance, and 10 mm diameter tool electrode at the indicated separations between tool and substrate.
Materials | Etching 15
discharge of the double layers for the cases with small separations.
Prediction of surface morphology was accomplished through use of the feature profile evolution simulation. Here, the level set method was used for interface tracking, as this technique allows for robust handling of complicated etch and deposition features as well as simple manipulation of surface properties. This was particularly important for accurately estimating the resistances of connections to the workpiece from nodes in the electrolyte. In addition, the ability to resolve the properties of features at sizes below the length of the mesh allows for the use of less dense meshes, significantly increasing computational speed and allowing for the extension of this model into three dimensions. The implementation of the level set method used here implemented narrow banding to further increase computational efficiency. All forms of the profile evolution tool follow the same general algorithm. The tool location and level set description of the surface were used to form the simulation domain for the charging simulation (as the discharge during the pause period does not contribute to the modification of the tool or workpiece, it was ignored). Following the charging simulation, the transient dissolution current information was integrated for all capacitors at the workpiece for the duration of the pulse and divided by the length of the pulse and pause cycle to obtain average dissolution currents at each capacitor. These currents were then compared to a dissolution current for which the etch rate was known, obtained through an iterative process to match an experimental resolution. This generated the necessary surface velocities for the level set method, which were extended to the rest of the narrow band. The surface is then evolved for a set duration, and the tool is moved as appropriate. This overall process is repeated until a desired etch depth is reached (vertical etching) or a steady-state resolution downstream of the tool is found (lateral etch). To evaluate the etch model, simulations of shallow trench formation have been performed for varying pulse durations and the resolutions compared with those found experimentally. Experimental data for these features, formed by a short vertical etch followed by a lateral etch, showed a linear increase in etch resolution with pulse duration over a wide range of electrolyte resistivity. Again, experimental parameters were matched in the simulation. Owing to the uncertainty in tool diameter, two different sizes are examined, 10 and 30 mm. As summarized in Figure 4, greater resolutions were found for the larger diameter tool as expected because of the longer exposure of the trench sidewalls to etching
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Figure 4 Calculated resolution for a 0.1 mol L1 CuSO4/0.075 H2SO4 electrolyte using 10 and 30 mm diameter tools.
conditions, and these resolutions were roughly linear with respect to the pulse duration, matching the experimental trend. For the smaller diameter tool, however, the increase in resolution was sublinear with increasing pulse duration. The difference in the behavior of the resolution with tool diameter led to a study of the relationship among the tool diameter, pulse duration, and resolution, given in Figure 5. For the systems where the tool diameter is large relative to the resolution, the system was essentially 1-D. The linear increase in resolution with pulse duration seen was consistent with an analytical derivation of a 1-D model based on circuit elements. As the resolution grew larger relative to tool diameter, its value was no longer dependent simply on the pulse duration. The relationship among tool diameter, pulse duration, and resolution was found for a 2-D system consisting of a stationary cylindrical tool electrode concentrically arranged with a hollow cylindrical substrate, with electrolyte in the annular region. Subsequent 2-D computer simulations of a tool moving through a substrate closely matched the values predicted by this derivation, making it a useful method of estimating resolution for given system parameters. The variation in surface morphology with electrolyte concentration has also been considered through computational model, as shown in Figure 6. Profiles of a shallow lateral etch are given, demonstrating the decrease in the size of the trench formed with decreasing acid composition in a mixed electrolyte. In general, this trend has been found to hold down to electrolyte concentrations of roughly 0.05 mol L1, at which point dissolution of the substrate no longer occurs. The use of complex tool electrodes (templates) was also investigated, considering the creation of two deep holes in close proximity, as illustrated in Figure 7. Through a systematic study of profile evolution varying pulse durations and separations between features, the degree to
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Materials | Etching
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Figure 5 (a) Predicted resolutions from a two-dimensional (2-D) stationary model with pulse duration and tool radius. (b) Normalized resolution ratios with pulse duration and tool radius, illustrating 1-D and 2-D behavior. Reproduced with permission from Kenney JA and Hwang GS (2006) Etch trends in electrochemical machining with ultrashot voltage pulses: Predictions from theory and simulation. Electrochemical and Solid-State Letters 9(1): D1–D4, Figure 3.
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Figure 7 Profiles of the resulting holes formed by etching with (a) a simple cylindrical tool in two passes and (b) two properly spaced cylindrical tools in a single pass. Additional etching in the midpoint region occurs in the latter case due to the exponential nature of the dissolution current with respect to overpotential.
Figure 6 Surface morphologies following a shallow lateral etch using the same pulse characteristics and a mixed electrolyte of 0.1 mol L1 CuSO4 and (a) 0.025 mol L1 H2SO4, (b) 0.050 mol L1 H2SO4, and (c) 0.075 mol L1 H2SO4.
which the performance of these templates is decreased relative to that of a simple tool electrode was quantified. Both the discrepancies and the separation at which the maxima occur were found to be reduced with decreasing pulse duration. These discrepancies are explained by examining for the first time the effects of intra-tool electrode interactions on the overpotential and dissolution current at the substrate electrode. These studies have revealed that overpotential is largely additive among the
individual components of a tool electrode above a critical separation between components, whereas the exponential nature of the dissolution current with respect to overpotential leads to increased dissolution when using a complex tool electrode. Therefore, care must be taken when designing tool templates to account for the pulse duration to be used, ensuring that features in close proximity to each other do not merge.
Nomenclature Symbols and Units iDC RLow
RHigh
transient current lower resistance of shorter current path through electrolyte in pulsed ECM systems higher resistance of longer current path through electrolyte in pulsed ECM systems
Materials | Etching
Abbreviations and Acronyms CD ECD ECF ECJM ECM EJD ESD JED RC STEM STM
capillary drilling electrochemical drilling electrochemical fine drilling electrochemical jet machining electrochemical machining electrojet drilling electrostream drilling jet electrolytic drilling resistor–capacitor shaped tube electrochemical machining scanning tunneling microscope
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See also: Electrochemical Theory: Double Layer; Electrokinetics; Electrolytes: Overview.
Further Reading ASM Handbook Committee (1994) ASM Handbook, Vol. 5: Surface Engineering. Metals Park, OH: ASM International. ASM Handbook Committee (1995) ASM Handbook, Vol. 16: Machining. Metals Park, OH: ASM International. Kenney JA and Hwang GS (2006) Etch trends in electrochemical machining with ultrashot voltage pulses: Predictions from theory and simulation. Electrochemical and Solid-State Letters 9(1): D1--D4. Schuster R (2007) Electrochemical microstructuring with short voltage pulses. ChemPhysChem 8: 34--39. Schuster R, Kirchner V, Allongue P, and Ertl G (2000) Electrochemical micromachining. Science 289: 98--101.