Confined Chemical Etching for Electrochemical Machining with

Article pubs.acs.org/accounts

Confined Chemical Etching for Electrochemical Machining with Nanoscale Accuracy Published as part of the Accounts of Chemical Research special issue “Nanoelectrochemistry”. Dongping Zhan,* Lianhuan Han, Jie Zhang, Kang Shi, Jian-Zhang Zhou, Zhao-Wu Tian,* and Zhong-Qun Tian State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

CONSPECTUS: In the past several decades, electrochemical machining (ECM) has enjoyed the reputation of a powerful technique in the manufacturing industry. Conventional ECM methods can be classified as electrolytic machining and electroforming: the former is based on anodic dissolution and the latter is based on cathodic deposition of metallic materials. Strikingly, ECM possesses several advantages over mechanical machining, such as high removal rate, the capability of making complex three-dimensional structures, and the practicability for difficult-to-cut materials. Additionally, ECM avoids tool wear and thermal or mechanical stress on machining surfaces. Thus, ECM is widely used for various industrial applications in the fields of aerospace, automobiles, electronics, etc. Nowadays, miniaturization and integration of functional components are becoming significant in ultralarge scale integration (ULSI) circuits, microelectromechanical systems (MEMS), and miniaturized total analysis systems (μ-TAS). As predicted by Moore’s law, the feature size of interconnectors in ULSI circuits are down to several nanometers. In this Account, we present our perseverant research in the last two decades on how to “confine” the ECM processes to occur at micrometer or even nanometer scale, that is, to ensure ECM with nanoscale accuracy. We have been developing the confined etchant layer technique (CELT) to fabricate three-dimensional micro- and nanostructures (3D-MNS) on different metals and semiconductor materials since 1992. In general, there are three procedures in CELT: (1) generating the etchant on the surface of the tool electrode by electrochemical or photoelectrochemical reactions; (2) confining the etchant in a depleted layer with a thickness of micro- or nanometer scale; (3) feeding the tool electrode to etch the workpiece. Scavengers, which can react with the etchant, are usually adopted to form a confined etchant layer. Through the subsequent homogeneous reaction between the scavenger and the photoor electrogenerated etchant in the electrolyte solution, the diffusion distance of the etchant is confined to micro- or nanometer scale, which ensures the nanoscale accuracy of electrochemical machining. To focus on the “confinement” of chemical etching reactions, external physical-field modulations have recently been introduced into CELT by introducing various factors such as light field, force field, hydrodynamics, and so on. Meanwhile, kinetic investigations of the confined chemical etching (CCE) systems are established based on the finite element analysis and simulations. Based on the obtained kinetic parameters, the machining accuracy is tunable and well controlled. CELT is now applicable for 1D milling, 2D polishing, and 3D microfabrication with an accuracy at nanometer scale. CELT not only inherits all the advantages of electrochemical machining but also provides advantages over photolithography and nanoimprint for its applicability to different functional materials without involving any photocuring and thermoplastic resists. Although there are some technical problems, for example, mass transfer and balance, which need to be solved, CELT has shown its prospective competitiveness in electrochemical micromachining, especially in the semiconductor industry.

Received: June 30, 2016 Published: September 26, 2016 © 2016 American Chemical Society

2596

DOI: 10.1021/acs.accounts.6b00336 Acc. Chem. Res. 2016, 49, 2596−2604

Article

Accounts of Chemical Research

diffusion distance of electrogenerated etchant is “confined” at nanoscale through a subsequent homogeneous reaction. Therefore, nanoscale machining accuracy is achieved. Based on the “confinement” principle, CELT is now developed as a powerful ECM method for 1D milling, 2D polishing, and 3D microfabrication due to the introduction of external physical-field modulations, including the hydrodynamic field, light field, and force field and their synergetic effects.27

1. INTRODUCTION In manufacturing industries, ECM is widely used as a powerful technique for various applications. The origination can be traced back to 19th century when Michael Faraday discovered Faraday’s law of electrolysis. At the beginning, ECM was known as electroplating and electropolishing.1 In 1929, Gusseff, a Russian scientist, filed the first patent in ECM.2 Remarkably, Burgess published the first research paper in 1941 and defined the differences of ECM from the mechanical machining process.3 In 1959, ECM was first commercialized by Anocut Engineering Company, USA.3 Since then it has been extended to almost all manufacturing fields, especially aerospace, automobiles, and electronics.4 According to the electrochemical principles, ECM can be classified into two categories: (i) electrolytic machining based on anodic dissolution and (ii) electroforming based on cathodic deposition.5 During ECM processes, a sufficient potential is applied between the metal workpiece and the tool electrode. The metal workpiece can act as the anode or cathode, and it has to be separated from the tool electrode by the working electrolyte solution. Due to the noncontact working mode, the tool wear problem is avoided compared to mechanical machining techniques.5 Since the kinetic rate of the metal electrode process is very fast, ECM has a rapid material removal rate regardless of the hardness of metal materials.5 Thus, ECM has garnered attention among researchers due to its high efficiency. The electrochemical reactions usually occur in an ambient environment; thus, there is no thermal or mechanical stress on the workpiece surface.6 The most distinct advantage of ECM is its ability to fabricate complex 3D structures (e.g., the cooling microholes of turbine blades in aerospace and aircraft industries) that are impossible to fabricate by conventional machining techniques.7 Nowadays, miniaturization and integration of functional components are becoming significant in ULSI circuits,8 MEMS,9 and μ-TAS.10−12 As predicted by Moore’s law, the feature size of copper interconnectors in ULSI circuits are decreased to several nanometers.13 These emerging high-tech industries bring bold innovations into ECM. Microelectroforming techniques were developed to deposit metal materials in the 3D-MNS fabricated by photolithography, such as the Damascene processes in ULSI circuits as well as the LIGA14−17 and EFAB18,19 in MEMS. Besides the template method, an electrochemical fountain pen method was proposed to fabricate metal nanowires with a nanoscale pipet.20,21 By using a nanoelectrode as a tool electrode, ultrashort voltage pulses (USVPs) are adopted for electrochemical nanostructuring on metal surfaces based on anodic dissolution.22−24 There are three basic challenges for ECM in current industrial processes: nanoscale accuracy, complex structure, and mass production.25 However, it is difficult to fabricate complex 3D-MNS by LIGA and EFAB due to the limitations of photolithography. Meanwhile, the direct writing techniques are inefficient for mass production. Another limitation is that ECM is only applicable to metal materials with good conductivity because, as mentioned above, the workpiece has to act as either the anode or the cathode. The open and fundamental question is how to break the limitation of material applicability: Can the workpiece be semiconductors or insulators? In 1992, we proposed a confined etchant layer technique (CELT) to meet the above-mentioned requirements and also to extend the material applicability.26 In fact, CELT is an electrochemically induced chemical etching technique, in which the

2. THE PRINCIPLE OF CONFINED CHEMICAL ETCHING FOR ECM In CELT, the diffusion distance of the electrochemically generated etchant is confined to microscale or nanoscale simply by a subsequently homogeneous reaction, that is, an EC reaction system. The depletion layer of the electrogenerated etchant is defined as the confined etchant layer (CEL). When the tool electrode approaches the workpiece until the CEL contacts the workpiece, the chemical etching reaction will occur. Through feeding the tool electrode, the 3D-MNS on the tool electrode will be duplicated onto the substrate through the “confined” chemical etching reaction. Referring to Figure 1, the procedures of CELT can be formulated as follows: (1) Generating the chemical etchant on the surface of a tool electrode by electrochemical, photochemical or photoelectrochemcial reaction R → O + ne

or

R + hν → O( +ne)

(2) Confining the thickness of CEL at micro- and nanometer scale by a subsequent coupling reaction O+S→R+Y

or

O→Y

(3) Etching the workpiece for 1D milling, 2D polishing or 3D machining

O+M→R+P Where R is the precursor of the etchant, O is the etchant, S is the scavenger that can react with O in the working solution, Y is the product of S, M is the workpiece material, and P is the product of M. The etchant can also be the radicals generated through a photochemical reaction. Since the lifetime of radicals is very short, the diffusion distance is confined by the decomposition of the radicals. Here, we introduce a useful estimation methodology in electrochemical research. When the concentration of the scavenger is much higher than the etchant precursor, defining the pseudo-first-order rate constant of a confinement reaction with respect to O as k and the diffusion coefficient of O as D, the thickness of CEL (δ) will be estimated as D/k . If k is 10−9 s−1 and D is 10−5 cm2 s−1, δ will be 1 nm.26,29 Thus, in principle, CELT has an excellent machining accuracy. Although the thickness of CEL can be confined sufficiently to nanoscale in the bulk solution, the hypothesis of pseudo-first reaction will be inapplicable during the etching process because of (i) the hindered mass transfer between the ultrathin CEL and the bulk solution and (ii) the coupling effect of reaction 3. Since the feature size determines the performance of the microdevice, it is crucial to ensure the machining accuracy of CELT at nanoscale. For this purpose, it is essential to establish an evaluation and screening method for the confined chemical reaction system, that is, to figure out the reaction kinetics of the three reactions, the coupling effect among them, and the technical factors in the machining process. Finite element analysis methods have been 2597


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Figure 1. Principle of CELT. (1) Nonconfined etchant layer formed on the tool electrode surface; (2) the confined etchant layer formed on the tool electrode; (3, 4) schematic diagrams for 1D milling and 2D polishing, respectively; (5−9) protocols for 3D machining on the surface of the workpiece: (5) the mold electrode with a confined etchant layer and the workpiece before machining; (6) feeding the mold electrode to workpiece surface to start the CCE process; (7) the CCE process on the workpiece surface; (8) finishing the CCE process; (9) separating the mold electrode from the workpiece. The yellow color therein presents the confined etchant layer.

conversion, and developed it as the CELT system for a GaAs workpiece:34

developed recently to solve the physical model of the confined chemical reaction system.30,31 For metals and metal alloys, protons are generated to change the local acidity and hydroxides are adopted as the scavenger. For example, nitrite is oxidized to nitric acid to react with metals or metal alloys to fabricate the 3D-MNS.32,33 With CCE, CELT is applicable to semiconductors and insulators besides metallic materials. In the case of gallium arsenate (GaAs), we studied a CELT reaction system as follows:31 16Br − → 8Br2 + 16e−

E:

→ 2RSO3H + 10Br− + 10H+

(1)

k1

3+

+ AsO33 ‐ + 6Br − + 6H+

Br2 + 2Fe 2 + → 2Br − + 2Fe3 +

C2 :

3Br2 + GaAs + 3H 2O → Ga 3 + + AsO33 − k2

k1

(5)

(6)

The tool electrode is made of TiO2, Br− is adopted as the etchant precursor and hole acceptor, and Fe3+ is adopted as the electron acceptor. The characteristic of this system is the special confinement reaction 5, by which the mass transfer loops of Br− and Fe3+ are formed at the illuminated GaAs/solution interface. Due to the synergetic effect, the kinetic rate of interfacial charge transfer is improved one order higher in magnitude than that adopting oxygen as electron acceptor based on the SECM investigation and finite element analysis. Correspondingly, the material removal rate is promoted 10 times higher as proved by the drilling experiments on GaAs substrate.34 In summary, “confinement” is crucial to ensure the spatial resolution of the chemical etching reaction. Aside from the artificial confinement reactions, other methods can also realize the confinement effect. For example, the diffusion distance of the photogenerated hydroxide radicals (OH•) on the surface of an illuminated TiO2 photoanode is confined by the decomposition of the radicals themselves.35 The extensional “confinement” principles will be introduced in the following section of applications. Moreover, SECM is proven as a fascinating experimental technique for screening and optimizing the CELT system; meanwhile, the simulations based on finite element analysis are valuable to obtain the theoretical and technical parameters.30,31

(2)

k2

TiO2 , hν ,e−

2Fe3 + ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2Fe 2 +

C1:

+ 6Br − + 6H+

3Br2 + GaAs + 3H 2O

C2 :

TiO2 , hν ,h+

8Br − ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 4Br2,

(4)

5Br2 + RSSR + 6H 2O

C1:

→ Ga

PE:

(3)

where E is the electrochemical reaction to generate the etchant Br2, C1 is the confinement reaction, and C2 is the etching reaction; RSSR refers to the scavenger L-cystine. The reactive rate constants of C1 and C2 are defined as k1 and k2. Here the confinement reaction C1 is considered as a second-order reaction considering the mass balance of the CELT system. With the tip generation/substrate collection (TG/SC) mode of scanning electrochemical microscopy (SECM), the reactive rate constant of C1 is determined as (8.0 ± 1.0) × 103 dm3 mol−1 s−1. The heterogeneous E−C2 process is studied by the feedback mode of SECM, and the reaction rate of C2 is determined as (3.2 ± 0.5) × 10−2 cm s−1. Adopting these kinetic parameters, the coupling effect between C1 and C2 was investigated by a deformed geometry finite element model. The results show that the theoretical simulations can predict the machining profiles of CELT very well.31 Aside from the electrochemical methods, the etchant can be generated by photochemical or photoelectrochemical reactions. We found a synergetic effect in photoelectrochemical 2598


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3. THE ECM INSTRUMENT FOR CONFINED CHEMICAL ETCHING For all ultraprecision machining, a nanomanipulation system is the core component of the instrument, including both the precise positioning and the information feedback.28,36 To ensure the nanoscale positioning resolution in a large travel range, a macro− micro dual driven positioning strategy is adopted in the Z-axis positioning platform, which includes a stepper motor (resolution, 40 nm) and a piezo motor (resolution, 0.5 nm). The ball screw and the cross-roller guide of the Z-stage are both preloaded to realize antibacklash and holding stability. To ensure the stability of X and Y positioning platforms, two air-bearing stepper motors as well as an air-bearing rotary motor are adopted. Each positioning platform is equipped with a close-loop controller to ensure the repeatability. The X and Y platforms can reach a positioning resolution of 5 nm and a repeatability less than 0.5 μm/100 mm. In brief, as depicted in Figure 2, the homemade

Figure 3. Schematic diagram of substrate leveling based on the SECM current feedback mode.

CELT removes workpiece materials by CCE reactions while mechanical machining does so by mechanical cutting force. A GaAs wafer is too fragile to be machined by traditional mechanical methods. However, microgrooves were milled on a GaAs wafer surface by reactions 1-3 (Figure 4);28 with reactions 4−6,

Figure 2. Schematic diagram of the homemade CELT instrument.

nanomanipulation system comprises four-axis coordinated linkages made up of three linear platforms (X, Y, Z) and one rotary platform (ω). The X−ω platforms are assembled on a vibration isolation platform, while the Z−Y axis positioning platform is equipped on a marble bridge structure. For ECM, a special electrolytic cell is equipped on the workpiece stage to perform the electrochemical modulations and to avoid corrosion risk to the instrument. Alignment and levelness are also important for machining quality. It is important to note that the ECM instrument is qualified for any experiments of SECM; we proposed a unique leveling method based on the current feedback mode of SECM, which is a monotonic function of the tip−substrate distance and presents a periodic change when the workpiece stage is rotating (Figure 3).37 Importantly, decreasing the amplitude can improve the levelness of the workpiece stage. In addition, a force sensor is equipped on the Z-axis positioning platform to determine the zero point of machining before experiments and, in some cases, to control the contact force between the tool electrode and the workpiece.

Figure 4. Optical image of the grooves fabricated by 1D machining and their lateral profiles.

micro holes were drilled on a GaAs wafer surface.34 Besides GaAs, Br2 is applicable to silicon and other semiconductors.38,39 To improve the solubility of Si(IV), fluoride anions are added as the complexing agent.40−43 Recently, we proposed a new CELT system based on the photoelectric effect of semiconductors in which strong oxidants (e.g., Br2) are no longer needed. Electron−hole pairs would be produced on illuminated semiconductors. If there were electron acceptors in the solution, the illuminated area would be oxidized by the accumulated photogenerated holes. In this case, the confinement reaction is the recombination of photogenerated charges. The machining accuracy depends on the recombination reaction rate and the diffusion of photogenerated holes, which can be estimated as (δ = D hole /k recomb ). With this method, a GaAs grating with a period of 14 μm was fabricated. ECM was also performed on a Cu workpiece by a pulse potential method.44 The 1D machining processes such as drilling and milling are actually direct-writing techniques combining CELT with mechanical machining operations. It is highly efficient for

4. THE APPLICATION OF CONFINED CHEMICAL ETCHING IN ECM 4.1. One Dimensional (1D) Machining

The 1D machining is derived directly from the traditional operations of mechanical machining by using a cylinder platinum electrode as the cutting tool. It inherits the operations of traditional mechanical machining. The distinct difference lies in that 2599


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linear velocity along the radial direction (Figure 5c). Suppose the etchant generation reaction is occurring homogeneously along the linear tool electrode; the same amount of etchant will cover a smaller area in the central region than that in the outer region. Thus, the etch depth in the central region is higher than that in the outer region. Unlike the point-by-point removal in traditional mechanical machining, it is one-step shaving process for 2D machining. Moreover, the coupling effect provides a profile modeling method for the 2D machining of irregular surface. In addition, we also find that hydrodynamics plays an important role in the electropolishing of Ti and Ti alloys.45 The hydrodynamic field caused by the mechanical motion can improve not only the potential distribution but also the mass transfer at the workpiece/electrolyte solution interface. The surface roughness of a Ti6Al4V alloy workpiece is obtained lower than 1.9 nm (Figure 5d).

metal workpieces but not for semiconductors because of the relatively low etching rate. Thus, 1D machining is adopted to screen and optimize the CELT systems. Actually, the thickness of CEL is controllable experimentally by tuning the concentration ratio of the etchant precursor over the scavenger in the working solution. The machining accuracy can be controlled in the region from a few micrometers to one nanometer. 4.2. Two Dimensional (2D) Machining

In traditional mechanical machining, 2D machining is finished by controlling the cutting clearance smaller than the tool size. To ensure sufficient cutting force, the tool size has to be small. However, CELT can perform 2D machining by using a linear tool electrode, which works like a shaving razor sweeping through the workpiece surface.27,28 The application of CELT in 2D machining is to produce supersmooth surfaces, that is, surface polishing or planarization. We polished a GaAs workpiece using Br2 as the etchant and L-cystine as the scavenger (Figure 5a). The surface roughness was decreased from 500 to 23 nm.28 Alternatively, a planar tool electrode made up of ITO supported TiO2 nanoparticles is employed to produce OH• by photoelectrochemistry.35 Since the lifetime of OH• (τ) is as short as 10−9 s, the thickness of CEL can be estimated as 1 nm (δ = DOH•τ ) supposing the diffusion coefficient DOH• is 10−5 cm2/s. To enhance the confinement effect, 1 wt % glycine was added in the working solution. Through the photoinduced CELT, the surface roughness of a Cu workpiece was achieved as low as 5.3 nm (Figure 5b). We believe radical chemistry will play important roles in 2D machining. It should be noted that there is a coupling effect between the CCE reactions and the mechanical motion, which is caused by the hydrodynamic effect on the mass transfer processes. If the workpiece stage is rotating while the linear tool electrode is kept still, a curved surface can be obtained due to the difference of the

4.3. Three Dimensional (3D) Machining

The most attractive feature of CELT is its ability in the fabrication of 3D-MNS because of their importance in USLI circuits, MEMS, and NEMS, and μ-TAS. Since the CEL thickness can be controlled to micrometer or nanometer scale, CEL can keep the shape of the 3D-MNS on the large-area mold electrode surface. Thus, the 3D-MNS can be transferred precisely onto the workpiece surface. For metal workpieces, redox metal complexes are adopted as the etchant or the local pH is modulated in alkali electrolyte solution. For semiconductor workpieces, Br− is chosen as the etchant precursor, while ascorbic acid and cystine are used as the scavengers. To promote the solubility of produced Si(IV), Ti(IV), and other cations, F− is adopted as the ligand. Because the working electrolyte in the CEL is limited and difficult to refresh, the precipitation or hydrolysis of etching products will halt the 3D machining processes. Compared with 1D

Figure 5. (a) Optical image of the polished GaAs surface by 2D machining, Ra = 23 nm; (b) AFM images of the polished Cu surfaces by the photogenerated hydroxide radicals, Ra = 5.3 nm; (c) optical image of a curved surface obtained on a GaAs workpiece with a linear tool electrode in the workpiece rotating mode; (d) AFM images of the electropolished Ti6Al4V surface under a hydrodynamic mold, Ra = 9 nm. 2600


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Figure 6. (a, b) optical images of the eight-phase microlens arrays fabricated by CELT; (c,d) AFM images of the corresponding area of the conductive mold and the fabricated microstructure on GaAs wafer.

Figure 7. Hierarchical Fresnel nanostructures fabricated by ECBM on the GaxIn1−xP workpiece with a 20 mN contact force. (a) Confocal laser microscope image showing 23 concentric nanorings. Inset shows its 3D image. (b) High-resolution SEM images of the nanorings. (c) Topography profile of panel a. (d) Change in radius and space as a function of the first 10 nanorings from the center outward. Solid symbols represent the experimentally determined radii and spaces. Open symbols represent the FEM simulated results.

higher than the estimated CEL thickness in the bulk solution. Thus, CEL keeps the shape of the microlens very well. When the mold electrode is fed to the GaAs substrate, the etching process should have very good spatial resolution. The machining accuracy is about 50 nm, very close to the estimated CEL thickness (i.e., 41 nm).46 Recently, external force fields were introduced into CELT to fabricate hierarchical 3D-MNS on a GaAs substrate. Different from the conventional CELT machining, a contact force is applied between the mold electrode and the GaAs substrate. Orderly coaxial nanorings are formed on the surface of the

and 2D machining, the mass transfer and balance is a serious problem in 3D machining. Nevertheless, CELT is a powerful method for the fabrication of anaglyphic 3D-MNS.38,46 An eight-phase diffractive microlens array has been fabricated on the surface of a GaAs wafer (Figure 6). In the bulk solution, the CEL thickness is estimated to be approximately 41 nm.38 The outmost radii of the concentric rings on poly(methyl methacrylate) (PMMA) is 230 μm, and the total depth of seven steps on the PMMA is around 1.58 μm. The average height of each step is 227 nm, much 2601


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Accounts of Chemical Research convex microlens through the buckling effect due to the difference in the elastic modulus between the plastic PMMA substrate and the rigid Pt film. Thus, the complementary concave microlens with coaxial nanorings is obtained on the GaxIn1−xP thin film coated GaAs wafer (Figure 7). The height of the nanoring detectable by AFM is 50 nm, the smallest feature size obtained by CELT so far. The termed electrochemical buckling microfabrication (ECBM) is prospective in the fabrication of hierarchical 3D-MNS without involving photolithography or energy beam techniques.47

(2) Potential distribution: In most cases, the workpiece does not act as the electrode. Thus, the potential distribution at the large-area mold electrode/electrolyte interface is not uniform, which results in a nonuniform etching rate on the local workpiece surface. This phenomenon is very serious when the mold size reaches to the inch scale.

6. PERSPECTIVES As pointed out in section 5, mass transfer and potential distribution are the main problems to be solved immediately. Thus, the future developments should be focused as follows: (1) As for the mass transfer problem, hydrodynamic methods have to be introduced into CELT. In 1D and 2D machining, the electrolyte in CEL can be refreshed through mechanical motions. Coupling effects between CCE processes and mechanical motions have to be investigated to improve the surface accuracy and decrease the surface roughness. Moreover, coupling effect can help us to design or control the surface type through the profile modeling of the tool electrode. For 3D machining, microfluidics technology will be introduced in the designs of mold electrode and the electrolytic cell. The flowing electrolyte will supply the reactants and take away the products. (2) As for the potential distribution problem, one solution is to integrate the reference and counter electrodes into the mold electrode; the other is to generate a bipolar reaction on the local region of the workpiece by external physical fields. Since the traditional three-electrode or two-electrode system is no longer needed, external physical field modulation may be the best solution to the potential distribution problem. (3) The kinetic properties of the CELT system should be discovered, especially the coupling effect between the CCE system and the external physical fields, including the hydrodynamic field, electric field, light field, and so on.

5. ADVANTAGES AND DISADVANTAGES It is well-known that wet chemical etching (WCE) is one of the first techniques introduced for the fabrication of 3D-MNS.48,49 Proceeding along the special crystal plane, nanostructures made by anisotropic WCE onto the surface of single crystalline materials are facet dependent.50,51 Meanwhile, the spatial resolution of isotropic WCE is ruined by the same etching rate in all directions.52,53 In brief, the WCE process is uncontrollable. That is why WCE is underused as a wafer cleaning process in semiconductor industries. We have declared that confinement is the key issue of chemical etching for micro- and nanomachining.28,46,54 We demonstrated that the EC reaction is sufficient to confine the diffusion distance of the etchant produced at the electrode/electrolyte interface and developed CELT as an electrochemical machining method. Besides the electrochemical system, confined photochemical and photoelectrochemical systems were proposed for the 1D, 2D, and 3D machining. Both the feature size and the machining accuracy was achieved at submicrometer and nanometer scale. Based on the principle of CCE, CELT has its own featured advantages: (1) CELT is suitable for mass production, not only for the fabrication of complex 3D-MNS but also for surface polishing and planarization. (2) Due to the chemical removal principle, CELT has no critical requirement of the initial roughness of the workpiece surface as a distance sensitive technique. (3) Since the tool electrode does not have to contact the workpiece, CELT can work on flexible, fragile, or fissile materials, even materials harder than the machining tool. (4) Compared to conventional ECM, CELT is applicable for not only metals but also semiconductors and insulators. (5) Compared to mechanical machining and nanoimprint technologies, CELT has no heat effect and no tool wear problems. (6) Compared to the energy beam techniques, CELT can avoid physical damage and chemical denaturalization of the workpiece surface. (7) Compared to LIGA and EFAB, CELT is a one-step technique without any auxiliary processes, such as photolithography, planarization, and multilayer alignments. Due to the existence of CEL, CELT also has some disadvantages to be solved including (1) Mass transfer: Since the CEL is ultrathin, mass transfer between the CEL and the bulk solution is difficult. Thus, the etchant precursors and the scavengers will be exhausted. In 1D and 2D machining systems, this problem is not serious in the mechanical motion mode. However, at present, only anaglyphic 3D-MNS can be fabricated in 3D machining with a large-area mold electrode.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

All authors have given approval to the final version of the manuscript. The authors declare no competing financial interests. Notes

The authors declare no competing financial interest. Biographies Dongping Zhan was graduated from Harbin Engineering University (B.Sc.) and Wuhan University (Ph.D.), China. After postdoctoral fellowships at Peking University, the University of Texas at Austin, and Queens College, the City University of New York, he joined Xiamen University where he has been a Professor since 2013. His interests are in electrochemistry at nanoscale, including nanoelectrodes, electrochemical micro- and nanomachining, scanning electrochemical microscopy, and precise electrochemical instruments. Lianhuan Han received his Ph.D. at Xiamen University under the supervision of Dongping Zhan on electrochemical micromachining. He is now a postdoctoral fellow at Harbin Institute of Technology. 2602


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Jie Zhang received his Ph.D. at Xiamen University under the supervision of Dongping Zhan on electrochemical micromachining. He is moving to Hunan University of Science and Technology as an assistant professor. Kang Shi received his B.Sc. at Xiamen University and Ph.D. at Hong Kong Baptist University. He joined Xiamen University where he has been a Professor since 2011. His research interest focuses on the interfacial electrochemistry and bio-electroanalysis. Jian-Zhang Zhou received his B.Sc. and Ph.D. at Xiamen University where he is now an associate professor. His research interest focuses on photoelectrochemistry of nanomaterials. Zhao-Wu Tian received his B.Sc. at Xiamen University in 1949 and became a Professor in 1978 therein. He is a Member of the Chinese Academy of Sciences since 1980 and the Third World Academy of Sciences since 1996. His research interests cover electrochemistry from instruments through methodology to theory, including electrochemical micromachining. Zhong-Qun Tian received his B.Sc. at Xiamen University in 1982 and Ph.D. at University of Southampton in 1987. He has served at Xiamen University as a Professor since 1991 and was elected as Member of the Chinese Academy of Sciences in 2005 and the Elected President of the International Society of Electrochemistry in 2016. His research interests cover electrochemistry, surface-enhanced Raman spectroscopy, and nanochemistry.

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ACKNOWLEDGMENTS The financial support of the National Natural Science Foundation of China (Grants 21327002, 91323303, 21573054, 21321062, 91023006, 91023047, and 91023043) is appreciated.

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ABBREVIATIONS ECM, electrochemical machining; ULSI, ultralarge scale integration; LIGA, lithographie, galvanoformung, abformung (lithography, electroplating, and molding); MEMS, microelectromechanical systems; μ-TAS, miniaturized total analysis systems; CELT, confined etchant layer technique; 3D-MNS, three-dimensional micro- and nanostructures; CCE, confined chemical etching; USVP, ultrashort voltage pulses; CEL, confined etchant layer; SECM, scanning electrochemical microscopy; OH•, hydroxide radicals; ECMM, electrochemical mechanical machining; PMMA, poly(methyl methacrylate); ECBM, electrochemical buckling microfabrication; WCE, wet chemical etching

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