Confined Chemical Etching for Electrochemical Machining withNanoscale AccuracyPublished 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 powerfultechnique in the manufacturing industry. Conventional ECM methods can be classified as electrolytic machining andelectroforming: 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 makingcomplex three-dimensional structures, and the practicability for difficult-to-cut materials. Additionally, ECM avoids tool wear andthermal or mechanical stress on machining surfaces. Thus, ECM is widely used for various industrial applications in the fields ofaerospace, 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 byMoore’s law, the feature size of interconnectors in ULSI circuits are down to several nanometers. In this Account, we present ourperseverant research in the last two decades on how to “confine” the ECM processes to occur at micrometer or even nanometerscale, that is, to ensure ECM with nanoscale accuracy. We have been developing the confined etchant layer technique (CELT) tofabricate 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 byelectrochemical or photoelectrochemical reactions; (2) confining the etchant in a depleted layer with a thickness of micro- ornanometer scale; (3) feeding the tool electrode to etch the workpiece. Scavengers, which can react with the etchant, are usuallyadopted to form a confined etchant layer. Through the subsequent homogeneous reaction between the scavenger and the photo-or electrogenerated etchant in the electrolyte solution, the diffusion distance of the etchant is confined to micro- or nanometerscale, which ensures the nanoscale accuracy of electrochemical machining.To focus on the “confinement” of chemical etching reactions, external physical-field modulations have recently been introducedinto CELT by introducing various factors such as light field, force field, hydrodynamics, and so on. Meanwhile, kineticinvestigations of the confined chemical etching (CCE) systems are established based on the finite element analysis andsimulations. Based on the obtained kinetic parameters, the machining accuracy is tunable and well controlled. CELT is nowapplicable for 1D milling, 2D polishing, and 3D microfabrication with an accuracy at nanometer scale. CELT not only inherits allthe advantages of electrochemical machining but also provides advantages over photolithography and nanoimprint for itsapplicability to different functional materials without involving any photocuring and thermoplastic resists. Although there aresome technical problems, for example, mass transfer and balance, which need to be solved, CELT has shown its prospectivecompetitiveness in electrochemical micromachining, especially in the semiconductor industry.

Received: June 30, 2016Published: September 26, 2016

Article

pubs.acs.org/accounts

© 2016 American Chemical Society 2596 DOI: 10.1021/acs.accounts.6b00336Acc. Chem. Res. 2016, 49, 2596−2604

1. INTRODUCTIONIn manufacturing industries, ECM is widely used as a powerfultechnique for various applications. The origination can be tracedback to 19th century whenMichael Faraday discovered Faraday’slaw of electrolysis. At the beginning, ECM was known aselectroplating and electropolishing.1 In 1929, Gusseff, a Russianscientist, filed the first patent in ECM.2 Remarkably, Burgesspublished the first research paper in 1941 and defined thedifferences of ECM from the mechanical machining process.3

In 1959, ECM was first commercialized by Anocut EngineeringCompany, USA.3 Since then it has been extended to almost allmanufacturing fields, especially aerospace, automobiles, andelectronics.4

According to the electrochemical principles, ECM can beclassified into two categories: (i) electrolytic machining based onanodic dissolution and (ii) electroforming based on cathodicdeposition.5 During ECM processes, a sufficient potential isapplied between the metal workpiece and the tool electrode. Themetal workpiece can act as the anode or cathode, and it has to beseparated from the tool electrode by the working electrolytesolution. Due to the noncontact working mode, the tool wearproblem is avoided compared to mechanical machiningtechniques.5 Since the kinetic rate of the metal electrode processis very fast, ECM has a rapid material removal rate regardless ofthe hardness of metal materials.5 Thus, ECM has garneredattention among researchers due to its high efficiency. Theelectrochemical reactions usually occur in an ambient environ-ment; thus, there is no thermal or mechanical stress on theworkpiece surface.6 The most distinct advantage of ECM is itsability to fabricate complex 3D structures (e.g., the coolingmicroholes of turbine blades in aerospace and aircraft industries)that are impossible to fabricate by conventional machiningtechniques.7

Nowadays, miniaturization and integration of functional com-ponents are becoming significant in ULSI circuits,8 MEMS,9 andμ-TAS.10−12 As predicted by Moore’s law, the feature size ofcopper interconnectors in ULSI circuits are decreased to severalnanometers.13 These emerging high-tech industries bring boldinnovations into ECM. Microelectroforming techniques weredeveloped to deposit metal materials in the 3D-MNS fabricatedby photolithography, such as the Damascene processes in ULSIcircuits as well as the LIGA14−17 and EFAB18,19 in MEMS.Besides the template method, an electrochemical fountain penmethod was proposed to fabricate metal nanowires with ananoscale pipet.20,21 By using a nanoelectrode as a tool electrode,ultrashort voltage pulses (USVPs) are adopted for electro-chemical nanostructuring on metal surfaces based on anodicdissolution.22−24

There are three basic challenges for ECM in current industrialprocesses: nanoscale accuracy, complex structure, and massproduction.25 However, it is difficult to fabricate complex3D-MNS by LIGA and EFAB due to the limitations of photo-lithography. Meanwhile, the direct writing techniques areinefficient for mass production. Another limitation is that ECMis only applicable to metal materials with good conductivitybecause, as mentioned above, the workpiece has to act as eitherthe anode or the cathode. The open and fundamental question ishow to break the limitation of material applicability: Can theworkpiece be semiconductors or insulators?In 1992, we proposed a confined etchant layer technique

(CELT) to meet the above-mentioned requirements and also toextend the material applicability.26 In fact, CELT is an electro-chemically induced chemical etching technique, in which the

diffusion distance of electrogenerated etchant is “confined” atnanoscale through a subsequent homogeneous reaction. There-fore, nanoscale machining accuracy is achieved. Based on the“confinement” principle, CELT is now developed as a powerfulECM method for 1D milling, 2D polishing, and 3D micro-fabrication due to the introduction of external physical-fieldmodulations, including the hydrodynamic field, light field, andforce field and their synergetic effects.27

2. THE PRINCIPLE OF CONFINED CHEMICAL ETCHINGFOR ECM

In CELT, the diffusion distance of the electrochemicallygenerated etchant is confined to microscale or nanoscale simplyby a subsequently homogeneous reaction, that is, an EC reactionsystem. The depletion layer of the electrogenerated etchant isdefined as the confined etchant layer (CEL). When the toolelectrode approaches the workpiece until the CEL contacts theworkpiece, the chemical etching reaction will occur. Throughfeeding the tool electrode, the 3D-MNS on the tool electrode willbe duplicated onto the substrate through the “confined” chemicaletching reaction. Referring to Figure 1, the procedures of CELTcan be formulated as follows:

(1) Generating the chemical etchant on the surface of a toolelectrode by electrochemical, photochemical or photo-electrochemcial reaction

ν→ + + → +n h nR O e or R O( e)

(2) Confining the thickness of CEL at micro- and nanometerscale by a subsequent coupling reaction

+ → + →O S R Y or O Y

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

+ → +O M R P

Where R is the precursor of the etchant, O is the etchant, S is thescavenger that can react with O in the working solution, Y is theproduct of S, M is the workpiece material, and P is the product ofM. The etchant can also be the radicals generated through aphotochemical reaction. Since the lifetime of radicals is veryshort, the diffusion distance is confined by the decomposition ofthe radicals. Here, we introduce a useful estimation methodologyin electrochemical research. When the concentration of thescavenger is much higher than the etchant precursor, defining thepseudo-first-order rate constant of a confinement reaction withrespect to O as k and the diffusion coefficient of O as D, thethickness 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-firstreaction will be inapplicable during the etching process becauseof (i) the hindered mass transfer between the ultrathin CEL andthe bulk solution and (ii) the coupling effect of reaction 3. Sincethe feature size determines the performance of the microdevice,it is crucial to ensure the machining accuracy of CELT at nano-scale. For this purpose, it is essential to establish an evaluationand 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 themachining process. Finite element analysis methods have been

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developed recently to solve the physical model of the confinedchemical reaction system.30,31

For metals and metal alloys, protons are generated to changethe local acidity and hydroxides are adopted as the scavenger. Forexample, nitrite is oxidized to nitric acid to react with metals ormetal alloys to fabricate the 3D-MNS.32,33 With CCE, CELT isapplicable to semiconductors and insulators besides metallicmaterials. In the case of gallium arsenate (GaAs), we studied aCELT reaction system as follows:31

→ +− −E: 16Br 8Br 16e2 (1)

+ +

→ + +− + k

C : 5Br RSSR 6H O

2RSO H 10Br 10H1 2 2

3 1 (2)

+ +

→ + + ++ ‐ − + k

C : 3Br GaAs 3H O

Ga AsO 6Br 6H2 2 2

33

32 (3)

where E is the electrochemical reaction to generate the etchantBr2, C1 is the confinement reaction, and C2 is the etchingreaction; RSSR refers to the scavenger L-cystine. The reactive rateconstants of C1 and C2 are defined as k1 and k2. Here theconfinement reaction C1 is considered as a second-order reactionconsidering the mass balance of the CELT system. With the tipgeneration/substrate collection (TG/SC) mode of scanningelectrochemical microscopy (SECM), the reactive rate constantof C1 is determined as (8.0 ± 1.0) × 103 dm3 mol−1 s−1. Theheterogeneous E−C2 process is studied by the feedback mode ofSECM, and the reaction rate of C2 is determined as (3.2± 0.5) ×10−2 cm s−1. Adopting these kinetic parameters, the couplingeffect between C1 and C2 was investigated by a deformedgeometry finite element model. The results show that thetheoretical simulations can predict the machining profiles ofCELT very well.31

Aside from the electrochemical methods, the etchant can begenerated by photochemical or photoelectrochemical reac-tions. We found a synergetic effect in photoelectrochemical

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

⎯ →⎯⎯⎯⎯⎯⎯⎯⎯⎯ ⎯ →⎯⎯⎯⎯⎯⎯⎯⎯⎯ν ν− + +

+ −

PE: 8Br 4Br , 2Fe 2Feh hTiO , ,h

23 TiO , ,e 22 2

(4)

+ → ++ − + kC : Br 2Fe 2Br 2Fe1 22 3

1 (5)

+ + → +

+ +

+ −

− + k

C : 3Br GaAs 3H O Ga AsO

6Br 6H2 2 2

33

3

2 (6)

The tool electrode is made of TiO2, Br− is adopted as the

etchant precursor and hole acceptor, and Fe3+ is adopted as theelectron acceptor. The characteristic of this system is the specialconfinement 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 chargetransfer is improved one order higher in magnitude than thatadopting oxygen as electron acceptor based on the SECMinvestigation and finite element analysis. Correspondingly, thematerial removal rate is promoted 10 times higher as proved bythe drilling experiments on GaAs substrate.34

In summary, “confinement” is crucial to ensure the spatialresolution of the chemical etching reaction. Aside from the artifi-cial confinement reactions, other methods can also realize theconfinement effect. For example, the diffusion distance of thephotogenerated hydroxide radicals (OH•) on the surface of anilluminated TiO2 photoanode is confined by the decompositionof the radicals themselves.35 The extensional “confinement”principles will be introduced in the following section of appli-cations. Moreover, SECM is proven as a fascinating experimentaltechnique for screening and optimizing the CELT system; mean-while, the simulations based on finite element analysis arevaluable to obtain the theoretical and technical parameters.30,31

Figure 1. Principle of CELT. (1) Nonconfined etchant layer formed on the tool electrode surface; (2) the confined etchant layer formed on the toolelectrode; (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 startthe 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.

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3. THE ECM INSTRUMENT FOR CONFINED CHEMICALETCHING

For all ultraprecision machining, a nanomanipulation system isthe core component of the instrument, including both the precisepositioning and the information feedback.28,36 To ensure thenanoscale positioning resolution in a large travel range, a macro−micro dual driven positioning strategy is adopted in the Z-axispositioning platform, which includes a stepper motor (reso-lution, 40 nm) and a piezo motor (resolution, 0.5 nm). The ballscrew and the cross-roller guide of the Z-stage are both preloadedto realize antibacklash and holding stability. To ensure thestability of X and Y positioning platforms, two air-bearing steppermotors as well as an air-bearing rotary motor are adopted. Eachpositioning platform is equipped with a close-loop controller toensure the repeatability. The X and Y platforms can reach apositioning resolution of 5 nm and a repeatability less than0.5 μm/100 mm. In brief, as depicted in Figure 2, the homemade

nanomanipulation system comprises four-axis coordinatedlinkages made up of three linear platforms (X, Y, Z) and onerotary platform (ω). The X−ω platforms are assembled on avibration isolation platform, while the Z−Y axis positioningplatform is equipped on a marble bridge structure.For ECM, a special electrolytic cell is equipped on the

workpiece stage to perform the electrochemical modulations andto avoid corrosion risk to the instrument. Alignment and level-ness are also important for machining quality. It is important tonote that the ECM instrument is qualified for any experiments ofSECM; we proposed a unique leveling method based on thecurrent feedback mode of SECM, which is a monotonic functionof the tip−substrate distance and presents a periodic changewhen the workpiece stage is rotating (Figure 3).37 Importantly,decreasing the amplitude can improve the levelness of the work-piece stage. In addition, a force sensor is equipped on the Z-axispositioning platform to determine the zero point of machiningbefore experiments and, in some cases, to control the contactforce between the tool electrode and the workpiece.

4. THE APPLICATION OF CONFINED CHEMICALETCHING IN ECM

4.1. One Dimensional (1D) Machining

The 1D machining is derived directly from the traditionaloperations of mechanical machining by using a cylinder platinumelectrode as the cutting tool. It inherits the operations of tradi-tional mechanical machining. The distinct difference lies in that

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

micro holes were drilled on a GaAs wafer surface.34 Besides GaAs,Br2 is applicable to silicon and other semiconductors.38,39 Toimprove the solubility of Si(IV), fluoride anions are added as thecomplexing agent.40−43

Recently, we proposed a new CELT system based on thephotoelectric effect of semiconductors in which strong oxidants(e.g., Br2) are no longer needed. Electron−hole pairs would beproduced on illuminated semiconductors. If there were electronacceptors in the solution, the illuminated area would be oxidizedby the accumulated photogenerated holes. In this case, theconfinement reaction is the recombination of photogeneratedcharges. The machining accuracy depends on the recombinationreaction rate and the diffusion of photogenerated holes, whichcan be estimated as (δ = D k/hole recomb). With this method, aGaAs grating with a period of 14 μm was fabricated. ECM wasalso performed on a Cu workpiece by a pulse potential method.44

The 1D machining processes such as drilling and milling areactually direct-writing techniques combining CELT withmechanical machining operations. It is highly efficient for

Figure 2. Schematic diagram of the homemade CELT instrument.

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

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

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metal workpieces but not for semiconductors because of therelatively low etching rate. Thus, 1D machining is adopted toscreen and optimize the CELT systems. Actually, the thickness ofCEL is controllable experimentally by tuning the concentrationratio of the etchant precursor over the scavenger in the workingsolution. The machining accuracy can be controlled in the regionfrom a few micrometers to one nanometer.

4.2. Two Dimensional (2D) Machining

In traditional mechanical machining, 2D machining is finishedby 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 lineartool electrode, which works like a shaving razor sweepingthrough the workpiece surface.27,28 The application of CELT in2Dmachining is to produce supersmooth surfaces, that is, surfacepolishing or planarization. We polished a GaAs workpiece usingBr2 as the etchant and L-cystine as the scavenger (Figure 5a). Thesurface roughness was decreased from 500 to 23 nm.28

Alternatively, a planar tool electrode made up of ITOsupported TiO2 nanoparticles is employed to produce OH• byphotoelectrochemistry.35 Since the lifetime of OH• (τ) is asshort as 10−9 s, the thickness of CEL can be estimated as 1 nm(δ τ= •DOH ) supposing the diffusion coefficient DOH

• is10−5 cm2/s. To enhance the confinement effect, 1 wt % glycinewas added in the working solution. Through the photoinducedCELT, the surface roughness of a Cu workpiece was achieved aslow as 5.3 nm (Figure 5b). We believe radical chemistry will playimportant 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 bythe hydrodynamic effect on the mass transfer processes. If theworkpiece stage is rotating while the linear tool electrode is keptstill, a curved surface can be obtained due to the difference of the

linear velocity along the radial direction (Figure 5c). Suppose theetchant generation reaction is occurring homogeneously alongthe linear tool electrode; the same amount of etchant will cover asmaller area in the central region than that in the outer region.Thus, the etch depth in the central region is higher than that inthe outer region. Unlike the point-by-point removal in traditionalmechanical machining, it is one-step shaving process for 2Dmachining. Moreover, the coupling effect provides a profilemodeling 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 canimprove not only the potential distribution but also the masstransfer at the workpiece/electrolyte solution interface. The sur-face roughness of a Ti6Al4V alloy workpiece is obtained lowerthan 1.9 nm (Figure 5d).

4.3. Three Dimensional (3D) Machining

The most attractive feature of CELT is its ability in the fabri-cation of 3D-MNS because of their importance in USLI circuits,MEMS, and NEMS, and μ-TAS. Since the CEL thickness can becontrolled to micrometer or nanometer scale, CEL can keep theshape of the 3D-MNS on the large-area mold electrode surface.Thus, the 3D-MNS can be transferred precisely onto theworkpiece surface. For metal workpieces, redox metal complexesare adopted as the etchant or the local pH is modulated in alkalielectrolyte solution. For semiconductor workpieces, Br− ischosen as the etchant precursor, while ascorbic acid and cystineare used as the scavengers. To promote the solubility of producedSi(IV), Ti(IV), and other cations, F− is adopted as the ligand.Because the working electrolyte in the CEL is limited anddifficult to refresh, the precipitation or hydrolysis of etching pro-ducts 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 thephotogenerated hydroxide radicals, Ra = 5.3 nm; (c) optical image of a curved surface obtained on a GaAs workpiece with a linear tool electrode in theworkpiece rotating mode; (d) AFM images of the electropolished Ti6Al4V surface under a hydrodynamic mold, Ra = 9 nm.

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and 2D machining, the mass transfer and balance is a seriousproblem in 3D machining.Nevertheless, CELT is a powerful method for the fabrica-

tion of anaglyphic 3D-MNS.38,46 An eight-phase diffractivemicrolens array has been fabricated on the surface of a GaAswafer (Figure 6). In the bulk solution, the CEL thickness isestimated to be approximately 41 nm.38 The outmost radii of theconcentric rings on poly(methyl methacrylate) (PMMA) is230 μm, and the total depth of seven steps on the PMMA isaround 1.58 μm. The average height of each step is 227 nm,much

higher than the estimatedCEL thickness in the bulk solution. Thus,CEL keeps the shape of the microlens very well. When the moldelectrode is fed to the GaAs substrate, the etching process shouldhave very good spatial resolution. The machining accuracy is about50 nm, very close to the estimated CEL thickness (i.e., 41 nm).46

Recently, external force fields were introduced into CELT tofabricate hierarchical 3D-MNS on a GaAs substrate. Differentfrom the conventional CELT machining, a contact force isapplied between the mold electrode and the GaAs substrate.Orderly coaxial nanorings are formed on the surface of the

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 conductivemold 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 lasermicroscope image showing 23 concentric nanorings. Inset shows its 3D image. (b) High-resolution SEM images of the nanorings. (c) Topographyprofile of panel a. (d) Change in radius and space as a function of the first 10 nanorings from the center outward. Solid symbols represent theexperimentally determined radii and spaces. Open symbols represent the FEM simulated results.

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convex microlens through the buckling effect due to thedifference in the elastic modulus between the plastic PMMAsubstrate and the rigid Pt film. Thus, the complementary concavemicrolens with coaxial nanorings is obtained on the GaxIn1−xPthin film coated GaAs wafer (Figure 7). The height of thenanoring detectable by AFM is 50 nm, the smallest feature sizeobtained by CELT so far. The termed electrochemical bucklingmicrofabrication (ECBM) is prospective in the fabrication ofhierarchical 3D-MNS without involving photolithography orenergy beam techniques.47

5. ADVANTAGES AND DISADVANTAGESIt is well-known that wet chemical etching (WCE) is one of thefirst techniques introduced for the fabrication of 3D-MNS.48,49

Proceeding along the special crystal plane, nanostructures madeby anisotropic WCE onto the surface of single crystallinematerials are facet dependent.50,51 Meanwhile, the spatialresolution of isotropic WCE is ruined by the same etching ratein all directions.52,53 In brief, the WCE process is uncontrollable.That is why WCE is underused as a wafer cleaning process insemiconductor industries.We have declared that confinement is the key issue of chemical

etching for micro- and nanomachining.28,46,54 We demonstratedthat the EC reaction is sufficient to confine the diffusion distanceof the etchant produced at the electrode/electrolyte interfaceand developed CELT as an electrochemical machining method.Besides the electrochemical system, confined photochemical andphotoelectrochemical systems were proposed for the 1D, 2D,and 3D machining. Both the feature size and the machiningaccuracy was achieved at submicrometer and nanometer scale.Based on the principle of CCE, CELT has its own featuredadvantages:

(1) CELT is suitable for mass production, not only for thefabrication of complex 3D-MNS but also for surfacepolishing and planarization.

(2) Due to the chemical removal principle, CELT has nocritical requirement of the initial roughness of theworkpiece surface as a distance sensitive technique.

(3) Since the tool electrode does not have to contact theworkpiece, CELT can work on flexible, fragile, or fissilematerials, even materials harder than the machining tool.

(4) Compared to conventional ECM, CELT is applicable fornot only metals but also semiconductors and insulators.

(5) Compared to mechanical machining and nanoimprinttechnologies, CELT has no heat effect and no tool wearproblems.

(6) Compared to the energy beam techniques, CELT canavoid physical damage and chemical denaturalization ofthe workpiece surface.

(7) Compared to LIGA and EFAB, CELT is a one-steptechnique without any auxiliary processes, such as photo-lithography, planarization, and multilayer alignments.

Due to the existence of CEL, CELT also has somedisadvantages to be solved including

(1) Mass transfer: Since the CEL is ultrathin, mass transferbetween the CEL and the bulk solution is difficult.Thus, the etchant precursors and the scavengers will beexhausted. In 1D and 2Dmachining systems, this problemis not serious in the mechanical motionmode. However, atpresent, only anaglyphic 3D-MNS can be fabricated in 3Dmachining with a large-area mold electrode.

(2) Potential distribution: In most cases, the workpiece doesnot act as the electrode. Thus, the potential distribution atthe large-area mold electrode/electrolyte interface is notuniform, which results in a nonuniform etching rate on thelocal workpiece surface. This phenomenon is very seriouswhen the mold size reaches to the inch scale.

6. PERSPECTIVESAs pointed out in section 5, mass transfer and potentialdistribution 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 methodshave to be introduced into CELT. In 1D and 2D machin-ing, the electrolyte in CEL can be refreshed throughmechanical motions. Coupling effects between CCEprocesses and mechanical motions have to be investigatedto improve the surface accuracy and decrease the surfaceroughness. Moreover, coupling effect can help us to designor control the surface type through the profile modeling ofthe tool electrode. For 3D machining, microfluidicstechnology will be introduced in the designs of moldelectrode and the electrolytic cell. The flowing electrolytewill supply the reactants and take away the products.

(2) As for the potential distribution problem, one solution isto integrate the reference and counter electrodes into themold electrode; the other is to generate a bipolar reac-tion on the local region of the workpiece by externalphysical fields. Since the traditional three-electrode ortwo-electrode system is no longer needed, externalphysical field modulation may be the best solution to thepotential distribution problem.

(3) The kinetic properties of the CELT system should bediscovered, especially the coupling effect between theCCE system and the external physical fields, including thehydrodynamic field, electric field, light field, and so on.

■ AUTHOR INFORMATIONCorresponding Authors

*E-mail: dpzhan@xmu.edu.cn.*E-mail: zwtian@xmu.edu.cn.

Author Contributions

All authors have given approval to the final version of themanuscript. The authors declare no competing financialinterests.

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 postdoctoralfellowships at Peking University, the University of Texas at Austin, andQueens College, the City University of New York, he joined XiamenUniversity where he has been a Professor since 2013. His interests are inelectrochemistry at nanoscale, including nanoelectrodes, electro-chemical micro- and nanomachining, scanning electrochemicalmicroscopy, and precise electrochemical instruments.

Lianhuan Han received his Ph.D. at Xiamen University under thesupervision of Dongping Zhan on electrochemical micromachining. Heis now a postdoctoral fellow at Harbin Institute of Technology.

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Jie Zhang received his Ph.D. at XiamenUniversity under the supervisionof Dongping Zhan on electrochemical micromachining. He is moving toHunan University of Science and Technology as an assistant professor.

Kang Shi received his B.Sc. at Xiamen University and Ph.D. at HongKong Baptist University. He joined Xiamen University where he hasbeen a Professor since 2011. His research interest focuses on theinterfacial electrochemistry and bio-electroanalysis.

Jian-Zhang Zhou received his B.Sc. and Ph.D. at Xiamen Universitywhere he is now an associate professor. His research interest focuses onphotoelectrochemistry of nanomaterials.

Zhao-Wu Tian received his B.Sc. at Xiamen University in 1949 andbecame a Professor in 1978 therein. He is a Member of the ChineseAcademy of Sciences since 1980 and the Third World Academy ofSciences since 1996. His research interests cover electrochemistry frominstruments through methodology to theory, including electrochemicalmicromachining.

Zhong-Qun Tian received his B.Sc. at Xiamen University in 1982 andPh.D. at University of Southampton in 1987. He has served at XiamenUniversity as a Professor since 1991 and was elected as Member of theChinese Academy of Sciences in 2005 and the Elected President of theInternational Society of Electrochemistry in 2016. His research interestscover electrochemistry, surface-enhanced Raman spectroscopy, andnanochemistry.

■ ACKNOWLEDGMENTSThe financial support of the National Natural ScienceFoundation of China (Grants 21327002, 91323303, 21573054,21321062, 91023006, 91023047, and 91023043) is appreciated.

■ ABBREVIATIONSECM, electrochemical machining; ULSI, ultralarge scaleintegration; LIGA, lithographie, galvanoformung, abformung(lithography, electroplating, and molding); MEMS, micro-electromechanical systems; μ-TAS, miniaturized total analysissystems; CELT, confined etchant layer technique; 3D-MNS,three-dimensional micro- and nanostructures; CCE, confinedchemical etching; USVP, ultrashort voltage pulses; CEL,confined etchant layer; SECM, scanning electrochemicalmicroscopy; OH•, hydroxide radicals; ECMM, electrochemicalmechanical machining; PMMA, poly(methyl methacrylate);ECBM, electrochemical buckling microfabrication; WCE, wetchemical etching

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