Three-Dimensional Micromachining On Ni-Ti Alloy, Fused Quartz And ZnO Nanowires Array Using CELT

The development of the micro-electro-mechanical systems (MEMS),micro-optics, microchips helps the advancement of micromachining technology that isthe hotspot of the recent research and the core of MEMS. The requirements for a newapproach of micromachining include the ability to fabricate complex 3Dmicrostruetures, high output and batch process. The Confined Etchant LayerTechnique (CELT) as a new approach that can be applied to fabricate 3Dmicrostructures was proposed by Prof. Zhao-Wu Tian et al. in 1992. CELT is adistance-sensitive, maskless chemical etching process, and can be applied to fabricatemany kinds of materials including conductor, semiconductor or insulator. In the recentyears, further analysis on the CELT theory was carried out and the related apparatushas been set up. Arbitrary 3D microstructures were replicated on Ni, Cu, Si, GaAs byCELT, which demonstrates that CELT can be applied to fabricate 3D microstructures.However, most of the previous works were focused on the microstructures onsemiconductor and metal of single component. Here, this thesis concentrates on themicromachining on new materials including Ni-Ti alloy, fused quartz and ZnOnanowires array. Therefore, this thesis is divided into three main parts and abbreviatedas follows:Three- dimensional micromachining on nitinol using CELTNitinol is an alloy composed of near-equiatomic proportions of nickel andtitanium. This alloy shows a very high elastic deformation and a shape memory effect,which are not shown by other types of conventional metallic alloys. These propertiesalong with their superior ductility, fatigure strength and corrosion resistance haveresulted in many applications in MEMS.According to the principle of CELT, the etching solution should normallyinclude a precursor of an etchant and a scavenger. NaNO₂+KF were chosen asprecursors to generate HF and HNO₃. NaOH used as scavenger and Na₂C₄H₄O₆ was added to prevent the formation of Ni(OH)₂ precipitates in the base solution. Therefore,the etching solution contains NaNO₂, KF, NaOH, Na₂C₄H₄O₆.Oxidation of NO₂^- can generate H⁺ and then changes the pH value of the solutionnearby the conductive mold electrode. The thickness of confined etchant layer that isequal to the micromachining precision can be estimated by measuring the profile ofpH of electrode surface. However, the profile of pH is difficult to be measuredaccurately in such a small volume. Cyclic voltammetry was employed to study theelectrochemical behavior of the etching solution in order to choose an appropriatepotential for the etchant generation on the electrode. The results showed that theelectrochemical behavior especially the reductive current was strongly dependent onthe pH value of the solution. The oxidative current of NO₂^- in the base solution ismuch lower than that in acid and neutral solution. Since the reduction rate of NO₃^-that has been generated by the electrooxidation of NO₂^- at a positive potential wasstrongly dependent on pH, the reductive current at a negative potential was alsostrongly dependent on the pH value. In a strong acid environment, the reductivecurrent was increased since HNO₂ was decomposed as NO⁺ that can catalyse thereduction of HNO₃ in the solution. In a base environment, no reductive current wasobserved due to the absence of NO⁺. Therefore, the reductive current indicates thechange of pH value of the solution near the electrode surface. The cyclic voltammetriccurves in 1.5 M NaNO₂+ 0.5 M KF + 0.1 MNa₂C₄H₄O₆+ X M NaOH (X=0,0.2,0.4,0.6) etching solution with various concentration of NaOH show that no reductivecurrent was observed when the concentration of NaOH is as high as 0.4 M.A well defined Pt microcylindrical electrode with a diameter ca. 250μm wasused as a mold to fabricate microstructures on Ni-Ti alloy in 1.5 M NaNO₂+ 0.5 MKF + 0.1 M Na₂C₄H₄O₆ solution with different concentration of NaOH. The resultsshowed that the micromachining resolution could reach into the submicrometer rangewhen the concentration of NaOH was increased to 0.6 M. The optimum etchingsolution and most suitable etching parameter were determined by both cyclicvoltammetry and the micromachining experiments using the microcylindricalelectrode as a mold. The complex microstructures of "XMU" were fabricated on the nitinol in 1.5 M NaNO₂ + 0.5 M KF + 0.1 M Na₂C₄H₄O₆ + 0.4 M NaOH. The etchedpatterns on the nitinol matched approximately the size of the patterns on the mold andretained the complex shape of the mold.Three- dimensional micromachining on fused quartz using CELTFused quartz is made of nearly 100% pure silicon dioxide (SiO₂). It is fused athigh temperatures into amorphous material. The high optical purity and its thermal,chemical and mechanical stability make fused quartz excellent choice for manycritical applications in high quality optical systems. The conventional micromachiningtechnologies for quartz are based on photolithography followed by the subsequenttransfer of the resist profile into the substrate surface by chemical etching or reactiveion etching (RIE). The conventional micromachining methods are time-consumingand the processes for machining of complex 3D microstructures are especiallycomplicated.In principle, CELT can be applied to the workpiece regardless of its conductivity.One of the key issues in CELT is searching for a suitable etching solution. In thispaper, we report an investigation on the micromachining of fused quartz using CELT.HF was used as the etchant that was generated nearby the electrode surface byelectrooxidation of NO₂^- to produce proton. The protons then combine with the F^-that exists in the etchant solution to produce HF on the mold. NaOH as an excellentscavenger confines the HF in a thin layer nearby the electrode surface. The solutionthat contains KNO₂, KF, NaOH is suitable for etching quartz. A well-defined Ptmicrocylindrical electrode with 250μm diameter was used as a mold in the solutionsof 0.5 M KNO₂+1 M KF+ X M NaOH (X=0, 0.1, 0.2, 0.3). The results showed thatthe resolution and roughness became better with the increase of the concentration ofNaOH in the etching solution. The fluorosilicates with low solubility easily depositedin the etched holes because of high concentration of K⁺, Na⁺ in the solution. The pulsepotential was used in the electrochemical generation of the etchant in order to preventthe fluorosilicates from depositing in the etched holes by promoting the diffusion of fluorosilicates. The etching precision was improved to about several micrometers bythe increase of NaOH concentration and by replacing the chronoamperometry withpulse potential method.Preliminary study of three- dimensional micromachining on ZnO nanowiresarray using CELTZnO, a direct wide band gap semiconductor (～3.3 eV), is the focus of muchresearch for its potential application in fabricating light emitting diodes, sensors andUV laser diodes due to its large excitation binding energy (～60 meV) which is morethan two times higher than the thermal energy at room temperature. It also has use asa piezoelectric and transparent conducting material, and potential as a dilutedmagneticsemiconductor (DMS). In recent years, much research interest focuses on themicromachining of nanowire or nanorod arrays to form certain patterns because oftheir special physical properties and promising applications in field effect transistors,single electron transistors and sensor. According to the principle of CELT, theuniqueness of this technique in the application of micromachining on the nanowirearray can be summarized as: 1) it can be applied to three-dimensional complexpatterns which are negative copies of those on the mold ; 2) It is a distance sensitivetechnique and independent on the initial roughness of the substrate; 3) It can beapplied in a batch production with a fast speed; 4) It can be combined with theother top-down approaches. Here, the CELT has been preliminarily applied tofabricate a 3D pattern on ZnO nanowire array.ZnO nanowire array was prepared on Si substrate by microwave PlasmaEnhanced Chemical Vapor Deposition (PECVD). The Zn powder was used as thesource and the flow rate of pure air was controlled constantly in the depositingprocess. The morphology of ZnO is dependent on the supersaturation of ZnO. Theresults showed that the gas flow, quantity of Zn and the size and location of Siinfluenced the morphology of ZnO altogether. When ZnO grows under a highsupersaturation level, bulk crystal growth occurs instead of anisotropic nanostructure growth, and the morphology of ZnO is irregular with a secondary nucleation.However, well-aligned nanowires of ZnO grow on the substrate of Si under a lowsurpersaturation level and the growth direction of the ZnO wires is in the c-direction,which can be confirmed by the characterization of SEM and XRD.In the experiment, HNO₃ was used as an etchant, which was generated locally onthe conductive mold by electrochemical oxidation of 0.12 M NaNO₂. In order toconfine the diffusion of nitric acid, 0.16 M Tris (Tris(hydroxymethy) aminomethane)was used as a scavenger to decrease its diffusion distance. Using the microcylindricalelectrode as a mold , the best resolution was achieved when the concentration of triswas triple of that of NaNO₂. Compared with the method that produces the etchantunder a constant potential, pulsed potential was applied to achieve bettermicromachining precision. In the solution of 0.06 M NaNO₂+ 0.12 M Tris, themicropatterns that are the negative copy of the mold have been fabricated on ZnOnanowires array. Compared with the morphology of the ZnO nanowires withoutetching, the nanowires in the etched pattern were uniform and with a same length.However, the resolution is still in a micrometer dimension and the further experimentsto improve the resolution are ongoing in our lab.