| The research on microelectromechanical system(MEMS)has received more and more attention in recent years.The development of novel techniques to fabricate the micro-or nano-structures is one of the key problems for the advance of MEMS.At present,the dominant techniques to fabricate microstructures,such as IC and LIGA,all are based on the photolithography.However,they are suitable only for the fabrication of some simple microstructures since the cross sections of different depth of these structures perpendicular to the light beam are similar to the 2-dimensional mask used.Hence these structures are limited to "2.5-dimensional" and are different from the structures of truly "3-dimensional"(3D).Therefore,many extremely complex,arbitrary 3D micro-devices are still not available in batch fabrication and with a low cost.Electrochemical methods represent a significant contribution to the process of 3D micromachining and have resulted in a variety of techniques.Among these methods,two distance-sensitive techniques based on a 3D mold have been developed recently.Schuster and his co-workers invented a technique that is termed electrochemical micromachining,in which the microelectrode behaves as a milling cutter to engrave the workpiece electrode when an ultra-short voltage pulse is applied to it,and produces a 3D structure.Our group led by Professor Z.W.Tian proposed and developed another effective electrochemical technique for 3D micromachining,which is named as confined etchant layer technique(CELT).The working principle is described as follows.An active etchant is generated electrochemically at the surface of a mold electrode(the working electrode) with a 3D microstructure in a three-electrode cell.As the solution contains a designed scavenger,the etchant is consumed rapidly during its diffusion away from the surface of the mold electrode into the solution because of its rapid reaction with the scavenger. Therefore,the etchant is confined within an extremely thin diffusion layer around the surface,thus the contour of the etchant layer profile can keep the shape of the microstructure of the mold with a high resolution.A complete negative copy of the 3D microstructure of the mold can be fabricated if the mold is continuously approaching the substrate.Therefore,in principle,CELT can be applied to micromachine different kinds of substrates,including metals and semiconductors,regardless of the conductivity of the substrates.In this dissertation,â… investigated how to fabricate the complex three-dimensional microstructures on several metallic materials with CELT.The research work can be described as follows:1.The micromachining of copper with CELTFeCl2 was used as a precursor of generating etchant Fe3+for the micromachining of copper.The SnCl2 and ascorbicacid were used as scavengers.2,2-bipyridine was used as a useful additive for micromachining of Cu.We also developed another procedure in which NaNO2 was used as a precursor of generating the etchant HNO3 for etching copper. In this case,NaOH was used as scavenger.Citric acid was used as an additive to avoid forming Cu(OH)2 precipitate in the micromachining processes.3D complex microstructures were replicated on copper surface successfully.The spatial resolution of this machining was about 0.95μm and the optimized processing condition was proposed: constant current method was employed,the current density i=1×10-2～2.5×10-2A/cm2, temperature T=35～40℃.The reactive mechanism was investigated for the etching system including FeCl2 and SnCl2.The role of complex ligands and scavengers were analyzed in detail.The effects of generating rate of etchant on the morphology of the etched surface were studied.The faster the generating rate of etchant was,the easier to get uniform surface. However,the very high current density will cause the reduction of the spatial resolution of the etched morphology.Furthermore it could lead to evolution of oxygen,which will interfere the etchant's diffusion layer.The influences of ligands on the morphology of the etched surface were also investigated.When different ligands were used,the way in which the etchant aggress (etch)the copper atom of the substrate was very different owing to the different configurations of the complexes,and hence leading to different surface morphology.2.The micromachining of nickel with CELTThe NaNO2 was used as precursor of generating etchant HNO3.The NaOH was used as a scavenger.The replication of 3-dimensional complex microstructure on nickel surface was successfully finished using this system with the addition of the ligand of tartaric acid.The etching resolution reached at sub-micrometer scale.The etching mechanism was investigated about the etching system containing NaNO2.The effects of scavenger NaOH on the generation of etchant were studied through the cyclic voltammograms.The through-hole micromachining on a nickel foil with a thickness of 50μm was carried out.The etching error was around 4μm when the fabricated microhole was about 89μm in diameter.The high error was related to the repeating precision of the CELT instrument.The influences of concentration of different composition in the etching system and the current density on the micromachining resolution were investigated.The optimized processing condition was:current density i=5×10-3A/cm2～2.5×10-2A/cm2,at room temperature. 3.The micromachining of titanium and its alloys with CELTHF was selected as an etchant for titanium,and the "etchant-scavenger" system was designed.Both NaNO2 and NaClO3 were used as precursors to generate protons.After initiating of electrolysis the generated H+ associated with F- that was supplied by fluoride (e.g.,NaF)in the solution to form HF acid for etching titanium.The NaOH was selected as the scavenger.The etching mechanism of "NaNO2+NaClO3+NaF+NaOH" system was also studied.The interaction between NaNO2 and NaClO3 was investigated.The influence of concentration of NaNO2 and NaClO3 on etching resolution was studied.The experiment results suggest that the NaNO2 can prevent the diffusion of etchant to a certain extent.In the solution containing "NaNO2+NaClO3+NaF+NaOH",the microholes and 3D complex microstructure was fabricated on titanium surface successfully.The etching resolution reached at the sub-micrometer scale.The effect of surfactant on the resolution of etching titanium was investigated.Suitable operation conditions were suggested as follows:the current density i=1.25×10-2～5.0×10-2mA/cm2,the temperature T=35～45℃.4.The micromachining of aluminum,magnesium and cadmium with CELTNaNO2 was used as precursor of generating etchant HNO3.NaOH was used as scavenger.Tartaric acid was used as a useful additive.Na2SiO3 or sodium polyphosphate was added as an inhibitor against spontaneous corrosion.3-dimensional complex microstructure on aluminum surface was replicated successfully.The etching resolution reached at the sub-micrometer scale.The etching mechanism was investigated.The pH change near the working electrode surface during the electro-oxidation of NO2- was explored.The effect of current density on the diffusion layer of H+ was also investigated.The limitations of the etching system with NaOH as the scavenger were discussed.Available etching systems for magnesium were screened through the measurement on the change of the solution pH near the electrode surface and etching rate.Selecting NaNO2 as precursor for electrochemically generating etchant HNO3,NaOH as scavenger,and small amount of Na2SiO3 as inhibitor,we can replicate 3-D microstructure of the mold onto the magnesium alloy surface. Sub-micrometer scale resolution was obtained.The etching process on cadmium was studied by using NaNO2 as a precursor of generating etchant and NaOH as a scavenger.The very preliminary result indicates that the micromachining on cadmium by CELT was feasible.Overall,the comprehensive study shows that in order to obtain the good quality and the high spatial resolution,the corrosion rate for the generated etchant must be sufficiently high.It is better to avoid generating the gas as the co-product at the etched surface,It is necessary to add some additives if the gas product is unavoidable in the process. |
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