Micro/nanomachining On Semiconductors And Metals By New Efficient Electrochemical Methods And Their Applications

The development of the micro-electro-mechanical systems(MEMS), micro-optics,micro-chips helps the advancement of micromachining technology that is the hotspot of the recent research and the core of MEMS.The requirements for new approaches of micro/nanomachining include the ability to fabricate complex 3D microstructures,high output and batch process.Recently,the electrochemical machining method is considered to be hopeful and environmentally friendly due to its mild working conditions,low cost and easy controllability and has a good potential ability to fabricate complex 3D micro/nano structures.Here,this thesis concentrates on developing new electrochemical micromachining methods and applying them to fabricate effective 3D micro/nanostrucutures on semiconductors and metals.The application of these methods to produce a diffractive microlens array and cell patterns was preliminarily explored.This thesis is divided into seven chapters.Chapterâ… introduces the process and characteristics of the main approaches for micro/nanomachining.The application of the techniques especially those in the fabrication of microlens array and cell patterns are introduced in detail.Two electrochemical micromachining methods developed in our group were recommended in Chaperâ… .The aim and main task of this thesis are presented.Chapterâ…¡introduces the experimental reagents and characterization tools. In chapterâ…¢and chapterâ…£,the Confined Etchant Layer Technique(CELT) has been applied to achieve effective 3D micromachining on n-GaAs and p-Si.In chapter V,CELT was applied to fabricate large-scale diffractive microlens array on n-GaAs. The PMMA/Ti/Pt mold with complex micro/nano structures was fabricated by traditional hot embossing technique and radio frequency magnetron sputtering.After two-step precisely replication,the binary diffractive microlens array on the quartz was transferred to the n-GaAs.Chapterâ…¥introduces a new technique named as electrochemical wet-stamping method(E-WETS),Localization of anodic dissolution using patterned agarose has been employed to fabricate micro-structures on metals (Ni,Cu and Au/ITO) and p-Si.In addition,the patterned Au/ITO substrate was applied to pattern Hela cells by further modification.In chapterâ…¦,some unsolved questions were discussed and the prospective plan was presented.The main results of this work are listed as follows:(1) The confined etchant layer technique has been applied to achieve effective three-dimensional(3D) micromachining on n-GaAs and p-Si.This technique operates via an indirect electrochemical process,and is a maskless,low-cost technique for microfabrication of arbitrary 3D structures in a single step.Br2 was electrogenerated at the mold surface and used as an efficient etchant for n-GaAs and p-Si;L-cystine was used as a scavenger,for both substrates.The resolution of the fabricated microstructure depended strongly on the composition of the electrolyte,and especially on the concentration ratio of L-cystine to Br^-.A well-defined,polished Pt microcylindrical electrode was employed to examine the deviation of the size of the etched spots from the real diameter of the microelectrode.The thickness of the confined etchant layer can be estimated and thus the composition of the electrolyte can be optimized for better etching precision.The etched patterns were approximately negative copies of the mold,and the precision of duplication could reach the micrometer level for p-Si and the sub-micrometer level for n-GaAs.Although the same etchant(Br₂) and scavenger(L-cystine) were used in the etching solutions for GaAs and Si,the etching process,or mechanism,is completely different in the two cases.Compared with the fast etching process on GaAs in an etching solution with a concentration ratio of 3:1 of L-cystine to Br^-,the concentration ratio needs to be 50:1 for etching of Si.For the micromachining of Si,the addition of a cationic surfactant (cetyltrimethylammonium chloride,CTAC1) is necessary to reduce the surface tension of the substrate and hence reduce the influence of evolution of the by-product H₂.The function of the surfactant CTAC1 in comparison with an anionic surfactant(sodium dodecyl sulfate) was studied in contact angle experiments and micromachining experiments and then is discussed in detail.(2) A large-scale diffractive microlens arrays on n-GaAs has been fabricated by using an efficient electrochemical technique named CELT(confined etehant layer technique).This microlens array is an eight-phase level diffractive optic device with eight concentric rings and seven steps in one lenslet.When appropriate chemical solutions and etching conditions are chosen,an approximate copy of the diffractive microlens array on the quartz is transferred onto the n-GaAs.In this thesis,attention has been paid to the electromicromachining process that critically relates to the practical application.The feeding of the workpiece makes the refreshment of the solution in a very small volume mandatory;the heterogeneous reaction between the etchant and the workpiece enhances the confinement of the scavenger and thus the etching resolution in the horizontal direction reaches tens of nanometers.These studies will definitely help the electrochemical method come into mass production for micro-optic component arrays on GaAs.When the influence of the heterogeneous reaction on the etching resolution or the thickness of CEL needs to be taken into account,a mathematic model is proposed to illustrate the concentration gradient under different conditions with and without the influence of the substrate,considering different distance between the mold and the workpiece.Therefore,the pressure between the mold and the workpiece is kept at a constant and small value to ensure the nearest distance between them.Moreover,the concentration of the precursor HBr is increased to increase the resolution of micromachining and the etching rate and thus increase the efficiency of the micromachining.(3) Localization of electrochemical polishing using pattemed agarose has been employed to fabricate microstructures on p-Si and metals by electrochemical wet stamping method(abbreviated as E-WETS).The patterns were first transferred from a master to an agarose stamp,and then the microstructures were fabricated by limiting electrochemical polishing in the small contact area between the stamp and the workpiece.The gel stamp acts as the current flow channel between the working electrode and the counter electrode,simultaneously directing the electrolyte to the preferential parts of the workpiece.When the microstructures are fabricated by partial anodic dissolution on p-Si,they are approximately the same as those on the master. Lateral deviation of the fabricated microstructures from those on the master is approximately 2.6%and the electrochemical etching rate in HF is around several micrometers in an hour.This newly developed technique can be used as a low-cost and simple approach to fabricate microstructures on p-Si with high fidelity at a fast rate.This method has also been applied to micromachining metals,such as nickel, copper and Au film.Patterned Au/ITO substrate was immersed in mPEG-SH (methoxy(poly-(ethylene glycol))thiol) solution,and a cell-resistant self-assembled monolayer formed on gold islands.The gold islands coated with SAMs became cell-resistant and thus Hela cells only adhered on the exposed ITO surface.