Experimental Research On Micro-electroforming Technology Under A Magnetic Field

The Micro Electro Mechanical System (MEMS) is one of the hot spots in today’sscientific and technological research. It not only contains a huge economic benefits, butalso reflects the important position in the national strategic high-tech, and MEMS hasgradually grown into a huge field. As one of the main means of achieving of micro-devices and micro-structures, the micro-electroforming technology is taken seriously byindustry because of process flexible, high precision replication and the processingcapacity of aspect ratio3D microstructures. However, for the difficulty of the liquidphase mass transfer, the micro-electroforming products usually emerge lots of defectssuch as pinholes, knobbles and hydrogen embrittlements, and its application and devel-opment can be limited. So completely solving the problem of mass transfer limitationhas become one focus of many researchers at home and abroad.An innovation implementing the micro-electroforming under a vertical magneticfield was propounded to solve the liquid phase mass transfer limitation in thisdissertation. The microfluid within a microspace simulation was once made in thepresence of magnetic field as electrodeposition was in progress by our group. The resultshowed that the almost stagnant electrolyte was stirred when the external magnetic fieldwas superimposed, and it was better to achieve the exchange of substances within themicro space and the bulk solution than that without magnetic field. This paper carriedout micro electroforming experiment under an external magnetic field for the first time,and studied the influence of the magnetic field effects on the electroforming processesand validated the numerical result.This dissertation was supported by Nation Nature Science Foundation (50805077)China Post Doctor Science Foundation (200801373), and Henan Provincial KeyLaboratory of Precision Manufacturing Technology and Engineering.This paper summarized the micro-electroforming technology and its research status,and gave an overview on magnetoelectrochemical deposition technology. The mechani-sms of magnetoelectrochemical deposition were researched and summarized intensively,and considered that magnetihydrodynamic effect (MHD) played an important role toimprove the liquid phase mass transfer, the gradient magnetic force effect also made some contribution to mass transfer and the magnetizing force effect was more importantto the orientation of electrocrystallization.An experimental device on micro-electroforming in an external magnetic field wasdeveloped in this dissertation, which included magnetic field unit, electrodeposition unitand electrolyte circulation unit. And the design of the magnetic field unit was key anddifficult, and its source is a permanent magnetic field. The shape and dimensionalparameters of the permanent magnet were optimised by an image method, and thegathering magnet structure’s design was optimized seriously. The central magnetic fluxdensity of the end product is up to0.867T when the long of the working air-gap was30mm, and the different magnetic intensity could be obtained via a bolt.This dissertation overall investigated the influence of a perpendicular magneticfield (B=0.82T) on the macro electroforming processes for the first time. In theexperiments, there were two different electrolyte witch contained wetting agent or not,and they were named as Model II and Model I electrolyte. Taking use of Model IIelectrolyte, this paper studied the impact of the magnetic field (with or without) onelectrotype layer, and the results showed that the magnetic effects thinned the diffusionlayer, enhanced the liquid phase mass transfer, raised the limit current density, excludedeffectively hydrogen bubbles, refined grains, and the morphology quality, density andhardness of the deposit layer were improved. Other test adopting Model I electrolytethat was circulated quickly was practiced under the magnetic field or not. The resultsindicated, after magnetic field was superimposed, the grain size of the electroforminglayer was smaller and more uniform, and an amorphous layer perhaps was acquiredwhen the current density was3A/dm~2. This could be a great discovery.The process status of the micro electroforming was overviewed in this dissertation,and based on the macro magnetic-electroforming test and the simulation of the microfluid within a micro space the micro electroforming experiments under a magnetic fieldperpendicular to the electric field were carried out for the first time. The dimensionalparameters of the core mould were700μm in width,1750μm in height and2.5in aspectratio. The researches indicated that there were some pinholes coming out withoutmagnetic field as the current density was up to7A/dm~2. When the virtual magnetic fieldwas superimposed, the morphology quality of the micro electroforming devices and the limit current density were improved significantly. And some devices with good surfaceprofile, fewer defects and higher hardness were fabricated under9A/dm~2in the currentdensity. The numerical result was validated well. At last the micro electroformingdevices with typical high-aspect-ratio structure characteristic and good morphologyquality were manufactured at high current density.