Fundamental Research On Micro Electroforming Of Metal Microstructures By Using A Movable Mask

Electroforming is a specialised electroplating process based on the metallic ion electrochemical deposition on a cathode. Due to the workpiece material is accreted at atomic size, this micro-deposition process brings electroforming tremendous potential in the micro-manufacturing field. Besides, electroforming is advantageous for micro machining because there are no heat affected zones, cutting force and tool wear. The Micro/Meso processing technologies have been the focus of concern and become the hot spot of research in industrialized countries because the Micro/Meso system is the most closely related to high-tech product and national defence field.For fabricating Micro/Meso metal microstructures, a novel electroforming technology is presented in this paper, and relative researches have been carried out. The dissertation consists of seven sections, including:(1) A novel electroforming technology is developed. An insulating mask with patterned features for localizing the deposition region moves periodically along the metal growth direction during electrodeposition instead of the traditional patterned photoresist mask keeping still during electrodeposition. In principle, the microstructures with unlimited aspect ratio can be fabricated by the proposed technology.(2) On the basis of analyzing the characteristics of the proposed technology, the experimental system is developed. It consists of servo-control feed unit, electrolyte circulating system, electrodeposition unit and process monitoring unit. The developed system fulfills the requirements of the proposed technology. The functions of the system include x-y-z axes motion control, deposition status monitoring and deposition thickness measurement in real time, electrolyte heating and circulating, and electrolyte refreshing periodically. A special fixture is developed to hold the mask for good mating between the mask and the cathode. The control software is programmed by LabVIEW graphic language. It is very easy to control the x-y-z axes by the software. The electric signals such as current, cell voltage are acquired in real time to monitor the electrodeposition status. The control system can quickly take corresponding actions according to the acquired signals.(3) UV lithography is employed to fabricate the shielded anode in the paper. The parameters of UV lithography process including spincoating, pre-bake, exposure, post exposure bake and development have been studied to pattern SU-8 photoresist spincoated on a metal substrate. Using orthogonal array technique, an optimization experiment is performed to characterize the processing of SU-8 photoresist. The optimized process parameters of pre-bake time, exposure dose, post exposure bake time and development time are obtained and then analyzed by BP neutral network. The process parameters with better resolution is predicted by the BP neutral network and verified by experiment. Some SU-8 moulds with good definition, good topography, fewer defects and excellent adhesion with metal substrate have been produced successfully.(4) Based on the theories of electric field and electrochemistry, a model of current distribution between the shielded anode and cathode is build. Finite Element Method is then used to solve the model. For better understanding the process and obtain good processing parameters, the deposition localization is analyzed and numerical simulation is carried out based on the model.(5) The key factors influencing the electroforming process by using shielded anode have been studied experimentally. For selecting appropriate anode material, several kinds of material such as dissolution and undissolution anode, Copper-Phosphorus and copper are studied. The range of current density is also analyzed and studied. The performance of the shielded anode under chemical, electrochemical and mechanical effect is also studied and method for prolong the shield anode's life is proposed. Some metal microstructures such as micro gears, micro bars have been produced successfully by different movement type of the shielded anode. (6) The current density distribution and mass transfer in the proposed micro electroforming process by using a movable mask are studied theoretically and experimentally. Typical high-aspect-ratio metallic microparts such as micro static electrical combs and micro electrodes in the scale of several hundred microns have been produced. The proposed technique could expand the nontraditional micromachining techniques. Applying this technique to the fabrication of micro products would speed up the development of MEMS.