Study On The Methods And Mechanism Of Improving The Thickness Uniformity Of Micro Electroforming Layer

With the rapid development of MEMS technology,metal micro devices have good applications in many fields,and the demand for metal micro devices is gradually increasing.Micro electroforming is one of the main methods to fabricate metal micro devices.However,during the electroforming process,the uneven current density distribution produces on electroformed surface due to the effects of current edge effect,ion distribution,feature size of electroformed mould and so on.The uneven current density distribution causes the problem of thickness nonuniformity of electroforming layer.This problem affects surface quality,dimensional accuracy,yield and usability of the electroformed micro devices.So,the thickness nonuniformity of electroforming layer was investigated in this dissertation.Two methods of improving the thickness uniformity of micro electroforming layer were explored,including sacrificial structure-style auxiliary cathode and megasonic micro electroforming.And the mechanism and application of improving the thickness uniformity of micro electroforming layer by megasonic agitation were studied.The specific research contents are as follows:The method of improving the thickness uniformity of micro electroforming layer by the sacrificial structure-style auxiliary cathode was investigated.This method designs the sacrificial structure-style auxiliary cathode around electroformed micro devices by finite element method simulation,and improves the thickness uniformity of micro electroforming layer by improving the primary current distribution.As a research object,the thickness uniformity of metal mold used for injection molding of microfluidic chip was studied by using the finite element simulation method.The factors that affect thickness uniformity were investigated,such as the dimensions and placement of auxiliary cathode,electroforming current density,electroforming time.Based on the simulation,the micro electroforming experiments were performed,and the metal molds were fabricated by the method of sacrificial structure-style auxiliary cathode.The experiment results are in good agreement with the simulation results.Under the preferred parameter combination,the thickness uniformity of metal mold with the sacrificial structure-style auxiliary cathode is improved by 49.1%compared with the result without auxiliary cathode.The method of improving the thickness uniformity of micro electroforming layer by megasonic agitation was studied.Based on the relationships among sound field,flow field,mass transfer and electrodeposition process,a computational model of megasonic micro electroforming process was established.And the effects of megasonic vibration mode and megasonic power density on the thickness uniformity of micro electroforming layer were studied by using the simulated method.Then,the megasonic micro electroforming experiment was performed to verify the validity of simulation model.The simulated results show that megasonic agitation improves the ion concentrations and tertiary current density distribution on cathode surface,and then improves the thickness uniformity.Under the effects of bidirectional,bidirectional alternating and single directional megasonic vibration modes,the improvement effects of three megasonic vibration modes on thickness uniformity are as follow:bidirectional megasonic,bidirectional alternating megasonic,single directional megasonic.Under the vibration mode of bidirectional megasonic,the improvement effect of thickness uniformity increases with the increase of power density.The experiment results are in good agreement with the simulation results.The experimental results show that compared with the result under non-megasonic condition,the thickness uniformity of micro electroforming layer under the megasonic power density of 2.4 W/cm2 are increased 45.3%.From the perspectives of throwing power and mass transfer,the mechanisms of improving the thickness uniformity of micro electroforming layer by megasonic agitation were studied.(1)The effect of megasonic vibration on throwing power was studied.Throwing power experiments,conductivity measured experiments and electrochemical experiments were performed with and without megasonic.The results reveal that megasonic cavitation effect increases the number of conductive ions,enhances the thermal motion of conductive ions,and then increases the conductivity of electroforming solution.Meanwhile,the acoustic streaming and cavitation effect decrease the cathodic polarization,and thus the throwing power and thickness uniformity are improved.And throwing power and thickness uniformity increase with the increase of power density.(2)The effect of megasonic agitation on mass transfer was studied.A computational model was developed,and the factors that affect mass transfer were quantitatively investigated by using mass transfer coefficient,such as power density,aspect ratio,vertical and horizontal distance between the microchannel and transducer center.The simulated results show that when the megasonic power density increases,the mass transfer coefficient of micro electroforming increases.In addition,under the same megasonic power,mass transfer coefficient increases with decreasing aspect ratio and vertical and horizontal distance between the microchannel and transducer center.In order to verify the validity of simulated method,electrochemical experiments were performed.The experiment results are in good agreement with the simulation results.The mechanisms of improving the thickness uniformity of micro electrofonning layer by megasonic agitation are that the megasonic vibration can increase the conductivity of electroforming solution,decrease the cathodic polarization,improve the throwing power,and thus improve the thickness uniformity.Meanwhile,the megasonic vibration can enhance the mass transfer of micro electroforming,improve the ions and tertiary current density distribution on cathode surface,and thus improve the thickness uniformity.Finally,to prove the validity and feasibility of method of megasonic micro electroforming on metal micro devices,the applied research for fabrication of metal micro devices was investigated.Three typical metal micro devices were fabricated by megasonic micro electroforming,including the metal mold used for injection molding of microfluidic chip in biomedical field,the micro-channel heat sink baseplate used in electronic equipment field and micro-column array electrode used in mechanical engineering.And the thickness uniformity of metal micro devices was investigated.(1)The result indicates that megasonic micro electroforming can improve the thickness uniformity of metal micro devices.(2)Compared with the result under non-megasonic condition,when the power density is 1.6 W/cm2,the electroforming current density and time are 150 A/m2 and 2 h,respectively,the thickness uniformity of metal mold is improved by 40.9%.And the thickness uniformity of micro-channel heat sink baseplate is improved by 34.5%when the power density,electroforming current density and time are 1.6 W/cm2,200 A/m2 and 2 h,respectively.(3)Megasonic agitation can enhance the mass transfer of micro electroforming,solve the problems of pits and electroforming failure for micro-column array electrode,and thus improve the quality and thickness uniformity of electroforming layer.