Study On The Methods And Mechanism Of Megasonic Assisted Through-Mask Electrochemical Machining For Metal Micro Structures

With the development of the micro-electromechanical system(MEMS)technology,metal micro structures of MEMS devices have been used in many fields.There are a variety of manufacturing methods for metal micro structures.Among them,through-mask electrochemical machining(TMEMM)technology has the advantages of low cost,little pollution,no wear of tool electrode,high processing efficiency,which has a wide application prospect in the manufacturing of micro structures.However,in the process of TMEMM,lateral etching is unavoidable in the machining process.The lateral etching leads to excessive material removal in the unmachined area,which severely reduces the localization of TMEMM and limits the application of TMEMM technology in the fabrication of metal micro structures.In order to solve this problem,this paper proposed the megasonic assisted TMEMM(MA-TMEMM)method and the megasonic with particles assisted TMEMM(MP-TMEMM)method to improve the etching localization of the TMEMM.The research contents are as follows:The relationship between the megasonic intensity and the anodic etching rate was studied.The simulation model of the fabrication of micro pit structure in MA-TMEMM was established.Firstly,the coupling relationship of the acoustic field,the flow field,mass transfer process and the electrolytic process in MA-TMEMM was analyzed.The influence rule of megasonic intensity on anodic etching rate was determined.The simulation model of fabricating micro pits in MA-TMEMM process was established.Secondly,the COMSOL multiphysics software was used to simulate the anode profile evolution in MA-TMEMM process.Acoustic field,gas-liquid two-phase flow field,mass transfer process and electrolytic process were analyzed by simulation.The simulation results showed that with the increase of megasonic intensity the sound pressure in the electrolyte increased,the gas concentration in the electrolyte decreased,and the conductivity of the electrolyte increased.The etching rate and localization of the micro pit improved to a certain extent with megasonic wave.It can be seen that the megaconic wave promotes the anode reaction,improves the deep etching ability,and improves the localization of the micro pits.The vibration mode and the transducer performance of the megasonic transducer were studied,and the experimental equipment for MA-TMEMM was set up.Based on the theory of elastomer vibration,the thickness mode of the megasonic transducer was analyzed and the half-wave matching of the radiation terminal of the transducer was defined.The mode analysis and harmonic response analysis were used to determine the vibration mode and response amplitudes of the quartz half-wave matched megasonic transducer.The impedance analyzer and acoustic radiation force balance were used to measure the performance of the megasonic piezoelectric transducer.The test results showed that the megasonic transducer has the lowest impedance and the highest radiated acoustic power when the resonance frequency of the piezoelectric ceramic is 1 MHz and the thickness of the matching quartz layer is 3 mm.The electro-acoustic efficiency of transducer is up to 67.3%.Based on the design of the megasonic transducer,a megasonic electrolyzer with side acoustic source was made.The other functional components such as the plate agitation part,electrolyte circulation system,water heating bath and device body were integrated.An experimental device for MA-TMEMM was developed.The experimental equipment has the characteristics of high integration,good corrosion resistance,large radiated sound power,uniform distribution of sound energy.The basic process parameters of TMEMM were carried out.Based on the optimization of the basic process parameters of TMEMM,the MA-TMEMM experiments were carried out under different megasonic intensity.The mechanism of megasonic on material etching rate and localization were analyzed.Firstly,the influences of different electrolytic voltages,electrolyte components,pH value of electrolyte and pulse frequency on etching results without megasonic were studied,and the basic process parameters suitable for TMEMM in this paper were determined.Secondly,the MA-TMEMM experiments of micro pits were carried out under different megasonic intensity.The experimental results showed that with the increase of megasonic intensity,the material removal rate and etching localization of micro pits were improved.When the megasonic intensity was 0 W/cm2、1.6 W/cm2、4.8 W/cm2 and 8.0 W/cm/respectively,the etching factor(EF)obtained by the experiment was 1.56,1.75,1.91 and 2.35,respectively.Under the effect of megasonic,the etching localization of micro pits increased by 12.2%,22.4%and 50.6%,respectively.According to the experimental results,the mechanism of megasonic on improving the etching rate and etching localization of TMEMM was analyzed from the aspect of removal of electrolytic products.The relationship between the solid electrolytic products and etching rate was investigated and the simulation model of MA-TMEMM was revised.Based on the MA-TMEMM method,in order to further improve the etching localization,a novel method of MP-TMEMM was proposed.The mechanism of the improving effect of the etching localization in the fabrication of micro structures in MP-TMEMM was revealed.Firstly,The TMEMM experiments were carried out with different SiC particle diameters,particle phase contents and megasonic intensity.The original pit diameter of the mask was 100 μm and the center distance was 200 μm.The experimental results showed that under the same conditions,the MP-TMEMM method can get better etching localization than the traditional TMEMM and MA-TMEMM.When the etching time was 120 s,the SiC diameter was 40 μm,the dispersed phase content was 6 g/L,and the megasonic intensity was 8.0 W/cm2,the average micro pit diameter was 148.7 μm,the depth was 59.9 μm,and the EF was 2.50.Compared with the traditional TMEMM and MA-TMEMM,the EF of micro pits etched by MP-TMEMM method increased 58.2%and 35.9%,respectively.Secondly,the erosion process of MP-TMEMM was analyzed,and a semi-rational formula of the particle erosion on solid electrolytic products was established.Thirdly,electrochemical impedance spectroscopy(EIS)was used to test the impedance properties of the solid electrolytic products after MP-TMEMM and the mechanism of MP-TMEMM on improving the etching localization of micro structures was revealed.In order to verify the effectiveness and feasibility of the MP-TMEMM in the fabrication of metal micro devices,the fabrication study of three kinds of typical devices with MP-TMEMM method was carried out.Three kinds of micro devices were micro convex array structure,micro channel array structure and fine micro filter line grid plate structure.Besides,their etching effects were evaluated.The EF of the micro convex array under the traditional TMEMM and MP-TMEMM were 2.07 and 2.87 respectively;the EF of the micro channel arrays were 2.77 and 4.02,respectively;The EF of the grid plates at 120 s were 0.93 and 1.79,respectively.The synergistic effect of megasonic and particles can effectively improve the etching localization and replication precision of micro structures.