Study on the Mechanism and Methods of Reducing the Residual Stress in Micro Electroforming Layer

Author:Song Chang

Supervisor:du li qun


Degree Year:2018





With the rapid development of MEMS technology,metal microdevices have obtained widely attention and applications.Micro electroforming is one of the main techniques for fabricating the metal microdevices.During the micro electroforming process,residual stress inevitably produces in the electroforming layer due to the non-uniform electrocrystallization.Excessive residual stress may lead to the warpage,delamination or even debonding of the electroforming layer,which cannot be ignored and seriously influences the dimensional accuracy,fabrication yield,mechanical properties and service life of metal microdevices.In this paper,three methods of reducing the residual stress in micro electroforming layer have been proposed based on the research of the simulation study and measurement method of residual stress.The detailed research work contains the following five parts:The residual stress simulation model in micro electroforming layer was established based on the study of residual stress composition.Based on the“Equivalent Reference Temperature(ERT)" method,the residual stress in micro electroforming layer was simulated by a fictitious temperature load.By using the method of combining experiment measurement and simulation calculation,the mathematical relationship between the residual stress measurement results and the fictitious temperature load was studied,and the range of ERT for electroforming layer was obtained.Take the common electroforming material Ni as an example,the residual stresses were simulated by five different ERT values.The ERT for Ni electroforming layer satisfying the accuracy requirement of the simulation model was determined to be 235.15K.This model is suitable for the simulation study of residual stress distribution in electroforming metal microstructure with micron order feature size.For exploring the measurement of residual stress in electroforming metal microstructure,the interactive relationship between the residual stress in electroforming layer and the stress in solid particle during the composite electrodeposition process was investigated based on the Guglielmi model.The residual stress measurement method of metal microstructure by Micro Raman Spectroscopy was proposed by codeposition of Raman active solid particles with metal materials.In this paper,β-SiC particle with a strong Raman activity was selected as the composite phase.The stress calculation method of P-SiC particle was established based on the principle of MRS,Scherrer equation and Hooke’s law.The reliability of the method for measuring the residual stress in metal microstructure based on MRS was verified by the XRD residual stress method.This method is suitable for measuring the residual stress in metal microstructure with micron order feature size.In order to improve the uniformity of electrocrystallization,reduce the generation of residual stress in essentially,the design method of presetting stress release factor in metal microstructure was proposed based on the simulation model of residual stress in micro electroforming layer.The simulation results were studied experimentally by combining with the UV-LIGA technology and the self-presented MRS residual stress measurement method.Experiment results were consistent with the simulation results,which showed that presetting stress release factor can effectively reduce the average residual stress in metal microstructure,decrease the stress concentration factor and improve the residual stress uniformity.The best effect was obtained by presetting annular groove assisted rivet shape stress release factors,which reduced the average residual stress by 37%.This method is suitable for designing the metal microstructure that has no strict requirements on the structural weight and morphology under the premise of ensuring the structural stiffness and satisfying the operating function.Based on the study of the residual stress measurement in metal microstructure by MRS,the method of reducing the residual stress in micro electroforming layer by doping β-SiC particles was proposed from the view of improving the electroforming mode.The influence of four composite electroforming process parameters on the residual stress in electroforming layer was studied by the L9(34)orthogonal test.Experimental investigation showed that under the experimental conditions in this paper,the process parameters of electroformed with the minimum residual stress were β-SiC particles mass concentration of 20 g/L,current density of 1 A/dm2,magnetic stirring speed of 600 rpm and electroforming temperature of 50℃.The residual stress obtained under these process parameters was reduced by 42%.By studying the relationships between the intercalation capacities of β-SiC particles,grain size and residual stress,the mechanism of reducing the residual stress in electroforming layer by doping β-SiC particle was revealed.Doping hard particles can improve the nucleation of metal materials and refine the grain size,and thus reduce the residual stress.This method provides an idea for the study of reducing the residual stress in electroforming layer by adopting particles that can promote nucleation.Based on the research of megasonic can improve the electroforming process,the method of reducing the residual stress by megasonic assisted electroforming was proposed from the perspective of introducing the external action.The micro electroforming experiments with and without megasonic agitation were designed on the self-developed experimental platform.Experiment results showed that with the increase of megasonic power density,the residual stress firstly decreased and then increased a little.The best effect was obtained with the megasonic power density of 2 W/cm2,which reduced the residual stress by 60%.From the points of dislocation theory and steady cavitation,the mechanism of reducing the residual stress in micro electroforming by megasonic was revealed.During the electroforming process,the application of megasonic can increase the dislocation density in electroforming layer,promote the lattices to be growing with a balance state,provide stress release points on the surface of electroforming layer by cavitation bubbles,and thus reduce the residual stress.This method is suitable for fabricating most microstructure,especially for the microstructure with high thickness or large high aspect,has the advantages of wide application range,high efficiency and simple operation.