Fabrication Of MEMS Safety & Arming Devices With Low Residual Stress Based On Micro Electroforming Process

With the development of MEMS technology,metal micro-devices fabricated by UV-LIGA technique are increasingly used in milltary,aerospace and other fields.However,the deformation and delamination phenomena of electroformed layer often appeared due to the effect of residual stresses,which seriously reduce the dimensional accuracy,mechanical properties and service life of the micro-devices.In this paper,the fabrication process and difficulties of a MEMS safety & arming(S&A)device was discussed.Aiming at fabricating the low-stress micro-devices,the main research contents are as follows:MEMS S&A devices consist of framework,explosion-proof slider,centrifugal safety lock and setback safety lock and micro-spring.According to the characteristics of these microstructures,the fabrication process was developed and the effect of residual stress on the framework and setback safety lock was analyzed.The simulation analysis of the residual stress shows that the method of adding stress release structure is helpful to reduce and homogenize the residual stress in the framework microstructure.However,as the main function structure of MEMS S&A devices,the deformation problem of setback safety lock caused by the residual stress cannot be solved by the method of adding stress release structure.The effects of aging treatment and megasonic-assisted micro electroforming on the setback safety lock are explored separately.According to the experiment results,the aging treatment can effectively reduce the bending deformation of the microstructure due to residual stress,but this method will deteriorate other properties such as toughness and surface quality of the microstructure.The megasonic-assisted micro electroforming method can effectively reduce the residual stress in the micro electroforming layer during the stress generation phase.Therefore,megasonic-assisted micro electroforming method can be used to fabricate the setback safety lock with low stress,but the method of measuring bending deformation can only characterize the effect of this method on residual stress in microstructures indirectly.In order to further quantitatively characterize the residual stress in the setback safety lock,the residual stress in the samples with or without megasonic-assisted micro electroforming was tested by nanoindentation technique and X-ray diffraction method.The Suresh model and the Lee model in the nanoindentation technique were used to calculate the residual stress in the samples,and the influence of different calculation models on the stress test results werediscussed.The results show that the residual stress of nickel coating calculated by Lee model is closer to the residual stress obtained by XRD.According to the results obtained by Lee model,the mean compressive stress of samples with megasonic has a decrease of 57.84%,compared with that of samples without megasonic.These results indicate that the megasonic-assisted micro electroforming method can effectively reduce the residual stress in the electroforming layer and avoid excessive macroscopic deformation of the microstructure,which contributes to improving the fabrication success rate of microstructures.According to the above research contents,the fabrication of microstructures and the assembly of MEMS S&A devices were carried out.The results show that the lamination and bending of the casting layer are not found in the framework microstructures and the setback safety lock microstructures,which verifies the feasibility of adding stress release structure and using megasonic-assisted micro electroforming method to fabricate low stress microstructures.In addition,the key dimensions of the MEMS S&A devices are measured and the source of dimensional error of the MEMS S&A devices after assembly were analyzed.