Research On Mechanical Properties And Size Effects In Mems

Some key problems of mechanical properties and size effects in MEMS (Micro Electro Mechanical Systems) are investigated theoretically, systematically and deeply in this paper.MEMS is a increasingly growing technology of multidisciplinary interaction,primarily dealing with fabrication, design and application of micro machines (MM) and micro mechanical assemblies (MMA) with major dimensions on the order of microns. Material in the range of micro scopes displays strong size effects upon many mechanical properties, and the impotance of those with characteristic length of micro devices and micro systems in proportion to high power of size decreases relatively, such as inertial force (L4) and electromagnetic force (L3), while the impotance of those with characteristic length in proportion to lower power of size increases correspondingly, such as elastic force (L2), surface tension (L1) and electrostatic force (L0) etc., meanwhile, the ratio of surface area to volume and surface friction increase rapidly, and the thermal conduction and chemical reaction accelerate largely. At present, the technology of micro machining grows up increasingly, but the research on fundamental theory of MEMS falls behind and turns into"the bottleneck problem"stunting further development of the discipline, in which the study of material mechanical properties lags far behind of the research on electrology properties, and material mechanical properties should be investigated accurately, upon which size effect stand out especially.Size effects are investigated systematically, of which the connotation is also analyzed in detail and types are distinguished in terms of different physical quantities. Size effects in both broad and narrow sense are presented, and the mathematical model of size effects in narrow sense is founded for functional analysis, which is analyzed in view of absolute value, relative value and sensitivity and made clear using applied examples. Positive, negative and zero size effects are presented and the criterions distinguishing the three effects are given, keeping the continuity and integrality of size effects from macro to micro scope. The known research on size effects is limited to qualitative analysis of single mechanical property, while the mathematical model founded exceeds the previous limitation, for it can be used in functional analysis of size effects upon many different mechanical properties and other different physical quantities, furthermore, the quantitative analysis can be carried out accurately.Six groups of micro-bridge beams specimens made of SCS (Single Crystal Silicon) with differentsizes were fabricated using photolithography technology, of which micro beams with trapezoidal section could be representative of those with rectangle and square section. Bending tests of specimens were carried out using nano indentation method, in which some important experimental data of load and displacement was obtained, and the measurement method used here could be applied to both elastic and plastic materials, and many mechanical properties, such as elastic modulus, hardness and bending strength, could be calculated simultaneously. Theoretical analysis shows that the average elastic modulus of single crystal silicon is 170.295±2.4850GPa, showing no size effects, but the bending strength ranges from 3.24GPa to 10.15GPa, displaying strong size effects, and the average hardness is 9.4967±1.7533 GPa, upon which no obvious size effects are observed. The Weibull analysis of bending strength of micro-bridge beams is made, and the fracture characterization of micro beams is also investigated using Griffith's theory, finally, the criterion of strain design is given, useful for reliability design of micro silicon devices.The integrated model of QFD/TRIZ/FUZZY/HQQ is founded based on QFD demand for product creative design, in which the material strength and deformation parameters are selected as primary technology contradiction for searching optimal size of micro beams structure in micro force sensor design for friction test. The theoretical knowledge of management discipline is applied to product design of MEMS in the method used, strongly showing multidisciplinary interaction characterization of MEMS.The residual stress in MEMS is investigated, in which the thermal mismatch and intrinsic stresses are discussed in detail. Some primary measurement methods and their basic work principles are compared, from which both advantage and disadvantage of each method are concluded. Finally, the release and control mechanism of residual stress is discussed, logically explaining composite toughening phenomenon of phase transformation materials. The research results are valuable enough to consider residual stress factor in design and application of MEMS products.