MEMS

Microelectromechanical Systems (MEMS) and/or Nanoelectromechanical Systems (NEMS) is a newly rising interdisciplinary technology. The mechanical design of MEMS and/or NEMS is one of the frontiers of mechanical engineering. The traditionary mechanical design methods and theories can not be used in MEMS design due to surface effect and size effect. The up-to-date development of the study of MEMS has brought about varies kinds of micro-components. When these micro-components are used to build up a microelectromechanical system to obtain a definite function, the theory of MEMS design should be developed. There are some existing problems of the fundamental design issues of MEMS about microtribology, microthermodynamics, microhydrodynamics, micromechnics of materials and micromechanisms. These problems have heavily restricted the development of MEMS, and the fundamental design issues of MEMS should be studied as quickly as possible [14].

As the influences of surface forces on the micromachines are very large due to the surface effect with an increase in the ratio of surface area to volume and the small clearance between micro-components, the analysis of forces on micromachines is different from that on marcomachines. Some neglected influences in marcomachines should be considered in the design of micromachines. On the other hand, the change of material properties and mechanical behavior should be considered duo to the size effect on material strength. Obviously, the surface effect and the size effect are two key problems of many fundamental design issues to be studied in MEMS. These two kinds of effect affect each other and they can not be studied separately. So, the two representative problems about the understanding of the mechanical behavior of MEMS materials and the unwanted typical “sticking” phenomenon in MEMS should be theoretically and experimentally studied [18]. As discussed above, the study of these two relating problems are firstly brought forward in this dissertation and this dissertation takes up two key points among the existing problems in the development of MEMS, the mechanical behavior of MEMS materials and the stiction phenomenon in MEMS, for investigating theoretically and experimentally in detail. New micromechanical models and approaches are proposed. The specific works finished and main innovative contributions of this dissertation are as follows:



Firstly, a novel, direct and convenient method for micromechanical property measurements by beam bending using a nanoindenter has been developed, and a microtensile test device with a magnet-solenoid actuator and a fibre-optical displacement sensor has been designed and constructed to investigate the tensile mechanical properties of polysilicon films. Based on the investigation of mechanical properties of coating/substrate system by nanoindentation [30], the data obtained from nanoindentation are certified unsuitable to be used in the study of deformation of microcantilever because of the emergence of phase transformation under indentation [37]. Thus, a method for micromechanical property measurements by beam bending using a nanoindenter has been demonstrated [20]. A systematic analysis of the measurement principle and influences has been made [11]. In order to achieve the net deformation properties of a thin coating, both load-penetration depth and load-penetration depth squared plots have been demonstrated to be necessary if a more complete understanding of coating/substrate system behavior is to be gained [1], which has been suggested to be a rule to evaluate the mechanical properties of thin solid films[15]. In the meantime, in the deflection of microbeams, the influence of the indenter tip pushing into the top of the microbeams and the curvature across its width must be considered [21]. With these influences considered, the mechanical properties of microcantilever deformation can be obtained [23]. In order to investigate the mechanical properties of microstructures under purely tensile pulling procedures, the optimum configuration and geometry for a magnet-solenoid force actuator has been presented, enabling its application to the design of microtensile test device [16]. The electromagnetic force actuators have distinct advantages for applications involving very small displacements, which include linear operation, low hysteresis, no friction and direct electrical control. The magnet within a coil has a low stiffness relative to the mechanism and will readily be self-align to the primary translation axis and so all but eliminate crossaxis forces. This low stiffness coupling also reduces the transmission of mechanical vibration between the stage and the frame. The displacement of the sample is measured with two groups of optical fibers bundled together having a realizable accuracy of 50nm in stable thermal conditions. This microtensile test device is used to measure the mechanical properties of polysilicon films. During the test, the tensile stress linearly increases with the displacement of the precise stage, and the stress curve manifests no yield point until fracture. The specimen has no necking, and no plastic deformation is observed. The Young's modulus are obtained by calculating the slope of the stress-strain curve using a least-squares fit [22、4]. The Young’s modulus of polysilicon films are also calculated by either the isotropic model of Reuss-Voigt model, Shtrik-Hashin model or texture model. The bounds for the {110} in-plane elastic constants are modeled to be in good agreement with the results of the tensile experiments. It has been found that the measured average values of Young’s modulus by tensile tests falls within the theoretical bounds of texture model [31]. The test data accounts for the uncertainties of the reported Young’s modulus mean values obtained for polysilicon. This data may be used in the future reliability design of polysilicon MEMS.

Secondly, the size effect of the typical manufactured component on bending strength of polysilicon microcantilever beams and tensile strength of polysilicon thin films has been systematically investigated. Statistical analysis of bending strength [7] and tensile strength [8] for various specimen sizes predict the size effect of specimen length, width，cross-section area，surface area, and volume. Especially，the size effect of specimen width on tensile strength has been theoretically and experimentally studied. It shows that the result concluded by Toshiyuki tsuchiya, which was stated that statistical analysis of tensile strength predicted no size effect of specimen width, is incorrect. The Weibull distribution probability function is adopted to represent such tensile strength variations for polysilicon brittle materials [24]. Statistical analysis of the specimen size effects on the tensile strength predicts the size effects on the length, the surface area and the volume of the specimens due to microstructural and dimensional constrains. The fracture strength increases with the increase of the ratio of surface area to volume. In such cases the size effect can be traced back to the ratio of surface area to volume as the governing parameter [13]. The test data accounts for the uncertainties in mechanical properties and may be used in the future reliability design of polysilicon MEMS [26].

Thirdly, the rupture physics and crack origin of polysilicon thin films are analyzed by means of AFM, SEM and TEM [36]. The maximum-likelihood method is also applied to analyze the fracture origin of polysilicon films combining with the cross-section photo of fracture face [24] .The fracture behavior indicates that the fracture origin of polysilicon thin films is located on the surface of the specimens. The surface roughness is proved to be the main fracture origin of polysilicon thin films for the first time and the effect of surface roughness on tensile strength has been theoretically and experimentally studied, which can be expressed by fracture mechanics [38]. For brittle materials the strength is governed by the maximum flaw size, typically at the surface. Clearly the processing route and resulting surface roughness have a strong influence on strength. The valley is a stress concentration and a flaw at the surface. Furthermore, the mechanical properties of polysilicon thin films coated with OTS film are also investigated. It is found that tensile strengths of polysilicon thin films are enhanced greatly when they are coated with OTS films. It is clearly that not only the tribology properties and anti-stiction properties of microstructures can be improved but also the mechanical properties of microstructures can be enhanced. These results would work as guidance for the design of MEMS materials.

Lastly, base on the investigation of the mechanical properties of microstructures the influence of capillary forces on sticking of a surface micromachined polysilicon microcantilever in ambient environment or the rinse liquid and the influence of quantum mechanical effect such as the Casimir effect on sticking and stability of a micro polysilicon membrane strip cavity structure in vacuum are investigated. Mechanical stability and sticking are the troublesome problems in microfabrication and operation processes when separations of components in MEMS are in the sub-micrometer regime. Some mechanical effect, including quantum mechanical effect should be taken into account for solving the problems. Capillary forces will play a dominant role on sticking in ambient environment or the rinse liquid. On the other hand, the magnitude of Casimir force is significant when the membranes work in vacuum without the effect of capillary forces. The Casimir effect may play an important role if the separations are small enough [9]. The corrections of roughness, conductivity, and temperature to the Casimir force must be considered for accurate account of their effects [32]. With nothing other than the Casimir force loading the strip, there exist a stable static equilibrium state and an unstable static equilibrium state, depending on the value of a dimensionless constant K [2、3]. The membrane strip will collapse if the value of K is larger than the critical value. Thus, the influence of capillary forces and the Casimir effect on sticking and stability of microstructures are investigated [6]. The study on the design of anti-sticking structures under different forces shows that sticking and stability of microcantilevers and micro membrane strip cavities has something to do with Young’s modulus of materials, surface properties, length of structures, thickness of structures and separation between the fixed surface and the deflecting component. But, it is independent of width of structures [39]. A map of the size design of anti-sticking structures has been proposed for the first time, which provides a way to check if a system with given dimensions and material properties will be in a stable equilibrium [34]. These results are expected to be useful for the design of MEMS, and of significance for establishing a framework of the design theory of MEMS.