Fundamental Research On Design And Fabrication Of Silicon Microball Bearings

The microball bearing is considered as a best mechanical support for MEMS owing to its lower friction over planar contact bearing, increased stability over noncontact bearing and easy integration with rotary MEMS like micro-engine, micro-motor and micro-bump. However, the friction and wear of bearings can be easily produced by the direct contact between the microballs and the raceway due to the problems with the structure design and the manufacturing process of microball bearings, which influence the bearings’ reliability and lifetime and make it impossible for the bearing to support robustly. The main contents of this thesis are as follows.A design scheme of the structure of the microball bearing is proposed according to the wear-prone location of the microball bearing’s raceway and the difficulty of clamping and aligning silicon wafer encountered in the bond process, in which the structure of the raceway, the number of the rolling elements and the pathway of the bearing chamber of the microball bearing are investigated emphatically. This design decreases the friction and wear caused by the contact between the microball and the bond interface and also reduces the possibility of that the microballs will tend to go out of the raceway because the low-weight microballs are susceptible to vibration in the bond process.The contact characteristics of the microball bearing which has a raceway coated with a hard film are studied theoretically. The effects of the elastic modulus and the thickness of the films as well as the material and the diameter of the microball on the contact between the microball and the raceway in the microball bearing are inspected. The results show that higher elastic modulus and thicker film can reduce the maximum radial tensile stress along the raceway surface and the maximum shear stress along the film/substrate interface, but increase the maximum contact pressure. The maximum von Mises stress along the axial direction and the stress gradient along the film/substrate interface in the bearing can be reduced by decreasing the elastic modulus or increasing the thickness of the film. Compared with the Si3N4 and ZrO2 microball, the 440 C stainless steel microball can improve the contact characteristics of the microball bearing. For the microball bearing with same material micrlballs, the larger diameters of microballs are, the better contact characteristics of the bearing is. The radial stress of the film surface exhibits the tensile stress at the leading edge of the contact area, and then transforms into the compressive stress along the direction of the friction force under larger friction coefficients. As the friction coefficient increases, the maximum von Mises stress increases and its axial position moves to the surface of the raceway. What’s more, the radial tensile and compressive stresses suffered by the surface are increasing gradually with the increase of friction coefficient.As for the difficulty in microfabrication of the microball bearing with various characteristic dimensions, the process flow which employs Halo Etch and Nest Etch methods is presented. It improves the uniformity of etch rate and the precision of patterns, and thus reduces the imbalance of rotor and increases the stability of microball bearing at high speed. Moreover, to encapsulate the bearing chamber, the process flow which combines the Intermediate-Layer Bonds with Shadow Mask technology is presented. It reduces the thermal deformation caused by the extremely high temperature and avoids the clogging of bearing chamber and the ball-raceway adhesion caused by the deposited intermediate materials.The package design is proposed for the microball bearing being designed and microfabricated in this study, and the micro rolling friction measurement system is developed as well. The real-time measurement of the rotational speed, the angular displacement, the normal force, the supply pressure and the mass flow rate of the microball bearing are realized, and thus the quantitative relationship between the normal force and the friction torque are obtained. The three-dimensional profiler, the scanning electron microscope and the energy dispersive spectrometer are adopted to analyze the wear of the microball bearing’s raceway. The results show that the main reason of the microball bearing wear includs two points. The contact area between the microballs and the raceway coated with a SiC film suffers from a high stress at high instantaneous temperatures, so that the adhesive wear is produced.