Effects Of Phosphorus Doping On The Fatigue Properties Of Single Crystal Silicon Micro-structure

Silicon is one of the most widely used materials for micron-scale structures ofMicroelectromechanical Systems (MEMS). The fatigue properties of silicon are ofparticular importance for the reliability of silicon-based MEMS. It is commonly appliedthat studying fatigue properties of silicon through fatigue experiments, which, however,hasn’t worked as well as expected, owing to the abundance of factors that affect thefatigue properties. The fatigue properties of silicon have been studied for two decades.Great development has been made in some sub-domains such as fatigue testing, fatiguelifetime modeling and fatigue mechanism. But there is no extensively accepted theoryfor silicon fatigue. This dissertation, in another angle of view, aims to investigate theeffects of the single factor, i.e. phosphorus doping, on the fatigue properties of singlecrystal silicon (SCS). The main work and significant results in this dissertation are asfollows:1. According to the characteristics of anisotropic wet etching, an original andeconomical micro-structure is designed. Its dimensions are determined with FEA. Sixgroups of test samples of different doping concentration are manufactured by bulkmicromachining. An off-chip bending test system is designed, whose performance,through adjusting and optimizing, can meet the testing demand in this work.2. The effect of phosphorus doping upon the bending strength of SCS isinvestigated based on the bending strength test data. It is found that phosphorus, insome extent, can enhance the strength of SCS. Furthermore, the mechanism probably isthat phosphorus doping can increase the difficulty of the fracture of some atom bondsand the energy consumed for creating new surfaces.3. The effect of phosphorus doping upon the fatigue properties of SCS isinvestigated. Firstly, according to the strength information of each group, fatigue testsare properly performed. Secondly, the probability model of fatigue fracture is abtainedby mathematical deduction. Thirdly and the most importantly, the material constants Cand n are achieved by fitting the probability model and fatigue test data. By analyzingthe tendency of C and n along with doping concentration, we find that phosphorus canevidently improve the fatigue properties of SCS. The mechanism is that phosphorusdoping can hinder the growth and propagation of fatigue cracks.From the research above, we know how phosphorus doping affects the bendingstrength and fatigue properties of SCS, which embodys the significance for reliabilitydesign of SCS-based MEMS.