Stress Investigation Of Silicon Nanomembranes In Bending Flexible Thin Film P-intrinsic-n Diodes On Plastic Substrates

High performance flexible electronics with extreme mechanical properties(e.g.bendability)have drawn great attention in the past few years with their unique advantages,such as high flexibility,light weight,resistance to impact.Since the low intrinsic carrier mobility of the materials used in flexible electronics,the research of the flexible electronics mainly stays in the low-speed field.However,with the realization of transferable monocrystalline silicon nanomembrane(SiNM),flexible electronics demonstrate great potentials for high speed and high performance applications such as portable wireless devices,long-distance RFIDs,optoelectronics,steerable antennas etc.High-speed flexible devices are required to work in the repeated deformation conditions.So the stress will inevitably have an impact on the electrical properties of devices,especially in high-speed high-frequency applications,the parasitic effect is more obvious.Therefore,the study of the mechanical properties of the flexible devices which need to commonly operate under various mechanical conditions is essential.In this work,we introduce the complete fabrication process of the flexible devices,and present the radio frequency(RF)characterization of flexible microwave single-crystalline SiNM P-type-Intrinsic-N-type(PIN)diodes with different bending radii.An rf strain equivalent circuit model is developed to analyze the underlying mechanism and reveals device parameters change with bending radii.Focusing on the stress distribution on such devices with various structures and convex bending radii,we have explained how the mechanical bending strain affects the performance of flexible devices.PIN diodes(diode areas:40,80,and 240?m~2)and SPST switches[series/shunt diodes(D1/D2)areas:240/40?m~2 and 40/40?m~2]have been used for analysis in this paper.A 3-D model is developed employing the Finite Element Method(FEM)to help us explore the stress distribution on the devices under bending conditions.The single crystal Si thin films are the important objects of the stress distribution study.The Von Mises equivalent stress data of the cross-sections of Si films have been obtained,as well as the three principal stresses in X,Y and Z directions.The tensile stress along the sample length direction(X direction in the finite element model)which is the main component of the equivalent stress has been studied in details.