Numerical study on the influence of buried oxide layer of SOI wafers on the terminal characteristics of a micro/nano cantilever biosensor with an integrated piezoresistor

Abstract

Piezoresistive micro/nano cantilever sensors have been extensively utilized in the detection and quantification of miniscule forces generated due to the physical interaction of biomolecules. Due to the ultra-sensitive nature of piezoresistive cantilever sensors, they have found numerous applications, especially in the development of point-of-care testing systems for healthcare applications. Over the years, silicon-on-insulator (SOI) wafers have been widely utilized to realize silicon based piezoresistive cantilever sensors due to their performance and fabrication related advantages. Treatise encompasses examples where researchers have retained the buried oxide (BOX) layer of SOI wafers to realize piezoresistive cantilever devices. In this paper, we investigate the impact of BOX layer of SOI wafers on the performance of composite piezoresistive cantilever biosensors. Typically, the response of piezoresistive cantilever sensors is evaluated by considering only the electro-mechanical response. However, the design of such sensors is a multi-variant problem due to (i) their composite structure and (ii) interdependent nature of the electrical, mechanical and thermal characteristics. Therefore, in the present study, we devise a new figure of merit (FoM) defined as the product of sensitivity ratio and the square of resonant frequency of the sensor. The sensitivity ratio represents the ratio of the electrical sensitivity due to surface stress and thermal sensitivity of the sensor. The proposed FoM takes into account the interplay between the electrical, mechanical and thermal response of the sensor. Finite element method based numerical simulations are performed to model and investigate the influence of BOX layer on the thermo-electro-mechanical response of a square shaped silicon piezoresistive cantilever sensor. Simulation results show that the presence of the supporting BOX layer enhances the mechanical stability i.e. spring constant and resonant frequency of the sensor. In addition, it is found that the BOX layer reduces the zero bias deflection of the cantilever due to difference in TCE of the constituent layer materials. Furthermore, it is shown that by dimensional optimization of the thicknesses of the constituent layers, sensitivity ratio of the sensor with BOX layer can be improved up to 2.96.