The analysis and structural design of micro SOI pressure sensors

Abstract

j'square (max) 0.308pa 2 (1) h 2 (j' circu la r (m ax) = 3 pr 2 (2) 4""h"2 II. STRUCTURE DESIGN In the pressure sensors, a square diaphragm is used because of its higher sensitivity compared to a circular diaphragm with corresponding thickness h . A measure for the sensitivity of a pressure sensor is the maximum mechanical stress at the edge of the diaphragm. The maximum mechanical stress of a square and a circular diaphragm is given by the following equations respectively:Where (j'square(max) is the maximum mechanical stress for a square diaphragm; (j'circular(max) is the maximum mechanical stress for a circular diaphragm; p represents the applied pressure; h is the diaphragm thickness; a is the diaphragm width/length for square and r is the diaphragm radius for circular respectively.When the parameters of square and circular diaphragm are same thickness and width (a =2 r ), there is the relationship between them under same pressure input.This shows that the maximum stress at the edge of a square diaphragm is considerably higher than at the edge of a circular diaphragm, thus yielding a pressure sensitivity which is approximately 60% higher.The structure of pressure sensor is shown in Fig 1. The pressure sensor is composed of a square diaphragm and a deep cavity with 54.7°inclined wall. The square diaphragm acts as the sensing component that senses the measured pressure directly. The applied pressure is transformed to the strain and the stress of the square diaphragm. A vacuum condition in the sensor becomes a necessity for the measurement of absolute pressure. This structure is obtained by the way of anodic bonding with Pyrex glass in vacuum condition. The size of structure can be determined as TABLE I . It was utilized the anisotropy characteristic along different orientation of single crystal silicon. The piezoresisitance characteristic was used to (3) (j' square(max) = 1.64 (j' circular(max) Abstract -A kind of micro piezoresistive pressure sensor with stable performances under high temperature is designed based on the silicon on insulator (SOl). Through analyzing the stress distribution of diaphragm by finite element method (FEM), the model of structure was established. The fabrication operated on SOl wafer, which can be used in extreme high temperature environments, and applied the technology of anisotropy chemical etching. Performances of this kind of SOl piezoresistive sensor, including size, sensitivity, and temperature were investigated. The result shows that the precision is O.57 % FS. Therefore, this kind of design not only has smaller size, simplicity preparation but also has high sensitivity, temperature coefficient and accuracy.

Design and fabrication of a tiny micro-pressure sensor with improved linearity and sensitivity

“…The sensor structure is integrated into a (100) substrate of N-type SOI wafer, which includes a square pressure-sensitive diaphragm composed of monocrystalline silicon, silicon dioxide barrier layer and piezoresistors for electro-mechanical conversion. According to elastic theory, under the condition of the same size, the maximum stress of square diaphragm is 1.64 times that of circular diaphragm, and greater stress is conducive to improving the sensitivity of pressure sensor [17]. Two piezoresistors in the maximum stress zone of the edge of the square diaphragm are optimized, which match with two resistors to form a half active Wheatstone bridge.…”