Measurement of fracture stress, young's modulus, and intrinsic stress of heavily boron-doped silicon microstructures

Abstract

Heavily boron-doped silicon microstructures fabricated using deep-boron diffusions and boron etch-stops have been widely used in a variety of integrated sensors and actuators. For many applications, having knowledge of the mechanical properties such as Young's modulus, intrinsic stress, and fracture stress of these films is very important in predicting the response parameters of the sensors and actuators that utilize them. These parameters include mechanical resonant frequency, sensitivity, bandwidth, linearity, and operating range. This paper describes the measurement of fracture stress, Young's modulus, and intrinsic stress of boron-doped silicon microstructures at doping concentrations above 5 x 1019 cm-3. The measurement of fracture stress is performed using 15 microm thick cantilever beams of widths ranging from 20 to 150 microm. The beams were bent to fracture and the maximum fracture stress was measured to be [approximate] 1.8 x 1010 dyne cm-2 which is a factor of about six higher than silicon structures with larger dimensions (bulk silicon). Young's modulus and intrinsic stress were measured using a novel custom-designed doubly supported beam (bridge) structure. The measurement technique uses the characteristic pull-in voltage of the beam as electrostatic voltage is applied across an air gap capacitor in the middle of the beam, which causes the bridge to collapse. The Young's modulus for (110)-oriented silicon was measured to be [approximate] (2-2.2) x 1012 dyne cm-2 which is 20%-30% higher than undoped silicon. The measured intrinsic stress of 1.83 x 108 dyne cm-2 agrees well with the measured pressure-deflection characteristics of thin diaphragms.