A new electrical residual stress characterization using bent beam actuators

In this paper we propose a new class of sensitive and compact passive stress measurement pattern that uses the piezoresistivity of polysilicon. The proposed measurement pattern utilizes the resistance change of the beam that connects the structures moving in opposite directions. The unit structure is composed of a pair of bent beam actuators with apexes at their mid-points connected by a link beam. The bent beam actuators are 15 μm wide and 200 μm long, and are bent by 0.1 rad (0.7◦). The link beam is 5 μm wide and 264 μm long. The unit structure composed of bent beam actuators amplifies and transforms deformations caused by residual stress into elongation or contraction of the linking beam. Hence, the residual stress can be evaluated by measuring the resistance change of the proposed pattern. It is shown that tensile and compressive residual stress levels of about 10 MPa, corresponding to strains below 6 × 10−5, can be measured by using a 40 μm thick test pattern of polysilicon. The sensitivity of the proposed test pattern is 0.83% for the stress of 10 MPa. Additionally, electrical measurements allow this pattern to be suitable for post-packaging stress monitoring. (Some figures in this article are in colour only in the electronic version)