Addressing nanowire stress during mechanical characterization

Silicon nanowires are well suited for many applications, including transistors, resonators, pressure sensors, and accelerometers. To use them in these devices, researchers must first characterize their mechanical properties. This is typically done while the nanowires are suspended, with in situ loading tests using scanning electron microscopy (SEM).

However, the effects of the electron beam used in SEM are not fully understood. After exposing a variety of nanowires to an electron beam, Spejo et al. found the irradiation gradually charges the samples. This electrical charge generates electrostatic forces, putting additional mechanical stress on the nanowire that can induce fracture.

The nanowires in this work were fabricated on silicon-on-insulator wafer substrates. The insulating material between the nanowire and the substrate is responsible for the charging effect during electron beam exposure.

Nanowire samples that lack an insulating layer allow the incident charge to leave, avoiding additional stress. But in some applications an insulator is unavoidable. Transistors based on silicon-on-insulator wafer substrates, for example, must have their nanowires characterized on the same substrate as used in final fabrication.

To prevent additional mechanical stress in such applications, the researchers proposed making all regions of the nanostructure the same potential during sample preparation. This simple solution would reduce potential gradients to prevent electrostatic effects on the suspended nanowire during SEM.

“Our work reinforces the importance of proper sample preparation during nanomechanical tests to avoid electrostatic effects that might be harmful or even create hidden artifacts on nanowires during the SEM characterization,” said author Lucas Spejo.

Source: “In situ electron-beam-induced mechanical loading and fracture of suspended strained silicon nanowires,” by Lucas B. Spejo, José L. Arrieta-Concha, Marcos V. Puydinger dos Santos, Angélica D. Barros, José A. Diniz, and Renato A. Minamisawa, JVST: B (2023). The article can be accessed at https://doi.org/10.1116/6.0002246 .