Non-uniformities in MOSFET-array characteristics caused by probe-induced mechanical stress

Abstract

We study the impact of probe-induced mechanical stress on the characteristics of MOSFETs in transistor arrays by measuring our samples with different probe overdrive, and, thus, different amounts of probe-induced mechanical stress. We demonstrate that, despite the relatively high thickness of the back-end-of line (BEOL) layers of the sample (~10 μm), the mechanical stress can propagate from the pads, through the BEOL and down to the test transistors. This vertical compressive stress enhances the ON-state characteristics of the NFETs located below the probing pads, whereas it degrades those of the PFETs located at the same location. On the other hand, the transistors that are not below the probing pads remain unaffected. We perform finite element simulation of the propagation of mechanical stress through the BEOL layers, which corroborates that the apparent spatial non-uniformity of the transistor characteristics within the array can be attributed to probe-induced mechanical stress. These results highlight the importance of accounting for the impact of probe-induced mechanical stress even when measuring samples with a considerably thick BEOL. Finally, we propose the possibility of exploiting the impact of probe-induced mechanical stress on MOSFET characteristics as a detection mechanism versus probing attacks in the context of hardware security.