Feasibility study of photo thermal acoustic imaging of buried nanowires in Gate-All-Around (GAA) nanowire FETs

With the ongoing drive to integrate more functionality and processing power on the same semiconductor area, the device structures have become 3D. Such structures, like FinFETs and their successors Gate-All-Around (GAA) nanowire FETs, bring on new challenges to measure their geometry and material properties non-destructively at the nanometer scale. Photo Thermal Acoustic Imaging (PTAI) is a potential metrology method. Here, the sample is heated locally by a pulsed optical pump beam. This generates a propagating elastic wave in the sample, which is scattered by inclusions in the sample. Part of the scattered energy travels to the sample surface, where the surface deformations are sensed by an optical probe beam using interferometric detection. Thus, no physical contact for actuation or sensing is required. Moreover, PTAI allows for metrology through optically opaque layers. This work investigates the feasibility of PTAI for the detection and imaging of nanowires. In PTAI a pulsed pump beam locally heated up a volume of the material — the absorption of optical energy and its conversion into heat were calculated analytically. The temperature distribution generated by the heat, the creation of a stress wave due to thermal expansion and the propagation and scattering of the resulting elastic waves were calculated using finite element modeling. The probe beam was modeled by spatially averaging the sample surface displacement. The datasets were imaged using a wavenumber frequency domain mapping. The simulation results showed that the detection of nanowires should be feasible using PTAI, however the imaging of nanowires is limited by the acoustic frequency and optical probe beam width.