Integrated GHz Scanning Acoustic Microscope for 3D nano-imaging
Microelectronics is on the verge of disappearing as we are fully entering the nanoelectronics era. This trend sees also the end of, or at least a departure from, Moore's law in the sense that the typical 2D scaling of transistor technologies is slowing down. Novel approaches however emerge to extend a comparable scaling paradigm, like the introduction of 3D structures or the definitions of complex feature devices instead of general-purpose transistors (e.g., FINFETS, MRAMs...). Along with these new approaches come the introduction of complex and hybrid processes involving new materials and stacks that require novel metrology and characterization techniques to ensure their manufacturability and success. In particular, modern nanoelectronics requires the development of novel 3D imaging techniques taking advantage of alternative transparency windows to peer through otherwise-reflecting or absorptive layers. In that perspective, extending imaging from the visible to the near infrared is tempting but comes with the disadvantage of the use of longer wavelengths and principle loss of resolution. An alternative strategy is required that relies on another type of wave, itself characteristic of another physical principle. Ultra- or hyper-sound imaging, based on the use of a rich variety of acoustic waves, e.g. shear or longitudinal, offers splendid opportunities. On the one hand, the sound velocity is typically 5 orders of magnitude smaller than the speed of light so that one can image using acoustic nanometric waves at GHz frequencies, typical of mm-wave RaDARs nowadays widely used. On the other hand, as acoustic (hypersound) waves are supported by and propagate through solids and fluids, they address totally different transparency windows than electromagnetic waves, and are even compatible with immersion lithography. These considerations lead to the interest in developing GHz scanning acoustic microscopes (SAM). GHZ SAM systems exist but are bulky and expensive. Originally developed for imaging biological tissues, they remain difficult to access. The target of this PhD is to develop a novel generation of these systems, using ultrasound transducers developed at imec and co-integrated with CMOS in a SoDAR architecture. To that purpose, various acoustic waves and imaging strategies need to be modelled and assessed. The final goal of the PhD is to determine the ultimate limits of integrated multi-pixels GHz SAM, in terms of resolution and penetration depth.