Thesis title: Emerging non-volatile memories: new proposals and existing technologies engineering
Reliability and scaling are the main challenges in Non-Volatile Memories (NVM). Although one-transistor (1Tr) memory cell is a well-established technology in embedded Flash (eFlash) memories, alternative solutions are spreading in the NVM market. Goals of the present thesis are to investigate existing NVM technologies, alternative to 1Tr, and to examine new proposals for the implementation of new NVM strategies.
The work can be divided in two parts, both concerning possible solutions in the NVM field of application. The first part deals with the optimization of standard operations as read and, especially, program and erase in terms of efficiency, speed and reliability in Embedded SuperFlash (ESF) technology. Preliminary simulations of the ESF cell are carried out in order to extract key parameters for the following test-on-chip phase. Specifically, electrostatic dielectric stresses on ESF cell target areas were speculated and the influence of specific electrical waveforms in read, program and erase operations are studied. Simulations results are then validated on actual ESF cells, paving the way to specific algorithms implementation. Erase and program operations are then deeply investigated, looking for effective solutions in terms of efficiency, always taking into account reliability aspects. Thermal impact on elected algorithms are also explored, proving their efficacy at different operative
temperatures. Moreover, algorithms influence on variability over the whole ESF array is quantified. Finally, algorithm performances for an elevated number of program/erase cycles are tested, validating the effectiveness of the proposed solutions. The second part concerns the analysis of a new nanostructured material suitable for NVM applications. Such a challenging research is most likely compensated by the potential outcomes connected to future technological developments. Preliminary inspections of Microwave (MW) synthesized Silicon Nanowires (SiNWs) suggested
that a switching resistive structures for crystallographic phase change memories is possible. In particular, presence of an exotic silicon phase (Si-BC8) in SiNWs upon MW treatment was predicated on previous works from Sapienza research group. Hence, further investigations are performed on the MW-treated SiNWs. Morphological, elemental and structural characterizations are carried out in order to validate the hypothesis of Si-BC8 formation in the SiNWs. Template fabrication for SiNWs vertical growth is also realized. Optimization of the MW-based synthetic procedure is accomplished. Manufactured material band structure is inspected by optical characterization, which reveals its actual optoelectronic properties. Metal catalyst impact on the synthetic route is explored by inspecting prepared samples with different metal species. Compositional and chemical state sample analyses are performed to clarify the mechanism underneath the sample structural and optical modifications. Identification of phase modifications inside tin nanoparticles rather than in silicon is attained, hindering potential NVM applications. Finally, actual applications of the resulting synthesized material are evaluated on the basis of its identified properties.