Resistive memory for Space applications - a Radiation-Hardening By Design Approach for Non-volatile Memories

Emerging memories have been investigated for years, but, to date, they occupy a marginal place in the memory applications scenario. NAND Flash memories are still the undisputed winner in the market competition for their low “cost per bit”. However, in the scope of the space applications, the major requirement is not the cost, but a high robustness against radiation effects and emerging memories gain an important position in that field, as they turn out to be really profitable for those applications. Their physical storage mechanism is intrinsic robust to radiations, while the periphery circuitry does not need charge pumping, so that normal radiation hardening by layout (RHBL) techniques can be really effective. Hence, it seems highly profitable for space applications to consider a radiation-hardened design of an oxide resistive memory. This work describes the radiation hardened design of a stand alone, nonvolatile memory device implemented with oxide resistive devices. In Chapter 1 and Chapter 2 the main physical phenomena related to the space environment and their effects on silicon devices are described and discussed, dealing with the current physical models and definitions involved. Chapter 3 is entirely dedicated to the radiation effects on memories, showing problems related to the different technologies and architectures. Chapter 4 offers a full overview on emerging technologies focusing the attention on the resistive oxide RAMs describing the main physico-chemical processes involved in the resistance change. Moreover, an approximated behavioral model suited for memory applications is proposed and compared to the actual electric and physical models presented in literature. The main radiation hardening by design (RHBD) techniques are treated in Chapter 5, while Chapter 6 shows how these techniques have been implemented in the design of the non-volatile resistive device. Some peculiar circuit and design solutions are presented. In Chapter 7 are described the radiation tests carried out on the devices: destructive failure tests under Xe heavy ions and protons, total ionizing dose (TID) tests under Co-60 have been conducted in the facilities of the University of Jyväskylä, Finland, in order to characterize the devices under radiation to check the effectiveness of the design solutions. Then, some single event effect (SEE) phenomena have been noticed and a physical interpretation has been proposed. In Chapter 8 some conclusions and technical considerations are, finally, reported.