The development of a radiation hard non‐volatile memory technology by using standard CMOS silicon processing was the main goal of this project. Since standard silicon memories, such as flash memories tend to fail under irradiation, a new approach is envisaged: the development of a specific memory technology, so called resistive random‐access memory (RRAM), which is able to sustain heavy ions and other charged particles. The radiation-induced leakage effects were minimized by using edge-less transistors (ELT). The non-volatile 1 Mbit memory chip was fabricated in IHP’s 250nm CMOS process line; the total area occupancy is 64 mm2. The 1T-1R memory cells are constituted by a select ELT transistor, which also sets the current compliance, whose drain is in series to a Metal-Insulator- Metal (MIM) stack. The MIM cell integrated on the metal line 2 of the CMOS process is a TiN/HfO2/Ti/TiN stack. The MIM cells area is 700 x 700 nm2. The relevant electrical parameters were defined for the RRAM memory and monitored during reliability characterization. The reliability assessment was carried out through a specific set of measurements performed either with a dedicated testing instrument called RIFLE (Research Instrument for FLash Evaluation) for the array testing. Array testing allows obtaining large statistical information about writing/reading behaviors of the memory.
The 1Mbit test vehicle was the result of the ensemble of eight 128kbit modules each having its own decoding scheme, ATD (Address Transition Detection) and sense amplifier. The test vehicle allowed a systematic characterization of the technology before, during and after the exposure to radiation. TID, proton and heavy ion characterization was performed by using a protected mother board and a daughter board hosting the 1Mbit test vehicle. The total ionizing dose (TID) can result in device failures. In TID irradiation campaign, the dose was ramped up until 500 krad, without any loss of data. Two additional types of radiation have been used to evaluate the radiation hardness: Xenon ions with 1200 MeV energy and protons with 52 MeV energy. Caused by the high fluence of 1011 cm-2 at 52 MeV (protons) and 106 cm-2 at 1200 MeV (Xenon) for 9 respectively 15 min, the LRS states of all cells were switched to HRS. After the readout procedure, the LRS states could be recovered completely by a standard set process. During the proton and heavy ion irradiation campaign, no ruptures have been observed in the memory circuitry.
