Electronic switching in phase-change memories Pirovano, A.; Lacaita, A.L.; Benvenuti, A. ...
IEEE transactions on electron devices,
03/2004, Letnik:
51, Številka:
3
Journal Article
Recenzirano
A detailed investigation of electronic switching in chalcogenide-based phase-change memory devices is presented. An original bandgap model consistent with the microscopic structure of both ...crystalline and amorphous chalcogenide is described, and a physical picture of the switching mechanism is proposed. Numerical simulations provide, for the first time, a quantitative description of the peculiar current-voltage curve of a Ge/sub 2/Sb/sub 2/Te/sub 5/ resistor, in good agreement with measurements performed on test devices.
A detailed investigation of the time evolution for the low-field resistance R/sub off/ and the threshold voltage V/sub th/ in chalcogenide-based phase-change memory devices is presented. It is ...observed that both R/sub off/ and V/sub th/ increase and become stable with time and temperature, thus improving the cell readout window. Relying on a microscopic model, the drift of R/sub off/ and V/sub th/ is linked to the dynamic of the intrinsic traps typical of amorphous chalcogenides, thus providing for the first time a unified framework for the comprehension of chalcogenide materials transient behavior.
In this paper, a 90-nm 128-Mcell non-volatile memory based on phase-change Ge 2 -Sb 2 -TeB alloy is presented. Memory cells are bipolar selected, and are based on a /xtrench architecture. ...Experimental investigation on multi-level cell (MLC) storage is addressed exploiting the chip MLC capability. To this end, a programming algorithm suitable for 2 bit/cell storage achieving tightly placed inner states (in terms of cell current or resistance) is proposed. Measurements showed the possibility of placing the required distinct cell current distributions, thus demonstrating the feasibility of the MLC phase-change memory (PCM) storage concept. Endurance tests were also carried out. Cumulative distribu tions after 2-bit/cell programming before cycling and after 100 k program cycles followed by 1 h/150 degC bake are presented. Experimental results on MLC endurance are also provided from a 180-nm 8-Mb PCM demonstrator with the same mutrench cell structure.
The threshold switching mechanism in amorphous chalcogenides is investigated, showing experimental data that once and for all demonstrate its electronic nature. The physical mechanisms responsible ...for the switching to the highly conductive state are discussed and the impact of cumulative read-out pulses is also investigated, showing that phase-change transitions induced by usual reading operations in phase-change memory cells are completely negligible.
The phase transformation in chalcogenide-based nonvolatile memories is studied by cell electrical characterization. The cell state (amorphous, crystalline, or mixed) is changed by applying electrical ...pulses, then the cell resistance R and the current-voltage characteristics are measured. From the analysis of the electrical parameters of the cell, we provide evidence for a stacked-like phase distribution in the active layer. Results are discussed with reference to the thermal profile during the program pulse in the chalcogenide layer.
We studied the programming dynamics in phase change memory cells. It is shown that programming in stand-alone cells is strongly affected by the parasitic capacitance in the measurement setup, leading ...to a current overshoot after threshold switching of the amorphous chalcogenide. This results in a parasitic melting and quenching of the active material, affecting the current distribution during program and the final phase distribution in the active material. The relevance of this artefact for real-device operation is discussed with reference to the value of the parasitic capacitance.
Single Event Effects in 90-nm Phase Change Memories Gerardin, S.; Bagatin, M.; Paccagnella, A. ...
IEEE transactions on nuclear science,
2011-Dec., 2011-12-00, 20111201, Letnik:
58, Številka:
6
Journal Article
Recenzirano
Single event effects are investigated in 90-nm phase change memories. The cells are shown to be insensitive to heavy-ion strikes and will likely remain so for a few more generations. Possible ...physical mechanisms leading to upsets in future generations are discussed. Errors and functional interrupts, in addition to single event latch-up, were observed during read, word and buffer program due to strikes in the peripheral circuitry.
Emerging phase-change memory (PCM) technology for non-volatile applications presents many potential advantages in terms of scalability, endurance and program/read speed. While several integration ...issues have still to be solved before achieving volume-production stage, the fundamental physics of chalcogenide switching and phase-change behaviour has still to be comprehensively understood. This paper provides an in-depth analysis of the switching and programming transient in PCM cells. It is shown that the cell parasitic capacitance can lead to a marked current overshoot in the programming transient. As evidenced by experiments, this overshoot is able to melt and quench the active material as in a reset operation. The parasitic reset results in a series distribution of crystalline and amorphous phases after program. The analysis of array cell capacitance instead indicates that no parasitic reset is to be expected, allowing for a localized crystallization during program, as previously obtained by numerical simulations.
We study proton and heavy ion irradiation effects on phase change memories (PCM) with MOSFET and BJT selectors and the effect of the irradiation on the retention characteristics of these devices. ...Proton irradiation produces noticeable variations in the cell distributions in PCM with MOSFET selectors mostly due to leakage currents affecting the transistors. PCM with BJT selectors show only small variations after proton irradiation. PCM cells do not appear to be impacted by heavy-ion irradiation. Using high temperature accelerated retention tests, we demonstrate that the retention capability of these memories is not compromised by the irradiation.
In order to validate phase change memory (PCM) technology, the programming reliability, in terms of reading window between the programmed and erased state, must be guaranteed at array level with an ...error less then 1 part-per-billion. The reset distribution is significantly influenced by the quenching operation Mantegazza D, Ielmini D, Pirovano A, Gleixner B, Lacaita A L, Varesi E, et al. Electrical characterization of anomalous cells in phase change memory arrays. IEDM Tech Dig 2006:53–56. In this paper this phenomenon is explained in terms of PCM active material crystallization statistics. A significant spread in the crystallization times among PCM cells is detected both in the write-reset operation from the melted-amorphous state (quenching) and in the erase-set operation from the solid-amorphous state. At statistical level, a correlation between set at high and low temperatures and quenching behavior of cells is found, allowing to describe the programming distributions uniquely in terms of crystallization times statistics.
