Fabio Pellizzer

Electronic switching in phase-change memories

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.

Reliability study of phase-change nonvolatile memories

A detailed investigation of the reliability aspects in nonvolatile phase-change memories (PCM) is presented, covering the basic aspects related to high density array NVM, i.e., data retention, endurance, program and read disturbs. The data retention capabilities and the endurance characteristics of single PCM cells are analyzed, showing that data can be stored for 10 years at 110/spl deg/C and that a resistance difference of two order of magnitude between the cell states can be maintained for more than 10/sup 11/ programming cycles. The main mechanisms responsible for instabilities just before failure as well as for final device breakdown are also discussed. Finally, the impact of read and program disturbs are clearly assessed, showing with experimental data and simulated results that the crystallization induced during the cell read out and the thermal cross-talk due to adjacent bits programming do not affect the retention capabilities of the PCM cells.

Low-field amorphous state resistance and threshold voltage drift in chalcogenide materials

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.

Analysis of phase distribution in phase-change nonvolatile memories

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.

4-Mb MOSFET-selected /spl mu/trench phase-change memory experimental chip

A /spl mu/trench Phase-Change Memory (PCM) cell with MOSFET selector and its integration in a 4-Mb experimental chip fabricated in 0.18-/spl mu/m CMOS technology are presented. A cascode bitline biasing scheme allows read and write voltages to be fed to the addressed storage elements with the required accuracy. The high-performance capabilities of PCM cells were experimentally investigated. A read access time of 45 ns was measured together with a write throughput of 5 MB/s, which represents an improved performance as compared to NOR Flash memories. Programmed cell current distributions on the 4-Mb array demonstrate an adequate working window and, together with first endurance measurements, assess the feasibility of PCMs in standard CMOS technology with few additional process modules.

Internal Temperature Extraction in Phase-Change Memory Cells During the Reset Operation

The phase-change memory technology is based on a chalcogenide compound able to reversibly switch between two stable states, namely, an amorphous high-resistive state and a crystalline low-resistive one, enabling the storage of the logical bit. Such phase transition is made by electrical pulses delivered to the memory cell, able to force a temperature raise within the material and to allow the temperature conditions required for the phase change. The cell internal temperature needs accurate control, and the evaluation of the thermal resistance of the memory cell represents a milestone to develop thermally efficient cell architectures and to gain insights into the thermal properties of the phase-change material. An experimental method for cell internal temperature evaluation has been developed and then supported by the electrothermal simulation of the cell behavior during the program operation, allowing for scaling predictions.

Electrical and Thermal Behavior of Tellurium Poor GeSbTe Compounds for Phase Change Memory

The phase change active material exploration represents an important stage in order to further strengthen the know-how on the Phase Change Memory (PCM) technology. This work reports a path for PCM material exploration toward the tellurium poor region of the GeSbTe (GST) ternary compound system. Data retention enhancement is reported and associated to a factor 1.5 increase of the crystallization activation energy. A detailed description and discussion of the remarkable electrical and thermal parameters of the PCM cell as a function of GST composition is provided.

“Phase-Change Memories for nano-scale technology and design”

In this paper we will review the evolution of Phase-Change Memories (PCM) through the last decade, starting from the first electrical results on single cells and ending with the latest news of multi-Gb chips. Entering into the sub-30nm realm, PCM is demonstrating the capability to enter the broad memory market and to become a mainstream technology.

Non-Volatile semiconductor memories for nano-scale technology

In this paper we will review the evolution of Non-Volatile Memories (NVM) through the last decades, driven by a continuous introduction of new materials that helped the reduction of the single cell area and so the cost of the memory. As we entered into the sub-20nm realm, limitations are appearing for Flash memories and alternative physical mechanisms are proposed to store information in high density media. In this scenario, Phase Change Memory (PCM) is demonstrating the capability to enter the broad memory market and to become a mainstream technology.

Phase-Change Memory Device Architecture

In this chapter we review the main categories of device architectures that have been studied and realized in order to exploit the phase-change mechanism in electronic devices.