Sandrine Lhostis

Electrical Behavior of Phase-Change Memory Cells Based on GeTe

In this letter, we present a study on the electrical behavior of phase-change memories (PCMs) based on a GeTe active material. GeTe PCMs show, first, extremely rapid SET operation (yielding a gain of more than one decade in energy per bit with respect to standard GST PCMs), second, robust cycling, up to 1 × 105, with 30-ns SET and RESET stress time, and third, a better retention behavior at high temperature with respect to GST PCMs. These results, obtained on single cells, suggest GeTe as a promising alternative material to standard GST to improve PCM performance and reliability.

Evidence of Germanium precipitation in phase-change Ge1−xTex thin films by Raman scattering

In situ annealing Raman scattering is used to evidence germanium postcrystallization in the crystalline phase of Ge-rich phase-change GeTe thin films. Both reflectivity and Raman scattering show that the crystallization temperature of the as-deposited amorphous phase increases with increasing Ge content going from Tc=180 °C (Ge0.5Te0.5) to 360 °C for Ge-rich Ge0.76Te0.24. The crystallized phase adopts the rhombohedral α-GeTe phase structure, and whatever the starting composition. For Ge-rich GeTe we observe a second characteristic temperature around 375 °C, which signs the crystallization of a precipitated cubic Ge phase and thus the presence of two distinct phases.

Comparative Assessment of GST and GeTe Materials for Application to Embedded Phase-Change Memory Devices

This work presents a thorough comparative assessment of undoped GST and GeTe active phase-change (PC) materials for application to embedded memory devices (in particular consumer and automotive products). The material screening and qualification is performed through optical reflectivity and 4-probes resistivity measurements. Electrical performances are then investigated through tests of lance-cell analytical PC memory cells. Reset current densities of GST and GeTe are comparable, while GeTe data-retention at high- temperature is significantly improved compared to GST, suggesting that GeTe-based compounds are promising candidates for embedded PC memory applications.

Chemical interface analysis of as grown HfO2 ultrathin films on SiO2

The quality of the interface between a HfO2 high-k gate dielectric and the Si substrate directly influences its electrical properties. The chemical composition of the interfacial region of HfO2 deposited on a SiO2∕Si(100) substrate by pulsed liquid injection metal organic chemical vapor deposition at 430 and 550°C was investigated by medium energy ion scattering, angular resolved x-ray photoemission spectroscopy analysis, and high resolution transmission electron microscopy. It is shown that the HfO2∕SiO2 interface is abrupt with low roughness and no silicate. The interface roughness with SiO2 is found to be close to that generally measured in silicon technology (silicon oxide above silicon substrates) [E. A. Irene, Solid-State Electron., 45, 1207 (2001)]. The analysis of the experimental results indicates that the deposition technique does not lead to the formation of an extended silicate layer at the HfO2∕SiO2 interface.

Angular and polarization properties of cross-holes nanostructured metallic filters

It has been shown in literature that cross-shaped holes arrays can be made insensitive to polarization at normal incidence, and can even feature good stability for off-normal incidence. In this work we look for the optimal design rules to obtain high spectral stability conditions in the visible for those structures, through a complete review of all geometrical parameters using CMOS-compatible materials. Rigorous Coupled Wave Analysis (RCWA) simulations have been used to identify the most-impacting parameters and to determine typical ranges allowing for the realization of low-color errors image sensors whatever the light incidence. It appears that the two main parameters are the ratio of the arm width to the arm length of the crosses and the distance between crosses, which both have to be low to ensure stable responses of the filters. We demonstrate the results with CIE chromaticity diagrams reporting the responses of a RGB filter designed with the established rules under various illumination conditions.

Reliable 300 mm Wafer Level Hybrid Bonding for 3D Stacked CMOS Image Sensors

3D Stacked Image sensor is the stacking of a Back-Side Illuminated (BSI) CMOS Image Sensor on a logic die. It enables compact size, higher performances and additional functionalities compared to standard BSI sensors. The highest footprint reduction is obtained with 3D hybrid bonding with metal interconnects between top and bottom tiers. Hybrid bonding process with oxide / copper direct bonding allows the highest scalability of interconnect pitch. In this study we present the morphological and electrical characterizations of a test vehicle. The hybrid bonding of wafers from two different technology nodes is performed using a dual damascene integration for the hybrid bonding level. The main parameters to assess the bonding interface quality are analyzed such as the influence of the pad design, the impact of reworkability and wafer -- to-wafer overlays. The process robustness is studied through reliability tests and electromigration measurements.

On Carbon doping to improve GeTe-based Phase-Change Memory data retention at high temperature

This paper investigates material and electrical properties of a new chalcogenide alloy for Phase-Change Memories (PCM): Carbon-doped GeTe (named GeTeC). First, several physico-chemical, optical and electrical analyses have been performed on full-sheet chalcogenide depositions in order to understand the intrinsic GeTeC phase-change behavior, and to characterize structure and composition of amorphous and crystalline states. Then, GeTeC with two different Carbon doping (4% and 10%) has been integrated in pillar-type analytical PCM cells. Physico-chemical and electrical data indicate that GeTeC is characterized by a much more stable amorphous phase compared to undoped GeTe. Thus, GeTeC offers a slower programming speed versus GeTe, but an improved data retention at high temperature. Finally, we argue that GeTeC alloy is a promising candidate for future developments of PCM technologies for embedded applications.

Effect of ion implantation energy for the synthesis of Ge nanocrystals in SiN films with HfO2/SiO2 stack tunnel dielectrics for memory application.

Ge nanocrystals (Ge-NCs) embedded in SiN dielectrics with HfO2/SiO2 stack tunnel dielectrics were synthesized by utilizing low-energy (≤5 keV) ion implantation method followed by conventional thermal annealing at 800°C, the key variable being Ge+ ion implantation energy. Two different energies (3 and 5 keV) have been chosen for the evolution of Ge-NCs, which have been found to possess significant changes in structural and chemical properties of the Ge+-implanted dielectric films, and well reflected in the charge storage properties of the Al/SiN/Ge-NC + SiN/HfO2/SiO2/Si metal-insulator-semiconductor (MIS) memory structures. No Ge-NC was detected with a lower implantation energy of 3 keV at a dose of 1.5 × 1016 cm-2, whereas a well-defined 2D-array of nearly spherical and well-separated Ge-NCs within the SiN matrix was observed for the higher-energy-implanted (5 keV) sample for the same implanted dose. The MIS memory structures implanted with 5 keV exhibits better charge storage and retention characteristics compared to the low-energy-implanted sample, indicating that the charge storage is predominantly in Ge-NCs in the memory capacitor. A significant memory window of 3.95 V has been observed under the low operating voltage of ± 6 V with good retention properties, indicating the feasibility of these stack structures for low operating voltage, non-volatile memory devices.

Crystallization study of “melt quenched” amorphous GeTe by transmission electron microscopy for phase change memory applications

In situ transmission electron microscopy (TEM) observations were performed for a better understanding of the “melt quenched” GeTe crystallization mechanism. The evolution of the crystallite morphology observed during annealing shows a growth-dominated crystallization behavior. Scanning transmission electron microscopy—electron dispersive x-ray spectroscopy and high resolution electron microscopy experiments were also performed on cycled GeTe devices, showing that void formation is responsible for the cell failure after 107 cycles.

Carbon-doped GeTe Phase-Change Memory featuring remarkable RESET current reduction

In this paper we present a study of Phase-Change non-volatile Memory (PCM) devices integrating carbon-doped GeTe as chalcogenide material. Carbon-doped GeTe, named GeTeC, remarkably lowers the RESET current and features very good data retention properties as well. In particular, GeTe PCM with 10% carbon inclusions (named GeTeC10%) yields about 30% of RESET current reduction with respect to pure GeTe and GST. Furthermore, our GeTeC10% memory cells are expected to guarantee a 10-years-lifetime-temperature of about 127°C, which is one of the highest ever reported for PCM. The outstanding properties of GeTeC make this material promising for non-volatile memory technologies.