Characterization of high-K/metal gate using picosecond ultrasonics

Transition from oxide gates to replacement metal gates is well underway and performance benefits have been demonstrated in state-of-the-art microprocessors. High-K/metal gate combination is important for all emerging new applications that require high-performance and low gate-leakage including all silicon and non-silicon nanoelectronic transistors. Picosecond ultrasonic measurements are used as checkpoints during various stages of development and integration of high-K/metal gate. The small spot, non-destructive nature of this technology allows for measurements directly on product wafers and on various line array structures in small measurement sites (30 μm × 30 μm). The technique has shown excellent correlation with cross-section TEM, demonstrating capability for monitoring advanced gate stacks. Picosecond ultrasonics provides high-throughput and can be used for in-line monitoring after the process is transferred to high volume manufacturing.     

Electronic properties of GST for non-volatile memory

Impacts of annealing temperature and film thickness to the resistivity of Ge₂Sb₂Te₅(GST) have been studied. The resistivity of GST drops when the annealing temperature reaches 180 °C, rises above 360 °C and the thicker film crystallized more easily. Electronic device of phase change memory also has been fabricated with metal sidewall technology using 5 μm lithographic technology. The device was successfully programmed by 100 ns of 5 V pulse for SET and 10 ns of 10 V pulse for RESET. More than 100 times on/off ratio has been reached.     

Phase change materials and their application to random access memory technology

Phase change materials can exist in two different phases, the amorphous and the crystalline phase, which exhibit distinctly different physical properties. It is possible to repeatedly switch the state of these materials, from the amorphous phase to the crystalline phase by heating the material above its crystallization temperature, and from the crystalline to the amorphous phase by melt-quenching. Phase change materials have been utilized very successfully in all modern optical re-writable storage media such as CDs, DVDs and Blu-ray disks. Recently, they have also been applied to solid-state memory devices where their large difference in electrical resistivity is used to store information. This paper reviews the unique properties of phase change materials in particular as they are important for their application to these devices.     

嵌入式相变存储器的技术特点和研究现状

从2001年Intel在IEDM发表第一篇相变存储器的论文以来,相变存储器的发展十分迅猛.相变存储器由于具有非易失性、循环寿命长、元件尺寸小、功耗低、可多级存储、高速读取、抗辐射、耐高低温、抗振动、抗电子干扰和制造工艺简单等优点,被认为最有可能取代目前的FLASH和DRAM而成为未来半导体存储器主流产品.文中系统地介绍了嵌入式相变存储器的存储机理及其主要工作特点,从相变材料,器件结构,存储阵列等方面分析国内外研究现状,并讨论了器件失效与可靠性问题.     

Fabrication process not limited by the lithography resolution of lateral phase change memory

To improve the performance of phase change memory (PCM) and reduce the cost of fabrication, we propose a new lateral PCM structure based on the technology of angle evaporation to define the critical dimension controllable, not limited by the limitation of lithography resolution. The fabrication process is cost-effective. PCM cells featured 80 nm×100 nm were successfully demonstrated, although the resolution of the aligned used was 1 µm only. Compared with the traditional lateral PCM structure, finite element simulation results show that the new structure has better thermal stability.     

Fabrication of high density nano-pillar type phase change memory devices using flexible AAO shaped template

In this study, the high density nano-pillar type phase change memory was fabricated using duplicating nano-patterns of the anodic alumina oxide (AAO) by nanoimprint process. The high density nano-hole array of AAO template was transferred to the flexible PVC polymer template using hot embossing method. To use the flexible AAO shaped template for UV-NIL, the high density nano-pillar type Ge₂Sb₂Te₅ patterns were fabricated, and the electrical properties of the device were evaluated by conducting atomic force microscopy, connected electrical measurement system. To use the flexible AAO shaped template for UV-NIL, high density GST pattern could be fabricated even on the flexible polyimide (PI) substrate.     

A new extension method of retention time for memory cell on dynamic random access memory

Demands have been placed on dynamic random access memory (DRAM) to not only increase memory capacity and data transfer speed but also to reduce operating and standby currents. When a system uses DRAM, the restricted data retention time necessitates a refresh operation because each bit of the DRAM is stored as an amount of electrical charge in a storage capacitor. Power consumption for the refresh operation increases in proportion to memory capacity. A new method is proposed to reduce the refresh power consumption dynamically, when full memory capacity is not required, by effectively extending the memory cell retention time. Conversion from 1 cell/bit to 2^N cells/bit reduces the variation of retention times among memory cells. The proposed method reduces the frequency of disturbance and power consumption by two orders of magnitude. Furthermore, the conversion itself can be realized very simply from the structure of the DRAM array circuit, while maintaining all conventional functions and operations in the full array access mode.     

Improving the data retention of phase change memory by using a doping element in selected Ge_2Sb_2Te_5

The crystallization characteristics of a ubiquitous T-shaped phase change memory(PCM) cell, under SET current pulse and very small disturb current pulse, have been investigated by finite element modelling. As analyzed in this paper, the crystallization region under SET current pulse presents first on the corner of the bottom electron contact(BEC) and then promptly forms a filament shunting down the amorphous phase to achieve the low-resistance state, whereas the tiny disturb current pulse accelerates crystallization at the axis of symmetry in the phase change material. According to the different crystallization paths, a new structure of phase change material layer is proposed to improve the data retention for PCM without impeding SET operation.This structure only requires one or two additional process steps to dope nitrogen element in the center region of phase change material layer to increase the crystallization temperature in this confined region. The electrical-thermal characteristics of PCM cells with incremental doped radius have been analyzed and the best performance is presented when the doped radius is equal to the radius of the BEC.     

Nitrogen-doped Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films for nonvolatile memory

Nitrogen-doped Ge₂Sb₂Te₅ (GST) films for nonvolatile memories were prepared by reactive sputtering with a GST alloy target. Doped nitrogen content was determined by using x-ray photoelectron spectroscopy (XPS). The crystallization behavior of the films was investigated by analyzing x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Results show that nitrogen doping increases crystallization temperature, crystallization-activation energy, and phase transformation temperature from fcc to hexagonal (hex) structure. Doped nitrogen probably exists in the grain vacancies or grain boundaries and suppresses grain growth. The electrical properties of the films were studied by analyzing the optical band gap and the dependence of the resistivity on the annealing temperature. The optical band gap of the nitrogen-doped GST film is slightly larger than that of the pure GST film. Energy band theory is used to analyze the effect of doped nitrogen on electrical properties of GST films. Studies reveal that nitrogen doping increases resistivity and produces three relatively stable resistivity states in the plot of resistivity versus annealing temperature, which makes GST-based multilevel storage possible. Current-voltage (I-V) characteristics of the devices show that nitrogen doping increases the memory’s dynamic resistance, which reduces writing current from milliampere to microampere.