Publications

Fabrication and characterization of emerging nanoscale memory

Conventional solid state memory technologies such as flash memory, DRAM, and SRAM are facing scaling challenges due to fundamental limitations. Therefore, various new memory technologies are being widely researched and evaluated to continue the cost/performance improvement trend of solid state memory devices. To assess the potential scalability of emerging nanoscale memory beyond conventional limits, it is essential to characterize and understand how differently they perform at the nanoscale compared to known properties in the microscale. New nanoscale fabrication methods and new memory technologies offer a great opportunity for future memory device research. In this regard, we evaluated characteristics of nanoscale phase change memory and Ni oxide memory using nanofabrication technologies such as nanowire growth, nanocrystal synthesis, diblock copolymer patterning, and e-beam lithography. Evaluated characteristics include not only their device performance but also key material properties that might affect the ultimate device performance. The nanofabrication method for each memory material is also discussed due to its potential to overcome the difficulties of conventional semiconductor fabrication process.

Crystallization times of Ge-Te phase change materials as a function of composition

The crystallization times of Ge–Te phase change materials with variable Ge concentrations
29.5–
72.4 at. % were studied. A very strong dependence of the crystallization time on the composition
for as-deposited, amorphous films was confirmed, with a minimum for the stoichiometric
composition GeTe. The dependence is weaker for melt-quenched, amorphous material and
crystallization times are between one to almost four orders of magnitude shorter than for
as-deposited materials. This is promising for applications because recrystallization from the
melt-quenched phase is the relevant process for optical and solid state memory, and fast
crystallization and weak dependence on compositional variations are desirable.

Size-dependent phase transitions and morphological control over phase chane properties using colloidal nanoparticle building blocks

Colloidal nanocrystals have long been used to study the dependence of phase stability and transitions on size. Structural phase stability, in particular, can change dramatically in the few nanometer regime where the surface plays a significant role in determining the overall energy of the system. Implications include the well known depression of melting temperature with decreasing size and changes to the kinetics and thermodynamics of crystal-crystal phase transitions sometimes leading to extremely hysteretic behavior and metastable structures. Polytypism and crystalline phases not found in the bulk have likewise been observed in nanocrystals. We are studying the size-dependent phase stability of colloidal GeTe nanoparticles, synthesized alternatively in crystalline or amorphous form. GeTe is a
representative phase change memory material, exhibiting bistability. It is also the simplest ferroelectric material, undergoing a displacive transition from the high symmetry rock salt phase to a distorted, rhombohedral phase around 325 °C. Hence, the scaling behavior of the amorphous-crystalline and rhombohedral-cubic transitions at small sizes has direct bearing on understanding materials limits to scaling of phase change and ferroelectric random access memories. Using in-situ x-ray diffraction while heating, we observe the crystallization of our amorphous nanoparticles and the reversible rhombohedral-cubic phase transition of our nanocrystals. Remarkably, the amorphous phase is maintained above 300 °C, finally crystallizing at more than 150 °C above the bulk film crystallization temperature. At the same time, the resistivity of a film of these nanoparticles drops by orders of
magnitude upon heating, demonstrating that such solution-processed films retain the high resistivity contrast required for memory applications. By preparing films from nanoparticles of different sizes, from 2.2 to 4.5 nm in diameter, we show that the crystallization temperature depends strongly on size, reaching 400 °C for the smallest particles studied. Our long term goal is to utilize such size effects to tune the properties of phase change memory materials by manipulating nanostructured morphology as well as more conventional composition parameters.

Integrating Phase-Change Memory Cell with Ge Nanowire Diode for Crosspoint Memory - Experimental Demonstration and Analysis

In this paper, we demonstrate a novel phase-change memory cell utilizing a low-temperature in situ doped single crystalline germanium nanowire diode as a bottom electrode as well as a memory-cell selection device. The integrated memory cell shows promising characteristics such as low programming current, large set/reset resistance ratio, and rectifying behavior, which is required for high-density 3-D crosspoint memory. The small contact area determined by the diameter of nanowires enables low programming current below 200uA for reset and 50uA for set. The average resistance ratio of set/reset state programmed by repetitive pulse programming is 82, which is large enough for large-array operation. The heterojunction formed between in situ doped Ge nanowires and Si substrate provides isolation for crosspoint-memory operation.

Phase Change Nanodots Patterning Using a Self-Assembled Polymer Lithography and Crystallization Analysis

Crystallization behavior of scalable phase change materials can be studied on nanoscale structures. In this paper, high density ordered phase change nanodot arrays were fabricated using the liftoff technique on a self-assembled diblock copolymer template, PS-b-PMMA (polystyrene-poly(methyl-methacrylate)). The size of the nanodots was less than 15nm in diameter with 40nm spacing. This method is quite flexible regarding the patterned materials, and can be used on different substrates. The crystallization behavior of small scale phase change nanodot arrays was studied using time-resolved X-ray diffraction, which showed the phase transition for different materials, such as Ge15Sb85, Ge2Sb2Te5 and Ag and In doped Sb2Te. The transition temperatures of these nanodot samples were also compared with their corresponding blanket thin films and it was found that the nanodots had higher crystallization temperatures and crystallized over a broader temperature range.

Thickness and Stoichiometry Dependence of the Thermal Conductivity of GeSbTe Films

Thermal conduction in GeSbTe films strongly influences the writing energy and time for phase change memory (PCM) technology. This study measures the thermal conductivity of Ge2Sb2Te5 between 25 and 340 °C for layers with thicknesses near 60, 120, and 350 nm. A strong thickness dependence of the thermal conductivity is attributed to a combination of thermal boundary resistance (TBR) and microstructural imperfections. Stoichiometric variations significantly alter the phase transition temperatures but do not strongly impact the thermal conductivity at a given temperature. This work makes progress on extracting the TBR for Ge2Sb2Te5 films, which is a critical unknown parameter for PCM simulations.

Synthesis of Metal Chalcogenide Nanodot Arrays Using Block Copolymer-Derived Nanoreactors

Soluble metal chalcogenide precursors are used to fabricate arrays of metal chalcogenide nanodots by spin-coating. Nanodots are formed after thermal decomposition of the precursors, which are collected in patterned nanowell arrays. These arrays are derived from block copolymer patterns and may consist of the polymer itself or result from etching to transfer the pattern to an inorganic substrate. Etching provides enhanced control over nanowell shape and the morphology of the resulting metal chalcogenide array.

Generalized Phase Change Memory Scaling Rule Analysis

[Excerpt] Phase change memory (PCM) based on phase change material such as Ge2Sb2Te5 (GST) is one of the promising candidates to replace flash memory technology which will face significant reliability and scalability problems in the near future [1]. Not only does PCM have, or is expected to have, desirable characteristics such as fast read/write and good endurance but also it is expected to be highly scalable [2]. To understand and identify the requirements that future scaled phase change memory devices must meet, a feasible scaling scenario is essential. Most of the scaling scenarios are based on constant-voltage scaling [3, 4]. However, a thorough analysis of the theoretical basis of device scaling for phase change memory is not yet available. In this paper, we present a generalized scaling analysis for phase change memory in analytical forms which are verified by 3D finite-element electrothermal modeling. Our analytical solutions provide insights into the key device parameters that control the maximum temperature of the phase change memory cell and the minimum required programming voltage. ...

Diblock Copolymer Directed Self-Assembly for CMOS Device Fabrication

We present our recent work on using diblock copolymer directed self-assembly for the fabrication of silicon MOSFETs. Instead of using self-assembly to assemble the entire device, we plan to utilize self-assembly to perform one critical step of the complex MOSFET process flow in the beginning. Initial results of using PS-b-PMMA to define pores with hexagonal array having diameter of 20 nm for contact hole patterning will be described. Potential integration issues for making MOSFETs will also be addressed.

Transition Behavior of High Density Ordered Phase Change Nanostructure from Diblock Copolymer Template

An Integrated Phase Change Memory Cell With Ge Nanowire Diode For Cross-Point Memory

We demonstrate a novel phase change memory cell utilizing doped nanowire pn-junction diode both as a bottom electrode and a memory cell selection device for a cross-point memory array. Using an isolated vertical nanowire in each cell, the contact area is below the lithography limit. Very low SET programming current of 30 uA is achieved. RESET/SET resistance ratio is 100x. The diode provides 100x isolation between forward and reverse bias in the SET state.

The Synthesis and Characterization of Germanium Chalcogenide Nanoparticles via Single-Source Precursor and Co-precipitation

X-Ray Diffraction Studies of Phase Change Nanoparticles Produced by Self-Assembly-based Lithographic Techniques

An important parameter for the scaling behavior of phase change materials is the crystallization temperature as a function of phase change nanoparticle size. Phase change nanoparticles can be fabricated using electron-beam lithography, but this method is expensive and time-consuming. We have fabricated nanoparticles by various self-assembly-based lithographic processes and applied time-resolved X-ray diffraction to study their crystallization behavior. ...

Phase Transitions and Thermal Properties in GeSbTe (2:2:5)

Biomimetic Approaches for Fabricating High-Density Nanopatterned Arrays

A variety of alternative approaches to lithography have been investigated to address the many challenges currently associated with scaling of feature sizes. One of the more cost-effective techniques has been to use the intrinsic self-assemblying properties of AB diblock copolymers to make polymer thin films as nanometer etch masks. A more etch-resistant film can be fabricated through enrichment of domains within the block copolymer thin films with metals such as silicon. In contrast to previous methods for mineralizing polymer thin films, we demonstrate here a biomimetic approach for synthesizing two-dimensional silica nanopatterns at neutral pH and mild temperatures within polystyrene-b-poly-4-vinylpyridine thin films. We further show that one can employ these benign synthetic conditions to fabricate sub-20 nm features of phase change materials (PCM) while keeping the PCM film in its amorphous state in order to study the effect that scaling these materials has on their crystallization temperatures.

Analysis of Temperature in Phase Change Memory Scaling

We analyze constant-voltage isotropic and nonisotropic scaling issues for phase change memory (PCM) based on electrothermal physics. Various analytical and simulation models of general and typical PCM cells that support the analysis is also provided. The analysis shows that the maximum temperature in the PCM cell, which is a key parameter for PCM operation, is independent of geometrical sizes and depends only on the voltage and material properties. This leads to the minimum programming voltage concept, which is determined by material properties of the phase change material.

Phase Change Nanodot Arrays Fabricated Using a Self-Assembly Diblock Copolymer Approach

Self-assembling diblock copolymer, polystyrene-b-poly-4-vinylpyridine (PS-b-P4VP), was used as the template for fabricating phase change nanostructures. The high density GeSb nanodots were formed by etching into an amorphous GeSb thin film using silica hard mask which was patterned on top of polymer. The nanodot arrays are 15 nm in diameter with 30 nm spacing. This is smaller than most structures obtained by e-beam lithography. Time-resolved x-ray diffraction studies showed that the phase transition occurred at 235°C, which is 5°C lower than blanket GeSb film but higher than that of Ge2Sb2Te5 (150°C). GeSb showed good temperature stability for fabrication of small memory devices.