Sadegh M. Sadeghipour

Phase change random access memory, thermal analysis

Despite very encouraging progress in recent years, phase change random access memory (ovonic unified memory, OUM) still faces several problems, such as reliability (lifetime), power consumption and speed, which need to be resolved before it can be commercialized. There have been a number of attempts to address such problems, even through devising other alternatives such as line memory and thermal GST memory cells. However, a comprehensive thermal engineering of the OUM memory cell is missing from the literature, and yet can have a great impact on design and optimization of the device. Such an analysis can definitely serve the OUM technology to achieve the optimum design and can even be used as a guideline for defining the research path. This manuscript provides an insight into the thermal issues and phenomena in the phase change random access memory cell. I-structure is proposed for OUM which has the combined features of T-structure and the line memory cell

Heat Sinks With Enhanced Heat Transfer Capability for Electronic Cooling Applications

In a competition at Carnegie Mellon University, the mechanical engineering students designed and manufactured 27 heat sinks. The heat sinks were then tested for thermal performance in cooling a mock processor. A heat sink with three rows of 9, 8, and 9 dimpled rectangular fins in staggered configuration performed the best, while having the least total volume (about 25% less than the set value). Validation of the observed thermal performance of this heat sink by experimentation and numerical simulations has motivated the present investigation. Thermal performance of the heat sinks with and without dimples have been evaluated and compared. Results of both the measurements and simulations indicate that dimples do in fact improve heat transfer capability of the heat sinks. However, dimples cause more pressure drop in the air flow. Keeping the total volume of the heat sink and the height of the fins constant and changing the number of the fins and their arrangement show that there is an optimum number of fins for the best performance of the heat sink. The optimum fin numbers are different for inline and staggered arrangements.

Thermal characterization of thin film Cu interconnects for the next generation of microelectronic devices

With the dramatic scaling of the transistors, the important issues like RC delay, electromigration failure and heat dissipation emerge, which need to be addressed urgently. Substitution of copper for aluminum has been suggested to reduce the RC delay of interconnects. While the electrical and mechanical properties of thin copper films have been extensively investigated; their thermal characterizations have received less attention. The lateral thermal conductivity of a 144 nm thick copper film is measured using the electrical resistance Joule heating and thermometry in a suspended bridge. The thermal conductivities at 300 K and 450 K are 240 and 280 W/m-K, respectively, which is smaller than the corresponding bulk values. The impact of the interconnect dimension and thermal conductivity on the self-heating is investigated as a function of interconnect via density. It is concluded that for via separation distances less than 5 /spl mu/m, the combination of Cu interconnect and vias can significantly reduce the average temperature rise in multilayer interconnects.

Thermal characterization of dielectric and phase change materials for the optical recording applications

Advances in the phase change optical recording technology strongly depend on the optical and thermal optimizations of the metal/ZnS–SiO2/phase change multilayer structure, which requires accurate modeling and thermal characterization of the phase change media structure. In the present work, the thermal conductivities of the amorphous and crystalline Ge4Sb1Te5 phase change and ZnS–SiO2 dielectric layers of thicknesses in the range of 50–300nm have been measured using the transient thermoreflectance technique. The data are between factors of 2–4 different from the previously measured values for thin film and bulk samples. The thermal boundary resistance at a metal/ZnS–SiO2 interface is found to be around 7×10−8m2W−1. This might have serious implications for the future phase change recording application which attempts to achieve the high writing speeds by decreasing the thickness of a ZnS–SiO2 dielectric layer.

Modeling of Localized Heating Effect in Sub-Micron Silicon Transistors

The performance and reliability of sub-micron semiconductor transistors demands accurate modeling of electron and phonon transport at nanoscales. The continued downscaling of the critical dimensions, introduces hotspots, inside transistors, with dimensions much smaller than phonon mean free path. This phenomenon, known as localized heating effect, results in a relatively high temperature at the hotspot that cannot be predicted using heat diffusion equation. While the contribution of the localized heating effect to the total device thermal resistance is significant during the normal operation of transistors, it has even greater implications for the thermoelectrical behavior of the device during an electrostatic discharge (ESD) event. The Boltzmann transport equation (BTE) can be used to capture the ballistic phonon transport in the vicinity of a hot spot but many of the existing solutions are limited to the one-dimensional and simple geometry configurations. We report our initial progress in solving the two dimensional Boltzmann transport equation for a hot spot in an infinite media (silicon) with constant temperature boundary condition and uniform heat generation configuration.Copyright

Thermal Characterization of the 144 nm GMR Layer Using Microfabricated Suspended Structures

The performance and reliability of GMR heads are influenced by the level of temperature rise, which may occur in the device during the normal operation or during an electrostatic discharge (ESD) event. However, the reliable electro-thermal modeling of the GMR sensor to predict the temperature rise, demands an accurate knowledge of the thermal properties of its constituent materials such as Al2 O3 passivation and GMR layers. The lateral thermal conductivity of the GMR layer, which has not been measured previously, can largely influence the maximum temperature rise in the GMR sensor. The present effort will be directed at thermal characterization of the CoFe/Cu multilayer structures made of extremely thin periodic layers, using steady-state and frequency domain heating and thermometry in suspended bridges. The measurements are performed on several suspended structures with the lengths and widths in the range of 250 to 500 μm and 16 to 20 μm, respectively.Copyright

Thermal analyses of confined cell design for phase change random access memory (PCRAM)

This investigation is an effort to demonstrate the feasibility of confined cell design for a phase change random access memory (Ovonic Unified Memory, OUM) cell which benefits, respectively, from the low current and the simple fabrication process of a line memory cell and T- structure. Similar to OUM devices, the confined cell design is also highly scalable. The 2-D numerical simulation results of the programming process for confined cells show very high (104 - 105) reset/set resistance ratios. However, the proper selection of the reset current is very important for achieving a complete amorphization of the active region, free of crystalline regions. Formation of such crystalline residues can create low resistance paths for the electric current rendering unacceptably low reset/set resistance ratios (of order two) during read. Simple analytical expressions for the reset current and thermal tome constants of a confined cell were obtained using 2-D and 3-D heat conduction heat transfer analyses. The results agree well with those of 2-D thermal and crystallization modeling, which indicate that the analytical models can be used as a powerful preliminary design tool for PCRAM.

Thermal Characterization of the High-Thermal-Conductivity Dielectrics

It has been recognized that future improvements in performance and reliability of the microelectronic devices may only be possible through the use of new high-thermal-conductivity materials for thermal management in compact packaging systems. The diamond-like dielectric materials, in bulk form or thin film configurations are the likely choice, due to their high thermal conductivity and their excellent mechanical and electrical properties. However, the accurate thermal characterization of these materials has proven to be extremely challenging due to variations in fabrication processes and therefore their microstructures, as well as the practical difficulties in measuring small temperature gradients during the thermal characterization process. The variations in microstructure of these materials (e.g., CVD diamond) would manifest into anisotropic, nonhomogeneous, and thickness-dependent thermal properties that may vary by several orders of magnitude. As a result of these complications, a wide range of experimental techniques have been developed over the years, which may or may not be appropriate for thermal characterization of high-thermal-conductivity material of given microstructure and physical dimension. We will describe and critically review the existing thermal-characterization techniques for high-thermal-conductivity dielectric materials. In addition, we propose a number of techniques that are particularly tailored for accurate thermal characterization of diamond, silicon nitride (Si3N4), aluminum nitride (AlN), and silicon carbide (SiC) films and substrates. In each case, specific comments about the experimental technique and procedure, detailed description of the heat transfer process, and sensitivity analysis are provided.

Simulation of the writing on the patterned optical phase-change recording media

Irregularities in the edge of the bit marks is one of the most critical sources of the noise that lead to timing jitter in optical phase change recording. The maximum linear bit density achievable at a given wavelength and lens numerical aperture is often limited by jitter. Jitter is largely determined by the combined optical, thermal, and crystallization properties of the medium and by the optical quality and recording strategy of the laser beam. Media with a patterned phase-change layer are proposed for controlling the mark edge jitter by making distinct mark edges. The preliminary simulation results from writing on the transverse and longitudinal patterns look promising in producing rectangular bit marks with distinct boundaries. The bit marks written on patterned media also have a smaller size compared to those written on continuous media. An interesting and unexpected result is the disappearance of the crescent-shaped trailing edge in all the bit marks on the patterned media, the shape and existence ...

Nanoscale Energy Transport in Information Technology Research With an Application to High-Density Data Storage Devices and Systems

By all measures, the data storage industry is one of the most important components of the Information Technology (IT) revolution. In recent years, many of the emerging technologies rely heavily on energy transport at extremely short time and length scales as a mean to overcome the superparamagnetic limit - a serious impediment to future advancement of storage technology. Additionally, thermally induced failure and reliability issues at the nanoscale are becoming increasingly important due to rapid device miniaturization in data storage applications. Further advances in high-technology data storage systems will be difficult, if not impossible, without rigorous treatment of nanoscale energy transport. This manuscript reviews the thermal design issues and challenges in thermally assisted magnetic disk recording, thermally assisted scanned probe recording, phase change optical data recording, magnetoresistive random access memory (MRAM) and giant magnetoresistive (GMR) heads. Relevant thermally induced failures in GMR heads, write coil, interconnects and MRAM will be discussed as well.Copyright