Yizhang Yang

Nanoscale Calorimetry Using a Suspended Bridge Configuration

Abstract A new setup for small-scale differential scanning calorimetry (DSC) studies based on a suspended bridge configuration is presented. The new setup has three major advantages over previously reported DSC setups: 1) superior temperature uniformity in the bridge cross section; 2) less heat loss to the surroundings by at least two orders of magnitude; and 3) a faster transient response by three orders of magnitude. This paper includes a thermal analysis to support these improvements. A major contribution of the new thermal analysis over previous reports is the inclusion of the thermal mass of the substrate in calculations, which makes thermal design more detailed, dramatically affecting accuracy and sensitivity in measurements. Furthermore, the new thermal analysis more accurately accounts for heat loss to the substrate and the surroundings in efforts to resolve suspected inconsistencies in previously reported data. Experimental validation of the new setup is presented by measuring the specific heat of thin layers of Si02 and CoFe. The specific heat of Si02 was found to be 2.2 times 106 Jm -3 K-1 which is nearly 10% different from the literature values of bulk specimens. For CoFe, the specific heat value of 3.16 x 106 Jm -3 K-1 is obtained using differential Cu/Si02 and Cu/Si02/CoFe structures compared to the value of 3.5 times 106 Jm -3 K-1 obtained using single CoFe suspended structure.

Thermal and electrical characterization of Cu/CoFe superlattices

The present work is directed at thermal and electrical characterization of the Cu/CoFe multilayer, which is made of extremely thin periodic layers, using steady-state Joule heating and thermometry in suspended bridges in the temperature range of 50–300 K. The total thickness of the layer is ds=144 nm, while the thickness of individual repeats are 12 and 21 A for CoFe and Cu layers, respectively. The experimental data for thermal conductivity of a 144-nm-thick single Cu layer is also presented for comparison. The experimental data indicates that the spin-dependent electron scattering at the Cu/CoFe interface contributes to a strong reduction in thermal conductivity of these layers compared to the bulk values. The calculated Lorenz numbers (from the thermal and electrical conductivity data) varies by nearly a factor 2 from 4×10−8 W Ω K−2 at 50 K to 1.8×10−8 W Ω K−2 at 300 K and is different from the free electron value of L0=2.45×10−8 W Ω K−2. This implies that the Wiedemann-Franz law does not hold for Cu/C...

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.

High-Efficiency Transient Temperature Calculations for Applications in Dynamic Thermal Management of Electronic Devices

The highly nonuniform transient power densities in modern semiconductor devices present difficult performance and reliability challenges for circuit components, multiple levels of interconnections and packaging, and adversely impact overall power efficiencies. Runtime temperature calculations would be beneficial to architectures with dynamic thermal management, which control hotspots by effectively optimizing regional power densities. Unfortunately, existing algorithms remain computationally prohibitive for integration within such systems. This work addresses these shortcomings by formulating an efficient method for fast calculations of temperature response in semiconductor devices under a time-dependent dissipation power. A device temperature is represented as output of an infinite-impulse response (IIR) multistage digital filter, processing a stream of sampled power data; this method effectively calculates temperatures by a fast numerical convolution of the sampled power with the modeled systems impulse response. Parameters such as a steady-state thermal resistance or its extension to a transient regime, a thermal transfer function, are typically used with the assumption of a linearity and time-invariance (LTI) to form a basis for device thermal characterization. These modeling tools and the time-discretized estimates of dissipated power make digital filtering a well-suited technique for a run-time temperature calculation. A recursive property of the proposed algorithm allows a highly efficient use of an available computational resource; also, the impact of all of the input power trace is retained when calculating a temperature trace. A network identification by deconvolution (NID) method is used to extract a time-constant spectrum of the device temperature response. We verify this network extraction procedure for a simple geometry with a closed-form solution. In the proposed technique, the amount of microprocessor clock cycles needed for each temperature evaluation remains fixed, which results in a linear relationship between the overall computation time and the number of temperature evaluations. This is in contrast to time-domain convolution, where the number of clock cycles needed for each evaluation increases as the time window expands. The linear dependence is similar to techniques based on FFT algorithms; in this work, however, use of z-transforms significantly decreases the amount of computations needed per temperature evaluation, in addition to much reduced memory requirements. Together, these two features result in vast improvements in computational throughput and allow implementations of sophisticated runtime dynamic thermal management algorithms for all high-power architectures and expand the application range to embedded platforms for use in a pervasive computing environment.

Field-dependent thermal and electrical transports in Cu∕CoFe multilayer

This paper reports field-dependent thermal and electrical conductivity measurements of a 144 nm thick Cu∕CoFe giant magnetoresistive multilayer made of extremely thin periodic layers (12 and 21 A for CoFe and Cu layers, respectively), using steady-state Joule heating and electrical resistance thermometry in suspended bridges between 300 and 380 K. Large decreases in the electrical and thermal resistivities from antiparallel to parallel alignment of the magnetization in the film, referred to as the giant magnetoresistance (GMR) and giant magnetothermal resistance (GMTR), are observed. GMR ratios of 17% and 12% and large GMTR ratios of 25% and 58% are measured at 300 and 380 K, respectively. It is concluded that different electron scattering rates for charge and heat transports in the ferromagnetic CoFe layer are responsible for the difference between the GMR and GMTR ratios. While the previous works only reported the relative change in thermal conductance due to applied magnetic field, the present manuscri...

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.

Ballistic phonon transport and self-heating effects in strained-silicon transistors

In this manuscript, different aspects of nanoscale thermal transport in strained silicon transistors will be addressed. The two-dimensional Boltzmann transport equations for phonons in Si and SiGe alloy layers, along with the acoustic mismatch model for the interface, are used to capture the sub-continuum heat conduction effects in the device. It is shown that the lateral thermal conductivity of a 10-nm strained-Si layer grown on the SiGe underlayer can vary from 14 to 20W/m-K, depending on the interface specularity parameter. The resulting temperature distribution in the device is used to predict the impact of self-heating on performance of future generations of strained-Si devices. The analysis shows that the merits of strained-Si technology can be suppressed by excessive self-heating; therefore, additional considerations in the design of these devices need to be taken into account

Detailed modeling of temperature rise in giant magnetoresistive sensor during an electrostatic discharge event

With further miniaturization of the giant magnetoresistive (GMR) heads, the electrostatic discharge (ESD) failure has become the primary reliability issue in manufacturing of these sensors. In this article, the thermal response of the GMR read head to excessive current/voltage during an ESD event is investigated numerically, using a three-dimensional (3D) finite element analysis. Unlike the previous studies, the thermal properties of the GMR, Al2O3 gap, and shield layers used in the simulation are the experimentally measured values, which are different from the bulk values. The simulation results show that temperature in the GMR element sharply increases as the GMR dimensions are reduced, indicating the future GMR heads will be more susceptible to the ESD damages. In addition, thermal properties of the GMR elements and the gap materials play key roles in the accurate prediction of the temperature field in a GMR head during ESD events. The simulations are performed for both the current-in-plane (CIP) and p...

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

Transient Frequency-Domain Thermal Measurements With Applications to Electronic Packaging

Non-uniform power distribution, increased die-size, and multiple-chip modules present new challenges for the thermal management of modern integrated circuit (IC) packages. Thermal characterization techniques capable of resolving partial thermal resistances at the component level have received increased emphasis in development of advanced packaging technologies. This paper aims to develop a practical method for thermal characterization of IC packages using the frequency-domain measurement technique as a complementary technique to the widely used time-domain thermal transient measurement technique. This paper discusses practical implementation of the technique and demonstrates both thermal modeling and experimental results. Thermal impedances measured in frequency-domain yield the structure function, which describes the dynamic thermal response of the device based on thermal RC network analysis. Various applications of this technique in thermal characterization of the IC packages subjected to field conditions are also discussed.