Device Operating Temperature Research Articles

Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors

Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors

Analysis on Self-Heating Effect in 7 nm Node Bulk FinFET Device

The analyses on self-heating effect in 7 nm node non-rectangular Bulk FinFET device were performed using 3D device simulation with consideration to contact via and pad. From selfheating effect simulation, the position where the maximum lattice temperature occurs in Bulk FinFET device was investigated. Through the comparison of thermal resistance at each node, main heat transfer path in Bulk FinFET device can be determined. Selfheating effect with device parameter and operation temperature was also analyzed and compared. In addition, the impact of interconnects which are connected between the device on self-heating effect was investigated.

Dynamic Chemical Expansion of Thin Film Non-Stoichiometric Oxides at Extreme Temperatures

Thin film non-stoichiometric oxides are of interest as electrodes and electrolytes in electrochemical energy conversion devices including solid oxide fuel cells and electrolyzers because of their potential to lower device operating temperatures and as model systems to aid in investigating rate limiting kinetics. Many of these non-stoichiometric oxides undergo chemical expansion, or composition-dependent volume change, upon ‘breathing’ of oxygen that can result from variations in temperature, oxygen partial pressure, and electrical bias. However, thin film responses are not always accurately predicted based on bulk models or experiments. Understanding the time-resolved operando chemical expansion of non-stoichiometric oxide thin films is essential for modeling and predicting the development of stress and strain in actual devices during temperature excursions or gas interruption. In this work, we report a novel method for measuring chemical expansion of thin films of non-stoichiometric oxides at temperatures up to 650°C and demonstrate its application to the model fluorite-structured system (Pr, Ce)O2-δ (PCO). In this technique, we vary the oxygen activity of the material of interest by applying an oscillating electrical bias across a supporting yttria stabilized zirconia (YSZ) electrolyte, and use a high-temperature, depth-sensing instrumented indenter to detect the amplitude and phase lag of the material’s expansion response, allowing us to characterize nm-scale chemical expansion in PCO films with sub-second-scale resolution. We relate the observed frequency and temperature-dependent response to fundamental processes governing expansion kinetics and compare our results with available defect models for PCO. This ‘electrochemomechanical spectroscopy’ provides an accessible means of characterizing how factors such as film thickness, composition, or microstructure, influence the operando chemomechanical deformation of non-stoichiometric oxide-based devices on short time scales.

Heat evolution and dissipation in organic light-emitting diodes on flexible polymer substrates

We investigated the effect of the heat generated in organic light-emitting diodes (OLEDs) on their performance with a focus on the thermo-physical properties of the substrates. OLEDs were fabricated on polyethylene terephthalate (PET), polyimide (PI), and glass substrates, and the instantaneous and time-resolved performances of the OLEDs were compared. The device operation temperature (T) was predicted via heat transfer analysis using the finite element method (FEM). The value of T during operation was experimentally measured using an infrared (IR) camera, and the results were compared with the numerically calculated values. The effects of T on the time-resolved electroluminescence (EL) spectra of the OLEDs on the different substrates were also investigated. The experimental results of this study agreed well with the numerical predictions that the T of the OLEDs on the polymers are higher than the T of the OLEDs on glass during operation due to the relatively low thermal diffusivity (α) of the polymer substrates used in this study. The results also showed that the performance and reliability of the flexible OLEDs (f-OLED) are highly dependent on the heat extraction capabilities of the substrates; thus, the current density (J), luminescence (L), voltage (V) characteristics, and efficiencies (η) over time of the OLEDs on PET and PI are inferior to the OLEDs on the glass substrate.

Solid-cryogen-stabilized, cable-in-conduit (CIC) superconducting cables

This paper considers the use of a solid cryogen as a means to stabilize, both mechanically and thermally, magnesium diboride (MgB2) superconducting strands within a dual-channel cable-in-conduit (CIC) cable for use in AC applications, such as a generator stator winding. The cable consists of two separate channels; the outer channel contains the superconducting strands and is filled with a fluid (liquid or gas) that becomes solid at the device operating temperature. Several options for fluid will be presented, such as liquid nitrogen, hydrocarbons and other chlorofluorocarbons (CFCs) that have a range of melting temperatures and volumetric expansions (from solid at operating temperature to fixed volume at room temperature). Implications for quench protection and conductor stability, enhanced through direct contact with the solid cryogen, which has high heat capacity and thermal conductivity (compared with helium gas), will be presented. Depending on the cryogen, the conductor will be filled initially either with liquid at atmospheric conditions or a gas at high pressure (∼100 atm). After cooldown, the cryogen in the stranded-channel will be solid, essentially locking the strands in place, preventing strand motion and degradation due to mechanical deformation while providing enhanced thermal capacity for stability and protection. The effect of cryogen porosity is also considered. The relatively high heat capacity of solid cryogens at these lower temperatures (compared to gaseous helium) enhances the thermal stability of the winding. During operation, coolant flow through the open inner channel will minimize pressure drop.

Kinetics of Ion Transport in Perovskite Active Layers and Its Implications for Active Layer Stability.

Solar cells fabricated using alkyl ammonium metal halides as light absorbers have the right combination of high power conversion efficiency and ease of fabrication to realize inexpensive but efficient thin film solar cells. However, they degrade under prolonged exposure to sunlight. Herein, we show that this degradation is quasi-reversible, and that it can be greatly lessened by simple modifications of the solar cell operating conditions. We studied perovskite devices using electrochemical impedance spectroscopy (EIS) with methylammonium (MA)-, formamidinium (FA)-, and MA(x)FA(1-x) lead triiodide as active layers. From variable temperature EIS studies, we found that the diffusion coefficient using MA ions was greater than when using FA ions. Structural studies using powder X-ray diffraction (PXRD) show that for MAPbI3 a structural change and lattice expansion occurs at device operating temperatures. On the basis of EIS and PXRD studies, we postulate that in MAPbI3 the predominant mechanism of accelerated device degradation under sunlight involves thermally activated fast ion transport coupled with a lattice-expanding phase transition, both of which are facilitated by absorption of the infrared component of the solar spectrum. Using these findings, we show that the devices show greatly improved operation lifetimes and stability under white-light emitting diodes, or under a solar simulator with an infrared cutoff filter or with cooling.

High Thermal Stability of SiC Packaging with Sintered Ag Paste Die-attach combined with Imide-based Molding

We report that ultra-high thermal reliability can be achieved by combining Ag paste sintering die-attach and injection transfer molding with imide-based mold composite. Test specimens of SiC SBD in TO247 discrete-package are prepared; SiC dies bonded on Cu lead frame, Al ribbon wiring from electrode of SiC Schottky barrier diode (SBD) chip to the Cu leads, and injection transfer molding process. The hybrid nano+micro flake Ag paste realizes a low temperature bond process of 250°C in air, die-shear stress recorded 23 MPa without intensive pressure. The thermo-mechanical parameters like glass transition temperature, Young's modulus and CTE of the thermosetting composite are carefully tuned for both the manufacturing process targeting the device operating temperature of 250°C. The test pieces are tested by harsh thermal cycling between −50°C to 250°C, and compared with the other specimens made with usual Pb-5Sn soldering die-attach with the same molding composite. Interface failures are investigated by scanning acoustic tomography (SAT), and the electric properties of SBD are checked by I-V measurements. The sintered Ag die-attach survived over 500 cycles without any failures around the interfaces of SiC die, Cu lead-frame, and mold materials, and no obvious change is found in electronic properties. However, Pb-Sn die-attach joints are gradually damaged with increasing thermal cycles causing degraded I-V property. The SEM images indicate the microstucture of sintered Ag layer is unchanged after the thermal cycles, because of the impregnated imid-matrix into the porous Ag. Our results demonstrate that the combination of Ag sinter paste die-attach and imide-based thermosetting composite can be a massproduction-ready packaging technology to realize the high operation temperature of post-silicon power devices.

High-temperature and high-power-density nanostructured thermoelectric generator for automotive waste heat recovery

Given increasing energy use as well as decreasing fossil fuel sources worldwide, it is no surprise that interest in promoting energy efficiency through waste heat recovery is also increasing. Thermoelectric generators (TEGs) are one of the most promising pathways for waste heat recovery. Despite recent thermoelectric efficiency improvement in nanostructured materials, a variety of challenges have nevertheless resulted in few demonstrations of these materials for large-scale waste heat recovery. Here we demonstrate a high-performance TEG by combining high-efficiency nanostructured bulk materials with a novel direct metal brazing process to increase the device operating temperature. A unicouple device generates a high power density of 5.26Wcm−2 with a 500°C temperature difference between hot and cold sides. A 1kW TEG system is experimentally demonstrated by recovering the exhaust waste heat from an automotive diesel engine. The TEG system operated with a 2.1% heat-to-electricity efficiency under the average temperature difference of 339°C between the TEG hot- and cold-side surfaces at a 550°C exhaust temperature. The high-performance TEG reported here open up opportunities to use TEGs for energy harvesting and power generation applications.

Fabrication of thermoelectric materials – thermal stability and repeatability of achieved efficiencies

Thermoelectric properties of metastable metal chalcogenides degrade with thermal cycling.

Exergy analysis of a thermoelectric power generator: influence of bi-tapered pin geometry on device characteristics

The thermal characteristics of bi–tapered thermoelectric generator are investigated. The influence of the dimensionless shape parameter, associated with the device pin geometric configuration, on the first– and second–law efficiencies and device output power is examined. The study is extended to include the effects of the external load parameter and the device operating temperature ratio on the device characteristics. It is found that the first–law efficiency is not considerably influenced by the dimensionless shape parameter; however, the second–law efficiency and the device output power are significantly affected by the shape factor. The external load parameter has an adverse effect on the first– and the second–law efficiencies and the device output power. In this case, increasing external load parameter lowers the thermal performance of the thermoelectric generator. Reducing operating temperature ratio improves the first– and the second–law efficiencies and the device output power significantly.

Modeling of high composition AlGaN channel high electron mobility transistors with large threshold voltage

We report on the potential of high electron mobility transistors (HEMTs) consisting of high composition AlGaN channel and barrier layers for power switching applications. Detailed two-dimensional (2D) simulations show that threshold voltages in excess of 3 V can be achieved through the use of AlGaN channel layers. We also calculate the 2D electron gas mobility in AlGaN channel HEMTs and evaluate their power figures of merit as a function of device operating temperature and Al mole fraction in the channel. Our models show that power switching transistors with AlGaN channels would have comparable on-resistance to GaN-channel based transistors for the same operation voltage. The modeling in this paper shows the potential of high composition AlGaN as a channel material for future high threshold enhancement mode transistors.

Formation, migration, and clustering of point defects in CuInSe2 from first principles

The electronic properties of high-efficiency CuInSe2 (CIS)-based solar cells are affected by the microstructural features of the absorber layer, such as point defect types and their distribution. Recently, there has been controversy over whether some of the typical point defects in CIS—VCu, VSe, InCu, CuIn—can form stable complexes in the material. In this work, we demonstrate that the presence of defect complexes during device operational time can be justified by taking into account the thermodynamic and kinetic driving forces acting behind defect microstructure formation. Our conclusions are backed up by thorough state-of-the-art calculations of defect interaction potentials as well as the activation barriers surrounding the complexes. Defect complexes such as InCu−2VCu, InCu−CuIn, and VSe−VCu are shown to be stable against thermal dissociation at device operating temperatures, but can anneal out within tens of minutes at temperatures higher than 150–200 °C (VCu-related complexes) or 400 °C (antisite pair). Our results suggest that the presence of these complexes can be controlled via growth temperatures, which provides a mechanism for tuning the electronic activity of defects and the device altogether.

Small-Pitch HgCdTe Photodetectors

If we can make wavelength-sized detectors, we approach the limit at which smaller detectors have no further advantage for imaging focal plane arrays with practical (f/1-2) optics. Of course, this must be accomplished without compromising performance—a challenge for 5-μm devices for which the perimeter, the currents of which depend on passivation quality, is very large compared with the area of the device. This paper describes the development of small LWIR HgCdTe detectors and compares dark current performance with that of larger basic devices, as described by “Rule 07”, a well-known rule of thumb which gives the HgCdTe dark-current density characteristics of the best reported diodes as a function of device cut-off wavelength and operating temperature. Low cross-talk requires a fully-depleted absorber layer sufficiently thick to provide adequate quantum efficiency (QE). Preliminary results show dark-current densities are more than a factor of ten below the Rule 07 trend line. With these dark-current densities, the measured ∼40% non-anti-reflection-coated QE in the 8–10 μm region is more than adequate to achieve background-limited performance with the margin under tactical backgrounds for the fast (f/1), diffraction-limited optics required for the small pixels.

Wavelength conversion of incoherent light by sum-frequency generation.

In this paper, we reveal that some kinds of optical nonlinearities are further enhanced when incoherent light, instead of a laser, is used as a pump light. This idea was confirmed both theoretically and experimentally in the case of sum-frequency generation (SFG) using the optical second nonlinearity. The conversion efficiency of the SFG with incoherent light pumping increased as the bandwidth of the incoherent pump light decreased, finally reaching twice the conversion efficiency of conventional second harmonic generation (SHG) by laser pumping. This method dramatically relaxes the severe requirements of phase matching in the nonlinear optical process. The conversion efficiency became less sensitive to misalignment of the wavelength of pump light and also of device operation temperature when the bandwidth of the incoherent pump light was sufficiently broad, although the improvement of the conversion efficiency had an inverse relationship with the insensitivity to the phase-matching condition. The temperature tuning range was enhanced by more than two orders of magnitude in comparison with the conventional SHG method. As an example of a promising application of this new idea, we performed the generation of quantum entangled photon-pairs using cascaded optical nonlinearities (SFG and the subsequent spontaneous parametric down conversion) in a single periodically poled LiNbO3 waveguide device, in which the incoherent light was used as the pump source for both the parametric processes. We have achieved high fidelity exceeding 99% in quantum-state tomography experiments.

Tuning La0.67Sr0.33MnO3 surface magnetism using LaMnO3 and SrTiO3 caps

We have investigated how the surface magnetism of epitaxial La0.67Sr0.33MnO3 films on NdGaO3 substrates is modified by 2nm-thick caps of the undoped manganite LaMnO3 and the band insulator SrTiO3. Low-temperature photoemission electron microscopy images with x-ray magnetic circular dichroism contrast reveal large ferromagnetic domains, and a reduction of surface Curie temperature by 40K (LaMnO3 cap) and 70K (SrTiO3 cap). We use these negative results to argue that capped films of sub-optimally doped La1−xSrxMnO3 (x≠0.33) should improve the operating temperature of all-oxide and other spintronic devices.

Temperature Characteristics of Monolithically Integrated Wavelength-Selectable Light Sources

The temperature characteristics of monolithically integrated wavelength-selectable light sources are experimentally investigated. The wavelength-selectable light sources consist of four distributed feedback (DFB) lasers, a multimode interferometer coupler, and a semiconductor optical amplifier. The oscillating wavelength of the DFB laser could be modulated by adjusting the device operating temperature. A wavelength range covering over 8.0nm is obtained with stable single-mode operation by selecting the appropriate laser and chip temperature. The thermal crosstalk caused by the lateral heat spreading between lasers operating simultaneously is evaluated by oscillating-wavelength shift. The thermal crosstalk approximately decreases exponentially as the increasing distance between lasers.

Robust Irreversible Polarization Switching in Optically Injected VCSELs

We report experimental observations of robust irreversible polarization switching (IPS) in vertical cavity surface emitting lasers (VCSELs) subject to external optical injection. It is shown that, when operated in a two-mode regime, the VCSEL exhibits IPS for both parallel and orthogonal optical injection. The polarization switching is maintained for a wide range of values of control parameters, including frequency detuning, injected power $({\rm P}_{\rm inj})$ , bias current, and device operating temperature.

Structural insights into the formation and evolution of amorphous phase‐change materials

Abstractmagnified imageReduction of programming current is a major research goal in the development of phase‐change random‐access memory devices. The power‐limiting step is the amorphization of the phase‐change material (PCM), where a significant energy input is required to induce melting prior to amorphization. To address the challenge of reducing power consumption while retaining switching speed, a detailed understanding of the physics underpinning the amorphization process is required. As yet, little has been done to study the dynamics of the melt‐quench process at the atomic level. In this article, we report a detailed study of the melting mechanism and kinetics, and the effect of quench rate on the amorphization process in the prototypical PCM Ge2Sb2Te5, using ab initio molecular‐dynamics simulations. We also study the evolution of the amorphous phase under low‐temperature annealing, shedding light on the structural changes, which may occur after amorphization at device operating temperatures. Our results give microscopic insight into the amorphization of PCMs, and should inform future work to understand and resolve important issues in device engineering. magnified imageEffect of quench rate on the structure of Ge2Sb2Te5: While quenching at −5 K ps−1 leads to successful amorphization (left), quenching at a slower −1 K ps−1 leads to crystallization as the temperature is lowered (right).

Germanium-on-Glass solar cells: fabrication and characterization

We report on Germanium on Glass solar cells realized by wafer bonding, layer splitting and epitaxial regrowth. We provide a detailed description of the layer transfer process and discuss the material characterization. The solar cells are fabricated and tested to extract the most significant figures of merit, evaluating their performance versus device area and operating temperature. The cells exhibit typical conversion efficiencies exceeding 2.4% under AM1.5 irradiation and a maximum efficiency of 3.7% under concentrated excitation. This Germanium on Glass approach is promising in terms of added flexibility in multi-junction engineering and allows a significant cost reduction thanks to the re-usability of the Ge substrates.

Thermomigration in solder joints

In 3D IC technology, the vertical interconnection consists of through-Si-vias (TSV) and micro solder bumps. The size of the micro-bump is approaching 10μm, which is the diameter of TSV. Since joule heating is expected to be the most serious issue in 3D IC, heat flux must be conducted away by temperature gradient. If there is a temperature difference of 1°C across a micro-bump, the temperature gradient will be 1000°C/cm, which can cause thermomigration at the device operation temperature around 100°C. Thus thermomigration will become a very serious reliability problem in 3D IC technology. We review here the fundamentals of thermomigration of atoms in microbump materials; both molten state and solid state thermomigration in solder alloys will be considered. The thermomigration in Pb-containing solder joints is discussed first. The Pb atoms move to the cold end while Sn atoms move to the hot end. Then thermomigration in Pb-free SnAg solder joints is reviewed. The Sn atoms move to the hot end, but the Ag atoms migrate to the cold end. Thermomigration of other metallization elements, such as Cu, Ti and Ni is also presented in this paper. In solid state, copper atoms diffuse rapidly via interstitially to the cold end, forming voids in the hot end. In molten state, Cu thermomigration affects the formation of intermetallic compounds.