Low Thermal Conductivity Substrates Research Articles

Enhancing sp3 formation in DLC on low thermal conductivity substrate: Interval deposition minimizing ion energy sensitivity

Depositing sp3-rich diamond-like carbon (DLC) on low thermal conductivity substrates is difficult due to the high sensitivity of sp3 formation to ion energy. This sensitivity is attributed to the heating effect of ion bombardment during plasma-enhanced chemical vapor deposition (PECVD) processing. This study introduced an interval deposition method aimed at minimizing the ion energy sensitivity, thereby achieving sp3-rich DLC deposition and broadening the process window. The variation in sp3 content of deposited DLC showed that the interval deposition can enhance sp3 formation, which is particularly significant when the single deposition duration is ≤1 s and the ratio of interval duration to deposition duration is ≥3. Deposition experiments with varying bias voltages confirmed that the ion energy sensitivity of sp3 formation in DLC can be effectively reduced by the proposed method. The analysis of transient film temperature during the deposition process revealed that this method took effect by limiting the temperature elevation of the film. Performance comparison showed that the adhesion of films to the substrate was improved by 138 % using the interval deposition method, compared to the traditional continuous deposition. This user-friendly and high-tolerance deposition method can significantly advance the application fields of DLC coatings.

Linearity and Sensitivity Analysis of MEMS Thermal Wind Sensor via Analytical Model

In this article, an analytical model focusing on the linearity and sensitivity of micro-electromechanical systems (MEMS) thermal wind sensors with low thermal conductivity substrate (LTCS) and packaging is proposed. The influence of substrate material, substrate thickness, heater size, measurement distance, packaging material, and packaging size on the sensor performances are investigated to serve as design guidelines for future thermal wind sensors. For the first time, model results reveal that all the factors listed above have opposite effects on the sensitivity and linearity of the wind sensors. Furthermore, sensitivity and linearity are related to power consumption and measurement range respectively. Therefore, a compromise between sensitivity and linearity is required for high wind speed measurement applications. The real wind sensor fabricated with ceramic substrate and encapsulated by a plastic carrier was tested in a wind tunnel. The experimental results illustrated that the sensitivity of the sensor at 3.3 m/s is 157 mV/( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{W}\cdot \text{m}\cdot \text{s}^{-1}$ </tex-math></inline-formula> ). In addition, the fitting linearity parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$B_{m}$ </tex-math></inline-formula> is 0.424 in the range from 0 to 33 m/s.

Controlling of longwave oscillatory Marangoni patterns on a rhombic lattice

We apply nonlinear feedback control to govern the stability of long-wave oscillatory Marangoni patterns. We focus on the patterns caused by instability in thin liquid film heated from below with a deformable free surface. This instability emerges in the case of substrate of low thermal conductivity, when two monotonic long-wave instabilities, Pearson’s and deformational ones, are coupled. We provide weakly nonlinear analysis within the amplitude equations, which govern the evolution of the layer thickness and the temperature deviation. The action of the nonlinear feedback control on the nonlinear interaction of two standing waves is investigated. It is shown that quadratic feedback control can produce additional stable structures (standing rolls, standing squares and standing rectangles), which are subject to instability leading to traveling wave in the uncontrolled case.

Analysis of thickness and interfacial thermal resistance of Au microheater on glass or polyimide substrate

The 3ω method is widely used for measuring the thermal properties of a substrate or thin film on which a microheater is deposited. Recently, samples with non-formal shapes and has a thermal conductivity of 1 W/mK or below have been analyzed using the bidirectional 3ω method. In measuring the thermal conductivity of such samples, the measurement sensitivity and accuracy can be increased using a 3ω sensor fabricated on a low thermal conductivity substrate such as glass or polyimide. In this study, thermal characterization is conducted on a microheater deposited on glass and polyimide substrates for a 3ω sensor. The effects of the microheater thickness and the interfacial thermal resistance between the substrate and microheater, on the temperature amplitude and phase lag are analyzed. Results from analytic solutions considering the microheater thickness and the interfacial thermal resistance are compared with results from numerical analysis of two-dimensional conduction heat transfer and experiment.

Nonlinear feedback control of Marangoni wave patterns in a thin film heated from below

We investigate the effect of nonlinear feedback control on the oscillatory mode of the long-wave Marangoni instability. Thin liquid film heated from below with a deformable free surface atop a substrate of low thermal conductivity is in our spotlight. We apply nonlinear feedback control to change the excitation of convection from subcritical to supercritical, or to stabilize a definite kind of convective pattern. Nonlinear interaction of traveling waves is examined by means of weakly nonlinear analysis within the amplitude equations. The cases of single wave, of pair of counter-propagating waves, and of three-dimensional two-wave patterns are considered. It is shown that by means of quadratic feedback control it is possible to replace a subcritical bifurcation by a supercritical one, as well as extend the stability region of traveling rolls and traveling rectangles and produce additional structures (traveling squares).

Substrate dependent resistive switching in amorphous-HfOx memristors: an experimental and computational investigation

Long-range thermal environment makes significant impact on resistive switching in amorphous-HfOx (x ∼ 1.8) memristors; and the substrate of low thermal conductivity improved both the digital and analog switching performance.

Near-junction heat spreaders for hot spot thermal management of high power density electronic devices

Many high power (opto-) electronic devices such as transistors, diodes, and lasers suffer from significant hot spot temperature rises due to the high heat fluxes generated in their active area, which limits their performance, reliability, and lifetime. Employing high thermal conductivity materials near the heat source, known as near-junction heat spreaders, offers a low-cost and effective thermal management approach. Here, we present analytical heat spreader models and a methodology to evaluate their performance. Experimental demonstration of near-junction diamond heat spreaders on vertical GaN PiN diodes revealed significantly reduced spreading resistances, along with very low temperature gradients across the device. The findings in this work provide design guidelines and demonstrate excellent prospects, especially for the devices on low thermal conductivity substrates. The theoretical analysis of optimized diamond heat spreaders shows an 86% reduction of spreading resistance for GaN devices and 98% for Ga2O3 devices. In addition, our results show that a 3 μm-thick layer of high-quality CVD-deposited diamond heat spreaders on GaN-on-Si devices can provide better heat spreading than GaN-on-SiC devices and perform similar to GaN-on-diamond devices, highlighting the significant potential of heat spreaders as an effective and low-cost thermal management approach.

Marangoni instability in a heated viscoelastic liquid film: Long-wave versus short-wave perturbations.

We investigate the Marangoni instability in a thin layer of viscoelastic fluid, confined between its deformable free surface and a substrate of low thermal conductivity. Following a theoretical analysis, we study the stability of the present system for the case when the fluid layer is subjected to heating from below. Here, we use the Maxwell model to depict the rheology of the viscoelastic fluid. Linear stability analysis of the quiescent base state reveals that, in addition to the conventional short-wave mode, a long-wave instability can also emerge in this system. We demonstrate the appearance of both the long-wave monotonic and oscillatory instabilities in such a system. We study this long-wave mode analytically using the scaling k∼sqrt[Bi] (k is the wave number and Bi is the Biot number), whereas the short-wave mode is examined numerically. The influential role of elasticity of the fluid and the other involved parameters on the stability of the system is aptly discussed, and their ranges are identified within which a particular instability mode gets critical.

Sensitivity Enhancement of Silicon-on-Insulator CMOS MEMS Thermal Hot-Film Flow Sensors by Minimizing Membrane Conductive Heat Losses.

Minimizing conductive heat losses in Micro-Electro-Mechanical-Systems (MEMS) thermal (hot-film) flow sensors is the key to minimize the sensors’ power consumption and maximize their sensitivity. Through a comprehensive review of literature on MEMS thermal (calorimetric, time of flight, hot-film/hot-film) flow sensors published during the last two decades, we establish that for curtailing conductive heat losses in the sensors, researchers have either used low thermal conductivity substrate materials or, as a more effective solution, created low thermal conductivity membranes under the heaters/hot-films. However, no systematic experimental study exists that investigates the effect of membrane shape, membrane size, heater/hot-film length and (size) to (hot-film length) Ratio (MHR) on sensors’ conductive heat losses. Therefore, in this paper we have provided experimental evidence of dependence of conductive heat losses in membrane based MEMS hot-film flow sensors on MHR by using eight MEMS hot-film flow sensors, fabricated in a 1 µm silicon-on-insulator (SOI) CMOS foundry, that are thermally isolated by square and circular membranes. Experimental results demonstrate that: (a) thermal resistance of both square and circular membrane hot-film sensors increases with increasing MHR, and (b) conduction losses in square membrane based hot-film flow sensors are lower than the sensors having circular membrane. The difference (or gain) in thermal resistance of square membrane hot-film flow sensors viz-a-viz the sensors on circular membrane, however, decreases with increasing MHR. At MHR = 2, this difference is 5.2%, which reduces to 3.0% and 2.6% at MHR = 3 and MHR = 4, respectively. The study establishes that for membrane based SOI CMOS MEMS hot-film sensors, the optimum MHR is 3.35 for square membranes and 3.30 for circular membranes, beyond which the gain in sensors’ thermal efficiency (thermal resistance) is not economical due to the associated sharp increase in the sensors’ (membrane) size, which makes sensors more expensive as well as fragile. This paper hence, provides a key guideline to MEMS researchers for designing the square and circular membranes-supported micro-machined thermal (hot-film) flow sensors that are thermally most-efficient, mechanically robust and economically viable.

Effect of substrate on phase-change characteristics of GeSb thin films and its potential application in three-level electrical storage

Phase-change Ge8Sb92 films were deposited on the varied thermal-conductivity substrates by radio frequency sputtering and their crystallization behavior was investigated. Situ sheet resistance measurement and the X-ray diffraction spectra show a double stage phase transitions of Ge8Sb92 films on the low thermal-conductivity substrates while a single stage phase transition on the high ones with the increasing heating temperature. The first-stage phase transition is amorphous-to-crystalline transition and the second-stage phase transition is partial-to-complete crystalline transition. The results provide experimental basis for the optimization of Ge8Sb92 phase-change memory and the possibility application in three-level electrical storage with single layer GeSb-type phase-change materials.

Marangoni instability in a thin film heated from below: Effect of nonmonotonic dependence of surface tension on temperature.

We investigate Marangoni instability in a thin liquid film resting on a substrate of low thermal conductivity and separated from the surrounding gas phase by a deformable free surface. Considering a nonmonotonic variation of surface tension with temperature, here we analytically derive the neutral stability curve for the monotonic and oscillatory modes of instability (for both the long-wave and short-wave perturbations) under the framework of linear stability analysis. For the long-wave instability, we derive a set of amplitude equations using the scaling k∼(Bi)^{1/2}, where k is the wave number and Bi is the Biot number. Through this investigation, we demonstrate that for such a fluid layer upon heating from below, both monotonic and oscillatory instability can appear for a certain range of the dimensionless parameters, viz., Biot number (Bi), Galileo number (Ga), and inverse capillary number (Σ). Moreover, we unveil, through this study, the influential role of the above-mentioned parameters on the stability of the system and identify the critical values of these parameters above which instability initiates in the liquid layer.

Experimental Investigation of Jet Impingement Cooling With Carbon Dioxide at Supercritical Pressures

Jet impingement cooling is widely used in many industrial applications due to its high heat transfer capability and is an option for advanced high power density systems. Jet impingement cooling with supercritical pressure fluids could have much larger heat transfer rates combining with the large fluid specific heat near the pseudocritical point. However, the knowledge of its flow and heat transfer characteristics is limited. In this study, the flow and the local and average heat transfer characteristics of jet impingement cooling with supercritical pressure fluids were studied experimentally with carbon dioxide first. An integrated thermal sensor chip that provided heating and temperature measurements was manufactured using micro-electro-mechanical systems (MEMS) techniques with a low thermal conductivity substrate as the impingement cooled plate. The experiment system pressure was 7.85 MPa, which is higher than the critical pressure of carbon dioxide of 7.38 MPa. The mass flow rate ranged from 8.34 to 22.36 kg/h and the Reynolds number ranged from 19,000 to 68,000. The heat flux ranged from 0.02 to 0.22 MW/m2. The nozzle inlet temperature ranged from lower to higher than the pseudocritical temperature. Dramatic variations of the density at supercritical pressures near the heating chip were observed with increasing heat flux in the strong reflection and refraction of the backlight that disappeared at inlet temperatures higher than the pseudocritical temperature. The local heat transfer coefficient near the stagnation point increased with increasing heat flux while those far from the stagnation point increased to a maximum with increasing heat flux and then decreased due to the nonuniformity of jet impingement cooling. The heat transfer is higher at inlet temperatures lower than the pseudocritical temperature and the surface temperature is slightly higher than the pseudocritical temperature due to the dramatic changes in the fluid thermo-physical properties at supercritical pressures.

Temperature Dependent Thermal Conductivity and Elastic Properties of a-InGaZnO4 and a-In2Ga2ZnO7 Thin Films

Amorphous In-Ga-Zn-O is an important oxide semiconductor in advanced display technologies. Despite its importance, little has been reported on the thermal and elastic properties of this material. Here, the temperature dependence of the thermal conductivity, shear modulus, and internal friction of a-InGaZnO4 and a-In2Ga2ZnO7 films are presented. The thermal conductivity of a-In2Ga2ZnO7, measured from 100 K to room temperature, was found to be larger than that of a-InGaZnO4 over the entire temperature range. At room temperature the thermal conductivities were 1.9 W/m K and 1.4 W/m K for the a-In2Ga2ZnO7 and a-InGaZnO4 films, respectively. The shear modulus and internal friction of these films were measured in the temperature range of 340 mK to 65 K. At 4.2 K the shear modulus of the a-InGaZnO4 and a-In2 Ga2ZnO7 films was 44 GPa and 42 GPa, respectively. The internal friction of thin films at each composition exhibited a temperature dependence and magnitude that is in agreement with that observed in all amorphous solids. As the self-heating effect is of concern in the development of amorphous In-Ga-Zn-O based thin film transistors on low thermal conductivity substrates, a thermal model of such a device utilizing a-In2Ga2ZnO7 or a-InGaZnO4 as the active layer was explored. It was found that the temperature increase of the thin film transistor channel is essentially independent of the thermal conductivity of the active layer.

Thick Film Heater for Sensor Application

A thick film microheater was elaborated. The microheater is intended for fast heating of small volume samples under measurement in optical based system. Thermal analysis of microheater was carried out using finite element method (FEM) for heat transfer calculation as a function of time and space. A nodal heat transfer function was calculated in classical form including all basics mechanisms of heat exchange – heat conduction, convection and radiation were considered. Work focuses on the influence of some construction parameters (ex. length, thermal conductivity of substrate, substrate thickness) on microheater performance. The results show that application of thin substrate of low thermal conductivity and low thickness for miroheater construction and resistor of optimum dimensions leads to significant power consumption decrease and increase of overall optical measurement system performance.

Nonlinear electro-thermal modeling and field-simulation of OLEDs for lighting applications II: Luminosity and failure analysis

In recent years great effort has been put into development of lighting purpose Organic Light Emitting Diodes (OLEDs) worldwide. Among many concerns of developers is heat-removal from the thin film structure of the active layers of OLEDs which are typically realized on low thermal conductivity substrates such as glass or polymer foils. The other issue is to provide the OLEDs with transparent, yet high electrical conductivity electric power supply structure, therefore metallic shunting grids are added to the layer stack of OLEDs. These two major issues necessitate self-consistent electro-thermal simulation of large area OLEDs in which the temperature dependent I–V characteristics of the light emitting polymer layers are also considered. Our first article discussed the details of the algorithmic fundamentals of our nonlinear electro-thermal field solver. This second paper presents the new, temperature dependent radiance and luminance distribution map calculation extension of the algorithm. Using this feature the inhomogeneity of large area OLEDs in free convection environment can be estimated; such calculations for glass-based research OLED samples are also presented. Because of the operational degradation of the OLED, non-emissive “dark” spots can appear. Cause of these spots can be e.g. manufacturing problem, damage of the surface protection, etc. Electro-thermal and luminance simulation of this failure type is demonstrated and is compared with measured results. Thermal runaway as the possible physical phenomenon behind the appearances of dark spots is proved by simulations.

Long-wave Marangoni convection in a thin film heated from below

We consider long-wave Marangoni convection in a liquid layer atop a substrate of low thermal conductivity, heated from below. We demonstrate that the critical perturbations are materialized at the wave number K ∼ √Bi, where Bi is the Biot number which characterizes the weak heat flux from the free surface. In addition to the conventional monotonic mode, a novel oscillatory mode is found. Applying the K ∼ √Bi scaling, we derive a new set of amplitude equations. Pattern selection on square and hexagonal lattices shows that supercritical branching is possible. A large variety of stable patterns is found for both modes of instability. Finite-amplitude one-dimensional solutions of the set, corresponding to either steady or traveling rolls, are studied numerically; a complicated sequence of bifurcations is found in the former case. The emergence of an oscillatory mode in the case of heating from below and stable patterns with finite-amplitude surface deformation are shown in this system for the first time.

Parametric Study of Solid-Phase Axial Heat Conduction in Thermally Integrated Microchannel Networks

A parametric study is presented to highlight design challenges of thermally integrated microchannel networks for portable chemistry and/or fuels reforming. One-dimensional modeling analysis of heat transfer in a two-fluid system is presented for the case of (i) two nonreacting fluids (heat exchanger), (ii) a single exothermic reacting fluid and a second nonreacting fluid (regenerative combustor), and (iii) one exothermic reacting fluid and a second endothermic reacting fluid (heat exchanger reactor). In each case, the influence of solid-phase thermal conductivity and thermal packaging upon thermal efficiency, reaction conversion, and steady-state multiplicity is investigated. Results demonstrate the importance of both packaging and solid-phase axial thermal conduction upon system performance, with optimal performance obtained using low thermal conductivity substrates. Modeling analysis predicts steady-state multiplicity when employing low thermal conductivity materials, illustrating the need for future de...

Heat Dissipation in High-Power GaN Electronics on Thermally Resistive Substrates

The heat dissipation in GaN devices grown on low thermal conductivity lithium gallate (LGO) substrates was investigated. The thermal conductivity of single-crystal LGO was measured utilizing the 3/spl omega/ technique for temperatures ranging from 100 K-500 K. For the GaN layer, the thermal conductivity was estimated using a phonon transport model which included dislocation density and temperature dependence. These data were then used in a finite element program to determine the thermal behavior of a heterojunction field-effect transistor. Based on a maximum junction temperature of 500 K, it was found that devices with a power dissipation of 1 W/mm were possible if the primary heat dissipation path was through the low thermal conductivity substrate. However, in using a front side cooling scheme, results suggest that it may be possible to develop devices with power dissipation in the range of 10 W/mm.

Flip Chip Mounting for Improved Thermal Management of AlGaN/GaN HFETs

AbstractWe report on a temperature study of flip-chip mounted multi-finger AlGaN/GaN heterostructure field effect transistors (HFETs) using micro-Raman spectroscopy and infrared (IR) thermography. Flip-chip mounting can be used to improve thermal device management, in particular, for devices grown on low thermal conductivity substrates such as sapphire. In this study, we compare two flip-chip mounted HFETs of different flip-chip contact bump layout designs and a non flip-chip mounted HFET. Both temperature measurements and 3D temperature simulations are presented. The results show that minimizing the distance between the bumps and the active area of the HFET is essential for obtaining a low device operating temperature.

Surface flow visualization using thermal tufts produced by an encapsulated phase change material

A new thermal tuft method for flow visualization similar to the laser thermal tuft method is described. The earlier laser thermal tuft method involved heating a spot on a surface with a laser, producing a teardrop temperature distribution on a surface coated with thermochromic liquid crystals. In the present study, thermal tufts are produced by embedding small thermal masses in a low thermal conductivity substrate. When a model is exposed to wind tunnel flow, the embedded thermal masses remain (nearly) isothermal while the thermal tufts appear downstream. The advantage of using embedded thermal masses over previous methods is that no laser or heated base is needed. Multiple tufts can easily be achieved. By using an encapsulated phase change material the phase change between solid and liquid (ice) produces isothermal spots. Images of the surface temperature distributions associated with these thermal tufts were obtained using both liquid crystal and infrared thermography. Flow visualization using these thermal tufts is demonstrated for an impinging jet and on a flat surface in a wind tunnel with crossflow.