Thin-film Resistive Heater Research Articles

Modeling and Experiments on Temperature and Electrical Conductivity Characteristics in High-Temperature Heating of Carbide-Bonded Graphene Coating on Silicon.

A novel non-isothermal glass hot embossing system utilizes a silicon mold core coated with a three-dimensional carbide-bonded graphene (CBG) coating, which acts as a thin-film resistance heater. The temperature of the system significantly influences the electrical conductivity properties of silicon with a CBG coating. Through simulations and experiments, it has been established that the electrical conductivity of silicon with a CBG coating gradually increases at lower temperatures and rapidly rises as the temperature further increases. The CBG coating predominantly affects electrical conductivity until 400 °C, after which silicon becomes the dominant factor. Furthermore, the dimensions of CBG-coated silicon and the reduction of CBG coating also affect the rate and outcome of conductivity changes. These findings provide valuable insights for detecting CBG-coated silicon during the embossing process, improving efficiency, and predicting the mold core's service life, thus enhancing the accuracy of optical lens production.

Stem heating results in hydraulic dysfunction in Symplocos tinctoria: implications for post-fire tree death.

Fire-induced heating of stems can impair plant water transport by deforming xylem and increasing vulnerability to cavitation, but it is not clear whether these effects can result in tree death, or how quickly this may occur. In field experiments, we heated stems of Symplocos tinctoria (L.) L'Hér saplings to 90 °C using a thin-film resistive heater, and we monitored stomatal conductance, leaf water potential, sap flow and hydraulic conductivity until stem death. Sap flow and stomatal conductance declined quickly after heating, while whole-plant hydraulic conductance and leaf water potential remained high for the first week. In fact, leaf water potential increased during the first days after heating, indicating that stomatal closure was not initially caused by leaf water deficit induced by impaired water transport. After 1week, leaf water potential and whole-plant conductance declined below unheated controls, while stomatal conductance and sap flow continued declining, approaching zero after 2weeks. To better understand the cause of these declines, we directly measured hydraulic conductivity of heated stems. Stems underwent a progressive decline in conductivity after heating, and by the time that samples were severely wilted or desiccated, the heated portion of stems had little or no conductivity. Importantly, conductivity of heated stems was not recovered by flushing stems to remove embolisms, suggesting the existence of physical occlusions. Scanning electron micrographs did not reveal deformed cell walls, nor did it identify alternative causes of blockages. These results reveal that stem heating can result in xylem dysfunction and mortality, but neither response is immediate. Dysfunction was likely caused by wound responses rather than embolism, but improved understanding of the mechanisms of heat-induced hydraulic failure is needed.

A Sub-nL Chip Calorimeter and Its Application to the Measurement of the Photothermal Transduction Efficiency of Plasmonic Nanoparticles

Here, we introduce a microchip for differential measurements of aqueous samples in twin calorimetric arms with a sub-nL enclosed space. Each arm features a fluidic microchannel located in close proximity to a thin-film resistance thermometer and heater patterned on a low-stress SiN membrane held over a Si cavity with suspension beams. The chip exhibits a rapid response with a thermal time constant of less than 10 ms in atmosphere and a residual thermal conductance below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6{\mu }\text{W}{\cdot }\text{K}^{-1 }$ </tex-math></inline-formula> , one of the lowest reported and accounts for the excellent sensitivity registered in vacuum up to 11 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}{\cdot }\text{W}^{-1 }$ </tex-math></inline-formula> . These values are in good agreement with those ones predicted by modeling using finite element method, lumped-capacitance analysis, and calorimeter electrothermal circuit simulation together with the measurement system. Long suspension beams in each arm ensure uniform temperature maintained across its membrane as revealed by the fluorescence-based melting curve analysis. We share the specific heat capacity values of water, glycerin, and mineral oil accurately measured by the chip. Lastly, we demonstrate the chip calorimeter utility on the measurement of the photothermal transduction efficiency of plasmonic nanoparticles, which makes our study further unique in expanding the applications of chip calorimeters. [2021-0044]

Fabrication and embedded sensors characterization of a micromachined water-propellant vaporizing liquid microthruster

Rapid advances in micro/nanotechnology have enabled to achieve high levels of miniaturization, promoting the development of low cost and highly efficient microsystems for specific applications. In the space sector, the miniaturization of satellites has led to a renewed interest in the research and development of advanced micro-propulsion technologies able to generate small and accurate thrust forces and high specific impulse.This work presents the design and fabrication of a silicon-based water-propellant Vaporizing Liquid Microthurster (VLM) equipped with embedded microsensors for real-time monitoring of in-channel vapor/liquid fraction and fluid temperature during its operation. Anisotropic dry etching of silicon wafer and thermo-compressive bonding were chosen as key fabrication steps: the former process was used to better control the surface roughness on microchannels inner walls, the latter was used to guarantee the fluidic tightness and complete the fabrication process of the device. Borofloat 33 glass, used to seal the micromachined silicon wafer, allows the optical inspection of the fluid flow and vaporization within the different chambers during the device operation. A platinum resistive heater placed on the bottom of the chip was exploited for the heating of the propellant. A set of Resistive Temperature Detectors (RTDs) and, for the first time, capacitive sensors were designed and integrated inside the microthruster chip to add distributed sensing capabilities for flow instability control. Further, a secondary low-power platinum thin film resistive heater was placed inside each of the eight channels, in order to allow for localized precision fluid heating and flow control. The operational feasibility of the fabricated microthruster was assessed by means of a preliminary characterization of the embedded sensors, based on experimental tests supported by numerical investigations; the results demonstrated that the designed microsensor devices are able to maximize the microthruster efficiency, in terms of microtexture-enhanced in-channel water vaporization and reduced power consumption.

Modeling and experimental performance analysis of a novel heating system and its application to glass hot embossing technology.

Traditional glass molding involves infrared heating; however, the thermal cycle time is long. A molding technique based on three-dimensional carbide-bonded graphene (CBG) has been developed to mold boron silicon glass. This CBG coating on a wafer silicon die can serve as a thin-film resistance heater to heat up the sample surface very rapidly with a relatively low applied voltage. To improve the precision temperature control in hot embossing so as to enhance the process quality, a heating system with lower energy consumption, shorter cycle time, and much more precision control is proposed. We have used COMSOL to simulate the whole heating process, and the heating behavior of a CBG-coated silicon wafer was experimentally investigated. The results showed that the repeatability of the heating system is stable, and it is suitable for precision glass hot embossing. Finally, an example of a precisely fabricated microlens array (Schott P-SK57) is embossed by using this novel heating system.

A microscopic physical description of electrothermal-induced flow for control of ion current transport in microfluidics interfacing nanofluidics.

The phenomenon of electrothermal (ET) convection has recently captured great attention for transporting fluidic samples in microchannels embedding simple electrode structures. In the classical model of ET-induced flow, a conductivity gradient of buffer medium is supposed to arise from temperature-dependent electrophoretic mobility of ionic species under uniform salt concentrations, so it may not work well in the presence of evident concentration perturbation within the background electrolyte. To solve this problem, we develop herein a microscopic physical description of ET streaming by fully coupling a set of Poisson-Nernst-Planck-Navier-Stokes equations and temperature-dependent fluid physicochemical properties. A comparative study on a standard electrokinetic micropump exploiting asymmetric electrode arrays indicates that, our microscopic model always predicts a lower ET pump flow rate than the classical macroscopic model even with trivial temperature elevation in the liquid. Considering a continuity of total current density in liquids of inhomogeneous polarizability, a moderate degree of fluctuation in ion concentrations on top of the electrode array is enough to exert a significant influence on the induction of free ionic charges, rendering the enhanced numerical treatment much closer to realistic experimental measurement. Then, by placing a pair of thin-film resistive heaters on the bottom of an anodic channel interfacing a cation-exchange medium, we further provide a vivid demonstration of the enhanced model's feasibility in accurately resolving the combined Coulomb force due to the coexistence of an extended space charge layer and smeared interfacial polarizations in an externally-imposed temperature gradient, while this is impossible with conventional linear approximation. This leads to a reliable method to achieve a flexible regulation on spatial-temporal evolution of ion-depletion layer by electroconvective mixing. These results provide useful insights into ET-based flexible control of micro/nanoscale solid entities in modern micro-total-analytical systems.

On-chip LAMP-BART reaction for viral DNA real-time bioluminescence detection

The present paper describes the development of an integrated lab-on-chip, in which viral DNA amplification with real-time on-chip detection is carried out under constant temperature of 65 °C. The lab-on-chip is composed of a disposable 10-μL polydimethylsiloxane reaction chamber which is thermally and optically coupled to a glass substrate that hosts a thin-film metallic resistive heater and thin-film amorphous silicon diodes which act as temperature and radiation sensors. A loop-mediated isothermal amplification (LAMP) technique was optimized to specifically amplify parvovirus B19 DNA and coupled with Bioluminescent Assay in Real Time (BART) technology to provide real-time detection of target DNA. The experimental results demonstrate the ability of the proposed device to discriminate among different concentrations of viral DNA with an excellent agreement with standard off-chip methods.

Nacre Mimetic with Embedded Silver Nanowire for Resistive Heating

Nacre mimetics show great potential as lightweight, mechanically robust, and functional materials. Here, we introduce highly reinforced nacre-mimetic nanocomposite (NC), prepared via self-assembly of poly(vinyl alcohol) polymer-coated synthetic nanoclay from aqueous dispersions, as a transparent and mechanically robust substrate to prepare silver nanowire (AgNW) embedded thin-film resistive heater. AgNW is a promising substitute for widely used brittle and expensive indium tin oxide (ITO) due to its high electrical conductivity, excellent optical transparency, and good mechanical flexibility. However, current AgNW-based electrodes mostly suffer from limitations such as surface roughness and weak adhesion of AgNWs to flexible plastic substrates (e.g., poly(ethylene terephthalate), PET). We demonstrate that AgNW can be embedded in nacre-mimetic NC substrate by simple hot-pressing as compared to PET, leading to excellent stability against scotch tape peeling without any encapsulation layer. The AgNW/NC resistive heater shows uniform sheet resistance varying from 10 to 80 Ω/sq with 70 to 91% transmittance at 550 nm on decreasing AgNW density from 111 to 23 mg/m2. Moreover, AgNW/NC heater can generate rapid (10 s) and repetitive long-term heating response at low input voltages. It shows only small variation in temperature (4 °C) and sheet resistance during bending at 14 mm diameter for 2000 cycles as well as under various extreme mechanical deformations. Taking advantage of the conformability, mechanical deformability, and fast thermal response of the resistive heater, we demonstrated the in vitro temperature-triggered release of antibiotics (i.e.,vancomycin) from phase change material-coated, antibiotic-loaded hydrogels. Hence, we envision that the resistive heater can be introduced as a potential candidate into flexible electronics and wearable devices.

Hotspot Thermal Management via Thin-Film Evaporation—Part I: Experimental Characterization

The presence of hotspots with extreme heat fluxes and temperatures in high-performance electronics has led to severe thermal management challenges in the semiconductor industry. Our work experimentally investigates the potential of capillary-fed thin-film evaporation as a thermal management solution for devices where hotspots are superposed with mild background heating. The front side of our test device incorporates well-defined silicon micropillar arrays for passive fluidic transport via capillary wicking, and the backside is patterned with thin-film resistive heaters to emulate $640 \times 620~\mu \text{m}^{2}$ hotspots and $1 \times 1$ cm2 uniform background heating. Our micropillar wick design facilitates efficient fluidic and thermal transport and dissipated ${\approx} 6$ kW/cm2 from a single hotspot without background heating beyond which viscous losses exceeded the capillary pressure and dryout occurred due to insufficient liquid supply. While the capillary-limited hotspot dryout heat flux decreased by creating concurrent hotspots as well as by superposing a hotspot with mild uniform background heating, it was unaltered (within the measurement error) when the location of the hotspot was varied within the $1 \times 1$ cm2 microstructured area. The ultrahigh heat fluxes in our experiments were achieved by suppressing boiling by judiciously designing the fluidic transport where the microstructured surface maintains a thin liquid film and syphons only the required amount of liquid to sustain evaporation. Our experimental results show that thin-film evaporation is a viable thermal management solution for the next-generation high-performance power electronics, among others, where hotspots pose a significant challenge.

Carbide-bonded graphene coating of mold insert for rapid thermal cycling in injection molding

A novel carbide-bonded graphene coating on silicon insert was proposed to realize rapid thermal cycling (RTC) in injection molding. Continuous and dense carbon-bonded graphene coating was prepared on the surface of the silicon cavity through chemical vapor deposition. Serving as a thin-film resistance heater, the graphene coating was able to heat the mold cavity rapidly to above the glass-transition temperature of the polymer, even if the applied power source was of relatively low voltage. When the voltage was 240V, the coating was heated up to 145.6°C in a short period of 10s, that is, the average and transient heating rates were able to be as high as 11.6°C/s and 16.1°C/s, respectively. This injection molding technique of RTC was successfully implemented to produce plate samples with uniform sizes in thickness (600μm). The influence of RTC on weld line, internal stress, and replication fidelity was also investigated. Compared with conventional injection molding, our facile method was able to mold products with smaller weld mark, less internal stress, and better replication fidelity. The tensile strength and elongation at yield of the products were also enhanced by 37.77% and increased by 265.11%, respectively, with much less energy consumption.

Alternating current electrothermal micromixer with thin film resistive heaters

Alternating current electrothermal flow has been extensively used for mixing of fluids in microfluidic systems. This work presents a novel approach to mixing two laminar flow streams in microchannels. The micromixer consists of a pair of asymmetric planar alternating current electrodes disposed along a fluidic channel and a thin film resistive heater below the electrode. Multiphysics (fluid flow, temperature, electrical, and concentration fields) in the micromixer is studied by numerical simulation method. The results indicate that external heating by the thin film resister leads to flow body forces that are proportional to the temperature gradient and chaotic stretching and folding of fluid material lines. Enhancement of mixing is demonstrated by numerical simulation results. Effects of size and location of the thin film resistive heater on mixing efficiency are also investigated.

Enhanced electrothermal pumping with thin film resistive heaters

This work demonstrates the use of thin film heaters to enhance electrothermal pumping in microfluidic systems. Thin film heating electrothermal pumping is more efficient than Joule heating alone. Numerical simulations of an asymmetric electrode array are performed to demonstrate the advantages of incorporating thin film heaters. This specific simulation shows that thin film heater electrothermal pumping provides approximately two and one-half times more volumetric flow than Joule heating alone for the same input power to both systems. In addition, external heating allows for electrothermal pumping to be applicable to low conductivity media.

Experimental and numerical study of microchannel heater/evaporators for thermal phase-change actuators

The performance of thermal phase-change actuators is strongly affected by the design of their heater/evaporators. The use of microchannels to control the liquid working fluid via capillary forces promises one approach to optimize the operation of heater/evaporators and the performance of the actuators. An experimental and numerical study of heater/evaporators for phase-change actuators, based on open rectangular microchannels, is presented. The microchannel heater/evaporators consist of SU8 walls fabricated in a radial pattern on either silicon or silicon nitride membranes. The microchannels, of rectangular cross section, taper from a width of 80μm at their outer radius to a width of five microns at their inner radius, and have a constant depth of 40μm. The energy balance for the heater/evaporators is determined experimentally through measurements of electrical power dissipated as heat in a thin-film resistance heater, sensible heat transfer by conduction radially out of the evaporator, and latent heat transfer by evaporation from the microchannels themselves. A finite difference code is then used to calculate liquid flow pumped by capillary forces in the microchannels as well as sensible and latent heat transfer rates. The numerical code is shown to predict the experimental measurements well. The performance of this type of heater/evaporator for phase-change actuators is shown to be limited by two mechanisms: dry-out of the working fluid in the microchannels due to the capillary limit determines maximum evaporative mass transfer rates, while conduction heat losses out of the evaporators determines maximum evaporator efficiencies.

A MEMS Differential-Scanning-Calorimetric Sensor for Thermodynamic Characterization of Biomolecules

This paper presents a microelectromechanical systems sensor for differential scanning calorimetry (DSC) of liquid-phase biomolecular samples. The device consists of two microchambers, each of which is based on a freestanding polyimide diaphragm and surrounded by air cavities for thermal isolation. The chambers are each equipped with a thin-film gold resistive heater and temperature sensor and are also integrated with a thin-film antimony-bismuth (Sb-Bi) thermopile. For DSC measurements, the chambers are respectively filled with a biomolecular sample and a reference solution, with their temperatures varied at a constant rate. The thermopile voltage is measured to determine the differential power between the chambers for thermodynamic characterization of the biomolecules. The device is used to measure the unfolding of proteins in a small volume (1 μL) and at practically relevant concentrations (approximately 1 mg/mL). Thermodynamic properties, including the enthalpy change and melting temperature, during this conformational transition are determined and found to agree with published data.

MEMS-based microthruster with integrated platinum thin film resistance temperature detector (RTD), heater meander and thermal insulation for operation up to 1,000°C

We have fabricated microthruster chip pairs—one chip with microthruster structures such as injection capillaries, combustion chamber and converging/diverging nozzle machined using the deep reactive...

MEMS-based microthruster with integrated platinum thin film resistance temperature detector (RTD), heater meander and thermal insulation for operation up to 1,000°C

We have fabricated microthruster chip pairs—one chip with microthruster structures such as injection capillaries, combustion chamber and converging/diverging nozzle machined using the deep reactive ion etching process, the other chip with sputtered platinum (Pt) thin film devices such as resistance temperature detectors (RTDs) and a heater. To our knowledge, this is the first microelectromechanical systems-based microthruster with fully integrated temperature sensors. The effects of anneal up to 1,050°C on the surface morphology of Pt thin films with varied geometry as well as with/without PECVD-SiO2 coating were investigated in air and N2 and results will also be presented. It was observed that by reducing the lateral scale of thin films the morphology change can be suppressed and their adhesion on the substrate can be enhanced. Chemical analysis with X-ray photoelectron spectroscopy showed that no diffusion took place between neighboring layers during annealing up to 1 h at 1,050°C in air. Electrical characterization of sensors was carried out between room temperature and 1,000°C with a ramp of ±5 Kmin−1 in air and N2. In N2, the temperature-resistance characteristics of sensors had stabilized to a large extent after the first heating. After stabilization the sensors underwent up to eight further temperature cycles. The maximum drift of the sensor signal was observed for temperatures above 950°C and was less than 8.5 K in N2. To reduce the loss of combustion heat, chip material around microthruster structures was partially removed with laser ablation. The effects of thermal insulation were investigated with microthruster chip pairs which were clamped together mechanically. The heater was operated with up to 20 W and the temperature distribution in the chip pairs with/without thermal insulation was monitored with seven integrated RTDs. The experiments showed that a thermal insulation allows the maximum temperature as well as the temperature gradient within the microthruster chip pairs to be increased.

Optimal Thin-Film Topology Design for Specified Temperature Profiles in Resistive Heaters

In this paper, we optimized the topology of a thin-film resistive heater as well as the electrical potential of the electrodes on the boundaries. The objective was to minimize the difference between the actual and prescribed temperature profiles. The thin-film thickness was represented by 100 design variables, and the electrical potential at each electrode were also design variables. The topology optimization problem (inverse problem) has been solved with two methods, i.e., with a genetic algorithm (GA) and with a conjugate gradient method using adjoint and sensitivity problems (CGA). The genetic algorithm used here was modified in order to prevent nonconvergence due to the nonuniqueness of topology representation. The conjugate gradient method used in inverse conduction was extended to cope with our electrothermal problem. The GA and CGA methods started with random topologies and random electrical potential values at electrodes. Both the CGA and GA succeeded in finding optimal thin-film thickness distributions and electrode potential values, even with 100 topology design variables. For most cases, the maximum discrepancy between the optimized and prescribed temperature profiles was under 0.5°C, relative to temperature profiles of the order of 70°C. The CGA method was faster to converge, but was more complex to implement and sometimes led to local minima. The GA was easier to implement and was more unlikely to lead to a local minimum, but was much slower to converge.

Experimental pool boiling investigations of FC-72 on silicon with artificial cavities and integrated temperature microsensors

In this experimental study, fluorinert FC-72 is boiled on a silicon chip with artificial cavities and integrated microsensors. The horizontal silicon chip with dimensions of 39.5 × 19 × 0.38 mm is completely immersed in FC-72. The integrated nickel–titanium temperature microsensors on the back of the chip are calibrated individually and exhibit a near-linear increase of electrical resistance with temperature. The applied heat fluxes and the resulting wall superheat at the boiling surface are varied by means of an integral thin-film resistance heater (95% Al, 4% Cu and 1% Si), also on the back of the silicon chip. Artificial cylindrical cavities with a mouth diameter of 10 μm and depths of 40, 80 or 100 μm situated above the microthermometers serve as artificial nucleation sites, due to trapped vapour. Bubble growth rates, frequencies, departure diameters of bubbles and waiting times between bubbles from an isolated cavity for different wall superheats and pressures were obtained by analysing high-speed video images and the simultaneously measured temperature below the artificial cavity.

Quench Characteristics of an NbTi CICC With Non-Uniform Current Distribution

The effect of non-uniform current distribution (NUCD) on stability and quench characteristics of an NbTi/Cu cable-in-conduit conductor (CICC) with non-insulated or bare strands has been investigated by artificially introducing NUCD among the strands using a sophisticated current feeder system with thin-film resistive heaters. The stability margin or minimum quench energy (MQE) has been found to decrease significantly, especially in the transition region between so-called well-cooled and ill-cooled regions, due to the introduction of NUCD. The experimental results of stability margin with uniform current distributions have been compared with numerical calculations using the 1-D finite element code GANDALF, whereas the non-uniform current distribution cases have been treated by another code developed by the authors, which models the last two twisting stage of the conductor with uniform contact resistances between sub-cables. The numerical calculation results also simulate the similar behaviors as found in experiments. The quench propagation velocities deduced from voltage tap signals are also discussed.

Development of an electrically tuneable Bragg grating filter in polymer optical fibre operating at 1.55 µm

We present a thorough study on the development of a polymer optical fibre-based tuneable filter utilizing an intra-core Bragg grating that is electrically tuneable, operating at 1.55 µm. The Bragg grating is made tuneable using a thin-film resistive heater deposited on the surface of the fibre. The polymer fibre was coated via the photochemical deposition of a Pd/Cu metallic layer with the procedure induced by VUV radiation at room temperature. The resulting device, when wavelength tuned via Joule heating, underwent a wavelength shift of 2 nm for a moderate input power of 160 mW, a wavelength to input power coefficient of −13.4 pm mW−1 and time constant of 1.7 s−1. A basic theoretical study verified that for this fibre type one can treat the device as a one-dimensional system. The model was extended to include the effect of input electrical power changes on the refractive index of the fibre and subsequently to changes in the Bragg wavelength of the grating, showing excellent agreement with the experimental measurements.