bluuebunny/crossref_metadata_2025 huggingface dataset:
(Invited) Thermoelectric Parameters in Blends of Polymers with Slightly Offset Carrier Energies
Authors: Li Hui , Katz Howard E.
Published: February 2020

The mechanical flexibility, low temperature processing, potential printability, capability of blending to form composites, and use of common elements are attractive features for the use of polymers in thermoelectrics. The subject of this presentation is the use of polymers blended with dopants and polymer additives to tune the charge carrier density and Fermi energy-transport energy difference while maintaining charge carrier mobility. Two thiophene polymers, poly(bisdodecylquaterthiophene) and poly(bisdodecyl thioquaterthiophene) (PQT12 and PQTS12, respectively), were the principal polymers used. Cyclic voltammetry and current-voltage measurements indicated that the introduction of sulfurs into the side chains induces traps in films containing PQTS12. Doping the polymer with sulfur in side chains (PQTS12) with the strong oxidant nitrosonium tetrafluroborate (NOBF4), we obtained an especially high conductivity up to 350 S cm-1. Furthermore, the high conductivity is stable in air without extrinsic ion contributions that are associated with the commonly used conductive polymer poly(ethylenedioxythiophene) (PEDOT:PSS). The thermoelectric power factor compared favorably with prior reports for p-type polymers that were made by the alternative process of immersion of polymer films into dopant solutions, and fit the established models for thermoelectric polymers. Additional data obtained from thiophene copolymers containing the ethylenedioxithiophene and thieno[3,2-b]thiophene subunits supported these conclusions. Blend microstructure, assessed using grazing incidence X-ray scattering, and mobility evaluated in field-effect transistors, was not adversely affected by the blending. Experiments, calculations and simulations are consistent with the idea that blending and doping polymers with closely-spaced energy levels and compatible morphologies to promote carrier mobility favor increased power factors relative to the individual polymers.
(Invited) Phonon Engineering for Improved Seebeck Coefficient and Thermal Conductivity of Thermoelectric Nanolaminate Films Synthesized on Porous Templates
Authors: Baumgart Helmut , Chen Xin , Zhang Kai
Published: February 2020

Recently promising advances have been achieved with phononic crystal (PnC) nanostructures in thermoelectric materials, because the thermal conductivity of PnC samples is lower compared to non-patterned thermoelectric samples due to phonon-boundary scattering. In this study we report on the successful synthesis of PbTe and PbSe thermoelectric layers on patterned porous silicon substrates by a thermal ALD system. PbTe/PbSe nanolaminates grown on porous silicon membranes have higher Seebeck coefficients than the ones grown on regular planar silicon wafers. The higher Seebeck values result from the lower thermal conductivity k in porous structures, which in turn lead to a higher figure of merit ZT. We demonstrate the feasibility to enhance the figure of merit ZT further by modulating the size and periodicity of the pattern and the thickness of the thermoelectric film in relation to the mean free path (MFP) of the phonons of the thermoelectric material. In our study PbTe and PbSe thin films and nanolaminates were synthesized by Atomic Layer Deposition (ALD) technology on regular planar silicon wafers and on macro-porous silicon templates. ALD is a surface saturating, self-limiting, absolutely conformal thin film synthesis technique reacting two pulsed precursors that enable film growth of one atomic layer per growth cycle. Lead bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C11H19O2)2), (trimethylsilyl) telluride ((Me3Si)2Te) and (trimethylsilyl) selenide ((Me3Si)2Se) were employed as the chemical ALD precursors for lead, tellurium and selenium, respectively. The surface morphology observations indicate Volmer–Weber growth mechanism of PbTe/PbSe nanolaminates during the thermal ALD deposition process rather than classic layer-by-layer growth. The ALD PbTe/PbSe nanolaminates exhibited complete surface coverage on planar bulk silicon wafers and also inside the pores of microporous silicon membranes. For the case of porous Si templates, the Seebeck measurements in both the horizontal and vertical directions revealed significant enhancement in Seebeck coefficients for PbTe/PbSe nanolaminates synthesized inside strictly periodic lithographically defined porous silicon membranes.
(Invited) Planar Thermoelectric Micro Refrigerators Based on Nanograined SiGe Thin Films
Authors: Huang Baoling
Published: February 2020

Rapid development of modern miniaturized microelectronic and optical devices demands for low-power and fast-response on-chip refrigeration to reduce thermal noise and improve device lifetime. Thermoelectric microrefrigerators offer an attractive all-solid-state solution with many prominent merits. Conventional thermoelectric microrefrigerators widely adopt integrated-circuit (IC)-incompatible toxic heavy metal compounds as thermoelectric elements, rendering them unsuitable for on-chip integration. Here we developed free-standing planar thermoelectric microrefrigerators based on nanograined SiGe thin films. The novel support-free design and the record high performance of the nanograined SiGe thin films lead to significant improvement in the cooling performance. A maximum cooling temperature of 10.3 K together with a response time of 16 ms has been achieved in the single-stage microrefrigerator with a power consumption of 56 μW near room temperature while a maximum cooling temperature of 11.2 K can be achieved in the two-stage refrigerator with 0.41 mW input power. The cooling temperatures of the SiGe microrefrigerators also improve with the increasing ambient temperature, reaching up to 15 K and 17 K for the single and two-stage microrefrigerators respectively at 340 K. These SiGe thin-film devices can deliver cooling performances comparable to the records for traditional heavy-metal-based microrefrigerators while providing excellent IC compatibility and almost one to two orders of magnitude higher cooling temperature to power consumption ratios (184 K/mW), rendering them ideal candidates for low-power on-chip refrigeration.
Design of Electric Field-Induced High Thermal Conduction Routes in Polymer-Based Nanocomposites with Low Amount of Boron Nitride Nanosheets (≤20 vol%)
Authors: Cho Hong-Baek , Nakayama Tadachika , Choa Yong-Ho
Published: February 2020

Polymeric nanocomposites with high thermal conductivity with and without electrical insulating properties are great potential for thermal interface materials (TIMs) in semiconductor industry, in particular, in the field of high-power-density electronics and optoelectronics. Conventional materials require a large amount, over 70 vol%, of electrically conducting fillers such as carbon allotropes to attain thermal conductivities more than 1 W/mK. However, due to the incorporation of large quntities of expensive fillers and enevitible increase of viscosity are inefficient cost-effectiveness and processibility.

Here, we introduce electric field-induced application techniques to control the assembly of inorganic micro- or nanofillers at a low level of filler loading less than 20 vol% and to achieve enhancement of thermal conductivity in polymer matrix. Boron nitride (BN) nanosheets have been incorporated into electrically insulating polysiloxane matrix to generate high thermally conductive routes inside the nanocomposites. The homogeneous suspension of filler particles and prepolymer mixture was cast woth 200~300 μm thickness and subjected to diﬀerent electric ﬁelds. Analysis showed that BN nanosheets were fabricated in the hybrid ﬁlms with oriented or linear assembles perpendicular to the ﬁlm surface with high anisotropy. In particular, the thermal conductivity of composites with controlled assembly of BN nanosheets could be increased to 1.56 W/mK only using 15 vol% of BN fillers which was over 500% of composite with random distribution. Mechanisms for the ﬁeld-induced fabrication and structural variation of the two dimensional filler assembly in the polymer matrix are elucidated in relation to enhancement of the thermal properties. This technique provides a facile, controllable approach that can be extended to the synthesis of other TIMs in based on polymer-based nanocomposites.
(Invited) Temperature-Dependent Thermal Diffuse Scattering for Scanning Transmission Electron Microscope Thermometry
Authors: Wehmeyer Geoff
Published: February 2020

Scanning transmission electron microscopy (STEM) thermometry techniques open new opportunities for mapping temperature (T) with high spatial resolution. Existing STEM thermometry methods based on measuring thermally-induced strains must contend with small thermal expansion coefficients (<10 parts per million (ppm)/K) for some materials, as well as potentially non-local relationships between strain and temperature. In contrast, the well-known mechanism of thermal diffuse scattering (TDS) offers promise for inherently local T measurements, and Debye-Waller theory predicts that many materials should display temperature coefficients >1000 ppm/K at room temperature. This T-dependent TDS has not been leveraged for STEM thermometry, however, because the Debye-Waller effect on the Bragg peak intensity is typically overwhelmed by the effects of thermal tilts and thermal drift.

Here, we demonstrate quantitative TDS measurements using STEM by measuring the diffuse background intensity (rather than the Bragg peak intensity) in energy-filtered scanning electron nanodiffraction patterns. Applying virtual apertures to these diffraction patterns during post-processing allows us to quantify the T-dependent TDS in the diffuse background between the Bragg spots; previous TEM work (with the beam in flood mode) showed that this diffuse signal is relatively insensitive to thermal tilts and drift. Using this diffuse signal, we measure a position-averaged temperature coefficient of 2400±400 ppm/K for a single-crystal gold film averaged between T=100 K and T=300 K, and compare our results with the predictions of Debye-Waller theory. The measurements display typical temperature uncertainties of 8 K and temperature sensitivities of 51 K Hz-1/2. This TDS-based STEM thermometry demonstration provides a step towards the goal of non-contact nanoscale temperature mapping of thin nanostructures.

Figure 1
(Amazon Catalyst at ECS Grant Winner) Active Control of Heat Flow in Quantum Computing Applications through Piezoelectric Induced Mechanical Strain
Authors: Rajan Aravindh , Kommandur Sampath
Published: February 2020

Current technologies focused on manipulating heat flow primarily employ materials that exhibit metal-insulator transitions. However, for these materials to be efficient, the operating temperature of the device must be close to the transition temperature. Since the transition temperature is not easily tunable, the solution involves developing and studying new materials that have desirable transition temperatures which often is an inhibitor. Furthermore, as temperature and heat are strongly coupled, the ability to actively control heat flow can be limited.

We propose a solution for active control of heat flow that involves altering the thermal conductivity of a material through mechanical strain. Stretching (compressing) a material leads to an increase (decrease) in the spacing between the atoms, results in a decrease (increase) in thermal conductivity of the material. Our approach involves achieving the required strain by applying an electrical voltage, referred to as the piezoelectric effect. In this study, the relationship between mechanical strain and thermal conductivity is experimentally studied on thin films. Both inorganic and organic films, fabricated using solution processing and sputtering techniques will be used to explore this effect. Thermal conductivity measurements will be carried out using 3-omega, which is a well-established electro-thermal technique. There will also be a thrust to quantify anisotropy in thermal conductivities of the piezoelectric films. In this talk, we report data from preliminary experiments that demonstrate the reversible and tunable thermal conductivity of piezoelectric thin films such as ZnO.
(Invited) Impact of Stoichiometry and Interface Configuration on the Time Stability and the Speed-Limiting Step in Memristive SrTiO3 Cells
Authors: Dittmann Regina , Bäumer Christoph , Siegel Sebastian , Hensling Felix V. E. , Heisig Thomas , Gutsche Alexander , Menzel Stephan
Published: February 2020

Resistive switching oxides are promising candidates for future non-volatile memory and for functional units of neuromorphic computing. Epitaxial SrTiO3 thin films can be regarded as model system for resistive switching thin films due to their well-known defect chemistry and the absence of grain boundaries. We have recently shown that so-called eightwise switching in SrTiO3 can be attributed to the release and reincorporation oxygen at the interface to a Pt electrode [1,2]. In the light of this knowledge, we studied in detail the impact of the Sr/Ti stoichiometry on the filament formation process, the filament stability [3] and the switching kinetics. We observed that Sr rich thin films exhibit an improved retention of the low resistive state (LRS) at low current compliance values with respect to the stoichiometric thin films. In order to identify the underlying processes, we investigated the filament formation process of SrTiO3 thin films with different stoichiometry by photoelectron emission microscopy. Stoichiometric thin films form a stable LRS state as soon as the current compliance is sufficient to trigger SrO segregation at the surface [4]. We attribute this to the impeded reoxidation of the oxygen deficient filament by the presence of the SrO at the surface. We could mimic this effect by intentionally depositing additional SrO, or other oxygen blocking layers such as Al2O3 on the surface of stoichiometric thin films and were thereby able to improve the memory window as well as the LRS retention.

We furthermore investigated the switching kinetics of the SET process for different Sr/Ti stoichiometries and interface layers. We compared these results to analytical models for the switching kinetics based on ion movement, modeled by the Mott-Gurney-Law, and an oxygen exchange reaction at the interface, modeled by the Butler-Volmer-Equation, as the rate-determining steps in combination with simulated temperature and electrical potential data from finite element simulations, as shown in Figures 1 ((a) stoichiometric, (b) Sr rich). For stoichiometric SrTiO3 cells, oxygen transfer at the oxide-metal interface is determined as the rate-determining step. In contrast, for Sr-rich STO cells we propose oxygen diffusion within the SrO layer at the metal-STO interface as rate-limiting. We will discuss possible reasons for the different rate-determining steps, such as different vacancy concentrations and exchange probabilities.

References

[1] D. Cooper et al. Adv. Mat.. 29, 1700212 (2017)

[2] T. Heisig et al., ,Adv. Mater. 30, 1800957 (2018)

[3] F.V.E. Hensling, et al., Solid State Ionics 325, 247 (2018)

[4] Bäumer et al., Nature Commun. 6, 8610 (2015)

Figure 1
Resistive Switching in Atomic Layer Deposited HfO2/ZrO2 Nanolayer Stacks
Authors: Tang Lin , Maruyama Hiraku , Zhang Dou , Nino Juan C.
Published: February 2020

Resistive switching materials and devices have received unparalleled recent attention due their potential and demonstrated use as resistive random access memory (RRAM) and neuromorphic applications. Resistive switching or memristive materials can serve as two-terminal electrical components where the resistance is a function of the amount and direction of current previously experienced and therefore they can uniquely emulate biological synapse behavior. HfO2 is a well-known dielectric and memristive material that has been previously explored for RRAM applications. Here, to better understand the resistive switching in harfnia in the presence of insulating interfaces, the electrical properties of HfO2/ZrO2 nanolayer stacks are discussed. The stacks with total thickness of 16 nm were prepared on platinized Si substrates using atomic layer deposition (ALD). A capping layer of TiN (30 nm thick) and a Au top electrode (100 nm thick) were sputtered to complete the device. We will discuss the current-voltage behavior, pulse time mode measurements, retention and endurance tests of the devices. Resistive switching was observed in all multilayer samples, with the set voltage (Vset) decreasing with increasing number of layers (i.e., number of hafnia-zirconia interfaces). We will show that grazing incidence x-ray diffraction (GI-XRD) demonstrated the transition of the hafnia phase from monoclinic to orthorhombic structure during the post metallization annealing, and that shifts in the binding energy of the x-ray photoelectron spectroscopy spectra (XPS) imply the existence of hafnia suboxide (HfO2-δ). More importantly, we will show how memristive/insulating nanostructures like nanolayered HfO2/ZrO2 can help modulate the resistive switching of memristive-based devices, to include for example, a two-step electroforming process, and multistate resistive switching.
Electrical Pulse Driven Insulator-Metal Transition of VO2 Devices As Leaky-Integrate-and-Fire Artificial Neurons
Authors: Xu Zhen , Fan Zhaoyang
Published: February 2020

Both artificial neurons and synapses are needed to efficiently implement artificial neural networks directly in hardware. Memristors have been actively investigated to implement synaptic functions. In contrast, the implementation of artificial neurons are still limited to Si-based circuit components, while single device based artificial neuron has not been well explored.

In this study, we show that VO2 two-terminal devices, driven under electrical pulses, can be used to implement the leaky, integrate and fire function of spiking neurons. Since the thermally driven metal-insulator transition of VO2 is volatile and electrical pulses can produce Joule heating, while the heating and cooling have delay times, VO2 devices exhibit characteristic firing time and relaxation time, corresponding to timing difference of the exciting pulse and the resistance switching. In addition, the integrating effect of several small pulses can also trigger the firing from high resistance to low resistance transition. These characteristics suggest the potential using the volatile phase transition of VO2 for artificial spiking neuron. Experimental measurements are further elaborated by a 2-D heterogeneous resistor network based modelling. The insulator-metal transition in VO2 under electrical bias was simulated to confirm the dynamic formation of current filament between two electrodes, and the formation of this path is a flashing process during a single pulse in the pulse train when the integrate effect of these pulses arrives the threshold.
Probabilistic Silicon Synapse for Learning- and Energy-Efficient Neuromorphic System
Authors: Wang Gunuk
Published: February 2020

For sustainable advancements in electronics technology, the field of neuromorphic electronics; i.e., electronics that imitate the principle behind biological synapses with a high degree of parallelism has recently emerged as a promising candidate for novel computing technologies. In this study, we designed and fabricated a vertical form of a gate-tunable binary Si switching device using a SiO
x
memristor in combination with graphene barrister, which is inspired by tunable and probabilistic synaptic signaling processing of the rod-to-rod bipolar cells in the human visual system. The device architecture can function as a changeable probabilistic artificial synapse that can be utilized as the basic component of the sparse neural network capable of low-power and learning acceleration for recognizing images. We also developed a drop-connect algorithm that can reflect the sparse connectivity in the biological neural network, and then evaluated the recognition accuracy and the consuming energy for several fashion items according to the probabilistic degree of Si synaptic activity in the drop-connect neural network. In this result, our suggested probabilistic Si synapse and the drop-connect network could offer a distinctive and novel strategy of a neuromorphic computing system equipped with low-power and learning acceleration.
Parallel Programming of an Ionic Floating-Gate Memory Array for Scalable Neuromorphic Computing
Authors: Fuller Elliot James , Keene Scott T , Melianas Armantas , Wang Zhongrui , Agarwal Sapan , Li Yiyang , Tuchman Yaakov , James Conrad D. , Marinella Matthew J. , Salleo Alberto , Talin A. Alec
Published: February 2020

Neuromorphic computers could overcome efficiency bottlenecks inherent to conventional computing through parallel programming and read out of artificial neural network weights in a crossbar memory array. However, selective and linear weight updates and <10nA read currents are required for learning that surpasses conventional computing efficiency. We introduce an ionic floating-gate memory array [1] based upon a polymer redox transistor connected to a volatile conductive-bridge memory (CBM). Selective and linear programming of a transistor array is executed in parallel by overcoming the bridging threshold of the CBMs. Synaptic weight readout with currents <10nA is achieved by diluting the conductive polymer in an insulating channel to decrease the conductance. The redox transistors endure > 109 ‘read-write’ operations and support > 1MHz ‘read-write’ frequencies.

[1] Fuller et. al. Science, 2019 (in press)
Ultralow Power Dual Gated Sub-Threshold Oxide Neuristors: An Enabler for Higher Order Neuronal Temporal Correlations
Authors: John Rohit Abraham , Tiwari Nidhi , Basu Arindam , Mathews Nripan
Published: February 2020

Inspired by neural computing, the pursuit of ultralow power neuromorphic architectures with highly distributed memory and parallel processing capability has recently gained more traction. However, emulation of biological signal processing via artificial neuromorphic architectures does not exploit the immense interplay between local activities and global neuromodulations observed in biological neural networks, and hence are unable to mimic complex biologically plausible adaptive functions like heterosynaptic plasticity and homeostasis. Here, we demonstrate emulation of complex neuronal behaviours like heterosynaptic plasticity, homeostasis, association, correlation and coincidence in a single neuristor via a novel dual-gated architecture. This multiple gating approach allows one gate to capture the effect of local activity correlations and the second gate to represent global neuromodulations, allowing additional modulations which augment their plasticity and enabling higher order temporal correlations at a unitary level. Moreover, the dual-gate operation extends the available dynamic range of synaptic conductance while maintaining symmetry in the weight-update operation, expanding the number of accessible memory states. Finally, operating neuristors in the sub-threshold regime enables synaptic weight changes with high gain, while maintaining ultralow power consumption of the order of femto-Joules.
Synthesis and Electrical Properties of ALD Deposited TiO2-Based Memristor Structures
Authors: Zhang Kai , Lin Pengtao , Pradhan Aswini K , Baumgart Helmut
Published: February 2020

Memristor devices have been rapidly emerging as a promising candidate in the field of non-volatile memory devices. There are a wide range of applications such as non-volatile random access memory, dynamic random access memory, image processing, flash memory, neural networks, and artificial intelligence. In recent years, many new materials for memristors have been investigated including metal-oxides due to their potential for high-density integration, high endurance, fast switching behavior, low power consumption and simple structure. Among the promising binary transition metal oxide materials, such as nickel oxide, zirconium oxide, zinc oxide, hafnium oxide, and Ta2O5, TiO2 films are extensively used to fabricate nonvolatile memristor devices due to its simple structure and compatibility with CMOS integration processes. Recently, ZnO has been widely used for various applications due to its good electrical conductivity, wide band gape (3.37 eV), high exciton binding energy (~60 meV), low cost, nontoxicity, high mechanical and thermal stability. When doped with Aluminum ZnO grown via atomic layer deposition (ALD) has been reported to show resistivity values ranging from insulating to on the order of 10−3 Ω·cm, which is suitable for memory device electrodes.

Many methods are available for synthesis of thin films such as magnetron sputtering, pulsed laser deposition, thermal oxidation, electrodeposition, chemical vapor deposition, sol gel chemical reaction, and atomic layer deposition. ALD technique is considered promising for potential application in the preparation of memristors based on active TiO2 layers. The ALD is a self-limiting technique that allows one atomic layer growth for each growth cycle. ALD can precisely control the film layer thickness, stoichiometry, composition, uniformity, and sharp interfaces. ALD also shows perfect conformal coverage when it deposits thin film on complex surface structures. Therefore, ALD is considered as a novel and competitive method to deposit MIM memory structures. To create such MIM devices, first a transparent Al:ZnO film was grown on a Si wafer as a transparent bottom electrode followed by an active layer of TiO2 film. Then the top contact layer consisting of Al:ZnO was deposited in-situ on the TiO2 layer to complete the memristor structure. All the thin films in the memristor structure were synthesized sequentially by ALD technology.

Several physical characterization techniques have been utilized to analyze the ALD TiO2 thin films of memory devices. The crystal structure was determined by X-ray diffraction (XRD). The film morphology was studied by field emission scanning electron microscopy (FE-SEM). The surface roughness was measured by atomic force microscopy (AFM). The electric properties were tested by a semiconductor parametric analyzer. The results demonstrate promising memristive properties of synthesized ALD TiO2-Based Memristors sandwhiched between Al:ZnO electrical contacts.
(Invited) 3D Passive Devices and Through-Substrate Connections for Medical and Automotive Applications
Authors: Bunel Catherine , Lefeuvre E , Jacqueline S , Jatlaoui M
Published: February 2020

Murata has developed a unique technology in silicon for capacitors and interposers with integrated passive devices .The research was focused on advanced concepts with 3D trenches with extremely high aspect ratio and exquisite high K dielectrics to get to the highest capacitor density with excessive reliability. The technology is compliant with strong capability in terms of packaging enabling outstanding performances of miniaturized products and heterogeneous solutions.

This paper outlines two stacking technologies: one in medical where the performance and form factor parameter are becoming more and more important and the other one is in automotive where higher computing performance will be entailed. Both are subject to demanding requirements in terms of reliability.

The 3D integration schemes, one that doesn’t rely on traditional methods of interconnect and the other with Through Silicon Via (TSV), will be depicted in addition to the manufacturing processes.
Controllable Single Crystal Copper Morphology on Copper Foil By Electrodeposition
Authors: Chen Chia-Hsiang , Dow Wei-Ping , Lin Liang-Jie
Published: February 2020

The Copper foil have been developed for a long time in history. Nowadays, the requirements of the copper foil used for various electrical products and applications in our life. Electrodeposited (ED) copper foils with excellent electrical characteristics and a low cost would be a promising candidate to replace rolled copper foils. In this work, ED copper foils were fabricated in a bubble flow plating bath at 25°C. Three additives were used in the copper plating solution, which can control the surface morphology on the ED copper foil. The interaction of these additives directly influence the surface morphology with distinct structure and roughness. The surface morphologies, roughness and structure orientation of these ED copper foils were characterized by SEM, TEM and XRD, respectively. As a result, controllable copper morphology (i.e., copper nodules) deposited on the ED copper foil was single crystal copper, which possesses better electric and thermal conductivity. The special structure extends the applications of the ED copper foil in next generation.
Using Reduce Graphene Oxide (rGO) As a Conducting Layer for a PCB Metallization
Authors: Chang I-Hsuan , Dow Wei-Ping
Published: February 2020

Electroless (ELS) copper plating process has been commonly used for PCB metallization. However, the ELS copper process is not environment-friendly because it contains precious metal (i.e., Pd catalyst), formaldehyde and chelating agents. Also, ELS copper plating process gives rise to several problems on the metallization of through holes (THs) and microvias.

In this work, ELS copper deposition process was replaced by a reduced graphene oxide (rGO) process to form a conducting layer on the sidewalls of THs and microvias of a PCB for subsequent copper electroplating. rGO has an excellent conductivity and metal barrier capability. Also, the rGO process has more advantages such as environment-friendly, short process steps, no toxic chemicals and organic solvent. A high copper throwing power (> 90 %) in the THs with a high aspect ratio can be obtained using rGO as a conducting layer, even better than ELS copper plating. Moreover, the rGO process could result in good copper filling performance in the microvias without seam or void in the via center. After thermal reliability test, the rGO grafted on the sidewall of the through holes and microvias could pass critical thermal reliability criterion due to its excellent thermal stability. Therefore, brand-new and promising rGO process is hopeful to play a significant role in the future PCB fabrication.
Study on Electromigration of Nano-Silver Paste with Different Hot Pressing Sintering Conditions
Authors: Chen In Gann , Yang Chia Ming , Chio Yan Ruei , Hsu Lien Chung , Hsieh Po Tsung
Published: February 2020

Silver is suitable for use as a interconnect material due to its good thermal conductivity and electrical conductivity. Additionally, nano-sized silver particles (or pastes) can be rapidly sintered at related low temperatures (<250°C ) due to their high surface to volume ratio. The nano-sized silver particle paste developed in this study can be hot-pressed at 250°C and 10 Mpa to form a low porosity (6.2%) and low-resistivity (2.9*10-8 Ω-m) interconnect, which is comparable to that of commercial available products.

With the demand for higher packing density of integrated circuit, the interconnect line width decreases and the current density increases. In general, if the current density is increased to higher than 105 A/cm2, electro-migration phenomenon would become more serious and could cause unpredictable failure of interconnect. In order to understand the electro-migration mechanism of silver interconnect made by nano-paste followed by hot pressing sintering, the porosity of samples with different hot-pressing parameters was measured. Subsequently, current-stressing tests (at 250°C and 105 A/cm2 ) of interconnects were carried out. Through observations of changes in the surface morphology and cross-sectional microstructures during current-stressing tests, the failure mechanism and electro-migration phenomenon are investigated in detail. It is observed as shown in the figure that the electro-migration occurred in early stage (within about 1 day) that resulted in the decrease of porosity near cathode and increase of porosity near anode respectively.

Figure 1

Figure 1
Computational Modeling of Copper Deposition in through Silicon Via Structures
Authors: Braun Trevor Michael , Josell Daniel , Moffat Thomas P.
Published: February 2020

Copper electrodeposition is critical to integrated circuit manufacturing, ranging in scale from nanometer features in damascene plating to micrometer scale through-silicon via structures and even millimeter scale printed circuit board fabrication. Essential to this technology is electrodeposition of void-free copper in high aspect ratio cavities, which is achieved through various additives that enable bottom-up filling. In through-silicon-via structures, void-free filling can be realized with micro-to-milli molar concentrations of polyether suppressor and chloride additives. A variety of analytical measurements have shown chloride adsorption on copper is a necessary precursor to formation of a polyether blocking layer to suppress deposition. In this study, experimental measurements and computational models are presented exploring the dynamics of polyether and chloride adsorption during copper deposition. Microelectrode measurements eliminate solution iR effects, while demonstrating comparable results to rotating disk electrode systems at equivalent transport conditions. A two-additive co-adsorption model for polyether and chloride is presented, comparing simulations to experimental feature filling at conditions when chloride transport is limiting formation of the inhibiting ad-layer (< 80 μmol/L). Suppression breakdown is related to the local polyether coverage, which is in turn limited by the chloride coverage on the depositing interface. A passive-active transition of copper deposition is associated with limited flux of chloride in the feature. At higher chloride concentrations (≥ 80 μmol/L), sidewall breakdown during deposition occurs nearer to the bottom of the via, followed by a shift to bottom-up growth.
(Invited) Controlling Light with Phonons in Polar Semiconductors: Novel Approaches to Infrared Nanophotonics
Authors: Caldwell Joshua David
Published: February 2020

The field of nanophotonics is based on the ability to confine light to sub-diffractional dimensions. Up until recently, research in this field has been primarily focused on the use of plasmonic metals. One highly promising alternative are the surface phonon polaritons that can be supported with low-losses within polar dielectric and semiconductor species. However, these modes can only be supported within a relatively narrow, material specific spectral region referred to as the Reststrahlen band. Here, methods for broadening and shifting these Reststrahlen bands will be discussed, as well as the potential for expanding it using carrier-phonon coupling.
Electrical Double Layer Gated Field Effect Transistor Biosensors for the Quantitative Detection of Beta-Human Chorionic Gonadotropin
Authors: Smith B , Liao Liang-Wen , Chen Po-Hsuan , Wang Yu-Lin
Published: February 2020

In this research, we have developed biosensors to detect beta human chorionic gonadotropin (beta-HCG) which is a hormone widely used to test for pregnancy and also serves as a biomarker for risk assessment of certain types of cancers. The biosensor methodology is based on extended gate design of electrical double layer (EDL) gated field effect transistor (FET). In this study, we fabricated a sensor array chip and functionalized the gold electrode surface with antibodies to capture the antigen, beta-HCG. Each sensor in the sensor array chip consists of two gold electrodes, one connected to the gate voltage supply and the other connected to the gate terminal of the FET, which is located on isolated PCB such that the test solution does not directly contact the transistor active region. A hand-held sensor measurement unit is fabricated which can apply the biasing voltages, measurement the resulting drain current signal and display the output results in LCD monitor.

Fluorescence measurement technique is used to confirm and analyze the surface functionalization with antibodies to beta-HCG. The relation between beta-HCG antigen concentration and electrical signal is established by testing the sensor array chips using the hand-held device. The test samples are prepared by diluting purified stock proteins in 1X PBS, as it simulates the physiological conditions of human urine. Using our FET biosensor, we can directly detect beta-HCG in high ionic strength media like 1X PBS, without performing additional sample processing. Most pregnancy tests have a sensitivity of 25–50 IU/l (1.5-3 ng/ml), based on immunochromatography which is the most widely technique for detection of beta-HCG. Using our sensor methodology, we can obtain a sensor response curve with a dynamic range of antigen concentration between 0.03 to 30 ng/ml. As the antigen concentration approximately approaches 10 ng/ml, the current gain value is nearly saturated. The result shows that when beta-HCG antigen concentration increases, current gain value decreases. This sensing methodology can facilitate quantitative detection of beta-HCG in physiological conditions, whereas the current commercially available assays provide only qualitative test results. The sensor demonstrates a very wide dynamic range of detection, with a very low detection limit, which shows the potential of our sensor to detect pregnancies in very early stages. Furthermore, this sensor platform can enable the use of beta-HCG for risk stratification of cancers as well. The sensor can be batch produced and does not use additional labels/reagents or sample pre-processing techniques, which can greatly enhance the cost-effectiveness, comparable to the current commercialized beta-HCG assays.
Epitaxial Growth of Cu2O on Ag Substrate by Electrodeposition
Authors: Dai Dajin , Huang Pei-Yu , Chang Liuwen
Published: February 2020

Cuprous oxide (Cu2O) is a p-type semiconductor with a direct bandgap of 2.1 eV and high hole mobility at ambient temperature. Recently, the importance of using an epitaxial film to fabricate optoelectronic devices to improve the performances has been pointed out [Mahenderkar et al., Science, 355, 1203–1206 (2017)]. Investigation of epitaxial growth of Cu2O on, mostly, MgO substrate by molecular beam epitaxy or pulsed laser deposition has been carried out for a decade. However, these techniques require high-quality substrates and expensive facilities. Electrodeposition, on the other hand, provides another option with ease of operation. However, the substrate has to be electrically conductive and possesses a low lattice mismatch to Cu2O.

In this study, the epitaxial growth of Cu2O on Ag substrate by electrodeposition was studied by a combinatorial substrate approach mainly using the electron backscatter diffraction (EBSD) technique. The effect of substrate orientation on epitaxial growth can thus be explored efficiently. The lattice mismatch between Ag and Cu2O is 4.3 %. Prior to the deposition, the annealed, polycrystalline Ag substrate was electropolished and analyzed by EBSD over a marked region of about 500 x 500 μm2. Atomic force microscopy was used to characterize the surface morphology of the substrate grains of different orientations. Afterwards, Cu2O films were deposited galvanostatically on the substrates in an electrolyte containing copper sulfate and lactic acid. The crystal structure and orientation of the marked area was analyzed by EBSD again after electrodeposition. The orientation distribution of the films and the orientation relationship between the overlaid Cu2O and the Ag substrate were thus obtained. X-ray diffraction was also used for phase identification. Results indicated that the substrate grains exhibited similar surface roughness values of approximately 3 nm regardless the orientation. X-ray diffraction reults confirmed that the films deposited at different conditions contained solely the Cu2O phase. No diffraction peaks of Cu and CuO were found. At a low current density of 0.06 mA/cm2, the film has a rough surface composing of faceted islands of 5-20 mm in size. A portion of the islands exhibited the same orientation as the substrate grain. On the other hand, at a high current density of 0.25 mA/cm2, films with smooth surfaces were obtained. The Cu2O films all grew epitaxially on the Ag grains in two orientation relationships: a cube-on-cube one and a <100>/45o one. The origin of the <100>/45o orientation relationship was further studied and will be explained in detail. The deposited films were also analyzed by photoluminescence at room temperature and showed strong near band-edge emissions at 1.97-2.06 eV.
Electroanalytical Characterization of Electrochemical Capacitor Systems
Authors: Mavroudis Helena , Forghani Marveh , Donne Scott W
Published: February 2020

This study aims to investigate the behaviour of the electrode materials during charging and discharging processes using an indicator electrode. Generally, the capacitive behaviour of the electrochemical capacitors is associated with the electrical double layer at the surface of the electrode, and in some materials pseudo-capacitance, which results from redox processes. More importantly, it is possible that some oxidized or reduced species which are soluble in electrolyte diffuse or migrate from one electrode to another electrode. Hence, an indicator electrode can detect these ions movement during the charging and discharging processes.

In this study, each electrochemical cell has been made of two symmetrical activated carbon (AC) electrodes with the same mass of active material on each electrode. The AC electrochemical cells have been tested in the non-aqueous electrolyte (1 M TEABF4) and aqueous electrolyte (0.5 M K2SO4) using cycling voltammetry (CV) at 25 mV/s sweep rate between 0 and 2.3 V and 0 to 2 V respectively. It was followed by the step potential electrochemical spectroscopy (SPECS) experiment with a 10 mV potential step and 30 s of equilibration time. The SPECS method is based on applying a series of equal magnitude potential steps on a working electrode, with sufficient rest time to allow for equilibrium to be established for each step throughout an applied potential window. This slow scan rate allows the separation of the charge storage mechanisms, such as electrical double layer and diffusion-limited processes [1]. At the same time, the open circuit potential (OCP) of both electrodes have been measured with respect to the Ag/AgNO3 reference electrode during the CV and SPECS experiments. This technique allows one to see how an individual electrode behaves in the overall cell performance.

Moreover, the electroanalytical characterisation of the AC electrochemical cell has been investigated using the indicator electrode. In this method, the platinum mesh was located in the middle of the two separated papers between two electrodes and the Pt mesh potential was clamped at -± 0.5 V with respect to the Ag/AgNO3 reference electrode during the CV and SPECS experiments. This new technique enables one to detect the migration of oxidised or reduced species from each of the electrodes during charging/discharging processes. In an ideal capacitor, this indicator electrode should have zero current; however, it has been observed that the indicator current changes at both positive and negative potentials during charge storage mechanism. This indicates the emitting and migration of redox species between the two electrodes. Also, it can be seen that there are different effects of aqueous and non-aqueous electrolyte on the behaviour of the redox migration.

Reference: [1]. Marveh Forghani and Scott W. Donne, Journal of the Electrochemical Society 2018 165: A664-A673.
Supercapacitors from Waste: Converting Pulp and Paper Mill Waste to Nitrogen Doped Supercapacitors
Authors: Latham Kenneth G
Published: February 2020

The conversion of waste into energy storage materials has become an important topic in the field of sustainability. The challenge being that new materials from waste should perform better than those that are currently available, be cost effective, environmental friendly and scalable. The was streams from pulp and paper mills make an ideal candidate for upgrading, as they contain a wide range of biomaterials that have the potential to be converted into carbon materials for supercapacitors. The challenge with these waste streams is that they are wet and contain a range of inorganic materials.

To overcome this, hydrothermal carbonization was utilized to convert the waste streams into carbonaceous materials. Hydrothermal carbonization is a wet, low temperature, thermochemical carbonization technique that excels in converting wet biomass into carbon products that can be easily activated an converted into carbon electrodes for supercapacitors. Additionally, the carbon materials can be doped with nitrogen, phosphorus, boron and sulfur by simply adding the dopant to the raw material prior to carbonization. Doped carbon materials have displayed improved performance and stability over their non-doped counterparts, as the dopants provide additional charge storage sites, increasing capacitance.

In this work we have examined the potential to upgrade black liquor, an inexpensive by product of the pulping process, via hydrothermal carbonization for supercapacitors. These materials have been doped with a range of readily available nitrogen sources (NH4Cl, melamine, urea) to increase the stability and capacitance of the electrode. Additionally, a range of activation agents (KOH, K2CO3, KHCO3 and melamine) were investigated to improve the capacitance and stability. The electrodes were tested in aqueous electrolytes, 4M KOH and 0.5M Na2SO4, with stable voltage windows of 1.1V and 1.8V, respectively.

To examine the capacitance and cycle stability, cyclic voltametry and galvanostatic charge-discharge measurements were employed, finding that most materials had a capacitance <190 Fg-1 at 25 mV s-1. The electrochemical variations between the materials were further examined by Step Potential Electrochemical Spectroscopy, which allowed the decoupling of double layer and pseudocapacitance contributions. These were combined with in-depth X-ray Photoelecton Spectroscopy of the surface to draw correlations between pseudocapacitane from the carbon-oxygen/carbon-nitrogen functional groups and capacitance contributions from the double layer.