United States

SU-8 is widely used in a variety of applications for fabricating micro-scale or millimeter-scale structures due to its high viscosity, high optical transmission in the UV range, and excellent mechanical properties. The use of SU-8 for cryogenic applications has been limited because of possible internal stress in SU-8 films due to CTE mismatch with Si, the most commonly used material as a substrate, which can cause adhesion-related problems. We have developed a fabrication process of micro-scale SU-8 structures on a molybdenum (Mo) substrate using polyimide as an interfacial layer. Fabricated SU-8 structures showed good adhesion to the substrate from room temperature to cryogenic temperatures down to 4.2 K. Mo was chosen as a substrate due to its robustness, excellent mechanical strength, and compatibility with laser and CNC machining. SU-8 fabrication was done on 500-micron thick commercially available Mo wafers. The wafers were cleaned using an H2O2/H2SO4 “piranha” solution to remove organic material from the substrate. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) was done before and after cleaning. SU-8 2075 was used to fabricate 250-micron tall structures with different aspect ratios on the Mo substrate. HD-4100 series polyimide was evaluated as an interfacial layer to improve the adhesion of SU-8 to Mo. Thermal shock tests were done using LN2 to test the adhesion of SU-8 with and without the interfacial layer. Shear tests were performed to characterize the adhesion of the SU-8 pillars to the substrate. The Mo wafers were laser-cut using a femtosecond laser to singulate them into pieces with SU-8 structures. These pieces were used as part of a superconducting interconnection scheme for multi-channel DC superconducting flexible cables, wherein the SU-8 structures provided self-alignment and distributed mechanical pressure over the contact area of the flexible cables. This interconnect scheme was demonstrated to be functional over multiple thermal cycles from room temperature to 4.2 K. The results of these experiments provide guidelines and motivation for using SU-8 with a polyimide interfacial layer in cryogenic system applications. Acknowledgment: We gratefully acknowledge funding and technical guidance from Microsoft for this work. We thank the Alabama Micro/Nano Science and Technology Center (AMNSTC) for providing access to fabrication and characterization facilities used in this work.

EMC 2023

Fabrication of SU-8 Microstructures on Bulk Molybdenum Substrates for Cryogenic Applications

superconducting interconnects

adhesion improvement

su-8

molybdenum

cryogenic

**Welcome to ACL-IJCNLP 2021!**

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ACL-IJCNLP 2021

The great event is jointly organized by the Association for Computational Linguistics (ACL) and Asian Federation of Natural Language Processing (AFNLP).

technical paper

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC). This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. 

The need for higher power RF and energy conversion electronics has motivated the search for new device heterojunctions beyond what can be grown using conventional techniques. Instead, we can conceive of new material layer combinations that result in higher performance transport characteristics, e.g., channel conductivity, breakdown voltage, and heat dissipation. However, realizing a high-quality bonded interface has several challenges including the production and processing of low-defect thin material layers and reducing the interface trap density. Here, we have fabricated bonded n--GaN/p+-Si diodes to investigate the dependence of interface quality on surface pretreatments and post-bond annealing. Si-doped GaN films were grown on BN-sapphire templates [1] before being exfoliated using stressed Ni-spalling [2]. The GaN films were bonded top (Ga-polar)-side down onto (100) P-doped Si. IV and CV measurements were performed to characterize the transport across and the charge at the interface. Conducting scanning probe measurements were performed on the GaN surface to characterize the bond areal uniformity. These results are generally applicable to the optimization of interface properties for novel bonded junctions. [1] M. Snure, G. Siegel, D. C. Look, and Q. Paduano, J. Cryst. Growth 464, 168 (2017). [2] E. W. Blanton, M. J. Motala, T. A. Prusnick, A. Hilton, J. L. Brown, A. Bhattacharyya, S. Krishnamoorthy, K. Leedy, N. R. Glavin, and M. Snure, Small 17, 2102668 (2021).

Investigation of Bonded GaN-Si p-n Junction Interface Properties

Defying the isotropic nature of the conventional chemical etch, metal-assisted chemical etching (MacEtch) has allowed spatially defined anisotropic etching by using patterned metal catalyst films to locally enhance the etch rate of various semiconductors [1]. Significant progress has been made on achieving unprecedented aspect ratio nanostructures using this facile approach, mostly in solution. However, the path to manufacturing scalability remains challenging, because of the difficulties in controlling etch morphology including the minimization of porosity and aggregation, and etching uniformity. Taking MacEtch from liquid phase to vapor phase completely will eliminate surface tension, allow independent control of the partial pressure, injection timing and pattern of each etchant, while preventing the movement of the metal-catalyst films. Prior demonstrations of VP-MacEtch utilize thermally generated vapor by suspending the sample over a heated bath of either HF/H2O2 or HF with atmospheric O2 as the oxidant [2-4]. However, the presence of H2O2 in the vaporized solution serves to increase porosity with little impact on the etch rate compared to ambient oxygen [2,4]. The use of O2 as the oxidant limits the range of etching control, as the molar ratio may only be controlled through the dilution of HF. This approach leads to etch rates substantially lower (0.1-0.4 µm/min [1-3]) compared to the liquid phase (>3 µm/min [5]). Although increasing the concentration of HF does improve the etch rate, because of the fixed oxidant supply as the vapor flows across the sample the O2 becomes depleted leading to non-uniform etching and failure in pattern transfer. Here, we demonstrate a programmable vapor-phase MacEtch (VP-MacEtch) approach, with independent control of the etchant flow rates, injection and pulse time, chamber pressure and temperature, as well as UV irradiation. Silicon (Si) nanostructures are demonstrated with each of these parameters systematically varied synchronously or asynchronously, moving MacEtch closer to a manufacturing technique for versatile arrays of 3D Si micro and nanostructures.[6] Figure 1 presents a fully computer-controlled programmable VP-MacEtch system that operates under vacuum with an N2 carrier gas and representative structures produced, with unprecedented features. This work was supported in part by United States Army Research Office Award No. W911NF2110337 , National Science Foundation ECCS #2200651, and a gift from Lam Research. [1] X. Li and P.W. Bohn, “Metal-assisted chemical etching in HF/H2O2 produces porous silicon,” Appl. Phys. Lett., 77, 2572 (2000). [2] Hildreth, O. J., & Schmidt, D. R. (2014). Vapor Phase Metal-Assisted Chemical Etching of Silicon. Advanced Functional Materials, 24(24), 3827–3833. [3] Kim, J. D., Kim, M., Chan, C., Draeger, N., Coleman, J. J., & Li, X. (2019). CMOS-Compatible Catalyst for MacEtch: Titanium Nitride-Assisted Chemical Etching in Vapor phase for High Aspect Ratio Silicon Nanostructures. ACS Applied Materials and Interfaces, 11(30), 27371–27377. [4] Romano, L., Kagias, M., Vila-Comamala, J., Jefimovs, K., Tseng, L. T., Guzenko, V. A., & Stampanoni, M. (2020). Metal assisted chemical etching of silicon in the gas phase: A nanofabrication platform for X-ray optics. Nanoscale Horizons, 5(5), 869–879. [5] Balasundaram, K., Sadhu, J. S., Shin, J. C., Azeredo, B., Chanda, D., Malik, M., Hsu, K., Rogers, J. A., Ferreira, P., Sinha, S., & Li, X. (2012). Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires. Nanotechnology, 23(30), 305304. [6] Janavicius, L. L, Michaels, J. A, Chan, C., Sievers, D. J, Li, X. (2023). “Programmable Vapor-Phase Metal-Assisted Chemical Etching for Versatile High-Aspect Ratio Silicon Nanomanufacturing,” Apph. Phys. Rev. accepted.

Realization of Vapor-Phase MacEtch: Mechanism, Programmability and Scalability

Wireless interconnected devices have become ubiquitous, with many transformative applications emerging as sensor nodes that scale to smaller dimensions and higher node density. Energy harvesting for sensor systems at the millimeter-scale and microscale has become paramount, where functionality and system lifetime is determined by available power. Energy can be harvested through optical, radio-frequency, mechanical, piezo, thermoelectric and many diverse means [1]. Optical energy harvesting via photovoltaic cells scales efficiently down to the microscale, including sources from outdoor sunlight, indoor lighting, and near-infrared illumination (NIR ~ 850 nm, particularly for biomedical implantable sensors[2]). GaAs/AlAs photovoltaics (PV) provide high power conversion efficiencies for these sources across a broad range of optical spectra Furthermore, GaAs-based PV provides a platform for integrated optoelectronic devices that can be used for low-power optical communications. In this work, we present thin (<8 µm) GaAs PV cells for energy harvesting in microscale systems and heterogeneous integration approaches with low-power CMOS electronics. The PV cell structures consist of a single or dual-junction devices that were grown epitaxially by external foundries. Single junction devices are designed for optical response ranging from outdoor sunlight to 850 nm monochromatic illumination, where lateral series-connected devices may be fabricated to step up voltage output for energy harvesting. Dual junction “segmented” PV devices are designed for monochromatic illumination at 850 nm with target applications for bio-implantable systems. Devices were fabricated using previously established etching, photolithography, and metallization technologies. SiNx passivation layers are used on mesa devices, which also serve as electrical isolation for bonding pads metal interconnects. Fabricated devices are mounted epi-side down on glass substrates for substrate removal using wet chemical etching facilitated by an InGaP etch stop layer. A substrate transfer process is used to flip substrates to a sapphire substrate for device testing or flip chip bonding to silicon CMOS. Single junction and dual-junction PV devices show power density on the order of 1 µW/mm2 under mild 850 nm irradiation with an efficiency >20 %. Experiments using substrate removal utilized a new dual-junction epitaxial growth run where devices exhibited a diode blocking behavior attributed to an improper tunnel junction. Nevertheless, substrate removed devices exhibit power generation and integrity of electrical behavior for both frontside and substrate removed backside illumination. Further progress on substrate removed microscale PV harvesters and flip-chip bonding for device integration will be presented. The authors gratefully acknowledge the National Science Foundation award CBET-2129817 and the National Institute of Health award 1RF1NS128667. J. D. Phillips, Frontiers in Nanotechnology, 3, 633931, 2021 E. Moon et al, … J. D.Phillips, ACS Photonics 8, no. 5 (2021)

Substrate-Removed GaAs Photovoltaic Cells for Microscale Energy Harvesting

2D-layered metal-carbide MXenes have emerged as promising earth-abundant materials for electrocatalysis and energy storage due to their exceptional combination of high conductivity and high surface area. Large scale synthesis of MXenes offers the potential for low-cost device applications, but this goal demands integration with advanced manufacturing approaches such as 3D printing and methods for tuning MXenes’ electrical properties. Here we present two complementary strategies to address these practical limitations of MXenes. The low chemical activity of pristine MXenes such as Ti3C2Tx, and limited active site density inhibit MXenes as practical electrocatalysts for hydrogen evolution reaction (HER). We present a simple and scalable nitrosulfurization method for overcoming these limitations via interfacial doping and structural modification with nonmetallic electron donors (N and S) by heat treatment of 2D titanium carbide (Ti3C2Tx) MXene with thiourea. XPS measurements show substitution of N and S occurs at the basal plane with controllable compositions ranging from 2 - 4 at. % (Sulfur), and 5-15 at. % (Nitrogen) depending on the thermal annealing conditions. HRTEM, SEM, and AFM characterization illustrate simultaneous structural changes from planar nanosheets to hexagonal-nanoplate-decorated nanosheets. XRD reveals increased interlayer spacing after N,S doping, which is expected to increase the diffusivity of electrolytes into the layers and enhance electrocatalytic activity. The optimally N,S doped Ti3C2Tx MXene exhibits a high concentration of Ti-S and Ti-N bonds , yielding a low overpotential of –260 mV and a Tafel slope of 85 mV/dec at a current density of 10 mA/cm2 in 0.5 M H2SO4—3X lower than pristine Ti3C2Tx. This strategy to improve electrocatalytic activity of Ti3C2Tx MXene for hydrogen evolution could open the possibility for manipulating the electrocatalytic performance of a broad range of MXenes. Finally, towards the goal of facilitating device integration of MXenes, we have developed MXene coatings that, when applied on 3D printed conductive lattices, can significantly enhance the specific capacity of microscale porous carbon supercapacitor electrodes. Electrochemical impedance spectroscopy illustrates the capacity for these 3D MXene structures to offer high areal capacitance will delivering low electrode resistance for fast charging and discharge. Mechanical measurements of these electrodes show that integration of 2D MXenes with 3D lattices provides a means for enhancing the electrochemical performance while precisely controlling mass and device geometry.

Anionic Engineering and 3D Integration of 2D Ti3C2Tx MXene for Electrocatalysis and Energy Storage

The urgent need for high-efficiency, low-cost, and environmentally friendly solar cell technology compels new research into innovative solar materials that are stable, earth-abundant, and benign for the development of sustainable thin-film photovoltaics (PVs). Antimony selenide (Sb2Se3) is one such solar-absorbing material that has recently emerged as a promising alternative to CdTe (cadmium telluride) solar cells. CdTe solar cells represent the second-largest PV technology on the market, but contain toxic compounds that have been demonstrated to leach toxins into the soil and water table when left to decompose in landfills. In moderate quantities, the constituent components of Sb2Se3 are inexpensive to source and are non-toxic to humans and the ecosystem [2-3]. Similar to CdTe, Sb2Se3 is a simple binary compound with just one stable phase and stoichiometry – unlike the multivalent copper selenide (CuSe) or tin selenide (SnSe) binary systems. Sb2Se3 exhibits a direct bandgap of 1.1-1.3eV, which approaches the ideal Shockey-Queisser value for single-junction solar cells and demonstrates efficient light harvesting with an absorption coefficient of 105cm-1 in the near-infrared region below about 1000 nm. The >30% theoretical efficiency of Sb2Se3 cells suggests a comparable efficiency to existing CIGS and CdTe solar cells. Other research shows the efficiency of Sb2Se3 thin-film solar cells has improved from 0.66% in 2009 (Nair et al) to 6.6% and 7.6% in 2018 by improving the crystallinity of Sb2Se3 films (Choi et al, Zhou et al). In 2019, the implementation of a one-dimensional (1D) crystal structure improved construction of Sb2Se3 solar cells and resulted in an efficiency of 9.2%. However, these Sb2Se3 solar cells typically use CdS as a n-type partner and therefore are not environmentally friendly due to the toxicity of cadmium. For this reason, we developed and implemented a titanium dioxide (TiO2) n-type partner for the Sb2Se3 solar cells. This paper explores the process of spin-coating fabrication for TiO2 as an n-type partner using three different solutions on FTO-coated soda-lime glass substrates, the deposition of Sb2Se3 for the absorption layer, and nickel deposition by thermal evaporation to construct the complete solar cell. In addition to fabrication processes, we conducted optical-electrical and materials characterization of these TiO2/Sb2Se3 cells to determine their efficiency and investigate their effects on the solar cell performance.

Development of Sustainable Non-Toxic TiO2/Sb2Se3 Solar Cells for Renewable Energy Applications

Semiconductor lasers have a wide range of applications in industry and our daily life. Vertical-Cavity Surface-Emitting Lasers (VCSELs), developed from the 1980’s, have several advantages, compared to edge-emitting lasers, such as wafer-scale testing, uniformity, scalability, circular beam, single-mode, 2D array, etc. A VCSEL consists of a light-emitting active region sandwiched between a top Distributed Bragg Reflector (DBR) and a bottom DBR. In 2017, Apple released the iPhone X embedding VCSELs for face recognition. This was a game-changer. The market is expected to reach $1.9 billion in 2027. Current VCSELs on cell phones for facial recognition are based on 940 nm VCSELs consisting of GaAs/AlAs DBRs grown on GaAs(001) substrates. It is desirable to have longer-wavelength VCSELs, however, because the screen of a smart phone is transparent at longer wavelength (1380 nm) and because of eye safety. The maximum permissible exposure to the retina is higher for wavelength longer than 940 nm. Long-wavelength lasers beyond 1300 nm is commonly fabricated on InP substrates because those wavelengths cannot be accessed by the GaAs/AlGaAs material system. However, it is not practical to grow a DBR on InP because the difference of the index of refraction of InP-based DBR pair, such as InGaAs/InAlAs, is relatively small, compared to that of GaAs/AlAs DBR pair. Thus, many more pairs must be grown to achieve high reflectivity. Also, InP substrates are smaller, more fragile, and more expensive than GaAs substrates. There are several approaches to produce long-wavelength emission from heterostructures grown on GaAs substrates: (1) self-assembled InAs/(In)GaAs quantum dots (QDs), (2) dilute nitride GaInNAs(Sb)/GaAs quantum wells (QWs), (3) wafer bonding of InGaAsP/InP active layers to top and bottom GaAs/AlAs DBRs, and (4) type-II InGaAs/GaAsSb QWs. Each approach has its pros and cons. Here we report on the type-II InGaAs/GaAsSb QWs because QWs have more volume for light amplification than QDs, fewer defects than dilute nitrides, and simpler in processing than wafer bonding. The emission wavelength can be tuned independently by the composition and thickness of the electron and hole quantum wells. There are only a few reports on InGaAs/GaAsSb QWs grown on GaAs substrates. InGaAs and GaAsSb are both compressively strained to GaAs, and tensile-strained GaAsP can be grown to compensate InGaAs and GaAsSb. There is one report of MBE-grown strain-compensated GaAsP/GaAs/GaAsSb VCSELs on GaAs, emitting at 1266 nm. So far, there does not seem to be a report on strain-compensated GaAsP/InGaAs/GaAsSb QWs, let alone VCSELs, on GaAs. In this talk we report on molecular-beam epitaxy (MBE) and characterization of these QWs, specifically investigating the effects of strain compensation. The compositions and layer thicknesses are measured by high-resolution X-ray rocking curves and compared with simulations. Two series of samples were grown. One series have 3 InGaAs/GaAsSb samples grown in the same day: without the GaAsP strain-compensating layer, with GaP and with GaAs0.8P0.2 strain-compensating layers. We found that only the last sample with GaAs0.8P0.2 strain-compensating layers shows a strong low-temperature (LT) photoluminescence (PL) at 1100 nm. The second series also have 3 samples with 2, 4, and 6 nm In0.3Ga0.7As thickness while the corresponding GaAs0.8P0.2 thickness is also adjusted to maintain strain compensation and be within its own critical layer thickness. We found that only the 2 nm-thick In0.3Ga0.7As sample shows a strong LT PL at 1050 nm, indicating 4 nm thickness is already larger than the critical layer thickness. These samples do not have room-temperature (RT) PL, but later samples grown with a higher substrate temperature show RT PL. In conclusion, as expected, strain compensation is beneficial, and we are pursuing longer wavelength emission by adjusting the layer parameters through COMSOL and growth conditions.

The Effects of Strain Compensation in Type-II InGaAs/GaAsSb Quantum Wells Grown on GaAs(001) Substrates

Mid-wave infrared (MWIR) detectors (λcut-off>3μm) are commonly used in medical devices, remote sensing, and spectroscopy [1]. MWIR III-V semiconductors, such as InSb (λcut-off=5.3μm) are more readily fabricated as infrared focal plane arrays (FPA’s) because of their weaker ionic and stronger bonding property compared to II-VIs. III-V-based FPA’s also have the advantage of spatial uniformity, temporal stability, scalability and affordability. This is particularly true in the case of InSb FPAs.[2] To cover the MWIR spectrum and reach the long wave infrared (LWIR) region, type-II superlattices such as InAs/InAsSb, bulk InAsSb on GaSb and metamorphic buffers are used [3]. Metamorphic buffers allow for better coverage of the MWIR spectrum by allowing for increasing antimony percentage in InAsxSb1-x. However, it is challenging to achieve repeatability in the growth of these thick buffer layers (4μm). Previously, direct growth of GaSb on GaAs has been achieved with a threading dislocation density of 108 per cm3 and further reduction is possible with the use of defect filter layers [4]. In this research, we utilize interfacial misfit dislocations to grow fully relaxed InSb on InAs substrate as well as direct growth of AlxIn1-xSb buffer layers on InAs, similar to the GaSb/GaAs system. Developing a fully relaxed, low dislocation density buffer layer will be the basis for developing tunable InAsSb absorbers for MWIR and LWIR applications. The MBE growth of InSb growth on a InAs substrate starts with the removal of native oxide. To grow a fully relaxed InSb layer using interfacial misfit dislocation arrays, the excess arsenic on the InAs surface must be replaced with antimony. We achieve this by eliminating the Arsenic flux. The surface is monitored using reflection high energy electron diffraction (RHEED). The RHEED pattern turns to 4x2 from 2x4 in the absence of arsenic supply which indicates the transition from a group V rich to a group III rich surface. The process is controlled manually by observation of the RHEED pattern transition, and an extra 30 seconds is added after the transition to achieve uniformity over the substrate. The next step involves growth of 500 nm thick Al0.2In0.8Sb and Al0.1In0.9Sb buffer layers to optimize the buffer layers with different growth temperatures. Finally, InSb quantum wells (QWs) are grown for photoluminescence analysis. Both the directly grown AlInSb and InSb epilayers on InAs substrates are analyzed using HR-XRD ω-2θ scans as well as reciprocal space mapping to quantify the dislocation density of the buffer layers. HR-XRD indicates a relaxation of 99.4% for the direct growth of InSb on InAs, and a slightly reduced value for the Al0.1In0.9Sb and Al0.2In0.8Sb epilayers. By using reciprocal space mapping, we are able to determine the screw and mixed dislocation density for the layers. TEM analysis of the samples shows formation of misfit dislocation arrays at the AlInSb/InAs interface. InSb QWs grown with different AlxIn1-xSb barrier layers are investigated by photoluminescence (PL). We will present a detailed analysis of the buffer layers with respect to threading dislocation density using XRD scans as well as TEM images. We will also present a comprehensive analysis of the InSb QWs on the basis of PL and TRPL results. [1] W. L. Sarney, S. P. Svensson, Y. Xu, D. Donetsky, and G. Belenky, “Bulk InAsSb with 0.1 eV bandgap on GaAs,” J. Appl. Phys., vol. 122, no. 2, p. 025705, Jul. 2017. [2] A. Rogalski, “Next decade in infrared detectors,” in Electro-Optical and Infrared Systems: Technology and Applications XIV, Warsaw, Poland, 2017. [3] D. Z. Ting et al., “Advances in III-V semiconductor infrared absorbers and detectors,” Infrared Phys. Technol., vol. 97, pp. 210–216, Mar. 2019. [4] A. Mansoori et al., “Reducing threading dislocation density in GaSb photovoltaic devices on GaAs by using AlSb dislocation filtering layers,” Sol. Energy Mater. Sol. Cells, vol. 185, pp. 21–27, Oct. 2018.

MBE Growth of InSb Quantum Well on InAs (100) Using AlInSb Buffer Layer

In this research, multiple commercially available copper (Cu) nanoparticle conductive inks have been investigated, both as received and in modified form, for future flexible printed electronic device fabrication and its applications. This work reports the characterization and fabrication of the inkjet printed flexible Cu electrodes. Different fluid properties such as viscosity, surface tension, particle size measurement, and pH values were investigated along with their temperature dependance. Three commercial formulations, Ink 1: DM-CUI-5002 (Dycotec Materials Ltd. UK), Ink 2: Metalon® CI-004 (NovaCentrix, USA), Ink 3: Metalon® CI-005 (NovaCentrix, USA), and a fourth custom diluted version of the Dycotec ink (Ink 4) were investigated. The mean particle sizes of the different Cu nanoparticle inks are 56.2 nm for Ink 1/ Ink 4, 8.62 nm for Ink 2, and 11.5 nm for Ink 3. The viscosities at 25°C for Inks 1 through 4 are 17.14, 31.30, 13.52, and 15.02 cPs, respectively. The resistivity obtained for each nanoparticle ink (Ink 1-4) varied from 1.01× Ω/cm to 2.31× Ω/cm. Different fluid properties of the nanoparticle inks have been investigated and optimized for inkjet printing using Fujifilm Dimatix Materials Printer - DMP 2850. All the jetting recipes are developed based on both legacy printheads and recently launched (2021) Samba G3L printheads. This new printhead requires a lower ink viscosity which potentially renders all commercially available Cu inks unsuitable for use without modification. Hence, the optimized diluted Dycotec Cu ink has been modified with diethylene glycol monoethyl ether (5:1) to achieve the viscosity within the range of Samba G3L operating parameters. Inkjet printing parameters varied based on different printheads and formulations of inks i.e. platen temperature for both printheads were chosen as 35°C, cartridge temperature varied from 40 - 50°C, jetting voltage ranged from 28 to 35 V, jetting frequency and meniscus set point for legacy and Samba printheads were chosen as 5 kHz and 0.5 as well as 3.5 kHz and 5 respectively. The drop spacing used for printing different patterns with both print heads and all the types of Cu inks was chosen as 15 mm (1693.3 DPI) to ensure 70% overlapping of the two adjacent ink droplets. Polyimide has been chosen as a printing substrate as it offers excellent adhesion characteristic, mechanical stability, and flexibility over a wide range of temperature i.e., -269 °C to 400 °C. The optimization of the sintering process has also been investigated thoroughly. The required temperature to sinter the printed Cu films was settled at 300-400 °C. To remove oxidation 3% hydrogen gas in 97% argon was flowed in the vacuum chamber with ~4 Torr. Electrochemical impedance spectroscopy (EIS) measurement was performed for the inkjet printed Cu electrodes to evaluate their performance in terms of impedance. This process was performed using phosphate buffer saline with pH 7.43. The EIS was performed for two different designs (plain and interdigitated electrodes) for the chosen (based on their compatibility with Samba G3L printheads – DMP 2850) ink solutions (Ink 2 – plain: 575 Ω & interdigitated: 400 Ω and Ink 4 – plain: 418 Ω & interdigitated: 380 Ω). Based on our experiments/investigation (fluid properties) and reported optimized inkjet printing parameters (waveforms, platen temperature, and cartridge temperature), this research indicates the potential viability of utilizing commercially available Cu ink – Ink 3 for printing flexible electronics with both legacy and newly commercialized Samba G3L printheads. However, Ink 4 showed excellent performance with the lowest impedance (resistivity and EIS) making it a promising formulation for flexible inkjet printed Cu electronics. Honeywell Federal Manufacturing and Technologies, LLC operates the Kansas City National Security Campus for the United States Department of Energy / National Nuclear Security Administration under Contract Number DE-NA0002839.

Characterization and Fabrication of 3D Inkjet Printed Flexible Copper Electrodes

Recently, stretchable strain sensor is considered as the promising technology for various application, such as wearable electronics, healthcare monitoring, and robotic skin/arms, due to its mechanical stretchability and multi-axial sensing capabilities. The recent research for stretchable strain sensors have made tremendous efforts to significantly improve key deterministic parameters, including sensitivity, durability, sensing range, and response time. Although various high-performance strain sensors have been demonstrated using a range of mechanistic device structures, most conventional strain sensors are only capable of detecting uniaxial strain and lack the ability to measure directional variable motions. In order to overcome this challenge, novel sensor devices with geometrically engineered architectures, composed of multidimensionally stacked channels or cross-shaped detecting electrodes have been developed. These architectures were developed to address the challenge of achieving multidirectional strain sensing, by employing isotropic conducting pathways. While these advancements are significant, the complicated and intermixed pathways in the channel region can cause unsatisfactory linearity and make it difficult to determine the strain direction under complex stimulation environments. In addition, it is worth noting that many recent studies have utilized machine learning techniques, such as Neural network to classify data obtained by strain sensor. However, in order for Neural network to be applied effectively in industry, optimization processes such as model lightweighting are necessary. Here, we propose a simply designed array of wearable strain sensor capable of detecting and interpreting the direction and intensity of strain through a neural network algorithm trained through cognitive learning. The wearable strain sensor arrays comprises a heterogeneous surface architecture consisting of a stretchable substrate made of polydimethylsiloxane (PDMS), strain-insensitive rigid electrode regions made of polyethylene terephthalate (PET) film, and a strain-sensing channel region composed of multi-walled carbon nanotube (MWCNT) with carbon black (CB) nanofiber dispersed in a PDMS matrix. We utilized a mechanism based on the change in the number of conducting pathways between MWCNT and CB depending on the direction of strain. The use of a heterogeneous device architecture can reduce the negative effects of electrode strain on electrical interruptions, resulting in more reliable strain sensing characteristics. This wearable strain sensor arrays exhibited high sensitivity with a gauge factor of approximately 8, a wide strain sensing range of 0 to 35%, and excellent reliability after 3,000 stretching cycles. To detect strain from any direction, Strain sensor array was made using three sensors arranged at 45-degree tilted angles. Furthermore, to achieve high-accuracy detection of both the intensity and direction of strain, a machine learning algorithm known as a multi-output neural network was employed. A neural network was constructed for strain sensing, which showed an accuracy of over 95% for both strain intensity and direction. In addition, optimization of the strain sensing system’s neural network was done by adjusting the number of sensors and input data dimension to achieve a lightweight model. By employing this method, not only data obtained from the wearable strain sensor arrays be efficiently handled, but also the strain intensity and direction can be accurately classified.

Carbon Based Omnidirectional Wearable Strain Sensor Arrays with Optimized Multi-Output Neural Networks

Consisting of nearly 80 billion neurons and hundreds of trillions of synapses, the human brain is one of the most complex information-processing systems in the known universe. To understand how the brain functions, it is crucial to be able to measure the spatiotemporal dynamics of the generation and propagation of signals between each of these neurons. Measuring these signals can provide insight into normal function as well as shed light into the signatures or root causes of diseases such as epilepsy or Parkinson’s disease. However, measuring the electrical activity of neurons at such a large scale has proven to be an extremely challenging endeavor. Electrocorticography is one method that is commonly used in the clinic to measure aggregate neural activity. Cutting-edge electrocorticography systems may employ thin-film fabrication of flexible microelectrode arrays with dense wiring schemes to achieve higher channel-count systems. These microelectrode arrays are typically fabricated using polymers such as parylene-C and the microelectrodes are routed individually and conductively bonded to rigid silicon-based acquisition circuits for digitization and downstream signal processing. Recent efforts have demonstrated high-density microelectrode arrays which can achieve thousands of channels; however, it is a major challenge to form reliable interconnects over one thousand channels between the flexible microelectrodes and the rigid acquisition electronics. Taking inspiration from the high-definition flexible display industry, we have developed a 16x16 array of actively multiplexed microelectrodes using dual-gate thin film transistors made of indium gallium zinc oxide and fabricated on flexible polyimide substrates at low temperature. These neural interfaces use on-site multiplexing to help drastically reduce the interconnect bottleneck and thus improve the scalability of the system towards higher channel counts. Furthermore, this marriage of flexible substrates with active electronics is accomplished in a monolithic fashion, allowing for superior performance in a soft and wet biofluid environment. A major challenge with achieving reliability in such a system is the hermeticity of the encapsulation. Active electronics require biasing and control signals, thus any failure in encapsulation, if not properly designed for, can cause charge injection directly into cortical tissue. Thus, we have incorporated conformal atomic-layer-deposited ceramic biofluid barriers in addition to polymer encapsulation layers to help improve the operation of our active electronics in biofluid mediums. We are currently evaluating this encapsulation strategy in accelerated aging soak tests in elevated saline chambers, and have observed functional TFT microelectrode arrays after several weeks of soaking in saline. We expect that this strategy should be compatible with chronic experimental lifetimes of several months and plan to perform animal testing in the rat model after completing benchtop characterization. Another crucial aspect of this work is ensuring that the TFTs are robust over long periods of time. We have found that the contact resistance, mobility, and threshold voltage are all very sensitive to annealing and surface preparation conditions. By incorporating fluorine-based plasma treatment to the IGZO contact areas, and by optimizing the metal deposition conditions, we have found that we can lock in dopants in the IGZO film such that after curing the encapsulating polyimide films in nitrogen ambient, the annealing of these films results in stable and time-invariant electrical properties.

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