Radio-frequency Sputtering System Research Articles

Investigation of Resistive Switching of Insulating Hafnium Nitride for Nonvolatile Memory Applications

In this study, nitrogen-rich Hafnium Nitride (HfN) featuring insulating properties was investigated for achieving resistive switching, crucial for the functionality of resistive random-access memory (ReRAM) devices. Devices were fabricated with a 15-nm HfN resistive switching layer using a Radio Frequency (RF) sputtering system. The fabricated devices successfully exhibited a bipolar switching characteristic with a high On/Off ratio (up to 104). An interesting 2-step behavior was also observed during the formation of the conduction filament which was suspected to be tied to the migration of the nitrogen ions. This is the first attempt at using HfN as the resistive switching material for nonvolatile memory applications.

Aluminum-Doped Zinc Oxide Enzymatic Dopamine Biosensor Integrated With Potentiometric Readout Circuit Board

In this study, the standard industrial flexible printed circuit board (FPCB) was employed to fabricate the potentiometric enzymatic dopamine (DA) biosensor. The aluminum-doped zinc oxide (AZO) was used to make the sensing films of the DA sensor. This sensing film was prepared by a radio frequency (RF) sputtering system. The thickness and surface morphology of the AZO films were observed by the field emission scanning electron microscope (FE-SEM), and the elemental composition and oxidation state of the films were analyzed through X-ray electron spectroscopy (XPS). Tyrosinase (Tyr) was used as the enzyme of the sensor, which was immobilized on AZO film with (3-Aminopropyl)triethoxysilane (APTES) and glutaraldehyde. After the sensor was prepared, we integrated the sensor and the readout circuit board through the FPCB connecter and then measured the sensing characteristics of the DA sensor in the DA concentration range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1~\mu \text{M}$ </tex-math></inline-formula> –10 mM. The sensing characteristics mostly include average sensitivity and linearity, limit of detection (LOD), interference effect, response time, reproducibility, flexibility, and lifetime. Based on the experimental results, the DA sensor in this study has good sensing characteristics. Through the enzyme, the sensor has good selectivity for DA in the presence of other relatively high concentrations interfered with biomolecules.

Potentiometric Nonenzymatic Ascorbic Acid Sensor for Static and Dynamic Measurements

We fabricated a potentiometric nonenzymatic ascorbic acid (AA) sensor. The main sensing materials are molybdenum trioxide (MoO3) film and copper oxide nanoparticles (CuO NPs). Basic sensors and sensing materials were fabricated by the screen printing technique, the radio frequency sputtering system, and the green synthesis method. The average sensitivity, linearity, and interference effects of the AA sensor under static measurements were examined. In order to simulate dynamic measurements, we performed experiments by the microfluidic system and analyzed the best sensing characteristic at low flow rates (1.4– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.0~\mu \text{L}$ </tex-math></inline-formula> /min) and high flow rates (10– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50~\mu \text{L}$ </tex-math></inline-formula> /min). The best average sensitivities under static and dynamic measurements are 35.26 and 61.11 mV/decade, respectively. Ultimately, the AA sensor achieves better stability and accuracy with the help of a unity-gain frequency and low power consumption instrumentation amplifier (UGFPCIA). This study shows that the AA sensor can possibly achieve excellent sensing properties even without the help of enzyme and a conventional commercial chip (LT1167).

The Sensing Characteristics Investigation of a Potentiometric Ascorbic Acid Biosensor Integrated With Analog-to-Digital Converter

In this study, an enzymatic ascorbate oxidase/(AO) ruthenium dioxide (RuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) ascorbic acid (AA) biosensor was realized by a radio frequency (RF) sputtering system. The characteristics of the RuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> sensing membrane were analyzed by field-emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS). Moreover, electrochemical impedance spectroscopy (EIS) was performed to show that a higher concentration of AA caused a larger capacitance of the RuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> membrane. Therefore, more charges can be accumulated on the sensing membrane. To read out the AA sensing signals, an instrumentation amplifier (IA) integrated with an analog-to-digital converter (ADC) was utilized to substitute the traditional data acquisition (DAQ) device. Then, the analog signals were converted into binary codes and transmitted to the computer, and the binary codes were multiplied by the weighting and converted to decimal codes. Hence, the average sensitivity and the linearity were obtained which were 58.22 mV/decade and 0.991, respectively. The hysteresis effects of the AO/RuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> AA biosensor were also discussed. The hysteresis voltages in forward and reverse cycles were 3.77 mV and 4.59 mV, respectively.

ZnO and AZO Film Potentiometric pH Sensors Based on Flexible Printed Circuit Board

In this study, we deposited zinc oxide (ZnO) and aluminum-doped zinc oxide (AZO) on the electroless nickel immersion gold (ENIG) of a flexible printed circuit board (FPCB) as a potentiometric pH sensor. The sensing films of the pH sensor were fabricated by a radio frequency (RF) sputtering system and analyzed by field emission scanning electron microscope (FE-SEM) and X-ray photoelectron spectroscopy (XPS). In the pH 2 to 10 buffer solutions, it was observed that the characteristics of the pH sensor through the voltage–time (V-T) measurement system include average sensitivity and linearity, drift effect, and repeatability. According to the experimental results, the pH sensors in this study could exhibit good characteristics.

Resistive Switching and Synaptic Characteristics in ZnO/TaON-Based RRAM for Neuromorphic System.

We fabricated an ITO/ZnO/TaON/TaN device as nonvolatile memory (NVM) with resistive switching for complementary metal-oxide-semiconductor (CMOS) compatibility. It is appropriate for the age of big data, which demands high speed and capacity. We produced a TaON layer by depositing a ZnO layer on a TaN layer using an oxygen-reactive radio frequency (RF) sputtering system. The bi-layer formation of ZnO and TaON interferes with the filament rupture after the forming process and then raises the current level slightly. The current levels were divided into high- and low-compliance modes. The retention, endurance, and pulse conductance were verified with a neuromorphic device. This device was stable and less consumed when it was in low mode rather than high mode.

Post-annealing treatment in improving high dielectric constant MgO-based metal-oxide-semiconductor diodes

Metal-oxide-semiconductor (MOS) diodes with a high dielectric constant magnesium-oxide (MgO) insulating layer were fabricated using a magnetron radio frequency sputtering system. MgO has a high dielectric constant of approximately 11.2, which is three times higher than the dielectric constant (3.9) of silicon dioxide (SiO2), thereby ensuring a three times thicker gate oxide and reducing gate leakage current while maintaining the same capacitance density. Post-annealing treatment was employed on the MgO film to study how annealing treatment affects the electrical characteristics of MOS diodes. It was observed that the post-annealing treatment of MgO effectively diminished the gate leakage current by approximately one order, thereby increasing the rectification ratio from 8.5 × 103 to 6.8 × 104 for the MOS diodes with as-deposited and post-annealed MgO. In contrast to the MOS diodes with as-deposited MgO, the post-annealing treatment of MgO significantly decreased the flatband voltage shift from 7.8 to 1.3 V and reduced the fixed oxide charge density from 1.3 × 1012 to 2.3 × 1011 cm−2; also, the interface trap charge density was suppressed from 1.8 × 1013 to 3.2 × 1012 cm−2 eV−1. Large quantities of fixed oxide charge attracted more electrons accumulated at the Si surface, which decreased the barrier height from 0.85 to 0.81 eV for the MOS diodes with post-annealed and as-deposited MgO. Moreover, x-ray photoelectron spectroscopy showed that the oxide charges were caused by the defects inside MgO, particularly oxygen vacancies. The oxygen vacancies were compensated by the oxygen atoms introduced from the air during the post-annealing treatment.

ZINC SULFIDE ANTI-REFLECTIVE THIN FILM COATING FOR GERMANIUM OPTICAL WINDOWS

In this work, Anti-reflective thin film is made on Germanium (Ge) optical window, which is one of the most used materials in thermal imaging systems. ZnS material was used its optical transmittance between 2-14 m and due to the fact that it has a refractive index proportional to the refractive index of Ge. ZnS thin films have been prepared by Radio Frequency (RF) magnetron sputtering on Germanium (Ge) optical windows for anti-reflection coating (ARC). ZnS films were produced at different thicknesses using RF sputtering system working pressures under 3, 20 and 30 mTorr. The other RF systems parameters such as RF power, deposition temperature were kept constant for all depositions. Crystal structures, optical and surface properties of ZnS thin films were characterized with X-ray diffraction (XRD), Atomic force microscopy (AFM), Fourier transform ınfrared (FTIR) and UV-VIS transmission spectrometer. The characterization results of Ge optical windows coated ZnS thin films grown at 3 mTorr pressure show that high optical transmission and good crystallinity in ınfrared wavelength region (2-14 um).

Enhancing the Performance of NiO-Based Metal-Semiconductor-Metal Ultraviolet Photodetectors Using a MgO Capping Layer

Nickel oxide (NiO) based metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors (PDs) are fabricated using a radio-frequency (RF) magnetron sputtering system. In this study, the surface of NiO is capped with magnesium oxide (MgO) using the same RF sputtering system. The effects of the MgO capping layer on the characteristics of the prepared MSM-PDs are studied. Using X-ray diffraction, a smaller grain size and tensile strain on the MgO-capped NiO is observed, which is due to the introduction of O atoms from MgO. The MgO capping layer effectively reduced the dark current by three orders of magnitude compared to the MSM-PDs without the MgO capping layer. In the MSM-PDs without the MgO capping layer, the photo-to-dark current ratio is only 100, and the ratio is enhanced by three orders of magnitude upon capping the MSM-PDs with MgO. Both the MSM-PDs, with and without the MgO capping layer, exhibit a UV/visible rejection ratio as high as 800, demonstrating the fabricated MSM-PDs are good visible-blind photodetectors. X-ray photoelectron spectroscopy illustrates that the introduced Mg atoms from MgO react with NiO to form strong MgxNi1-xO and/or MgO bonding, which passivate the surface defects on NiO surface, thus significantly reducing the dark current and enhancing the performance of the prepared MSM-PDs.

Base-width modulation effects on the optoelectronic characteristics of n-ITO/p-NiO/n-ZnO heterojunction bipolar phototransistors

ITO/NiO/ZnO npn heterojunction bipolar phototransistors (HBPTs) with various base widths are fabricated using a radio-frequency sputtering system. The effects of base-width modulation on the optoelectronic characteristics of the prepared HBPTs are studied. The dark current of HBPTs decreases with increasing base width because the injected electrons from the emitter are recombined in the wide base region. The photocurrent increases with decreasing base width, which is attributed to higher emitter-base injection efficiency. The responsivity increases with the collector-emitter voltage (V CE) in the HBPTs with a 100 nm base width, whereas the responsivity sharply decreases at V CE > 4 V for the HBPTs with a thinner base width (80 nm) due to the punch-through effect. In contrast, the responsivity approaches saturation at large V CE for HBPTs with a thicker base width (120 nm). The responsivity and detectivity decrease with increasing incident light intensity, which is caused by an increase in the base recombination loss. The HBPTs with a base width of 100 nm exhibits the largest responsivity and detectivity; their detectivity is higher than that of HBPTs with base widths of 80 and 120 nm by approximately two and three orders, respectively.

Investigation of Flexible Arrayed Lactate Biosensor Based on Copper Doped Zinc Oxide Films Modified by Iron-Platinum Nanoparticles.

Potentiometric biosensors based on flexible arrayed silver paste electrode and copper-doped zinc oxide sensing film modified by iron-platinum nanoparticles (FePt NPs) are designed and manufactured to detect lactate in human. The sensing film is made of copper-doped zinc oxide (CZO) by a radio frequency (RF) sputtering system, and then modified by iron-platinum nanoparticles (FePt NPs). The surface morphology of copper-doped zinc oxide (CZO) is analyzed by scanning electron microscope (SEM). FePt NPs are analyzed by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The average sensitivity, response time, and interference effect of the lactate biosensors are analyzed by voltage-time (V-T) measurement system. The electrochemical impedance is analyzed by electrochemical impedance spectroscopy (EIS). The average sensitivity and linearity over the concentration range 0.2–5 mM are 25.32 mV/mM and 0.977 mV/mM, respectively. The response time of the lactate biosensor is 16 s, with excellent selectivity.

Innovative Infrared-pulsed laser assisted RF sputtered β-Ga2O3 thin film at low temperature process

Innovative new method for the pulsed laser assisted radio-frequency sputtering at low temperature process was successfully applied for the β phase Ga2O3 thin film as wide band gap materials. Thin film of wide bandgap material, β-Ga2O3, was successfully deposited on SiO2/Si and quartz substrates by employing radio-frequency sputtering system with assisting process of ytterbium-laser irradiation. Both sputtering system and infrared pulsed laser irradiation process were employed at the same time to increase the acquired energy densities for the adatoms on the thin film growth process. X-ray diffraction patterns (XRD) showed the prominent peaks of unique crystalline properties of Ga2O3 thin films in β phase. The optical energy bandgap of Ga2O3 thin films derived from spectral transmittance was measured in the range of 4.90–5.06 eV for a variation of laser power. High resolution X-ray photon spectroscopy (XPS) analysis showed that O 1s and Ga 3d position in core level. We confirm that infrared optical energy was effectively added to fabricate β-Ga2O3 thin film by employing the Yb-laser-assisted RF sputtering process.

Study of the Glucose Sensor Based on Potentiometric Non-Enzymatic Nafion/CZO Thin Film

This study proposed a potentiometric non-enzymatic electrochemical sensor for glucose detection based on copper doped zinc oxide (CZO) as a sensing film. In this article, a metal oxide film was used, which was able to react catalytically with glucose. The wires of the sensor were prepared by screen printing technology, and the CZO thin film was deposited on the sensor by the radio frequency (R. F.) sputtering system. In order to make the non-enzymatic sensor more stable, Nafion layer was added to the working electrodes. The characteristics of the sensor were measured using a voltage time (V-T) measurement system. Average sensitivity and linearity of the Nafion/CZO glucose sensor was tested in glucose concentrations ranging from 2 mM - 14 mM. After experiment, it was confirmed that the sensor had good performance. Experiments confirmed that the Nafion/CZO sensor exhibited characteristics including: good selectivity for glucose, average sensitivity of 6.45 mV/mM, a linearity of 0.998, a response time of 20 sec, and a limit of detection (LOD) of 0.51 mM. The device has a low relative standard deviation (RSD) and would be suitable for further development and eventual use in biomedical applications.

Analysis of CMOS Extended-Gate Field-Effect Transistor With On-Chip Window Based on Uricase/RuO2 Sensing Film

In this study, the extended-gate field-effect transistor (EGFET) with an on-chip sensing window under the TSMC 0.18μm 1P6M (one poly and six metals) CMOS process technology was fabricated as a biosensor. The sensing window was composed of six metal layers and functionalized with the ruthenium dioxide (RuO2) thin film and uricase for uric acid detection. The RuO2 thin film was deposited on the top metal layer of the sensing window using the radio frequency (RF) sputtering system. The silver probe was employed as a reference electrode to provide a voltage to the test solution with a micro-volume of 0.5 μL. The properties of the EGFET for uric acid detection were investigated through the semiconductor parameter analyzer. The EGFET had a voltage and the current sensitivity of 8.63 mV/(mg/dL) and 0.17 (μA)1/2/(mg/dL), respectively. The device worked with a supply voltage lower than 1.8V. Based on the results, the fabrication of the miniaturized biosensor device was a success. Due to its advantages such as the use of low voltage and its simple fabrication process, it could help realize the development of a wearable biosensor.

Fabrication of Silicon Dioxide by Photo-Chemical Vapor Deposition to Decrease Detector Current of ZnO Ultraviolet Photodetectors.

Zinc oxide (ZnO)-based semiconductor is a promising application for ultraviolet photodetectors (UV PDs). The performance of ZnO UV PDs can be improved in two orientations: by reduction of the dark current and by increasing the photocurrent. In the study, we used two processes to prepare ZnO UV PDs: photochemical vapor deposition to fabricate silicon dioxide as an insulator layer and a radio frequency sputter system to prepare the ZnO film as an active layer. The results show that the silicon dioxide layer can reduce the dark current. Moreover, a large photo–dark current ratio of the metal–insulator–semiconductor (MIS) structured PD is 200 times than the metal–semiconductor–metal (MSM) structured PD. When the silicon dioxide thickness is 98 nm, we can significantly enhance the rejection ratio. The silicon dioxide layer can reduce the noise effect and enhance the device detectivity. These results indicate that the insertion of a silicon dioxide layer into ZnO PDs is potentially useful for practical applications.

Opto-electrical characterization of quaternary sputtered copper indium gallium selenide nanorods via glancing angle deposition

Nanorod arrays have become an attractive alternative to their thin film and bulk counterparts in photovoltaic and photoconductivity research. This is mainly attributed to their superior optical and electrical properties. Light trapping and unique bandgap geometries in vertically aligned nanostructures result in high optical absorption and provide enhanced carrier collection by utilizing a fully depleted p-n junction between the anode and cathode via an isolated “capping” construction. The combination of these two features leads to the development of high efficiency nanostructured devices that can be utilized in solar cells and photodetectors. Optical absorption properties, geometry, and opto-electrical properties of nanorod arrays of CuInxGa(1−x)Se2 (CIGS), a p-type semiconductor with a wide bandgap ranging from 1.0 to 1.7 eV, are compared to their thin film counterparts. Utilizing a radio frequency sputtering system, a quaternary target, and glancing angle deposition technique, both isolated vertical arrays of CIGS nanorods and “core-shell” devices were fabricated, while conventional film devices were fabricated by normal incidence deposition. Scanning electron microscopy images indicated a successful growth of CIGS nanorods. Optical absorption and opto-electrical performance were found to be strongly improved by the presence of the isolated nanorod structures through spectroscopic reflectometry and responsivity testing under a solar simulator.

Investigation of Flexible Arrayed Urea Biosensor Based on Graphene Oxide/Nickel Oxide Films Modified by Au Nanoparticles

In this work, the flexible arrayed urea biosensor is composed of silver paste electrodes by screen printing technology. The sensing film is made of the nickel oxide (NiO) by radio frequency (R.F.) sputtering system and then modified by Au nanoparticles (Au NPs) and graphene oxide (GO). The average sensitivity, response time, interference effect, and electrochemical impedance of urea biosensors are analyzed by the voltage- time (V-T) measurement system and electrochemical impedance spectroscopy (EIS). The surface morphology of the biosensor is analyzed by scanning electron microscope (SEM). The results show that the flexible arrayed urea biosensor has excellent average sensitivity over a wide concentration range (0.01-100 mM) of 57.16 mV/decade, linearity of 0.998, a response time of 25 s, a limit of detection (LOD) of 7.64 μM, and an excellent selectivity.

Wafer-scale high-quality Ag thin film using a ZnO buffer layer for plasmonic applications

Realizing laterally continuous, ultraflat silver (Ag) single-crystal films is a significant technological challenge. Ag thin film grown on various hetero-substrates has been used in numerous applications, due to its superior electrical and optical properties. To exploit these properties without degradation and apply these films to high-precision patterning, surface plasmonics, and so on, a high-quality thin film having an ultraflat surface and few grain boundaries is needed. A zinc oxide (ZnO) buffer layer can be used to facilitate the growth of a single-crystalline Ag thin film on a sapphire (Al2O3) substrate. ZnO films deposited on Al2O3 substrates have grain boundaries; however, Ag films grown on ZnO are nearly grain-free and close to single-crystalline quality. This can be explained by the exceptionally small extended atomic distance mismatch, of ~0.08%, between Ag and Al2O3, in which the ninth Ag atom and the eighth Al atom are matched in terms of coherence and periodicity. A modified radio frequency sputtering system with a single-crystal Ag target enabled wafer-scale growth of ultraflat, grain-free Ag films. The proposed approach using a ZnO buffer provides a new method for fabricating Ag films with high adhesion, anti-oxidative stability, and superior optical properties, and allows for easy nano-patterning.

Analysis of Chloride Ion Sensor Modified by Graphene Oxide Under Microfluid Flow

In this paper, we used a thermal evaporation system and a radio-frequency sputtering system to fabricate aluminum (Al) electrodes and indium–gallium–zinc oxide (IGZO) sensing membranes on the polyethylene terephthalate substrate. The IGZO/Al chloride ion sensor modified by graphene oxide has array sensing windows to avoid sensor malfunction. We used a screen printing technology to fabricate the sensor that made sensor a flexible structure. It can be adapted to many environments. The chloride ion sensor was immersed in the different concentrations of NaCl solutions from $10^{-5}$ to 1 M to measure the sensing characteristics of the chloride ion sensor. From the static measurement, the average sensitivity and linearity of the chlorine ion sensor were 60.536 mV/pCl and 0.973, respectively. We integrated microfluidic framework to measure the dynamic characteristic. From the dynamic measurement, we could know that the best average sensitivity and linearity of the chloride ion sensor for the flow rate of $20~\mu \text{L}$ /min were 64.639 mV/pCl and 0.972, respectively. To confirm the non-ideal effect of chloride ion, the hysteresis voltages of chloride ion in the different concentrations of NaCl solutions in two different cycles were 2.700 and 2.789 mV, respectively.

Investigation of a Wireless Real-Time pH Monitoring System Based on Ruthenium Dioxide Membrane pH Sensor

This paper proposed a wireless real-time pH monitoring system. This system was associated ruthenium dioxide (RuO2) membrane pH sensor, microcontroller, liquid-crystal display (LCD), C/C++ program, and WiFi communication module to develop and use for ecosystem monitoring in the future. The ruthenium dioxide thin film of the pH sensor was deposited onto indium tin oxide (ITO) and polyethylene terephthalate (PET) substrates by the radio frequency (RF) sputtering system. The surface morphology of RuO2 thin film with scanning probe microscopy (SPM) was investigated. The electrochemical impedance analysis of RuO2 thin film was also studied.