Substrate Bias Research Articles

High-performance thermochromic YSZ/[formula omitted]/YSZ coatings for energy-saving smart windows

The reversible semiconductor-to-metal transition of vanadium dioxide (VO2) makes VO2-based coatings a promising candidate for thermochromic smart windows, reducing the energy consumption of buildings. We report on a scalable sputter deposition technique for fast preparation of strongly thermochromic YSZ/V0.986W0.014O2/YSZ coatings, where YSZ denotes Y-stabilized ZrO2, on conventional soda-lime glass at a relatively low substrate surface temperature (350 °C) and without any substrate bias voltage. The thermochromic V0.986W0.014O2 layers and the antireflection YSZ layers were deposited using a controlled high-power impulse magnetron sputtering of a single V–W and Zr–Y target, respectively. A coating design utilizing a second-order interference in the YSZ layers was applied to increase both the integral luminous transmittance (Tlum) and the modulation of the solar energy transmittance (ΔTsol). We present the phase composition (X-ray diffraction) and microstructure (high-resolution transmission electron microscopy) of the coatings and their optical properties (spectrophotometry and spectroscopic ellipsometry). The YSZ/V0.986W0.014O2/YSZ coatings exhibit a transition temperature of 33–35 °C with Tlum = 64.5% and ΔTsol = 7.8% for a V0.986W0.014O2 thickness of 37 nm, and Tlum = 46.1% and ΔTsol = 13.2% for a V0.986W0.014O2 thickness of 67 nm. The results constitute an important step to a cost-effective and high-rate preparation of large-area thermochromic VO2-based coatings for future smart-window applications.

Towards lowering energy consumption during magnetron sputtering: Benefits of high-mass metal ion irradiation

The quest for lowering energy consumption during thin film growth by magnetron sputtering techniques becomes of particular importance in view of sustainable development goals. As large fraction of the process energy is consumed in substrate heating for the purpose of providing high adatom mobility necessary to grow dense films, the most straightforward strategy toward more environment-friendly processing is to find alternatives to thermally activated surface diffusion. One possibility is offered by high mass metal ion irradiation of the growing film surface, which has been recently shown to be very effective in densification of transition metal nitride layers grown with no external heating, such that Zone 2 microstructures of the structure-zone model are obtained in the substrate temperature Ts range otherwise typical for Zone 1 growth. The large mass difference between the incident ion and the atoms constituting the film results in effective creation of low energy recoils, which leads to film densification at low Ts. Due to their high mass, metal ions become incorporated at lattice sites beyond the near-surface region of intense recoil generation leading to further densification, while preventing the buildup of residual stress. The practical implementation of this technique discussed in this Perspective employs heavy metal targets operating in the high-power impulse magnetron sputtering (HiPIMS) mode to provide periodic metal-ion fluxes that are accelerated in the electric field of the substrate to irradiate layers deposited from direct current magnetron sputtering (DCMS) sources. A key feature of this hybrid HiPIMS/DCMS configuration is the substrate bias that is synchronized with heavy metal ion fluxes for selective control of their energy and momentum. As a consequence, the major fraction of process energy is used at sputtering sources and for film densification, rather than for heating of the entire vacuum vessel. Model material systems include TiN and metastable NaCl-structure Ti1−yAlyN films, which are well-known for challenges in stoichiometry and phase stability control, respectively, and are of high relevance for industrial applications. This Perspective provides a comprehensive overview of the novel film growth method. After presenting basic concepts, time-resolved measurements of ion fluxes at the substrate plane, essential for selective control of metal ion energy and momentum, are discussed. The role of metal ion mass, energy, momentum, and concentration is described in more detail. As some applications require substrate rotation for conformal coating, a section is devoted to the related complexity in the implementation of metal-ion-synchronized growth under industrial conditions.

High-mass metal ion irradiation enables growth of high-entropy sublattice nitride thin films from elemental targets

Synthesis of high-entropy sublattice nitride (HESN) coatings by magnetron sputtering is typically done using custom-made alloyed targets with specific elemental compositions. This approach is expensive, requires long delivery times, and offers very limited flexibility to adjust the film composition. Here, we demonstrate a new method to grow HESN films, which relies on elemental targets arranged in the multicathode configuration with substrates rotating during deposition. TiVNbMoWN films are grown at a temperature of ∼520 °С using Ti, V, Nb, and Mo targets operating in the direct current magnetron sputtering mode, while the W target, operated by high power impulse magnetron sputtering (HiPIMS), provides a source of heavy ions. The energy of the metal ions EW+ is controlled in the range from 80 to 620 eV by varying the amplitude of the substrate bias pulses Vs, synchronized with the metal-ion-rich phase of HiPIMS pulses. We demonstrate that W+ irradiation provides dynamic recoil mixing of the film-forming components in the near-surface atomic layers. For EW+ ≥ 320 eV the multilayer formation phenomena, inherent for this deposition geometry, are suppressed and, hence, compositionally uniform HESN films are obtained, as confirmed by the microstructural and elemental analysis.

In-situ monitoring of stress evolution in high power impulse magnetron sputtering-deposited Ti-Al-N films: Effect of substrate bias and temperature

The origin of residual stress during film deposition is a topic of strong interest in the coating engineering community; it requires appropriate investigation tools to explain the different mechanisms acting on the evolution of stress concerning process conditions and material characteristics. In the present work, we used in-situ curvature stress probing combined with ex-situ techniques such as X-ray diffraction cos2α*sin2ѱ applied to the deposition of model Ti0.42Al0.58N coatings. The films were prepared at room temperature and 300 °C, and four substrate bias strategies were studied, namely two constant values of substrate bias (0 V and −75 V), and two systems with alternating high bias and no bias. At room temperature, the application of bias changed the stress from slightly tensile to compressive, while at 300 °C, the use of high bias increased the compressive intrinsic stress by 3 GPa along with hardness. Linking in-situ and ex-situ stress measurements permitted a direct assessment of thermal stress, which varied considerably with bias. The combined analysis revealed the presence of different contributors to stress generation, particularly at the grain boundaries. The relevance of each contributor to the final stress state was found to depend on the energetic growth conditions. Stress monitoring revealed significant differences between the growth of dense films on the porous ones compared to the opposite sequence, suggesting a straightforward pathway to stress mitigation. This is documented by films produced in the alternating −75/0/−75 V bias sequence that presented comparable mechanical properties as films deposited at a constant high (−75 V) bias, but with substantially lower residual stress.

Influence of Substrate Bias Voltage on the Electrical and Optical Properties of IWO Thin Films

Influence of Substrate Bias Voltage on the Electrical and Optical Properties of IWO Thin Films

Ultrathin superconducting TaCxN1−x films prepared by plasma-enhanced atomic layer deposition with ion-energy control

This work demonstrates that plasma-enhanced atomic layer deposition (PEALD) with substrate biasing enables the preparation of ultrathin superconducting TaCxN1−x films. By comparing with films grown without substrate biasing, the enhanced ion energies yield a hundredfold reduction in room-temperature resistivity: a comparably low value of 217 μΩ cm is obtained for a 40 nm film. The ion-energy control enables tuning of the composition, counteracts oxygen impurity incorporation, and promotes a larger grain size. Correspondingly, the critical temperature of superconductivity (Tc) displays clear ion-energy dependence. With optimized ion energies, a consistently high Tc around 7 K is measured down to 11 nm film thickness. These results demonstrate the high ultrathin-film quality achievable through PEALD combined with substrate biasing. This process is particularly promising for the fabrication of low-loss superconducting quantum devices.

Effects of Bias Voltage and Substrate Temperature on the Mechanical Properties and Oxidation Behavior of CrWSiN Films

CrWSiN films were prepared through the co-sputtering technique, and the process variables were substrate bias voltage and temperature. The mechanical properties of hardness and elastic modulus of the CrWSiN films were dominantly affected by their average crystallite size and by substrate bias voltage and temperature. Moreover, the effect of substrate temperature was more evident than that of substrate bias. The highest hardness and elastic modulus of 42.6 and 459 GPa, respectively, were obtained for the Cr20W28Si9N43 film fabricated at a substrate temperature of 400 °C, which exhibits an evident advantage over the 25.0 and 323 GPa values for the Cr21W28Si9N42 film fabricated at room temperature. In contrast, an increase in negative bias voltage to −100 V on the substrate decreased the mechanical properties compared to one prepared using a similar process without applying the negative bias voltage. The oxidation resistance of the Cr-enriched Cr37W4Si10N49 and Cr37W5Si10N48 films was superior to that of the Cr20W28Si9N43 films with near-equal Cr and W contents annealed at 900 °C in air. The formation of a surficial Cr2O3 layer plays a vital role in restricting subsequent oxidation for CrWSiN films.

Deposition of nanometer-thick amorphous carbon films by filtered cathodic vacuum arc under oblique ion incidence conditions

Filtered cathodic vacuum arc (FCVA) is a promising technique for synthesizing extremely thin carbon films exhibiting highly tetrahedral (sp3) carbon atom hybridization. However, most FCVA studies are for incident ions impinging perpendicular to the substrate surface. In this study, the effects of the ion incidence angle on the thickness, structure, and surface roughness of amorphous carbon (a-C) films deposited under different substrate bias voltages were examined in the light of computational and experimental results. Dynamic simulations and film thickness measurements showed a decrease in film thickness with increasing ion incidence angle, attributed to the decrease in carbon ion fluence on the film surface. Carbon atom hybridization analysis revealed a critical ion incidence angle for a given substrate bias voltage. Simulation and experimental results demonstrated that the enhancement of sp3 hybridization resulted from carbon-carbon collision cascades on the growing film surface and densification induced by direct and recoil implantation processes controlled by the ion incidence angle and substrate bias voltage. The strong dependence of a-C film growth on the carbon ion incidence angle exemplified by the results of this study has direct implications in the oblique deposition of ultrathin a-C films with predominant tetrahedral carbon atom hybridization. The present study provides a framework for optimizing the ion incidence angle to achieve the deposition of a-C ultrathin films with high sp3 contents.

Substrate bias effects on cathodic arc deposited Cr coatings

We investigate the effects of substrate bias on cathodic arc deposition of Cr coatings without external substrate heating at four different substrate bias levels, floating, −30, −50, and −70 V. After 10 min of deposition, the substrate temperature reaches 180, 210, 230, and 260 °C for floating potential, −30, −50, and −70 V, respectively. Time-of-flight energy elastic recoil detection analysis shows that all grown coatings are of high purity, with no coating containing more than 0.2 at.% of C and/or O. Increasing the substrate bias also reduces the number of macroparticles, steers the texture from preferred [110]- to [100]-orientation and induces a residual compressive stress of ∼450 MPa in the coatings. The hardness of the coatings remains at a constant 7.5 GPa irrespective of the substrate bias. The four-point probe resistivity of the grown coatings is 15–17 μΩcm regardless of substrate bias, close to the 12.9 μΩcm of bulk Cr.

SOI FinFET Design Optimization for Radiation Hardening and Performance Enhancement

This work proposes Total Ionizing Dose (TID) hardening techniques compatible with conventional 14-nm-node silicon-on-insulator (SOI) FinFETsf process flows through performing 3-dimensional (3-D) simulations based on technology computer-aided design (TCAD) tools. The simulation results reveal a significantly critical TID impact induced by trapped charges in the buried oxide (BOX) and the spacer with calibration against 14 nm SOI FinFETfs experimental data (error < 6%). Inspired by the physical interpretation, an optimization technique featuring an optimized gate structure, spacer length, and substrate bias is designed. The optimized gate structure is utilized to enhance the local gate-to-channel coupling at the bottom and reduce the generation and capture of electron-hole pairs. By reducing the spacer length, a lower sensitive volume in the spacer can effectively suppress the TID response. The setting of the negative substrate bias greatly improves the subthreshold characteristics, weakening the TID effect in the BOX. By adopting the combined optimization including these techniques, the threshold voltage shift (VTH) induced by a 5 Mrad(SiO2) irradiation can be reduced to 21 mV, whereas VTH is 45 mV with only gate structure optimized and 97 mV without hardening. Meanwhile, the ION/IOFF after radiation increases to 1 × 107, which is at least four orders of magnitude better than the original device. Meanwhile, subthreshold swing (SS) is reduced from 81 mV/dec to 71 mV/dec, and Drain Induced Barrier Lowering (DIBL) is reduced from 120 mV/V to 99 mV/V, respectively. The combined optimization design is demonstrated as an effective method to improve the tolerance against TID irradiation without compromising performance, promoting 14 nm SOI FinFET’s application in future harsh space environments.

Effect of Nitrogen Pressure and Substrate Bias Voltage on Structure and Mechanical Properties of Vacuum Arc Deposited VN Coatings

The binary vanadium–nitrogen (V–N) coatings were formed using cathodic arc evaporation. Two sets of coatings were produced using: (a) nitrogen pressure (pN2) from 0.001 Pa to 3 Pa at a constant substrate bias voltage (UB) of − 100 V and (b) a substrate bias voltage from − 50 to − 300 V at a constant nitrogen pressure of 1.5 Pa. The influence of the above parameters on the coating properties, in particular on the insufficiently investigated and described adhesion of the coatings to the substrate, was demonstrated. The phase transformation V → V + V2N → V + c-VN → h-VN → h-VN + c-VN and c-VN → h-VN occurs for coatings formed with increasing nitrogen pressure and substrate bias voltage, respectively. With the increase in pN2 and UB, an increase in coating hardness and adhesion to the substrate is observed, as well as an improvement in wear resistance.

Fabrication and Characterization of Self-Aligned WSe2 p-Type Field-Effect Transistor

Semiconducting transition metal dichalcogenides (TMDCs) have been growing interests as channel materials in field effect transistors (FET) for next generation low power digital electronics. [1]. Since a complementary metal oxide-semiconductor (CMOS) inverter constructed from pairs of n-type and p-type FETs is a fundamental building blocks in modern digital electronics [2], implementation of both n-type and p-type FETs with semiconducting TMDCs is of particular importance [3]. Among various semiconducting TMDCs, tungsten diselenide (WSe2) is a promising candidate for constructing a CMOS inverter because of its high mobility, symmetric electron and hole effective mass and ambipolar transport [4]. These prominent features of WSe2 is suitable for an application to CMOS inverter.One of the significant challenges is to develop a top gate and top contact FET structure with WSe2 for CMOS device integration on the same wafer. In addition, it is desirable to minimize the access region between channel and S/D contact because this access region served as a series resistance to channel resistance, resulting in the lower drain current in FET. This study reports a self-aligned process to fabricate top gate and top contact WSe2 p-type FET. In this self-aligned structure, the gate electrode and source/drain contact edges are automatically positioned and hence there are no overlap or access region between the gate and source/drain. The feature of this proposed method is that the gate stacks was utilized as a mask for self-aligned formation of WOx, which is source/drain contact for efficient hole injection [5]. The focus on this study is the impact of self-aligned structure of p-type WSe2 FET on the electrical characteristics. The effectiveness of proposed process was experimentally demonstrated by the fabrication and characterization of device.Figures 1 shows the self-aligned fabrication process of p-type WSe2 FET. 20 nm-thick SiO2 was formed by dry oxidation of p+-Si substrate. The mechanically exfoliated multi-layer WSe2 was transferred by PDMS stamp. Next, 15 nm-thick Al2O3 was prepared by ALD at 200 oC with H2O. Then, Nickel (Ni) metal was deposited by RF sputtering on Al2O3. Ni metal was patterned for gate electrode with conventional lithography process. After that, A2O3 layer was removed by wet etching. Subsequently, substrate was exposed to oxygen radicals to form WOx used as source/drain contact at surface of WSe2. Second layer of Al2O3 was deposited as an encapsulating passivation layer. After the contact hole opening, the electrical contact pad was fabricated. Finally, forming gas (N2 : H2 = 97 % : 3 %) annealing was performed at 200 oC for 30 min. The top gate length was 5 μm. The gate width was estimated to be 40 μm by optical microscope. The electrical characteristics were measured with a manual probe station in an atmospheric pressure without inert gas purge at room temperature using a precision semiconductor parameter analyzer (Agilent 4156 C).Figures 2 shows the Id–Vg characteristics of fabricated FETs for (a) back gate operation and (b) top gate operation. Representative Id–Vg characteristics of p-type FETs were observed irrespective of back gate and top gate operation. The on/off ratio of about 106 ~ 107 order were obtained for both back gate and top gate operation. Furthermore, the threshold voltage and sub-threshold slope were modulated by varying the substrate bias during gate sweep.This study opens up interesting directions for the research and development of TMDC-based devices.AcknowledgmentsThis study was supported by a JSPS Grant-in-Aid for Scientific Research (C) (Grant No. 20K04616), a research grant for Tokyo Tech Challenging Research Award, the Samco Foundation and a research grant for Suematsu Award.

Effects of Substrate Bias Voltage on Structural and Optical Properties of Co-Sputtered (AlxGa1–x)2O3 Films

Effects of Substrate Bias Voltage on Structural and Optical Properties of Co-Sputtered (AlxGa1–x)2O3 Films

The effect of substrate bias voltage on the mechanical and electrochemical corrosion properties of multilayered CrN/CrAlN coatings produced by cathodic arc evaporation

AbstractThe multilayered CrN/CrAlN film was deposited on the SS420 substrate by cathodic arc evaporation technique. The effect of substrate bias voltage was investigated on the microstructure, surface morphology, and mechanical and electrochemical corrosion properties of the deposited film. The deposited CrN/CrAlN coatings were characterized by X‐ray diffraction and field emission scanning electron microscope techniques. Mechanical properties, corrosion resistance, and surface morphology of the coated samples were studied by nano‐indentation, polarization test, electrochemical impedance spectroscopy, and atomic force microscopy, respectively. Adhesive and dense nanostructure CrN and CrAlN phases were successfully deposited on the substrate. The refinement of the coating crystallite was observed from 31.2 to 24.3 nm by increasing the substrate bias voltage, this improved the mechanical properties of the coating. The number of macroparticles, pinholes and porosities, and the surface roughness of coatings significantly decreased by increasing the substrate bias voltage. The CrN/CrAlN coating reduced the corrosion current density in contrast to the SS420 substrate. Better corrosion resistance was obtained for the coating deposited at 150 V of bias voltage due to lower defects in the film structure.

Polymorphism and crystal reorientation governing the magnetic anisotropy in Gd thin films

200 nm Gd films were prepared on Si (100) wafers under different conditions [substrate temperature (TS) and bias voltage on the substrate (prior to deposition)] to study the influence of Gd polymorphism and crystal orientations on their magnetic properties. The Gd hexagonal close-packed (Gd-hcp) structure had a crystalline reorientation from (002) to (100) preferential orientation concomitantly with appearance of (111) preferentially oriented Gd face-centered cubic (Gd-fcc) structure when TS increases from room temperature to 673 K. The crystallite orientations were spreader in the highest TS. All Gd films displayed thermal hysteresis of (6 ± 1) K near to room temperature associated with their magnetocaloric properties. The analysis of the ferromagnetic resonance results, taken at different temperatures, indicated that the in-plane magnetization is favored by the emergence of Gd-fcc phase and (100) preferential orientation of the Gd–hcp structure. This observation suggests that the bulk Gd spin reorientation effect is even a more complex issue in thin films, since the structural reorientation of the hcp c-axis may also emerge and internal strains favor a growth of a Gd-fcc structure that can strongly influence the effective anisotropy in sputtered Gd films. Therefore, our results bring new insights on structural and magnetic properties of Gd thin films that have aroused interest of the scientific community in recent decades.

On the surface biasing effectiveness during reactive high-power impulse magnetron sputter deposition of zirconium dioxide

In this study, we investigate the effect of surface biasing during high-power impulse magnetron sputter deposition of non-conductive zirconium dioxide films on substrates with different capacitances. We employ reactive high-power impulse magnetron sputtering to deposit ZrO2 films using a circular Zr target in a pure oxygen atmosphere. The deposition experiments are conducted on two types of substrates: high-capacitance (thin thermally grown SiO2 on Si) and low-capacitance (soda-lime glass). By varying the time shift between the target voltage pulse and the negative substrate bias voltage pulse, we analyze the impact on the preferred crystal orientation of the deposited ZrO2 films and the deposition rate. We measure the current and voltage waveforms during the deposition process and utilize a surface charging model to understand the charging behavior of the growing film. Additionally, time- and energy-resolved ion mass spectra are measured to gain insights into the ion behavior during film growth. Our results demonstrate that energetic ion bombardment and appropriate timing of substrate biasing can influence the crystal orientation of ZrO2 films, favouring specific orientations over others. We also explain the different behavior observed on low- and high-capacitance substrates.

Regiodivergent Synthesis of Brominated Pyridylthiophenes by Overriding the Inherent Substrate Bias

AbstractA regiocontrolled functionalization of a pyridylthiophene scaffold was accomplished. Regioselectivity for deprotonation of the pyridylthiophene was switched by changing the reaction conditions, including the metal amide base and the solvent. Subsequently, in situ transmetalation and halogen dance on the corresponding organometallic species were controlled by additives and the reaction temperature, as well as by the above reaction conditions. This method successfully enabled the synthesis of four iodinated constitutional isomers from a single starting material, 2-(5-bromo-2-thienyl)pyridine.

Accelerated optimization of chromium coating microstructures for nuclear fuel applications through electrochemical corrosion testing

Corrosion of zirconium alloy fuel cladding in nuclear reactors is one of the main limiting factors to component lifespan. Protective coatings may be applied to reduce the corrosion rate during regular operation and can have the added benefit of mitigating follow-on risks associated with loss-of-coolant accidents. This study considers the use of electrochemical testing to rapidly evaluate the corrosion protection of chromium coatings on zirconium alloys. By correlating these electrochemical testing results with the microstructural characteristics and mechanical properties of the coating, it is possible to qualitatively predict the performance of these coatings during long-term autoclave testing and eventual in-reactor use. A series of chromium coatings were deposited using cathodic arc physical vapor deposition (CA-PVD) with varying substrate bias. Electrochemical testing showed that the coatings provided improvements in corrosion protection over the uncoated baseline zirconium alloys, with further improvements observed for substrates to which voltage bias was applied. This is attributed to the increased adatom mobility during the deposition process, allowing for the development of a dense, uniform microstructure of the chromium coating. These results were compared to autoclave exposure data over a 56-day period to show the effectiveness of electrochemical testing towards predicting significant differences in corrosion resistance under regular operating conditions. This approach provided limited qualitative insight on long-term performance in its present state, but shows promise for further development.

Bias-supply timing tailored to the aspect ratio dependence of silicon trench etching in Ar plasma with alternately injected C4F8 and SF6

In semiconductor device fabrication, the feature profiles of a high-aspect-ratio (HAR) Si trench is needed to be controlled considering aspect-ratio-dependent etching (ARDE). This is achieved by a cyclic process in which Ar plasma is sustained while C4F8 and SF6 are alternately injected and a short-period supply of a substrate bias is provided. Presently, the transient behaviors of gaseous and surface reactions are dynamically revealed by measuring the plasma parameters using a surface wave probe and optical emission spectroscopy. When the etched surface is fluorinated during the cycle, an ARDE-free Si-trench feature profile can be fabricated by controlling the bias-supply timing.

Effect of pulse width on deposition of diamond-like carbon on high-power pulsed magnetron sputtering

A diamond-like carbon (DLC) film was deposited using high-power pulsed magnetron sputtering. The effect of pulse width on the deposition of DLC film was investigated under constant peak power density or average power density to clarify the densification mechanism of DLC film. The maximum hardness of 25 GPa analyzed by nanoindentation was obtained using Ar gas without negative substrate bias voltage at pulse width 30 μs and a peak power density of 1.5 kW cm−2. The flux and energy of C+ and Ar+ incident to the DLC film was evaluated by using energy-resolved and time-resolved mass spectrometry to clarify the relation between the input power to the target and the behavior of produced ions. The change in hardness is well correlated with the ion flux ratio C+/Ar+. This result indicates that a flux and energy of Ar+ as well as C+ is a key parameter to characterize the microstructure of DLC film.