Low Sputtering Pressure Research Articles

Superconducting Nb interconnects for Cryo-CMOS and superconducting digital logic applications

A 100 nm wide superconducting niobium (Nb) interconnect was fabricated by a 300 mm wafer process for Cryo-CMOS and superconducting digital logic applications. A low pressure and long throw sputtering was adopted for the Nb deposition, resulting in good superconductivity of the 50 nm thick Nb film with a critical temperature (T c) of 8.3 K. The interconnects had a titanium nitride (TiN)/Nb stack structure, and a double-layer hard mask was used for the dry etching process. The exposed area of Nb film was minimized to decrease the effects of plasma damage during fabrication and atmosphere. The developed 100 nm wide and 50 nm thick Nb interconnect showed good superconductivity with a T c of 7.8 K and a critical current of 3.2 mA at 4.2 K. These results are promising for Cryo-CMOS and superconducting digital logic applications in the 4 K stage.

The effect of DC power and annealing on the structural, optical, and electrical characteristics of sputtered amorphous WOX thin films

Tungsten oxide (WOX) is considered as one of the promising materials due to its unique properties in widespread optoelectronic and photonic applications. This study reports on the deposition of WOX thin films via reactive magnetron sputtering technique with various low DC power and constant sputtering pressure at room temperature. XRD results reveal that an amorphous state for all the as-deposited films, while annealed films at 650 °C have WO3 monoclinic structure. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies indicate all as-deposited films have a dense, homogeneous, and quite smooth surface with a root-mean-square roughness (σRMS) below 1 nm. Annealing significantly affects film morphology, the formation of large grains and increasing the surface roughness. Optical parameters were examined by ultraviolet–visible (UV–Vis) spectrophotometer. Increasing sputtering power leads to a decrease in transparency value, an increase in absorbance, a reduction in penetration depth, a red shift of the fundamental absorption edge, and a reduction in the band gap energy from 3.06 eV to 2.61 eV. The annealed films exhibit a smaller optical band gap compared to as-deposited films in the range of 2.44 eV–2.93 eV. The temperature dependence of the sheet resistance corresponds to the Arrhenius law for as-deposited films with DC power of 10 W–30 W, indicating semiconducting nature. A decreasing trend in activation energy (0.283–0.055 eV) was observed with increasing DC power. All the annealed films exhibit insulator nature with a resistance of more than 100 MΩ. Analysis of these results suggests that the desired structural, optical, and electrical properties can be achieved by controlling deposition conditions.

Characterization of the nanoparticle suspension by operando absorption spectrophotometry during sputtering onto liquids

Although the synthesis of nanoparticles (NPs) by low-pressure plasma-based sputtering onto liquids offers many advantages as compared to classic wet-chemistry-based approaches, not much is known about the nanoparticle formation mechanism in these conditions. Here, we report about an operando absorption spectrophotometry setup which allows recording time- and space-resolved spectra of the liquid into which the sputtered metal atoms are incorporated. The measurements are carried out during the plasma treatment but also afterward while keeping the sample inside the vacuum chamber. The device capabilities therefore allow studying the evolution of unstable systems, i.e., combination of sputtered atoms and host liquids for which aggregation may appear even during the plasma treatment time. As a model system, silver atoms are deposited onto a silicone oil which may be stirred or not. Our operando analysis provides qualitative and quantitative data and highlights that the nanoparticle formation and aggregation dynamics vary according to the process conditions.

Effect of argon sputtering pressure on the electrochemical performance of LiFePO4 cathode

The development of a novel small-scale electrical power source technology is essential in the design of microelectronic devices. In this sense, this study presents the optimization of the sputtering deposition parameters to grow a LiFePO4 cathode, a candidate material for the development of solid-state lithium batteries. Specifically, it was evaluated the effect of argon deposition pressure on the electrochemical properties of LiFePO4 cathode. By combining X-ray photoelectron spectroscopy with electrochemical characterization methods, it was revealed that argon deposition pressure modulated the degree of the re-sputtering effects. In addition, it was found that high argon deposition pressure leads to the growth of LiFePO4 films with oxygen and phosphor deficiency. The absence of these atoms caused the decrease in the LiFePO4 electrochemical activity. Conversely, deposition parameters based on low argon sputtering pressure significantly reduced the degree of the re-sputtering effects, which in turn favored the growth of stoichiometric LiFePO4 films with an enhanced electrochemical activity.

Heating of liquid substrate by low-pressure sputtering plasma

The heating of the liquid substrate by low-pressure (∼1 Pa) sputtering plasma has been investigated by in situ temperature measurements. The combination of “green” non-toxic solvent, castor oil, copper target, and direct current magnetron sputtering was chosen as a model system. The temperature increase induced by plasma was registered with two thermocouples placed immediately under the liquid surface and in the bulk solution. The effect of the working gas pressure and sputter power was studied. It was shown that the liquid temperature increases at a rate of up to 1 °C/min, depending on the sputtering conditions. The experimental data were compared with numerical calculations and COMSOL simulation. Provided information is essential data for the detailed explanation of the formation of nanoparticles during sputtering onto liquids, a clean approach for production of colloidal solutions of “naked” nanoparticles.

Oblique angle co-deposition of nanocolumnar tungsten thin films with two W sources: Effect of pressure and target current

Two series of tungsten thin films are sputtered on silicon and glass substrates by oblique angle co-deposition technique with an original configuration. Two opposite distinct tungsten targets are simultaneously used, both tilted with an oblique angle of 80°. The growth is performed at low (0.33 Pa) and high (1.5 Pa) argon sputtering pressure and the current intensity applied to the targets varies between 50 and 250 mA. The effect of these deposition parameters on the films microstructure and electrical properties is investigated by scanning and transmission electron microscopy, X-ray diffraction and pole figures, and van der Pauw method. Due to self-shadowing effect, all tungsten sputtered thin films are porous and columnar. At low pressure, the columnar tilt angle β can be tuned with the target current intensity until obtaining vertical columns. X-ray diffraction and pole figure analyses point out a A15 crystal structure (β-W) and a uniaxial fiber texture with a <100> growth direction. In contrast, tungsten thin films deposited at high pressure present a cauliflower structure and are poorly crystallized. Both deposition parameters also affect the films electrical resistivity and anisotropy. These behaviors are discussed and linked to the microstructure and crystallography.

Negative-ion bombardment increases during low-pressure sputtering deposition and their effects on the crystallinities and piezoelectric properties of scandium aluminum nitride films

Scandium aluminum nitride (ScAlN) films are being actively researched to explore their potential for use in bulk acoustic wave and surface acoustic wave resonators because of their good piezoelectric properties. Sputtering is commonly used in ScAlN film deposition. Unfortunately, it has been reported that film quality metrics such as the crystallinity and piezoelectric properties can deteriorate before the Sc concentration reaches 43% without an isostructural phase transition. One reason for this is bombardment with negative ions generated from carbon and oxygen impurities in the Sc ingots. Because the number of negative ions increases during low-pressure sputtering deposition, their effect on film quality may be considerable. In this study, we investigated negative-ion bombardment of the substrate during sputtering deposition and its effects on ScAlN crystallinity and piezoelectric properties. Negative-ion energy distribution measurements indicated that many more negative ions collide with the substrate during ScAlN film deposition than during AlN deposition. In addition, decreasing the sputtering pressure further increased the number of negative ions and their energies. It is well known that film quality improves at low pressures because increasing the mean free path reduces thermalization and scattering of sputtered particles. Although, AlN crystallinity and piezoelectric properties improved at low pressures, the properties of ScAlN films deteriorated dramatically. Therefore, the results indicated that ion bombardment increase at low pressure adversely effects ScAlN crystal growth, deteriorating crystallinity and piezoelectric properties. ScAlN films may be improved further by suppressing negative-ion bombardment of the substrate.

Amorphous NdIZO Thin Film Transistors with Contact-Resistance-Adjustable Cu S/D Electrodes

High-performance amorphous oxide semiconductor thin film transistors (AOS-TFT) with copper (Cu) electrodes are of great significance for next-generation large-size, high-refresh rate and high-resolution panel display technology. In this work, using rare earth dopant, neodymium-doped indium-zinc-oxide (NdIZO) film was optimized as the active layer of TFT with Cu source and drain (S/D) electrodes. Under the guidance of the Taguchi orthogonal design method from Minitab software, the semiconductor characteristics were evaluated by microwave photoconductivity decay (μ-PCD) measurement. The results show that moderate oxygen concentration (~5%), low sputtering pressure (≤5 mTorr) and annealing temperature (≤300 °C) are conducive to reducing the shallow localized states of NdIZO film. The optimized annealing temperature of this device configuration is as low as 250 °C, and the contact resistance (RC) is modulated by gate voltage (VG) instead of a constant value when annealed at 300 °C. It is believed that the adjustable RC with VG is the key to keeping both high mobility and compensation of the threshold voltage (Vth). The optimal device performance was obtained at 250 °C with an Ion/Ioff ratio of 2.89 × 107, a saturation mobility (μsat) of 24.48 cm2/(V·s) and Vth of 2.32 V.

Comparative Study of Pd/B4C X-ray Multilayer Mirrors Fabricated by Magnetron Sputtering with Kr and Ar Gas.

Ultrathin Pd/B4C multilayers are suitable X-ray mirrors working at the photon energy region of 7–20 keV. To further improve the layer structure, Pd/B4C multilayers with a d-spacing of 2.5 nm were fabricated by magnetron sputtering using the heavy noble gas Kr and compared with the conventional ones fabricated by Ar. Although the Kr-sputtering process can work at a lower pressure, the interface width—especially the interface roughness—is a little larger than that made by Ar. A stronger polycrystallization and a lower content of sputter gas atoms were found in the Kr-made sample, which can be explained by the joint effect from less recoiled particles and lower sputtering pressure. A good reflectance of 68% of the Kr made multilayer was measured at 10 keV, which is only slightly lower than that of the Ar made sample (71%).

The influence of substrate bias and sputtering pressure on the deposited aluminium nitride for magnetoelectric sensors

A residual stress of Aluminium nitride (AlN) thin films has been a problem in the magnetoelectric (ME) composites that are used in the AC magnetic field sensors. The present work aims to optimize the deposition process of AlN in order to fabricate a nearly-zero stress of AlN thin films as well as ME composites without losing the microstructural and piezoelectric properties. The influences of RF bias power and sputtering pressure on the residual stress, microstructure, and piezoelectric response have been investigated. Two different stacks are deposited on Si/SiO2 substrates: Ta/Pt/AlN and Ta/FeCoSiB/Ta/Pt/AlN. Pulsed DC reactive sputter depositions have been performed to deposit AlN films. With increasing the substrate bias, stress of the AlN films and the stacks with the magneto strictive layer are augmented. A variation of the sputtering pressure is a promising way to fabricate nearly zero stress of the AlN films and the stacks without the magneto strictive layer. A transition from tensile to compressive stress has been observed at the low sputtering pressure. Sputtering pressure also affects the stress of AlN films and the stacks with the magneto strictive layer. FWHM of AlN (0002) peaks are nearly constant within the ranges of the substrate bias. By reducing the sputtering pressure, FWHM is broadened due to lower ionization degree associated with AlN formation and greater number of micro-arcs. However, the magnitude of e31,f is increased due to a lower residual stress at the low sputtering pressure.

Oxygen Reduction Reaction Activity of Nanocolumnar Platinum Thin Films by High Pressure Sputtering

Nanocolumnar platinum thin films (Pt-TFs) were produced by high pressure sputtering (HIPS) and investigated as oxygen reduction reaction electrocatalysts for polymer electrolyte membrane fuel cells. Conventional high-density Pt-TF prepared by low pressure sputtering was also studied for comparison. Pt-TFs were deposited on a microporous layer (MPL)-like surface composed of carbon particles to mimic catalyst-coated gas diffusion electrodes. Electron microscopy imaging revealed that HIPS Pt-TFs developed a cauliflower-like columnar microstructure, which originated from a shadowing effect during HIPS. This shadowing effect is enhanced on the rough surface of the MPL-like carbon that leads to the nano-cauliflower formation. With this approach, we also aimed to relate the catalyst performance obtained by benchtop tests directly to membrane electrode assembly test results. The electrochemically active surface area of Pt-TFs increased from 10 to 19 m2 g−1 with increasing sputter pressure. Specific activity of conventional high-density and nanocolumnar films were similar at ∼600 μA cm−2, which is likely due to their similar crystal grain sizes, >5 nm. On the other hand, mass-specific (MA) activity values increased from ∼0.06 A/mgPt for conventional Pt-TF to ∼0.13 A/mgPt for HIPS Pt-TFs, which is consistent with the columnar microstructure of HIPS films providing a better catalyst utilization compared to conventional Pt-TF.

Growth and characterization of c-axis titled ZnO thin film by radio frequency magnetron sputtering

This work focuses on fabrication of c-axis tilted ZnO thin film which is used in FBAR biosensors to induce shear mode acoustic wave in the liquid environment. The ZnO thin film was deposited via using one-step method by radio frequency (RF) magnetron sputtering with fixed inclined angle between the normal of the target and substrates. All the samples were investigated by the SEM, XRD and UV visible spectrophotometer. It is found that the motion states of substrates and angle of tilted target decide the inclined angle of crystals growth. The sputtering pressure affects the quality of column structure. The high sputtering pressure destroy the growth of column structure. Besides, the procedure of annealing is very important for optimizing the quality and orientation of crystals growth. The thin film deposited on the static substrates at low sputtering pressure (1.0 Pa) has preferred tilted c-axis orientation column structure. The tilted angle of c-axis is 13.6° and close to the value (13.1°) of theory formula. As for optical transmittance of samples, the optical band gap is decreased as sputtering pressure reduces.

1J2-3 Effect of negative ion bombardment increased in low-pressure sputtering deposition on piezoelectric properties of ScAlN thin films

1J2-3 Effect of negative ion bombardment increased in low-pressure sputtering deposition on piezoelectric properties of ScAlN thin films

Sputtered Mo-bilayer thin films with reduced thickness and improved electrical resistivity

In this study, Mo-bilayer film, the thickness of which was reduced to approximately 270 nm with a very low resistivity of 14 μΩ.cm, was successfully grown by DC magnetron sputter. The Mo-bilayer, whose bottom and top layers were obtained by high pressure sputter (HPS) and low pressure sputter (LPS) respectively, demonstrates good adhesivity and crystalline properties, together with high reflectance. In order to obtain Mo-bilayer with these improved properties, we first determined the optimal growth temperature and pressure parameters by checking the structural and electrical properties respectively of Mo-single layers. As a result, we achieved a deposit of Mo-bilayer thin film that can be used as a good back contact layer in solar cell applications, both in terms of material cost saving and its superior properties, even at such low thickness.

Self-Supported Nanocolumnar Platinum Thin Film Catalysts for Oxygen Reduction Reaction

Nanocolumnar platinum thin films (Pt-TFs) were produced by high pressure sputtering (HIPS) and investigated as oxygen reduction reaction electrocatalysts for polymer electrolyte membrane fuel cells. Conventional high-density Pt-TF by low pressure sputtering was also studied for comparison. Pt-TFs were deposited on a microporous layer (MPL)-like surface composed of carbon particles in order to mimic the catalyst coated gas diffusion layer in a membrane electrode assembly (MEA). Scanning electron microscopy and transmission electron microscopy images revealed that HIPS Pt-TFs developed a cauliflower-like columnar microstructures, which originated from a shadowing effect. At high working gas pressure conditions of HIPS, sputtered Pt atoms go through several gas phase collisions, lose their directionality, and can arrive at the substrate surface at oblique angles that results in the formation of columnar islands that shadow the gaps among them from incident flux of atoms. This shadowing effect is believed to be enhanced on the rough surface of MPL-like carbon particles that leads to the formation of nano-cauliflower structures. Electrochemical characterization of the samples was performed by cyclic voltammetry and rotating disk electrode measurements on Pt-TF/MPL-like-layer/glassy-carbon-insert samples in aqueous perchloric acid electrolyte. With this approach, we also aimed to relate the catalyst performance obtained by benchtop tests directly to MEA results. Electrochemically active surface area (ECSA) of PT-TFs were found to be ranging from 10 m2/g to 19 m2/g with increasing sputter pressure, which was determined from the charge for hydrogen adsorption and desorption in the cyclic voltammograms. In addition, carbon monoxide (CO)-stripping of Pt-TF coated gas diffusion electrodes were performed in a fuel cell station and similar ECSA values were recorded (ranging from 11 to 15 m2/g). This indicated that successful mimicking of a gas diffusion electrode in an MEA can be achieved by utilizing an MPL-like substrate surface for benchtop tests in aqueous electrolyte environment as opposed to directly depositing Pt-TFs on glassy carbon inserts. Specific activity (SA) values for conventional high-density and nanocolumnar films were similar that was approximately ~600 μA/cm2, which is believed to correspond to grain sizes of >5 nm as obtained from TEM and XRD analysis. On the other hand, mass-specific (MA) activity values increased from 0.06 A/mgPt for conventional Pt-TF to 0.13 A/mgPt for HIPS Pt-TFs. Higher MA agrees with the columnar microstructure of HIPS thin films that increases the surface to volume ratio of the Pt layer that provides a better catalyst utilization compared to conventional Pt-TFs.

Magnetic properties of RF-sputtered NiO and Ni0.8Fe0.2O1-δ samples grown on SiO2/Si substrates

ABSTRACTMagnetic properties of RF (Radio Frequency)-sputtered NiO and Ni0.8Fe0.2O1-δ films made at low or high oxygen (O) sputtering pressures were studied and discussed. The X-ray diffraction pattern shows that all samples have a NaCl-type structure. The magnetic properties were measured as a function of magnetic field at different temperatures using the PPMS Model-6000. All samples contain a small magnetic moment resulting in a saturation magnetisation of 1–5 emu/cm3. A hysteresis curve at fields below 500 oersted (Oe), and a high-field magnetic slope were observed in all samples. Low-field hysteresis and high magnetic slope depend on the temperature. For a low O flow sputtered sample, the high-field slope increases with increasing temperature, and for a high O flow sputtered sample, the high-field slope decreases with increasing temperature. The low-field hysteresis increases for low O flow sputtered samples but increases for high O flow sputtered samples.

Tuning the photoluminescence, conduction mechanism and scattering mechanism of ZnSnN2

The high electron concentration and low mobility of ZnSnN2 hinder its potential applications in photocatalytic and optoelectronic devices. To reveal the mechanism, herein, ZnSnN2 thin films were prepared under different sputtering pressure. The results show that impurity band conduction, an electron density of above 1020cm−3 and a mobility of 2 cm2 V−1 s−1 dominated by variable-range hopping are observed in samples prepared at lower sputtering pressure, due to the unintentional incorporation of substitutional oxygen which is from residual vapour and which substitutes nitrogen, while conduction band conduction, an electron density of 1019 cm−3, a mobility of 24 cm2 V−1 s−1 limited by ionized impurity scattering and self-compensation ratio as well as an interband direct recombination emission are found in samples prepared at higher sputtering pressure, due to the decrease in substitutional oxygen doping.

Nucleation of titanium nanoparticles in an oxygen-starved environment. II: theory

The nucleation and growth of pure titanium nanoparticles in a low-pressure sputter plasma has been believed to be essentially impossible. The addition of impurities, such as oxygen or water, facilitates this and allows the growth of nanoparticles. However, it seems that this route requires such high oxygen densities that metallic nanoparticles in the hexagonal αTi-phase cannot be synthesized. Here we present a model which explains results for the nucleation and growth of titanium nanoparticles in the absent of reactive impurities. In these experiments, a high partial pressure of helium gas was added which increased the cooling rate of the process gas in the region where nucleation occurred. This is important for two reasons. First, a reduced gas temperature enhances Ti2 dimer formation mainly because a lower gas temperature gives a higher gas density, which reduces the dilution of the Ti vapor through diffusion. The same effect can be achieved by increasing the gas pressure. Second, a reduced gas temperature has a ‘more than exponential’ effect in lowering the rate of atom evaporation from the nanoparticles during their growth from a dimer to size where they are thermodynamically stable, *. We show that this early stage evaporation is not possible to model as a thermodynamical equilibrium. Instead, the single-event nature of the evaporation process has to be considered. This leads, counter intuitively, to an evaporation probability from nanoparticles that is exactly zero below a critical nanoparticle temperature that is size-dependent. Together, the mechanisms described above explain two experimentally found limits for nucleation in an oxygen-free environment. First, there is a lower limit to the pressure for dimer formation. Second, there is an upper limit to the gas temperature above which evaporation makes the further growth to stable nuclei impossible.

Sterilization for Bacillus Subtilis var. natto by Low Pressure Sputtering and Laser Ablation Plasma using Metal Powder Target

Sterilization for Bacillus Subtilis var. natto by Low Pressure Sputtering and Laser Ablation Plasma using Metal Powder Target

The role of interface between LiPON solid electrolyte and electrode in inorganic monolithic electrochromic devices

Transparent lithium phosphorus oxynitride (LiPON) thin films were deposited by RF sputtering at room temperature for electrochromic (EC) applications. We carried out a basic study from single layer, double layers, and eventually to devices. The interactions between LiPON and bottom EC electrode during sputtering were highlighted. The effects of N2 pressure on structural, morphological, chemical, and electrical properties of the LiPON films were investigated. The Li ionic conductivity of the films increased with decreasing N2 pressure. The results of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated a probable increase of nitrogen content in the films with decreasing N2 pressure. However, low pressure sputtering process induced the formation of a resistive layer between LiPON and bottom electrode. The electrochemical impedance measurements were conducted to evaluate the charge transfer processes across the interfaces. In the case of WO3 electrode, the charge transfer resistance at the WO3/LiPON interface could be considerably high up to 5100 Ω cm2. For the NiO/LiPON interface, the highest value was 1400 Ω cm2. The high charge transfer resistance significantly hindered the ion transport, thus leading to irreversible Li insertion/extraction processes and lowered charge capacity in the electrode layers. The cycling reversibility and optical contrast for the complete EC device were compromised accordingly. The device using a 130 nm-thick LiPON ion conductor showed an optical modulation of 40% at 550 nm driven by −1.5 V (coloration) and 1 V (bleaching) with switching time of 30 s.