High-rate Sputtering Research Articles

Materials Based on Amorphous Al2O3 and Composite W-Al2O3 for Solar Coatings Deposited by High-Rate Sputter Processes

In parabolic trough technology, the development of thermally and structurally stable solar coatings plays a key role in determining the efficiency, durability, and economic feasibility of tube receivers. A cermet-based solar coating is typically constituted by a thin film stratification, where a multilayer graded cermet is placed between an infrared metallic reflector and an antireflection filter. This work reports the realization of materials based on Al2O3 and W characterized by high structural and chemical stability in vacuum at high temperature, obtained through the optimization of high-deposition-rate processes. Al2O3 material, employed as the antireflection layer, was deposited through a reactive magnetron sputtering process at a high deposition rate. Cermet materials based on W-Al2O3 were deposited and employed as absorber layers by implementing reactive magnetron co-sputtering processes. An investigation into the stability of the realized samples was carried out by means of several material characterization methods before and after the annealing process in vacuum (1 × 10−3 Pa) at high temperature (620 °C). The structural properties of the samples were evaluated using Raman spectroscopy and XRD measurements, revealing a negligible presence of oxides that can compromise the structural stability. Spectrophotometric analysis showed little variations between the deposited and annealed samples, clearly indicating the high structural stability.

Influence of the method of gas supply to the chamber on the processes of reactive magnetron sputtering of Ti-Al composite target

The paper demonstrates study results of the processes of reactive magnetron sputtering of a Ti-Al composite target in an Ar/O2 gas mixture with various methods of gas supply to the chamber. It has been established that, regardless of the method of gas supply, when changing the oxygen concentration in the Ar/O2 gas mixture, the ratio of the Al and Ti content in the deposited Ti1-xAlxOy films changes. The metal content ratio in the deposited films varies in proportion to the change in the ratio of the control lines intensities of the optical emission of aluminum AlI (396.15 nm) and titanium TiI (395.82 nm) in plasma. This makes it possible to effectively use the method of optical emission spectroscopy to predict and control the content of metals in deposited films. Based on studies of the dielectric characteristics of deposited Ti1-xAlxOy films, it was discovered that with separate gas supply (Ar is supplied to the target region, O2 is supplied to the substrate region), in contrast to the joint gas supply (the Ar/O2 gas mixture is supplied to the target region), it is possible to deposit dielectric films in the transient mode at relatively high sputtering rates. In this case, the oxygen supply to the substrate region makes it possible to increase the oxygen content in the films up to 60-64%. Keywords: preactive magnetron sputtering, composite target, thin films, titanium-aluminum oxide, elemental composition, permittivity, dielectric loss tangent.

Remanufacturing of the waste refractory Mo–10Nb sputtering target by spark plasma sintering technology

Mo–10Nb refractory sputtering targets used in the flat panel display industry have attracted extensive attention, whereas the shortcoming of low target utilization is now urgently needed to be solved. This work is to repair the waste Mo–10Nb target by spark plasma sintering (SPS) at 1550 °C and 10 min, and the feasibility is demonstrated. It is shown that the repaired target retains identical phases and high purity. Clean and strong bonds are found at the repair interface. Regardless of the position, the repaired target exhibits finer grains as well as higher density, microhardness, and thermal conductivity compared with the original target. The two targets are then sputtered to obtain relevant films. Both films consist of the same phases and show columnar appearances. Mo, Nb, O, and C are present in both films, where Mo and Nb exist in the form of pure Mo, MoO2, MoO3, pure Nb, and Nb2O5. A higher sputtering rate, smaller film thickness deviation, grain size and surface roughness, less oxidation, and larger elastic modulus and nano-hardness are obtained from the film prepared by the repaired target. SPS is proven to be a well-suited method for producing the regenerated target.

Depth profiling of thin plasma-polymerized amine films using GDOES in an Ar-O2 plasma

Thin polymer films were deposited on polished stainless-steel samples by PECVD from a cyclopropylamine precursor and characterized by X-ray photoelectron spectroscopy, secondary-ion mass spectrometry and glow-discharge optical emission spectroscopy (GDOES) depth profiling. These depth profiles exhibited reasonable agreement. The GDOES involved the erosion of the polymer films in plasma sustained by an asymmetric RF capacitively coupled discharge using both Ar and Ar-O2 gases. The application of pure Ar caused unwanted effects, such as the broadening of the polymer-film/substrate interface, which were suppressed when using the mixture with oxygen. Another benefit of oxygen was a significant increase in the etching rate by a factor of about 15 as compared to pure argon. The mechanisms involved in the depth profiling using the mixture of gases were elaborated in some detail, taking into account plasma parameters typical for an asymmetric, capacitively coupled RF discharge in a small volume. The main benefit of using the Ar/O2 GDOES profiling with respect to XPS and SIMS depth profiling is the increased sputtering rate for polymer films. Comparing the GDOES depth profiling with the Ar/O2 mixture with profiling in pure Ar, the benefits are a higher sputtering rate and better depth resolution at the polymer/substrate interface.

Behavior and process of background signal formation of hydrogen, carbon, nitrogen, and oxygen in silicon wafers during depth profiling using dual‐beam TOF‐SIMS

The behavior and mechanism of background signals during depth profiling of atmospheric elements using dual‐beam time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) have been experimentally investigated for silicon wafers. The background signals of atmospheric elements were found to be inversely proportional to the sputtering rate. Most of the background signals are largely attributable to the accumulation of components through adsorption and ion bombardment in the pre‐equilibrium state. On the other hand, the contribution of real‐time adsorption during the instant after the last sputtering in the equilibrium state is negligible under the present experimental conditions. H2O is dominant in the background formation process of hydrogen and oxygen, which is supported by the higher adsorption coefficients. The background levels of carbon and nitrogen are lower than those of hydrogen and oxygen. Furthermore, the background signal of carbon with respect to the sputtering rate shows a different trend than the other elements. This could be attributed to accumulation in the pre‐equilibrium state. These results indicate that the background levels can be lowered close to those of dynamic‐SIMS by using an extremely high sputtering rate in dual‐beam TOF‐SIMS.

Neon and helium focused ion beam etching of resist patterns

Helium ion microscopy has attracted many applications in imaging, nanofabrication and analysis. One important field of study in nanofabrication using ion beam is the milling or etching of materials using a helium or neon focused ion beam (FIB), with and without chemical gas assistance. In particular, the neon FIB has a relatively high sputtering rate with a lower probability of swelling and less re-deposition issues compared to a helium FIB. Here, both neon and helium FIB etchings are investigated for milling and repairing electron-beam lithography (EBL) defined hydrogen silsesquioxane (HSQ) and polymethyl methacrylate (PMMA) resist patterns. Different dosages of neon FIB etching result in distinct etching profiles. Using the appropriate doses, arrays of uniform gap with aspect ratio more than 20 can be achieved on HSQ nanostructures. The neon FIB etching has a resolution of 20 nm on HSQ patterns. With XeF2 assistance, neon FIB etching can be enhanced for etching depth by a factor of ∼1.2. Whereas, helium FIB can also etch thick HSQ patterns, with much lower etch rates. But with XeF2 assistance, helium FIB etching depth can be enhanced significantly by a factor of around 5. Furthermore, both helium and neon FIB etching methods have been employed to selectively remove residual particles in deep and narrow trenches without affecting the resist patterns. The chemical analysis of these residual particle composition and resist patterns can be also performed using helium ion microscopy coupled with secondary ion mass spectrometry (SIMS) using neon FIB. Besides, a neon FIB can also effectively etch PMMA patterns which are commonly used in nanofabrication and the unwanted connections can be etched away.

Power supply efficiency in dual magnetron large area sputter coatings

In large area sputter coating of glass, the consumption of electric power is one of the major cost drivers. This applies especially to dielectric transparent coatings which are relatively thick and for which sputter yields and rates can be rather low. These materials are usually sputtered from dual magnetrons using medium frequency (sine wave) or bipolar (square wave) power supplies at frequencies up to 100 kHz. Frequencies at 50 kHz or higher are beneficial for suppressing arcing on the target surface, and the power required to achieve viable sputter rates is often high, making the power consumption a significant cost contribution. In this study we look at the effect of the power supply technology and frequency on the sputter rate and the sputtered layer thickness per electrical energy input. The tests were carried out with aluminum dual planar targets and with TiOx dual rotary targets. At low frequencies around 20 kHz, the bipolar generator can yield about 10% higher sputtering rates at the same input power. At about 40 kHz, which is often chosen to minimize arcing, the rates from the two power supplies are about equal.

Hardening the surface of metals with WC1-x coatings deposited by high-speed plasma spraying

The deposition of protective hardening coatings allows increasing the lifetime of important technical materials and pure metals. This work shows the possibility of depositing protective coatings based on cubic tungsten carbide (WC1-x) on metal substrates using a high-speed (~1 km/s) plasma jet generated by a coaxial magneto-plasma accelerator. In a plasma spraying process, the necessary conditions (high sputtering and crystallization rates) are created for synthesizing the high-temperature WC1-x phase and stabilizing it in the coating structure. The formation of WC1-x phase was confirmed by X-ray diffraction and energy dispersive X-ray spectroscopy. The substrate parameters such as linear dimensions and heat dissipation rate were found to directly affect the phase composition (WC1-x yield) and the structure of the deposited coating. The coatings with a higher WC1-x content are characterized by an increased hardness. The firstly made direct measurements of a Berkovich hardness (H = 33.0 ± 0.9 GPa) and Young's modulus (E = 401 ± 14 GPa) for bulk WC1-x grains, exceeding several micrometers in size, confirmed the suggestion about higher mechanical properties of the metastable WC1-x phase in comparison with hexagonal tungsten carbides WC and W2C.

Fabrication and growth of c-axis textured Nd2Fe14B thin films by high-rate sputtering

To apply Nd–Fe–B thin films for mass-produced heat-assisted magnetic recording media, we investigated the high-rate sputtering conditions required to obtain c-axis textured Nd2Fe14B thin films and analyzed the growth mechanism. Magnetization curves indicated that higher substrate temperatures and sputtering rates resulted in a higher degree of perpendicular magnetic anisotropy; a Nd–Fe–B layer deposited at a substrate temperature (Tsub_0) of 600 °C and a sputtering rate (Rsp) of 2.6 nm/s had the easy axis perpendicular to the film plane. The dependence of the magnetic properties on the sputtering rate was due to a decrease in the substrate temperature during sputtering; there was a threshold for obtaining a high squareness ratio. X-ray diffraction analysis and transmission electron microscopy (TEM) images showed that the c-axis textured Nd2Fe14B crystal phase was formed in the Nd–Fe–B layer deposited at Tsub_0 = 600 °C and Rsp = 2.6 nm/s, which resulted in the highly perpendicular magnetic anisotropy. In addition, the TEM images showed a layer of Nd2Fe14B with no obvious lattice fringes near the interface between the Nd–Fe–B layer and Mo underlayer, while the lattice fringes of the Nd2Fe14B phase were not parallel to this interface but gently curved along the Mo cap layer. We propose that the c-axis orientation was achieved by the rotation of the c plane, which has the lowest surface energy in the Nd2Fe14B phase, toward the vacuum-side surface during sputtering.

High-rate sputter deposition of chromium oxide thin films using water vapor as a reactive gas

The use of water vapor as a reactive gas under metallic mode for high rate sputter deposition of chromium oxide thin films have been studied in the course of this research. Chromium oxide (Cr2O3) thin films were deposited by an RF magnetron sputtering system with chromium metal as a target and argon (Ar) and water vapor (H2O) as the sputtering gas. Water was injected to the target or substrate surface at an H2O flow ratio of H2O/(H2O + Ar), varied from 0 to 100%. A liquid nitrogen cold trap was placed in the sputtering chamber to adsorb non-reacted H2O molecules.When the cold trap was inactive, the deposition rate and discharge voltage dropped and the transmittance of the films increased above an H2O flow ratio of 10%. Discharge voltage and plasma emission measurements showed that Cr2O3 films were deposited under compound target mode. In contrast, when the cold trap was active, the deposition rate increased with increase in H2O flow ratio and also a gradual increase in the transmittance of the films was observed. Hence, the target surface was considered to be almost in a metallic state and the oxidation proceeded gradually. Results of this study indicate that H2O sputtering using cold trap is a promising method for the formation of chromium oxide thin films at high deposition rates under metallic target mode.

Influence of the microstructure on the diffusion barrier performance of Nb-based coatings for cyclotron targets

The number of medical procedures involving the use of cyclotron-produced radionuclides is constantly growing year by year. The design and construction of the cyclotron targets appropriate for the production of the radionuclides of interest are the most challenging issues. The cyclotron targets for the medical radionuclide production suffer from two main corrosion problems: the corrosion due to proton-irradiated water and liquid metal embrittlement. The design of the target for radionuclide production limits the ability to select an ideal material that meets all of the following requirements: machinability or ease of construction, high melting temperature, high thermal exchange performance, excellent chemical inertness, etc. The use of thermally and mechanically suitable substrate materials protected by chemically resistant coatings can be a good compromise. These two corrosion problems can be attributed to the mechanism of diffusion by the aggressive particles through the protective coating. In this research, niobium has been chosen as the principal material for the design of thin film protective coatings. The coating microstructure was correlated to specific deposition parameters to provide chemical resistance to both proton-irradiated water corrosion and liquid metal embrittlement. Film densification and amorphization were pursued to achieve niobium-based thin films efficient as diffusion barriers to proton-irradiated water and liquid metal. The most important conclusions were that the performance of thin films as diffusion barriers varied dramatically based on various deposition parameters and deposition technologies. Among the configurations studied, only three are acceptable as anticorrosion coatings: niobium deposited on axis with unbalanced magnetron sputtering, niobium coated at a high sputtering rate and on a water-cooled sample holder, and the niobium-titanium alloy sputtered at a low argon pressure.

High-rate reactive high-power impulse magnetron sputtering of transparent conductive Al-doped ZnO thin films prepared at ambient temperature

Reactive high-power impulse magnetron sputtering was used for high-rate (deposition rate of 60 nm/min) deposition of conductive (resistivity of 3 × 10−3 Ωcm) and optically transparent (extinction coefficient at the wavelength of 550 nm of 0.01) ZnO:Al thin films at ambient temperature (< 40 °C). We used planar Zn:Al target (3.09 at.% of Al) with diameter of 100 mm and thickness of 6 mm mounted on strongly unbalanced magnetron. The films were deposited in argon‑oxygen atmosphere at constant argon and oxygen partial pressure of 2.0 Pa and 0.1 Pa, respectively. An average target power and voltage pulse length were kept constant to 1.9 Wcm−2 and 200 μs, respectively. A pulse-averaged target power density was varied in the range of 190 Wcm−2–940 Wcm−2. Optical emission spectroscopy was used for better understanding of correlations between plasma discharge properties and structural, electrical and optical properties of prepared films.

Dense Kondo behavior in the low-temperature resistivity and specific heat for amorphous Ce50Al50 alloy

We measured the low-temperature specific heat Cp, resistivity ρ, and magnetoresistance Δρ(H)/ρ(0) for amorphous Ce50Al50 synthesized by a DC high-rate sputter method. The low-temperature Cp (T &amp;lt; 2 K) decreases rapidly toward 0 K and has no indication of phase transition. The value of γ0 which is extrapolated down to 0 K of the Cp/T is 117 mJ/molK2. The temperature dependence of ρ increases with decreasing temperature down to 0.6 K. We found that both a weak localization effect and a coherent Kondo state might be realized in the low-temperature region for the present alloy from the conductivity analysis. Furthermore, in the low-temperature ρ, a T2 term with a very large coefficient A was observed. The ratio of A/γ02 is 0.63 ×10-5 (μΩcm/K2)/(mJ/molK2)2 and is near the value of typical Ce-based heavy-fermion compounds. The magnetoresistances Δρ(H)/ρ(0) at 0.5 K and 2 K are almost constant in the magnetic field region of H &amp;lt; 10 kOe. We considered that the negative magnetoresistance effect is due to the weak localization and the positive magnetoresistance to the heavy-fermion state in the present alloy.

High Initial Sputter Rate Found for Vaccinia Virions Using Isotopic Labeling, NanoSIMS, and AFM.

High-lateral-resolution secondary ion mass spectrometry (SIMS) has the potential to provide functional and depth resolved information from small biological structures, such as viral particles (virions) and phage, but sputter rate and sensitivity are not characterized at shallow depths relevant to these structures. Here we combine stable isotope labeling of the DNA of vaccinia virions with correlated SIMS imaging depth profiling and atomic force microscopy (AFM) to develop a nonlinear, nonequilibrium sputter rate model for the virions and validate the model on the basis of reconstructing the location of the DNA within individual virions. Our experiments with a Cs+ beam show an unexpectedly high initial sputter rate (∼100 um2·nm·pA-1·s-1) with a rapid decline to an asymptotic rate of 0.7 um2·nm·pA-1·s-1 at an approximate depth of 70 nm. Correlated experiments were also conducted with glutaraldehyde-fixed virions, as well as O- and Ga+ beams, yielding similar results. Based on our Cs+ sputter rate model, the labeled DNA in the virion was between 50 and 90 nm depth in the virion core, consistent with expectations, supporting our conclusions. Virion densification was found to be a secondary effect. Accurate isotopic ratios were obtained from the initiation of sputtering, suggesting that isotopic tracers could be successfully used for smaller virions and phage.

High-rate sputter deposition of electrochromic nickel oxide thin films using substrate cooling and water vapor injection

Nickel oxide is a promising electrochromic (EC) material because it changes color upon electrochemical oxidation and reduction. In this study, the authors developed a reactive sputtering technique using water vapor as a reactive gas to deposit highly hydrated oxide thin films. They used an Ar+H2O gas mixture, with H2O injected directly onto the substrate surface. Subsequently, the authors studied the effects of substrate cooling on the mechanism of Ni oxide thin film formation and how this process affected the EC properties of the resulting Ni oxide thin films. At substrate temperatures of −80 and −120 °C, amorphous hydrated Ni oxide thin films were deposited with a high deposition rate of approximately 35 nm/min, which was greater than that of metallic Ni films. It was surmised that the metallic target mode was achieved at low substrate temperatures in the Ar+H2O atmosphere. The Ni oxide films exhibited good EC properties with a large transmittance change. Consequently, substrate cooling and water vapor injection were found to be effective in the high-rate deposition of Ni oxide thin films with good EC properties.

Magnetic and transport properties of amorphous Ce-Al alloy

Amorphous (a-)Ce50Al50 has been prepared by DC high-rate sputter method. The structure of the obtained sample has been confirmed to have an amorphous structure because there are no Bragg peaks in the X-ray diffraction measurement and have a clear exothermic peak by the differential scanning calorimetry measurement. We have measured the resistivity ρ, magnetic susceptibility χ, specific heat Cp and thermoelectric power S for a-Ce50Al50. The temperature dependence of ρ exhibits a small temperature dependence less than 10% in the whole temperature region. χ follows a Curie-Weiss behavior in the high-temperature region of T>90K. The effective paramagnetic moment peff, estimated from C is 2.18 μB/Ce-atom. The low-temperature Cp/T increases rapidly with decreasing temperature and tends to a saturation. S(T) exhibits negative values in a wide temperature region. A minimum of S appear at around 60K, and S decreases linearly with decreasing temperature down to 10K. The low-temperature S is almost 0μV/K down to 2K. From these results, we have pointed out that present a-Ce50Al50 would be an incoherent Kondo material.

Review Article: Tracing the recorded history of thin-film sputter deposition: From the 1800s to 2017

Thin films, ubiquitous in today's world, have a documented history of more than 5000 years. However, thin-film growth by sputter deposition, which required the development of vacuum pumps and electrical power in the 1600s and the 1700s, is a much more recent phenomenon. First reported in the early 1800s, sputter deposition already dominated the optical-coating market by 1880. Preferential sputtering of alloys, sputtering of liquids, multitarget sputtering, and optical spectroscopy for process characterization were all described in the 1800s. Measurements of threshold energies and yields were carried out in the late 1800s, and yields in reasonable agreement with modern data were reported in the 1930s. Roll-to-roll sputter coating on flexible substrates was introduced in the mid-1930s, and the initial demonstration of sustained self-sputtering (i.e., sputtering without gas) was performed in 1970. The term magnetron dates to 1921, and the results of the first magnetron sputtering experiments were published in the late 1930s. The earliest descriptions of a parallel-plate magnetron were provided in a patent filed in 1962, rotatable magnetrons appeared in the early 1980s, and tunable “unbalanced” magnetron sputtering was developed in 1992. Two additional forms of magnetron sputtering evolved during the 1990s, both with the goal of efficiently ionizing sputter-ejected metal atoms: ionized-magnetron sputtering and high-power impulse magnetron sputtering, with the latter now being available in several variants. Radio frequency (rf) glow discharges were reported in 1891, with the initial results from rf deposition and etching experiments published in the 1930s. Modern capacitively-coupled rf sputtering systems were developed and modeled in the early 1960s, and a patent was filed in 1975 that led to pulsed-dc and mid-frequency-ac sputtering. The purposeful synthesis of metal-oxide films goes back to at least 1907, leading to early metal-oxide and nitride sputtering experiments in 1933, although the term “reactive sputtering” was not used in the literature until 1953. The effect of target oxidation on secondary-electron yields and sputtering rates was reported in 1940. The first kinetic models of reactive sputtering appeared in the 1960s; high-rate reactive sputtering, based on partial-pressure control, was developed in the early 1980s. While abundant experimental and theoretical evidence already existed in the late 1800s to the early 1900s demonstrating that sputtering is due to momentum transfer via ion-bombardment-induced near-surface collision cascades, the concept of sputtering resulting from local “impact evaporation” continued in the literature into the 1960s. Modern sputtering theory is based upon a linear-transport model published in 1969. No less than eight Nobel Laureates in Physics and Chemistry played major roles in the evolution of modern sputter deposition.

Optical emission spectroscopy during the deposition of zirconium dioxide films by controlled reactive high-power impulse magnetron sputtering

Time-resolved optical emission spectroscopy was performed near the sputtered Zr target and in a plasma bulk during a controlled high-rate reactive high-power impulse magnetron sputtering of stoichiometric ZrO2 films in argon-oxygen gas mixtures at the argon pressure of 2 Pa. The repetition frequency was 500 Hz at the deposition-averaged target power density of 52 W cm−2 with a peak target power density of 1100 W cm−2. The voltage pulse duration was 200 μs. From the time evolutions of the excited-state populations for the chosen atoms (Zr, Ar, and O) and ions (Zr+, Zr2+, Ar+, and O+), and of the excitation temperature during a voltage pulse, the trends in a time evolution of the local ground-state densities of these atoms and ions during the voltage pulse were derived. Near the target, a decrease in the ground-state densities of Ar and O atoms, caused by a gas rarefaction and intense electron-impact ionization, was observed in the first half of the voltage pulse. Simultaneous, very effective electron-impact ionization of sputtered Zr atoms was proved. A composition of particle fluxes onto the substrate during a film deposition was found almost independent of the instantaneous oscillating oxygen partial pressure.

Development of ZnTe as a back contact material for thin film cadmium telluride solar cells

Cadmium telluride (CdTe) is high-efficiency commercialised thin film photovoltaic technology. However, developing a stable low-resistivity back contact to the CdTe solar cells is still an issue. High work function and low level of doping of this material don't allow to create an ohmic contact with metals directly. Copper is commonly used to lower the back contact barrier in CdTe solar cells, but an excessive amount of copper diffusing through the cell is harmful for the device performance and stability. In this work a copper-doped ZnTe (ZnTe:Cu) buffer layer was incorporated in between CdTe and gold metal contact by high-rate pulsed DC magnetron sputtering. The back contact was then activated by rapid thermal processing (RTP) resulting in spectacular improvement in key device performance indicators, open circuit voltage (VOC) and fill factor (FF).

Mechanical and tribological properties of “substrate–material” multifunctional composite with shape memory effect

The properties of steel–TiNi, TiNiCu, NiAl alloy multifunctional composite with shape memory effect are studied. The system is obtained under high-energy exposure (argon arc and laser surfacing, plasma and high-rate gas flame sputtering) with the formation of a structure with fine-grained to nanoscale dispersity. The experimental studies reveal the efficiency of the elaborated technique of the synthesis of composites to increase the wear resistance, fatigue strength, and endurance at frictionally cyclic low-cycle loading of material. The increase in fatigue and wear characteristics are explained by the processes caused by the combined cyclic loading and reverse friction. As is shown, the friction and mechanical fatigue in a surface-modified layer of the material undergoing the shape memory effect in the friction domain causes an increase in temperature that favors the martensite–austenite transformation, whereas the pressure arising in friction induces the transformation plasticity effect owing to the formation of stress martensite.