Intrinsic stress of magnetron-sputtered niobium films

The stresses in niobium films were studied and the following preliminary results were obtained. (1) Niobium films can be prepared in any stress state (tensile, stress free or compressive) by varying the argon sputtering pressure. (2) As the bias voltage increases, more argon is incorporated into the film; both T_c and R/R₀ decrease; and the stress becomes more compressive and seems to saturate at about 1.5 × 10¹⁰ dyn cm^?2 at higher bias voltages (at an argon sputtering pressure of 1.9 Pa). (3) The lattice parameters show a close relation to the film stresses. (4) Lowering the sputtering rate results in a higher argon content in the bias-sputtered films. (5) The as-deposited film surface is smoother when deposited at lower pressures; the film has a columnar structure and intercolumnar gaps at higher pressures. (6) The film prepared at a higher bias voltage has a smoother as-deposited surface and a much smaller column size.From this study of the behavior of the stresses in niobium films, it appears that the stress is determined mainly by the microstructure and the energetic particle bombardment. Energetic particle bombardment may promote compressive stress by the incorporation of argon, by the formation of a more dense microstructure and by a “shot-peening” action.     

Magnetron sputter deposition of low-stress, carbon-containing cubic boron nitride films using Ar―N2―CH4 gas mixtures

Cubic boron nitride (c-BN) films produced by PVD and plasma-assisted CVD techniques typically exhibit undesired high compressive stresses. One of the effective and feasible methods to reduce stress and hence improve film adhesion has been a controlled addition of a third element into the film during deposition. In the present study, BN films were grown on to silicon substrates using reactive magnetron sputtering with a hexagonal BN target. An auxiliary flow of methane was mixed into argon and nitrogen as the working gas. The deposition was conducted at various methane flow rates at 400 °C substrate temperature, 0.2 Pa total working pressure, and − 250 V r.f. substrate bias. The microstructure of the deposited films was then examined in dependence of the methane flow rate. With increasing methane flow rate from 0 to approx. 2.0 sccm, the fraction of the cubic BN phase in the deposited films decreased gradually down to approx. 75 vol.%, whereas the film stress was reduced much more rapidly and almost linearly in relation to the methane flow rate. At 2.1 sccm methane, the stress became approx. 3 times reduced. Owing to the significantly decreased film stress, adherent, micrometer thick, cubic-phase dominant films can be allowed to form on silicon substrate. The microstructure of the films will be illustrated through FTIR and XRR.     

The effect of the backscattered energetic atoms on the stress generation and the surface morphology of reactively sputtered vanadium nitride films

During the reactive magnetron sputtering of transition metal nitrides in an Ar-N₂ ambient, Ar⁺ and N₂⁺ plasma ions are neutralized upon impingement on the target and are backscattered towards the growing film as neutral Ar and N species, respectively. Based on simulations, as well as on plasma and on film characterization techniques we manifest the relationship between the bombardment by the backscattered energetic atoms and the properties of reactively sputtered vanadium nitride (VN) films. Depending on the N₂ flow (q_N2) two bombardment regimes are established. In the first regime, (q_N2 < 20 sccm) the contribution of the N species to the energetic bombardment is insignificant. The major bombarding species in this regime are the backscattered Ar species, as well as positive plasma ions and sputtered atoms. These species have relatively low energies and subplantation ratios and thus, their energy is transferred to the surface of the growing film. In the second regime (q_N2 > 20 sccm) the backscattered N atoms are the major bombarding species and their flux to the growing film increases with increasing the N₂ flow. We argue that the backscattered N atoms have higher energy and subplantation ratio in comparison to the other bombarding species. As a result, a higher part of their energy is dissipated in the bulk of the film. The two bombarding regimes correlate well with the residual compressive stresses and the surface roughness of the films. Films grown at q_N2 < 20 sccm exhibit low compressive stresses and their roughness drops when q_N2 is increased. This consistent with the low subplantation ratio and the transfer of the energy of the bombarding species to surface the growing film. The compressive stresses of films grown at q_N2 > 20 sccm are higher, than those of the films grown in the first regime, and increase with increasing N₂ flow. This is attributed to the subplantation of the bombarding N species in the growing film.     

Effect of compressive stress inducing a band gap narrowing on the photoinduced activities of sol-gel TiO2 films

TiO₂ thin films grown on different kinds of substrates were obtained by sol-gel process. X-ray diffraction revealed that the TiO₂ lattice parameter c decreased continuously, indicating a continuous variation in the compressive stress, a negligible compressive stress of the film grown onto Soda-Lime Glass (SLG), medium compressive stress of the film grown onto BoroSilicate Glass (BSG) and large compressive stress of the film deposited onto the Quartz Substrate (QS). UV-Vis absorbance spectra exhibited a red-shift of the absorbance edge of the TiO₂ films suggesting a lowering of the band gap, which is a direct consequence of the increase of the compressive stress. X-ray photoelectron spectroscopy revealed that the surface composition of titania films was similar except for sodium-ion concentration. The rate observed during the photo-oxydation of the stearic acid on TiO₂/QS was twice as high as that of TiO₂/BSG and about 1000 times superior to that of TiO₂/SLG. The photoinduced wettability shows an identical dependence of the compressive stress. According to these results, the compressive stress could be used to tune the band gap of the titanium oxide in order to enhance the photoinduced properties.     

Optical properties of tungsten oxide thin films by non-reactive sputtering

Tungsten oxide thin films were grown on glass substrates by RF sputtering at room temperature using a tungsten trioxide target for several values of the argon pressure (P_Ar). The structural and morphological properties of these films were studied using X-ray diffraction and atomic force microscopy. The as-deposited films were amorphous irrespective of the argon pressure, and crystallized in a mixture of hexagonal and monoclinic phases after annealing at a temperature of 350 °C in air. Surface-roughness increased by an order of magnitude (from 1 nm to 20 nm) after thermal treatment. The argon pressure, however, had a strong influence on the optical properties of the films. Three different regions are clearly identified: deep blue films for P_Ar ≤ 2.67 Pa with low transmittance values, light blue films for 2.67 Pa < P_Ar < 6 Pa with intermediate transmittance values and transparent films for P_Ar ≥ 6 Pa with high transmittance values. We suggest that the observed changes in optical properties are due to an increasing number of oxygen vacancies as the growth argon pressure decreases.     

The effect of argon pressure, residual oxygen and exposure to air on the electrical and microstructural properties of sputtered chromium thin films

Chromium thin films were deposited on SiO₂/Si wafers using two sputtering systems with different levels of cleanliness, and at argon sputtering pressures varying between 0.13 and 0.93 Pa. Films from the two systems grown under identical sputtering conditions had significantly different resistivity values that are shown to be due to differences in residual oxygen in the chambers. Electrical transport measurements were conducted on the series of grown films to investigate the influence of argon pressure on film electrical resistivity. The films morphology, microstructure and composition were characterized using scanning electron microscopy and X-ray photoelectron spectroscopy. Significant differences were found in Cr thin films sputtered at different sputtering pressures; differences in resistivity performance and microstructure were noted. This change was shown to be due to the transition from porous structure to a denser microstructure. The Cr films sputtered at high pressure contained large quantities of oxygen when exposed to air. Some of the oxygen is added to the film during the deposition depending on the deposition rate and the base pressure of the sputtering system. The rest is incorporated into the film once it is exposed to air. The amount of oxygen added at this stage depends on the structure of the film and would be minimal for the films deposited at low sputtering pressures.     

Optimizing indium tin oxide thin films with bipolar d.c.-pulsed magnetron sputtering for electrochromic device applications

In this paper, indium tin oxide (ITO) films were prepared by bipolar d.c.-pulsed magnetron sputtering in a mixture of argon and oxygen onto unheated glass substrates. A target of ITO with 10 weight percent (wt %) tin was used. The influences of ratios of t^– _on/t⁺ _on (negative pulse-on time/positive pulse-on time) on the optical, electrical, and structural properties of ITO films have been investigated. The correlations between the deposition parameters and the film properties were discussed. An optimal condition based on reactive bipolar d.c.-pulsed sputtering for obtaining high transmittance, low resistivity, and low surface roughness of ITO films with high deposition rate is suggested. Then, ITO films grown at room temperature by bipolar d.c.-pulsed sputtering were used to form electrochromic devices of WO₃. Better electrochromic performances were found in comparison to those measured with commercially available ITO films on glass substrates.     

Stress and texture in HIPIMS TiN thin films

Titanium nitride (TiN) films in the thickness range of 0.013 µm to 0.3 µm were grown by high power impulse magnetron sputtering (HIPIMS) on silicon substrates in two deposition modes: a) the substrate was grounded and b) − 125 V bias was applied to the substrate. On the films we performed microstructure-, film texture- and film stress-analysis. The films deposited under − 125 V bias experienced a more energetic ion bombardment than the films deposited on grounded substrates. This difference in ion bombardment energy is reflected in the different microstructure. In contrast to previous results for TiN films grown by conventional reactive magnetron sputtering, we observe no major film stress gradient for increasing film thicknesses. We explain this observation from the absence of a 200-to-111 texture crossover during film growth.A moderate ion bombardment leads to TiN films with (111) texture, while an intense ion bombardment leads to films with (001) texture (Greene et al.; Appl. Phys. Lett. 67 (20) 2928-2930 (1995)). At the same time (001) oriented grains are much more susceptible to compressive stress generation by ion bombardment than (111) oriented grains.     

Characterization of bi-phased Zr2ON2–ZrO2 coatings deposited by RF magnetron sputtering

The aim of this work is to develop zirconium oxynitride coatings by RF magnetron sputtering on silicon substrates. The film properties were analyzed as a function of oxygen flux percentage in two different inert gas atmospheres namely argon and helium. At low oxygen flux percentage, Zr₂ON₂ and ZrO₂ phases are observed from the structural characterization by X-ray diffraction. The atomic ratio of nonmetallic to metallic atoms (N + O)/Zr content varies from 1.22 to 2.03 for zirconium oxynitride films deposited in argon atmosphere and from 1.43 to 2.33 for films deposited in helium atmosphere. The thickness of the film was measured by surface profiler and the growth rate decreases from 11.33 to 5.1 nm/min for films deposited in argon atmosphere and from 7.01 to 3.75 nm/min for films deposited in helium atmosphere with increase in oxygen flux percentage. The films deposited are hydrophobic and the contact angle was measured by contact angle measuring system. Higher surface roughness and maximum contact angle values of 100° and 103° are observed for films deposited in argon and helium atmosphere respectively at low oxygen flux percentage (2.5%). The surface energy of films was calculated by two methods: Owens-Wendt's geometric mean and Wu's harmonic mean approach. The elevated surface energy values were observed with increase in oxygen flux percentage. The stress measurements of the deposited films were done by sin²ψ X-ray diffraction method which depends on the variation of Zr₂ON₂ and m-ZrO₂ phases.     

Sputter deposition of ZnO thin films at high substrate temperatures

ZnO thin films have been deposited on GaN and ZnO substrates at substrate temperatures up to 750 °C by radio-frequency sputtering using ZnO ceramic targets in pure argon or in a mixture of argon and oxygen. By optimizing the sputter parameters, such as sputtering power, Ar/O₂ sputtering gas ratio and temperature of the substrates high quality films were obtained as judged from the X-ray rocking curve half width and luminescence line width. The crystallinity of the ZnO films increases with increasing substrate temperature. Yet there are distinct differences between films grown on GaN templates and on O- and Zn-polar ZnO substrates.     

Temperature effect on structure and surface morphology of indium tin oxide films deposited by reactive ion-beam sputtering

Indium tin oxide (ITO) films were deposited by reactive ion-beam sputtering. The relationship among the surface morphology, the resistivity ρ of the films, the substrate temperature T_S and the film thickness t_F was investigated. The heat power from the ion source during the sputtering was 265 W. T_S increased from 30 to 145 °C with an increase of t_F. The films thinner than 187 nm at T_S lower than 120 °C were amorphous, the film surface was as smooth as the substrate. The films deposited at T_S in the range between 135 and 145 °C were polycrystalline. So, the films thicker than 375 nm were in a multilayer structure of a polycrystalline layer on an amorphous layer. The surface of the polycrystalline films became rough. ρ of the films suddenly decreased at t_F of 375 nm, where the structure of the films changed. Next, the amorphous films with t_F of 39 nm were annealed in the atmosphere. The film structure changed to a polycrystalline structure at annealing temperature T_A of 350 °C. However, the surface roughness of all the films was almost same. As a result, the substrate temperature during the sputtering was important for the deposition of the films with a very smooth surface.     

Effect of substrate temperature and bias voltage on the crystallite orientation in RF magnetron sputtered AlN thin films

The crystal orientation and residual stress of AlN thin films were investigated using X-ray diffraction and substrate curvature method. The AlN films were deposited on Si(100) by RF magnetron sputtering in a mixed plasma of argon and nitrogen under various substrate negative bias V_s (up to − 100 V) and deposition temperature T_s up to 800 °C. The results show that lower temperature and moderate bias favor the formation of (002) plane parallel to the substrate surface. On the contrary, strong biasing beyond − 75 V and deposition temperature higher than 400 °C lead to the growth of (100) plane. At the same time nanoindentation hardness and compressive stress measured by substrate curvature method showed significant enhancement with substrate bias and temperature. The biased samples develop compressive stress while unbiased samples exhibit tensile or compressive stress depending on plasma power and temperature. The relationships between deposition conditions and crystallographic orientation of the films are discussed in terms of surface energy minimization and ion bombardment effects.     

Surface and electronic structure of Ga0.92In0.08N thin film investigated by photoelectron spectroscopy

The surface and electronic structure of Ga_0.92In_0.08N layers grown by metal organic chemical vapor deposition (MOCVD) have been investigated by means of photoemission. Stability of chemical composition of the surface subjected to Ar⁺ ion sputtering was proven by means of X-ray photoemission spectroscopy. The analysis of the relative intensities of In 3d, Ga 3p, and N 1s peaks showed that argon ion bombardment does not change significantly the relative contents of the layer constituents. Simultaneous efficient removal of the main contaminants (O and C) was observed during the sputtering procedure, proving that argon sputtering can be used as a method for preparation of clean Ga_1−xIn_xN surfaces.For a clean (0001)-(1×1) surface prepared by repeated cycles of Ar⁺ ion sputtering and annealing, electronic structure was investigated. The band structure was explored along the Γ-A direction of the Brillouin zone, measuring angle-resolved photoemission spectra along the surface normal. A similar set of data was also acquired for the same surface of GaN layer. Comparison of the collected data revealed an additional feature at the valence band edge, which can be ascribed to the presence of In in the layer.     

Some properties of r.f.-sputtered hafnium nitride coatings

Hafnium nitride coatings were deposited by reactive r.f. sputtering from a hafnium target in nitrogen and argon gas mixtures. The rate of deposition, the composition, the electrical resistivity and the complex index of refraction were investigated as functions of the target-to-substrate distance and the nitrogen fraction f_N2 in the sputtering atmosphere. It was found that the relative composition of the coatings is independent of f_N2 for values above 0.1. The electrical resistivity of the hafnium nitride films changes over 8 orders of magnitude when f_N2 changes from 0.10 to 0.85. The index of refraction is almost constant at 2.8(1?0.3i) up to f_N=0.40 then decreases to 2.1(1?0.1i) for higher values of f_N2.     

Optical waveguiding and birefringence properties of sputtered zinc oxide (ZnO) thin films on glass

Thin (≈1 μm), waveguiding and c-axis oriented ZnO films of good optical quality have been fabricated on corning glass substrates by r.f. magnetron reactive sputtering without substrate heating. The optical parameters of the films deposited in different O₂:Ar sputtering gas mixtures were found to vary. The extraordinary and ordinary refractive indices (n_e and n_o respectively) of the ZnO film grown in 60% O₂ and 40% Ar were found to be the highest (n_e=1.9876, n_o=1.9692) and closest to bulk single crystal values, and the birefringence of the film was ≈0.018. The films were annealed at 380 °C in air for 1 h, as a result of which the crystalline quality of the films was found to improve with increase in X-ray density and decrease in stress. While the refractive index decreased, the propagation loss was lowered substantially (3–5 dB/cm) as a result of annealing. The annealed zinc oxide film with minimum stress exhibited lowest loss (3 dB/cm) and highest birefringence (≈0.018). This indicated a correlation between propagation loss and stress.     

In situ grown superconducting YBCO films on buffered silicon substrates for device applications

Thin films of YBa₂Cu₃O_7?δ (YBCO) have been grownin situ on silicon single-crystal (100) substrate by using SrTiO₃ as a buffer layer. The deposition has been carried out by on-axis rf magnetron sputtering method. The deposition condition have been optimized by studying the plasma characteristics and correlating them with the superconducting performance of the film. Films deposited at substrate temperature in the range of 680–700°C from stoichiometric YBCO targets in an argon + oxygen mixture (3∶1) are superconducting and showc-axis epitaxy. Compositional confirmation has been carried out using Rutherford backscattering. Scanning tunneling microscopy of the films reveal formation of well-defined layered structure with some defects in the initial stages ofin situ growth of the films. Films grown on SrTiO₃ substrates have excellent crystalline quality (XRD), transition temperatureT _c0=81 K and the critical current densityJ _c >2×10⁵ A/cm² for unpatterned films at 77 K. On silicon substrates using buffer layers thein situ deposited YBCO films shows a higher transition width andT _c0 is also slightly less (71 K).     

Mössbauer spectroscopy study of amorphous TbFeCo films for perpendicular magnetic data recording

Amorphous films of the Tb _x Fe_{(100 − x − y)}Co _y system with a thickness of ∼200 nm were deposited by RF magnetron sputtering at various pressures of argon (P _Ar) in the working chamber. It is established that the easy axis in Tb₂₉Fe₆₁Co₁₀ films is oriented at an angle of 15° ± 4° relative to the normal to the film surface, whereas in Tb₂₆Fe₆₅Co₉ films this axis is parallel to the normal. The properties of the Tb₂₆Fe₆₅Co₉ films deposited at P _Ar = 5 mTorr meet the requirements to media for the perpendicular magnetic data recording.     

Stress stability and thermo-mechanical properties of reactively sputtered alumina films

The stability of residual stress inherent on deposition in reactively sputtered alumina films is studied during thermal cycling and annealing, simulating temperature excursions experienced by the films during device fabrication and subsequent operation. Increasing the magnitude of substrate bias applied during deposition acts to reduce the amount of argon incorporated in the films; more incorporated argon corresponds to smaller values of modulus and hardness and a larger coefficient of thermal expansion (CTE). Large, irreversible changes in film stress develop on heating, acting to decrease the compressive residual stress of films deposited on silicon substrates to a smaller, equilibrium value, whereas films deposited on Al₂O₃-TiC substrates behave differently. Thermal cycling and annealing have little effect on the modulus and CTE, but the hardness increases significantly and the threshold load for indentation crack initiation decreases precipitously during heat treatment. Possible mechanisms of irreversible stress development and mechanical property modifications are discussed.     

Influence of the argon flow rate on the arsenic fraction of a-GaxAs1−x sputtered at high substrate temperature

The experimental study of the chemical composition of amorphous gallium arsenide (a-Ga_xAs_1−x) versus argon flow rate, Q, by rf sputtering, shows that the As fraction of sputtered films is controlled by the argon flow rate. At the substrate temperature, T_s=500 °C, the films are stoichiometric when deposited under the argon flow rate between 8 and 22 sccm. These observations indicate that at low argon flow rate the As fraction of films is governed only by the preferential re-sputtering of As during the film growth. In addition, a correlation between the deposition rate R, and chemical composition x was deduced from these results.     

Synthesis and properties of thin film polymorphs of molybdenum trioxide

Thin film polymorphs of molybdenum trioxide have been synthesized by reactive r.f. sputtering in an atmosphere of argon plus oxygen. Films deposited on thermally insulated substrates are polycrystalline and exhibit preferred orientation. Depending on the oxygen partial pressure P_O2 during sputtering, the films crystallize in either the thermodynamically stable α-MoO₃ form (P_O2 ≈ 50%) (unique two-dimensional layered structure) or the newly discovered, metastable monoclinic β-MoO₃ phase (P_O2 ≈ 10%) (three-dimensional ReO₃-related structure). Metastable β films convert thermally to the α phase and the transformation appears topotactic. In contrast, films deposited on cooled substrates are amorphous. A correlation between the particular phase formed and adatom mobility is noted.