Compressive Stress In Films Research Articles

On the evolution of residual stress at different substrate temperatures in sputter-deposited polycrystalline Mo thin films by x-ray diffraction

The evolution of in-plane biaxial residual stress in polycrystalline Mo thin films deposited on Si at different substrate temperature were investigated using x-ray diffraction methods. The analyses were performed using both single and multi hkl reflection peak shifts methods in the asymmetric grazing incidence geometry. High compressive stress builds up in films synthesized at low substrate temperature while tensile stresses which are relatively low in magnitude manifest themselves at high substrate temperature. While such an evolutionary pattern is a direct consequence of competition between the intrinsic and thermal components of stress and varies as a function of temperature, the predominant cause of intrinsic compressive stress in Mo films deposited at low substrate temperature was traced to oxygen impurities incorporated in the film during the growth process.

Mechanical, tribological and corrosion performance of WBN composite films deposited by reactive magnetron sputtering

Abstract WBN composite films with various boron contents ranging from 25.1 at.% to 46.5 at.% were deposited by a multi-target magnetron sputtering system. The microstructure, mechanical, tribological and corrosion behavior of films were studied using XRD, SEM, FTIR, HRTEM, nano-indentation, Ball-on-disc dry sliding wear tester, Bruker 3D Profiler and compared to W2N. All the films exhibited face-centred cubic (fcc) structure W2N; bcc α-W phases appeared as the B content was 25.1 at.% and amorphous BN appeared as the B content was 31.9 at.%. The hardness and compressive stress of WBN films first increased and then decreased with increasing the B content. As the B content was 38.1 at.%, they reached the maximum values of 36.1 GPa and 2.6 GPa, respectively. The best wear resistance at room and elevated temperature was found for the film which was shown to exhibit the highest hardness and compressive stress. The corrosion resistance of the substrate which coated with W2N films or WBN films was superior to the uncoated substrate. The corrosion resistance of the substrate which coated with W2N films was improved slightly by doping some boron content.

Postdeposition treatment of IBS coatings for UV applications with optimized thin-film stress properties

Ion-beam-sputtering (IBS) single-layer and multilayer coating designs for UV applications were examined after the deposition process as well as after a defined postdeposition treatment. High internal compressive film stress as well as moderate absorption losses in the UV spectral range were measured at the as-deposited thin films. Due to a controlled postdeposition treatment process, the absorption losses and the high compressive stress can be reduced significantly. We show that the remaining thin-film stress of SiO2 and HfO2 multilayer designs can be specifically manipulated by the parameters of the postdeposition treatment. Even zero and tensile stress can be achieved for complex multilayer coatings.

Hard multifunctional Hf–B–Si–C films prepared by pulsed magnetron sputtering

Hf-B-Si-C films were deposited onto silicon and glass substrates using pulsed magnetron co-sputtering of a single B4C-Hf-Si target (at a fixed 15% Hf fraction and a varying 0–50% Si fraction in the target erosion area) in pure argon. We focus on the effect of the Si content in the films. The film structure changes from nanocolumnar (at 0–7at.% of Si) to nanocomposite (at around 10at.% of Si) to amorphous (at higher Si contents). Both nanocolumnar and nanocomposite HfB2-based films exhibit a hardness of up to 37GPa and a high H/E* ratio of around 0.15. The Si incorporation leads to a significant reduction of the compressive stress of films and improvement of their oxidation resistance (unmeasurable mass change after annealing up to 800°C at 35at.% of Si). All films exhibit a high electrical conductivity and very smooth defect-free surfaces with an average roughness below 1nm. Consequently, the films may be used as a new class of hard and electrically conductive protective coatings with a high oxidation resistance at elevated temperatures.

Effects of temperature-induced stress on the structural, electrical, and optical properties of ZnO:Ga thin films grown on Si substrates

Crystalline ZnO:Ga thin films with highly preferential c-axis oriented crystals were prepared on Si(001) substrates at different temperatures using the reactive magnetron sputtering technique. Effects of temperature-induced stress in ZnO:Ga films were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), electrical transport, and spectroscopic ellipsometry measurements. XRD results showed that the films were highly c-axis (out-of-plane) oriented and crystallinity improved with growth temperature. The residual compressive stress in films grown at low temperature relaxes with substrate temperature and becomes tensile stress with further increases in growth temperature. Resistivity of the films decreases with increasing stress, while the carrier concentration and mobility increase as the stress increases. The mechanism of the stress-dependent bandgap of ZnO:Ga films grown at different temperatures is suggested in the present work.

Effects of annealing temperature on mechanical durability of indium-tin oxide film on polyethylene terephthalate substrate

Effects of the annealing temperature on mechanical durability of indium-tin oxide (ITO) thin films deposited on polyethylene terephthalate (PET) substrates were investigated. The ITO films were annealed at the range from 150°C to 195°C after the DC sputtering deposition for the production of polycrystalline ITO layers on the substrates. The onset strains of cracking in the annealed ITO films were evaluated by the uniaxial stretching tests with electrical resistance measurements during film stretching. The results indicate that the onset strain of cracking in the ITO film is clearly increased by increasing the annealing temperature. The in-situ measurements of the inter-planer spacing of the (222) plane in the crystalline ITO films during film stretching by using synchrotron radiation strongly suggest that the large compressive stress in the ITO film increases the onset strain of cracking in the film. X-ray stress analyses of the annealed ITO films and thermal mechanical analyses of the PET substrates also clarifies that the residual compressive stress in the ITO film is enhanced with increasing the annealing temperature due to the considerably larger shrinkage of the PET substrate.

Erosive wear performance of boron-doped diamond films on different substrates

Diamond films have been considered as good candidate protective coatings on varieties of substrates in order to improve their erosive wear performance. In the present study, boron-doped diamond films with thicknesses of about 6–7 µm are first deposited on five different substrates using the hot filament chemical vapor deposition method, including SiC, WC-Co, Ta, Ti, and Si substrates. The residual stress and adhesive strength of the five boron-doped diamond films are estimated, respectively, by means of material properties of substrates and the diamond, Raman spectra, and static indentation results. Erosion tests are conducted on all the boron-doped diamond coated samples in an air–sand erosion rig with angular SiC sands as erodents. The results exhibit that the formation of ring cracks on the boron-doped diamond film in the erosion process has a strong link to the total residual stress. The higher residual compressive stress in the Ti-based boron-doped diamond film can slow down the formation of ring cracks to some extent. Nevertheless, the erosive wear performance, which is mainly evaluated based on the erosion rate and film removal, is much related to the film–substrate adhesion. As a result, the Ti-based boron-doped diamond film is quickly worn and removed because of the poorest adhesion. On the contrary, both the SiC- and Si-based boron-doped diamond films with favorable adhesion perform much better erosion resistance. Moreover, comparative erosion tests on thicker SiC- and WC-Co-based boron-doped diamond films (21–23 µm) and their applications on nozzles can both further manifest the same kind of relationship, the SiC-based boron-doped diamond film presenting longer steady erosive stage and slower film removal than the WC-Co-based one.

Atom insertion into grain boundaries and stress generation in physically vapor deposited films

We present evidence for compressive stress generation via atom insertion into grain boundaries in polycrystalline Mo thin films deposited using energetic vapor fluxes (<∼120 eV). Intrinsic stress magnitudes between −3 and +0.2 GPa are obtained with a nearly constant stress-free lattice parameter marginally larger (0.12%) than that of bulk Mo. This, together with a correlation between large compressive film stresses and high film densities, implies that the compressive stress is not caused by defect creation in the grains but by grain boundary densification. Two mechanisms for diffusion of atoms into grain boundaries and grain boundary densification are suggested.

Optimizing amorphous indium zinc oxide film growth for low residual stress and high electrical conductivity

With recent advances in flexible electronics, there is a growing need for transparent conductors with optimum conductivity tailored to the application and nearly zero residual stress to ensure mechanical reliability. Within amorphous transparent conducting oxide (TCO) systems, a variety of sputter growth parameters have been shown to separately impact film stress and optoelectronic properties due to the complex nature of the deposition process. We apply a statistical design of experiments (DOE) approach to identify growth parameter–material property relationships in amorphous indium zinc oxide (a-IZO) thin films and observed large, compressive residual stresses in films grown under conditions typically used for the deposition of highly conductive samples. Power, growth pressure, oxygen partial pressure, and RF power ratio (RF/(RF+DC)) were varied according to a full-factorial test matrix and each film was characterized. The resulting regression model and analysis of variance (ANOVA) revealed significant contributions to the residual stress from individual growth parameters as well as interactions of different growth parameters, but no conditions were found within the initial growth space that simultaneously produced low residual stress and high electrical conductivity. Extrapolation of the model results to lower oxygen partial pressures, combined with prior knowledge of conductivity–growth parameter relationships in the IZO system, allowed the selection of two promising growth conditions that were both empirically verified to achieve nearly zero residual stress and electrical conductivities >1480S/cm. This work shows that a-IZO can be simultaneously optimized for high conductivity and low residual stress.

Fracture toughness improvements of dental ceramic through use of yttria-stabilized zirconia (YSZ) thin-film coatings

ObjectivesThe aim of this study was to evaluate strengthening mechanisms of yttria-stabilized zirconia (YSZ) thin film coatings as a viable method for improving fracture toughness of all-ceramic dental restorations. MethodsBars (2mm×2mm×15mm, n=12) were cut from porcelain (ProCAD, Ivoclar-Vivadent) blocks and wet-polished through 1200-grit using SiC abrasive. A Vickers indenter was used to induce flaws with controlled size and geometry. Depositions were performed via radio frequency magnetron sputtering (5mT, 25°C, 30:1 Ar/O2 gas ratio) with varying powers of substrate bias. Film and flaw properties were characterized by optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Flexural strength was determined by three-point bending. Fracture toughness values were calculated from flaw size and fracture strength. ResultsData show improvements in fracture strength of up to 57% over unmodified specimens. XRD analysis shows that films deposited with higher substrate bias displayed a high %monoclinic volume fraction (19%) compared to non-biased deposited films (87%), and resulted in increased film stresses and modified YSZ microstructures. SEM analysis shows critical flaw sizes of 67±1μm leading to fracture toughness improvements of 55% over unmodified specimens. SignificanceData support surface modification of dental ceramics with YSZ thin film coatings to improve fracture toughness. Increase in construct strength was attributed to increase in compressive film stresses and modified YSZ thin film microstructures. It is believed that this surface modification may lead to significant improvements and overall reliability of all-ceramic dental restorations.

In situ test structures for the thermal expansion coefficient and residual stress of polysilicon thin films

In this research, micromachined devices consisting of four micro-rotating structures for the in situ determination of the thermal expansion coefficient (TEC), tensile and compressive residual stress of polysilicon thin films are studied. The structures are heated electrically and deflect due to the thermal expansion. The lateral displacements of the devices are related to the thermal stress and residual stress of the test beams. The micro-rotating structures are arranged, so that the lateral displacements are designed to be either a constant value which is used to determine the TEC of the thin film or a variable value that changes with the residual stress of the thin film. An analytical model of the test structure is presented. The finite element software ANSYS is used to verify the analytical model and provide guidelines for the structure design. Experimental results with a surface micromachined polysilicon thin film are used to demonstrate the proposed method. In the experiments, a current–voltage measurement system only is required. The TEC for the polysilicon thin film is obtained to be (2.61 ± 0.04) × 10−6 K−1 from 400 to 420 K and the residual stress is measured as −(10.15 ± 0.70) MPa.

Turbostratic-like carbon nitride coatings deposited by industrial-scale direct current magnetron sputtering

Carbon nitride thin films were deposited by direct current magnetron sputtering in an industrial-scale equipment at different deposition temperatures and substrate bias voltages. The films had N/(N+C) atomic fractions between 0.2 and 0.3 as determined by X-ray photoelectron spectroscopy (XPS). Raman spectroscopy provided insight into the ordering and extension of the graphite-like clusters, whereas nanoindentation revealed information on the mechanical properties of the films. The internal compressive film stress was evaluated from the substrate bending method. At low deposition temperatures the films were amorphous, whereas the film deposited at approximately 380°C had a turbostratic-like structure as confirmed by high-resolution transmission electron microscopy images. The turbostratic-like film had a highly elastic response when subjected to nanoindentation. When a CrN interlayer was deposited between the film and the substrate, XPS and Raman spectroscopy indicated that the turbostratic-like structure was maintained. However, it was inconclusive whether the film still exhibited an extraordinary elastic recovery. An increased substrate bias voltage, without additional heating and without deposition of an interlayer, resulted in a structural ordering, although not to the extent of a turbostratic-like structure.

Alucone Interlayers to Minimize Stress Caused by Thermal Expansion Mismatch between Al2O3 Films and Teflon Substrates

Alucone films were employed as interlayers to minimize stress caused by thermal expansion mismatch between Al(2)O(3) films grown by atomic layer deposition (ALD) and Teflon fluorinated ethylene propylene (FEP) substrates. The alucone films were grown by molecular layer deposition (MLD) using trimethylaluminum (TMA), ethylene glycol (EG), and H(2)O. Without the alucone interlayer, the Al(2)O(3) films were susceptible to cracking resulting from the high coefficient of thermal expansion (CTE) mismatch between the Al(2)O(3) film and the Teflon FEP substrate. Cracking was observed by field emission scanning electron microscopy (FE-SEM) images of Al(2)O(3) films grown directly on Teflon FEP substrates at temperatures from 100 to 160 °C and then cooled to room temperature. With an alucone interlayer, the Al(2)O(3) film had a crack density that was reduced progressively versus alucone interlayer thickness. For Al(2)O(3) film thicknesses of 48 nm deposited at 135 °C, no cracks were observed for alucone interlayer thicknesses >60 nm on 50 μm thick Teflon FEP substrates. For thinner Al(2)O(3) film thicknesses of 21 nm deposited at 135 °C, no cracks were observed for alucone interlayer thicknesses >40 nm on 50 μm thick Teflon FEP substrates. Slightly higher alucone interlayer thicknesses were required to prevent cracking on thicker Teflon FEP substrates with a thickness of 125 μm. The alucone interlayer linearly reduced the compressive stress on the Al(2)O(3) film caused by the thermal expansion mismatch between the Al(2)O(3) coating and the Teflon FEP substrate. The average compressive stress reduction per thickness of the alucone interlayer was determined to be 8.5 ± 2.3 MPa/nm. Comparison of critical tensile strains for alucone films on Teflon FEP and HSPEN substrates revealed that residual compressive stress in the alucone film on Teflon FEP could help offset applied tensile stress and lead to the attainment of much higher critical tensile strains.

Analysis of stress/strain in Electroless Copper Films

Polymer substrates were chemically coated with copper using various electroless copper baths and the internal strain/stress, as well as the adhesion quality, in the resulting copper films were studied during and after deposition as a function of the deposit thickness and the operation parameters of the electroless bath. The appearance of internal compressive stress in the copper film correlates to the probability of buckle driven delamination failure (blistering). Based on a simple theoretical concept we derived limits of allowed compressive stress in the copper film without inducing this failure mode. Furthermore depth-resolved X-ray diffraction (XRD) measurements in up to 1 μm thick electroless films indicate an approximately linear internal stress profile from about +200 MPa tensile stress at the substrate/adsorbate interface to −100 MPa compressive stress at the surface of the deposit. This will be explained in terms of a possible composition gradient of nickel in the copper film.

Impact of the Oxygen Amount of an Oxide Layer and Post Annealing on Forming Voltage and Initial Resistance of NiO-based Resistive Switching Cells

ABSTRACTForming voltage (VForm) and initial resistance of NiO-based resistive switching (RS) cells with various NiO films were investigated. Deposited NiO films were 〈111〉-oriented and the lattice constant was larger than that of bulk. It was revealed from XRD analyses that there were residual compressive stresses in NiO films. The magnitude of the residual stress was different among NiO films depending on their deposition conditions, and VForm monotonically increases with the increase in the magnitude of the residual stress. The relationship between VForm and the residual stress may be ascribed to the changes in the density of oxygen vacancies in NiO films. NiO films were also post annealed in Ar at 450°C. RS cells with annealed NiO films having small oxygen composition exhibited forming-free behavior, indicating the generation of conductive filaments by the annealing. The region whose lattice constant is smaller than that of bulk appeared after annealing only in such NiO films, suggesting that the small lattice-constant region may be linked to the generation of the filaments.

Strain in Electroless Copper Films: Analysis by in situ X-Ray Diffraction and Curvature Methods

Strain in chemically deposited copper films on polymer substrates was determined by means of in situ X-ray diffraction (XRD), deposit stress analyzer (DSA) and spiral contractometer (SC). The strain evolution of the films was studied as a function of copper film thickness and electroless copper bath parameters, during and after deposition. The results are not indicative of a preferred crystallite orientation or texturing in the deposit. The copper film stress is controllable over a wide range of some 100 MPa from compressive to tensile stress by appropriate variation of bath parameters (e.g. temperature, concentration of bath components such as nickel, stabilizer and formaldehyde). A higher tendency of blister generation for relaxed or compressively stressed films is apparent, which implies that a sufficient level of tensile stress throughout the deposition promotes film adhesion. An observable change from tensile to compressive film stress during the cooling of the sample from bath operation to rinse water temperature is discussed in terms of substrate-induced thermal stress to the copper film. In this context, the difference in the substrate materials required for XRD (polymer), DSA (copper) and SC (stainless steel) may be a significant factor contributing to the diverging measured stress behaviors of the methods. Moreover, it is questionable whether SC stress data can be compared with XRD and DSA stress data, due to the low resolution of the SC method (~60 MPa).

Influence of field-annealing on the microstructure, magnetic and microwave properties of electrodeposited Co0.3Fe0.7 films

Microstructure, magnetic properties and microwave characteristics of Co0.3Fe0.7 films fabricated by electro-deposition technique were investigated with post deposition field-annealing from 100 to 400°C. The crystallinity and grain size of film was enhanced with annealing temperature increment. The lattice spacing of crystallographic planes has decreased with increase in field-annealing temperature. This has induced compressive stress in field-annealed films. Coercivity and saturation magnetization values were increased with enhanced crystallinity of film. Ferromagnetic resonance was observed for as-deposited and field-annealed films. The ferromagnetic resonance frequency had shifted from 0.85 to 1.81GHz as the annealing temperature was increased from 100 to 400°C which was found to strongly correlate with the increase of saturation magnetization value.

Lateral Buckling of High Aspect Ratio Janus Nanowalls

AbstractIn recent years, the fabrication of Janus materials and their potential applications has been of much interest in Materials Science. Here, we report the fabrication of an entirely novel structure–Janus nanowalls and the phenomenon of lateral buckling in them. Polymeric nanowalls were prepared with the replica molding technique and metal films, of comparable thicknesses, were then deposited on one side of the polymer nanowalls by vacuum process. During the metal deposition, the nanowalls themselves buckle laterally; this buckling is induced by the compressive residual stress in the metal film and geometric confining constraints. The feature of wrinkle patterns resulting from the lateral buckling was theoretically investigated using the scaling analysis. Theoretical results are in good agreement with the experimental observations.

First principles investigation of interaction between impurity atom (Si, Ge, Sn) and carbon atom in diamond-like carbon system

The interaction between impurity atom (Si, Ge, and Sn) and carbon atom in diamond-like carbon (DLC) system was investigated by the first principles simulation method based on the density functional theory. The tetrahedral configuration was selected as the calculation model for simplicity. When the bond angle varied in a range of 90°–130° from the equivalent state of 109.471°, the distortion energy and the electronic structures including charge density of the highest occupied molecular orbital (HOMO) and partial density of state (PDOS) in the different systems were calculated. The results showed that the addition of Si, Ge and Sn atom into amorphous carbon matrix significantly decreased the distortion energy of the system as the bond angles deviated from the equilibrium one. Further studies of the HOMO and PDOS indicated that the weak covalent bond between Si(Ge, Sn) and C atoms was formed with the decreased strength and directionality, which were influenced by the electronegative difference. These results implied that the electron transfer behavior at the junction of carbon nano-devices could be tailored by the impurity element, and the compressive stress in DLC films could be reduced by the incorporation of Si, Ge and Sn because of the formation of weaker covalent bonds.

Mid-frequency PECVD of a-SiCN:H films and their structural, mechanical and electrical properties

The mid-frequency pulsed plasma enhanced chemical vapour deposition (PECVD) of hydrogenated amorphous silicon carbonitride (a-SiCN:H) was investigated to prove the suitability of these films as a mechanical stiff insulator for the integration of piezoelectric fibres in microstructured aluminium plates. For the a-SiCN:H deposition trimethylsilane (SiH(CH3)3; 3MS) and nitrogen in mixture with argon were used. The films were characterised regarding their deposition rate, elastic modulus and hardness (nanoindentation), mechanical stress, elemental composition (ERDA) and electrical insulating properties.The breakdown field strength of μm-thick a-SiCN:H films is in the range of 2–4 MV/cm. At pressures of a few Pa the deposition rate reached values up to 6 μm/h. It is limited by the power absorption in the 100 kHz bipolar-pulsed discharge. Varying the pressure from 2 Pa to 15 Pa has only little influence on the film composition. With increasing pressure during deposition the elastic modulus of the films decreases from about 146 GPa to 100 GPa and the compressive film stress from 1.2 GPa to 0.55 GPa. By reducing the 3MS flow rate from 50 sccm to 10 sccm (at 8 Pa deposition pressure), the carbon and the hydrogen concentrations in the films were reduced by about 10 at. %. The Si-content is only slightly reduced but the N-content is more than tripled. In contrast, the changes in the mechanical film properties are comparatively small. The mechanical properties of a-SiCN:H films are not simply correlated to the stoichiometry but are rather controlled by the ion bombardment during growth.