Deposition and characterization of ultra-high barrier coatings for flexible electronic applications

A barrier structure consisting of SiO_x and SiN_x films was deposited on the polymer substrate at 80 °C via plasma-enhanced chemical vapor deposition (PECVD). However, the low radius of curvature (R_c) of the barrier-coated substrate may cause the inconvenience of the following fabrication processes. By depositing a 150 nm-SiN_x film, the R_c of the barrier-coated polycarbonate (PC) substrate can increase from 80 to 115 mm without inducing any cracks in the barrier structure. Furthermore, the thermal stress of the barrier structure can be adjusted via extending the PECVD process duration in the chamber and replacing PC by the polyethersulone (PES) substrate. The R_c can increase to ∼356 mm by depositing the 150 nm-SiN_x film on the other side of the PES substrate. Finally, the calcium test result of the barrier films/PES/SiN_x sample was calculated to be around 3.05 × 10⁻⁶ g/m²/day, representing that the barrier structure did not fail after modification.     

Effect of NH3 flow rate on growth, structure and luminescence of amorphous silicon nitride films by electron cyclotron resonance plasma

In this paper, hydrogenated amorphous silicon nitride (a-SiN_x:H) films have been deposited using an electron cyclotron resonance chemical vapor deposition system. The effect of NH₃ flow rate R on the deposition rate, structure and luminescence were studied using various techniques such as optical emission spectroscopy, Fourier Transform Infrared absorption (FTIR), X-ray photoelectron spectroscopy (XPS) and fluoro-spectroscopy, respectively. Optical emission behavior of SiH₄ + NH₃ plasma shows that atomic Si radical concentration determines the film deposition rate. Structural transition of a-SiN_x film from Si-rich one to near-stoichiometric/N-rich one with R was revealed by FTIR and the two phase separation of a-Si and a-Si₃N₄ was also convinced in Si-rich SiN_x films by XPS. Either photo- or electroluminescence for all the SiN_x films with R > 3 sccm shows a strong light emission in visible light wavelength range. As R < 6 sccm, recombination of electrons and holes in a-Si quantum dots is the main mechanism of photo/electroluminescence for Si-rich SiN_x films, however, for photoluminescence, gap states' luminescence is also in competition; as R > 6 sccm, light emission of the SiN_x film originates from defect states in its band gap.     

The effect of composition on the bond structure and refractive index of silicon nitride deposited by HWCVD and PECVD

Silicon nitride (SiN_x) is a material with many applications and can be deposited with various deposition techniques. Series of SiN_x films were deposited with HWCVD, RF PECVD, MW PECVD and LF PECVD. The atomic densities are quantified using RBS and ERD. The influence of the atomic densities on the Si-N and Si-Si bond structure is studied. The density of N-N bonds is found to be negligible. New Si-N FTIR proportionality factors are determined which increase with increasing N/Si ratio from 1.2 · 10¹⁹ cm^− 1 for Si rich films (N/Si = 0.2) to 2.4 · 10¹⁹ cm^− 1 for N rich films (N/Si = 1.5). The peak position of the Si-H stretching mode in the FTIR spectrum is discussed using the chemical induction model. It is shown that especially for Si-rich films the hydrogen content affects the Si-H peak position. The influence of the composition on the refractive index of the films is discussed on the basis of the Lorentz-Lorenz equation and the Kramers-Kronig relation. The decreasing refractive index with increasing N/Si ratio is primarily caused by an increase of the band gap.     

Bulge testing and fracture properties of plasma-enhanced chemical vapor deposited silicon nitride thin films

The mechanical properties and fracture behavior of silicon nitride (SiN_x) thin film fabricated by plasma-enhanced chemical vapor deposition is reported. Plane-strain moduli, prestresses, and fracture strengths of silicon nitride thin films deposited both on a bare Si substrate and on a thermally oxidized Si substrate were extracted using bulge testing combined with a refined load-deflection model of long rectangular membranes. The plane-strain moduli and prestresses of SiN_x thin films have little dependence on the substrates, that is, for the bare Si substrate, they are 133 ± 19 GPa and 178 ± 22 MPa, respectively, while for the thermally oxidized substrate, they are 140 ± 26 GPa and 194 ± 34 MPa, respectively. However, the fracture strength values of SiN_x films grown on the two substrates are quite different, i.e., 1.53 ± 0.33 GPa and 3.08 ± 0.79 GPa for the bare Si substrate and the oxidized Si substrate, respectively. The reference stresses were computed by integrating the local stress of the membrane at the fracture over the edge, surface, and volume of the specimens and fitted with the Weibull distribution function. For SiN_x thin film produced on the bare Si substrate, the volume integration gave a significantly better agreement between data and model, implying that the volume flaws are the dominant fracture origin. For SiN_x thin film grown on the oxidized Si substrate, the fit quality of surface and edge integration was significantly better than the volume integration, and the dominant surface and edge flaws could be caused by buffered HF attacking the SiN_x layer during SiO₂ removal.     

Excellent passivation effect of Cat-CVD SiNx/i-a-Si stack films on Si substrates

We have demonstrated that the surface recombination velocity can be lowered to as low as 1.3 cm/s for n-type c-Si wafers and to 9.0 cm/s for p-type wafers by using amorphous Si (a-Si) and Si nitride (SiN_x) stacked films prepared by catalytic chemical vapor deposition (Cat-CVD). These values are much lower than those of c-Si wafers passivated by same stacked structures formed by low-damage remote plasma-enhanced CVD (PECVD). It is revealed that Cat-CVD a-Si insertion layers play an important role to improve interface quality, and also SiN_x films are also essential for reducing the surface recombination velocity down to such low levels.     

Thin film passivation of organic light emitting diodes by inductively coupled plasma chemical vapor deposition

The characteristics of an SiN_x passivation layer grown by a specially designed inductively coupled plasma chemical vapor deposition (ICP-CVD) system with straight antennas for the top-emitting organic light emitting diodes (TOLEDs) are investigated. Using a high-density plasma on the order of ∼ 10¹¹ electrons/cm³ formed by nine straight antennas connected in parallel, a high-density SiN_x passivation layer was deposited on a transparent Mg-Ag cathode at a substrate temperature of 40 °C. Even at a low substrate temperature, single SiN_x passivation layer prepared by ICP-CVD showed a low water vapor transmission rate of 5 × 10^− 2 g/m²/day and a transparency of ∼ 85% respectively. In addition, current-voltage-luminescence results of the TOLED passivated by the SiN_x layer indicated that the electrical and optical properties of the TOLED were not affected by the high-density plasma during the SiN_x deposition process.     

Annealing optimization of silicon nitride film for solar cell application

Hydrogenated films of silicon nitride (SiNx:H) is commonly used as an antireflection coating as well as passivation layer in crystalline silicon solar cell. SiNx:H films deposited at different conditions in Plasma Enhanced Chemical Vapor Deposition (PECVD) reactor were investigated by varying annealing condition in infrared (IR) heated belt furnace to find the optimized condition for the application in silicon solar cells. By varying the gases ratio (R = NH₃/SiH₄ + NH₃) during deposition, the SiNx:H films of refractive indices 1.85-2.45 were obtained. Despite the poor deposition rate, the silicon wafer with SiNx:H film deposited at 450 °C showed the best effective minority carrier lifetime. The film deposited with the gases ratio of 0.57 shows the best peak of carrier lifetime at the annealing temperature of 800 °C. The single crystalline silicon solar cells fabricated in conventional industrial production line applying the optimized film deposition and annealing conditions on large area substrates (125 mm × 125 mm) were found to have the conversion efficiencies as high as 17.05 %. Low cost and high efficiency single crystalline silicon solar cells fabrication sequence employed in this study has also been reported in this paper.     

Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays

A novel plastic substrate for flexible displays was developed. The substrate consisted of a polycarbonate (PC) base film coated with a gas barrier layer and a transparent conductive thin film. PC with ultra-low intrinsic birefringence and high temperature dimensional stability was developed for the base film. The retardation of the PC base film was less than 1 nm at a wavelength of 550 nm (film thickness, 120 µm). Even at 180 °C, the elastic modulus was 2 GPa, and thermal shrinkage was less than 0.01%. The surface roughness of the PC base film was less than 0.5 nm. A silicon oxide (SiO_x) gas barrier layer was deposited on the PC base film by a roll-to-roll DC magnetron reactive sputtering method. The water vapor transmission rate of the SiO_x film was less than 0.05 g/m²/day at 40 °C and 100% relative humidity (RH), and the permeation of oxygen was less than 0.5 cc/m² day atm at 40 °C and 90% RH. As the transparent conductive thin film, amorphous indium zinc oxide was deposited on the SiO_x by sputtering. The transmittance was 87% and the resistivity was 3.5 × 10^− 4 ohm cm.     

Influence of N2 flow rate on the mechanical properties of SiNx films deposited by microwave electron cyclotron resonance magnetron sputtering

Hydrogen-free amorphous silicon nitride (SiN_x) films were deposited at room temperature by microwave electron cyclotron resonance plasma-enhanced unbalance magnetron sputtering. Varying the N₂ flow rate, SiN_x films with different properties were obtained. Characterization by Fourier-transform infrared spectrometry revealed the presence of Si-N and Si-O bonds in the films. Growth rates from 1.0 to 4.8 nm/min were determined by surface profiler. Optical emission spectroscopy showed the N element in plasma mainly existed as N⁺ species and N₂⁺ species with 2 and 20 sccm N₂ flow rate, respectively. With these results, the chemical composition and the mechanical properties of SiN_x films strongly depended on the state of N element in plasma, which in turn was controlled by N₂ flow rate. Finally, the film deposited with 2 sccm N₂ flow rate showed no visible marks after immersed in etchant [6.7% Ce(NH₄)₂(NO₃)₆ and 93.3% H₂O by weight] for 22 h and wear test for 20 min, respectively.     

Microstructure and mechanical properties of CrAlN/SiNx nanostructure multilayered coatings

Multilayered CrAlN and SiN_x films were deposited periodically by radio frequency reactive magnetron sputtering. In the CrAlN/SiN_x multilayered coatings, the thickness of CrAlN layer was fixed at 4 nm, while that of SiN_x layer was adjusted from 4 nm to 0.3 nm. The dependence of the SiN_x layer thickness on the preferred orientation, crystalline behavior and mechanical properties of multilayered coatings were discussed with the aid of XRD patterns and HRTEM. It was demonstrated that amorphous SiN_x layer transformed to a crystallized one when the thickness decreased from 4 nm to 0.3 nm. The crystalline SiN_x layer grew epitaxially, formed the coherent interface with the CrAlN layer, and the columnar structure was exhibited. The critical layer thickness for the transition from amorphous SiN_x to a crystallized one was found to be around 0.4 nm, and maximum hardness of 33 GPa was revealed.     

Microwave characterization of laser ablated Y1Ba2Cu3O7−x thin films

In the present study we report the measurements of microwave surface resistance (R_s) of YBCO thin films on LaAlO₃ substrate as a function of temperature, thickness and magnetic field by microstrip resonator technique. The T_c(R = 0) of the films is 90 K and J_c > 10⁶ A/cm² at 77 K. The microwave surface resistance has been measured for films of various thicknesses. The value of R_s has been found to be initially decreased with increasing film thickness due to increase in number of defects. A minimum microwave surface resistance has been obtained for film thickness of about 300 nm. The increase of R_s with film thickness above 300 nm is possibly due to degradation of the film microstructure as observed with Atomic Force Microscopy. Temperature dependence of surface resistance has been studied for best quality films. The field induced variations of surface resistance are also investigated by applying dc magnetic field perpendicular to stripline structure and surface of the film. A general linear and square field dependence of R_s at low and high value of fields has been observed with critical field value of 0.4 T which confirms the microwave dissipation induced by flux flow in these resonators at 10 GHz frequency. The hysteresis of R_s in dc field observed for field value above critical field shows the higher value of surface resistance in decreasing field than in increasing field which is in agreement with one state critical model and is a characteristic of homogeneous superconductors.     

Application of plasma polymerized siloxane films for the corrosion protection of titanium alloy

Organosilicon film and SiO_x-like film are deposited on titanium alloy (Ti6Al4V) surfaces by atmospheric pressure (~ 10⁵ Pa) dielectric barrier discharge to improve its corrosion resistance in Hanks solution. Hexamethyldisiloxane (HMDSO) is used to be the chemical precursor. The organosilicon film deposited in Ar/HMDSO system has high growth rate (75 nm/min) and low surface roughness (3 nm), while the SiO_x-like film deposited in Ar/O₂/HMDSO system has lower growth rate (35 nm/min) and slightly higher surface roughness (9 nm). The potentiodynamic polarization tests show that both the two siloxane films coated Ti6Al4V samples have more positive corrosion potential and one order of magnitude lower corrosion current density than the substrate, indicating the corrosion resistance of Ti6Al4V can be improved by depositing siloxane film on its surface. In particular, as the surface is more compact and cross-linked, the SiO_x-like film has better corrosion resistance than the organosilicon film.     

Transmission electron microscopy characterization of TiN/SiNx multilayered coatings plastically deformed by nanoindentation

Plastic deformation of TiN_5 nm/SiN_0.5 nm multilayers by nanoindentation was investigated by transmission electron microscopy in order to identify deformation mechanisms involved in film failure resulting from severe plastic deformation. The TiN layers exhibited a crystalline fcc structure with a [002] preferential orientation; further crystal growth was interrupted by the amorphous SiN_x layers. After severe plastic deformation collective vertical displacement of slabs of several TiN/SiN_x-bilayers, which resulted from shear sliding at TiN/TiN grain boundaries, was observed. They are, together with horizontal fractures along the SiN_x layers, vertical cracks under the indenter tip following the TiN grain boundaries and delamination from the substrate, the predominant failure mechanisms of these coatings. The deformation behaviour of these films provides an experimental support for the absence of dislocation activity in grains of 5 nm size.     

Amorphous In-Ga-Zn-O thin-film transistor with coplanar homojunction structure

Amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) with a coplanar homojunction structure are demonstrated. The coplanar source and drain regions made of a-IGZO were formed by depositing a hydrogenated silicon nitride (SiN_X:H) layer onto the a-IGZO layer. The a-IGZO regions on which the SiN_X:H layer was directly deposited showed the low resistivity of 4.7 × 10⁻³  Ω cm and degenerated conduction. The fabricated TFT showed excellent transfer and output characteristics with a field-effect mobility of 11 cm² V^− 1 s^− 1, a subthreshold swing of 0.17 V decade^− 1, and an on-to-off current ratio larger than 1 × 10⁹. The width-normalized source-to-drain resistance (R_sdW) calculated using a channel resistance method was 51 Ω cm. This TFT also showed good stability over environment change and under electrical stress.     

Giant pyroelectric coefficient of Mn-doped PLT thin films epitaxially grown on (001) Pt/MgO

The c-axis-oriented epitaxial thin films of Mn-doped Pb_1−xLa_xTi_1−x/4O₃ (PLT) on (001) Pt/MgO substrates were prepared by rf-magnetron sputtering. To investigate the effect of the doped ion, 0-1.7 mol% MnO₂ added to the PLT target powder. The temperature dependence of the relative dielectric constant ?_r measurements and modified Curie-Weiss plots suggested that the increasing of diffuseness n was induced by high-La substitution and the diffuseness n of PLT thin films decreased by the addition of Mn, considerably. Inner stress and thermodynamic analysis were carried out and the results propose that the increasing of γ with Mn doping caused by increasing the misfit strain of the c-axis-oriented epitaxial PLT thin films and substrate. As a result, giant pyroelectric coefficient (γ = 15.8 × 10⁻⁸ C/cm² K) of Mn-doped epitaxial PLT thin film was achieved.     

Investigation of brittle failure in transparent conductive oxide and permeation barrier oxide multilayers on flexible polymers

An oxide multilayer structure—consisting of an indium zinc oxide (IZO) conductive layer, a silicon oxide (SiO_x, x = 1.8) water vapor permeation barrier, and an aluminum oxide (Al₂O₃) interlayer—coated on polyethylene terephthalate (PET) is proposed as a transparent flexible substrate for display and photovoltaic applications. Vital properties of the multilayer, such as the low water vapor impermeability of the SiO_x barrier and the high conductance of the IZO film, degraded considerably because of the crack formation in bend geometries, attributed to the large difference between elastic properties of the oxide films and polymers. In order to suppress the crack formation, a 10-nm-thick Al₂O₃ interlayer was sputtered on Ar ion-beam treated PET surfaces prior to a SiO_x plasma-enhanced chemical vapor deposition (PECVD) process. Changes in the conductance and water vapor impermeability were investigated at different bending radii and bending cycles. It was found that the increases in resistance and water vapor transmission rate (WVTR) were significantly suppressed by the ion-beam PET pretreatment and by the sputtered Al₂O₃ interlayer. The resistance and WVTR of IZO/SiO_x/Al₂O₃/PET systems could be kept low and invariable even in severely bent states by choosing the SiO_x thickness properly. The IZO (135 nm)/SiO_x (90 nm)/Al₂O₃ (10 nm)/PET system maintained a resistance of 3.2 × 10^− 4 Ω cm and a WVTR of < 5 × 10^− 3 g m² d^− 1 after 1000 bending cycles at a bending radius of 35 mm.     

Characterization of ZnO-In2O3 transparent conducting films by pulsed laser deposition

Transparent conducting oxide (TCO) films in the ZnO-In₂O₃ system were prepared by a pulsed laser deposition method. A target that consists of the mixture of ZnO and In₂O₃ powders was used. Influences of the target composition x (x = [Zn]/([Zn] + [In])) and heater temperature on structural, electrical and optical properties of the TCO films were examined. Introduction of oxygen gas into the chamber during the deposition was necessary for improvement in the transparency of the deposited films. The amorphous phase was observed for a wide range of x = 0.20-0.60 at 110 °C. Minimum resistivity was 2.65 × 10⁻⁴ Ω cm at x = 0.20. The films that showed the minimum resistivity had an amorphous structure and the composition shifted toward larger x, as the substrate temperature increased. The films were enriched in indium compared to the target composition and the cationic In/Zn ratio increased as the substrate temperature was increased.     

Effect of dopant concentration on the structural, electrical and optical properties of Mn-doped ZnO films

The Mn-doped ZnO (Zn_1 − xMn_xO) thin films with manganese compositions in the range of 0-8 at.% were deposited by radio-frequency (RF) magnetron sputtering on quartz glass substrates at room temperature (RT). The influence of Mn concentration on the structural, electrical and optical properties of Zn_1 − xMn_xO films has been investigated. X-ray diffraction (XRD) measurements reveal that all the films are single phase and have wurtzite structure with (002) c-axis orientation. The chemical states of Mn have been identified as the divalent state of Mn²⁺ ions in ZnO lattice. As the content of Mn increases, the c-lattice constant and the optical band gap of the films increase while the crystalline quality deteriorates gradually. Hall-effect measurements reveal that all the films are n-type and the conductivity of the films has a severe degradation with Mn content. It is also found that the intensity of RT photoluminescence spectra (PL) is suppressed and saturates with Mn doping.     

Tl2Ba2CaCu2O8 thin films on 2 in. LaAlO3(0 0 1) substrates

High quality Tl₂Ba₂CaCu₂O₈ (Tl-2212) superconducting thin films are prepared on both sides of 2 in. LaAlO₃(0 0 1) substrates by off-axis magnetron sputtering and post-annealing process. XRD measurements show that these films possess pure Tl-2212 phase with C-axis perpendicular to the substrate surface. The thickness unhomogeneity of the whole film on the 2 in. wafer is less than 5%. The superconducting transition temperatures T_cs of the films are around 105 K. At zero applied magnetic field, the critical current densities J_cs of the films on both sides of the wafer were measured to be above 2 × 10⁶ A/cm² at 77 K. The microwave surface resistance R_s of film was as low as 350 μΩ at 10 GHz and 77 K. In order to test the suitability of Tl-2212 thin films for passive microwave devices, 3-pole bandpass filters have been fabricated from double-sided Tl-2212 films on LaAlO₃ substrates.     

Microstructure and tribological performance of CNx–TiNx composite films prepared by pulsed laser deposition

CN_x–TiN_x composite films were prepared on high-speed steel (HSS) substrate by pulsed KrF excimer laser co-deposition process with graphite/Ti combined targets and a substrate temperature of 200 °C. The composition, morphology and microstructure of the films were characterized by energy dispersive X-ray spectrum (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The adhesion and tribological performance of the films were investigated using a conventional scratch tester and a ball-on-disk tribometer, respectively. In the graphite/Ti range of 0.5–2.0 of the target, TiN_x, a-CN_x and metallic Ti phase were found in the composite films. The TiN_x disappeared in the films at a high graphite/Ti ratio of the target. With increasing the graphite/Ti ratio of the target, the adhesion to substrate of the composite films deteriorated from 46 N to 26 N, and the friction coefficient decreased from 0.23 to 0.17. The composite film deposited at the graphite/Ti ratio of 1.0 showed a low friction coefficient, good adhesion and wear rate of 3.2 × 10⁻⁷ mm³/Nm in humid air.