Properties of polyimide/Al2O3 and Si3N4 deposited thin films

Polyimide (PI) nanocomposites with different proportions of Al₂O₃ were prepared via two-step reaction. Silicon nitride (Si₃N₄) was deposited on PI composite films by a RF magnetron sputtering system and used as a gas barrier to investigate the water vapor transmission rate (WVTR). The thermal stability and mechanical properties of a pure PI film can be improved obviously by adding adequate content of Al₂O₃. At lower sputtering pressure (4 mTorr), the PI/Al₂O₃ hybrid film deposited with Si₃N₄ barrier film exhibits denser structure and lower root mean square (RMS) surface roughness (0.494 nm) as well as performs better in preventing the transmission of water vapor. The lowest WVTR value was obtained from the sample, 4 wt.%Al₂O₃-PI hybrid film deposited with Si₃N₄ barrier film with the thickness of 100 nm, before and after bending test. The interface bonding, Al-N and Al-O-Si, was confirmed with the XPS composition-depth profile.     

Super H2O-barrier film using Cat-CVD (HWCVD)-grown SiCN for film-based electronics

“Super H₂O-barrier film” with a water vapor transmission rate (WVTR) less than 1 mg/m²/day has been developed. The barrier layer is a single layer of amorphous SiCN grown by organic Cat-CVD (O-Cat-CVD) with a thickness of 100 nm. SiCN has been grown by using a gas mixture of monomethylsilane (MMS; Si (CH₃)H₃), NH₃ and H₂ on polyethylene-naphthalate (PEN) film substrates. It has been found that the WVTR drastically depends on the W-filament temperature of O-Cat-CVD. The WVTR changed from 5 × 10⁻¹ to 1 × 10⁻³, corresponding to the W-filament temperature increase from 1100 to 1200 C. We have recently succeeded in developing the “super H₂O-barrier film” by the coating of single layers of SiCN for both sides of the PEN film without using the widely used polymer/inorganic multilayer coating. The both-side coating has been found to be crucial to avoid the H₂O penetration into PEN films and also to avoid the breakdown of the SiCN/PEN interface caused by the H₂O accumulation at the interface.     

Effects of argon and oxygen flow rate on water vapor barrier properties of silicon oxide coatings deposited on polyethylene terephthalate by plasma enhanced chemical vapor deposition

Plasma polymer coatings were deposited from hexamethyldisiloxane on polyethylene terephthalate (PET) substrates while varying the operating conditions, such as the Ar and O₂ flow rates, at a fixed radio frequency power of 300 W. The water vapor transmission rate (WVTR) of the untreated PET was 54.56 g/m²/day and was decreased after depositing the silicon oxide (SiO_x) coatings. The minimum WVTR, 0.47 g/m²/day, was observed at Ar and O₂ flow rates of 4 and 20 sccm, respectively, with a coating thickness of 415.44 nm. The intensity of the peaks for the Si-O-Si bending at 800-820 cm^− 1 and Si-O-Si stretching at 1000-1150 cm^− 1 varied depending on the Ar and O₂ flow rates. The contact angle of the SiO_x coated PET increased as the Ar flow rate was increased from 2 to 8 sccm at a fixed O₂ flow rate of 20 sccm. It decreased gradually as the oxygen flow rate increased from 12 to 28 sccm at a fixed Ar carrier gas flow rate. The examination by atomic force microscopy revealed a correlation of the SiO_x morphology and the water vapor barrier performance with the Ar and O₂ flow rates. The roughness of the deposited coatings increased when either the O₂ or Ar flow rate was increased.     

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.     

Atomic layer deposition on polymer based flexible packaging materials: Growth characteristics and diffusion barrier properties

One of the most promising areas for the industrial application of atomic layer deposition (ALD) is for gas barrier layers on polymers. In this work, a packaging material system with improved diffusion barrier properties has been developed and studied by applying ALD on flexible polymer based packaging materials. Nanometer scale metal oxide films have been applied to polymer-coated papers and their diffusion barrier properties have been studied by means of water vapor and oxygen transmission rates. The materials for the study were constructed in two stages: the paper was firstly extrusion coated with polymer film, which was then followed by the ALD deposition of oxide layer. The polymers used as extrusion coatings were polypropylene, low and high density polyethylene, polylactide and polyethylene terephthalate. Water vapor transmission rates (WVTRs) were measured according to method SCAN-P 22:68 and oxygen transmission rates (O₂TRs) according to a standard ASTM D 3985. According to the results a 10 nm oxide layer already decreased the oxygen transmission by a factor of 10 compared to uncoated material. WVTR with 40 nm ALD layer was better than the level currently required for most common dry flexible packaging applications. When the oxide layer thickness was increased to 100 nm and above, the measured WVTRs were limited by the measurement set up. Using an ALD layer allowed the polymer thickness on flexible packaging materials to be reduced. Once the ALD layer was 40 nm thick, WVTRs and O₂TRs were no longer dependent on polymer layer thickness. Thus, nanometer scale ALD oxide layers have shown their feasibility as high quality diffusion barriers on flexible packaging materials.     

Optical properties of evaporated poly-Si thin-films on glass

The optical properties of boron- and phosphorus-doped polycrystalline silicon films with light (~ 1 × 10¹⁶ cm⁻³), moderate (~ 5 × 10¹⁷ cm⁻³) and heavy doping (~ 1 × 10¹⁹ cm⁻³) were investigated in this work. The films were prepared by solid-phase crystallization of evaporated amorphous silicon films on borosilicate glass. Tauc-Lorentz models with one or two oscillators were used to model both reflection and transmission data collected by a spectrophotometer over the wavelength range of 400 nm-2000 nm. The results indicate that the crystal quality of the films is improved by phosphorus doping, while boron has a negligible impact on the crystal quality. The poly-Si films exhibit greater absorption than c-Si for visible wavelengths. This enhanced absorption is believed to be associated with defected a-Si material at the grain boundaries and intra-grain defects.     

Tunnelling through diamond-like carbon nanofilms deposited by electron-beam-induced deposition

The tunnelling properties in metal/diamond-like carbon (DLC)/semiconductor junctions and structural characteristics of thin DLC films produced using different electron beam conditions were studied. We show that under the same electron dose conditions, thicker DLC films were obtained using lower accelerating voltages (2 kV) than when using higher accelerating voltage (20 kV). However, under the settings used the thicker films showed worse insulating performance than the thinner films. We attribute this effect to the variation of tunnelling barrier height in DLC deposited using different accelerating voltages. DLC films with a tunnelling barrier height of up to 3.12 eV were obtained using a 20 kV electron-beam, while only 0.73 eV was achieved for 2 kV DLC films. The X-ray photoemission spectra of the C 1s core level in these films reveal components at 284.4 ± 0.1 eV and 285 ± 0.1 eV, which were identified as the sp² and sp³ hybrid forms of carbon. The sp³/sp² concentration ratio increased with increasing electron beam accelerating voltage. We show how this effect is responsible for the barrier height variation.     

Zinc cadmium oxide thin film transistors fabricated at room temperature

Zinc cadmium oxide (ZnCdO) transparent thin film transistors (TFTs) have been fabricated with a back-gate structure using highly p-type Si (001) substrate. For the active channel, 30 nm, 50 nm, and 100 nm thick ZnCdO thin films were grown by pulsed laser deposition. The ZnCdO thin films were wurtzite hexagonal structure with preferred growth along the (002) direction. All the samples were found to be highly transparent with an average transmission of about 80%~ in the visible range. We have investigated the change of the performance of ZnCdO TFTs as the thickness of the active layer is increased. The carrier concentration of ZnCdO thin films has been confirmed to be increased from 10¹⁶ to 10¹⁹ cm⁻³ as the film thickness increased from 30 to 100 nm. Base on this result, the ZnCdO TFTs show a thickness-dependent performance which is ascribed to the carrier concentration in the active layer. The ZnCdO TFT with 30 nm active layer showed good off-current characteristic of below ~ 10¹¹, threshold voltage of 4.69 V, a subthreshold swing of 4.2 V/decade, mobility of 0.17 cm²/V s, and on-to-off current ratios of 3.37 × 10⁴.     

Aluminum oxide barrier coating on polyethersulfone substrate by atomic layer deposition for barrier property enhancement

Aluminum oxide layers were deposited on flexible polyethersulfone (PES) substrates via plasma enhanced atomic layer deposition (PEALD) process using trimethylaluminum (TMA) and oxygen as precursor and reactant materials. Several process parameters in PEALD process were investigated in terms of refractive index and layer thickness. Number of process cycle increased the thickness and refractive index of the layer to enhance the barrier properties. Non-physisorbed TMA and unreacted oxygen were purged before and after the plasma reaction, respectively. Identical purge time was applied to TMA and oxygen and it was optimized for 10 s. Thinner and denser layer was formed as substrate temperature increased. However, the PES substrate could be deformed above 120 °C. Aluminum oxide layer formed on PES at optimized conditions have 11.8 nm of thickness and reduced water vapor transmission rate and oxygen transmission rate to below 4 × 10^− 3 g/m² day and 4 × 10^− 3 cm³/m² day, respectively. Polycarbonate and polyethylene naphthalate films were also tested at optimized conditions, and they also showed quite appreciable barrier properties to be used as plastic substrates.     

Tribological behavior of radio-frequency sputtering WS2 thin films with vacuum annealing

Thin films of tungsten disulfide (WS₂) were deposited on 3Cr13 martensitic stain less steel substrate by radio-frequency (RF) sputtering. The as-deposited films were annealed at 473, 673 and 873 K respectively for 2 h in 5 × 10^− 4 Pa vacuum. Composition of the films was inspected by energy dispersive spectroscopy. Surface morphology and structure properties were studied by scanning electron microscopy and X-ray diffraction techniques. Tribological behavior was also examined using tribometer. At 473 K, the films exhibited low crystallization structure and no significant improvement in the tribological performance. At 673 K, the tribological performance was improved and a transition from non-crystalline to hexagonal structure took in place. When the annealing temperature rose up to 873 K, the films cracked and fell off from the substrate. The results suggested that with suitable technical parameters vacuum annealing could promote crystallization and improve tribological performance of RF sputtering WS₂ films.     

Rapid ultraviolet-curing of epoxy siloxane films

The ultraviolet (UV) curable epoxy siloxane polymer is shown to cross-link at low UV dosages of 130 mJ/cm², making it desirable for use in nanoimprinting and the rapid fabrication of micro/nano-scaled patterns. In this paper, the dielectric and mechanical properties of this UV-cured epoxy siloxane polymer are investigated. The results of these tests show that the rapid UV-cured polymer films have a dielectric constant of 2.7 ± 0.13, leakage current density on the order of 10⁻⁹ A/cm² under 1 MV/cm, dielectric strength of greater than 5 MV/cm, and a reduced modulus of elasticity of 6.2 GPa characterized using nanoindentation. These properties indicate that the epoxy siloxane can be used to fabricate layers for functional device applications.     

Carrier transport in polycrystalline transparent conductive oxides: A comparative study of zinc oxide and indium oxide

Highly doped indium-tin oxide films exhibit resistivities ρ as low as  1.2 × 10^− 4 Ω cm, while for ZnO films resistivities in the range of 2 to 4 × 10^− 4 Ω cm are reported. This difference is unexpected, if ionized impurity scattering would be dominant for carrier concentrations above 10²⁰ cm^− 3. By comparing the dependences of the effective Hall mobility on the carrier concentration of ZnO and ITO it is found that grain barriers limit the carrier mobility in ZnO for carrier concentrations as high as 2 × 10²⁰ cm^− 3, independently, if the films were grown on amorphous or single crystalline substrates. Depending on the deposition method, grain barrier trap densities between 10¹² and 3 × 10¹³ cm^− 2 were estimated for ZnO layers. Also, crystallographic defects seem to reduce the mobility for highly doped ZnO films. On the other hand, for ITO films such an influence of the grain barriers was not observed down to carrier concentrations of about 10¹⁸ cm^− 3. Thus the grain barrier trap densities of ZnO and ITO are significantly different, which seems to be connected with the defect chemistry of the two oxides and especially with the piezoelectricity of zinc oxide.     

Cat-CVD SiN passivation films for OLEDs and packaging

A Roll-to-roll type catalytic chemical vapor deposition (Cat-CVD) apparatus was developed for the application to flexible organic light-emitting diode (OLED) displays and packaging. Silicon nitride (SiN_x) films were prepared by this roll-to-roll type apparatus at temperatures below 60 °C. It was found that these SiN_x films are highly moisture resistant, and the water vapor transmission rate (WVTR) on plastic substrates could be lowered to 0.01 g/m² day. Roll-to-roll type Cat-CVD is one of the most promising methods for the preparation of barrier films for OLED displays and packaging.     

Effect of inserting a buffer layer on the characteristics of transparent conducting impurity-doped ZnO thin films prepared by dc magnetron sputtering

In transparent conducting impurity-doped ZnO thin films prepared on glass substrates by a dc magnetron sputtering (dc-MS) deposition, the obtainable lowest resistivity and the spatial resistivity distribution on the substrate surface were improved by a newly developed MS deposition method. The decrease of obtainable lowest resistivity as well as the improvement of spatial resistivity distribution on the substrate surface in Al- or Ga-doped ZnO (AZO or GZO) thin films were successfully achieved by inserting a very thin buffer layer, prepared using the same MS apparatus with the same target, between the thin film and the glass substrate. The deposition of the buffer layer required a more strongly oxidized target surface than possible to attain during a conventional dc-MS deposition. The optimal thickness of the buffer layer was found to be about 10 nm for both GZO and AZO thin films. The resistivity decrease is mainly attributed to an increase of Hall mobility rather than carrier concentration, resulting from an improvement of crystallinity coming from insertion of the buffer layer. Resistivities of 3 × 10^− 4 and 4 × 10^− 4Ω cm were obtained in 100 nm-thick-GZO and AZO thin films, respectively, incorporating a 10 nm-thick-buffer layer prepared at a substrate temperature around 200 °C.     

Water permeation through organic-inorganic multilayer thin films

We prepared organic (self-assembled monolayer (SAM))-inorganic (TiO₂) multilayer barrier films on polyethylene terephthalate substrate using atomic layer deposition and molecular layer deposition methods in the same deposition chamber. The water permeation was mainly blocked by the inorganic TiO₂ layer. While the lag time was proportional to the thickness of the TiO₂ layer, the steady-state permeation rate was relatively independent of the thickness. The multilayer approach was effective in extending the lag time due to both the tortuous path effect and the internal desiccant effect. Water permeation occurred sequentially in the organic-inorganic multilayer barriers by water accumulation in the organic SAM layers. The water vapor transmission rate was 7.0 × 10^− 4 g/m²·day during the lag time of 155 h at 60 °C and a relative humidity of 85% with 5-dyad barrier film.     

Effect of deposition parameters and annealing temperature on the structure and properties of Al-doped ZnO thin films

Transparent conductive films of Al-doped ZnO (AZO) were deposited onto inexpensive soda-lime glass substrates by radio frequency (rf) magnetron sputtering using a ZnO target with an Al content of 3 wt%. The Taguchi method with a L₉ orthogonal array, signal-to-noise (S/N) ratio and analysis of variance (ANOVA) were employed to examine the performance characteristics of the coating operations. This study investigated the effect of the deposition parameters (rf power, sputtering pressure, thickness of AZO films, and substrate temperature) on the electrical, structural, morphological and optical properties of AZO films. The grey-based Taguchi method showed the electrical resistivity of AZO films to be about 9.15 × 10⁻³ Ω cm, and the visible range transmittance to be about 89.31%. Additionally, the films were annealed in a vacuum ambient (5.0 × 10⁻⁶ Torr) at temperatures of 400, 450, 500 and 600 °C, for a period of 30 min. It is apparent that the intensity of the X-ray peaks increases with annealing treatment, leading to improved crystallinity of the films. By applying annealing at 500 °C in a vacuum ambient for 30 min, the AZO films show the lowest electrical resistivity of 2.31 × 10⁻³ Ω cm, with about 90% optical transmittance in the visible region and a surface roughness of Ra = 12.25 nm.     

Chitosan tailor‐made films: the effects of additives on barrier and mechanical properties

With the aim of achieving ‘tailor‐made’ chitosan films, the effects of several variables on the properties of chitosan films were studied. These variables were chitosan concentration and molecular weight of thermally depolymerized chitosan, addition of lipids (palmitic acid, beeswax or carnauba wax) and plasticizer (glycerol). The water vapour transmission rate (WVTR) and mechanical properties of these films were measured. The innovative feature of this study is that it provides specific information to support the design of tailor‐made films. These can only be formulated when the effects of the important variables are well understood. It was found that WVTR was reduced by 57% in film made from chitosan that had been thermally treated for 7 h at 100°C (molecular mass 13.7 kDa), while in the emulsion films, the WVTR was increased by incorporation of palmitic acid, beeswax or carnauba wax incorporation. The mechanical properties (tensile strength and elongation at break) were improved when glycerol was used as plasticizer, resulting in more elastic films (increasing the elongation at break by 62%). Copyright © 2008 John Wiley & Sons, Ltd.     

Co-sputtered oxide thin film encapsulated organic electronic devices with prolonged lifetime

Effective top-side thin film encapsulation for organic light-emitting devices (OLEDs) was achieved by deposition of a multi-layer water diffusion barrier stack to protect the device against moisture permeation. The barrier stack was formed by alternative depositions of co-oxide and fluorocarbon (CF_x) films. The co-oxide layer was fabricated by magnetron co-sputtering of silicon dioxide (SiO₂) and aluminum oxide (Al₂O₃). While the CF_x layer was formed by plasma enhanced chemical vapor deposition. The water vapor transmission rate of the optimized diffusion barrier stack can be down to 10^− 6 g/m²/day. The OLEDs encapsulated with the multilayer stack have been shown to have operation lifetime of over 18,000 h which is nearly the same as devices with conventional glass-cover encapsulation.     

Broadband transmission masks, gratings and filters for extreme ultraviolet and soft X-ray lithography

Lithography and patterning on a nanometre scale with extreme ultraviolet (EUV) and soft X-ray radiation allow creation of high resolution, high density patterns independent of a substrate type. To realize the full potential of this method, especially for EUV proximity printing and interference lithography, a reliable technology for manufacturing of the transmission masks and gratings should be available. In this paper we present a development of broadband amplitude transmission masks and gratings for extreme ultraviolet and soft X-ray lithography based on free-standing niobium membranes. In comparison with a standard silicon nitride based technology the transmission masks demonstrate high contrast not only for in-band EUV (13.5 nm) radiation but also for wavelengths below Si L-absorption edge (12.4 nm).The masks and filters with free standing areas up to 1000 × 1000 μm² and 100 nm to 300 nm membrane thicknesses are shown. Electron beam structuring of an absorber layer with dense line and dot patterns with sub-50 nm structures is demonstrated. Diffractive and filtering properties of obtained structures are examined with EUV radiation from a gas discharge plasma source.     

Tensile strain engineering of Si thin films using porous Si substrates

Highly tensile strained (up to 2.2%) thin monocrystalline silicon (mc-Si) films were fabricated by a simple and low-cost method based on the in-plane expansion of meso-porous silicon (PS) substrates upon low temperature oxidation. To control the film thickness below 100 nm, an original “two wafer” technique was employed during the porosification process. This method enables the fabrication of a 60 nm thick mc-Si films on 250 μm thick meso-porous silicon substrates over areas as large as 2 in. with a surface roughness and cleanliness comparable to that of standard Si wafers. Crack-free 60 nm thick Si films can be strained up to 1.2% by controlled low temperature oxidation of the PS substrate. Structural and strain analysis of the PS/mc-Si structures performed by transmission electron microscopy and micro-Raman scattering spectroscopy are reported.