Oxidation of aluminium nitride substrates

The growth of oxide films on two types of aluminium nitride substrates of different origin has been studied as a function of temperature. At a given set of oxidation reaction parameters, the oxide layers grown on substrates with a relatively large grain size and high concentrations of Y-Al-O-based liquid sintering aid phases (type I substrates) were observed to be thicker and more diffuse than those obtained on substrates with an average particle size of approximately 3 m and low liquid sintering aid concentrations (type II substrates). The effects of the oxygen partial pressure variation on the oxide film growth have been investigated for the oxidation of type II AIN substrates. The kinetics of the growth of oxide films on such substrates were analysed and determined to fit best to a linear rate law. This type of rate law indicates that the rate-limiting step in the growth of oxide films on high-quality type II aluminium nitride substrates is an interface reaction-controlled process.     

Ion beam synthesis of aluminium nitride: characterisation of thin AIN layers formed in microelectronics aluminium

Abstract

Buried AlN thin layers have been formed by high dose N⁺ ion implantation into microelectronics grade Al films (containing 1 at.-%Si), which were deposited on Si wafers. The structures obtained have been characterised by spreading resistance measurements, transmission electron microscopy, secondary ion mass spectrometry, X-ray diffraction, and X-ray photoelectron spectroscopy. The results show the formation of buried dielectric precipitates of crystalline AlN at implantation doses below the threshold and a continuous polycrystalline AlN layer at doses above the threshold. The AlN grains have the wurtzite structure, sizes of about 10–15 nm, and a preferred orientation in relation to the Al matrix, namely, <110>_AIN parallel to <110>_Al. The data also show that, under certain conditions, the main impurities (Si and O) are gettered in the buried layer. Moreover, for thin Al films, the formation of a Si rich surface layer is observed. This surface layer is formed by Si diffusion from the substrate, probably due to the penetration of N⁺ ions into the Si substrate. The distribution and evolution of these impurities and the different phases formed are studied as a function of the thickness and grain size of the Al film, as well as of the annealing processes.

MST/3304     

Direct formation of wafer scale graphene thin layers on insulating substrates by chemical vapor deposition

Direct formation of high-quality and wafer scale graphene thin layers on insulating gate dielectrics such as SiO(2) is emergent for graphene electronics using Si-wafer compatible fabrication. Here, we report that in a chemical vapor deposition process the carbon species dissociated on Cu surfaces not only result in graphene layers on top of the catalytic Cu thin films but also diffuse through Cu grain boundaries to the interface between Cu and underlying dielectrics. Optimization of the process parameters leads to a continuous and large-area graphene thin layers directly formed on top of the dielectrics. The bottom-gated transistor characteristics for the graphene films have shown quite comparable carrier mobility compared to the top-layer graphene. The proposed method allows us to achieve wafer-sized graphene on versatile insulating substrates without the need of graphene transfer.     

The dielectric properties of aluminium nitride substrates for microelectronics packaging

The dielectric behaviour of sintered polycrystalline aluminium nitride substrates has been examined over the frequency range 500 Hz to 10 MHz and correlated with composition and microstructure. For pure, white AlN at 20 ° C both the permittivity () and dielectric loss () are frequency independent giving = 9.2±0.05 and tan = (2.1±0.1) × 10^–3. The permittivity is less than for pure alumina substrates ( = 10.2) but tan compares favourably, with that (1.4 × 10^–3) of alumina, which though used more widely has a thermal conductivity some eight times less than that of AlN. The addition of impurities, particularly iron, to give opaque black AlN causes large, frequency dependent increases in ; at 500 Hz the loss is seven times that of pure white AlN and is two times greater above 100 kHz. The temperature coefficient of permittivity [( – 1)( + 2)]^–1 [/T]_p between –180 and +180 ° C for pure white AlN is 1.05×10^–5 K^–1 which is similar to the value of 9×10^–6 K^–1 for pure Al₂O₃. For impure black AlN the coefficient below 20 ° C is the same but above 20 ° C there is a rapid, non-linear increase of with temperature. Below 180 ° C for pure white AlN and 20 ° C for impure black AlN the values of temperature coefficient are frequency independent at least up to 200 kHz.     

Breakdown conduction in carbon thin film devices with silver film contacts on glass substrates

The process in which a device changes irreversibly from a low to a high conducting state was followed by observing transient current-voltage (I_f−V_f) characteristics. The locus enveloped by a group of the I_f−V_f characteristics represented the current-controlled negative resistance (CCNR). It is shown that these events can be explained by a thermal activation mechanism.     

Integral method for echelles covered with lossless or absorbing thin dielectric layers

We make a generalization of the integral method in the electromagnetic theory of gratings to study diffraction by echelles covered with dielectric lossless or absorbing layers. Numerical examples are given that show that, as in the resonance domain, the diffraction efficiency is more complicated than being a simple product of lossless diffraction efficiency curves and plane surface reflectivity.     

Development of as-fired aluminium nitride substrates with smooth surface and high thermal conductivity

As-fired aluminium nitride (AIN) substrates with smooth and uniform surface have been developed by green sheet and firing technology. The effect of setting for firing on surface roughness was investigated. AIN substrates were fabricated by pressureless sintering of green sheets piled up and sandwiched between AIN plates in an AIN crucible. The thermal conductivity, surface roughness and bending strength of the substrate sintered at 1770 °C for 2 h under a pressure of 1 MPa nitrogen were 194 Wm^–1 K^–1, 0.15 (m and 353 MPa, respectively.     

Evaporated insulating layers for thin film circuits

The basic properties of a new glass-forming dielectric material were studied with regard to its application in thin film circuits. Direct evaporation by electron bombardment leads to highly insulating layers with good reproducibility. Thin film capacitors are deposited in a high vacuum onto glass substrates or glazed ceramics through metal masks.

The dielectric constant of =3.7 and the high breakdown field of about 4–7 MV cm⁻¹ are important for conductor cross-overs. This material is also of interest as a highly stable capacitor dielectric because of its low loss (tan δ=0.2%) and a temperature coefficient of about +25 ppm K⁻¹.     

Influence of interface layers on Ag thin film growth

We prepared Ag thin films on SiO2/Si substrates, with and without a subnanometer-thick organic interface layer of 3-mercaptopropyltrimethoxysilane (MPTMS). The surface morphology and electrical resistivity of these films were investigated and compared. Ag films grown with an MPTMS layer were relatively flat, resulting in a smaller critical thickness. This was probably because migration of Ag atoms on the substrate was suppressed by interactions between the Ag atoms and the thiol moiety. The deposition rate and terminating group of the organic interface layer also influenced the Ag film growth.     

All graphene-based thin film transistors on flexible plastic substrates

High-performance, flexible all graphene-based thin film transistor (TFT) was fabricated on plastic substrates using a graphene active layer, graphene oxide (GO) dielectrics, and graphene electrodes. The GO dielectrics exhibit a dielectric constant (3.1 at 77 K), low leakage current (17 mA/cm(2)), breakdown bias (1.5 × 10(6) V/cm), and good mechanical flexibility. Graphene-based TFTs showed a hole and electron mobility of 300 and 250 cm(2)/(V·s), respectively, at a drain bias of -0.1 V. Moreover, graphene TFTs on the plastic substrates exhibited remarkably good mechanical flexibility and optical transmittance. This method explores a significant step for the application of graphene toward flexible and stretchable electronics.     

Microstructural studies of polytype formation in oxygen-containing aluminium nitride

Oxygen-doped AlN polycrystals were investigated by transmission electron microscope, and different stages of multilayered polytype formation during pressureless sintering were fixed. The decomposition of polytypes was found to take place during quasihydrostatic compression of samples at high temperatures. A model of multilayered polytypes appearing is proposed. The process of polytype formation is represented as isostructural delamination of the AlN-O solid solution at the expense of oxygen extraction on stacking faults, causing the oxygen-rich interlayer formation.     

Study of adhesion of carbon nitride thin films on medical alloy substrates

The adhesion improvement of biocompatible thin films on medical metal alloy substrates commonly used for joint replacement implants is studied. Diamond-like carbon (DLC) and carbon nitride (CN) thin films are, because of their unique properties such as high hardness, wear resistance and low friction coefficient, candidates for coating of medical implants. However, poor adhesion on substrates with high thermal expansion coefficient limits their application. We deposited CN films by pulsed DC discharge vacuum sputtering of graphite target on CoCrMo and Ti6Al4V substrates. Surface nitridation of the substrate, changing the deposition parameters and use of interlayer led to improved adhesion properties of the films. Argon and nitrogen gas flow, thickness of the film and frequency of the deposition pulses had significant influence on the adhesion to the substrate. Properties of deposited films were analyzed using Scanning Electron Microscopy, Raman spectroscopy and tribology tests.     

Corrosion of aluminium nitride substrates in acid,alkaline solutions and water

Aluminium nitride substrates were immersed in acid, basic solutions and deionized water for 1–120 h at room temperature. The corrosion rates are higher in basic solutions (NaOH and KOH) than those in acid solutions (CH₃COOH, HCOOH, HNO₃, HCl and H₂SO₄) and deionized water. The weight loss of AIN corroded in alkali aqueous reaches 70% and results in an increase in surface roughness ranging from 10 nm to 7 m after 3 days corrosion. However, the weight loss in acid solution is only 1/700 of the alkali case. Violent chemical reactions between AIN and basic solutions were observed. Na₂O, or Na₂Al₂O₄·6H₂O, is the intermediate product, and NaOH is a catalytic agent of the reaction. The surface morphology of the AIN etched by alkaline solutions is coral-like in microscopic view and appears like hills. In contrast, only several atomic layers of AIN surface are etched off in acid solutions and in deionized water. The lightly etched surface is mirror-like and flat, and the shapes of the grains are visible under the microscope, as the corrosion rate of each AIN grain varies with different crystal orientations. Consequently, after etching in acid solutions, the resulting microscopic surface morphology looks like a map of a jigsaw puzzle.     

Mechanical spectroscopy of thin layers of PPV polymer on Si substrates

The vibrating reed technique with electro“static” excitation and optical detection has been applied to investigate thin layers of poly-phenylene-vinylene, deposited by spin coating onto microfabricated Si cantilevers, during temperature cycling programs between 90 and 540 K at a rate of 1 K/min. From the vibration frequencies the Young’s modulus of the film can be estimated to be about 10 MPa at room temperature in the precursor phase (if prepared from a solution in toluene), which increases by conversion to the conjugate bonded polymer to about 50 MPa. The temperature dependence of internal friction reveals the processes of γ relaxations (crankshaft motion of side branches in the precursor) and β-relaxation (movements of a few monomer blocks in the polymer chain), as well as peaks indicating the structural transformations during conversion, and possibly a glass transition in the amorphous precursor phase. After conversion only the β-relaxation persists.     

Low temperature silicon nitride by hot wire chemical vapour deposition for the use in impermeable thin film encapsulation on flexible substrates

High quality non porous silicon nitride layers were deposited by hot wire chemical vapour deposition at substrate temperatures lower than 110 degrees C. The layer properties were investigated using FTIR, reflection/transmission measurements and 1:6 buffered HF etching rate. A Si-H peak position of 2180 cm(-1) in the Fourier transform infrared absorption spectrum indicates a N/Si ratio around 1.2. Together with a refractive index of 1.97 at a wavelength of 632 nm and an extinction coefficient of 0.002 at 400 nm, this suggests that a transparent high density silicon nitride material has been made below 110 degrees C, which is compatible with polymer films and is expected to have a high impermeability. To confirm the compatibility with polymer films a silicon nitride layer was deposited on poly(glycidyl methacrylate) made by initiated chemical vapour deposition, resulting in a highly transparent double layer.     

The plasma anodization to exhaustion of thin aluminium films on silicon substrates

Plasma anodization is a low temperature process that is of interest in the preparation of insulating films on semiconductors because of its effect in reducing the possibility of dissociation of the semiconductor.It has previously been applied to the formation of aluminum oxide layers on silicon and it is now demonstrated that the onset of substrate oxidation can be recognized electrically during the anodization. A model for the anodic process is presented in terms of electron and ion transport through the growing layer. Chemical information is presented and sources of contamination are identified. The films have high resistivities and breakdown strengths and MOS measurements are presented.It is expected that this monitoring technique could be applied to other material systems.     

Anodic film formation on high strength aluminium alloy FVS0812

Barrier-type film growth on the high strength aluminium alloy FVS0812 has been studied by a combination of transmission electron microscopy and Rutherford backscattering spectroscopy. The film is composed mainly of amorphous anodic alumina, but is contaminated with iron species incorporated into the film from the alloy. The film may also be contaminated with silicon and vanadium species at levels below the detection limit of the present experiments. The contaminant species are primarily incorporated locally into the film during oxidation of Al₁₃(Fe, V)₃Si dispersoids and the resulting film material is of reduced resistivity compared with anodic alumina of high purity. As a consequence of the presence of regions of film material of differing resistivities, the film is of irregular thickness. The average thickness corresponds to a nm/V ratio of about 1.3. Iron species incorporated into the film migrate outwards at roughly 2.1 times the rate of Al³⁺ ions. The iron species are not ejected in significant amounts to the electrolyte on reaching the film/electrolyte interface and hence, a thin layer of film material highly enriched in iron species develops at the film surface. The layer may also be enriched in vanadium species, if these are incorporated into the film and migrate more rapidly than Al³⁺ ions. Enrichment of iron, and possibly other alloying element atoms, is found in a thin layer of alloy immediately beneath the anodic film, paralleling enrichments of alloying element atoms found following anodic oxidation of other aluminium alloys. The enrichments at both the alloy/film and film/electrolyte interfaces do not appear to be continuous across the macroscopic surface of the specimens, probably due to the non-uniformity of film growth on the two-phase substrate. The maximum voltage for the selected conditions of anodizing was limited to 68 V as a result of oxygen generation at flaws which are present extensively in the anodic film. This revised version was published online in November 2006 with corrections to the Cover Date.     

Modelling of boron nitride: Atomic scale simulations on thin film growth

Molecular-dynamics simulations on ion-beam deposition of boron nitride are presented. A realistic Tersoff-like potential energy functional for boron nitride, which was specially fitted to ab initio-data, has been used. The impact of energetic boron and nitrogen atoms on a c-BN target is simulated with energies ranging from 10 to 600 eV. The structural analysis of the grown films shows that a loose, dominantly sp²-bonded structure arises at high ion flux. In no case the formation of a sp³-bonded phase is observed, but the obtained films partially reveal textured basal planes as found in experiment. Two different growth regimes are identified for ion energies above and below 100 eV.