Anti-diffusion Barrier Research Articles

Development of intermediate layer systems for direct deposition of thin film solar cells onto low cost steel substrates

The functionalisation of low-cost steel over large areas with low cost intermediate layers (ILs) for utilisation as substrates in thin film solar modules is reported. Three approaches for the deposition of ILs are demonstrated and evaluated; a thick SiOx sol–gel based on a one-step acidic catalysis applied by spray technique, a commercial screen-printable dielectric ink, and an epoxy-based material (SU8) deposited by screen printing or bar coating. These ILs demonstrated the properties of surface levelling (quantified by mechanical profilometry), electric insulation (tested using breakdown voltage and leakage current) and acted as an anti-diffusion barrier (demonstrated with glow discharge mass spectrometry). Moreover, the performances of amorphous silicon (a-Si:H) and organic photovoltaic (OPV) thin film solar cells grown on carbon and stainless steels (a-Si:H: 5.53% and OPV: 2.40%) show similar performances as those obtained using a reference glass substrate (a-Si:H: 5.51% and OPV: 2.90%). Finally, a cost analysis taking into account both the SiOx sol–gel and the dielectric ink IL was reported to demonstrate the economic feasibility of the steel/IL prototypes.

Laser-assisted stress reduction in molybdenum microstructures for CMOS compatible MEMS integration

The fusion of micro-cantilevers over complementary-metal-oxide-semiconductor (CMOS) wafers (with driving electronics) is essential integration requirement for the development of portable sensing devices. In general, the stress gradient in metal cantilevers (after release) lead to structure warping and limits the performance and stability of the final fabricated devices. This work is focused to solve the requirement of (i) having a stress-free metal based micro-electro-mechanical structures (MEMS) without using high-temperature annealing technique and (ii) process compatibility with vapor HF release. We demonstrate a laser-assisted stress controlling scheme to achieve free-hanging metal cantilever without affecting the underneath (essential requirement for CMOS-MEMS integration) deposited layers. The molybdenum (Mo) metal is deposited by physical vapor deposition (PVD) technique at the temperature of 200 °C and then patterned to obtain the microcantilever beams of different length. High-pulse-energy excimer laser (wavelength =248 nm with pulse duration =24 ns) is used to anneal these metal cantilevers and their residual stress is measured by analyzing the final displacement of the released microbeams. Different laser energy density ranging from 200 × 10−3 J/cm2 to 450 × 10−3 J/cm2 is used to study the optimized parameters in the annealing process. Proposed pulsed laser-annealing (PuLA) technique reduces not only the stress gradient but also aid stress-tuning ability that has numerous advantages especially in electrostatic and piezoelectric MEMS devices. Furthermore, we also present the impact of laser action on the grain microstructures and electrical conductivity. Results show that molybdenum microstructures with localized laser heat treatment achieve a reduced surface roughness (from 7.5 nm to 3.8 nm) and improved electrical conductivity. Results demonstrate that the proposed PuLA scheme is a promising technique for (i) eliminating stress in metal interconnects in CMOS-MEMS integrations, (ii) building a stress-free and smooth surface micromechanical platform for N/MEM (resonators, sensors and actuators) devices and (iii) establishing molybdenum as an anti-diffusion barrier in Cu interconnect technology owing to its excellent bulk property.

Exploring the Origin of Contact Destruction in Tetradymite-Like-Based Thermoelectric Elements

Polycrystalline thermoelectric elements of n-type Bi2(Te0.95Se0.05)3 and p-type (Bi0.25Sb0.75)2Te3 were fabricated by modified Bridgman technique, followed by electrospark cutting. A thermoelectric micromodule, consisting of 12 legs with an Ni anti-diffusion barrier and high-temperature SnSb solder between them, was assembled. To investigate thermoelements under conditions close to operating ones, the micromodule was annealed at 443 K for 1000 h. It was revealed that after annealing for more than 40 h, the near-contact zone of the n-type legs degraded, leading to a complete break of the micromodule junctions. Our results revealed that during annealing, the SnSb solder flowed into the unprotected side surface of the legs, and contacted with the cleavage planes along which tin can diffuse into the thermoelement volume. In contrast, the p-type legs were not affected by the contact with the solder. Different impacts of the solder on n- and p-type legs were explained in the framework of density functional theory (DFT) calculations. Substitution, diffusion and thermodynamic stability calculations showed that Bi to Sn substitution is energetically more beneficial than Sb to Sn substitution. Additionally, it was calculated that for the Bi2Te3 + Sn system, it is more beneficial to form a TeSn and Bi phase, while an Sb2Te3 + Sn system is thermodynamically stable.

Impact of (111)-oriented TiN conductive layers for the growth of vanadium dioxide films and the effect of grain boundary diffusions

We report on advantage of (111)-oriented TiN conductive layers for realizing low temperature crystalline growth of vanadium dioxide (VO2) films with phase transition. Comparison between (111)-oriented TiN and random polycrystalline TiN layers was performed to clarify the effect of oriented TiN for succeeding VO2 growth. Oriented growth of VO2 (010) on (111)-oriented TiN suggests feasibility of most dense TiN (111) surface with pseudo-hexagonal lattice fitted against VO2 (010) surface with monoclinic angle β of 122°. Abrupt resistance change nearly three orders of magnitude was achieved in the films prepared at substrate temperatures of 250 ～ 300 °C. XPS depth profiles clearly showed abrupt interface between VO2 and TiN at deposition temperatures lower than 300 °C. On the other hand, it was found that (111)-oriented TiN layers enhance diffusions of Ti and O atoms through the grain boundaries, which act as direct path for diffusion at temperatures higher than 350 °C. Strictly unchanged composition of TiN layers confirms the superior property as anti-diffusion barrier material, however, grain boundaries arrangements are supposed to be crucial factors for the maintanance of layerd structure. Obtained results tell us the impact of (111)-oriented TiN layers not only for succeeding growth of VO2 films, but also for design of layerd device introducing TiN as electrodes.

Fabrication and characterization of single-pair thermoelectric generators of bismuth telluride using silver-sintering technology

Silver sintering technology was investigated as a method for interconnect formation of thermoelectric generators (TEGs). Using silver sintering we were able to assemble commercial bismuth telluride pellets and copper legs to fabricate TEGs, which are designed to work at elevated temperatures. Nickel was deposited electroless for 30, 60, and 90 minutes on the bismuth telluride pellets to serve as an anti-diffusion barrier between Ag and Bi2Te3. The impact of different fabrication conditions (i.e. sintering temperatures from 210 to 265°C and pressures from 5 to 7 MPa, respectively) on the properties of single-pair TEGs was investigated. Lowest specific contact resistances of (5.07 ± 3.00) × 10-5 Ω·cm2 were obtained which agree well with state of the art for soldered contacts of bismuth telluride. With TEGs sintered at 6 MPa and 265°C long-term operation tests were performed at temperature gradients across the TEG of 190°C for 360 hours. The Seebeck power remained unchanged showing values around 0.3mV/K, which slightly depended on the anti-diffusion-layer thickness.

Multilayer antidiffusion barrier schemes for Schottky and ohmic contact metallisations to InAlN/GaN HEMTs

ABSTRACTThe reported work focuses on developing antidiffusion barriers capable to increase the thermal stability of metal contacts above 700 C. In the chosen approach, such an antidiffusion barrier consists of several bilayers of materials with different crystalline structures. It has been demonstrated that an interface between such materials effectively blocks the atomic interdiffusion. In this work the following groups of materials were used as the bilayers: ZrB2 and ZrN and TaSiN and TiN. The materials were deposited by means of room temperature sputtering from elemental and compound targets in inert Ar and reactive Ar+N2 atmospheres. The structures were characterised using secondary ion mass spectroscopy depth profiling and scanning electron microscopy cross sectional imaging directly after deposition and after degradation. I-V characteristics were measured and contact resistivities were determined from the circular transmission line method.

Multilayered mirrors based on La/B 4C(B 9C) for X-ray range near anomalous dispersion of boron ( λ≈6.7 nm)

Multilayered mirrors based on lanthanum and boron carbides for wavelength near 6.7 nm are manufactured and studied by reflectometry and electron microscopy. Normal incidence mirrors demonstrate reflection coefficient near 40% what is nearly 20% less than theoretical prediction. It seems that the main problem here is interlayer diffusion caused by La high chemical activity, which we propose to solve by implying antidiffusion barrier layers.

Towards the Integration of Epitaxially Grown KTN Thin Films in Silicon Technology

The use of a high quality SrTiO 3 buffer layer was investigated in order to give access to the integration in standard silicon technology of KTa 1 − x Nb x O 3 thin films, grown by pulsed laser deposition. The buffer layer was previously epitaxially grown onto Si substrates by molecular beam epitaxy, using a process implying seed and sacrificial layers of strontium. The 40 nm thick SrTiO 3 underlayer acts together as a matching layer, a seed layer and an anti-diffusion barrier, preventing the formation of the undesired competing pyrochlore phase. High crystalline quality perovskite single-phase epitaxial KTa 1 − x Nb x O 3 thin films were reproducibly obtained by the use of this process.

Measuring the diffusion of Ti and Cu in low-k materials for microelectronic devices by EELS, EFTEM and EDX

The need to reduce RC delay and cross talk in Cu interconnects means that ultra low-k dielectrics such as porous SiCOH are being integrated into microelectronic devices. Unfortunately porous materials lead to integration issues such as metal diffusion into the porosity of the dielectric, especially when chemical vapour deposition (CVD) methods are used for metal deposition. In our case, the copper anti-diffusion barrier used before Cu deposition is MOCVD TiN. Without an appropriate surface treatment (pore sealing) of the low-k the TiN may diffuse in the porosity. The presence of Ti or Cu in the low-k is deleterious as it can raise the dielectric constant and the leakage current. EFTEM EELS and EDX have been used to map Ti, Cu, O and C as a function of process conditions.

Influence of Ta antidiffusion barriers on the thermal stability of tunnel junctions

Previous work on spin-dependent tunnel junctions based on NiFe/CoFe/Al2O3/CoFe/MnIr reported structural thermal stability upon anneals up to 300 or 320 °C. Above 320 °C, the tunneling magnetoresistance (TMR) starts to decrease, and interdiffusion occurs at the MnIr/CoFe interface. MnIr diffuses into CoFe and reduces its magnetic moment. It was proposed that this mechanism could be partially responsible for the observed loss of TMR, due to the decrease in interface polarization. In this letter, tunnel junctions were prepared with a 2 Å-thin Ta antidiffusion barrier inserted at the CoFe/MnIr interface, to stop MnIr diffusion into the pinned electrode. Structural and magnetization measurements indicate that the CoFe and MnIr layers remain unchanged with anneals up to 440 °C, and that the antidiffusion barrier is working. However, the TMR degradation above 320 °C was not avoided, which suggests that improving the thermal stability of the junctions will probably require changes in the barrier itself.

Enhancement of the critical current density in Chevrel phase superconducting wires

The critical current densities Jc of Chevrel phase wires with niobium as an antidiffusion barrier were measured in magnetic fields up to 24 T. At 20 T and 1.9 K, Jc reaches 5.4×108 A/m2 and decreases slightly down to 3.1×108 A/m2 at 24 T. A wire with a 20% superconducting cross section has been successfully drawn and its overall critical current density Jcov exceeds 100 A/mm2 at 1.9 K up to a magnetic field slightly above 20 T. This demonstrates the ability of Chevrel phase wires to be used in high magnetic field applications. Moreover, some parts of the coil have certainly higher Jc, since Jc is very often limited by a thermal excursion of the entire coil. The effective upper critical field μ0Hc2*, deduced from the magnetic field dependence of Jc, is too low compared to the expected bulk value, indicating that superconducting properties at the grain boundaries are still degraded. If the bulk μ0Hc2 can be restored at the grain surfaces, Jcov should be higher than 100 A/mm2 up to at least 30 T.

Characterization of titanium-aluminum nitride thin films by ion beam techniques and X-ray diffraction

Ternary nitride thin films have found widespread use in recent years as a coating that offers superior performance either as an anti-diffusion barrier in semiconductor technology or as protection against corrosion and wear in fine metallurgy processes. Film performance has been seen to depend very strongly on structure and composition. The aim of this work was to establish well-defined correlations between the deposition parameters and characteristics of the films in order to provide a way to deposit films with predetermined usable features. In the present case titanium-aluminum nitride thin films were deposited by reactive sputtering using both the dc and the rf power sources. In fact, by varying the deposition conditions, films with different thicknesses, stoichiometries and crystalline structures were obtained. The analytical techniques used to perform such characterization were Rutherford backscattering spectrometry, nuclear reaction analysis ( 14N(p,γ) 15O and 27Al(p,γ) 28 Si resonant reactions) and X-ray diffraction. Ternary crystalline phases not detected previously at room temperature were observed here by X-ray diffraction.

Pb(Zr, Ti)O3 thin film growth on yttrium-treated Si(100)

Abstract Pulsed laser ablation has been used to deposit ferroelectric Pb(Zr, Ti)O3 (PZT) thin films on Si(100) and on yttrium-treated Si(100) substrates. The yttrium (Y) treatment of a Si surface followed by oxidation resulted in formation of a very thin, Y-enhanced SiO2 antidiffusion barrier layer, thereby suppressing the undesirable PZT/Si interdiffusion. The best PZT film grown on Y-treated Si(100) had a breakdown voltage of 0.6 MV/cm, a coercive field of 71 KV/cm, and a remanent polarization of 18 μC/cm2.

Niobium antidiffusion barrier reactivity in tin-doped, in situ PbMo 6S 8-based wires

The reactivity of the niobium antidiffusion barrier at the time of heat treatment is presented in the case of NbCu-sheathed, in situ PbMo 6S 8 (PMS-) based wires doped with 2.6 at.% Sn. As a result of this reactivity, a lamellar layer grows axially towards the centre of the wire, consuming a significant amount of sulphur at the expense of the formation of the superconducting PMS phase. Investigation of the growth kinetics by scanning electron microscopy under variable experimental conditions indicates that slightly decreasing the heat treatment temperature or increasing the powder densification can considerably minimize the niobium reactivity. In the light of results from electron microprobe analysis together with X-ray powder diffraction data, the possible existence of a new Pb 0.15Nb 2S 3 compound is discussed.

Photoenhanced oxidation of stainless steels: An Auger electron spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy study

The photoenhanced oxidation of several types of stainless steels under high flux (about 2 MV m −2) visible-UV irradiation has been investigated. Oxidation rates were studied under photoirradiation with both a broad-band xenon discharge source and a variety of lasers. Substantial enhancement of oxidation was observed for all photoirradiated samples as compared with reference samples oxidized under similar conditions but with IR heating. A quadratic dependence on photon flux was obtained for the oxide thickness over the flux range 0–2 MW m −2. A threshold wavelength for the oxidation enhancement was found at about λ = 410 nm. The enhanced oxidation under photoirradiation is believed to result from the photoreduction of the antidiffusion barrier oxides Cr 2O 3 and FeCr 2O 4 otherwise found under IR heating. This photoreduction of the barrier oxides was found to occur at wavelengths consistent with the threshold wavelengths ( λ = 400–425 nm) for the photodesorption of CO and CO 2 from stainless steels, indicating that the two events are intimately related.