Effective Diffusion Barrier Research Articles

A kinetic study on the mechanism of hydrogen evolution in Ni–P coated titanium hydride powder

In this work, titanium hydride powder (TiH2) was coated by an electroless method through an aqueous coating bath consisting of nickel-phosphorus compounds. Energy Dispersive Spectroscopy (EDS) was utilized for characterization of nickel compounds in the coated layer. Also, DTA, XRD and TGA patterns were determined in order to take different stages of hydrogen release process into consideration. In conformity with the results, only 1.25wt% Ni–P coating was deposited on the as-received TiH2, so that its great majority includes Ni5P2 compound. Also, the hydrogen gas release process, in this case, occurs at seven stages, exactly similar to that in TiH2 heat treated in air. The coating causes various stages of hydrogen evolution to undergo a temperature delay roughly equal to 400°C. It means that the coating displaces various stages of hydrogen release process to higher temperatures thereby delaying efficiently the starting time of the reaction approximately for 40min. It implies that Ni–P coating can act as an effective diffusion barrier to hydrogen gas evolution. Also, activation energy values for these seven stages were calculated. It was shown that only active mechanism in Ni–P coated TiH2 is internal diffusion, i.e. the diffusion of hydrogen atoms into titanium lattice.

Recent trends of patents on front TCOs for highly efficient thin-film Si photovoltaic devices.

The recent trends of US patents on the front transparent electrode of thin-film silicon (Si) photovoltaic (PV) devices are reviewed. The various transparent conductive oxide (TCO) materials have been invented to satisfy a multifunctional prerequisite for the front electrode: high electrical conductivity, high optical transparency, effective light trapping, anti-reflection effect, and diffusion barrier. The recent surge of filed patents reflects the great importance of the front TCO technology for high efficiency thin-film Si PV devices. Among the TCO materials, properties of commercially available F-doped tin oxide (SnO2:F)-coated glass substrates are compared. SnO2:F-coated glass substrates share 20-30% of the cost for production of thin-film Si PV modules - evaluated values from mass production at KISCO. Therefore, the cost and technological innovation must be established for cost-effective mass production of large-area thin-film Si multijunction PV modules.

Evaluation of catalytic activity of Ag and Au dendrimer-encapsulated nanoparticles in the reduction of 4-nitrophenol

Langmuir–Hinshelwood parameters were determined with the use of gold and silver dendrimer-encapsulated nanoparticles (Au-DENs and Ag-DENs) as catalysts. Dendrimers were used as templating and stabilizing agents to ensure that there is no aggregation of metal nanoparticles. The synthesis of DENs using amine terminated generation 4 poly(amido)amine dendrimer (G4-PAMAM-NH2) was successfully demonstrated. These synthesized DENs were characterized using UV–visible spectrophotometry (UV–vis), electron dispersion X-ray analysis (EDX), and Fourier transformed infrared spectrophotometry (FTIR). High resolution transmission electron microscopy (HRTEM) was used to determine the average particle sizes, and the average particle sizes were found to be 1.23±0.13 and 1.17±0.13nm for Au- and Ag-DENs, respectively. Comparison of catalytic activity between the two metal nanoparticles was done in terms of thermodynamic parameters obtained from Langmuir–Hinshelwood fits at 298K. 4-Nitrophenol (NP) reduction by sodium borohydride (NaBH4) in the presence of the synthesized DENs as homogeneous catalysts was used by virtue of being a model reaction. Effect of diffusion barrier in all kinetic runs was addressed by calculations of the second Damköhler number and by the application of the second Fick's law of diffusion. The Langmuir–Hinshelwood model proved to be an efficient model for thermodynamic comparison of the two metal nanoparticles.

Stability and Microstructure Characterization of Barrierless Cu (Sn, C) Films

Thermal stability, adhesion and electronic resistivity of the Cu alloy films with diffusion barrier elements (large atom Sn and small atom C) have been studied. Ternary Cu (0.6 at.% Sn, 2 at.% C) films were prepared by magnetron co-sputtering in this work. The microstructure and resistivity analysis on the films showed that the Cu (0.6 at.% Sn, 2 at.% C) film had better adhesion with the substrate and lower resistivity (2.8 μΩ·cm, after annealing at 600 °C for 1 h). Therefore, the doping of carbon atoms makes less effect to the resistivity by decreasing the amount of the doped large atoms, which results in the decreasing of the whole resistivity of the barrierless structure. After annealing, the doped elements in the film diffused to the interface to form self-passivated amorphous layer, which could further hinder the diffusion between Cu and Si. So thus ternary Cu (0.6 at.% Sn, 2 at.% C) film had better diffusion barrier effect. Co-doping of large atoms and small atoms in the Cu film is a promising way to improve the barrierless structure.

Redox-responsive biodegradable PEGylated nanographene oxide for efficiently chemo-photothermal therapy: A comparative study with non-biodegradable PEGylated nanographene oxide

Nanographene oxide (NGO) with a non-sheddable poly(ethylene glycol) (PEG) coating has been used for chemo-photothermal therapy. However, the drug release of PEGylated NGO (NGO-PEG) with an amine bond is adversely affected by the diffusion barrier effect of PEG shells. Here, we developed a simple new method for the preparation of biodegradable PEGylated NGO conjugates (NGO-SS-PEG) with cleavable disulfide bonds for rapid drug release and more efficiently chemo-photothermal therapy. The glutathione (GSH)-induced and photothermal-mediated intracellular release of doxorubicin (DOX) from NGO-SS-PEG was studied in A549 cells using confocal laser scanning microscopy and flow cytometry analysis. In vivo cytotoxicity experiments were performed on chemo-photothermal therapy. Furthermore, we presented a comparative study of intracellular drug release and biological efficacy between NGO-SS-PEG/DOX and NGO-PEG/DOX. The results demonstrated that the rapid drug release from the NGO-SS-PEG conjugates with sheddable PEG was triggered upon the stimulus of high GSH levels inside A549 cells. Interesting, the DOX release mediated by the photothermal effect from the NGO-SS-PEG conjugates was found to be more obvious than that for NGO-PEG. Additionally, NGO-SS-PEG showed a higher efficacy than NGO-PEG for anti-tumor therapy compared with NGO-PEG. Thus, NGO-SS-PEG can improve therapeutic efficacy and is an attractive drug nanocarrier.

Effects of solvent on the formation of the MUA monolayer on Si and its diffusion barrier properties for Cu metallization

We investigated the effects of solvents, such as ethanol and isooctane, on self-assembly of the mercaptoundecanoic acid (MUA) monolayer on Si and its diffusion barrier properties for Cu metallization. The use of isooctane as a solvent produced MUA self-assembled monolayers (SAMs) (~1.3 nm thick) on Si. These acted as an effective diffusion barrier against Cu diffusion up to 200°C. In contrast, the MUA SAMs produced by ethanol allowed the diffusion of Cu to a MUA-Si interface at 200°C, stimulating the out-diffusion of Si into Cu and thus resulting in the degraded diffusion barrier properties. This was possibly due to the partial formation of interplane hydrogen bonding between the terminal groups of the bound acid and free thiol groups. This provided less dense thiol surface groups, thus leading to poor adhesion of Cu to MUA SAMs. The fabricated Cu/isooctane-assisted MUA source/drain electrode a-Si:H thin film transistors with a channel length of 10 µm exhibited an excellent electron mobility of 0.74 cm /V-s, threshold voltage of −0.51 V, I /I ratio of 3.25 × 10 , specific contact resistance of 4.24 Ω-cm after annealing at 200°C.

Formation of an Alkali Diffusion Barrier and Stain‐Resistant Glass Surface

As it is well known, the structures of silica and soda‐lime‐silica glasses are subjected to alkali ion diffusion. This diffusion can lead to staining of the glass surface, thus detrimentally affecting its optical functions. In this work, an effective alkali diffusion barrier layer is created by boron diffusion into the top 10 to 50‐nm thick layer of glass surface. In this layer, alkali mobility is electrochemically arrested by a coordination change of boron. As a result, the glass surface stays pristinely clear, even under sustained exposure to hot and humid conditions. Structural studies indicate that when three‐coordinated boron diffuses into a pre‐existing glass network, it converts nonbridging oxygens, associated with alkali ions, into bridging oxygens by changing its coordination from 3 to 4. This forces alkali ions to change their bond association from nonbridging oxygens to the charge sphere of boron due to the required charge neutrality. This new bond arrangement immobilizes the alkali ions at these sites. As a result of the alkali ion immobilization, and the reduction in nonbridging oxygens in the network, the glass surface becomes resistant to nucleophillic attack, as well as resistant to stain formation, and crystallization.

Tungsten-Doped Indium Oxide Thin Film as an Effective High-Temperature Copper Diffusion Barrier

Tungsten-Doped Indium Oxide Thin Film as an Effective High-Temperature Copper Diffusion Barrier

Interface reaction between an electroless Ni–Co–P metallization and Sn–3.5Ag lead-free solder with improved joint reliability

To address the reliability challenges brought about by the accelerated reaction with the implementation of lead-free solders, an electrolessly plated Ni–Co–P alloy (3–4wt.% P and 9–12wt.% Co) was developed as the solder metallization in this study. Three compounds layers, (Ni,Co)3Sn4, (Ni,Co)3P and (Ni,Co)12P5, are formed at the reaction interface. Nano-sized voids are visible in the (Ni,Co)3P layer under transmission electron microscopy, but no large voids are found under scanning electron microscopy. This is an indication of effective diffusion barrier performance by the Ni–Co–P metallization compared with the binary Ni–P metallization. The influence of interfacial reaction on the solder joint reliability was reported through the evaluation of the tensile strength of solder micro-joints. Upon aging at 180°C for 600h, the tensile strength of Ni–Co–P/Sn–3.5Ag solder joint remains high, and the failure is caused by the bulk solder necking and collapse. As a comparison, the tensile strength of the Ni–P/Sn–3.5Ag solder joint drops significantly after aging for 400h at 180°C, and the fracture mode has shifted from ductile failure in the bulk solder to brittle failure at the solder joint interface. The Ni–Co–P metallization, having a much slower consumption rate and improved resistance to joint strength degradation during long-term aging treatment, is a potential candidate for future microelectronic solder metallization materials.

Nano-scale surface morphology optimization of the ohmic contacts and electrical properties of AlGaN/GaN high electron mobility transistors using a rapid thermal annealing dielectric protection layer

Ohmic contacts with a low contact resistance and low surface roughness are essential to ensure the optimal device performance of AlGaN/GaN high electron mobility transistors. A tantalum metal layer and Si3N4 or SiO2 thin films deposited by plasma enhanced chemical vapor deposition are here used as an effective diffusion barrier and protection layer, respectively, in the standard Ti/Al/metal/Au ohmic metallization scheme to obtain high-quality ohmic contacts. The Si3N4 or SiO2 thickness significantly affects the surface roughness. The optimal Ti/Al/Ta/Au ohmic contact is prepared with a 40-nm thick Ta barrier layer and a 50-nm thick SiO2 rapid thermal annealing protection layer. Compared with conventional ohmic contact stacks, the optimal contact exhibits a low contact resistance of approximately 0.05Ω·mm and a nano-scale surface roughness, with a root-mean-square deviation of approximately 9nm. Lastly, the ohmic contact was used to optimize AlGaN/GaN high electron mobility transistor with a 0.5-μm gate length, exhibiting a maximum drain current density of 760mA/mm, a peak transconductance of 420mS/mm, a unity-gain cut-off frequency of 24.6GHz, and a maximum frequency of oscillation of 45.4GHz. The X-band power performance has an output power density of 7.4W/mm, an output power of 32dBm, and a power added efficiency of 38% at an input power of 21dBm.

Selective atomic layer deposition of zirconia on copper patterned silicon substrates using ethanol as oxygen source as well as copper reductant

The authors report a new chemical approach for the selective atomic layer deposition of ultrathin layers of zirconium oxide (ZrO2) on copper patterned silicon surfaces. Instead of using common atomic layer deposition (ALD) oxygen sources such as water, oxygen, or ozone, the authors use ethanol, which serves as oxygen source for the ALD on the silicon side and as effective reducing agent on the copper side, thereby selectively depositing ZrO2 film on the silicon surface of the substrate without any deposition on copper up to at least 70 ALD cycles. The resulting ZrO2 nanofilm is found to be an effective copper diffusion barrier at temperatures at least up to 700 °C.

Trapping of interstitial defects: filling the gap between the experimental measurements and DFT calculations

We show that any impurity will slow the diffusion of oxygen in Nb. Using a first-principles plane-wave pseudopotential method and the supercell model, we calculated the interaction energies between substitutional atoms (SA) (X = Ti, V, Ta, Zr, and Hf) and interstitial oxygen in a Nb matrix. All impurities act as traps for oxygen: undersized SA (Ti and V) have strongest binding at the nearest octahedral interstice, while for oversized SA (Zr and Hf), the strongest trapping site is the second-nearest octahedral interstice. We evaluated the diffusion coefficients of O in the Nb–X alloys using kinetic Monte Carlo (KMC) modeling based in the transition state theory, using our calculated oxygen migration energies. From this, the effective (average) X–O interaction energies were extracted using the Oriani model (Oriani 1970 Acta Metall. 18 147–57). The effective X–O interaction energies are close to the strongest interaction energies between X and O obtained from the direct supercell calculations. The phenomenological effective diffusion barrier obtained from the KMC modeling is close to the energy difference between the most stable configuration and the highest saddle point along the diffusion path. Both results demonstrate that the weaker trapping site has negligible influence on the diffusion of O.

Electro-migration of impurities in TlBr

We observed the electro-migration of Cu, Ag, and Au impurities that exist in positive-ion states in TlBr detectors under electric field strengths typically used for device operation. The migration occurred predominantly through bulk- and specific-channels, which are presumed to be a network of grain and sub-grain boundaries. The electro-migration velocity of Cu, Ag, and Au in TlBr is about 4–8 × 10−8 cm/s at room temperature under an electric field of 500–800 V/mm. The instability and polarization effects of TlBr detectors might well be correlated with the electro-migration of residual impurities in TlBr, which alters the internal electric field over time. The effect may also have been due to migration of the electrode material itself, which would allow for the possibility of a better choice for contact material and for depositing an effective diffusion barrier. From our findings, we suggest that applying our electro-migration technique for purifying material is a promising new way to remove electrically active metallic impurities in TlBr crystals, as well as other materials.

Diamond CVD film formation onto WC–Co substrates using a thermally nitrided Cr diffusion-barrier

Diamond film deposition onto WC-Co substrates exhibits several limitations regarding the final diamond quality in the film and its adhesion due to the chemical interaction between the Co in the substrate and the diamond CVD environment. In the present study, the use of a ~1.5μm thermally nitrided Cr interlayer was examined as an effective diffusion barrier throughout the CVD process. Nitridation of the Cr PVD layer in NH3 environment resulted in the formation of a graded CrN/Cr2N layer comprised mainly of the CrN phase, accompanied with the formation of a porous ‘net-like’ microstructure at the surface. During both thermal nitridation and exposure to the CVD environment up to 360min, the diffusion of C and Co from the substrate into the interlayer was limited to the region adjacent to the Cr–N interlayer/WC–Co substrate interface, which contained the Cr2N phase. In this region, the Co interacted with the Cr lattice to form a CoCr phase, which was suggested to enhance the chemical binding between the interlayer and the substrate. The region containing the CrN phase was suggested to act as an effective diffusion barrier due to its fully occupied interstitial sites and relatively high crystalline density compared to the underlying Cr2N phase. It was evident that the deleterious effects of Co during the CVD process were successfully suppressed using the Cr–N interlayer and the deposited diamond film exhibited improved adhesion and higher diamond quality.The formation of phases within the interlayer during nitridation and the diamond CVD process, and diamond quality evaluation in the deposited films were investigated by complementary techniques: SEM, XRD, XPS, SIMS and Raman spectroscopy.

Analysis and optimisation of ohmic contact resistance and surface morphology of a Ta-based diffusion barrier layer in AlGaN/GaN HEMTs on Si (1 1 1) substrates

Various rapid thermal annealing (RTA) conditions for the Ti/Al/Ta/Au ohmic contact process and the resulting surface analysis have been investigated to characterise AlGaN/GaN high-electron-mobility transistors (HEMTs) on Si (111) substrates. The use of Ta as an effective diffusion barrier in Ti/Al/metal/Au ohmic metallisation yields better ohmic contacts than other metal formations. To achieve low ohmic contact resistance (Rc) and good surface morphology, we tested different Ta layer thicknesses (20,40,60,and80nm) in a deoxidation process with smooth O2/H2 plasma-etching pre-treatments comprised of five annealing temperatures (700,750,800,850,and900°C) and two annealing times (15 and 30s). The best ohmic resistance, 0.03Ωmm (7.27×10−7Ωcm2), is obtained for a Ta barrier layer thickness of 40nm, annealing temperature of 850°C and annealing time of 30s. In atomic force microscopy (AFM) images, nano-scale surface morphology with a root-mean-square (RMS) deviation of 6.9nm is observed. Through comparative energy dispersive spectrometry (EDS) analysis of the surface morphologies at the bulge and sunken areas at 800°C annealing temperature, we found that the surface degradation phenomenon is easily generated at 800°C annealing temperature by a significant reaction of the Au–Al alloy. As a result, Au and Al are diffused into the Ta barrier layer, and a bulge and sunken are generated on the ohmic contact surface. From the EDS spectra, the Au ratio continually decreases from the optimised area (63%) to the sunken area (32%) due to the diffusion of Au through the Ta barrier layer. Conversely, the Au ratio continually increases from the optimised area (63%) to the bulged area (90%) due to the inability of Au to diffuse downward; Al diffuses upward and causes the Au bulge.

Selective emitter formation by laser doping of spin-on sources

Formation of selective emitter (SE) structures with a controlled dopant profile by laser (KrF Excimer laser at 248nm) annealing of spin-on dopant sources is presented. Different barrier layers (BL) like spin-on glass (SoG), PECVD deposited SiN and SiOx were used as semi-transparent barrier layers for the dopant diffusion. The presence of BL at the interface between silicon substrate and the layer of dopant source controls the dopant profile of shallow emitter (ShE) during thermal diffusion, so that the etch back step could be avoided. This method allows the realization of shallow and selective emitters using a single layer of dopant source. The dopant concentration and depth with respect to the laser parameters and barrier thickness were analyzed using secondary ion mass spectrometry. It was found that the doping profile of phosphorous was precisely controlled in the shallow region upto 200nm with a suitable emitter sheet resistance. Also, the SiN and SoG layers acted as effective phosphorous diffusion barriers for both shallow and selective emitters. On the other hand, the SiOx barrier layers, relatively lower thickness, resulted in the best electrical results at comparably lower laser fluences. In addition, laser induced damage in the silicon crystal at moderate laser fluences is nominal, and is found to be considerable at higher energies due to the enhanced energy absorption of silicon. Periodic structures were observed on the surface of laser treated silicon at the moderate laser fluences. The results were presented in detail in terms of physical behavior of the dopant diffusion with respect to laser fluence in the presence of barrier layer.

Resistive switching characteristics of Pt/TaOx/HfNx structure and its performance improvement

The refractory transition metal nitride (TMN) film Hafnium nitride (HfNx) was successfully prepared on silicon-based substrates as bottom electrodes for resistive random access memory (RRAM) cells in Pt (top)/metal oxide/ HfNx (bottom) sandwich structure. The reproducible resistive switching (RS) characteristics of the memory cells were studied systematically for RRAM applications. The advantages of adopting HfNx instead of Pt as bottom electrode material were demonstrated, including the improvement of the low resistive state value, the RS endurance and the uniformity of RS parameters. The composition and chemical bonding states of the prepared HfNx was analyzed by X-ray photoelectron spectroscopy (XPS) technique. The nitrogen content in the HfNx and the Gibbs free energy of the corresponding metal oxide formation has great influences on the RS properties. The oxygen reservoir ability and diffusion barrier effect of the HfNx play a key role in the RS performance improvement of the RRAM devices.

Mechanism of drug release from double-walled PDLLA(PLGA) microspheres

The drug release and degradation behavior of two double-walled microsphere formulations consisting of a doxorubicin-loaded poly(d,l-lactic-co-glycolic acid) (PLGA) core (∼46 kDa) surrounded by a poly(d,l-lactic acid) (PDLLA) shell layer (∼55 and 116 kDa) were examined. It was postulated that different molecular weights of the shell layer could modulate the erosion of the outer coating and limit the occurrence of water penetration into the inner drug-loaded core on various time scales, and therefore control the drug release from the microspheres. For both microsphere formulations, the drug release profiles were observed to be similar. The degradation of the microspheres was monitored for a period of about nine weeks and analyzed using scanning electron microscopy, laser scanning confocal microscopy, and gel permeation chromatography. Interestingly, both microsphere formulations exhibited occurrence of bulk erosion of PDLLA on a similar time scale despite different PDLLA molecular weights forming the shell layer. The shell layer of the double-walled microspheres served as an effective diffusion barrier during the initial lag phase period and controlled the release rate of the hydrophilic drug independent of the molecular weight of the shell layer.

Effects of surface finishes and current stressing on the interfacial reaction characteristics of Sn–1.2Ag–0.7Cu–0.4In solder bumps

The effects of surface finishes on the in situ interfacial reaction characteristics and electromigration (EM) reliabilities of ball grid array (BGA) Sn–1.2Ag–0.7Cu–0.4In lead-free solder bumps were investigated under annealing and EM test conditions of three different temperatures, 135 °C, 150 °C, and 170 °C, with a constant electrical current density of 5.0 × 103 A/cm2. The activation energies of total intermetallic compounds (IMCs) were found to be 0.87 eV for electroless nickel–immersion gold (ENIG) and 0.86 eV for organic solderability preservatives (OSP). The ENIG finish appeared to act as an effective diffusion barrier between the Cu substrate and the solder, which leads to better EM reliability in comparison with Cu-based pad systems. The failure mechanisms were explored in detail via in situ EM tests.

Experimental and Thermokinetic Simulation Studies on the Formation of Deleterious Zones in Dissimilar Ferritic Steel Weldments

The methods to predict and prevent the formation of hard and soft zones in dissimilar weldments of 9Cr-1Mo and 2¼Cr-1Mo ferritic steels during high-temperature exposure are examined in this article. The computational studies have been carried out using multicomponent diffusion model incorporated in Dictra and validated by experimental methods using EPMA and TEM. Carbon concentration profiles across the interface of the weld joint between the two ferritic steels were simulated in the temperatures ranging from 823 K to 1023 K (from 550 °C to 750 °C) for various time durations using “diffusion in dispersed phase model” in Dictra. When precipitation and diffusion were incorporated into the calculations simultaneously, the agreement was better between the calculated and the experimentally measured values of carbon concentration profiles, type, and volume fractions of carbides in the hard zone and diffusion zone, width, and the activation energy. Calculation results of thermodynamic potentials of carbon in 2¼Cr-1Mo and 9Cr-1Mo steels suggested that the diffusion is driven by the activity gradient of carbon across the joint. The effectiveness of nickel-based diffusion barrier in suppressing the formation of hard and soft zones is demonstrated using calculations based on the cell model incorporated in Dictra.