Characterization of TiN/TiSi2 bilayer for application to ULSI

The properties of TiN/TiSi₂ bilayer formed by rapid thermal annealing (RTA) in an NH₃ ambient after the titanium film is deposited on the silicon substrate is investigated. It is found that the formation of TiN/TiSi₂ bilayer depends on the RTA temperature and a competitive reaction for the TiN/TiSi₂ bilayer occurs at 600°C. Both the TiN and TiSi₂ layers represent titanium-rich films at 600°C anneal. The TiN layer has a stable structure at 700°C anneal while the TiSi₂ layer has C₄₉ and C₅₄ phase. Both the TiN and TiSi₂ layers have stable structures and stoichiometries at 800°C anneal. When the TiN/TiSi₂ bilayer is formed, the redistribution of boron atoms within the TiSi₂ layer gets active as the anneal temperature is increased. According to secondary ion mass spectroscopy analysis, boron atoms pile up within the TiN layer and at the TiSi₂−Si interface. The electrical properties for n⁺ and p⁺ contacts are investigated. The n⁺ contact resistance increases slightly with increasing annealing temperature but the p⁺ contact resistance decreases. The leakage current indicates degradation of the contact at high annealing temperature for both n⁺ and p⁺ junctions.     

Investigation of RuTiN and RuTiO diffusion barrier suggested by anew design concept for future high-density memory capacitors

As a new design concept for high-density memory capacitors, we proposed both RuTiN (RTN) and the RuTiO (RTO) materials as sacrificial oxygen diffusion barriers. The sheet resistance of the newly developed RuTiN and RuTiO barriers after oxidation was much lower than that of polycrystalline nitride and ternary amorphous barriers reported by others. For the Pt/barrier/TiSi_x/n⁺⁺poly-plug/n ⁺ channel layer/Si contact structure, contact resistance was below 5 kohm even after annealing up to 750°C. Correspondingly, the new RuTiN and RuTiO films, as diffusion barriers for oxygen, are very promising materials for high-density capacitors     

Thermal Stability of Nitride-Based Diffusion Barriers for Ohmic Contacts to <Emphasis Type="Italic">n</Emphasis>-GaN

The use of TaN, TiN, and ZrN diffusion barriers for Ti/Al-based contacts on n-GaN (n ∼ 3 × 10¹⁷ cm⁻³) is reported. The annealing temperature (600–1,000°C) dependence of the Ohmic contact characteristics using a Ti/Al/X/Ti/Au metallization scheme, where X is TaN, TiN, or ZrN, deposited by sputtering was investigated by contact resistance measurements and Auger electron spectroscopy (AES). The as-deposited contacts were rectifying and transitioned to Ohmic behavior for annealing at ≥600°C. A minimum specific contact resistivity of ∼6 × 10⁻⁵ Ω-cm⁻² was obtained after annealing over a broad range of temperatures (600–900°C for 60 s), comparable to that achieved using a conventional Ti/Al/Pt/Au scheme on the same samples. The contact morphology became considerably rougher at the high end of the annealing range. The long-term reliability of the contacts at 350°C was examined; each contact structure showed an increase in contact resistance by a factor of three to four over 24 days at 350°C in air. AES profiling showed that the aging had little effect on the contact structure of the nitride stacks.     

Thermo-Sensitive Ba0.64Sr0.36TiO3 Thin Film Capacitors for Dielectric Type Uncooled Infrared Sensors

Ba_0.64Sr_0.36TiO₃ (BST) thin films are prepared on Pt/Ti/SiO₂/Si₃N₄/SiO₂/Si substrates by a sol-gel method. Thermo-sensitive BST thin film capacitors with a Metal-Ferroelectrics-Metal (M-F(BST)-M) structure are fabricated as the active elements of dielectric type uncooled infrared sensors. XRD are employed to analyze the crystallographic structures of the films. AFM observations reveal a smooth and dense surface of the films with an average grain size of about 35 nm. Rapid temperature annealing (RTA) process is a very efficient way to improve crystallization quality. The preferable annealing temperature is 800°C for 1 min. The butterfly shaped C-V curves of the capacitors indicate the films have a ferroelectric nature. The dielectric constant and dielectric loss of the films at 100 kHz are 450 and 0.038, respectively. At 25°C, where the thermo-sensitive capacitors work, the temperature coefficient of dielectric constant (TCD) is about 5.9 %/°C. These results indicate that the capacitors with sol-gel derived BST thin films are promising to develop dielectric type uncooled infrared sensors.     

Optoelectronic Properties of Quasi‐Linear,Self‐Assembled Platinum Complexes: Pt–Pt Distance Dependence

Charge‐carrier mobilities of various self‐assembled platinum complexes were measured by time‐resolved microwave conductivity techniques in the temperature range –80 to +100 °C. Eight compounds were investigated in the present study, including the original Magnus' green salt ([Pt(NH₃)₄][PtCl₄]) and derivatives with the general structure [Pt(NH₂R)₄][PtCl₄], where R denotes an alkyl side chain. In one instance, the chlorines were substituted with bromines. For these complexes, which all consist of a linear backbone of platinum atoms with Pt–Pt distances, d, varying from 3.1 to ≥ 3.6 Å, a strong, inverse correlation was found between d and the one‐dimensional charge‐carrier mobility, Σμ_1D. The highest value of Σμ_1D at room temperature was observed for R = (S)‐3,7‐dimethyloctyl (dmoc) with Σμ_1D ≥ 0.06 cm² V^–1 s^–1. Almost all materials exhibited a charge‐carrier mobility that was relatively independent of the temperature over the range studied. One exceptional compound (R = (R)‐2‐ethylhexyl) showed a pronounced negative temperature dependence of the charge‐carrier mobility; upon decreasing the temperature from +100 °C to –80 °C the charge‐carrier mobility increased by a factor of about ten.     

Bottom‐Up Engineering of Subnanometer Copper Diffusion Barriers Using NH2‐Derived Self‐Assembled Monolayers

A 3‐aminopropyltrimethoxysilane‐derived self‐assembled monolayer (NH₂SAM) is investigated as a barrier against copper diffusion for application in back‐end‐of‐line (BEOL) technology. The essential characteristics studied include thermal stability to BEOL processing, inhibition of copper diffusion, and adhesion to both the underlying SiO₂ dielectric substrate and the Cu over‐layer. Time‐of‐flight secondary ion mass spectrometry and X‐ray spectroscopy (XPS) analysis reveal that the copper over‐layer closes at 1–2‐nm thickness, comparable with the 1.3‐nm closure of state‐of‐the‐art Ta/TaN Cu diffusion barriers. That the NH₂SAM remains intact upon Cu deposition and subsequent annealing is unambiguously revealed by energy‐filtered transmission electron microscopy supported by XPS. The SAM forms a well‐defined carbon‐rich interface with the Cu over‐layer and electron energy loss spectroscopy shows no evidence of Cu penetration into the SAM. Interestingly, the adhesion of the Cu/NH₂SAM/SiO₂ system increases with annealing temperature up to 7.2 J m^?2 at 400 °C, comparable to Ta/TaN (7.5 J m^?2 at room temperature). The corresponding fracture analysis shows that when failure does occur it is located at the Cu/SAM interface. Overall, these results demonstrate that NH₂SAM is a suitable candidate for subnanometer‐scale diffusion barrier application in a selective coating for copper advanced interconnects.     

High Molar Extinction Coefficient Ion‐Coordinating Ruthenium Sensitizer for Efficient and Stable Mesoscopic Dye‐Sensitized Solar Cells

Ru(4,4‐dicarboxylic acid‐2,2′‐bipyridine) (4,4′‐bis(2‐(4‐(1,4,7,10‐tetraoxyundecyl)phenyl)ethenyl)‐2,2′‐bipyridine) (NCS)₂, a new high molar extinction coefficient ion‐coordinating ruthenium sensitizer was synthesized and characterized using ¹H NMR, Fourier transform IR (FTIR), and UV/vis spectroscopies and cyclic voltammetry. Using this sensitizer in combination with a nonvolatile organic‐solvent‐based electrolyte, we obtain a photovoltaic efficiency of 8.4 % under standard global AM 1.5 sunlight. These devices exhibit excellent stability when subjected to continuous thermal stress at 80 °C or light soaking at 60 °C for 1000 h. An electrochemical impedance spectroscopy study revealed that device stability is maintained by stabilizing the TiO₂/dye/electrolyte and Pt/electrolyte interface during the aging process. The influence of Li⁺ present in the electrolyte on the device photovoltaic parameters was studied, and the FTIR spectral and photovoltage transient study showed that Li⁺ coordinates to the triethyleneoxide methylether side chains on the K60 sensitizer molecules.     

Crystallization behavior and ferroelectric properties of PbTiO<Subscript>3</Subscript>/Ba<Subscript>0.85</Subscript>Sr<Subscript>0.15</Subscript>TiO<Subscript>3</Subscript>/PbTiO<Subscript>3</Subscript> sandwich thin film on Pt/Ti/SiO<Subscript>2</Subscript>/Si substrates

A PbTiO₃/Ba_0.85Sr_0.15TiO₃/PbTiO₃ (PT/BST15/PT) sandwich thin film has been prepared on Pt/Ti/SiO₂/Si substrates by an improved sol-gel technique. It is found that such films under rapid thermal annealing at 700°C crystallize more favorably with the addition of a PbTiO₃ layer. They possess a pure, perovskite-phase structure with a random orientation. The polarization-electric field (P-E) hysteresis loop and current-voltage (I-V) characteristic curves reveal that a PT/BST15/PT film exhibits good ferroelectricity at room temperature. However, no sharp peak, only a weak maximum, is observed in the curves of the dielectric constant versus temperature. The dielectric constant, loss tangent, leakage current density at 20 kV/cm, remnant polarization, and coercive field of the PT/BST15/PT film are 438, 0.025, 1.3 × 10⁻⁶ Acm⁻², 2.46 μCcm⁻², and 41 kVcm⁻¹, respectively, at 25°C and 10 kHz. The PT/BST15/PT film is a candidate material for high sensitivity elements for uncooled, infrared, focal plane arrays (UFPAs) to be used at near ambient temperature.     

Low-temperature CVD TiN as a diffusion barrier between gold and silicon

Low-temperature (200°C), atmospheric pressure chemical vapor deposited (APCVD) titanium nitride films are shown to be effective diffusion barriers for the Au/TiN/Si contact scheme. The samples were analyzed by Rutherford backscattering spectroscopy (RBS), and by optical microscopy. It was found that a pure TiN layer constitutes an effective barrier for 40 min at 550°C, at which point the TiN cracks and peels. Even thin layers of TiN (200Å thick) can significantly reduce the amount of interdiffusion between the gold and silicon.     

Characterization of an Oxidized Porous Silicon Layer by Complex Process Using RTO and the Fabrication of CPW‐Type Stubs on an OPSL for RF Application

This paper proposes a 10‐µm thick oxide layer structure that can be used as a substrate for RF circuits. The structure has been fabricated using an anodic reaction and complex oxidation, which is a combined process of low‐temperature thermal oxidation (500 °C, for 1 hr at H₂O/O₂) and a rapid thermal oxidation (RTO) process (1050 °C, for 1 min). The electrical characteristics of the oxidized porous silicon layer (OPSL) were almost the same as those of standard thermal silicon dioxide. The leakage current density through the OPSL of 10 µm was about 10 to 50 nA/cm² in the range of 0 to 50 V. The average value of the breakdown field was about 3.9 MV/cm. From the X‐ray photo‐electron spectroscopy (XPS) analysis, surface and internal oxide films of OPSL prepared by a complex process were confirmed to be completely oxidized. The role of the RTO process was also important for the densification of the porous silicon layer (PSL) oxidized at a lower temperature. The measured working frequency of the coplanar waveguide (CPW) type short stub on an OPSL prepared by the complex oxidation process was 27.5 GHz, and the return loss was 4.2 dB, similar to that of the CPW‐type short stub on an OPSL prepared at a temperature of 1050 °C (1 hr at H2O/O2). Also, the measured working frequency of the CPW‐type open stub on an OPSL prepared by the complex oxidation process was 30.5 GHz, and the return was 15 dB at midband, similar to that of the CPW‐type open stub on an OPSL prepared at a temperature of 1050 °C (1 hr at H₂O/O₂).     

Microgenerator Using BiSbTe-Pt Thermopile and Pt-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Ceramic Combustor

The performance of a microcombustor thermoelectric generator device based on a thermopile using p-type Bi_0.3Sb_1.7Te₃ (BST) and n-type Pt films has been investigated. The BST films were prepared by two different methods—pulsed laser deposition (PLD) and sputter deposition—on Si₃N₄/SiO₂ multilayers on Si substrate. The ceramic catalyst combustor was patterned on the thermopile end on a thin membrane fabricated by back-side bulk etching of the silicon substrate. At 138°C the thermoelectric power factors of the PLD and sputter-deposited films were 3.6 × 10⁻³ W/mK² and 0.22 × 10⁻³ W/mK², respectively. The power from the generator with the sputter-deposited film was 0.343 μW, which was superior to that of the device with the PLD film, which provided 0.1 μW, for combustion of a 200 sccm flow of 3 v/v% hydrogen in air.     

Resistive Switching Memory Integrated with Nanogenerator for Self‐Powered Bioimplantable Devices

Resistive random access memory (ReRAM) devices powered by piezoelectric nanogenerators (NGs) have been investigated for their application to future implantable biomedical devices. Biocompatible (Na_0.5K_0.5)NbO₃ (NKN) films that are grown at 300 °C on TiN/SiO₂/Si and flexible TiN/Polyimide (TiN‐PI) substrates are used for ReRAM and NGs, respectively. These NKN films have an amorphous phase containing NKN nanocrystals with a size of 5.0 nm. NKN ReRAM devices exhibit typical bipolar switching behavior that can be explained by the formation and rupture of oxygen‐vacancy filaments. They have good ReRAM properties such as a large ratio of R_HRS to R_LRS as well as high reliability. The NKN film grown on flexible TiN‐PI substrate exhibits a high piezoelectric strain constant of 50 pm V^?1. The NKN NG has a large open‐circuit output voltage of 2.0 V and a short‐circuit output current of 40 nA, which are sufficient to drive NKN ReRAM devices. Stable switching properties with a large ON/OFF ratio of 10² are obtained from NKN ReRAM driven by NKN NG.     

Electrically Programmable Bistable Capacitor for High‐Frequency Applications Based on Charge Storage at the (Ba,Sr)TiO3/Al2O3 Interface

Hysteresis is induced in paraelectric (Ba,Sr)TiO₃ (BST) thin‐film capacitors by inserting an Al₂O₃ barrier layer of a few nanometers in thickness between the BST layer and the electrode. The observed hysteresis is explained by ambipolar charge carrier injection through the Al₂O₃ layer and charge storage at the BST/Al₂O₃ interface. The magnitude of the hysteresis can be directly adjusted by manipulating the thickness ratio between BST and Al₂O₃. Taking into account the low loss of (Ba,Sr)TiO₃ capacitors, the observed switching and retention characteristics are suitable for application as non‐volatile programmable high‐frequency devices, e.g., in radio‐frequency identification.     

Large area tunnel oxide passivated rear contact n‐type Si solar cells with 21.2% efficiency

This paper reports on the implementation of carrier‐selective tunnel oxide passivated rear contact for high‐efficiency screen‐printed large area n‐type front junction crystalline Si solar cells. It is shown that the tunnel oxide grown in nitric acid at room temperature (25°C) and capped with n⁺ polysilicon layer provides excellent rear contact passivation with implied open‐circuit voltage iV_oc of 714 mV and saturation current density J_0b′ of 10.3 fA/cm² for the back surface field region. The durability of this passivation scheme is also investigated for a back‐end high temperature process. In combination with an ion‐implanted Al₂O₃‐passivated boron emitter and screen‐printed front metal grids, this passivated rear contact enabled 21.2% efficient front junction Si solar cells on 239 cm² commercial grade n‐type Czochralski wafers. Copyright © 2016 John Wiley & Sons, Ltd.     

Nitrogen‐Doped Nanoporous Carbon/Graphene Nano‐Sandwiches: Synthesis and Application for Efficient Oxygen Reduction

A zeolitic‐imidazolate‐framework (ZIF) nanocrystal layer‐protected carbonization route is developed to prepare N‐doped nanoporous carbon/graphene nano‐sandwiches. The ZIF/graphene oxide/ZIF sandwich‐like structure with ultrasmall ZIF nanocrystals (i.e., ≈20 nm) fully covering the graphene oxide (GO) is prepared via a homogenous nucleation followed by a uniform deposition and confined growth process. The uniform coating of ZIF nanocrystals on the GO layer can effectively inhibit the agglomeration of GO during high‐temperature treatment (800 °C). After carbonization and acid etching, N‐doped nanoporous carbon/graphene nanosheets are formed, with a high specific surface area (1170 m² g^?1). These N‐doped nanoporous carbon/graphene nanosheets are used as the nonprecious metal electrocatalysts for oxygen reduction and exhibit a high onset potential (0.92 V vs reversible hydrogen electrode; RHE) and a large limiting current density (5.2 mA cm^?2 at 0.60 V). To further increase the oxygen reduction performance, nanoporous Co‐N_x/carbon nanosheets are also prepared by using cobalt nitrate and zinc nitrate as cometal sources, which reveal higher onset potential (0.96 V) than both commercial Pt/C (0.94 V) and N‐doped nanoporous carbon/graphene nanosheets. Such nanoporous Co‐N_x/carbon nanosheets also exhibit good performance such as high activity, stability, and methanol tolerance in acidic media.     

Water‐Dispersible,Ligand‐Free,and Extra‐Small (<10 nm) Titania Nanoparticles: Control Over Primary,Secondary, and Tertiary Agglomeration Through a Modified “Non‐Aqueous” Route

Non‐aqueous routes to inorganic nanoparticles are supposedly based on the absence of water; here, this view is partially challenged, showing that the presence of water (or moisture) is probably necessary, and is surely useful to achieve a precise control over the growth/aggregation phenomena leading to titanium dioxide nanoparticles. This study is focused on the preparation of size‐controlled and ligand‐free titania (anatase) nanoparticles in water dispersion. This is achieved through a three‐step process: 1) production of primary (3–4 nm) nanoparticles from titanium alkoxides (Ti(OnPr)₄, Ti(OnBu)₄ or Ti(OⁱPr)₄) in benzyl alcohol through the controlled addition of water; 2) thermal growth phase, where the aggregation of primary nanoparticles at 80 °C leads to secondary nanoparticles with a typical fractal dimension of 2.2–2.4; the primary particles are still identifiable as the individual crystallites composing the secondary nanoparticles; 3) precipitation/re‐dispersion in water, where secondary nanoparticles further agglomerate to yield tertiary nanoparticles. The size of the latter and their photocatalytic efficiency is primarily controlled by the nature of residual alkoxide chains; in particular, isopropoxide groups allow to produce anatase nanoparticles with an average size of 7–8 nm in water dispersion and in the absence of any stabilizing ligand, which is an unprecedented result.     

Integration of high dielectric Ba0.5Sr0.5TiO3 films into amorphous TaSiN barrier layer structures

The high-k dielectric material (Ba,Sr)TiO₃ has been intensively investigated for possible applications in dynamic random access memory circuits. During the BST deposition process in O₂ ambient, typically at 650°C, the diffusion of oxygen through the bottom electrode into the poly Si plug must be prevented. Amorphous TaSiN films are excellent candidates as oxygen barrier layers. Ba_0.5Sr_0.5TiO₃ (BST) films with thickness of 100 nm were deposited on the electrode structure SiO₂/TaSiN/Pt. The sol–gel method was used to grow the BST films. The barrier effect for oxygen diffusion is studied in TaSiN layers with thickness of 50 nm, which were deposited by a reactive sputter process. X-ray photoemission spectroscopy results confirm that this amorphous material is a suitable barrier against oxygen diffusion at 650°C. The BST films, deposited at 650°C and post-annealed at 650°C show a dielectric constant of 100 at 100 kHz and a dissipation factor of less than 5%.     

High‐Performance Micro‐Solid Oxide Fuel Cells Fabricated on Nanoporous Anodic Aluminum Oxide Templates

Micro‐solid oxide fuel cells (μ‐SOFCs) are fabricated on nanoporous anodic aluminum oxide (AAO) templates with a cell structure composed of a 600‐nm‐thick AAO free‐standing membrane embedded on a Si substrate, sputter‐deposited Pt electrodes (cathode and anode) and an yttria‐stabilized zirconia (YSZ) electrolyte deposited by pulsed laser deposition (PLD). Initially, the open circuit voltages (OCVs) of the AAO‐supported μ‐SOFCs are in the range of 0.05 V to 0.78 V, which is much lower than the ideal value, depending on the average pore size of the AAO template and the thickness of the YSZ electrolyte. Transmission electron microscopy (TEM) analysis reveals the formation of pinholes in the electrolyte layer that originate from the porous nature of the underlying AAO membrane. In order to clog these pinholes, a 20‐nm thick Al₂O₃ layer is deposited by atomic layer deposition (ALD) on top of the 300‐nm thick YSZ layer and another 600‐nm thick YSZ layer is deposited after removing the top intermittent Al₂O₃ layer. Fuel cell devices fabricated in this way manifest OCVs of 1.02 V, and a maximum power density of 350 mW cm^?2 at 500 °C.     

Characterization of a rapid thermal anneal TiNxOy/TiSi2 contact barrier

In this paper, the physical and electrical properties of a TiN_xO_y/TiSi₂ dual layer contact barrier are reported. The TiN_xO_y/TiSi₂ barrier was formed by rapidly annealing a Ti thin film on Si in an N₂ ambient. During this process, the Ti film surface reacts with N₂ to form a TiN_xO_y skin layer and the bulk of the Ti film reacts with Si to form an underlying TiSi₂ layer. The influences of rapid thermal anneal (RTA) conditions on the TiN_xO_y layer were investigated by varying the RTA temperature from 600 to 1100° C and cycle duration from 30 to 100 s. It is found that the resulting TiN_xO_y and TiSi₂ layer thicknesses are dependent on RTA temperature and the starting Ti thickness. For a starting Ti thickness of 500Å, 150Å thick TiN_xO_y and 800Å thick TiSi₂ are obtained after an RTA at 900° C for 30 s. The TiN_xO_y thickness is limited by a fast diffusion of Si into Ti to form TiSi₂. When a Ti film is deposited on SiO₂, Ti starts to react with SiO₂ from 600° C and a significant reduction of the SiO₂ thickness is observed after an RTA at 900° C. The resulting layer is composed of a surface TiN_xO_y layer followed by a complex layer of titanium oxide and titanium suicide. In addition, when Ti is depos-ited on TiSi₂, thicker TiN_xO_y and TiSi₂ layers are obtained after RTA. This is because the TiSi₂ layer retards the diffusion of Si from the underlying substrate into the Ti layer. NMOSFETs were fabricated using the TiN_xO_y/TiSi₂ as a contact barrier formed by RTA at 900° C for 30 s and a significant reduction of contact resistance was obtained. In addition, electromigration test at a high current density indicated that a significant improvement in mean time to failure (MTF) has been obtained with the barrier.     

Layer‐by‐Layer Conjugated Extension of a Semiconducting Polymer for High‐Performance Organic Field‐Effect Transistor

A donor–acceptor (D–A) semiconducting copolymer, PDPP‐TVT‐29, comprising a diketopyrrolopyrrole (DPP) derivative with long, linear, space‐separated alkyl side‐chains and thiophene vinylene thiophene (TVT) for organic field‐effect transistors (OFETs) can form highly π‐conjugated structures with an edge‐on molecular orientation in an as‐spun film. In particular, the layer‐like conjugated film morphologies can be developed via short‐term thermal annealing above 150 °C for 10 min. The strong intermolecular interaction, originating from the fused DPP and D–A interaction, leads to the spontaneous self‐assembly of polymer chains within close proximity (with π‐overlap distance of 3.55 Å) and forms unexpectedly long‐range π‐conjugation, which is favorable for both intra‐ and intermolecular charge transport. Unlike intergranular nanorods in the as‐spun film, well‐conjugated layers in the 200 °C‐annealed film can yield more efficient charge‐transport pathways. The granular morphology of the as‐spun PDPP‐TVT‐29 film produces a field‐effect mobility (μ _FET) of 1.39 cm² V^?1 s^?1 in an OFET based on a polymer‐treated SiO₂ dielectric, while the 27‐Å‐step layered morphology in the 200 °C‐annealed films shows high μ _FET values of up to 3.7 cm² V^?1 s^?1.