Traps with near-midgap energies at the bonded Si/SiO<Subscript>2</Subscript> interface in silicon-on-insulator structures

Capture centers (traps) are studied in silicon-on-insulator (SOI) structures obtained by bonding and hydrogen-induced stratification. These centers are located at the Si/SiO₂ interface and in the bulk of the split-off Si layer. The parameters of the centers were determined using charge deep-level transient spectroscopy (Q-DLTS) with scanning over the rate window at fixed temperatures. Such a method allows one to study the traps near the Si midgap at temperatures near 295 K. It is shown that the density of traps with a continuous energy spectrum, which are located at the bonded Si/SiO₂ interface, decreases by more than four orders of magnitude at the mid-gap compared with the peak density observed at the activation energy E _a ≈0.2–0.3 eV. The capture centers are also found in the split-off Si layer of the fabricated SOI structures. Their activation energy at room temperature is E _a =0.53 eV, the capture cross section is 10^?19 cm², and the concentration is (0.7–1.7)×10¹³ cm^?3. It is assumed that these capture centers are related to deep bulk levels induced by electrically active impurities (defects) in the split-off Si layer close to the Si/SiO₂ interface.     

Growth of CsLiB6O10 thin films on Si substrate by pulsed laser deposition using SiO2 and CaF2 as buffer layers

CsLiB₆O₁₀ (CLBO) thin films are grown on Si (100) and (111) substrates using lower index SiO₂ and CaF₂ as buffer layers by pulsed KrF (248 nm) excimer laser ablation of stoichiometric CLBO targets over a temperature range of 425 to 725°C. A CaF₂ buffer layer is grown on Si by laser ablation while SiO₂ is prepared by standard thermal oxidation. From extended x-ray analysis, it is determined that CaF₂ is growth with preferred orientation on Si (100) at temperatures lower than 525°C while on Si (111) substrate, CaF₂ is grown epitaxially over the temperature range; this agrees well with observed reflection high energy electron diffraction patterns. X-ray 2θ-scans indicate that crystalline CLBO are grown on SiO₂/Si and CaF₂/Si (100). Analysis of reflectance spectra from CLBO/SiO₂/Si yields the absorption edge at 182 nm. Surface roughness of the CaF₂ and CLBO/CaF₂/Si film are 19 and 15 nm, respectively. This relatively rough surface caused by the ablation of wide bandgap CaF₂ and CLBO limits the application of CLBO for waveguiding measurement.     

The effects of gamma irradiation on low-pressure chemical-vapor-deposited silicon dioxide metal-oxide-silicon structures

The effects of gamma irradiation on as-deposited, oxygen-annealed, and dual-dielectric gate (undoped polysilicon/oxide) low-pressure chemical-vapor-deposited (LPCVD) silicon dioxide (SiO₂) metal-oxide-silicon (MOS) structures were investigated. As-deposited LPCVD SiO₂ MOS structures exhibit the largest shift in flatband voltage with gamma irradiation. This is most likely due to the large number of bulk oxide traps resulting from the nonstochiometric nature of as-deposited LPCVD SiO₂. Dual-dielectric (undoped polysilicon/annealed LPCVD SiO₂) MOS structures exhibit the smallest shift in flatband voltage and increase in interface state density compared to as-deposited and oxygen-annealed LPCVD SiO₂ MOS structures. The interface state density of dual-dielectric MOS structures increases from 5 × 10¹⁰ eV cm⁻² to 2–3 × 10¹¹ eV cm⁻² after irradiation to a gamma total dose level of 1 Mrads(Si). This result suggests that the recombination of atomic hydrogen atoms with silicon dangling bonds, either along grain boundaries or in crystallites of the undoped polysilicon layer in dual-dielectric (undoped polysilicon/annealed LPCVD SiO₂) MOS structures, probably reduces the number of atomic hydrogen atoms reaching the Si/SiO₂ interface to generate interface states.     

Synchrotron study of the formation of nanoclusters in Al2O3/SiO x /Al2O3/SiO x /…/Si(100) multilayer nanostructures

Al₂O₃/SiO _x /Al₂O₃/SiO _x /…/Si(100) multilayer nanoperiodic structures (MNS) are studied by X-ray absorption near-edge structure spectroscopy (XANES). Experimental XANES spectroscopy spectra are obtained using synchrotron radiation. The formation of Si nanoclusters in the surface layers of the structures during their high-temperature annealing is observed. The structures featured intense size-dependent photoluminescence in the wavelength region near 800 nm. At the same time, it is shown that the formation of aluminum silicates is possible. The inversion effect of the intensity of the XANES spectra during the interaction of synchrotron radiation with MNSs is revealed.     

Roughness and texture correlation of Al films

Titanium films prepared by standard direct-current (DC) magnetron physical vapor deposition (PVD) and ionized metal plasma PVD (I-PVD), with Al (0.5wt.%Cu) films on them, were studied. The surface roughness, reflectivity, and crystalline texture of Ti on SiO₂/Si and Al on TiN/Ti/SiO₂/Si were investigated with the same thickness of Al, TiN, and Ti. The surface roughness of Al films with Ti/TiN underlayers was found to be capable of monitoring Al(111) texture. So, the reflectivity of Al/TiN/Ti film stack can be used as a quick monitor for the electromigration (EM) lifetime.     

Growth and electrical properties of the (Si/Ge)-on-insulator structures formed by ion implantation and subsequent hydrogen-assisted transfer

Systematic features of endotaxial growth of intermediate germanium layers at the bonding interface in the silicon-on-insulator structure consisting of buried SiO₂ layer implanted with Ge⁺ ions are studied in relation to the annealing temperature. On the basis of the results for high-resolution electron microscopy and thermodynamic analysis of the Si/Ge/SiO₂ system it is assumed that the endotaxial growth of the Ge layer occurs via formation of a melt due to enhanced segregation and accumulation of Ge at the Si/SiO₂ interface. Effect of germanium at the bonding interface on the Hall mobility of holes in silicon layers with nanometer-scale thickness is studied. It is found that the structures including the top silicon layer with the thickness 3–20 nm and incorporating germanium feature the hole mobility that exceeds by a factor of 2–3 the hole mobility in corresponding Ge-free silicon-on-insulator structures.     

Structural and electrical characterization of CoNiO monolayer as copper diffusion barrier in integrated circuits

Diffusion barrier properties of CoNiO monolayer, deposited by Langmuir Blodgett (LB) technique, were studied against the diffusion of copper through SiO₂. Cu/CoNiO/SiO₂/Si and Cu/SiO₂/Si test structures were prepared and compared for this purpose. These test structures were annealed at temperatures starting from 100 °C up to 650 °C in vacuum. Samples were characterized using Energy Dispersive X-ray Spectroscopy (EDS), Atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscope (SEM), four probe resistivity measurement, Capacitance-Voltage (C‒V), Current-Voltage (I‒V) characterization techniques. EDS and AFM confirmed the composition and structure of the deposited monolayer. Thermal stability was studied using X-ray diffraction (XRD), Scanning Electron Microscope (SEM) and four probe techniques. Results indicated that structure with barrier was stable up to 600 °C whereas its counterpart could sustain only up to 300 °C. Sheet resistance of Cu/SiO₂/Si structure starts increasing at 300 °C and that of Cu/CoNiO/SiO₂/Si test structure was almost unchanged up to 600 °C in. SEM analysis of samples annealed at different temperatures also confirmed the XRD and four probe results. Biased Thermal Stress (BTS) was applied to the samples and its effect was observed using C‒V analysis. C‒V curves showed that in the presence of CoNiO barrier layer there was no shift in the C‒V curve even after 120 min of BTS while in the absence of barrier there was a significant shift in the C‒V curve even after 30 min of BTS. Leakage current density (j_L) was plotted against the BTS duration under same BTS conditions. It was found that the Cu/CoNiO/SiO₂/Si stack could survive about two times more than the Cu/SiO₂/Si stack.     

Electroluminescence from Au/Si/SiO2/p-Si structure

Si/SiO₂ films have been grown using the two-target alternation magnetron sputtering technique. The thickness of the SiO₂ layer in all the films was 8 nm and that of the Si layer in five types of the films ranged from 4 to 20 nm in steps of 4 nm. Visible electroluminescence (EL) has been observed from the Au/Si/SiO₂/p-Si structures at a forward bias of 5 V or larger. A broad band with one peak 650–660 nm appears in all the EL spectra of the structures. The effects of the thickness of the Si layer in the Si/SiO₂ films and of input electrical power on the EL spectra are studied systematically.     

Electrical characterization of high-k gate dielectrics fabricated using plasma oxidation and post-deposition annealing of a Hf/SiO2/Si structure

High permittivity (high-k) gate dielectrics were fabricated using the plasma oxidation of Hf metal/SiO₂/Si followed by the post-deposition annealing (PDA), which induced a solid-phase reaction between HfO_x and SiO₂. The oxidation time and PDA temperature affected the equivalent oxide thickness (EOT) and the leakage current density of the high-k dielectric films. The interfacial structure of the high-k dielectric film/Si was transformed from HfO_x/SiO₂/Si to HfSi_xO_y/Si after the PDA, which led to a reduction in EOT to 1.15 nm due to a decrease in the thickness of SiO₂. These high-k dielectric film structures were investigated by X-ray photoelectron spectroscopy. The leakage current density of high-k dielectric film was approximately four orders of magnitude lower than that of SiO₂.     

Design and fabrication of a novel high damage threshold HfO2/TiO2/SiO2 multilayer laser mirror

This paper describes a new method to design a laser mirror with high reflectivity, wide reflection bandwidth and high laser-induced damage threshold. The mirror is constructed by three materials of HfO2/TiO2/SiO2 based on electric field and temperature field distribution characteristics of all-dielectric laser high reflector. TiO2/SiO2 stacks act as the high reflector (HR) and broaden the reflection bandwidth, while HfO2/SiO2 stacks are used for increasing the laser resistance. The HfO2/TiO2/SiO2 laser mirror with 34 layers is fabricated by a novel remote plasma sputtering deposition. The damage threshold of zero damage probability for the new mirror is up to 39.6 J/cm2 (1064 nm, 12 ns). The possible laser damage mechanism of the mirror is discussed.     

Confinement of Thermoresponsive Hydrogels in Nanostructured Porous Silicon Dioxide Templates

A thermoresponsive hydrogel, poly(N‐isopropylacrylamide) (poly(NIPAM)), is synthesized in situ within an oxidized porous Si template, and the nanocomposite material is characterized. Infiltration of the hydrogel into the interconnecting nanoscale pores of the porous SiO₂ host is confirmed by scanning electron microscopy. The optical reflectivity spectrum of the nanocomposite hybrid displays Fabry–Pérot fringes characteristic of thin film interference, enabling direct, real‐time observation of the volume phase transition of the confined poly(NIPAM) hydrogel. Reversible optical reflectivity changes are observed to correlate with the temperature‐dependent volume phase transition of the hydrogel, providing a new means of studying nanoscale confinement of responsive hydrogels. The confined hydrogel displays a swelling and shrinking response to changes in temperature that is significantly faster than that of the bulk hydrogel. The porosity and pore size of the SiO₂ template, which are precisely controlled by the electrochemical synthesis parameters, strongly influence the extent and rate of changes in the reflectivity spectrum of the nanocomposite. The observed optical response is ascribed to changes in both the mechanical and the dielectric properties of the nanocomposite.     

Scanning tunneling microscopy investigation of the microtopography of SiO2 and Si surfaces at the Si/SiO2 interface in SIMOX structures

The microtopography of silicon and silicon oxide surfaces in SIMOX structures is investigated by scanning tunneling microscopy. A method of using scanning tunneling microscopy to study Si/SiO₂ interfacial roughness is developed for this purpose. It is shown that the relief of the silicon surface in SIMOX structures is smoother than that of the oxide surface. The observed Si/SiO₂ interfacial roughness is due to oxygen ion implantation in the silicon single crystal. The roughness of the SiO₂ and Si surfaces at the Si/SiO₂ interface is compared for the standard and high-temperature oxidation of the silicon single crystal. Fiz. Tekh. Poluprovodn. 33, 708–711 (June 1999)     

Electron tunneling through a double barrier in a reverse-biased metal-oxide-silicon structure

A number of effects in metal/(tunnel-thin SiO₂)/p ⁺-Si structures associated with electron tunneling from the valence band of bulk Si into a metal have been studied. The tunneling occurs through two successively arranged tunnel-transparent barriers: that of the depleted space charge region in Si and the SiO₂ barrier, with the possible intermediate involvement of a quantum well formed by the Si conduction band. The current-voltage characteristics of the structures are calculated in terms of a simple model that considers these mechanisms for the purely depletion mode, i.e., with the inversion layer charge neglected. The relationship between the structure parameters (p-Si doping level and oxide thickness) and the relative contributions of nonresonant and resonant (via quantum-well levels in the Si conduction band) tunneling to the overall current through an MOS structure is discussed. The conditions most favorable for the observation of resonance effects are formulated.     

Reduction in absorption in quartz/Si,quartz/Si/SiO<Subscript>2</Subscript>, and SiC/Si/SiO<Subscript>2</Subscript> structures on laser treatment

The effect of laser radiation on the optical absorption spectra of the quartz/Si, quartz/Si/SiO₂, and SiC/Si/SiO₂ systems is studied. The effects of modification of the transparency of the structures are clarified.     

Breakdown properties of metal/NIDOS SiO2/silicon structures

NIDOS/SiO₂/silicon structures have been annealed in a nitrogen (N₂) ambient and X-ray photoelectron spectroscopy (XPS) characterization has been performed in order to definitively demonstrate the nitrogen atoms out-diffusion from the nitrogen doped silicon (NIDOS) film towards the buried oxide layer. The nitridation of the SiO₂ layer is related to the competition between nitrogen atoms out-diffusion phenomena on one side into the underlying oxide layer and on the other side into an oxynitride layer grown during annealing. In order to analyse and optimize the corresponding MIS process, different structures such as metal/SiO₂/silicon, metal/(NH₃-‘nitrided’)SiO₂/silicon, metal/(N₂O-nitrided)SiO₂/silicon, metal/poly-Si*/SiO₂/silicon (* indicates deposited from disilane Si₂H₆) and metal/NIDOS/SiO₂/silicon have been realized and compared by capacitance–voltage, current–voltage and ageing under constant current injection experiments. The optimization of the NIDOS-nitridation process gives the highest charge-to-breakdown for the lowest nitridation level or even for no intentional nitridation. The dielectric breakdown improvement should therefore not be related to the nitridation phenomena alone but also to the intrinsic properties of the polysilicon layer itself.     

Improvement in Performance of Carbon Nanotube Field-Effect Transistors on Patterned SiO<Subscript>2</Subscript>/Si Substrates

Horizontally aligned single-walled carbon nanotubes (SWNTs) were fabricated on patterned SiO₂/Si substrates with groove-and-terrace structures, which were obtained using electron-beam lithography and reactive ion etching. Scanning electron microscopy observation revealed that SWNTs were aligned in the direction parallel to the groove-and-terrace structures and were preferentially grown along the edges of terraces. Using aligned SWNTs as multichannels, carbon nanotube field-effect transistors (CNTFETs) were fabricated on the patterned SiO₂/Si substrates. This method will be promising to control the direction of SWNTs on SiO₂/Si substrates for fabrication of high-performance CNTFETs with high current outputs.     

Study of ultrathin vanadium nitride as diffusion barrier for copper interconnect

Ultrathin Vanadium nitride (VN) thin film with thickness around 10 nm was studied as diffusion barrier between copper and SiO₂ or Si substrate. The VN film was prepared by reactive ion beam sputtering. X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy and current-voltage (I-V) technique were applied to characterize the diffusion barrier properties for VN in Cu/VN/Si and Cu/VN/SiO₂ structures. The as-deposited VN film was amorphous and could be thermal stable up to 800 °C annealing. Multiple results show that the ultrathin VN film has good diffusion barrier properties for copper.     

Transformation of interface states in silicon-on-insulator structures under annealing in hydrogen atmosphere

Changes induced by annealing the spectrum of states on a Si/SiO₂ interface obtained by direct bonding and on a Si(substrate)/〈thermal SiO₂〉 interface in silicon-on-insulator (SOI) structures were investigated by charge-related deep-level transient spectroscopy. The structures were formed by bonding silicon wafers and slicing one of the wafers along a plane weakened by hydrogen implantation. The SOI structures were annealed at 430°C for 15 min in hydrogen, which corresponded to the conventional mode of passivation of the Si/SiO₂-interface states. The passivation of interface states by hydrogen was shown to take place for the Si/〈thermal SiO₂〉 interface, as a result of which the density of traps substantially decreased, and the continuous spectrum of states was replaced by a band of states in the energy range E _c=0.1–0.35 eV within the entire band. For the traps on the bonded Si/SiO₂ interface, the transformation of the centers occurs; namely, a shift of the energy-state band is observed from E _c=0.17–0.36 to 0.08–0.22 eV. The trapping cross section decreases by about an order of magnitude, and the density of traps observed increases slightly.     

Silicon-on-insulator structures with a nitrogenated buried SiO<Subscript>2</Subscript> layer: Preparation and properties

Electrical properties of silicon-on-insulator (SOI) structures with buried SiO₂ layer implanted with nitrogen ions are studied in relation to the dose and energy of N⁺ ions. It is shown that implantation of nitrogen ions with doses >3 × 10¹⁵ cm⁻² and an energy of 40 keV brings about a decrease in the fixed positive charge in the oxide and a decrease in the density of surface stares by a factor of 2. An enhancement of the effect can be attained by lowering the energy of nitrogen ions. The obtained results are accounted for by interaction of nitrogen atoms with excess silicon atoms near the Si/SiO₂ interface; by removal of Si-Si bonds, which are traps of positive charges; and by saturation of dangling bonds at the bonding interface of the SOI structure.     

Electroforming of Si/SiO2 /W Structures with an Exposed Nanometer-Thick SiO2Layer

An experiment is presented concerning the electroforming of exposed-insulator Si/SiO₂/W structures, i.e., the current-induced formation of a carbonaceous conducting medium on the exposed area of the insulator and the self-organization of a nanometer-sized nonconducting gap in the medium. It is shown that the electroforming of Si/SiO₂/W structures greatly differs from that of Al/Al₂O₃/W structures. This difference is mainly due to the high level of the spreading resistance of the Si electrode and the high initial conductivity on the exposed area. The current–voltage characteristics of the electroformed structures are examined. It is argued that the structures could serve as a basis for memory arrays with a high density of data.