Film thickness effect on the properties of interconnection between YBCO and Si for superconductor and semiconductor integration

YBa₂Cu₃O_7−δ (YBCO) films were grown on Si using a buffer layer of yttria-stabilized zirconia (YSZ) by pulsed laser deposition. Interconnections between the YBCO film and the Si substrate using Au have been fabricated by lithography. The YBCO films showed zero-resistance critical temperatures in the range of 80–85 K and were found to be grown in the c–axis orientation. The structural properties of the films of various thicknesses were analyzed by XRD, SEM and Raman spectroscopy. Because of very different thermal expansion coefficients of Si and YBCO, there is a tendency to crack formation for films. The crack formation interrupts the current flow and increases the normal state resistance of the films. The effect of cracks on the contact resistances between YBCO films of various thicknesses and Si substrates through Au interconnections is reported.     

A study of Pd2Si films on silicon using Auger electron spectroscopy

Auger electron spectroscopy (AES) combined with in situ sputter etching is used to quantitatively evaluate the growth kinetics of thin films of Pd₂Si on 〈111〉 Si substrates. The growth of Pd₂Si is found to be diffusion limited and to be characterized by an activation energy of 1·4±0·2 eV in close agreement with previous results obtained on thicker Pd₂Si films. AES is also used to uniquely identify the silicide phase Pd₂Si. Measurement of the expansion occurring during Pd₂Si film formation indicates the Pd₂Si film is about 20% thicker than expected. Correlation of the AES results with careful electrical measurements of the Pd₂Si/Si interface reveals that the contact barrier energy φ_Bn decreases slightly with increasing thickness of the Pd₂Si film. A 3% decrease in φ_Bn was observed for complete conversion of 500 Å of Pd to Pd₂Si.     

Growth and characterization of superconducting V3Si on Si and Al2O3 by molecular beam epitaxial techniques

A study of the growth parameters governing the nucleation of metastable superconducting A15 V₃Si on Si and A1₂O₃ is presented. Nominally, 500Å films of V_1-xSi_x were produced through codeposition of V and Si onto heated (111) Si and (1102) A1₂O₃ substrates. Samples were prepared in a custom-built ultrahigh vacuum (UHV) chamber containing dual e-beam evaporation sources and a high temperature substrate heater. V and Si fluxes were adjusted to result in the desired average film composition. V_0.75Si_0.25 films prepared at temperatures in excess of 550° C on Si show significant reaction with the substrate and are nonsuperconducting while similar films grown on A1₂O₃ exhibit superconducting transition temperatures(@#@ T_c @#@) approaching bulk values for V₃Si (16.6-17.1 K). Codeposition at temperatures between 350 and 550° C results in superconducting films on Si substrates while growth at lower temperatures results in nonsuperconducting films. Lowering the growth temperature to 400° C has been shown throughex situ transmission electron microscopy (TEM) and Auger compositional profiling to minimize the reaction with the Si substrate while still activating the surface migration processes needed to nucleate A15 V₃Si. Variation of film composition aboutx = 0.25 is shown to result in nonsuperconducting films for highx and superconducting films with T_c approaching the bulk V value (5.4 K) for lowx. Finally, lowering the V_0.75Si_0.25 deposition rate is shown to raise T_c.     

CO2 detection with CNx thin films deposited on porous silicon

In recent years, porous silicon (PSi) has attracted a great deal of attention for sensing applications. However, the high reactivity of PSi surfaces causes serious problems of stability. In this work, we developed new thin films that can serve as stabilizer of PSi for CO₂ gas sensors development. PSi surface was coated with carbon nitride (CN_x) film which is one of the most important interfering to stabilize the PSi layer. CN_x film was deposited by pulsed laser ablation. The effect of CO₂ gas on the sensor response was investigated for different polarization voltages. The electrical properties of (Al/CN_x/PSi/Si) structure were modified in the presence of the gas. The device shows a high sensitivity against CO₂ gas. Furthermore, the current variation of the sensor as a function of time has been investigated. The results show that the Al/CN_x/PSi/Si structure becomes stable after the first two weeks.     

The influence of dielectric layers on the CW-Laser Annealing of polysilicon

Polysilicon films were deposited on the following four dielectric layers: 0.10 μm Si0₂, 0.10 μm Si₃N₄, 1.0 μm Si0₂ and a dual layer of 0.10 μm Si₃N₄ over 1.0 μm Si0₂. The films were doped with As or P and then annealed with a scanning CW-argon laser. The resulting sheet resistance at a given laser power depends upon both the dielectric material and its thickness. The threshold power, that causes complete melting of the polysilicon, is inversely proportional to the thermal resistance of the underlying dielectric layers. With certain laser parameters and substrate film combinations, excessive thermal stresses cause cracking of the polysilicon films. The laser annealing process is also influenced by the net optical reflectance of the polysilicon/ dielectric multilayer films.     

Control of the Ti diffusion in Pt/Ti bottom electrodes for the fabrication of PZT thin film transducers

For the achievement of microactuators based on piezoelectric thin films, a Pt/Ti/Si bottom electrode is widely used. This study presents the experimental results for Ti out-diffusion in Pt and Si for both sputtered Pt/Ti and Pt/TiO_x electrodes. These results have been compared before and after a rapid thermal annealing (RTA). The diffusion has been characterized by secondary ion mass spectroscopy (SIMS) analysis using Cs⁺ as a primary ion source. The Pt orientation has been observed by XRD measurements. Ti thin films (20 nm) have been sputtered in pure Ar whereas TiO_x films have been obtained by reactive sputtering in a mixture of Ar/O₂ (90/10). Finally, the Pt (100 nm) has been sputtered without vacuum breaking. After RTA (400°C, 30 s, in N₂), the Pt film exhibited a (1 1 1) orientation for both Ti and TiO_x adhesion films. The roughness of the Pt film measured by AFM with TiO_x underlayer was 80% less than that of the Pt/Ti bi-layer. The TiO_x film, as shown by SIMS analysis, has drastically reduced the diffusion of Ti in both Pt and Si. This phenomenon is accompanied by a very low Pt roughness. These results are analyzed in terms of diffusion and regrowth mechanisms inside the Pt film.     

Growth of AgGaTe2 Layers by a Closed-Space Sublimation Method

AgGaTe₂ layers were deposited on Si substrates by the closed-space sublimation method. Multiple samples were deposited with various source temperatures and holding times, and constant temperature differential. Variation of the source temperature was used primarily to improve the stoichiometry of the film. Deposited films were evaluated by the θ–2θ method of x-ray diffraction (XRD) and transmission electron microscopy. These results confirmed that the deposited films were stoichiometric (after optimizing the above parameters). From XRD, it was also clear that films deposited on Si (111) have strong preference for (112) orientation.     

Adhesion study of tetra methyl cyclo tetra siloxanes (TMCTS) and tri methyl silane (3MS)-based low-k films

Chemical vapor deposited (CVD) low-k films using tri methyl silane (3MS) precursors and tetra methyl cyclo tetra siloxanes (TMCTS) precursors were studied. Films were deposited by means of four processes, namely, O₂, O₂ + He process and CO₂, CO₂ + O₂ process for 3MS and TMCTS precursors, respectively. Interfacial adhesion energy (G_c), of low-k/Si samples, as measured by a 4-point bending test displayed a linear relationship with film hardness and modulus. Fractography studies indicated two possible failure modes with the primary interface of delamination being either at low-k/Si or Si/epoxy interface. In the former, once delamination initiated at the low-k/Si interface, secondary delamination at the Si/epoxy and epoxy/low-k interfaces was also observed. Films with low hardness (<5 GPa) displayed a low G_c (<10 J/m²) with an adhesive separation of Si/epoxy, epoxy/low-k, and low-k/Si interfaces. Whereas, films of high hardness (>5 GPa) displayed interfacial energies in excess of 10 J/m² with separation of Si/epoxy and epoxy/low-k interfaces, thus indicating excellent adhesion between the Si and low-k films. Films with high hardness have less carbon in the system causing it to be more “silicon dioxide” like and exhibiting better adhesion with the Si substrate.     

Effect of the film thickness on the crystal structure and ferroelectric properties of (Hf0.5Zr0.5)O2 thin films deposited on various substrates

In this work, the effect of the film thickness on the crystal structure and ferroelectric properties of (Hf_0.5Zr_0.5)O₂ thin films was investigated. The thin films were deposited on (111) Pt-coated SiO₂, Si, and CaF₂ substrates with thermal expansion coefficients of 0.47, 4.5, and 22×10⁻⁶/°C, respectively. From the X-ray diffraction measurements, it was found that the (Hf_0.5Zr_0.5)O₂ thin films deposited on the SiO₂ and CaF₂ substrates experienced in-plane tensile and compressive strains, respectively, in comparison with the films deposited on the Si substrates. For films deposited on all three substrates, the volume fraction of the monoclinic phase increased with increasing film thickness, with the SiO₂ substrate having the lowest monoclinic phase volume fraction at all film thicknesses tested. The grain size of the films, which is an important factor for the formation of the ferroelectric phase, remained almost constant at about 10 nm in diameter regardless of the film thickness and type of substrate utilized. Ferroelectricity was observed for the 17 nm-thick films deposited on SiO₂ and Si substrates, and the maximum remanent polarization (P_r) value of 9.3 µC/cm² was obtained for films deposited on the SiO₂ substrate. In contrast, ferroelectricity with P_r=4.4 µC/cm² was observed only for film on SiO₂ substrate in case of 55 nm-thick films. These results suggest that the films under in-plane tensile strain results in the larger ferroelectricity for 17 nm-thick films and have a ferroelectricity up to 55 nm-thick films.     

Electrical Properties of (110)-Oriented Nondoped Mg2Si Films with p-Type Conduction Prepared by RF Magnetron Sputtering Method

Magnesium silicide (Mg₂Si) thick films with (110) orientation were fabricated on (001) sapphire substrate using radiofrequency magnetron sputtering. Stoichiometric Mg₂Si films with composition Si/(Mg + Si) = 0.33 were achieved over a range of vacuum from 10 mTorr to 140 mTorr and 300°C. On postannealing the film at 500°C, the out-of-plane lattice parameter shifted to lower values and the electrical conductivity increased by two orders of magnitude. A room-temperature Seebeck coefficient of 517 μV K^?1 was observed and found to decrease with increasing temperature; the Seebeck coefficient remained at a constant positive value of 212 μV K^?1 at 500°C. This can be related to the possibility of p-type conduction in this temperature range.     

Origination of misfit dislocations at the surface during the growth of GeSi/Si(001) films by low-temperature (300–400°C) molecular-beam epitaxy

The method of the molecular-beam epitaxy, at comparatively low temperatures (300–400°C), was used to grow Ge_xSi_{1 ? x} /Si(001) films with a constant composition (x = 0.19–0.32) across a film and as well as two-layer heterostructures with the Ge content at the upper layer no lower than 0.41. Using transmission electron microscopy, it is shown that the main cause of an increase in the density of threading dislocations with increasing Ge fraction in the plastically relaxed films is the origination of the dislocation half-loops at the film surface; in turn, these dislocation half-loops are generated owing to the formation of a three-dimensional profile at the surface of the growing or annealed film.     

Photoluminescent and electronic properties of nanocrystalline silicon doped with gold

Doping nanocrystalline silicon (nc-Si) films grown by laser ablation with gold leads to a considerable suppression of the nonradiative recombination of the charge carriers and excitons, an increase in the intensity and stability of the visible photoluminescence, and enhancement of the low-energy (1.5–1.6 eV) photoluminescence band. In Au-doped samples, the magnitude of the photovoltage and the rate of electron capture by traps in the film are reduced, and the density of boundary electron states and the concentration of deep electron traps at the single-crystal silicon (c-Si) substrate are decreased as well. The observed effect of doping on the photoluminescent and electronic properties of nc-Si films and nc-Si/c-Si structures is caused by the passivation of dangling Si bonds with Au and by the further oxidation of silicon at the surface of nanocrystals, which results in the formation of high-barrier SiO₂ layers.     

Growth of diamond films on si(100) with and without boron nitride buffer layer

Diamond films were grown on Si(100) and boron nitride deposited Si(100) substrates using hot filament chemical vapor deposition (HFCVD) technique. Microstructure and morphology of diamond films have been investigated systematically as a function of CH₄ and H₂ ratio and the ambient pressure. The deposited films were characterized by employing techniques such as scanning electron microscopy (SEM) and laser Raman spectroscopy. The average size and growth rate of diamond particles were found to increase with the CH₄ to H₂ ratio and decrease with the ambient pressure. Maximum growth rate of synthetic diamond deposited on Si(100) was found to be &#x223C;3.5 &#x00B5;m/hr for the film deposited at 20 Torr with CH₄:H₂ &#x223C; 1.5:100 (substrate temperature &#x223C;850&#x00B0;C). In most of these depositions, the morphology of the diamond crystals was cubic with significant secondary nucleation at higher methane concentrations and ambient pressure. The diamond film deposited on Si(100) with BN buffer layer shows an improvement in growth rate and the coverage, and the secondary nucleation was found to be substantially reduced, resulting in relatively smooth morphology. MicroRaman investigations show less amorphous graphite formation and better structural quality of diamond film than the one deposited without the BN buffer layer. On leave from Department of Physics University of Poona, Pune-411007 INDIA     

Fabrication and characterization of PZT-PMWSN thin film using pulsed laser deposition

Thin PZT films are being developed for use in sensor and actuator application in micromechanical systems. For the use as sensor and actuator, it is desirable to combine high mechanical quality factor (Q_m) with high piezoelectric constant (d) and high electro-mechanical coupling factor (k_p). We fabricated PbZr_xTi_1−xO₃-Pb(Mn,W,Sb,Nb)O₃ (PZT-PMWSN) targets with variations in the Zr/Ti ratio. The dielectric and piezoelectric properties of PZT-PMWSN ceramics were investigated as a function of Zr/Ti ratio. At the Zr/Ti ratio of 0.52/0.48, the electrical coupling factor (k_p) and the mechanical quality factor (Q_m) showed a maximum value of 0.56 and 2344, respectively. The PZT-PMWSN thin film has been prepared on a Pt/Ti/SiO₂/Si substrate using pulsed laser deposition method. The structural property was characterized with XRD, SEM and ferroelectric hysteresis loop measurement were used to characterize the electrical properties of PZT-PMWSN thin film.     

Degradation pattern of thin HfO2 films on Si(100) under ultrahigh-vacuum annealing: An investigation by x-ray photoelectron spectroscopy and low-energy ion scattering

The evolution of microstructure and phase structure of ultrathin HfO₂ films on Si(100) under ultrahigh-vacuum annealing is investigated in situ by x-ray photoelectron spectroscopy (XPS) and low-energy ion scattering (LEIS). The onset temperature of degradation is found to depend on film thickness. It is established that, for HfO₂ (4 nm)/SiO₂ (1 nm)/Si(100) specimens, 5-min annealing at about 900°C causes silicon (LEIS evidence) to appear on the surface, the silicon being uncombined with oxygen or the metal (XPS evidence). A longer annealing at the same temperature produces HfSi_x; annealing at 950°C converts the entire HfO₂ film into polycrystalline silicide whose grains are partly oriented as the Si substrate. With respect to annealing in a low-oxygen environment, the experimental results support a model whereby the degradation of an ultrathin HfO₂ film starts with the formation of nanopores by clustering of oxygen vacancies, whose density increases sharply due to partial desorption of oxygen; HfO_x with x < 2 then forms in the vicinity of vacancy clusters. It is concluded that the formation of hafnium silicide, the end product of HfO₂ degradation, starts in Si surface areas at the bottom of nanopores.     

Electrical properties of low-dielectric-constant films prepared by PECVD in O2/CH4/HMDSO

Low-dielectric constant (low-k) films have been prepared by plasma-enhanced chemical vapor deposition (PECVD) from hexamethyldisiloxane (HMDSO) mixed with oxygen or methane. The films are analyzed by ellipsometry, infrared absorption spectroscopy while their electrical properties are deduced from C–V, I–V and R–f measurements performed on Al/insulator/Si structures. For an oxygen and methane fraction equal to 50% and 22%, respectively, the dielectric constant and losses are decreased compared with those of the film prepared in a pure HMDSO plasma. The effect of adding 22% of CH₄ in HMDSO plasma increases the Si–CH₃ bonds containing in the polymer film and as the constant of methyl groups in the film increased the dielectric constant of the film decreases. For this film, the dielectric constant is 2.8, the dielectric losses at 1 kHz are equal to 2×10⁻³, the leakage current density measured for an electric field of 1 MV/cm is 3×10⁻⁹ A/cm² and the breakdown field is close to 5 MV/cm.     

Local crystal structural modifications in pulsed laser deposited high-k dielectric thin films on silicon and germanium

The thin film growth conditions are correlated with the local structures formed in Hf_xZr_1−xO₂ (x=0.0–1.0) high-k dielectric thin films on Si and Ge substrates during deposition. Pulsed laser deposition (PLD) technique has been used in the synthesis of the thin films with systematic variations of substrate temperature, Zr content of the targets and substrate selection. The local structural information acquired from extended X-ray absorption spectroscopy (EXAFS) is correlated with the thin film growth conditions. The response of the local structure around Hf and Zr atoms to growth parameters was investigated by EXAFS experiments performed at the National Synchrotron Light Source of Brookhaven National Laboratory. The competing crystal phases of oxides of Hf were identified and the intricate relation between the stabilized phase and the parameters as: the substrate temperature; Hf to Zr ratio; have been revealed. Specifically, HfO₂ thin films on Si(1 0 0) exhibit a tetragonal to monoclinic phase transformation upon increase in the substrate temperature during deposition whereas, HfO₂ PLD films on Ge(1 0 0) substrates remain in tetragonal symmetry regardless of the substrate temperature.     

Effect of titanium dioxide (TiO2) seed layer on the phase composition of 0.7Pb (Mg1/3Nb2/3)O3-0.3PbTiO3 film prepared by pulsed laser deposition

Lead-magnesium niobate-lead titanate (PMN-PT) thin films with and without the TiO₂ seed layer were deposited on Pt/Ti/SiO₂/Si substrates through pulsed laser deposition. The study aimed to characterize the effect of the TiO₂ seed layer on the phase composition and properties of PMN-PT film. Without the TiO₂ seed layer, the pure perovskite phase could be obtained in the thinner PMN-PT film while with the TiO₂ seed layer, the pure perovskite phase was formed in the thicker PMN-PT film. The ferroelectric properties of PMN-PT films with the TiO₂ seed layer were exhibited. As a result, the maximum amount of remnant polarization reached the amount of 32 μC/cm² for the PMN-PT thin film with the TiO₂ seed layer.     

Effect of chemical treatment on photoluminescence spectra of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> layers with built-in Si nanocrystals

The effect of chemical treatment in saturated vapors of ammonia and acetone on the spectral composition and intensity of photoluminescence in porous SiO _x films containing Si nanocrystals (nc-Si) is studied. The porosity of the SiO _x films is provided by oblique vacuum deposition of thermally evaporated silicon or silicon monoxide on polished silicon substrates. The kinetics of adsorption of the vapors is monitored by variations in the frequency of a quartz oscillator on which the films to be studied are deposited. As a result of chemical treatment followed by high-temperature annealing of the SiO _x films at the temperature 950°C, a new band, absent from the as-prepared films, appears in the photoluminescence spectrum at shorter wavelengths. The peak position and intensity of the band depend, correspondingly, on the composition of the film and on the time duration of the treatment. It is found that the new photoluminescence band is quenched upon exposure to laser radiation at the wavelength 488 nm. The quenching is more pronounced at the band peak. The possibility of controlling the characteristics of photoluminescence of the porous structures by chemical treatment is shown.     

Study of the I–V characteristics of nanostructured Pd films on a Si substrate after vacuum annealing

The I–V characteristics of nanostructured Pd films on a Si substrate are investigated. The nanostructures (nanoislands) are formed by the vacuum annealing of continuous ultrathin Pd films sputtered onto a substrate. The shape of the I–V characteristics of the investigated Si substrate-Pd film system is shown to be heavily dependent on the degree of film nanostructuring. The surface morphology of the films is studied using scanning electron microscopy.