A single quality factor for the deposition process of reactively sputtered thin a-C:H:N films

The carbon bonding modifications, produced by the different deposition conditions in nitrogenated a-C:H films (a-C:H:N) prepared by reactive-sputtering of a graphite target, are investigated by quantitatively analysing the evolution of the D- and G-bands in the Raman spectra. The film C content is evaluated and shown to depend on the many variables involved into the a-C:H:N film growth through a single quality factor, dimensionless combination of the dimensional process-variables. The film structural changes observed are understood in terms of the decreasing sp³:sp² ratio achieved with the diminishing film C content. The quality factor introduced, able to indicate how the variable-configuration can eventually change without significantly affecting the result in terms of C content and resulting film properties, represents a simple scaling law for the a-C:H:N film deposition, whose validity is preliminarily demonstrated for variations of rf power, total pressure and reactive-gas flow-rate in the ranges from 180 to 300 W, from 20 to 38 mTorr and from 5 to 27 sccm, respectively. A very simple model is therewith proposed accounting for the particular variable combination ultimately effective in determining the final issue of the deposition process. The generality of the proposed method is finally demonstrated.     

Effect of barium contamination on gate oxide integrity in high-k dram

Barium impact on the gate oxide breakdown was studied using E-ramp and constant current stress (CCS) charge to breakdown. Wafers were contaminated with Ba after a 7.5 nm gate oxide growth and 300 nm poly-silicon deposition. The measurements were done on capacitors having areas of 0.1, 1, 4, and 16 mm². Up to a contamination level of 4×10¹⁴ atoms/cm² no degradation in oxide integrity was observed either by E-ramp or CCS. Time of flight (ToF)-SIMS measurement of Ba diffusion profile at 800 °C shows a diffusion of Ba over distances of some tens of nanometers, thus Ba does not reach the gate oxide region. The effect on the gate oxide breakdown can be correlated with the slow diffusion of Ba in poly-Si. Therefore, no major concern of yield and reliability due to Ba contamination is seen for the integration of Ba containing dielectrics into memories.     

Non-linear dielectric properties of metal-amorphous-tantalum pentoxide-metal structures

High dielectric constant (high K) materials are becoming important due to their potential applications in embedded dynamic random access memory and radio frequency capacitors. Among high K materials that can replace silicon dioxide for these applications, tantalum pentoxide (Ta₂O₅) is the candidate of choice since it is already available in the semiconductor industry. One of the advantages of Ta₂O₅ is that it can be deposited at low temperature (<500 °C). This makes it suitable for the fabrication of metal-oxide-metal structures that require low thermal budget. The use of Ta₂O₅ for silicon device applications requires good electrical performances in terms of leakage current and capacitance dispersion. The roughness of the bottom electrode, as determined by atomic force microscopy measurements, is found to impact the leakage current. Capacitance voltage measurements exhibit a non-linear variation with respect to the applied bias. We suggest a model that well explains the observed capacitance variations. This model is based on the relaxation of the free carriers and the Kerr effect     

Influence of Nd2O3 on the thermal and dielectric properties of Pb-based lead borosilicate glasses

Neodymium has been employed in various Pb-based glass compositions containing ZnO, MgO, CaO, and BaO, to obtain low melting glasses for use as dielectric materials. Thermal analysis (DTA, TMA) was applied for both qualitative and quantitative thermal properties (T_g, CTE) of the glasses, and impedance analysis of the dielectric properties (ε_r, tanδ) was performed. It was found that with increasing Nd₂O₃, the glass transition temperature (T_g), the dilatometer softening point (T_d) and dielectric constant (ε_r) became higher. Also, a peak shift suggesting neodymium oxide introduced structural changes was found with IR spectra and XPS. These seem to support the conclusion that the addition of increased amounts of Nd₂O₃ can lead to a more rigid glass structure.     

Deviations from square-root distributions of electronic states in hydrogenated amorphous silicon and their impact upon the resultant optical properties

We study the distributions of conduction band and valence band electronic states associated with hydrogenated amorphous silicon. We find that there are substantial deviations from square-root distributions, particularly deep within the bands and within the gap region. The impact of these deviations is assessed through a determination of the spectral dependence of both the joint density of states function and the imaginary part of the dielectric function. These deviations are found to have a considerable effect upon the determination of the corresponding Tauc optical gap, the optical gap obtained for the case of hydrogenated amorphous silicon being 220 meV lower than the energy difference between the valence band and conduction band band edges. We suggest that the standard interpretation for the Tauc optical gap, as the energy difference between these band edges, should be reconsidered in light of these results.     

Dielectric properties and structural features of barium-iron phosphate glasses

The dielectric constant of barium-iron phosphate glasses with the general composition (40−x)BaO · xFe₂O₃ · (60−x)P₂O₅ has been investigated at two fixed frequencies (100 kHz and 9.0 GHz). The dielectric constant measured using microwave technique, and the ratio O/P of these glasses increase with increasing Fe₂O₃ content. The structure and valence states of the iron ions in these glasses were investigated using Mössbauer spectroscopy, infrared spectroscopy and differential thermal analysis. Both Fe(II) and Fe(III) ions present in these glasses in octahedral coordination act as permanent dipoles, and the increase of the iron concentration increase these permanent dipoles, contributing to the dielectric constant.     

Influence of annealing on the physical properties of filtered vacuum arc deposited tin oxide thin films

Tin oxide (SnO₂) thin films were deposited on UV fused silica (UVFS) substrates using filtered vacuum arc deposition (FVAD). During deposition, the substrates were at room temperature (RT). As-deposited films were annealed at 400 and 600 °C in Ar for 30 min. The film structure, composition, and surface morphology were determined as function of the annealing temperature using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The XRD patterns of the SnO₂ thin films deposited on substrates at RT indicated that the films were amorphous, however, after the annealing the film structure became polycrystalline. The grain size of the annealed films, obtained from the XRD analysis, increased with the annealing temperature, and it was in the range 8-34 nm. The AFM analysis of the surface revealed an increase in the film surface average grain size from 15 nm to 46 nm, and the surface roughness from 0.2 to 1.8 nm, as function of the annealing temperature. The average optical transmission of the films in the visible spectrum was >80%, and increased by the annealing ∼10%. The films’ optical constants in the 250-989 nm wavelength range were determined by variable angle spectroscopic ellipsometry (VASE). The refractive indexes of as-deposited and annealed films were in the range 1.83-2.23 and 1.85-2.3, respectively. The extinction coefficients, k(λ), of as-deposited and annealed films were in the range same range ∼0-0.5. The optical energy band gap (E_g), as determined by the dependence of the absorption coefficient on the photon energy at short wavelengths, increased with the annealing temperature from 3.90 to 4.35 eV. The lowest electrical resistivity of the as-deposited tin oxide films was 7.8 × 10⁻³ Ω cm, however, film annealing resulted in highly resistive films.     

Dielectric properties of Zr-Sn-Ti-O thin films prepared by pulsed laser deposition

Zr_0.26Sn_0.23Ti_0.51O₂ (ZSTO) films with a dielectric constant of about 40 have been prepared directly on silicon substrates by pulsed laser deposition at 600 °C. TEM observation showed that the as-deposited films are amorphous. Differential thermal analysis showed that the ZSTO films crystallize at about 620 °C. Capacitance-voltage (C-V) characteristics of metal-oxide-semiconductor (MOS) composed of Pt/ZSTO/Si prepared at different deposition temperature have been measured. The EOT of the MOS structures with the same ZSTO physical thickness increased slightly when the deposition temperature increased. The EOT is about 4.2 nm for the 40 nm ZSTO deposited at 600 °C. The leakage current characteristics of ZSTO films for the as deposited, post-annealed in oxygen ambient and post-annealed in nitrogen ambient by rapid thermal annealing have been studied comparatively. The films post-annealed in nitrogen ambient have the lowest leakage current and the as-deposited films have the largest leakage current characteristics. It is proposed that amorphous Zr-Sn-Ti oxide stabilized at 600 °C is a potential dielectric material for dynamic random access memory and high k dielectric gate applications.     

Er-doped hydrogenated amorphous silicon: structural and optical properties

The effect of erbium-doping on the structural and optical properties of hydrogenated amorphous silicon (a-Si:H) is investigated. Optical absorption and Raman spectra indicate that erbium doping introduces defect states, and that above a concentration of 0.27 at.%, induces strong structural disorder. The photoluminescence measurements show that erbium doping introduces non-radiative decay paths for carriers in a-Si:H, leading to decrease in both the Er³⁺ and intrinsic a-Si:H luminescence intensity when the Er concentration is increased to more than 0.04 at.%. The results are compared to that of Er-doped crystalline Si, and the possible excitation mechanisms of Er in a-Si:H are discussed.     

Influence of thermal annealing on the composition and structural parameters of DC magnetron sputtered titanium dioxide thin films

Titanium dioxide films have been deposited using DC magnetron sputtering technique. Films were deposited onto RCA cleaned p‐silicon substrates at the ambient temperature at an oxygen partial pressure of 7 × 10^–5 mbar and sputtering pressure of 1 × 10 ^–3 mbar. The deposited films were annealed in the temperature range 673–873 K. The structure and composition of the films were confirmed using X‐ray diffraction and Auger electron spectroscopy. The structure of the films deposited at the ambient was found to be amorphous and the films annealed at 673 K and above were crystalline with anatase structure. The lattice constants, grain size, microstrain and the dislocation density of the film are calculated and correlated with annealing temperature.     

Structure and properties of lithium thio-boro-germanate glasses

Structural studies of the ternary xLi₂S + (1 − x)[0.5B₂S₃ + 0.5GeS₂] glasses using IR, Raman, and ¹¹B NMR show that the Li₂S is not shared proportionately between the GeS₂ and B₂S₃ sub-networks of the glass. The IR spectra indicate that the B₂S₃ glass network is under-doped in comparison to the corresponding composition in the xLi₂S + (1 − x)B₂S₃ binary system. Additionally, the Raman spectra show that the GeS₂ glass network is over-modified. Surprisingly, however, the ¹¹Boron static NMR gives evidence that ∼80% of the boron atoms are in tetrahedral coordinated. A super macro tetrahedron, B₁₀S₁₈⁻⁶ is proposed as one of the structures in these glasses in which can account for the apparent low fraction of Li₂S present in the B₂S₃ sub-network while at the same time enabling the high fraction of tetrahedral borons in the glass.     

Experimental observations of the thermal stability of high-k gate dielectric materials on silicon

High-k dielectric materials including zirconium oxide and hafnium oxide produced by atomic layer deposition have been evaluated for thermal stability. As-deposited samples have been compared with rapid thermal annealed samples over a range of source/drain dopant activation temperatures consistent with conventional complimentary metal oxide semiconductor polysilicon gate processes. Results of this initial investigation are presented utilizing analyses derived from X-ray diffraction (XRD), X-ray reflectometry (XRR), medium energy ion spectroscopy, high resolution transmission electron microscopy (HRTEM), tunneling atomic force microscopy, scanning electron microscopy, Auger electron spectroscopy and secondary ion mass spectroscopy. Changes in interface and surface roughness, percent crystallinity and phase identification for each material as a function of anneal temperature have been determined by XRD, XRR and HRTEM. Finally, high-k wet etch issues are presented relative to subsequent titanium silicide blanket film resistivity values.     

Characterization of ultrathin gate dielectrics by grazing X-ray reflectance and VUV spectroscopic ellipsometry on the same instrument

Precise characterization of high k gate dielectrics becomes a challenging task due to the very thin thickness (<3-4 nm), which will be needed in the next generation of integrated circuits. Conventional techniques such as spectroscopic ellipsometry (SE) in the UV-visible range becomes difficult to use alone because of the great correlation between thickness and optical indices. To overcome this problem a new versatile instrument integrating SE in the VUV spectral range and grazing X-ray reflectance (GXR) has been developed recently by SOPRA. Both kinds of measurements can be made at the same location on the sample and at the same time. The analysis is made with a common optical model adjusting the layer thickness and the surface and interface roughness on the GXR data and the optical indices and other parameters like surface or interface roughness or inter-diffusion on the SE data. The paper describes some experimental results obtained with this system on ZrO₂, HfO₂ and La₂O₃ films. Results are correlated with other experimental techniques in some cases.     

Band offsets of high dielectric constant gate oxides on silicon

High dielectric constant oxides will soon be needed to replace silicon dioxide as the gate dielectric material in complementary metal oxide semiconductor technology. The oxides must have band offsets with silicon of over 1 eV for both electrons and holes in order to have low leakage currents. We have calculated the band offsets for many candidate oxides using the method of charge neutrality levels. Ta₂O₅ and SrTiO₃ have small or vanishing conduction band offsets on Si. La₂O₃, Y₂O₃, Gd₂O₃, ZrO₂, HfO₂, Al₂O₃ and silicates like ZrSiO₄ have offsets over 1.4 eV for both electrons and holes, making them better gate dielectrics. Zirconates are better than titanates as they have wider gaps.     

Study of CaO-WO3-P2O5 glass system by dielectric properties, IR spectra and differential thermal analysis

Dielectric constant ε^′, loss tanδ and ac conductivity σ_ac of 40CaO-xWO₃-(60−x)P₂O₅ (with 0?x?15) glasses are studied over a range of frequencies and temperature. The dielectric breakdown strength of these glasses is also determined at room temperature. The values of dielectric parameters, viz., ε^′, tanδ and σ_ac of CaO-P₂O₅ glasses are found to decrease with the introduction of WO₃ up to 3 mol% and increase with further increase in the concentration of WO₃; the probable reasons for such an increase are identified and explained with the aid of IR spectra and differential thermal analysis of these glasses. The variation of tanδ with temperature at different frequencies of CaO-P₂O₅ glasses has exhibited dielectric relaxation effects with decreasing relaxation intensity with increase in the concentration of WO₃ from 0 to 3 mol%; such relaxation effects seem to have been absent in glasses containing WO₃ beyond 3 mol%. The relaxation phenomenon has been analysed by a pseudo-Cole-Cole plot method and the possible mechanism responsible for such relaxation effects has been suggested.     

Heat treatment induced structural and optical properties of rf magnetron sputtered tantalum oxide films

Rf magnetron sputtering technique was employed for preparation of tantalum oxide films on quartz and crystalline silicon (111) substrates held at room temperature by sputtering of tantalum in an oxygen partial pressure of 1x10^‐4 mbar. The films were annealed in air for an hour in the temperature range 573 – 993 K. The effect of annealing on the chemical binding configuration, structure and optical absorption of tantalum oxide films was systematically studied. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Method of measuring depth profile of hydrogen in soda-lime glass

The depth profile of hydrogen in glass, as an indicator for the presence of water, has been determined by two independent techniques. Secondary ion mass spectroscopy (SIMS) gives high spatial resolution measurements of the variation of hydrogen concentration as a function of depth into the glass. These measurements are not absolute, however. Elastic recoil detection analysis (ERDA) provides absolute measurements of the variation of hydrogen concentration with depth, although the spatial resolution is only in the order of 50 nm. The combination of the measurements obtained by these two techniques yields absolute high-resolution data of the depth dependence of hydrogen in glass.     

Simulation of hydrogen evolution from nano-crystalline silicon

The temperature dependent H-evolution from nano-crystalline silicon is simulated with molecular dynamics. The nano-crystalline silicon model consists of a heterogeneous dispersion of nano-crystallites in an amorphous silicon matrix. An excess H-density occurs at the nano-crystallite surface. Simulations find a low temperature evolution peak at 250-400 °C, where the H-evolution occurs from the surface of the nano-crystallite. In addition there is a higher temperature peak at 700-800 °C. The two-peak feature agrees with H-evolution measurements of H-diluted a-Si:H grown near the phase boundary of crystalline growth. We find that H is released into mobile molecular and bond-centered like configurations.     

Interface engineering of photoelectrochemically prepared Si surfaces

The oxide of Si(1 1 1) formed by electropolishing in dilute ammonium fluoride solution is analysed by photoelectron spectroscopy using synchrotron radiation. The oxidic layer is about 3.1 nm thick and contains Si-F_x species as well as oxyfluorides. The oxyfluorides are found preferentially at the electropolishing layer surface. SiOH species are concentrated at the oxidic film/substrate interface. The full width half maximum of the Si 2p line indicates that the Si/electropolishing oxide interface is smoother than the Si/natural oxide interface.