High temperature characterization of high-κ dielectrics on SiC

We have fabricated thin catalytic metal–insulator–silicon carbide based structure with palladium (Pd) gates using TiO₂ as the dielectric. The temperature stability of the capacitor is of critical importance for use in the fabrication of electronics for deployment in extreme environments. We have evaluated the response to temperatures in excess of 450 °C in air and observed that the characteristics are stable. Results of high temperature characterization are presented here with extraction of interface state density up to 650 °C. The results show that at temperatures below 400 °C the capacitors are stable, with a density of interface traps of approximately 6×10¹¹ cm² eV⁻¹. Above this temperature the C–V and G–V characteristics show the influence of a second set of traps, with a density around 1×10¹³ cm² eV⁻¹, which is close to that observed for slow states near the conduction band edge. The study of breakdown field as a function of temperature shows two distinct regions, below 300 °C where the breakdown voltage has a strong temperature dependence and above 300, where it is weaker. We hypothesize that the oxide layer dominates the breakdown voltage at low temperature and the TiO₂ layer above 300 °C. These results at high temperatures confirms the suitability of the Pd/TiO₂/SiO₂/SiC capacitor structure for stable operation in high temperature environments.     

Investigation of strontium tantalate thin films for high-k gate dielectric applications

Strontium tantalate (STO) films were grown by liquid-delivery (LD) metalorganic chemical vapor deposition (MOCVD) using Sr[Ta(OEt)₅(OC₂H₄OMe)]₂ as precursor. The deposition of the films was investigated in dependence on process conditions, such as substrate temperature, pressure, and concentration of the precursor. The growth rate varied from 4 to 300 nm/h and the highest rates were observed at the higher process temperature, pressure, and concentration of the precursor. The films were annealed at temperatures ranging from 600 to 1000 °C. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and ellipsometry indicated that the as-deposited and the annealed films were uniform and amorphous and a thin (>2 nm) SiO₂ interlayer was found. Crystallization took place at temperatures of about 1000 °C. Annealing at moderate temperatures was found to improve the electrical characteristics despite different film thickness (effective dielectric constant up to 40, the leakage current up to 6×10⁻⁸ A/cm², and lowest midgap density value of 8×10¹⁰ eV⁻¹ cm⁻²) and did not change the uniformity of the STO films, while annealing at higher temperatures (1000 °C) created voids in the film and enhanced the SiO₂ interlayer thickness, which made the electrical properties worse. Thus, annealing temperatures of about 800 °C resulted in an optimum of the electrical properties of the STO films for gate dielectric applications.     

dc-Conduction mechanism in lanthanum–manganese oxide films grown on p-Si substrate

Thin films of (La–Mn) double oxide were prepared on p-Si substrates for electrical investigations. The samples have been characterised by X-ray fluorescence (XRF) and X-ray diffraction (XRD) methods. The XRF spectrum was used to determine the weight fraction ratio of Mn to La in the prepared samples. The XRD study shows the formation of grains of LaMnO₃ compound through a solid-state reaction for annealing at 800 °C. Samples used to study the electrical characteristics of the prepared films were constructed in form of a metal–oxide–Si MOS structures. Those MOS structures were characterised by the measuring their capacitance as a function of gate voltage C(V_g) in order to determine the oxide charge density Q_ox, the surface density of states D_it at the oxide/Si interface, and to extract the oxide voltage in terms of gate voltage. The extracted dielectric constant of the double oxide film is lower than that of pure La₂O₃ film and larger than that of pure Mn₂O₃ film, but the formation of LaMnO₃ grains by a solid-state reaction at 800 °C increases the relative permittivity to 11.5. These experimental conclusions might be useful to be used in the field of Si-oxide alternative technique. The leakage dc current density vs. oxide field J(E_ox) relationship for crystalline films follow the mechanism of Richardson–Schottky (RS), from which the field-lowering coefficient and the dynamic relative permittivity were determined. Nevertheless, the leakage current density measured in a temperature range of (293–363 K) was not controlled by the RS mechanism. It was observed that the temperature dependence of the leakage current in crystalline (La–Mn) oxide insulating films has metallic-like temperature behaviour, which might be important in the technical applications.     

Post deposition UV-induced O2 annealing of HfO2 thin films

UV-assisted annealing processes for thin oxide films is an alternative to conventional thermal annealing and has shown many advantages such as low annealing temperature, reducing annealing time and easy to control. We report in this work the deposition of ultra-thin HfO₂ films on silicon substrate by two CVD techniques, namely thermal CVD and photo-induced CVD using 222 nm excimer lamps at 400 °C. As-deposited films of around 10 nm in thickness with refractive indices from 1.72 to 1.80 were grown. The deposition rate measured by ellipsometry was found to be about 2 nm/min by UV-CVD, while the deposition rate by thermal CVD is 20% less than that by UV-CVD. XRD showed that the as-deposited HfO₂ films were amorphous. This work focuses on the effect of post deposition UV annealing in oxygen on the structural, optical and electrical properties of the HfO₂ films at low temperature (400 °C). Investigation of the interfacial layer by FTIR revealed that thickness of the interfacial SiO₂ layer slightly increases with the UV-annealing time and UV annealing can convert sub-oxides at the interface into stoichiometric SiO₂, leading to improved interfacial qualities. The permittivity ranges in 8–16, are lower than theoretical values. However, the post deposition UV O₂ annealing results in an improvement in effective breakdown field and calculated permittivity, and a reduction in leakage current density for the HfO₂ films.     

Dielectric thin films for MEMS-based optical sensors

Nanoindentation and optical measurements have been employed in order to investigate the mechanical properties of low-temperature (50–330 °C) plasma-enhanced chemical vapour deposited (PECVD) SiN_x, as well as thermally evaporated SiO_x and Ge thin films for applications in micro-electro-mechanical systems (MEMS) fabricated on temperature sensitive, non-standard substrates. The temperature of the SiN_x deposition process is found to strongly influence Young’s modulus, hardness, and stress, with a critical deposition temperature in the 100 °C to 150 °C range which depends on the details of other deposition conditions such as chamber pressure and RF-power. The properties of PECVD SiN_x films deposited above this critical temperature are found to be suitable for MEMS applications, whereas films deposited at lower temperatures exhibit low Young’s modulus and hardness, as well as environment-induced stress instabilities. The investigated thin films have been incorporated into a monolithic integrated technology comprising low-temperature (125 °C) MEMS and HgCdTe IR detectors, in order to realize successful prototypes of tuneable IR microspectrometers.     

Dielectric tunable properties of (Ba1−xSrx)TiO3 thin films on LaAlO3 substrate

(Ba_1−xSr_x)TiO₃ (1−x=0.8, 0.7, 0.6 and 0.5) thin films were prepared on (0 0 1) LaAlO₃ substrates by sol–gel method. The films were found to be crystallized in preferential (0 0 1) orientation after post-deposition annealing at 750°C for 1.5 h and 1100°C for 2 h in air, respectively. We investigated the dependence of tunability and dissipation factor on annealing temperature and different Ba/Sr ratios. It was found that the tunability increased dramatically and dissipation factor decreased obviously with increasing annealing temperature, and Ba_0.6Sr_0.4TiO₃ thin films annealed at 1100°C for 2 h have a tunability of 46.9% at 80 kV/cm bias filed and a dissipation factor of 0.008 at 1 MHz.     

Properties of low temperature Sn–Ag–Bi–In solder systems

The metallurgical and mechanical properties of Sn–3.5 wt%Ag–0.5 wt%Bi–xwt%In (x = 0–16) alloys and of their joints during 85 °C/85% relative humidity (RH) exposure and heat cycle test (−40–125 °C) were evaluated by microstructure observation, high temperature X-ray diffraction analysis, shear and peeling tests. The exposure of Sn–Ag–Bi–In joints to 85 °C/85%RH for up to 1000 h promotes In–O formation along the free surfaces of the solder fillets. The 85°C/85%RH exposure, however, does not influence the joint strength for 1000 h. Comparing with Sn–Zn–Bi solders, Sn–Ag–Bi–In solders are much stable against moisture, i.e. even at 85 °C/85%RH. Sn–Ag–Bi–In alloys with middle In content show severe deformation under a heat cycles between −40 °C and 125 °C after 2500 cycles, due to the phase transformation from β-Sn to β-Sn + γ-InSn₄ or γ-InSn₄ at 125 °C. Even though such deformation, high joint strength can be maintained for 1000 heat cycles.     

Characterization of rare earth oxides based MOSFET gate stacks prepared by metal-organic chemical vapour deposition

We have prepared rare earth oxides based MOSFET gate stacks using metal-organic chemical vapour deposition, MOCVD. Gd₂O₃, La₂O₃, Nd₂O₃ and Pr₆O₁₁ films with thickness 3–20 nm were deposited on silicon substrate at 500 °C. The films were characterized by X-ray diffraction, X-ray reflectivity, transmission electron microscopy and X-ray photoelectron spectroscopy. As a next step, Ru films were grown on the dielectric films at 300 °C as a gate electrode. Electrical characterization of the MOS structures was performed by capacitance–voltage measurements. The structures annealed at 430 °C in forming gas (90% N₂+10% H₂) exhibited dielectric constant ranging from 12 to 14. Typically, the films showed high values of fixed oxide charge density, . Fixed oxide charges can be decreased by post-deposition annealing in forming gas and in oxygen.     

Study of dc conduction mechanisms in dysprosium–manganese oxide insulator thin films grown on Si substrates

Samples of amorphous and crystalline (Dy–Mn) oxide thin films have been prepared on Si(p) substrates. The crystal structure of the oxide film annealed under different conditions was investigated by the X-ray diffraction method (XRD). The percentage weight composition of the compound-oxide films was determined by the X-ray fluorescence (XRF) spectroscopy method. It was observed that Dy oxide and Mn oxide prevent each other to crystallize alone or making a solid solution even at 600 °C, but a compound of DyMnO₃ was formed through the solid-state reaction at T > 800 °C. Samples in form of Al/oxide/Si MOS structures were characterised by measuring their capacitance as a function of gate voltage C(V_g) in order to determine the fixed and interface charge densities as well as the oxide voltage in terms of gate voltage. The total surface charge density was in the device-grade of 10¹⁰–10¹¹ cm⁻². The dc measurements at room temperature show that the main mechanism controlling the current flow is the Richardson–Schottky (RS) mechanism. The parameters of the RS model like the field lowering coefficients and the dynamic relative permittivity were determined. The leakage current density of the samples was studied as a function of temperature in a range of (293–380 K). It was observed that the temperature dependence of crystalline (Dy–Mn) oxide films has a property that higher temperature reduces the current, which may be important in the application in circuits that operate under extreme conditions. Thermal activation energies of electrical conduction were determined.     

Optical and electrical characterization of hafnium oxide deposited by liquid injection atomic layer deposition

Optical and electrical properties of a set of high-k dielectric HfO₂ films, deposited by liquid injection atomic layer deposition (LI-ALD) and post deposition annealed (PDA) in nitrogen (N₂) ambient at various temperatures (400–600 °C), were investigated. The films were prepared using the cyclopentadienyl of hafnium precursor [Cp₂Hf(CH₃)₂] with water deposited at 340 °C. The spectroscopic ellipsometric (SE) results show that the characteristics of the dielectric functions of these films are strongly affected by annealing temperatures. I–V results show that N₂-based PDA enhances the average energy depth of the shallow trapping defects from Poole–Frenkel conduction fitting. This also correlated with the measured increase in MOS capacitance–voltage hysteresis.     

Recrystallization and dielectric properties of CaHfOx films on Si

The recrystallization and dielectric behavior for amorphous CaHfO_x films on Si substrates has been investigated. Upon conventional annealing in air, the CaHfO_x films remain amorphous up to an annealing temperature of 800 °C for annealing times of 1 h. This recrystallization temperature is significantly higher than that reported for HfO₂ subjected to rapid thermal annealing. Metal–insulator–semiconductor structures with Pt gate electrodes were fabricated with various CaHfO_x film thickness for capacitance–voltage and leakage current measurements. From this, the permittivity of CaHfO_x was determined, along with interface layer capacitance for films on Si. The enhanced stability against polycrystalline grain growth, along with the thermodynamic stability of both CaO and HfO₂ in contact with Si, suggests that CaHfO_x may be an attractive gate dielectric for future generation metal–oxide–semiconductor field-effect transistor applications.     

Characterisation and passivation of interface defects in (1 0 0)-Si/SiO2/HfO2/TiN gate stacks

The density and energy distribution of electrically active interface defects in the (1 0 0)Si/SiO₂/HfO₂ system are presented. Experimental results are analysed for HfO₂ thin films deposited by atomic layer deposition and metal-organic chemical vapour deposition on (1 0 0)Si substrates. The paper discusses the origin of the interface states, and their passivation in hydrogen over the temperature range 350–550 °C.     

Low temperature processed hafnium oxide: Structural and electrical properties

In this work hafnium oxide (HfO₂) was deposited by r.f. magnetron sputtering at room temperature and then annealed at 200 °C in forming gas (N₂+H₂) and oxygen atmospheres, respectively for 2, 5 and 10 h. After 2 h annealing in forming gas an improvement in the interface properties occurs with the associated flat band voltage changing from −2.23 to −1.28 V. This means a reduction in the oxide charge density from 1.33×10¹² to 7.62×10¹¹ cm⁻². After 5 h annealing only the dielectric constant improves due to densification of the film. Finally, after 10 h annealing we notice a degradation of the electrical film's properties, with the flat band voltage and fixed charge density being −2.96 V and 1.64×10¹² cm⁻², respectively. Besides that, the leakage current also increases due to crystallization. On the other hand, by depositing the films at 200 °C or annealing it in an oxidizing atmosphere no improvements are observed when comparing these data to the ones obtained by annealing the films in forming gas. Here the flat band voltage is more negative and the hysteresis on the C–V plot is larger than the one recorded on films annealed in forming gas, meaning a degradation of the interfacial properties.     

Correlation between infrared transmission spectra and the interface trap density of SiO2 films

This work is an attempt to estimate the electrical properties of SiO₂ thin films by recording and analyzing their infrared transmission spectra. In order to study a big variety of films having different infrared and electrical properties, we studied SiO₂ films prepared by low pressure chemical vapor deposition (LPCVD) from SiH₄ + O₂ mixtures at 425 °C and annealed at 750 °C and 950 °C for 30 min. In addition thermally grown gate quality SiO₂ films of similar thickness were studied in order to compare their infrared and electrical properties with the LPCVD oxides. It was found that all studied SiO₂ films have two groups of Si–O–Si bridges. The first group corresponds to bridges located in the bulk of the film and far away from the interfaces, the grain boundaries and defects and the second group corresponds to all other bridges located near the interfaces, the grain boundaries and defects. The relative population of the bulk over the boundary bridges was found equal to 0.60 for the LPCVD film after deposition and increased to 4.0 for the LPCVD films after annealing at 950 °C. Thermally grown SiO₂ films at 950 °C were found to have a relative population of Si–O–Si bridges equal to 3.9. The interface trap density of the LPCVD film after deposition was found equal to 5.47 × 10¹² eV⁻¹ cm⁻² and decreases to 6.50 × 10¹⁰ eV⁻¹ cm⁻² after annealing at 950 °C for 30 min. The interface trap density of the thermally grown film was found equal to 1.27 × 10¹¹ eV⁻¹ cm⁻² showing that films with similar Si–O–Si bridge populations calculated from the FTIR analysis have similar interface trap densities.     

Optical characterization of Si-rich silicon nitride films prepared by low pressure chemical vapor deposition

An investigation of the optical properties of Si-rich silicon nitride films prepared by low pressure chemical vapor deposition (LPCVD) from dichlorosilane (SiH₂Cl₂, DCS) and ammonia (NH₃) mixtures has been performed. From TEM analysis, it was found that the excess Si forms nanocrystals the size of which depends on the temperature. The real and the imaginary part of the refractive index of the films were calculated using spectroscopic ellipsometry by fitting the ellipsometric data in the range 1000–250 nm using the Tauc–Lorentz model. It was found that the optical constants of the films mainly depend on their chemical composition which can be controlled by the DCS/NH₃ flow ratio. Annealing at temperatures up to 1100 °C for 4 h does not considerably affect the refractive index of the films. Depending on their stoichiometry and the annealing conditions applied after growth, some of the films emitted light in the visible at room temperature. This was attributed to the quantum confinement of carriers in the Si nanocrystals contained in the films.     

Effect of rapid thermal annealing of sputtered aluminium nitride film in an oxygen ambient

Aluminium nitride (AlN) thin films were deposited by radio frequency (RF) magnetron sputtering on p-type silicon (Si) substrate of (1 0 0) orientation using only argon (Ar) gas at substrate temperature of 300 °C. In order to achieve improved electrical properties, we performed post-deposition rapid thermal annealing (RTA). Sputtered AlN films were annealed in an oxygen ambient at temperatures of 600, 700, and 800 °C using RTA for 30 min. The orientation of the AlN crystal in the film was investigated using X-ray diffraction (XRD). The characteristic spectra by functional group were analyzed by Fourier transformation infrared (FTIR) spectroscopy. The electrical properties of the AlN thin films were studied through capacitance–voltage (C–V) characteristics in metal–insulator–semiconductor (MIS) device using the films as insulating layers. The flatband voltages (V_FB) in C–V curves were found to depend on crystal orientations. Negative V_FB was found in the case when AlN (1 0 0) peak was found. Also, when AlN (1 0 3) peak was observed upon increasing the annealing temperature, the value of V_FB was positive and after annealing at 700 °C, AlN (1 0 3) peak intensity was found to be maximum and V_FB was as high as+6.5 V.     

Vibrational spectroscopy characterization of low-dielectric constant SiOC:H films prepared by PECVD technique

Low-dielectric constant SiOC:H films were prepared by plasma enhanced chemical vapour deposition (PECVD) from trimethyl-silane (H–Si–(CH₃)₃) and ozone (O₃) gas mixture. The samples were preliminarily annealed at 400 °C in N₂ atmosphere and then in N₂+He plasma. Afterwards, they were treated in vacuum at some fixed temperatures in the range between 400 and 900 °C. Structural investigations of the annealed films were carried out by means of vibrational spectroscopy techniques. FT-IR spectrum of a preliminarily treated sample shows absorption bands due to stretching modes of structural groups like Si–CH₃ at 1270 cm⁻¹, Si–O–Si at 1034 cm⁻¹ and C–H_x in the region between 2800 and 3000 cm⁻¹. No significant spectral change was observed in the absorption spectra of samples annealed up to 600 °C, indicating that the preliminarily treated film retains a substantial structural stability up to this temperature. Above 600 °C, absorption spectra show a strong quenching of H-related peaks while the band due to Si–O–Si anti-symmetric stretching mode shifts towards higher energy, approaching the value observed for thermally grown SiO₂. Raman spectra of samples treated at temperatures T500 °C exhibit both D and G bands typical of sp²-hybridised carbon, due to the formation of C–C bonds within the film which is accompanying the release of hydrogen. The intensity of D and G bands becomes more pronounced in samples annealed at higher temperatures, thus suggesting a progressive precipitation of carbon within the oxide matrix.     

Low-temperature carbon monoxide gas sensors based gold/tin dioxide

Tin dioxide nanocrystals were synthesized by a precipitation process and then used as the support for 2 wt.% gold/tin dioxide preparation via a deposition–precipitation method, followed by calcination at 200 °C. Thick films were fabricated from gold/tin dioxide powders, and the sensing behavior for carbon monoxide gas was investigated. The gold/tin dioxide was found to be efficient carbon monoxide gas-sensing materials under low operating temperature (83–210 °C). The Au/SnO₂ sensor with SnO₂ calcined at 300 °C exhibited better CO gas-sensing behavior than the SnO₂ calcined at other temperatures. The experimental results indicated the potential use of Au doped SnO₂ for CO gas sensing.     

Electrical properties of SiO2/TiO2 high-k gate dielectric stack

The trapping/detrapping behavior of charge carriers in ultrathin SiO₂/TiO₂ stacked gate dielectric during constant current (CCS) and voltage stressing (CVS) has been investigated. Titanium tetrakis iso-propoxides (TTIP) was used as the organometallic source for the deposition of ultra-thin TiO₂ films at low temperature (<200 °C) on strained-Si/relaxed-Si_0.8Ge_0.2 heterolayers by plasma-enhanced chemical vapor deposition (PECVD) in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of 66.67 Pa. Stress-induced leakage current (SILC) through SiO₂/TiO₂ stacked gate dielectric is modeled by taking into account the inelastic trap-assisted tunneling (ITAT) mechanism via traps located below the conduction band of TiO₂ layer. The increase in the gate current density observed during CVS from room temperature up to 125 ^oC has been analyzed and modeled considering both the buildup of charges in the layer as well as the SILC contribution. Trap generation rate and trap cross-section are extracted. A capture cross-section in the range of 10⁻¹⁹ cm² as compared to 10⁻¹⁶ cm² in SiO₂ has been observed. A temperature-dependent trap generation rate and defects have also been investigated using time-dependent current density variation during CVS. The time dependence of defect density variation is calculated within the dispersive transport model, assuming that these defects are produced during random hopping transport of positively charge species in the insulating high-k stacked layers. SILC generation kinetics, i.e. defect generation probability under different injected fluences for various high-constant stress voltages in both polarities have been studied. An empirical relation between trap generation probability and applied stress voltage for various injected fluences has been developed.     

Ballistic electron emission microscopy studies of the temperature dependence of Schottky barrier height distribution in CoSi2/n-Si(1 0 0) diodes formed by solid phase reaction

Ballistic electron emission microscopy (BEEM) and ballistic electron emission spectroscopy have been performed on polycrystalline and epitaxial CoSi₂/n-Si(1 0 0) contacts at temperatures ranging from −144°C to −20°C. The ultra-thin CoSi₂ films (10 nm) were fabricated by solid state reaction of a single layer of Co (3 nm) or a multilayer of Ti (1 nm)/Co (3 nm)/amorphous-Si(1 nm)/Ti (1 nm) with a Si substrate, respectively. The spatial distribution of barrier height over the contact area obeys a Gaussian function at each temperature. The mean barrier height increases almost linearly with decreasing temperature with a coefficient of −0.23±0.02 meV/K for polycrystalline CoSi₂/Si diodes and −0.13±0.03 meV/K for epitaxial diodes. This is approximately equal to one or one-half of the temperature coefficient of the indirect energy gap in Si, respectively. It suggests that the Fermi level is pinned to different band positions of Si. The width of the Gaussian distribution is about 30–40 meV, without clear dependence on the temperature. The results obtained from conventional current–voltage and capacitance–voltage (I–V/C–V) measurements are compared to BEEM results.