D. Spassov

X-ray photoelectron spectroscopy of thermal thin Ta2O5 films on Si

Abstract X-ray photoelectron spectra (XPS) have been recorded for thermally grown thin tantalum pentoxide films on Si. The peak decomposition technique was employed to identify the composition and the chemical states throughout the film. It is established that stoichiometric Ta2O5 detected at the surface of the layer is reduced to tantalum suboxides in its depth which amount increases in the depth of Ta2O5 film. The results show that even the films obtained at low oxidation temperature (673 K) have a two-layer structure (i.e. silicon dioxide at the interface and tantalum oxide above it) suggesting oxidation of silicon substrate in addition to oxidation of the tantalum film on Si. The interface between Si and this ultra thin SiO2 is not abrupt and the coexistence of Si–O and Ta–O bonding states in close proximity to the interface is found. The thicknesses of both, the tantalum oxide and the SiO2, as well as the width of the Si–SiO2 interface transition region are evaluated.

Influence of oxidation temperature on the microstructure and electrical properties of Ta2O5 on Si

The effect of the oxidation temperature (673 - 873 K) on the microstructural and electrical properties of thermal Ta 2 O 5 thin films on Si has been studied. Auger electron spectroscopy and X-ray photoelectron spectroscopy results revealed that the films are non-stoichiometric in the depth; an interfacial transition layer between tantalum oxide and Si substrate, containing presumably SiO 2 was detected. It has been found by X-ray diffraction that the amorphous state of Ta 2 O 5 depends on both the oxidation temperature and the thickness of the films-the combination of high oxidation temperature (> 823 K) and thickness smaller than 50 nm is critical for the appearance of a crystal phase. The Ta 2 O 5 layers crystallize to the monoclinic phase and the temperature of the phase transition is between 773 and 823 K for the thinner layers (< 50 nm) and very close to 873 K for the thicker ones. The electrical characterization (current/voltage; capacitance/voltage) reveals that the optimal oxidation temperature for achieving the highest dielectric constant (∼32) and the lowest leakage current (10 -8 A/cm 2 at 1 MV/cm applied field) is 873 K. The results imply that the poor oxidation related defects are rather the dominant factor in the leakage current than the crystallization effects.

Constant voltage stress induced current in Ta2O5stacks and its dependence on a gate electrode

Response of 8 nm Ta2O5 stacks with different gates (Al, W and Au) to voltage stress at gate injection is studied by probing under various voltage/time conditions at room temperature and at 100 °C. A stress-induced leakage current (SILC) is detected in all samples and reveals gate dependence. It is established that the pre-existing traps actually govern this response, and the impact of gate-induced defects is stronger. The Au-gated devices are the most susceptible to the stress degradation. Two processes—electron trapping at pre-existing traps and positive charge build-up—are suggested to be responsible for generation of SILC. It is concluded that despite some gate effects, the origin of CVS degradation in this particular high-k dielectric is different from that in SiO2.

Impact of Si substrate nitridation on electrical characteristics of Ta2O5 stack capacitors

A comparison of the effect of rapid thermal nitridation (RTN) of the Si surface in N2O and NH3 ambient at different temperatures (700?850??C) on the dielectric and electrical characteristics of thin (~20?nm) Ta2O5 stacks has been made. The electrical parameters of capacitors (film permittivity, oxide charge, densities of bulk traps, interface and slow states, leakage current) are discussed in terms of the impact of N incorporation in the interface region. The films on both types of RTN-treated Si exhibit ~100 times lower leakage current than Ta2O5 on bare Si, but among the two RTN processes NH3 nitridation is more beneficial since only it simultaneously increases also the stack permittivity. This improvement in parameters is suggested to be due to a real nitridation of Si surface which occurs under the NH3 rapid thermal process. RTN in N2O does not produce resistance to the oxidation substrate and it could explain the observed lack of stack dielectric constant improvement. The composition of the interfacial layer under NH3 RTN appears to be TaSi-oxinitride-like, while the interface region at N2O-nitrided Si seems to be SiO2-like. Each RTN process, however, modifies the Si surface and constitutes a specific interface layer different from that at the bare Si substrate. The composition of this layer defines parameters of the traps close to the substrate, the barrier height at the Ta2O5/interface layer and influences the conduction mechanisms in the stacks.

Electrical behaviour of Ti-doped Ta2O5 on N2O- and NH3-nitrided Si

The influence of process parameters: doping approach, Si surface nitridation ambient (NH3 and N2O), type of the gate (Al and W) and its technology of deposition on the electrical characteristics (capacitance–voltage, temperature-dependent current–voltage curves), and the mechanism of conductivity of Ti-doped stacks (6 nm) have been investigated. Among the three factors studied, the surface engineering appears to be with the greatest impact on the film permittivity and stack charges. It is shown that the Ti incorporation through the surface of Ta2O5 deposited on NH3-rapid thermally nitrided Si is effective in achieving films with high permittivity. The interface metal/doped high-k layer is a critical factor in leakage current behaviour. The evaporated Al is a good candidate as a top electrode of Ti-doped stacks giving satisfactory level of leakage current while radiation defects introduced in the stack during W sputtering cause current deterioration, making sputtered W less favourable as a metal electrode of Ta2O5-based capacitors. The conduction mechanism in Ti-doped Ta2O5 stacks is controlled by the gate electrode processing rather than by both the substrate nitridation and the doping approach. The energy levels of the traps responsible for the current transport are estimated.

Temperature dependence of leakage currents in Ti-doped Ta2O5 films on nitrided silicon

Conduction mechanisms and electrically active defects in rf sputtered Ti doped Ta2O5 thin films have been investigated by analysing the impact of temperature on the I–V curves. The films (about 6 nm thick) were grown on silicon substrates nitrided in N2O or NH3 ambient. Two different methods of Ti incorporation were used—a surface doping, where a thin Ti layer is deposited on the top of Ta2O5, and a bulk doping, where the Ti layer is sandwiched between two layers of Ta2O5. The I–V characteristics were measured in a temperature range from 30 to 100 °C. Current is found to be governed mainly by the modified Poole–Frenkel (PF) effect, with a temperature dependent degree of compensation. Two types of trap centres involved in the PF conduction process have been identified: shallow traps located at about 0.3–0.4 eV below the conduction band edge of the dielectric, present in all doped films, and deeper centres at 0.65 eV, present only in the bulk-doped films. The origin of these trap centres is also discussed.

Tailoring the Electrical Properties of HfO2 MOS-Devices by Aluminum Doping

In this work dielectric and electrical properties of Al-doped HfO2 layers deposited by plasma-enhanced atomic layer deposition in dependence on the thickness and the added Al amount in the films have been investigated. Special attention is dedicated to C-V and I-V hysteresis analysis as a measure for trapping phenomena in the films. A detailed study of conduction mechanisms in dependence on the composition of the layers has also been performed. The densities and spatial and energy positions of traps have been examined. It is found that only a small amount of Al-doping decreases the trapping which is assigned to a reduction of oxygen vacancy-related traps in HfO2. On the contrary, higher amounts of Al introduced in HfO2 films increase the trapping ability of the stacks which is due to the introduction of deeper Al2O3-related traps. The results imply that by adding a proper amount of Al into HfO2 it is possible to tailor dielectric and electrical properties of high-k layers toward meeting the criteria for particular applications.

Nanomechanical properties of pure and doped Ta2O5 and the effect of microwave irradiation

The nanomechanical properties of pure and doped Ta2O5 films (100 nm) on Si, and the effect of short time (10 s) microwave irradiation are studied by nanoindentation testing. The local mechanical parameters as determined by the force measuring ability of atomic force microscopy are compared with the data from both the Oliver–Pharr nanoindentation technique and the continuous stiffness measurements. The impact of the dopant type (Hf and Al) on the surface morphology, elastic modulus and hardness of the films is determined. The results reveal an increase in elastic modulus and hardness after the doping. The irradiation produces a little lower values of mechanical parameters. The results are discussed in juxtaposition with the established previously strong effect of irradiation on the electrical properties of Ta2O5-based stacks. From device perspective point of view Hf-doped Ta2O5 is noteworthy for micro-electro-mechanical system applications.

Doped Ta2O5 and mixed HfO2–Ta2O5 films for dynamic memories applications at the nanoscale

Abstract The doping of Ta 2 O 5 films with a proper element or its mixing with another high- k dielectric as a breakthrough to extend the potential of Ta 2 O 5 toward meeting the criteria for future technological nodes is discussed. Essential issues in the engineering of storage capacitor parameters for dynamic memories based on Ti-doped Ta 2 O 5 , Hf-doped Ta 2 O 5 and mixed HfO 2 –Ta 2 O 5 layers are presented. The benefits and the disadvantages of these modified Ta 2 O 5 stacks are discussed.

Doping of Ta 2 O 5 as a way to extend its potential for DRAM applications

The doping of Ta<inf>2</inf>O<inf>5</inf> films with a proper element or its mixing with another high-k dielectric as a breakthrough to tailor Ta<inf>2</inf>O<inf>5</inf> properties towards meeting the criteria for future technological nodes are discussed. Essential in the engineering of advanced DRAM storage capacitors parameters and characteristics of Ti-doped Ta<inf>2</inf>O<inf>5</inf>, Hf-doped Ta<inf>2</inf>O<inf>5</inf> and mixed HfO<inf>2</inf>-Ta<inf>2</inf>O<inf>5</inf> layers as an illustration of both the benefits and the disadvantages of modified Ta<inf>2</inf>O<inf>5</inf>-based stacks are presented.