Work Function Of TiN Research Articles

Controlling the TiN Electrode Work Function at the Atomistic Level: A First Principles Investigation

The paper reports on a theoretical description of work function of TiN, which is one of the most used materials for the realization of electrodes and gates in CMOS devices. Indeed, although the work function is a fundamental quantity in quantum mechanics and also in device physics, as it allows the understanding of band alignment at heterostructures and gap states formation at the metal/semiconductor interface, the role of defects and contaminants is rarely taken into account. Here, by using first principles simulations, we present an extensive study of the work function dependence on nitrogen vacancies and surface oxidation for different TiN surface orientations. The results complement and explain a number of existent experimental data, and provide a useful tool to tailoring transport properties of TiN electrodes in device simulations.

Mechanisms of temperature dependence of threshold voltage in high-k/metal gate transistors with different TiN thicknesses

The change in temperature coefficient of the threshold voltage (=dVth/dT) for poly-Si/TiN/high-k gate insulator metal–oxide–semiconductor field-effect transistors (MOSFETs) was systematically investigated with respect to various TiN thicknesses for both n- and p-channel MOSFETs. With increasing TiN thickness, dVth/dT shifts towards negative values for both n- and p-MOSFETs. A mechanism that changes dVth/dT, depending on TiN thickness is proposed. The main origins are the work function of TiN (ΦTiN) and its temperature coefficient (dΦTiN/dT). These are revealed to change when decreasing the thickness of the TiN layer, because the crystallinity of the TiN layer is degraded for thinner films, which was confirmed by ultraviolet photoelectron spectroscopy (UPS), transmission electron microscopy (TEM) and X-ray diffraction (XRD).

Role of point defects and HfO2/TiN interface stoichiometry on effective work function modulation in ultra-scaled complementary metal–oxide–semiconductor devices

We investigate the physical properties of a portion of the gate stack of an ultra-scaled complementary metal-oxide-semiconductor (CMOS) device. The effects of point defects, such as oxygen vacancy, oxygen, and aluminum interstitials at the HfO2/TiN interface, on the effective work function of TiN are explored using density functional theory. We compute the diffusion barriers of such point defects in the bulk TiN and across the HfO2/TiN interface. Diffusion of these point defects across the HfO2/TiN interface occurs during the device integration process. This results in variation of the effective work function and hence in the threshold voltage variation in the devices. Further, we simulate the effects of varying the HfO2/TiN interface stoichiometry on the effective work function modulation in these extremely-scaled CMOS devices. Our results show that the interface rich in nitrogen gives higher effective work function, whereas the interface rich in titanium gives lower effective work function, compared to a stoichiometric HfO2/TiN interface. This theoretical prediction is confirmed by the experiment, demonstrating over 700 meV modulation in the effective work function.

Titanium nitride as promising gate electrode for MOS technology

AbstractTitanium nitride (TiN) films have been grown on Si (100) substrates by DC. reactive magnetron sputtering from a titanium (Ti) metallic target at different nitrogen partial pressures (80–10 sccm) and different power (500–1500 W). The effects of the nitrogen pressure and power on structural and electrical properties of TiN films were investigated by measuring their X‐ray diffraction (crystal orientation), Atomic Force Microscopy (surface roughness), four‐probe technique (electrical resistivity) and film thickness. It is show that deposition rate (13–65 nm/min) decrease with an increase of nitrogen pressure. The films have the grain size (0.001–0.027 μm2) along the sample surface and the size of the grain increase with an increase of nitrogen partial pressure. The X‐ray diffraction measurement show that films were crystalline (with preferred orientation (311)), but some of them were polycrystalline ((311) and (111) preferred orientation). These films have been used as gate electrodes in MOS capacitors, which were fabricated with TiN and SiNxOy as gate electrode and dielectric, respectively. With 20 minutes of annealing time, the extracted TiN work function and flat‐band voltage were about 4.65 eV and ‐0.29 V, which indicates that TiN can be used as mig‐gap electrode for MOS technology. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Work function engineering in low-temperature metals

Semiconductor devices require conducting electrodes with disparate work functions for their operation. Of recent interest are fluidic processing approaches for large-area devices, which present unique challenges in the identification of materials having disparate work functions but similar melting temperatures. Such materials may be engineered by alloying with low-melting temperature metals. As a demonstration, the work function of tin and four binary tin alloys is measured by ultraviolet photoemission spectroscopy and Kelvin probe method. We demonstrate the control of metal work function by 600 meV through alloying while keeping the melting temperature within a 140 °C range.

First-principles calculation of the TiN effective work function onSiO2and onHfO2

This paper reports density functional theory (DFT) electronic structure calculations of the valence band offsets (VBO) between TiN and cristobalyte $\mathrm{Si}{\mathrm{O}}_{2}$ and between TiN and monoclinic $\mathrm{Hf}{\mathrm{O}}_{2}$ for various interface chemical compositions. To investigate the impact of species interdiffusion on the effective TiN work function, we considered modifications of the TiN and dielectrics composition within the first monolayer from the interface. We found that the calculated VBO's depend on the stoichiometry of the interface: they are the smallest for oxygen/nitrogen rich interfaces and increase for reduced interfaces where metal-metal bonds are formed. The impact of the interface stoichiometry on the VBO for the assumed interface models can be as large as $0.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the $\mathrm{Ti}\mathrm{N}∕\mathrm{Si}{\mathrm{O}}_{2}$ and $\mathrm{Ti}\mathrm{N}∕\mathrm{m}\text{\ensuremath{-}}\mathrm{Hf}{\mathrm{O}}_{2}$ interfaces, respectively. We also found that species interdiffusion within our model systems does not affect the VBO significantly. All the calculated VBO's obtained from stoichiometric interfaces and their structural modifications are smaller than expected from available data. Finally we considered possible schemes for calculating the effective metal work function on a dielectric. We conclude that the rather poor accuracy of such work function calculations (stemming from the underestimated VBO's) may be explained by the crudeness of our model interface, which is limited by the lack of experimental data on the interfaces atomistic structures. Our analysis indicates that with the introduction of a transition region between the metal and the dielectric instead of the sharp interfaces and their slight variations studied here, or by overoxidizing the interface, the calculated VBO's may improve. On the other hand, the accuracy problem may have a more fundamental origin, namely the local density approximation (LDA) of DFT which leads to a severe band gap underestimation. Scaling our calculated VBO's by the corresponding experimental band gaps yields better agreement with measured TiN work functions on $\mathrm{Hf}{\mathrm{O}}_{2}$ for stoichiometric or near-stoichiometric interfaces. However, for TiN on $\mathrm{Si}{\mathrm{O}}_{2}$ the scaled VBO is still considerably smaller than experimental data suggests. The inclusion of $GW$ corrections or self-interaction corrected pseudopotentials improves the agreement with experimental data significantly, which strongly suggests that the VBO is underestimated by LDA/DFT as well as the band gap.

Metal Electrodes Work Function Measurement at Deca-Nanometer Scale using Kelvin Probe Force Microscope: a Step Forward to the Comprehension of Deposition Techniques Impact on Devices Electrical Properties

AbstractIn this letter, we report on Work Function (WF) measurements performed at deca-nanometer scale on various metals using Kelvin probe Force Microscope (KFM). We first demonstrated the relationship between the WF value and the grain crystallographic orientation by combining KFM and Electron Back Scattered Diffraction (EBSD) performed over the same Cu area. Once this relationship was established, KFM was used to provide, in addition to WF value, crystallographic properties of TiN PVD films grown on various substrates. Finally we characterized the effect of N2/H2 plasma treatment on the WF of TiN grown by CVD. In the latter case, the modification of the bulk chemical potential by post-treatment was proposed.

Growth mechanism of TiN film on dielectric films and the effects on the work function

We have investigated the growth mechanism of ALD-TiN film on different dielectrics and the resulting effective work function value. TiN nucleation rate and growth rate are found to be dependent on the dielectric films. TiN growth mechanism changed from 3-D type on SiO 2 to layer-by-layer type on HfO 2. The minimum TiN thickness required for a complete surface coverage varies according to the growth mechanism. Capacitor (MOSCAP) characterization revealed that the effective work function of TiN is dependent not only on dielectric films but also on the TiN thickness. The behavior of work function and fixed charge correlated with the growth mechanism of TiN on dielectric films.

Variable work function in MOS capacitors utilizing nitrogen-controlled TiN x gate electrodes

A substantial shift in the work function of TiN x by as much as 0.7 eV is achieved by varying the nitrogen gas flow during the reactive sputter deposition of the metal gate, which indicates tunability for replacing poly-Si in a CMOS process. TiN x MOS capacitors having multiple SiO 2 thicknesses have been evaluated and the work function of TiN x can be altered from 4.2 to 4.9 eV depending on the nitrogen content. The values are stable after RTP annealing up to 600 °C in nitrogen gas for 30 s, although annealing at 800 °C changes the work function for the different compositions towards a mid-gap value. No variation in EOT with annealing temperature is observed for the TiN x /SiO 2 stacks deposited at high nitrogen gas flow. The change in work function appears not to be correlated to the crystalline orientation of the TiN x . The work function is instead believed to be affected by extrinsic states in the metal/dielectric interface.

Investigation of the Thermal Stability of Reactively Sputter-Deposited TiN MOS Gate Electrodes

The effective work function of TiN, deposited by reactive magnetron sputtering, was found to be unaltered at /spl sim/5 eV after rapid thermal processing (RTP) annealing in nitrogen atmosphere at temperatures below 700/spl deg/C. However, further increase in the RTP temperature lowered the extracted work function by 0.4-0.5 eV to midgap values. In this brief, RTP anneals of TiN/SiO/sub 2//p-Si MOS capacitors were evaluated by extracting the metal gate TiN work function from capacitance-voltage measurements of MOS capacitors with multiple SiO/sub 2/ thicknesses. The RTP anneals were performed in nitrogen between 600/spl deg/C and 1000/spl deg/C for 30 s. The effective oxide charge density in the capacitors increased by a factor of five at RTP temperatures exceeding 800/spl deg/C. The resistivity seems to decrease slightly with increasing RTP temperature. The crystallographic orientation of the TiN films remain (111) after annealing up to 900/spl deg/C and is apparently not responsible for the change in work function. Analysis by X-ray photoelectron spectroscopy indicates no significant change in the binding states of titanium and nitrogen in the TiN/SiO/sub 2/ interface with increasing temperature.

Improvement of threshold voltage deviation in damascene metal gate transistors

The metal gate work function deviation (crystal orientation deviation) was found to cause the threshold voltage deviation (/spl Delta/V/sub th/) in the damascene metal gate transistors. When the TiN work function (crystal orientation) is controlled by using the inorganic CVD technique, /spl Delta/V/sub th/ of the surface channel damascene metal gate (Al/TiN or W/TiN) transistors was drastically improved and found to be smaller than that for the conventional polysilicon gate transistors. The reason for the further reduction of the threshold voltage deviation (/spl Delta/V/sub th/) in the damascene metal gate transistors is considered to be that the thermal-damages and plasma-damages on gate and gate oxide are minimized in the damascene gate process. High performance sub-100 nm metal oxide semiconductor field effect transistors (MOSFETs) with work-function-controlled CVD-TiN metal-gate and Ta/sub 2/O/sub 5/ gate insulator are demonstrated in order to confirm the compatibility with high-k gate dielectrics and the technical advantages of the inorganic CVD-TiN.