A. Shanware

Application of HfSiON as a gate dielectric material

Physical and electrical properties of HfSiON that make this material desirable as the gate dielectric in a standard metal–oxide–semiconductor flow are reported. Sputtering was used to deposit films with minimal low dielectric constant interface layers, equivalent oxide thicknesses below 13 A, and leakage current density at least two orders of magnitude lower than SiO2. The presence of nitrogen in the film enhances the thermal stability relative to HfSiO, and no crystallization was observed for anneals up to 1100 °C.

Thermochemical description of dielectric breakdown in high dielectric constant materials

A thermochemical/molecular model is developed for breakdown in high dielectric constant materials and the model suggests that a fundamental relationship exists between dielectric breakdown strength (Ebd) and dielectric constant (k). The model indicates that Ebd should show an approximate (k)−1/2 dependence over a wide range of high dielectric constant materials. The model also predicts that the field-acceleration parameter (γ), from time-dependent dielectric breakdown (TDDB) testing, should increase with dielectric constant. TDDB and Ebd data are presented for model support. The thermochemical model suggests that the very high local electric field (Lorentz-relation/Mossotti-field) in high-k dielectrics tends to distort/weaken the polar molecular bonds making them more susceptible to bond breakage by standard Boltzmann processes and/or by hole capture and thus lowers the breakdown strength.

Trends in the ultimate breakdown strength of high dielectric-constant materials

The ultimate breakdown strength E/sub bd/ of a dielectric material is found to decrease as the dielectric-constant k increases. A thermochemical description of the ultimate breakdown strength of high-k dielectrics suggests that E/sub bd/ should reduce approximately as (k)/sup -1/2/ over a wide range of dielectric materials while the field-acceleration parameter /spl gamma/ should increase in similar but inverse manner. New time-dependent dielectric breakdown (TDDB) data are presented over a wide range of dielectric materials and E/sub bd/ was found to decrease as (k)/sup -0.65/ while /spl gamma/ increases as (k)/sup 0.66/. The good agreement between thermochemical theory and high-k TDDB observations suggests that the very high local electric field (Lorentz-relation/Mossotti-field) in high-k dielectrics tends to distort/weaken the polar molecular bonds making them more susceptible to bond breakage by standard Boltzmann processes and/or by hole-capture and thus lowers the breakdown strength.

Advanced CMOS transistors with a novel HfSiON gate dielectric

We report for the first time on short channel transistors fabricated using HfSiON, a new high-k gate dielectric material. HfSiON has superior electrical characteristics such as low leakage current relative to SiO/sub 2/, low interfacial trap density, electron and hole carrier mobilities /spl sim/80% of the universal curve at E/sub eff/>0.8 MV/cm and scalability to equivalent oxide thicknesses of less than 10 /spl Aring/. This material is also thermally stable up to 1100/spl deg/C in contact with poly Si, and exhibits boron blocking significantly better than SiO/sub 2/ and SiON. The results indicate that this material is a promising high-k gate dielectric with good transistor characteristics.

Proposed universal relationship between dielectric breakdown and dielectric constant

Dielectrics with high dielectric constant k will likely be required for future replacement of conventional SiO/sub 2/ gate dielectric. Physical models are needed which describe the reliability tradeoffs associated with the high-k material selection. In this paper we report on a fundamental relationship existing between the dielectric breakdown E/sub bd/ and dielectric constant k. An approximate E/sub bd/ /spl sim/ (k )/sup -1/2/ relation is found and seems to be universal, i.e., the relation holds over nearly two decades of dielectric constant. A physics-based model has been developed to understand this critically important relationship. The good fit of the physical model (with no adjustable parameters) to the experimental data suggests that the local electric field (Lorentz-relation/Mossotti-field) in these high-k materials plays a very important role in the observed E/sub bd/ /spl sim/ (k)/sup -1/2/ behavior. The very high local electric field (in high-k materials) tends to distort/weaken polar molecular-bonds thereby lowering the enthalpy of activation required for bond breakage by standard Boltzmann processes.

Characterization and comparison of the charge trapping in HfSiON and HfO/sub 2/ gate dielectrics

Charge trapping in HfSiON and HfO/sub 2/ gate dielectrics was studied using both DC and pulsed I/sub D/-V/sub G/ characterization techniques. The data shows a significant amount of hysteresis in HfO/sub 2/ but negligible instability in HfSiON. Constant voltage stress measurements of HfO/sub 2/ and HfSiON films show that the threshold voltage shift in HfO/sub 2/ films is as much as 10 times higher than that of HfSiON. Further, modeling of the time dependence of the threshold voltage shows that the trap capture cross section responsible for the instability of HfO/sub 2/ films is 10 times higher than the trap capture cross section of HfSiON. Our data indicate that amorphous HfSiON has better electrical stability than HfO/sub 2/.

Thermal stability of hafnium–silicate and plasma-nitrided hafnium silicate films studied by Fourier transform infrared spectroscopy

Structure and bonding changes in ultrathin hafnium–silicate (HfSiO) and plasma-nitrided HfSiO (HfSiON) films as a result of thermal annealing are presented. To track these changes, attenuated total reflection Fourier transform infrared spectroscopy (FTIR) and high-resolution transmission electron microscopy were used. It is shown that for films with a given Si content, HfSiON films have superior thermal stability compared to the corresponding HfSiO films. It is also demonstrated that besides giving chemical state changes for the thin-film constituents, FTIR can also be used to track interfacial SiO2 growth as well as phase separation in ultrathin high-κ films resulting from thermal annealing.

Low-frequency noise characteristics of HfSiON gate-dielectric metal-oxide-semiconductor-field-effect transistors

High dielectric constant materials are being developed as possible replacements for SiO2 as the gate dielectric. Although these materials do overcome the issue of gate leakage current because of increased thickness for a given equivalent capacitance, several other problems arise, such as degraded carrier mobility and higher low-frequency noise due to increased fixed charges and traps in the high-k film. HfSiON gate-dielectric metal-oxide-semiconductor field-effect transistors (MOSFETs), presented here, offer lower 1∕f noise compared to other high-k materials, but the noise levels are relatively higher than in SiO2 devices. Oxide-trap-induced correlated carrier number-mobility fluctuations dominate in all of these devices. Measured noise characteristics as well as extracted oxide trap density values are discussed for various geometries and sizes. The latter, measured to be 1.5×1019–1.6×1020cm−3eV−1, is higher than that for SiO2 MOSFETs with similar dimensions (4.1×1016–7.8×1016cm−3eV−1). This work represen...

Evaluation of the positive biased temperature stress stability in HfSiON gate dielectrics

Electrical instability due to charge trapping in high-k materials is a primary concern for the usefulness of these films in future CMOS devices. This paper reports the effect of charge trapping on the threshold voltage and transistor drive current of devices made with HfSiON gate dielectric. Our results show that the physics of the charge trapping in HfSiON is unique and follows logarithmic dependence with time rather than usual exponential dependence. NMOS devices fabricated with HfSiON films show acceptable electrical stability for 10 years without substantial degradation of either the threshold voltage or the drive current.

Impact of interfacial layer on low-frequency noise of HfSiON dielectric MOSFETs

Low-frequency noise measurements and analysis were performed on n-channel MOSFETs with HfSiON as the gate-dielectric material. The role of SiON interfacial-layer thickness was investigated. It was observed that these fluctuations can be described by the unified flicker-noise model that attributes noise to correlated carrier-number/mobility fluctuations due to trapping states in the gate dielectric. The model was modified to include the effect of different gate stack layers on the observed noise. The carrier-number fluctuations were found to dominate over the correlated mobility fluctuations in the measured bias range and more so at the lower gate overdrives. The noise magnitude showed a decrease with increasing SiON interfacial-layer thickness. Furthermore, an inverse-proportionality relationship was revealed between the effective oxide trap density and SiON thickness.