S. J. O'Shea

Effect of Tip Size on Force Measurement in Atomic Force Microscopy

An atomic force microscope (AFM) has been used to study solvation forces at the solid-liquid interface between highly oriented pyrolytic graphite (HOPG) and the liquids octamethylcyclotetrasiloxane (OMCTS), n-hexadecane (n-C16H34), and n-dodecanol (n-C11H23CH2OH). Oscillatory solvation forces (F) are observed for various measured tip radii (Rtip=15-100 nm). It is found that the normalized force data, F/Rtip, differ between AFM tips with a clear trend of decreasing F/Rtip with increasing Rtip.

Liquid Atomic Force Microscopy: Solvation Forces, Molecular Order, and Squeeze-Out

We review the use of atomic force microscopy (AFM) in liquids to measure oscillatory solvation forces. We find solvation layering can occur for all the liquids studied (linear and branched alkanes) but marked variations in the force and dissipation may arise dependent on: a) the temperature, b) the tip shape/radius of curvature, and c) the degree of molecular branching. Several findings (e.g., the strong temperature dependence in measured solvation forces, solvation oscillations using branched molecules) differ from those observed using the Surface Force Apparatus, because of the nanoscale area probed by AFM. Conduction AFM is used to explore how liquid is squeezed out of the tip–sample gap, and enables the change in contact area of the tip–sample junction to be monitored and compared to mechanical models. We find elastic models provide a good description of the deformation of ordered, solid-like solvation layers but not disordered, liquid-like layers.

Single vacancy defect spectroscopy on HfO2 using random telegraph noise signals from scanning tunneling microscopy

Random telegraph noise (RTN) measurements are typically carried out at the device level using standard probe station based electrical characterization setup, where the measured current represents a cumulative effect of the simultaneous response of electron capture/emission events at multiple oxygen vacancy defect (trap) sites. To better characterize the individual defects in the high-κ dielectric thin film, we propose and demonstrate here the measurement and analysis of RTN at the nanoscale using a room temperature scanning tunneling microscope setup, with an effective area of interaction of the probe tip that is as small as 10 nm in diameter. Two-level and multi-level RTN signals due to single and multiple defect locations (possibly dispersed in space and energy) are observed on 4 nm HfO2 thin films deposited on n-Si (100) substrate. The RTN signals are statistically analyzed using the Factorial Hidden Markov Model technique to decode the noise contribution of more than one defect (if any) and estimate t...

Oscillatory Forces in Liquid Atomic Force Microscopy.

By analysing oscillatory type solvation forces acting at the tip-surface interface in atomic force microscopy (AFM) in liquids it is possible to study interactions arising from purely physical effects. The observation of oscillatory forces with sharp tips indicates a high geometric symmetry of the tip-liquid-surface system. As such, liquid mediated forces may prove general for high resolution non-contact imaging in liquids. The effective viscosity (damping) increases markedly as the tip apex samples the liquid solvation layers closest to the surface and this suggests that strong damping effects (low Q) may always be present in non-contact imaging in liquids. The magnitude of solvation effects (both interfacial stiffness and viscosity) is less than that observed using the surface force apparatus (SFA) and this is probably related to tip roughness (asymmetry) or to the more limited volume over which liquid is confined in AFM.

Nanoscale electrical and physical study of polycrystalline high-κ dielectrics and proposed reliability enhancement techniques

Grain boundaries (GBs) in polycrystalline high-κ (HK) dielectric materials affect the electrical performance and reliability of advanced HK-based metal-oxide-semiconductor (MOS) devices. In this work, we present a localized study comparing the electrical conduction through grains and GBs for CeO2 and HfO2-based HK dielectrics using scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) at the nanometer scale, in conjunction with macroscopic MOS capacitor device level analysis. Nanoscale STM conduction analysis clearly reveals faster degradation at GB sites and their vulnerability to early percolation. Multi-layer HK dielectric stacks (capping of La2O3 on CeO2 and dual-layer ZrO2/HfO2) are proposed as an effective technique to significantly enhance the time-dependent dielectric breakdown (TDDB) robustness of advanced HK metal gate (MG) stacks.

Localized degradation and breakdown study of cerium-oxide high-к gate dielectric material using scanning tunneling microscopy

In this work, we use scanning tunneling microscopy (STM) to study the localized degradation, breakdown and post-breakdown of a high-к (HK) gate dielectric material, cerium oxide (CeO2) deposited directly on a silicon substrate. The novelty of the study lies in analyzing the breakdown phenomenon from a macroscopic metal-oxide-semiconductor (MOS) capacitor level to a very localized nanoscale breakdown location. The physics of failure for these polycrystalline cerium oxide HK films is also discussed.

Nanoscale physical analysis of localized breakdown events in HfO 2 /SiO X dielectric stacks: A correlation study of STM induced BD with C-AFM and TEM

The study of scanning tunneling microscopy (STM) induced localized degradation and polarity dependent breakdown (BD) of HfO2/SiOx dielectric stacks is presented in this work, together with a correlated investigation of the BD locations by transmission electron microscopy (TEM). The localized dielectric BD events are also analysed using conductive-atomic force microscopy. The analysis of the degradation and breakdown phenomenon has been performed from a macroscopic (device) level to a localized nanometer scale BD location. A new technique is adopted to induce the degradation and BD of the HfO2/SiOx dielectric stacks locally using a combined STM/scanning electron microscopy nano-probing system. The BD locations were identified on blanket wafers and gate electrode area of the dielectric, and the sample containing these regions was prepared using focused ion beam for the physical analysis using TEM. This method of analysis is very useful in studying the nature of the BD events in dielectrics with and without the gate electrode, elucidating the role of the gate electrode in dielectric BD events.

Localized characterization of charge transport and random telegraph noise at the nanoscale in HfO2 films combining scanning tunneling microscopy and multi-scale simulations

Charge transport and Random Telegraph Noise (RTN) are measured successfully at the nanoscale on a thin polycrystalline HfO2 film using room temperature Scanning Tunneling Microscopy (STM). STM is used to scan the surface of the sample with the aim of identifying grains and grain boundaries, which show different charge transport characteristics. The defects responsible for charge transport in grains and grain boundaries are identified as positively charged oxygen vacancies by matching the localized I-V curves measured at the nanoscale with the predictions of physics-based multi-scale simulations. The estimated defect densities at grains and grain boundaries agree with earlier reports in the literature. Furthermore, the current-time traces acquired by STM at fixed bias voltages on grains show characteristic RTN fluctuations. The high spatial resolution of the STM-based RTN measurement allows us to detect fluctuations related to individual defects that typically cannot be resolved by the conventional device-...

Leakage current and structural analysis of annealed HfO2/La2O3 and CeO2/La2O3 dielectric stacks: A nanoscopic study

Grain boundaries in the polycrystalline microstructure of post-annealed high-κ (HK) dielectrics are a major limitation in the reliability of HK dielectrics used for advanced CMOS technologies. Another challenge in the field of HK dielectrics is to ensure higher drain drive current in CMOS, while maintaining low leakage current. In this work, the authors demonstrate enhanced performance of HfO2 and CeO2 dielectrics by incorporating lanthanum. The resulting stacks show promising dielectric characteristics with reduced leakage current and uniform (amorphous) crystal structure. The improved HK characteristics were shown to occur even over nanometer-length scales using scanning probe microscopy and transmission electron microscopy, in agreement with previous studies based on micron-scale device-level measurement.

An SEM/STM based nanoprobing and TEM study of breakdown locations in HfO 2 /SiO x dielectric stacks for failure analysis

Abstract The formation of conductive percolation path in high-κ (HK)/interfacial layer (IL) dielectric stack is accompanied by dynamic changes in the electrical and chemical properties at the nanometer length scale. It is therefore essential to study these breakdown (BD) events using high-precision nanoscale characterization tools to investigate the physical mechanisms of failure for advanced HK dielectric based devices. In this work, we carry out a new method for electrical nanoprobing of HfO2/SiOx (x