Session G17: Focus Session: Emerging Research Devices and Materials for Microelectronics Industry I

The scanned-probe microscope as nano-metrology tool

Metrology is an essential requirement in the microelectronics industry. As features in computing and memory devices (and also in the flat-panel, hard disc, and CD/DVD industries) reach farther into the nanoscale, their metrology becomes increasingly difficult. Scanned--probe microscopes (SPMs) offer potential solutions. SPMs can produce images with resolution down to the atomic level. However, because of inherent nonlinearities, conventional SPMs possess poor \underline {metrology }capabilities. Nanometrology requires closed-loop scanning, high throughput, and long-term stability, with subnanometer lateral and vertical resolution and extreme scan flatness over 100s of $\mu $m. We have developed metrology scanners suited for high-precision scanning and positioning applications. They have ultralow out-of-plane motion error ($<$1 nm over 100 $\mu $m scan area). A DSP-based controller enhances the scanner performance. Advanced control algorithms improve dynamic characteristics of the system significantly by reducing phase lag and settling time. The motion control system routinely achieves sub-nanometer resolution and accuracy with high working bandwidth and long-term stability. Using the performance of these metrology scanners we propose a vision of a complete SPM-based CD metrology tool that will enable nanometrology of future generations of electronic devices.   

Optical metrology of sub-wavelength critical dimensions of lines on Si wafers

As semiconductor manufacturing progresses toward the 65 nm technology node, non-destructive characterization is required for the repeatable fabrication of devices. Optical metrology offers an advantageous solution in both cost and throughput, as the use of visible or uv light is less expensive to implement than scanning probe techniques and will offer greater flexibility as a parallel measurement process. The major impediment to using optics in the critical dimension (CD) metrology of features within the sub- wavelength regime is often thought to be the diffraction limit. However, comparisons of data to theoretical modeling can yield quantitative linewidth values. Simulations indicate that sensitivity to changes in CD below 20 nm is accessible by using structured illumination, for example reducing the illumination numerical aperture (INA). To increase the rigor of this analysis, we have imaged single, isolated lines of Si on Si with \textit{$\lambda $}=546 nm light with an INA=0.11. Although the lines measured in this case range in width nominally from 200 nm to 950 nm, the engineered illumination demonstrated here is directly applicable to the extensibility of optical metrology to technologically relevant dimensions. We also demonstrate that CD sensitivity can be enhanced further by collecting images as a function of focus position, mining additional linewidth information contained in the three-dimensional interference field above the sample.   

Measurement of Thicknesses of High-$\kappa $ Gate-Dielectric Films on Silicon by Angle-Resolved XPS

We report on the use of a new NIST database for the Simulation of Electron Spectra for Surface Analysis (SESSA) in measuring thicknesses of candidate high-$\kappa $ gate-dielectric materials (HfO$_{2}$, HfSiO$_{4} $, ZrO$_{2}$, and ZrSiO$_{4})$ on silicon by angle-resolved XPS. For conventional measurements of film thicknesses, effective attenuation lengths (EALs) have been computed for these materials from SESSA as a function of film thickness and photoelectron emission angle (i.e., to simulate the effects of tilting the sample). These EALs are believed to be more accurate than similar EALs obtained from the transport approximation because realistic cross sections are used for both elastic and inelastic scattering in the film and substrate materials. We also present ``calibration curves'' showing calculated ratios of selected photoelectron intensities from thin films of HfO$_{2}$ on Si with an intermediate SiO$_{2}$ layer. These ratios provide a simple and convenient means of determining the thicknesses of SiO$_{2}$ and HfO$_{2}$ films for particular measurement conditions.   

Terahertz Spectroscopy as a non contact estimation technique of defect states in high dielectric constant materials

A large number of gate dielectric materials have been examined during the past few years to replace Silicon dioxide in the MOSFET industry to reduce gate leakage currents for microfabrication of devices. Among them, Hafnium based materials have become a very promising candidate. In the reported work, the effect of Hafnium dioxide films on p-type silicon substrates has been investigated and compared with conventional dielectric material, Silicon dioxide, using CW visible pump/THz probe spectroscopy. Drude analysis of the experimentally obtained differential transmission spectra evaluates the electric permittivity of the interfacial layer and the calculated defect density is found to be higher for HfO$_{2 }$than for SiO$_{2 }$which agrees with Hall measurements. Additional measurements on Silicon Nitride deposition and photoresist coated p+ Silicon on p-type silicon wafers without any oxide gave an interfacial defect density 50 times higher than that of SiO$_{2}$/p+ interface. Results indicate that the mobility of the layer underneath Hafnium is less than that of Silicon. Hence the present study emphasizes the advantage of THz spectroscopy as a non-contact tool for semiconductor metrological applications.   

3D-Imaging of Non-spherical Silicon Nanoparticles Embedded in Silicon Oxide by Plasmon Tomography

We apply plasmon tomography to construct three-dimensional images of silicon nanoparticles in a silicon dioxide matrix, a materials system of interest for optical and storage devices, and at a level of detail and resolution not possible by conventional microscopies. We find that silicon particles with complex morphologies and high surface to volume ratios are dominant rather than the commonly assumed near-spherical structures. These results would affect quantum-confined excitons and interface density of states and, thus, the optical properties of this material. Our findings might explain some of the puzzles related with this material, including the broad photoluminescence band.   

Metrology for new microelectronic materials.

Traditional scaling of the CMOS Field-Effect-Transistor (FET) has been the basis of the semiconductor industry for 30 years. The 15 year horizon of the International Technology Roadmap for Semiconductors (ITRS) is reaching a point which ``challenges the most optimistic projections for the continued scaling of CMOS (for example, MOSFET channel lengths of roughly 9 nm).'' As silicon CMOS technology approaches its limits, new device structures and computational paradigms will be required to replace and augment standard CMOS devices for ULSI circuits. These possible emerging technologies span the realm from transistors made from silicon nanowires to heteroepitaxial layers for spin transistors to devices made from nanoscale molecules. One theme that pervades these seemingly disparate emerging technologies is that the electronic properties of these nanodevices are extremely susceptible to small perturbations in structural and material properties such as dimension, structure, roughness, and defects. The extreme sensitivity of the electronic properties of these devices to their nanoscale physical properties defines a significant need for precise metrology. This talk will provide an overview of emerging devices and materials, and, through example, an overview of the characterization needs for these technologies. .   

Internal dielectric interface: SiO$_{2}$- HfO$_{2}$

Hafnia is the leading candidate to replace silica as the gate dielectric in CMOS technology. Typically, hafnia films are deposited by atomic layer deposition (ALD) on the oxidized surface of a silicon wafer. The oxide could be native or thermally grown. Therefore, the high-k dielectric film is not in direct contact with Si, but rather with silicon dioxide. We investigate theoretically the atomic structure of the SiO$_{2}$-HfO$_{2}$ interface, its energretics, and thermodynamic stability with respect to oxygen exchange across the interface. We have examined the electronic properties of the interface including the band discontinuity using density functional theory. To model the interface we build a supercell structure by connecting $\beta $-crystobalite (crystalline silica polymorph) and cubic hafnia. This model, while being obviously rather simplistic allows for systematic study of the dielectric thickness effects, and consistent placement of defects with respect to the interface. The striking atomic feature of the calculated interface structure is three-fold coordinated interfacial oxygen atoms connected to one Si and two Hf neighbors. The Si-O and Hf-O bond lengths are 1.62 and 2.1 {\AA}, respectively. The energy of the interface is estimated to be in the range of 900-4000 erg/cm$^{2}$ depending on the oxygen chemical potential. The structure has no states in the gap, and the estimated valence band offset agrees well with the MIGS theory. We discuss the effect of vacancies on the band alignment, and possible implications of our results to Si-SiO$_{2}$-HfO$_{2}$-Metal stacks.   

Nanometer-resolution measurement and modeling of lateral variations of the effective work function at metal-bilayer /oxide interfaces

Future CMOS technology will require metal gates with a ``tunable'' effective work function (EWF) to precisely adjust the transistor turn-on voltage. It was shown using \textit{macroscopic} $C-V$ measurements that this could be done by adjusting the thickness of a very thin low-EWF metal covered by a high-EWF metal film (or vice-versa) on a SiO$_{2}$ film [1,2]. We are using nm-resolution Ballistic Electron Emission Microscopy (BEEM) and Internal Photemission (Int-PE) to investigate whether a 5-nm Pt/1.4-nm Al/SiO$_{2}$ gate stack has a \textit{laterally inhomogeneous} energy barrier at the metal/SiO$_{2}$ interface, produced by nm-sized pinholes in the thin Al film. Initial measurements of the average BEEM threshold voltage vs. applied oxide bias do in fact suggest an inhomogeneous energy barrier at the interface. Further BEEM and Int-PE measurements and finite-element electrostatic simulations are on-going, and will be discussed. Work supported by the Semiconductor Research Corp. [1] Gao \textit{et al}., Mat. Res. Soc. Symp. Proc. 765, D1.4.1-6 (2003). [2] I.S. Jeon \textit{et al}., IEDM-04, 303 (2004).   

Is the oxygen vacancy the dominating charge trap in hafnia based MOSFETs?

One of the factors affecting performance of high-k based MOSFETS is the instability of the threshold voltage, V$_{T}$, attributed to a high concentration of the carrier traps in the dielectric stack. Bulk oxygen vacancies in HfO$_{2}$ had been suggested as dominating electric traps, although this has not been unambiguously proved. We present ab initio calculations of the vacancies and divacancies in the monoclinic HfO$_{2}$ with the charge states +2,+1,0,-1,-2. Recent electrical I-V measurements probing V$_{T}$ relaxation under pulsed electrical stress [1] infer that: 1. The charge trapping under the applied stress is significantly faster than de-trapping following stress release suggesting significance of lattice relaxation upon trapping. 2. The kinetics of this V$_{T}$ relaxation is multiexponential with dominating activation energies in the range and 0.25-0.45 eV. We juxtapose the experimental data with the energetic parameters obtained from the calculations and consider whether the charge trapping and de-trapping by the variously charged vacancy levels accounts for all available experimental trends. [1] R. Choi, S.C. Song, C.D. Young, G. Bersuker, B.H. Lee, Appl. Phys. Lett. 87, 122901 (2005).   

Resistive Switching of Individual Dislocations in Insulating Perovskites -- A Potential Route Towards Nanoscale Non-Volatile Memories.

Electrically controlled resistive switching effects have been reported for a broad variety of binary and multinary oxides in recent years. In particular, titanates, zirconates, and manganites have been in the focus of the studies. In many cases, the mechanism of the switching and the geometrical extension of the phenomenon (filaments vs. bulk) are still under discussion. In this work, we present evidence for a redox-based switching mechanism and we indicate a potential route towards highly scalable non-volatile memories based on this switching effect. The challenge our work is to utilize resistive switching mechanism with the aim to construct \textit{active} electronic elements on a real nanoscale level, here by reversibly switching the electrical properties of individual dislocations by electrical stimuli. We demonstrate that standard undoped SrTiO$_{3}$ single crystals, utilized as a model system, exhibit a switching behavior along filaments based on dislocations, mediated by oxygen transport. For this, we employed a three-step procedure: the crystals were, at first, annealed at elevated temperatures under reducing conditions, then exposed to 200mbar O$_{2}$ pressure at room temperature, and finally subjected to an electric field under ultrahigh vacuum (electroformation). This treatment induced in a metal-insulator (SrTiO$_{3})$-metal (MIM) system a transition to metallic state. A hysteretic behavior appears after dynamical polarization of the MIM structure at the maximum electroforming currents. The shape of the I/V curve has the typical signature for bi-stable switching known for these types of perovskites. The positive temperature dependence of the resistance of the low- (LRS) and the high-resistance (HRS) state clearly identifies both states to be metallic in character. The inhomogeneity of the electrical transport becomes directly evident from a simple optical inspection and the conductivity maps as measured by LC-AFM of a planar structure. One can trace the formation of the filaments, emerging from the cathode and propagating towards the anode during the electroformation process. These filaments are well-oriented along the $<$100$>$-axis of the crystal and show a discrete and granular substructure on the nano-scale. The similarity in lateral distribution of exit points (spots) of conducting nano-filaments with respect to the distribution of etch pits suggests that the electrical transport along dislocations determines the micro- and meso-scopic electrical transport phenomena. Our results suggest that a dedicated contact arrangement is required to handle the filamentary conduction in a practical way by using macroscopic electrodes. At the same time, it emphasizes the need to control the relevant processes on the level of individual dislocations. With LC-AFM it is possible to specifically address single dislocations crossing the surface with adequate spatial resolution and use the conducting cantilever as the nano-electrode through galvanic point contact. We succeeded to initiate the local electroformation process for a single dislocation by applying a dc bias to the tip of the cantilever. Such nano-prepared dislocations reveal bi-stable switching behavior between a linear and a non-linear $I/V$-characteristics. The dynamic range of the electrical resistance covers at least 3 to 4 orders of magnitude at read-out voltages of 0.1 V. In order to develop a microscopic model for the filament, we performed first-principles calculations of extended, linear defects in SrTiO$_{3}$. Our analyse of electronic structure for extended defects with TiO enrichment establish that already subtle changes in O-content are sufficient to modulate the electronic properties and provide the necessary self-doping capability with a reversible transition between non-metallic and metallic behaviour. We propose a model for the resistive switching in SrTiO$_{3}$ based on the modulation of the electrical properties through electrical stimuli in a small segment of an orthogonal network of dislocations. Switching in our case corresponds then to an electrochemical ``closing'' or ``opening'' of the single dislocation in the uppermost portion of the network. Our results show that the switching behaviour in single-crystalline SrTiO$_{3}$ is an inherent property of the material and can be easily activated by external stimuli. Due to the availability of dislocation densities up to 10$^{12}$ cm$^{-2}$ in single crystals and thin film, one can even envisage to approach the Tbit regime, as long as the dislocations can be successfully arranged into registered superstructures. In summary, evidence is given that the electrical conductance of individual dislocations in a prototype perovskite, SrTiO$_{3}$, can be switched between a low and a high conducting state by the application of an electrical field. We demonstrate on the basis of \textit{ab initio} calculations and measurements with a scanning probe microscope SPM that the modulation of the electrical properties is related to the induced change in oxygen stoichiometry and the self-doping capability with a local insulator- metal transition along the core of the dislocations. A model is presented based on a three-dimensional network of such a filamentary structure to analyze the bi-stable resistive switching in the macroscopic metal-insulator-metal (MIM) structure. Our results show that electrically addressing individual dislocations in single crystals as well as epitaxial thin films provides a dynamic range for switching between low and high conducting states which covers several orders of magnitude in resistance and can be of technological interest for the application in Tbit non-volatile memory devices..   

Electrical characteristic of metal-oxide-semiconductor with NiSi$_{2}$ nanocrystals embedded in oxide layer

The nano-structured electronic devices have received more attention recently. Metal-oxide-semiconductor structure with NiSi$_{2}$ nanocrystals embedded in the oxide layer, HfO$_{2}$/SiO$_{2}$, has been fabricated. Comparing with conventional ones, it could be operated under lower voltage and faster program/erase speed and has better endurance and retention. We have measured the temperature-dependent tunneling V-I curve on these HfO$_{2}$/SiO$_{2}$ nano-structured devices for the temperature from 1.2K to 300K. The results show an abnormal electrical characteristic. The tunneling V-I characteristics appear a new threshold voltage in the low temperature region, from 30K to 100K, while applied a negative voltage. The abnormal threshold voltage disappears when the temperature higher than 150K or lower than 30K. We attribute the new threshold voltage to the discrete quantum states of NiSi$_{2}$ nanocrystals in the oxide layer.   

Photoemission Studies of Nitrided Hafnium Silicates for High-$\kappa$ Dielectrics

Nitrided hafnium silicates are strong candidate materials for replacing the SiO$_2$ gate dielectric in transistors for low standby power applications. Integrating these materials with the silicon substrate of the channel or metal gates open up a variety of interfacial issues. Photoelectron spectroscopy with its sensitivity to local chemical bonding is an invaluable tool for investigating these interfaces. Hafnium silicates were deposited using Atomic Layer Chemical Vapor Deposition and subsequently nitrided. A maximum entropy based algorithm was used to non-destructively reconstruct concentration profiles as a function of depth from angle resolved photoemission data and a good correlation was obtained from depth profile data obtained using Medium Energy Ion Scattering. Nitrogen is seen to diffuse towards the gate stack/silicon interface at higher temperatures. Trends in the nitrogen and oxygen profiles suggest replacement of the oxygen with the nitrogen during nitridation. These films were rapid thermally annealed to study their phase stability, and shifts in the photoelectron spectra reveal behavior that is not entirely consistent with what would be expected with previously reported phase segregation of these films into pure HfO$_2$ and SiO$_2$. Interfacial charge associated with these systems is reflected in the photoemission spectra, and this complements observations from other techniques.