HfO2 as gate dielectric on Si and Ge substrate

Hafnium oxide HfO2 has been considered as an alternative to silicon dioxide SiO2 in future nano-scale complementary metal-oxide-semiconductor (CMOS) devices since it provides the required capacitance at the reduced device size because of its high dielectric constant. HfO2 films are currently deposited by various techniques. Many of them require high temperature annealing that can impact device performance and reliability. In this research, electrical characteristics of capacitors with HfO2 as gate dielectric deposited by standard thermal evaporation and e-beam evaporation on Si and Ge substrates were investigated. The dielectric constant of HfO2 deposited by thermal evaporation on Si is in the range of 18-25. Al/HfO2/Si MOS capacitors annealed at 450°C show low hysteresis, leakage current density and bulk oxide charges. Interface state density and low temperature charge trapping behavior of these structures were also investigated. Degradation in surface carrier mobility has been reported in Si field-effect-transistors with HfO2 as gate dielectric. To explore the possibility of alleviating this problem we have used germanium (Ge) substrate as this semiconductor has higher carrier mobility than Si. Devices fabricated by depositing HfO2 directly on Ge by standard thermal evaporation were found to be too leaky and show significant hysteresis and large shift in flatband voltage. This deterioration in electrical performance is mainly due to the formation of unstable interfacial layer of GeO2 during the HfO2 deposition. To minimize this effect, Ge surface was treated with the beam of atomic nitrogen prior to the dielectric deposition. The effect of surface nitridation, on interface as well as on bulk oxide, trap energy levels were investigated using low temperature C-V measurements. They revealed additional defect levels in the nitrided devices indicating diffusion of nitrogen from interface into the bulk oxide. Impact of surface nitridation on the reliability of Ge/HfO2/Al MOS capacitors has been investigated by application of constant voltage stress at different voltage levels for various time periods. It was observed that deeper trap levels in nitrided devices, found from low frequency and low temperature measurements, trap the charge carrier immediately after stress but with time these carriers detrap and create more traps inside the bulk oxide resulting in further devices deterioration. It is inferred that though nitrogen is effective in reducing interfacial layer growth it incorporates more defects at interface as well as in bulk oxide. Therefore, it is important to look into alternative methods of surface passivation to limit the growth of GeO2 at the interface

Reliability Analysis of Hafnium Oxide Dielectric Based Nanoelectronics

With the physical dimensions ever scaling down, the increasing level of sophistication in nano-electronics requires a comprehensive and multidisciplinary reliability investigation. A kind of nano-devices, HfO2-based high-k dielectric films, are studied in the statistical aspect of reliability as well as electrical and physical aspects of reliability characterization, including charge trapping and degradation mechanisms, breakdown modes and bathtub failure rate estimation. This research characterizes charge trapping and investigates degradation mechanisms in high-k dielectrics. Positive charges trapped in both bulk and interface contribute to the interface state generation and flat band voltage shift when electrons are injected from the gate under a negative gate bias condition.A negligible number of defects are generated until the stress voltage increases to a certain level. As results of hot electrons and positive charges trapped in the interface region, the difference in the breakdown sequence is attributed to the physical thickness of the bulk high-k layer and the structure of the interface layer. Time-to-breakdown data collected in the accelerated life tests are modeled with a bathtub failure rate curve by a 3-step Bayesian approach. Rather than individually considering each stress level in accelerating life tests (ALT), this approach derives the change point and the priors for Bayesian analysis from the time-to-failure data under neighborhood stresses, based on the relationship between the lifetime and stress voltage. This method can provide a fast and reliable estimation of failure rate for burn-in optimization when only a small sample of data is available

Resistive switching devices with improved control of oxygen vacancies dynamics

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Energetic deposition of dielectric metal oxide coatings

This thesis examines the optical and electronic properties of wide-bandgap metal oxides grown using energetic deposition methods. The films have also been incorporated in metal/oxide/metal devices for use as high-κ gate oxide materials and memristive devices. This study aimed to assess the potential advantages of energetic deposition methods compared to conventional physical vapour deposition techniques for depositing high quality dielectric thin films. In particular, the influence of dopants, native defects and impurities on the performance of optical and electrical devices incorporating energetically deposited metal oxides was explored Firstly, thin films of HfO2 were energetically deposited from a filtered cathodic vacuum arc (FCVA) at room temperature and found to exhibit higher density and significantly lower current leakage than HfO2 deposited by conventional, low energy physical vapour deposition techniques. Capacitance-voltage measurements performed on the FCVA HfO2 film revealed a low threshold voltage shift (ΔVFB = 0.60 V) corresponding to low fixed oxide charge density (8.3 × 1011 cm−2). A wide optical bandgap of 6.0 eV and high refractive index of >2.1 in the visible spectrum were also observed. The excellent optical and electrical properties of the FCVA deposited HfO2 are attributed to film densification caused by the energetic depositing flux. Secondly, HfO2-xNx films were energetically deposited by high-power impulse magnetron sputtering (HiPIMS). The passivation of oxygen vacancies (Ov) defects by nitrogen and local bonding of the resultant films was investigated by X-ray absorption spectroscopy (XAS). It was found that Ov densities in HfO2-xNx films decrease with increasing N2 partial pressure during deposition. Passivation was achieved by atomic nitrogen substitution and N2 interstitials at the Ov site. The incorporation of low mobility interstitial species inhibited crystallisation. These effects combine to greatly reduce electron leakage paths in the films resulting in high breakdown strengths. The refractive indices of HfO2-xNx films were found to be dependent on N2 pressure during deposition. Increases in the refractive index were attributed to high densities of N2 interstitials. The substitution of atomic nitrogen for Ov did not significantly reduce the optical bandgap, allowing sufficient band offsets for the HfO2-xNx films to be used as an effective gate dielectric. The resistive switching mechanisms in memristors formed on monoclinic HfO2 and HfO1.86N0.14 films deposited by HiPIMS were investigated. Gradual conductance modulation, short-term plasticity (STP) and long-term potentiation (LTP) were implemented in HfO2 memristors with high Ov densities using voltage-spike stimulation, suggesting suitability for electronic synaptic applications. The switching dynamics of the HfO2 memristor were explained by the interactions of Ov with grain boundaries. The passivation of Ov defects by nitrogen in the HfO1.86N0.14 film was confirmed by XAS. The HfO1.86N0.14 memristor exhibited threshold switching and current-controlled negative differential resistance (CC-NDR). Numerical modeling showed this behaviour to be Joule heating-induced. These findings suggest simple bilayer selector/memory cells could be fabricated from hafnia heterolayers. Lastly, the neuronal properties of a zinc tin oxide/silver oxide memristor fabricated by FCVA were demonstrated. STP, LTP and spike-timing dependent plasticity learning/memory functions have been described and explained by Ag+/O- electromigration across a dielectric interfacial layer. Importantly, CC-NDR induced oscillations were observed in the same device above room temperature. This discovery paves the way for transistor-free neuromorphic architectures based solely on memristors, thereby enabling the next generation of non-von Neumann computing architectures

Analysis of Performance Instabilities of Hafnia-Based Ferroelectrics Using Modulus Spectroscopy and Thermally Stimulated Depolarization Currents

The discovery of the ferroelectric orthorhombic phase in doped hafnia films has sparked immense research efforts. Presently, a major obstacle for hafnia's use in high-endurance memory applications like nonvolatile random-access memories is its unstable ferroelectric response during field cycling. Different mechanisms are proposed to explain this instability including field-induced phase change, electron trapping, and oxygen vacancy diffusion. However, none of these is able to fully explain the complete behavior and interdependencies of these phenomena. Up to now, no complete root cause for fatigue, wake-up, and imprint effects is presented. In this study, the first evidence for the presence of singly and doubly positively charged oxygen vacancies in hafnia–zirconia films using thermally stimulated currents and impedance spectroscopy is presented. Moreover, it is shown that interaction of these defects with electrons at the interfaces to the electrodes may cause the observed instability of the ferroelectric performance

Understanding the Electrochemical Behavior of Monolithic, Bilayer, and Electrically Coupled Nano-Scaled Films in Different Environments

Advances in integrated circuit (IC) fabrication has led to microelectronic devices with sub-micrometer size features. These small features have been used to enhance the functionality of devices including faster processing. At the same time, this feature miniaturization has also caused unintended reliability concerns from corrosion. While the susceptibility of materials to degradation has not changed, the small length scales have meant that a shorter time of or amount of corrosion can lead to device failure. Prior work in understanding corrosion in microelectronic devices has included classifying failure mechanisms in microelectronic devices. These works indicated that the primary factors contributing to corrosion events included moisture in the operating environment, ionic contaminants diffusing into the devices, small distances between metallic lines that created large electric fields, and electric contact between dissimilar materials. While there has been substantial research on bulk material corrosion mechanisms, more work is needed to understand the corrosion of films and features. The intention of this study was to understand the electrochemical performance of Ti and TiNx films. This material system was selected due to its wide appearance in microelectronic devices as adhesion layer and diffusion barrier layer for the metallization. Ti/TiN layered systems also appear in other coating applications such as wear-resistant coatings, where the architectural arrangement of layers has been of interest to researchers. These layered configurations could lead to both galvanic and interfacial corrosion. The aims of the research were to (1) understand how the pH of electrolyte influences the electrochemical behavior of Ti and TiNx thin films independently; (2) evaluate the stability of electrochemical performance of the Ti and TiNx films during 105 days of exposure to each electrolyte; (3) study effect of galvanic coupling on electrochemical performance of Ti and TiNx; and (4) determine if macro-scale corrosion tests can show a considerable difference between the behavior of Ti/TiN multilayered and electrically coupled samples. Monolithic Ti and TiN, bilayered Ti/TiN, and nanolaminated Ti/TiN with 100 nm thick layers in all cases were deposited onto (100) Si wafers using sputtering deposition. Electrolytes were 3 wt. % chloride ions with different pH values, ranging from acidic to basic. While all wafers in a process chamber were considered a single sample, we will report the tests of the replicates made from each sample. Four (4) sample replicates were kept in each electrolyte and tested independently. Results showed more negative potentials for monolithic and electrically coupled systems in the basic electrolyte. The range of OCP values in the basic cell was -0.2 V to -0.8 V, while it was -0.4 V to +0.4 V in the neutral and acidic cells. Some instabilities were observed in basic electrolytes in terms of fluctuations in OCP curves. We expected to see some differences between the behavior of monolithic Ti and TiN because of having oxide layers of different stoichiometry and composition, and the difference between the two was pronounced in the acidic electrolyte with an order of magnitude higher polarization resistance for TiN. Electrically coupling Ti and TiN affected the behavior of Ti in acidic cell. Macro-scale corrosion tests such as OCP and LPR did not show a considerable difference between the electrochemical performance of bilayered Ti/TiN and electrically coupled Ti/TiN. However, Mott-Schottky analysis showed different flat-band potentials (-0.08 V for bilayer and -2.34 for electrically coupled system). EIS technique showed that the peaks in the low frequency range of Bode phase diagrams appeared at different frequencies for bilayer and electrically coupled films

Optimization of performance and reliability of HZO-based capacitors for ferroelectric memory applications

In an era in which the amount of produced and stored data continues to exponentially grow, standard memory concepts start showing size, power consumption and costs limitation which make the search for alternative device concepts essential. Within a context where new technologies such as DRAM, magnetic RAM, resistive RAM, phase change memories and eFlash are explored and optimized, ferroelectric memory devices like FeRAM seem to showcase a whole range of properties which could satisfy market needs, offering the possibility of creating a non-volatile RAM. In fact, hafnia and zirconia-based ferroelectric materials opened up a new scenario in the memory technology scene, overcoming the dimension scaling limitations and the integration difficulties presented by their predecessors perovskite ferroelectrics. In particular, HfₓZr₁₋ₓO₂ stands out because of high processing flexibility and ease of integration in the standard semiconductor industry process flows for CMOS fabrication. Nonetheless, further understanding is necessary in order tocorrelate device performance and reliability to the establishment of ferroelectricity itself. The aim of this work is to investigate how the composition of the ferroelectric oxide, together with the one of the electrode materials influence the behavior of a ferroelectric RAM. With this goal, different process parameters and reliability properties are considered and an analysis of the polarization reversal is performed. Starting from undoped hafnia and zirconia and subsequently examining their intermixed system, it is shown how surface/volume energy contributions, mechanical stress and oxygen-related defects all concur in the formation of the ferroelectric phase. Based on the process optimization of an HfₓZr₁₋ₓO₂-based capacitor performed within these pages, a 64 kbit 1T1C FeRAM array is demonstrated by Sony Semiconductor Solutions Corporation which shows write voltage and latency as low as 2.0 V and 16 ns, respectively. Outstanding retention and endurance performances are also predicted, which make the addressed device an extremely strong competitor in the semiconductor scene

Total Ionizing Dose Response of High-k Dielectrics on MOS Devices

As advanced Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) technology continues to minimize the gate oxide thickness, the exponential increase in gate leakage current poses a major challenge for silicon dioxide (SiO2) based devices. In order to reduce the gate leakage current while maintaining the same gate capacitance, alternative gate insulator materials with higher dielectric constant (high-k) became the preferred replacement of SiO2 gate dielectrics. Germanium (Ge) MOSFETs have been regarded as promising candidates for future high-speed applications because they possess higher carrier mobility when compared to silicon based devices. At present, advanced microelectronics devices and circuits are used in aerospace engineering, nuclear industry, and radiotherapy equipment. These applications are unavoidably exposed to space-like radiation, which has a relative low radiation dose rate at 10-2-10-6 rad(Si)/s. For these reasons, it is necessary to understand the low-dose-rate radiation response of high-k materials based on Si and Ge MOS devices. The radiation response of high-k materials such as radiation-induced oxide and interface trap density have been typically examined by carrying out off-site capacitance-voltage (CV) measurements. However, the conventional and off-site radiation response measurements may underestimate the degradation of MOS devices. In this study, a semi-automated laboratory-scale real-time and on-site radiation response testing system was developed to evaluate the radiationresponse. The system is capable of estimating the radiation response of MOS devices whilst the devices are continuously irradiated by -rays raysrays. Moreover, the complete CV characteristics of MOS capacitors were measured in a relatively short time. The pulse CV measurement reduces the impact of charge trapping behavior on the measurement results, when compared to conventional techniques. The total ionizing dose radiation effect on HfO2 dielectric thin films prepared by atomic layer deposition (ALD) has been investigated by the proposed measurement system. The large bidirectional ΔVFB of the irradiated HfO2 capacitor was mainly attributed to the radiation-induced oxide trapped charges, which were not readily compensated by bias-induced charges produced over the measurement timescales of less than 5 ms. Radiation response of Ge MOS capacitors with HfO2 and HfxZr1-xOy gate dielectrics was also investigated. It was found that radiation-induced interface traps were the dominant factor for Flat-band Voltage shift (ΔVFB) in HfO2 thin films, whereas the radiation response for Zr-containing dielectrics under positive bias was mainly affected by oxide traps. Under positive biased irradiation, the Zr-doped HfxZr1-xOy exhibited smaller ΔVFB than that of HfO2. This is attributed to the de-passivation of Ge-S bonds in capacitors incorporating HfO2 thin films, resulting in the build-up of interface traps. Under negative biased irradiation, ΔVFB was attributed to the combined effect of the net oxide trapped charges and the passivation of Ge dangling bonds at the Ge/high-k interface