Browsing by Author "Gerald Lucovsky, Committee Co-Chair"

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Breakdown and Reliability of CMOS Devices with Stacked Oxide/Nitride and Oxynitride Gate Dielectrics Prepared by RPECVD
(2003-06-09) Lee, Yi-Mu; Carlton Osburn, Committee Co-Chair; Gerald Lucovsky, Committee Co-Chair; John Hauser, Committee Member; Veena Misra, Committee Member
Remote-plasma-enhanced CVD (RPECVD) silicon nitride and silicon oxynitride alloys have been proposed to be the attractive alternatives to replace conventional oxides as the CMOS logic and memory technology node is scaled beyond 100 nm. This dissertation is focused on the degradation and breakdown of RPECVD stacked oxide/nitride (O/N) and oxynitride gate dielectrics under constant-current stress (CCS) and constant-voltage stress (CVS). By monitoring the time-to-breakdown of the dielectrics, the device reliability can be determined and further used to evaluate the dielectric quality and the scaling limits of the dielectric thickness. It is found that the breakdown behavior of the gate oxide and RPECVD gate dielectrics is influenced by the degree of boron penetration, which in turn leads to increases in the gate leakage current. During electrical stresses, positive charges and hole trapping are generated at the Si/SiO2 interface and also in the dielectric layer, resulting in device degradation and final breakdown. We successfully use the RPECVD technique to incorporate an ultrathin (~0.6 nm) interfacial oxide layer and one monolayer of nitrogen in the gate stacks to improve the interface properties. Therefore, the stress-induced charges and trapping are suppressed and the device performance including SILC, threshold voltage instability, drive current and switching characteristics is improved. In addition, shorter-channel devices show more degraded electrical properties compared to longer-channel devices due to the increased damaged region in the gate-drain overlap near the channel. The TDDB reliability and lifetime of MOS devices with RPECVD O/N gate dielectric for the foreseeable mobile application are also investigated. This study is the first to reveal the trend of Weibull slopes and activation energy of O/N gate stacks. It has been found that Poisson area scaling is valid for O/N gate stack, indicating that the intrinsic breakdown is a random process and can be explained by the percolation model. Also, the voltage and temperature acceleration parameters are determined from TDDB. The projection of device lifetime based on total chip area and low percentile failure rate is demonstrated. The maximum tolerable operating voltage for a total gate area of 0.1 cm2 and 0.01% failure rate at 125° C is projected to be 1.9 V for 2.07 nm stacked O/N gate dielectrics.
EXAFS Studies of Ge-Sb-Te Alloys for Phase-Change Applications
(2007-12-07) Baker, David Andrew; David Aspnes, Committee Member; Michael A. Paesler, Committee Co-Chair; Gerald Lucovsky, Committee Co-Chair; Gerald Iafrate, Committee Member
Studies of amorphous (a-) semiconductors have been driven by technological advances as well as fundamental theories. Observation of electrical switching, for example, fueled early interest in a-chalcogenides. More recently a-chalcogenide switching has been applied successfully to programmable memory devices as well as DVD technology where the quest for the discovery of better-suited materials continues. Thus, switching grants researchers today with an active arena of technological as well as fundamental study. Bond constraint theory (BCT) and rigidity theory provide a powerful framework for understanding the structure and properties of a-materials. Application of these theories to switching in a-chalcogenides holds the promise of finding the best composition suited for switching applications. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is an ideally suited technique to investigate the switching properties of these materials. Films of amorphous Ge2Sb2Te4, Ge2Sb2Te5, and Ge2Sb2Te7 exhibit differing bonding structures and bond statistics, which result in different electronic and optical properties. Results of new EXAFS experiments on these three critical compositions in the Ge-Sb-Te system are presented in light of BCT and rigidity theory.
GaN-dielectric interface formation for gate dielectrics and passivation layers using remote plasma processing
(2003-07-03) Bae, Choelhwyi; Gerald Lucovsky, Committee Co-Chair
In previous studies, device quality Si-SiO2 interfaces and dielectric bulk films (SiO2) were prepared using a two-step process; i) remote plasma-assisted oxidation (RPAO) to form a superficially interfacial oxide (~0.6 nm) and ii) remote plasma enhanced chemical vapor deposition (RPECVD) to deposit the oxide film. The same approach has been applied to the GaN-SiO2 system. After a 300 oC remote N2/He plasma treatment of the GaN surface, residual C and Cl were reduced below Auger electron spectroscopy (AES) detection, and the AES peak ratio of O KLL and N KLL was ~0.06 or ~0.1 monolayer of oxygen. RPAO of GaN surfaces using O2, N2O, and N2O in N2 source gases were investigated by on-line AES to determine the oxidation kinetics and chemical composition of the interfacial oxide. Without an RPAO step, subcutaneous oxidation of GaN takes place during RPECVD deposition of SiO2, and on-line AES indicates a ~0.6-0.8 nm subcutaneous oxide. Compared to single step SiO2 deposition, significantly reduced interface state density (Dit) was obtained at the GaN-SiO2 interface by independent control of GaN-Ga2O3 interface formation by thin RPAO oxide (~1 nm) and SiO2 film deposition by RPECVD. High-low frequency method and conductance method indicate that Dit of GaN Metal-Oxide-Semiconductor (MOS) sample without RPAO is ~5 times larger than that of the sample with RPAO. For the GaN MOS structure with remote plasma oxidation and nitridation, Dit determined at DCmax was low-to-mid x 1011 cm-2eV-1. Also, we report on high temperature and photo-assisted capacitance-voltage (C-V) characteristics.
The Local Structure and Kinetics of Ge-Sb-Te Phase Change Materials for use in Solid State Applications
(2010-04-12) Washington, Joseph St. Paul; Michael A. Paesler, Committee Chair; Nicholas C.M. Fuller, Committee Member; Eric A. Joseph, Committee Member; John E. Rowe, Committee Member; David E. Aspnes, Committee Member; Gerald Lucovsky, Committee Co-Chair
Recent interest in phase change materials (PCMs) for non-volatile memory applications has been fueled by the promise of scalability beyond the limit of conventional DRAM and NAND flash memory. Typical PCMs such as Ge2Sb2Te5 (GST) require significant nitrogen doping to shift their crystallization temperature (Tx) sufficiently above standard CMOS device operation ranges (~ 80ÂºC ) but also well below the melting point for thermal stability. Reactive ion etching (RIE) in an Ar/Cl2/CHF3 plasma chemistry is another crucial step en-route to fabricating energy efficient, high density, nano-scaled PCM memory devices, yet it can lead to unfavorable, irreversible modification of the GST material. Chalcogens such as Te in GST can easily diffuse and interact unfavorably with the adjacent materials in the device structure thus negatively impacting the lifetime of a PCM device cell. In light of these implications for the final solid state device, it is necessary to understand and articulate the nature and structural implications of doping/alloying GST, the local structural changes that occur post etch processing, and the nature of the Ge-Sb binary and the Ge-Sb-Te ternary alloys. Fourier Transform Infrared Spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine spectroscopy (XAFS), and in-situ, time resolved, X-ray diffraction (XRD) is used to understand the local structure of nitrogen in GST, and results point to the formation of preferential Ge-N bonding in a chemically ordered germanium nitride local bonding environment in as-deposited and annealed films. XAFS of various GeSb and Ge-Sb-Te glasses in the binary and ternary systems, in conjunction with time resolved XRD, show that Te in thin films of GeSb with gradually increasing Te atomic concentration prevents phase segregation, promotes stability, and induces nucleation. A multi-edge refinement of as-deposited thin films of Ge2Sb2Tex (x=4, 5, 6, 7), shows that Ge-Sb bonds are present in Ge2Sb2Te4 and Ge2Sb2Te5 from EXAFS fits, and these Ge-Sb bonds can also be isolated in the Ge near edge spectra in light of inelastic losses, i.e. shake-up / shake-off effects. The chemical and structural effects of RIE on the crystallization of N-GST is examined via XPS, XAFS, time resolved laser reflectivity and XRD. Time resolved laser reflectivity and XRD show that exposure to various etch and ash chemistries significantly reduces the crystallization speed, while the transition temperature from the rocksalt to the hexagonal phase is increased. XPS and XAFS attribute this to the selective removal and oxidization of N, Ge, Sb, and Te, thus altering the local bonding environment to the detriment of device performance.