Browsing by Author "John R. Hauser, Committee Member"

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Device Fabrication and Characterization for Alternative Gate Stack Devices
(2003-06-04) Kim, Indong; Carlton M. Osburn, Committee Chair; John R. Hauser, Committee Member; Veena Misra, Committee Member; Jon-Paul Maria, Committee Member
Aggressive scaling has continued to improve MOSFET transistor performance. An effective oxide thickness (EOT) less than 1.0nm is required for future technology nodes. However, tunneling currents of SiO2 become quite prominent below 1.5nm, leading to high leakage current. High-K dielectrics are required to reduce this leakage. A thicker dielectric reduces the probability of electron and/or hole tunneling through the gate dielectric and therefore the tunneling current. The use of metal gate electrodes is one of the technologies assumed in ITRS roadmap to circumvent the high sheet resistance and depletion associated with poly-Si gates. This dissertation covers the following research areas. First, projections of gate leakage currents for future ITRS nodes were made. High-K dielectrics which dramatically lower leakage will be needed for low standby power applications around year 2005. Secondly, NMOS and PMOS devices with alternative gate stacks were fabricated and evaluated using a new non-self aligned process. PVD HfO2 with an equivalent oxide thickness of 1.2 nm had a channel mobility comparable to SiO2. The effect of post metallization annealing of devices having PVD HfO2 was studied. Forming gas (10% H2 / 90% N2) annealing at 400° C enhanced drive current and channel mobility for devices having 1.2nm HfO2 gate dielectrics by eliminating interface states. PMA using 10% deuterium for 1.2nm HfO2 gate dielectrics resulted in larger enhancement drive currents and device channel mobility as compared to forming gas anneals. The stability of poly-Si gated HfO2 (~1.2nm EOT) dielectrics was also assessed after constant current stressing of the gate. The changes in device properties were measured as a function of stress time and stress current. With forming gas annealed HfO2, positive shifts in the threshold voltage exhibited a power law dependence on the injected charge (ΛVt ∝ QINJ 0.1). Finally, the properties of dilute Hf silicate were studied. A leakage minima was found at an intermediate Hf silicate (45~75% HfO2) composition. Nitirdation inhibited oxygen diffusion through Hf silicate dielectrics, and resulted in lower EOTs (10% lower) for nitrided samples.
Germanosilicide Contacts to Ultra-shallow P+N Junctions of Nanoscale CMOS Integrated Circuits by Selective Deposition of In-situ Doped Silicon-Germanium Alloys
(2003-04-21) Liu, Jing; Mehmet C. Ozturk, Committee Chair; John R. Hauser, Committee Member; Gregory N. Parsons, Committee Member; Carlton Osburn, Committee Member
One of the key challenges for future CMOS technology nodes is to form source/drain junctions with very small parasitic series resistance values. This requires fundamentally new junction and contact formation technologies to produce ultra-shallow junctions with super-abrupt doping profiles, above equilibrium dopant activation and contact resistivity values near 10⁻⁸ ohm-cm². Recently, this laboratory demonstrated a new junction formation technology based on selective deposition of heavily doped Si[subscript 1-x]Ge[subscript x] alloys in source/drain regions isotropically etched to the desired depth. The results to date indicate that the technology has the potential to meet all junction and contact requirements of future CMOS technology nodes. Of particular interest to this thesis is the smaller bandgap of Si[subscript 1-x]Ge[subscript x] resulting in a smaller metal-semiconductor barrier height, which is a key advantage in reducing the contact resistivity of a metal-semiconductor contact. In this work, formation of germanosilicide contacts to heavily boron doped Si[subscript 1-x]Ge[subscript x] alloys was studied with the intention to find a contact solution for future CMOS technology nodes beyond 100 nm. During the early stages of the research project, germanosilicides of Ti, Co, Ni, Pt, W, Ta, Mo and Zr were studied to identify the most promising candidates as source/drain contacts. The first set of experiments showed that Zr, Ni and Pt may have advantages over other candidates. Of the three germanosilicides, zirconium di-germanosilicide, Zr(Si[subscript 1-x]Ge[subscript x])₂ exhibited the best thermal stability but suffered from a high resistivity and excessive substrate consumption. Ni and Pt germanosilicides were considered attractive because they were both mono-germanosilicides and consumed much less Si[subscript 1-x]Ge[subscript x] than Zr(Si[subscript 1-x]Ge[subscript x])₂. Additionally, both had resistivity values lower than that of Zr germanosilicide which could be reached at temperatures as low as 300 °C. Of the three germanosilicides, NiSi[subscript 1-x]Ge[subscript x] was found to be the only one capable of yielding the desired contact resistivity of ˜ 10⁻⁸ ohm-cm² on both p⁺ and n⁺ Si[subscript 1-x]Ge[subscript x]. Unfortunately, NiSi[subscript 1-x]Ge[subscript x] was found to suffer from Ge out-diffusion, which had a direct negative impact on its thermal stability. NiSi[subscript 1-x]Ge[subscript x] formed at temperatures above 450 °C exhibited high sheet resistance and a rough germanosilicide⁄Si[subscript 1-x]Ge[subscript x] interface. Below this temperature, ultra-shallow p⁺-n juntions with self-aligned NiSi[subscript 1-x]Ge[subscript x] contacts were formed with excellent reverse bias junction leakage characteristics. It was also observed that the thermal stability of NiSi[subscript 1-x]Ge[subscript x] formed on heavily boron doped Si[subscript 1-x]Ge[subscript x] was noticeably better. A new approach was proposed to form ultra-thin Ni Si[subscript 1-x]Ge[subscript x] layers with enhanced thermal stability. By inserting a thin Pt interlayer between Ni and Si[subscript 1-x]Ge[subscript x], the thermal stability of NiSi[subscript 1-x]Ge[subscript x] was found to be significantly improved. On boron doped Si[subscript 1-x]Ge[subscript x], the material was found to be stable at least up to 700 °C with a total starting metal thickness of 10 nm. Pt incorporation was also found to result in better surface and interface roughness. This work has shown that high quality boron doped Si[subscript 1-x]Ge[subscript x] source⁄drain junctions with NiSi[subscript 1-x]Ge[subscript x] contacts can be formed. The junctions exhibit contact resistivity values near 10⁻⁸ ohm-cm², which satisfies the requirements of future CMOS technology nodes.
Redox-active Organic Molecules on Silicon and Silicon Dioxide Surfaces for Hybrid Silicon-molecular Memory Devices.
(2006-11-17) Mathur, Guruvayurappan; Jonathan S. Lindsey, Committee Member; Eric Rotenberg, Committee Member; John R. Hauser, Committee Member; Veena Misra, Committee Chair
The focus of this dissertation is on creating electronic devices that utilize unique charge storage properties of redox-active organic molecules for memory applications. A hybrid silicon-molecular approach has been adopted to make use of the advantages of the existing silicon technology, as well as to study and exploit the interaction between the organic molecules and the bulk semiconductor. As technology heads into the nano regime, this hybrid approach may prove to be the bridge between the existing Si-only technology and a future molecule-only technology. Functionalized monolayers of redox-active molecules were formed on silicon surfaces of different doping types and densities. Electrolyte-molecule-silicon test structures were electrically characterized and studied using cyclic voltammetry and impedance spectroscopy techniques. The dependence of the oxidation and reduction processes on the silicon doping type and density were analyzed and explained using voltage balance equations and surface potentials of silicon. The role played by the silicon substrate on the operation of these memory devices was identified. Multiple bits in a single cell were achieved using either molecules exhibiting multiple stable redox states or mixed monolayer of different molecules. Self-assembled monolayers of redox-active molecules were also incorporated on varying thickness of silicon dioxide on n- and p- silicon substrates in an attempt to create non-volatile memory. The dependences of read/write/erase voltages and retention times of these devices were correlated to the SiO2 thickness by using a combination of Butler-Volmer and semiconductor theories. The region of operation of the silicon surface (accumulation, depletion or inversion) and the extent of tunneling current through the silicon dioxide were found to influence the charging and discharging of the molecules in the monolayer. Increased retention times due to the presence of SiO2 can be useful in realizing non-volatile memories. Polymeric films of molecules were formed on Si and SiO2 substrates and exhibited very high surface densities. Metal films were deposited directly on these films and the resultant devices were found to exhibit redox-independent behavior. A combination of metal gate and dielectric was deposited on molecules in an attempt to create solid-state hybrid silicon-molecular devices. The metal gate and dielectric can replace the electrolyte and electrolytic double-layer to create an electronic cell instead of an ionic cell. The redox properties of the molecules were retained after the deposition of dielectric and metal, which augurs well for a solid-state device. FET type structures were fabricated and molecules incorporated on them in order to modulate the characteristics of the FETs by charging and discharging the molecules. Drain current and transfer characteristics of electrolyte-gated "moleFETs" were modulated by oxidizing and reducing molecules on the channel region. Hybrid moleFET devices may be ideal tools for creating non-volatile FLASH type memory devices. This work has recognized the interaction of organic molecules and bulk silicon and utilized the advantages of current CMOS technology along with the unique properties of molecules, such as discrete quantum states, low voltage operation etc., to create a class of hybrid memory devices. A way to create solid-state molecular devices retaining the inherent properties of molecules has been proposed and demonstrated. This work might be useful in providing a smooth transition from silicon electronics to molecular electronics.