R-3 Repository :: Browsing by Author "Wilson, William L., Jr."

Browsing by Author "Wilson, William L., Jr."

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A study of the photodissociation dynamics of dimethylzinc and the implications for the growth of zinc selenide films
(1993) Elias, Joseph A.; Wilson, William L., Jr.
The dynamics of the ArF photodissociation of DMZn (dimethylzinc) have been studied. The implications of the results for the growth of ZnSe thin films are discussed. Time-resolved absorption profiles of ground state Zn and methyl radicals have been acquired using a plasma emission source. Time-resolved fluorescence from excited CH radicals has also been studied. The results indicate that the Zn concentration remains constant after the dissociating ArF pulse, indicating the system is a good source for free Zn atoms. The CH$\sb3$ radicals that are created recombine to form ethane, and simulations of the kinetics indicate that the concentration of methyl radicals reaching the substrate is insignificant. There was no indication of monomethylzinc formation. The CH radical is likely to combine with H$\sb2$ to form CH$\sb3$ within several microseconds. Thus the major sources for carbon contamination in the growth process (CH$\sb3$, MMZn, CH) are unlikely to reach the substrate.
Development of a 500 MW, one-microsecond, multi-kiloampere relativistic klystron amplifier
(1996) Haynes, William Brian; Wilson, William L., Jr.
This work presents research on the one-microsecond, L-band relativistic klystron amplifier (RKA) project conducted at Los Alamos National Laboratory. A collection of theoretical analyses is presented for rf cavities, intense electron beams, beam-cavity interactions, and small-signal klystron bunching. Electromagnetic field simulations were done for three dimensional cavity structures using HFSS with very accurate results. Particle-in-cell simulations of the complete RKA were done using the two dimensional code ISIS. Extraction efficiency for intense modulated beams is discussed and verified in simulations. Designs for input and idler cavities are reviewed. Extremely low-Q, single-gap, output cavities are investigated for coupling rf power from very low-impedance, modulated, electron beams. Output cavities with a Q less than 4 have been designed, measured, and tested. Methods were implemented for designing 2D equivalent output cavity structures to model 3D structures in 2D codes. A technique for ex-situ rf conditioning of the output cavity gap pieces is presented. A beam-pipe center conductor, intended to reduce the space-charge potential depression of the beam, is discussed. Diagnostics for intense-beam and high-power rf measurements are presented. A coaxial directional coupler and load, capable of handling more than 500 MW at 1300 MHz, were designed. Mode conversion from coax to waveguide is discussed for $>$100 MW power levels. Methods for determining the gap voltage in an operating cavity are presented. Pulse-shortening of the rf in the RKA is also discussed. A 650 kV, 5 kA, one-microsecond, annular beam has been produced from a stainless-steel, explosive-field-emission cathode. The beam current was modulated up to 70% $(I\sb1 /I\sb0$ = 70%) using a two-cavity bunching section operating at 1300 MHz. RKA structures simulated in ISIS have extracted up to 250 MW. This number was consistent with the extracted power actually measured in the equivalent experiment. Overall energy extraction was as high as 160 J per pulse. The average rf output power coupled into the 6-inch-diameter coax transmission line was approximately: 300 MW for 300 ns, 250 MW for 500 ns, and 100 MW for 1 $\mu$s. Peak power levels as high as 475 MW have also been produced.
Electrical and thermal modeling of electrostatic discharge protection structures for submicron VLSI
(1989) Stiegler, Harvey J.; Wilson, William L., Jr.
A modeling technique has been developed which simulates a semiconductor device subjected to electrostatic discharge (ESD) stress according to the human body model (HBM). To accomplish this, a computer program was developed which solves the electron and hole continuity equations, Poisson's equation, and the heat flow equation in one dimension. The program has been applied to npn structures typical of the parasitic bipolar devices found in MOS output stages. Profiles from lightly-doped drain (LDD), double-diffused drain (DDD), and graded drain (GD) device structures were investigated. The performance of these various profiles under ESD stress has been compared in order to understand their functioning and to determine the important design parameters. It is found that device heating is reduced for structures in which the doping profile rises steeply to a high concentration in the drain region near the metallurgical junction. The rate of heating is related to reduced carrier saturation velocity due to local heating and its effects on charge distribution, electric field, and total potential drop across the reverse-biased junction. The modeling technique presented gives results which are in reasonable agreement with measured data. This technique should be a useful tool for evaluating new device structures, fabrication processes, or process changes before committing to the costly and time-consuming process of actual device fabrication.
Electrical characterization of EEPROM test structures
(1996) Elias, Joseph Andrew; Wilson, William L., Jr.
Electrically Erasable Programmable Read Only Memory (EEPROM) test structures have been studied using Fowler-Nordheim (FN) tunneling and low-frequency noise measurements, both before and after electrical stress. Non-linear FN curves have been observed, which can yield insights into the failure mechanisms of the EEPROMs. A model is proposed where the barrier width is locally widened due to charge trapping in the oxide. This model is based on the interpretation of the non-linear FN curves, which leads to a failure mechanism that may be dependent on the tunneling current density. As the EEPROM test structure is stressed, the trapped charge causes the barrier width between the cathode conduction band and the oxide conduction band to distort. This distortion widens the tunneling barrier locally, and forces current into other regions with narrower barrier widths. This reduces the effective tunneling area of the structure and leads to an increased tunneling current density, which may be responsible for device failure. Asperities in the regions of high current density may also affect the tunneling current. Curve fits to the data must take into account the area and field enhancement dependencies. The curve fits allow for variations in tunneling area, field enhancement, and current density to be used as metrics for comparison of device lifetimes. Noise measurements were done to attempt to correlate tunneling current noise with the device lifetime. The results were inconclusive, as burst noise was dominant and this noise was not a direct function of stress on the device. A model is proposed which is consistent with the data observed with the FN and noise measurements.
ESD circuit synthesis and analysis using TCAD and SPICE
(1999) Rodriguez, Juan Antonio; Wilson, William L., Jr.
This thesis describes the development of a SPICE sub-circuit model for an avalanche triggered SCR used for ESD protection in integrated circuits. The purpose of this work was to develop a model that accurately predicts the terminal characteristics of the SCR operating under steady state and transient conditions. Process and device simulations (using Technology Computer Aided Design tools, or TCAD tools) were used to gain insight into the dynamics of the complex SCR behavior prior to and during the latchup triggering. Test structures of the SCR, as well as its sub-components, were fabricated for characterization and modeling data collection. The TCAD results gave us access to internal physical quantities at key points along the I--V characteristics, which explicitly indicated the dynamics leading to latchup triggering. These analyses and the test structure characterization were necessary to properly formulate the SPICE model. The SPICE model development approach is presented, as well as methods to validate the model including steady state and fast rise time transient measurements on the actual ESD circuit. This is the first SPICE model presented for an avalanche triggered SCR demonstrating accurate terminal behavior under both steady state and transient triggering conditions. It is intended for use in a design environment for examining ESD circuit behavior at the chip level. The model allows a way to synthesize new circuits in a simulation environment without the need to fabricate test circuits and variations in silicon. Furthermore, the physical insight gained from the models will become more important as process technologies scale into deep submicron feature sizes.
Laser-assisted metalorganic chemical vapor deposition of zinc sulfide
(1997) Stokes, Scott Wilson; Wilson, William L., Jr.
Laser-assisted metalorganic chemical vapor deposition (MOCVD) was used to grow crystalline zinc sulfide at temperatures as low as 200$\sp\circ$C. The metalorganic sources, dimethylzinc and diethylsulfide, were photodissociated with radiation from a 193 nm ArF excimer laser passing parallel to the substrate. Epitaxial films were grown on gallium arsenide by both the thermal and laser-assisted MOCVD methods. Zinc sulfide films grown on silicon substrates were polycrystalline possibly due to a reaction between sulfur and the silicon substrate creating an amorphous compound at the interface. X-ray diffraction and photoluminescence results indicate that films grown by laser-assisted MOCVD below 500$\sp\circ$C on either substrate were not as high quality as those grown above 500$\sp\circ$C. It is postulated that increased hydrocarbon contamination in the low temperature laser-assisted MOCVD grown films is responsible for the decrease in film quality.
Low-frequency noise in spin valve sensors
(1999) Stokes, Scott Wilson; Wilson, William L., Jr.
Low frequency noise in giant magnetoresistive spin valves has been studied as a means of optimizing signal to noise ratios and characterizing device performance. The devices studied were sputter deposited NiFe/Cu/NiFe/FeMn spin valves with D R/R ∼ 4%. Static measurements demonstrated a strong dependence of the magnetic coupling and giant magnetoresistance (GMR) ratio on the thickness and quality of the Cu spacer layer and the bottom NiFe layer (free layer). These parameters were varied to determine how the noise in spin valve sensors would be affected. Noise power spectra were measured in patterned spin valves. The noise was observed to have a 1/f slope at low frequencies. The fluctuation-dissipation relation relating thermal fluctuations in magnetization to the resistance fluctuations was used to explain the origin of the 1/f noise. The noise was found to be sensitive to the anisotropy and defect density of the free layer. The noise was minimized for spin valves operating with parallel anisotropy axes and an applied field aligned along the hard axis of magnetization. Dynamic fields were used to measure the Barkhausen noise in the sense layer of the spin valve. The low frequency noise in the presence of dynamic fields was much greater than the 1/f noise background. Clustering of Barkhausen jumps was used to explain the observed dependence of the noise power on the magnitude and frequency of the applied field. Higher frequency signals resulted in lower Barkhausen noise. The noise was reduced when the applied field was aligned along the hard axis of magnetization.
Methods of electrically characterizing zinc selenide epitaxial layers on gallium arsenide substrates
(1994) Haynes, William Brian; Wilson, William L., Jr.
A number of different methods for electrically characterizing ZnSe thin films are presented. These include the Hall effect, current-voltage profiling, and capacitance-voltage profiling. The planar Schottky technique is used to analyze p-type ZnSe. The conductance method of Nicollian and Brews is applied for the first time to the ZnSe/GaAs MIS system to find the surface state density profile and the time constants associated with particular states. A novel photowash technique is used to make the GaAs surface gallium rich before ZnSe growth. Electron Paramagnetic Resonance is discussed in the context of probing thin film semiconductors. Room temperature mobilities for undoped, 1 $\rm\mu m,$ ZnSe films grown by Laser-assisted Metal Organic Chemical Vapor Deposition are as high as 309 $\rm cm\sp2/V$-s. Measured mobilities at 77 K are low due to hole conduction in p-type GaAs at the interface. Heterojunction barrier heights are found to be in the range of 0.6-0.9 eV and are most likely due to interface traps. Schottky diode n-values are found to be high ($>$30) because of the heterojunction barrier. P-type conduction in the nitrogen-doped samples has not been found. Undoped ZnSe is n-type and is typically depleted of carriers. Surface state densities for both untreated and Ga-rich ZnSe/p-GaAs interfaces are found to be in the range of $10\sp{12}$ $\rm cm\sp{-2}$-$\rm eV\sp{-1}.$
MOCVD growth of p-doped thin-film zinc selenide
(1994) Borthakur, Apolak; Wilson, William L., Jr.
Zinc Selenide (ZnSe) is a II-VI semiconductor with a band gap of 2.67 eV. The large bandgap makes it a promising material for the development of blue-green electro-optic devices and for the improvement of the storage density of memories. The major research problem associated with ZnSe is the difficulty in doping it p type. While there have been reports of good p doped ZnSe grown by molecular beam epitaxy (MBE), there have not been any reliable reports of heavily doped p type ZnSe grown by metal organic chemical vapor deposition (MOCVD). MOCVD is a much cheaper process than MBE. It has high throughput and can be used for commercial production. Thus it is of great interest to investigate the development of ZnSe by MOCVD. We report here a MOCVD technique for obtaining high quality p doped ZnSe. Growth parameters have been optimized to yield p type ZnSe doped to as much as $\rm 7.9\times10\sp{17}/cm\sp{3},$ which should be adequate for development of ZnSe blue-green LEDs.
Numerical application of concepts from confocal microscopy to holography and other coherent imaging systems
(1994) Byrd, Marc Jeston; Wilson, William L., Jr.; Blanchard, Andrew J.
A novel methodology based on the scanning confocal microscope is presented which enables a general solution of the depth resolution problem in holography and other coherent imaging schemes. The method does not depend on a priori information about the object. Background and historical perspective are provided, as well as some review of the Rayleigh-Sommerfeld diffraction formulation and other pertinent physical optics topics. Some signal processing and numerical methods specific to the simulation of the propagation and diffraction of light are presented and applied. Holograms were simulated, providing the initial test bed for these concepts. Collimated confocal reconstruction of holograms, is discussed and demonstrated on a simulated hologram. The apertured scanning version is then discussed, and shown to have depth and lateral discrimination properties similar to those of the scanning confocal microscope. The first application of confocal processing to real holographic data is presented, demonstrating the expected depth discrimination and contrast improvement. Frequency diverse microwave holograms were used as input, and therefore background and characterization of that system are provided. In addition, some improvements in computational reconstruction of spherical shell microwave holograms are presented. As a demonstration for apertured scanning confocal hologram reconstruction, data from a typical microwave experiment was used as input. The experiment, using a 1/16 scale tank model, involved a scan of a full 360 degrees, and frequency diversity from 10 to 26 GHz. The results of confocal processing clearly demonstrate the desired effects of improved depth discrimination and contrast over conventional reconstruction. Details on the top of the tank become visible when strong returns from below the plane of interest, which are prominent in conventional reconstruction, are removed by the depth discrimination effect associated with the confocal arrangement. By demonstrating computational implementation of the concepts associated with the confocal microscope, opportunities are provided for imaging in many regimes where lenses and/or mirrors of high quality are not available. Extension of confocal processing to these systems is briefly discussed. Also, many opportunities to apply recent advances in scanning confocal microscopy are recognized, which may be implemented computationally. These include edge traversal and detection, automatic refocusing, and super-resolving methods.
Polyimide-based field effect transistor structures
(1999) Steele, Jennifer Marie; Wilson, William L., Jr.
Because of the electrical insulating properties of polyimide, it is widely used in the packaging of semiconductor chips. However, it has been documented that the electrical resistivity can be decreased by several orders of magnitude to about 0.1--1 O-cm when irradiated with ultraviolet laser light. This unique property of polyimide can be exploited to fabricate devices. In this study, capacitor structures on silicon wafers were created using a laser-modified layer of conducting polyimide as the top plate and a still intact layer unmodified polyimide as the insulator. This work quantifies the properties and quality of these capacitor structures and optimizes the procedure for fabricating them. First, the capacitor-insulator transition of thin film polyimide on silicon wafers were characterized and compared to bulk experiments. Then capacitor structures were fabricated and studied. Finally, an unexpected frequency response was discovered originating from the conductivity of the laser-modified polyimide. This frequency response was successfully modeled in PSPICE.
Process design and circuit model development
(1995) Rodriguez, Juan Antonio; Wilson, William L., Jr.
Process design for integrated circuit manufacturing has traditionally been implemented with little simulation prior to fabrication. As with circuit design of a decade ago, the available simulation tools were mainframe-based, often incompatible, and lacked accurate physical models. Recent developments in process and device simulation allow accurate process modeling which reflect actual fabrication plant capabilities. A highly structured simulation environment implemented for development of Texas Instruments' PRISM$\rm\sp{TM}$ technology is described, together with results of a simulation approach to circuit model development for a new class of silicon power transistors. A new analytical model for field effect transistor modeling is also proposed. This new model preserves continuity of both the drain current and conductance over all bias conditions. It also accurately models the effects of substrate bias on device behavior.
Study of degradation in tunneling oxide thin films in EEPROM and FLASH EEPROM test stuctures
(1997) Chen, Chun; Wilson, William L., Jr.
Degradation of the tunneling oxide film in EEPROM and FLASH memory test structures has been studied. Two models have been used to characterize the tunneling induced degradation of the oxide thin film. They are the effective tunneling area model and the effective oxide field model. These two models correspond to the two extreme cases of oxide charge trapping. Our study not only suggested an accelerated reliability test method for the tunneling oxide film, but also provided a description of the oxide charge trapping process during Fowler-Nordheim tunneling. The generation mechanism of the oxide trapped charge is discussed.

Browsing by Author "Wilson, William L., Jr."

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