We have developed a tractable computational approach, PRISM theory (polymer Reference Interaction Site Model), for modeling structure and thermodynamics of polymer liquids and alloys. PRISM theory allows one to predict the effect of polymer architecture and monomer structure on the intermolecular packing in the condensed phase. Three applications of this method are discussed: phase behavior of polymer blends, solubility of gases in polymers, and structure of polymers near walls and interfaces. In these applications, nonrandom mixing effects (not included in previous theories) play an important role in the macroscopic properties of importance to the materials scientist.
Three different methods are discussed for computing the sensitivity of the temperature field to changes in material properties and initial-boundary condition parameters for heat conduction problems. The most general method is to derive sensitivity equations by differentiating the energy equation with respect to the parameter of interest and numerically solving the resulting sensitivity equations. An example problem in which there are twelve parameters of interest is presented and the resulting sensitivity equations are derived. Numerical results are presented for thermal conductivity and volumetric heat capacity sensitivity coefficients for heat conduction in a 2-D orthotropic body. The numerical results are compared with the analytical solution to demonstrate that the numerical method is second order accurate as the mesh is refined spatially.
An electrodynamic shaker is modeled as a mixed electrical/mechanical system with an experimentally derived two port network characterization. The model characterizes the shaker in a manner that the performance of the shaker with a mounted load (test item and fixture) can be predicted. The characterization depends on the measurements of shaker input voltage and current, and on the acceleration of the shaker armature with several mounted loads. The force into the load is also required, and can be measured directly or inferred from the load apparent mass.
The most commonly used solder for electrical interconnections in electronic packages is the near eutectic 60Sn-40Fb alloy. This alloy has a number of processing advantages (suitable melting point of 183C and good wetting behavior). However, under conditions of cyclic strain and temperature (thermomechanical fatigue), the microstructure of this alloy undergoes a heterogeneous coarsening and failure process that makes the prediction of solder joint lifetime complex. A viscoplastic, microstructure dependent, constitutive model for solder, which is currently under development, was implemented into a finite element code. With this computational capability, the thermomechanical response of solder interconnects, including microstructural evolution, can be predicted. This capability was applied to predict the thermomechanical response of a mini ball grid array solder interconnect. In this paper, the constitutive model will first be briefly discussed. The results of computational studies to determine the thermomechanical response of a mini ball grid array solder interconnects then will be presented.
We examine the threshold characteristics of selectively oxidized VCSELs as a function of the number, thickness, and placement of the buried oxide apertures. The threshold current density for small area VCSELs is shown to increase with the number of oxide apertures in the cavity due to increased optical loss, while the threshold current density for broad area VCSELs decreases with increasing number of apertures due to more uniform current injection. Reductions of the threshold gain and optical loss are achieved for small area VCSELs using thin oxide apertures which are displaced longitudinally away from the optical cavity. We show that the optical loss can be sufficiency reduced to allow lasing in VCSELs with aperture area as small as 0.25 micrometer2.
A new constitutive model is proposed for the modeling of penetration and large stress waves in concrete. Rate effects are incorporated explicitly into the damage evolution law, hence the term "brittle failure kinetics." The damage variable parameterizes a family of Mohr-Coulomb strength curves. The model, which has been implemented in the CTH code, has been shown to reproduce some distinctive phenomena that occur in penetration of concrete targets. Among these are the sharp spike in deceleration of a rigid penetrator immediately after impact. Another is the size scale effect, which leads to a nonlinear scaling of penetration depth with penetrator size. This paper discusses the theory of the model and some results of an extensive validation effort.
A Reusable Interface Technology is presented for application to thermal parameter estimation problems. It is applied to the estimation of thermal conductivity of compacted Al2O3 powder without binder. As temperature increases, the thermal conductivity of Al2O3 powder without binder decreases.
Fabrication of group-III nitride devices relies on the ability to pattern features to depths ranging from approximately 1000 angstroms to >5 μm with anisotropic profiles, smooth morphologies, selective etching of one material over another, and a low degree of plasma-induced damage. In this study, GaN etch rates and etch profiles are compared using reactive ion etch (RIE), reactive ion beam etching (RIBE), electron cyclotron resonance (ECR), and inductively coupled plasma (ICP) etch systems. RIE yielded the slowest etch rates and sloped etch profiles despite dc-biases >-900 V. ECR and ICP etching yielded the highest rates with anisotropic profiles due to their high plasma flux and the ability to control ion energies independently of plasma density. RIBE etch results also showed anisotropic profiles with slower etch rates than either ECR or ICP possibly due to lower ion flux. InN and AlN etch characteristics are also compared using ICP and RIBE.
Long-wavelength vertical cavity surface emitting lasers (VCSELs) are attractive for a variety of application but one of the major obstacles in implementing these structures is the lack of sufficiently large refractive index contrast (Δn) in the mirror layer pairs that can be lattice matched to InP. In order to realize a monolithic device, a AlGaAsSb/AlAsSb material system (Δn to approximately 0.52) lattice matched to InP is utilized as a means of forming highly reflecting distributed Bragg reflectors (DBRs) with relatively few mirror pairs. The structure is grown by molecular beam epitaxy. The active region consists of 2 λ thick bulk InGaAs, whereas top and bottom DBR are made up of 15 and 20 periods of AlGaAsSb/AlAsSb mirror pairs respectively.
The most commonly used solder for electrical interconnections in electronic packages is the near eutectic 60Sn-40Pb alloy. This alloy has a number of processing advantages (suitable melting point of 183C and good wetting behavior). However, under conditions of cyclic strain and temperature (thermomechanical fatigue), the microstructure of this alloy undergoes a heterogeneous coarsening and failure process that makes the prediction of solder joint lifetime complex. A viscoplastic, microstructure dependent, constitutive model for solder, which is currently under development, was implemented into a finite element code. With this computational capability, the thermomechanical response of solder interconnects, including microstructural evolution, can be predicted. This capability was applied to predict the thermomechanical response of various leadless chip carrier solder interconnects to determine the effects of variations in geometry and loading. In this paper, the constitutive model will first be briefly discussed. The results of computational studies to determine the effect of geometry and loading variations on leadless chip carrier solder interconnects then will be presented.
Flashlamp pumped lasers use pulse power switches to commute energy stored in capacitor banks to the flashlamps. To lower the total cost of these switches, Sandia National Laboratories has a research program to evaluate large closing switches. The National Ignition Facility (NIF) is one of the applications of the program designed by Lawrence Livermore National Laboratory, Los Alamos National Laboratory, and Sandia National Laboratories. The target value of the energy switched by single devices is 1.6 MJ, from a 6 mF, 24 kV capacitor bank. The peak current is 500 ka. The lifetime of the NIF facility is 24 thousand shots. The goal of the experiment in Sandia is to test switches with the full NIF wave shape, and the correct voltage.
Quad-level polysilicon surface micromachining technology, comprising three mechanical levels plus an electrical interconnect layer, is giving rise to a new generation of micro-electromechanical devices and assemblies. Enhanced components can now be produced through greater flexibility in fabrication and design. New levels of design complexity that include multilevel gears, single-attempt locks, and optical elements have recently been realized. Extensive utilization of the fourth layer of polysilicon differentiates these latter generation devices from their predecessors.1 This level of poly enables the fabrication of pin joints, linkage arms, hinges on moveable plates, and multi-level gear assemblies. The mechanical design aspects of these latest micromachines will be discussed with particular emphasis on a number of design modifications that improve the power, reliability, and smoothness of operation of the microengine.2 The microengine is the primary actuation mechanism that is being used to drive mirrors out of plane and rotate 1600-μm diameter gears.3 Also discussed is our most advanced microme chanical system to date, a complex proof-of-concept batch-fabricated assembly that, upon transmitting the proper electrical code to a mechanical lock, permits the operation of a micro-optical shutter.
A project to estimate well costs in regions of current geothermal activity has been initiated. Costs associated with commonly encountered drilling problems will be included. Activity-based costing techniques will be employed to allow the identification of cost drivers and the evaluation of the economic effects of new technologies and operational procedures on well costs. The sensitivity of well costs to a number of parameters such as rate-of-penetration and daily operating costs will be examined. Additional sensitivity analyses and trade-off studies will evaluate the efficiency of various operational practices and preventive, as well as remedial, actions. These efforts should help provide an understanding of the consumption of resources in geothermal drilling.
An amp-hour counting battery charge control algorithm has been defined and tested using the Digital Solar Technologies MPR-9400 microprocessor based photovoltaic hybrid charge controller. This work included extensive laboratory and field testing of the charge algorithm on vented lead-antimony and valve regulated lead-acid batteries. The test results have shown that with proper setup amp-hour counting charge control is more effective than conventional voltage regulated sub-array shedding in returning the lead-acid battery to a high state of charge.
We developed a self-aligned emitter etchback technique that requires only a single emitter diffusion and no alignments to form self-aligned, patterned-emitter profiles. Standard, commercial, screen-printed gridlines mask a plasma-etchback of the emitter. A subsequent PECVD-nitride deposition provides good surface and bulk passivation and an antireflection coating. We succeeded in finding a set of parameters which resulted in good emitter uniformity and improved cell performance. We used full-size multicrystalline silicon (mc-Si) cells processed in a commercial production line and performed a statistically designed, multiparameter experiment to optimize the use of a hydrogenation treatment to increase performance. Our initial results found a statistically significant improvement of half an absolute percentage point in cell efficiency when the self-aligned emitter etchback was combined with a 3-step PECVD-nitride surface passivation and hydrogenation treatment.
Gear systems rotating on hubs have been operated to failure using Sandia's microengine as the actuation device. Conventional failure modes such as fatigue induced fracture did not occur, indicating that the devices are mechanically extremely robust. The generic route to failure observed for all rotating devices involves sticking of structures that are in sliding contact. This sticking evidently results from microscopic changes in the sliding surfaces during operation. The rate at which these changes occur is accelerated by excessive applied forces, which originate from non-optimized designs or inappropriate drive voltages. Precursors to failure are observed, enabling further understanding of the microscopic changes that occur in the sliding surfaces that ultimately lead to failure.
A multi-level polysilicon surface-micromachining technology consisting of 5 layers of polysilicon is presented. Surface topography and film mechanical stress are the major impediments encountered in the development of a multilayer surface-micromachining process. However, excellent mechanical film characteristics have been obtained through the use of chemical-mechanical polishing for planarization of topography and by proper sequencing of film deposition with thermal anneals. Examples of operating microactuators, geared power-transfer mechanisms, and optical elements demonstrate the mechanical advantages of construction with 5 polysilicon layers.
This work considers the problem of causing multiple (100's) autonomous mobile robots to converge to a target and provides a "follow-the- leader" approach to the problem. Each robot has only a limited-range sensor for sensing the target and also larger but also limited-range robot-to-robot communication capability. Because of the small amount of information available to the robots, a practical approach to improve convergence to the target is to have a robot follow the robot with the best quality of information. Specifically, each robot emits a signal that informs in-range robots what its status is. A robot has a status value of 0 if it is itself in range of the target. A robot has a status of 1 if it is not in range of the target but is is communication range of a robot that is in range of the target. A robot has a status of 2 if it is not in range of the target but is within range of another robot that has status 1, and so on. Of all the mobile robots that any given robot is in range of, it follows the one with the best status. The emergent behavior is the ant-like trails of robots following each other toward the target. If the robot is not in range of another robot that is either in range of the target or following another robot, the robot will assign -1 to its quality-of-information, and will execute an exhaustive search. The exhuastive search will continue until it encounters either the target or another robot with a nonnegative quality-of-information. The quality of information approach was extended to the case where each robot only has two-bit signals informing it of distance to in-range robots.
This work considers the problem of maximum utilization of a set of mobile robots with limited sensor-range capabilities and limited travel distances. The robots are initially in random positions. A set of robots properly guards or covers a region if every point within the region is within the effective sensor range of at least one vehicle. We wish to move the vehicles into surveillance positions so as to guard or cover a region, while minimizing the maximum distance traveled by any vehicle. This problem can be formulated as an assignment problem, in which we must optimally decide which robot to assign to which slot of a desired matrix of grid points. The cost function is the maximum distance traveled by any robot. Assignment problems can be solved very efficiently. Solution times for one hundred robots took only seconds on a Silicon Graphics Crimson workstation. The initial positions of all the robots can be sampled by a central base station and their newly assigned positions communicated back to the robots. Alternatively, the robots can establish their own coordinate system with the origin fixed at one of the robots and orientation determined by the compass bearing of another robot relative to this robot This paper presents example solutions to the multiple-target-multiple-agent scenario using a matching algorithm. Two separate cases with one hundred agents in each were analyzed using this method. We have found these mobile robot problems to be a very interesting application of network optimization methods, and we expect this to be a fruitful area for future research.
This paper describes how variable structure control can be used to describe the overall behavior of multiple autonomous robotic vehicles with simple finite state machine rules. The importance of this result is that we can then begin to design provably asymptotically stable group behaviors from a set of simple control laws and appropriate switching points with variable structure control. The ability to prove convergence to a goal is especially important for applications such as locating military targets or land mines.
Three methods for fiber end-face preparation based on the availability of exceptionally good cleaved surfaces from a commercial vendor were discussed. A few breakdown and damage processes were studied for this purpose. Results were also compared to previous measurements obtained from fibers which were mechanicallly polished using an optimized polishing. The mean values for maximum transmitted energy before breakdown or damage for the cleaved-only and cleaved-plus-flame polished fibers were a bit higher than the corresponding value for mechanical polished fibers.
Sandia National Laboratories has refined a process for developing inherently safer system designs, based on methods used by the Laboratories to design detonation safety into nuclear weapons. The process was created when the Laboratories realized that standard engineering practices did not provide the level of safety assurance necessary for nuclear weapon operations, with their potential for catastrophic accidents. A systematic approach, which relies on mutually supportive design principles integrated through fundamental physical principles, was developed to ensure a predictably safe system response under a variety of operational and accident based stresses. Robust, safe system designs result from this thematic approach to safety, minimizing the number of safety critical features. This safety assurance process has two profound benefits: the process avoids the need to understand or limit the ultimate intensity of off normal environments and it avoids the requirement to analyze and test a bewildering and virtually infinite array of accident environment scenarios (e.g., directional threats, sequencing of environments, time races, etc.) to demonstrate conformance to all safety requirements.
A temperature end point method was developed for tungsten CMP (WCMP) processing in the Sandia Microelectronics Development Laboratory (MDL), a facility which develops and prototypes a variety of silicon based devices including ASIC, memory, radiation hardened CMOS and microelectromechanical systems. A large product variety and small production lot size prevents process recipe optimization or standardization for each mask level and product. Rigorous product reliability requirements and prohibitively expensive hardware qualifications essentially require that a single process and consumable set be established for all products, with minimal opportunity for adjustment. A timed process was not suitable without significant potential for manual inspections and rework. Over several weeks of processing on an IPEC 472, the temperature end point method gave a 7.7% 1-sigma end point time distribution. This enabled a 50% reduction in daily process qualification wafers, and allowed minimization of yield loss, rework, and oxide erosion.
Mass spectrometry of the plasma effluent during Reactive Ion Beam Etching (RIBE) of GaAs using an Inductively Coupled Plasma (ICP) source and a Cl{sub 2}/Ar gas chemistry shows that AsCl{sub 3}, AsCl{sub 2} and AsCl are all detected as etch products for As, while GaCl{sub 2} is the main signal detected for the Ga products. The variation in selective ion currents for the various etch products has been examined as a function of chuck temperature (30--100 C), percentage Cl{sub 2} in the gas flow, beam current (60--180 mA) and beam voltage (200--800 V). The results are consistent with AsCl{sub 3} and GaCl{sub 3} being the main etch product species under their conditions, with fragmentation being responsible for the observed mass spectra.
As a first step toward the development of a new remote sensing technique that the authors call holographic correlation spectroscopy, they demonstrate that diffractive optics can be used to synthesize the infrared spectra of real compounds. In particular, they have designed, fabricated, and characterized a diffractive element that successfully reproduces the major features f the spectrum of gaseous HF in the region between 3,600 cm{sup {minus}1} and 4,300 cm{sup {minus}1}. The reflection-mode diffractive optic consists of 4,096 lines, each 4.5 {micro}m wide, at 16 discrete depths relative to the substrate (from 0 to 1.2 {micro}m), and was fabricated on a silicon wafer using anisotropic reactive ion-beam etching in a four-mask-level process. The authors envision the use of diffractive elements of this type to replace the cumbersome reference cells of conventional correlation spectroscopy and thereby enable a new class of compact and versatile correlation spectrometers.
