Gilbert A. Hawkins

The effect of rapid thermal annealing on the precipitation of oxygen in silicon

We report observations of the effect of heat pulses on the precipitation of oxygen in single‐crystal silicon. In our experiments, heat pulses are applied for various durations at a fixed temperature of 1200 °C prior to a two‐step precipitation sequence. When no pulse is applied, precipitation is at a maximum and is close to that expected from considerations of solid solubility, provided account is taken of the loss of oxygen due to evaporation. For very short pulses (e.g., 2 s), precipitation is suppressed by several fold. As the length of the pulse is increased to roughly 100 s, the precipitation recovers to its initial value. For very long pulses (greater than 10 000 s), precipitation decreases to nearly zero. There are no further changes in the precipitation characteristics for pulse times up to 50 000 s. We interpret these results in terms of a model in which the thermal pulses modify an initial distribution of heterogeneous nucleation sites. The fraction of heterogeneous sites that survive the two‐st...

A new method for deflecting liquid microjets

A new method is reported for deflecting a microscopic jet emanating from a nozzle away from the nozzle’s axis of symmetry. It relies on putting energy into the jet through an asymmetric heater embedded in the nozzle. This novel phenomenon is probed theoretically. It is shown that jet deflection is set by the competition among three effects. Two of these can be attributed to the variation with temperature of surface tension and the third to that of viscosity. Whether the contact line is fixed or free is shown to profoundly impact the extent of jet deflection at a given flow rate.

Micro-jet nozzle array for precise droplet metering and steering having increased droplet deflection

We present the architecture and fabrication method of a fluidic device with increased droplet deflection. The device is capable of producing picoliter size droplets precisely and steering them. The precision is a consequence of the reproducibility of the nozzles that are made using VLSI technology and tools. In addition, the droplet size is determined by the precise timing of applied heat pulses. We present both experimental and modeling results.

Method for reduction in surface generation current in polycrystalline‐silicon‐gate metal‐oxide‐semiconductor devices

Generation current from interface states at the Si/SiO2 interface is a dominant noise source in metal‐oxide‐semiconductor type solid‐state imagers. A simple method to reduce the interface state density of the solid‐state imagers is described. The method involves annealing at 450 °C in the presence of an unpatterned aluminum film over the layer of passivation oxide of a completed device. In comparing this method to the conventional process of annealing with patterned aluminum, this method results in an order of magnitude reduction in surface generation current. It is believed that Al reacts with trace water in the chemical‐vapor‐deposition oxide and generates active hydrogen. Hydrogen diffuses through the oxide and polycrystalline‐silicon gate to the Si/SiO2 interface and passivates the interface states.

MEMS-Based Microfluidic Devices

Micro-Electro-Mechanical Systems (MEMS) technology can be integrated with microfluidic functionality to enable the generation of microdrops with unprecedented throughput and precise control of drop volume, speed, and placement. The most prominent examples of microdrop generators are in the field of inkjet printing where printheads with thousands of nozzles produce steady streams of microdrops at kilohertz repetition rates. In this paper, we discuss a proposed MEMS-based microfluidic drop generator that operates on the basis of a thermally induced Marangoni effect. We describe the physics of droplet generation and discuss operating performance relative to the fluid rheology, thermal modulation, and wavelength dependencies.© 2010 ASME

Monte Carlo Simulation of Precipitate Nucleation and Growth: Time Dependent Results

A three-dimensional Monte Carlo computer program has been developed to study the heterogeneous nucleation and growth of oxide precipitates during the thermal treatment of crystalline silicon. In the simulations, oxygen atoms move on a lattice with randomly selected lattice points serving as nucleation sites. The change in free energy that the oxygen cluster would experience in gaining or losing one oxygen atom is used to govern growth or dissolution of the cluster. All the oxygen atoms undergo a jump or a growth decision during each time step of the anneal. The growth and decay kinetics of each nucleation site display interesting fluctuation phenomena. The time dependence of the cluster size generally differs from the expected 3/2 power law due to the fluctuations in oxygen arrival at and incorporation in a precipitate. Competition between growing sites and coarsening are observed.

Oxygen Precipitation in Silicon: Numerical Models

We present a numerical model that simulates the evolution of precipitates and the diffusion of interstitial oxygen in Czochralski silicon. The growth and/or dissolution of each precipitate and the local concentration of interstitial oxygen with which the precipitates interact are followed as a function of time. We treat realistic densities of discrete, interacting precipitates and determine how the precipitate density influences the extent of the precipitation. The model also treats oxygen outdiffusion and the formation of precipitate-free or denuded zones. We apply the model to previous experimental data on the time dependence of precipitate growth and to the development of denuded zones during intrinsic gettering.

Oxygen Precipitation in Silicon: Monte Carlo and Deterministic Studies

Interstitial oxygen precipitates in silicon during thermal treatment. The amount precipitated increases in an S-shaped fashion as a function of increasing initial interstitial oxygen concentration. A likely hypothesis for this behavior is that the number of nucleation sites that develop into precipitates (successful sites) varies with the initial interstitial oxygen concentration as well as with the precipitation rate at each site. In this paper, a deterministic precipitate growth model is first used to show that a fit to the present data requires the precipitate density to increase by more than a factor of 10 when the oxygen concentration goes from 24 to 40 ppma. Three-dimensional Monte Carlo calculations are then used to show how the nucleation site survival probability depends on the initial number of oxygen atoms at the site and the oxygen concentration. The program treats oxygen diffusion, growth at nucleation sites by the addition of oxygen atoms, and loss at nucleation sites by the escape of oxygen atoms.

Numerical Studies of Precipitate Coarsening Phenomena

A three-dimensional Monte Carlo computer program is used to study the time-dependent coarsening of a precipitate population in a solid. The emphasis is on the influence of the escape rate of solute atoms from a precipitate on the coarsening behavior. Three models for the escape attempt frequency are compared. The time when coarsening is observed and the extent of coarsening depend on the model as does the percentage of solute precipitated in a given time. The qualitative results of the present calculations are compared with earlier findings.