This research program is focusing on the demonstration and development of rare-earth-doped fiber and planar waveguide lasers operating in the visible and ultraviolet. Recently, the first ultraviolet fiber laser pumped by any means was demonstrated. Also, lasing in the deep violet (412 nm) was realized. Based on upconversion in neodymium-doubled fluorozirconate glass fiber, these lasers operate at room temperature and power outputs at 421 nm of ~0.5 mW have been obtained. Short (<50ps) pulses of coherent and UV radiation have been generated by producing radial wavepackets in rubidium vapor. The wavepackets are modulated at terahertz frequencies determined by energy level spacings in the atom.

Experiments are in progress to develop a visible-emitting fiber laser pumped by red or near-IR semiconductor diodes. Emphasis is placed on the holmium-doped fluorozirconate glass (ZBLAN) fiber laser that operates in the green (lambda ~ 549 nm). By optimization of the pump wavelength and fiber length, the pump power threshold of the laser has been reduced below 30 mW, which brings the pumping requirements for the device within the realm of commercially available red diodes.

The focus of these experiments is the development of planar waveguides in rare earth-doped LiNbO₃ or fluoride glass that are capable of generating or amplifying coherent visible or UV radiation. The optical quality of LiNbO₃ or glass films produced by sol-gel processes are being examined by laser techniques. The ultimate goal of this work is the development of waveguides suitable for being driven by semiconductor laser diodes.

The ability to a priori engineer new materials, and thin films in particular, critically depends on being able to select the precursor atoms and radicals. This is difficult to do in conventional plasma and thermal chemical vapor deposition apparatus. We have proposed a new plasma deposition and etching technique in which the flux of radicals to the substrate can be more selectively controlled. This technique combines a pulsed electron beam with a bulk plasma in a radio frequency discharge. The goals are to sequentially deposit single epitaxial layers of semiconductor materials or control the flux of radicals and ions to the substrate in a plasma etching reactor.

Plasmas are becoming increasingly important in high-technology manufacturing. Plasmas are used to deposit films, harden materials, modify surfaces, and synthesize bulk materials. We are supporting the University of Wisconsin NSF ERC for Plasma-Aided Manufacturing by developing computer models for systems of interest to the manufacture of microelectronics and index gradable materials. We have developed nonequilibrium electron and ion kinetics models of reactive ion etching (RIE) and inductively coupled plasma (ICP) reactors; and cleanup of plasma source ion implantation (PSII) using Fourier transform mass spectroscopy (FTMS). We are also addressing the consequences of particulate contamination in polymerizing plasmas.

One of the greatest causes of reduced yields during manufacturing of microelectronics devices is contamination of the wafer by particles (sizes 10s nm to many µm). The contamination of wafers during plasma etching results from particles being driven to the wafer by ``ion wind,'' electrostatic, fluid drag, and thermophoretic forces. We are developing comprehensive computer models of plasma etching tools that include the forces on and transport of dust particles. We have quantified the manner in which electrode-topography may lead to electrostatic traps for particles in the plasma. We are collaborating with semiconductor manufacturers to minimize particle contamination.

The microelectronics fabrication industry is developing plasma-etching tools that scale to wafers exceeding 12 in. in diameter. We are investigating these scaling issues by developing two-dimensional simulations of plasma-etching reactors. The models address low-pressure, high plasma density devices driven by inductively coupled electromagnetic fields. We are optimizing antenna designs, gas mixtures, and flow patterns to maximize the uniformity of processing. This work is being performed collaboratively with Sematech, LAM Research Inc., and Applied Materials Inc.

One of the tasks of Sematech, the national consortium of semiconductor manufacturers, is to conduct equipment improvement programs (EIPs) on manufacturing tools used in the fabrication of microelectronic devices. In this project, we are providing computational support for an EIP on an inductively coupled plasma reactor for etching of metal interconnects. We have developed computer models for plasma etching reactors that are capable of addressing a variety of physical phenomena. We are applying these models to investigate issues related to uniformity of etching and aspect ratio dependent etching.

Nonplanar structures in contact with a plasma can cause perturbations in the electric potentials and plasma densities. Plasma etching reactors for fabrication of microelectronic devices are carefully designed to minimize these perturbing effects. It is less well known that nonuniform structures below the wafer can also perturb the plasma. Observations of dust particle traps and nonuniform etching have been correlated with dielectric structures beneath the wafer. In this research program, we are developing plasma equipment models to investigate the effects of electrode topography (above and below the wafer) on the uniformity of plasma generation, ion fluxes to the wafer, and dust particle traps.

Increasing environmental awareness and regulatory controls have motivated research into new methods to cleanse toxic gases from air streams. We are computationally investigating the use of plasmas for cleansing of NO_x (oxides of nitrogen) from diesel exhaust. Reducing (or oxidizing) radicals, which react with NO_x to produce less harmful substances, are readily and efficiently produced by plasmas. Production of these radicals can be easily and rapidly controlled by plasma sources such as dielectric barrier discharges or coronas, thereby providing a practical method to adapt to changing road conditions. In this work, we are developing multidimensional computer models of the plasma chemistry and gas dynamics during the remediation process.

More stringent regulations on the allowable levels of toxic gases in the exhausts from internal combustion engines and industrial processes have motivated research into more efficient methods to treat those gas streams. We are developing multidimensional plasma chemistry models to investigate plasma remediation as a method to remove toxins from atmospheric pressure gases. In plasma remediation, electron impact reactions produce oxidizing or reducing radicals which either convert the toxin to a harmless gas or to a gas for which conventional remediation methods can be used. We are investigating reaction mechanisms and the plasma hydrodynamics in dielectric barrier and corona discharges for remediation of NO_xa and VOCs (volatile organic compounds). Scaling laws for energy efficiency are being developed.

Of the hundreds of processing steps in the manufacture of silicon microelectronics devices such as memory chips and microprocessors, approximately one-third use plasmas for etching, deposition, cleaning, or passivation. As wafer sizes continue to increase, the need for highly uniform, particle-free plasma-processing equipment also increases. In this project we are developing plasma equipment models (PEMs) to study important plasma generation and transport processes and to investigate methods to scale plasma reactors to process larger wafers. The PEMs are geometrically flexible and are able to address a variety of chemistries. Particular attention is being paid to processes which generate particle contamination of the wafers.

As wafer sizes continue to increase, improvements in plasma processing for microelectronics fabrication will soon require that real-time control be applied to stabilize and improve manufacturing steps. We are investigating real-time control methodologies for obtaining spatially uniform etch rates and to signal the end point of the etch. This work is being performed in collaboration with the University of Michigan, where experiments on real-time control of plasma etching reactors are being performed. We are developing computer models of those processes to determine the physical processes, sensor outputs, and control points that are best monitored to achieve these goals.