Pauline Ho

Photodissociation of formaldehyde in a molecular beam

Formaldehyde photopredissociation was studied under collisionless conditions by the technique of crossed laser and molecular beams. Detection of the molecular product CO after excitation of H2CO near the S1 origin gives strong support to the sequential coupling model for fast nonradiative decay of S1 states through broadened S0 levels to the continuum. For H2CO excitation at 283.9 nm, formation of the radical product HCO dominates dissociation to molecular products by at least one order of magnitude.

Modeling the plasma chemistry of C2F6 and CHF3 etching of silicon dioxide, with comparisons to etch rate and diagnostic data

A detailed chemical reaction mechanism is reported that describes the C2F6 and CHF3 plasma etching of silicon dioxide, which is widely used in the fabrication of microelectronic devices. The gas-phase part of the C2F6 mechanism involves 28 species and 132 reactions, while the surface part involves 2 materials, 6 species, and 85 reactions. Rate parameters are generally taken from independent studies in the literature, or estimated from rates measured for related species. Zero-dimensional simulations using these mechanisms compare well with a large body of etch rate and diagnostic measurements in three different high-density plasma reactors. The diagnostic measurements include electron and negative ion absolute densities, CF, CF2, and SiF densities, gas temperatures, and ion current densities. An analysis of the dominant reaction paths shows the importance of gas-phase electron impact reactions and the need to include reactions of the etch-product species. On the surface, the etching reactions are dominated...

Chemical Reaction Mechanisms for Modeling the Fluorocarbon Plasma Etch of Silicon Oxide and Related Materials

As part of a project with SEMATECH, detailed chemical reaction mechanisms have been developed that describe the gas-phase and surface chemistry occurring during the fluorocarbon plasma etching of silicon dioxide and related materials. The fluorocarbons examined are C{sub 2}F{sub 6}, CHF{sub 3} and C{sub 4}F{sub 8}, while the materials studied are silicon dioxide, silicon, photoresist, and silica-based low-k dielectrics. These systems were examined at different levels, ranging from in-depth treatment of C{sub 2}F{sub 6} plasma etch of oxide, to a fairly cursory examination of C{sub 4}F{sub 8} etch of the low-k dielectric. Simulations using these reaction mechanisms and AURORA, a zero-dimensional model, compare favorably with etch rates measured in three different experimental reactors, plus extensive diagnostic absolute density measurements of electron and negative ions, relative density measurements of CF, CF{sub 2}, SiF and SiF{sub 2} radicals, ion current densities, and mass spectrometric measurements of relative ion densities.

Laser Probes and Numerical Modeling as Process Diagnostics in Chemical Vapor Deposition

The chemical vapor deposition process consists of a chemically reacting flow in which the fluid mechanics and chemical kinetics are strongly coupled. Laser probes such as Raman spectroscopy and laser induced fluorescence can be used to measure gas temperature fields and chemical species concentrations, but often the interpretation of such data is difficult because several interacting chemical and physical phenomena are occurring simultaneously. Detailed numerical modeling of the experimental system under study provides valuable insights into these interactions and allows one to make useful comparisons between experiment and the model to gain a fundamental understanding of the CVD process. Examples of this approach are given for silicon deposition from silane and fluid mechanics diagnostics in a rotating disk CVD reactor. 19 refs., 8 figs.

Gas-phase silicon atom densities in the chemical vapor deposition of silicon from silane

Silicon atom number density profiles have been measured using laser-induced fluorescence during the chemical vapor deposition of silicon from silane. Measurements were obtained in a rotating-disk reactor as a function of silane partial pressure and the amount of hydrogen added to the carrier gas. Absolute number densities were obtained using an atomic absorption technique. Results were compared with calculated density profiles from a model of the coupled fluid flow, gas-phase and surface chemistry for an infinite-radius rotating disk. An analysis of the reaction mechanism showed that the unimolecular decomposition of SiH{sub 2} is not the dominant source of Si atoms. Profile shapes and positions, and all experimental trends are well matched by the calculations. However, the calculated number density is up to 100 times smaller than measured.

Laser Studies of the Interaction of SiO with the Surface of a Thin Film

The interaction of SiO radicals from a SiCl 4 /O 2 plasma with the surface of a depositing thin film is studied with the IRIS (Imaging of Radicals Interacting with Surfaces) technique, which combines spatially-resolved laser-induced fluorescence with molecular beam methods. In contrast to previous results for SiH, SiO appears not to react at the surface of the depositing film, but desorbs with a cosine spatial distribution for a wide range of substrate temperatures. No evidence is observed for specular scattering of the molecules.

Laser studies of the SiH radical/surface interaction during deposition of a thin film

This paper presents a new method for studying the interaction of radicals with the surface of a depositing film using a combination of laser spectroscopy and molecular beam techniques. The reactivity of SiH molecules with the surface of a depositing a-Si:H film is measured to be at least 0.95, with no significant dependence on rotational state. 9 refs., 3 figs.