Joseph S. Choi and Dr. David Allred, Physics and Astronomy
As current microtechnology advances toward building nanosize components for computers and aerospace devices, oxidation of thin films can have dramatic effects on the electronic and optical properties of these devices. In order to understand optical properties of thin film materials and their oxides in a high purity environment, we have been building an ultrahigh vacuum deposition chamber to achieve a base pressure under 10-9 torr. We have incorporated evaporation and sputtering capabilities to create thin films in vacuum, and an in situ ellipsometer to analyze the optical properties and oxidation of the films without exposure to atmosphere. This combined with an oxygen leak allows for control over the amount of oxidation on the films. Ruthenium was initially investigated with the chamber in high vacuum to understand initial oxidation of thin films, and the applicability of our chamber.
Ultrahigh vacuum (UHV) is defined as pressures below 10-9 torr. (Atmospheric pressure is 7.6×10+2 torr) Achieving UHV pressures in a vacuum chamber requires more elaborate cautions to be taken than high vacuum chambers (pressures around 10-6 torr), which are common in the labs at BYU. To achieve UHV pressures requires a high temperature bake out, careful choice of material, vacuum-safe machining, and cleaning of the inside components.
Building our UHV deposition chamber took most of my research time and efforts. After a year of building together with several other undergraduates, we were able to pump down the chamber in April 2001. Our initial attempt allowed us to achieve a base pressure of 1.5×10-7 torr, in the low high vacuum range. We hope that improved bake out methods, cleaning, and upgrade of components will allow us to eventually achieve the UHV range.
In May 2001, a ruthenium thin film, with thickness of about 28 nanometers, was successfully created using a sputter gun at a base pressure of 1.0 x 10-6 torr. The film was then analyzed with the in situ (measurement inside the vacuum chamber) ellipsometer. The ellipsometer emits a polarized light that reflects off of the film. This reflection causes a change in the polarization of the light, which can then be analyzed to obtain optical properties and thickness of the layers of the film. Ellipsometric analysis was performed on the ruthenium film with a computer software to improve the known optical constants for ruthenium, and measure the oxidation rate and amount of the ruthenium thin film. The improved optical constants for ruthenium are shown in Table 1. (The unit for the wavelengths is angstrom, equivalent to 0.1 nanometer) Our analysis showed that the initial 24 hours of oxidation of ruthenium thin film in atmospheric air is negligible, ellipsometric measurements being about 0.3 nanometers thick.
Many publishable experiments can be carried out in the future with the UHV deposition chamber that has been built. Optical constants of ruthenium and other materials of interest can be made.
Research on the initial and continued oxidation of various thin films may be the greatest advantage that this deposition chamber can offer.