Thin Film Materials Research
The example photograph is a time of flight spectrometer for medium energy ion beam analysis of surfaces. This system, developed at Vanderbilt, is one of the highest resolution systems in the world for analysis of ultra-thin films such as those needed for future generations of semiconductor devices.
Thin films made of alloys that could replace the silicon dioxide currently used in transistors and other microelectronic devices that are the heart of computers and telecommunications devices. The computer industry is searching for materials to replace silicon dioxide because it will not function properly as computer components continue to shrink in size.A layer of silicon dioxide is used to make the part of a computer transistor within an integrated circuit that plays the pivotal role in switching the transistor on or off. As the transistor is reduced in size, the silicon dioxide layer must also be reduced in thickness, ultimately becoming too thin to function adequately to control the transistor's electrical current.
One of the most important methods for the analysis of semiconductor thin film structures, both during develop-ment and for quality control, has been Rutherford backscattering spectrometry (RBS). However, the depth resolution of RBS is typically about 10 nm in cases of practical interest.
Using state-of-the-art fast timing electronics and a patented spectrometer design, re-searchers in the Accelerator Laboratory at Vanderbilt have developed an alternative medium energy backscattering technique (MEBS) which, in principle, is capable of approximately 1 nm near-surface depth resolution and which presently functions within a factor of two of this theoretical op-timum.
MEBS has been applied in studies ranging from the absorption of oxygen by thin films in Earth orbit to basic research on radiation effects on optical coating. However, it has found its most important recent applications in the meas-urement of dielectric and other special pur-pose thin films on silicon. It is particularly useful in assessing the stoichiometry and uniformity of SiO2, TiN, and silicon oxynitride films and has been used to assess the results of effects of growth conditions on metallic silicides. Because of the relative low energy of the ion beam, 270 keV typi-cally, as compared with 1-2 MeV for conventional RBS, MEBS may also be used to analyze fragile organic thin films and is presently being used by Accelerator Laboratory researchers in conjunction with studies on organic light emitting diodes.
MEBS may be applied to any surface analytical problem for which conventional Rutherford back-scattering analysis is appropriate, and it will generally provide higher quality data about the first few tens of nanometers of the surface. Examples include all classes of planar semiconductor struc-tures, optical coatings, shallow ion implants, trace element analysis, and studies of thin-film crystallinity via ion channeling. Accelerator Laboratory researchers welcome the opportunity for collaboration in applying MEBS and other ion beam analytical techniques at our disposal to these or other materials problems from any engineering discipline.
Research is also being done at Vanderbilt on certain thin films made of alloys strong enough at the molecular level to replace silicon dioxide as transistors are made smaller. To study these extremely thin films, a unique ultra-high-vacuum chemical vapor deposition reactor that deposits the alloy was designed to allow researchers to study their properties in situ before they are exposed to contaminants in the air. A spectroscopic ellipsometer, recently purchased through a grant from the U.S. Defense University Research Instrumentation Program, alllows researchers to study alloy films of aluminum oxide and zirconium oxide. The laser equipment will be installed in the reactor and will allow the researchers to analyze the materials by studying their response to varying wavelengths and angles of laser light.
- Medium Energy Backscattering Technique (MEBS)
- Ion Beam Analysis