Multilayer materials made up of a plurality of layers of different material are become increasingly important and are finding use in many different areas. These areas include, for example, X-ray optics coatings, superconducting materials, decorative and wear resistant coatings, catalytic materials and semiconductor materials. The layers in such materials may range, for example, from several angstroms (10,000 angstroms=1 micrometer) to several hundred angstroms or more in thickness. Such multilayer materials may comprise a regular repetition of layers where, for example, repeating layers have the same composition, thickness and spacing. The multilayer material may comprise an irregular repetition of layers where repeating layers have a different layer thickness.
Determining the structure of such multilayer materials is necessary to assess performance, to check reliability and for quality control, for example, and for otherwise evaluating these materials. It is desirable to determine the number of layers, layer thickness, whether the structure has been degraded as a result of heating and whether the layer spacing is uniform, for example. Thus, an effective, rapid and low cost analytical technique is needed for the analysis of such multilayer materials.
While several methods are known which can be used to analyze multilayer structures, each has limitations rendering its use impractical in many applications.
Auger spectroscopy is a surface technique in which electrons bombard the surface of the material to be analyzed, inside an ultra high vacuum chamber. In response, the surface emits characteristic electrons. These characteristic electrons are then measured with an energy sensitive sensor. The Auger technique can be combined with ion sputter etching to remove material at the surface and analyze the new surface by the Auger technique. However, this technique is subject to certain limitations and inaccuracies because mixing and resputtering at the surface of the sample can result in compositional changes. In addition, preferential sputter etching can occur which also changes the surface composition.
Secondary ion mass spectroscopy (SIMS) is similar to the Auger technique except that ions are utilized instead of electrons. SIMS is thus another surface technique and requires ion sputter etching of the surface with the same attendant problems previously described.
X-ray fluorescence can be utilized to provide an average composition and can be used in conjunction with X-ray diffraction techniques. However, conventional X-ray equipment requires a total sample thickness of several thousand angstroms.
Rutherford backscattering is a process by which helium atoms are bombarded at very large energies into a sample. Some of these ions are backscattered and by a complex computer unfolding process it is possible to obtain a depth profile. Complicated and expensive equipment is required for this process.
A need exists for a multilayer profile technique that is simple, inexpensive, rapid and provides excellent resolution.