With over 100 years research using x-rays, the unstrained structure of most materials has been determined. However, new materials are being formed constantly, and as materials are used in thinner and thinner layers, changes in the structure can either be beneficial or deleterious. We can use x-rays, especially from a bright source such as a synchrotron, to determine in what ways the structure has changed. Even for bulk materials, the structure close to the surface may have been overlooked by researchers, but it is this structure which first interacts with the atmosphere and other materials. There are two complementary techniques used for studying near-surface structures. They are Grazing Incidence X-ray Scattering (GIXS) and Crystal Truncation Rod Scattering (CTR). Depending on the question being asked, one or the other (or both) can be used.
Anomalous Scattering from Amorphous Materials
Although amorphous materials do not exhibit long-range order, they do have short-range order, and in some cases intermediate range order. X-ray scattering can be used to determine this order. Using the tuneable radiation from the synchrotron, we can obtain additional information about the environment around a specific atomic species. This technique is known as Anomalous X-ray Scattering. Very close to the absorption edge of an element both the real and imaginary corrections to the index of refraction change over a relatively small energy range. By collecting scattering data both near and away from an absorption edge, the structure surrounding that particular element can be derived.
Total-reflection X-Ray Fluorescence (TXRF)
Metal impurities on silicon wafer surfaces result in decreased yield. Tracking the impurity level non-destructively can be addressed using total-reflection x-ray fluorescence (TXRF), In this technique, x-rays incident on a wafer at a very grazing angle (<0.1 degree) excite contaminant atoms on the surface which re-radiate the incident energy as characteristic fluorescent photons. These photons are detected using a solid-state Si(Li) detector. By calibrating on a wafer with a known contamination level the concentration on an unknown wafer can be determined. At present, we are able to detect Ni atoms present on a wafer at a concentration of <1x10^8 atoms/cm^2, which is considerably better than any other non-destructive technique. For comparison, there are ~10^15 atoms/cm^2 of silicon on the silicon surface, so the technique is sensitive to 1 impurity atom/10 million substrate atoms. At present the Semiconductor Industry Association (SIA) roadmap calls for sensitivity of ~10^9 atoms/cm^2, so we are well below what is required for the foreseeable future.