Anderson Materials Evaluation, Inc. can determine the elemental composition of a wide range of materials using two different and somewhat complementary quantitative analytical spectroscopies:
When making a decision about which technique to use for a given analysis, it is important to bear in mind these essential characteristics of each technique:
XPS or ESCA Surface Analysis
- The analyzed depth is a very surface-sensitive depth of about 10 nm. A quantitative elemental analysis with this strong a sensitivity to the surface is extraordinarily important in obtaining highly quantitative measurements of surface contamination, causes of adhesive bonding problems or thin film delamination problems, issues relating to surface reactions and the initiation of corrosion problems, the characterization of surface oxides and other surface treatments, the segregation of some elements to a surface in a metal alloy, the operation of catalysts, and many more technological problems. But, while the surface composition of many fully reacted materials may be the same as the bulk composition, it also is very different in many materials.
- The area of analysis is an ellipse, when samples are mounted in the usual way, of variable size, but the most common size has a major axis of 1000 nm and a minor axis of 500 nm. 600 x 300 nm and 300 x 150 nm are also available, but the signal strength drops considerably upon using smaller areas.
- In many cases, a material can be argon ion beam etched to provide quantitative elemental composition results as a function of depth from the surface. Some materials, such as many polymers, are too fragile to perform ion beam etching on without decomposing them. But this technique is very useful with metal alloys, semiconductor materials, and many glass, ceramic, inorganic, and mineral materials.
- XPS allows the quantitative measurement of all the elements except hydrogen and helium. There are cases in which two elements have overlapping photoelectron peaks and this may degrade the quantitative accuracy of the analysis. Other non-overlapping peaks for the same elements may offer an alternative accurate analysis.
- The XPS quantitative analysis of the material at the surface is highly quantitative in most cases. It tends to be more accurate quantitatively than EDX or EDS is when the surface chemistry is the same as the bulk chemistry. This is an important qualification, however.
- XPS can provide a separate quantitative elemental analysis for each of two or three chemical phases occupying the analysis area when high energy resolution spectra for the elements present are also acquired. EDX will only provide an average elemental composition in the volume it analyzes, though that volume iscommonly about 3000 times smaller. In most cases, XPS analysis samples many particles or many grains, while EDX analysis may sample only one particle or one grain. EDX analysis may require more sets of data to achieve an accurate average.
- XPS analysis works well on insulating materials, as well as on conducting or semiconducting materials. This avoids coating interference problems that may accompany EDX analysis.
EDX or EDS Analysis
- The analysis depth is commonly about 1000 to 2000 nm.
- Due to subsurface scattering of the electron beam, the analyzed volume is shaped like a Florence flask with the narrow neck shortened and reaching down into the bulk material before bulging out into the somewhat spherical subsurface volume from which most of the elemental composition data comes. The lateral dimensions tend to be about 1000 nm. One can raster the electron beam and obtain the average composition over larger to much larger areas, which can relieve the problem of getting accurate average results when all the signal with a point analysis might come from a single particle or metallic grain.
- Our EDX detects all elements from carbon and heavier elements. The quantitative results for carbon, nitrogen, oxygen, fluorine, neon, and sodium progressively improve as the element becomes heavier. Hydrogen, helium, lithium, beryllium, and boron are not detected with our EDX. Lithium, beryllium, and boron are detected and can be quantitatively analyzed with XPS. The XPS accuracy of analysis for carbon, nitrogen, and oxygen is very high.
- The quantitative analysis changes as the electron beam energy is changed, since the scattering depth and lateral range change and change differently in different materials. The most accurate quantitative analysis is obtained when using standard materials similar to the material to be analyzed.
- Because of the charged excitation electron beam, rather than neutral x-rays used in XPS analysis, EDX analysis on some insulating materials is challenging and may be of reduced usefulness. The volume of analysis may have to be much larger to get useful data. One can raster the electron beam over a very large area and acquire the x-ray spectrum from that large area since less charge is built-up in the smaller fraction of the time the electron beam is being dumped into a given volume of material.
- If one is trying to identify which metal alloy a metal is, EDX is usually much preferred to XPS because some elements in the metal may have a strong preference to segregate to the surface to bond with oxygen, carbon dioxide, or water vapor; or because its surface energy is less than its lattice energy, or due to prior heat treatment of the alloy. The greater depth of analysis with EDX is very helpful in this case, though even when using EDX the surface needs to be cleaned, polished, and cleaned before analysis to establish the metal alloy.
It is often a good idea to talk to Charles, Lorrie, or Kevin about which analysis is most useful for a particular materials characterization or problem-solving task. In many cases, both techniques will provide useful and even essential information.