EDX vs SIMS: Detection Limit, Depth Resolution and Quantification
EDX and SIMS both tell you which elements a sample contains and how much, but they are good at very different things. EDX identifies elements from the energy of the characteristic X-rays emitted when an electron beam hits the sample, so it can quickly and almost non-destructively analyze the spot you are viewing in an SEM or TEM. Its detection limit, however, is typically about 0.1 wt%, so it suits major constituents and particle identification. SIMS sputters the sample with primary ions and mass-analyzes the secondary ions that come off; it can detect some elements down to the ppb level and measures impurity profiles with depth. In exchange, it is destructive, and accurate quantification needs reference standards.
| Aspect | EDX | SIMS |
|---|---|---|
| Principle | An electron beam knocks out inner-shell electrons, and a detector measures the energy of the characteristic X-rays emitted | Primary ions of cesium or oxygen sputter the sample, and the ejected secondary ions are mass-analyzed |
| Typical detection limit | Typically about 0.1 wt% | Typically ppm to ppb, depending on the element and conditions |
| Measurable elements | Generally elements heavier than boron or carbon; light elements have poor sensitivity and quantification, hydrogen cannot be measured, and some peaks overlap (such as silicon and tungsten) | Almost all elements including hydrogen, and isotopes; watch for interferences between ions of similar mass (such as ³⁰SiH and ³¹P) |
| Depth information | An average over the volume the electrons spread into (roughly several hundred nm to a few µm in bulk samples in an SEM); depth profiles are hard to get | Measures while sputtering down from the surface, giving depth profiles; depth resolution is typically a few nm |
| Lateral resolution | Roughly micrometer scale in an SEM because the electrons spread; nanometer level on thin samples in a TEM or STEM | In depth profiling, typically an average over an area tens to hundreds of µm across |
| Destructive or not | The analysis itself is nearly non-destructive (some materials are damaged by the beam); TEM analysis needs a thin lamella cut out, for example with a FIB | Destructive: the analyzed material is sputtered away |
| Quantification | Correction calculations give an approximate composition of major constituents on the spot; errors are large for light and trace elements | The fraction that becomes ions varies strongly with the surrounding material (matrix effect), so signals are converted to concentration with standards in the same matrix |
| Speed and convenience | A detector attached to an SEM or TEM, typically giving a spectrum within a few minutes | Needs a dedicated instrument, and even a single depth profile takes time |
| Typical uses | Identifying particles and failure sites, film and alloy composition, elemental maps in a TEM | Ion implantation and doping profiles, junction depth, trace contamination in films and at interfaces (including hydrogen, carbon and oxygen) |
Figures are typical orders of magnitude and vary by process generation and product.