The way an electron microscope differs from a visible light microscope can be reasonably inferred from the names of the two techniques; where visible light microscopy focuses the rays of light that our eyes perceive normally using optical glass lenses, electron microscopy uses strong electromagnetic fields to produce, shape, and focus a beam of electrons onto the surface of a sample. As the electron beam interacts with the sample, several phenomena occur; for the microscopist, the most important of these phenomena is the generation of secondary and backscattered electrons.
By scanning the electron beam across the surface of a sample and collecting the secondary and backscattered electrons, the electron microscope can construct an image of the sample. Since an electron has a far shorter wavelength than a photon of visible light, the diffraction limit of the tool is much smaller; resolutions of several angstroms can be achieved, where visible light is limited to roughly two tenths of a micron. This increased resolution makes it possible for an analyst with access to a good electron microscope in his or her lab to find nano-scale defects like gate oxide pinholes or crystalline dislocations.
Tuning the electron microscope detector to gather mostly secondary or mostly backscattered electrons can produce different data, showing greater implied topography or accentuating elemental differences, respectively. Electron microscopes also boast a much greater depth of field than optical microscopes, making it possible to keep large three-dimensional structures in focus across larger distances - a benefit when performing inspections of circuit assemblies or deprocessed integrated circuits.
Electron microscopy services are not limited to imaging; in addition to the generation of secondary and backscattered electrons, bombarding a part with a high energy electron beam also produces characteristic x-rays as a result of the excitation and relaxation of the electrons orbiting the atoms of the sample. The energies of these characteristic x-rays are uniquely tied to the element from which they are emitted; by using an energy dispersive spectrometer (EDS), these x-rays can be collected and the material composition of the sample can be identified. The EDS can be used to positively determine the makeup of contaminants, measure the constituents of an alloy to be compared to a specification or other reference, or generate an elemental “map” showing where certain elements are concentrated on a sample.
The electron microscope can also be used as an isolation tool for certain types of defects. The electrons that make up the focused beam of the tool are negatively charged, and therefore will experience some degree of attraction or repulsion depending on the charge present on a sample. By intentionally placing a charge on a sample (for example, connecting a voltage source to a failing signal on an integrated circuit), it is possible to change the way that the electron beam interacts with the device, creating differences in image contrast that can highlight a defect. This technique, known as "charge contrast" or "voltage contrast", can be invaluable in finding certain types of anomalies, especially those that cause open circuits. Indeed, certain defects may not require any additional setup at all; the passive charge contrast resulting from the electron beam itself may be enough for an analyst to pinpoint a defect.
Electron microscopy allows our electronic failure analysts to take incredible images of a huge variety of defects. From melted silicon to cracked metallization and all points between, the electron microscope is an invaluable tool for inspecting any anomaly. Electron microscopy services are, of course, only one part of a successful failure analysis; though an electron microscope picture might be the culminating piece of data for a failure analysis report, it takes experience and skill to ensure that the electron microscope picture is, in fact, of the defect at the root cause of failure.