Electron Microscopy - Revealing the Invisible Defect
While many different tools and techniques are used in performing failure analysis and assembling a report, the crux of the analysis is a clear, sharp photograph of the defect that lies at the root of the failure. Indeed, not only in failure analysis, but in any of the sciences, it can be said that "seeing is believing": a detailed picture can remove any shadow of doubt as to the nature of an object. In the case of failure analysis, a good image can help to identify the type of corrective action that must be implemented to resolve a recurring problem. For larger defects, an image taken with an optical microscope is often sufficient; however, given the infinitesimally small geometries used in modern semiconductors, a defect that may be catastrophically huge in terms of circuit performance may still be so small that it is effectively invisible to a traditional microscope - some defects are so miniscule, it is physically impossible to image them accurately with any sort of visible light optics. In these cases, electron microscopy is more than capable of peeling away the cloak of invisibility enshrouding a defect, providing crisp, detailed images at magnifications far beyond the limits of a traditional microscope.
Rather than using rays of light that have been focused by glass lenses, electron microscopy bombards a sample with a beam of electrons that has been shaped and focused by electromagnetic fields. When this focused electron beam interacts with a sample, several different phenomena occur. Auger electrons and x-rays can be generated, which can be used for elemental analysis techniques; most importantly to the operation of an electron microscope, however, is the generation of secondary and backscattered electrons. By scanning the electron beam across the surface of a sample, and measuring the energy of these electrons at multiple points, 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.
The unique operation of an electron microscope also lends itself to the isolation of certain types of electrical failure. 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.
Without electron microscopy, it would be impossible to find and photograph defects on many modern devices. Electron microscopy services are, of course, only one part of a successful failure analysis; while the electron microscope may be able to take a jaw-dropping picture, one must still identify the proper jaw dropping picture to take.
Derek Snider is a failure analyst at Insight Analytical Labs, where he has worked since 2004. He is currently an undergraduate student at the University of Colorado, Colorado Springs, where he is pursuing a Bachelors of Science degree in Electrical Engineering.