Electron Microscopy – Knowing the Limitations
The scale of features on a modern semiconductor device is so infinitesimally small as to be hard to even conceptualize. A cutting edge transistor from a powerful processor may have a channel length as small as 35 nanometers – a size dwarfed even by microorganisms like bacteria or viruses. In order to study these miniscule movers of the electronic world, electron microscopy is a necessity for any failure analysis lab; however, an electron microscope is not a panacea for imaging ailments, and must be applied with proper knowledge of device characteristics and the limitations of the tool.
Electron microscopy works by bombarding a sample with high energy electrons (usually accelerated through thousands of volts), then detecting electrons given off by the sample (in the form of either secondary or backscattered electrons). After some intense signal processing, these collected electrons are translated into a black and white image that an analyst can study in order to better understand the smallest details of a given device. Due to the exceptionally short wavelength of the travelling electrons, spatial resolution is greatly increased over an optical microscope, which is limited by diffraction to resolving features of a few hundred nanometers. This increased spatial resolution is what makes electron microscopy so attractive; however, the act of assaulting a sample with an electron beam is not without its potential pitfalls.
Often, a electronic failure analyst may be interested in examining a non-conductive or dielectric sample. These types of samples add an additional layer of complexity to an analysis, as, in addition to emitting secondary and backscattered electrons that are used to construct an image, the sample will also absorb part of the incoming electron beam entirely and begin to accumulate a charge. This charging effect is very detrimental to imaging – the charged area may deflect the beam slightly, distorting the appearance of features. Charging can also be a contributing factor to noise in an image, making interpretation much more difficult, as adding random pixels to an image can obscure smaller defects. In some instances, the beam can even burn or melt these types of samples, as they simply can’t hold up to the extremely high energy levels generated by the microscope.
Greasy, organic, or otherwise liquid samples also pose a problem for electron microscopy. In order to collect the greatest number of electrons to create an image, it is necessary to place the sample under a very high vacuum to prevent any emitted electrons from scattering off of any gas molecules between the sample and detector. Unfortunately, this high vacuum takes its toll on the aforementioned liquid samples, causing them to outgas (essentially “boiling off”, depending on the type of liquid). This makes it nearly impossible, for example, to image any sort of particulate in a solution (at least, without specially designed environmental electron microscopes).
Electron microscopy is undoubtedly vital to the failure analysis process; however, it is not without its limitations. Fortunately, with the proper training, an analyst can overcome these limitations, even using them to their advantage in techniques like passive voltage contrast imaging. The trick, naturally, is finding a lab with the proper experience to image even the most troublesome samples!
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.