Electron Microscopy – Knowing the Limitations
A modern semiconductor device has features that are so minute they are difficult to even imagine. Even microorganisms like bacteria or viruses are dwarfed by the size of a cutting-edge transistor from a potent processor, which can have a channel length as short as 35 nanometers. Any semiconductor and integrated circuit failure analysis lab must have an electron microscope to study these tiny components of the electronic universe. However, an electron microscope is not a cure-all for imaging issues and must be used with the correct understanding of device characteristics and the tool’s limitations.
By blasting a sample with high-energy electrons (often accelerated through thousands of volts), followed by the detection of electrons released by the sample, electron microscopy operates (in the form of either secondary or backscattered electrons). These gathered electrons are converted into a black-and-white image after extensive signal processing so that an analyst can examine it and learn more about the tiniest characteristics of a particular item. Compared to an optical microscope, which is constrained by diffraction to resolving features of a few hundred nanometers, spatial resolution is significantly improved by the travelling electrons’ extraordinarily short wavelength. The greater spatial resolution of electron microscopy is what makes it so appealing, yet bombarding a subject with an electron beam has risks of its own.
A non-conductive or dielectric sample is frequently of interest to an electronic failure analyst. These kinds of samples of electronic components increase the complexity of analysis because they emit secondary and backscattered electrons that are utilised to build images as well as absorb a portion of the incoming electron beam completely and start to build up a charge. The charged area may deflect the beam slightly, altering the appearance of features, which makes the charging effect particularly harmful for taking pictures. Adding random pixels to an image can hide lesser flaws, which makes interpretation much more challenging. Charging can also contribute to the noise of an image. As they can’t withstand the extremely high energy levels produced by the microscope, the electron microscopy beam may occasionally even burn or melt certain kinds of samples.
Samples that are liquid, greasy, organic, or otherwise problematic provide challenges for electron microscopy. Place the sample under an extremely high vacuum to prevent any emitted electrons from scattering off of any gas molecules between the sample and detector to capture the most electrons possible to form an image. Regrettably, the aforementioned liquid samples suffer from this high vacuum, which results in them outgassing (or, depending on the type of liquid, literally “boiling off”). For instance, it is practically hard to imagine any kind of particulate in a solution because of the high vacuum (at least, without specially designed environmental electron microscopes).
Although electron microscopy is unquestionably essential to the process of analysing electronic failures, it is not without its drawbacks. However, an electrical failure analyst can get beyond these restrictions with the right training and even benefit from them in methods like passive voltage contrast imaging. Finding a microelectronics failure analysis lab with the necessary expertise to photograph even the trickiest samples is the challenge, of course!
