Typically, emission microscopy is performed following the chemical decapsulation of a part, after the plastic encapsulant material has been removed from over the surface of the semiconductor die. When viewed this way, the junctions and transistors of the device (the most likely sources of photoemission) lie beneath all the metal traces on the die. While this is not necessarily an issue on older technologies, modern semiconductors with more densely packed metal can pose a problem by obscuring any emission sites from the view of the camera. To combat this effect, emission microscopy can be performed from the back side of the semiconductor, through the silicon substrate. The die must first be polished until it is exceptionally thin, to allow the photoemissions to pass through to the camera easily. Because silicon is moderately transparent to infrared light, an IR light source is used in conjunction with the emission microscope (which has an IR-sensitive camera) in order to image the die, then photoemissions are captured. When viewed this way, the active regions of the device are the “top” layer, and the metal traces on the die lie beneath; therefore, there is nothing to obscure any photoemissions from the camera, allowing even the most densely wired parts to be analyzed with ease.
In addition to the typical integrated circuit applications, emission microscopy can also be used for failure analysis of optoelectronics. All optoelectronics use light in their application, whether they are solid state relays that use light to control a switch or laser diodes that emit single-wavelength light. Since emission microscopy is designed around the detection of light, it is a perfect fit for analyzing these types of devices. By setting a fixed acquisition time, the emission microscope can even be used to determine whether a device is emitting enough light by comparing images from both a functional and a nonfunctional device.
Emission microscopy is a versatile, effective tool for failure analysis. It can be applied to many different types of devices, in many different ways; however, it takes the training of emission microscopy experts and a thorough understanding of the tool in order to turn the results from an emission microscope into a successful failure analysis.
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.