Scanning Electron Microscopy (SEM)


Scanning electron microscopy is used for inspecting topographies of specimens at very high magnifications using a piece of equipment called the scanning electron microscope.


SEM magnifications can go to more than 300,000 X but most semiconductor manufacturing applications require magnifications of less than 3,000 X only. SEM inspection is often used in the analysis of die/package cracks and fracture surfaces, bond failures, and physical defects on the die or package surface.


During SEM inspection, a beam of electrons is focused on a spot volume of the specimen, resulting in the transfer of energy to the spot. These bombarding electrons, also referred to as primary electrons, dislodge electrons from the specimen itself. The dislodged electrons, also known as secondary electrons, are attracted and collected by a positively biased grid or detector, and then translated into a signal. 


To produce the SEM image, the electron beam is swept across the area being inspected, producing many such signals. These signals are then amplified, analyzed, and translated into images of the topography being inspected. Finally, the image is shown on a CRT.


The energy of the primary electrons determines the quantity of secondary electrons collected during inspection. The emission of secondary electrons from the specimen increases as the energy of the primary electron beam increases, until a certain limit is reached. Beyond this limit, the collected secondary electrons diminish as the energy of the primary beam is increased, because the primary beam is already activating electrons deep below the surface of the specimen. Electrons coming from such depths usually recombine before reaching the surface for emission.


Aside from secondary electrons, the primary electron beam results in the emission of backscattered (or reflected) electrons from the specimen. Backscattered electrons possess more energy than secondary electrons, and have a definite direction. As such, they can not be collected by a secondary electron detector, unless the detector is directly in their path of travel. All emissions above 50 eV are considered to be backscattered electrons.


Fig. 1.  Example of a SEM photo of a contaminated area on a leadframe; EDX analysis is usually performed to identify such contaminants


Backscattered electron imaging is useful in distinguishing one material from another, since the yield of the collected backscattered electrons increases monotonically with the specimen's atomic number. Backscatter imaging can distinguish elements with atomic number differences of at least 3, i.e., materials with atomic number differences of at least 3 would appear with good contrast on the image. For example, inspecting the remaining Au on an Al bond pad after its Au ball bond has lifted off would be easier using backscatter imaging, since the Au islets would stand out from the Al background.


A SEM may be equipped with an EDX analysis system to enable it to perform compositional analysis on specimens.  EDX analysis is useful in identifying materials and contaminants, as well as estimating their relative concentrations on the surface of the specimen.


When performing SEM inspection, the following must be observed:

1) The
EHT must be high enough to provide a good image but low enough to prevent specimen charging.

2) To maximize
contrast due to material differences, use as low an EHT as possible.


3) If possible, sputter-coat the specimen to prevent specimen charging. Sputter-coating is considered destructive. Never sputter-coat units that still need to undergo electrical testing, curve tracing, EDX analysis, inspection, etc.


4) The probe current must be set to its default value, unless a higher probe current is needed to focus the point of interest properly.


Fig. 2.  Two examples of Scanning Electron Microscopes


Failure Mechanisms/Attributes Used For: Die/Package Cracks, Die Attach Failures/Defects, Bonding Failures/Defects, Wire Defects/Fractures, Lead Defects/Failures, Foreign Materials on Die/Package, Die Surface Defects, Seal Cracks/Defects, etc.


Transmission Electron Microscopy (TEM)


Transmission Electron Microscopy (TEM) is a technique used for analyzing the morphology, crystallographic structure, and even composition of a specimen. TEM provides a much higher spatial resolution than SEM, and can facilitate the analysis of features at atomic scale (in the range of a few nanometers) using electron beam energies in the range of 60 to 350 keV. 


Unlike the SEM which relies on dislodged or reflected electrons from the specimen to form an image, the TEM collects the electrons that are transmitted through the specimen.  Like the SEM, a TEM uses an electron gun to produce the primary beam of electrons that will be focused by lenses and apertures  into a very thin, coherent beam.  


This beam is then controlled to strike the specimen. A portion of this beam gets transmitted to the other side of the specimen, is collected, and processed to form the image. 


For crystalline materials, the specimen diffracts the incident electron beam, producing local diffraction intensity variations that can be translated into contrast to form an image.  For amorphous materials, contrast is achieved by variations in electron scattering as the electrons traverse the chemical and physical differences within the specimen. 


The greatest consideration when performing TEM analysis is sample preparation.  The quality of sample preparation contributes greatly to whether the micrograph will be good or not, so analysts are required to exercise the necessary diligence in preparing the sample for TEM analysis.


See Also:  Failure AnalysisAll FA Techniques Optical InspectionEDX/WDX Analysis

Auger AnalysisEBICFA Lab EquipmentBasic FA Flows Package FailuresDie Failures




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