Scanning Electron Microscopy (SEM)

Operation

Under high vacuum (10-6 Torr), an electron beam varying in intensity up to 30 keV is rastered over the sample surface, creating secondary electrons. These are extracted from the sample and imaged to create a high resolution, high depth of field, secondary electron image at magnifications to 250k x. Interactions of the sample atoms with the primary electron beam also result in inner core ionization of the atoms with the subsequent emission of quantized X-ray photon. Wavelength or energy analysis of the emitted X-rays provides qualitative elemental identification (because each element radiates at a specific wavelength) and the relative intensities of these X-rays are used for quantitative analysis.

In semiconductor materials, electron beam bombardment also creates electron-hole pairs, resulting in an electron beam induced current (EBIC), which can be imaged to correlate with other sample features. Various stains (over 600) and sample preparation techniques are utilized to enhance the areas of interest

Applications

SEMs are used in the characterization of microstructural surface topography, step coverage, grain size, oxide slope, construction parameters, sample composition, contamination, structural defects, bonding defects, intermetallic formation and degree of wire bond deformation. Digital X-ray maps of up to eight elements may be

Energy Dispersive X-Ray Spectroscopy (EDXS)

The interaction between the energetic monochromatic electrons from an impinging electron beam and the electrons in the atoms of the specimen results in the generation of X-rays. Characteristic X-rays are always of a specific energy or wavelength and identify the elements in a specimen. The same principle is used to create the X-ray source for X-ray radiography, except the current of the electron beam is very high and the goal is the generation of a strong source of X-rays.

The electron microprobe instrument utilizes a stationary beam of electrons to excite X-rays from the sample area of interest. The X-rays are characterized by their specific wavelength with a crystal diffraction grating. The electron microprobe suffers from its selectivity and method of analysis. Various crystals are required to efficiently diffract different wavelengths of X-rays. In addition, a given crystal can diffract only one wavelength at a time. Therefore, ranging over all the wavelengths in the periodic chart is a very tedious and time consuming task. The more common instrument is an energy dispersive X-ray analyzer (EDXA) attachment to the SEM. This tool and its capabilities makes use of the known binding energy in a doped wafer of silicon, which is used as a detector.

The generated X-rays impinge on the silicon detector surface. The silicon is doped with lithium. The penetration depth of the X-ray into the silicon is a direct function of the energy of the X-rays. Interactions occur along the penetration track between the X-rays and the silicon atoms creating hole-electron pairs. The generation of each hole-electron pair requires a specific amount of energy. Therefore a weak X-ray of a shallow penetration depth will generate a smaller current pulse than a more energetic X-ray of a longer penetration track.