Figure 1

SEM

Microscopists have developed an array of sample preparation techniques that have extended the applicability of SEM. However these preparations impose an additional burden in time, cost, complexity and difficulty. Equally important, all preparations may modify the sample in some way, introducing a layer of complexity and uncertainty to the interpretation and use of the information they contain.

Figure 2

Low-Vacuum SEM and ESEM

The primary imaging signals used in SEM are secondary electrons (SE), which are electrons from sample atoms ejected by interactions with the primary electron beam, and backscattered electrons (BSE), which are electrons from the primary beam that have been scattered back out of the sample by the nuclei of sample atoms. Secondary electrons have very low energy and can escape to be detected only if they are created very close to the sample surface. The SE signal has high spatial resolution and is very sensitive to surface topography. BSE have higher energy and can travel greater distances through the sample. The BSE signal has lower resolution and carries information about sample composition (atomic number). Conventional (Everhardt-Thornley) SE detectors cannot function in low-vacuum conditions so some LVSEMs rely on BSE as the primary imaging signal. Though the BSE has less resolution, it is also less sensitive to charging and does permit imaging of less conductive samples without conductive coatings.

ESEM extends SEM capability to much higher chamber pressures, providing high-resolution SE imaging and a means for neutralizing charge even on highly insulating samples. Higher chamber pressures are achieved by differentially pumped vacuum stages with decreasing pressure levels between the sample and the electron source. The stages are separated by pressure-limiting apertures (PLA), which may or may not serve also as beam-limiting apertures in the electron optical system. Depending on the specific application for which the detector is designed, the final PLA may be positioned close to the sample surface, beyond the final optical aperture, thus reducing beam scatter by shortening the distance the beam must travel through the final high-pressure region.

The secondary electron detector actually uses the gas in the sample vicinity in the amplification process. Secondary electrons emerging from the sample surface are accelerated toward the detector by an imposed electric field. The accelerated electrons ionize gas molecules to create additional electrons and positive ions. This process repeats several times between the sample and the detector, resulting in a cascade amplification of the secondary electron signal. Equally important, the positive ions created in the cascade are available to neutralize any charge accumulating at the sample surface.

Because ESEM does not require intact conductive coatings and is insensitive to gas or contamination generated by the sample, it can be used to investigate dynamic processes that cannot be observed in conventional SEM. The emerging field of in situ SEM actually manipulates the sample environment as an experimental parameter, observing changes in the sample in response to changes in the environment. For example, ESEM can be used to monitor coating behavior through heat and humidity cycles, or quite literally, to “watch the paint dry” as solvent evaporates.

Figure 3

X-ray Microanalysis

Figure 4

DualBeam

A “Slice & View” technique takes three-dimensional imaging to the next level. It cuts a sequence of cross sections through the sample at designated intervals, automatically acquiring and saving an image of each section. The stack of images is then reconstructed into a virtual model of the sample. The model can be rotated to any viewing angle, virtually resectioned, and rendered with surfaces that follow contrast contours to emphasize structural details.

FIB can also deposit material with nanoscale control. In the deposition process, a volatile precursor material is introduced through a fine needle in the vicinity of the beam and sample surface. The beam decomposes the precursor into a nonvolatile component that deposits on the surface and a volatile component that is pumped away by the vacuum system. A variety of materials are available, including metals and insulators. The same kind of injector system can be used to introduce gases that enhance FIB milling speed.

FIB should not be neglected as an imaging technique in its own right. FIB images are particularly sensitive to channeling effects and are very useful for visualizing grain structures.

Figure 5

SEM Applications in Failure Analysis

Figure 3 shows a zoom series of SEM images that are shot at progressively higher magnifications. The low magnification image at upper left demonstrates the large field of view and excellent depth of field available. The series zooms in on a defect. The image at the lower right was shot at a magnification of 4000x. In it the defect has been cross sectioned by the ion beam and the sample retilted to show the wedge shaped FIB pit from above. The rectangular feature on top of the defect is a protective layer about 2 microns thick deposited by the FIB prior to cross sectioning to protect any delicate surface structure from erosion by the “tails” of the ion beam during the milling process.

Figure 4 shows three images that are a secondary electron SEM image (left), a FIB-induced secondary electron image (middle) and an X-ray map overlaid on the SEM image. The defect appears to be a particle inclusion. The FIB-deposited protective layer overlays the particle. The bright, “liquid” appearance of the particle in the left image is an artifact of charging. The variations in contrast among the grains in the lower layer of the middle image show the sensitivity of FIB images to grain orientation due to channeling effects. The X-ray map overlaid in the right image was set to display Ca in red and Si in green. The composition of the inclusion often helps to identify the source.

In Figure 5 we see three images that illustrate the Slice & View technique. The system automatically cuts a sequence of cross sections through the region of interest and acquires a high-resolution image of each one. The image stack (left) is combined to reconstruct a three-dimensional model (middle) of the region. The model can be rotated, resectioned and rendered to reveal internal structure.

Figure 6 contains images of the same multilayer automotive paint sample. The secondary electron image on the left illustrates the sensitivity of the SE signal to surface topography. The backscattered electron image in the middle shows the sensitivity of the BSE signal to the varying composition of each layer. Both images were acquired in low vacuum conditions, the SE image with a large field gaseous detector (ESEM) and the BSE with a solid state detector, and show no charging despite the insulating nature of the sample. The X-ray spectrum (bottom) was acquired from the top layer. The energy levels (x-axis) of the spectral peaks indicate the elemental identity of the emitting atoms. A quantitative determination of the concentration of each element can be derived from the intensity (y-axis) of the peaks.

Figure 6

Conclusion

For further information, contact FEI Electron Optics b.v. Building AAE, Achtseweg Noord 5, P.O. Box 80066, 5600 KA Eindhoven / Acht, The Netherlands, www.FEI.com or William.Neijssen@fei.com; Laurent.Roussel@fei.com.