Affordable, compact scanning electron microscopes (SEMs) are providing fast, direct access to high- resolution imaging to an expanding number of engineers and technicians. A new generation of personal electron microscopes delivers usable magnifications up to 24,000X, more than 20 times better than conventional light microscopes, and a depth of focus (up to 400 microns) for better imaging of complex, 3-D objects. In addition, a personal electron microscope allows novices to quickly make high-resolution images and analyses, immediately confirming expectations or exploring new possibilities suggested by the results. Personal electron microscopes are adding value in a growing number of industrial and R&D applications.
Designing a personal electron microscope requires much more than simply scaling down the size of an existing design.
Traditional electron microscopes present the operator with a large set of choices and parameters to optimize. One development in the new personal microscope is the touch-screen user interface. Although other small, inexpensive tabletop SEMs have been introduced, their widespread acceptance has been hindered by their operational complexity.
All operations, including navigation with the motorized stage, are highly automated and controlled through a touch-screen user interface and a rotary knob controller. By eliminating variables, automating adjustments and using creative software design, the personal microscope simplifies SEM operations to the barest essentials: driving the stage and changing the magnification. Digital images are immediately saved to a media USB Flash drive or a network storage location for additional off-line examination.
Conventional SEMs require a high vacuum in the large sample chamber, typically requiring more than five minutes to pump down. New personal microscopes cut that time to less than 30 seconds by permitting relatively low vacuum (higher pressure) in the sample chamber. Lower vacuum levels also permit imaging of nonconductive samples with less risk of charging artifacts.
The electron column creates and focuses the scanning electron beam. Instead of the tall electron column and large electromagnetic lenses of a conventional SEM, the personal electron microscope column can literally fit in the palm of a hand, and its lenses use permanent magnets to focus the electron beam. In addition to being less costly, the smaller, lighter column is far less sensitive to interference from acoustic and mechanical vibrations that are abundant in engineering and classroom environments. Eliminating the need for vibration isolation and facility requirements, such as special cooling and high-vacuum equipment, reduce the cost and complexity of the system.
PerformanceThe key determinants of image quality in an SEM are resolution and signal-to-noise ratio. In simple terms, these factors are determined by the choice of electron source and the accelerating voltage of the electrons. SEMs use one of three types of electron source. Field emission sources produce very high resolution but require expensive high-vacuum systems inconsistent with the cost and sample requirements of a personal electron microscope.
At the other extreme, tungsten sources have the lowest vacuum requirements but need to operate at relatively high accelerating voltages (for example, 15 kilovolts) to provide acceptable signal-to-noise characteristics. But increasing the accelerating voltage decreases surface detail visibility as a result of electron penetration into the sample.
A third choice, hexaboride crystals (LaB6 or CeB6), has properties and requirements that lie between those of field emission and tungsten sources. The unique properties of hexaboride crystals provide stable electron-emitting media with work functions near 2.5 kiloelectron volts (keV). The low work function yields higher currents at lower cathode temperatures than tungsten, which means greater brightness (or current at beam focus) and longer cathode lifetime. Typically, these cathodes exhibit 10 times the brightness and more than 10 times the service life of tungsten cathodes. In electron microscope applications, these characteristics translate to more current in a smaller spot at the sample, improved resolution and less frequent cathode replacement.
ApplicationsThe high-resolution imaging capabilities of a personal electron microscope can detect errors in processes and perform quality control checks. The large depth of focus enables engineers, technicians and students to image materials and compounds that are not visible with optical microscopes. As manufacturers move to smaller dimensions, SEM images can be used to maintain narrower processing parameters. The personal electron microscope can be used for applications such as failure analysis, defect detection, particle morphology, size analysis, fiber analysis and metrology.
One such application is the inspection of semiconductor component leads for potential defects. Government regulations have forced the adoption of lead-free solder in many semiconductor and electronics applications. Lead-free solders can exhibit tin whiskers, crystalline-like growths believed to result from residual stress on the surface of the leads.
These crystalline growths create a potential source of electrical shorts and arcing in chips. They can range in length from 10 nanometers to 10 millimeters and can actually cause device and system failures. Analysts predict that these issues will become more frequent in this new solder-free semiconductor era.
Another application that benefits from rapid, 3-D imaging is the characterization and distribution of micro-structural features in metals. Rapid examination of common engineering alloys-for example, Al, Ti, Fe and Ni-can be performed with a personal electron microscope in areas such as routine metallurgical analysis, quality control, failure analysis and research studies.
Metallography provides information about an alloy, linking its composition and processing to its properties and performance. Certain distinct features on metals can exceed optical resolution limits. These features can be distinguished with a personal electron microscope.
Two SEM images of titanium each can be shown with distinct information not easily attained or impossible with optical inspection techniques. Precise quantitative measurement of the a-laths is important for input into emerging models that are capable of predicting alloy properties. The presence of sub-micron secondary nucleation sites indicates a strengthened form of the alloy. This type of information is valuable in both process development and production control settings.
Supporting Future AdvancementsAs technology advances toward nanoscale structures and manufacturing, electron beam-based examination techniques will be required. Everyday inspections currently performed with optical techniques can benefit from personal electron microscope technology.
Personal electron microscopes will find new applications in many industries and educational environments. Their use will promote continued advancement in areas such as process control, materials characterization, failure analysis and quality control during the coming years. Q
Quality OnlineFor information on microscopy, visit www.qualitymag.com to read these articles:
- “Integrated Microscopy”
- “Quality 101: “Microscopy Use Will Grow”
- “Quality Innovations: “Mainstream Microscopy”