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Tuesday, August 28, 2007

Closing the Imaging Gap Between Optical and Electron Microscopy

A new tabletop SEM combines the high magnification of electron microscopy with the ease of use of optical microscopy to improve performance in a benchtop instrument.

A radical new breed of microscope fills the critical gap between optical and electron microscopy. Optical microscopes are easy to use but generally limited to useful magnifications of 1,000x or less. Electron microscopes routinely operate at magnifications of 100,000x but can also be orders of magnitude more difficult to operate. The new microscopes, known commonly as tabletop or benchtop scanning electron microscopes (SEM), provide useful magnifications up to 20,000x and are as easy to use as the typical laboratory-grade optical microscopes.

The new instruments could not have come at a better time since the performance gap they fill corresponds to the ability to resolve features with sizes between 5 nm and 100 nm, a range that is critical in the booming field of nanotechnology. Microelectronics, microelectromechanical systems (MEMS), composite materials, and pharmaceuticals are but a few of the most obvious technologies with a pressing need for fast, easy access to structural and morphological characterization in this size range.

A typical SEM

The tabletop Phenom. All images: FEI Co.

The most striking development in the new tabletop SEM is its ease of use. Although small, inexpensive SEMs have been introduced more than once over the half century history of the technology, their widespread acceptance has been hindered by their operational difficulty. One of the new tabletop SEMs, the Phenom, from FEI Co., Hillsboro, Ore., is as easy to use as an optical microscope and accepts virtually any sample that will fit into the sample holder. Achieving this level of operational simplicity required much more than simply scaling down existing SEM technology; it required a redesign of many of the microscope's core components.

Back to basics
Optical microscopes use transparent lenses to focus light from the specimen into a real image, either directly onto the retina of the eye or into a camera or digital imaging system for capture and storage. SEMs create a virtual image by scanning a finely focused beam of electrons over the sample surface and mapping the intensity of various signals emitted at each point into an image array that is captured and displayed electronically.

Operating an optical microscope requires little more than placing the sample on the stage and focusing the image and is usually accomplished in a matter of seconds. Conventional SEMs, which require a high vacuum in the sample chamber, typically require several minutes to pump down the sample chamber in addition to any preparation required to make the sample compatible with the vacuum (cleaning, drying, coating techniques etc.). The time required to get an SEM image can easily become many minutes or hours. The Phenom cuts away the time, difficulty, and expense of the conventional SEM. The operator simply places the sample in the sample holder on the microscope. The automatically focused image is displayed less than 30 seconds later, with the resolution and depth of field typical of full-size SEMs.

Tabletop SEMs can be used in forensic analysis, showing traces of materials found on clothing, such as this diatom.

A quick look with a tabletop SEM at bulk particles can show their morphology. This sample has primarily spheroidal morphology.

Improving performance
To achieve this level of performance, engineers focused on a few basic requirements: size (including facility requirements), image quality, sample requirements, and easy operation.

Size -Successful imaging at high magnification in a conventional SEM requires a quiet environment. The presence of general lab equipment and a lively, vocal workforce in the same room as the SEM causes vibrations that distort the image. It is not unusual for a customized SEM room to cost as much as the SEM itself.

The SEM's sensitivity to vibration is a function of the resonant frequency of the column and sample holder, determined primarily by its length and diameter. The Phenom's miniaturized column is approximately 10 times smaller than a conventional SEM column and is energized by permanent magnets rather than the commonly used electromagnets. The sample holder is also smaller and rigidly mounted to the SEM column. The result is a dramatic increase in the resonant frequency and a system that is virtually unaffected by the noise and vibration of a typical lab environment.

Image quality -The key determinants of image quality in an SEM are resolution and signal-to-noise ratio. In simple terms, these factors are themselves determined by the choice of electron source and the accelerating voltage of the electrons. SEMs use one of three types of electron sources. Field emission sources produce very high resolution but require expensive high vacuum systems inconsistent with the cost and sample requirements of a tabletop SEM. At the other extreme, tungsten sources have the lowest vacuum requirements but need to operate at relatively high accelerating voltages to provide acceptable signal-to-noise characteristics. Unfortunately, increasing the accelerating voltage decreases the image resolution as a result of electron penetration into the sample. The third choice, LaB6, has properties and requirements that lie between those of field emission and tungsten sources. As a result, this type of source can be run at 5kV, providing the optimal combination of resolution and signal-to-noise in a tabletop SEM.

Sample requirements -The vacuum requirements in an SEM's sample chamber affect many aspects of its operation. Higher sample chamber vacuum requirements increase pump down and sample exchange time and impose tighter constraints on sample type and condition. The Phenom's sliding vacuum seal reduces chamber volume and sample exchange time. Reduced vacuum levels in the chamber also contribute to faster sample exchange, but, more importantly, relax the constraints on sample type. FEI pioneered the development of low vacuum (ESEM) technologies that allow just about any sample that will fit in the holder, with little or no need for coating, cleaning, drying or other preparations.

Ease of use-Perhaps the most important characteristic of a lab instrument is its ease of use. A traditional electron microscope has an often bewildering user interface with an extremely large set of choices and parameters to optimize. A large majority of the variables of a traditional SEM are not needed in a workhorse tool. By eliminating variables, automating adjustments, and creative software design, the Phenom has reduced operating the microscope to driving the stage and changing the magnification. With this kind of software interface, training a novice to use the system takes a matter of minutes- the instrument becomes productive almost immediately.

This image of a fruit fly shows the underside of the head. One of the compound eyes, the mandible, and two palps are visible.

Industrial applications SEMs are used in many fields of industry, such as pharmaceuticals, composite materials, and life science. These industries, as well as many others, will benefit from the new tabletop SEMs in characterizing structures and morphologies in the nanometer range easily and quickly in the lab. Some industrial applications include:

Particle characterization: Size, distribution, and morphology of particles and powders are critical parameters for industries such as pharmaceuticals, composites, cosmetics, and catalysts.

Often these measurements are derived from bulk analyses such as laser scattering; however, interpreting the data requires some knowledge of the particles themselves, i.e., are they spherical, rod-shaped, a mix of large and very small particles, etc. Tabletop microscopes with magnification ranges up to 20,000x are ideal for this requirement. A "quick look" at these materials can greatly improve the efficiency of analysis and characterization. Measuring the dimensions and uniformity of coatings is also of critical importance in materials research.

Quality assurance in MEMS: MEMS are miniaturized components commonly used in high volume, low-cost applications, such as the automotive and electronic industries. One of the most common applications is the use of a MEMS accelerometer inside automotive air bags. The reliability of this class of device is paramount so inspection and quality assurance are frequent steps in the manufacturing process. Images of 3-D objects at magnifications in the range 100-5,000 are required, which is a task ideally suited for a tabletop SEM microscope.

Crime scene investigation: Forensic scientists have long used microscopic images as an aid to their investigations, and the trend in this field is to look at finer and finer levels of detail. Traces of foreign material found on clothing can often be used to help establish where the clothing has been. For example, the presence of a specific species of pollen can potentially indicate a particular area where the clothing must have been. Similarly, the presence of specific species of algae attached to clothing can indicate conclusively that the clothing has been in water and potentially which body of water.

Building the future technology workforce
It is a common complaint that increasingly fewer students are enrolling in science and technology degree programs. Of the many reasons for this decline, one seems to be perennial: the difficulty for students to become excited about science when it is so hard to visualize and internalize the basic concepts. The opportunity to have an easy to learn and easy to use use tabletop electron microscope inside the classroom could help mitigate this problem.

This new class of tabletop microscope that goes far beyond the resolution capabilities of a light microscope, but does not require all the cost and complexity of using a typical scanning electron microscope, will find applications in many industries and educational environments. It is not unreasonable to speculate that this capability could have the same kind of effect on the efficiency of development of new methods and materials as the PC had on the productivity of the office.

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