What follows is a list of some physical perspectives on the electrons used routinely for transmission electron microscopy. Without knowing it, you may on a daily basis be putting to practical use things, like the wave nature of electrons, that were inconceivable in the early part of this century. In fact, some of the properties of these electrons may be only marginally conceivable today!

Molecular microspectroscopy is the application of infrared or Raman spectrascopy, combined with light microscopy, for chemical analysis on the microscopic scale. For over a decade infrared microspectroscopy (ISM) has been an expanding technology both scientifically and commercially. Micro- Raman spectroscopy has a long history of scientific accomplishments with limited commercial success. Both of these techniques give spectral data that can be related to the bonding, crystalline state, isotopic content and molecular orientation of a material, but not its elemental composition.

This month's installment completes our primer series on magnetic fields by defining conditions where remedial EM site shielding is in fact technically feasible and by illustrating examples of both passive and active shielding methods (or attaining lower site fields. For those readers who may have missed one or more previous articles, we have thus far presented some basic physics describing magnetic source fields [Microscopy Today, November, 1995), examined magnetic survey equipment and methods relevant to EM interference thresholds (Ml, January, 1996), and, in Part IV (“Survey Data Analysis”, MT, May, 1996), suggested techniques for interpretation of typical EM site survey data.

I was one told by a Nobel Laureate in Chemistry that light microscopy was simply a service foundation. By this he meant to class the microscope with computers, gas chromatographs, infrared spectrophotometers, x-ray diffractometers, mass spectrometers, etc. With all due respect to this gentleman and to these other instruments, there is a vital difference between the polarized light microscopes (PLM) and each of these instruments. First, a trained microscopist requires far more training than a qualified operator of, and interpreter of data from these other instruments. Second, there is considerably more basic physical and chemical information observable and measurable with PLM.

Over the past decade the achievable resolution of the scanning electron microscope (SEM) in secondary electron (SE) imaging mode has improved by about one order of magnitude. In fact, instruments capable of demonstrating a resolution of one nanometer and exhibiting a probe size of less than 0.6 nm are not available. Continued improvements in electronoptics, electron sources, and in electronic and mechanical stability promise even smaller probe sizes that still contain adequate current for imaging, it is therefore relevant to consider what resolution might ultimately be achievable with an SEM in SE mode.

Did I ever tell you about my first optical enginsering project? It was a video projector which used many innovative ideas, including one of the first liquid crystal video displays. It was dssigned to be the brightest projector ever made and was used in a helmet mounted display prototype. My first job out of college was to gather the parts, put it together, and measure its characteristics. To make a long story short, I had to turn the room lights out to see the image! What made a 20,000-footlambert design into a 20- footlambert display? Illumination! I learned a lot about illumination in those few short weeks, and everything I learned is directly applicable to microscopes. In fact the whole science of illumination was developed by microscapists. Electron microscopes deal with exactly the same concepts.

High Angle Dark Field Imaging, or Z contrast imaging, is an Imaging method. It takes advantage of the useful fact that if one uses the high angle scattering intensities and eliminates the elastic scattered (diffracted) beams from the image (by using a Howie type angular dark field detector), the remaining image will be characterized by, if the probe used is on the order of the atomic dimensions, intensity modulations that reveal atom positions and relative atomic number. In simple terms, the image will display Z-contrast at the atomic level and can differentiate columns of heavy atoms from columns of lighter ones.

There seems to be a consensus among both the scientific and lay communities in according the highest rank, in terms of prestige, to theoretical scientists. This may seem justified because theoreticians understand the thought processes and activities of the experimentalists, the laypeople, and even the artists, excepting perhaps poets, but the reverse is not the case. Virtually nobody can understand the theoretician. Thus, in order of prestige, after the theoreticians come the experimental scientists and then the technologists, but the lowest on the totem pole are the designers of hardware, of scientific instruments. This towly rank does not seem to bother them, and they are happy in their field. As a matter of fact, many who earn royalties or actually manufacture the hardware are laughing ail the way to the bank.

The Small Angle Cleavage Technique (SACT) developed by John McCaffrey is an extremely easy and cost-effective technique for producing superior cross sectional TEM samples from semiconductor and other single crystal materials. In the original technique, a special jig is required to be made in order to make too cuts in a 2 x 0.5 mm slotted capper TEM grid to create vertical tabs to mount the cleaved samples. This jig is not commercially available and the actual cutting and bending of the grids require a significant amount of manual dexterity. (Fat fingered fellows, like myself, need not apply.) Early on, John proposed a design for a commercial grid that could easily be bent into the required shape with no cutting, but the electron microscopy supply houses passed on the idea. Until now, microscopists wanting to experiment with the technique had to invest in the cost of having one of these jigs made.

In order to perform quantitative X-ray microanalysis many parameters, representing the various stages of X-ray generation and transport through the specimen at the chosen experimental conditions, must be known for all of the elements that might be encountered. Although ideally quantification is done by reference to standards so that only the functional variation of these parameters is required, in current practice it is increasingly necessary to work in situations where standardization is impossible and consequently where absolute magnitudes must be known. The quality and quantity of data that is now available varies widely.

An innovative technique has been developed which allows simultaneous cleaning of a sample and a specimen stage which minimizes and, in some cases eliminates, contamination of inorganic specimens analyzed via SEM.TEM, STEM and AEM.

The technology involves subjecting the specimen and the specimen stage to a reactive plasma gas which efficiently removes a wide range of contaminants from critical surfaces. The procedure may be carried out prior to inserting the specimen and specimen stage into the EM via an externally produced accessory unit and can be extended for use on all types of electron microscopes where the sample and stage are loaded external to the instrument. Such an instrument was produced and implemented first by Dr. Nestor Zaluzec at Argonne National Laboratory. His system modified a commercially available pfasma etching system which included adapting it to accept a side entry holder.

Often times one is asked the question "how can the optical microscope be used to make quantitative measurements?" The microscope certainly lets one observe the sample {whatever the sample may be) and make qualitative statements about what is it one is looking at. It has been used for various purposes from just visual observation in brightfield, reflection and the one that is near and dear to my heart, observations under crossed polarizers. This certainly allows one to identify materials as being isotropic or anisotropic. If one works on materials like liquid crystals, an optical microscope is a tool that is used all the time. It certainly allows for identification of various phases of liquid crystals based on the defects and textures that can be observed. All of this is somewhat qualitative in nature. By that I mean that the observations do not lend themselves to obtain, say for example, refractive index or birefringence of the material. I would like to describe a technique that will allow one to do just that.

After the trauma of birth, a baby is suddenly in a strange new world. The first order of the day is to make sense out of the jumbie of signals in order to form an image of what is out there. This image building proceeds with increased accuracy throughout adolescence. For those who labor in science the process never stops. They continue to try to refine their mental image of nature throughout their professional lives. In this effort, vision is the most valuable natural sense. In a happy blend of poetry and science, Diane Ackerman in her book, A Natural History of Senses, calls eyes “the great monopolist of our senses.” It is, in fact, hard to imagine experimental science without the power of vision. By the same token, any enhancement of visual acuity will promote the precision of our image of nature - the very essence of science.

Specimen support grids are now almost universally 3.05 mm in diameter, except for a few high resolution stages and some very old instruments. They are available in a vast range of materials and designs. One catalogue lists 86 types in a total of 10 materials. The reason for this proliferation is to enable one to control the following:

1. (a) the amount of support the specimen needs (unsupported areas range from 20 pm to 1 mm in extent);

2. (b) the material of the grid, so that it neither interferes with X-ray analysis nor reacts with the specimen;

3. (c) the labeling of specific regions of a specimen (many grids have identification marks for relocation of interesting fields).

The cheapest most widely used supports are copper grids at a spacing of 100 bars in"1. Most grids have a shiny side and a dull side. Opinions differ as to the best side on which to mount the specimen but if a consistent practice is adopted it is always known which way up in the microscope the specimen was mounted.

This is the first in a series of articles that serve to identify what scanning Cathodoluminescence can offer the materials technologist. Whilst begining with Electronic Materials. Geological,Ceramic and Diamond applications will be covered over the next installments concluding with a reveiw of instrumentation needed to perform the studies covered in the final chapter.

Cathodoluminescence (CL) is the most important optical phenomenon used by all electron microscopists. It is the process by which electrons are converted into light in the Everhart Thornley secondary electron detector and also in the screens and cameras of TEMs.

Industrial laboratories have traditionally used 4” x 5” (100 x 125 mm) photomicrographs. Polaroid 4” x 5” prints have largely replaced glass plate and sheet film contact prints in industrial laboratories. High resolution 2000 by 3000 pixel imaging systems should be able to match the resolution of the 4” x 5” photomicrographs, typically 6 cycles per mm at a magnification of 500 times N.A., with a significantly larger field size which better utilizes modern, wide field optics. CCD cameras with 1024x 1280 pixel sensor arrays should be able to essentially match Polaroid 4” x 5“ photomicrograph prints in field size and resolution.

In my last article, I discussed the need for scheduled system checks on scanning electron microscopes. The article used an experience at a local SEM laboratory to point out the importance of monitoring your instrument by performing simple but regular system performance evaluations.

In this article I will discuss some principles behind the performance of imaging systems such as scanning electron and scanning probe microscopes. We will talk about the concepts of image linearity and dimensional calibration.

Opticai and election microscopy have been used extensively as problem solving tools in the graphic arts industry. Atomic force microscopy (AFM) is a complementary tool to optical and electron microscopy - it provides a high resolution view of areas 150 mm by 150 mm and smaller, images may be viewed in three dimensions and imaging can take place in either air, under liquid, or in a vacuum.

AFM can be used for more than just the three-dimensional imaging of nanoscale features on a sample surface; it can detect friction, adhesion, compliance, thermal properties, and even magnetic signals. AFM images are stored digitally and so the data can be enhanced and viewed in many rays. Algorithms allow the display of a line profile across the image surface at any desired location along with quantitative height information. Average roughness data can also be calculated.

The ability of scanning beam x-ray microanaiysis to show the spatial distribution of elements was recognized quite early. At first it was done one element at a time by wavelength dispersive spectroscopy (WDS), but when energy dispersive spectroscopy was developed, that became the technique of choice. Because there have been several significant improvements in the technology, and also because maps are used so universally, I thought a review of the technology from the early days to the latest technology might be interesting. Though modern maps are ail digital, the original analog “dot” maps are used by a few even to this day. Other names for the technique are sometimes used such as “x-ray image”, which is correct but can be confused with radiography. Compositional image has also been used, but the term is broad and includes more than x-ray maps. Consequently, the universally recognized term “maps” will be used.