As with other scanning electron microscopy, the JSM-840 examines structure by bombarding the specimen with a scanning beam of electrons and then collecting slow moving secondary electrons that the specimen generates. These are collected, amplified, and displayed on a cathode ray tube (CRT, typically a slower version of the picture tube of a television set) although now, most are driven by PCs and these computer-generated images are displayed on LCDs. The electron beam is scanned using a raster pattern so that an image of the surface of the specimen is formed. Specimen preparation typically includes drying the sample and making it conductive to electricity, if it is not already. Photographs are taken at a very slow rate of scan in order to boost the signal and capture greater resolution. All our SEMs use digital imaging. SEM is typically used to examine the external structure of objects that are as varied as biological specimens, rocks, metals, ceramics and almost anything that can be observed in a dissecting light microscope. The JEOL JSM-840 is used for conventional samples of normal sizes using the airlock. These specimens are usually mounted on multiple "stubs" (up to 4 specimen stubs fit) and each are observed individually. An alternative for large specimens is to vent the column an use the specimen drawer, which allows silicon disks and other objects to be viewed, but takes longer to pump as the whole specimen viewing area is brought to room pressure.
JEOL JSM-840 SEM Characteristics
This microscope joined the lab in 2009 and is used for general observation and also for the Oklahoma Ugly Bug contest (see http://www.uglybug.org/). The JEOL JSM-840 is among the last analog SEMs made, but is fully updated for digital operation.
The JEOL JSM-840 is a specialized SEM that may be driven as an electron beam (e-beam) device that focuses to a fine spot that can be scanned in a predetermined fashion on a specimen surface. Typically the target surface is a fine-grained photoresist material that is used for lithography. The produced high contrast lithographs may be used to generate electronic components or other products typically produced by a photographic method. SEM generation of the pattern allows greater resolution and thus miniaturization of scans in comparison with light generated patterns, as the wavelength of electrons may be 1000 times finer than photons. Thus this SEM can generate among some of the finest resolution photoresist patterns; this will eventually be exceeded by the Zeiss Neon SEM, which is on order.
The JEOL 840 scanning electron microscope has a LaB6-gun and thus provides higher resolution than the Zeiss 960 or Hitachi TM-3000, but less than the JEOL JSM-880 or Zeiss Neon.
The JEOL JSM-840 accepts conventional specimens that are mounted on stubs, which are 1 cm wide discs that have a pin-mount on the base of the disc. Specimens are mounted on to the stub before being made conductive -- usually by sputter coating the specimens with gold/palladium. Because of the palladium content, specimens typically appear silverish. Although the coating is coarse by TEM or JSM-880 standards, the grains can be barely resolved in the JSM-840.
Below are some examples of stubs used for the DSM-960 from the Oklahoma Uglybug Contest which is an annual outreach program for OU's SRNEML, as well as other EM labs around the State. Specimens are kept in a desiccator under vacuum and often with silica beads to keep them dry so that they do not change or decay. Mothballs are used as an effective deterrent for biological activity when sheets of specimens are too large to allow storage in a vacuum.
Specimen storage for conventional SEM is facilitated by using commercially available Plexiglass sheets with holes for the specimen pins. Hundreds of specimens can be mounted, indexed and stored on such a sheet.
Here are some insects from the Oklahoma Uglybug contest at higher magnification.