Scanning Electron Microscopes
Secondary Electron Microscope or SEM was developed in 1938 to image the surface of samples with high resolution. Both a SEM and a light microscope apply the same principle; however, instead of using visible light, the SEM use electrons for imaging. Nonetheless, the wavelength of visible light limits resolution of the images from the optical microscope, while accelerated electrons in the SEM, which has much shorter wavelength, make it possible to investigate features in microscale to nanoscsale with high resolution. Therefore, this instrument has opened doors in numerous fields such as, physics, materials science, biology, chemistry etc.
To create an image, a beam of incident electrons(or primary electrons) is generated at the top of the microscope by a thermal emission source, for example, a heated tungsten filament, or a field emission cathode. The energy of the incident electrons can be varied from 100 eV to 30 keV depending on the evaluation objectives. The electron beam follows a vertical path through the vacuum chamber. The beam also passes through electromagnetic lenses which focus the beam down toward the specimen. When the beam hit the specimen, secondary emissions such as electrons, and X- rays are emitted from the specimen to chamber. Secondary electrons, backscattered electrons and X-rays are detected and converted to into a signal to the screens by detectors .
However, the primary limitation of the SEM is that a sample has to be clean, dry and electrically conductive. For non-conductive specimens, they must be coated with a conductive film to prevent charging. Moreover, a high vacuum chamber is required during operating to reach high resolution because the incident beam may interact with atoms in air before hitting the sample.
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Field Emission Gun (FEG) SEM
The field emission scanning electron microscope(FE-SEM) is an electron microscope which produces the images of the sample by scanning with a high-energy beam of electrons in a raster scan pattern. In conventional SEM, an electron beam is generated by heating a tungsten filament, thus the size of the resulting beam is limited by the geometry of the filament, which limits magnification. In contrast, in the FE-SEM, A cold field emission source generates a electron beam by applying a high voltage to a very sharp point, which is as small as 10 nm, thus the FE-SEM offers high performance and high resolutions. As a result, this FEG-SEM provides higher spatial resolution, better reliability and higher performance than other conventional scanning electron microscopes. However,the FE-SEM requires extremely high vacuum conditions in the chamber .
Environmental (EVO) SEM
The Environment Scanning Electron Microscope (ESEM) was developed in the mid eighties to overcome the constrains of the conventional SEM that requires high vacuum and dry samples. The ESEM allows researchers to investigate features of a sample in a range of pressure and temperature. The wet, dirty or non-conductive sample can be examined in their natural state without any preparation. The ESEM provides high resolution secondary electron imaging, at pressure as high as 50 Torr, and temperatures as high as 1800 K.
Rather than using a single pressure limiting aperture in conventional SEM, ESEM uses multiple pressure limiting apertures to separate chamber from the column; therefore, pressure as high as 50 Torr can be sustained in the chamber, but the column is still in high vacuum condition. Moreover, the ESEM uses Environmental Secondary Detector(ESD), which can be operated in non-vacuum environment, instead of using Everhart-Thornley(ET) in conventional SEM.
The ESD applies the principle of gas ionization. A positive charge is applied to the detectors to attract secondary electrons emitted by the sample when interacting with a beam of primary electron. The secondary electrons are accelerated in the detector field, thus the they hit gas molecules. The secondary electrons signal are amplified and positive ions are created as a result of gas ionization. For non-conductive samples, the sample surface effectively attracts the positive ions caused by gas ionization as charge accumulates from the beam of primary electron. These positive ions help to prevent charging artifacts .