Vacuum pump used in SEM

Scanning Electron Microscope (abbreviated as SEM), referred to as Scanning Electron Microscope. It is an electron microscope that uses a focused electron beam to scan the surface of the sample to produce an image of the sample surface.
Scanning electron microscope (SEM), also known as SEM analysis or SEM microscope, is very effective for microanalysis and failure analysis of solid inorganic materials. The electron microscope is performed at high magnification to produce high-resolution images and accurately measure very small features and objects.

Scanning Electron Microscope

working principle
Electrons interact with atoms in the sample to generate various signals containing information about the surface mapping topography and composition of the sample. The electron beam is usually scanned in a raster scan pattern, and the position of the beam is combined with the detected signal to produce an image.
The scanning electron microscope emits an electron beam (about 50um in diameter) from an electron gun, which is converged by a magnetic lens system under the action of an accelerating voltage to form an electron beam with a diameter of 5 nm, which is focused on the surface of the sample, between the second condenser and the objective lens Under the action of the deflection coil, the electron beam scans the sample in a raster shape, and the electrons interact with the sample to generate signal electrons. These signal electrons are collected by the detector and converted into photons, and then amplified by an electrical signal amplifier, and then imaged on the display system.

The reason for using vacuum is mainly based on the following two reasons:

1.The filament in the electron beam system will rapidly oxidize and become invalid in the ordinary atmosphere, so in addition to the vacuum when using the SEM, it is also necessary to fill the entire vacuum column with pure nitrogen or inert gas at ordinary times.
2.In order to increase the mean free path of electrons, so that more electrons are used for imaging.

Since electrons travel very slowly in the air, the vacuum of the electron microscope must be maintained by the vacuum system, otherwise, the molecules in the air will obstruct the emission of the electron beam and cannot be imaged.
Two types of vacuum pumps are connected in series to obtain the vacuum in the electron microscope barrel. When the electron microscope is started, the first-stage rotary vane vacuum pump obtains low vacuum as the pre-vacuum of the second-stage pump; the second stage uses oil diffusion pump. The pump obtains a high vacuum.

 first-stage rotary vane vacuum pump

Generally, an electron microscope is composed of three parts: a power supply system, a vacuum system, and an imaging system. Important components such as the electron gun, electron optical system, and imaging analysis system of the electron microscope are all installed in the vacuum system, and the vacuum pump is the core part of the vacuum system. Its function is to effectively remove gas from the sealed system to achieve the purpose of maintaining vacuum. The commonly used vacuum pumps for analytical instruments are rotary mechanical pumps, oil diffusion pumps, and molecular turbo pumps.

second stage uses oil diffusion pump

As a pre-pump for medium and low vacuum systems or medium and high vacuum systems. The oil diffusion pump relies on high-speed oil vapor jets to carry gas for pumping. The ultimate vacuum of the molecular turbo pump can reach 10-5 Pa or higher, and it is phasing out the oil diffusion pump.
The vacuum pump must be quieter than the computer of the machine, and the vibration should be as small as possible, so as not to affect the image.

molecular turbo pumps

For low-resolution imaging, a majority of the SEM samples can be sputtered or carbon-coated in the pressure range of 1 x 10−1 to 1 x 10−2 mbar. A higher vacuum lower than 1 x 10−3 mbar base pressure allows the sputtering of oxidizing metals. These metals have a lower grain size ideal for high-resolution imaging. Likewise, lower scattering with the background gases enables high purity, high density, amorphous carbon films.

turbo pumps

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