What are the requirements of scanning electron microscopy for test samples

Date：2023-08-04 15:37:00Views：249

SEM does not obtain the sample information by direct observation, but by emitting electron beam to excite the sample, and using backscatter or Secondary electrons detector to analyze the composition and morphology of the observed object. To obtain accurate results, the test sample must meet certain requirements. The following are the requirements for testing samples using scanning electron microscopy:

1. The morphology should be resistant to high vacuum

Scanning electron microscopy uses an electron beam to scan the surface of an object for imaging. The presence of air can deform the electron beam and affect the scanning effect, so the tested sample is more resistant to high vacuum.

2. The surface of the sample should not contain organic grease pollutants

Oil stains can easily decompose into carbon hydrides under the action of electron beams, causing significant pollution to the vacuum environment. The surface details of the sample are covered by hydrocarbons; Hydrocarbons reduce the yield of imaging signals; The adsorption of hydrocarbons on the electron beam path causes significant scattering; Hydrocarbons are adsorbed on the surface of the detector crystal, reducing the efficiency of the detector. Severe interference to electron beams with low acceleration voltage.

3. If the sample is dry

Water vapor will accelerate the evaporation of the cathode material of the electron gun, greatly reducing the lifespan of the filament; Water vapor will scatter the electron beam, increasing the energy dispersion of the electron beam, thereby increasing large color difference and reducing resolution.

4. The sample surface can conduct electricity

The surface of the sample should be conductive, but if it cannot, it should be gilded to increase conductivity. In most cases, the number of charges of the primary electron beam is greater than the sum of the number of backscattered electrons and Secondary electrons, so the excess electrons should be introduced into the ground, that is, the sample surface potential should be kept at 0. If the surface of the sample is not conductive or the sample ground wire is broken, then there is a static charge on the surface of the sample, causing the surface negative potential to continuously increase, leading to charging effects, causing image distortion, and slowing down the incident electron beam. At this point, the sample is like an electron plane mirror. For non-conductive samples, it is recommended to use ion sputtering coater to spray metal, which can significantly improve the quality of images observed by Scanning electron microscope.

5. Considerations for Special Sample Preparation

Scanning electron microscopy generally does not recommend observing magnetic materials. If it is necessary to observe such substances, it is recommended to demagnetize the material first. If you want to detect and observe the weak contrast mechanism, you need to eliminate the strong contrast mechanism (such as morphology contrast), otherwise it is difficult to detect the weak contrast. When the backscattered Electron diffraction contrast (EBSD), type I and II magnetic contrast or other weak contrast mechanisms are desired, the magnetic domain characteristics of magnetic materials must eliminate the morphology of the samples. Chemical polishing and Electropolishing are used to make the sample produce a mirror surface that almost eliminates the appearance.

Before testing, it is necessary to ensure that the sample meets the above requirements in order to obtain accurate test results. For more professional knowledge related to third-party testing, the editor will continue to edit, organize, and publish it in the industry information column to share with you.