What Is Sample Preparation?

Sample preparation plays an important role in transmission electron microscopy. According to the working principle of TEM, the sample for TEM analysis must be transparent to the electron beam. Generally, the thickness of the observation area of the sample is preferably controlled to about 100-200 nm. In addition, the samples produced must also be representative to truly reflect some of the characteristics of the material being analyzed. Therefore, these characteristics must not be affected during sample preparation. If the effects have occurred, the manner and extent of the effects must be known. TEM sample preparation is a topic that covers a wide range of topics and has many methods. Which method you choose depends on the type of material and the information you want to obtain. TEM samples can be divided into indirect samples and direct samples. This chapter only introduces widely used sample preparation techniques such as replica, electrolytic dual spray, and ion thinning.

Sample Preparation

There are many methods for preparing direct samples, but in general, the overall production process is divided into the following steps:

(1) Initial thinningpreparing a thin sheet with a thickness of about 100 to 200 m;

(2) Cut a f3 mm wafer from sheet E;

(3) Pre-thinning-thinning the central area of the wafer to several feet from one or both sides of the wafer;

(4) Final thinning.

1. Initial thinning-thin sheet preparation from block samples

For ductile materials, such as metal, in order to avoid mechanical damage to the material (for example, to study the structure and density of defects in the material), EDM is usually used to obtain slices with a thickness of about 200 m from block samples. In addition, the material can also be rolled into flakes, and then the rolling defects can be eliminated by annealing. For some brittle materials (such as Si, GaAs, NaCl, MgO), they can be cleaved along the cleavage plane with a blade and repeated cleavage until it reaches the level of electron transparency. If the slice is not parallel to the cleavage plane, diamond saw can be used. In addition, there are some special methods, such as cutting rock salt with a wire saw using water as a solvent. In addition, an ultra-thin slicer can also be used to cut samples from block samples that can be directly observed by transmission electron microscopy.

2. Wafer cutting

If the plasticity of the material is good and the requirements for mechanical damage are not very strict, a special small punch can be used to directly f3mm wafers from the sheet. For brittle materials, there are 3 basic methods to choose from, namely EDM cutting, ultrasonic drilling and abrasive drilling. EDM is used for conductor materials, the latter two are commonly used for ceramic and semiconductor materials.

3 Pre-thinning

The purpose of pre-thinning is to further thin the central area of the wafer, so as to ensure that the central part of the wafer is finally perforated (the area near the edge is available for observation). Pre-thinning usually uses a special mechanical grinding machine to reduce the thickness of the central area to about 10 m. With the help of microprocessor-controlled precision grinding, the thickness of the electron beam can be obtained (<1 m). Sometimes chemical methods are also used for pre-thinning.

4 Final thinning

There are two commonly used final thinning methods, namely electrolytic polishing and ion bombardment. Electrolytic thinning can only be used for conductive samples. It is fast and does not cause mechanical damage, so it is widely used for metal and alloy sample preparation. Figure 9-15 is a schematic diagram of an electrolytic thinning device. Ion thinning is suitable for the preparation of samples of refractory metals, hard alloys and non-conductive materials. This method has complex equipment, long thinning time, and difficult to master the later stages of thinning.

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