There are three reasons for choosing dimpling as the primary step in the preparation of transmission electron microscope (TEM) specimen:
(1) Dimpling significantly minimizes the time required to carry out final thinning for electron transparency, saving more time specifically when the final polishing step is comparatively slow, e.g., in the case of ion polishing)
(2) Less specimen movement occurs during tilting and less x-ray shadowing occurs during energy dispersive x-ray spectroscopy (EDS) analyses since the electron transparent area is always formed at the center of the specimen disc
(3) Fragile TEM specimens can be easily handled because of the thick and supportive rim of the material that is formed during dimpling.
Dimpling Ductile Materials
The difficulty often faced while choosing a method for dimpling ductile materials such as solders and pure metals is that the cutting rate is very low when compared to the cutting rate for harder materials such as semiconductors and metal oxides. This is because the commonly used polishing grit, i.e. aluminum oxide or diamond, gets embedded in the soft material during dimpling, and consequently, instead of the specimen, grinding of the dimpling wheel is initiated.
This issue can be overcome by using cubic boron nitride (CBN) as the polishing compound. As CBN particles have a rounded shape, they are not easily embedded in the materials that are polished.
The recommended starting thickness for the ductile material ranges from 100 to 150 µm. Specimen discs of 3 mm diameter are cut from this material by using the model 659 disc punch manufactured by Gatan.
A phosphor bronze wheel with a diameter of 15 or 20 mm is fit in the model 656 dimple grinder, and then the 3mm discs are dimpled down to an approximate thickness of 10 µm by using a 4 – 6 µm CBN grit. A felt wheel is installed and then polishing is continued initially with 4-6µm and subsequently with 0 – 2 µm CBN grit such that the coarse marks formed upon grinding by using the 4 – 6 µm grit are entirely removed.
Sperm whale oil substitute is used as a lubricant to prevent drying of the CBN paste during the grinding as well as the polishing steps. Figure 1 shows a scanning electron microscope (SEM) image of a 316 stainless steel TEM specimen disc that is ground and polished with the CBN grit.
Figure 1. SEM image of a 316 stainless steel TEM specimen disc after grinding (left) and polishing (right) with CBN.
Measurement of Cutting Rates
Aluminum, stainless steel, copper, and a Pb-Sn eutectic solder can be thinned from a thickness of around 100 µm to less than 10µm by using the following criteria: a dimpling load of 10 g, a wheel diameter of 15 mm, a wheel speed of 150 rpm, and 4 – 6 µm CBN paste. Figure 2 depicts a bar graph that illustrates the different cutting rates achieved during the dimpling process.
Figure 2. Dimpling rates for some common ductile materials using CBN paste.
As can be seen from the figure, the CBN cuts soft materials at a rate of approximately 2 – 6 µm/min, which is similar to the rate at which diamond cuts silicon under identical conditions.
The CBN paste is provided by Gatan in easy-to-handle 5g syringes:
- 0-2µm CBN: 656-04-010
- 4-6µm CBN: 656-04-011
This information has been sourced, reviewed and adapted from materials provided by Gatan, Inc.
For more information on this source, please visit Gatan, Inc.