In order to achieve electron transparency (less than 100 nm in thickness), which is needed in order to do TEM, different sample preparation techniques are used. The chosen technique depend on the type of sample that will be investigated, and what information you are interested in acquiring.
Powder samples: These samples are the easiest ones to prepare. The powder have to be less than 100 nm in thickness. If the powder is larger, it will have to be mortared. This can is usually carried out in ethanol, but can also be done in liquid nitrogen (N2), in order to achieve brittle fracture if the sample is ductile. The sample is then deposited onto a holey carbon film, supported by a copper (Cu, can also be Mo, Ni etc..) grid, and wait till the ethanol evaporates.
Cross section samples: Ex. investigating the cross-section of thin films samples. First a sandwich of four sample pieces (with the film surface against each other) are glued together. Then the sandwich is ground down from each side (perpendicular to the substrate surface) using silicon carbide, till the sample is about 20 – 40 micrometer. The thin sample is the glues onto a Cu slot (can also use Ni, Ti, Mo etc..).
If wanting to reduce the presence of Cu, the sandwich can be embedded into a Cu tube, by cutting a cylindrical piece of the sandwich out and glue it inside a Cu tube. Small round pieces can then be cut out and ground down to 20 – 40 micrometer.
The thin samples can then be made electron transparent by ion-milling with argon (Ar). For this we have three instruments available at the research park, Gatan PIPS, Gatan PIPS II, and a Fischione ion mill. This can also be done with cooling done to liquid N2 temperature in order to reduce the damage by the ion milling.
In order to do less ion milling, dimpling can be carried out on the thin sample. For that, we have a Gatan dimpler. A small round hole is ground out in the middle of the sample, reducing the sample thickness in this area.
Wedge samples: Sometimes you want to avoid ion-milling (or at least reduce it), or look at
a thin film from the top (plane-view sample). Then you can make a wedge sample. You make either a sandwich (for a cross-section sample) or take a thin film sample and glue it on a pod with the film down (so you grind from the back). Then you can use f.ex. a Multiprep machine. By first grinding down, and then introduce an angle, you will create a wedge with a certain angle. With this possibility, we can grind / polish down to about 100 nm thickness, thereby avoiding ion-milling the sample.