Since an investigation via transmission electron microscope (TEM) requires the electron beam to pass through the sample, the sample needs to be sufficiently thin to permit the transmission of electrons. The necessary thickness for this is in the range of 100 nm and below. While thin films prepared on membranes can fulfil this requirement from the start, most samples need to be carefully prepared for TEM investigation.

Our preparation lab provides different methods of preparing a sample for TEM investigation: either by classical preparation or by focused ion beam (FIB). Further, the lamella created by these processes can either show a cross-section or a plane view of the sample, depending on the desired investigation. Lamellae with different angles to the sample surface can also be prepared, but this requires the crystallographic orientation of the sample to be known for classical preparation. In the FIB setup, an EBSD detector allows to align the sample in the desired direction prior to cutting, so no prior information is needed.

Classical preparation:

Firstly, the sample is sawed into small pieces with a wire saw. For cross-section preparation, two pieces are then glued together film-to-film. The glued piece is then sawed and polished further until it is in the shape of a disc of around 150 µm thickness, with the cross-section of the two films in the middle. Next, the disk is thinned with a dimpler until the middle is only around 20 µm thick. The last step of thinning is done via a precision ion polishing system (PIPS), which further removes material from the middle until a hole starts to form. The hole itself is used for alignment and focusing in the TEM, while the remaining sample contains four areas around the perimeter of the hole where the film has a suitable thickness for TEM investigation. For a plane view preparation, the sample is directly dimpled and polished, so that only the substrate is removed. 

Fig. 1: Classical preparation of a cross-section sample for TEM investigation.

Focused Ion Beam (FIB):

Using a focused beam of Ga-ions, a small area is cut out of the sample surface and welded to a small manipulator. After removal of the sample piece it can be further cut into shape by the ion beam, thus creating a cross-section or plane view lamella. The lamella is then attached to the finger of a lamella holder and thinned further until the desired thickness is reached. Because large amounts of charged particles impinge on the sample surface, the sample should be electrically conductive.

Fig. 2. FIB-preparation of a plane-view sample.

Cross-section:

A cross-section through the different layers of the sample. This geometry is useful for investigating layer thickness and composition, epitaxial relation, and crystallinity as well as surface or interface profile, especially of multilayer samples.

Plane view:

A flat cutout of the sample film plane. Because long sides of the lamella are within the film plane, this geometry can be used for investigating the lateral distribution of composition or crystallinity as well as distance and alignment of micro- or nanoparticles in the film plane, but without depth resolution.