Manufacturing and application of a 2 µm dark field aperture in
TEM
Authors: Selve Sören , Berger Dirk 
Published: December 2016

For an entire TEM characterization of many materials, it is necessary to achieve selected area electron diffraction (SAED) patterns of smallest regions with assigning the reflexions to their origins in the real image. In a previous work we showed that we were able to successfully reduce the field of view by a customized SAED aperture to a 15 nm range [1]. Though it gives us very local information about the samples structure, in daily work it is not always satisfying, since the real image is as important to understand the correlation between certain Bragg spots and the real structure, e.g. given by a series of dark field images. Especially for closely neighboured reflexions, commercial objective apertures are too large and do not allow the separated selection of these spots. Since our conventional TECNAI is equipped with the standard aperture‐stripe, we are limited to a smallest size of 10 µm (12 µm in reality) which delivers a field of view of ca. 7.5 mrad inside the back focal plane. The smallest commercially available aperture has a diameter of 5 µm.
Figure 1a displays a section of a polycrystalline fcc diffraction pattern. The marked large circle represents the standard 10 µm objective aperture, while the smaller one represents our custom made aperture with a diameter of 2 µm or 1.5 mrad inside the back focal plane. This example shows, with standard apertures it is impossible to select the (311) reflexions without overlap of their neighboured (220) or (222).
Therefore, we reworked the present PtIr aperture‐stripe by focused ion beam (FIB) in two steps [2]. At first an existing hole of the stripe – there are 2 rows of holes, one provides smaller and the other one larger diameters, which are seldom used – was closed by ion beam‐induced Pt‐deposition. As a second step, a centred opening was sputtered into that layer by using of circular masks up to 2‐µm in diameter. To minimize a conical shape of the opening, at low ion beam current (280 pA) with a high aspect ratio is used and the hole is successively milled from both sides. If the Pt‐deposition is too thin, there is a high risk that scattered electrons in the TEM will not be entirely blocked by the new aperture and create artefacts and distortions in the images. Therefore, it has a thickness of around 6.5 µm. First investigations with TEM proved that the deposited layer is not transparent for 200 kV electrons anymore and thermally stable as well.
Figure 1b‐d shows an application of the new objective aperture on a multi‐twinned system of polycrystalline diamonds. Although the twinned areas are in the range of 5–10 nm it becomes possible to correlate the chosen diffraction spots with their origins in the real image. The adjustment of the new objective aperture has to be done very carefully, it can easily outshine the observation screen or the CCD camera, so one can easily lose the designated position, but the selection of certain diffraction spots requires a very accurate positioning. Other than at larger apertures where slight drifts are not critical because of the visibility of the selected area and therefore easier readjustments, slightest drifts must be avoided.
In conclusion, the new 2‐µm objective aperture can be very helpful for the understanding and structural characterization of samples according their crystallinity, their growth behaviour or even defect studies.