Nanospectroscopy beamline - sample suitability

Samples must be conductive. Insulators cannot be imaged due to surface charging under e-beam or photon beam irradiation.
Semiconductor substrates must be doped, so that their conductivity is few ohm·cm.
Oxides might not be suitable for experiments, depending on the thickness of the film and the material under study. Increasing the sample temperature might reduce charging under irradiation with photons or electrons. Please note that many oxides do reduce under irradiation with the photon beam, an aspect which needs to be considered when evaluating the experiment feasibility.
Samples must be flat, without tips. Please consider that tips may act as field emitters, which may result in a continuous HV leakage to the objective lens. In the worst cases, tips can prodice severe arcing between sample and objective lens.
In general, faceted samples might be difficult to deal with, because it is often impossible to align the e-beam normal to one the crystallographic faces. Nonetheless, it is possible to align the beam to the optical face.

The samples must fulfill some basic geometric requirements that are given by the hosting sample cartirdges. Square sample must have a minimum size of 7 mm x 7 mm; best size is 10 mm x 10 mm, maximum is 11 mm x 11 mm. Round samples must have a diameter between 6 mm and 13 mm. The sample thickness has to be smaller than 4 mm, but for efficient heating at temperatures higher than 1000°C a thickness of less than 0.5 mm is recommended.

A photo of the standard cartridge is shown in Figure 1. The sample is sandwiched between the cartridge body and a Mo cap. Caps with 4 (left) and 7 mm hole (right) diameter are available.

Figure 1a: SPELEEM sample cartridge and cap holders.

The cartridge is equipped with a filament for radiative sample heating. Furthermore, the filament can be biased to allow sample heating by electron bombardment. Using thin samples one can reach a max temperature of 1600°C in rapid flashes. A type C thermocouple (W5%Re/W26%Re) is fixed to the cartridge in close proximity to the sample, to allow measuring the sample temperature. The calibration parameters for the thermcouple can be found here. A calibration table is also available. Notice that the thermocouple is not spotwelded on the sample, but on a Mo ring in contact with the sample. For this reason the temperature readout may not be correct (plus or minus 50°C than actual sample temperature).

For special applications, we can provide a special sample holder which allows applying voltage/current to electrodes onto the sample (shown in Figure 2). For this purpose, the filament and the thermocouple leads are used. This implies that with the modified sample holder, the sample cannot be heated.

modifed sample cartridge
Figure 2: Modified sample cartridge for applying a bias voltage to the sample.

Designing patterned samples
The maximum field of view of the SPELEEM is only 50µm, and one can only move plus minus 2 mm from the manipulator central position. As it is impossible to scan the entire sample area, the patterned nanostructures must be placed in proximity to the cartridge centre. Thus, when mounting the sample on the cartridge, it is essential to be able to see where the nanostructures are, so that they can moved towards the centre. In order to do this, it is useful to put visible markers on the sample. The schemes shown below were contributed by our collaborators, and you're invited to copy and adapt the idea to your case.

sample cartridge layout
Figure 3: how to design a patterned sample.

Left hand side of Figure 3: a metal (typically Au) film is evaporated on one quadrant on the sample; holes are created in the film by lift-off technique or by etching, which leaves the sample surface free. The nanostructures must be placed inside the most central holes. The main advantages of this procedure are two: it is easy to find the pattern, because produces contrast in XPEEM and LEEM. The edge of the quadrant can be found quickly. From here, one can use the micrometers to move and find the structures (remember to bring a map of the sample!). If different nanostructures are placed in different holes, there will be no need to exchange samples.

Right hand side of Figure 3: mounting the sample so that the target is centerd to the hole in the Mo spider cap is easyly done. In the microscope, one moves the X coordinate until finding the on streight line parallel to the Y axis. Then one moves along Y, follwoing the pattern edge-line and eventually finding the nanostructures!