Figure 2.17: Principle steps of TEM preparation technique used in the conventional cross-section preparation method
Ar+or Xe+ ions beams. The parameters used for etching of different material vary. For II-VI materials is used a 3.5 KeV voltage and the direction of the beams are at±4with respect to the upper surface plane of the specimen. The etching time is around one and a half hours until the transparency is reached.
2.5.1.2 Tripod-method
Using the conventional preparation method it is difficult to control the depth of the dimple which leads to a higher ion milling time. A long ion milling time gives rise to amorphitisation, which is undesirable, and one of the biggest problem for HRTEM imaging of crystalline materials. Therefore, an improvement of the sample preparation is necessary. By using the tripod method, a large transparent area can be obtained in both cross-section and plan-view geometries and the ion milling time is reduced con-siderably, to minutes. This method can also be used as preliminary thinning step when FIB will be employed.
In this case, the sample of interest is embedded by using the epoxy glue in two silicon pieces and a thin piece of glass, as a mark for the film position. The formed sandwich is cut into slices (2 mm wide and 300-500µmthick) with the diamond wire saw. Then, the thinning of the slide is performed by mounting the specimen on a planar glass holder, attached to the tripod polisher.
The tripod polisher is equipped with micrometers screws that allow a tilting of the specimen at the certain angle with respect to the polishing plane of the planar glass plate of the Struers machine. Thinning is then performed gradually, using the diamond pads with grains from 30 µmto 0.5 µm , at different rotation velocities from 150 rpm to 30 rpm. The thinning is carried out planar on both sides of the specimen, until a thickness of 30 µm is reached. Afterwards, in order to obtain a large transparent area a wedge angle is grinded until a thickness of 10 to 15µmis reached in the area of interest.
This can be deduced from the degree of transparency of the silicon seen under an op-tical microscope, which changes considerably when a thickness below 50µmis reached.
The next step is to mount the specimen on a copper grid with a slot (2mm x 1mm). This
2.5 Sample preparation for TEM
Figure 2.18: Principle steps of TEM preparation technique used in the tripod method
can be done by using the epoxy glue on a hot plate for about 20 minutes. In the final stage, the sample is ion etched for less then 45 minutes, until a small hole develops at the interface. The ion milling time is in case of preparing GaAs/ZnSe structures, almost half of the time that is used for a sample prepared with the conventional method. In conclusion, the tripod method has the advantage of obtaining good quality TEM speci-mens avoiding dimpling, with larger electron transparent areas and reduced ion etching time with minimized amorphization.
2.5.1.3 FIB prepared TEM specimen
The newest method of TEM specimen preparation employs the Focus Ion Beam etch-ing in a so-called H configuration. The advantage of this method, in comparison with the other methods is its capability of producing a site specific area of interest (10 µm) in a relatively short time. Moreover, one can obtain specimens with higher mechani-cal stability and allow a possibility of post-preparation. Nevertheless, the method has also some disadvantages. The etching using Ga ions is performed at a relatively higher energy (30 kV) which can damage the sample and point defects can be produced. The redeposition of sputtered material is also possible. Moreover, the thickness of the amor-phous layer deposited during the FIB processing could be up to 2 µm. Therefore, for obtaining high quality specimens, a combination of different preparation methods is favourable.
2.5.2 Plan-view geometry
For plan-view TEM specimens preparation, typically 3×3mm2pieces are cleaved or cut with a diamond wire from the wafer and grinded until a thin specimen with a thickness of about 80µmis achieved. Afterwards, the sample is dimpled down to 60µm, so that the thickness in the center is about 20µm. The specimen is then fixed on a copper grid, with the diameter of the slot of about 1.5 mm by using epoxy glue.
In the last step of preparation, plan-view specimens require also ion etching which can produce thick amorphous layers, and a small and non-uniform transparent area.
Alternatively, a better quality of the plan-view specimens can be achieved using two other procedures: chemical etching or FIB preparation.
Figure 2.19: Principal steps of the plan view TEM specimen preparation methods [56]
For chemical etching, the ensemble is mounted on a glass holder in a way that the epitaxial film is protected. Afterwards, it is dipped into a chemical solution ofNaOH : H2O2 = 5 : 1) which has a higher etching rate for III-V than for II-VI materials. Then, the sample is removed from the chemical solution when a small hole is visible in the centre of the specimen. The last step is cleaning the sample with acetone and ethanol to remove the remaining solution and the wax. The plan-view TEM specimen preparation procedure is shown in Fig. 2.19. Using FIB, plan-view TEM preparation is similar to the cross-section one, showing the same advantages and disadvantages.
In conclusion, a high quality TEM specimen can be obtained using the synergy of complimentary techniques. The goal is the preparation of artifact-free specimens.