Tilted specimen

Figure 1. A modem high-resolution TEM operating at 200 kV (JEOL JEM-2010). The spherical aberration of the objective lens is 0.5 mm and the resultant point resolution is about 0.20 nm. The double-tilt specimen holder can rotate to +20°.

Special correction programs must be used, for example, for tilted specimens, thin film coatings, small particles, and biological specimens. 

To tilt specimen to the right orientation using a double-tilt holder is always the difficult part for a TEM beginner. Different users have different ways to double-tilt specimen. Here is just one way that can be used. 

In practice it is found that operation at or near to 2 with an insulating sample always reduces charging considerably, but for an entirely stable image it may still be necessary to apply a thin conductive coating. As both 6 and 11 increase if the sample is tilted (see Section 3.2.3), the crossover energy will depend on sample tilt as well as the compmition [60, 71, 73]. A tilted specimen surface releases more... 

Tilt. The flat impactor must be delivered onto the specimen with a minimum of tilt between the impacting surfaces. Long projectiles help to minimize tilt. 

A projectile with a flat noseplate perpendicular to its motion impacts a larger flat specimen plate. However, the specimen plate is not perfectly aligned, resulting in a slight tilt at impact. 

The beam next arrives at the final lens-aperture combination. The final lens does the ultimate focusing of the beam onto the surface of the sample. The sample is attached to a specimen stage that provides x- and j>-motion, as well as tilt with respect to the beam axis and rotation about an axis normal to the specimen s surface. A final z motion allows for adjustment of the distance between the final lens and the sample s surface. This distance is called the working distance. 

Fig. 2. Evolution of an ED pattern on tilting the specimen about an axis perpendicular to the lube axis. (a.b,c) The spots A and B as well as C and D approach one another. In (d) the spots A and B coalesce. In (f) the spots C and D form a single symmetrical streak. The positions of the spots oo.l remain unchanged. On moving the spots A and B as well as C and D describe arcs of the same circles centred on the origin [9].

Channelling effects can provide two types of information in RBS experiments. If a detector is adjusted to have an energy window corresponding to a chosen atomic species, a specimen tilt-through over a channelled direction brings information on the perfection of crystallinity of the target and also on the lattice location of dopants or impurities. The yields vs. tilt-through curve has a minimum in the channelled direction, and the smaller this minimum yield, the more perfect is the crystal. 

Fig. 14. Influence of specimen orientation (catalyst B) relative to the electron beam on the appearance of the electron-micrograph. Angle of tilt (a) 0°20 (b) 4°30 (c) 7°20 (d) 16°.

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