3.5.3. Take-off Angle

The angle between the surface of the sample and the detecting crystal is termed the "take-off" angle, f (Figure 3.5.3).

Figure 3.5.3. Schematic illustration of the take-off angle. For a given angle of electron incidence, the length of the absorption path is directly proportional to the cosecant of the take-off angle, f (after Kevex Corporation 1983).

High take-off angles decrease absorption of the X-rays and other effects produced in the sample and result in higher count rates. Low take-off angles increase absorption because the X-rays must traverse more material. Thus, a low take-off angle is more sensitive to topographic effects. For this reason, low angles are used to enhance the topographic resolution of secondary electron images. Absorption increases rapidly for take-off angles under 25°, but is relatively constant for angles greater than 35°. All Cameca microprobes (including the MBX) have a take-off angles of 40°, ARL machines have angles of 52.5°, and MAC and ETEC machines angles of 38.5°. Count rates on ARL microprobes are about 30% higher than on MAC microprobes simply due to the higher take-off angle.