Handbook of Modern Coating Technologies

Positron microscopy and microprobing

The experimental methods considered in the preceding sections have recently been supple-mented by new positron spectroscopic technologies, even though some of them are still in
the development phase or have restricted applications to defect studies. In what follows, we briefly consider them to give an idea of their potential value.
The application of the slow positron beam technique (Section 5) is restricted only by its spatial resolution. The development of positron beams with enhanced brightness and focus¬ing precision would permit studying materials with the help of a positron microscope or microprobe (see Section 5.7).
Rich and van House in 1988 created the first transmission positron microscope (SST) [133,137]. When transmitting positrons through a polymer foil, they reached an increase of 55. Compared with a transmission electron microscope (TEM), TPM can have many new applications. Comparison of contrast formation in TPM and TEM can provide valuable infor-mation about the given elements in the effective reflection region. A much more effective screening of nuclei creates a reduced small-angle scattering of positrons. Between electron and positron microscopy there is a strongly Z-dependent (Z—atomic number) difference in amplitude contrast. Comparison of contrast mechanisms for both methods can provide important chemical information on the properties of materials.
Based on the effects of the negative work function of the positron on certain surfaces, a positron reemitting microscope (PRM) has been created (see Schultz and Lynn, 1988 [156]). A positron beam with an energy of several kiloelectron-volts is directed to the surface of the sample. Many of the positrons reach the surface, some of them are thermalized and diffuse. To form an image using a position-sensitive detector, the lateral distribution of reemitted positrons is used. A multichannel plate [153] is used as a detector. Due to the fact that near¬surface defects impede positron emission, a dark contrast appears in the image. Thus for detecting active defects by positrons, PRM is an extremely sensitive tool both in the near¬surface region and on the surface. Van House and Rich created such a microscope in the geometry of reflection and achieved a spatial resolution of 2.3 pm (see references cited in the book [133]).
The main disadvantages of positron microscopy are as follows: the absence of a positron beam with sufficient brightness; since point sources of positrons are not available, spatial resolution is limited. Therefore technically positron microscopy is poorly developed.
The combination of positron microprobe and SEM was created at the University of Bonn, Germany. SEM images were used to select regions on the sample surface for subsequent positron measurements with higher lateral resolution. For this purpose, a monoenergetic positron source was integrated into the optical system of a commercially available SEM (see references cited in the monograph [133]) and was replaced by a magnetic prism to filter the energy of positrons was installed instead of the electron gun. Through a prism with the same magnetic field, positron and electron beams were directed to the optical axis of the micro¬scope (Fig. 5—14C).