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Positron Probes

Some materials diagnostic applications of positron beams include:

Positron reemission spectroscopy (PRS)-This technique is based on the phenomenon that positrons implanted near the surface of a solid can thermalize and be reemitted because many solids possess a negative work function for positrons. The energy of the reemitted positrons can be analyzed to yield the types of contrast that are not available with conventional scanning electron microscopy. The technique has the ability to distinguish non-uniform film thickness, varying crystal orientations, differences in bulk defect density, concentrations of adsorbed molecules, and contaminant layers.

Positron annihilation induced Auger electron spectroscopy (PAES)-This technique is analogous to electron induced Auger electron spectroscopy (AES), except that the core hole, which leads to the ejection of the Auger electron, is created by positron annihilation rather than electron impact. For this technique, positrons are injected at low energy into the surface to be analyzed. The ejected electrons are analyzed in the usual way using an electron energy spectrometer, but the measurement is substantially simplified because of the absence of background high-energy secondary electrons.

Re-emitted Positron Energy Loss Spectroscopy (REPELS)-In this process, low energy monoenergetic positrons bombard the surface to be studied, and those that are reflected inelastically are energy analyzed. Energy is lost by transfer to vibrational modes and electronic state transitions of the surface and surface-absorbed molecules.

Low-Energy Positron Diffraction (LEPD)-For this technique, a crystalline sample is bombarded with low-energy (0-300 eV) monoenergetic positrons. Because of the low energy, there is relatively little penetration into the sample, and the diffracted positrons backscatter, producing spots on a fluorescent screen. The positions of the spots are a measure of the sample's diffraction sites. This information can be used to determine the crystal structure of a clean substrate or to analyze an adsorbed layer.

Positron Induced Ion Desorption Spectroscopy (PIIDS)-This relatively new technique uses time-of-flight mass spectrometry to measure the mass spectrum of ions desorbed from surfaces by the injection of positron pulses. The ion desorption rate due to positron injection is much larger than that for photodesorption.

Positron Annihilation Lifetime Spectroscopy (PALS)-Positrons injected into surfaces can be trapped and subsequently annihilate in vacancy-type defects. For high-energy positrons obtained directly from ²²Na, the lifetime, can be measured by recording the time delay between the prompt 1.2 MeV gamma ray that is emitted by the nucleus simultaneously with the positron, and the 511 keV annihilation gamma rays. This technique has been extensively applied to the study of bulk properties of solids. One of the most important current applications of lifetime spectroscopy is the analysis of microvoids in semiconductors and polymers. This technique is the most sensitive one available for studying voids in solids, and can provide information about both the size and concentrations of voids. The technique has been applied to characterizing the properties of semiconductors, such as ion-implanted silicon to study, for example, stress voiding and electromigration. One of the most important current areas of research is the study of the properties of polymers. Positron life-time spectroscopy is capable of measuring the free volume fraction and microscopic size distribution of voids, which determine such properties as impact strength, gas permeability and aging characteristics. Another important topic is the development of low-k dielectrics in microelectronic fabrication. Such dielectrics are essential for increasing CPU speeds, and can be characterized using lifetime spectroscopy in a way that is not possible using any other available technique.

Variable Energy Positron Lifetime Spectroscopy (VEPLS)-The power of the PALS technique can be substantially enhanced by implementing it using a monoenergetic beam source rather than a radioactive source. By varying the beam energy, positrons can be implanted to varying depths so that a depth profile of void size and concentration can be obtained. Furthermore, if the beam diameter is small, it can be scanned across the surface, so that three-dimensional information can be obtained. The technique requires pulse widths that are short compared to typical annihilation times in materials (~100 ps).

Positron Annihilation Spectroscopy (PAS)-This technique measures the Doppler-broadening of the 511 keV gamma-ray line resulting from the annihilation of positrons implanted into solids. The required information is contained in the gamma-ray lineshape. PAS can provide the same type of information about defects as PALS and VEPLS.

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