Dopant Mapping of Semiconductors with Scanning Electron Microscopy

Compound semiconductor devices, as typified by laser diodes, photo diodes, and high electron mobility transistors, are used for optical communication systems, satellite communications, and cellular base stations to meet a growing demand for large-capacity and high-speed communication systems that play a key role in the infrastructure of modern society. The properties and reliability of semiconductor devices are thus becoming increasingly important. Therefore, in addition to electrical evaluation, physical analysis to observe the configuration and composition of these devices is required in order to improve device performance. Physical analysis also provides important information on the electrically active dopant distribution and concentration, which determine the properties of semiconductor devices and therefore require precise control. Therefore, a large number of dopant profiling techniques have been developed. One of the most attractive techniques that can meet industrial demands for rapid data measurement and quick sample preparation is a scanning electron microscopy (SEM). Dopant contrast observation using SEM was hardly reported until the 1990s; however, substantial progress was made with this technique after Perovic et al. demonstrated that the contrast of the observed SE intensity was not only dependent on the type of doping, but was also sensitive to the doping concentration levels(1). In 1998, a linear dependence was reported between the observed contrast and the logarithm of the dopant concentration in p -type Si(2). This was confirmed for dopant concentrations in the range from 1016 to 1020 cm−3(3). In 2006, it was demonstrated that dopant contrast could be observed from even focused ion beam (FIB)-prepared silicon p -n junctions(4). Thus, dopant contrast observation using SEM is a very important method for the process development and failure analysis of semiconductor devices because it is a quick and highly sensitive technique. However, there are problems to be overcome, such as the reported reversal of contrast with different primary beam energy from Si with a thick oxide layer(5). A decrease in contrast is also observed during SEM observation, which causes surface contamination*1(6). It was revealed that SEM was sensitive to the dopant concentration in the p -type region, but not to that in the n -type region(7). In addition, the interpretation of III-V semiconductor devices at the interface of heterostructures is more complicated than that for silicon homostructures(8), and accordingly it is difficult to observe accurate dopant distributions. Therefore, we have focused on solving such problems to achieve accurate dopant mapping. In this paper, we report the decrease in contrast due to contamination and propose methods to solve the problem.