Development of time-of-flight neutron depth profiling at Penn State University

Neutron depth profiling (NDP) is a nondestructive, near-surface technique, which utilizes a thermal/cold neutron beam to determine the concentration of specific light elements versus the depth in materials. The depth distribution is obtained by measuring the energy loss spectrum of protons, alphas, or recoil atoms in the substrate materials. For conventional NDP, the depth resolution is highly dependent on the limited ability of the detectors and associated electronics. A novel technique, the time-of-flight (TOF) method that is based on a completely different energy-measurement mechanism, can greatly improve the depth resolution for the accurate measurement of the dopant depth profile in especially shallow junction devices. Such a set of TOF NDP facility is being constructed at the 1 MW Breazeale Nuclear Reactor at Penn State University, Radiation Science and Engineering Center. In the TOF-NDP, a timing start signal is obtained from electrons emitted simultaneously with a neutron-induced recoil particle leaving the surface of the sample. The same particle generates the subsequent stop signal, whereby the residual energy of the particle is much more precisely determined from the particle flight time than currently obtained by the use of surface barrier detectors. In this paper, the Penn State conventional NDP measurement results will be presented and TOF-NDP facility will be described.