Advances in optical carrier profiling through high-frequency modulated optical reflectance

Abstract

As indicated by the ITRS roadmap, obtaining accurate information on the electrically active dopant profile for sub-30-nm structures is a key issue. Presently, however, there is no conventional, probe-based (destructive) technique available satisfying the ITRS targeted depth (3%) and carrier level (5%−10%) reproducibility and accuracy. In this work, the authors explore the promising capabilities of nondestructive photomodulated optical reflectance (PMOR) techniques, based on the localized (micrometer beam size) detection of variations in the reflectivity of the sample, due to thermal and plasma (excess carrier) effects as can be generated by a modulated pump laser such as the Therma-Probe® (TP) system. Earlier and more recent work using low modulation (1 kHz) frequencies has shown that it is possible, but rather tedious, to extract the electric junction depth (at about 1018 cm−3) and carrier concentration of chemical vapor deposition grown (CVD) (boxlike) structures based on so-called power curves (where the reflected power of the probe laser is plotted versus the power of the pump laser). In this work the authors focus on high-frequency (1 MHz) PMOR, which gives two (instead of one) independent signals, i.e., the amplitude (A) and phase angle (ϕ) of the reflected probe beam. It has been proposed earlier and is confirmed in this work that a single simple measurement allows for the direct and easy extraction of the junction depth (Xj) and carrier concentration (N) of boxlike profiles. Furthermore, the shape of the so-called three-dimensional PMOR offset curves (A and ϕ versus offset), where the distance of the pump relative to the probe beam is varied over several micrometers, might help to obtain information on more complex profiles. The principles allowing for the extraction of arbitrary carrier profiles, with nanometer depth resolution and carrier concentrations between 1018 and 1021 cm−3, from offset curves will be discussed and evidence for the proposed ideas will be given for homogeneously doped material and CVD boxlike structures based on FSEM device simulations.