Photoelectron emission microscopy of ultrathin oxide covered devices

Photoelectron emission microscopy (PEEM) has been used to investigate simple device structures buried under ultrathin oxides. In particular, we have imaged Au-SiO2 and p-type Si-SiO2 structures and have demonstrated that PEEM is sensitive to these buried structures. Oxide overlayers ranging up to 15.3 nm were grown by systematically varying the exposure time of the structures to a plasma-enhanced chemical-vapor deposition process. The change in image contrast as the oxide thickness increases was used to quantify the inelastic mean-free path of low-energy photoelectrons (similar to1 eV) in amorphous silicon dioxide. For Au structures we find that the dominant mean-free path for photoelectrons in the overlying oxide is about 1.18 +/- 0.2 nm. Yet, we find a residual observable signal from the buried Au structure through roughly 13 oxide attenuation lengths. The signal attenuation from the Au can be explained by the spread of the photoelectron energies and the energy dependence of the electron-phonon interaction. Similar intensity attenuation behavior is also seen from heavily p-doped silicon (10(20) cm(-3)) regions, but the signal is only observable through roughly 3.0 mm of oxide, and the signal from the 10(18) cm(-3) regions is not detectable through the thinnest oxide layer of approximately 2.5 nm. Here, the energy spread (similar to2.0 eV) is more narrowly distributed about the phonon loss energies, leading to the observed attenuation behavior from heavily p-doped silicon. (C) 2002 American Vacuum Society.