Electron-Enhanced Atomic Layer Deposition of Titanium Nitride Films Using an Ammonia Reactive Background Gas

Abstract

Electron-enhanced atomic layer deposition (EE-ALD) of titanium nitride (TiN) films was achieved using sequential exposures of tetrakis(dimethylamido)titanium (TDMAT) and low energy electrons in the presence of a continuous NH3 reactive background gas (RBG). Performing EE-ALD concurrently with a RBG is a new ALD film growth technique. The TiN EE-ALD was performed utilizing a hollow cathode plasma electron source (HC-PES). The HC-PES can deliver a high electron flux into background gases at pressures up to several mTorr. The TiN EE-ALD was conducted at temperatures of 30–70 °C using an electron acceleration voltage of 100 V and a NH3 pressure of ∼1 mTorr. The incident electron flux promotes electron stimulated desorption (ESD) and facilitates rapid nucleation and low temperature film growth. The TiN EE-ALD film growth was achieved on a variety of substrates including a native oxide on silicon, a SiO2 thermal oxide, and in situ silicon nitride films grown using electron-enhanced chemical vapor deposition (EE-CVD). Growth rates of 0.75 to 1.8 Å per cycle were measured using in situ four-wavelength ellipsometry for different TDMAT precursor exposures. Ex situ X-ray photoelectron spectroscopy (XPS) studies indicated that the TiN films were high purity and slightly nitrogen-rich. The in situ ellipsometry also measured low resistivities of ∼120 μΩ cm for the TiN films with thicknesses of ≥60 Å. These low resistivities were confirmed by ex situ four-point probe measurements and ex situ spectroscopic ellipsometry. X-ray diffraction investigations determined that the TiN EE-ALD films were crystalline. X-ray reflectivity studies also indicated that the thin TiN films had densities similar to bulk films. The high quality of the TiN EE-ALD films is attributed to the NH3 RBG. Interaction between the low energy electrons and the NH3 RBG is believed to form •NH2 and •H radical species that react with the surface during EE-ALD and improve film purity. The reactive •NH2 and •H species likely lead to nitridation and carbon removal from the films. RBGs greatly expand the possibilities for tuning film composition and properties during EE-ALD. In addition, TiN EE-ALD was accomplished on insulating substrates such as an SiO2 thermal oxide. The TiN EE-ALD is believed to be possible on insulating substrates because the secondary electron yield for electron energies of ∼100 eV is greater than unity.