Thermal gas-phase etching of titanium nitride (TiN) by thionyl chloride (SOCl₂)

In this work, thermal based gas-phase etching of titanium nitride (TiN) is demonstrated using thionyl chloride (SOCl₂) as a novel etchant. A single etchant is utilised in a pulsed fashion to etch TiN. This type of etching technique may also be considered as a chemical gas-phase or dry etching. The removed TiN amount was measured by various techniques like spectroscopic ellipsometry (SE), weighing balance and in some cases X-ray reflectometry (XRR). Additionally, the post-etch surfaces were analysed with X-ray photoelectron spectroscopy (XPS) and bright field transmission electron microscopy (BF-TEM). The surface roughness and morphology of before and after etching TiN films were measured using atomic force microscopy (AFM). The etch per cycle (EPC) was calculated and is plotted as a function of SOCl₂ pulse time, purge time after SOCl₂ exposure, number of etch cycles and etch temperature (T_etch). An increase in EPC with an increase in SOCl₂ pulse time as well as etch temperature was observed. SOCl₂ is able to etch TiN starting from 270 °C with an EPC of about 0.03 Å to almost 1.2 Å at 370 °C. Arrhenius plot determined the activation energy (E_a) of about 25 kcal/mol for TiN etching by SOCl₂. In addition, the etch selectivity between different substrates such as silicon dioxide (SiO₂), silicon nitride (Si₃N₄) and aluminum oxide (Al₂O₃) was investigated on blanket as well as 3D structures. Moreover, thermodynamic calculations were performed for various possible etch reactions. Titanium from TiN is proposed to be etched in the form of either titanium trichloride (TiCl₃) or titanium tetrachloride (TiCl₄). Nitrogen from TiN films may form volatile by-products such as diatomic nitrogen (N₂), nitrous oxide (N₂O) and nitrogen dioxide (NO₂).