Patterning of Titanium Nitride Films by Dry Reactive Ion Etching in Inductively Coupled Plasma

Titanium nitride (TiN) is a key material in the semiconductor industry due to its compatibility with CMOS devices. It is characterised by many interesting properties such as high thermal stability, good electrical conductivity, including matching lattice structure and good adhesion with the high-k materials [1] (e.g. HfO₂, ZrO₂, Al₂O₃). TiN has found use in many applications such as tunable work-function metal gates in field-effect-transistors (FETs), diffusion barriers, metal hard masks, anti-reflective coatings, extreme ultraviolet patterning, sub-bandgap and Schottky photodetectors [2]. However, in order to form the FET structure of TiN gates, an etching process is required.

Various etching processes have been developed for TiN, including wet etching processes, conventional dry reactive ion etching (RIE) processes using Cl- or F-based chemistries, or plasma atomic layer etching. Wet etching processes typically exhibit high surface roughness, making it difficult to control the etch rate at the nanometre scale. Although RIE can control the etch rate with anisotropic patterns, there are many studies that contradict the etching mechanism of TiN thin films [3-5]. Another problem with TiN etching in Cl- and F-based plasmas is related to the formation of non-volatile by-products, which can degrade the properties of the final etched structures. Therefore, a thorough study is needed to optimise the etching process to achieve the required etch rate, etch profile and selectivity to the mask material.

In the present work, we have investigated the dry reactive ion etching of TiN films using an Oxford Plasmalab 100 inductively coupled plasma (ICP) system in fluorine-based chemistry. The influence of the CF₄/Ar gas mixture ratio, the effect of the ICP power, and the pressure were investigated. The etching rate and the etch profile were evaluated from surface profilometer and SEM measurements. It was found that increasing the ICP power increases the selectivity of the photoresist mask to TiN (from 0.22 to 0.63), and also improves the etch profile (~51° tilt angle for 200 W of ICP power compared to ~17° tilt angle for pure RF plasma only). The etching rate of TiN was varied from 2 to 40 nm/min. The lowest etching rate was observed for the Ar-rich RF plasma, proving the dominance of the chemical etching over the physical sputtering process. The uniformity of the etching process was also investigated. The best uniformity was achieved at 15 mTorr pressure, where the difference in etch rate measured over a 2" substrate was only ~1.2 nm/min. In addition, XPS measurements were performed on the samples in order to analyse the non-volatile by-products formed on the etched TiN structures.