PhD in surface physics (M/F) – Study and development, using a local approach, of atomically controlled cold plasma etching processes for titanium nitride (TiN)

Titanium nitride (TiN) is a compound used in many fields: biomedical, batteries and microelectronics. Because of its superconductivity, TiN is also being considered as an active material in quantum devices (microwave resonators, qubits). In addition, the development of 3D CMOS devices imposes to be able to etch TiN deposited on complex substrate geometries. These new applications and changes in integration paradigm require etching processes that enable to: (i) precisely control of the etched TiN thickness down to critical dimensions in the range few tens of nanometres, (ii) maintain the initial stoichiometry, (iii) avoid the introduction of morphological defects (roughness, loss of crystallinity), (iv) anisotropically or isotropically etch depending on the requirements, (v) while limiting temperature variations during the process.

Plasma enhanced Atomic Layer Etching (ALE) using, as a cyclic etching process controlled at the atomic layer scale, provides a technological solution to the specifications defined for the shaping of TiN.

We propose to elucidate the various plasma/surface interaction mechanisms within the TiN plasma ALE and to develop processes at low temperatures (≤100°C) kept constant. The study will rely on a local approach to understanding the mechanisms at work in the modification and activation steps (elementary stages of an ALE cycle), based on qualitative and quantitative analyses of plasmas and TiN surface using in situ diagnostics and ex situ characterisations.

The project will explore three plasma modification strategies for the TiN surface: direct fluorination, oxidation conversion followed by fluorination and hydrogen-inhibited chlorination; and the development of a hybrid activation step combining substrate heating (≤100°C) and controlled medium-energy ion bombardment.

The main difficulties encountered in etching are being able to: precisely control the etched depth at scales of less than a few nanometres, etch laterally or etch without the TiN undergoing degradation of its properties (electrical or chemical).

In this exploratory thesis project, we propose to elucidate the various plasma/surface interaction mechanisms within the plasma ALE of TiN and to develop processes at low temperature (T°≤ 100°C) kept constant. This topic will provide scientific and technological resources for a material (TiN) with high economic potential (CMOS, quantum) and an innovative plasma process (ALE). The subject is also of undoubted interest for national sovereignty, as it will help to consolidate French expertise in these two areas.