The dry and anisotropic patterning of metals and metallic alloys is a challenge for multiple aspects of CMOS nanotechnology. The main requirements are first the use of non-halogenated chemistries, leading typically to corrosion issues which a concern to materials and device reliability, and second the need to create volatile compounds, in order to avoid the presence of non-volatile residues contaminating some part of the wafer and the chamber walls. Up till now, most attempts used low temperature CxHy-based continuous wave plasmas or noble gas ion beam technology. The pure plasma approach suffers from low etch rate, poor selectivity and undercut issues as well as poor volatility of the formed compound; the ion beam approach is not compatible with tight pitch and dimensions, causes mask sputtering at almost same rate as the target layer, and lead to severe residues deposits and damage to the remaining layers. There is therefore a strong need to explore new chemistries and methods allowing to pattern metals with directionality and leading to the formation of volatile products that can be easily removed (pumped away) from the surface.
The proposed PhD work will explore new approaches for etching metallic elements and compounds using preferentially non-halogenated chemistries. The target materials will be at start pure elements such as Rhodium and Iridium, then will be extended to alloys such as the MACs (metal aluminum carbides) and/or magnetic alloys such as CoFeB. The principle of atomic layer etching will be applied, where the process is cyclic with time-separated steps. As possible sequence is as follows: first, pre-treatment of the surface so as to enhance its reactivity with the subsequent step; in a second stage the surface is exposed to organic chemicals in the vapor phase leading to energetically favorable formation of volatile metal-organic products. The key requirement for each step is its self-limiting nature and/or the ability to control it at the sub-nanometer level (atomic layer etching).
An example of sequence is the following. It has been shown recently that the chemical state of the metal is important for its reactivity; for instance high oxidation states favor the subsequent reactions with organic compounds . Changing the oxidation state can be achieved by exposure to oxidative plasmas. Subsequently, the exposure to organic vapor (commodity molecules) can lead to the formation of volatile metalorganic species, for instance thermally activated reaction with acetylacetone (ACAC). This phenomenon can be enhanced by using ion implantation techniques, for instance by implantation of H leading to weakening of the metal-oxygen bond . If needed, the catalytic effect of S-free surface fluorination will be explored.
The PhD work will explore the different reactions paths and options for this technique. The PhD work will be separated into four different activities: 1) pre-screening of best metal-oxide/organic combination through wet dip into organic solutions; 2) once a starting database will be established, the reaction paths will be theoretically modelled by ab-initio calculation coupled to thermodynamic generator leading to the determination of ΔG(T,P), ΔS(T,P) for each reaction over technologically relevant T and P ranges; 3) transfer of acquired knowledge to a vapor etching system enabling in-situ plasma oxidation and organic vapor exposure; 4) development of sidewall passivation techniques so as to enable anisotropic pattern transfer. The work will screen the selectivity of optimal process conditions towards materials of reference used in the concerned process flow (formation of interconnects, MRAM stack).
The PhD candidate must have excellent hands-on skills, have basic knowledge in organo-metallic chemistry, physics and chemistry of plasma systems.
 Jack Kun-Chieh Chen, Nicholas D. Altieri, Taeseung Kim, Thorsten Lill and Meihua Shen and Jane P. Chang, J. Vac. Sci. Technol. A 35, 05C304 (2017)
 Hu Li, Kazuhiro Karahashi, Pascal Friederich, Karin Fink, Masanaga Fukasawa, Akiko Hirata, Kazunori Nagahata, Tetsuya Tatsumi, Wolfgang Wenzel, and Satoshi Hamaguchi, J. Vac. Sci. Technol. A 35, 05C303 (2017)
Required background: chemistry
Type of work: 5% literature and technology study, 70% experimental work, 25% modelling
Supervisor: Stefan De Gendt
Daily advisors: Jean-Francois de Marneffe, Geoffrey Pourtois
The reference code for this PhD position is STS1712-37. Mention this reference code on your application form.