WS2 device fabrication using DUV lithography and dry resist

Monolayer MX₂ (transition metal dichalcogenides) are considered as alternative channel materials for advanced CMOS devices. The construction of CMOS transistors requires to develop a contact module, which aims at establishing an electrical connection between the MX₂ layer and the outer world. In our lab flow, we use the lift-off approach for contacts on WS₂, where resist is spun directly on the WS₂, patterned by e-beam, contact metal is sputtered then the resist is developed. During development, the metal coated on the resist is removed (lift-off) and only the metal in contact with WS₂ remains.

There are two main difficulties of the contact lithography module: first, the presence of residual polymers coming from the transfer step, hindering good electrical connection between the MX₂ film and the deposited metal. First, the presence of residual resist which might not be developed correctly. Second, the adverse effect of solvents used for spinning and development of the resist. Fortunately, using the e-beam approach solves these two issues since the solvents used are acetone and anisole which does not modify the WS₂. However, e-beam is a slow technique which requires significant resources. There is a significant interest to move to DUV resist based lithography, enabling faster and simpler device manufacturing. The goal of the master thesis is twofold: first, to set up protocol for DUV-based contact ‘lift-off’ lithography on WS₂, using a specially designed dry resist. Second, the processing damage to the electrical properties of the WS₂ will be studied using WS₂ devices made by the new protocol.