Periodic Reporting for period 1 - PULSE2D (Pulsed plasma technology for 2D materials integration)
Reporting period: 2017-09-01 to 2019-08-31
While exceptional properties of TMDs have been demonstrated on mechanically exfoliated flakes (using the Nobel-winning scotch tape technique) large scale integration of these materials into sophisticated devices remains very challenging. Due to the extreme sensitivity and fragility of 2D crystals in comparison to classical bulk materials, all processing technologies used in semiconductor fabs (e.g. deposition, etching, cleaning) have to be reexamined. The goal of the PULSE2D project is to develop plasma-based technology for cleaning, functionalization and etching of TMD materials with atomic-scale precision (Figure 1).
Application of developed processes will allow integration of 2D TMDs on full wafers in semiconductor fabs leading to fabrication of new generations of high-performance devices.
WP2 focused on the application of atomic layer etching (ALE) technique for wafer-scale integration of TMDs in collaboration with Oxford Instruments Plasma Technology (OIPT). Selective etching of gate dielectrics is a critical step for contacting the TMD channel in field-effect transistors. ALE of high-k dielectric (ZrO2, HfO2, Al2O3) layers in BCl3/Cl2 chemistry has been investigated. We have shown that by using ALE gate dielectric materials can be removed selectively to SiO2 deposited at the TMD – high-k interface. This allowed for the first time formation of top contacts to 2D layers in a fab-compatible manner.
WP3 deals with the improvement of film closure during atomic layer deposition of gate dielectrics on MoS2 (WS¬2). We have shown that remote hydrogen plasma pre-treatment combined with molecular doping with Cl2 and OCS can be used to improve ALD nucleation on TMD surface. However, the improved nucleation comes at the expense of material damage (nucleation sites are associated with surface defects). Therefore an alternative ALD process with extended exposure of the TMD surface to precursor molecules has been proposed. This new approach is being investigated at the moment when this report is being written.
Overall, plasma processes developed in the framework of the project together with the fundamental understanding of plasma-surface interactions with 2D materials might be applied in the coming years for fabrication of new devices based on 2D materials for logic, memory and optoelectronic application.