2D materials for piezoresistive strain gauges and membrane based nanoelectromechanical systems
Wagner, Stefan; Lemme, Max Christian (Thesis advisor); Mokwa, Wilfried (Thesis advisor); Niklaus, Frank (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen University, 2018
Layered two-dimensional (2D) materials possess outstanding electronic, optical, chemical and me-chanical properties for a wide range of potential future nanoelectronic devices. Besides the carbon-based graphene, many other 2D materials are being investigated, like phosphorene, hexagonal boron nitride and transition metal dichalcogenides (TMDs). Large-area growth of these materials is now possible and transfer to arbitrary substrates could enable future volume production for commercial wafer-based applications. Graphene consists of only one atomic layer of carbon with a high tensile strength, but is at the same time elastic, impermeable to gas and shows a piezoresistive effect under strain. These properties make graphene an interesting material for nanoelectromechanical systems (NEMS). The TMD platinum diselenide (PtSe2) was only recently discovered and therefore little is known about its mechanical properties. This semi-metal can be grown at a temperature of 400 °C by thermally assisted conversion of platinum thin-film films. The low growth temperature makes it compatible with back-end-of-line (BEOL) manufacturing processes and it could be directly integrated with silicon technology, making it a very promising candidate for future nanoelectronic devices.In the presented work, the potential of graphene and PtSe2 as as a material for NEMS sensors is investigated. Graphene was chosen because it is extensively investigated. PtSe2 on the other hand is a recently discovered material with mostly unknown properties showing a high potential for NEMS devices. Strain sensors were manufactured from both materials, characterized by a bending beam setup and compared with commercial metallic strain gauges. The structure was further used to determine the gauge factor of these 2D materials. For graphene a gauge factor between 1.4 and 1.7 was determined and for PtSe2 approx. -85. Piezoresistive NEMS pressure sensors were also manufactured using graphene and PtSe2 as membrane material as well as a direct readout mechanism. Substrates with blind holes were fabricated and five different transfer methods were investigated. The yield of the resulting 2D material based suspended membranes was compared. Furthermore, Raman spectroscopy has been developed as a characterization method to study suspended graphene and PtSe2 membranes. In the course of the work, for the first time three-dimensional Raman tomography images of nanoelectronic components were recorded. Electrical measurements were carried out in-situ on these components in a home-built pressure chamber. The graphene sensors corresponds to literature values and exceed the sensitivity of many other NEMS pressure sensors when normalized by their membrane area, which is one to four orders of magnitude smaller than that of other sensors. PtSe2 membrane based pressure sensors, with an equally small membrane area, outperform comparable NEMS pressure sensors with a one to four orders of magnitude higher sensitivity. These results suggest a very great potential for future miniaturization of sensors and the development of other NEMS applications based on 2D materials.
- Chair of Electronic Devices