Development of nanocrystalline graphite for MEMS and membranes

Thin carbon films such as graphite and graphene are promising materials for micro- and nanoelectromechanical systems (MEMS and NEMS) applications, including resonators and switches, and also to be used as membranes for molecular sieving. The potential of these materials to be used in such applications is due to their electrical and mechanical properties, and also because they may be used as ultra-thin films. However, the most widely-used methods to synthesise these films have a tendency to cause poor performance in MEMS and NEMS. The most common technique, chemical vapour deposition (CVD) onto a catalyst, is followed by a physical transfer of the film to a separate substrate. This causes wrinkling and a variation in strain across the film, which leads to variable performance of MEMS and NEMS. Therefore, plasma-enhanced CVD (PECVD), which can be used to deposit nanocrystalline graphitic films directly onto large-area substrates, such as 6-inch silicon wafers, is a promising route to overcome this issue. Doubly-clamped beams and square micromechanical membranes were fabricated using nanographite films of 300 to 400 nm thickness, on silicon substrates. The compressive built-in stress of the film, measured as 436 MPa, caused these microstructures to buckle out-of-plane when they were released. The buckling behaviour of both structures was used as a characterisation tool to measure the Young’s modulus of nanographite, which is a key mechanical parameter of materials, and was measured as 23 GPa. To demonstrate the use of nanographite in a MEMS application, cantilever and doubly-clamped beam resonators of thickness around 300 nm were fabricated. Despite the built-in compressive stress of the film, the doubly-clamped beams were fabricated without buckling, and were under an effective tensile stress. This was achieved through the fabrication procedure, by using a 25 to 30 μm isotropic etch undercut of the 200 μm-wide anchors. The stress gradient in the film caused deflection of the anchors which ‘pulled’ the beams tight through the application of tensile stress. The devices were actuated electrostatically and the vibration response was measured using laser Doppler vibrometry. Cantilevers were measured with fundamental natural frequencies between 5 and 25 kHz, and for doubly-clamped beams natural frequencies were measured between 245 and 640 kHz. Quality factors under ambient pressure were around 5 to 10 for cantilevers and 20 to 30 for doubly-clamped beams, and around 1800 under 30 mTorr vacuum for a doubly-clamped beam. An ultrahigh vacuum system with mass spectrometer was constructed to measure the permeance of gases through nanographite thin films. The permeance of He, H2, Ne, CO2 and O2 through 350 nm thick nanographite membranes was measured. The permeance of He, H2 and Ne at 150 °C was 5.1, 4.0 and 0.08 × 10-10 mol / (m2 ·s · Pa), respectively, but the permeance of CO2 and O2 was below the limit of detection of the mass spectrometer. An estimation of the maximum permeance of CO2 and O2 respectively were 1.12 × 10-12 and 4.76 ×10-15 mol / (m2 ·s · Pa). The low permeance of Ne, CO2 and O2 relative to H2 and He showed that nanographite is a promising membrane material for molecular sieving.