Polycrystalline Diamond Micro/Nano-Electro-Mechanical Systems

Micro/nano-electro-mechanical systems (MEMS/NEMS) incorporating miniature scale mechanical elements into electronic circuits are of interest in applications ranging from atomic resolution mass spectrometers¹ to heated stages for <i>in-situ </i>material characterisation². For devices utilising resonating structures, the frequency of operation is proportional to the acoustic velocity of the material from which they are fabricated, making diamond with its unrivalled value of 18000 m/s ideally suited for use in high frequency NEMS with minimal dissipation from scaling induced losses. While offering the benefits of diamond but in thin-film form, the use of polycrystalline diamond (PCD) for smaller scale NEMS however is often prohibited by its considerable columnar growth induced roughness, complicating fabrication and resulting in significant surface associated dissipation³. In addition, the concentration of strain at the attachment points of typically used geometries is expected to radiate energy away from the device and lead to substantial loss that scales with frequency⁴. Through combining the use of chemical mechanical polished (CMP) stock with advanced geometries it is therefore expected that dissipation can be minimised in thin-film diamond based NEMS⁵. The addition of high mechanical strength, high thermal conductivity, and tailorable conductivity meanwhile also makes diamond a suitable candidate for the fabrication of ultra-high temperature and single material micro-hotplates, with electromigration or structural failure limiting temperatures to 1500 °C with routinely used materials². The aim of this work is to therefore investigate the utilisation and optimisation of PCD in the aforementioned devices, enabling the next generation of diamond NEMS/MEMS.<br/>To this end, metallised doubly clamped and ‘free-free’ geometry resonators incorporating flexural supports were fabricated from intrinsic PCD films with and without CMP. The devices were then placed in a Quantum Design Physical Property Measurement System at cryogenic temperatures, and actuated magnetomotively through sweeping the frequency of an applied AC signal while in the presence of a static magnetic field^6,7. Upon comparing the resulting resonances, reductions in dissipation were observed upon both switching from rough to polished films and the addition of 2^nd mode flexural supports. In addition, suspended type PCD micro-hotplates with dimensions of the order of 100 μm were fabricated on SOI substrates and tested at pressures below 10^-4 mbar. The spectral emission and composition of the resulting devices were studied with spectrometry and <i>in-situ</i> Raman spectroscopy, with temperatures of up to 1900 °C reached through self-heating at power consumptions of less than 200 mW.<br/>¹ K. Jensen <i>et al</i>., Nat Nano <b>3</b>, 533–537 (2008).<br/>² R. G. Spruit <i>et al</i>., J Microelectromech Syst. <b>26</b>, 1165–1182 (2017).<br/>³ X. M. H. Huang <i>et al</i>., New J. Phys. <b>7</b>, 247 (2005).<br/>⁴M. Imboden <i>et al</i>., Appl. Phys. Lett. <b>90</b>, 173502 (2007).<br/>⁵E. L. H. Thomas <i>et al</i>., Carbon <b>68</b>, 473–479 (2014).<br/>⁶ K. L. Ekinci <i>et al</i>., Appl. Phys. Lett. <b>81</b>, 2253–2255 (2002).<br/>⁷T. Bautze <i>et al.</i>, Carbon <b>72</b>, 100–105 (2014).