Fabrication of nanomechanical sensors monolithically integrated on CMOS by full wafer nanostencil lithography

Abstract:
Mechanical resonators with deep submicron dimensions are a specific type of nanoelectromechanical systems (NEMS) and have a strong interest in two areas: as high sensitivity sensors and as building blocks for high frequency telecommunication systems. Practical applications of mechanical resonators require on-chip signal processing, where optimal performance is achieved in case of monolithic integration with CMOS. Such NEMS/CMOS are very promising systems which combine outstanding sensing attributes, provided by the mobile mechanical part, with the possibility to electrically detect an enhanced output signal. Indeed, regarding the electrical response, a monolithic integration effectively provides two major advantages: (i) reduction of parasitic loads at resonator output, and (ii) ‘onchip’ amplification and conditioning of the resonance signal. In terms of technology, combining fabrication processes of silicon nanomechanical resonators with a standard CMOS technology is challenging, the limiting factor being mainly lithography. In this paper, we present a novel wafer-scale technological process based on post-processing CMOS circuits using nanostencil lithography (nSL). As it will be shown, we have succeeded in the parallel definition and fabrication of thousands of silicon nanomechanical resonators at the 200 nm scale which are monolithically integrated with CMOS circuits. Beyond process fabrication aspects, this paper presents results of electrical characterization demonstrating the functionality of NEMS/CMOS resonators whose resonance spectrum is electrically measured through a dedicated CMOS circuitry. Finally, we propose a specific application of NEMS/CMOS that illustrates their good performance as mass sensors: nanocantilevers placed in a high-vacuum evaporation chamber are used to monitor in-situ the deposition of sub-nm thick gold layers.