First, the mechanical and electrical behavior of electrostatically actuated nano/microresonators (cantilevers, bridges and quad-beams) embedded in a capacitive detection scheme have been analyzed. In such a scheme, the main issue comes from parasitic stray capacitances that can drastically degrade the performance of the transduction. Additionally, output parasitic capacitances arising from the measurement instrumentation can further reduce the available signal levels. In this sense, the advantages and the feasibility of a monolithic integration with CMOS circuitry have been studied. Indeed NEMS/CMOS are very promising systems which combine outstanding sensing attributes, thanks to the mobile mechanical part, with the possibility to electrically detect the output signal in enhanced conditions. Regarding the electrical response, such integration provides two major advantages: (i) reducing all the parasitic loads at the resonator output, and (ii) amplifying and conditioning ‘on-chip’ the resonance signal. Hence, a specific low-power CMOS readout circuit, whose function is to read out the capacitive current generated by a resonating nano/micromechanical device, has been designed. It is basically a transimpedance amplifier whose architecture is based on a second generation current conveyor. Its topology and the corresponding layout have been described and the circuit behavior (intrinsic and coupled to the NEMS) has been fully simulated. According to simulation results, the detection of the resonance of nano/microresonators is greatly enhanced through the CMOS integration.
Then, NEMS/CMOS devices have been fabricated combining a standard CMOS technology (CNM one) with emerging nanopatterning techniques, in particular with nanostencil lithography (nSL), of which the resolution and the conditions of applications have been optimized. Our works demonstrate the potential of nSL as a parallel, straightforward and CMOS compatible patterning technique to define at full wafer scale nanodevices on CMOS. These results represent the first time that an emerging nanolithography technique has been used to pattern multiple N-MEMS devices on a whole CMOS wafer in a parallel, potentially low-cost approach. The same strategy could be extended to other examples of nanodevices, such as single electron transistors on CMOS, for which there is at present no affordable technological process that fulfill the requirements of high resolution processing at wafer scale and CMOS compatibility.
After their fabrication, fully integrated nanomechanical resonators (cantilevers and quad-beams) have been extensively characterized electrically. Their mechanical resonance has been successfully sensed by the CMOS circuitry. Cantilevers and quad-beams have exhibited quality factors in vacuum up to 9500 and 7000 respectively. The resonance frequency could be tuned by varying the driving voltage and interesting hysteretic non-linear behaviors have been observed either in air or in vacuum
Finally, these resonators have been implemented as ultra-sensitive mass sensors in four different applications: in this way the extreme versatility and the high performance of such sensors has been demonstrated. Indeed, such ultra-sensitive nanosensors open up new possibilities of exploring new physical or chemical phenomena previously unattainable with any other tools. In the first experiment, wetting mechanisms of sessile droplets have been explored at very small scales (volumes in the femtoliter range) implementing the resonators as nano/microbalances. Such phenomena could not have been analyzed with traditional quartz microbalances whose mass resolution is more limited. In the second experiment, a new architecture of resonator based on a double nano/microcantilever has been designed and tested: this new device allows making reliable measurements under ambient conditions by providing a direct estimation of the measurement uncertainty.
The fact that NEMS-based mass sensors provide an unprecedented mass sensitivity and a very high spatial resolution inherent to their small size makes of them interesting devices for industrial applications as well. With regard to this matter, another experiment has consisted in monitoring in-situ the deposition of ultra-thin gold layers both with NEMS/CMOS and quartz-crystal microbalances. When measuring in real time the mass of these uniform deposits of thicknesses inferior to sub-monolayer, silicon nano/microresonators have exhibited a mass sensitivity better than QCM by between two and three orders of magnitude. This is very promising with regard to the possibility of replacing QCM in the semiconductor industry as a tool to monitor the deposition of thin layers. These outstanding mass sensing attributes have led us to apply such sensors as positioning sensors according to an innovative concept. In fact, CNM and EPFL are presently developing a ‘quasi-dynamic’ stencil lithography system. This system consists in performing successive depositions of several materials through a nanostencil shadow mask which is displaced in-between each deposition: in this way high-purity and structured multi-deposits can be obtained. In this context, NEMS/CMOS mass sensors are used as positioning sensors for the in-situ alignment between the movable nanostencil and the substrate to be patterned.