Marc Sansa

Nanomechanical Mass Sensor for Spatially Resolved Ultrasensitive Monitoring of Deposition Rates in Stencil Lithography

Keywords: mass sensors ; nanoelectromechanical systems ; nanolithography ; nanomechanical sensors ; High-Frequency Applications ; Cmos-Mems ; Devices Reference LMIS1-ARTICLE-2009-006doi:10.1002/smll.200990007View record in Web of Science Record created on 2009-01-28, modified on 2017-05-10

High-sensitivity linear piezoresistive transduction for nanomechanical beam resonators

Highly sensitive conversion of motion into readable electrical signals is a crucial and challenging issue for nanomechanical resonators. Efficient transduction is particularly difficult to realize in devices of low dimensionality, such as beam resonators based on carbon nanotubes or silicon nanowires, where mechanical vibrations combine very high frequencies with miniscule amplitudes. Here we describe an enhanced piezoresistive transduction mechanism based on the asymmetry of the beam shape at rest. We show that this mechanism enables highly sensitive linear detection of the vibration of low-resistivity silicon beams without the need of exceptionally large piezoresistive coefficients. The general application of this effect is demonstrated by detecting multiple-order modes of silicon nanowire resonators made by either top-down or bottom-up fabrication methods. These results reveal a promising approach for practical applications of the simplest mechanical resonators, facilitating its manufacturability by very large-scale integration technologies.

Enabling electromechanical transduction in silicon nanowire mechanical resonators fabricated by focused ion beam implantation.

We present the fabrication of silicon nanowire (SiNW) mechanical resonators by a resistless process based on focused ion beam local gallium implantation, selective silicon etching and diffusive boron doping. Suspended, doubly clamped SiNWs fabricated by this process presents a good electrical conductivity which enables the electrical read-out of the SiNW oscillation. During the fabrication process, gallium implantation induces the amorphization of silicon that, together with the incorporation of gallium into the irradiated volume, increases the electrical resistivity to values higher than 3 Ω m, resulting in an unacceptably high resistance for electrical transduction. We show that the conductivity of the SiNWs can be restored by performing a high temperature doping process, which allows us to recover the crystalline structure of the silicon and to achieve a controlled resistivity of the structures. Raman spectroscopy and TEM microscopy are used to characterize the recovery of crystallinity, while electrical measurements show a resistivity of 10(-4) Ω m. This resistivity allows to obtain excellent electromechanical transduction, which is employed to characterize the high frequency mechanical response by electrical methods.

Dynamic stencil lithography on full wafer scale

In this paper, the authors present a breakthrough extension of the stencil lithography tool and method. In the standard stencil lithography static mode, material is deposited through apertures in a membrane (stencil) on a substrate which is clamped to the stencil. In the novel dynamic mode, the stencil is repositioned with respect to the substrate inside the vacuum chamber and its motion is synchronized with the material deposition. This can be done either in a step-and-repeat or in a continuous mode. The authors present the first results proving the accurate x-y-z in situ positioning and movement of our stages during and in between patterning.

Electrical transduction in nanomechanical resonators based on doubly clamped bottom-up silicon nanowires

The frequency response of double-clamped bottom-up grown silicon nanowires is measured electrically by means of a frequency modulation (FM) detection scheme. In comparison with other electrical methods, FM detection is simpler and it allows the use of smaller actuation signals. We have been able to resolve the first three mechanical resonance modes up to frequencies higher than 350 MHz. The FM detection scheme relies on a transduction mechanism that presents a linear dependence of the change of conductance with the nanowire deflection/actuation signal. The modeling of the system corroborates that two different transduction mechanisms (linear and quadratic) co-exist.

Horizontally patterned Si nanowire growth for nanomechanical devices

We report a method to pattern horizontal vapor-liquid-solid growth of Si nanowires at vertical sidewalls of Si microstructures. The method allows one to produce either single nanowire structures or well-ordered nanowire arrays with predefined growth positions, thus enabling a practical development of nanomechanical devices that exploit the singular properties of Si nanowires. In particular, we demonstrate the fabrication of doubly clamped nanowire resonators and resonator arrays whose mechanical resonances can be measured by optical or electrical readout. We also show that the fabrication method enables the electrical readout of the resonant mode splitting of nanowire resonators in the VHF range, which allows the application of such an effect for enhanced nanomechanical sensing with nanowire resonators.

Monolithic CMOS-MEMS oscillators with micro-degree temperature resolution in air conditions

This paper reports the thermal characterization of MEMS resonator-based oscillators fabricated using a conventional 0.35-µm CMOS process. The MEMS resonators are sub-micrometer scale metal beams which resonance frequency is highly dependent on temperature (up to −2055 ppm/°C). Thanks to the monolithic integration of the oscillator electronics, the oscillator frequency stability is better than 1 ppm allowing temperature resolutions up to 0.00019°C for 0.1 s averaging time, which is at least one order of magnitude better than the best previously reported.

Top-down silicon microcantilever with coupled bottom-up silicon nanowire for enhanced mass resolution

A stepped cantilever composed of a bottom-up silicon nanowire coupled to a top-down silicon microcantilever electrostatically actuated and with capacitive or optical readout is fabricated and analyzed, both theoretically and experimentally, for mass sensing applications. The mass sensitivity at the nanowire free end and the frequency resolution considering thermomechanical noise are computed for different nanowire dimensions. The results obtained show that the coupled structure presents a very good mass sensitivity thanks to the nanowire, where the mass depositions take place, while also presenting a very good frequency resolution due to the microcantilever, where the transduction is carried out. A two-fold improvement in mass sensitivity with respect to that of the microcantilever standalone is experimentally demonstrated, and at least an order-of-magnitude improvement is theoretically predicted, only changing the nanowire length. Very close frequency resolutions are experimentally measured and theoretically predicted for a standalone microcantilever and for a microcantilever-nanowire coupled system. Thus, an improvement in mass sensing resolution of the microcantilever-nanowire stepped cantilever is demonstrated with respect to that of the microcantilever standalone.

15.6 A 30-to-80MHz simultaneous dual-mode heterodyne oscillator targeting NEMS array gravimetric sensing applications with a 300zg mass resolution

The extreme sensitivity of nano electro mechanical system (NEMS) to atomic scale physical variations has led to the breakthrough development of NEMS-based mass spectrometry systems capable of measuring a single molecule [1]. Parallel sensing using thousands of devices will help to circumvent the small effective sensing area while opening new perspectives for applications that require spatial mapping. While the development of NEMS CMOS co-integration technology [2] is of paramount importance to achieve high density sensor arrays (>1000 devices), the readout circuitry capable of tracking NEMS resonator frequency shifts is still the limiting factor for the very large scale integration of individually addressed sensors. Moreover, in order to resolve the mass and position of an adsorbed analyte, single particle mass sensing applications require to track simultaneously and in real time at least two modes of the resonators. This requirement adds complexity to the design of the overall system. To respond to the size, power consumption and resolution constraints linked to NEMS array measurement, we propose a compact heterodyne self-oscillator analog front-end IC which performs 1ms simultaneous dual-mode frequency tracking compatible with a “pixel-based” readout scheme. We report less than 1mA power consumption with a 300zg mass resolution for 26000µm2 size.

Evaluating the compressive stress generated during fabrication of Si doubly clamped nanobeams with AFM

In this work, the authors employed Peak Force tapping and force spectroscopy to evaluate the stress generated during the fabrication of doubly clamped, suspended silicon nanobeams with rectangular section. The silicon beams, released at the last step of fabrication, present a curved shape that suggests a bistable buckling behavior, typical for structures that retain a residual compressive stress. Both residual stress and Youngs modulus were extracted from experimental data using two different methodologies: analysis of beam deflection profiles and tip-induced mechanical bending. The results from the two methods are compared, providing an insight into the possible limitations of both methods.