Arrays of suspended silicon nanowires defined by ion beam implantation: mechanical coupling and combination with CMOS technology.

We present the fabrication, operation, and CMOS integration of arrays of suspended silicon nanowires (SiNWs). The functional structures are obtained by a top-down fabrication approach consisting in a resistless process based on focused ion beam irradiation, causing local gallium implantation and silicon amorphization, plus selective silicon etching by tetramethylammonium hydroxide, and a thermal annealing process in a boron rich atmosphere. The last step enables the electrical functionality of the irradiated material. Doubly clamped silicon beams are fabricated by this method. The electrical readout of their mechanical response can be addressed by a frequency down-mixing detection technique thanks to an enhanced piezoresistive transduction mechanism. Three specific aspects are discussed: (i) the engineering of mechanically coupled SiNWs, by making use of the nanometer scale overhang that it is inherently-generated with this fabrication process, (ii) the statistical distribution of patterned lateral dimensions when fabricating large arrays of identical devices, and (iii) the compatibility of the patterning methodology with CMOS circuits. Our results suggest that the application of this method to the integration of large arrays of suspended SiNWs with CMOS circuitry is interesting in view of applications such as advanced radio frequency band pass filters and ultra-high-sensitivity mass sensors.

Thermal scanning probe lithography for the directed self-assembly of block copolymers.

Thermal scanning probe lithography (t-SPL) is applied to the fabrication of chemical guiding patterns for directed self-assembly (DSA) of block copolymers (BCP). The two key steps of the overall process are the accurate patterning of a poly(phthalaldehyde) resist layer of only 3.5 nm thickness, and the subsequent oxygen-plasma functionalization of an underlying neutral poly(styrene-random-methyl methacrylate) brush layer. We demonstrate that this method allows one to obtain aligned line/space patterns of poly(styrene-block-methyl methacrylate) BCP of 18.5 and 11.7 nm half-pitch. Defect-free alignment has been demonstrated over areas of tens of square micrometres. The main advantages of t-SPL are the absence of proximity effects, which enables the realization of patterns with 10 nm resolution, and its compatibility with standard DSA methods. In the brush activation step by oxygen-plasma exposure, we observe swelling of the brush. This effect is discussed in terms of the chemical reactions occurring in the exposed areas. Our results show that t-SPL can be a suitable method for research activities in the field of DSA, in particular for low-pitch, high-χ BCP to achieve sub-10 nm line/space patterns.

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.

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.

Fast on-wafer electrical, mechanical, and electromechanical characterization of piezoresistive cantilever force sensors.

Validation of a technological process requires an intensive characterization of the performance of the resulting devices, circuits, or systems. The technology for the fabrication of micro and nanoelectromechanical systems (MEMS and NEMS) is evolving rapidly, with new kind of device concepts for applications like sensing or harvesting are being proposed and demonstrated. However, the characterization tools and methods for these new devices are still not fully developed. Here, we present an on-wafer, highly precise, and rapid characterization method to measure the mechanical, electrical, and electromechanical properties of piezoresistive cantilevers. The setup is based on a combination of probe-card and atomic force microscopy technology, it allows accessing many devices across a wafer and it can be applied to a broad range of MEMS and NEMS. Using this setup we have characterized the performance of multiple submicron thick piezoresistive cantilever force sensors. For the best design we have obtained a force sensitivity Re(F) = 158µV/nN, a noise of 5.8 µV (1 Hz-1 kHz) and a minimum detectable force of 37 pN with a relative standard deviation of σ(r) ≈ 8%. This small value of σ(r), together with a high fabrication yield >95%, validates our fabrication technology. These devices are intended to be used as bio-molecular detectors for the measurement of intermolecular forces between ligand and receptor molecule pairs.

Metal microelectromechanical oscillator exhibiting ultra-high water vapor resolution.

Water vapor sensing characterization of a metal resonator fabricated with an industrial 0.35 µm CMOS technology is reported. The resonator frequency is ∼13.2 MHz and exhibits a sensitivity magnitude of ∼3.5 kHz per %RH without requiring any additional hygroscopic coating layer. An on-chip integrated oscillator circuit enables an unprecedented resolution of 0.005 %RH.

Full-wafer fabrication by nanostencil lithography of micro/nanomechanical mass sensors monolithically integrated with CMOS.

Wafer-scale nanostencil lithography (nSL) is used to define several types of silicon mechanical resonators, whose dimensions range from 20 µm down to 200 nm, monolithically integrated with CMOS circuits. We demonstrate the simultaneous patterning by nSL of ∼2000 nanodevices per wafer by post-processing standard CMOS substrates using one single metal evaporation, pattern transfer to silicon and subsequent etch of the sacrificial layer. Resonance frequencies in the MHz range were measured in air and vacuum. As proof-of-concept towards an application as high performance sensors, CMOS integrated nano/micromechanical resonators are successfully implemented as ultra-sensitive areal mass sensors. These devices demonstrate the ability to monitor the deposition of gold layers whose average thickness is smaller than a monolayer. Their areal mass sensitivity is in the range of 10(-11) g cm(-2) Hz(-1), and their thickness resolution corresponds to approximately a thousandth of a monolayer.

Mechanical detection of carbon nanotube resonator vibrations.

Bending-mode vibrations of carbon nanotube resonators were mechanically detected in air at atmospheric pressure by means of a novel scanning force microscopy method. The fundamental and higher order bending eigenmodes were imaged at up to 3.1 GHz with subnanometer resolution in vibration amplitude. The resonance frequency and the eigenmode shape of multiwall nanotubes are consistent with the elastic beam theory for a doubly clamped beam. For single-wall nanotubes, however, resonance frequencies are significantly shifted, which is attributed to fabrication generating, for example, slack. The effect of slack is studied by pulling down the tube with the tip, which drastically reduces the resonance frequency.

A femtogram resolution mass sensor platform based on SOI electrostatically driven resonant cantilever. Part II: sensor calibration and glycerine evaporation rate measurement.

This paper presents mass measurements of glycerine beads performed by means of laterally resonant micro-cantilevers. The transducer architecture is based on a resonant cantilever electrostatically coupled by two parallel placed electrodes. Previous to glycerine measurements, a calibration of the mass sensor has been performed by measuring a standard mass based on latex spheres. From these measurements, a value of the mass responsivity is deduced. In addition, a study of the transducer phase noise has been carried out in order to determine the minimum detectable mass. Mass measurements experiments have been performed by detecting the change on the resonance frequency of the on-plane cantilever resonant mode, produced by locally deposited mass. Additionally, the mass losses detected on the calibrated transducer after glycerine drop deposition allowed determining its evaporation rate.

A femtogram resolution mass sensor platform, based on SOI electrostatically driven resonant cantilever. Part I: electromechanical model and parameter extraction.

A microcantilever based platform for mass detection in the femtogram range has been integrated in the doped top silicon layer of a SOI substrate. The on-plane fundamental resonance mode of the cantilever is excited electrostatically and detected capacitively by means of two parallel placed electrodes in a two port configuration. An electromechanical model of the cantilever-electrodes transducer and its implementation in a SPICE environment are presented. The model takes into account non-linearities from variable cantilever-electrode gap, fringing field contributions and real deflection shape of the cantilever for the calculation of the driving electrostatic force. A fitting of the model to the measured S(21) transmitted power frequency response is performed to extract the characteristic sensor parameters as Young modulus, Q factor, electrical parasitics and mass responsivity.

Thermal AFM: a thermopile case study.

In this work, an atomic force microscope (AFM) with an integrated thermal sensor has been used to obtain the local spatial distribution of temperatures in a micromachined thermopile with submicron resolution. In this communication, we will show how the dimensional, structural and functional characteristics of a thermopile suits well with the requirements for AFM thermal imaging, and how a deeper insight of the thermopile operation can be gained with the aid of these advanced scanning probe-based tools.

On the electromechanical modelling of a resonating nano-cantilever-based transducer.

An electromechanical model for a transducer based on a lateral resonating cantilever is described. The on-plane vibrations of the cantilever are excited electrostatically by applying DC and AC voltages from a driver electrode placed closely parallel to the cantilever. The model predicts the static deflection and the frequency response of the oscillation amplitude for different voltage polarization conditions. For the electrostatic force calculation the model takes into account the real deflection shape of the cantilever and the contribution to the cantilever-driver capacitance of the fringing field. Both the static and dynamic predictions have been validated experimentally by measuring the deflection of the cantilever by means of an optical microscope.

Atomic force microscope characterization of a resonating nanocantilever.

An atomic force microscope (AFM) is used as a nanometer-scale resolution tool for the characterization of the electromechanical behaviour of a resonant cantilever-based mass sensor. The cantilever is actuated electrostatically by applying DC and AC voltages from a driver electrode placed closely parallel to the cantilever. In order to minimize the interaction between AFM probe and the resonating transducer cantilever, the AFM is operated in a dynamic non-contact mode, using oscillation amplitudes corresponding to a low force regime. The dependence of the static cantilever deflection on DC voltage and of the oscillation amplitude on the frequency of the AC voltage is measured by this technique and the results are fitted by a simple non-linear electromechanical model.

AFM lithography of aluminum for fabrication of nanomechanical systems.

Nanolithography by local anodic oxidation of surfaces using atomic force microscopy (AFM) has proven to be more reproducible when using dynamic, non-contact mode. Hereby, the tip/sample interaction forces are reduced dramatically compared to contact mode, and thus tip wear is greatly reduced. Anodic oxidation of Al can be used for fabricating nanomechanical systems, by using the Al oxide as a highly selective dry etching mask. In our experiments, areas as large as 2 micro m x 3 micro m have been oxidized repeatedly without any sign of tip-wear. Furthermore, line widths down to 10nm have been routinely obtained, by optimization of AFM parameters, such as tip/sample distance, voltage and scan speed. Finally, AFM oxidation experiments have been performed on CMOS processed chips, demonstrating the first steps of fabricating fully functional nanomechanical devices.

Electrochemical platinum coatings for improving performance of implantable microelectrode arrays.

The formation and properties of electrochemical platinum films grown on platinum contacts contained in implantable flexible microelectrodes were investigated. The resulting platinum deposits were obtained by applying cyclic voltammetry to baths containing concentrations around 70 mM of chloroplatinic acid. A pre-activation step was necessary before the platinum-electroplating step in order to achieve good adhesive properties. The benefits of this process were ascribed to higher corrosion resistance, lower impedance and improved adhesion to the sputtered platinum. These improvements can make the application of this electrochemical technique highly useful for increasing the lifetime of implantable microelectrode arrays, such as cuff structures (IEEE Trans. Biomed. Eng. 40 (1993) 640). These medical devices, obtained by semiconductor technology could be used for selective stimulation of nerve fascicles, although, poor long-term performance has been achieved with them. The dissolution rate for platinum thin-film microelectrodes under fixed corrosion test conditions was 38.8 ng/C. Lower rates were observed for electroplated microelectrodes, obtaining a dissolution rate of 7.8 ng/C under analogous experimental ageing conditions. The corrosion behaviour of the electroplated platinum during stimulation experimental conditions was estimated by electrochemical impedance spectroscopy.

Comparison of highly efficient absorbing boundary conditions for the beam propagation method.

Numerical experiments using the paraxial finite-difference beam propagation method have been performed with the following boundary conditions: perfectly matched layer, Higdon absorbing boundary conditions, complementary operators method, and extended complementary operators method. We have shown that Higdon operators must be modified for the paraxial wave equation to take into account the spectrum of incident rays on the boundaries of the computational domain. Reflection coefficients, accuracy, numerical dissipation/ gain, memory requirements, and time computation are compared and discussed for these absorbing techniques.

Characterization of antiresonant reflecting optical waveguide devices by scanning near-field optical microscopy.

Silicon-based antiresonant reflecting optical waveguide (ARROW) devices were studied by means of a scanning near-field optical microscope. Various structures such as a Y junction of a Mach-Zehnder interferometer and a directional optical coupler were characterized, showing the propagation of the light inside the devices simultaneously with the topography. Scattering on the splitting point of the Y junction was shown, as well as a partial coupling of the light between the two branches of the coupler. Measurements on the decay length of the evanescent field were also performed to study the use of the ARROW waveguide for sensor purposes.