Projet ANR-10-NANO-0021 – PIGE : La piézorésistance géante dans les structures de silicium pour les microsystèmes
Programme : Nanotechnologies et Nanosystèmes (P2N) 2010
Année de financement : 2010
Coordinateur du projet : Monsieur Alistair ROWE (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION REGIONALE ILE-DE-FRANCE SECTEUR OUEST ET NORD)
Partenaires : Centre national de la recherche scientifique; Centre national de la recherche scientifique; Schola Genevensis
Montant : 469653.0 euros
Date de début : 2011
Durée : 36 mois
Résumé : Piezoresistance (PZR), the change of electrical resistivity with an externally applied mechanical stress, is well known and was first measured in silicon almost 60 years ago. In addition to revealing details of the band structure, PZR is exploited in a number of diverse stress sensing applications. Recent literature shows an increasing interest in the use of PZR for the detection of micro- and nanosystems movements. In most cases it is desirable to have the largest sensitivity possible, either to enable the detection of nanoscale movements or to reduce device power consumption. Clearly silicon is the preferred material since integrable devices are possible in a material with highly attractive mechanical properties. However the PZR coefficients in silicon cannot exceed -104 x 10-11 Pa-1.  While sufficient for many applications, this is too small for a number of others including those addressed below. Recent reports of “giant” PZR in other materials,  or in silicon micro- and nanostructures,  including our own,  have therefore been hailed as offering a way over the -104 x 10-11 Pa-1 barrier that will enable a number of new PZR technologies.
The main objectives of the current project are threefold: i) understand the origin of a giant PZR observed in silicon nanowires,  ii) investigate a two-terminal version of a geometric effect previously reported in four-terminal silicon microstructures, and iii) demonstrate the utility of this approach for two specific sensing applications.
The origin of the PZR in suspended silicon nanowires, more than 30 times larger than in bulk silicon is not clear. Suggested explanations include an enhanced bulk effect, a quantum mechanical effect  and an electrostatic surface depletion effect first described by us. For use in eventual applications, and from a purely scientific viewpoint, it is necessary to further investigate this phenomenon. It is our intention to test the electrostatic model via lateral PZR measurements on surface depletion layers in silicon. In order to respectively rule out the first two possibilities, longitudinal and transverse stress measurements on micrometer sized devices will be performed. The electrostatic effect should also depend on the surface Fermi energy, which can be modified via surface chemical functionalization. The effect of chemical treatments on the PZR will also be investigated.
Recently the current project partners reported an enhanced PZR in artificially structured silicon resistors. This effect was demonstrated in four-terminal devices, making them difficult to integrate with standard electronic bridge circuits and reducing the sensitivity.  The general concept, stress-induced current switching away from a metallic short circuit, should however be valid for two-terminal devices at large uniaxial stresses of the order of several hundred MPa where a divergence of the resistance with applied stress is expected. In this case, large incremental PZR coefficients for small stress changes about a large offset stress should be possible. This idea, which will be tested in a mechanical press, was the basis of a recent patent submission that has attracted the interest of potential commercial partners.
Either of these concepts could prove useful for the nanoscale movement detection in microsystems. To this end, in a second phase of the project we wish to develop a process to integrate sensors based on one or both of these concepts onto membranes or cantilevers. The resulting devices will be compatible with the applications envisaged in the final phase of the project. Two specific applications will be targeted, i) all-electrical atomic force microscopy (AFM) and ii) ultra-sensitive silicon membrane pressure sensors. Evaluation of prototype performance will be via comparison with the current state-of-the-art devices and techniques, in particular optical detection of cantilever motion in AFM and pressure sensitivity of commercial silicon pressure sensors.