Active, artificial hair cells for biomimetic sound detection based on active cantilever technology.

We aim at building and studying artificial hair cells (AHC) based on MEMS technology to understand the extraordinary sound perception of the human ear and build a sensor system with similar properties. These perception properties, i.e. detecting six orders of sound pressure level and simultaneously frequency differences of only 3-5 Hz, are obtained mainly due to the sophisticated biological sensors in the inner ear, called hair cells, which convert the acoustic waves into electric signals. They amplify weak inputs and compress larger ones, known as compressive nonlinearity, thus enabling this impressive dynamic range, typically not captured by current engineering solutions. We tackle this demand by building artificial hair cells on the basis of smart, self-actuated and self-sensing mechanical resonator beams with suitable actuation feedback. Thereby, we take advantage of the fact that the compressive nonlinearity arises naturally in dynamical systems tuned to a bifurcation point. This tuning is achieved by an appropriate feedback loop inspired by physiological models. Initial results on the detection properties of a single AHC will be shown demonstrating amplification and a decreased width of the resonance peak.

Contact atomic force microscopy using piezoresistive cantilevers in load force modulation mode.

Scanning probe microscopy (SPM) encompasses several techniques for imaging of the physical and chemical material properties at nanoscale. The scanning process is based on the detection of the deflection of the cantilever, which is caused by near field interactions, while the tip runs over the sample's surface. The variety of deflection detection methods including optical, piezoresistive, piezoelectric technologies has been developed and applied depending on the measurement mode and measurement environment. There are many advantages (compactness, vacuum compatibility, etc.) of the piezoresistive detection method, which makes it very attractive for almost all SPM experiments. Due to the technological limitations the stiffness of the piezoresistive beams is usually higher than the stiffness of the cantilever detected using optical methods. This is the basic constraint for the application of the piezoresistive beams in contact mode (CM) atomic force microscopy (AFM) investigations performed at low load forces (usually less than 20 nN). Drift of the deflection signal, which is related to thermal fluctuations of the measurement setup, causes that the microscope controller compensates the fluctuations instead of compensating the strength of tip-surface interactions. Therefore, it is quite difficult to keep near field interaction precisely at the setpoint level during the whole scanning process. This can lead to either damage of the cantilever's tip and material surface or loosing the contact with the investigated sample and making the measurement unreliable. For these reasons, load force modulation (LoFM) scanning mode, in which the interaction at the tip is precisely controlled at every point of the sample surface, is proposed to enable precise AFM surface investigations using the piezoresistive cantilevers. In this article we describe the developed measurement algorithm as well as proposed and introduced hardware and software solutions. The results of the experiments confirm strong reduction of the AFM entire setup drift. The results demonstrating contactless tip lateral movements are presented. It is common knowledge that such a scanning reduces tip wear.

Use of self-actuating and self-sensing cantilevers for imaging biological samples in fluid.

In this paper, we present a detailed investigation into the suitability of atomic force microscopy (AFM) cantilevers with integrated deflection sensor and micro-actuator for imaging of soft biological samples in fluid. The Si cantilevers are actuated using a micro-heater at the bottom end of the cantilever. Sensing is achieved through p-doped resistors connected in a Wheatstone bridge. We investigated the influence of the water on the cantilever dynamics, the actuation and the sensing mechanisms, as well as the crosstalk between sensing and actuation. Successful imaging of yeast cells in water using the integrated sensor and actuator shows the potential of the combination of this actuation and sensing method. This constitutes a major step towards the automation and miniaturization required to establish AFM in routine biomedical diagnostics and in vivo applications.

Integration of scanning probes and ion beams.

We report the integration of a scanning force microscope with ion beams. The scanning probe images surface structures non-invasively and aligns the ion beam to regions of interest. The ion beam is transported through a hole in the scanning probe tip. Piezoresistive force sensors allow placement of micromachined cantilevers close to the ion beam lens. Scanning probe imaging and alignment is demonstrated in a vacuum chamber coupled to the ion beam line. Dot arrays are formed by ion implantation in resist layers on silicon samples with dot diameters limited by the hole size in the probe tips of a few hundred nm.

Quantum size aspects of the piezoresistive effect in ultra thin piezoresistors.

Proximal probe sensors with an ability to detect extremely small forces (10(-15)-10(-18)N) play significant role in scanning probe microscopy applications. The detection of extremely low forces, require producing micromachined cantilevers with as small as possible spring constants, which is considered by the optimization of the sensor design. In the last year many papers describing the fabrication process of producing ultrathin cantilevers (below 100nm) with integrated piezoresistors for deflection read-out have been published. In the case of such cantilevers the required thickness of piezoresistors is in the range of 50nm. From a quantum mechanical point of view, an electrical carrier transport confinement in direction perpendicular to the cantilever surface can be expected and in this manner we have to consider the quantum size effect. The goal of the project described in this paper is to calculate and determine the piezoresistive coefficients in p type Si thin (under 50nm) piezoresistors taking into account the quantum size effect and to compare them with the corresponding coefficients for bulk material. The calculation of the band structure will use the mathematical apparatus of an exact analytical diagonalization six-band k.p model, modified with the envelope function approximation. The behaviour of the thin piezoresistors employed as integrated deflection read-out will be also discussed. Moreover, critical issues in the realization of piezoresistors formed by MOS transistor channel will be presented.