Pascal Vairac

Scanning microdeformation microscopy in reflection mode

A scanning microdeformation microscope based on a vibrating tip with piezoelectric detection in a transmission operating mode is upgraded to operate in reflection mode. Images which reveal subsurface inhomogeneities in silicon and metallic samples are presented. As in the first configuration the lateral resolution and the apparent penetration depth is related to the tip diameter.

Optical detection for scanning microdeformation microscopy

Scanning microdeformation microscopy is a kind of ac contact force microscopy sensitive to the variations of the local elastic constants of the investigated material. In this letter a new optical interferometer designed for detecting the small displacement of the cantilever is reported. The setup is described and some applications of the laser probe to scanning microdeformation microscopy are demonstrated.

Study of photothermal vibrations of semiconductor cantilevers near the resonant frequency

The elastic vibrations of semiconductor cantilevers, which were excited with a frequency-modulated laser, were studied theoretically and experimentally in this paper. The carrier density, temperature and elastic displacement distributions were obtained by using the theoretical model including plasma and thermoelastic effects. The results showed that the plasma wave had a significant influence on the vibrations. Using an interferometric setup the photothermal signals were investigated near the resonant frequency. The results showed that the experimental results were in good agreement with the theoretical ones.

Near-field acoustic densimeter and viscosimeter

We have developed a vibrating microsensor allowing simultaneous measurement of density and viscosity in liquids. A small-sized spherical oscillating probe is immersed in the liquid and is connected to a microcantilever driven in flexural vibration by a piezoelectric transducer. Cantilever resonance parameters (frequency and damping) are used to measure the probe–fluid interaction. The developed analytical model enables us to compute fluid characteristics from measured data by solving the inverse problem. Measurements performed for various fluids are compared to literature data. Investigation depth of the sensor is determined by studying the effect of the presence of an immersed wall in the vicinity of the probe.

Detection and High-Precision Positioning of Liquid Droplets Using SAW Systems

The capability to accurately handle liquids in small volumes is a key point for the development of lab-on-chip devices. In this paper, we investigate an application of surface acoustic waves (SAW) for positioning micro-droplets. A SAW device based on a 2 times 2 matrix of inter-digital transducers (IDTs) has been fabricated on a (YXl)/128deg LiNbO3 substrate, which implies displacement and detection in two dimensions of droplets atop a flat surface. Each IDT operates at a given frequency, allowing for an easy addressing of the active channel. Furthermore, very low cross-talk effects were observed as no frequency mixing arose in our device. Continuous as well as pulsed excitations of the IDTs have been studied, yielding, respectively, continuous and step-by-step droplet displacement modes. In addition, we also have used these two excitation types to control the velocity and the position of the droplets. We also have developed a theoretical analysis of the detection mode, which has been validated by experimental assessment.

Ultra-wide acoustic band gaps in pillar-based phononic crystal strips

An original approach for designing a one dimensional phononic crystal strip with an ultra-wide band gap is presented. The strip consists of periodic pillars erected on a tailored beam, enabling the generation of a band gap that is due to both Bragg scattering and local resonances. The optimized combination of both effects results in the lowering and the widening of the main band gap, ultimately leading to a gap-to-midgap ratio of 138%. The design method used to improve the band gap width is based on the flattening of phononic bands and relies on the study of the modal energy distribution within the unit cell. The computed transmission through a finite number of periods corroborates the dispersion diagram. The strong attenuation, in excess of 150 dB for only five periods, highlights the interest of such ultra-wide band gap phononic crystal strips.

Electronic and thermal generation of vibrations of optically excited cantilevers

The results of the theoretical analysis of the dynamic effects in the optically excited cantilevers were given. Theoretical model for dynamic elastic bending for two-layer cantilevers was derived including electronic and thermal elastic deformation effects which have the main influence on the dynamics of the cantilevers. The influence of the carrier transport characteristics (the carrier diffusion coefficient, the lifetime of photogenerated carriers, and the carrier recombination velocities) to the elastic vibrations of cantilevers was analyzed. Theoretical model was verified by comparing with the experimental results. The results of these investigations are important for sensors, actuators, and resonators based on the cantilevers.

MEASUREMENT OF FLUID PROPERTIES WITH A NEAR-FIELD ACOUSTIC SENSOR

The proposed microsensor is derived from the electromechanical resonator of our ac force microscope: the scanning microdeformation microscope (SMM). A submillimetric spherical probe immersed in the fluid sample replaces the tip usually used in SMM. This sphere is connected to a cantilever, which is excited at the resonance frequency. The measurement of the resonance frequency and of the damping enables the quantitative characterization of the investigated fluid. A theoretical approach allows to model the system behavior. The model takes into account two aspects: the mechanical vibration of the cantilever and the fluid mechanics. The values predicted by the model are in good agreement with the experimental measurements performed for various viscous media.

Quantitative thermal microscopy using thermoelectric probe in passive mode

A scanning thermal microscope working in passive mode using a micronic thermocouple probe is presented as a quantitative technique. We show that actual surface temperature distributions of microsystems are measurable under conditions for which most of usual techniques cannot operate. The quantitative aspect relies on the necessity of an appropriate calibration procedure which takes into account of the probe-to-sample thermal interaction prior to any measurement. Besides this consideration that should be treated for any thermal contact probing system, the main advantages of our thermal microscope deal with the temperature available range, the insensitivity to the surface optical parameters, the possibility to image DC, and AC temperature components up to 1 kHz typically and a resolution limit related to near-field behavior.

In-plane optical measurement of vibrations of MEMS: gradient methods using interferometry and image processing

Optical probing is a non invasive tool useful to characterize the vibrations of the small moving components of microsystems (MEMS/MOEMS). This paper presents two complementary methods that can sense in-plane components of the vibration. The first one is a heterodyne interferometer, commonly used for out-of-plane component detection. The edge of the sample partially occults the laser beam, and, consequently, the intensity is amplitude modulated when the sample vibrates. The electronics has been modified so that both phase and amplitude of the output signal are extracted. Actual sensitivity is about 10-11 m/√Hz. In the second gradient method, a parallel acquisition of synchronous images is performed with a camera and a microcomputer, which stores the successive images for subsequent processing. Before digital lock-in processing, the images sequence is inter-correlated and interpolated to increase the accuracy of the method. This simple processing technique allows nanometer sensitivity. Both techniques are presented, analyzed and compared from theoretical and experimental point of view.