Benoit Charlot

Scanning thermal imaging of microelectronic circuits with a fluorescent nanoprobe

We have developed a scanning thermal imaging method that uses a fluorescent particle as a temperature sensor. The particle, which contains rare-earth ions, is glued at the end of an atomic force microscope tip and allows the determination of the temperature of its surrounding medium. The measurement is performed by comparing the relative integrated intensity of two fluorescence lines that have a well-defined temperature dependence. As an example of application, we show the temperature map on an operating complementary metal-oxide-semiconductor integrated circuit.

Bistable nanowire for micromechanical memory

We present a micromechanical device designed to be used as a non-volatile mechanical memory. The structure is composed of a suspended slender nanowire (width: 100 nm, thickness: 430 nm, length: 8 to 30 ?m) clamped at both ends. Electrodes are placed on each side of the nanowire to (1) actuate the structure during the data writing and erasing mode and (2) determine its position by measuring the capacitive bridge in the reading mode. The structure is patterned by electron beam lithography on a pre-stressed thermally grown silicon dioxide layer. When later released by plasma etching, the stressed material relaxes and the beam buckles by itself to a position of lower energy. These symmetric bistable Euler beams exhibit two stable deformed. This paper presents the microfabrication process and analysis of the static buckling of nanowires. Snapping of these nanowires from one stable position to another by mechanical or electrical means will also be discussed.

Wireless sensor network node with asynchronous architecture and vibration harvesting micro power generator

This paper presents recent advances in the development of a microsystem designed to be part of a wireless sensor network. This microsystem is developed with two particular technologies: asynchronous circuits and ambient energy harvesting power generator. Asynchronous technologies offer several advantages allowing a global decrease in the power consumption of the node. In addition, the presence of an ambient energy scavenger allows the system to power itself, thus reducing maintenance and increasing the lifetime of the node.

A Full Fingerprint Verification System for a Single-Line Sweep Sensor

This paper presents a full fingerprint verification system. It is composed of a tactile fingerprint sensor, integrated read out and conversion circuits, and dedicated recognition algorithms. The sensor is a single-line sweep mode sensor, e.g., it is made of a single line of sensing elements, thus covering the minimum surface of silicon. Compared with cm2 sized touch sensors, it offers a large cost reduction and possibility of easy integration into portable devices. The use of a single line to measure a fingerprint requires the user to sweep its finger along the sensor. This sensing scheme produces fingerprint images with several distortions that needs further image processing to allow efficient fingerprint recognition. This is why we developed and present here specific algorithms to take care of the sensors specifications. This paper will present measurement results, as well as a performance evaluation of the entire verification system.

High damping electrostatic system for vibration energy scavenging

Advances in low power electronics and microsystems design open up the possibility to power small wireless sensor nodes thanks to energy scavenging techniques. Among the potential energy sources, we have focused on mechanical surrounding vibrations. To convert vibrations into electrical power we have chosen mechanical structures based on electrostatic transduction. Thanks to measurements and in agreement with recent studies [1], we have observed that most of surrounding mechanical vibrations occurs at frequencies below 100 Hz. We report here global simulations and designs of mechanical structures able to recover power over a large spectrum below 100 Hz. Contrary to existing structures tuned on a particular frequency [2], we have investigated conversion structures with a high electrical damping. Mathematica analytical models have been performed to determine the mechanical and electrical parameters that maximize the scavenged power for a wide number of applications. Two prototypes of mechanical structures have been designed.

Electrically induced stimuli for MEMS self-test

A major problem for applying self-test techniques to MEMS is the multi-domain nature of the sensing parts that require special test equipment for stimuli generation. In this work we describe, for three different types of MEMS that work in different energy domains, how the required nonelectrical test stimuli can be induced onchip by means of electrical signals. This provides the basis for adding BIST strategies for MEMS parts embedded in the coming generation of integrated systems. The first case corresponds to an accelerometer as a review of a classical example. The last two cases correspond to piezoresistive and infrared sensors that we use in innovative applications under development in our Laboratory, and for which the self-test methods are new to our knowledge. The last case is also illustrated as a complete application that corresponds to an infrared imager. The on-chip test signal generation proposed requires only slight modifications and allows production test of the imager with a standard test equipment, without the need of special infrared sources and the associated optical equipment. The test function can also be activated off-line in the field for validation and maintenance purposes.

Generation of Electrically Induced Stimuli for MEMS Self-Test

A major task for the implementation of Built-In-Self-Test (BIST) strategies for MEMS is the generation of the test stimuli. These devices can work in different energy domains and are thus designed to sense signals which are generally not electrical. In this work, we describe, for different types of MEMS, how the required non-electrical test stimuli can be induced on-chip by means of electrical signals. This provides the basis for adding BIST strategies for MEMS parts embedded in the coming generation of integrated systems. The on-chip test signal generation is illustrated for the case of MEMS transducers which exploit such physical principles as time-varying electrostatic capacitance, piezo-resistivity effect and Seebeck effect. These principles are used in devices such as accelerometers, infrared imagers, pressure sensors or tactile sensors. For implementation, we have used two major MEMS technologies including CMOS-compatible bulk micromachining and surface micromachining. We illustrate the ability to generate on-chip test stimuli and to implement a self-test strategy for the case of a complete application. This corresponds to an infrared imager that can be used in multiple applications such as overheating detection, night vision, and earth tracking for satellite positioning. The imager consists of an array of thermal pixels that sense an infrared radiation. Each pixel is implemented as a suspended membrane that contains several thermopiles along the different support arms. The on-chip test signal generation proposed requires only slight modifications and allows a production test of the imager with a standard test equipment, without the need of special infrared sources and the associated optical equipment. The test function can also be activated off-line in the field for validation and maintenance purposes.

Extending fault-based testing to microelectromechanical systems

As stable fabrication processes for MicroElectroMechanical Systems (MEMS) emerge, research efforts shift towards the design of systems of increasing complexity. The ways in which testing is going to be performed for large volume complex devices embedding MEMS are not known. As in the microelectronics industry, the development of cost-effective tests for larger systems may well require test stimuli targeting actual faults, developing fault lists and fault models for realistic manufacturing defects and failure modes, and using fault simulation as a major approach for assessing testability and dependability. In this paper, we illustrate how fault-based testing can be extended to MEMS, both for bulk and surface micromachining technologies, making possible the reuse of analog testing techniques.

FAULT SIMULATION OF MEMS USING HDLS

This paper describes an approach to fault simulation of MEMS using an analog Hardware Description Language (HDL). HDL languages facilitate the description of mixed-domain devices, providing powerful representation capabilities which are not limited to the use of the traditional equivalent electrical modes. This is exploited in this paper for fault simulation of MEMS, showing the advantages of using an HDL for this task. An electro-thermal converter is used as test vehicle, for which an equivalent electrical more is readily obtained. Typical defects and failure mechanisms which can affect these devices fabricated using CMOS-compatible bulk micromachining are shown. These defects are used for illustrating the fault simulation approach which appears to be more comprehensive and systematic than previous approaches.

A sweeping mode integrated fingerprint sensor with 256 tactile microbeams

This paper reports recent advances in the development of a tactile fingerprint sensor made by a CMOS compatible front side bulk micromachining technology. This device enables the measurement of a fingerprint by the way of a mechanical scanning principle of the finger roughness. While this sensing principle has shown good results on a first prototype of reduced width, we present here the design, fabrication and test of a new sensor. This sensor contains 256 pressure sensitive microbeams for a total length of 1.28 cm and is fully integrated with analog and mixed signal electronics. In this paper we will detail the general working principle of the tactile fingerprint sensor and the two prototypes that have been manufactured and tested with a special focus on the electronic architecture and the test results of the second prototype.