E. Lora-Tamayo

Thermal and mechanical aspects for designing micromachined low-power gas sensors

Metal oxide gas sensors have to be heated during operation. Ideally they should have low power consumption and a uniform temperature distribution over the sensitive area. Thermal isolation is achieved by locating the sensitive element on thin free-standing membranes fabricated by means of micromachining of silicon. Uniformity of the temperature is helpful for discriminating different gases and, thus, increasing the selectivity. This can be achieved by careful design of the heater or by additional structures. Micromachined test structures have been fabricated, their mechanical strength and thermal behaviour have been determined and the results have been correlated with extensive FEM studies.

Resonant silicon accelerometers in bulk micromachining technology-an approach

The fabrication and characterization of resonant silicon accelerometers, made in bulk micromachining technology, is presented. The devices consist of a silicon mass, coupled axially to a strain-sensitive vibrating silicon beam. The beam is driven electrothermally and sensed piezoresistively by means of implanted piezoresistors. Two different accelerometer types are shown, differing in the complexity of the respective fabrication processes and in performances. Closed-loop operation of the devices is demonstrated. Also in the closed loop, static and dynamic measurements of prototypes have been performed. The sensor types presented are compared, and the resonant acceleration sensor concept is discussed.

Wafer level packaging of silicon pressure sensors

In this paper, a new pre-packaging technique for silicon pressure sensors on the wafer level is presented. It is based on the use of UV photopatternable silicone which is deposited over the whole wafer by means of a novel device suitable for low-viscosity material coating and mask alignment. The process consists of the exposure of the deposited layer to UV light leading to cross-linking of the polymer according to the pattern of a chrome mask, which leaves the membrane area as well as the bonding pads free from silicone. After development and dicing, the chips are packaged and conventional wire bonding is performed. The area surrounding the UV photopatternable silicone pattern can then be filled with soft silicone gel for protection of the electrically active components against aggressive media. Results are presented for silicon piezoresistive pressure sensors on which 1.5-mm high octagonal wall structures of silicone elastomer have been patterned. Despite the thickness of the deposited layer, good resolution and aspect ratio are obtained, and the temperature dependence of the packaged pressure sensors is not influenced by the polymer layers.

New bulk accelerometer for triaxial detection

Abstract A new structure for a piezoresistive triaxial accelerometer has been designed and fabricated. The FEM simulations show a null crosssensitivity for the x - and y -detections and a very low-level one for the z -direction (

Simple technology for bulk accelerometer based on bond and etch back silicon on insulator wafers

Abstract A simple technology for bulk-micromachined accelerometers based on bond and etch back silicon on insulator (BESOI) wafers is presented. This technology is an easy combination of bulk- and surface-micromachining technology using the buried oxide as a sacrificial layer, allowing a precise control of the thickness of the beams and the fabrication of complex structures. Cantilever-beam, quad-beam, twin-mass [ Chr. Burrer, J. Esteve, J.A. Plaza, M. Bao, O. Ruiz, J. Samitier, Fabrication and characterization of a twin-mass accelerometer, Sensors and Actuators A 43 (1994) 115–119] and a new triaxial accelerometer [ J.A. Plaza, J. Esteve, E. Lora Tamayo, Acelerometro triaxial, Spanish Patent No. 9 701 154 (28 May, 1997) ] have been fabricated and their results are presented. Over-range structures have been included without any additional process step. The devices are anodically bonded to a glass wafer in order to reduce the package stresses and to control the damping of the structures.

Effect of silicon oxide, silicon nitride and polysilicon layers on the electrostatic pressure during anodic bonding

The quality of the anodic bonding between glass and silicon wafers coated with silicon oxide, silicon nitride and polysilicon layers has been investigated. We have used an electrostatic test to study the effect of these layers on the quality of the bond. The electrostatic test shows how the electrostatic pressure decreases with the increase of the thickness of the silicon oxide layer. In the bonds with silicon wafers coated with silicon nitride layers, electrostatic pressure problems are not observed but the uniformity of the bond is very bad. The deposition of a polysilicon layer over the silicon oxide and silicon nitride layers increases the electrostatic pressure and, in consequence, gives a high-quality bond.

Real time protein recognition in a liquid-gated carbon nanotube field-effect transistor modified with aptamers.

The combination of optimized and passivated Field Effect Transistors (FETs) based on carbon nanotubes (CNTs) together with the appropriate choice and immobilization strategy of aptamer receptors and buffer concentration have allowed the highly sensitive and real time biorecognition of proteins in a liquid-gated configuration. Specifically we have followed the biorecognition process of thrombin by its specific aptamer. The aptamer modified device is sensitive enough to capture a change in the electronic detection mechanism, one operating at low protein concentrations and the other in a higher target concentration range. The high sensitivity of the device is also sustained by the very low detection limits achieved (20 pM) and their high selectivity when other target proteins are used. Moreover, the experimental results have allowed us to quantify the equilibrium constant of the protein-aptamer binding and confirm its high affinity by using the Langmuir equation.

Surface micromachining technology applied to the fabrication of a FET pressure sensor

This paper presents a novel surface-micromachined pressure sensor based in a FET device. The diaphragm acts as the gate of the transistor and the gate - source voltage varies in a nearly linear form with pressure when keeping constant the current along the channel. Both theoretical characteristics and the technological process are analysed. Finally the advantages over existing microsensors are examined.

Interaction of biomolecules sequentially deposited at the same location using a microcantilever-based spotter.

A microspotting tool, consisting of an array of micromachined silicon cantilevers with integrated microfluidic channels is introduced. This spotter, called Bioplume, is able to address on active surfaces and in a time-contact controlled manner picoliter of liquid solutions, leading to arrays of 5 to 20-μm diameter spots. In this paper, this device is used for the successive addressing of liquid solutions at the same location. Prior to exploit this principle in a biological context, it is demonstrated that: (1) a simple wash in water of the microcantilevers is enough to reduce by >96% the cross-contamination between the successive spotted solutions, and (2) the spatial resolution of the Bioplume spotter is high enough to deposit biomolecules at the same location. The methodology is validated through the immobilization of a 35mer oligonucleotide probe on an activated glass slide, showing specific hybridization only with the complementary strand spotted on top of the probe using the same microcantilevers. Similarly, this methodology is also used for the interaction of a protein with its antibody. Finally, a specifically developed external microfluidics cartridge is utilized to allow parallel deposition of three different biomolecules in a single run.

Photolithographic packaging of silicon pressure sensors

Abstract The packaging technique presented here provides direct interaction between the sensing element and the physical or chemical variable to be measured as well as hermetic insulation and mechanical protection of the silicon sensor and its electrically active components. The sensors are embedded in a mechanically decoupling shell of silicone elastomer with low Youngs modulus, leaving the sensing area free from covering. The process can be considered as a synthesis of conventional soft-mounting techniques and photolithographically controlled partial encapsulation. It has been applied to package piezoresistive silicon pressure sensors for use in humid environments and the results obtained show the suitability of the concept. Due to its modularity and unlike application-specific packaging methods, it can potentially be applied to a variety of microelectronic sensors.