High-temperature low-power performing micromachined suspended micro-hotplate for gas sensing applications

Sensors and Actuators B 114 (2006) 826–835

High-temperature low-power performing micromachined suspended micro-hotplate for gas sensing applications J. Cerd`a Belmonte a,∗ , J. Puigcorb´e a , J. Arbiol a , A. Vil`a a , J.R. Morante a , N. Sabat´e b , I. Gr`acia b , C. Can´e b a

Electronic Materials and Engineering, Electronics Department, Faculty of Physics, University of Barcelona. C/Mart´ı i Franqu`es 1, E-08028 Barcelona, Spain b Department of Silicon Technologies and Microsystems, National Center of Microelectronics, CNM-CSIC, Campus UAB, E-08193 Bellaterra, Spain Received 22 May 2004; received in revised form 22 July 2005; accepted 25 July 2005 Available online 22 September 2005

Abstract Micromachined gas sensors that work at high temperatures, namely 700 ◦ C or more, are necessary for some specific applications but they present a thermal cross-talk when they are integrated with other micromachined devices. In this paper we describe the fabrication steps and the implementation of a suspended micro-hotplate device for high temperature gas sensing applications, which presents a very low-power consumption. Simulations with the finite element model tool show that its integration with other micromachined devices is appropriate. The sensor device has been tested with BaSnO3 under oxygen and CO atmospheres. © 2005 Elsevier B.V. All rights reserved. Keywords: Micro-hotplate; Micromachining; Gas sensor; BaSnO3 ; Microdropping; Oxygen

1. Introduction Chemical gas sensors are playing a more important role day-by-day in our lives due to their successful applications. At the present time, many efforts are focused on silicon microsensors as chemical sensors, which consist basically in a layer of a gas sensitive material, some electrodes, a heater element and a silicon-based micromachined substrate. These sensors have some advantages such as very small dimensions, low weight, low manufacturing cost, pulsed or modulated mode of operation of the heater element, they have the possibility of being integrated together with other devices and, if the fabrication process is CMOS based they can also take advantage of the integrated circuits (IC) microtechnology [1].

∗ Corresponding author at: Imperial College of London, Department of Materials, Dr Molly Stevens Lab., Exhibition Road, London Sw7 2AZ, UK. Tel.: +34 934021148; fax: +34 934021147. E-mail addresses: [email protected], [email protected] (J.C. Belmonte).

0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.07.057

Within the field of chemical gas sensors different materials for detecting and identifying gas particles can be used. If we focus on material oxide semiconductor materials (MOS materials) it is possible to establish three basic groups depending on their sensing mechanism. In the first group belong electron conductor materials such SnO2 , WO3 , In2 O3 , etc. These materials are normally used to monitor oxidizing and reducing gases and hydrocarbons. The sensing mechanism of a sensor based on an electron conductor material is affected by the sensitive film thickness, the particle size of the MOS material, the addition of catalytic metals such as Pt, Pd, Au, etc., the geometry of the electrodes and the operation temperature which ranges from 150 up to 450 ◦ C. A second group of materials is this based on mixed conductors. Common materials, which fit into this category, are TiO2 , Nb2 O3 , Ga2 O3 , perovskite-like oxides, etc. In this case, both electron as well an ion-conduction take place and the predominant effect depends on the operation temperature. At temperatures below 500 ◦ C, they behave mainly as electron conductors. At temperatures between 500 and 700 ◦ C, the sensing mechanism

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is a mixture of that of electron conductors and of that of ion conductors. At temperature above 700 ◦ C, the major sensing mechanism is based on the ion conductor one. The selectivity and sensitivity of sensors based on these materials can be improved in the same way as it has been described in the previous material category. The last type of materials is the ion conductor. The most widely used is ZrO2 for the O2 monitoring. Devices based on ZrO2 have operating temperatures higher than 700 ◦ C. In spite of this large variety of gas sensing materials for chemical sensors, not all of them are used on silicon micromachined structures due to their high operation temperature. Nowadays, there are many silicon micromachined microhotplates, which stand easily temperatures between 350 and 500 ◦ C for a long time [2–4], but there are no commercial sensors with structures that can stand up to 700 ◦ C. Then, only those materials belonging to the electron conductor group are appropriate to be used due to the restriction of the substrate temperature. A micromachined sensor with a micro-hotplate that can stand such high temperatures, namely 700–800 ◦ C, would provide an important range of high temperature performing materials to be used as the sensing layer which right now are strongly related to alumina substrates and to the restrictions that theses substrates carry out. For example, in alumina-based arrays the integration of two or more sensors in the same substrate when at least one sensor is working at high temperature, i.e. 500 ◦ C or higher, is not possible due to the thermal cross-talk between the sensors. Nevertheless, gas sensor arrays based on alumina substrates present others disadvantages such as very high power consumption and the non-possibility of the integration of signal-conditioning, signal-processing and control circuitry onto the same alumina sensor substrate. A way to overcome these limitations is using silicon-based micromachined technology to fabricate the gas sensor devices and arrays [5,6]. For most of the applications it is necessary to have a sensor device with very low thermal conductivity both to avoid the heat cross-talk with the other devices and the corresponding integrated circuits that compose the whole device and to lower the power consumption of the heater element to allow battery-power operation in portable detectors, if needed. When the sensor device is used up to 500 ◦ C, the low thermal inertia is achieved by a reduction of the thickness of the dielectric membranes. This reduction involves some difficulties due to the limitations introduced by the stress when combining layers of different materials. If the gas sensor device should perform at higher temperatures than 600 ◦ C more stress in introduced in the dielectric membrane due to the thermal effects, so the design for devices working up to 500 ◦ C is not valid in this case. Then, the thermal properties of the thin films that compose the dielectric sensor membrane and their effects are important to be studied to achieve a good optimization and performance of the device [7]. One way to minimize the thermal conductivity is by a proper design of the geometry of the dielectric membrane.

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Many papers show a suspended membrane for this purpose. The geometry of the membrane and of the arms that stand it and its number can be varied. Different designs can be found in several papers, for example, D¨ucs¨o et al. [8] show a 5 ␮m thick silicon membrane suspended by four silicon nitride arms which achieve 200 ◦ C with P = 15 mW, Semancik et al. [9] present an array of suspended micro-hotplates that are based on a SiO2 plate, four arms and can operate to temperatures up to 500 ◦ C due to the Al metallization, Solzbacher et al. [10] propose a Si C micro-hotplate, which consists on a square membrane suspended by six arms and achieves 400 ◦ C with P = 35 mW, and Lee et al. [11] report a micro-hotplate which is totally suspended in air by Pt bonding wires and P = 100 mW at 400 ◦ C. In prior papers [12,13], BaSnO3 was synthesized in order to be used as a possible oxygen gas sensor when it works as an ionic conductor at high temperature. Therefore, structures that can stand these high temperatures and have good thermal isolation are claimed. In this paper, a BaSnO3 gas sensing layer will be deposited over interdigited platinum electrodes, a SiO2 layer as an insulating layer, a platinum heater with a double spiral shape and a Si3 N4 dielectric layer suspended by four bridges of the same material. The area of the suspended membrane is 400 ␮m × 400 ␮m, the width of each bridge is of 60 ␮m and the membrane size is of 1 mm × 1 mm. Also, thermo-mechanical FEM simulations were performed to minimize edge effects, to avoid stressed singularities, to optimise the heater power consumption and to achieve a homogeneous temperature distribution. Gas sensor devices based on these suspended micro-hotplates offer additional benefits in contrast to those that are not suspended due to the thermally isolation of the hotplate. Then, the device is characterized by a rapid thermal response and its power consumption is decreased in comparison to other gas sensor structures.

2. FEM simulations The design of the device has been supported using thermal and mechanical simulations with the FEM analysis tool ANSYS. Temperature gradient simulations and mechanical stress simulations of the sensor micro-hotplate are required in order to establish the geometrical characteristics of the structure. In this way, the temperature distribution on the microhotplate and the regions around it has been simulated. For the simulation only convective energy losses have been taken into account since the irradiative energy losses are very small due to the small area of the heated plate. In Fig. 1 it is straight forward deduced that the temperature drops down radically at the anchors of the bridges. When the temperature of the microhotplate is set to 700 ◦ C in the simulation tool, the regions around the structure have a temperature around 150 ◦ C. From the simulation picture it can also be seen that a good uniformity of the temperature is achieved in the heater active area.

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Fig. 1. FEM simulation of temperature distribution in a quarter of the microhotplate and the surrounding regions. Fig. 3. FEM simulation of the vertical displacement in a quarter of the microhotplate and the surrounding regions.

Then, the present sensor device can be integrated with other devices that work at temperatures of 250 ◦ C or higher, as it is considered that a difference of 100 ◦ C between chip temperature and membrane centre are enough for preventing heat cross-talk. The mechanical stress and the vertical displacement of the micro-hotplate at 700 ◦ C and without the gas sensitive layer are shown in Figs. 2 and 3, respectively. In the mechanical stress distribution figure, it is possible to observe that along the arms a moderate-high stress accumulation. The maximum stress is observed at the junction between the Si3 N4 dielectric bridge and the silicon substrate (edge effect). The vertical displacement is of more than −7 ␮m for the suspended hot plate. Taking into account these results it is possible to conclude that the present structure is relatively fragile.

Fig. 2. FEM simulation of the mechanical stress in a quarter of the microhotplate and the surrounding regions.

3. Device fabrication The micromachined structure was fabricated on a double side polished p-type 1 0 0 Si wafer. The thickness of the silicon substrates was 300 ␮m. The basic technological steps for the device fabrication, shown in Fig. 4, can be summarised as follow: (1) A SiO2 layer of 500 nm of thickness was thermally grown at 1100 ◦ C. Then, a layer of 300 nm of Si3 N4 was deposited on each side of the silicon substrate by low-pressure chemical vapour deposition (LPCVD) at 800 ◦ C. The nitride was implanted with boron ions (dose = 4E15 cm−2 , energy = 100 keV) to reduce the inherent stress of the dielectric membrane (Fig. 4A). (2) A bi-layer of Ti/Pt (20 nm/200 nm) for the heater deposited by sputtering. The geometry of the heater was defined by the lift-off technique. The resulting sheet resistance of the heater was of 1.6
/
(Fig. 4B). (3) A SiO2 layer 800 nm thick was deposited as electrical insulator between the heater and the electrodes by plasma enhanced chemical vapour deposition (PECVD) at 380 ◦ C. Then, the contact opening was done by dry etch through the insulating layer (Fig. 4C). (4) The Ti/Pt (20 nm/200 nm) electrodes and heater bonding pads were patterned by lift-off and deposited by sputtering as for the heater. The width of the electrodes was of 10 ␮m and the separation between them of 100 ␮m. After the stabilization of the electrodes, the Ti and the Pt form a new chemical compound that is stable. For this reason, the recalibration of the TCR of the heater element will be needed (Fig. 4D). (5) An etch mask was patterned in the back of the substrate and a window in the Si3 N4 layer was defined. After that, front side nitride layer that defines the suspended

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Fig. 4. Process sequence of the bulk-micromachined suspended membrane: (A) SiO2 thermally grown layers and implantation of Si3 N4 layers, (B) pattern and deposition of the Ti/Pt (20/200 nm) heater and (C) deposition of an insulating layer of SiO2 between the heater and the electrodes. Pattern and etching of the heater through the insulating layer (D) pattern and deposition of the Ti/Pt (20 nm/200 nm) electrodes and heater contacts. Definition of the geometry of the heater and of the contacts by lift-off (E) silicon bulk micromachining in the backside and definition by RIE of the suspended micro-hotplate in the front side.

micro-hotplate structure was patterned. Then, the backside silicon was etched with KOH at T = 70 ◦ C. After the silicon bulk micromachining, the suspended microhotplate was defined in the front side by reactive ion etching (RIE) (Fig. 4E). The total number of masks used in the process is 6. After this process the micromachined wafer was diced, chips were mounted on TO package and four wire bonds (two for the heater contacts and two for the electrodes contacts) were made. Then, the devices were ready for the sensing layer deposition. The obtained structures are shown in Fig. 5.

4. Power consumption and thermal characterization The temperature characteristics of the heater element and the power consumption of the device have been studied. To calculate the temperature coefficient resistance characteristic (TRC) of the platinum heater a calibration curve has been

obtained. For that, the whole sensor device was introduced in an oven together with a temperature reference sensor. Then, the data of the temperature sensor and the data of the heater resistance of the gas sensor when a constant current is applied were acquired automatically by a computer. The device was heated up to 180 ◦ C, point at which the effects of the tin bondings of the studied structure (lost of linearity) are manifested due to the fact that we have introduced the whole device in the furnace. The value obtained of the TCR is 1.9E−3
K−1 , which is in good agreement with the literature for a platinumbased element (see Fig. 6). In Fig. 7 the power consumption of the device versus the temperature of the heater is shown. We can deduce from this graphic that the present device is a low power consumption sensor device since a temperature as high as 400 ◦ C can be reached applying 50 mW. While the experiment for the power consumption versus temperature was performed, the platinum resistance placed on the silicon substrate (see Fig. 5) and on the right of the suspended micro-hotplate was measured. The variations on

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Fig. 7. Power consumption of the sensor vs. the variation of the temperature of the micro-hotplate.

Fig. 5. SEM image of the 400 ␮m × 400 ␮m nitride suspended microhotplate with Pt electrodes and Pt heating element from (A) primary electrons and (B) secondary electrons. On the right of the sensor structure, a resistance element made of platinum is placed for studies of heat dissipation through the silicon substrate.

Fig. 6. Temperature coefficient resistance change from room temperature to 175 ◦ C, divided by the resistance at room temperature.

resistance are induced by a change of temperature. Therefore, the temperature of the silicon substrate was measured at the same time as the micro-hotplate temperature was increased from room temperature up to 400 ◦ C. Fig. 8 shows that the change in temperature of the silicon substrate is almost insignificant. Although the micro-hotplate has a temperature of 400 ◦ C, the silicon substrate has a temperature of less than 38 ◦ C, when the room temperature is of 22.6 ◦ C. The transient response of the temperature has also been measured (see Fig. 9). It takes around 10 ms to heat the microhotplate from room temperature up to T = 400 ◦ C, so the thermal inertia is low. This fact enables the possibility of using a modulated temperature mode of operation. A small peak can be observed when the temperature pulse is applied. We assume that this peak is due to the heat transfer to the heater

Fig. 8. Variation of temperature in the silicon substrate close to the microhotplate when it is heated from room temperature to 400 ◦ C. The temperature is measured by means of a reference resistance of platinum.

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Fig. 9. Transient response of a temperature pulse of the heater element from room temperature to 400 ◦ C.

element surroundings. The physical phenomenon takes some milliseconds to achieve the equilibrium.

5. BaSnO3 gas sensing film preparation In this study BaSnO3 nanoparticles have been chosen as the sensing layer. BaSnO3 is a cubic perovskite-like material (Fig. 10) which shows a mixed conduction at temperatures between 500 and 650 ◦ C and ionic conduction at temperatures around 700 ◦ C and higher [14,15]. It is stable at high

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temperatures up to 1000 ◦ C [16,17]. Thus, it is a very promising material to detect gases at high temperatures. The synthesis of the material is based on process with only two chemical reagents, namely Ba(OH)2 and K2 SnO3 ·3H2 O. The most noteworthy properties of this procedure, in comparison to the methods used in the literature [18–27], are the simplicity of the method and the reduced sintering time. Other remarkable characteristics are repeatability, mass-production and the low cost of the new method. Both the synthesis and the structural and the electrical characterization of BaSnO3 nanopowders obtained by this wet chemical route are presented and discussed in previous papers [12,13]. Pastes prepared with these powders are suitable to be implemented in integrated gas sensor devices based on silicon technology [28]. The BaSnO3 gas sensitive layer is deposited onto the suspended micro-hotplate taking advantage of the sensing layer deposition method called microdropping (or drop-coating) [28–34]. Briefly, the microdropping technology is based on the use of a paste prepared with nanopowders of different ceramic oxides, previously stabilized and having already incorporated the catalytic element, if needed. Then, the paste is deposited on the membrane of a micromachined silicon substrate using a microdeposition drop technique. Finally, the deposited sensitive layer is fired in order to remove the organic solvent used in the paste preparation. This procedure allows, easily, the fabrication of gas sensors. It takes advantage of micromachined silicon-based sensors, i.e. low thermal inertia, low power consumption, optional pulsed operation mode, etc. Moreover, it takes also advantage of thick film nanomaterial printable methods, i.e. use of wet chemical routes, easily stabilisation process, high porous degree, etc. In Fig. 11 a scan electron microscope picture of the sensor structure and the sensitive material deposited on it by dropcoating is shown.

6. Electrical characterization

Fig. 10. Structure of cubic BaSnO3 (perovskite structure). The cubic perovskite structure has the general chemical formula ABO3 , where A is a divalent cation and B is a tetravalent cation.

The gas test of the gas sensor has been carried out in a sensor test system. Briefly, a desired gas concentration is obtained in a stainless steel test chamber – where the sensors are placed – by means of several mass flow controllers (Bronkhorst F201C). Acquisition boards (Computer boards CIO-DAC08/16 and CIO-DAS1602/16) of a computer perform the gas sensor data acquisition and the mass flow control. BaSnO3 has been tested in oxygen and CO atmospheres, in which the heater element has been set at temperatures of 600 and 700 ◦ C (Fig. 12). In spite of one of the main features of the micromachined gas sensor structures is the possibility of pulsing or modulating the temperature of the heater element, we have not taken advantage of this feature and constant temperature has been used. The reason is that we expect to design an O2 micromachined gas sensor using a gas sensitive material that is an ionic conductor at temperatures as

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Fig. 11. Photograph of the BaSnO3 gas sensing material deposited by microdropping onto the suspended micro-hotplate.

high as 700 ◦ C, and for this bulk-based sensing mechanism no modulation of the temperature is needed. BaSnO3 is an n-type perovskite-like oxide. Its more characteristic property as a gas sensitive material is that it is stable under reductive environments at elevated temperatures. At high temperatures an equilibrium between the oxygen partial pressure in the atmosphere and the oxygen in the lattice of the material is achieved and the sensing dominant mechanism is bulkcontrolled. When the material bulk is in contact with low oxygen pressures the oxigen from the crystal leaves the bulk lattice and then an oxygen vacancy defect appears. Due to the increase in oxygen vacancies, there are more free electrons in the lattice and consequently the conductivity increases and the measured resistance of the sensor decreases. For high oxygen pressures, the efect is the opposite, so the resistance of the material increases. In Fig. 12A it is shown that the oxygen dependency in the graphic is of the type: σ ∝ Ppartial (O2 )−1/m

(1)

where m = −0.01 for T = 600 ◦ C and m = −0.05 for T = 700 ◦ C. These results that are in close agreement with those presented by Fleischer and Meixner [35]. In a previous work [13] the ionic and electronic conduction mechanisms of thick films of BaSnO3 on alumina substrates in O2 and CO atmospheres are studied. The experiments showed that at temperatures of 700 ◦ C and over, the dominant mechanism is bulk-controlled. Since the detection mechanism of CO is based on the electronic conduction at low temperatures, at 700 ◦ C and over the sensor detects O2 but CO2 . To prove it with a thin film and a micromachined

Fig. 12. Gas sensor response at two different working temperatures to (A) different oxygen concentrations from 1% to 21% in nitrogen and (B) a CO containing atmosphere in synthetic air with concentrations ranging from 50 up to 500 ppm.

structure, the cross-sensitivity between O2 and CO of the sensor has been tested out. In Fig. 12B, different CO concentrations ranging from 50 to 500 ppm have been measured in air as carrier gas. It is clear that the sensitivity of the device to this interfering gas is almost negligible at T = 700 ◦ C as it was expected. So we can conclude that at the experiment temperature the ionic conductivity sensing mechanism is the predominant while the electronic conduction is almost hindered.

7. Conclusions and further work In this paper a gas sensor device that consists on a suspended micro-hotplate for high temperature applications, which presents a very low-power consumption and low thermal inertia, has been studied. The device has been fabricated by a CMOS compatible process. Both the suspended plate and the four bridges are made of a layer of 300 nm of Si3 N4 , and the heater element and the interdigited electrodes are

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Table 1 New designs for the suspended micro-hotplate to be investigated Design

Layout

Silicon nitride

Silicon 1 ␮m

Silicon 5 ␮m

1

2

3

4

5

6

made of platinum. Thermal simulations (FEM) show that the device is thermally isolated and therefore it is suitable to be integrated together with other devices in order to be part or compose an array of micromachined devices. However, the sensor has some drawbacks related to its fragility. As it is shown in the simulations, the stress in the bridges was considerable and the vertical displacement of the plate was quite large. So, when the gas sensitive layer was deposited on top of the electrodes the small arms broke after some time without any electrical used of the device. Since the devices resisted all the experiments made without gas sensitive layer we can think in to alternatives to be studied in the near future: (1) The first one would be to change the material deposition method in order to obtain a thin film. The material deposition technique used in this work gives a layer that has a non-flat surface [28]. This fact together with the

large thickness of the drop enhances the probability of fracture of the dielectric bridges. Then, the use of other deposition techniques such as chemical vapour deposition which give a very thin film in the range of nanometres and with a flat surface would help to avoid the breakage of the arms of the structure. (2) The second alternative is to redesign the structure. That can be made in two ways: or changing the design of the arms or instead of Si3 N4 using another material for the bridges and the hot plate. Both alternatives have been tested giving the results shown in Table 1. Microhotplates with different shapes in the arms to avoid the accumulated mechanical stress where fabricated. The materials used were Si3 N4 and silicon with two different thicknesses: 1 and 5 ␮m. The designs numbered 1 and 3 present four arms. The ones numbered 2, 4 and 5 have two arms and number 6 one arm. The fabrication of the Si3 N4 -based structures was not very successful

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compared to the fabrication of the silicon-based structures as it can be seen in the pictures. This may be due to the fact that the Si3 N4 is a deposited material what involves and intrinsic mechanical stress, while the silicon is a growth material. This stress entails a bending of the hot plate. The structures made with silicon, which are very robust, have the very important drawback: high temperature cross-talk. So they cannot be integrated with other devices. Due to the fragility of the sensor studied in this paper, making gas test has been a real challenge. As said before, most of the structures broke after depositing the sensitive layer. The ones that survive, once they were used as a gas sensor, they last for very few tests. However, in this report we could show that BaSnO3 exhibits a bulk-controlled gas sensing mechanism at temperatures as high as 700 ◦ C, where the response to CO is almost negligible compared to the response towards oxygen.

Acknowledgements The Spanish FEDER program 2FD97-1804-C03-01 and the Spanish CICYT program MAT1999-0435-C02-01 financed the project.

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Biographies Dr. Judith Cerd`a Belmonte graduated in physics at the University of Barcelona in 1999 and she received her PhD in physics in the same university in 2003. At present, she is postdoctoral fellow at the Department of Materials of the Imperial College, London. Her current interests involve the study and synthesis of self-assembled monolayers on nanoparticles for electronic devices and medical applications. Dr. Jordi Puigcorb´e graduated in physics from the University of Barcelona in 1997 and he received his PhD in physics in the same university in 2003. His interests arc the thermomechanical study of different micromachined gas sensor substrates based on closed and suspended membrane combining coupled electro-thermo-mechanical threedimensional finite element method simulations with different experimental techniques such as those used in Microsystems characterization (thermoelectrical, thermography, AFM, XRD, confocal microscopy, Auger..). Dr. Jordi Arbiol graduated in physics from the University of Barcelona in 1997, received his European PhD in physics in 2001, and obtained the PhD Extraordinary Award of the Electronics Department. He joined the Electronics Department in 1997, and in 2000 he was appointed as Assistant Professor in this department. His current research activities are centered in the structural, compositional and morphological characterization of nanosized materials and devices by means of TEM related techniques (HRTEM, EELS, EFTEM, Z-contrast, Electron Tomography,...).

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Dr. Ana Vil`a graduated in physics in 1989 from the University of Barcelona (Spain). She worked on the structural characterization of semiconducting materials, particularly using TEM and HREM techniques, and obtained her PhD in 1995 from the same university. Afterwards, her work centered on sensors and hybrid technology, and now is specially involved in the simulation, fabrication, characterization and testing of gas sensors. She has been full-time professor at the university since June 2001. Prof. Dr. Joan R. Morante was born in Mataro (Spain). He obtained the Bachelor degree in 1977 from the University of Barcelona. In 1980, he received the PhD degree in Physics from the same university. Since 1986 he is full professor of Electronics and director of the Electronic Materials and Engineering group, EME. Now, he is the head of the EME (Electronic Materials and Engineering) research group, co-director of the CEMIC (Centre for Engineering of Microsystems) from 1999 and director of the CeRMAE (Centre of reference of Advanced Materials for Energy) from 2003. His activity is devoted to the electronic materials and technology, physics and chemical sensors, actuators, and microsystems. He has special interest in nanoscience and nanotechnologies. Dr. Neus Sabat´e received her BSc degree on physics from Barcelona University (Spain) in 1998. In 1999 she joined the Microsystems Department of Centro Nacional de Microelectronica in Barcelona and she obtained her PhD in physics in 2003, working on the development of gas and flow sensor devices and microsystem. In 2004 she joined the Electronics Department of the University of Barcelona to work in MEMS applications for the gas sensing field. She is currently working in MEMS reliability issues and particularly in stress investigations at the Micro Materials Center at IZM Fraunhofer in Berlin. Dr. Isabel Gr`acia joined the National Microelectronic Center (CNM) in 1986 working on potolitography. In 1993, she received the PhD degree in physics from Autonomous University of Barcelona (UAB), Spain, working on chemical sensors, that is also her current research field. Dr. Carles Can´e is Telecommunication Engineer since 1985 and he got the PhD in 1989. Since 1990 he is full time senior researcher at the National Microelectronics Centre in Spain CNM-CSIC and has been working in the development of CMOS technologies and also on mechanical and chemical sensors, MEMs and microsystems. His current expertise is in mechanical and chemical sensors and its integration with CMOS electronics.