L. Fonseca

Soft breakdown conduction in ultrathin (3-5 nm) gate dielectrics

Prior to any attempt to model a charge transport mechanism, a precise knowledge of the parameters on which the current depends is essential. In this work, the soft breakdown (SBD) failure mode of ultrathin (3-5 nm) SiO/sub 2/ layers in polysilicon-oxide-semiconductor structures is investigated. This conduction regime is characterized by a large leakage current and by multilevel current fluctuations, both at low applied voltages. In order to obtain a general picture of SBD, room-temperature current-voltage (I-V) measurements have been performed on samples with different gate areas, oxide thicknesses and substrate types. An astounding matching between some of these I-V characteristics has been found. The obtained results and the comparison with the final breakdown regime suggest that the current flow through a SBD spot is largely influenced by its atomic-scale dimensions as occurs in a point contact configuration. Experimental data are also presented which demonstrate that specific current fluctuations can be ascribed to a blocking behavior of unstable SBD conduction channels.

Development of a CMOS-compatible PCR chip: comparison of design and system strategies

In the last decade research in chips for DNA amplification through the polymerase chain reaction (PCR) has been relatively abundant, but has taken very diverse approaches, leaving little common ground for a straightforward comparison of results. Here we report the development of a line of PCR chips that is fully compatible with complementary-metal-oxide-semiconductor (CMOS) technology and its revealing use as a general platform to test and compare a wide range of experimental parameters involved in PCR-chip design and operation. Peltier-heated and polysilicon thin-film driven PCR chips have been produced and directly compared in terms of efficiency, speed and power consumption, showing that thin-film systems run faster and more efficiently than Peltier-based ones, but yield inferior PCR products. Serpentine-like chamber designs have also been compared with standard rectangular designs and with the here reported rhomboidal chamber shape, showing that serpentine-like chambers do not have detrimental effects in PCR efficiency when using non-flow-through schemes, and that chamber design has a strong impact on sample insertion/extraction yields. With an accurate temperature control (±0.2 °C) we have optimized reaction kinetics to yield sound PCR amplifications of 25 µl mixtures in 20 min and with 24.4 s cycle times, confirming that a titrated amount of bovine albumin serum (BSA, 2.5 µg µl−1) is essential to counteract polymerase adsorption at chip walls. The reported use of a CMOS-compatible technological process paves the way for an easy adaption to foundry requirements and for a scalable integration of electro-optic detection and control circuitry.

Detection of low NO2 concentrations with low power micromachined tin oxide gas sensors

Abstract Semiconductor gas sensors integrated on silicon substrates with thermally isolated structures are presented and technological processing steps of their fabrication are described. Tin oxide sensitive layers have been deposited by reactive sputtering technique due to the compatibility with IC fabrication. The active area has a size of 500×500 μm2 and is supported by a membrane of silicon nitride. Polysilicon is used as heating material and the power consumption is below 50 mW at the operating temperature of 350°C for every sensor prepared. Good isolation among chip devices was guaranteed from FEM thermal simulations [A. Gotz, I. Gracia, C. Cane, E. Lora-Tamayo, M.C. Horrillo, J. Getino, C. Garcia, J. Gutierrez, A micromachined solid state integrated gas sensor for the detection of aromatic hydrocarbons, Sensors and Actuators B 44 (1997) 483–487.]. Very low concentrations of NO2 have been detected with such type of device obtaining good sensitivity and short response time for various thin-film thicknesses of tin oxide.

Detection of gases with arrays of micromachined tin oxide gas sensors

Abstract A good detection of NO 2 , CO and toluene at low concentrations has been carried out by using a micromachined gas sensor array composed of three devices working at different temperatures. The structure is fabricated using standard microelectronic technologies and tin oxide layers as sensitive material. The total power consumption of the array is in the range of 150 mW and a good uniformity of temperature is achieved, thanks to a silicon plug placed under the active area of each sensor. With this device type, it is possible to discriminate gases in a mixture when each array microsensor is heated at a proper temperature.

Full ceramic micro solid oxide fuel cells: towards more reliable MEMS power generators operating at high temperatures

Batteries, with a limited capacity, have dominated the power supply of portable devices for decades. Recently, the emergence of new types of highly efficient miniaturized power generators like micro fuel cells has opened up alternatives for continuous operation on the basis of unlimited fuel feeding. This work addresses for the first time the development of a full ceramic micro solid oxide fuel cell fabricated in silicon technology. This full-ceramic device represents a new generation of miniaturized power generators able to operate at high temperatures, and therefore able to work with a hydrocarbon fuel supply. Dense yttria-stabilized zirconia free-standing large-area membranes on micromachined silicon were used as the electrolyte. Thin-film porous electrodes of La0.6Sr0.4CoO3−δ and gadolinia-doped ceria were employed as cathode and anode materials, respectively. The electrochemical performance of all the components was evaluated by partial characterization using symmetrical cells, yielding excellent performance for the electrolyte (area specific resistance of 0.15 Ω cm2 at temperatures as low as 450 °C) and the electrodes (area specific resistance of the cathode and anode below 0.3 Ω cm2 at 700 °C). A micro solid oxide fuel cell with an active area of 2 mm2 and less than 1 micrometer in thickness was characterized under fuel cell conditions, using hydrogen as a fuel and air as an oxidant. A maximum power density of 100 mW cm−2 and 2 mW per single membrane was generated at 750 °C, having an open circuit voltage of 1.05 V. Impedance spectroscopy of the all-ceramic membrane showed a total area-specific resistance of ∼3.5 Ω cm2.

Silicon nanowire arrays as thermoelectric material for a power microgenerator

A novel design of a silicon-based thermoelectric power microgenerator is presented in this work. Arrays of silicon nanowires, working as thermoelectric material, have been integrated in planar uni-leg thermocouple microstructures to convert waste heat into electrical energy. Homogeneous, uniformly dense, well-oriented and size-controlled arrays of silicon nanowires have been grown by chemical vapor deposition using the vapor–liquid–solid mechanism. Compatibility issues between the nanowire growth method and microfabrication techniques, such as electrical contact patterning, are discussed. Electrical measurements of the nanowire array electrical conductivity and the Seebeck voltage induced by a controlled thermal gradient or under harvesting operation mode have been carried out to demonstrate the feasibility of the microdevice. A resistance of 240 Ω at room temperature was measured for an array of silicon nanowires 10 µm -long, generating a Seebeck voltage of 80 mV under an imposed thermal gradient of 450 °C, whereas only 4.5 mV were generated under a harvesting operation mode. From the results presented, a Seebeck coefficient of about 150–190 µV K−1 was estimated, which corresponds to typical values for bulk silicon.

Assessment of the final metrological characteristics of a MOEMS based NDIR spectrometer through system modelling and data processing

A model of a miniaturised NDIR (Non Dispersive Infrared) gas analysis system, aiming to predict the final system specifications, is presented. It comprises the different elements of the NDIR detector, including a surface micromachined Fabry-Perot tuneable filter. These models have been used to estimate the response of the NDIR system to different gas mixtures. Multivariate regression methods like Partial Least Squares (PLS) allow the recovery of the true sample composition from the IR absorption spectra measured with the NDIR system, despite the limited selectivity of the filter. Combining model and data processing permits one to predict the effect on the final system specification of design parameters. Here we compare the effect of the technology used for the filter on the system errors.

Stress-profile characterization and test-structure analysis of single and double ion-implanted LPCVD polycrystalline silicon

Abstract A very low stress gradient across the polysilicon layers is required for the fabrication of large micromechanical structures based on surface-micromachining technologies. In this work the residual stress and the stress gradient of 2 μm thick LPCVD polysilicon layers are presented as a function of deposition, doping and annealing conditions. Low stress gradients are obtained by optimizing the doping profile using a two-step deposition and implantation process. The results obtained by mechanical test structures are corroborated by micro Raman measurements. The effects of the polysilicon stress gradient on surface-micromachined accelerometers are analysed. Polysilicon layers with low tensile stress and a stress gradient lower than 5 MPa μm −1 are required for the fabrication of surface-micromachined z -accelerometers.

Assessment of the final metrological characteristics of a MOEMS-based NDIR spectrometer through system modeling and data processing

A model of a miniaturized non-dispersive infrared (NDIR) gas analysis system, aiming to predict the final system specifications, is presented. It comprises the different elements of the NDIR detector, including a surface micromachined Fabry-Perot tunable filter. These models have been used to estimate the response of the NDIR system to different gas mixtures. Multivariate regression methods like partial least squares allow recovering the true sample composition from the IR absorption spectra measured with the NDIR system, despite the limited selectivity of the filter. Combining model and data processing permits to predict the effect on the final system specification of design parameters. Here, we compare the effect of the technology used for the filter on the system errors.

Towards a full integration of vertically aligned silicon nanowires in MEMS using silane as a precursor

Silicon nanowires present outstanding properties for electronics, energy, and environmental monitoring applications. However, their integration into microelectromechanical systems (MEMS) is a major issue so far due to low compatibility with mainstream technology, which complicates patterning and controlled morphology. This work addresses the growth of 〈111〉 aligned silicon nanowire arrays fully integrated into standard MEMS processing by means of the chemical vapor deposition-vapor liquid solid method (CVD-VLS) using silane as a precursor. A reinterpretation of the galvanic displacement method is presented for selectively depositing gold nanoparticles of controlled size and shape. Moreover, a comprehensive analysis of the effects of synthesis temperature and pressure on the growth rate and alignment of nanowires is presented for the most common silicon precursor, i.e., silane. Compared with previously reported protocols, the redefined galvanic displacement together with a silane-based CVD-VLS growth methodology provides a more standard and low-temperature (<650 °C) synthesis scheme and a compatible route to reliably grow Si nanowires in MEMS for advanced applications.