Massimiliano Decarli

Batch fabrication of metal oxide sensors on micro-hotplates

We report the parallel fabrication of miniaturized chemical sensors by the direct integration of nanostructured transition metal oxide films onto micro-hotplate platforms based on micromachined suspended membranes. This has been achieved by local deposition on a 10 × 10 membrane wafer of a supersonic cluster beam through a microfabricated auto-aligning silicon shadow mask. The sensing properties of the obtained devices were tested with respect to various gaseous species. For reducing and oxidizing species such as ethanol and NO2, very good performance in terms of linearity and sensitivity was observed. These results demonstrate the feasibility of the coupling of a bottom-up nanofabrication technique such as supersonic cluster beam deposition to a top-down microfabricated platform for a direct and parallel integration methodology of nanomaterials in MEMS.

A low-cost microsystem for noninvasive uroflowmetry

A microsystem composed of a micromachined resistive flow sensor and a signal conditioning CMOS IC is proposed for biomedical applications. The device can be adapted to noninvasively monitor urinary dysfunctions in male patients. The flow sensor, thermally simulated with ANSYS, is based on the hot-film principle: A thin film of gold laid on a suspended micromachined silicon membrane is heated while the fluid under test flows through the duct mounted above the membrane. The flow rate is sensed by measuring the temperature difference between two of the four polysilicon temperature sensors realized on the membrane. Simulations of the flow sensor with flow rates within 0.1-18 ml/s evidence a maximum temperature difference of 20degC between the temperature sensors. Characterization of the fabricated flow sensor shows temperature coefficient of resistance (TCR) values of -1930 ppm/degC for the polysilicon resistors, i.e., a resistance variation of about 4% at high flow rates. The CMOS readout designed for the flow sensor is a resistive bridge-to-duty cycle converter based on a relaxation oscillator. The digital output of the circuit is duty-cycle modulated by the change in resistance of the flow sensors elements. Experimental tests on the CMOS interface, conducted with a setup of 1% precision resistors, report a maximum nonlinearity below 0.9% and a resolution of 7 bits over the full range of 4% resistance variation. The CMOS integrated readout circuit, provided with a digital output, allows simple signal interfacing towards any standard PC for periodical data transfer and storage

Long-term outdoor reliability assessment of a wireless unit for air-quality monitoring based on nanostructured films integrated on micromachined platforms.

We have fabricated and tested in long-term field operating conditions a wireless unit for outdoor air quality monitoring. The unit is equipped with two multiparametric sensors, one miniaturized thermo-hygrometer, front-end analogical and digital electronics, and an IEEE 802.15.4 based module for wireless data transmission. Micromachined platforms were functionalized with nanoporous metal-oxides to obtain multiparametric sensors, hosting gas-sensitive, anemometric and temperature transducers. Nanoporous metal-oxide layer was directly deposited on gas sensing regions of micromachined platform batches by hard-mask patterned supersonic cluster beam deposition. An outdoor, roadside experiment was arranged in downtown Milan (Italy), where one wireless sensing unit was continuously operated side by side with standard gas chromatographic instrumentation for air quality measurements. By means of a router PC, data from sensing unit and other instrumentation were collected, merged, and sent to a remote data storage server, through an UMTS device. The whole-system robustness as well as sensor dataset characteristics were continuously characterized over a run-time period of 18 months.

Influence of Etching Potential on Convex Corner Anisotropic Etching in TMAH Solution

Anisotropic etching with tetramethylammonium hydroxide (TMAH) water solutions is a simple and CMOS-compatible way to obtain geometrical patterns in single-crystal silicon wafers. The fabrication of trenches and other features is although limited by the need to compensate convex corners which tend to be etched very fast. Such compensation produces footings at the bottom edge of the etched walls, yielding a complex and scarcely predictable geometry which might affect the performance of devices in applications such as fluidics. The etch rates for different crystal planes are affected not only by the TMAH concentration and etching temperature but also by the etching reaction potential. In this work, shallow TMAH etching of single-crystal silicon wafers in 25-wt% TMAH at 90°C is examined up to a depth of 30 m, at potentials ranging from -1 to -2 V, using etching masks to obtain compensated convex corners. Identification of the sidewalls as {311} planes is performed by angle measurement on SEM and optical images. The etch ratio of the (100) crystal plane versus both (311) and (111) planes is measured at the various potentials. Morphological differences between cathodic and anodic potentials with respect to the open-circuit potential (OCP) are examined: Cathodic etching (between OCP and -2 V) yields footing patterns and a pronounced undercut, while anodic etching (between -1 V and OCP) produces smooth sidewalls with no footing, an increased (100)/(311) etch ratio, and a decreased (100)/(111) etch ratio.

Developing a genomic-based point-of-care diagnostic system for rheumatoid arthritis and multiple sclerosis

In this paper the methodology of designing a genomic-based point-of-care diagnostic system composed of a microfluidic Lab-On-Chip, algorithms for microarray image information extraction and knowledge modeling of clinico-genomic patient data is presented. The data are processed by genome wide association studies for two complex diseases: rheumatoid arthritis and multiple sclerosis. Respecting current technological limitations of autonomous molecular-based Lab-On-Chip systems the approach proposed in this work aims to enhance the diagnostic accuracy of the miniaturized LOC system. By providing a decision support system based on the data mining technologies, a robust portable integrated point-of-care diagnostic assay will be implemented. Initially, the gene discovery process is described followed by the detection of the most informative SNPs associated with the diseases. The clinical data and the selected associated SNPs are modeled using data mining techniques to allow the knowledge modeling framework to provide the diagnosis for new patients performing the point-of-care examination. The microfluidic LOC device supplies the diagnostic component of the platform with a set of SNPs associated with the diseases and the ruled-based decision support system combines this genomic information with the clinical data of the patient to outcome the final diagnostic result.

A silicon micromachined alcoholometer

This research work was aimed at finding an innovative process based on MEMS technologies to measure the alcoholic strength of hydro-alcoholic solutions. A new Si micromachined alcoholometer was developed. A microhotplate, based on a dielectric thin membrane, is used to heat and measure the temperature of droplets of hydro-alcoholic solution dispensed by a tiny capillary during an evaporation cycle. It was found that the alcoholic strength is correlated with the integration of the temperature of the droplet over time during its evaporation. Opposed to the old measurement methods, this new procedure takes advantage of many properties of the hydro-alcoholic solutions such as: superficial tension, latent heat of evaporation, boiling point and heat capacity. All these issues contribute together to give a good response in terms of good resolution and accuracy over a wide range of alcoholic degree.