Vittorio Guarnieri

Microstructures etched in doped TMAH solutions

Tetra-methyl ammonium hydroxide, or TMAH, is an anisotropic silicon etchant that is gaining more and more attention in the fabrication process of mechanical microstructures and device isolation, as an alternative to the more usual KOH and EDP etchants [1]: because of its high compatibility with conventional IC processes, due to the absence of metal ions in it. The possibility to passivate the aluminum metalisation in properly saturated TMAH solution has also been demonstrated by doping the solution with appropriate amounts of silicon or silicic acid [2, 3]. This increases the range of application of these etchants, simplifying both the post processing and the etch set-up configuration. In this paper we present some different technological solutions adopted for the fabrication of 3D structures using as anisotropic etchant doped TMAH solutions. The effects of the additives on etch uniformity and surface roughness are studied. Using the etching results under different doping conditions of the TMAH solutions, we obtained silicon bulk-micromachined structures with controlled surface roughness. Furthermore the aluminium passivation permits to etch devices with no protection of the metalisation layers as some applications require (i.e. bolometers).

Sensors Based on Technology “Nano-on-Micro” for Wireless Instruments Preventing Ecological and Industrial Catastrophes

The problem of gas analyzers compatible with wireless networks can be solved by using sensors based on the “nano-on-micro” technology. The basis of this technology consists in nano-composite sensing metal oxide semiconductor or thermocatalytic materials deposited on a microhotplate fabricated using silicon or alumina microelectronic technology. As a result, the sensor combines the advantages of both technologies: on the one hand, high stability and sufficient selectivity of nano-composite materials, and, on the other hand, microprocessor compatibility, low-cost, mass-production possibilities, and low power consumption of microelectronic substrates. Two methods for the fabrication of microhotplates are the most promising: the silicon based technology of silicon oxide/silicon nitride membranes and the CeraMEMS technology of thin alumina films (TAF). The first technology enables the fabrication of microheaters with a power consumption around 20 mW for an operating temperature below 450°C. Advantages of CeraMEMS platforms are: (1) operation at temperature up to 600°C and, potentially, up to 800°C; (2) robustness compared with silicon chip with thin membrane; (3) perfect Pt and sensing layer adhesion without any adhesive layers; (4) low cost of middle scale production (104–107 chips per year) compared with the silicon technology. The CeraMEMS platform can be used for the fabrication of semiconductor and thermocatalytic gas sensors, as a source of IR radiation for optical gas sensors and as bolometers. The sensor withstands ∼7 × 106 on-off cycles. Heater resistance drift is below 3% per year at 550°C.

Platinum metallization for MEMS application: focus on coating adhesion for biomedical applications.

The adherence of Platinum thin film on Si/SiO2 wafer was studies using Chromium, Titanium or Alumina (Cr, Ti, Al2O3) as interlayer. The adhesion of Pt is a fundamental property in different areas, for example in MEMS devices, which operate at high temperature conditions, as well as in biomedical applications, where the problem of adhesion of a Pt film to the substrate is known as a major challenge in several industrial applications health and in biomedical devices, such as for example in the stents.1-4 We investigated the properties of Chromium, Titanium, and Alumina (Cr, Ti, and Al2O3) used as adhesion layers of Platinum (Pt) electrode. Thin films of Chromium, Titanium and Alumina were deposited on Silicon/Silicon dioxide (Si/SiO2) wafer by electron beam. We introduced Al2O3 as a new adhesion layer to test the behavior of the Pt film at higher temperature using a ceramic adhesion thin film. Electric behaviors were measured for different annealing temperatures to know the performance for Cr/Pt, Ti/Pt, and Al2O3/Pt metallic film in the gas sensor application. All these metal layers showed a good adhesion onto Si/SiO2 and also good Au wire bondability at room temperature, but for higher temperature than 400 °C the thin Cr/Pt and Ti/Pt films showed poor adhesion due to the atomic inter-diffusion between Platinum and the metal adhesion layers.5 The proposed Al2O3/Pt ceramic-metal layers confirmed a better adherence for the higher temperatures tested.The adherence of Platinum thin film on Si/SiO2 wafer was studies using Chromium, Titanium or Alumina (Cr, Ti, Al2O3) as interlayer. The adhesion of Pt is a fundamental property in different areas, for example in MEMS devices, which operate at high temperature conditions, as well as in biomedical applications, where the problem of adhesion of a Pt film to the substrate is known as a major challenge in several industrial applications health and in biomedical devices, such as for example in the stents. We investigated the properties of Chromium, Titanium, and Alumina (Cr, Ti, and Al2O3) used as adhesion layers of Platinum (Pt) electrode. Thin films of Chromium, Titanium and Alumina were deposited on Silicon/Silicon dioxide (Si/SiO2) wafer by electron beam. We introduced Al2O3 as a new adhesion layer to test the behavior of the Pt film at higher temperature using a ceramic adhesion thin film. Electric behaviors were measured for different annealing temperatures to know the performance for Cr/Pt, Ti/Pt, and Al2O3/Pt metallic film in the gas sensor application. All these metal layers showed a good adhesion onto Si/SiO2 and also good Au wire bondability at room temperature, but for higher temperature than 400 °C the thin Cr/Pt and Ti/Pt films showed poor adhesion due to the atomic inter-diffusion between Platinum and the metal adhesion layers. The proposed Al2O3/Pt ceramic-metal layers confirmed a better adherence for the higher temperatures tested.

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

A Cr-doped WO 3 sensor for chromatographic systems in wine quality applications

In this work, the performances of a Cr-doped WO3 sensor array are investigated for the application of this device as detector in gas chromatographic systems. Chromatographic separation modules combined with detectors based on semiconducting metal oxides enable the implementation of systems with both high selectivity and high sensitivity. Comparison of results of the sensor array and mass spectrometer detectors validates the use of this class of devices for low-cost systems for the determination of the volatile organic compounds contents in wines for quality assessment of alcoholic beverages.

Microhotplate-based silicon gas sensor arrays with linear temperature gradient for wine quality monitoring

In this work, we describe the design implementation, validated by experimental results, of an innovative gas sensor array for wine quality monitoring. The main innovation of this integrated array deals with the simultaneous outputs, from a single chip on TO-12 socket, of 8 different signals coming from a WO3 thin film structure heated in a linear temperature gradient mode, allowing an overall evaluation of gas sensing properties of the material in a 100°C-wide window, typically from 300 to 400°C. The implemented sensitive layer is a WO3 film deposed by RF-sputtering. Preliminary tests of gas sensing showed good responses to the target analytes for the specific application (1-heptanol, 3-methyl butanol, benzaldehyde and ethyl-hexanoate).

Optimization of TMAH etching for MEMS

Tetra-methyl ammonium hydroxide (TMAH) is an anisotropic silicon etchant that is gaining considerable use in silicon sensor micromachining due to its excellent compatibility with CMOS processing, selectivity, anisotropy and relatively low toxicity, as compared to the more used KOH and EDP etchants. In this paper, the influence of temperature and concentration of the TMAH solution together with oxidizer additions is studied in order to optimize the anisotropic silicon etching for MEMS fabrication. In particular this optimized etchant formulation has been employed at ITC-Irst in the development of a basic fabrication process for piezoresistive pressure sensors based on a silicon membrane and four resistors connected in a Weatherstone bridge configuration. The active element of the sensor, i.e. the thin silicon membrane, is formed by etching anisotropically from the backside of the wafer. Both process and etching have to be tuned and matched in order to obtain an optimum fabrication sequence. Some improvements such as higher etch rate and better surface finish have been obtained by the addition of ammonium peroxidsulfate as oxidizing agent under different conditions. This simplifies both the post processing and the tech set-up. The process parameters and the thermo-electro-mechanical characteristics of the pressure sensors were tested and are compared with the analytical and numerical simulations.

Monitoring System for Under-Water Pipe Line

The growing interest in new under-water pipe lines for the delivery of natural gas (in Baltic Sea, Mediterranean region, Caspian Sea, Black Sea, etc.) needs a new instruments for the monitoring of leakage and/or possible destruction of the pipes, which could not only disturb fragile ecological systems of Baltic and other seas, but also can lead to the technological catastrophes. We plan to develop a prototype of the system consisting of pervaporation membrane and gas sensors. Overall system will be immersed into water and will be fabricated in two versions: as a stationary instrument dipped into sea water and as an instrument towed along the pipe line. Micromachined metal oxide semiconductor gas sensors used for the detection of methane concentrations is designed and fabricated using “nano-on-micro” approach. Overall system is optimized from the point of view of minimum power consumption, which is necessary to assure its long term operation under autonomous conditions.

Development of silicon microheaters for chemoresistive gas sensors

We report on the design, fabrication, and characterization of a microheater module for chemoresistive, metal-oxide semiconductor gas sensors, consisting of a dielectric stacked membrane, micromachined from bulk silicon and with an embedded polysilicon resistor heater. Fabricated structures exhibit excellent heating efficiency, requiring only 30 mW to achieve a temperature of 500 C. Measured electrothermal characteristics are in good agreement with the outcomes of 3D numerical simulations.

Development of MEMS-based liquid chromatography modules for agrofood applications

This work presents the realization of a MEMS-based miniaturized system for liquid chromatography focused on agrofood applications, and in particular on the detection of wine defects. The main modules of the systems are: i.) a Si-based separation column with inlet/outlet for fluidic connections; ii.) a three-microelectrode voltammetric sensor. Moreover, a Platinum heater has been realized on the back side of the chip containing the Si column in order to operate at temperatures greater than the room temperature. The realized device consists of a Silicon/Pyrex structure realised by anodic bonding. Microchannels and inlet/outlet have been fabricated by Deep Reactive Ion Etching (DRIE) and Tetra Methyl Ammonium Hydroxide (TMAH) wet etching respectively. The column has been functionalised with n-octyltriethoxysilane (C8-TEOS). A lift-off technique has been developed for realizing the Pt heater and the Pt microelectrodes on-chip. In order to separately characterize the main modules of the device, a package of the system has been realized following a modular approach; appropriate tubing and nanovolume connections have been used in order to minimize dead volumes. Then other packages approaches have been considered in order to minimize dead volumes and to avoid leakage issues. Preliminary characterization tests of the two main modules have been performed. The capability of the system to correctly retain and detect Acetic acid has been tested.