Csaba Dücső

Thermo-mechanical design and characterization of low dissipation micro-hotplates operated above 500C

This work describes the results of a systematic investigation of micro-hotplates capable of operating up to 600?C both in static and dynamic modes. The goal of development is to form a reduced power consumption micro-pellistor for portable devices. For the selection of optimum device geometry and the membrane layer structure, alternatives FEM analysis was applied. The materials considered were Si3N4, SiO2, TiO2/Pt, Al2O3 and their combination in various multilayer structures. To reduce the chip size DRIE was selected for the release of the membrane. Experimental characterization of the hotplates was carried out by various techniques; the average hotplate temperature was deduced from the resistance of the applied Pt heater and verified by micro-melting point measurements. Buckling of the membranes was tested by means of optical methods and the cumulative stress of the multilayer structure was quantified by Makyoh-topography. Pulsed mode cyclic heating revealed the dynamic properties and also served for accelerated stability tests. For demonstration, micro-heaters with heat dissipation up to 23?C/mW and t90%<3ms were constructed. The hotplates were coated with Pt catalyst to form a combustive type gas sensor operated at elevated temperature.

Mats of Functionalized Carbon Nanotubes for Gas/Vapor Sensing

Gas sensing properties of different carbon nanotube (mostly multiwall, MWCNT) mats, based on electrical resistance measurement were investigated in a simple arrangement and found that the sensitivity for different gases or vapors highly depends on the pre-treatment and functionalization of nanotubes. The selectivity of the sensing was demonstrated by building a vapor recognition system based on an array of multitube sensors made of differently functionalized MWCNTs.

Microwave-CVD Diamond Protective Coating for 3D Structured Silicon Microsensors

In this work a multi-step nucleation-deposition MW-CVD process is pr e ented developed for contiguous covering of 3 dimensional Si bulk micromachined structures. A c orrelation between the nucleation rate and the electric field distribution over the 3D surf ace was established. The layer integrity was characterised by scanning electron and atomic force microsc opy. The pinhole density of the layers was measured by conventional alkaline underetching method. This techniq ue is primarily suitable for the characterisation of layers deposited on flat surfaces and was use d as a reference for the electrochemical method applied for the non-planar surfaces. Application of the protective c oating was demonstrated on the surface of a Si capacitive pressure sensor chip. Introduction Microsensors operate in harsh environment such as aggressive chemical s or abrasive media, require robust encapsulation in order to fulfil general reliability and lifetime requirements. Besides the extra costs involved, the required special packaging often limits sensor sensitivity, or compatibility with the sensing principle. The operation conditions of the device can be extended with a properly selected thin film coating, deposited directly on the exposed surface of the sensor-chip at a re sonable price. Diamond films exhibit unique mechanical, thermal, optical and electrical properties in combinat ion with excellent chemical stability. The main concern in their use as protective layer in sensors is the ne ed for the formation of pinhole-free surface coating on the typically 3 dimensional structures. The objective of the experiments is to define the technology of Micro-Wave Chemical Vapour Deposition (MW-CVD) for manufacturing graphite free diamond films on 3D surfaces with vertical feature size of hundred micrometers. The pinhole densi ty <1/cm should be realised for the effective protection of the surface of a 5 x 5 mm 2 capacitive pressure sensor chip, which was selected for the demonstration. The pressure sensor can be operated even in abrasive fluids or in chemically reactive environment with proper encapsulation of the diamond coated chip. The elaboration of the coati ng method opens the way towards other 3D structured sensor applications. Experimental The test structure The test structure is a capacitive pressure sensor chip formed by wafer bonding of two silicon elements. The two parts are electrically separated by a silicon-diox i e layer grown on the bottom Si substrate. The electrode on the rigid substrate can be formed either from Pt or polycrystalline silicon. This technique makes the high temperature diamond deposition process feasible. The fl exible membrane was formed by electrochemical etch stop (ECES) technique in KOH solution. Wafer and membrane thickness are 380 μm and up to 20 μm respectively. ( Fig. 1). Materials Science Forum Online: 2003-01-15 ISSN: 1662-9752, Vols. 414-415, pp 69-74 doi:10.4028/www.scientific.net/MSF.414-415.69

Electrical Characterization of Tungsten Nanowires Deposited by Focused Ion Beam (FIB)

The deposition of nanowires for interconnects in nanoelectronic devices werestudied morphologically by scanning electron microscopy (SEM), atomic force microscopy (AFM) and by in-situ resistance measurements. The deposition and basic characterization of nanometer size tungsten wires by gas injection (GIS) and focused ion beams (FIB) was carried out in-situ in a LEO 1540 XB workstation. The I(V) measurement showed that the deposited W wires have ohmic characteristic. The variation of the resistance during an ex-situ heating was linear with a low thermal coefficient (4% of the pure metallic W).

The critical impact of temperature gradients on Pt filament failure

Abstract This work established the correlation between the location of temperature gradients and the positions where breakdown is observed on different Pt filament layouts in cantilever and full membrane type micro-hotplates. Focusing on practical aspects like in real operation, self-heating was applied to investigate the limitations of high temperature application and to reveal the fatal failure mechanisms. Besides electromigration, another phenomenon playing dominant role in the breakdown of the filaments, the temperature gradient driven thermomigration of Pt was identified. This limits the local allowable temperature gradient to

The Role of Phase Changes in TiO 2 /Pt/TiO 2 Filaments

This work analyses the role of phase changes in TiO2/Pt/TiO2 layer stacks for micro-heater application regarding their stability and reliable operation. The polycrystalline Pt layer wrapped in a TiO2 adhesion layer underwent a continuous recrystallisation in a self-heating operation causing a drift in the resistance (R) versus temperature (T) performance. Simultaneously, the TiO2 adhesion layer also deteriorates at high temperature by phase changes from amorphous to anatase and rutile crystallite formation, which not only influences the Pt diffusion in different migration phenomena, but also reduces the cross section of the Pt heater wire. Thorough scanning electron microscopy, energy dispersive spectroscopy, cross-sectional transmission electron microscopy (XTEM) and electron beam diffraction analysis of the structures operated at increasing temperature revealed the elemental structural processes leading to the instabilities and the accelerated degradation, resulting in rapid breakdown of the heater wire. Owing to stability and reliability criteria, the conditions for safe operation of these layer structures could be determined.