Csaba Dücsö

Deposition of Tin Oxide into Porous Silicon by Atomic Layer Epitaxy

The deposition of conformal coatings into porous silicon layers was successfully demonstrated. Tin oxide films were formed from SnCl 4 and H 2 O precursors by atomic layer epitaxy. The influence of the porous substrate structure on the deposition parameters was analyzed from the viewpoint of formation mechanism, growth rate, and layer composition. The SnO x covered porous substrates were characterized by means of Rutherford backscattering, secondary ion mass spectrometry, cross-sectional transmission electron microscopy, and ellipsometry. The mesoporous structure of the Si substrate uniquely determines the gas-phase diffusion and physisorption of the precursors. The processing parameters favoring chemisorption are more critical for porous silicon than those for a flat surface. Even a small decrease in the deposition temperature results in a considerable increase in the growth rate through gas-phase reactions, and the process becomes chemical vapor deposition-like. Conformal step coverage was obtained on extremely high (140 :1) aspect ratio pores if the deposition conditions were chosen such that chemisorption was the growth rate determining step in the process.

Characterization of an Integrable Single-Crystalline 3-D Tactile Sensor

Porous-Si-micromachining technique was used for the formation of single-crystalline force-sensor elements, capable of resolving the three vector components of the loading force. Similar structures presented so far are created from deposited polycrystalline Si resistors embedded in multilayered SiO2/Si3N4 membranes, using surface micromachining technique for a cavity formation. In this paper, the authors implanted four piezoresistors in an n-type-perforated membrane, having their reference pairs on the substrate in order to form four half bridges for the transduction of the mechanical stress. They successfully combined the HF-based porous-Si process with conventional doping and Al metallization, thereby offering the possibility of integration with readout and amplifying electronics. The 300times300 mum2 membrane size allows for the formation of large tactile arrays using single-crystalline-sensing elements of superior mechanical properties. They used the finite-element method for modeling the stress distribution in the sensor, and verified the results with real measurements. Finally, they covered the sensors with different elastic silicon-rubber layers, and measured the sensors altered properties. They used continuum mechanics to describe the behavior of the rubber layer

Three dimensional single crystalline force sensor by porous Si micromachining

A porous Si micromachining technique was used for the formation of single crystalline force sensor elements, capable of resolving the three vectorial components of the load. Similar structures presented so far, are formed from deposited polycrystalline Si resistors embedded in multilayered SiO/sub 2//Si/sub 3/N/sub 4/ membranes, using a surface micromachining technique for cavity formation. In the present work, in the n-type perforated membrane, four implanted piezoresistors were fabricated with their reference pairs on the substrate, in order to form 4 half-bridges for the transduction of the mechanical stress. The HF based porous Si process was successfully combined with conventional doping and Al metallization, thereby offering a possible integration of read-out and amplifying electronics. The 300/spl times/300 /spl mu/m/sup 2/ membrane size allows the formation of large area arrays for tactile sensing using single crystalline sensing elements of superior mechanical properties.

Determination of deformation induced by thin film residual stress in structures of millimeter size

The deformation of Si/SiN x based square membrane structures are measured by Makyoh topography. The results are compared to finite-element modelling of the structure, and the thermal expansion coefficient of SiN x is extracted.

A novel micro-structured reference material for microbeam analysis

Abstract In order to determine the beam spot size and the linear or raster scanning properties of microprobe analytical systems, a novel certified reference material (CRM) has been developed by IRMM, consisting of permalloy (81% Ni, 19% Fe) strip patterns of different widths on a silicon substrate. The general layout of this micro-structured reference material with pattern sizes ranging from 2 to 100 μm, fabricated with production schemes of microelectronics circuitry on silicon wafers, is discussed. The large size range of the individual pattern structures makes the material equally applicable to very fine and less focused microbeams. Several long distances between characteristic patterns as well as broad line widths of selected structures are certified for each individual chip. First chips of this material were investigated with high-energy ion microprobes as well as with X-ray microprobes with capillary optics. Due to the very good definition of the metal lines and their edge profiles, line scan results of XRF, PIXE or RBS can be directly converted to spot size and microbeam profile. A special set of micro-structures on the CRM chips allows to obtain quantitative information about the “skirt” of microbeams.

3D force sensors for laparoscopic surgery tool

3D force sensors were developed to further integration in laparoscopic heads of surgery robots. The Si sensors operate with piezoresistive transduction principle by measuring the stress induced signals of the symmetrically arranged four piezoresistors in a deforming membrane. As the chip size has to be reduced to a few mm2, the conventional anisotropic alkaline etching technique was replaced by deep reactive ion etching (DRIE) for membrane formation. Moreover, DRIE enables to form any geometry of the membrane and offers the formation of monolith force transfer rod protruding over the chip surface. This rod increases shear sensitivity of the structure, thereby plays crucial role in tactile sensing. The technology applies SOI (silicon on insulator) wafers of appropriate device layer thickness, which provide highly uniform membranes and reproducibility of the process. According to the medical and functional requirements the sensors must be covered by biocompatible and sterilisable elastic polymers. As the elastomer drastically effect on the performance of the device, the proposed sensor structures were modelled by coupled finite element simulation to determine the appropriate geometric parameters meet the functional requirements. Sensors were covered with spherically shaped PDMS (polydimethylsiloxane) polymer and the effect of the elastic coating was also studied in terms of sensitivity and response time. Finally, the design of the laparoscopic head with the integrated 3D MEMS force sensors is also demonstrated.

Optimisation of low dissipation micro-hotplates - Thermo-mechanical design and characterisation

This work describes the results of a systematic investigation of micro-hotplates on thin isolating membranes capable of operation up to 600 °C both in static and dynamic mode. For the selection of optimum device geometry and the 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 quantified by Makyoh-topography. Pulsed mode cyclic heating revealed the dynamic properties and also served for accelerated stability tests. For demonstration microheater devices with heat dissipation up to 23 °C/mW and t90 <; 3ms were constructed to form the basis of combustive type gas sensors operated at elevated temperature.

Heavy water in gate stack processing

The high reactivity of the free silicon surface and its consequence: the “omnipresent” native silicon dioxide hinders the interface engineering in many processing steps of IC technology on atomic level. Methods known to eliminate the native oxide need in most cases vacuum processing. They frequently deteriorate the atomic flatness of the silicon. Hydrogen passivation by a proper DHF (diluted HF) treatment removes the native silicon oxide without roughening the surface while simultaneously maintains a “quasi oxide free” surface in a neutral or vacuum ambient for short time. Under such circumstances the last thermal desorption peak of hydrogen is activated at around 480-500°C where the free silicon surface suddenly becomes extremely reactive. In this study we show that deuterium passivation is a promising technology. Due to the fact that deuterium adsorbs more strongly on Si surface than hydrogen even at room temperature, deuterium passivation does not need vacuum processing and it ensures a robust process flow.

A Novel Micro-Structured Reference Material for Ion and X-Ray Microbeam Analysis

Abstract. In order to determine the beam spot size and the linear or raster scanning properties of microprobe analytical systems, a novel reference material has been developed by IRMM, consisting of permalloy (81% Ni, 19% Fe) strip patterns of different widths on a silicon substrate. The general layout of this micro-structured reference material with pattern sizes ranging from 2 to 100 μm, fabricated with production schemes of microelectronics circuitry on silicon wafers, is discussed. The large size range of the individual pattern structures makes the material equally applicable to very fine and less focused microbeams. Due to the choice of substrate and pattern materials, these samples exhibit a high elemental contrast suitable for analysis with X-ray and electron detection and with ion scattering techniques. First chips of this material were investigated with both X-ray tube based as well as synchrotron radiation based X-ray microprobes with capillary optics. Due to the very good definition of the metal lines and their edge profiles (0.5 μm high, shape irregularities and undulations < 0.1 μm) line scan results of for example XRF can be directly converted to spot size and microbeam profile.

Carbon Nanotubes - Towards Artificial Nose Implementation

Functionalization of multiwall carbon nanotubes (MWCNTs) allows the outer wall of the nanotubes to be turned into chemically selective surfaces. As in the electric transport along the nanotube axis the outer wall plays a dominant role, the modulation of this transport lends itself to direct application in gas sensing. In our work the modification of electrical conduction was used as a detection principle in random networks (mats) of MWCNT. In comparison with conventional adsorption type (e.g. Taguchi) sensors these MWCNT networks exhibited very fast response times (<10 s) to all gases and vapors applied, and showed acceptable long-term stability.