István Bársony

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.

Porous silicon bulk micromachining for thermally isolated membrane formation

A novel low thermal budget technique is proposed for the preparation of thermally isolated silicon membranes. The selective formation of porous silicon in a p-type silicon wafer results in an undercut profile below the implanted n-type silicon regions. The sacrificial porous layer is subsequently removed in a dilute KOH solution. A non-stoichiometric LPCVD nitride layer combination forms the suspension of the single-crystalline silicon membranes. This technique eliminates the need for epitaxial substrates and backside alignment, and proves to be very efficient in the realization of a high-temperature micro-hotplate operating with minimum power consumption for the purpose of integrated gas sensors.

Characterization of different porous silicon structures by spectroscopic ellipsometry

Abstract The results of multiparameter fitting of spectroscopic ellipsometric (SE) spectra on porous silicon layers (PSL) were connected with the processing parameters (oxidation, etching time, porosity, argon implantation dose). Two optically different types of silicon forms, a bulk-type silicon (c-Si) and polycrystalline-like silicon with enhanced absorption in the grain boundaries (p-Si) needed to be mixed with voids in the appropriate ratio, and the PSL had to be divided in depth in several different sections in order to obtain the best fit. The sectioning reflects the effect of upper and lower interfaces or inhomogeneity in depth. The effective porosity and the sublayer thicknesses are determined with high precision. In the case of argon implantation, we used the dielectric function of c-Si and implanted amorphous silicon (a-Si) mixed with voids. The regression analysis of SE spectra clearly shows the effect of different doses of implantation. The sectioning reflects that to the full range of argon ions the porous silicon became amorphous and denser. The overall thickness of the originally porous layer also significantly reduced (from 670 nm to 320 nm) due to the argon implantation.

Materials and processing for realization of micro-hotplates operated at elevated temperature

Micro-pellistors and conductivity-type gas sensors are among the most promising candidates for the monolith integration of gas-sensing elements in olfactory gas detection. Both types have to be operated at elevated temperatures of 200–600 °C; therefore, the formation of thermally isolated integral micro-hotplates is the key element in the development of the array. In this paper, the alternative processes are discussed with emphasis on thermal isolation, selection of the appropriate structural materials, formation of stable contacts to the filaments and deposition of gas-sensitive layers. The results of thermal and mechanical model calculations were confirmed by experimental measurements.

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

Large area self-assembled masking for photonic applications

Ordered porous structures for photonic application were fabricated on p- and n-type silicon by means of masking against ion implantation with Langmuir-Blodgett (LB) films. LB films from Stober silica spheres [J. Colloid Interface Sci. 26, 62 (1968)] of 350nm diameter were applied in the boron and phosphorus ion-implantation step, thereby offering a laterally periodic doping pattern. Ordered porous silicon structures were obtained after performing an anodic etch and were then removed by alkaline etching resulting in the required two-dimensional photonic arrangement. The LB silica masks and the resulting silicon structures were studied by field emission scanning electron microscope analysis.

Thermal investigation of micro-filament heaters

The present work is a study of the thermal properties of integrable micro-filaments. The realised structure is the key element of the targeted integrable micro-pellistor for catalytic gas sensing, therefore, its thermal characterisation is of crucial importance. Direct measurement of temperature in the micro-heater is unfortunately troublesome due to the small dimensions used. In this work the temperature of the micro-filament was measured by two different methods, based upon the resistance alteration of the filament, and analysis of the thermal radiation of the heated micro-filament. The reason for the differences was explained by inherent features of the methods, and confirmed by thermal simulation.

Thermopile antennas for detection of millimeter waves

A thermopile structure is proposed for the detection of microwave/millimeter wave radiation. The thermopairs in the suggested linear arrangement function as antennas. 5.58 V/W responsivity was achieved at 100 GHz with 40 serial connected thermopairs. The experimentally observed polarity and frequency dependence convincingly verify the proper detector operation.

Investigation of electrical properties of Au porous Si Si structures

Results of capacitance-voltage (C-V) and capacitance-time measurements performed on Au/porous silicon (PS)/Si diodes having different porosity and morphology of the obtained porous layer are presented. The C-V characteristics of the studied Au/PS/Si diodes are similar to those of poor metal-insulator-semiconductor capacitors. The porosity dependence of the «insulator» capacitance is interpreted by a model assuming two parallel phases. The results obtained indicate that the C-V measurements may be useful for checking (i) the long-term stability of the metal/PS/Si junctions, (ii) the lateral homogeneity of the wafers, and (iii) the thickness and porosity of the porous layer

Efficient catalytic combustion in integrated micropellistors

This paper analyses two of the key issues of the development of catalytic combustion-type sensors: the selection and production of active catalytic particles on the micropellistor surface as well as the realization of a reliable thermal conduction between heater element and catalytic surface, for the sensing of temperature increase produced by the combustion. The report also demonstrates that chemical sensor product development by a MEMS process is a continuous struggle for elimination of all uncertainties influencing reliability and sensitivity of the final product.