D. Randjelovic

Adsorption–desorption noise in micromechanical resonant structures

Abstract In this paper we analyze adsorption–desorption (AD) noise in resonant microelectromechanical structures. This noise is generated by instantaneous differences in the rates of adsorption and desorption of contaminant molecules to and from the resonator surface, which cause mass fluctuations and consequentially frequency fluctuations. Contrary to the usual statistical approach, we used the analogy between AD processes in resonant structures and generation–recombination processes in semiconductors. Starting from that general analogy and using the theory of Langmuir’s isotherm, since it is the most convenient for this case, we derived an exact expression for the AD fluctuations-induced phase noise for the case of one adsorption layer. Our numerical results show that AD induced phase noise is comparable to other noise sources and becomes dominant for very small resonator beam dimensions. The presented theory is applicable to the determination of the limiting performance of various micromechanical resonators and microcantilever sensors and also for the theoretical studying of AD processes in general. In addition, phase noise measurements and their comparison with the theory are convenient for the investigation of catalytic AD processes.

Vacuum MEMS sensor based on thermopiles - simple model and experimental results

This paper presents a simple model of MEMS thermopile based vacuum detector and experimental results obtained for sensors designed and fabricated at IHTM-IMTM. Sensors contain two thermopiles, each with 30 multilayer p+Si/Al thermocouples and p+Si or Al heater. Thermal isolating membrane has sandwich structure consisting of sputtered SiO2 and residual n-Si layer. Dependence of output thermopile voltage on pressure was measured for sensors with different membrane thickness. Tests were performed in pressure range (10-3-105) Pa. Experimental results were compared with theoretical predictions.

Silicon UV flame detector utilizing photonic crystals

In this paper we propose a silicon UV flame detector for combustion systems. In gas burners the relative intensity of flame radiation is dominant in the UV region. In the visible and IR regions the relative intensity of radiation of the incandescent surfaces is several orders of magnitude greater than the gas flame radiation intensity. Therefore it is required that the flame detector has a much greater sensitivity in the UV region. The propose detector is formed on n-type silicon on isolator wafer. In order to suppress sensitivity in the visible and the IR regions, the absorption region of the detector is greatly reduced, and a UV filter utilizing photonic crystal is designed. The p-n junctions are formed by very shallow diffusion of impurities. The contacts are made after the deposition of a thin oxide layer. The UV filter is then sputtered on the detector surface. The filter consists of a thin silver film, and a 1D photonic crystal made of twelve pairs of NaF/Y2O3 layers. The photonic band gaps of the crystal should suppress the propagation of the light with wavelengths greater than 0.35 micrometers . For the detector active area of 5 mm2, the thickness of the silver layer of 0.13 micrometers and a dark current of 1 nA, the noise equivalent power at 0.32 micrometers is 4.23 10-13 W/Hz1/2. The calculated flame signal to total signal ratio is 0.52.

Enhancement of radiative lifetime in semiconductors using photonic crystals

Abstract In this work we present a structure consisting of a semiconductor sample immersed between two one-dimensional photonic crystals, designed with the aim to investigate the possibility to increase radiative lifetime in infrared photodetectors, i.e., to decrease the background radiation limit. Since we used a HgCdTe detector for the detection of CO2 laser radiation, we formed a defect in the front-side photonic crystal with the transmission peak on 10.6 μm. Our numerical calculations show that the 1-D photonic crystal structures at the front and at the back side of the semiconductor sample significantly increase the radiative lifetime in the photodetector.

Integrated multicolor detector utilizing 1D photonic bandgap filter with wedge-shaped defect

We propose a single-chip multicolor photodetector for micrometers range based on a linear IR semiconductor detector array with an integrated 1D photonic bandgap (PBG) filter. A wedge- shaped defect slab is introduced into the filer instead of one of the layers. The bandgap of the photonic crystal coincides with the spectral sensitivity range of the photodetector array, while the built-in defect gives a transmission peak within the same range. The defect thickness varies along the array length and thus shifts the transmission peak wavelength. The optimized photonic bandgap filter including defect is designed using the transfer matrix method. The peak frequency is tuned by choosing the geometrical parameters of the wedge-shaped defect. In our experiments, thin alternating Si and SiO2 films are sputtered onto the array surface, thus forming a 1D PBG structure. The defect is fabricated by gradually changing the middle Si layer thickness over the width of the array. Its wedge-forming is performed by micromachining or, alternatively, by in-situ oblique deposition within the sputtering system and, possibly, subsequent chemomechanical polishing. The characteristics of the finished PBG structure are measured using an IR spectrophotometer. An increase of the number of PBG layers improves the confinement of transmission peaks and thus decreases the crosstalk between the array elements. Although our multicolor detector is designed for the (3-5) micrometers atmospheric window, it can be straightforward redesigned for any other optical range.

Transport Parameters of Inkjet Printed Nanoparticle Silver on Polyimide Substrate Measured at Room and Liquid Nitrogen Temperatures

This brief presents the transport parameters of silver layers on a polyimide substrate fabricated by an inkjet printing technology using nanoparticle inks with 20 wt% and 40 wt% silver. All the electrical characteristics of the samples were measured by Hall effect measurement system at 0.37 T. The electron mobility in inkjet printed silver layer was 7.07 and 4.58 cm²/V s for 20 wt% silver, 29.6 and 21.3 cm²/V s for 40 wt% silver at 77 and 300 K, respectively. The resistivity in inkjet printed silver layer with 20 wt% is 20.6×10^-6 and 27.1×10^-6Ω cm, and 4.81×10^-6 and 8.27×10^-6Ω cm for 40 wt% silver at 77 and 300 K, respectively. The surface morphology and profiles of the samples were obtained with an atomic force and scanning electron microscope.

Multipurpose thermal sensor based on Seebeck effect

This paper reports a multipurpose thermal sensor based on the Seebeck effect. The sensor has thermocouples with a multilayer structure consisting of one thermocouple strip laying on the insulating membrane and the other one under the first thermoelement and the membrane. The vertical arrangement allows a greater number of thermocouples to be placed on a given chip area. Central and lateral thermistors are placed near hot and cold thermocouple junctions, respectively, and serve for determination of the temperature difference established on the chip. Experimental results confirm that the same structure could be used as a thermal converter and a gas flow meter.

Monolayer gas adsorption in plasmonic sensors: Comparative analysis of kinetic models

The analysis of adsorption kinetics is given, assuming no dissociative adsorption takes place and no multiple binding sites exist. The phenomenological nonlinear model and the simple linear model are considered from the point of view of surface plasmon resonance (SPR) chemical sensors. A simple method for analytical calculation of the necessary rate constants is proposed. A comparative analysis of the described models is performed. Two conditions that affect the validity of the linear model are found. First one, referring to the ratio between the overall number of particles and the number of binding sites, and the second one, that guarantees the validity of linear model in much broader spectrum of practical situations, referring to the ratio of rate constants. A method for avoiding the accumulation of numerical errors in calculus is also proposed. The presented approach is general and can be used in various applications, including other types of chemical sensors, membranes for filtering, different catalytic systems, resonant micro or nano-electro-mechanical structures.

Analysis of radiation absorptance in silicon ultraviolet detector

Novel silicon ultraviolet flame detector structures are described. The first detector structure is fabricated in a thin silicon layer on a SOI wafer. The second structure is made on a silicon wafer which is back-side etched under the active detector area. We also proposed a designed UV filter utilising 1-D photonic crystal. The analysis of radiation absorptance is performed and the results presented.

ICTM pressure transmitter in the feed water system of the thermal power plant

This paper presents implementation of pressure transmitters, developed at Institute of Chemistry, Technology and Metallurgy (ICTM), within the feed water system existing in thermal power plant TENT-A located in Obrenovac, Serbia. For this purpose, high pressure transmitters (up to 400 bar), with standard current output (4-20) mA, digital display for in-situ reading and incorporated HART protocol for remote reading, are used. This type of monitoring enables higher degree of reliability, safety and energy efficiency of the feed water system. In order to optimize the energy efficiency aspect, special attention is focused on accuracy and precision of the measurements.