A. Kovacs

LPCVD against PECVD for micromechanical applications

After discussion of the basic aspects of CVD and its reaction kinetics LPCVD and PECVD will evolve as techniques commonly used at high temperature and lower temperature , respectively. Films deposited by these two techniques differ in several aspects, i.e., thickness, uniformity, purity, density, electrical properties, adhesion, step coverage, etc. Reactor designs are discussed in brief for optimization of the process parameters to yield optimized film properties. Then each of the major film materials such as polysilicon, SiN, , , SiC and some exotics such as diamond films are discussed with respect to their application in microstructures and their film properties in dependence on the deposition technique and follow-on processing, e.g., internal stresses due to imperfection in structure and composition or clamping, film density, pinhole density, and etchability. The discussion then moves to the application of LPCVD and PECVD in microstructures. A few typical examples will be presented for functional layers: films for membranes, cantilevers, etc in mono- and heterostructures, or ion sensitive films including passivation films as used in many sensors (e.g., microphones) and actuators (e.g., micromotors), especially such as fabricated by surface micromachining. Some room is also given to SiC, a new micromechanical material. A summary and weighting of the two CVD techniques is given.

Porous silicon-based humidity sensor with interdigital electrodes and internal heaters

A novel design of a one wafer side processed porous silicon-based humidity sensor with interdigital electrodes is presented. An integrated heater element over the porous layer provides the effective heating and the low power consumption of the device. Reliable contacts between metal and porous Si are formed via crystalline n-Si islands within the porous layer, formed by exploiting the selectivity of the electrochemical etching process. The effects of the electrode and heater geometry and also the parameters of the porous matrix are investigated with special emphasis on response and recovery time. To ensure the adequate thermal conditions sensor structures and packaging techniques were also investigated. The applied heater geometry results in faster recovery at a cost of reduced power consumption.

Structural parameter sensitivity analysis of cantilever- and bridge-type accelerometers

The paper deals with the analytical estimation of bandwidth and device sensitivity of cantilever- and bridge-type monolithic piezoresistive and capacitive accelerometers using different beam models and of their design sensitivity due to the change of geometrical parameters. Using a simplified two-beams model, closed-form formulae have been derived to give the relationship between the rates of length, width and thickness of the beams and the lowest eigenfrequency characterizing the bandwidth as well as the physical sensitivity of the accelerometers. By means of symbolic derivation, a structural design sensitivity analysis has been carried out to obtain information for the optimal selection of the geometrical parameters.

Mechanical analysis of polycrystalline and single-crystalline silicon microstructures

Abstract Polycrystailine silicon (polysilicon) microstructures are fabricated with surface micromachining using a sacrificial layer technique. Due to the internal residual stress in LPCVD polysilicon layers, only a free-standing length of up to a maximum value for such microstructures, e.g., cantilevers, bridges, membranes, can be obtained. Single-crystalline silicon microstructures are fabricated with bulk micromachining techniques using appropriate etchstop techniques. Unstructured and structured single-crystal membranes have been fabricated. Due to the lower residual stress in single-crystalline membranes, a different range of geometries and hence of mechanical properties is available. For both polysilicon and bulk silicon structures, finite-element analysis with the ANSYS® simulation program has been used to determine the mechanical sensitivity and resonance frequency.

Numerical implementation of prager's kinematic hardening model in exactly integrated form for elasto-plastic analysis

Abstract An exact solution of the Prandtl-Reuss equation for Pragers kinematic hardening model is described in the paper. As compared with numerical integration methods, the method described permits stress calculations to be made quickly and accurately in the case of solution of time-independent elastic-plastic problems using the finite element method. The FORTRAN code for the case of plane stress and plane strain as well as for axisymmetrical problems is given.

Design of best management practice applications for diffuse phosphorus pollution using interactive GIS

The paper presents a complex environmental engineering tool, which is appropriate to support decision making in watershed management. The PhosFate tool allows planning best management practices (BMPs) in catchments and simulating their possible impacts on immissions. The method has two parts: (a) a simple phosphorus (P) fate model to calculate diffuse P emissions and their surface transport, and (b) an interactive tool to design BMPs in small catchments. The fate model calculates diffuse P emissions via surface pathways. It is a conceptual, distributed parameter and long-term (annual) average model. The model also follows the fate of emitted P from each cell to the catchment outlets and calculates the field and in-stream retention. The fate model performed well in the Zala River catchment as a case study. Finally, an interactive design tool was developed to plan BMPs in the catchments and simulate their possible impacts on diffuse P fluxes. Different management scenarios were worked out and their effects evaluated and compared to each other. The results show that the approach is suitable to test BMP scenarios at small catchment scale.

Impacts of the climate change on runoff and diffuse phosphorus load to Lake Balaton (Hungary).

The paper outlines a multi-component assessment of the impacts of the climate change on runoff and total phosphorus loads to the large shallow Lake Balaton in Hungary. Present hydrological cycle of the lake catchment has been examined using the rainfall-runoff model WetSpa. Particular phosphorus concentration in runoff was estimated on the basis of the simulated streamflow using an empirical power equation. Dissolved phosphorus concentrations were determined as a function of landuse and soil type of the corresponding sub-catchment. The model was calibrated and validated against daily observations manually at monitoring sites of sixteen inflowing streams around the lake. Runoff stemming from shoreline urban developments was calculated by the urban runoff simulation model SWMM. Phosphorus concentrations in urban runoff were calculated by an empirical relationship derived from field measurements. The model was henceforward run for climate change scenario analysis. Present weather data were modified by the climate change scenarios imported from the results of the CLIME project. The results indicate that the impact of the climate change on runoff and phosphorus load appears in the change of the distribution within a time period rather than in the total volume. However, due to the high uncertainties in climate models, the presented calculations are possible assumptions rather than established statements.

Thermal stresses in a shrink fit due to an inhomogeneous temperature distribution

SummaryBased on Trescas yield condition and its associated flow rule, a semianalytical method is presented for the calculation of thermal stresses due to steady-state thermal loading in an assembled shrink fit. The calculation is evaluated assuming plane stress conditions, linear elastic-perfectly plastic materials, and linearly temperature dependent yield stresses. Depending on the temperature gradient, different combinations of pure elastic and plastic zones arise in the shaft and in the hub.

Etch-stop behaviour of depletion layers

The authors demonstrated that the thickness of a diaphragm using the electrochemical etch-stop technique can be monitored with the applied reverse bias voltage on individual pn-junctions. It is thus possible to produce diaphragms of differing thickness on one wafer using different reverse bias voltage values. This effect is attributed to an etch-rate reduction of approximately a factor of ten when the etch front reaches the depletion layer of the pn-junction. Since the depletion-layer thickness is voltage dependent different diaphragm thicknesses can be obtained. For a metallurgical junction depth of 4.4 mu m diaphragm thicknesses of 5.5 mu m, 6.5 mu m and 9.5 mu m were obtained for respective reverse bias values of 2 V, 8 V and 20 V. Since depletion layers can also be obtained with metal oxide silicon semiconductor (MOS) structures, MOS-capacitors with n+ source diffusions were biased with 10 V gate to substrate and 3 V source to substrate voltages. Electrochemical etching yielded membranes of typically 3 mu m thickness. They are lightly p-doped in this case (3*1015 cm-3) and completely stress free. For MOS-structures no final etch-stop can be obtained, however, etch-through monitors are used as etch end-point detectors. Very thin, single-crystal diaphragms with thicknesses below 2 mu m are of importance in several applications. Such thin diaphragms were made with shallow implanted pn-junctions of 1.2 mu m junction depth and an electrical activation anneal. The resulting diaphragm thickness obtained was 2 mu m by sufficient overetching.

Characterization of porous silicon based optical sensor system for biosensor applications

Porous silicon based multilayer structures for optical sensors have been simulated, fabricated and tested. The properties of optical sensors using porous silicon multilayer can be adjusted by appropriate substrate material, morphology, process parameters in the pore formation process and by surface treatment (thermal oxidation). Heavily and lightly doped p-doped substrates have been used to realize porous silicon layers with different morphology, porosity (30–80%), pore size (mesoporous range) and specific surface area (200–700m2/cm3). Thermal oxidation stabilizes the surface and results in hydrophilic surfaces for effective adsorption of liquid analytes. Oxidation reduces the porosity and the pore size but improves the wetting behavior of liquid analytes in the porous volume. Different multilayer structures using native and oxidized porous silicon and corresponding concepts of optical sensor systems have been proved for aqueous and organic analytes. Sensors using small pore size (2–4nm) and high porosity (70–80%) have been realized and characterized. A simple, low cost optical sensor system based on multilayer, a tunable light source and a detector has been realized.