Ulrich Mescheder

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.

Local Laser Bonding for Low Temperature Budget

A new bonding process for Si-wafer has been developed. The bonding is provided through intermediate layers such as Al or Au forming an eutectic alloy with silicon. A focused laser beam is used to heat up the contact site locally to temperatures well above the eutectic temperature of the corresponding alloys. Depending on the laser wavelength used the bond partner might be pyrex or silicon. This bonding process is especially suitable for bonding wafers containing devices with low temperature budget. The bonding strength of about 40 MPa is comparable to that of anodic bonding. The presented technique allows for a considerable reduction of the area needed for proper bonding. Furthermore, it provides for electrical contacts between the cap wafer and the device wafer so that new functions can be integrated into the cap.

Properties of SiO2 electret films charged by ion implantation for MEMS-based energy harvesting systems

This paper presents the results of charging SiO2 thin film electret by ion implantation. A maximum charge density of 16 mC m−2 has been shown using 500 nm thermal oxide. Charge is reproducible and stable in time. Two types of ions were used for ion implantation: phosphorus (P+) and boron (B+). Directly after implantation, it was found that charging with B+ resulted in more stable surface potential compared to charging with P+, but after 50 days the difference is negligible. The extrapolated long-term stability of SiO2 electrets charged by ion implantation is around 1 year. SiO2 is a promising electret for energy harvesting, which can be charged in a reproducible and stable way by ion implantation. The results are compared to corona charging of SiO2. The developed process can be used to realize vibration-based capacitive energy harvester systems using CMOS-compatible processes.

Ultrastable assembly and integration technology for ground- and space-based optical systems.

Optical metrology systems crucially rely on the dimensional stability of the optical path between their individual optical components. We present in this paper a novel adhesive bonding technology for setup of quasi-monolithic systems and compare selected characteristics to the well-established state-of-the-art technique of hydroxide-catalysis bonding. It is demonstrated that within the measurement resolution of our ultraprecise custom heterodyne interferometer, both techniques achieve an equivalent passive path length and tilt stability for time scales between 0.1 mHz and 1 Hz. Furthermore, the robustness of the adhesive bonds against mechanical and thermal inputs has been tested, making this new bonding technique in particular a potential option for interferometric applications in future space missions. The integration process itself is eased by long time scales for alignment, as well as short curing times.

Micromachined Hotplate Platform for the Investigation of Ink-Jet Printed, Functionalized Metal Oxide Nanoparticles

This paper describes a novel micromachined platform serving as an interface between nanosized, gas sensitive metal oxide particles, and the macroscopic world. Through a combination of ink-jet printing and microelectromechanical systems technologies, it thus becomes possible to quickly test and characterize new nanosized metal oxide particles with respect to their gas sensitivity. Within the framework of this report, we describe the design considerations, thermal finite-element method simulations, processing, characterization, and utilization of the platform. Due to the low-power consumption, the hotplate provides an experimental platform to test nanoparticle-based metal oxide gas sensors for mobile systems.

Odor-Sensing System to Support Social Participation of People Suffering from Incontinence.

This manuscript describes the design considerations, implementation, and laboratory validation of an odor sensing module whose purpose is to support people that suffer from incontinence. Because of the requirements expressed by the affected end-users the odor sensing unit is realized as a portable accessory that may be connected to any pre-existing smart device. We have opted for a low-cost, low-power consuming metal oxide based gas detection approach to highlight the viability of developing an inexpensive yet helpful odor recognition technology. The system consists of a hotplate employing, inkjet-printed p-type semiconducting layers of copper(II) oxide, and chromium titanium oxide. Both functional layers are characterized with respect to their gas-sensitive behavior towards humidity, ammonia, methylmercaptan, and dimethylsulfide and we demonstrate detection limits in the parts-per-billion range for the two latter gases. Employing a temperature variation scheme that reads out the layer’s resistivity in a steady-state, we use each sensor chip as a virtual array. With this setup, we demonstrate the feasibility of detecting odors associated with incontinence.

Optoelectrical Detection System Using Porous Silicon-Based Optical Multilayers

Porous silicon-based multilayer structures for optical sensors have been simulated, fabricated and tested. The properties of optical sensors using porous silicon multilayers 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 (200-700 m2/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 (2-4 nm) and high porosity (70%-80%) have been realized and characterized. A simple, low cost optical sensor system based on multilayer, a LED-based illumination system providing discrete wavelengths (RGB) and a wide band detector has been realized and tested.

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.

3D capacitive vibrational micro harvester using isotropic charging of electrets deposited on vertical sidewalls

In this paper the design and fabrication of an integrated micro energy harvester capable of harvesting electrical energy from low amplitude mechanical vibrations is presented. A specific feature of the presented energy harvester is its capability to harvest vibrational energy from different directions (3D). This is done through an innovative approach for electrets placed on vertical sidewalls and thereby allowing for miniaturization of 3D capacitive energy harvester on monolithic CMOS substrates. A new simple electret charging method using ionic hair-dryers/hair ionizers is reported and shown that it can be effectively used for electrets-based micro energy harvesters.

MEMS-Based Air Quality Sensor

This paper presents a novel sensor array based on MEMS-technology for quantitative measurement of the air quality and a novel model based measurement strategy. The sensor array consists of SnO2 - gas sensors, a porous silicon based humidity sensor and a Pt-temperature sensor monolithically integrated on a single chip. It was fabricated using SOI-technology, Pt-metallization and bulk micromachining. Good characteristics of the sensor array in terms of thermal properties, sensitivity, selectivity and time response have been achieved. The values of gas content and atmospheric humidity evaluated based on the developed sensor models are in a good consistence with the actual values.