Rainer Buchner

A Temperature Compensation Circuit for Thermal Flow Sensors Operated in Constant-Temperature-Difference Mode

This paper presents a new temperature compensation technique for thermal flow sensors that are operated in a constant-temperature-difference (CTD) mode by means of a simple analog circuit. The resistive heater of a thermal flow sensor is maintained at a constant temperature some tens of Kelvins above fluid temperature with the help of a Wheatstone bridge circuit. In case of a change in media temperature, an adjustment of the heater temperature is necessary; otherwise, the temperature difference falls/rises with respect to the temperature change, and the sensor output signal deviates from its calibration. Temperature compensation can be performed by the use of an additional resistive temperature sensor. The circuit design presented here includes a potentiometer that is capable of changing the resistance of the temperature sensor and its temperature coefficient of resistance (TCR) for an easy adjustment for temperature compensation. This gives the freedom to use any material such as platinum, aluminum, or, in our case, an alloy of tungsten and titanium (WTi) for the temperature sensor, regardless of its resistance value and TCR with respect to the heater of a thermal flow sensor.

Toward Flexible Thermoelectric Flow Sensors: A New Technological Approach

A new technological approach on thin flexible sensors is presented. As proof of concept, a thermoelectric flow sensor on a 10-mum-thick polyimide foil has been realized. The advantages of silicon as a thermoelectric material and the stability of low-pressure chemical vapor deposition (LPCVD)-silicon nitride as a protective coating are combined with the flexibility of polymer substrates. The thermoelectric flow sensor is fabricated on a standard silicon wafer for handling purposes. Only the functional layers that are embedded in 600 nm of LPCVD-silicon nitride are transferred onto a 10-mum-thick polyimide. The bulk silicon has been removed using deep reactive ion etching. Samples have been fabricated and tested, proving the potential of this new technological concept. The first characterization results show that the sensor layout has to be adapted to the properties of the polymer substrate.

Miniaturised Thermal Flow Sensors for Rough Environments

Two concepts of miniaturised thermoelectric flow sensors for rough pressurized environments are presented, based on our high-temperature thermopile fabrication process. Membrane based thermal flow sensors show high sensitivity and stability against aggressive fluids using a LPCVD silicon nitride (SiN) passivation layer but are weak against high pressure. We realised pressure stable sensors using either quartz as substrate or by stabilising the membrane using polymers. The sensors have been fabricated and characterised. The sensors with the polymer stabilised membrane show good sensitivity and dynamic behaviour while the quartz based sensors have superior stability towards pressure and aggressive media. In comparison the quartz based sensors have a far less complex fabrication process than those with a stabilised membrane.

Temperature stability improvement of thin-film thermopiles by implementation of a diffusion barrier of TiN

A new generation of thin-film thermopiles is presented with improved temperature stability due to the implementation of a diffusion barrier of titanium-nitride. Commonly thermocouples are made of aluminium and polysilicon or semi-metals materials as Sb, Te and Bi. Exposure to elevated temperature in the range of a few hundred degrees centigrade would affect the functionality of the devices and leads to system failure. Prior to this work an approach for the realisation of high-temperature stable thermopiles made of tungsten-titanium alloy and polysilicon has been reported. Due to implementation of a diffusion barrier of TiN at the thermoelectric contacts the diffusion processes could be suppressed to a great extend and the temperature stability of these thermoelectric devices has been further improved. Functionality of the diffusion barrier was verified using TOF-SIM and the temperature stability achieved was investigated by detailed measurements.

Thermoelectric Flow Sensors with Monolithically Integrated Channel Structures for Measurements of Very Small Flow Rates

A thermal flow sensor with monolithically integrated channel structures for measurements of flow rates down to 40 nlmin-1 is presented. The sensor is based on a fabrication process using a high-temperature silicon nitride as protective coating. The channel is realised by means of surface micromachining and consists of SU-8. For chemical stability and to gain hydrophilic properties, the channel is coated on the inside by silicon-oxide and silicon-nitride as a moisture barrier. Sensor systems have been fabricated and characterised.

Thermoelectric flow sensors on flexible substrates and their integration process

A thermoelectric mass flow rate sensor on a 10 µm thick polyimide foil and its integration process on planar as well as non-planar surfaces has been developed. Flow sensors, fabricated by MEMS technology, usually have a height which is directly coupled to the wafer thickness (typically 525 µm or 380 µm). They are not bendable and an integration process is always complex when steps in flow channels have to be avoided. A fabrication process has been developed where the functional layers were removed from the silicon substrate and transferred onto a 10 µm thick polyimide foil. The resulting flexible flow sensor has been integrated on different materials by means of flip chip mounting and tested on its electrical and mechanical characteristics. It could be shown that thermoelectric flow sensors on polymer foils have comparable characteristics to flow sensors on silicon substrates but higher diversity in system integration.

Pressure stable thermoelectric flow sensors by means of membrane perforation

A commonly used method for measurements of small liquid flows are membrane based thermoelectric flow sensors. High temperature stable flow sensors have been developed and fabricated at IMSAS. A pressure stable version of these sensors has been developed by creating a membrane with perforation. Thereby a backpressure compensation is achieved. Here we present the fabrication process, first measurements and simulation results showing pressure stability to at least 8 bar.

NDIR humidity measurement

Danfoss IXA is developing a multi gas sensor that utilizes NDIR spectroscopy to simultaneously measure various gasses such as CO2 and humidity. NDIR spectroscopy is vastly ignored in commercial humidity sensing although humidity is absorbing radiation in large parts of the infrared spectrum, seemingly making it an obvious measurand for NDIR spectroscopy. The present work elaborates on the reason for this contradiction and explains why NDIR, despite all, is suitable for humidity measurements and specifically beneficial in aggressive environments where an optical approach allows for non-contact measurement which might enhance the long term stability. It is shown that the performance of the developed NDIR spectroscopy-based prototype can compete with commercially available humidity sensors in the ranges of interest for indoor climate control.

New electrical connection technology for microsystems using inktelligent printing ® and functional nanoscaled INKS

A new electrical connection technology for highly miniaturized systems based on functional printing has been studied. INKtelligent printing® with its ability to print structures in micrometer size uses electrically conductive inks with functional nanoparticles to provide MEMS devices with electrical contacts. Due to large dimensions of conventional packaging technologies like flip chip or wire bonding, INKtelligent printing® will dramatically increase size of assembled systems. Here, an aerosol beam technology - also known as Maskless Mesoscale Material Deposition (M3D®) or Aerosol Jet® - is presented. Tests have been done to characterize conductivity, contact resistance, long term stability and temperature dependency as also applications with flow sensors.

Simple Modeling of the Thermal Behavior of a Tiny Liquid Splat on a Thermal Microsensor

In this paper, a thermal method to analyze the features of tiny drops is investigated. The explanation is based on the fundamental laws of heat transfer and phase transition. This simple approach is to divide the thermal behavior of a droplet upon settling on the surface of a thermopile sensor into two successive phases, so that the process could be modeled by means of a set of partial differential equations with one space variable. The theoretical predictions for two different liquids are confirmed with the experimental data. The good agreement between them paves the way for optimizing the geometry of the sensor and enhances its potential in practical applications