Overload-proof LTCC differential pressure sensors for high temperature applications

Motivation: Requirements in the automotive, aerospace and in the chemical industry increase and include applications with higher temperatures up to 300 ° C and more. Pressure sensors have been used since long time to monitor gas pressures in the automotive industry or to control processes in the industry. Extensive efforts have been made in recent years to measure process parameters directly in the process area of reaction chambers (chemical industry) or close to exhaust gases. Large problems of present solutions are insufficient high operating temperature of the pressure transmitter and their comparatively low overload resistance (usually max. 3 ... 4 times the rated pressure). For buffering sudden differential pressure shocks the measuring membranes of the ceramic sensors are oversized, which leads to reduction in measurement sensitivity. Furthermore, there is the wish of cost-effective or suitable solution for measuring the relevant parameters of pressure and temperature together at the measuring location. The aim of our development is an economically viable solution of a high-temperature and overload-proof ceramic pressure transmitter for measurement data at high temperatures (> 300 °C). To make these signal data available on the spot a high temperature analog signal conditioning for the offset and temperature of the measuring bridge will be developed. / Method: The basic idea of the desired solution is to develop a new generation of high-temperature-stable, overload-resistant, miniaturized and cost-reduced pressure transmitters based on an enhanced mass-production-capable LTCC multilayer technology. Therefore a new layer deposition processes is developed with special screen-printable dielectrics, sacrificial materials and direct writing of functional layers. The overall concept of such a solution can be seen in Figure 1. The enhanced multilayer technology allows the combination of differentially structured ceramic layers. With the newly developed dielectric paste and printing technology additive structures can be created and positioned with high precision and resolution, which was previously not possible. Dielectric layers were printed by screen printing and the free standing geometries were achieved using sacrificial pastes. Terminations and thick-film conductors, which are also embedded in the dielectric deformation element (membrane), are sintered together with the membrane and generate the strain-sensitive measuring bridge. The bridge in turn generates the pressure - related analog output signal (bridge voltage) which is transferred to a signal conditioning circuit. The technology chain for the production of such LTCC differential pressure sensors is shown in Figure 2. / Results: The presented research results arose from the cooperation between Fraunhofer IKTS (ceramic multilayer technology), with the company ADZ Nagano GmbH (high-temperature signal conditioning) and with the company LUST Hybrid-Technik GmbH (packaging technology). The present state of development of an overload resistant LTCC differential pressure sensor for high temperature applications will be shown. In the field of sensor technology, novel miniaturized ceramic differential pressure sensors were developed and optimized for their properties in terms of overload resistance (10 to 12 times the rated pressure) and high-temperature stability (up to 300°C). An LTCC technology process chain enhanced due to a combination of the sacrificial paste technique and direct writing technology will be shown. In this way self-supporting microstructures were produced (figure 2). The achievable free standing layer thicknesses of 10 to 20 microns double the structure resolution of the conventional LTCC process and also allow for a significant increase of the structural resolution in the lateral dimensions. Thus, cantilevered sensor membranes and small cavities were created, which allow the membrane to be touch on the cavity bottom and thus to be protected against destruction in the event of pressure shocks and overloads. A cross section of manufactured differential pressure sensor with one deformation element (membrane) und two cavities can be seen in figure 3.