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A New Concept for LTCC-Based Pressure Sensors
Keywords: LTCC, pressure sensors, strain sensitive thick film resistors
LTCC (Low Temperature Cofired Ceramic) is a well known technology for highly integrated, reliable, high frequency suited, high temperature stable 3-D electronic packages for mobile communication, automotive, airborne/ space or medical applications. Because of its 3-D-shape (integration of diaphragms, channels and cavities) and its linear stress/ strain behaviour LTCC complies all requirements for the integration of mechanical structures e.g. for pressure sensors. LTCC-based pressure sensors have many advantages in comparison with classic steel or ceramic-based pressure sensors. LTCC is a high volume technology which helps to make the sensors cost effective. Different diaphragm thicknesses can be achieved by using different tape thicknesses. LTCC-based diaphragms can be used as fired. There is no need to grind or polish the diaphragm surface after firing. Furthermore all components of the sensor system (sensor body, electronics) can be integrated in one LTCC-based multilayer substrate. The paper describes different aspects of the development and characterization of piezoresistive pastes (co- and postfiring) for LTCC-based pressure sensors. The goal of this investigations was to find an optimum compromise between strain-sensitiviy, stability and noise behaviour. The new sensor concept encloses a modified solution for the pressure connection of the sensor. The sensor cell is embedded mechanically stress free and is fixed by thin LTCC-cantilevers. These cantilevers contain micro-channels for the pressure connection. Using the discussed piezoresistive pastes LTCC-based pressure sensors were developed and tested. The sensor development encloses a FEM-simulation (Finite Elements Method) based sensor design having regard to the mechanical characteristics of LTCC (Youngs Modulus, fracture strength) as well as the development of the manufacturing technology. Different types of pressure sensors (absolute, differential and relativ pressure) for different pressure ranges were developed. Diaphragm thicknesses down to 50 m (cofired in a differential pressure sensor) could be achieved for mbar-sensors. The sensors were characterized to detect their sensitivity, non-linearity, hysteresis, temperature behaviour and long time stability. All sensors showed a very linear pressure dependent signal behaviour (non-linearity and hysteresis <0.05%FS). Together with their high temperature and long time stability LTCC-based pressure sensors could be a real alternative to conventional sensor solutions.
Uwe Partsch, Group Leader MLC
Fraunhofer IKTS
Dresden, Saxony 01277,
Germany


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