Integrated overvoltage protection for improved reliability of LTCC microsystems

LTCC (Low Temperature Co-fired Ceramics) technology offers many advantages in electronics, such as miniaturization through multilayer circuits, thermal stability, robustness in harsh operating conditions, or a high integration density of various active and passive components.

Protecting the components from voltage peaks is very important for the reliability of circuits embedded in multilayer ceramic substrates. Until now, such overvoltage protection cannot be integrated into multilayer circuits. These resistors, called varistors (variable resistors), are therefore only integrated outside the monolithic circuit through packaging and connection techniques. If a critical voltage is exceeded, they disspate current without damaging the circuit’s sensitive functional elements. This special switching function is made possible by grain boundary effects in doped zinc oxide.

In collaboration with VIA electronic GmbH, Fraunhofer IKTS has succeeded in maintaining this functionality even after integration by co-firing. This means, that the varistor is printed onto the LTCC first and then sintered with the circuit. The challenge lies in the various interactions between the glass-ceramic LTCC substrate and the components of the varistor element, which form liquid components during the joint firing process. In particular, a liquid phase containing Bi₂ O₃ and Sb₂O₃ between the semiconducting zinc oxide grains of the varistor reacts with phases of the LTCC substrate even below the firing temperature. This molten phase containing bismuth and antimony is then no longer available for the formation of potential barriers at the grain boundaries and therefore offers no over-voltage prote.

By optimizing the varistor composition and selecting suitable metallizations, LTCC structures have been created with integrated varistor elements that withstand co-firing without losing the functionality of overvoltage protection (Fig. 2). The structure of the varistor shows that the decisive grain boundary phases are retained even after the joint firing process (appearing bright in the ground contrast, Fig. 3).

Based on this promising groundwork, integrations for different switching voltages are currently being developed in the “Hybrid Microsystems” department. The number of grain boundaries between the electrodes of the varistor is crucial here. Low voltages require varistor thicknesses between 10 and 50 µm, which is achieved through screen printing. Other designs are available for higher switching voltages. The diverse possibilities and the reliability of LTCC technology are significantly enhanced by this integrable functionality.

The authors would like to thank the German Federal Ministry of Education and Resarch BMBF for financial support as part of the RUBIN funding initiative (support code: 03RU1U162E).