Printed heater elements for smart sensor packages in LTCC

Abstract

Gas sensor technology requires a heating element, which has a good thermal coupling to the active layer, providing thus a high local temperature increase without affecting the surrounding package. The use of Low Temperature Cofired Ceramics (LTCC) based packages allows the assembly of buried heaters in the near vicinity of the sensing layer. The heater must meet various requirements. Its resistance value should be in a reasonable range in order to allow the use of common power supply and the tolerances should be narrow to guarantee a stable working point without the need of additional control circuits. LTCC technology provides two ways for manufacturing of heaters: screen printing of thick film resistors or conducting lines formed as meander. For the first approach, a variety of pastes is available, but thick film resistors entail tolerances of 20% or more due to printing result and firing. Conducting lines are more stable with regard to tolerances, but their low sheet resistance results in low heater resistance if the area is limited. The use of resinate pastes as a potential solution of this problem is investigated in this work. These pastes have high organic portion and the resulting layers achieve a thickness in the range of few hundred nanometers after firing. As consequence, the sheet resistance of the conductor paths is increased and reasonable heater resistance values are achievable at small areas. The used of this pastes as an alternative approach for heater integration in LTCC packages will be discussed.