Structuration of zero-shrinkage LTCC using mineral sacrificial materials

Recently, LTCC (low-temperature co-fired ceramic) technology has increasingly found applications beyond pure electronics, in fields such as microfluidics, sensors and actuators, due to the ease of shaping the tapes in the green (unfired) state. Accurate control of hollow structures such as channels, membranes, cavities and gaps below cantilevers has remained difficult, however, although carbon-based sacrificial materials and adhesive/solvent-assisted low-pressure lamination techniques are adequate for several uses. Mineral sacrificial pastes (MSP), introduced by several groups including our laboratory, allow in principle much better control of open structures such as bridges and cantilevers, as they are removed only after the firing step. In practice, accurate dimensional control has been limited by deformation of the LTCC during sintering, due to shrinkage mismatch with the MSP. Attempts to eliminate this problem have met with limited success, as it is very difficult to perfectly match the shrinkage curve of the MSP (which must retain open porosity) to that of the LTCC substrate. Therefore, in this work, we endeavour to investigate MSP materials on self-constraining "zero-shrinkage" LTCC tape, which is therefore compatible with a low degree of sintering of the MSP. We present results of optimising the MSP formulation accordingly, to achieve reasonable consolidation, low deformation of LTCC and easy removal in weak acid solutions. Important topics such organic vehicle formulation and complete release processes (etching, rinsing and drying) of thin structures are also addressed.