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- 2013 (7) (entfernen)
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- LTCC (2)
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- MnZn-ferrite (1)
- Strain gauge (1)
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Eingeladener Vortrag
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Co-firing of low temperature co-fired ceramics (LTCC) and functional ceramics like ferrites is a promising approach to increase the level of integration in future microsystems, and to create new applications for LTCC technology. Besides the development of compatible material combinations, the configuration of the sintering process is an important issue for successful co-firing. A method is presented to derive the linear shrinkage mismatch of a material combination based on density data calculated from the master sintering curves (MSCs) of the individual materials. The influence of the firing profile on the constraint in the combined multilayer can be anticipated using this method. To investigate and improve the co-firing of ferrite and dielectric tape, the shrinkage mismatch with respect to heating rate was studied. A significant reduction of shrinkage mismatch was found for increased heating rates. The calculated results are verified by lateral shrinkage measurements on combined laminates.
Recent advances in the development of high gauge factor thin films for strain gauges prompt the research on advanced substrate materials. A glass ceramic composite has been developed in consideration of a high coefficient of thermal expansion (9.4 ppm/K) and a low modulus of elasticity (82 GPa) for the application as support material for thin-film sensors. In the first part, constantan foil strain gauges were fabricated from this material by tape casting, pressure-assisted sintering, and subsequent lamination of the metal foil on the planar ceramic substrates. The accuracy of the assembled load cells corresponds to accuracy class C6. That qualifies the load cells for the use in automatic packaging units and confirms the applicability of the low-temperature co-fired ceramic (LTCC) substrates for fabrication of accurate strain gauges. In the second part, to facilitate the deposition of thin-film sensor structures to the LTCC substrates, pressure-assisted sintering step is modified using smooth setters instead of release tapes, which resulted in fabrication of substrates with low average surface roughness of 50 nm. Titanium thin films deposited on these substrates as test coatings exhibited low surface resistances of 850 Ω comparable to thin films on commercial alumina thin-film substrates with 920 Ω. The presented material design and advances in manufacturing technology are important to promote the development of high-performance thin-film strain gauges.
For co-firing of MnZn-ferrite tapes and LTCC dielectric tapes, the sintering shrinkage curves and the coefficient of thermal expansion of ferrite and dielectric tapes were matched. Highly densified embedded ferrite without any cracks could be manufactured by co-firing at 900 °C in nitrogen atmosphere. However, the permeability of MnZn-ferrite co-fired between dielectric tapes is significantly reduced (µ´=100) compared to that of the separately sintered ferrite (µ´=500). Changes in the phase stability and microstructure of MnZn-ferrite were investigated to explain the permeability reduction in the embedded ferrite. It is supposed that early densification of the dielectric tapes on the top and bottom of the ferrite layer prevent the gas exchange during sintering which is necessary for (Mn,Zn)Fe2O4 spinel formation. As a result, high amount of Fe2O3 secondary phase and a Mn-rich spinel phase with low permeability remain in the embedded ferrite layer.
Plane LTCC substrates can be produced in zeroshrinkage technology by pressure-assisted sintering. The application of axial pressure during sintering supports densification, enhances cofiring of different materials, and restrains deformation. However, the sintered surfaces are affected by the release tapes and by the setters used in the pressure-assisted sintering process. In this study, the effect of release tape particle size on the surface properties of sintered substrates was investigated. Moreover, the substitution of release tape by the use of a vitreous carbon setter was tested. Surface roughness and sheet resistances of metallic thin films on the respective samples were determined. It was found that the roughness of the substrates is not directly correlated with the adequacy of the surface for thin-film deposition. Compared with a freely sintered sample with an average roughness (Ra) of 0.31 μm, pressure-assisted sintering created smoother surfaces using finegrained release tapes (Ra = 0.18 μm) and rougher surfaces using coarse release tapes (Ra = 0.46 μm). Still, sheet resistances were higher on all sinter-pressed samples. The vitreous carbon setter created unusual trenched surface structures, characterized by varying roughness values and intermediate sheet resistances on the LTCCs tested.