Hansu Birol

Application of graphite-based sacrificial layers for fabrication of LTCC (low temperature co-fired ceramic) membranes and micro-channels

Fabrication of sensors and micro-fluidic structures from low temperature co-fired ceramic (LTCC) sheets is a growing interest in the micro-packaging community. Such devices usually have inner cavities, whose production is quite complicated. The most elegant method to build such structures so far achieved is by a fugitive phase that is introduced into the multilayer and removed during firing. This paper, therefore, is aimed to introduce the graphite-based sacrificial paste developed for this purpose, and it is constructed in two sections: (i) selection of paste and determination of LTCC open-porosity elimination temperature, and (ii) fabrication and characterization of pressure sensitive LTCC membranes. In the former section, it is shown that increased heating rates (and decreasing tape thickness) shift the open porosity elimination temperature of LTCC by 20 °C, which is small compared to the shift of graphite oxidation temperature (about 100 °C). In the latter section, three parameters affecting the balance between the graphite oxidation and LTCC sintering are studied: heating rate, graphite phase thickness and width of the membrane inlet/outlet channels. As expected, larger heating rates and narrow inlet/outlet channels are found to hinder the oxidation of graphite and evacuation of the resulting products, which results in swollen membranes. Large graphite thickness, through the increased channel height, results in lower swelling in spite of the larger amount of graphite to be oxidized. Membranes with low swelling are found to exhibit excellent pressure sensing characteristics, whereas those with high swelling display hysteretic behavior.

Modification of Thick-Film Conductors Used in IP Technology for Reduction of Warpage during Co-Firing of LTCC (Low Temperature Co-Fired Ceramic) Modules

LTCC technology offers low temperature firing (<900 °C) of a materials system, which is based on LTCC sheets/tapes and (ideally) compatible thick-film components. Screen-printed materials on LTCC tapes, such as conductor, resistor, inductor thick-films are co-fired (simultaneously fired), providing a highly-functional package. This comes along with additional benefits such as ease of LTCC tape structuring, fabrication of hermetic and complex 3-D structures, etc. The major difficulty encountered arises from the differential shrinkage rate of LTCC tape and thick-film components, which has to be avoided for fabrication of warpage-free, flat surfaces that is vital for membranes, beams, etc. Therefore the goal of this study is the reduction of deformation, by matching the shrinkage rate of conductor with that of LTCC, which is achieved by mixing the commercially-available paste with selected additives.

Millinewton force sensor based on low temperature co-fired ceramic (LTCC) technology

Fabrication of a millinewton force sensor, which is based on LTCC technology as an alternative to the widely used alumina (Al2O3) is described. The new sensor, integrated with piezoresistor thick-film, has shown an increase in the sensitivity by two times compared to the alumina-based version in the first attempt. This is ascribed to the smaller elastic modulus and finer substrate thickness attainable by LTCC. Moreover design flexibility is also an important factor, which contributes to the increased sensitivity of the sensor. It is strongly expected that the device performance can further be improved, by modifying the co-fired LTCC components such as LTCC tape and the thick-film terminations.

Structuration and Fabrication of Sensors Based on LTCC (Low Temperature Co-Fired Ceramic) Technology

The purpose of this paper is to demonstrate sensors and structures fabricated using the LTCC technology, which has been addressed and employed increasingly as a smart packaging approach for several applications. The focus will be on inclination and cantilever force sensors and micro-fluidic structures. Motivation for selection of LTCC for these applications in addition to fabrication and structuring of the devices will be explained in details. TGA (thermo-gravimetric analysis), dilatometer analysis, SEM (scanning electron microscopy), electronic equipment for measuring sensor performance will be extensively used for explanation of the results. It will also be shown that, compared to classical thick-film technology on alumina, LTCC allows a considerable increase in sensitivity, and is therefore better suited for the sensing of minute forces and pressures.