Functional Ceramic Micro Components for Microsystems Technology

Thin film ceramic components and layers will become an integral part of microsystems technology in the future. Colloidal dispersed oxide particles as used in classical ceramic powder processing have now been considered as building blocks for microdevices.

Ceramics have several advantages over other materials also in microsystems, e.g. heat resistance, hardness, corrosion resistivity, large specific surfaces, functional properties such as semiconduction, piezo- and pyroelectricity, catalytic activity and even superconductivity.

Figure 1. Ceramic microline structures on a microhotplate compared with a human hair (Diameter 100 µm) lying across the sensor lines.Ceramic microline structures on a microhotplate compared with a human hair (Diameter 100 µm) lying across the sensor lines.

Ceramic fabrication techniques have been investigated to miniaturize ceramic components to the scale of several microns using nano scaled oxide powders, monosized powders and nanutubes. These new ceramic shaping methods allow to structure surfaces by microcontact printing, molding in photoresist structured surfaces, micromolding in capillaries (MIMIC) and embossing.

These technigues allow gas sensors that are barely visible by the human eye cover only 4 by 10 µm² which is two orders of magnitude smaller than most of today’s microsensor designs or templates for photonic band gap materials. The miniaturized sensors could be integrated on state-of-the-art silicon semiconductor device including microhotplates using standard silicon micromachining. The techniques, which make possible the direct use of a colloidal dispersion for generating functional ceramic microstructures, are derived from soft lithography and modern colloidal chemistry. In micromolding in capillaries (MIMIC) polydimethylsiloxane elastomers with microchannels served as molds which then were spontaneously filled with ceramic suspensions owing to capillary forces. The resulting micro-thickfilm structures have a height of several micrometers and therefore differ from usual ceramic thin films. Small lines of 1-2 µm width for several mm in length could easily be prepared. Micropatterns of ceramic powders can also be obtained by selective wetting of microcontact-printed surfaces using organic templates. Aqueous colloidal dispersions adhered only to the hydrophilic micropatterns whereas they are repelled by the hydrophobic surroundings in a simple dip coating process. Printing and selective wetting enable a feature resolution of 5 µm. The third microfabrication approach is photoresist casting. Ceramic microstructures with cross sectional areas of 5 by 10 µm² are feasible using conventional photolithography.

Examples of microdevices are a continuous flow enzyme reactor inside PDMS channels with a two-fold hierarchical mixing structure. Compared to a microchannel system without mixing struts, a tremendously increased chemical product formation is possible.

Another example is a tin oxide gas sensor.These sensors showed responses for reducing gases obtained from an active sensing area of only 4 by 10 µm².

Figure 2. Template for photonic band gap material (diameter of sphere 300 nm).

In order to minimize power consumption of such miniaturized gas sensors, they were integrated on 250 nm thin microhotplates that consume only mW heating power to reach 800°C. The new shaping techniques allow complete gas sensor arrays consisting of more than 20 sensors on one hotplate. Individual doped sensors in a sensor array are envisaged to mimic an artificial nose on a micron scale in future. Many of these techniques enable the shaping and successful integration of ceramics with a wide verity of properties in micro sensor and MEMS technology.

Figure 3. Tin oxide sensor lines (black, 5 µm width) on an integrated heating meander.

Figure 4. Tin oxide sensor lines (black) on a micro hotplate (Si₃N₄, 300x300 µm²).

Figure 5. Ceramic In₂O₃ sensor array integrated on a Si substrate (8x8 mm²).

Authors

M. Heule, U. Schönholzer, S. Vuillemien, L. Gauckler
Nonmetallic Inorganic Materials, Department of Materials, ETHZ

References

• Schönholzer, U. P., Stutzmann, N., Tervoort, T. A., Smith, P. and Gauckler, L. J. (2002); "Micropatterned Ceramics by Casting into Polymer Molds", J. Am. Ceram. Soc. 85(7): 1885-1887.
• Heule, M. and Gauckler, L. J. (2001); "Microfabrication of Ceramics based on Colloidal Suspensions and Photoresist Masks", J. Photopolym. Sci. Technol. 14(3): 449-452.
• Heule, M. and Gauckler, L. J. (2001); "Gas Sensors Fabricated from Ceramic Suspensions by Micromolding in Capillaries", Adv. Mater. 13(23): 1790-1793.