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A Reactive-Ion Etch for Patterning Piezoelectric Thin Film

Gaseous mixtures BCl3 and Cl2 are highly selective for etching PbZr1-xTixO3 films.

Reactive-ion etching (RIE) under conditions described below has been found to be a suitable means for patterning piezoelectric thin films made from such materials as PbZr1-xTixO3 or BaxSr1-xTiO3. In the original application for which this particular RIE process was developed, PbZr1-xTixO3 films 0.5 μm thick are to be sandwiched between Pt electrode layers 0.1 µm thick and Ir electrode layers 0.1 μm thick to form piezoelectric capacitor structures. Such structures are typical of piezoelectric actuators in advanced microelectromechanical systems now under development or planned to be developed in the near future.

RIE of PbZr1-xTixO3 is usually considered to involve two major subprocesses: an ion-assisted-etching reaction, and a sputtering subprocess that removes reactive byproducts. RIE is favored over other etching techniques because it offers a potential for a high degree of anisotropy, high-resolution pattern definition, and good process control. However, conventional RIE is not ideal for patterning PbZr1-xTixO3 films at a thickness as great as that in the original intended application. In order to realize the potential benefits mentioned above, it is necessary to optimize process conditions — in particular, the composition of the etching gas and the values of such other process parameters as radio-frequency power, gas pressure, gas-flow rate, and duration of the process. Guidelines for determining optimum conditions can be obtained from experimental determination of etch rates as functions of these parameters.

Etch-gas mixtures of BCl3 and Cl2, some also including Ar, have been found to offer a high degree of selectivity as needed for patterning of PbZr1-xTixO3 films on top of Ir electrode layers in thin-film capacitor structures. The selectivity is characterized by a ratio of Å10:1 (rate of etching PbZr1-xTixO3 ÷ rate of etching Ir and IrOx). At the time of reporting the information for this article, several experiments on RIE in BCl3 and Cl2 (and sometimes Ar) had demonstrated the 10:1 selectivity ratio, and further experiments to enhance understanding and obtain further guidance for optimizing process conditions were planned.

This work was done by Eui-Hyeok Yang and Larry Wild of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Manufacturing category.

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Refer to NPO-30349, volume and number of this NASA Tech Briefs issue, and the page number.

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