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Stress and piezoelectric properties of aluminum nitride thin films deposited onto metal electrodes by pulsed direct current reactive sputtering

Abstractc‐Axis oriented aluminum nitride (AlN) thin films are successfully prepared on amorphous polyimide films by radiofrequency magnetron reactive sputtering at room temperature. Structural analysis shows that the AlN films have a wurtzite structure and consist of c‐axis oriented columnar grains about 100 nm wide. The full width at half maximum of the X‐ray diffraction rocking curves and piezoelectric coefficient d33 of the AlN films are 8.3° and 0.56 pC N–1, respectively. The AlN films exhibit a piezoelectric response over a wide temperature range, from –196 to 300 °C, and can measure pressure within a wide range, from pulse waves of hundreds of pascals to 40 MPa. Moreover, the sensitivity of the AlN films increases with the number of times it was folded, suggesting that we can control the sensitivity of the AlN films by changing the geometric form. These results were achieved by a combination of preparing the oriented AlN thin films on polyimide films, and sandwiching the AlN and polymer films between top and bottom electrodes, such as Pt/AlN/polyimide/Pt. They are thin (less than 10 μm), self powered, adaptable to complex contours, and available in a variety of configurations. Although AlN is a piezoelectric ceramic, the AlN films are flexible and excellent in mechanical shock resistance.

The authors have investigated the influence of oxygen concentration in sputtering gas on the piezoelectric response of aluminum nitride (AlN) thin films prepared on silicon substrates. The piezoelectric response strongly depends on the oxygen concentration, and changes from +6.8to−7.0pC∕N with increasing oxygen concentration from 0% to 1.2%. The polar direction drastically inverts from the Al polarity to N polarity. When the oxygen concentration in sputtering gas was 1.2%, the oxygen concentration in the AlN films was 7at.%. Furthermore, the growth rate of the AlN films gradually decreases with increasing oxygen concentration in sputtering gas.

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The authors have investigated the influence of sputtering pressure on the polarity distribution of aluminum nitride (AlN) films. They have found that sputtering pressure strongly influences the polarity distribution of AlN films prepared on molybdenum electrodes. The polarity distribution of the AlN films was observed by piezoresponse force microscopy. The polarity orientation is decided with respect to each fine grain constituting the AlN films, and polarity conversion from Al polarity to N polarity is observed with increasing sputtering pressure. The piezoelectric response of the films changes from +3.7to−4.4pC∕N with increasing sputtering pressure from 0.36to4.0Pa.

The authors have investigated the influence of the crystal orientation and the polar distribution on the piezoelectric response of aluminum nitride (AlN) thin films prepared on Si, Mo∕Si, and Mo∕AlN-interlayer (IL)/Si substrates with increasing sputtering power. The crystal orientation of films prepared on Si improves with increasing sputtering power. On the other hand, the crystal orientation of films prepared on Mo∕Si and Mo∕AlN-IL∕Si hardly changes. However, the piezoelectric response of all the films drastically changes from negative to positive values and the predominant polarity changes from N polarity to Al polarity. We found the proportional relationship between the polar distribution and piezoelectric response.

The authors have investigated a combination of high and low modulus materials in diaphragms for high sensitivity response. High and low modulus materials are aluminum nitride (AlN) thin films and polyimide films, respectively. AlN was sputtered deposited. The AlN films consist of columnar crystal grains and indicate c-axis orientation. The diaphragm indicates a high sensitivity response of 37200pC∕N, although the piezoelectric coefficient d33 of the AlN film is 2.6pC∕N. The sensitivity response is fifty times as high as that of diaphragms consisted of AlN films and superalloy foils.