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 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.

Yukari Morofuji ＊ , Non-member C-axis-oriented zinc oxide (ZnO) thin films were prepared on Inconel substrates by rf magnetron sputtering technique. The ZnO film characteristics were evaluated under pressures of 10.0 to 300.0 MPa and frequencies of 0.1 to 30 Hz at room temperature. The deviation from the linearity of charges with pressures for the ZnO films was within 0.52 % of a full scale at 300.0 MPa, which indicated a good linearity between 10.0 and 300.0 MPa. Furthermore the ZnO films were evaluated under combustion pressure in a single-piston 90 cc engine rotating at 1900 rpm. The waveform of the ZnO films was similar to those of a commercial sensor. It is demonstrated that the ZnO films have a good possibility as combustion pressure sensor elements.