Impact of annealing temperature on piezoelectric and morphological properties of aluminium nitride thin films

Aluminium nitride (AlN) is a promising material for high temperature applications in harsh environments. Reactively sputter-deposited AlN from an aluminium target is an interesting piezoelectric thin film material with high CMOS compatibility. The properties of thin films differ from bulk material, especially at high temperatures. Therefore, the influence of different annealing temperatures up to 1000°C on the microstructure of AlN thin films with a typical thickness of about 500 nm was investigated. The degree of c-axis orientation is unaffected for temperature between room temperature and 400°C. By further increasing the annealing temperature up to 800°C, the intensity of the (0 0 2) peak decreases slightly. At 1000°C, the peak intensity decreased by a factor of about 1.5. The grain size increased with increased annealing temperature, where the grain size was about 40 nm in “as deposited” state and increased to 50 nm at 800°C and to 100 nm at 1000°C. At 1000°C annealing temperature the columnar growth is disturbed by the formation of pinholes inside the film. Finally, the piezoelectric coefficients d₃₃ and d₃₁ were determined experimentally by laser scanning vibrometry. Effective values for temperatures between room temperature and 400°C were 3.0 and -1.0 pm/V, respectively. By further increase of the annealing temperature, the piezoelectric coefficients decreased. The effective d₃₃ and d₃₁ values for films annealed at 800°C were 1.8 pm/V and -0.77 pm/V, respectively. As for films annealed at 1000°C, no displacement profile was detected by the laser vibrometer, which indicates poor film structure.