Improved piezoelectric constants of sputtered aluminium nitride thin films by pre-conditioning of the silicon surface

Abstract

The group III–V material aluminium nitride (AlN) is frequently used in micro-electromechanical devices and systems (MEMS) due to its piezoelectric properties, its high thermal and electrical stability as well as its compatibility with CMOS technology. But, the trend towards miniaturization of MEMS devices requests a continuous decrease in geometrical dimensions of the active AlN thin film, thus demanding at least the same piezoelectric properties at lower film thickness. In this work, two different approaches are applied to measure the piezoelectric coefficients, using the direct as well as the converse piezoelectric effect. The first approach utilizes laser doppler vibrometry measurements in combination with finite element analysis, allowing the determination of d33 and d31. For the second method, an oscillating force is applied to the thin film and the generated charge is measured. A surface-near substrate conditioning step applying sputter etching is used in order to improve the piezoelectric coefficients over a wide thickness range (i.e. 40 nm to 400 nm) by about 20% compared to samples without pre-treatment. Basically, the coefficients remain constant for a film thickness of 100 nm and above, thus allowing the application of thin active layers of aluminium nitride without any reduction in the sensing and actuation potential.