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

Abstract

Polycrystalline aluminum nitride thin films were deposited onto platinum, aluminum, and titanium electrodes by reactive magnetron sputtering in the pulsed direct current mode. The films exhibited all a columnar microstructure and a c-axis texture. The built-in stress and the piezoelectric properties of these films were studied as a function of both the processing conditions and the electrode material. Stress was found to be very much dependent on the growth conditions, and values ranging from strong compression to high tension were observed. The piezoelectric d33,f coefficient was shown to rely on substrate quality and ionic bombardment: The nucleation surface must be stable with regard to the nitrogen plasma and present a hexagonal symmetry and, on the other hand, enough energy must be delivered to the growing film through ionic bombardment.

Low temperature aluminum nitride thin films for sensory applications

“…The magnitude of this potential increases as the deposition progresses, initially depending on the process parameters and afterwards on the increasing plasma resistance. 3 According to Figure 4, the self-induced bias increases from −18 V to −20 V (−25 V) for 250 nm (2 µm) thick samples. This self-induced voltage further accelerates the positive ions in the plasma towards the substrate, enhancing the surface diffusion and decreasing the defect density.…”

“…Ar + , N 2 + and N + . 3 The former occurs when some of the ions that are accelerated towards the cathode are neutralized by electron acquisition and get reflected back from the target surface towards the substrate. If the sputtering pressure is sufficiently low, these energetic neutrals experience very few collisions during their flight towards the substrate and hit the growing film surface with most of their energy conserved.…”

“…adding to the bombardment of the growing surface promoting a better texture quality. 3,23,33 Further investigations focus on the piezoelectric and dielectric properties of the deposited films with respect to the film thickness. Clamped (effective) piezoelectric coefficients as a function of the AlN film thickness are depicted in Figure 7(a).…”

“…is extensively investigated in the literature. 3,26 In this work, Pt was chosen as the seed layer. An epitaxial orientational relationship between Pt and AlN is known to promote the columnar and (002) oriented growth of AlN.…”

“…1,2 Despite having lower piezoelectric coefficients compared to popular piezoelectric materials, such as ferroelectric lead-zirconate-titanate (PZT) or ZnO, AlN is preferred in many MEMS applications due to its lower dielectric loss tangent and higher signal-to-noise ratio (SNR). [2][3][4][5][6][7][8][9] Most common applications include bulk resonators, ultrasonic transducers, sensors, actuators, bandpass filters and energy harvesters. 10 Non-epitaxial AlN films are usually prepared by means of chemical and/or physical vapor deposition (CVD and PVD) techniques.…”