Paper TF-ThP7
Deposition of Relatively Thin Highly C-axis Oriented AlN Films for High Frequency Electro-Acoustic Devices

Thursday, October 21, 2010, 6:00 pm, Room Southwest Exhibit Hall

Thin film bulk acoustic resonators (FBAR) and bulk acoustic wave (BAW) filters based on piezoelectric aluminum nitride (AlN) thin films are widely employed for different wireless applications. As the resonance frequency of the resonator is determined by the thickness of the AlN layer, there is essential interest of using ultra-thin 100-200 nm films to extend the current technology from 1-2 to 5-10 GHz range. Reactive magnetron sputtering is a method of choice enabling formation of the films with a high degree of c-axis texture; however it is challenging to deposit such thin films with acceptable piezoelectric coefficients due to their drastically downward crystallinity compared to 500-2000 nm thick films required for lower frequency devices.

In this study, we use reactive sputtering technology with a dual cathode ac powered S-Gun magnetron [1]. The quality of the c-axis crystal orientation is characterized by the full width at half maximum (FWHM) of the AlN (0002) X-ray diffraction peak. AlN films deposited by the S-Gun on low-doped Si wafers exhibit a strong crystal orientation, which is improved with increasing film thickness. It is more sophisticated to achieve the same crystal orientation when the film is deposited on a metal bottom electrode, as it is required for the FBAR and BAW devices. Therefore, formation of well-textured underlying electrodes is essential, especially if the AlN films are relatively thin.

The crystal orientation of common electrode metals and alloys deposited on Si, SiO₂, SiC, and diamond-like substrates is greatly enhanced by depositing a thin AlN seed layer underneath the metal. In the paper, we will discuss the critical conditions and process steps required for producing ultra-thin highly textured AlN films. For this purpose, we have developed a two-step sputter deposition process enabling better conditions for AlN nucleation. After preliminary rf plasma treatment of the substrate surface, the first 20-50 nm thick AlN layer is deposited at elevated temperature with higher nitrogen concentration in Ar-N₂ gas mixture, stimulating growth of higher quality columnar grains with the increase of AlN film thickness.

The results obtained for 100 and 200 nm thick AlN films deposited onto different metal electrodes (Mo, Cr, Ir) are presented in the paper.

The sputter technology has demonstrated high efficiency in producing very thin (100 nm) AlN films exhibiting superior crystallinity with FWHM < 2.5˚on Mo electrode, which is equal to or even better than the value for the deposition on Si substrates.

[1] V. V. Felmetsger, P. N. Laptev, and S. M. Tanner, Surf. & Coat. Technol., 204 (2009) 840-844.