Manufacturability of highly doped aluminum nitride films

Abstract

There have been several investigations [1], [2], [3], that demonstrated benefits of adding dopants such as (Sc) or combination of other materials, like Zr/Mg for example, to the aluminum nitride (AlN) films in order to increase coupling coefficient (kt^2) of the Bulk Acoustic Wave (BAW) devices. For concentrations below 10% atomic Sc, it is possible to use a single composite target with a standard magnetron design [4]. Most R&D systems that performed initial investigations on AlScN films with high concentration of Sc dopant, used two separate targets with two separate magnetrons: one with pure Al and one with pure Sc with different applying power to compensate for the large difference in sputtering rates of the two materials and get stoichiometric composition. Unfortunately, depositing from two different targets is only viable for low volume R&D experiments. The system described in this article uses standard dual conical magnetron with AC deposition source. Targets are cut into multiple segments as shown in Figure 1 [5]. Based on simple geometry of target's surface, deposited film composition is proportional to the surface of specific pieces of target material. Unfortunately, Al is eroded at much higher rate than Sc at the same potential and same magnetic field. Over the target life, concentration of Sc increases in the deposited films. In order to maintain same Sc composition over the entire target life, it is necessary to vary magnetic field locally over the surface of the Al and Sc pieces to provide same erosion rate of Al vs. Sc at the same target potential. Adjusting magnetic field for each segment of both Al and Sc allows for constant deposited film composition over the entire target life solves this problem.