Compressive stress reduction in sputter-deposited yttrium aluminum nitride (Y₀.₂Al₀.₈N) thin films for BAW resonators with high electromechanical coupling

Abstract

In the past decade, rare earth elements have found increasing interest in enhancing the piezoelectric coefficient of aluminum nitride (AlN) thin films. Until now, scandium has been the most successful rare earth element to enhance the piezoelectric coefficient (d₃₃) up to 600 %. In this research paper, we present a 250 % increase in d₃₃ to 12.5 pC/N for sputter-deposited yttrium alloyed aluminum nitride (Y₀.₂Al₀.₈N) thin films compared to pure AlN. However, this increase in d₃₃ comes with a substantial increase in compressive layer stress well above 1 GPa. Therefore, a chamber pressure sweep technique is introduced, resulting in a 40 % reduction in the stress level without significantly affecting the crystallographic film parameters to facilitate device integration. Even more, high compressive stress in thin films is a critical concern in piezoelectric MEMS devices such as bulk acoustic wave (BAW) resonators, as it can adversely affect the piezoelectric material properties and hence, the electromechanical performance. For this purpose, we demonstrate that the pressure sweep technique improves the electromechanical coupling factor of Y₀.₂Al₀.₈N to 7.02 %, being substantially above the reference value of 6.02 % set by pure AlN. In summary, the findings from this study may stimulate further effort to integrate YₓAl₁₋ₓN thin films in BAW filter devices for future telecommunication applications.