Epitaxial aluminium scandium nitride on molybdenum electrodes for bulk acoustic wave resonators

The demand for high-performance radio frequency (RF) filters, particularly in the 3.3 GHz to 5.9 GHz range driven by 5G telecommunications, has led to an interest in bulk acoustic wave (BAW) resonators exhibiting high effective electromechanical coupling coefficient (k 2 eff) and high quality factor (Q). While aluminum nitride (AlN) has conventionally served as the piezoelectric layer in BAW resonators, the superior piezoelectric response (d33) and intrinsic electromechanical coupling coefficient (k 2 t ) of aluminum scandium nitride with 30% Sc concentration (Al0.7Sc0.3N) promises further improvement in resonator performance. In addition, electrical, physical, and acoustic properties of the electrode material are vital for BAW resonator performance. Among possible options, molybdenum (Mo) emerged as the preferred electrode.
This study focuses on assessing resonator performance by employing a high overtone bulk acoustic wave resonator (HBAR) and exploring the impact of the top electrode material and crystallographic texture of sputtered materials. Magnetron sputtering was utilized to deposit all layers, with AlN acting as a seed layer on silicon (Si) substrates to facilitate epitaxial growth of Mo and Al0.7Sc0.3N.
Epitaxial growth of AlN on Si substrates at 700°C was observed with improved crystal quality (XRD ω-FWHM = 1.3°), reduced oxygen impurities, and sub-band gap absorption. The AlN films were determined to be mixed polar, and a growth model explaining the origin of mixed polarity is presented. To obtain uni-polar AlN, an aluminum (Al) nucleation layer was used. However, Al was found to grow as islands, promoting growth of mixed polar AlN. Ideal growth conditions required to obtain thin but closed Al layers are presented. Introducing ammonia (NH3) during sputtering resulted in quasi-2D-growth of AlN, leading to a 35% reduction in surface roughness, a 78% increase in average grain size, and a 76% increase in thermal conductivity.
Resistivity analysis revealed that epitaxial Mo (6.6 ± 0.06 µΩcm) has a lower resistivity than fiber-textured Mo (7.29 ± 0.06 µΩcm). Further improvement in resistivity (5.95 ± 0.06 µΩcm) was observed when Mo was grown on AlN sputtered in an NH3 atmosphere. The observed improvement in resistivity was mostly related to an increase in grain diameter. Mo grown on epitaxial AlN was found to have three rotational domains; a growth model explaining this phenomenon is presented. AlScN grown at high total sputter power resulted in a reduction of abnormally oriented grains. Al0.7Sc0.3N grown on epitaxial Mo limited the rotation of grains about the c-axis and improved its crystal quality (XRD ω-FWHM = 1.9°). A comparison iii between HBAR resonators with gold (Au) and Mo as the top electrode showed that the fundamental resonance frequency shifts to a higher frequency with Mo as the top electrode. HBARs fabricated using epitaxial material stacks exhibited a 25% improvement in performance compared to fiber-textured equivalents, highlighting the potential of epitaxial materials to enhance BAW resonator performance and, consequently, RF filter efficiency.