Low-Temperature Epitaxial Growth of AlN Thin Films on a Mo Electrode/Sapphire Substrate Using Reactive Sputtering

Abstract

High-crystalline aluminum nitride (AlN) thin films are essential for device applications, and epitaxial growth is a promising approach to improve their crystalline quality. However, a high substrate temperature is usually required for the epitaxial growth, which is not compatible with the complementary metal-oxide-semiconductor (CMOS) process. Furthermore, it is very difficult to obtain epitaxial AlN thin films on the deposited metal layers that are sometimes necessary for the bottom electrodes. In this work, epitaxial AlN thin films were successfully prepared on a molybdenum (Mo) electrode/sapphire substrate using reactive sputtering at a low substrate temperature. The structural properties, including the out-of-plane and in-plane relationships between the AlN thin film and the substrate, were investigated using X-ray diffraction (XRD) 2θ-ω, rocking curve, and pole figure scans. Additional analyses using scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) were also carried out. It was shown that highly c-axis-oriented AlN thin films were grown epitaxially on the Mo/sapphire substrate with an in-plane relationship of AlN [112¯0]//sapphire [101¯0]. This epitaxial growth was attributed to the highly ordered and oriented Mo electrode layer grown on the sapphire substrate. In contrast, the AlN deposition on the Mo/SiO2/Si substrate under the same conditions caused poorly oriented films with a polycrystalline structure. There coexisted two different low-crystalline phases of Mo (110) and Mo (211) in the Mo layer on the SiO2/Si substrate, which led to the high mosaicity and polycrystalline structure of the AlN thin films.

An 8-inch commercial aluminum nitride MEMS platform for the co-existence of Lamb wave and film bulk acoustic wave resonators

“…Although predefining the release cavity on the interposer layer provides multiple benefits for both FBAR and LWR devices, the extra layers added compared with the conventional design may influence the AlN thin film, hence affecting its performance. Typically, the quality of AlN thin film is highly correlated to the quality of the Mo bottom electrode underneath [47] and the AlN thickness [14] (thicker AlN tends to yield a lower FWHM value). Although the FWHM measured in this work is not as good as some prior arts [16] due to thinner AlN thickness, the performance (k 2 t , Q) delivered is still comparable with the state-of-the-art.…”

Structural Analysis of Mo Thin Films on Sapphire Substrates for Epitaxial Growth of AlN

“…Microelectromechanical systems (MEMS) based on micromachining technology are promising for applications in various industries including electronics, energy, automotive, biomedical, aerospace, and robotics. Many types of materials have been investigated for MEMS applications, and among them, aluminum nitride (AlN) has received considerable attention owing to its versatile and unique properties [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ]. AlN is a III–V compound semiconductor with a direct wide bandgap of 6.2 eV.…”

Strong (110) Texturing and Heteroepitaxial Growth of Thin Mo Films on MoS2 Monolayer

“…In AlN-based BAW resonators and high-frequency filters, − out-of-plane (110)-orientated Mo thin films are typically used as bottom electrodes because the (110) texture of the Mo films enables the growth of highly c -axis (0001) crystal orientated AlN active layers, which are required for achieving the highest piezoelectric constant and electroacoustic coupling. − It was revealed that the growth of (0001)-orientated AlN on (110)-textured Mo (or vice versa ) is possible thanks to the presence of the local heteroepitaxial relationship of AlN(0001)­[21̅1̅0]||Mo(110)[1̅11] with a small lattice mismatch of 4.9% and the effective reduction of the crystallization energy. , The degree of the c -axis (0001) crystal orientation in the AlN growth crucially depends on the quality of the (110)-texturing and the surface morphology of the Mo bottom electrode. − Sputtering or physical vapor deposition is the most frequently used method to deposit Mo thin films with resistivities lower than 10 –6 Ω m, which are normally required to reduce the ohmic loss . Although some reported improved (110)-textures in Mo films by adjusting the working pressure, the discharge power, and the deposition rates in magnetron sputtering, the thickness of the Mo bottom electrodes typically needs to be a few hundred nanometers (e.g., ∼200–300 nm) to achieve a high degree (110)-texture evolution. , This may pose a serious challenge for further development of the AlN-based BAW technology, as the resonance frequency of a BAW device is inversely proportional to the thickness of the AlN layer and its (top and bottom) electrodes .…”