Growth of C-Axis Textured AlN Films on Vertical Sidewalls of Silicon Microfins

• Published in: IEEE transactions on ultrasonics, ferroelectrics, and frequency control Vol. 68; no. 3; pp. 753 - 759
• Main Authors: Ramezani, Mehrdad, Felmetsger, Valeriy V., Rudawski, Nicholas G., Tabrizian, Roozbeh
• Format: Journal Article
• Language: English
• Published: United States IEEE 01-03-2021; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: 3-D integration; Acoustic waves; Aluminum nitride; aluminum nitride (AlN); Aspect ratio; Crystal structure; Crystallinity; Deposition; Film thickness; fin bulk acoustic wave resonator (FinBAR); high-quality-factor (<italic xmlns:ali="http://www.niso.org/schemas/ali/1.0/" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">Q ); III-V semiconductor materials; Magnetron sputtering; Platinum; Resonators; Rough surfaces; sidewall piezoelectric transducer; Silicon; silicon (Si) micromechanical resonator; Single crystals; Surface layers; Surface roughness; Surface treatment; Texture; Wireless communication systems; Wireless communications
• ISSN: 0885-3010; 1525-8955
• DOI: 10.1109/TUFFC.2020.3013111

A fabrication process is developed to grow <inline-formula> <tex-math notation="LaTeX">{c} </tex-math></inline-formula>-axis textured aluminum nitride (AlN) films on the sidewall of single-crystal silicon (Si) microfins to realize fin bulk acoustic wave resonators (FinBARs). FinBARs enable ultradense integration of high-quality-factor (<inline-formula> <tex-math notation="LaTeX">{Q} </tex-math></inline-formula>) resonators and low-loss filters on a small chip footprint and provide extreme lithographical frequency scalability over ultra- and super-high-frequency regimes. Si microfins with large aspect ratio are patterned and their sidewall surfaces are atomically smoothened. The reactive magnetron sputtering AlN deposition is engineered to optimize the hexagonal crystallinity of the sidewall AlN film with <inline-formula> <tex-math notation="LaTeX">{c} </tex-math></inline-formula>-axis perpendicular to the sidewall of Si microfins. The effect of bottom metal electrode and surface roughness on the texture and crystallinity of the sidewall AlN film is explored. The atomic-layer-deposited platinum film with (111) crystallinity is identified as a suitable bottom electrode for deposition of <inline-formula> <tex-math notation="LaTeX">{c} </tex-math></inline-formula>-axis textured AlN on the sidewall with <inline-formula> <tex-math notation="LaTeX">{c} </tex-math></inline-formula>-axis orientation of 88.5° ± 1.5° and arc-angle of ~12° around (002) diffraction spot over film thickness. A 4.2-GHz FinBAR prototype is implemented showing a <inline-formula> <tex-math notation="LaTeX">{Q} </tex-math></inline-formula> of 1574 and effective electromechanical coupling (<inline-formula> <tex-math notation="LaTeX">{k}_{\text {eff}}^{{2}} </tex-math></inline-formula>) of 2.75%, when operating in the 3rd width-extensional resonance mode. The lower measured <inline-formula> <tex-math notation="LaTeX">{Q} </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">{k}_{\text {eff}}^{{2}} </tex-math></inline-formula> compared to simulations highlights the effect of granular texture of sidewall AlN film on limiting the performance of FinBARs. The developed <inline-formula> <tex-math notation="LaTeX">{c} </tex-math></inline-formula>-axis textured sidewall AlN film technology paves the way for realization and monolithic integration of multifrequency and multiband FinBAR spectral processors for the emerging carrier aggregated wireless communication systems.