Measurement of Intrinsic Mechanical Loss in Aluminum Films from 3 to 25 GHz by HBAR Spectroscopy

In this work we measure, for the first time, mechanical loss in an Al thin film from 3 GHz to 25 GHz through the use of high overtone bulk acoustic resonator (HBAR) spectroscopy. This is made possible by scaling down a HBAR to use a thin (200 nm) aluminum nitride (AlN) piezoelectric transducer and replacement of the thick substrate with a sputtered, suspended metal film (~1 µm). Resonant overtones are transduced piezoelectrically while >90% of the mechanical energy is confined to the Al, allowing Al mechanical loss to be accurately determined from quality factor at series resonance (Qs). Measured Qs for 7 overtones ranging from 3-25 GHz with values from 140 to 50 was found to be ~30% less than predicted by analytical models considering thermoelastic and phonon phonon damping. Additionally, measured frequency dependence of Q (f
^-0.46
) aligned well with the dependence predicted by thermoelastic and Landau Rumer regime phonon phonon damping (f
^-0.56
) and was well above the f
^-1
dependence predicted by thermoelastic and Akhiezer regime phonon phonon damping. This technique can be applied to measure mechanical loss in metal thin films up to K band, allowing loss characterization and enabling informed decision making for design of many high frequency acoustic devices.