*Article* **Effect of Substrate-RF on Sub-200 nm Al0.7Sc0.3N Thin Films**

**Michele Pirro \* , Xuanyi Zhao, Bernard Herrera, Pietro Simeoni and Matteo Rinaldi**

Electrical and Computer Engineering Department, Northeastern University, Boston, MA 02115, USA; zhao.xuan@northeastern.edu (X.Z.); herrerasoukup.b@northeastern.edu (B.H.); p.simeoni@northeastern.edu (P.S.); m.rinaldi@northeastern.edu (M.R.)

**\*** Correspondence: pirro.m@northeastern.edu

**Abstract:** Sc-doped aluminum nitride is emerging as a new piezoelectric material which can substitute undoped aluminum nitride (AlN) in radio-frequency MEMS applications, thanks to its demonstrated enhancement of the piezoelectric coefficients. Furthermore, the recent demonstration of the ferroelectric-switching capability of the material gives AlScN the possibility to integrate memory functionalities in RF components. However, its high-coercive field and high-leakage currents are limiting its applicability. Residual stress, growth on different substrates, and testing-temperature have already been demonstrated as possible knobs to flatten the energy barrier needed for switching, but no investigation has been reported yet on the whole impact on the dielectric and ferroelectric dynamic behavior of a single process parameter. In this context, we analyze the complete spectrum of variations induced by the applied substrate-RF, from deposition characteristics to dielectric and ferroelectric properties, proving its effect on all of the material attributes. In particular, we demonstrate the possibility of engineering the AlScN lattice cell to properly modify leakage, breakdown, and coercive fields, as well as polarization charge, without altering the crystallinity level, making substrate-RF an effective and efficient fabrication knob to ease the limitations the material is facing.

**Keywords:** scandium-doped aluminum nitride; ferroelectric; MEMS; substrate-RF; residual stress; coercive field; leakage current
