Study of ferroelectric/dielectric multilayers for tunable stub resonator applications at microwaves

• High tunability and significant global loss of resonator printed on KTa0.5Nb0.5O3 layer.
• Resonator tunability decreases with the increase of the Bi1.5Zn0.9Nb1.5O7−δ (BZN) layer thickness.
• Resonator global loss decreases with the increase of the BZN layer thickness.
• The figures of merit of resonators are similar, with and without the BZN layer.

Tunable coplanar waveguide stub resonators deposited on various ferroelectric/dielectric heterostructures are studied in the 10-GHz band. A frequency tunability of up to ~ 45% is achieved under a moderate biasing field (Ebias < 100 kV/cm) when the resonator is printed on KTa0.5Nb0.5O3 (KTN) ferroelectric thin film alone: this comes from the large permittivity agility of the KTN material (εr(KTN) varies from ~ 700 to ~ 200). Nevertheless this also leads to significant insertion loss due to the dielectric loss of the ferroelectric material itself (tanδr(KTN) ≈ 0.15–0.30 at 10 GHz). In this paper, an original route has been considered to reduce the device loss while keeping up a high frequency tunability. It consists in associating the KTN film with a dielectric film to elaborate ferroelectric/dielectric multilayers. The Bi1.5Zn0.9Nb1.5O7−δ (BZN) oxide material is selected here for two main reasons, namely its low dielectric loss (tanδr(BZN) ≈ 0.005–0.0075) and its moderate relative permittivity (εr(BZN) ≈ 95–125) at 12.5 GHz. The relevance of this approach is studied numerically and experimentally. We compare numerically two different heterostructures for which the ferroelectric film is grown on the dielectric film (KTN/BZN), or vice versa (BZN/KTN). A stub resonator printed on the most relevant heterostructure has been fabricated, and experimental data are discussed and compared to the numerical results.