Processing and On-Wafer Measurements of Ferroelectric Interdigitated Tunable Microwave Capacitors
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Processing and On-Wafer Measurements of Ferroelectric Interdigitated Tunable Microwave Capacitors Alexander M. Grishin, Jang-Yong Kim, Sergey I. Khartsev, Department of Condensed Matter Physics, Royal Institute of Technology, SE-164 40 Stockholm-Kista, SWEDEN
ABSTRACT Na0.5K0.5NbO3 (NKN) and Pb(Zr0.53Ti0.47)O3 (PZT) films have been grown by rf-magnetron sputtering and pulsed laser deposition techniques, correspondingly, on sapphire (Al2O3-0112, rcut), YAlO3 + 1% Nd (Nd:YAlO3-001), and quartz (Y+36°-cut) single crystal substrates. Interdigital capacitor (IDC) of coplanar waveguide (CPW) structures were defined by a standard lift off technique in a Au(0.5µm)/Cr(10nm) electrode electron beam evaporated on ferroelectric film surface. IDCs consisted of five pairs of fingers separated by 2 and 4 µm gap. On-wafer microwave characterization was performed using a workbench equipped with a coplanar probe station (Cascade Microtech) with G-S-G (Ground-Signal-Ground) Picoprobe, a network analyzer (Agilent Technologies E8364A) operating in 45 MHz to 40 GHz range and programmable power supply for dc DUT (Device Under Test) biasing. Assumed equivalent circuit for the IDC/CPW structure contains planar capacitor under test C, the coplanar line with a complex impedance ς and a parasitic capacitance Cp between the signal and ground lines. The de-embedding technique has been employed to determine all six complex parameters C, ς and Cp from S-parameter measurements performed for three different device structures: device, open and thru. NKN film interdigital capacitors on sapphire show superior performance in this microwave range: the frequency dispersion was as low as 18%, voltage tunability = 1 – C(40V)/C(0) (40 V, 200 kV/cm) about 14%, loss tangent ~ 0.11, K-factor = tunability/tanδ from 131% @ 10 GHz to 56% @ 40 GHz. The reliability of the de-embedding procedure is clearly proved by analysis of the frequency dependences of the parasitic capacitance and loss tangent as well as impedance of the coplanar line. Within the accuracy of experimental data and de-embedding calculations these values appear to be voltage independent: Cp ~ 70 fF, tan δp changes from 0.07@10 GHz to 0.15@40 GHz; real and imaginary part of interconnect impedance increases with frequency from 0.16 Ω@10 GHz to 0.36 Ω@40 GHz and from 1.6 Ω@10 GHz to 5.84 Ω@40 GHz respectively. INTRODUCTION Electrically tunable ferroelectric films for microwave devices have become the subject of intense research efforts in recent years. Dielectric permittivity in ferroelectric films can be decreased by nearly an order of magnitude by applying dc bias electric field. This changes the length of electromagnetic wave in the microwave device which can be exploited for various microwave devices: phase shifters, variable capacitors (varactors) for voltage controlled oscillators, tunable filters, electronically steerable antennas, etc. Recent advances in epitaxial ferroelectric film growth promise miniature, low voltage, low-distortion microwave devices with frequency-variable
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