Thick electrodes for high frequency high Q tunable ferroelectric thin film varactors

Abstract

Thin film barium strontium titanate (BST) shows great promise for voltage tunable dielectric devices for use at RF and microwave frequencies. An MOCVD process has been developed for production of BST, resulting in films with very low losses (as low as 0.002-0.004) and tunabilities over 50% at low operation voltages. With these values of BST loss, overall device quality factors at RF (100 MHz+) frequencies are primarily limited by losses in the thin metal electrodes, such as Pt, normally used for ferroelectric thin films. The bottom electrode in parallel plate capacitor structures is particularly challenging, since it must provide a good growth surface for BST and be stable at high (>600 "C) growth temperatures in an oxidizing atmosphere yet have high conductivity and compatibility with Si or SiO2/Si substrates. These challenges have previously prevented use of Pt thicknesses over 0.1-0.2 pm. Our solution to this problem, involves combinations of adhesion layers at the Pt/SiO2 interface and embedded stabilization layers to make functioning Pt bottom electrodes as thick as 2 pm. Devices with dielectric Q factors over 150 at 100 MHz (tan 6 -0.006 as measured and modeled by S-parameters) and overall device Q factors over 50 at 30 MHz are described. We have also inserted these devices into tunable filters, achieving tunabilities of 50% and low insertion losses (0.3 dB) at RF frequencies.

Silicon substrate integrated high Q-factor parallel-plate ferroelectric varactors for microwave/millimeterwave applications

“…In comparison with co-planar design 1,3 the devices with parallel-plate electrodes 2,4 offer lower control voltage and higher tuneability. The Q-factor of BST varactors on Pt/Si substrates is limited mainly by the losses in bottom Pt electrodes 5,6 which is usually 100-200 nm thick. 2,7 To reduce these losses bottom electrodes made of thick Pt (up to 2 µm) 5 , Au 8 , and Cu 9 have been proposed.…”

“…The Q-factor of BST varactors on Pt/Si substrates is limited mainly by the losses in bottom Pt electrodes 5,6 which is usually 100-200 nm thick. 2,7 To reduce these losses bottom electrodes made of thick Pt (up to 2 µm) 5 , Au 8 , and Cu 9 have been proposed. However, no Q-factor or/and tuneability of these varactors at microwave frequencies are reported explicitly.…”

Thin Film Ba0.25 Sr0.75 TiO3 Varactors on Au Bottom Electrode for Microwave Applications

“…The BSTO thin film ferroelectric varactors reveal performance superior to semiconductor analogues at frequencies higher than 10-20 GHz [4]. To reduce the losses highly conductive metal bottom electrodes made of thick Pt (up to 2 tm) [5], Pt/Au [4], [6] and Cu [7] have been proposed. However, the bottom electrode (template) strongly affects the crystallinity of the BSTO film and often produces specific interfacial regions.…”

Dependence of luminescence efficiency on dopant concentration and sintering temperature in the erbium-doped Ba0.7Sr0.3TiO3 thin films

“…[5][6][7][8][9][10][11][12][13][14][15][16][17] The high dielectric constant of BST offers the potential of producing denser memories with simpler capacitor structures than those fabricated with conventional silicon dioxide or silicon nitride dielectrics. Combining with the merits of electricity, BST:Er will be a promising candidate for optoelectronic devices.…”