PhD Thesis Arne Lüker final version V4 - Cranfield University

Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications
PST directly on SiO2
Ferroelectric thin films like PZT and BST often need a metallic like layer on top of the
substrate to promote a crack-free and highly orientated growth. The most common
metallic layer is Pt due to its high chemical resistance, relative good lattice match to the
ferroelectric film, and its stability to the high temperatures required for the growth of
ferroelectric thin films. Pt is a precious material and therefore a not insignificant part of
the cumulative overall costs of ferroelectric devices and applications. Moreover,
sometimes it is beneficial from the technological site of view to have the ability to work
with a patterned bottom electrode rather than a continuous one.
Fig. 7.1 and Fig. 7.2 show examples of realised applications with PST on a patterned Pt
bottom electrode. The Ti/Pt layer was first deposited on a thermal oxidised Si substrate
like described in section 3.1.1. Standard
photolithography and a liftoff process were
used to define the bottom electrode prior to
the deposition of 500 nm
Pb0.4Sr0.6Mn0.03Ti0.97O3. To form the specific
capacitor areas and provide access to the
bottom electrode, the PST film was then
patterned using a deionized water diluted HF
and HCl etch solution. To protect the relevant
PST areas, a Shipley S18181 photoresist mask
was used, again using standard
photolithography. After etching, the resist
layer was dissolved with acetone. A 1.7 µm thick Nickel layer serves as top electrodes.
Fig. 7.1 shows a simple design of a varactor produced this way while Fig. 7.2 shows a
more sophisticated varactor design, a slotted varactor, to minimize the variation of the
capacitor area due to mask alignment errors [1].
SiO2
PST
Pt
Ni
Fig. 7.1: PST on SiO2 and a patterned Pt bottom
electrode
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