Reaney- Tunable Thin Film Capacitors

Question 1

What does the application of a large electric field on a ferroelectric material affect?

Answer 1

Strongly affects the temperature of the phase transition and the magnitude of the permittivity maximum. A larger field suppresses the peak in ε and shifts it to higher temperatures. Field can have a greater effect in some directions than others. The room temperature value of permittivity decreases with increasing bias field

Question 2

Effect on permittivity by applied field use in capacitors

Answer 2

The capacitance of the device alters on the application of an applied field. This principle of voltage dependent capacitance has led to research into tuneable filters

Question 3

Desirable properties in microwave circuit technology

Answer 3

High dielectric tunability with applied field (Δε/ε > 20%)
Low dielectric loss at microwave frequencies (tanδ < 0.005)
Low to moderate relative permittivity (30 to 1500)
Highly insulating properties

Question 4

Properties of materials chosen for microwave circuit technology

Answer 4

They have ferroelectric phase transitions below room temperature and so are paraelectric at room temperature. Most room temperature research into Ba1-xSrxTiO3 (BST) based materials where Tc decreases linearly with increasing Sr content at a rate of 3.3K/at% allowing accurate tailoring of the transition temperature

Question 5

Figure of merit for a tuneable dielectric

Answer 5

K=(εro-εrv)/εroTan(δο)
Where K is the figure of merit
εsubro is the permittivity at zero DC bias field voltage
εsubrv is the permittivity at maximum DC bias field voltage
Tanδsubo is the dielectric loss at zero DC bias

Question 6

Layout of a thin film capacitor based on BST

Answer 6

Square of BST, top electrode is a small square on top surface. Under is a ground plane and then the silicon substrate. Top and bottom electrodes usually Pt

Question 7

How is the dielectric for a planar thin film capacitor deposited?

Answer 7

Typically sol-gel, MOCVD or sputtering as used in NvRAMs

Question 8

Relative permittivity of BST vs DC voltage

Answer 8

Peak at 0V and exponential decrease either side

Question 9

Why are thin films required?

Answer 9

It is their integration into silicon which will eventually give rise to a mass market. Also the field strength V/m is what induces the changes in permittivity and needs to be high using only small voltages (3V) so need thin layers (1-200nm)

Question 10

Main problem associated with tuneable filter devices

Answer 10

Allocations require the filter is used at high frequencies (>1GHz and fields). BST has a high loss at these frequencies (0.005) and this is made worse when a field is applied to tune the device.

Question 11

Problems with tunability measurements

Answer 11

Rarely performed at high operating frequencies and the literature only quotes the low frequency data. Also dielectric loss not factored in and only %tunability considered.

Question 12

Which deposition method is the only one to give the required crystalline quality of BST thin layers?

Answer 12

MOCVD as the dielectric loss for sputtered or sol-gel films is too high to give K factors close to those necessary for applications

Question 13

Other materials that could be used instead of BST

Answer 13

Single crystals of SrTiO3 have excellent tunability and low dielectric loss.
Another candidate is CaTiO3 for cryogenic temperatures
As is cadmium pyroniobate Cd2Nb2O7
KTaO3 and (Ln0.5 Na0.5)TiO3 also
(Sr1-xPbx)TiO3 system, Bi2O3-ZnO-Nb2O5 and PbO-Nb2O5 based films

Question 14

SrTiO3 alternative to BST

Answer 14

For single crystals, εr can be reduced from 20000 to about 2000 at low temperatures by application of 10kV/cm DC field and tanδ around 10^-4. However only recently possible to fabricate films exhibiting similar losses

Question 15

Cadmium pyroniobate as a BST alternative

Answer 15

Without an applied field the temperature dependence of εr is characterised by occurrence of peaks around 80 and 180K. By applying DC bias peaks are greatly suppressed and finally eliminated at 15kV/cm. So high electric field tunability of permittivity with low dielectric loss at radio frequencies

Question 16

KTaO3 as a BST alternative

Answer 16

Possesses high tunability (50%) at 20kV/cm and low dielectric loss tanδ (0.003)

Question 17

(Ln0.5 Na0.5)TiO3 as a BST alternative

Answer 17

Shows low tunability (<1%) at 20kV/cm but a small dielectric loss (0.0004) over a wide T range 10-150K

Question 18

(Sr1-xPbx)TiO3 system as a BST alternative

Answer 18

At low frequencies Sr0.7Pb0.3TiO3 exhibits a respectable tunability level (70%) associated with a low dielectric loss (0.001)

Question 19

Future with 5G

Answer 19

Have mm wavelengths and 30GHz frequencies. This mobile signal will be increasingly adsorbed by the atmosphere and buildings. Higher powered transmission environmentally unacceptable so devices will require antennas that can be steered electronically to the signal source. Sequence of tuneable devices of BST-like materials can facilitate formation of phased array antenna that can be steered at relatively low voltages and solve signal absorption without increasing the power of mobile mast transmissions.