Gasto de los visitantes a la America´s Cup Match (año 2007)

Atomic Force Microscopy (AFM) was used to perform the surface characterization of the sputtered BST thin films deposited at different Ar/O2 ratios. All

the films were deposited at a substrate temperature of 600ºC. The two dimens ional surface plots of the BST thin films deposited at 90/10 and 70/30 Ar/O2 ratios are shown

in Figure 4.6. The BST films deposited at 90/10 Ar/O2 have a surface roughness of 3.4

nm while the BST films deposited at 70/30 Ar/O2 ratio ha ve a surface roughne ss of 7 nm.

The surface roughness increases with the oxygen content as the cation mobility on the growth surface reduces with higher oxygen pressures. Also, the sputtered BST thin films have a much lower surface roughness than the pulsed laser deposited BST thin films.

(a) (b)

4.2.3 XRD Analysis of Sputtered BST Films on MgO, Al2O3 and LaAl2O3

For the fabrication of interdigital capacitors, BST thin films were deposited on three different substrates like MgO, Al2O3 and LaAl2O3 using RF sputtering. A very good

control over the structural properties of the BST films deposited on these substrates has been achieved. All the films were deposited at a substrate temperature of 650 °C, RF power of 150 W and Ar/O2 ratio of 90/10 sccm. The optimized XRD results are shown in

Figure 4.7.

Figure 4.7 X-ray Diffraction Patterns of Sputtered BST Thin Film on MgO, Al2O3 and

LaAl2O3

BST/MgO

BST/Al2O3

As shown in Figure 4.7, epitaxial films were deposited on both MgO(100) and LaAl2O3(100) substrates and they were highly crystalline, whereas polycrystalline films

were deposited on alumina substrates. The intensities of the diffraction peaks of BST films on different substrates are tabulated in Table 4.5

Table 4.5 Intensity of All the Diffraction Peaks of BST on Different Substrates

Sample Plane Bragg’s Angle

[°2?] Intensity [cts] FWHM [°2Th.] d-spacing [Å] 100 21.9797 3529.52 0.1574 4.04404 MgO - A 200 44.7994 14176.20 0.3542 2.02311 100 21.8089 2057.97 0.2362 4.07532 LaAl2O3 -B 200 44.4467 17842.50 0.3149 2.03834 110 31.5592 173.99 0.1968 2.83498 Al2O3- C 200 45.3074 33.74 0.6298 2.00160

4.2.4 AFM Analysis of Sputtered BST Films on MgO and Al2O3

The two dimensional surface plots of the BST films deposited at 90/10 Ar/O2

ratios on both MgO and Al2O3 substrates are shown in Figure 4.8. The BST film

deposited on MgO has a surface roughness of 1.14 nm while the BST film deposited on Al2O3 has a surface roughness of 3.43 nm.

Sol - gel BST Solution

500rpm x 3sec 2000 rpm x 20 sec

Drying

(Hot Plate – 300 - 400 ° C/min)

Crystallization (Furnace/RTA - 600 -800 ° C)

BST Thin Films Repetition

Sol - gel BST Solution

500rpm x 3sec 2000 rpm x 20 sec

Drying

(Hot Plate – 300 - 400 ° C/min)

Crystallization (Furnace/RTA - 600 -800 ° C)

BST Thin Films Repetition

Figure 4.8 Surface View of BST Films Sputtered on MgO and Al2O3

4.3 XRD Analysis of Sol-gel Deposited BST Thin Films

BST thin films were deposited by sol gel on MgO substrates and the process flow for the deposition is shown in Figure 4.9.

Sol gel solution were bought from Mitsubishi inc., and the steps depicted in process flow was followed. The XRD spectra of the sol gel deposited BST films show that they are epitaxial and (100) dominated and it’s shown in 4.10. Annealing was done at 700 °C for two hours in oxygen ambience.

BST(100)

BST(200) BST(100)

BST(200)

Figure 4.10 X-ray Diffraction Patterns of Sol- gel Deposited BST Thin Film on MgO

4.4 Summary

The deposition conditions have a significant impact on the structural and electrical properties of BST thin films. Optimization of the deposition conditions for high tunability and low dielectric loss BST thin films has been accomplished for both parallel plate and interdigital structures. The structural characterization of the BST thin films deposited using pulsed laser deposition, sputtering and sol- gel has been performed by XRD and AFM. Pulsed laser deposited BST thin films on Pt/TiO2/SiO2/Si turn

polycrystalline with increase in temperature. An improvement in crystallinity has been observed with increasing oxygen content with well defined grains boundaries. High laser energy and smaller substrate to target distance cause incomp lete surface coverage with cluster formation. Sputtered BST thin films Pt/TiO /SiO /Si have better crystallinity at

higher temperatures. Changes to the Ar/O2 ratio affect the crystallinity of the films.

Increasing oxygen concentration changes the films from being polycrystalline to monocrystalline and increases the surface roughness of the films. BST thin films were also deposited on MgO, Al2O3 and LaAl2O3 substrates by sputtering. Epitaxial films were

obtained on MgO and LaAl2O3 substrates while polycrystalline films were obtained on

Al2O3 substrate. BST thin films were deposited by sol gel deposition method on MgO

CHAPTER FIVE

ELECTRICAL CHARACTERIZATION

Electrical characterization of BST thin film is necessary to integrate BST capacitors in RF and microwave applications. Polarization mechanism relates the variation of dielectric constant across the frequencies and variation in dielectric loss. The total polarizability is dependent on electronic, ionic, dipole and space charge. The objective is to study the dependence of dielectric constant (tunability) and dielectric loss tangent on the applied field in RF frequencies, since they are the important factors for the realization of device.

Tunability is defined as the ratio of maximum dielectric constant to its minimum value and is given as follows:

o v o v o T ? ? ? ? ? : . 100 (%) ? ? ? (5.1) where e0 is the dielectric constant at zero bias voltage and ev is the dielectric constant at

maximum bias voltage. The quality factor and loss tangent of BST thin film is related as

p s m BST R C Q Q Q ? ? ? tan? ? ? 1 1 1 (5.2)

where, Rp is the parallel resistance which accounts for BST losses, C is the capacitance of

the dielectric, tan d is the loss tangent and Q is the total quality factor and Rs is the series

the losses in lumped circuit elements. Quality factor can be defined as the ratio of stored energy to the average energy dissipated in the system per cycle. Dielectric loss is attributed to the damping of the polarization mode by impurities. Another important factor to be considered is the Figure of merit. The Figure of merit is given by

? ? ? ? tan . o v o k ? ? (5.3)

For tunable high frequency applications, the objective is to obtain high figure of merit, which can be achieved by high tunability and low dielectric loss. To attain this objective, it is very important to have accurate measurements of the devices.