Quantitative Imaging of Permittivity and Tunability with a Near-Field Scanning Microwave Microscope

We have developed a near-field scanning microwave microscope to quantitatively image the dielectric permittivity and tunability of thin-film dielectric samples on a length scale of 1 μm. We have demonstrated this technique with permittivity images and local hysteresis loops of a 370 nm thick Ba_0.6Sr_0.4TiO₃ thin film at 7.2 GHz. We also observe the role of annealing in the recovery of dielectric tunability in a damaged region of the thin film. We can measure changes in relative permittivity ε_r as small as 2 at ε_r = 500, and changes in dielectric tunability dεr/dV as small as 0.03 V^-1.

References:

D. E. Steinhauer, et al., Appl. Phys. Lett., 75,3180 (1999)., Cond-mat

D. E. Steinhauer et al., Rev. Sci. Instrum. 71, 2751 (2000).

Figures:

Fig. 1

Schematic of the near-field scanning microwave microscope. The open-ended coaxial probe has a sharp tip (see inset) which is held in gentle contact with the sample. The graph shows the frequency shift (Δf) as a function of dielectric permittivity (ε_r) for a series of 500 μm thick bulk samples at 7.2 GHz. The data points indicate experimental results, while the line shows model results.

Fig. 2

Images and hysteresis loops at 7.2 GHz of a (370 nm Ba_0.6Sr_0.4SrO₃) / (70 nm La_0.95Sr_0.05CoO₃) / (500 mm LaAlO₃) thin-film sample (see inset to Fig. 1). All images show the same 20 x 20 Μm² area of the sample. (a) Permittivity image. (b) Hysteresis loop taken at the location marked "+" in (a). (c) Dielectric tunability image. (d) Dielectric tunability hysteresis loops. The solid line corresponds to the "+" in (c), while the dashed line corresponds to the "o" in (c). (e) Dielectric tunability image taken after the sample was annealed at T = 650^o C in air for 20 minutes; note the change from (c). (f) Atomic force microscope topographic image. The milled vertical line marked by the arrow in (f) appears ragged in (a) and (c) because of drift in the microscope during scanning.

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