Quantitative Sheet Resistance Imaging with a Near-Field Scanning Microwave Microscope

We have developed a technique for quantitative imaging of the sheet resistance of metallic thin films by monitoring frequency shift and quality factor in a resonant scanning near-field microwave microscope. This technique allows fast acquisition of images at approximately 10 ms per pixel over a frequency range from 0.1 to 50 GHz. In its current configuration, the system can resolve changes in sheet resistance as small as 0.6 Ω/ o for 100 Ω/o films. We demonstrate its use at 7.5 GHz by generating a quantitative sheet resistance image of a YBa₂Cu₃O_7-8(YBCO) thin film on a 5 cm diameter sapphire wafer.

References:

D. E. Steinhauer et al., Applied Physics Letters, 71 (12), p. 1736 (1997). Cond-mat

D. E. Steinhauer et al., Applied Physics Letters, 72 (7), p. 861 (1998). Cond-mat

Figures:

Fig. 1.

Schematic of the near-field scanning microwave microscope. The sample is scanned beneath the open-ended coaxial probe, which is connected to a resonant transmission line. The transmission line is coupled to the microwave source via a decoupler. A diode detector outputs a voltage proportional to the microwave power escaping from the resonator. The inset shows the probe tip and sample, with the probe-sample capacitance C_X and the sample resistance R_X. The feedback circuit keeps the microwave source locked onto a resonant frequency of the transmission line resonator. Two voltage outputs of the microscope are Δf, which is proportional to frequency shift, and V_2fFM, which is related to the Q of the microscope resonator.

Fig. 2(a).

The symbols indicate the resonant frequency shift as a function of sheet resistance R_X of the sample. The lines indicate results of a model based on microwave circuit analysis of the system. A probe with a 500 μ m center conductor was used, at a frequency of 7.8 GHz.

Fig. 2(b).

Frequency shift as a function of probe-sample separation. The inset is a diagram of a model for the probe's interaction with a bare substrate, with a fringe capacitance C', and an air gap capacitance C(h).

Fig. 2(c).

Unloaded quality factor (Q₀) of the resonator as a function of sample sheet resistance R_X and probe-sample separation, at 7.5 GHz. The solid line is the result of our simple model for a height of 50 μ m.

Fig. 3(a).

Optical photograph of NIST standard 3.2 lines/mm resolution target. The target consists of a pattern of chromium thin film lines on glass.

Fig. 3(b).

A false-color frequency shift image obtained with the scanning microwave microscope. The color bar indicates the frequency shift corresponding to different colors in the image. This image was obtained at a frequency of 10.7 GHz, with a probe with a 200 μ m diameter center conductor, at a height of 5 μ m.

Fig. 4.

Room temperature images of a variable-thickness YBCO thin-film on a 5 cm diameter sapphire wafer, where the film is the thickest at the center. The tick marks are 1 cm apart for the images of (a) frequency shift relative to the resonant frequency (7.5 GHz) when the probe is far away from the sample, (b) unloaded Q, and (c) sheet resistance (R_X). The arrows in (c) point to small semi-circular regions where clips held the wafer during deposition, and thus no film is present. The labels indicate values at each contour line. A probe with a 500 μ m diameter center conductor was used at a height of 50 μ m, at a frequency of 7.5 GHz. The data in (c) was obtained by converting the Q data in (b) to R_X, using the relationship shown by the blue "+" signs in Fig. 2(c).

---

Center for Superconductivity Research, University of Maryland, College Park, MD 20742-4111
Phone: 301.405.6129 Fax: 301.405.3779
Copyright © 2001 University of Maryland
Contact us with comments, questions and feedback