We describe 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/✷ for 100 Ω/✷ films. We demonstrate its use at 7.5 GHz by generating a quantitative sheet resistance image of a YBa2Cu3O 7−δ thin film on a 5 cm-diameter sapphire wafer.

We describe near-field imaging of sample sheet resistance via frequency shifts in a resonant coaxial scanning microwave microscope. The frequency shifts are related to local sample properties, such as surface resistance and dielectric constant. We use a feedback circuit to track a given resonant frequency, allowing measurements with a sensitivity to frequency shifts as small as two parts in 10 6 for a 30 ms sampling time. The frequency shifts can be converted to sheet resistance based on a simple model of the system.There is a growing need to develop non-destructive microscopy techniques to quantitatively measure the microwave properties of materials on a length scale much less than the free space wavelength. For example, spatially resolved measurements of complex conductivity would be of significant utility for evaluating oxide superconducting and ferroelectric thin film samples. Sensitivity to microwave and millimeter wave surface resistance and dielectric constant have been previously demonstrated; for example, Bryant and Gunn 1 used a coaxial resonator to measure semiconductor resistivities on a 1 mm length scale. Waveguides 2 and coaxial geometries 3−7 have also been used to image conductivity and dielectric constant contrast. In this letter, we describe the use of a microwave microscope with an open-ended coaxial probe to quantitatively map the surface resistance of a metallic thin film.The key element in our system 7,8 is a 2 meter-long resonant coaxial transmission line (see Fig. 1). One end of the line is connected to an open-ended coaxial probe and the other end is weakly coupled to a microwave source via a capacitor C D . Near-field microwave energy at the exposed tip of the probe center conductor is coupled to the sample. As the sample is scanned beneath the probe tip, the resonant frequencies and quality factor Q of the open transmission line shift depending on the surface properties of the region of the sample closest to the probe's center conductor. 7,8 We measure the microwave power reflected back up the transmission line with a diode detector. 9 By using a fixed frequency source near one of the resonances f R , and scanning a sample underneath the probe, one can map the reflected power and generate an image. 7,8 However, this results in a convolution of two distinct contrast mechanisms: the frequency shift of the standing wave resonances and the change in Q.To disentangle these effects, we have developed a frequency-following feedback circuit, as shown in Fig. 1. We frequency modulate the source at a rate f F M ≈ 3 kHz with a deviation of about ±3 MHz and use a feedback loop to keep the average microwave source frequency locked to a specific resonant frequency f R (t) of the microscope. To accomplish this, the diode detector output voltage is amplified and sent to a lock-in amplifier referenced at the frequency f F M . The lock-in output is time-integrated; this voltage signal V out is added to the f F M oscillator signal and fed to the frequency-control input of the microwave source, (see Fig. 1...

We describe the use of 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 demonstrate this technique with permittivity images and local hysteresis loops of a 370 nm thick Ba0.6Sr0.4TiO3 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 ǫr as small as 2 at ǫr = 500, and changes in dielectric tunability dǫr/dV as small as 0.03 V −1 .

We describe the use of a near-field scanning microwave microscope to image the permittivity and tunability of bulk and thin film dielectric samples on a length scale of about 1 m. The microscope is sensitive to the linear permittivity, as well as to nonlinear dielectric terms, which can be measured as a function of an applied electric field. We introduce a versatile finite element model for the system, which allows quantitative results to be obtained. We demonstrate use of the microscope at 7.2 GHz with a 370 nm thick Ba 0.6 Sr 0.4 TiO 3 thin film on a LaAlO 3 substrate. This technique is nondestructive and has broadband ͑0.1-50 GHz͒ capability. The sensitivity of the microscope to changes in permittivity is ⌬⑀ r ϭ2 at ⑀ r ϭ500, while the nonlinear dielectric tunability sensitivity is ⌬⑀ 113 ϭ10 Ϫ3 ͑kV/cm͒ Ϫ1 .

We describe the operation of a simple near-field scanning microwave microscope with a spatial resolution of about 100 μm. The probe is constructed from an open-ended resonant coaxial line which is excited by an applied microwave voltage in the frequency range of 7.5–12.4 GHz. We present images of conducting structures with the system configured in either receiving or reflection mode. The images demonstrate that the smallest resolvable feature is determined by the diameter of the inner wire of the coaxial line and the separation between the sample and probe.