Surface resistance imaging with a scanning near-field microwave microscope

Abstract

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...

Review on Microwave Metamaterial Structures for Near‐Field Imaging

“…Thus, in principle, subwavelength spatial resolution becomes possible by reducing the aperture size and raster scanning the sample surface [17]. The concept was first experimentally demonstrated by Ash and Nicholls [26] (see the next section for a survey on near-field technique), who resolved a λ/60 grating structure by using λ = 3 cm microwave (10 GHz).…”

Broadband dielectric microwave microscopy on micron length scales

“…17 Basic details of the microscope system used in the present work were described elsewhere. 18,19 Here we summarize the additional features of the broadband microscope. The microscope is based on a half-wavelength coaxial transmission line resonator of length 25 cm with the inner conductor terminated by an Scanning Tunneling Microscope (STM) tungsten tip with about 1 µm-radius apex (Fig.…”

Low-temperature-compatible tunneling-current-assisted scanning microwave microscope utilizing a rigid coaxial resonator

“…A commercial STM controller (RC4 and SC4 by SPECS Surface Nano Analysis GmbH) is used to control tunnel current feedback. The quality factor, Q, and resonant frequency shift, ∆f , are measured either by acquiring the transmission spectrum using a PNA network analyzer or by the frequency feedback circuit (FFC) [30]. In the latter method, a frequencymodulated (FM) microwave with a frequency expressed as f c +D cos(2πf m t) (where f c , f m , and D are the carrier frequency, modulation rate, and frequency deviation, respectively) is used ( fig.…”