| In a near-field scanning microwave microscope, the interaction of microwaves
with a sample are investigated and imaged. The microscope usually
consists of an electromagnetic resonator which scans the sample under test.
Inhomogeneities in the sample's conductivity or permittivity change the
resonant frequency and/or quality factor of the resonator and can thus
be imaged. Although the wavelength of the electromagnetic radiation
we use is of the order of centimeters, the spatial resolution can be much
better: the interaction between resonator and sample takes place only in
the vicinity of the resonator end which is close to the sample. The
spatial resolution thus depends on the separation between the open end
of the resonator and the sample, and also on the shape of the resonator.
The principle of operation of our microscope is shown in Fig. 1. The microscope consists of a half-wavelength microstrip resonator which is scanned across the sample. Changes in the surface conductivity and permittivity of the sample are measured by recording the amount of power reflected from the resonator. The end of the resonator which is brought into close contact with the sample is a relatively narrow tip with a radius of about 10 to 20 mm. The other end is coupled through a capacitor to a 50-ohm semi-rigid coaxial cable leading to the readout electronics. A phase-locked oscillator with a tuning range between 2 and 2.5 GHz is coupled through a directional coupler to the resonator. The rf voltage across the resonator is amplified in a way we can measure the amplitude and the phase of the rf voltage developing across the resonator.
Figure 1. Schematic diagram of our scanning microwave microscope. |
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