GHz range magnetic field measurement of a coplanar waveguide with a magnetic force microscope tip by exploiting a beat signal between the coplanar waveguide and an exciting coil

This paper describes a GHz range magnetic field measurement of a coplanar waveguide (CPW) by using a magnetic force microscope (MFM) tip to explore a beat signal between a CPW with a 5-μm-wide signal line and a 50-μm-wide ground line and a semicircular exciting coil with a 100μ m diameter. The beating field is produced by supplying a signal current to the CPW and a reference current to the exciting coil. The signal current is fixed at a specific frequency between 0.1 and 7.0 GHz, while the reference is adjusted to produce a beat with a frequency near the cantilever resonance frequency. As the MFM tip is excited by the beat, the gradient of magnetic field can be successfully observed above the CPW surface for a signal frequency up to 7.0 GHz. The gradient of magnetic field is minimized near the two gaps, but is maximized near the center of the signal line. These results demonstrate that the tip interacts closely with the RF magnetic near-field of the CPW through the beating field, and exploring a reference RF field can detect the distribution of RF magnetic fields radiated from an RF circuit component with a very high spatial resolution.