Study on the measurement of microscopic RF field distribution with a MFM tip exploiting a beat signal between a CPW and an exciting coil

This paper describes a measurement of microscopic radio frequency (RF) field distribution with a magnetic force microscope (MFM) tip exploring a beat signal from a coplanar waveguide (CPW). This CPW has a 5 μm wide signal line and a 50 μm wide ground line assuming a basic model of power/ground lines in radio frequency integrated circuit (RFIC) chips. To produce a beating field near the CPW, a signal current with a fixed frequency in the GHz range is supplied to the CPW and a reference current with a slightly different frequency from a signal frequency is supplied to a semicircular exciting coil. The reference is adjusted to produce the beat with frequency in the vicinity of the cantilever resonance frequency. The mechanical resonance of the cantilever responding to the beat field above the CPW surface was successfully detected with signal frequency in the GHz range. This result provides a basic principle that, by exploring a reference RF field, RF magnetic fields radiated from signal lines, ground lines and power sources in RFIC chips are detectable with very high spatial resolution.