  We demonstrate dispersive charge sensing of Si/SiGe single and double quantum
dots (DQD) by coupling sub-micron floating gates to a radio frequency
reflectometry (rf-reflectometry) circuit using the tip of an atomic force
microscope (AFM). Charge stability diagrams are obtained in the phase response
of the reflected rf signal. We demonstrate single-electron dot-to-lead and
dot-to-dot charge transitions with a signal-to-noise ratio (SNR) of 2 and
integration time of $\tau~=~2.7~\mathrm{ms}$ and $\tau~=~6.4~\mathrm{ms}$,
respectively. The charge sensing SNR compares favorably with results obtained
on conventional devices. Moreover, the small size of the floating gates largely
eliminates the coupling to parasitic charge traps that can complicate the
interpretation of the dispersive charge sensing data.
