Scanning Capacitance Microscopy (SCM) is an Atomic Force Microscopy (AFM) based technique that simultaneously records topography and local capacitance with high spatial resolution. This tool is based on the high frequency MOS capacitor theory, and is routinely used in failure analysis to discern the 2D carrier profiles and/or defects in insulator layers of semiconductor devices. An ac voltage induces a dynamic change in capacitance formed by the SCM tip and oxidized semiconductor sample surface. Because of the small contact area, sensitivity of the capacitance measurements must be lower than 10-18 F in a 1 kHz bandwidth. SCM sensors capable of such sensitivity are commonly based on the Radio Corporation of America (RCA) capacitance sensor, and rely on the detection of the frequency shift of a resonator. High operating frequency for the resonator significantly improves the measurement sensitivity. In this article, we describe a sensor for SCM with sub-zeptofarad (< 10-21 F) sensitivity based on the designs of Tran et. al., but realized using a phase – sensitive detection system. This results in improved low frequency noise in the capacitance measurement. This design has an operating frequency of 3 GHz when unloaded and a resonator Q around 110, resulting in an improvement of the system sensitivity over the conventional RCA CED sensor, and may be used in a commercial AFM system. The performance of this sensor is discussed and two-dimensional dopant profile from a semiconductor structure is presented. The limitations of bulk resonator SCM systems are discussed and the prospects for monolithic sensors are described in the context of a 0.35 µm SiGe BiCMOS process.