Electrical impedance characterization at the nanoscale is a challenge for beyond CMOS investigations and for understanding the electronic properties of nanomaterials. Among the various scanning probe microscopes, Scanning Microwave Microscope (SMM) is of particular interest because it combines nanometric lateral resolution of atomic force microscopes and sub-fF capacitance sensitivity. In particular, using an interferometer, such sensitivity may be further increased, but for reaching aF scale capacitance sensitivity, the probe parasitic capacitance starts to be the limitation. For example, sub-100 nm diameter metallic pads evaporated on 100 nm-thick thermal SiO2 are not distinguished on S11 amplitude signal. In order to overcome this limitation, but also to address clearly the lateral electrical resolution of SMMs, we have fabricated two types of devices: 1- An "on chip calibration kit" composed of patterned metallic pads evaporated on 100 nm-thick SiO2 that provides reference capacitors. 2-"gold nanodots" are nanoscale bottom gold electrodes (from 5 nm to 200 nm) with an ohmic contact to a highly doped silicon subtrate, covered either by self-formed SiO2 during annealing process or by atomic layer deposition of Al2O3 (5 Å and 5 nm). The top electrode is the SMM Pt tip. Nanodot-based Capacitance estimation by finite element analysis with a given load applied on the tip and measured capacitance by SMM (with consideration of the calibration kit) are corroborated. Capacitances as small as 10 aF are measured with a lateral resolution of 30 nm. Finally, we address the role of tip shape, atmosphere (presence of N2), impact of hydrophobic molecules and discuss models when tunnel current is large.