Scanning microwave microscopy (SMM) is a scanning probe technique, that measures the interactions of a microwave from a sharp tip with the sample. The microwave reflection coefficient (S11 parameter), which is the ratio between the microwave power sent to the tip and the one received back after being reflected at the tip-sample contact, is used to deduce the local tip-sample microwave impedance. The microwave impedance yields information about the local capacitance from which one can deduce the dielectric constant and the dopant density. In terms of the dopant density measurement, SMM is similar to scanning capacitance microscopy (SCM), but offers wider range of measurements. SMM can be used for measurements of not only semiconductors, but also dielectric materials and metals, since it is not solely relying of the modulation of the depletion capacitance in the sample.

Understanding the S11 Parameter

The S11 parameter is defined as a function of the complex impedances of reference impedance (Z0) and load (ZL) – Eq.1. Impedance is a complex value and can be represented as a sum of real and imaginary parts – Eq.2. The real part of Z is resistance (R) and the imaginary part can be considered as capacitance (C) -Eq.3. Depending on the sample, resistance and capacitance are functions of conductivity, dielectric constant and carrier density.