Materials characterization by near-field scanning microwave microscopy

Abstract

Near-field scanning microwave microscope (NFSMM) has potentials to achieve characterizations of materials with high spatial resolution and sensitivity. A wide range of applications have been addressed such as dielectric constant measurement, impedance analysis of metals and semiconductor materials/devices, gas and chemical sensing, etc. [1, 2]. In this work, we propose a home-made NFSMM to perform materials nondestructive characterization. This system includes a vector network analyzer (VNA), a matching network and an evanescent microwave probe (EMP) [3, 4]. The configuration of the system is given in Fig. 1. The broadband matching network is composed of a hybrid coupler and an impedance tuner. This matching circuit permits the measurements with high sensitivity and accuracy in the frequency range from 2 GHz to 18 GHz. It is well known that the evanescent wave is concentrated around the probe apex and decays rapidly with the distance away from the probe tip. In this study, an EMP with apex of 66 µm is used to achieve a very local characterization of materials with a sub-wavelength resolution when the tip-material separation is in the order of the apex size. The electromagnetic coupling between the probe and the material under test is also investigated using numerical simulations (ANSYS/HFSS™) based on finite element method (FEM). Thanks to the fact that microwave penetrates into the material, the electric properties (impedance and dielectric constant) of materials can be quantitatively determined based on the wave reflected by the material. The proposed microscopy platform thus allows a straightforward estimation of material properties rather than just topographic information. Furthermore, another advantage of the system is the nondestructive characterization possibility, which avoids injuring the sample surface.