Giovanni Maria Sardi

Analysis of a transmission mode scanning microwave microscope for subsurface imaging at the nanoscale

We present a comprehensive analysis of the imaging characteristics of a scanning microwave microscopy (SMM) system operated in the transmission mode. In particular, we use rigorous three-dimensional finite-element simulations to investigate the effect of varying the permittivity and depth of sub-surface constituents of samples, on the scattering parameters of probes made of a metallic nano-tip attached to a cantilever. Our results prove that one can achieve enhanced imaging sensitivity in the transmission mode SMM (TM-SMM) configuration, from twofold to as much as 5× increase, as compared to that attainable in the widely used reflection mode SMM operation. In addition, we demonstrate that the phase of the S21-parameter is much more sensitive to changes of the system parameters as compared to its magnitude, the scattering parameters being affected the most by variations in the conductivity of the substrate. Our analysis is validated by a good qualitative agreement between our modeling results and experimen...

Optimization of the imaging response of scanning microwave microscopy measurements

In this work, we present the analytical modeling and preliminary experimental results for the choice of the optimal frequencies when performing amplitude and phase measurements with a scanning microwave microscope. In particular, the analysis is related to the reflection mode operation of the instrument, i.e., the acquisition of the complex reflection coefficient data, usually referred as S11. The studied configuration is composed of an atomic force microscope with a microwave matched nanometric cantilever probe tip, connected by a λ/2 coaxial cable resonator to a vector network analyzer. The set-up is provided by Keysight Technologies. As a peculiar result, the optimal frequencies, where the maximum sensitivity is achieved, are different for the amplitude and for the phase signals. The analysis is focused on measurements of dielectric samples, like semiconductor devices, textile pieces, and biological specimens.

A broadband toolbox for scanning microwave microscopy transmission measurements

In this paper, we present in detail the design, both electromagnetic and mechanical, the fabrication, and the test of the first prototype of a Scanning Microwave Microscope (SMM) suitable for a two-port transmission measurement, recording, and processing the high frequency transmission scattering parameter S21 passing through the investigated sample. The S21 toolbox is composed by a microwave emitter, placed below the sample, which excites an electromagnetic wave passing through the sample under test, and is collected by the cantilever used as the detector, electrically matched for high frequency measurements. This prototype enhances the actual capability of the instrument for a sub-surface imaging at the nanoscale. Moreover, it allows the study of the electromagnetic properties of the material under test obtained through the measurement of the reflection (S11) and transmission (S21) parameters at the same time. The SMM operates between 1 GHz and 20 GHz, current limit for the microwave matching of the cantilever, and the high frequency signal is recorded by means of a two-port Vector Network Analyzer, using both contact and no-contact modes of operation, the latter, especially minded for a fully nondestructive and topography-free characterization. This tool is an upgrade of the already established setup for the reflection mode S11 measurement. Actually, the proposed setup is able to give richer information in terms of scattering parameters, including amplitude and phase measurements, by means of the two-port arrangement.

Modeling of a metallic truncated cone for electromagnetic capacitive sensors

This paper presents an electromagnetic modeling of a grounded metallic truncated cone to be used for calibration purposes of a microwave imaging system. The basic idea is to demonstrate the equivalence between the cone and a cylinder having a suitable radius, in order to simplify the computation of its capacity with respect to ground. A mathematical expression for the capacitance of the uniform cylinder is presented, and its validity is confirmed by comparing the data provided by this formula with numerical values given by a commercial simulator. Starting from this analytic result, the model of the cone is presented, and a procedure for the choice of the cylinder radius is discussed in detail. This methodology can be applied to calculate the contribution to the stray capacitance of a metallic tip used for scanning probe microscopy, and specifically for microwave sensing applications. In particular, the capacitance due to the conic part of the probe can be quantified, an operation that is usually a difficult task when trying to separate it from contribution of the experimental setup. In our opinion, this issue is very important to improve the accuracy of system calibration in the scanning microwave microscopy technique.

Transmission microwave spectroscopy for local characterization of dielectric materials

In this work, the authors present a technique for the local characterization of the dielectric properties of materials. More in details, a setup will be described, and the related measurement modeling will be discussed. In this way, it is possible to obtain a calibrated and nondestructive determination of the dielectric constant in a submicrometric region; the detection of any surface or buried metallization is a straightforward application for microelectronics. The analysis is performed as a function of the frequency in the microwave range and, further on, the data can be transformed in time domain for one dimensional tomography. The authors will show that microwave spectroscopy can be performed by means of standard coaxial pins employed as probes for measurements both in reflection and transmission mode, giving the information of the frequency dependent properties of the exploited material or structure by means of the measured impedance. Experiments are performed in the range between 1 and 18 GHz, and d...

A coplanar waveguide microfluidic sensor for a micrometric local spectroscopy of liquid solutions

In this paper, we present a sensor suitable for performing the spectroscopy on a localized micrometric volume of a generic liquid, in the spectral range comprised between 1 GHz and 20 GHz. The sensor is based on two ungrounded, open-ended coplanar waveguides, acting as Input/Output ports, with a microfluidic channel passing through them to perform also transmission through the liquid analysis. The sensor has been designed and numerically simulated to obtain the electromagnetic response in different conditions: an empty channel for having a zero-reference response and for observing variations as a function of liquids filling the channel.

Microwave broadband characterization of aging of SU-8 polymer as CPW substrate

In this paper we present the methodology and the numerical results related to the analysis of aging of the SU-8 polymer when used as a primary layer for the realization of Coplanar Waveguide (CPW) structures. As test devices, we used a set of transmission lines with different lengths and T-shaped open stubs shunt resonators; by using these geometries, we are able to acquire the data in a broadband range, in principle between 1 GHz and 40 GHz. We conduct the analysis by comparing two different technology run: the first wafer with a deposited layer by a 12-year-old SU-8 and the second wafer, with the same photolithographed metallic geometries, with a brand-new processed SU-8 photoresist.

Analytical evaluation of the capacitance of a conical sensor for micro-nano imaging techniques

The analytical modelling of a grounded truncated metallic cone is presented in this work as a contribution to the de-embedding and calibration of a scanning microwave system based on capacitance measurements for imaging and spectroscopy purposes. First, an expression for the capacitance of a uniform cylinder is derived, and successively a procedure to determine an effective uniform cylinder radius for the truncated cone is developed. The truncated cone was chosen as a suitable geometry for the calculation of the stray capacitance versus ground of a metallic tip used for scanning probe microscopy and, more specifically, microwave sensing. An accurate calculation of the aforementioned capacitance is of outmost importance for system calibration in scanning microwave microscopy (SMM) technique.