Microwave Evanescent Probe Microscopy for Materials Analysis.

S.J. Stranick, C.A. Michaels and S.W. Robey.

Objective: Develop robust microwave-frequency microscopy methodologies and data reduction strategies for dielectric imaging and high throughput screening of chemically textured surfaces of thin films and semiconductors. The question of “accuracy” in the measurement of smaller and smaller sample volumes will be addressed as well as issues relevant to high throughput experimentation (namely, when is enough analysis enough).

Problem: The dielectric constant of a material determines if it will be a high loss material – making it ideal for shielding and anechoic applications – or a low loss material – making it ideal for waveguide, insulating, antenna, and device interconnect applications. However, the electromagnetic response of a material (dielectric response) must be accurately measured/screened in order for the material to be skillfully utilized in a given application. This presents a challenge when attempting to take advantage of combinatorial synthetic strategies for new materials discovery. While thousands of new materials can be made in parallel the screening process often has to be carried out in a serial, hands-on fashion. Certainly, the use of a conventional, serial product characterization would represent a significant bottleneck.

Approach: We are developing measurement approaches that exploit the use of a non-contact dielectric probe of a sample to accelerate the screening of process: microwave-frequency near-field microscopy and spectroscopy. A near-field microscope takes advantage of the non-propagating electromagnetic fields present at a sample’s surface when exposed to an electromagnetic wave. The tip of a near-field microscope causes the non-propagating fields to be focused at its apex. This results in an improvement in the spatial resolution to well below 100 nm. We have previously developed near-field microscopes that operate from the visible to the infrared regions of the electromagnetic spectrum. We now extend this measurement capability to longer wavelengths: microwaves. The importance of going to microwave frequencies is to gain access to information concerning the dielectric nature of materials. As stated above, the dielectric behavior of a material is often the most important characteristic in determining its potential utility.

Results and Future Plans: Microwave radiation up to 20 GHz is coupled evanescently to the sample surface using a sharp proximal probe that is part of a resonant cavity or a transmission line structure. Analysis of reflected and transmitted signals is performed using a microwave frequency, vector network analyzer allowing the extraction of dielectric constant information. The probe-sample separation is controlled using shear-force feedback giving the microscope the ability to map out the topographic structure of the sample surface. Our current focus is on the evaluation of probe designs and on the fabrication of geometry controlled calibration artifacts. At present, cavity and transmission line structures are being developed to achieve maximum sensitivity and frequency agility. We have fabricated, tested and integrated probe structures that have measured quality factors (Q) approaching 10³. Shown in Figure 1 is a representative single port, network analysis of one of the modes of a probe resonator. By monitoring the magnitude and frequency of this resonance mode as the tip is scanned across the surface, a dielectric response map of the sample can be acquired. This capability will have an impact on key applications in materials science, nanotechnology, molecular electronics, and high throughput experimentation.

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