THE RELIABILITY OF SPLIT RING RESONATOR BASED METAMATERIALS IN PLASMA ENVIRONMENT

Open Access
Author:
Zhao, Jing
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 08, 2016
Committee Members:
  • Clive A Randall, Thesis Advisor
  • Michael T Lanagan, Committee Member
  • Dinesh K Agrawal, Committee Member
Keywords:
  • metamaterials
  • split ring resonator
  • CST simulation
  • plasma
  • negative permittivity
Abstract:
Metamaterials are the artificial materials with a focus on structure more than composition, and they have customized properties that are not found in nature, such as negative refractive index. Negative refractive index is achieved through negative values of the permittivity and permeability in a certain range of frequency. Currently, the metamaterials work well in the range of microwave. In this range, the negative permeability can be readily achieved by using a structure of split-ring resonator (SRR). At the same time, plasma at the gap of SRR is a novel method to provide the negative permittivity. It is straightforward to fabricate SRRs in the laboratory by conventional processes. In this work, silver ring patterns were painted onto the alumina followed by firing at 850°C in a belt furnace. The geometry of the SRR was designed by the analytical equations and corroborated by the simulation software from CST Corporation. Plasmas in the ring gap were created by exposing the SRR to high power (300W) at the frequency of 2.45GHz in air atmosphere. However, the plasma only lasted for five minutes with serious silver erosion at the gap. Therefore, a protective dielectric layer is proposed in this thesis for SRR working in a harsh plasma environment. The development path begins with evaluating the reliability of the dielectric substrate in a plasma environment. Several oxide substrates were investigated including alumina, silica and a commercial low temperature co-fired ceramic (LTCC). The effect of plasma treatment on surface morphology and chemistry was characterized by optical profilometry, energy dispersive X-ray spectroscopy (EDS), and scanning electron microscopy (SEM). A network analyzer with a split-cavity resonator was used to monitor the microware dielectric properties before and after plasma treatment. It has found that small amounts of carbon residues will reduce the Q-factor. Commercial 96%Al2O3 is proven to be a suitable substrate for the SRR. Since most serious erosion occurs at the gap of the silver section in SRR, a protective dielectric layer was investigated to prevent degradation of the resonator performance. Several approaches were tried to protect the silver ring resonator including thin coatings made through thick film deposition. All of the SRR structures were tested in a high power microwave chamber which had a 1kW power supply at 2.45GHz. All of the tests were carried out in air and it was found that a plasma could not be ignited for thick dielectric protective coatings. To support the experimental result, simulations by CST software were carried out and the results show that the electric field intensity is substantially lower for thick dielectric coatings. These results suggest that substantially higher microwave power are needed to generate sufficient electric fields to ignite and sustain a plasma as the dielectric layer thickness increases. Comparisons between the uniform dielectric coating and a protective layer with a gap show that the partially protected SRR should work much better. Experimental results show that the SRR sample with partial protection works in a microwave environment for at least 5 hours with no evidence of failure.