Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Reexamination Certificate
2000-09-20
2003-07-22
Oda, Christine (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
C324S646000, C324S690000, C324S754120, C250S306000
Reexamination Certificate
active
06597185
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to measurement techniques. In particular this invention directs itself to a technique for highly localized measurements of complex microwave permittivity of materials.
More in particular, the present invention relates to a probe for non-destructive determination of complex permittivity of a material based on a balanced two-conductor transmission line resonator which provides confinement of a probing field within a sharply defined sampling volume of the material under study to yield a localized determination of the material's complex permittivity.
BACKGROUND OF THE INVENTION
One of the main goals of the near-field scanning microwave microscopy is to quantitatively measure a material's complex microwave permittivity (dielectric constant and conductivity) with a high sensitivity of lateral and/or depth selectivity (i.e. to determine the material's property over a small volume while ignoring the contribution of that volume's surrounding environment). This is particularly important in measurements on complex structures, such as semiconductor devices or composite materials, where, for example, the permittivity of one line or layer must be determined without knowledge of the properties of the neighboring lines or underlying layers.
In microwave microscopy the basic measurement is a determination of the reflection of a microwave signal from a probe positioned in close proximity to a sample. Phase and amplitude of the reflected signal may be determined directly by using a vector network analyzer or by determination of the resonant frequency and quality factor of a resonator coupled to the probe.
In many cases, the phase of the reflected signal correlates to a large degree with the real part of the sample permittivity, whereas magnitude is dominated by the imaginary part of the permittivity (i.e., the microwave absorption of the sample). Measurements of the microwave transmission from the probe through the sample are also possible, however, such an arrangement generally does not yield a localized determination of a sample's complex permittivity.
Many conventional approaches in microwave microscopy employ a coaxial probe geometry. An alternative to the rotationally-symmetric arrangement of the coaxial probes are planar structures such as a co-planar wave-guide or a strip-line wave-guide. Such an apparatus yields an imaging resolution on the order of the diameter or radius of curvature of the central conductor tip.
It is obvious, however, from considerations of classic electrodynamics that the volume of space over which such an apparatus determines the electrical properties of a sample is determined not by the local dimensions of the central conductor tip alone, but rather by a length scale given by the separation between the central conductor tip and the ground (outer) conductor or shield.
Therefore, in order to determine quantitatively the microwave properties of a material these properties must be devoid of non-uniformities on length scales at least a few times larger than the distance between the probe tip and the ground conductor while sufficient imaging contrast on length scales comparable to the radius of curvature of the tip can be easily achieved.
Furthermore, the inherent unbalanced character of the exposed part of the probe complicates any of the above-mentioned geometries due to the dipole-like current-flow in this area. The amount of radiation is critically dependent on the environment, i.e., the sample's complex permittivity and the probe-to-sample distance, and thus affects the amplitude of the reflected signal (reflection measurement) or quality factor of the resonator (resonant technique). The result is a potentially erroneous determination of the sample's microwave absorption.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a technique for selective localized determination of a complex permittivity of a material.
It is another object of the present invention to provide a novel probe for the non-destructive determination of a sample's complex permittivity based on a balanced two-conductor transmission line resonator which is symmetric with respect to an exchange of signal between the conductors that makes it possible to confine the probing field within the desired sampling volume which significantly reduces dependency of measurements on the sample volume's environment.
It is a further object of the present invention to provide a measurement technique applicable in the frequency domain up to about 100 GHz in which the sample's complex permittivity is determined with high accuracy either by a measurement of the phase and magnitude of a microwave or millimeter-wave signal reflected from the sample, as well as by a measurement of a resonant frequency and quality factor of a resonator formed by (or coupled to) a two-conductor transmission line, or by the capacitance measured between the two conductors of such a transmission line.
Furthermore, it is an object of the present invention to provide an apparatus for highly accurate determination of the complex permittivity of a sample which employs a probe capable of sharply localized measurements which can be easily controlled for modification of sampling volume as well as for the depth profiling.
In accordance with the principles thereof, the present invention is a novel probe for non-destructive measurements which includes a two-conductor transmission line comprising a pair of spatially separated, symmetrically arranged electrical conductors of circular, semi-circular, rectangular, or similar cross-section contour. One end of the transmission line (also referred to herein as the “probing end”) is brought into close proximity to the sample to be measured and may be tapered (or sharpened) to an end having very small spatial extent. A signal is fed through the transmission line toward the sample, and a signal reflected from the sample is measured. For this purpose, the opposite end of the transmission line is connected to electronics for the determination of the reflected signal's phase and magnitude. Measurements of the phase and magnitude of the reflected signal are broadband in frequency.
Preferably, for highly sensitive and accurate measurements, while employing less expensive electronics, a resonator is formed by a portion of the two conductor balanced transmission line with the conductors separated by air or another dielectric medium, and measurements of the resonant frequency and quality factor of the resonator are made. For example, such a dielectric medium may include a circulating fluid for temperature stabilization, or a high dielectric constant material for size reduction. In this type of embodiment, the opposite end of the transmission line is coupled to a terminating plate. Coupling to the resonator can then be provided by a conducting loop positioned close to the resonator. It is to be noted that an optional second coupling loop may be used for the measurement electronics.
Typically, the transmission line is operated in the odd mode, i.e., in a mode in which the current flow in one of the two conductors is opposite in direction to that in the other conductor.
The transmission line or the resonator may be partially enclosed by a metallic sheath. If a conducting sheath is used, the transmission line also supports an even mode, similar to that observed in a coaxial transmission line. When operated in the even mode, the interaction between the sample and the probe is similar to the coaxial symmetries.
When the probe is operated as a resonator, the two modes (odd and even) will in general result in two different resonant frequencies (due to dispersion), and can therefore be easily separated in the frequency domain to be powered and monitored independently. In order to enhance the dispersion, a piece of dielectric material is sandwiched between the conductors of the resonator.
The spacing between the two conductors of the resonator and their cross-sections must be properly cho
Christen Hans M.
Moreland Robert
Talanov Vladimir Vladimirovich
Deb Anjan K.
Neocera, Inc.
Oda Christine
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