Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
1999-05-27
2004-01-13
Moller, Richard A. (Department: 2856)
Measuring and testing
Liquid level or depth gauge
C324S200000, C324S228000, C324S229000
Reexamination Certificate
active
06675645
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the non-contact monitoring of electromagnetic parameters of thin films and bulks (conducting and non-conducting materials). More specifically, the present invention relates to an electromagnetic method and apparatus for measuring resistivity, permeability and permittivity, including artificial dielectric permittivity. The monitoring of these parameters permits the measurement of thickness of semiconductor and dielectric films and substrates, by determining the distance to a buried layer or its conductivity. This provides the possibility of non-destructive monitoring of quality in the process.
BACKGROUND OF THE INVENTION
The usefulness of application of RF or microwave field for monitoring electric parameters of different materials is recognized by the prior art. Such devices can operate with microwave excitation. When a monitored material is placed in a microwave electromagnetic field, for example inside a circular cavity resonator, the resonance frequency and Q-factor are dependent on the material permittivity and resistivity, see U.S. Pat. No. 3,458,808 Apparatus for Measuring the Properties of a Material by Resonance Techniques/N. B. Agdur, 1969.
Permittivity of a dielectric material can be monitored when the material is placed in the transmission line, for example a waveguide. The phase delay of a microwave signal is used for permittivity measurement, see Adrian D. Green and Wayne S. Holmes, “Dielectric Properties of Fresh Peas at Frequencies from 130 MHz to 4 GHz.”
Proceedings
31
Microwave Power Symposium,
1996, Boston, Mass., pp. 1-4.
The electromagnetic method of measuring resistivity in semiconductor substrates is also known. Directed by a tapered parallel plate antenna, the microwave radiation reflects from the semiconductor wafer, the reflection factor correlating with a wafer's resistivity, see S. Bothra and J. M. Borrego “Spatially Resolved Resistivity Measurements in Semiconductor Wafers Using Microwave Techniques”
Proceedings of
20
th
European Conference Vol.
1, 1990, pp. 990-994.
In particular, the state of the art is shown in Yu. N. Pchelnikov publications, disclosing a slow-wave structure application for permittivity measurement, see Yu. N. Pchelnikov “Possibility of Using a Cylindrical Helix To Monitor the Continuity of Media”,
Measurement Techniques, Vol.
38, # 10, 1995, pp. 1182-1184 and in the review on slow-wave structure-based sensors, see Yu. N. Pchel'nikov et al “Primary Measuring Transducers Based on Retardation Systems”,
Measuring Techniques, Vol.
37, # 5, 1994, pp. 506-510. These publications show that the change in the monitored parameters in the measuring volume leads to a change in signal phase delay in the slow-wave structure. A change in a delay alteration can be converted into a change in an oscillator's frequency.
Slowed electromagnetic waves and slow-wave structures are also well known in the field of microwave engineering, See Dean A. Watkins “Topics in Electromagnetic Theory”, New York, John Wiley & Sons, Inc., p. 1, These waves are electromagnetic waves propagating in one direction with a phase velocity &ngr;
p
that is smaller than the light velocity c in a vacuum. The relation c/&ngr;
p
is named slowing or deceleration and is designated as n. In the most practically interesting cases, slowed electromagnetic waves are formed in slow-wave structures by coiling one or two conductors (for example, into a helix, as it is shown in
FIG. 1
(Prior Art), where the other conductor is a cylinder), which increases the path length traveled by the wave, or by successively connecting resonant elements or cells, energy exchange between which delays the phase of the wave, or by using an electrodynamically dense medium (usually a dielectric), or a combination of these methods. Additional deceleration was also obtained due to positive electric and magnetic coupling in coupled slow-wave structures. See V. V. Annenkov, Yu. N. Pchelnikov “Sensitive Elements Based on Slow-Wave Structures”
Measurement Techniques, Vol.
38, # 12, 1995, pp. 1369-1375.
Slow-wave structure-based sensitive elements are known in the art, see Yu. N. Pchelnikov, I. A. Uvarov and S. I. Ryabtsev “Instrument for Detecting Bubbles in a Flowing Liquid”,
Measurement Techniques, Vol.
22, # 5, 1979, pp. 559-560. Slowing of the electromagnetic wave leads to a reduction in the resonant dimensions of the sensitive elements, and this enables one, by using the advantages of electrodynamic sues to operate at relatively low frequencies, which are more convenient for generation and are more convenient for primary conversion of the information signal, but sufficiently high to provide high accuracy and high speed of response. The low electromagnetic losses at relatively low frequencies (a few to tens of megahertz) also help to increase the accuracy and sensativity of the measurements. The slowing of the electromagnetic wave leads also to energy concentration in the transverse and longitudinal directions which results in an increase in sensitivity, proportional to the slowing down factor n. See V. V. Annenkov, Yu. N. Pchelnikov “Sensitive Elements Based on Slow-Wave Structures”
Measurement Techniques, Vol.
38, # 12, 1995, pp. 1369-1375.
Most slow-wave structures were made as two-conductor periodic transmission lines (see Dean A. Watkins “Topics in Electromagnetic Theory”, John Willy & Sons, Inc. Publishers). A version is possible when a slow-wave structure contains three or more different conductors. In all cases the slowed wave is excited in the electrodynamic element between different combinations of the two conductors. The coiled conductors increasing the wave path are named “impedance conductors”, and conductors with simple configuration such as rods, tapes, etc., stretched along the wave propagation direction are named “screen conductors”, see V. V. Annenkov, Yu. N. Pchelnikov “Sensitive Elements Based on Slow-Wave Structures”
Measurement Techniques, Vol.
38, # 12, 1995, pp. 1369-1375.
Both the prior art and the present invention measure one or more parameters of an electromagnetic field. Some of the prior methods and present invention use an electrodynamic element which is made as a section of an electromagnetic transmission line. The electrodynamic element is connected to an external RF or microwave signal generator which is used to excite an electromagnetic field. The change in, for example, resistivity, causes a shift in the characteristics of the electromagnetic field in the electrodynamic element. The shift in characteristics correlates to a change, for example, in the electromagnetic parameters of a monitored material. However, the prior art employs antennas, cavity resonators, wave-guides and two-conductor transmission lines.
Devices used in the prior art exhibit several problems overcome by the present invention. The previous design employs antennas, cavity resonators, wave-guides and two-conductor transmission lines. The monitoring by reflection and penetration factors measurement of the prior art requires very frequency, lying in particular, in a millimeter or an optical range, wherein measurements are possible only when optically transparent materials are involved. The electric parameters monitored by an electrodynamic element made as a section of a wave-guide in the prior art can not be made at relatively low frequency at which electromagnetic losses are small and the cost is also low. The previous methods that were based on wave-guide application are expensive, inconvenient and are not sufficiently accurate. The first is due to complexity of the microwave measuring circuits, the second is due to restricted volume inside the wave-guide; the third is due to radiation and electromagnetic losses in conductors, and due to cavity resonators' frequency dependence upon the environment temperature.
Thus, there is a need in the art for an electromagnetic method and apparatus for monitoring thin film and bulks electric parameters that is more convenient, has better sensitivit
Moller Richard A.
MTS Systems Corporation
Ostfeld David M.
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