Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2003-09-05
2004-10-05
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S662000
Reexamination Certificate
active
06801044
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of measurement of film properties, more specifically, to detecting and measuring local non-uniformities in thin conductive and non-conductive films on various conductive or non-conductive substrates. In particular, the invention may find use in detecting and measuring microinclusions contained inside and on the surfaces of thin films and topography of . . . patterns in multilayered structures. The invention may find use in the production of semiconductor devices and instruments.
BACKGROUND OF THE INVENTION
There exists a great variety of methods and apparatuses used in the industry for measuring thickness of coating films and layers applied or laid onto substrates. These methods and apparatuses can be classified in accordance with different criteria. Classification of one type divides these methods into direct and indirect. An example of a direct method is measurement of a thickness in thin metal coating films by means of so-called X-ray reflectivity. One of these methods is based on a principle that X-rays and gamma-rays are absorbed by matter. When a beam of rays passes through a material, the amount of the beam absorbed depends on what elements the material consists of, and how much of the material the beam has to pass through. This phenomenon is used to measure the thickness or density of a material. The advantage of measuring in this way is that the gauge does not have to touch the material it is measuring. In other words, in thickness measurement, the surface of a web or strip product will not be scratched. The instrument for this method is e.g., RMS1000 Radiometric System produced by Staplethorne Ltd (UK). The instrument uses a suitable radiation source and one or more radiation detectors installed in a mechanical housing which also provides high quality radiological shielding. The source may be an X-ray tube or a radioactive source. The instrument also uses a set of beam defining collimators and one or more radiation detectors. The detectors measure the radiation absorbed within the object or flow being measured and output the signal data to a computer. For thickness gauging, the collimators usually define a single, narrow beam. This gives optimum spatial resolution.
A disadvantage of radiation methods is the use of X-ray or gamma radiation that requires special safety measures for protection of the users against the radiation. The instruments of this type are the most expensive as compared to metrological equipment of other systems.
Another example of direct measurement is a method of optical interferometry, described e.g., by I. Herman in “Optical Diagnostics for Thin Film Processing”, Academic Press, 1996, Chapter 9. Although the optical interferometry method produces the most accurate results in measuring the thickness of a coating film, it has a limitation. More specifically, for conductive films, to which the present invention pertains, this method is limited to measurement of extremely thin coating films which are thin to the extent that a nontransparent material, such as metal, functions as transparent. In other words, this method is unsuitable or is difficult to use for measuring conductive films thicker than 200 Å to 500 Å.
Another example of direct measurement methods is measuring thickness of a film in situ in the course of its formation, e.g., in sputtering, magnetron target sputtering, CVD, PVD, etc. These methods, which are also described in the aforementioned book of I. Herman, may involve the use of the aforementioned optical interferometry or ellipsometry. However, in this case measurement is carried out with reference to both the surface of the substrate and the surface of the growing layer. Therefore, this method is applicable to measuring thickness of the film that has been already deposited with considerable limitations.
In view of the problems associated with direct methods, indirect non-destructive methods are more popular for measuring thickness of ready-made films. An example of a well-known non-destructive indirect method used for measuring thickness of a film is the so-called “four-point probe method”. This method is based on the use of four contacts, which are brought into physical contact with the surface of the film being measured. As a rule, all four contacts are equally spaced and arranged in line, although this is not a compulsory requirement. Detailed description of the four-point probe method can be found in “Semiconductor Material and Device Characterization” John Wiley & Sons, Inc., N.Y., 1990, pp. 2-40, by D. Schroder. The same book describes how to interpret the results of measurements. This method is classified as indirect because the results of measurement are indirectly related to the thickness of the film. It is understood that each measurement of electric characteristics has to correlated with the actual thickness of the film in each particular measurement, e.g., by cutting a sample from the object and measuring the thickness of the film in a cross-section of the sample, e.g., with the use of an optical or electron microscope. Nevertheless, in view of its simplicity, low cost, and convenience of handling, the four-point probe method is the most popular in the semiconductor industry.
However, the four-point method has some disadvantages. The main problem associated with the aforementioned four-point probe method consists in that in each measurement it is required to ensure reliable contact in each measurement point. This is difficult to achieve since conditions of contact vary from sample to sample as well as between the four pointed contact elements of the probe itself in repeated measurement with the same probe. Such non-uniformity affects the results of measurements and makes it impossible to perform precision calibration.
The authors of the present invention have developed a new approach for measuring characteristics of thin conductive and non-conductive films based on the use of oscillating circuits that interact with the object to be measured under specific resonance condition. The authors named the aforementioned new approach as Resonance Sensor Technology (hereinafter referred to as RST).
This technology has been developed by Miltimetrixs, CA in the beginning of 1999 and is currently being improved.
For better understanding the principle of RST, it would be advantageous to shortly describe the structure and operation of conventional inductive sensors for measuring, e.g., film thickness.
For example, U.S. Pat. No. 6,072,313 issued in 2000 to L. Li et al. describes in-situ monitoring and control of conductive films by detecting changes in induced eddy currents. More specifically, the change in thickness of a film on an underlying body such as a semiconductor substrate is monitored in situ by inducing a current in the film, and as the thickness of the film changes (either increase or decrease), the changes in the current are detected. With a conductive film, eddy currents are induced in the film by generating an alternating electromagnetic field with a sensor, which includes a capacitor and an inductor. The main idea of the apparatus of U.S. Pat. No. 6,072,313 consists of using a resistor and a capacitor in a parallel resonance circuit. The resonance is caused by means of an oscillator. The inductive coupling between the oscillation circuit and the Eddy current inducted in the coating is used for improving a signal
oise ratio and can be used for improving quality of measurements. In fact, this is a method well known in the radioelectronics for measuring under conditions of the electrical resonance. The above patent describes the aforementioned inductive method for measuring thickness of a film in chemical mechanical polishing (CMP).
A similar inductive method, which was used for measuring thickness of a slag, is disclosed in U.S. Pat. No. 5,781,008 issued in 1998 to J. Muller et al. The invention relates to an apparatus for measuring the thickness of a slag layer on a metal melt in a metallurgical vessel. The apparatus comprises a first induc
Kesil Boris
Velikov Leonid
Vorobyev Yuri
Le N.
Nguyen Hoai-An D.
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