Optics: measuring and testing – Dimension – Thickness
Reexamination Certificate
1999-05-24
2003-05-27
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Dimension
Thickness
C356S625000, C356S600000, C356S237200
Reexamination Certificate
active
06570662
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to techniques for determining conditions of surfaces, and, more specifically, to doing so by measuring a characteristic of the reflectivity and/or emissivity of surfaces. An example application is the measurement of the thickness of a film on a substrate, such as a film that is formed or removed from a semiconductor wafer during the fabrication of integrated circuits. The thickness measurement is made either simultaneously with the film processing (in situ) or thereafter (in line). More specific applications include in situ measurements of the thickness of a film being reduced or removed by techniques such as wet etching, plasma etching, spin etching or chemical-mechanical-polishing (CMP).
As a result of the development of new semiconductor processing techniques and a steadily shrinking minimum semiconductor element size, a need exists to constantly improve techniques of monitoring and measuring the results of processing, and also to develop new ones. The trend is to make as many measurements of semiconductor wafers as possible in situ, which is usually more difficult to do than as a separate step after the processing. An example of one recent development is described in U.S. Pat. No. 5,769,540, wherein the reflectivity of a surface is measured, from which its emissivity and/or temperature can be determined without contacting the surface. The emissivity measurement is also usable to determine the thickness of a film carried by the substrate. These techniques are particularly useful for making in situ measurements during rapid thermal processing (RTP). Another development, described in U.S. Pat. No. 5,695,660, measures the thickness or level of planarization of both dielectric and metal layers in situ by optical or thermal techniques during etching or CMP, including making the measurements through the back side of the wafer. When applied to CMP, an optical signal communicates with a wafer being processed through an optical window provided in one of the moving elements such as the wafer carrier. In published international (PCT) application no. WO 97/25660, multiple sensors are carried by a moving component of a CMP machine, with a wireless communication of measurements and control signals provided between the sensors and a host control station. Other patent documents of interest include U.S. Pat. Nos. 5,138,149, 5,190,614, 5,166,525, 5,292,605, 5,308,447, 5,362,969, 5,717,608 and 5,786,886, and PCT publication no. WO93/25893. Each of the foregoing patent publications is from Luxtron Corporation of Santa Clara, Calif., the assignee hereof, and is incorporated herein in its entirety by this reference.
It is a principal object of the present invention to provide further improvements to methods and instruments for optically measuring characteristics of surfaces, such as surfaces of circuit structures partially formed on semiconductor wafers or flat panel displays.
It is a more specific object of the present invention to provide such further improvements to monitor the effects CMP processing.
It is another object of the present invention to provide improved optical methods and instruments for measuring the thickness of layers of dielectric, semiconductor or metal materials carried by a substrate.
It is a further object of the present invention to carry out the foregoing objects simultaneously with processing the surface or layer being monitored (in situ).
It is an even more specific object of the present invention to accurately measure the changing thickness of a layer carried by a substrate, such as a semiconductor wafer, while being processed (in situ) to increase or decrease the layer thickness.
SUMMARY OF THE INVENTION
These and additional objects of the present invention are realized by the various aspects of the present invention, which are briefly and generally summarized.
A surface being monitored is illuminated by optical radiation with rays of the incident radiation spread over a wide angle to form a radiation field modified by the surface that is also collected over a wide angle and detected by a sensor. The angles of optical radiation illumination and collection are made sufficiently wide so that variations in an optical radiation path and/or of the surface being monitored, other than of the surface optical characteristic of interest, that occur over time or between different copies of the surface are minimized or substantially eliminated in order to improve the accuracy of the resultant measurements. In typical applications, the incident radiation is preferably spread over an angle of at least 45 degrees and up to 180 degrees when striking the surface being monitored, and is also preferably collected over an angle of 45 degrees or more, and up to 180 degrees. As the illumination angle is increased, the collection angle can be made narrower. The collected radiation is detected, and the detected radiation is processed to monitor a desired characteristic of the surface. Specific structures of sensors include use of an optical radiation spreading element, such as a diffuser or multi-pass reflector, positioned near to the surface being monitored, and an optical collection element, such as an end of an optical fiber or a lens, is positioned to receive the spread radiation after being modified by the surface, such as by reflection from the surface. Random or pseudo-random scattering of the illumination radiation is preferred, such as occurs when the optical radiation incident on the surface being monitored has passed through ground glass.
The wide angle illumination and detection significantly reduces the effects of variations in scattering of the optical signals that can occur independently of the quantity desired to be measured. The incident optical radiation, and that modified by the surface being monitored, can be scattered varying amounts that depend upon the surface, angles that the radiation strikes the surface and optical elements, changes over time, and other causes. If narrow angles are viewed, for example, any variation in the amount of incident radiation that is scatted into the narrow viewing angle because of differences in scattering properties across the surface being monitored or between different surfaces, versus that which is scattered over wider angles out of view, causes the detected optical signal to vary. Significant variations can also occur when the surface is being viewed through a liquid layer, such as an etchant or slurry, that changes its thickness and other characteristics over time. These factors often cause the measurement signal to have significant amounts of undesired noise. But if the surface is viewed over most or all the angles through which the incident radiation can be scattered by the surface, any liquid etchant on it and by the optical elements, this source of noise is significantly reduced. It is reduced further when the surface is illuminated over wide angles. Illumination and detection over a full hemisphere is ideal but significant improvements are made when lesser angles in the range given above are utilized.
The term “optical radiation” is used in this application to mean electromagnetic radiation in a continuous wavelength range including visible and near visible infrared and ultraviolet wavelengths, generally considered to extend from about 0.2 to about 4 microns. Monitoring the optical radiation modified by a surface usually also includes monitoring the level of radiation incident upon the surface so that the reflectivity or emissivity of the surface, or a related quantity, can be calculated either as the ultimate surface characteristic to be determined or as an intermediate quantity used to calculate some other surface characteristic. By making the measurements in a defined radiation wavelength range and with the geometric constraints discussed above, resulting calculations of the reflectivity or emissivity of the surface are highly meaningful because they are independent of changing conditions unrelated to surface reflectivity and emissivity. The calculation of emiss
Hoang Ahn N.
Schietinger Charles W.
Font Frank G.
Luxtron Corporation
Punnoose Roy M.
Skjerven Morrill LLP
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