Optics: measuring and testing – By polarized light examination – With polariscopes
Reexamination Certificate
1999-09-16
2001-08-14
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With polariscopes
C356S364000, C356S368000, C356S369000, C359S489040, C359S494010
Reexamination Certificate
active
06275291
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a micropolarimeter with a retarder, an analyzer disc located downstream of that and a photo-detector matrix as well as an ellipsometer with a light source, polarizer and polarimeter and to an ellipsometer with a light source, polarizer, polarimeter and reflected light microscope with lens and eyepiece, whereby the polarizer and polarimeter are integrated into the reflected light microscope.
In industrial applications, light is frequently used as a contactless probe to measure the characteristics of a specimen. The change in the properties of the beam following interaction with the specimen is used to make the evaluation. Polarimetry and ellipsometry use the information contained in the polarization characteristics and the changes in that information as a result of the interaction with the specimen. In fully polarized light, these characteristics are the ellipticity, the position of the major axis in three dimensions (azimuth) and the direction of rotation of the field strength vector. For partly polarized light, the degree of polarization is also included. These variables are described by the four elements of the Stokes vector which are designated the Stokes parameters (see R. M. Axxam, Bashara, Ellipsometry and Polarized Light, North Holland, Amsterdam, 1988).
Devices for the measurement of the parameters of polarized light are called polarimeters. The combination of a polarized light source with a polarimeter for the measurement of the characteristics of thin films and surfaces is called an ellipsometer. From the basic ellipsometric variables, and by using appropriate mathematical algorithms, it is possible to calculate the characteristics of the specimens, such as the thickness and the refractive index of films. To obtain information on the characteristics of the specimen at different locations on the specimen, the specimen is conventionally moved by a displacement device.
Information of this type is of great significance for quality control in the thin film and microelectronics industries. In the glass industry, a polarimeter can be used to analyze the stresses in a sheet of glass. As the diameter of the specimens increases, however, the scanning of the surface with an ellipsometer or polarimeter that takes only spot measurements becomes a very time-consuming process. The precision movement of very large specimens also requires to good deal of effort.
In ellipsometry and polarimetry, systems that contain mechanically moved polarizers or retarders (e.g. &lgr;/4 wafers) are frequently used (see R. M. Axxam, Bashara, “Ellipsometry and Polarized Light”, North Holland, Amsterdam, 1988).
A light source delivers a collimated bundle which is linearly pre-polarized by means of a polarizer. The polarization of the beam changes as a result of its interaction with the specimen. This change can be detected photometrically by means of a retarder and a downstream analyzer with a detector. For this purpose, either the retarder or the analyzer is rotated and the timed periodic signal that occurs is evaluated (see FIG.
1
). Such an ellipsometric system supplies the information only for the area of the specimen that is measured in the detection channel. This method is very slow and requires the use of very precise and expensive mechanical tables for the displacement of the specimen.
DE 197 08 036 describes an ellipsometric microscope that combines the construction of a reflected light or even a transmitted microscope with the construction of an ellipsometer, and thereby makes it possible to obtain a direct image of a surface, and simultaneously to evaluate the light reflected by the specimen in terms of polarization and intensity. For this purpose, movable ellipsometer components are used. In particular the procedure attempts to achieve the highest possible lateral resolution for a given angle of incidence between 0° and approximately 90°.
WO 86/07631 describes a photopolarimeter for the simultaneous measurement of all four Stokes parameters, in which three detectors are used for the analysis. In this system, of course, there are no moving parts, and only one point of the specimen is imaged.
U.S. Pat. No. 5,335,066 describes a similar device without moving parts.
EP 0 632 256 A1 describes an array polarimeter. In this case, however, the elements of the array are used for the determination of the polarization characteristics of a measurement beam that images only one point of the specimen.
U.S. Pat. No. 5,166,752 uses a CCD array in an ellipsometric system. The purpose of this system is to use upstream optics to measure a range of different angles of incidence simultaneously, and thus to increase the number of independent measurements for a given point on the specimen.
There is no provision for the simultaneous surface measurement of the polarization characteristics.
DE 195 47 553 C1 describes a device that does not have any moving components, and which, for the simultaneous determination of the polarization status of the electromagnetic beam, has a detector which has fields that are arranged in a matrix-like manner. Polarization films that have different azimuths are attached to the matrix fields of the CCD matrix of the detector. For example, three polarizers with different azimuths are combined into one image element, which corresponds to three angular positions of a rotating analyzer.
A similar system with polarization pixels is described in U.S. Pat. No. 4,286,843. Although a simultaneous measurement of the surface of the specimen is possible, the disadvantage of these systems is that they are very difficult to use in practical terms. The reason for the difficulty is the size of the pixels of the associated photo-detector array, which is in the magnitude of micrometers. The dimensions of these pixels are typically in the range of 20×20 &mgr;m
2
. The manufacture of such small pieces from a polarizer film that is typically 300 &mgr;m thick and their installation on the detector array is a technically very difficult and very expensive procedure. In fact, the author is unaware of any industrial realization of the system described in U.S. Pat. No. 4,286,843. An additional disadvantage is the design of the system in the form of a polarizer array. Thus not all of the Stokes parameters can be determined. In particular, it remains impossible to determine the direction of rotation of the polarization.
SUMMARY OF THE INVENTION
The object of the invention is therefore to create a compact, simultaneously operating imaging micropolarimeter for the simultaneous superficial determination of the film and geometric characteristics of a specimen, whereby the compact size of the system makes it easy to use in process equipment and in ellipsometers. The object of the invention also relates to a compact ellipsometer, by means of which the characteristics of the specimen can be measured easily.
This object is accomplished with a micropolarimeter in which the retarder is a one-piece retarder array with at least one pixel group that has at least three pixels, the major axis orientations of which are distributed over an angular range of 360°. In this case, it is logical to distribute the major axis orientations uniformly over the entire angular range.
The invention teaches that the simulation of the rotation of the retarder can be replaced by an array of retarder pixels that can be read by means of a photo-detector matrix, in particular by means of a CCD camera. Each pixel thereby corresponds to an angular position of the retarder. The polarization characteristics of the pixels in the group vary from pixel to pixel such that from the totality of the photometric information, the polarization of a partial beam of light striking this pixel group can be determined The number of pixels necessary for the polarization analysis is combined into a pixel group, which thereby forms a micropolarimeter.
The advantage of the system of retarder pixels claimed by the invention over systems of polarization pixels of the prior art is that the retarder technique deliver
Abraham Michael
Eberhardt Matthias
Font Frank G.
Hudak & Shunk Co. L.P.A.
NanoPhotonics AG
Nguyen Sang H.
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