Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
2003-05-20
2004-11-23
Toatley, Jr., Gregory J. (Department: 2877)
Optics: measuring and testing
By polarized light examination
Of surface reflection
C356S368000
Reexamination Certificate
active
06822739
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the field of quantum ellipsometry, which relies on the use of non-classical optical sources in conjunction with a coincidence-detection scheme. One of the age of old questions in the field has been: how does one measure reliably the reflection or transmission coefficient of an unknown sample? The reliability of these measurements heavily depend on the reliability of the source and detector used in the measurements. In the ideal condition, both the source and detector are absolutely calibrated. In practice, this condition is never met. However, high precision measurements are often required, thus, a multitude of experimental techniques have been developed. Two of those techniques, are the null and interferometric approach, allow getting around the imperfections of devices used in the measurements.
In the field of ellipsometry, high precision measurements are necessary in which the polarization of light is used to determine the properties of various optical samples. Ellipsometers have demonstrated to be useful also in other fields that require high precision measurement, such as biomedical applications.
In an ideal ellipsometer, the light emitted from a reliable optical source is directed into an unknown optical system (which may be an unknown sample that reflects the impinging light) and thence into a reliable detector. The practitioner keeps track of the emitted and detected radiation, and can infer information about the optical system. A device may be used as an ellipsometer if the source can emit light in any specified state of polarization. A sample is characterized by two parameters &psgr; and &Dgr;. The quantity &psgr; is related to the magnitude of the ratio of the sample's eigenpolarization complex reflection coefficients, r
1
and r
2
, via
tan
ψ
=
&LeftBracketingBar;
r
~
1
r
~
2
&RightBracketingBar;
;
&Dgr; is the phase shift between them.
FIG. 1
illustrates the traditional null ellipsometer arrangement. A sample
7
is illuminated with a beam of light that is polarized by a linear polarizer
4
from a source
2
. The reflected light from the sample
7
is generally elliptically polarized, is then analyzed. The polarization of the incident beam is adjusted by a linear polarization analyzer
6
for the change in the relative amplitude and phase, introduced by the sample, between the two eigenpolarization, such that the reflected beam is linearly polarized. Once the reflected beam passed through an orthogonal linear polarizer
6
, the linearly polarized beam will yield a null measurement at the optical detector
8
.
As stated above, the null ellipsometer does not require a calibrated detector since it does not measure intensity, but records a null. The principal drawback of null measurement techniques is the need for a reference to calibrate the null. For example, to define an initial location (the rotational axis of reference at which an initial null is obtained), and then to compare subsequent locations upon inserting the sample. Thus, eliminating the problem of an unreliable source and detector but necessitating the use of a reference sample. The accuracy and reliability of the measurement results depend on the information regarding the reference sample used. In this instant, the measurements are a function of &psgr;, &Dgr;, and other essential parameters of the reference sample.
The inteferometric ellipsometer requires a configuration in which light from the source follows more than one path, usually created by the aid of beam splitters before reaching a detector. A sample is placed on one of those paths. Thus, the efficiency of the detector can be measured by performing measurements when the sample is removed from the interferometer. The problem of an unreliable detector is eliminated, however, the reliability of the source and other components (beam splitters, mirrors, etc.) still remain. The accuracy of the measurements are limited by the information known regarding the parameters characterizing these optical components. The stability of the optical arrangement is also of importance to the performance of such a device.
SUMMARY OF THE INVENTION
Accordingly, the invention presents a novel interferometric technique to perform reliable ellipsometric measurements. This technique relies on the use of a non-classical optical source in conjunction with a coincidence-detection scheme. The ellipsometric measurements acquired with this scheme are absolute, and they neither require neither source nor detector calibration, nor do they require a reference.
According to one embodiment of the invention, a system for measuring ellipsometric data from a sample is provided. The system includes a source for providing a monochromatic light beam. The system also includes a nonlinear crystal for converting the monochromatic light beam into photon pairs by disintegrating photons from the monochromatic light beam, such that each of the photon pairs exhibits entanglement properties, wherein one of the photons of the pair is directed to the sample and the other of the photons of the pair is not directed to the sample. The system further includes a circuit for calculating the coincidence of one of the photons of the photon pair reflected from the sample and the other of the photons of the photon pair, wherein the measurements of the sample are obtained by analyzing the coincidence and the entanglement properties between one of the photons of the photon pair reflected from the sample and the other of the photons of the photon pair.
According to another aspect of the invention, a method of measuring ellipsometric data from a sample is provided. The method includes providing a monochromatic light beam, and converting the monochromatic light beam into photon pairs by disintegrating photons from the monochromatic light beam, such that each of the photon pairs exhibits entanglement properties, wherein one of the photons of the pair is directed to the sample and the other of the photons of the pair is not directed to the sample. The method further comprises calculating the coincidence of one of the photons of the photon pair reflected from the sample and the other of the photons of the photon pair, wherein the measurements of the sample are obtained by analyzing the coincidence and the entanglement properties between one of the photons of the photon pair reflected from the sample and the other of the photons of the photon pair.
According to another aspect of the invention, a system for measuring ellipsometric data from a sample is provided. The system includes a source for providing a monochromatic light beam, and a nonlinear crystal for converting the monochromatic light beam into photon pairs and creating a first beam that includes photon-pairs from disintegrated photons from said monochromatic beam. The system further includes a first beam splitter for splitting the first beam into a second beam and third beam, wherein the second beam includes photons from the photon-pairs directed to the sample and the third beam includes photons from the photon-pairs not directed to the sample. The system also comprises a second beam splitter for combining reflected photons from the sample of the second beam and third beam into a recombined beam and splitting the recombined beam into a fourth and fifth beam. The system also includes a coincidence circuit for calculating the coincidence of the fourth and fifth beam, wherein measurements on the sample are obtained by analyzing the coincidence and entanglement properties of the photons in the fourth and fifth beam.
According to another aspect the invention, a system for measuring ellipsometric data is provided. The system includes a source for providing a monochromatic light beam, and nonlinear crystal for converting the monochromatic light beam into photon pairs and creating a first beam that includes photon-pairs from disintegrated photons from said monochromatic beam. The system also includes a first beam splitter for splitting the first beam into a second and third beam,
Abouraddy Ayman F.
Saleh Bahaa E. A.
Sergienko Alexander V.
Teich Malvin C.
Toussaint, Jr. Kimani C.
Gauthier & Connors LLP
Lauchman Layla
Toatley , Jr. Gregory J.
Trustees of Boston University
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