Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
2000-11-17
2002-05-07
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
By polarized light examination
Of surface reflection
Reexamination Certificate
active
06384916
ABSTRACT:
BACKGROUND OF THE INVENTION
a). Technical Field
This invention relates generally to parallel detecting spectroscopic ellipsometer/polarimeter instruments that are capable of determining the polarization state of light over a wide range of wavelengths, in particular, after the light has interacted with a sample. The parallel detecting spectroscopic ellipsometer/polarimeter provides spectroscopic polarization information by simultaneously measuring four specific polarization states of the light with an optical configuration that requires no moving parts. The spectroscopic polarization information, which is so collected, can be used to provide real-time information about the sample through the use of advanced interpretive algorithms.
b). Background Art
The polarization effects of light reflected from surfaces has been studied since the early 19
th
century. In general, the term ellipsometry usually applies to analysis of light reflected from a surface, where the reflected light must be in a well defined pure state of elliptical polarization. More specifically, the term “ellipsometer,” is based on the phenomenon that the electric field vector of light reflected from a sample surface forms an ellipse in a time resolved wave due to the polarized light components parallel and perpendicular to the sample surface interacting in different ways and usually applies to analysis of reflected light where the light must be in a well defined pure state of elliptical polarization,. The term “polarimeter” is less well defined, but is usually applied to analysis of transmitted or scattered light. In general polarimetry determines the complete polarization state of light, including the capability to detect non-polarized components. The change in the polarization state of light measured by an ellipsometer after interacting with a material, is extremely sensitive to the properties of the material, including the thickness of a film, its electronic energy states, its surface roughness and morphology, its composition, and defect densities.
In a typical ellipsometer installation, light in a collimated beam passes through a linear polarizer that is oriented so that the optical electric field has components parallel and perpendicular to the plane of incidence of the material sample with which it will interact. After the interaction of the light with the sample, the relative amplitudes and phases of the components parallel and perpendicular to the plane of incidence of the material sample are changed. An ellipsometer measures changes in the relative amplitudes and phases between the parallel “p” and perpendicular “s” electric field components of a light beam, and more specifically of a polarized light wave, as it reflects from a sample surface. These parameters are traditionally expressed as &psgr; and &Dgr;, which are related to the ratio (&rgr;) of the reflectance coefficients for the p and s optical electric fields, ie. &rgr;=tan(&psgr;)e
i&Dgr;
=r
p
/r
s
. Unlike a reflectance/transmittance measurement, which only provides the ratio of reflected/transmitted to incidence irradiances, an ellipsometer can extract both real and imaginary parts of the dielectric function of the sample, (&egr;
1
,&egr;
2
), as a function of photon energy, hv, from the &psgr; and &Dgr;. However, due to the huge computational demands and the extremely tedious measurements, polarimetry and ellipsometry did not find a large number of applications until the second half of the 20
th
century, when automation and computers become readily available. A good history of ellipsometry in presented by R. M. A. Azzam, “Selected Papers on Ellipsometry,” SPIE Milestone Series, Vol. MS 27, SPIE, Bellingham, Wash. (1991).
Since the 1960's, literally thousands of ellipsometry/polarimetry papers and patents have been written discussing hundreds of applications and instrument designs. The advent of spectroscopic ellipsometry (D. E. Aspnes, J. B. Theeten, F. Hottier, “Investigation of the effective-medium models of microscopic surface roughness by spectroscopic ellipsometry,” Phys. Rev. B, 1979) and variable incident angle measurements (O. Hunderi, “On the problems of multiple overlayers in ellipsometry and a new look at multiple angle of incidence ellipsometry,” Surface Science 1976) has greatly added to the large interest as well. From an instrumentation standpoint, three basic approaches have been followed to measure the polarization state of light, “null”, “rotating analyzer” and “polarization filter” ellipsometers. The first instruments that measured polarization used true nulling methods that required a phase shifter as well as a polarizer to determine the light's polarization state. Modern day equivalent instruments determine the complex reflectance change, &rgr;, by sampling the intensity of the reflected light after it passes through a second polarizer, called the “analyzer”, whose orientation changes in a continuous fashion. The detected signal is a sinusoid as a function of the orientation angle (R. W. Collins, I. An, H. V. Nguyen, and Y. Lu, “Real-time spectroscopic ellipsometry for characterization of nucleation, growth, and optical functions of thin films”, Thin Solid Films, V. 233, (1993), p. 244). Although phase and amplitude of that sinusoid can be analyzed to extract &psgr; and &Dgr;, the data needs to be acquired as a function of time, with each analyzer orientation taking a finite time interval to acquire. Even with a continuously rotating analyzer and triggered detection system, data are acquired serially or sequentially as a function of the phase angle of the analyzer. This type of system can acquire spectroscopic data, primarily by passing incident white light through a monochrometer that provides a single wavelength of light. This single wavelength of incident light directed through a polarizer is reflected off of the sample, then through an analyzer and is detected with a photodetector. Additional wavelengths are selected with a monochrometer, and the ellipsometry data is serially acquired as a function of those wavelengths. For either single wavelength or multiple wavelength (spectroscopic) cases, the main problem with the use of such “rotating analyzer” instrinents is that the data is acquired serially, and typically takes several seconds to measure a data point at a single wavelength.
Polarization filter instruments primarily related to polarimetry, divide the light into multiple components to enable a complete or partial determination of the polarization state. In the 1970's photopolarimeters were designed for astronomy applications in which modulation of polarizing optics were used to determine polarization states of the light. Several photopolarimeter instruments based on simultaneous measurement of different polarizations of the light have been designed since then, including R. M. A. Azzam. “Division of amplitude photopolarimeter for the simultaneous including R. M. A. Azzam, “Division of amplitude photopolarimeter for the simultaneous measurement of all four Stokes parameters of light,” Optica Acta Vol. 29, pg. 685 (1982), G. E. Jellison, “Four channel polarimeter for time resolved ellipsometry, Opt. Lett., Vol. 12, Pg. 766 (1985), Azzam U.S. Pat. No. 4,681,450; Siddiqui U.S. Pat. No. 5,081,348; Berger et al. U.S. Pat. No. 5,102,222; Yamada et al. U.S. Pat. No. 5,335,066, and Lacey et al. U.S. Pat. No. 5,793,480. The basic polarization filter designs involve polarization dependent splitting of the light into several components that are measured simultaneously with multiple detectors. The collected intensities are then analyzed to determine the polarization state of the incoming light.
Azzamn and Jellison, supra, have developed photopolarimeters that split the light to be analyzed into different beams using appropriately coated optics. The output signals are collected with detectors, primarily photodetectors that do not discriminate frequency, and some or all of the parameters that characterize the polarization state of the monochromatic light is calculated using the measured intensitie
Margolis Donald W.
Pham Hoa Q.
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