Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
2000-08-04
2002-11-19
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
By polarized light examination
Of surface reflection
Reexamination Certificate
active
06483585
ABSTRACT:
INTRODUCTION
This invention pertains to instruments of simple design and operation used to measure the thickness of thin films as a function of changes to or attenuation of incident light. The devices and methods of this invention are improvements of a fixed polarizer ellipsometer where film thickness can be related as a function of the degree of ellipticity in polarized light, or rotation of polarized light, that is reflected by a thin film. More specifically, the invention relates to improvements in ellipsometry that speed measurement acquisition time and reduce the cost of ellipsometry devices for use in specific binding assays and other applications. The invention also relates to instruments and methods where measurement of light attenuation by a thin film is no longer dependent on the generation of elliptically polarized light.
BACKGROUND OF THE INVENTION
The following description of the background of the invention is provided simply as an aid in understanding the invention and is not admitted to describe or constitute prior art to the invention.
Optical measurements are commonly used to determine the thickness of thin films. Ellipsometers provide this information by determining the degree of ellipticity in polarized light that is reflected from a thin film. Ellipsometers typically include a light source, a polarizer, an analyzer, an optical compensator or quarter wave plate, and a detector. For example, U.S. Pat. No. 5,936,734 discloses using singly, partially, and/or multiply polarized electromagnetic radiation for ellipsometrically measuring regions of a patterened sample system, while U.S. Pat. No. 5,946,098 discloses a modified ellipsometer comprising a retarder element in the form of a prism.
Complex mathematical calculations are conventionally used to determine the film thickness. To use these mathematical calculations, the ellipsometer must have precise alignment of the rotating components for the measurement of signal intensity at the detector. Expensive and precise optical components must be used to provide for optimization of the detector signals in performing film thickness measurements. Measurement time is slow but the ellipsometer can provide precise thickness and refractive index determinations.
The rotation of ellipsometer components can be used to provide a sinusoidal plot of intensity at the detector as a function of time and angular velocity of the rotating component, as disclosed in U.S. Pat. No. 5,581,350. Measurements are made at two or more analyzer angles to determine the angle of the polarizer's optical axis and the offset of the actual analyzer angle relative to its nominal angle. Measurements provide the angle of the analyzer's optical axis and the offset of the polarizer angle relative to its nominal angle. This information is used to calibrate the ellipsometer, but the device operates according to time-consuming traditional principles that require precise alignment of rotating components for purposes of measuring film thicknesses. Similarly, U.S. Pat. No. 5,877,859 discloses rotating compensator ellipsometry methods, relying on a rotating compensator to produce a signal having a dc component, a two omega component, and a four omega component.
According to U.S. Pat. No. 3,985,447 it is also possible to rotate both the optical compensator and the polarizer at different angular speeds to measure the resultant transmitted optical intensity as a function of time. A Fourier analysis is used to determine the Stokes parameters of light that is reflected by the thin film. The film thickness and refractive index of the film can also be calculated in this manner based upon the Stokes parameters. A disadvantage of this device is that the system requires additional components including a time dependent rotating compensator. These additional components increase the expense and complexity of the system.
U.S. Pat. No. 4,725,145 discloses an instrument and method of use for measuring the state of polarization. The instrument contains only a photodetector. The photodetector has a partially specular surface and is placed at an oblique angle relative to the incident light source. The light adsorbed by the photodetector generates an electrical signal that is detected and related to the polarization of the light. The detector may be rotated to determine if the light contained any elliptical character. In the preferred mode of operation the entire system is rotated. The improvement is that the instrument does not include any wave retarders or polarizers. The system may contain one or more photodetectors. The amount of light adsorbed is a fraction of the incident radiation and is dependent on the incident light source and the azimuthal orientation of the plane of incidence. The detector surface rotates in a conical manner. Thus the plane of incidence is a plane which revolves around and through the incident light. The electrical output is modulated by the rotation and thus the modulation is a measure of the state of polarization of the light incident on the detector.
U.S. Pat. No. 5,552,889 discloses a method for measuring changes in polarized light that is independent of temperature. The method examines the AC and DC components of the light separately. The method requires an instrument design where two or more polarizers are arranged so that they are not orthogonal to each other. The modulation of the polarized signal is then measured at one or more photodetectors. The intensity of a constant component of the polarization signal is related to the position of the average plane of polarization. The alternating component of the polarization signal is normalized to the constant component and then the phase, amplitude, and position of the polarization is determined. The polarization signal is exactly linearized. The method requires a beam splitter to produce two beams of light.
U.S. Pat. No. 5,625,455 discloses an ellipsometer and an ellipsometric method. In the method the complex dielectric constant, the complex index of refraction, the transmittance, the reflectance, the adsorption coefficient, the optical density, and other optical properties may be measured by reflectance of a monochromatic light source. The instrument and method of use provide a direct measure of the optical and spectroscopic properties of the sample without numerical approximation or wavelength frequency scans. The light source should be elliptically polarized and the angle of incidence should be between 0° and 90°. The digitized intensity data reflected or transmitted from the sample is analyzed using integrals or sums. The integrals eliminate noise and allow the method to start and stop at any analyzer angle.
In certain applications, it is possible to eliminate some of the components from an ellipsometer-like device for purposes of reducing the cost of manufacture while still providing an acceptable level of accuracy. U.S. Pat. No. 5,494,829 to Sandstrom et al. describes a device that operates according to the principles of ellipsometry, but has a fixed polarizer and a fixed analyzer with the additional cost advantage of having no optical compensator or other complex optical components. This device is used for binding assay analysis to determine whether a biochemical reaction has provided a thin film analyte indicating the presence of bacterial infection in a patient.
According to the '829 patent, an antigen or an antibody is bound to a substrate and is incubated with an analyte solution that is prepared to include a body fluid specimen from a patient who is being tested for infection. A biochemical reaction grows a thin film on the substrate if the corresponding antibody or antigen is present in the solution. A positive test result is indicated by signal intensity at the detector relative to a delimiting threshold or background value. The device is typically calibrated to measure thickness in films having been produced by a particular antigenic reaction.
A number of spectrophotometric systems have been designed to analyze the thickness of films, in particular photoresist films. T
Foley & Lardner
Stafira Michael P.
Thermo Biostar Inc.
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