Thin film measuring device and method

Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer

Reexamination Certificate

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C356S340000

Reexamination Certificate

active

06236459

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to a thin film measuring device and method, and more particularly to a thin film measuring device and method that provides real time and/or spatially extended thin film thickness measurements.
2. Description of the Related Art
There are a large number of conventional devices which use interference effects to measure physical properties of various materials or bodies. Fizeau fringes have long been known to provide information regarding the thickness of thin films which are at least partially transparent to the illumination radiation. Fizeau fringes are produced by the interference of electromagnetic waves that are reflected from a first incident surface with electromagnetic waves reflected from a second incident surface of the thin film. When the two waves are coherent, a series of bright and dark bands are produced, often referred to as a fringe pattern, which correspond to contours of constant optical thickness of the thin film. If the thin film is illuminated with coherent monochromatic light, a fringe pattern of light and dark bands are produced. On the other hand, if the thin film is illuminated with white light or light that has a plurality of spectral bands, then a pattern of colored bands are produced, provided that the light source is sufficiently coherent on the scale of the film thickness. In the case of color fringes, contours of constant color indicate regions of constant thin film thickness. Examples of color fringes in everyday experience are the color fringes produced by light reflected by soap bubbles or light reflected from a puddle of water with a thin film of oil floating on top.
There are a large variety of conventional devices available for producing and recording interference patterns from various thin films. However, one is then left with an image of a pattern that may extend over a region of the thin film and may be dynamic in that the pattern changes in time according to changes in the film thickness over time.
An example of such a spatially extended and dynamic thin film is that of the tear film of a subject's eye. The tear film is believed to be approximately a two-layer film covering the cornea of the eye. The outer layer of the tear film is a very thin oily layer known as the lipid layer. The stability of the pre-corneal tear film is thought to result from interactions between its three major components: mucus glycoprotein, aqueous phase, and superficial Meibomain oils. Clinicians can gather useful information from observations of different aspects of the tear film. Examination of the lipid layer can be helpful in establishing the prognosis of prospective contact lens patients, anticipating special contact-lens-related problems, and in the analysis of symptoms of non-contact lens wearing patients.
The thickness of the lipid layer is believed to be a very informative cue of its stability which is useful in uncovering disorders. Examples of conventional devices for measuring static and dynamic properties of tear films are provided in Josephson, J. E., “Appearance of the Preocular Tear Film Lipid Layer”, American Journal of Optometry and Physiological Optics, vol. 60, no. 11, pages 883-887, 1993 and Hamano et al., “Bio-differential Interference Microscope Observation on Anterior Segment of the Eye”, Journal of Japanese Contact Lens Society vol. 21, pages 229-246, 1979. Such prior art devices are specially adapted to forming interference fringes from the tear film of a subject's eye. However, such prior art devices merely record the interference fringes as a video image. This leaves one with the task of extracting the quantitative film thickness information from the video image, over an extended spatial region of the image at a given time, at a given point in the image over time, or both over an extended spatial region for a period of time to produce film thickness contours which may change over time.
Another problem encountered in measuring the lipid layer of a subject's eye, is that the lipid layer does not normally produce color fringe patterns even though it is illuminated with sufficiently coherent white light. However, color patterns are observed when the lipid layer is thickened during blinking. This indicates that the lipid layer is typically very thin relative to visible light. In this case, the fringes appear to vary in intensity, without varying in color. Consequently, a problem with extracting information from interference patterns of the lipid layer is that one cannot determine the thicknesses in the range of interest based purely on the color of the fringes. On the other hand, a monochromatic interference pattern does not provide the additional information obtainable with a white light, or multiband source for the thicker thin films.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide an interferometric thin film measuring device which provides quantitative thin film thickness determinations over a spatially extended region of a thin film.
It is another object of this invention to provide an interferometric thin film measuring device that provides real time thin film thicknesses of a thin film.
It is another object of this invention to provide an interferometric thin film measuring device that provides real time thin film thicknesses over a spatially extended region of a thin film for a plurality of different times.
It is another object of this invention to provide an interferometric thin film measuring device that efficiently extracts quantitative thin film thicknesses from a thin film interference image.
It is another object of this invention to provide an interferometric thin film measuring device which extracts quantitative thin film thicknesses based on combined intensity and spectral information from a thin film interference image.
It is another object of this invention to provide a method of measuring thin films using any combination of devices according to the above noted objects.
It is another object of this invention to provide an interferometric method of determining a thin film thickness by generating and accessing a calibrated look-up table that summarizes measured or known properties of a thin film material.
The above and related objects of this invention are realized by providing an interferometric thin film measuring device with an interferometer, a detector, a system calibration unit, a weight vector calculating unit, a look-up table storage unit, a weight vector comparing unit and an output/storage unit. The interferometer has at least a light source which illuminates a thin film with light having substantially preselected spectral and coherence characteristics. More preferably, the interferometer includes optical components to collimate and focus the illumination light, to further select coherence and/or polarization properties, and a beam splitter to redirect the interference pattern to the detector. In the preferred embodiment, the detector digitizes at least a portion of the interference pattern in each of a plurality of image channels. The preferred embodiment uses a filter wheel that has a plurality of color filters arranged therein such that the detector digitizes image data in only one image channel at a given time. However, the broader concept of the invention includes digitizing the interference image either sequentially or simultaneously.
A system calibration unit is in communication with the detector. The system calibration unit calibrates the interferometer and detector system using a thin film with substantially known reflectance properties. In the preferred embodiment, the system calibration unit is implemented on a personal computer or work station. However, the invention includes implementing the system calibration unit as a dedicated hardware component. The system calibration unit is in communication with a weight vector calculating unit. The weight vectors provide approximations to the intensity of the light detected in each of the image channels as a function of thin film

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