Abstract:
An ellipsometer is used to analyze each of a plurality of sample portions that each include a substrate portion with a coating portion thereon, the substrate portions corresponding to respective spaced portions of a part with a curved surface. For each sample portion, the analysis includes: directing onto the coating portion a beam of radiation that includes first and second components with different polarizations; detecting energy of each of the first and second components reflected by the sample portion; and generating data that includes, for each of a plurality of different wavelengths, information regarding a change caused by the sample portion to a relationship between the first and second components.

Description:
FIELD OF THE INVENTION 
   This invention relates in general to analysis of coatings and, more particularly, to analysis of coatings on curved surfaces. 
   BACKGROUND 
   When fabricating optical components such as lenses, a coating is often formed on a surface of the component, to provide desired optical or physical properties. For example, the coating may provide an anti-reflective (AR) characteristic, a filtering characteristic, physical protection for the component, some other characteristic, or a combination of two or more characteristics. These coatings often include multiple layers of different materials that collectively provide the desired characteristic(s). 
   The layers in a coating should ideally have a uniform thickness, and the optical and mechanical characteristics of the layer should ideally be uniform throughout the layer. But as a practical matter, this is not always the case. For example, where a coating is formed on a relatively highly curved surface, it is common for a given layer within the coating to have a peripheral region that is as much as 30% to 50% thinner than a central region of that layer, or even more than 50% thinner. Further, different layers in the same coating often have different degrees of variation in thickness. For example, one layer may be 30% thinner in a peripheral region than in a central region, while another layer may be 50% thinner in the peripheral region that in the central region. Thus, even assuming that the layers of a coating all have the proper thicknesses in the central region, the thicknesses in the peripheral region will typically not be correct and, moreover, the ratios of thickness in the peripheral region will not be correct. As a result, the coating will provide the desired optical and mechanical characteristics in its central region, but may fail to provide these desired characteristics in its peripheral region, or may at least exhibit a degradation of these characteristics in the peripheral region. 
   One existing approach for analyzing a coating is to use a spectrophotometer to measure transmissivity and/or reflectance of the coating, at different locations on the coating. While existing approaches of this type have been generally adequate for their intended purposes, they have not been satisfactory in all respects. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawing, in which: 
       FIG. 1  is a block diagram of a coating analysis apparatus that embodies aspects of the invention, and shows an optical component that can be analyzed by the illustrated apparatus. 
       FIG. 2  is a diagrammatic front view of the optical component of  FIG. 1 , taken in the direction of arrow  56  in  FIG. 1 . 
       FIG. 3  is a diagrammatic sectional view taken along section line  3 - 3  in  FIG. 2 . 
       FIG. 4  is a diagrammatic front view of a “dummy” optical component. 
       FIG. 5  is a diagrammatic sectional view taken along section line  5 - 5  in  FIG. 4 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram of a coating analysis apparatus  10  that embodies aspects of the invention. The apparatus  10  includes an ellipsometer  14  of a known type. In the disclosed embodiment, the ellipsometer  14  is a model SOPRA GES5 ellipsometer obtained commercially from Sopra Inc. of Palo Alto, Calif. Alternatively, however, it would be possible to use any other suitable device. 
   Since the ellipsometer  14  is a device of a known type, it is described here only briefly, for the purpose of facilitating an understanding of the present invention. More specifically, the ellipsometer  14  includes a radiation source  16 , and the source  16  emits radiation that propagates along a path  16 , and passes through a polarizer  18 . After leaving the polarizer  18 , the radiation has two components with different polarizations. These two different polarizations are commonly referred to as “P” and “S” polarizations. In the disclosed embodiment, when these two components leave the polarizer, they have substantially the same amplitude and substantially the same wavelength (frequency), and are substantially in phase with each other. 
   The polarized radiation traveling along the path  17  impinges on an optical component  19 . The component  19  is not part of the ellipsometer  14 , and is discussed in more detail later. The component  19  reflects at least some of the polarized radiation from the ellipsometer. The reflected radiation travels along a path of travel  21  through an analyzer  22 , to a radiation detector  23 . 
   The ellipsometer  14  takes the information that it collects with the detector  23 , and looks for changes introduced into the polarized components by the optical component  19 . For example, the ellipsometer looks for changes in the amplitude of one component relative to the amplitude of the other component, and also looks for changes in the phase of one component relative to the phase of the other component. More specifically, for each of several different wavelengths within a range of interest, the ellipsometer  14  calculates a ratio of the amplitude of one of the polarized components relative to the amplitude of the other polarized component, as measured at the detector  23 . Second, for each of the different wavelengths in the range, the ellipsometer determines a value representing a phase difference between two polarized components, as measured at the detector  23 . All of this information is then supplied at  29  to a computer  31 . 
   The computer  31  includes computer hardware in the form of a conventional, commercially-available computer system of the type commonly known as a personal computer. In the disclosed embodiment, the hardware is a personal computer obtained commercially from Dell Inc. of Round Rock, Tex. However, the computer  31  could alternatively contain any other suitable computer hardware. The hardware of computer  31  executes a software program that facilitates the design and modeling of optical coatings. In the disclosed embodiment, the software is a program obtained commercially under the tradename FILMWIZARD from Scientific Computing International of Carlsbad, Calif. However, the software could alternatively be any other suitable program. The computer  31  is operated by a human operator  32 . The operator  32  is a person of ordinary skill in the art of coating design, who is familiar with use of the software executed by the computer  31 . 
   The apparatus  10  includes a support  41 , and a positioning mechanism  42  that supports the support  41  for pivotal movement about a pivot axis  43  that extends perpendicular to the plane of  FIG. 1 . The pivotal movement of the support  41  is indicated diagrammatically in  FIG. 1  by a double-headed arrow  44 . The positioning mechanism  42  is also capable of releasably holding the support  41  in any selected pivotal position. 
     FIG. 2  is a diagrammatic front view of the optical component  19 , taken in the direction of arrow  56  in  FIG. 1 .  FIG. 3  is a diagrammatic sectional view of the optical component  19 , taken along section line  3 - 3  in  FIG. 2 . As best seen in  FIG. 3 , the optical component  19  has a substrate  61 , with a highly-curved concave inner surface  62 , and a highly-curved concave outer surface  63 . In the disclosed embodiment, the substrate  61  is made of zinc selenide (ZnSe), but it could alternatively be made of any other suitable material. 
   An optical coating  66  is provided on the curved outer surface  63  of the substrate  61 . It would be possible for the coating  66  to be only a single layer of a single material. But in the disclosed embodiment, the coating  66  includes a plurality of different layers (not separately depicted), involving the use of one material for some layers, another material for other layers, and so forth. By interleaving different layers of different materials having different thicknesses, the coating  66  can be given certain desired optical and/or mechanical characteristics. 
   The layers of the coating may, for example, include silicon (Si) and silicon monoxide (SiO). It is possible that the multiple layers in the coating  66  would all have the same thickness. Typically, however, some layers are intentionally designed to be thicker or thinner than other layers. A separate consideration is that it is desirable for the thickness of each layer be relatively uniform throughout that layer. But as a practical matter, this can be difficult to achieve, especially where the outer surface  63  of the substrate  61  is highly curved. For example, one common coating technique is to place the substrate  61  in a not-illustrated vacuum chamber, with a conventional evaporation source disposed below the substrate  61 , and then successively evaporate different materials with the evaporation source in order to successively form the layers of the coating  66 . When the layers of a coating are formed in this manner, each layer of the resulting coating will tend to be thicker in its central region than in its peripheral edge region. 
   Different layers made of different materials may experience different degrees of variation in thickness. For example, a layer made of one material may have a peripheral edge region that is 35% thinner than its central region, whereas a different layer made of a different material may have an edge region that is 45% thinner than its central region. Since most or all layers in the coating  66  will be thinner in the edge region than in the central region, the coating will have an overall thickness in the edge region that is thinner than the overall thickness in the central region. In this regard, it is common for a coating to be 30% to 50% thinner in its edge region than in its central region. In the case of an optical component such as that illustrated at  19  in  FIGS. 1-3 , this variation in layer thicknesses and coating thickness can affect the optical performance of that component. 
   For example, assume hypothetically that the coating  66  is intended to pass radiation from a laser that has a wavelength of 1064 nm. More specifically, assume that the coating is configured to efficiently pass radiation within a waveband of 1040 nm to 1090 nm (where 1064 nm is approximately in the center of this waveband), that the coating has the proper thickness in its central region, and that the coating is about 35% thinner in its edge region that in its central region. Since the thickness changes by 35%, the effective operating range will also change by about 35%. For example, the coating may have the desired waveband of 1040 nm to 1090 nm in its central region, but a 35% variation in thickness could cause the effective operating waveband in the peripheral region of the coating to be about 676 nm to 709 nm (and it will be noted that the 1064 nm wavelength of interest is not within this altered waveband of 676 nm to 709 nm). Consequently, in order to evaluate a coating such as that shown at  66 , it is desirable to be able to accurately measure characteristics of the coating, such as variations in thickness, optical characteristics, and/or density. 
     FIG. 3  shows three integral portions  101 ,  102  and  103  of the optical component  19  that have been selected to be sample portions. The sample portion  103  is located in the center of the optical component  19 , the sample portion  101  is located near an outer edge of the component  19 , and the sample portion  102  is located between the sample portions  101  and  103 . Although  FIG. 3  shows three sample portions  101 - 103 , it would alternatively be possible to have a larger or smaller number of sample portions. Moreover, the locations of the sample portions could be different. 
   The sample portion  101  includes a substrate portion  111  that is part of the substrate  61 , and a coating portion  112  that is part of the coating  66 . The sample portion  102  includes a substrate portion  113  that is part of the substrate  61 , and a coating portion  114  that is part of the coating  66 . Similarly, the sample portion  103  includes a substrate portion  116  that is part of the substrate  61 , and a coating portion  117  that is part of the coating  66 . 
   Referring to  FIGS. 1 and 3 , the positioning mechanism  42  can selectively position and hold the optical component  19  so that the ellipsometer beam traveling along path  17  impinges on any selected one of the sample portions  101 ,  102  and  103 . Initially, for example, the substrate  41  and the optical component  19  can be positioned so that the radiation impinges on the sample portion  101 . Based on the reflected radiation received at the detector  23  from the path of travel  21 , the ellipsometer determines, for each of several different wavelengths of interest, a ratio of the amplitudes of the two polarized components in the reflected radiation, and a phase difference between these two components. This information is provided at  29  to the computer  31 . 
   The positioning mechanism  42  is then used to move the support  41  and the optical component  19  until the ellipsometer beam traveling along path  17  impinges on the sample portion  102  ( FIG. 3 ). The ellipsometer  14  then again determines a ratio of amplitudes and also a phase difference for each of the different wavelengths, and supplies this information at  29  to the computer  31 . Next, the positioning mechanism  42  positions the support  41  and the optical component  19  so that the ellipsometer beam traveling along path  17  impinges on the sample portion  103  ( FIG. 3 ). The ellipsometer  14  then determines a ratio of amplitudes and a phase difference for each of the different wavelengths, and supplies this data at  29  to the computer  31 . 
   The software running on the computer  31  can then, under direction of the operator  32 , use the data received at  29  from the ellipsometer  14  to evaluate characteristics of the coating  66 , such as thickness, optical properties, and material density. The software can also be used in a known manner to model potential adjustments to the design of the coating  19 , in an attempt to improve and optimize the optical and mechanical characteristics of the multi-layer coating  66 . 
   In  FIG. 3 , the coating  66  is provided on the convex outer surface  63  of the substrate  61 . It is alternatively possible to provide a coating on the concave inner surface  62  of the substrate  61 . However, if the inner surface  62  is sufficiently highly curved, it may be difficult or impossible to position the optical component so that the radiation beam from the ellipsometer  14  ( FIG. 1 ) will have the clearance needed to travel without obstruction to and be reflected by some or all portions of the coating, and then travel without obstruction to the detector  23 . Accordingly, a different approach can be used. 
   In this regard,  FIG. 4  is a diagrammatic view that is similar to  FIG. 2 , but that shows a “dummy” optical component  219 .  FIG. 5  is a diagrammatic sectional view of the dummy optical component  219 , taken along the section line  5 - 5  in  FIG. 4 . The component  219  includes a fixture or substrate  261  with a highly curved concave inner surface  262 , and a highly curved convex outer surface  263 . The surfaces  262  and  263  are equivalent in shape to the surfaces  62  and  63  in  FIG. 3 . The fixture  261  could optionally be made from an optical material such as zinc selenide. In the disclosed embodiment, the fixture  261  is made from stainless steel. But the fixture  261  could alternatively be made from any other suitable material, including but not limited to aluminum or regular steel. 
   The fixture  261  has three spaced circular openings  267 ,  268  and  269  that extend radially therethrough, and that have approximately equal diameters. The opening  269  is in the center of the fixture  261 , the opening  267  is near a peripheral edge of the fixture  261 , and the opening  268  is between the openings  267  and  269 . The number and configuration of the openings  267 - 269  is exemplary. The number of openings could be larger or smaller, the shape of the openings could be different, and/or the relative positions of the openings could be different. 
   Three substrate portions  272 ,  273  and  274  are fixedly mounted in the openings  267 ,  268  and  269 , respectively. The substrate portions  272 - 274  are approximately planar. Each substrate portion has a planar inner surface that, along its peripheral edge, is approximately flush with the inner surface  262  of the fixture  261 . In the disclosed embodiment, the substrate portions  272 - 274  are each made from exactly the same material as the substrate of the actual optical component that will be coated. For example, if the substrate of the optical component is made from zinc selenide (ZnSe), then the substrate portions  272 - 274  are each made from zinc selenide. Alternatively, however, the substrate portions  272 - 274  could be made from some other suitable material. 
   Using known evaporative techniques, a multi-layer optical coating  277  is formed on the inner surface  262  of the fixture  261 , and on the inner surfaces of each of the substrate portions  272 - 274 . As a result, the substrate portion  272  has thereon a portion  281  of the coating  277 , the substrate portion  273  has thereon a portion  282  of the coating  277 , and the substrate portion  274  has thereon a portion  283  of the coating  277 . The substrate portion  272  and the coating portion  281  together form a sample portion  286 , the substrate portion  273  and the coating portion  282  together form a sample portion  287 , and the substrate portion  274  and the coating portion  283  together form a sample portion  288 . The sample portions  286 - 288  may alternatively be referred to as “witness pieces”. 
   After the dummy component  219  of  FIGS. 4 and 5  has been fabricated and coated, the sample portions  286 ,  287  and  288  are each removed from the fixture  261 . The sample portion  272  is fixedly mounted on the support  41  of  FIG. 1  (in place of the optical component  19 ), and then the ellipsometer  14  is used to analyze the coating portion  281  of the sample portion  286 , in the same manner discussed above in association with the sample portions  101 - 103 . The sample portion  286  is then removed from the support  41  and replaced with the sample portion  287 , and the coating portion  282  of sample portion  287  is analyzed. Then, the sample portion  287  is removed from support  41  and replaced with the sample portion  288 , and the coating portion  283  of sample portion  288  is analyzed. The resulting data is supplied at  29  to the computer  31 , and is processed by the computer  31  and the operator  32  in a manner similar to that discussed above for the coating  66  of  FIG. 1-3 . 
   In the embodiment of  FIG. 1 , the support  41  can pivot about the axis  43 , under control of the positioning mechanism  42 . Alternatively, however, the support  41  and the positioning mechanism  42  could be omitted. The optical component  19  could initially be supported on a first non-movable and not-illustrated support that stationarily positions the optical component so that the ellipsometer beam impinges on the sample portion  101  ( FIG. 3 ). Then, the optical component  19  could be stationarily supported on a second non-movable and not-illustrated support that stationarily positions the optical component  19  so that the ellipsometer beam impinges on the sample portion  102  ( FIG. 3 ). Thereafter, the optical part  19  could be supported on a third non-movable and not-illustrated support that stationarily positions the optical component  19  so that the ellipsometer beam impinges on the sample portion  103 . 
   The apparatus  10  of  FIG. 1  permits measurement of variations in thickness, optical characteristics and material density across the radius of a component, with a high degree of accuracy and reliability. These measurements can then serve as a basis for improving and/or optimizing the optical and mechanical characteristics of a coating. 
   Although selected embodiments have been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.