Patent Publication Number: US-2018045507-A1

Title: Method And System For Real-Time In-Process Measurement Of Coating Thickness

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/781,457 filed Sep. 30, 2015, entitled “Method &amp; System For Real-Time In-Process Measurement Of Coating Thickness,” which claims priority to PCT Application No. PCT/US2014/027980 filed Mar. 14, 2014, entitled “Method and System For Real-Time In-Process Measurement of Coating Thickness,” published as WO 2014/143838, which claims priority to U.S. Provisional Patent Application No. 61/792,689 filed Mar. 15, 2013, entitled “Method and System for Inline Real-Time Measurement of Thin Film Thickness.” The content of each of the aforementioned applications is incorporated by reference herein in its entirety. 
    
    
     FIELD 
     The present disclosure generally relates to methods and systems for measuring the thickness of coatings or thin films on various substrates. Embodiments include methods and systems for real-time in-process measurement of coating thickness, and more particularly methods and systems for real-time in-process measurement of a coating thickness on a moving substrate or a coated product. 
     BACKGROUND 
     The measurement of thin films and coatings is appreciated in manufacturing settings. For example, regulating the application of a thin film or coating to a product within a preferred thickness range allows manufacturers to ensure that a film or coating is applied with sufficient thickness to prevent manufacturing defects while also avoiding wasteful application of film or coating in excess of a required thickness, thereby minimizing materials costs. In the context of manufacturing processes, measurement of thin layers of lubricious coatings on metals allows manufacturers to ensure that sufficient coating is applied to prevent substantial damage to expensive manufacturing and processing equipment. Furthermore, it is preferable to periodically perform measurements of thin films or coatings in real-time as the films or coatings are applied before the coated substrate proceeds further through the manufacturing process. 
     There are several currently known techniques for measuring thin films or coatings. However, the known methods have various limitations that significantly undermine their respective usefulness in industrial applications. For example, standard reflectometry based measurement techniques become unreliable when the thickness of the subject coating/film under consideration is below 200 nanometers (1 nanometer=0.001 microns). Known modeling-based reflectometry techniques are not well-suited and not robust enough for use in industrial production environments. 
     In addition, modeling-based reflectometry techniques for measuring film or coating thicknesses of less than 0.2 microns have typically focused on measurement of coatings and films on semiconductor substrates. However, applications of coatings or films in the semiconductor manufacturing process are performed on static (non-moving) substrates. A significant limitation of the currently known modeling-based reflectometry techniques capable of measuring coatings of less than 0.2 microns is that they require a static substrate on which to perform measurements of coatings or materials deposited thereon. 
     The present disclosure is directed to methods and systems for real-time in-process measurement of thin films or coatings, including films or coatings of less than 0.2 microns, on a moving substrate, and therefore overcomes the limitations of known methods and systems. 
     SUMMARY 
     The present disclosure generally relates to a method comprising providing and directing light waves of varying wavelengths toward a moving substrate comprising a coating, linearly polarizing the light waves, converting the linearly polarized light waves to circularly polarized light waves, analyzing elliptically polarized light waves reflected by the moving substrate, capturing analyzed light waves, generating light wave data based on the captured light waves, and determining a thickness of the coating based on the light wave data. In another embodiment, the method further comprises detecting a disturbance in the movement of the substrate based on the light wave data and adjusting an orientation of an analyzer and or an orientation of a detector based on the detected disturbance. 
     The present disclosure is also generally related to a system comprising a processor, a light source in communication with the processor, the light source configured to provide and direct light waves of varying wavelengths toward a moving substrate comprising a coating, a polarizer positioned between the light source and the moving substrate, a wave plate positioned between the polarizer and the moving substrate, an analyzer positioned to receive light waves reflected by the moving substrate, a detector, in communication with the processor, positioned to capture light waves reflected by the analyzer and configured to generate light wave data based on the captured light waves, and a memory in communication with the processor, wherein the memory comprises computer program code executable by the processor to determine a thickness of the coating based on the light wave data. In another embodiment, an orientation of the analyzer is adjustable and an orientation of the detector is adjustable. In still another embodiment, the memory further comprises computer program code executable by the processor to: detect a disturbance in the movement of the moving substrate based on the light wave data; and adjust the orientation of the analyzer or the orientation of the detector based on the detected disturbance. 
     Illustrative embodiments disclosed herein are mentioned not to limit or define the invention, but to provide examples to aid understanding thereof. Illustrative embodiments are discussed in the Detailed Description and further description of the invention is provided therein. Advantages offered by various embodiments of this invention may be further understood by examining this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages according to the present disclosure are better understood when the following Detailed Description is read with reference to the accompanying figures, wherein: 
         FIG. 1A  is a block diagram of a system for measuring thin film/coating thickness according to one embodiment of the present disclosure. 
         FIG. 1B  is an illustration of a configuration comprising components of system  100  according to one embodiment of the present disclosure. 
         FIG. 2  is a flow diagram of a method for measuring thin film/coating thickness according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments according to this disclosure provide methods and systems for inline real-time measurement of thin film or coating thickness, and more particularly to methods and systems for inline real-time measurement of thin film or coating thicknesses, including films or coatings of less than 0.2 microns, on a moving substrate. 
     Illustrative Embodiment 
     In one illustrative embodiment, a manufacturer employs the systems and methods of the present invention to measure the thickness of a lubricious coating applied to thin metal sheeting to ensure a sufficient layer of lubricious coating is present on the thin metal sheeting—used by the manufacturer to create its products—to prevent damage to expensive manufacturing equipment that processes the metal sheeting. In the illustrative embodiment, the manufacturer incorporates a broadband light source that directs light waves through polarizers and wave plates onto the surface of the thin metal sheeting containing the lubricious coating as it is moving through the equipment. The manufacturer further incorporates detectors that capture reflected light that passes through rotating analyzers. A computer controlling the manufacturing process is in communication with the light sources and the detectors and is programmed to configure the light sources to generate light within a particular spectrum range based on the particular metal and the particular lubricious coating being measured. The computer is further programmed to receive light wave data from the detectors. The computer quantifies the phase shift and polarization state changes of the reflected light, compared to the light waves generated by the light source, and then uses that information to evaluate and validate thickness of the lubricious coating at various locations on the metal sheeting as it is moving through the equipment. 
     In addition, rotating analyzers and detectors are coupled to an adjustment mechanism in communication with the computer. The computer operates to detect flutters, vibrations, or other disturbances in the movement of the metal sheeting and automatically adjusts the positions of the analyzers and detectors to ensure accuracy of the thickness measurements. 
     In the illustrative embodiment, a preferred thickness range, warning thickness level and a critical thickness threshold are defined and programmed into the computer. If the measured thickness of the lubricious coating is well within the preferred thickness range, the computer allows the manufacturing process to continue. If the computer detects that the lubricious coating is outside of the preferred thickness range, the computer provides feedback to the system controlling the application of the lubricious coating. In response, that system adjusts the application of the lubricious coating to bring it back within the preferred thickness range. In the event that the coating thickness reaches the warning thickness level, the computer alerts the equipment operators of a potential malfunction in the lubricious coating application system. The operators may then choose whether to shut down the manufacturing process to investigate or to continue the process. 
     Illustrative System 
       FIG. 1A  is a block diagram of a system for measuring thin film/coating thickness according to one embodiment of the present disclosure.  FIG. 1A  illustrates a configuration comprising components of system  100  according to one embodiment of the present disclosure. System  100  may comprise one of a variety of form factors. In one embodiment, system  100  may be a self-contained system comprising a single housing. In one embodiment, the system  100  may comprise a portable housing. In other embodiments, the system  100  may be integrated directly into manufacturing or testing equipment. In still another embodiment, system  100  may comprise a number of components in separate physical locations, but coupled through wired and/or wireless communications and/or networking well known to those having ordinary skill in the art. 
     Embodiments of the present disclosure can be implemented in combination with, or may comprise combinations of: digital electronic circuitry, computer hardware, firmware, software, light sources, optical equipment, and/or optical sensors. The system  100  shown in  FIG. 1  comprises a processor  102 . The processor  102  receives input signals and generates signals for communication, display, and processing sensor readings to measure thicknesses of thin films/coatings. The processor  102  includes or is in communication with one or more computer-readable media, such as memory  104 , which may comprise random access memory (RAM). 
     The processor  102  executes computer-executable program instructions stored in memory  104 , such as executing one or more computer programs for providing a user interface and/or processing sensor readings to measure thicknesses of thin films/coatings. Processor  102  may comprise a microprocessor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), one or more field programmable gate arrays (FPGAs), or state machines. The processor may further comprise a programmable electronic device such as a PLC, a programmable interrupt controller (PIC), a programmable logic device (PLD), a programmable read-only memory (PROM), an electronically programmable read-only memory (EPROM or EEPROM), or other similar devices. 
     Memory  104  comprises a computer-readable media that may store instructions, which, when executed by the processor  102 , cause it to perform various steps, such as those described herein. Embodiments of computer-readable media may comprise, but are not limited to, an electronic, optical, magnetic, or other storage or transmission device capable of providing the processor  102  with computer-readable instructions. Other examples of media comprise, but are not limited to, a floppy disk, CD-ROM, magnetic disk, memory chip, ROM, RAM, ASIC, configured processor, all optical media, all magnetic tape or other magnetic media, or any other medium from which a computer processor can read. Also, various other devices may include computer-readable media, such as a router, private or public network, or other transmission device. The processor  102 , and the processing, described may be in one or more structures, and may be dispersed through one or more structures. 
     Referring still to  FIG. 1 , the system  100  also comprises one or more user input devices  108  in communication with the processor  102 . For example, in some embodiments a user input device  108  may comprise a keyboard, mouse, trackball, touchscreen, touchpad, voice recognition system or any other input device known to one having ordinary skill in the art. 
     The system  100  also comprises a display  106 . Display  106  is in communication with processor  102  and is configured to display output from the processor  102  to the user. For instance, in one embodiment, display  106  is a standard computer monitor such as an LCD display or a cathode ray tube (CRT). In another embodiment, system  100  may comprise a touch-screen LCD that operates both as a display  106  and a user input device  108 . Various sizes of LCD displays may be used. 
     Referring now to  FIGS. 1A and 1B , the system  100  further comprises a light source  110 . For example, in one embodiment light source  110  is a broadband light source capable of generating light waves of multiple wavelengths. For example, the wavelengths of the illuminated light could be in the ultraviolet (UV), visible, or near infrared (NIR) regions. In one embodiment, light source  110  is capable of generating light waves having wavelengths in the UV and visible spectrum regions. In one such embodiment, the light source  110  comprises a xenon arc lamp. In another embodiment, light source  110  is capable of generating light waves having wavelengths in the visible and NIR spectrum regions. In one such embodiment, the light source  110  comprises a tungsten halogen lamp. In still another embodiment, light source  110  is capable of generating light waves having wavelengths in the UV, visible, and NIR spectrum regions. In one such embodiment, the light source  110  comprises a xenon arc lamp and a tungsten halogen lamp. 
     In one embodiment, light source  110  is coupled to processor  102  to allow the processor  102  to control the output of the light source  110 . For example, the processor  110  may communicate with the light source  110  to turn the light source  110  on or off, or to specify the type of light waves to be provided. In one embodiment, the processor communicates with the light source to specify light waves within the UV, visible, and/or NIR spectrum regions, or subsets thereof. In one embodiment, the processor  102  controls the amount of light generated to ensure the light levels are not saturated. In one embodiment, the light source  110  may be positioned to directly emit light towards a substrate  120 , as shown in  FIG. 1B . In other embodiments, the light guides may be used to direct light emitted from a light source  110  located at another position, such as within a housing comprising the processor  102  and memory  104 , towards a substrate  120 . 
     The system  100  further comprises a polarizer  118 . Polarizer  118  is an optical device that functions to convert unpolarized light waves passing through it, such as light waves provided by light source  110 , into linearly polarized light waves. In one embodiment, a Glan Taylor polarizer with an extinction coefficient of 10 5 :1 is used to convert the non-polarized light beam into linearly polarized light beam. In the embodiment illustrated in  FIG. 1B , polarizer is positioned such that light waves provided by light source  110  pass through polarizer  118 . 
     The system  100  further comprises a wave plate  112 . In one embodiment, the wave plate  112  is a quarter-wave plate Wave plate  112  functions to alter the polarization state of light waves passing through it. For example, a quarter-wave plate converts linearly polarized light waves passing through it into circularly polarized light waves. In the embodiment illustrated in  FIG. 1B , the wave plate  112  is positioned to receive light waves from a light source  110  that first passes through a polarizer  118 . In the embodiment illustrated in  FIG. 1B , the light waves passing through the wave plate  112  are incident light waves to a substrate  120  comprising a coating or film  122  on the top surface. In some embodiments, the wave plate  112  is rotatable and comprises a mechanism (e.g. an electric motor) for rotating the wave plate  112 . In one such embodiment, processor  102  communicates with wave plate  112  to control whether and at what speed the wave plate  112  is rotating. 
     The system  100  further comprises an analyzer  114 . In one embodiment, analyzer  114  is a rotating analyzer that receives elliptically polarized light reflected by substrate  120  and/or coating or film  122 . The rotating analyzer  114  functions to reflect light from various angular positions. The analyzer  114  is the same component as the polarizer  118  except that it is used to analyze the polarization state of the light wave instead of altering the polarization state of the incident light beam. In some embodiments, a rotating analyzer  114  comprises a mechanism (e.g. an electric motor) for rotating the analyzer  114 . In one such embodiment, processor  102  communicates with the rotating analyzer  114  to control whether and at what speed the analyzer  114  is rotating. 
     The system  100  further comprises a detector  116 . Detector  116  operates to detect the reflected light generated from various angular positions of the rotating analyzer. In one embodiment, the detector  116  comprises a spectrometer. In some embodiments, different detectors may be used for different wavelength ranges. In one embodiment, the detector  116  may be positioned to directly receive the reflected light generated from various angular positions of the rotating analyzer  114 , as shown in  FIG. 1B . In other embodiments, a probe connected to a light guide may be used to capture and direct the reflected light generated from various angular positions of the rotating analyzer  114  to a detector  116  located at another position, such as within a housing comprising the processor  102  and memory  104 . In one such embodiment, the probe comprises a fiber optic probe. The detector  116  operates to convert captured light waves into light wave data. In one embodiment, light wave data may be a voltage signal waveform that corresponds to the captured light wave. In another embodiment, light wave data comprises a data structure containing information that describes the capture light waves. 
     The detector  116  is in communication with the processor  102  and provides the light wave data to the processor  102 . The processor  102  is programmed to validate and evaluate the light wave data. In one embodiment, the processor  102  quantifies the phase shift and polarization state changes, compared to the light waves generated by the light source  110 , and then uses that information to evaluate and validate thickness of the coating/film  122  on the substrate  120 . In one embodiment, the processor  102  broadly calculates the polarization state change and the phase shift of the incident light waves on the sample to that of the reflected light waves emanating from the analyzer  114 . In one embodiment, theoretical models are developed for the given substrate coating combination and the Levenberg-Marquardt algorithm is used to calculate the best fit to match the light wave data with a theoretical model to determine thickness. In some embodiments, triangular smoothing techniques are applied to light wave data to optimize the quality of spectral response before it is evaluated. Furthermore, in some embodiments, techniques for determining signal quality and detecting noise are used to validate light wave data corresponding to individual measurements. In one embodiment, signal quality of the light wave data is determined by using predetermined coating specific spectral signatures to validate individual measurements. In still another embodiment, the processor  102  determines the strength and quality of the light waves based on the light wave data and dynamically adjusts the light intensity provided by light source  110 . 
     In some embodiments, system  100  may comprise two or more sets of light sources  110 , polarizers  118 , wave plates  113 , analyzers  114  and detectors  116 . In one such embodiment, the system  100  may simultaneously measure the thickness of the coating/film  122  at multiple locations on substrate  120 . In another embodiment, a single light source  110  and/or a single detector  116  may be used in conjunction with two or more sets of polarizers  118 , wave plates  113 , and analyzers  114 . In one embodiment, an optical switch and light guides coupled thereto may be used to provide light from a single light source to multiple locations on substrate  120 . In another embodiment, an optical switch with light guides attached thereto and probes coupled to the light guides may be used to capture the reflected light generated from various angular positions of multiple rotating analyzers  114  and direct the capture light to a single detector  116 . 
     While shown as individual components in  FIG. 1B , in some embodiments two or more of light source  110  (or the emission point of a light guide coupled to a light source  110 ), polarizer  118  and wave plate  112  may reside in a single housing. Similarly, in other embodiments, analyzer  114  and detector  116  (or a probe coupled to a detector  116  through a light guide) may reside in a single housing. 
     In one embodiment, analyzer  114  and detector  116  (or a probe coupled to a detector  116  through a light guide) are coupled to one or more adjustment mechanisms  124  in communication with processor  102  for adjusting the position of the analyzer  114  and detector  116  (or a probe coupled to a detector  116  through a light guide). In another embodiment, polarizer  118  and analyzer  114  are coupled to one or more adjustment mechanisms  124  in communication with processor  104  for adjusting the position of the polarizer  118  and analyzer  114 . In another embodiment, polarizer  118 , analyzer  114 , and detector  116  are coupled to one or more adjustment mechanisms  124  in communication with processor  102  for adjusting the position of the polarizer  118  and analyzer  114 . The one or more adjustment mechanisms  124  may use electric motors, linear actuators, sliding tracks, gimbal mechanisms, or any other components known to one having ordinary skill in the art. 
     Substrates and Coatings/Films 
     The present disclosure contemplates using the disclosed systems and methods to measure the thickness of a wide variety of coatings/films  122  on a wide variety of substrates  120 . Contemplated substrates comprise all manner of metals (e.g. aluminum, copper, nickel, titanium, steel, tin plate and other metals employed as components of or in the fabrication of products or processing of materials), a variety of films (e.g. thin stretched films, thin coatings on PET film substrates, Polyethylene Film Substrates, etc.), glass, plastics, rubber, latex, silicon (e.g. circuit boards, wafers), and solar cells. Contemplated coatings comprise all manner of lubricants, waxes, liquids (e.g. water), silicone, thin films, UV coatings, nanometric coatings, adhesives, cold and hot end glass container spray coatings, printed electronics, anti-reflective (AR) coatings, CdTe coatings, and CdS coatings. The present disclosure contemplates all methods for applying such coatings to such substrates known to one of ordinary skill in the art. For example, some coatings may be sprayed onto a substrate. Other coatings may be rolled onto the substrate. 
     Operation of an Illustrative System 
       FIG. 2  shows a flow diagram illustrating the operation of a system according to one embodiment of the present disclosure. In particular,  FIG. 2  shows steps performed by a system to perform inline real-time measurement of thin film thickness on a moving substrate. To aid in understanding how each of the steps may be performed, the following description is provided in the context of the illustrative diagrams of embodiments of the system shown in  FIGS. 1A and 1B . However, embodiments according to the present disclosure may be implemented in alternative embodiments. 
     Beginning at step  202 , light waves are generated and directed toward a coated substrate  120 . For example, the processor  102  communicates with the light source  110  to generate light waves within a one or more particular spectrum ranges to be directed at a coated substrate  120 . In one embodiment, a user interface provided by the processor  102  and displayed on display  106  permits a system operator to identify the substrate and/or coating material and the processor  102  then determines the appropriate light waves to select based on materials information stored in memory  104 . In another embodiment, the processor  102  is programmed to take into account the traits of the substrate and/or coating materials specified by a system operator to select the appropriate region of the wavelength spectrum for analysis. In still another embodiment, the user interface permits a system operator to manually configure the wavelength range for analysis to be used for the measurement process. 
     At step  203 , the light waves provided by the light source  110  are linearly polarized by passing through a polarizer  118 . In addition, the linearly polarized light waves are converted to circularly polarized light waves by passing through wave plate  112 . In one embodiment, wave plate  112  is a rotating quarter-wave plate. In one such embodiment, the processor  102  communicates with the wave plate  112  to configure the speed of the rotation of the rotating wave plate  112 . 
     At step  204 , elliptically polarized light waves reflected by the film/coating  122  and/or the substrate  120  are analyzed by an analyzer  114 . In one embodiment, the analyzer  114  is a rotating analyzer. In one such embodiment, the processor  102  communicates with the analyzer  114  to configure the speed of the rotation of the rotating analyzer  114 . 
     At step  206 , the detector  116  captures light waves reflected by the by the coated substrate  120  and then further reflected from various angular positions by analyzer  114 . 
     At step  208 , the detector  116  operates to convert the captured light waves into light wave data for communication to processor  102 . In one embodiment, light wave data may be a voltage signal waveform that corresponds to the captured light wave. In another embodiment, light wave data comprises a data structure containing information that describes the captured light waves. Once the light wave data is generated, it is communicated to the processor  102 . 
     At step  210 , the light wave data is processed by processor  102  to determine the thickness of the film/coating  122  on the surface of substrate  120 . In one embodiment, processor  102  quantifies the phase shift and polarization state changes, compared to the light waves provided by the light source  110 , and then uses that information to evaluate and validate thickness of the coating/film  122  on the substrate  120  using techniques disclosed herein. In other embodiments, the processor  102  may process the light wave data to determine other optical parameters of the film/coating  122  such as refractive index, surface roughness, and extinction coefficient. 
     At decision point  212 , it is determined whether the light wave data was valid. In one embodiment, the system determines whether there are vibrations, fluttering, or other disturbances in the movement of a moving substrate  120  that require adjustment of system components to obtain accurate measurements. If present, disturbances such as vibrations and flutter may impact the plane of incidence and the reflection of the light waves. In another embodiment, the system determines whether the light source  110  is providing too little or too much light. In one embodiment, disturbances and/or light level defects are identified based on the light wave data detected by detector  116  during the performance of previous iterations of the presently-described method. In some embodiments, the quality of the light wave data is validated using one or more techniques disclosed herein. 
     If the light wave data is validated, indicating that there are no vibrations, fluttering, or other disturbances in the movement of substrate  120  and no light level defects that require adjustment then the method proceeds to step  202  to perform another iteration of the method. However, if the validation process determines that the data is not valid, the measurement based on the light wave is discarded and the method proceeds to step  214 . 
     At step  214 , adjustments are made to compensate for detected disturbances and/or light level defects. In one embodiment, the orientation and/or position of the optics such as analyzer  114  and polarizer  118  are adjusted in real-time to accommodate for any changes in the plane of incidence. In another embodiment, the orientation and/or position of the analyzer  114  and detector  116  are adjusted in real-time to accommodate for any changes in the plane of incidence. In another embodiment, the orientation and/or position of polarizer  118 , analyzer  114 , and/or detector  116  are adjusted in real-time to accommodate for any changes in the plane of incidence. In one embodiment, processor  102  communicates commands to one or more adjustment mechanisms  124  to cause adjustment of the polarizer  118 , analyzer  114 , and/or the detector  116  based on the detected vibrations, fluttering, or other disturbances in the movement of substrate  120 . In another embodiment, the processor  102  communicates with light source  110  to adjust the intensity of the light. 
     General 
     The foregoing description of some embodiments of the disclosure has been presented only for the purpose of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Numerous modifications and adaptations thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention. 
     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, operation, or other characteristic described in connection with the embodiment may be included in at least one implementation of the invention. The invention is not restricted to the particular embodiments described as such. The appearance of the phrase “in one embodiment” or “in an embodiment” in various places in the specification does not necessarily refer to the same embodiment. Any particular feature, structure, operation, or other characteristic described in this specification in relation to “one embodiment” may be combined with other features, structures, operations, or other characteristics described in respect of any other embodiment.