Abstract:
Continuous on-line thin film measurements employ a sensor having a spectrometer for interferometric measurements and a stack of single channel detectors for adsorption measurements. The stack is separated from the spectrometer, which analyzes radiation that emerges (transmitted pass or reflected from) the film, whereas the stack analyzes radiation that has passed through the film multiple times. The spectrometer is (i) positioned directly opposite the source of radiation so that it detects transmitted radiation or (ii) disposed on the same side of the film as is the source of radiation so that the spectrometer detects radiation that is specularly reflected from the film. The sensor includes a broadband radiation source emitting visible to far infrared light which propagates through a measurement cell defined by reflective surfaces exhibiting Lambertian-type scattering. The sensor is capable of measuring thin plastic films with thicknesses down to 1 micron or less.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to sensors for measuring thin films using a combination of interferometry and near infrared absorption at fixed wavelengths. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the manufacture of sheet materials, it is well known that various sheet properties can be detected “on-line,” that is, while a sheet making machine is operating. On-line measurement devices measure sheet properties such as thickness, basis weight, moisture content, chemical composition and the like. Typically, such on-line devices employ sensors that periodically traverse, or scan, the moving sheets in the cross direction, which is perpendicular to the machine direction of sheet travel. 
         [0003]    Visible, near-IR and mid-IR sensors share a common need for large spectral range, high spectral resolution and high signal-to-noise ratio. A large spectral range is needed for the sensor to address a wide number of applications whereas high spectral resolution and signal-to-noise insure sensor accuracy and repeatability. However, these sensor attributes are usually mutually exclusive. For example, a single detector and filter combination affords high signal-to-noise ratio and potentially good spectral resolution but does not provide adequate spectral range. Conversely, a compact spectrometer provides high spectral range and resolution but sacrifices throughput. Additionally, while spectrometers provide high spectral range and resolution, a single unit does not cover the entire range between the visible and mid-IR due to practical and technical considerations. 
         [0004]    Plastic and paper industrial applications require versatile detectors with the above combination of characteristics for thickness measurements. Currently, a single sensor is employed to measure the thickness of thin plastic films on biax lines. The very thin films are measured using interferometry in the visible or near-IR where absorption is weak whereas the thicker films (&gt;15-20 microns) are measured using adsorption further out in the near-IR. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is based in part on the development of a sensor for robust, continuous on-line measurements of thin films wherein the sensor includes a spectrometer and a set or stack of single channel detectors. The stack is separated from the spectrometer, which is configured to analyze radiation that emerges (that is, transmitted through or reflected from) the film, whereas the single channel detectors are configured to analyze radiation that passed through the film multiple times. In this novel arrangement, the spectrometer is either (i) positioned directly opposite the source of radiation so that it detects radiation that passes through the film (that is, only 1 pass through the film) or (ii) disposed on the same side of the film as is the source of radiation so that the spectrometer detects radiation that is specularly reflected from the film. The single channel stack is offset from the source such that the stack detects reflected radiation that has passed multiple times through the film. The sensor is particularly suited for measuring thin plastic films especially films with thicknesses down to 1 micron or less. 
         [0006]    Accordingly, in one aspect the invention is directed to an apparatus for sensing a layer of material that includes: 
         [0007]    a broadband radiation source, disposed on a first side of the layer of material, that directs a beam of incident radiation into the layer of material; 
         [0008]    a spectrometer that detects (i) transmitted radiation that passes through the layer of material or (ii) reflected radiation that is reflected from the layer of material; 
         [0009]    a radiation receiver that detects at least a portion of a reflected beam that propagates through the layer of material; and 
         [0010]    one or more members that define a measurement cell with a path for the layer of material and wherein the measurement cell is configured to cause radiation to be reflected through the layer of material a plurality of times before being detected by the radiation receiver. In a preferred embodiment, the members (such as plates) that form the measurement cell exhibit near perfect Lambertian scattering which means that the angle at which light leaves the plate is independent of the angle at which the light impinges on the plate. This is in contrast to specular (mirror) reflection where the incident and exit angles are identical. With a Lambertian scattering surface, the angle of distribution follows a cosine law with the highest probability for the light to reflect at an angle normal to the plate but also a non-negligible probability to reflect at much smaller angles (for example, 45 degrees) therefore allowing the light to travel across the entire area of the plates. The Lambertian-type light scattering that is generated allows the light to interact multiple times with the layer(s) of material, thus, the radiation receiver&#39;s sensitivity to selected components within the layer is enhanced. 
         [0011]    In another aspect, the invention is directed to a method for measuring a plurality of characteristics of a flat sheet product which includes: 
         [0012]    (a) emitting broadband radiation that ranges from visible to far infrared radiation towards the flat sheet product; 
         [0013]    (b) analyzing reflected radiation that has propagated through the flat sheet product a plurality of times to measure characteristics using adsorption techniques; and 
         [0014]    (c) analyzing (i) transmitted radiation that passes through the flat sheet product or (ii) reflected radiation that is reflected from the flat sheet product to measure characteristics using a spectrometer when interference is detected. 
         [0015]    The spectrometer measures the optical thickness of the sheet using conventional thin film interferometry when interferences are detected. The visibility (amplitude) of the thickness fringes have to be large enough so the measurement is robust. Visibility of the interference pattern is defined as (Max−Min)/(Max+Min) where Max and Min are the maximum and minimum values of the interference spectrum. When or if the fringe visibility is too low, the sensor measures the product thickness using the multi-wavelength absorption technique. 
         [0016]    Both interferometry and adsorption measurements can be performed simultaneously to generate a thickness profile that consist of one or both measurements. In this fashion, physical characteristics such as film thickness can be ascertained continuously even if the film thickness fluctuates. The thickness measurement obtained from interferometry by employing the spectrometer is typically superior to the measurement obtained by absorption when the visibility is greater than a certain limit. The appropriate limit can be found experimentally. It can be in the range of 0.1 to 5%. It can also be a dynamic limit based on the signal-to-noise ratio of the measurement or of the 2-sigma accuracy of the fit to the interference pattern. 
         [0017]    Typically, the spectrometer analyzes radiation in a first radiation range and the radiation receiver analyzes radiation in a second radiation range with the spectrometer analyzing visible, a near infrared, mid-infrared spectral range, or far infrared spectral range. The spectrum characteristic of the spectrometer and the spectrum characteristic of one of said plurality of channel detectors can be designed to overlap or not overlap. 
         [0018]    The absorption measurement can also be used to decide which of absorption or interferometry is better to use. In this case, the calculated thickness by absorption should be greater than a certain value (typically around 10-12 microns) for absorption to be used and optical thickness by interferometry to be ignored. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIGS. 1, 2, 3, and 4  depict spectrometer/adsorption thickness sensors of the present invention; 
           [0020]      FIG. 5  illustrates a light receiver; and 
           [0021]      FIG. 6  shows a sheetmaking system implementing the sensor in a dual head scanner. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]      FIG. 1  illustrates a non-contacting optical sensor  2  that includes enclosures  4  and  6  (each also called “scanner head” or “head”) that house sensor components for measuring qualities, characteristics or features of a moving web  24  that can be monitored which include, but are not limited, to single and multi-layered compositions, coatings, films, webs or sheets. While the sensor will be illustrated in measuring characteristics in paper and plastic, it is understood that the sensor can be employed to detect a variety of components in a number of different materials including, for example, coated materials, fabrics, and the like. Sensor  2  is particularly suited for measuring the thickness or weight of a layer of light transmissive material  24  moving in the machine direction (MD). Scanner  2  includes a radiation or light source  8  that is positioned in head  4  and spectrometer  41  and a radiation receiver or detector  10  that are both positioned in head  6 . An upper diffuse reflector plate assembly  14 , which is secured to operative surface  12  of head  4 , comprises a reflective element  16 , such as a specular mirror, that is covered with a layer or plate  18  of calcium fluoride (CaF 2 ), sapphire or quartz glass. A specular mirror can comprises an aluminum coating formed on a polyimide which is available as KAPTON film. Outer surface  22  of layer  18  is preferably polished to make it easier to clean and to render it more resistant to moisture whereas inner surface  20  is highly roughened to serve as a diffusive surface. Similarly, a lower diffuse reflector plate assembly  34 , which is secured to operative surface  32  of head  6 , comprises a reflective element  46 , such as a specular mirror, that is covered with a layer or plate  48  of calcium fluoride, sapphire or quartz glass. Outer surface  42  of layer  48  can also be polished whereas inner surface  40  is highly roughened to serve as a diffusive surface. These reflective and diffusive plates work well in the 300 nm to 5 micron radiation range. In a preferred embodiment, each of the upper and lower diffuse reflector plates assemblies  14 ,  34  comprises a specular reflective surface with a diffusive layers consisting of microporous polytetrafluoroethylene (PTFE) covered with quartz glass. 
         [0023]    The upper and lower scanner heads  4 ,  6  are aligned so that planar polished surface  22  of upper scanner head  4  is parallel with and faces planar polished surface  42  of the lower scanner head  6 . Apertures  26 ,  37  and  36  provide access to light source  8 , spectrometer  41  and receiver  10 , respectively. Apertures  26  and  37 , which are configured on opposite sides of moving web  24 , are aligned so that spectrometer  41  detects radiation that is transmitted through web  24 . Apertures  26  and  36 , which are also configured on opposite sides of moving web  24 , are not aligned, that is, as shown, light source  8  and receiver  10  define respective axes of radiation that are laterally offset from one another along the MD path of moving web  24 . In this fashion, the arrangement of upper and lower diffuse reflector plates  14 ,  34  define a measurement window or cell through which web material  24  travels. 
         [0024]    In operation of sensor  2 , optics  9  such as a focusing lens in light source  8  focuses incident radiation  38  through aperture  26  toward moving web  24 . Optics  39  such as a collimating or conditioning lens is positioned to collection radiation  43  that is transmitted through web  24  and optics  11  such as a collimating or conditioning lens is positioned to collect radiation  28 , which is diffusively reflected from diffuse reflector plate  14 , through aperture  36 . Movement of the upper and lower scanner heads  4 ,  6  in the cross direction, which is traverse to the MD, is coordinated so that light is diffused and reflected by plate assemblies  14 ,  34  as radiation  44  propagates through layer of material  24  multiple times before being detected by receiver  10 . 
         [0025]    Light diffusing elements that scatter or diffuse light generally function in one of three ways: (a) as a surface light diffusing element utilizing surface roughness to scatter light in a number of directions, (b) as a bulk light diffusing element with flat outer surfaces and embedded light-scattering elements, or (c) as a combination of elements (a) and (b). The bulk diffuser diffuses the light within the material. Diffusion is achieved by light scattering as it passes through materials with varying indexes of refraction. The term “diffuser” or “diffuser member” means any material that is able to diffuse specular light (light with a primary direction) to a diffuse light (light with random direction). The term “light” means electromagnetic radiation having wavelength in ranges that are suited for measuring properties of a layer material with sensors of the present invention. Near infrared and/or mid-infrared radiation is particularly suited for measuring physical characteristics of paper and plastic products. 
         [0026]    Calcium fluoride, sapphire, and quartz glass are transparent to near and mid-infrared radiation. The randomly roughened surfaces  20 ,  40  can be produced by electric discharge techniques, mechanical grinding, or etching to create a plurality of randomly oriented and spaced facets and cavities for diffusively reflecting incident near and mid infrared radiation. 
         [0027]    Light source  8  can comprise, for instance, a Quartz Tungsten Halogen lamp to irradiate material  24  with radiation having wavelengths in at least first and second separate wavelength regions of the electromagnetic spectrum that are referred to as reference and measurement wavelength bands as further described herein. 
         [0028]    Spectrometer  41  can comprise, for instance, a grating based or linear variable filter (LVF) based array spectrometer. Acousto-Optic Tunable Filter (AOTF) spectrometer, Fourier Transform InfraRed (FTIR) spectrometer and Fabry-Perot spectrometer can also be employed. 
         [0029]    In the arrangement of radiation source  8 , radiation receiver  10  shown in  FIG. 1 , reflected light  44  travels in a direction that is parallel to the MD so that the cross direction (CD) resolution of sensor  2  is maintained. Although reflected radiation  44  shown in  FIG. 1  is depicted as traveling “downstream” in the opposite machine direction as web  24 , this feature is not critical to the sensor&#39;s function. In other words, sensor  2  will operate even if web  24  moves in the opposite direction so that the reflected radiation is moving “upstream” relative to the web; the critical feature is that incident radiation  38  that emitted from light source  8  travel along a path that is parallel to that of moving web  24  as reflected radiation  44  moves toward receiver  10 . 
         [0030]      FIG. 2  illustrates a non-contacting optical sensor  52 , which includes scanner head  54  that houses light source  58  and receiver or detector  60  and scanner head  56  that houses spectrometer  91 . Sensor  52  measures physical qualities, characteristics or features of a layer of light transmissive material  74  moving in the MD. An upper diffuse reflector plate assembly  64 , which is secured to operative surface  62  of head  54 , comprises a reflective element  66 , such as a specular mirror, that is covered with a layer or plate  68  made of alumina (Al 2 O 3 ). Similarly, a lower diffuse reflector plate assembly  84 , which is secured to operative surface  82  of head  56 , comprises a reflective element  96 , such as a specular mirror, that is covered with a layer or plate  98  of alumina. 
         [0031]    The upper and lower scanner heads  54 ,  56  are aligned so that planar surface  72  of alumna plate  68  is parallel with and faces planar surface  92  of alumina plate  98 . Apertures  76 ,  87  and  86  provide access to light source  58 , spectrometer  91  and receiver  60 , respectively, and they can be equipped with a window material, which can be roughened on one side or not, such as calcium fluoride, sapphire or quartz glass. The upper and lower diffuse reflector plates  64 ,  84  form a measurement window or cell through which web material  74  travels. In operation of sensor  52 , optics  59  in light source  58  focuses incident radiation  88  through aperture  76  toward moving web  74 . Optics  89  captures radiation  93  into spectrometer  91  and optics  61  collects radiation  78  that is reflected from surface  92  through aperture  86 . Movement of the upper and lower scanner heads  54 ,  56  in the cross direction is coordinated so that light is diffused and reflected between plate assembles  64 ,  84  as radiation  94  propagates through layer of material  74  multiple times before being detected by receiver  60 . Alumina, which is translucent to near and mid infrared radiation, serves as a bulk light-diffusing element. The alumina layer is typically smooth on both sides. 
         [0032]      FIG. 3  illustrates another non-contacting optical sensor  102 , which includes scanner head  104  that houses light source  108  and receiver or detector  110  and scanner head  106  that houses spectrometer  148 . Sensor  102  measures physical qualities, characteristics, or features of a layer of light transmissive material  124  moving in the MD. An upper diffuse reflector plate assembly  114 , which is formed on operative surface  112  of head  104 , comprises a reflective element consisting of a roughened operative surface that is coated with a metallic reflective coating. Alternatively, the reflective element consists of a diffusively reflective metallic surface. Similarly, a lower diffuse reflector plate assembly  134  has an operative surface  142  on head  106  that has a reflective element of the same construction. Suitable metallic coatings can be formed, for example, from gold, silver, and aluminum by electrochemical plating. 
         [0033]    The upper and lower scanner heads  104 ,  106  are aligned so that surface  112  of upper scanner head  104  is parallel with and faces surface  142  of lower scanner head  106 . Apertures  126 ,  147  and  136  provide access to light source  108 , spectrometer  148  and receiver  110 , respectively; the apertures can be optionally equipped with a calcium fluoride, sapphire or quartz glass window, which is roughened on one side or not. The upper and lower diffuse reflector plates  114 ,  134  define a measurement window or cell through which web material  124  travels. In operation of sensor  102 , optics  109  in light source  108  focuses incident radiation  138  through aperture  126  toward moving web  124 . Optics  149  collects radiation  153  into spectrometer  148  and optics  111  collects radiation  128  that is reflected from surface  142  through aperture  136 . Movement of the upper and lower scanner heads  104 ,  106  in the cross direction is coordinated so that light is diffused and reflected between plate assemblies  114  and  134  as radiation  144  propagates through layer of material  124  multiple times before being detected by receiver  110 . In this sensor  102 , the roughened metallic coating (or the diffusively reflective metallic surface) functions both as diffuser and reflective elements. 
         [0034]      FIG. 4  illustrates an embodiment of a non-contacting optical sensor where the spectrometer and the source of radiation are located on the same side of the moving web  174 . In this fashion, the spectrometer detects radiation that is reflected specularly from moving web or sheet  174 . Optical sensor  152  scanner head  154  houses light source  158 , receiver or detector  160 , and spectrometer  198  with spectrometer  198  being positioned upstream of radiation source  158 . Sensor  152  measures physical qualities, characteristics, or features of a layer of light transmissive material  174  moving in the MD. An upper diffuse reflector plate assembly  164 , which is formed on operative surface  162  of head  154 , comprises a reflective element consisting of a roughened operative surface that is coated with a metallic reflective coating. Alternatively, the reflective element consists of a diffusively reflective metallic surface. Similarly, a lower diffuse reflector plate assembly  184  has an operative surface  192  on head  156  that has a reflective element of the same construction. Suitable metallic coatings can be formed, for example, from gold, silver, and aluminum by electrochemical plating. 
         [0035]    The upper and lower scanner heads  154 ,  156  are aligned so that surface  162  of upper scanner head  154  is parallel with and faces surface  192  of lower scanner head  156 . Apertures  176 ,  197  and  186  provide access to light source  158 , spectrometer  198  and receiver  160 , respectively; the apertures can be optionally equipped with a calcium fluoride, sapphire or quartz glass window, which is roughened on one side or not. The upper and lower diffuse reflector plates  164 ,  184  define a measurement window or cell through which web material  174  travels. In operation of sensor  152 , optics  159  in light source  158  focuses incident radiation  188  through aperture  176  toward moving web  174 . Optics  199  collects radiation  193  into spectrometer  198  and optics  161  collects radiation  178  that is reflected from surface  192  through aperture  186 . Movement of the upper and lower scanner heads  154 ,  156  in the cross direction is coordinated so that light is diffused and reflected between plate assemblies  164  and  184  as radiation  194  propagates through layer of material  174  multiple times before being detected by receiver  160 . In this sensor  152 , the roughened metallic coating (or the diffusively reflective metallic surface) functions both as diffuser and reflective elements. 
         [0036]      FIG. 5  illustrates a suitable receiver that includes a detector assembly  200  that houses a six-channel sensor for measuring three properties in a layer of material. In this arrangement, there are three measurement filter/detectors  204 A,  206 A and  208 A and three corresponding reference filter/detectors  204 B,  206 B, and  208 B. A separate infrared band pass filter is positioned before each detector; in this fashion, each of the infrared detectors measures the intensity of only the portion of the infrared beam spectrum that falls within the band pass of the associated filter. A broadband infrared source of energy (not shown) directs incident radiation onto the layer of material to be analyzed and reflected radiation  202  is wavelength-analyzed by passing the beam through beam splitters  210 ,  212 ,  214  and the appropriate filters to the individual detectors. As is apparent, additional pairs of measure and reference detector/filters can be incorporated as needed. 
         [0037]      FIG. 6  illustrates one particular implementation of the sensors that are shown in  FIGS. 1, 2, 3, and 4 . In particular, the radiation source and detector are housed in a dual head scanner  258  of scanner system  240  which can be employed to measure the moisture content in paper or the concentration of polymer films. Upper scanner head  250  moves repeatedly back and forth in the CD across the width of the moving sheet  246 , which moves in the MD, so that the characteristics of the entire sheet may be measured. Scanner  258  is supported by two transverse beams  242 ,  244  on which are mounted upper and lower scanning heads  250 ,  252 . The operative faces of the lower and upper scanner heads  250 ,  252  define a measurement window or cell that accommodates sheet  246 . The lower scanner head  252  may include a sheet stabilization system such as an air-bearing stabilizer (not shown) to maintain the sheet on a consistent plane as it passes through the measurement cell. The movement of the dual scanner heads  250 ,  252 , is synchronized with respect to speed and direction so that they are aligned with each other. 
         [0038]    One technique of monitoring the thickness of a plastic film measures the concentration(s) (weights per unit area, typically measured in grams per square meter, gsm) of the particular polymer(s) that form the film. Multilayer films typically comprise a plurality of layers that are laminated together. Preferably, in the multilayer structure, adjacent layers are formed of different polymer materials. By employing different polymers with different physical properties, the multilayer film may have a combination of physical attributes not present in a single layer film. For example, the multilayer film may be moisture resistant, abrasion resistant, and yet remain pliable. The sensor of the present invention, among other things, is effective in controlling the production of multilayer films to assure that each layer in the film has the proper thickness or weight (gsm) so that the multilayer film has the right combination of properties. 
         [0039]    If the density of a particular polymer component in the multilayer film is known the thickness of the film component can be determined. The thickness can be calculated with a computer. The film thickness may not always be calculated and the weight (gsm) of the component is all that is required by the user for quality control. In the production of mono-polymers, film thickness is typically calculated. 
         [0040]    The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be considered as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.