BACKGROUND 
       [0001]    Within the field of chemical analysis of materials, optical computing has developed as an alternative to conventional spectrometry. Optical computers used to perform such analysis incorporate an integrated computational element or ICE (also sometimes referred to as a multivariate optical element or MOE). In contrast to a spectrometer, which separates light reflected off of or refracted through a sample of interest for subsequent analysis, an ICE is uniquely tuned to a specific pattern of a material of interest. Optical computers are generally capable of producing results of comparable quality and accuracy as laboratory grade spectroscopic systems, but without the delays associated with the multivariate analysis performed by a digital computer on the measured spectrum provided by a spectrometer. 
         [0002]    But ICEs are only as accurate and reliable as the manufacturing methods used to produce them. One significant factor that must be monitored closely in the production of ICEs is the thickness of each layer within an ICE&#39;s multilayer stack. Each layer&#39;s thickness can be monitored at different stages of the deposition process using an analytical instrument such as, for example, an ellipsometer or an infrared spectrometer. At predetermined stopping points within the production process, the ICE substrate is rotated to a position where a spectrum generated by a light beam reflected off of the ICE can be collected and measured. The measured spectrum may then be used to determine a layer&#39;s thickness or optical constants, allowing for corrections should process deviations be detected. 
         [0003]    During the thickness measurement described above, it is possible for a portion of the incident or reflected light to be blocked or clipped (i.e., partially blocked). Blocking and/or clipping can result, for example, from improper or inconsistent rotation and positioning of the substrate during measurements or from contaminants present on the surface of the ICE. Such blocking/clipping can skew the analysis, resulting in a misinterpretation of the loss of signal as an intrinsic layer absorption, and thus erroneous thickness and/or optical constant determinations, producing a defective ICE. Further, even if the blocking/clipping is recognized, current production methods do not provide for any sort of corrective action during the analysis of the optical data, other than to abort the production run, correct the problem and start a new production run. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Accordingly, there is disclosed herein a novel method for correcting optically blocked and/or clipped integrated computational element (ICE) layer measurements. In the drawings: 
           [0005]      FIG. 1  shows an illustrative ICE manufactured using the disclosed methods and systems. 
           [0006]      FIGS. 2A and 2B  show examples of chemical analysis systems implemented using illustrative ICEs manufactured using the disclosed methods and systems. 
           [0007]      FIGS. 3A and 3B  illustrate a layer thickness measurement of an illustrative ICE manufactured using the disclosed methods and systems, both without and with a contaminant obstructing the optical path. 
           [0008]      FIG. 4  shows a flow diagram of an illustrative method for measuring an ICE layer thickness during manufacturing that accounts for optical clipping and/or blocking. 
           [0009]      FIG. 5  shows an illustrative computer controlled ICE manufacturing system suitable for implementing the disclosed methods for the manufacture of an ICE. 
           [0010]      FIG. 6A  shows a plot of both a modeled and an actual ICE layer optical transmittance measurement without any correction to the modeled response. 
           [0011]      FIG. 6B  shows a plot of both a modeled and an actual ICE layer optical transmittance measurement using the disclosed methods to correct the modeled response. 
       
    
    
       [0012]    It should be understood that the drawings and corresponding detailed description do not limit the disclosure, but on the contrary, they provide the foundation for understanding all modifications, equivalents, and alternatives falling within the scope of the appended claims. 
       DETAILED DESCRIPTION 
       [0013]    The disclosed systems and methods are best understood by first describing an illustrative integrated computational element (ICE), a type of optical computing element produced by such methods and systems, as well as various device configurations that incorporate such ICEs. Accordingly,  FIG. 1  shows an illustrative ICE  118  integrated within an optical computer  100 , together with an exploded view of the internal structure of the ICE  118 . Illustrative ICE  118  includes multiple layers  102  and  104 , shown respectively in  FIG. 1  as silicon (Si, with a high index of refraction) and quartz (SiO 2 , with a low index of refraction). Other materials, such niobia, niobium, germanium, germania, magnesium fluoride and silicon oxide, are known in the art and may also be used to form the various layers  102  and  104  of an ICE, as well as to form the outer layer  108 . In at least some illustrative embodiments, the ICE is built up upon a substrate 106  made of BK-7 optical glass. The number and thickness of each of the layers is determined based upon the spectral attributes of the material of interest that the ICE is designed to identify. 
         [0014]    Continuing to refer to  FIG. 1 , light  120  reflected off of or transmitted through a sample of a material of interest is directed towards optical computer  100  and ICE  118  within it. In the example shown, the light with the spectrum of interest  110  passes through ICE  118  and continues on towards detector  112 , while the remaining wavelengths of light  114  are reflected by ICE  118  towards detector  116 . Detector  112  is typically configured to produce an output signal indicating the presence and/or concentration of a particular material of interest within the sample. 
         [0015]      FIGS. 2A and 2B  show various example configurations of optical computer  100  and its ICE suitable for identifying a material of interest within a flowing fluid (e.g., hydrocarbons provided by an oil and gas production well). In  FIG. 2A , a light source  202  provides a beam of light that is focused by lens  204  on sample window  210 . Sample window  210  allows the light to be reflected off of a fluid  208  flowing within pipe  206  and towards optical computer  100  and ICE  118   a.  The light directed towards the optical computer will have a spectrum reflecting the component materials within fluid  208 . ICE  118   a  of  FIG. 2A  is configured to reflect the spectrum of interest towards detector  112  while allowing the remaining wavelengths of light to pass through it towards detector  116 . 
         [0016]    In the example of  FIG. 2B , the light beam provided by light source  202  is focused by lens  204  so as to pass through two sample windows  210 , as well as fluid  208  within pipe  206 , and on towards optical computer  100  and ICE  118   b.  ICE  118   b,  unlike ICE  118   a,  is configured to allow the spectrum of interest to pass through the ICE towards detector  112 , while reflecting the remaining wavelengths of light towards detector  116 . In the examples of both  FIGS. 2A and 2B , detector  112  outputs a signal indicative of the presence and concentration of the material of interest which the ICEs are designed to identify. 
         [0017]    Because the thickness and composition of the ICE layers are factors that determine the specific spectral attributes that will be detected by the ICE, the fabrication of ICE layers must be closely monitored. As previously noted, a measurement light beam can be projected onto an ICE at various stages of production and the reflected light used to determine the thickness of a deposited layer.  FIG. 3A  shows an illustrative example of the layer of an ICE  300  being measured in this manner. A light source  302  projects a focused incident measurement light beam  304  onto the surface of ICE  300  after a layer has initially been deposited. Reflected light beam  308   a  is detected and measured by detector  306 , and a signal produced by the detector is sampled and processed (e.g., by a digital computer) to determine the thickness of the layer that corresponds to the intensity and spectral characteristics of the reflected light beam  308   a.    
         [0018]    However, if a contaminant or other foreign material partially or completely blocks the optical path of the light beams, the light detected and measured by detector  306  will not accurately reflect the thickness of the deposited layer. This situation is illustrated in  FIG. 3B , where an obstruction  310  partially blocks incident measurement light beam  304 , causing reflected light beam  308   b  to be of a lower intensity than would be produced by the deposited layer, given its actual thickness. As a result, the deposition of the layer will be stopped too soon, resulting in a layer that is too thin and thus a defective ICE, or the layer may be identified as too thick and the ICE discarded as defective, even though it may not be. 
         [0019]    The above-described ICE defect scenarios may be addressed by modeling the measurement light beam reflected off of the ICE layer and comparing the model results with the measured results of the light used to determine the thickness of the ICE layer. Such a model simulates the geometric model of the light path, as well as the characteristics of the ICE layers and substrate, for a spectral window corresponding to that of the sampled light. A discrepancy present between the measured spectra and the modeled spectra is indicative of an optical path obstruction. One approach to accounting for this discrepancy is by incorporating a virtual neutral density (VND) layer within the modeling of the measurement light beam reflection. The VND layer accounts for the amount of light that is blocked or clipped. In at least some illustrative embodiments, the optical density or absorption α VND  of this VND layer is computed using the formula, 
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         [0000]    wherein ΔI(λ) represents the difference or discrepancy between the actual reflected or transmitted spectra intensity I a (λ) and the modeled reflected or transmitted spectra intensity I m (λ), normalized to the incident spectra intensity I 0 (λ). 
         [0020]    In at least some instances, where the thickness of the obstruction exceeds the light penetration depth and the light is completely blocked, the VND optical density value α VND  is spectrally independent, i.e., it is not a function of wavelength. This reflects the fact that the blocking/clipping represented by the equation is homogeneous across the spectra of the sampled light. This is in contrast to the various materials used to form the ICE layers, which have absorption properties that change across the spectral window of the sampled light. In other instances, where the obstruction allows light of at least some wavelengths to pass through but blocks and/or reflects other wavelengths, the optical density value α VND  is spectrally independent for one or more wavelength ranges, but not over the entire spectral window. 
         [0021]    By using equation (1) to quantify and assess discrepancies between measured and modeled measurement light beam values, it is possible to identify and correct for optical path obstructions and ensure that the ICE layers are manufactured to the correct thickness.  FIG. 4  shows a flow diagram for an illustrative ICE manufacturing method  400  that incorporates this approach, and  FIG. 5  shows an illustrative ICE manufacturing system  500  suitable for implementing method  400 . ICE manufacturing system  500  includes a manufacturing control system  502  suitable for monitoring and controlling the manufacturing of an ICE within ICE manufacturing chamber  590 . Data is presented to a user via display  592 , and the user may further interact with the system via keyboard  596  and pointing device  594  (e.g., a mouse) to control the manufacturing process. If desired, manufacturing control system  502  can be programmed to send such commands automatically in response to automated processing measurements, thus partially or fully automating the ICE manufacturing process. 
         [0022]    Located within manufacturing control system  502  is a display interface  552 , a telemetry transceiver  554 , a processor  556 , a peripheral interface  558 , an information storage device  560 , a network interface  562  and a memory  570 . Bus  564  couples each of these elements to each other and transports their communications. Telemetry transceiver  354  enables the manufacturing control system  502  to communicate with the ICE manufacturing chamber  590 , and network interface  362  enables communications with other systems (e.g., a central data processing facility via the Internet). In accordance with user input received via peripheral interface  558  and program instructions from memory  570  and/or information storage device  560 , processor  556  processes telemetry information received via telemetry transceiver  554  to monitor the ICE manufacturing process and issue appropriate control signals. Storage device  560  may be implemented using any number of known non-transitory information storage media, including but not limited to magnetic disks, solid-state storage devices and optical storage disks. 
         [0023]    Various software modules are shown loaded into memory  570  of  FIG. 5 , where they are each accessed by processor  556  for execution. These modules include: process control module  572 , which monitors and controls the actual processing steps performed within ICE manufacturing chamber  590  to produce an ICE; ICE positioning and illumination control module  574 , which positions the ICE being manufactured into the proper positions for manufacturing and performing layer measurements, as well as controlling the illumination and measurement of the ICE layers; illumination model  576 , which models the light and layers as previously described; α calculation module  578 , which performs the optical density calculation of equation (1); spectrum analysis module  580 , which analyzes the sampled and modeled light, identifies any discrepancies between the measured and modeled results, and corrects the sampled light measurements if necessary; and layer thickness calculation module  582 , which determines the thickness of a deposited layer from the sampled/corrected measurement light. 
         [0024]    Referring now to both  FIGS. 4 and 5 , manufacturing of an ICE begins by depositing an initial ICE layer on a substrate, or continues by depositing an ICE layer onto an existing completed ICE layer (block  402 ; process control module  572 ). The ICE is then rotated into a predetermined position so as to allow a measurement light beam to be focused and projected onto the newly deposited layer (block  404 ; ICE position and illumination control module  574 ). The measurement light beam is projected at an angle that causes the beam to be reflected off of the ICE and back towards a detector, where the light is sampled, processed and measured (block  404 ; spectrum analysis module  580 ). The measurement light beam passing through the newly deposited layers is also modeled based on the expected thickness and known optical properties of the material used to form the layer to produce a modeled expected response of the measurement light (block  406 ; illumination model  376 ). The above-described absorption (i.e., optical density value) is calculated (block  408 ; α calculation module  578 ) to quantify any difference between the actual measurement light beam magnitude and the modeled measurement light beam magnitude across one or more wavelength windows of the sampled light. If such a difference exists and exceeds an error acceptance value across the spectral window, e.g., the average value of α VND  exceeds 0.01 (block  408 ; spectrum analysis module  580 ), blocking/clipping has been detected and the actual light measurement is adjusted based upon the calculated optical density value (block  410 ; spectrum analysis module  580 ). 
         [0025]    Once the actual light measurement has been adjusted (if necessary), the actual light measurement (raw or adjusted) is used to calculate the thickness of the deposited layer (block  412 ; layer thickness calculation module  582 ). If the layer is not at the desired thickness (block  414 ; layer thickness calculation module  582 ), the thickness is adjusted (block  418 ; process control module  572 ), for example by adding more material to the deposited layer if it is too thin. After the adjustment is performed, blocks  404 - 414  and  418  are repeated as needed until the layer is within a tolerance range of the target thickness (e.g., ±0.1% of target thickness). Once the target layer thickness is achieved (block  414 ; layer thickness calculation module  582 ), the entire method is repeated for subsequent layers until the last layer is completed (block  416 ; process control module  572 ), ending the method (block  420 ). 
         [0026]    The results of the above described method is best illustrated by graphing the measured (actual) and modeled transmittances of an ICE layer as a function of wavelength, as shown in the graphs of  FIGS. 6A and 6B . In the example shown, the spectral window selected is between 1500 and 2500 nanometers.  FIG. 6A  shows both the reflected light predicted by the illumination model (the solid line) and the actual measured reflected light. As can be seen, there is a significant discrepancy between the two. This discrepancy can be accounted for by comparing the optical densities computed using equation (1). The computed optical density difference quantifies the discrepancy and provides a basis for adjusting the predicted reflected light, as illustrated in  FIG. 6B . 
         [0027]    Numerous other modifications, equivalents, and alternatives, will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, although the examples presented describe correcting for the effects of an obstructed measurement light beam, errors/offsets induced in the measurement light beam intensity may be due to other cause, such as a misalignment of the ICE when rotated into position for the measurement operation. The described methods and system may be used to correct for these and other errors/offsets. Further, the corrections applied by the disclosed methods and systems are not limited to thickness measurements, and may also be applied to the determination of optical constants of the measured layer. Additionally, although the described measurement beam configuration reflects the light beam off of the ICE, other embodiments of the disclosed systems and methods may use measurement light beams that instead project the beam through the ICE. It is intended that the following claims be interpreted to embrace all such modifications, equivalents, and alternatives where applicable.
Methods and systems for manufacturing optical computing elements, including a method for correcting element layer thickness measurements during manufacturing that includes depositing an element layer on a glass substrate or a previously deposited layer, illuminating the deposited layer and sampling reflected or transmitted light produced by said illuminating, detecting and measuring an actual magnitude of the sampled light as a function of wavelength, and modeling the sampled light to produce a predicted magnitude of the sampled light. The method further includes determining a discrepancy between the actual and predicted magnitudes, adjusting the actual magnitude based on said discrepancy, calculating the thickness of the deposited layer based upon the adjusted actual magnitude of the sampled light, and adjusting the deposited layer's thickness if the calculated thickness is not within a tolerance range of a target thickness.