Patent Publication Number: US-2004046969-A1

Title: System and method for monitoring thin film deposition on optical substrates

Description:
FIELD  
       [0001] The present invention relates generally to thin film deposition on optical substrates, and more particularly, relates to monitoring and controlling layer thickness during thin film deposition.  
       BACKGROUND  
       [0002] Optical coatings may be used in the production of laser mirrors, optical filters, and other devices. A series of thin film layers are deposited onto an optical substrate to produce the optical coatings. A variety of thin film deposition methods, such as chemical vapor deposition (CVD) and physical vapor deposition (PVD), may be used to deposit the layers onto the optical substrate. The number of layers deposited on the substrate depends on the desired characteristics of the optical coatings. For example, the optical coating for an optical interference filter may consist of hundreds of layers.  
       [0003] The thickness of each of those layers may be critical to the performance of the final product. Some optical coatings require a high degree of wavelength accuracy and may be fractions of a nanometer thick. When mass-producing the optical coatings, it may be difficult to maintain thickness uniformity for each of the layers needed to produce a consistent final product.  
       [0004] To achieve coating uniformity, the optical substrates may be mounted on a substrate holder, or “planet,” located in a vacuum coating chamber. The substrate holders may be located in the chamber on a mechanical carousel, termed the “planetary system.” The planetary system may have a central axis around which additional planets having a sub-axis can rotate. Each substrate holder rotates and revolves about the vacuum chamber, reaching speeds up to 2000 rotations per minute. As the substrates are rotating in the vacuum coating chamber, thin films may be deposited onto the substrates to form the coating. Many factors may affect the optical properties of the coating, such as vacuum quality, deposition rate, temperature, gas flows, and ion source characteristics.  
       [0005] Optical monitors may be used during the thin film deposition process to monitor and control the layer thickness. An optical monitor uses light to measure film thickness. Some optical monitors monitor the thin film deposition process on a witness or test glass, and not on the actual optical substrate in which the thin film is being applied. Witness glasses may be needed for applications in which light cannot pass through the substrate, such as applications requiring frosted coatings. However, optical monitors that monitor witness glasses are not as accurate as those monitoring the actual substrate, as the witness glass is not located in the same position as the actual optical substrates. As a result, the coating deposited on the actual optical substrates has different optical properties than the coating deposited on the witness glass.  
       [0006] Other optical monitors may monitor an actual substrate while it is stationary. While these systems can achieve good results, the results are limited to the single substrate located where the optical monitoring is occurring. Furthermore, the coating uniformity may be limited to a small area on the substrate since the substrate is stationary. These limitations make this type of optical monitor impractical for mass-production applications.  
       [0007] Therefore an optical monitor capable of continually monitoring the actual optical substrate, while the substrate is moving during thin film deposition, would be beneficial. Such an optical monitor could be used in mass-production applications that desire a uniform and repeatable final product.  
       SUMMARY  
       [0008] A system and method for continuously monitoring layer thickness during thin film deposition of optical coatings is provided. A light source operable to generate a light beam is directed towards an optical substrate moving in a coating chamber. The light beam passes through the optical substrate and is detected by a detector. The detector is operable to detect light and provide an output representative of an amount of light detected. The amount of light detected is related to thickness of thin film layers deposited on the optical substrate. A control panel generates optical transmission data from the output representative of the amount of light detected by the detector.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] Presently preferred embodiments are described below in conjunction with the appended drawing figures, wherein like reference numerals refer to like elements in the various figures, and wherein:  
     [0010]FIG. 1 is a block diagram of a coating system employing an optical monitor, according to an exemplary embodiment;  
     [0011]FIG. 2 is a graphical representation of a typical path of a substrate moving in a planetary system, according to an exemplary embodiment;  
     [0012]FIG. 3 is a block diagram of an optical monitor, according to an exemplary embodiment;  
     [0013]FIG. 4 is a graphical representation of an optical monitor signal, according to an exemplary embodiment; and  
     [0014]FIG. 5 is a flow chart of a method of monitoring thin film deposition of optical coatings, according to an exemplary embodiment.  
    
    
     DETAILED DESCRIPTION  
     [0015]FIG. 1 is a block diagram of a coating system  100  employing an optical monitor according to an exemplary embodiment. The coating system  100  depicted in FIG. 1 illustrates ion beam sputter deposition, but any chemical or physical vapor deposition process may be used. For example, the thin film deposition process may be AC magnitron, RF sputtering, or Electron Beam processing.  
     [0016] The coating system  100  may include a coating chamber  106 . The coating chamber  106  may be a vacuum chamber that includes the equipment needed to perform thin film deposition. In this example, an ion gun  108  may be used to provide a beam of ions  110 . The ion gun  108  may direct the beam of ions  110  at a rotating or non-rotating ion target  112 . The ion target  112  may be a grounded metallic or dielectric sputtering target. As the beam of ions  110  hits the ion target  112 , material may be sputtered within the coating chamber  106 . The material sputtered from the ion target  112  may be deposited on one or more substrates  114  forming a coating.  
     [0017] The one or more substrates  114  may be held on a substrate holder  116 , also known as a “planet.” The substrate holder  116  may hold one or more substrates depending on the application. In this example, the substrate holder  116  is depicted in FIG. 1 as holding four substrates. However, the substrate holder  116  may hold more or less than four substrates.  
     [0018] The substrate holder  116  may be located in the coating chamber  106  on a mechanical carousel, known as a “planetary system.” The planetary system may provide single or dual axis rotation in the coating chamber  106 . The planetary system rotates and revolves the substrates  114  in the coating chamber  106 . The rotating and revolving of the substrates  114  while the material is being sputtered in the coating chamber  106  may cause a substantially similar thin film deposit on each of the one or more substrates  114  located on the substrate holder  116 . While one substrate holder  116  is shown in FIG. 1, the coating chamber  106  may include more than one substrate holder, such as in a dual-axis planetary system.  
     [0019] As the coating is formed on the one or more substrates  114 , a light source  102  may be directed towards the one or more substrates  114 . A light beam from the light source  102  may be directed into the coating chamber  106  with an adjustable mirror or prism through a glass view-port  120 . The adjustable mirror or prism is not shown in FIG. 1.  
     [0020] The adjustable mirror or prism may allow the light beam to be directed at different substrates  114  located on the substrate holder  116 . The angle that the light beam makes with the substrate can be set to angles other than normal incidence. While near normal incidence is shown in FIG. 1, other angles may be used. The glass view-port  120  may be designed to minimize distortion of the light beam as it travels through the view-port  120  and enters into the coating chamber  106 . Additionally, the glass view-port  120  may be positioned on a wall of the coating chamber  106  in a manner that minimizes deposition of sputtered material on the view-port  120 .  
     [0021] The light beam may then pass through the one or more substrates  114 . The amount of light that passes through the substrates  114  may be representative of the thickness of the thin film layers deposited on the substrates. A detector  104  held in the coating chamber  106  with a mounting fixture  118  may detect the light that passes through the one or more substrates  114 .  
     [0022] The light source  102  may generate a monochromatic light beam; however, monochromatic light is not required. In a preferred embodiment, the light source  102  is a laser diode. For example, the light source  102  may be a laser diode designed to generate a light beam with a wavelength of substantially 635 nanometers at substantially 3 milliwatts. However, other sources of light and light beam characteristics may be used.  
     [0023] The detector  104  may be any device operable to detect light, such as a photodiode. In a preferred embodiment, the detector  104  is a photodiode with part to number PIN-44DI from UDT Sensors, Inc. However, other photodiodes or light detectors may also be used. The detector  104  may detect light and provide an output signal of current representative of the amount of light detected.  
     [0024] The detector  104  may be mounted inside the coating chamber  106  using the mounting fixture  118 . The mounting fixture  118  may be a metal mounting post that is fastened to a coating chamber wall. The use of the metal mounting post may protect the detector  104  from excessive heat due to conduction in the coating chamber wall. Additionally, for thin film deposition processes that require high temperatures, the detector  104  may be water-cooled.  
     [0025] In an alternative embodiment, the detector  104  may be located outside the coating chamber provided that the light that passes through the one or more substrates  114  is directed out of the coating chamber  106  through a glass view-port using one or more mirrors or prisms. The glass view-port may be substantially the same as the glass viewExpress port  120  that allows the light beam from the light source  102  to enter the coating chamber  106 .  
     [0026] In a preferred embodiment, the light source  102  and the detector  104  may be located away from the center of the coating chamber  106 . While the light source  102  and the detector  104  may be placed in the center of the coating chamber  106 , placing the light source  102  and the detector  104  towards the sides of the chamber  106  may result in more optical measurements, providing more control over the thin film deposition process.  
     [0027] For example, FIG. 2 depicts a typical path of a substrate moving many times around the planetary system. Position  202  and position  204  represents two possible locations for the light source  102  and the detector  104  in the coating chamber  106 . Other positions are possible. Position  202  is located away from the center of the coating chamber  106 , while position  204  is located closer to the center of the coating chamber  106 . As shown in FIG. 2, locating the light source  102  and the detector  104  at position  202  may result in the substrate crossing the light beam more frequently than at position  204 .  
     [0028]FIG. 3 is a block diagram of an optical monitor  300 , according to an exemplary embodiment. The optical monitor  300  may monitor the deposition process of any thin film, and may be especially beneficial for monitoring the deposition of thin films used to produce optical coatings that require a high level of layer thickness accuracy. The coatings may include, but are not limited to, antireflective coatings, beam splitting coatings, notch filter coatings, and laser mirror coatings.  
     [0029] The optical monitor  300  may use optical transmission to take substantially continuous light measurements from a light beam  314  passing through the one or more substrates  114 . The light measurements may be taken without moving the substrates  114  from where the substrates  114  are positioned during the deposition process. The optical monitor  300  may include a light source  302 , a detector  304 , a control panel  306 , and a computer  308 . The light source  302  may be substantially the same as the light source  102  as depicted in FIG. 1. The detector  304  may be substantially the same as the detector  104  as depicted in FIG. 1.  
     [0030] The control panel  306  may contain an amplifier  310  and a microcontroller  312 . The amplifier  310  may be operable to convert the current signal from the output of the detector  304  into a voltage signal that can be measured. For example, the amplifier may be an operational amplifier or any other device capable of converting a current signal into a voltage signal.  
     [0031] The voltage signal may then be processed by the microcontroller  312 . An output of the microcontroller may be transmission data, which may be used to determine layer thickness. The microcontroller  312  may receive voltage signals from the amplifier  310  when the light beam  314  from the light source  302  is (1) unobstructed; (2) blocked by the substrate holder  116 ; or (3) passes through the one or more substrates  114 . FIG. 4 provides a graphical representation of an optical monitor signal depicting the voltage levels representative of these three scenarios.  
     [0032] When the light beam  314  is unobstructed, the detector  304  may detect substantially a maximum amount of light. The light beam  314  may be unobstructed when the light beam  314  travels between moving substrate holders. The current signal generated by the detector  304  in response to the light beam  314  may be converted into a 100% reference signal  402  by the amplifier  310 . The 100% reference signal  402  may represent the maximum amount of light that the optical monitor  300  is designed to detect. For example, the optical monitor  300  may be limited to detecting the maximum amount of light that the light source  302  can generate.  
     [0033] When the substrate holder  116  blocks the light beam  314 , the detector  304  may detect substantially a minimum amount of light. The current signal generated by the detector  304  in response to the blockage may be converted into a stray light signal  404  by the amplifier  310 . The stray light signal  404  may represent the background level of light in the coating chamber  106 . The stray light signal  404  may be greater than zero volts due to background light. For example, the ion gun  108  may give off light in the coating chamber  106 .  
     [0034] When the light beam  314  passes through the one or more substrates  114 , the detector  304  may detect an amount of light that is representative of the thickness of the thin film layer on the substrate. The current signal generated by the detector  304  in response to amount of light passing through the substrate  114  may be converted into the transmission signal  406  by the amplifier  310 . The transmission signal  406  may be used in calculating the layer thickness and controlling the deposition process.  
     [0035] The control panel  306  may also receive signals from position sensors located in the coating chamber  106 . (The position sensors are not shown in FIG. 1.) The control panel  306  may transmit the optical monitor data and the position sensor data from the microcontroller  312  to the computer  308 . For example, the microcontroller  312  may send data to the computer  308  on a periodic basis, such as every six seconds.  
     [0036] The computer  308  is shown in FIG. 3 as a stand-alone component of the optical monitor  300 ; however, the computer  308  may be co-located with the control panel  306 . The computer  308  may be any combination of hardware, software, and firmware that is capable of generating layer thickness information from the data supplied by the control panel  306 . For example, the computer  308  may be a main frame computer, a desk top computer, a lap top computer, or an integrated circuit.  
     [0037] The computer  308  may determine the optical properties of the deposited layers using the information obtained from the microcontroller  306  and known optical principles. The computer  308  may include analytical software capable of determining layer thickness, and comparing actual layer thickness to desired layer thickness. The computer  308  may display the results on a monitor or screen.  
     [0038] In addition, the computer  308  may provide correction information that may be used to adjust the thickness of a layer being deposited or subsequent layers. The correction information may take the form of different layer times or different process parameters, such as ion beam current or process gas flow amounts to use. The correction information may used to adjust the deposition process automatically. Alternatively, the correction information may be provided to an operator that can alter the deposition process in response to the information. The deposition process may be stopped earlier than expected or may continue for a longer period of time based on the correction information.  
     [0039] In an alternative embodiment, a witness glass may be monitored. The witness glass may be a test glass in which light can pass through. The witness glass may be needed for a frosted coating, an odd shaped substrate, or other applications in which the light beam  314  from the light source  302  cannot pass through the one or more substrates  114 . The witness glass may be positioned on the substrate holder  116  in a location that would provide a representative thickness measurement of the one or more substrates  114 .  
     [0040]FIG. 5 is a flow chart of a method  500  of monitoring thin film deposition of optical coatings. Step  502  is directing a light beam at an optical substrate. Thin films may have been deposited on the optical substrate. Alternatively, the light beam may be directed at a witness glass. The light beam may be generated by a light source located outside the coating chamber. The light beam is directed into the coating chamber through a glass view-port using an adjustable mirror or prism.  
     [0041] Step  504  is detecting light from the light beam after it passes through the optical substrate. A detector, such as a photodiode, detects the light. The detector may also detect when the light beam is unobstructed and when the substrate holder blocks the light beam. The detector may generate a current signal that is representative of the amount of light detected.  
     [0042] Step  506  is calculating layer thickness based on the amount of light detected. The amount of light that passes through the optical substrate may be related to the thickness of the thin film layer deposited on the substrate. The current signal from the detector may be converted into a voltage signal that can be used to calculate the layer thickness.  
     [0043] Step  508  is providing correction information. Using the thickness data, the deposition process may be adjusted to alter the amount of material deposited on the thin film layer currently being deposited or the thickness of a layer to be deposited in future processing of the optical coating. The deposition process may be adjusted automatically or manually.  
     [0044] By providing an optical monitor in the coating chamber that can continuously monitor layer thickness on actual moving substrates during thin film deposition, optical coatings may be uniformly produced in high volumes. The deposition of the coatings may be monitored from start to finish, which is especially beneficial for critical coatings requiring a high degree of wavelength accuracy.  
     [0045] It should be understood that the illustrated embodiments are exemplary only and should not be taken as limiting the scope of the present invention. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.