Patent Publication Number: US-2002008891-A1

Title: Substrate fixture for high-yield production of thin film based dense wavelength division multiplexers

Description:
[0001] This application claims priority to U.S. Provisional Patent Application Serial No. 60/217,115, entitled SUBSTRATE FIXTURE FOR HIGH-YIELD PRODUCTION OF THIN FILM BASED DENSE WAVELENGTH DIVISION MULTIPLEXERS, filed on Jul. 10, 2000 and U.S. Provisional Patent Application Serial No. 60/217, 060, entitled HIGH THROUGHPUT HIGH-YIELD VACUUM DEPOSITION SYSTEM FOR THIN FILM BASED DENSE WAVELENGTH DIVISION MULTIPLEXERS, filed on Jul. 10, 2000. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] A. Field Of The Invention  
       [0003] The present invention relates to a high speed rotational fixture assembly that has been designed to enable high yield production of thin film based demulitiplexers for DWDM (Dense Wavelength Division Multiplexer) systems. The fixture utilizes a dedicated thin film thickness monitor and shutter to allow individual thickness control of coatings on substrates positioned at various locations in a vacuum deposition system. The individual control compensates for variations in deposition rate, which are inherent in all deposition processes used to produce filters for high quality optics and telecommunication hardware components. Proper implementation of such fixtures should enable production yields of narrow band pass filters to improve significantly over yields currently achieved by conventional tooling.  
       [0004] DWDM systems enable information to be delivered inside fiber-optic cables at multiple wavelengths. The increase in the bandwidth is limited only by the number of wavelengths which can be superimposed on the fiber. Current state-of-the-art DWDMs can multiplex/demultiplex approximately 40 channels. Ultimately more than 1000 channels will be possible. During transmission, information is packaged within pulse-modulated carriers at specific wavelengths and superimposed (multiplexing) on the fiber. During reception, the carriers must be separated (demultiplexing). Optical component technology such as DWDM is critical in order to achieve the bandwidth necessary for future interactive services, such as “video on demand.” 
       [0005] The most widely used technology for DWDM multiplexer (mux) and demultiplexer (demux) devices is thin film-based. Multilayered, thin dielectric coatings are comprised of 150-200 layers with an individual optical layer thickness equal to multiples of ¼ of the wavelength to be transmitted (known as dielectric interference filters.) A collection of such filters, coupled together, each differing slightly in design to allow light transmission of different wavelengths, and “connected” to fiber-optic cable, enables the multiplexing (superposition) and demultiplexing (separation) of multiple wavelengths of laser light containing digital information.  
       [0006] B. Description Of The Related Art  
       [0007] Thin film coatings designed to permit light transmission/reflection over narrow (0.1-25 nm) and broad (&gt;25 nm) pass bands are typically comprised of multiple layers of two or more optically matched materials of “high” and “low” indices of refraction. The individual layer thickness, and number of layers, will ultimately define the optical performance of the filter. Typical “high performance” narrow band filters may have more than 100 individual layers.  
       [0008] Thickness uniformity is critical in any optical filter application. Optical coating systems are typically designed to produce coatings with thickness uniformity of approximately 0.1 percent variation over the substrate area. This level of thickness control is insufficient for multilayered coatings designed for DWDM. Layer thickness determines wavelength and amplitude (loss) of transmitted light therefore, accurate thickness determination and reproducibility are crucial. Thickness non-uniformity of 0.1 percent will lead to filters that do not meet required specifications.  
       [0009] In practice, tens of substrates (approximately  6  inch square or round) are coated with multilayered filters, designed for DWDM in “traditional” IBSD (ion beam sputter deposition) or IAD (ion-assisted deposition) systems. A typical IAD production coating system is approximated by a 60-inch cube, with a fixture assembly located at the top of the vacuum chamber as shown schematically in FIG. 1. The planetary fixture assembly is designed for thickness uniformity described above and can accommodate approximately 24 6-inch square substrates. As many as 5 QCMs (quartz crystal monitor), and an optical monitor, are positioned about the chamber to monitor deposition rate and optical layer thickness. The quartz monitors are calibrated prior to production. Deposition rate incident on the substrate assembly is determined by sampling each monitor and averaging.  
       [0010] The substrates are diced into thousands of ˜1 mm squares (called dies or chips). Every coated die is tested for performance to determine which ones (if any) meet requirements. Presently, major manufactures, such as OCLI®, are reporting production yields of less than 5 percent. The demand for such filters is approaching 1,000,000 per month. This demand will not be met with current system configurations without a significant increase in capital equipment to increase capacity. Customers for the filters have relaxed requirements and settled for inadequate performance to continue with installation of DWDM systems.  
       [0011] Quartz crystal monitors are extremely sensitive to minute changes in thickness. The device is based on changes in frequency in the quartz oscillator resulting from increased mass present on the surface. Thickness can be determined from a relationship approximated by the equation: 
         T (nm)˜ C Δν(Hz)/ρ(g/cm 3 ) 
       [0012] Where T is the film thickness in nanometers, Δν is the change in oscillation frequency in Hz, ρ is the density of the deposited material in g/cm 3  and C is a constant, effected by geometric properties of the deposition environment, and thermal and mechanical properties of the deposited material. With proper calibration, the QCM can accurately resolve differences in thickness less than 0.01 nm.  
       [0013] Deposition control processors are programmed with material data to allow the QCM to accurately determine thickness for identified materials. The material data included in the processor memory, or found in material handbooks, is often derived from ideal materials and do not necessarily reflect the properties of the thin film coating. In addition, the change in oscillation frequency is dependent upon the amount of material already present on the quartz crystal and the behavior of that material in the deposition environment. For these reasons, QCMs are not regarded as the preferred method of thickness and rate determination during optical filter production.  
       [0014] Currently, thin film filters for DWDM muxes and demuxes are produced with accepted yields of less than 5 percent, due to the complexity and uniformity requirements of the coatings designs. Coating equipment used for complex optical coatings is not optimally tooled to provide necessary uniformity for this application. Optical thickness monitors employed in most optical coating systems are not capable of resolving variations in thickness on the sub-angstrom level. Quartz crystal thickness monitors are more sensitive to changes in thickness, but are typically used inefficiently or improperly. This results in decreased accuracy of thickness determination vs. deposition time. The fixture design described in this document will increase accepted yields of thin film demultiplexers from 1 percent to 25-75 percent.  
       SUMMARY OF THE INVENTION  
       [0015] In accordance with one aspect of the present, a high yield fixture for the production of demux filters for DWDM systems includes a disk, the disk adapted to be rotatable at greater than 2400 rpm during operation, a dedicated multi-crystal quartz crystal thickness monitor, an optical thickness monitor, a clam shell shutter, a magnetic induction rotation mechanism, and multiple substrates, the substrates located concentrically about the quartz crystal monitor.  
       [0016] In accordance with another aspect of the present invention, a high yield fixture for production of optical filters includes a thickness monitor, a rotating member, shuttering means for shuttering the fixture, at least one substrate, and rotating means for rotating the fixture, wherein the rotating member is a disk adapted to be rotated at greater than 500 rpm, wherein the thickness monitor is a dedicated quartz crystal monitor, wherein the shuttering means is a clam shell shutter.  
       [0017] In accordance with another aspect of the present invention, the fixture also includes multiple substrates, the substrates located concentrically about the monitor, wherein the substrate is divided into a grid of dies, wherein the rotating means is a magnetic induction rotation mechanism, wherein the rotating member is a disk adapted to be rotated at greater than  
       [0018] 2400  rpm  
       [0019] In accordance with still another aspect of the present invention, a high speed substrate assembly for use in a line-of-sight deposition process includes multiple independent fixtures having at least one substrate, at least one thickness monitor, shuttering means for shuttering the fixture, and rotating means for rotating the fixture.  
       [0020] In accordance with yet another aspect of the present invention, the monitor includes a dedicated quartz crystal monitor and an optical thickness monitor.  
       [0021] In accordance with still another aspect of the present invention, the fixtures includes a rotatable disk, the at least one substrate and the monitors being located on the disk, wherein the at least one substrate is multiple substrates, the substrates being concentrically located about the quartz crystal monitor, wherein the shuttering means is a clam shell shutter, wherein the rotating means is a magnetic induction rotation mechanism.  
       [0022] In accordance with another aspect of the present invention, a method for creating substantially uniformly thick optical filters includes the steps of providing at least one evaporator, providing multiple independent fixtures, each of the fixtures having at least one substrate, at least one thickness monitor, shuttering means for shuttering the fixture, and rotating means for rotating the fixture, independently rotating the fixtures at greater than 500 rpm, independently monitoring layer thickness for each of the fixtures using the at least one thickness monitor, and independently shuttering the fixtures to ensure uniform deposition.  
       [0023] In accordance with yet another aspect of the present invention, the method includes the step of utilizing pulsed deposition to finish a layer.  
       [0024] In accordance with still another aspect of the present invention, the method includes the steps of independently rotating the fixtures at greater than 2400 rpm, providing multiple independent fixtures, each of the fixtures having multiple substrates, a quartz crystal monitor, an optical thickness monitor, shuttering means for shuttering the fixture, and rotating means for rotating the fixture, the substrates being concentrically located about the quartz crystal monitor, and independently monitoring layer thickness per revolution for each of the fixtures using the optical thickness monitor.  
     
    
    
     [0025] Still other benefits and advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed specification.  
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0026] The invention is illustrated in the following drawings:  
     [0027]FIG. 1A is a perspective view of a prior art IAD vacuum deposition system;  
     [0028]FIG. 1B is a bottom view of a prior art planetary substrate assembly;  
     [0029]FIG. 2A is a top view of the inventive substrate fixture, showing the QCM, the substrates, and the rotational mechanism;  
     [0030]FIG. 2B is a side view of the inventive fixture;  
     [0031]FIG. 3A is a side view of the fixture with the clam shutter in the open position;  
     [0032]FIG. 3B is a side view of the fixture with the clam shutter in the closed position;  
     [0033]FIG. 3C is a top view of the fixture with the clam shutter in the open position;  
     [0034]FIG. 3D is a top view of the fixture with the clam shutter in the closed position;  
     [0035]FIG. 4 is a top view of a dense high yield fixture array, showing the inventive fixtures in both the open and closed positions; and, FIG. 5 is a perspective view of the inventive deposition system. 
    
    
     DESCRIPTION OF THE INVENTION  
     [0036] Referring now to the drawings, which are for purposes of illustrating at least one embodiment of the invention only, and not for purposes of limiting the invention, FIGS. 2A and 2B are a representation of a high yield fixture  30 , call the Vomado™, which has been designed to produce demultiplexer filters for DWDM systems with greater than 25 percent accepted yield. The design is comprised of a disk  34  (which in this embodiment is approximately 8.5 inches in diameter) with a concentric multi-crystal QCM  20  and a dedicated shutter arrangement. In this embodiment, the disk  34  rotates at greater than 1000 rpm (the disk  34  may rotate as slowly as 500 rpm) during operation to ensure uniform deposition of material at typical coating deposition rates of 0.2-0.5 nm/s. Under these conditions, the disk  34  would perform equal to or greater than 20 revolutions for each atomic layer (monolayer) of deposited material. This ensures that angular variation of thickness is less than or equal to {fraction (1/20)} th  of a monolayer or approximately 0.02 nm.  
     [0037] With continuing reference to FIGS. 2A and 2B, the fixture  30  includes multiple substrates  18 , rotation mechanism  36 , a QCM  20 , and a fixture diameter  28 . The substrates  18 , which are divided into multiple dies (shown but not referenced), are located concentrically about the QCM  20 . In this embodiment, the rotating mechanisms  36  are magnetic induction mechanisms, and are located on either end of the fixture  30 , as shown in FIG. 2A. This rotation mechanism  36  is not limited to this configuration. Rotation can be accomplished in any way which does not interfere with the line of sight from the deposition source to the substrate  18 , and is chosen using sound engineering judgment.  
     [0038] FIGS.  3 A- 3 D show the fixture  30  with a “clam shell” type shutter  38 . Any shutter arrangement, commonly used for vacuum coating applications, and chosen using sound engineering judgment, would be acceptable. It is desirable to minimize the area occupied by the entire fixture assembly  44 , since maximum throughput is achieved with a dense array of fixtures  30  as shown in FIG. 4. FIGS. 3A and 3C show the fixture  30  in the open position. The clam shell shutter  38  occupies less space than other shutter arrangements, and covers all sides, as well as the face, of the fixture  30 .  
     [0039] However, it is to be understood that any means for closing the fixture  30  can be used as long as chosen using sound engineering judgment. In this invention “shuttering” is intended to encompass any means of restricting access, or the line of sight, to the substrate  18 . FIGS. 3B and 3D show the fixture  30  with the clam shell shutter  38  in the closed position. When in the closed position, the shutter  38  prevents further deposition, as access to substrate  18  is blocked. The ability to shutter the fixture  30  allows the uniform deposition of layers.  
     [0040] Since the QCM  20  can resolve sub-angstrom thickness changes, materials deposited onto the substrates  18 , held less than 2 inches away, are monitored with high precision. Geometrical calibration of the QCM  20 , to compensate of the position for the substrates  18 , is straightforward. Thermal fluctuations resulting from heat capacity of the deposited materials and second order effects from mechanical stress must be determined. This is accomplished with standard analysis techniques for material characterization. From this data, it is straightforward to develop an algorithm to maintain accurate thickness information from layer to layer.  
     [0041] Instability in the QCM  20  occurs when it is initially exposed to the electron gun  14  due to thermal flux generated by the heated source material. A quartz window (not shown) can be embedded into the clam shutter  38 , which would allow the quartz crystal to come to thermal equilibrium with the flux before deposited material must be monitored. Once thermal equilibrium was achieved, clam shutters  38  would open. Windows of this type would become less effective as the deposition progressed due to accumulation of evaporant, but would serve to improve the overall performance of the filters.  
     [0042] Thin filters are intended to be produced in the following way. Deposition is carried out with, but is not limited to, one of several conventional processes described above. Fixtures  30  are positioned approximately as shown in FIG. 4. During system calibration, the vertical position of each fixture  30  is individually adjusted to compensate for variations in depositions rate vs. chamber location.  
     [0043] As shown in FIGS. 4 and 5, the fixtures  30  are independent of one another, and can be in both an open  42  and closed  40  position. The fixtures  30  are rotatable independently of one another, and are also shuttered independently of one another. The independent nature of each of the fixtures  30  allows uniform deposition of the material onto the substrates  18 .  
     [0044] As the thickness of an individual layer approaches the target value, as measured by the individual fixture QCM  20 , the clam shutter  38  will close prior to achieving the target thickness. Individual fixtures  30  will be shuttered at different times, since like thicknesses will not be achieved simultaneously due to geometrical factors and nonuniform variations in deposition rate at different locations in the chamber  10 . Each fixture  30  will be independently reopened to a low rate pulsed deposition process to achieve the target thickness. The low rate pulsed process may take as much time as the initial “bulk” coating.  
     [0045] The fixture  30  can be adapted to more advanced deposition processes proposed for DWDM systems, such as epitaxial growth and pulsed molecular beam deposition. With the implementation of these processes the QCM  20  is replaced with Reflection High Energy Electron Diffraction (RHEED) or interferometric thickness monitoring techniques, depending on the morphology of the deposited film. The basic concept of high-speed rotation remains unchanged and the result is a significant improvement in acceptable yield.  
     [0046] The invention has been described with reference to at least one embodiment. Obviously, modifications and alterations will occur to others upon a reading and understanding of the specification. It is intended by applicant to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.  
     [0047] Having thus described the invention, it is now claimed: