Abstract:
A method of preparing a multi-layer, crack resistant sol-gel glass derived coating on a substrate by removing the outer periphery of each layer before curing the layer and depositing any succeeding layer.

Description:
FIELD OF THE INVENTION 
     This invention relates to integrated optics devices fabricated from sol-gel derived glass. 
     BACKGROUND OF THE PRIOR ART 
     Integrated optics is a term used to describe a rigid structure which has a plurality of waveguides defined therein, such as in the well-known arrayed waveguide grating (AWG). Such arrayed waveguide grating devices are commercially available and are typically fabricated by well-known photo-lithographic techniques used to configure film overlays on a silicon substrate. Copending application Ser. No. 09/574,840 filed May 19, 2000 for the inventors of the invention of the present application and assigned to the assignee of the present application, discloses a process for configuring thin films of sol-gel into a variety of structures useful for transmission of optical energy. 
     At the present time, films prepared from sol-gel with the required thickness and flatness to create integrated optic waveguiding structures can be produced by conventional spin coating techniques using standard semiconductor type coating equipments, such as those commercially available from Silicon Valley Group (SVG), Suss Micro Tec, Inc., or other manufacturers. While the sol-gel is in liquid form, centrifugal force acts as a surface leveling agent on the dispensed liquid. Unfortunately, the tendency of the liquid film toward uniformity does not apply at the outer edge of a spinning disk because the surface of the liquid film must curve and intersect the substrate somewhere in the vicinity of the edge. An effect may be produced where the outer periphery of the sol-gel has a different thickness than the rest of the surface, in the industry sometimes referred to as “edge bead” formation. This variation on the sol gel film thickness at the edge of the wafer often results in the formation of micro-cracks at the edge of the film. These micro-cracks further propagate onto the film structure during the subsequent heat treatment steps resulting in complete catastrophic failure of the film. Once the sol-gel film has cracked, all of the effort that has gone into the production of high-definition, sub-microns structures is wasted as the wafer becomes useless and must then be discarded. Furthermore, if a contact photo mask is to be used with a photosensitive sol gel to create a waveguide structure, this rim must be removed as otherwise there will be gaps between the mask and the photosensitive surface through which light may enter and substantially degrade the image quality and resolution of the photolithographed waveguide pattern. 
     It would be of great advantage to be able to fabricate glass integrated optical devices using a sol-gel process that provided extremely flat surfaces onto which submicron dimensioned features could be defined without the danger of the sol-gel film cracking or absorbing efficiency-reducing amounts of power. 
     SUMMARY OF THE INVENTION 
     The invention is based on the recognition that thin sol gel films tend to crack when formed in layers sufficiently thick to be useful for forming optical waveguide structures. The invention is further based on the realization that even when films of a useful thickness are formed by a succession of relatively thin films, cracking still occurs. But cracking can be avoided by forming a succession of relatively thin films of decreasing surface area resulting in a film side profile similar to that of a “ziggurat”. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The foregoing and other features of the present invention may become more apparent from a reading of the ensuing description together with the drawing, in which: 
         FIGS. 1-4  are successive views of the fabrication of a sol-gel film in a spin coating chamber in accordance with the principles of this invention; 
         FIG. 5  is a side view of a substrate having the crack-resistant sol-gel film of the invention; and 
         FIG. 6  is a flow diagram of the process steps for forming the sol gel film of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the ensuing description references will be made to  FIGS. 1-4  and the corresponding steps of flow diagram FIG.  6 . 
       FIG. 1  shows a spin coating chamber  100 , such as the “Falcon” spin coater manufactured by Suss Micro Tec, having a spin table  102  and a dispensing arm  101  which moves from the outer periphery of the spin table towards its center under program control to dispense various fluids. As is well known, the atmosphere within chamber  100  may be controllably specified. It is desired to use chamber  100  and dispensing arm  101  to apply various layers of coatings, especially sol-gel derived glass, onto substrate  200 . Substrate  200  may be glass, silicon or any other conventionally useful material. 
     An (optional) first step in the fabrication, as shown in  FIG. 6  at  601 , is to clean the substrate in any conventional manner. Then, the first solution to be applied to the substrate, typically a sol-gel buffer solution  201  is mixed at  602 . When buffer solution  201  is sufficiently mixed, it is dispensed onto substrate  200 , as shown in  FIG. 1 , by arm  101  while the spin table is rotated at a comparatively low speed. When buffer solution  201  has sufficiently coated substrate  200 , the rotational speed of spin table  102  is increased and maintained at a speed to produce a buffer coating of uniform thickness. See step  603 , FIG.  6 . 
     In accordance with an aspect of the invention, after the desired thickness and uniformity of buffer coating  201  has been achieved, a portion of its outer periphery will be removed, (see step  604 , FIG.  6  and FIG.  2 ). 
     Referring now to  FIG. 2 , the step of removing the outer periphery of buffer layer  201  is shown. Dispensing arm now travels only a short distance radially inward of the spin table and dispenses a sufficient quantity of solvent, illustratively isopropyl alcohol, to dissolve away the outer “rim” of coating  201 , illustratively approximately 1 mm. After the outer periphery of buffer coating  201  has been removed, the buffer layer is baked onto substrate  200 , (step  605 , FIG.  6 ). When the baking has completed, the buffer layer is inspected, step  606 , and the solution for the next coating is mixed, step  607 . 
     Referring to  FIG. 3 , the next coating  204 , typically a “core” layer, is dispensed by arm  101  over the buffer layer while the spin table is rotated. Then the spin table speed is increased to achieve a core layer  204  of uniform thickness over the buffer layer, step  608 , FIG.  6 . However, the outer periphery core layer  204  will overlap the edge of buffer layer  201  and a small “ring” layer of solution  204  will be applied at the outer edge of substrate  200 . The portion of layer  204  that overlaps buffer layer  201  will be removed in the next step,  609 , together with an outer peripheral portion of core layer  204  will be removed, see  FIG. 6 , and FIG.  4 . 
     In  FIG. 4 , dispensing arm  101  now travels a slightly greater distance radially inward than it did in  FIG. 2 , and again dispenses a solvent to remove the outer peripheral portion of core layer coating  204 . After the outer peripheral portion of core layer  204  has been removed, the coated substrate is baked, step  610 , FIG.  6 . Again, the baked core layer is inspected, step  611 ,  FIG. 6 , and if the inspection step is passed, the solution for the next coating, typically a cladding solution, is mixed at step  612 . The cladding solution is dispensed in a manner similar to step  608  and its outer periphery is removed in a manner similar to that described for step  609 . 
     The result of the fabrication process is a multilayer film shown in  FIG. 5  having a succession of thicknesses suitable for the formation of waveguides useful for the transmission of optical energy. 
     EXAMPLES 
     In one illustrative embodiment, the following procedures were employed: 
     For the Buffer and Cladding Layers: 
     a. Add 95.5 g (100 ml) of methyl trimethoxysilane to a 500 ml plastic bottle; 
     b. Add 93.3 g (100 ml) of tetraethyl orthosilicate to the bottle; 
     c. Add 40 g (40 ml) of 0.05N hydrochloric acid to the bottle; 
     d. Mix at 6000 RPM in a water bath at 25° C. for 45 minutes. 
     e. Measure viscosity (15±2 cP). 
     For the Core Layer: 
     Same as for the buffer layer, except add 3.6% per initial volume of trimethyl tin iodide and stir for 40 minutes before using. 
     The foregoing is deemed to be descriptive of the principles of the invention. Further and other modifications will be apparent to those skilled in the art and may be made without, however, departing from the spirit and scope of the invention.