Abstract:
Thermally-assisted organometallic sol-gel derived glasses have been found to permit fabrication of thin films sufficiently thin for telecom components. Inclusion of a photosensitizer in the film permits light of controlled intensity to modify refractive indices in the film to form useful structures.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 09/574,841, filed May 19, 2000, which is hereby incorporated by reference in its entirety.  
         [0002]     This application is also related to a companion application Ser. No. 09/574,840, filed on May 19, 2000, and assigned to the assignee of the present application. 
     
    
     FIELD OF INVENTION  
       [0003]     This invention relates to a process for producing photosensitive thin films of sol-gel derived glass and to such films of a thickness useful for integrated optic devices produced thereby.  
       BACKGROUND OF THE INVENTION  
       [0004]     The doctoral thesis by the application herein entitled “Photolithography of Integrated Optic Devices in Porous Glass,” City University of New York, 1992, describes an organometallic system of inclusions in a thermally-assisted, porous glass bulk material. The process for fabricating the glass requires introduction of a photosensitizer, exposure to light through a mask and two heat treatments. The doctoral thesis states that sol-gel techniques can be used to make the porous glass bulk material.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005]     The invention is based on the realization that the porous glass techniques for bulk materials using thermally assisted, organometallic, sol-gel derived glass can be extended to thin films suitable for telecom components and virtually free of lateral shrinkage. Consequently, a variety of highly desirable integrated optic components can be made by such a technique. Specifically, a technique for the photolithographic fabrication of integrated optic structures in thin films of photosensitive sol-gel glasses is described here. This technique involves the formation of a photosensitive sol-gel thin film including an organometallic photosensitizer, on a suitable substrate (glass, silicon, or any other support material). Next, the photosensitive film is exposed to white or ultraviolet light inducing a photochemical reaction in the photosensitive sol-gel glass network with the end photoproduct being a metal oxide. The photodeposited metal oxide is permanently bound to the sol-gel film glass network as a glass modifier during a heat treatment process, which in turn induces a permanent refractive index increase in the glass. The refractive index increase is dependent on the concentration of the photosensitizer and on the light energy used in the exposure process. Therefore, a spatially varying light intensity during exposure results in a spatially varying refractive index profile. This refractive index profile induced in the film can be designed to guide light.  
         [0006]     Exposure of the photosensitive sol-gel film to white or ultraviolet light induces the unbinding of the metal from the photoliabile moiety component of the photosensitizer followed by the binding of the metal to the sol-gel film. The exposed regions of the sol-gel film are converted to a metal oxide silica film by first and second step heatings at a low temperature and high temperature, respectively. The low temperature drives out the unexposed (unbound) photosensitizer and the unbound photolabile moiety. The higher temperature step unbinds the organic component from the bound photosensitizer and drives it off. This step also permanently binds the metal to the silica film forming a metal oxide glass modifier. If the sol-gel film is deposited on a glass or silicon substrate, a metal oxide doped silica region of Si—O-M-O—Si is formed in the exposed regions acting as a glass modifier which in turn modifies the refractive index. The unexposed photosensitizer is driven off during the heat treatment steps. Since no material is removed from the sol-gel film in this process, as in the case of prior-art processes, the resulting top surface is planar, thus leading to a simpler process for producing devices and for achieving increased lifetime of resulting devices. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIGS. 1, 2 ,  4  and  5  are schematic side views of thin films in accordance with the principles of this invention; and  FIG. 3  is a block diagram of the steps for fabricating a structured thin film in accordance with this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]      FIG. 1  is a side view of film  11  of a sol-gel film with R-M-X constituents dissolved therein. The film is shown formed (usually by a well known spinning technique) on the SiO 2  surface layer  12  on a silicon substrate  13 . The R constituents are taken from a class of volatile organic materials consisting of CH 3 , CH 3 —CH 2 , CH 3 —CH 2 —CH 2 , the M constituents (metals) are taken from the class of metals consisting of group IVB metals Ge, Sn and Pb, Group VIB including Se and Te, Group VIIIA including Fe, Co, Ni, and Group IVA including Ti and Zr and rare earth metals, and the X constituents (photolabile moiety) are taken from the class consisting of chlorine, iodine, fluorine, bromine, and carbonyls.  
         [0009]      FIG. 2  shows an alternate embodiment where the sol-gel film  20  is formed on a glass substrate  21 .  FIGS. 1 and 2  represent the initial sol-gel solution formed on appropriate substrates of silicon ( FIG. 1 ) and glass ( FIG. 2 ). The process of forming the sol-gel solution into useful film structures is discussed in connection with  FIG. 3 .  
         [0010]     Specifically,  FIG. 3  is a block diagram of the process for fabricating structured films from the sol-gel solution of  FIGS. 1 and 2 . Block  31  of  FIG. 3  represents the step of forming a sol-gel film with inclusions of R-M-X on a suitable substrate (as shown in  FIG. 1  or  FIG. 2 ). Block  32  represents the exposure of the film through a mask to light in a range of wavelengths from ultraviolet (UV) through the visible range. This step unbinds the photolabile moiety (X) and binds the metal (M) to the silicon oxide.  
         [0011]     Block  33  of  FIG. 3  represents the step of heating the film to about 300 degrees C. for a time to bind the metal permanently to the SiO 2 . This step also drives off the unexposed organometallic photosensitizer from the entire sol-gel layer and the unbound photolabile moiety (X) from the exposed portions of the sol-gel layer. Block  34  of  FIG. 3  represents the final heating step to about 900 degrees C. for unbinding the organic component (R) from the bound photosensitizer and driving off that component. This step also permanently binds the metal to the silica sol-gel film forming a metal oxide glass modifier.  
         [0012]      FIG. 4  shows the structure of  FIG. 1  with a mask  40  in place. Mask  40  is opaque to the incident light (arrow  41 ) in regions  42  and  43  and is transparent to light in region  44 . The result of exposure to light is a structured film (in excess of 1 micron) where the exposed region of the film includes Si—O-M-O—Si and the unexposed regions include SiO 2 .  
         [0013]     The concentration of photodeposited metal oxide determines the index of refraction of the exposed region which can be made relatively high compared to that of adjacent regions. If we visualize region  44  extending away from the view as indicated by the broken lines in  FIG. 5 , the resulting structure can be seen to represent a waveguide with the “core” being buried as indicated.  
         [0014]     In one specific embodiment, a sol-gel film 1-10 microns thick was formed on a silicon substrate 1 cm×0.5 cm×0.1 cm thick with a SiO 2  surface layer&lt;2 microns thick thereon. The sol-gel film included Sn (M) 2%, 1 (X) 2%, and (CH 3 ) 3  (R) 2%. Region  44  has a width of 10 microns, exposed to light with a wavelength of 254 nm for 30 minutes. The exposed region had an index of refraction of 1.55 and the unexposed regions had indices of refraction of 1.45. The film has a thickness of 1-10 microns after processing and has unchanged lateral dimensions.  
         [0015]     In another embodiment, a sol-gel film 1-10 microns thick was formed on a glass substrate 1 cm×0.5 cm×0.1 cm thick. The sol-gel film included Ti (M) 2%, Cl (X) 4%, and Cp (R) 4% where Cp is cyclopentadienyl. Region  44  has a width of 10 microns, exposed to light with a wavelength of 514 nm for 120 minutes. The exposed region had an index of refraction of 1.75 and the unexposed region had indices of refraction of 1.45. The film had an final thickness of 1-10 microns with the lateral dimensions thereof being unchanged.