Abstract:
An optical waveguide is made by forming a layer of SiO 2  on a substrate and implanting a region of the SiO 2  layer with Si ions to define a region containing a stoichiometric excess of Si which defines a region having an elevated refractive index surrounded by a region having a lower refractive index. The resulting optical waveguide is stable at the high temperatures required for many semiconductor processing methods, and is useful for optical interconnection in integrated optical and optoelectronic devices.

Description:
This is a divisional of Ser. No. 07/363,006, filed June 8, 1989, Now U.S. Pat. No. 4,934,774. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to optical waveguides and methods for their manufacture. 
     BACKGROUND OF THE INVENTION 
     Planar optical waveguides are required for optical interconnection in integrated optical and optoelectronic devices. The methods used to manufacture such waveguides must be compatible with semiconductor processing methods used to manufacture other parts of the integrated devices. 
     Planar optical waveguides have been made by depositing a photosensitive monomer on a substrate and selectively exposing the deposited monomer to ultraviolet (UV) radiation. The UV radiation polymerizes the exposed monomer to provide polymer regions having a relatively high refractive index bounded by monomer regions having a relatively low refractive index. A further monomer layer is generally deposited over the partially polymerized layer for protection against surface flaws and contaminants which could couple light out of the polymerized regions. Unfortunately, the waveguides made by this method are unstable at the high temperatures which are used in some semiconductor processing methods. Consequently, all high temperature processing steps must be completed before the waveguides are defined. Moreover, this method generally requires two or more deposition steps. 
     Planar optical waveguides have also been made by depositing or growing a first layer of Si0 2  on a substrate, depositing a layer of Si 3  N 4  on the first layer of Si0 2 , depositing a second layer of Si0 2  on the Si 3  N 4  layer, and selectively removing a partial thickness of the second Si0 2  layer in selected regions to lower the effective refractive index of the underlying Si 3  N 4  layer in those regions. This method requires three deposition or growth steps and one etch back step, all of which must be carefully controlled for satisfactory results. 
     Silicon-based planar optical waveguides have also been made by depositing or growing a first layer of undoped Si0 2  on a substrate, depositing P-doped Si0 2  on the layer of undoped Si0 2 , selectively removing regions of the P-doped Si0 2  layer to expose regions of the first layer of undoped Si0 2 , and depositing a second layer of undoped Si0 2  on the exposed regions of the first layer of undoped Si0 2  and on the remaining regions of P-doped Si0 2 . The regions of P-doped Si0 2  have a higher refractive index than the surrounding regions of undoped Si0 2 . This method also requires three deposition or growth steps and one etch back step, all of which must be carefully controlled for satisfactory results. 
     In U.S. Pat. No. 4,585,299, Robert J. Strain discloses a method for making silica-based planar optical waveguides in which boron, phosphorus, arsenic or germanium is implanted into a silicon substrate through a first mask and the substrate is oxidized through a second mask to provide a patterned Si0 2  layer which incorporates the implanted dopant. The implanted dopant raises the refractive index of a central region of the Si0 2  layer to provide a waveguide. This patent suggests that migration of the dopant during the oxide growth may be a problem. 
     Silicon-based planar optical waveguides have also been made by depositing or growing a layer of Si0 2  on a substrate and selectively bombarding the Si0 2  with H or B ions to define regions having a relatively high refractive index bounded by regions having a relatively low refractive index. The implantation process causes localized compaction of the Si0 2  which locally increases the refractive index of the Si0 2 . The presence of the implanted H or B ions may also modify the refractive index of the implanted Si0 2 . Unfortunately, the Si0 2  is decompacted and the implanted H or B ions are redistributed by diffusion in the Si0 2  layer if the waveguides are subjected to subsequent high temperature processing steps. The decompaction of the Si0 2  and the migration of the implanted H or B ions degrades the refractive index profile defined by the implantation process. Consequently, all high temperature processing steps must be completed before the waveguides are defined. 
     SUMMARY OF THE INVENTION 
     This invention seeks to obviate or mitigate problems with known planar optical waveguides and methods for their manufacture as described above. 
     One aspect of the invention provides an optical waveguide comprising a substrate and a layer of Si0 2  on the substrate. The layer of Si0 2  comprises a region containing a stoichiometric excess of Si which defines a region having an elevated refractive index surrounded by a region having a lower refractive index. 
     Another aspect of the invention provides a method for making an optical waveguide. The method comprises the steps of forming a layer of Si0 2  on the substrate and implanting a region of the Si0 2  layer with Si ions to define a region having an elevated refractive index surrounded by a region having a lower refractive index. 
     The optical waveguide according to the invention is stable at the high temperatures required for many semiconductor processing methods. Sample waveguides were annealed at 1100 degress Celsius in a non-oxidizing ambient for 12 hours without loss of definition of the refractive index profile. However, high temperature processing in an oxidizing ambient does cause loss of definition of the refractive index profile. 
     The method according to the invention requires only a single deposition or growth step, and no etch back step. Consequently this method is relatively simple and easy to control. Moreover, the method is compatible with standard semiconductor processing methods, and can be performed using readily available semiconductor processing equipment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are described below by way of example only. The description refers to the attached drawings, in which: 
     FIGS. 1a, 1b and 1c are cross-sectional views of an optical waveguide according to an embodiment of the invention at successive stages of its manufacture by a method according to a first embodiment of the invention; 
     FIG. 2 is a plot of refractive index versus depth for the optical waveguide of FIG. 1; and 
     FIGS. 3a, 3b, and 3c are cross-sectional views of an optical waveguide according to an embodiment of the invention at successive stages of its manufacture by a method according to a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In a method according to a first embodiment of the invention for making an optical waveguide, a layer 10 of Si0 2  is grown by steam oxidation of a &lt;100&gt; Si substrate 12 to form the structure shown in FIG. 1a. The steam oxidation is performed at 950 degrees Celsius and at atmospheric pressure to provide an Si0 2  layer approximately 710 nm thick. 
     A layer 14 of Si 3  N 4  approximately 2 microns thick is deposited on the Si0 2  layer, and defined using conventional photolithographic techniques to provide an opening 16 through the Si 3  N 4  layer 14 where a waveguide channel is desired. The resulting structure, shown in FIG. 1b, is inserted into conventional ion implantation equipment, where it is subjected to a dose of Si ions 18 at an implant dose of 4×10 16  cm -2  and an implantation energy of 40 keV. The Si 3  N 4  layer 14 acts as an ion implantation mask to provide selective implantation of the Si ions 18 into the Si0 2  layer 14 only through the opening 16. 
     The Si 3  N 4  layer 14 is removed using conventional techniques for the selective removal of Si 3  N 4  to leave the layer 10 of Si0 2  which now comprises an implanted region 20 containing a stoichiometric excess of Si as shown in FIG. 1c. The stoichiometric excess of Si as a function of depth approximates a Gaussian distribution function. The refractive index of the implanted region 20 is elevated by the presence of the excess Si in proportion to the local concentration of the excess Si. Thus, the excess Si defines a graded refractive index profile which defines a region having an elevated refractive index surrounded by a region having a lower refractive index. 
     FIG. 2 illustrates the refractive index profile of the implanted optical waveguide which may be measured using conventional etch back techniques combined with conventional ellipsometric refractive index measurements. 
     Waveguides made by methods similar to the method described above have been annealed in an inert ambient at 1100 degrees Celsius for 12 hours without detectable changes in the refractive index profile. These results indicate that although a minor proportion of the refractive index increase may be due to compaction of the Si0 2 , a mechanism which is reversed at high temperatures, most of the refractive index increase must be due to a different mechanism which is stable at high temperatures. It is believed that the increased refractive index of the Si-implanted Si0 2  is primarily due to the formation of Si--Si bonds which are stable at high temperatures. Thus, high temperature semiconductor processing steps which are conducted in an inert ambient may follow the formation of waveguides by the above method without degradation of the waveguide structure. 
     However, exposure of the implanted layers to high temperature processing in an oxidizing ambient reverses the refractive index increase due to implantation, probably because the presence of excess oxygen at elevated temperatures disrupts Si--Si bonds formed during implantation to form further Si0 2 . This effect can be used in an alternative method for making an optical waveguide as described below. 
     In a method according to a second embodiment, an Si0 2  layer 10 is grown as in the first embodiment. The implantation masking Si 3  N 4  layer 14 of the first embodiment is omitted, and the entire Si0 2  layer 10 is implanted with Si ions to form a refractive index profile, as shown in FIG. 3a. A layer 14 of Si 3  N 4  is then deposited on the Si0 2  layer 10 and defined using conventional photolithographic techniques so that the Si 3  N 4  layer 14 remains only over regions of the Si0 2  layer 10 where a waveguide is desired, as shown in FIG. 3b. The resulting structure is then heated in an oxidizing ambient to oxidize the implanted Si in regions of the Si0 2  layer 10 which are not covered by the remaining Si 3  N 4  layer 14 to erase the refractive index profile in those regions, as shown in FIG. 3c. The Si 3  N 4  layer 14 acts as an oxidation-resistant mask to prevent oxidation of the implanted Si and erasure of the refractive index profile in the regions where a waveguide is desired. 
     The methods described above may be modified by growing the Si0 2  layer 10 on Si substrates of different orientations and at different temperatures of pressures. Pressures exceeding atmospheric pressure may be required where a thick Si0 2  layer is desired. The SiO 2  layer may be formed on Si substrates or on substrates of materials such as III-V semiconductors by processes other than thermal growth such as chemical vapour deposition. 
     The Si0 2  thickness, the implantation energy and implantation dose may be modified to change the depth and refractive index profile of the resulting waveguide. For example, the implantation energy may range from 3 keV to 400 keV, and the implantation dose may range from 1×10 14  cm -2  to 2×10 17  cm -2 . 
     Non-Gaussian refractive index profiles can be obtained by performing a series of implantations at different implantation energies and optionally at different implantation doses. Successive implantations can be performed through different implantation masks to provide different refractive index profiles in different regions of the Si0 2  layer 10. A series of implantations through a common implantation mask can be used to provide a high refractive index well which extends to the surface of the Si0 2  layer 10 for surface coupling of a waveguide to an optical fiber or an optical device. 
     Other masking materials, such as polysilicon or Al may be used during implantation, and the thickness of the masking material should be selected to be at least three to five times the projected range of Si ions in the selected masking material at the selected implantation energy. These and other modifications are within the scope of the invention as claimed below.