Optical waveguide and method for its manufacture

Abstract: An optical waveguide is made by forming a layer of SiO₂ on a substrate and implanting a region of the SiO₂ 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: 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 SiO₂ on a substrate, depositing a layer of Si₃N₄ on the first layer of SiO₂, depositing a second layer of SiO₂ on the Si₃N₄ layer, and selectively removing a partial thickness of the second SiO₂ layer in selected regions to lower the effective refractive index of the underlying Si₃N₄ 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 SiO₂ on a substrate, depositing P-doped SiO₂ on the layer of undoped SiO₂, selectively removing regions of the P-doped SiO₂ layer to expose regions of the first layer of undoped SiO₂, and depositing a second layer of undoped SiO₂ on the exposed regions of the first layer of undoped SiO₂ and on the remaining regions of P-doped SiO₂. The regions of P-doped SiO₂ have a higher refractive index than the surrounding regions of undoped SiO₂. 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 US Patent 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 SiO₂ layer which incorporates the implanted dopant. The implanted dopant raises the refractive index of a central region of the SiO₂ 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 SiO₂ on a substrate and selectively bombarding the SiO₂ 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 SiO₂ which locally increases the refractive index of the SiO₂. The presence of the implanted H or B ions may also modify the refractive index of the implanted SiO₂. Unfortunately, the SiO₂ is decompacted and the implanted H or B ions are redistributed by diffusion in the SiO₂ layer if the waveguides are subjected to subsequent high temperature processing steps. The decompaction of the SiO₂ 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. US 4 521 443 discloses a similar process of planar waveguide manufacture that involves the implantation of N ions in a SiO₂ layer. The specification specifically explains that, though this N ion implantation process can withstand annealing for 30 minutes at a temperature in the range 450 to 500°C, it is not capable of withstanding annealing for 3 to 5 hours at a temperature in the range 600 to 1000°C. JP-A-63 02328 relates to a process intended to form Si segregates of controlled grain size in a SiO₂ film by Si ion implantation in the film, followed by an annealing step that promotes the segregation. Much of the specification is concerned with semiconductor device applications in which use is made of the effects of the process in modifying the conductivity and breakdown characteristics of the film. However, refractive index effects are also described, particularly in the context of the use of the process in the formation of a condenser lens in the film, though also in the context of its use in the creation of optical attenuators and filters. It will be noted that in these optical applications the light is directed through the thickness of the film rather than in any waveguide type application in which light propagates in the plane of the film. 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. According to one aspect, the invention provides a planar optical waveguide, comprising a channel region having an elevated refractive index surrounded by a region having a lower refractive index, which waveguide is formed in a layer of SiO₂ supported by a substrate, characterised in that the channel region is provided by a region of the SiO₂ layer containing a stoichiometric excess of Si. According to another aspect, the invention provides a method of making a planar optical waveguide comprising a channel region having an elevated refractive index surrounded by a region having a lower refractive index, the method comprising forming a layer of SiO₂ on a substrate, and characterised in that, in a region of the SiO₂ layer, Si ions are implanted to provide a region containing a stoichiometric excess of Si that constitutes said channel region. 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 degrees Celsius in a non-oxidizing ambient for 12 hours without loss of definition of the refractive index profile. However, high temperatures 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: Figures 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; Figure 2 is a plot of refractive index versus depth for the optical waveguide of Figure 1; and Figures 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 SiO₂ is grown by steam oxidation of a <100> Si substrate 12 to form the structure shown in Figure 1a. The steam oxidation is performed at 950 degrees Celsius and at atmospheric pressure to provide an SiO₂ layer approximately 710 nm thick. A layer 14 of Si₃N₄ approximately 2 microns thick is deposited on the SiO₂ layer, and defined using conventional photolithographic techniques to provide an opening 16 through the Si₃N₄ layer 14 where a waveguide channel is desired. The resulting structure, shown in Figure 1b, is inserted into conventional ion implantation equipment, where it is subjected to a dose of Si ions 18 at an implant dose of 4x10¹⁶ cm⁻² and an implantation energy of 40 keV. The Si₃N₄ layer 14 acts as an ion implantation mask to provide selective implantation of the Si ions 18 into the SiO₂ layer 14 only through the opening 16. The Si₃N₄ layer 14 is removed using conventional techniques for the selective removal of Si₃N₄ to leave the layer 10 of SiO₂ which now comprises an implanted region 20 containing a stoichiometric excess of Si as shown in Figure 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. Figure 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 SiO₂, 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 SiO₂ 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 SiO₂. 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 SiO₂ layer 10 is grown as in the first embodiment. The implantation masking Si₃N₄ layer 14 of the first embodiment is omitted, and the entire SiO₂ layer 10 is implanted with Si ions to form a refractive index profile, as shown in Figure 3a. A layer 14 of Si₃N₄ is then deposited on the SiO₂ layer 10 and defined using conventional photolithographic techniques so that the Si₃N₄ layer 14 remains only over regions of the SiO₂ layer 10 where a waveguide is desired, as shown in Figure 3b. The resulting structure is then heated in an oxidizing ambient to oxidize the implanted Si in regions of the SiO₂ layer 10 which are not covered by the remaining Si₃N₄ layer 14 to erase the refractive index profile in those regions, as shown in Figure 3c. The Si₃N₄ 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 SiO₂ layer 10 on Si substrates of different orientations and at different temperatures or pressures. Pressures exceeding atmospheric pressure may be required where a thick SiO₂ layer is desired. The SiO₂ 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 SiO₂ 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 1x10¹⁴ cm⁻² to 2x10¹⁷ 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 SiO₂ 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 SiO₂ 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.

Claims: A planar optical waveguide, comprising a channel region (20) having an elevated refractive index surrounded by a region (10) having a lower refractive index, which waveguide is formed in a layer (10) of SiO₂ supported by a substrate (12), characterised in that the channel region is provided by a region (20) of the SiO₂ layer (10) containing a stoichiometric excess of Si. An optical waveguide as defined in claim 1, wherein the stoichiometric excess of Si defines a graded refractive index profile. An optical waveguide as defined in claim 2, wherein the stoichiometric excess of Si is a function of depth that approximates to a Gaussian distribution function. A method of making a planar optical waveguide comprising a channel region (20) having an elevated refractive index surrounded by a region (10) having a lower refractive index, the method comprising forming a layer (10) of SiO₂ on a substrate (12), and characterised in that, in a region (20) of the SiO₂ layer (10), Si ions are implanted to provide a region (20) containing a stoichiometric excess of Si that constitutes said channel region (20). A method as defined in claim 4, wherein the forming of the layer (10) of SiO₂ on the Si substrate (12) comprises growing a layer of SiO₂ on the Si substrate. A method as defined in claim 5, wherein the growing of the layer (10) of SiO₂ on the Si substrate (12) comprises growing a layer of SiO₂ on the Si substrate by steam oxidation. A method as defined in claim 6, wherein the steam oxidation is performed at a pressure exceeding atmospheric pressure. A method as defined in claim 4, wherein the step of forming of the layer (10) of SiO₂ on the substrate (12) comprises depositing a layer of SiO₂ on a substrate by chemical vapour deposition. A method as defined in any claim of claims 4 to 8, wherein the implanting of the region (20) of the SiO₂ layer (10) with Si ions comprises forming an ion implantation mask (14) over the SiO₂ layer (10), the ion implantation mask having openings therethrough over regions of the SiO₂ layer (10) where implantation of Si ions is desired, and selectively implanting Si ions into the SiO₂ layer (10) through the openings of the ion implantation mask (14). A method as defined in any claim of claims 4 to 9, wherein the implanting of the region (20) of the SiO₂ layer (10) with Si ions comprises implanting the Si ions with an implantation dose between 1x10¹⁴ cm⁻² and 2x10¹⁷ cm⁻², and with an implantation energy between 3 keV and 400 keV. A method as defined in any claim of claims 4 to 10, wherein the implanting of the region (20) of the SiO₂ layer (10) with Si ions comprises performing a series of implantation steps at different implantation energies and/or different implantation doses. A method as defined in claim 11, wherein successive implantation steps are performed through different implantation masks. A method as defined in any claim of claims 4 to 12, wherein the implanting of the Si ions to provide the channel region (20) in the SiO₂ layer (10) comprises the steps of implanting Si ions in the SiO₂ layer (10), of forming an oxidation-resistant mask (14) selectively to cover the region (20) where the channel region is desired, and of heating the SiO₂ layer (10) in an oxidizing ambient to oxidize the implanted Si selectively in the regions not protected by the oxidation-resistant mask (14). A method as defined in any claim of claims 4 to 12, the method further comprising annealing the implanted SiO₂ layer (10) at a temperature exceeding 1000 degrees Celsius in a non-oxidizing ambient.

Inventors: BOOTHROYD ALBERT ROY, ELLUL JOSEPH PAUL, KALNITSKY ALEXANDER, BOOTHROYD, ALBERT ROY, ELLUL, JOSEPH PAUL, KALNITSKY, ALEXANDER
Applicants: NORTHERN TELECOM LTD, NORTHERN TELECOM LIMITED
Application year: 1990
Main classification: G02B6
IPC codes: G02B6