The present invention is related generally to fiberoptic cables and waveguides, and optical coupling therebetween. More specifically, the present invention relates to a method for making the endfaces of a plurality of fiberoptic waveguides on a substrate have a smooth surface for substantially reducing light scattering, and providing high-efficiency coupling with the end of a fiberoptic cable or optical fiber.
It is known in the art to use semiconductor substrates for providing a support for an optical waveguide formed on a substrate. Typically, one end of the waveguide is optically coupled to an end of a fiberoptic cable or optical fiber for permitting light waves to travel therebetween. Coherent light waves such as produced by lasers, are employed in fiberoptic communication systems, for example, for transmitting data signals through use of optical waveguides to couple the light waves to and from fiberoptical cables to photodetector and other circuitry.
A major problem providing high-efficiency optical coupling between optical waveguides and fiberoptic cables or optical fibers. It is important to achieve as near perfect alignment near the end of the fiberoptic cable and the endface of the waveguide for maximizing the transfer of light therebetween. Also, it is important to avoid any surface irregularity on the endface of the waveguides that may cause light scattering.
It is known in the prior art to form one or more V-grooves in  less than 100 greater than  silicon wafers, for retaining an optical fiber or fiberoptic cable, for coupling to the endfaces of waveguides cores formed in an opposing portion of the substrate. Note that although silicon (Si) is typically used for the substrate, other substrate materials include but are not limited to Gallium Arsenide (GaAs), and Indium Phosphide (InP).
In forming optical waveguides on various substrates, it is known in the art to employ different fabrication processes depending on the substrate material. The fabrication process may incorporate any one of chemical vapor deposition (CVD), fire flame deposition, and so forth. In such fabrication, V-grooves may be formed via chemical etching on one portion of the substrate. In this regard, anisotropic etchants for silicon substrates include potassium hydroxide (KOH), KOH and alcohol, ethylene diamine, and so forth. Etchant solutions for providing an anisotropic etching of GaAs or InP substrates include solutions of sulphuric acid (H2SO4), hydrochloric acid (HCl), bromic acid (HBr), and so forth. During such processing, masking is required for producing the proper formations, whereby the mask must have a relatively low etch rate. Examples of methods for forming V-grooves and optical waveguides in different portions of semiconductor substrates, and methods for the use of such grooves in retaining a portion of an optical fiber or cable, and aligning the same with an associated waveguide having an endface opposite the groove are shown in U.S. Patents of Carney U.S. Pat. No. 4,466,696, Welbourn U.S. Pat. No. 5,342,478, Menigaux et al. U.S. Pat. No. 5,518,965, Kitamura et al. U.S. Pat. No. 5,548,673, Yamane et al. U.S. Pat. No. 5,557,695, and Mizuta et al. U.S. Pat. No. 5,961,869. The teachings of these references are incorporated herein to the extent that they do not conflict herewith.
Much effort has been made in the prior art to develop better methods for coupling optical fibers or fiberoptic cables to waveguides formed on a semiconductor substrate. Examples of methods and apparatus for accomplishing such coupling are provided in many U.S. Patents, including Boivin U.S. Pat. No. 3,774,987, Blonder U.S. Pat. No. 4,966,433, Stein U.S. Pat. No. 5,046,809, Bossler U.S. Pat. No. 5,357,593, Bresby U.S. Pat. No. 5,371,818, and Harpin et al. U.S. Pat. No. 5,787,214. The teachings of the previously identified U.S. Patents are incorporated herein by reference to the extent that they do conflict herewith.
The teachings of Menigaux U.S. Pat. No. 5,518,965, entitled xe2x80x9cProcess For Producing A Structure Integrating A Cleaved Optical Guide With An Optical Fiber Support For A Guide-Fibre Optical Coupling,xe2x80x9d issued on May 21, 1996. This reference teaches the formation of a V-groove in one end portion of a semiconductor substrate in one embodiment, and also in the opposing end portion in another embodiment. An optical waveguide is formed on the portion of the substrate not containing a V-groove. A method is taught for cleaving the ends of the waveguide for forming the endface or endfaces thereof, for coupling to the ends of optical fibers.
It is known in the art to provide an integrated optic chip with one or more V-grooves formed in one portion of a substrate, and one or more waveguides formed in another portion of the substrate, with endfaces of the waveguide cores opposing respective ones of the V-grooves. An example of a substrate formed with a plurality of V-grooves and waveguide cores, as indicated, is shown in FIG. 1. More specifically, as shown, a substrate 1 has a plurality of juxtaposed V-grooves formed in a top first portion of the substrate 1. In an opposing top second portion of the substrate 1, a cladding layer 5. A plurality of waveguide cores 7 are formed within the cladding, as shown, with their longitudinal axes parallel to one another and to the parallel axes of the V-grooves 3. In the example shown, a dicing saw is used to form a transverse rectangular open slot 9 between the portions of the substrate 1 containing the V-grooves 3 and the waveguide cores 7. In this manner, the sloping endface walls of the V-grooves 3 are removed to provide vertical walls, for insuring that the ends of optical fibers 11 carried in the V-grooves 3 can abut endfaces 13 of the waveguide cores 7. However, such use of a dicing saw produces a rough surface on the endfaces of the waveguides 7, making it difficult to obtain high-efficiency coupling between the waveguide cores and optical fibers 11 carried in the V-grooves 3. Good coupling is precluded by light scattering caused by the rough surface on the waveguide endfaces 13 and sidewall 15 of the substrate 1. Polishing of the endfaces 13 of the waveguide cores 7 can serve to reduce the light scattering, and improve the efficiency of optical coupling thereto. However, such polishing is extremely difficult.
An object of the invention is to provide an efficient method for forming a smooth endface for a plurality of waveguides formed on a semiconductor substrate along with opposing V-grooves.
Another object of the invention is to provide a method for simultaneously cleaving a portion of the coupling ends of a plurality of waveguide layers or cores formed on a semiconductor substrate, in a manner avoiding cleaving of the substrate.
Yet another object of the invention is to provide a method for efficiently and simultaneously cleaving coupling end portions of a plurality of waveguide cores formed in cladding on a semiconductor substrate, for providing a smooth endface on the waveguide cores, thereby providing high-efficiency coupling to the ends of associated optical fibers or fiberoptic cables, respectively.
With the problems of the prior art in mind, the above-identified objects and other objects of the invention are satisfied in one embodiment by a method including the steps of initially forming a plurality of V-grooves in one top portion of a semiconductor substrate, with a plurality of waveguide cores equal in number to the number of V-grooves formed within a cladding layer or layers on another top portion of the semiconductor substrate, with endfaces of the waveguide cores opposing respective ones of ends of the V-grooves. A dicing saw or other means is used to form an open slotway between the portion of the substrate V-groove or grooves, and the portion of the substrate with the waveguide cores. In one embodiment, after using a dicing saw to form the open slotway, the portion of the substrate with the V-grooves is masked along with a portion of the open slotway. Then, an etchant is used to undercut through the sidewall of the portion of the substrate immediately below the cladding and waveguide core layers for undercutting the latter to provide a cantilevered waveguide portion. The mask material is removed, and a scribe line is formed across the transverse portion of the top surface of the cladding and waveguide core layers, with the scribe line preferably being parallel to the edge of the undercut portion of the substrate immediately underlying the cladding and waveguide core layers. Next, a downward force directed toward the V-grooves is applied to the cantilevered portion of the waveguide forward of the scribe line. The force cleaves the cladding and waveguide core layers, providing a smooth endface for the waveguide cores opposing respective V-grooves.
In a second embodiment of the invention, the open slotway is cut into the substrate at an angle via a dicing saw, followed by the aforesaid masking, etching, scribing, and cleaving steps.
In a third embodiment of the invention, the scribe line is formed on the top cladding layer forward of the top edge of the underlying substrate proximate the undercut portion of the substrate. In this third embodiment, a force is applied to the cantilevered portion of the cladding and waveguide core layers, causing these layers to breakaway leaving the endfaces Of the cladding and waveguide core layers at an angle.
In a fourth embodiment of the invention, the angled slotway is formed as previously indicated, but the scribe line is formed on the top cladding layer in alignment with the top edge of the underlying substrate proximate the undercut portion. This step is followed by masking, and then etching for forming the undercut. Next, the scribe line is formed in alignment with the top edge of the underlying substrate proximate the undercut portion, followed by cleaving to leave a smooth and vertically oriented endface for the cladding and waveguide core layers.
In a fifth embodiment of the invention, the cleaving force is provided by gluing the end of an object to the end of the cantilevered portion of the cladding and waveguide core layers. Setting of the glue is followed by pulling the object to apply a tension force on the cantilevered end (the force is longitudinally directed away from this end). Simultaneously a downward directed force can also be applied to the object proximate the region where it is connected to the cantilevered end of the waveguide and cladding layers, for cleaving the latter.
In a seventh embodiment of the invention, a high stress layer is formed on top of the cladding, with the front edge of the high stress layer being aligned with the edge of the substrate proximate the undercut region thereof. The high stress layer can be formed from an appropriate metal, such as nickel. Thereafter, the cantilevered portion of the cladding and waveguide layers can be cleaved by applying a downward force. In the latter embodiment, it is preferred but not absolutely necessary to use a scribe line before cleaving.