1. Field of the Invention
The invention relates generally to photonic structures. More particularly, the invention relates to photonic waveguide structures with enhanced performance.
2. Description of the Related Art
Microelectronic devices have conventionally included purely electronic devices such as transistors, resistors, capacitors and diodes that may be fabricated within and upon a semiconductor substrate. Such purely electronic microelectronic devices are often connected and interconnected with electrical conductors, to thus provide electrical circuits upon the semiconductor substrate.
As microelectronic fabrication technology has evolved and advanced, the advent of optoelectronic fabrication technology, that includes photonic devices and photonic structures also located upon a semiconductor substrate, has evolved. Photonic devices may include, but are not limited to photoemissive devices such as but not limited to conventional light emitting diodes and laser diodes. Photonic devices may also include, but are also not limited to, photoreceptive devices such as photodiodes and charge coupled devices. In contrast with microelectronic fabrication where microelectronic devices are connected and interconnected with electrical conductors, photonic devices are generally connected and interconnected using photonic coupling devices and photonic waveguides. Photonic waveguides in particular are often linear photonic structures that have specific cross-sectional dimensional requirements and index of refraction requirements for effective transmission of a particular photonic wavelength.
While photonic device fabrication, including in particular photonic waveguide structure fabrication, provides genuine advances and advantages with respect to conventional microelectronic device fabrication including patterned conductor layer fabrication, photonic device fabrication including photonic waveguide structure fabrication, is not entirely without problems. In particular, insofar as photonic waveguide structures are often conventionally located and fabricated upon an exposed surface of a substrate, for purposes of ease in photonic coupling to a photonic waveguide structure, photonic waveguide structures are often quite susceptible to environmental exposure that may lead to compromised performance of a photonic circuit that includes the photonic waveguide structure.
In addition, for photonic waveguide devices to be integrated into a fully developed chip technology, multiple layers of waveguides, active devices, and wiring must be formed in multiple layers. Thus a robust technology is needed for encapsulating waveguides in good quality cladding material and for achieving accurate planarization to facilitate later lithography steps. This is particularly important for antenna-coupled tunnel junction devices, where metal antennas are closely juxtaposed with the top surface of a planarized silicon waveguide, with a thin dielectric layer in-between.
Various, photonic waveguide and related structures having desirable properties, and methods for fabricating the photonic waveguide and related structures, are known in the optoelectronic fabrication art.
For example, McNab et al., in “Ultra-low noise photonic integrated circuit with membrane-type photonic crystal waveguides,” Optics Express, Vol. 11 (22), pp. 2927-39 (3 Nov. 2003), teaches a photonic waveguide structure comprising a silicon material. In particular, the photonic waveguide structure may be efficiently fabricated from a silicon-on-insulator (SOI) semiconductor substrate while using conventional semiconductor fabrication methodology.
In addition, Lin et al., in “Multi-User Hybrid Process Platform for MEMS Devices Using Silicon-on-Insulator Wafers,” 18th IEEE International Conference on Micro Electro Mechanical Systems, MEMS 2005, pp. 516-19, teaches a hybrid surface and bulk micromachining process using silicon-on-insulator substrates for fabricating various structures, including photonic waveguide structures. The process uses doped polysilicon for non-critical structures and single crystal silicon for critical structures.
Further, Hattori et al., in “Heterogeneous Integration of Microdisk Lasers on Silicon Strip Waveguides for Optical Interconnects,” IEEE Photonics Technology Letters, Vol. 18(1), Jan. 1, 2006, pp. 223-26, teaches an optical interconnect between a compound laser and a silicon-on-insulator based strip waveguide.
Finally, Kimerling et al., in Electronic-photonic integrated circuits on the CMOS platform,” Proc. SPIE 2006, Jan. 25-26, 2006, teaches integrated electronic and photonic structures, including waveguide structures, that may be fabricated using CMOS process technology.
Due to their potential for ease in integration with purely electronic microelectronic devices, as well as their reduced size and their high routability, photonic devices and photonic structures are likely to be of continued importance as microelectronic fabrication technology advances. In light of such continued importance, desirable are photonic devices and photonic structures, such as in particular photonic waveguide structures, that may be readily fabricated with enhanced performance.
The invention comprises a photonic waveguide structure readily fabricated with enhanced performance. Within the photonic waveguide structure, a photonic waveguide layer that is located over a substrate is sidewall cladded with a sidewall cladding layer that is also located over the substrate. The sidewall cladding layer does not cover a top surface of the photonic waveguide layer. In addition, a top surface of the photonic waveguide layer is typically no higher than a top surface of the sidewall cladding layer. The photonic waveguide structure in accordance with the invention thus provides a photonic waveguide structure where a photonic waveguide layer does not stand alone and rise above a substrate. When such a photonic waveguide layer within a photonic waveguide structure does stand alone and rise above a substrate absent an adjoining sidewall cladding layer, the photonic waveguide layer within the photonic waveguide structure is susceptible to environmental induced damage that may compromise the performance (i.e., including but not limited to yield and reliability) of the photonic waveguide structure.
The photonic waveguide structure in accordance with the invention may be fabricated while using a hard mask layer that serves sequentially as: (1) an etch mask for etching a photonic waveguide material layer located over a substrate to form a photonic waveguide layer located over the substrate; and (2) a planarizing stop layer when planarizing a blanket sidewall cladding layer located over the hard mask layer and the photonic waveguide layer to form the sidewall cladding layer that clads the sidewall of the photonic waveguide layer, but does not cover the top surface of the photonic waveguide layer or the hard mask layer.
A particular photonic waveguide structure in accordance with the invention includes a first photonic waveguide layer located over a substrate. This particular photonic waveguide structure also includes a sidewall cladding layer located cladding a sidewall of the first photonic waveguide layer. A top surface of the first photonic waveguide layer is not covered by the sidewall cladding layer.
A particular method for fabricating the photonic waveguide structure includes etching a photonic waveguide material layer located over a substrate, while using a hard mask layer located upon the photonic waveguide material layer as an etch mask, to provide a photonic waveguide layer located interposed between the hard mask layer and the substrate. This particular method also includes forming a blanket cladding layer over the substrate and covering the hard mask layer and the photonic waveguide layer. This particular method also includes planarizing the blanket cladding layer, while using the hard mask layer as a planarizing stop layer, to form a sidewall cladding layer that clads a sidewall of the photonic waveguide layer but does not cover a top surface of the photonic waveguide layer.
The objects, features and advantages of the invention are understood within the context of the Description of the Preferred Embodiment, as set forth below. The Description of the Preferred Embodiment is understood within the context of the accompanying drawings, that form a material part of this disclosure, wherein:
The invention, which includes a photonic waveguide structure, and a related method for fabricating the photonic waveguide structure, is understood within the context of the description provided below. The description provided below is understood within the context of the drawings described above. Since the drawings are intended for illustrative purposes, the drawings are not necessarily drawn to scale.
The base substrate 100 (which is optional within the instant embodiment when the dielectric layer 101 has a thickness sufficient to provide independent structural integrity to a photonic waveguide structure in accordance with the instant embodiment) may comprise any of several substrate materials. Non-limiting examples of substrate materials include conductor materials, semiconductor materials and dielectric materials. Further non-limiting examples of semiconductor materials include silicon, germanium, silicon-germanium alloy, silicon-germanium-carbon alloy and compound semiconductor materials. Further non-limiting examples of compound semiconductor materials include gallium arsenide, gallium phosphide and indium phosphide semiconductor materials.
The dielectric layer 101 generally comprises a conventional dielectric material, and in particular a conventional dielectric material that has an index of refraction less than a material from which is comprised the photonic waveguide material layer 102. Typically, such an index of refraction will be less than about 3 and more preferably from about 1.4 to about 1.5. Oxides of silicon are preferred dielectric materials for the dielectric layer 101, but such dielectric materials by no means limit the invention. Thus, oxides of other elements, as well as other elemental compositions, alternatively may be used. Examples of such alternative materials of appropriate dielectric constant include silicon nitride. Typically, the dielectric layer 101 has a thickness from about 300 to about 300000 angstroms.
The photonic waveguide material layer 102 is intended to comprise a photonic waveguide material. Candidate photonic waveguide materials may include at least some of the several semiconductor materials from which may be comprised the base substrate 100. Other suitable photonic waveguide materials (i.e., that typically have a refractive index greater than about 2 and more preferably from about 3.4 to about 4.0) are not excluded. Specific non-limiting examples of photonic waveguide materials include silicon materials and indium phosphide materials, of which a silicon photonic waveguide material is preferred. Typically, the photonic waveguide material layer 102 has a thickness from about 300 to about 30000 angstroms.
Finally, also illustrated in
In a first instance, the photonic waveguide structure whose schematic plan-view diagram is illustrated in
Similarly, the photonic waveguide structure whose schematic plan-view diagram is illustrated in
The particular taper of the tapered photonic waveguide layer 102″ may be effected using sidewall etching and undercutting of a waveguide layer that is initially formed beneath a hard mask layer that itself is tapered in an aerial dimension. Such sidewall etching may be effected using a wet chemical isotropic etchant or a dry plasma trim etch process.
The second photonic waveguide layer 105 may comprise any of several optically compatible photonic waveguide materials that properly optically mate with the first photonic waveguide layer 102″, typically by having a refractive index intermediate between those of the first photonic waveguide layer 102″ and the sidewall cladding layer 104′. Such photonic waveguide materials may include, but are not limited to inorganic waveguide materials, organic waveguide materials and composite waveguide materials that have an index of refraction in an appropriate range for optimal optical coupling. Typically, the second photonic waveguide layer 105 comprises a polymer waveguide material that has a thickness T and a width W that also may in-part be defined by an intended end use of the photonic waveguide structure in accordance with the invention. Again, end uses in a 1.55 micron photonics communication band may be common, but are by no means limiting.
The preferred embodiment is illustrative of the invention rather than limiting of the invention. Revisions and modifications may be made to methods, materials, structures and dimensions of a photonic waveguide structure in accordance with the preferred embodiment, while still providing a photonic waveguide structure in accordance with the invention, further in accordance with the accompanying claims.