Vertical mirror in a silicon photonic circuit

[0001] Embodiments of the present invention are directed to photonic circuits and, more particularly, to a vertical mirror in a photonic circuit for directing light normal to the surface of the circuit.

[0002] Silicon photonic circuits generally route optical signals in planar waveguides, and it is difficult to provide a path for light to enter/exit the circuits vertically. Routing light in or out of the wafer surface can be valuable for several reasons, such as coupling into a normal-incidence photodetector on the wafer surface, for wafer-level optical test/characterization, or other potential applications.

[0003] Of particular interest is the integration of planar silicon waveguides with Ge-based photodetectors. This is being addressed in several ways, all of which have various challenges. Planar photodetectors, in which the Ge is grown on top of the Si waveguide are quite large, because the optical coupling is inefficient and a long distance is needed for sufficient coupling of light to occur from the Si to the Ge.

[0004] To avoid this difficulty, trench sidewall photodetectors have been proposed, where the waveguide is terminated by a vertical facet. In this case, a facet with sufficient smoothness is difficult to form, and the epitaxial growth of the Ge-based photodetector can be very challenging.

[0005] Another area of interest is that of wafer-level optical testing. Currently, silicon photonic die must be singulated and have the edges polished in order to do optical testing; this is an expensive and time consuming process. The vertical mirror would enable rapid and inexpensive optical testing at the wafer level, dramatically reducing development times and testing costs.

[0014] Described is an apparatus and method comprising a total internal reflection (TIR) mirror that reflects light from planar silicon waveguides into the direction normal to the wafer surface (and vice-versa), enabling many such applications.

[0015] Embodiments described herein comprise a vertical TIR mirror and fabrication method thereof. The mirror may be made by creating a re-entrant profile using crystallographic silicon etching. Starting with an silicon on insulator (SOI) wafer, a deep silicon etch is used to expose the buried oxide layer, which is then wet-etched, opening the bottom surface of the Si device layer. This bottom silicon surface is then exposed so that in a crystallographic etch, the resulting shape is a re-entrant trapezoid with facets approximately 54° to the normal direction. These facets may be used in conjunction with planar silicon waveguides to reflect the light upwards based on the TIR principle. Alternately, light can be coupled into the silicon waveguides from above the wafer.

[0016] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0017] Referring now to FIG. 1, there is shown a top view and a side view of one embodiment of the inventive device in the first stages of fabrication. A device fabrication wafer (100) 100, comprising a silicon on insulator (SOI) wafer, sits atop a buried oxide layer (BOX) 102 on a handle wafer 104. The handle wafer 104 may be heavily boron-doped (˜10²⁰ cm⁻³) by implantation to prevent later etching.

[0018] FIG. 2 shows the device wafer 100 having a nitrite layer 200 patterned on its surface. As shown in FIG. 3, the Si wafer 100 may be etched down to the buried oxide (BOX) 102 to create a trench 300 using, for example a deep reactive ion etch (DRIE). In FIG. 4, the oxide layer 102 may be etched, such as by, for example, a wet etch in hydrofluidic acid (HF) to create an undercut area 400 beneath the Si device layer 100.

[0019] Referring now to FIG. 5, the wafer 100 may be immersed in a crystallographic etchant such as potassium hydroxide (KOH). In some embodiments, ammonium hydroxide (NH₄OH), ethylene diamene pyrocatechol (EDP) or tetramethyl ammonium hydroxide (TMAH) may be used. The bottom of the device layer 100 is exposed to the etchant, creating an inverted trapezoidal shape with a 54.7° angle to the normal. This angle may also be tuned by changing the etch conditions, such as composition or temperature, potentially creating a 45° facet. The etch terminates at the top silicon nitride layer 200. Assuming the handle wafer 104 is boron-doped as described earlier, it will not etch during this step; otherwise there will be a similar trapezoidal recess going downward into the handle wafer 100.

[0020] As shown in FIG. 6, the nitride layer 200 may be optionally removed to eliminate any overhang as shown in FIG. 5. The wafer 100 may be re-planarized by depositing and reflowing a thick oxide layer 600, for example. Alternately a polymeric material may be used for replanarization. Embodiments avoid the above challenges discussed above by creating an ultra-smooth mirror 500 defined by a re-entrant (111) crystal plane of the Si wafer 100, which reflects the light upward; thus optical signals can be coupled efficiently to the wafer surface.

[0021] At this point, the re-entrant TIR mirror 500 has been made, and the silicon photonic devices can be tested at the wafer level as illustrated in FIG. 7A. These mirrors 500 can be used in conjunction with planar silicon waveguides to reflect the light upwards based on the TIR principle. Alternately, light can be coupled into the silicon waveguides from above the wafer. Wafer-level optical test systems 701 can be utilized for batch testing without the need for cleaving and polishing before optical tests can be performed.

[0022] Alternately, as shown in FIG. 7B, a photodetector 700 may be formed on the surface of the wafer 100, positioned above the mirror 500. Such compact Ge photodetectors can be fabricated using standard surface processing, and furthermore a mirror 702, such as made from an aluminum or gold layer, may easily be fabricated on top of the photodetector, enabling a doubling in the detection efficiency as light passes through the Ge layer twice.

[0023] The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.

[0024] These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.