Reliable optical pathways are a crucial prerequisite for optoelectronic integration. Light can be coupled into conventional optical fibers or allowed to travel in free space, but the alignment and fabrication problems associated with these approaches make them unsuitable for short-distance interconnects. And although the integration of waveguides onto wafers could provide an answer, such optoelectronic circuits are typically difficult to fabricate. Recently, however, researchers at the University of Southampton (England) have developed a technique that allows construction of waveguides using standard VLSI (very-large-scale integration) processes.
The new method involves etching directly into the silicon wafer.1 Layers of thermal and low-temperature oxide are grown to a total thickness of 2.5 µm and then patterned using photoresist and a dry etch. To see the effect of the process at a range of different widths, Southampton researchers used 60-mm lines varying from 1.25 to 7.25 µm thick. The oxide that remains after patterning is used as a mask to etch the silicon below. After etching, the wafer consists of 8.5-µm-tall silicon ridges topped by a layer of silicon dioxide. The entire area is then coated with 160 nm of silicon nitride, most of which is immediately removed by plasma etching.
What remains are the nitride sidewalls of the ridge and the dioxide layer—now rounded—on top, both of which are used as masks to allow the bases of the ridges to be undercut. The silicon is then dry-etched, bathed in ortho-phosphoric acid to remove the nitride, and oxidized just enough to isolate the waveguide from the substrate. Oxidation produces a rounding of the edges everywhere except at the base, where it leaves a spike. This does not seem to affect waveguide performance, and, if not closed up, it could act as a communications channel with optoelectronics on the chip.
Using computer modeling, the researchers demonstrated that the 4-µm-thick, 1-µm-cladding-fabricated waveguide is multimode, but could be made single-mode by reducing its dimensions. A HeNe laser at 1.528 µm was used to test the structure, which guided light successfully, although the waveguides were too short to get meaningful loss measurements from them. Researchers Greg Parker and Ian Johnston say they are working on longer waveguides and should be able to release loss measurements soon. Parker expects losses will be less than 1 dB/cm.
Besides being easy to manufacture, Southampton researchers say it is possible that their waveguides will be compatible with optical fibers. The sidewalls are angled at just 10° so their reflectivity is likely to be as low as 30%, depending on the quality of the etching. This may mean that the waveguides could be used without cleaving or polishing. One possible obstacle to this, however, is the oxide layer that covers the end face of the waveguide. Depending on how difficult it is to accurately reduce this layer to an antireflection thickness, it might be necessary to remove this covering.
REFERENCE
1. I. R. Johnston and G. J. Parker, Proc. IEEE Optoelectron. 143 (1), (Feb. 1996).