Generating nitride waveguides

Polysilicon formed over an underlying insulator may be highly selectively etched. Therefore, polysilicon may be selectively etched using tetraalkylammonium hydroxide or NH4OH to define a nitride waveguide. The resulting nitride waveguide may have smoother surfaces resulting in less loss of light intensity as light travels through the nitride waveguide.

BACKGROUND

This invention relates generally to forming nitride waveguides.

Nitride waveguides may be utilized to communicate information in the form of light signals between a transmitter and a receiver. For example, two integrated circuits on the same circuit board may be coupled through a nitride waveguide to communicate with one another.

The existing nitride waveguides have suffered diminution of intensity due to the irregular configuration of the nitride waveguide. The irregular configuration of the nitride waveguide and, primarily, its sidewalls is largely due to the techniques used to form the waveguide. Conventional etching techniques result in a fairly rough exterior surface of the resulting nitride waveguide.

The effect of the rough exterior surface of the nitride waveguide is to cause losses in the course of transmitting light information through the waveguide. As light propagates along existing waveguides, light is scattered and refracted out of the waveguide, diminishing the intensity of the light that exits the waveguide.

Thus, there is a need for ways to reduce the loss of light intensity as light propagates through nitride waveguides.

DETAILED DESCRIPTION

Referring toFIGS. 1 through 4, a nitride waveguide with reduced line edge roughness may be prepared in one embodiment of the present invention using highly selective polysilicon etching. The resulting structures may exhibit reduced light loss, for example, for use in connection with optical interconnects.

Referring toFIG. 1, a structure may include a silicon substrate30covered by a dielectric layer28, in turn covered by polysilicon26in accordance with one embodiment of the present invention. The layer28may be any dielectric including oxide. An opening32may be patterned in the polysilicon26without adversely affecting the underlying dielectric28. The etch utilized to remove polysilicon26to form the opening32, as shown inFIG. 2, may be any conventional etch, such as a dry etch with a lithographically patterned photoresist layer on top of the polysilicon26.

Thereafter, the opening32may be filled with a nitride material34as shown inFIG. 3. Apolishing step may be utilized to polish the nitride34back to the surface of the polysilicon26. After the formation of the nitride material34, the surrounding polysilicon26may be removed, as shown inFIG. 4, using a highly selective etch such as tetraalkylammonium hydroxide or an NH4OH etch. For example, the tetraalkylammonium hydroxide may be tetramethylammonium hydroxide or tetraethylammonium hydroxide, as two examples. In some embodiments, a 25 percent solution of tetramethylammonium hydroxide may be used.

In other embodiments NH4OH at 24° C., together with sonic energy, may be utilized. Thus, a room temperature etching process may be utilized with ultrasonic or megasonic energy.

Hydroxide-based etches of silicon are normally done at high temperatures which reduces the selectivity of silicon versus nitride and/or underlying dielectric. At low temperatures the NH4OH is ineffective because hydrogen gas bubble formation at the structure being etched generates a protective silicon-hydrogen coating that shuts down the etch. However, by sonicating the chemical bath, H2bubble formation may be reduced or eliminated, dissipating the H2into solution upon formation allowing completion of the etch.

The selective etch does not attack the nitride which will become the waveguide or the underlying dielectric layer28.

In some embodiments a dry etch may be utilized to pattern the polysilicon and a highly selective wet etch may be utilized to remove the polysilicon. A waveguide formed of nitride may be created with nearly orthogonal edges and smooth walls, as shown inFIG. 4, in some embodiments of the present invention.

Referring toFIG. 5, a final optically compatible encapsulation layer36may be completed using an interlayer dielectric material36. The layer36may be deposited, for example, and the structure may be polished to form the smooth upper surface shown in FIG.5. By using sacrificial polysilicon to define the sidewalls of the nitride waveguide34, the dry etch/patterning of polysilicon is smoother and more consistent. Therefore, when the nitride is deposited and subsequently polished back to the desired level, the resulting nitride structure may have smoother sidewalls, leading to reduced scattering compared to patterning a nitride layer directly with a dry etch or negatively patterning an insulating material into which the nitride is deposited.