Silicon Photonics: Fundamentals and Devices

13.6 Lithography

The method of forming an epitaxial layer has been described in Section 13.3.2. A planar waveguide structure may be realized by this process. However, to fabricate a 3D waveguide, the layer should be etched to form a rib or ridge structure. The width of the rib waveguide is primarily determined by the lithographic definitions. The same process is also needed to define patterns for other active and passive devices.

The process of pattern transfer is known as lithography. Basically it is a two-step process. In the first stage a mask, normally a glass plate containing an opaque pattern in chromium metal, is made. In the second stage the features on this master are copied onto the substrate by a form of printing.

13.6.1 Mask Fabrication

The mask fabrication process by using e-beam lithography is described here. First, the design dimensions of the device are specified, and its salient features are assigned to one of a number of mask levels.

13.6.2 Pattern Transfer

In the second stage, the pattern is transferred from the mask to the substrate using an optical printing step known as photolithography. It is based on the properties of light-sensitive organic materials or photoresists (PRs), whose behavior is qualitatively similar to the electron resists that have been described. Both negative and positive optical resists exist. In the former, exposure to ultraviolet (UV) light polymerizes the material, and in the latter, cross-links are broken by the exposure. Figure 13.14 ...