Silicon Photonics: Fundamentals and Devices

13.2 Planar Processing

The starting point in planar processing is a suitably oriented silicon wafer, polished flat and cleaned, to be used as the substrate. Thin layers of the same or different materials are then deposited on this substrate, and the desired pattern is then delineated on the grown layer. The deposited layer may be either amorphous or crystalline, and, depending on its nature, different techniques are employed to grow them. In an alternative scheme, the surface property of the substrate can be modified by diffusion or ion implantation of different impurities. To define a pattern on the top layer, different etching techniques are employed. The pattern definition in a specified region on the surface may be accomplished by a process known as lithography, which itself involves the deposition and removal of materials.

The dimensions involved in most optoelectronic devices are of the order of microns; nearly all the processing steps are carried out in a clean room, in which filtered air is circulated to remove dust particles. The rooms are kept at constant temperature and humidity to ensure process reproducibility and reliability. Similarly, the services to the room (e.g., water and gases) are also highly purified and workers inside wear special clothing to keep the contaminants to the minimal level. These precautions greatly improve the yield and reliability of the devices.

After completion of the process steps, the devices are packaged and sealed and electrical contacts ...