Epitaxy

Epitaxy or epitaxial growth is the process of depositing a thin layer (0.5 to 20 microns) of single crystal material over a single crystal substrate, usually through chemical vapor deposition (CVD).

In semiconductors, the deposited film is often the same material as the substrate, and the process is known as homoepitaxy, or simply, epi. An example of this is silicon deposition over a silicon substrate.

Silicon epitaxy is done to improve the performance of bipolar devices. By growing a lightly doped epi layer over a heavily-doped silicon substrate, a higher breakdown voltage across the collector-substrate junction is achieved while maintaining low collector resistance. Lower collector resistance allows a higher operating speed with the same current.

Epitaxy has also recently been used in CMOS VLSI circuits. By fabricating the CMOS device on a very thin (3-7 microns) lightly doped epi layer grown over a heavily-doped substrate, latch-up occurrence is minimized.

Aside from improving the performance of devices, epitaxy also allows better control of doping concentrations of the devices. The layer can also be made oxygen- and carbon-free. The disadvantages of epitaxy include higher cost of wafer fabrication, additional process complexities, and problems associated with defects in the epi layer.

The chemical vapor deposition of silicon epitaxy is usually achieved using an epitaxial reactor (Fig. 1) that consists of a quartz reaction chamber into which a susceptor is placed. The susceptor provides two things: 1) mechanical support for the wafers and 2) an environment with uniform thermal distribution. Epitaxial deposition takes place at a high temperature as the required process gases flow into the chamber.

Fig. 1. Example of an Epitaxial Reactor

Wafer Fab Links: Incoming Wafers; Epitaxy; Diffusion; Ion Implant; Polysilicon;

Dielectric; Lithography/Etch; Thin Films; Metallization; Glassivation; Probe/Trim

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