Silicon crystallograpy

Lattice Structure

The atoms in the crystal structure of the silicon lattice is arranged in a ‘diamond cubic’ formation. In this orientation, each silicon atoms has four neighbouring silicon atoms with which it is bonded with. The lattice constant—the distance between atoms in the atomic unit cell—is 5.431 Å, and the atomic density of the material is 5.02×1022 atoms/cm3.

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Figure 1: Schematic of the silicon diamond cubic lattice structure.

Identification of wafer Crystal Orientations for Semiconductor Wafers

The orientation of the surface crystal plane of a silicon wafer, as well as the doping polarity (i.e. either n-type or p-type) on wafers are practically impossible to visually identify. Therefore,  manufacturers sometimes impose wafers with distinct cuts and markings to help distinguish between p-type and n-type wafers of different orientations. This is particularly common for round wafers with a 4” diameter. The orientation and doping are particularly important to know for processing, such as chemical etching, where the etching process is highly dependent on the exposed crystal planes. Figure 2 below demonstrates one such technique for identifying wafers, noting that different variations may be used by different manufacturers. It should be noted that monocrystalline silicon wafers generally are selected to have a <100> surface crystal orientation as this enables the formation of microscopic pyramid formation during alkaline texturing processes.

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Figure 2: Schematic illustration of the primary and secondary flats used for the identification of silicon wafers.