Crystal Defects in Silicon - Page 1 of 3

Crystalline Defects in Silicon

Like anything else in this world, crystals inherently possess imperfections, or what we often refer to as 'crystalline defects'. The presence of most of these crystalline defects is undesirable in silicon wafers, although certain types of 'defects' are essential in semiconductor manufacturing. Engineers in the semiconductor industry must be aware of, if not knowledgeable on, the various types of silicon crystal defects, since these defects can affect various aspects of semiconductor manufacturing - from production yields to product reliability.

Crystalline defects may be classified into four categories according to their geometry. These categories are: 1) zero-dimensional or 'point' defects; 2) one-dimensional or 'line' defects; 3) two-dimensional or 'area' defects; and 4) three-dimensional or 'volume' defects. Table 1 presents the commonly-encountered defects under each of these categories.

Table 1. Examples of Crystalline Defects

Defect Type	Examples
Point or Zero-Dimensional Defects	Vacancy Defects Interstitial Defects Frenkel Defects Extrinsic Defects
Line or One-Dimensional Defects	Straight Dislocations (edge or screw) Dislocation Loops
Area or Two-Dimensional Defects	Stacking Faults Twins Grain Boundaries
Volume or Three-Dimensional Defects	Precipitates Voids

There are many forms of crystal point defects. A defect wherein a silicon atom is missing from one of these sites is known as a 'vacancy' defect. If an atom is located in a non-lattice site within the crystal, then it is said to be an 'interstitial' defect. If the interstitial defect involves a silicon atom at an interstitial site within a silicon crystal, then it is referred to as a 'self-interstitial' defect. Vacancies and self-interstitial defects are classified as intrinsic point defects.

If an atom leaves its site in the lattice (thereby creating a vacancy) and then moves to the surface of the crystal, then it becomes a 'Schottky' defect. On the other hand, an atom that vacates its position in the lattice and transfers to an interstitial position in the crystal is known as a 'Frenkel' defect. The formation of a Frenkel defect therefore produces two defects within the lattice - a vacancy and the interstitial defect, while the formation of a Schottky defect leaves only one defect within the lattice, i.e., a vacancy. Aside from the formation of Schottky and Frenkel defects, there's a third mechanism by which an intrinsic point defect may be formed, i.e., the movement of a surface atom into an interstitial site.

Extrinsic point defects, which are point defects involving foreign atoms, are even more critical than intrinsic point defects. When a non-silicon atom moves into a lattice site normally occupied by a silicon atom, then it becomes a 'substitutional impurity.' If a non-silicon atom occupies a non-lattice site, then it is referred to as an 'interstitial impurity.' Foreign atoms involved in the formation of extrinsic defects usually come from dopants, oxygen, carbon, and metals.

The presence of point defects is important in the kinetics of diffusion and oxidation. The rate at which diffusion of dopants occurs is dependent on the concentration of vacancies. This is also true for oxidation of silicon.

Ion Implant; Gettering; Crystal Growing

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