Crystal Orientation and Defects in Silicon Wafers

What Defines the Crystal Orientation of Silicon?

The basic crystal unit cell of monocrystalline silicon is the zinc blende structure, in which each silicon atom bonds chemically with four neighboring silicon atoms. This structure is also found in monocrystalline carbon diamonds. 

Figure 2: Unit Cell of Monocrystalline Silicon Structure

Crystal orientation is defined by Miller indices, representing directional planes at the intersection of the x, y, and z axes. Figure 2 illustrates the <100> and <111> crystal orientation planes of cubic structures. Notably, the <100> plane is a square plane as shown in Figure 2(a), while the <111> plane is triangular, as depicted in Figure 2(b).

Figure 2: (a) <100> Crystal Orientation Plane, (b) <111> Crystal Orientation Plane

Why is the <100> Orientation Preferred for MOS Devices?

The <100> orientation is commonly used in the fabrication of MOS devices.

Figure 3: Lattice Structure of the <100> Orientation Plane

The <111> orientation is favored for manufacturing BJT devices due to its higher atomic plane density, making it suitable for high-power devices. When a <100> wafer breaks, fragments typically form at 90° angles. In contrast, <111> wafer fragments appear in 60° triangular shapes.

Figure 4: Lattice Structure of the <111> Orientation Plane

How is Crystal Direction Determined?

Visual Identification: Differentiation through morphology, such as etch pits and small crystal facets.

X-ray Diffraction: Monocrystalline silicon can be wet-etched, and defects on its surface will form etch pits due to a higher etching rate at those points. For <100> wafers, selective etching with KOH solution results in etch pits resembling a four-sided inverted pyramid, as the etching rate on the <100> plane is faster than on the <111> plane. For <111> wafers, etch pits take the shape of a tetrahedron or a three-sided inverted pyramid.

Figure 5: Etch Pits on <100> and <111> Wafers

What are the Common Defects in Silicon Crystals?

During the growth and subsequent processes of silicon crystals and wafers, numerous crystal defects can occur. The simplest point defect is a vacancy, also known as a Schottky defect, where an atom is missing from the lattice. Vacancies affect the doping process since the diffusion rate of dopants in monocrystalline silicon is a function of the number of vacancies. An interstitial defect forms when an extra atom occupies a position between normal lattice sites. A Frenkel defect arises when an interstitial defect and a vacancy are adjacent.

Dislocations, geometric defects in the lattice, may result from the crystal pulling process. During wafer manufacturing, dislocations relate to excessive mechanical stress, such as uneven heating or cooling, dopant diffusion into the lattice, film deposition, or external forces from tweezers. Figure 6 displays examples of two dislocation defects.

Figure 6: Dislocation Diagram of Silicon Crystal

The density of defects and dislocations on the wafer surface must be minimal, as transistors and other microelectronic components are fabricated on this surface. Surface defects in silicon can scatter electrons, increasing resistance and impacting component performance. Defects on the wafer surface reduce the yield of integrated circuit chips. Each defect has some dangling silicon bonds, which trap impurity atoms and prevent their movement. Intentional defects on the wafer’s backside are created to capture contaminants within the wafer, preventing these mobile impurities from affecting the normal operation of microelectronic components.**

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