2.3.1 Crystalline Silicon

In semiconductor devices mainly two layers are made of crystalline silicon. On the one hand the initial wafer substrate. It is produced either by the Czochralski crystal pull method [19] or by the floating-zone crystal growth technique [19]. Impurities (dopants) are added to the silicon in order to set the resistivity of the wafer in a range from 0.1 $\Omega$cm - 50 $\Omega$cm. On the other hand often an epitaxial layer (same crystal structure as the underlying wafer) is grown on the substrate by a high temperature CVD process. These epitaxial layers are used to form buried layers or to put a lightly doped layer on top of a heavily doped substrate [19].

The atoms in crystalline silicon are arranged in a diamond lattice structure with a lattice constant of 5.4307Å. Fig. 2.13 and Fig. 2.14 show a model of the silicon crystal seen along the $<$110$>$ and the $<$100$>$ directions [45]. Along these crystalline directions the lattice atoms form channels with a diameter of approximately 3.3Å(0.6 of the lattice constant) and 1.6Å(0.3 of the lattice constant). These channels can be used by implanted ions to penetrate rather deep into the target (channeling effect), because the scattering probability is reduced for a particle moving along a channel.

Along a random direction, for example by tilting the wafer by 7 $\,^\circ\;$no channel can be recognized. Therefore these ion beam directions are preferred to generate shallow doping profiles.

Figure 2.13: Model of a silicon crystal seen along the $<$110$>$ direction.

Figure 2.14: Model of a silicon crystal seen along the $<$100$>$ direction.

The most important physical properties relevant for ion implantation are presented in Tab. 2.2 and discussed in depth in [19], [25], [66], [83].

Table 2.2: Physical properties of silicon at 25 $\,^\circ\;$C

Atomic number	14
Lattice constant	5.4307	Å
Atom radius	1.18	Å
Atomic density	5.02 $\cdot$1022	cm-3
Density	2328	kg/m-3
Thermal conductivity	0.46	W/cm K