Damage formation and annealing studies of low energy ion implants ...

Chapter 3 Damage and Annealing processes
3.1 Introduction
Ion implantation into Si results in the displacement of Si atoms, i.e. damage to
the Si lattice. While simple atomic displacement can be described in terms of the atomic
theory summarised in Chapter 2, the role of energy deposition density in determining
the nature of the damage is more complex. This chapter describes the damage effects
produced during implantation and different types of model for damage build up and
amorphisation. To produce the electrically active doped regions, post implantation
annealing is required to restore the crystal lattice and enable the dopants to take up
substitutional lattice sites. The process by which the crystal lattice structure is restored
during annealing in most device production, i.e. solid phase epitaxial regrowth (SPER),
is described. Defect evolution also occurs during annealing and a brief summary of
these aspects is given. Defects still present after annealing can be detrimental to
transistor device performance in that they can introduce unwanted leakage currents and
decreasing carrier mobility, i.e. the proportionality constant between the electric field
and the electron drift velocity (1). Diffusion of the implanted ions to greater depths
during annealing needs to be minimised for shallow junction depths (Xj). The evolution
of the defects at the end of the range (EOR) of the implant can cause an anomalous
enhancement of dopant diffusion called transient enhanced diffusion (TED). The effects
discussed in this chapter shows that optimised annealing conditions must be a
compromise between conditions that result in high levels of electrical activation with
minimal defects and conditions that minimise the dopant diffusion.
3.1.1 Si crystal structure and its properties
The Si crystal lattice has a diamond – type structure. This can be considered to
be composed of two interpenetrating fcc lattices offset by a ¼ lattice cell along each of
the [100], [010] and [001] axes (2, 3). Each Si atom is covalently bound to four others
in a tetrahedral arrangement sharing two valence electrons of the sp 3 shell per bond (3)
with a typical bond angle of 109.47° (4). The conventional unit cell contains 8 atoms
and has a lattice constant, a, of 5.4 Å (2, 3). The distance of nearest neighbour spacing is
√3/4a = 2.4 Å and the next nearest neighbour is 3.8 Å (3). The Si unit cell is shown in
Figure 3.1. The atomic density of Si is 5.032 x 10 22 atoms/cm 3 at room temperature (3).
The (100), (110) and (111) planes are shown in Figure 3.2 respectively. Si has three
isotopes, 28 Si, 29 Si and 30 Si, with 28 Si the most abundant isotope with a natural
30