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The incident ion beam could sometimes be perpendicular to one of the crystallographic planes in Si. In this case, due to less scattering along the path, the ion may penetrate much deeper than the normal depth predicted by random
Fig. 9.15. Channeling in single crystal Si.
scattering (Fig. 9.15). To avoid channeling, the beam may be sent at 3-7° to the normal.
Crystalline structure of Si is damaged by implants as silicon atoms are scattered randomly and displaced from their normal positions. This damage is to the extent that silicon becomes amorphous in a short span. Hence a post-implant anneal becomes essential to give enough thermal energy to Si to restore original crystal structure. Normally, an anneal of about 30 minutes at 850-900°C is adequate to remove the crystalline damage and activate dopants by moving them into substitutional sites. Nowadays, rapid thermal annealing (RTA), spike anneal or flash anneal is often used as thermal budget is quite limited for short channel MOSFETs. RTA for 30 seconds at around 980°C is typically sufficient. This anneal, however, will redistribute the impurities by diffusion and some of the implant advantages are adversely affected.
The removal of the damage alone is not the only reason for the thermal step. The dopant activation is also carried out in this step. The dopant activation is normally limited by the solid solubility of the dopant in Si and increases with increasing temperature. The highest achievable activation is normally on the order of 1020 cm-3.
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