epdf.pub_nanotechnology-and-nanoelectronics-materials-devic

3.3 Applications of Nanodefects in Crystals 33
Early solutions to bulk isolation have been epitaxial deposition of the active
layer, e.g., n-type on p-type substrate, silicon-on-sapphire (SOS), and silicon-onoxide
(SOI). The latter includes versions like (i) oxygen implantation and SiO 2
formation, (ii) deposition of amorphous Si on SiO 2 and recrystallization, and (iii)
wafer bonding. All show specific pro and cons.
A newly established SOI procedure is based on the formation of point defects
(Fig. 3.21). A first wafer, A, is oxidized and implanted with hydrogen through the
oxide.
The implantation energy is selected in such a way that the ions come to rest under
the SiO 2 /Si interface after a few micrometers. The wafer is now placed headfirst
on a second wafer, B, so that the oxide comes in close contact with wafer B.
By suitable annealing, this contact is intensified and simultaneously the wafer A
splits at the place where the ions come to rest. After removing the main part from
wafer A, a configuration of silicon (wafer B), oxide, silicon-on-oxide (a remaining
thin layer of wafer A) remains. The active circuits are then manufactured on the
thin layer. The applied doses are about 10 17 cm 2 . It is shown [32] that plasma
hydrogenation reduces the required dosed by a factor of 10 (Fig. 3.22). This procedure
is called soft cut.
After hydrogen implantation the wafer is subsequently annealed (1000 °C, H 2
atmosphere) and, more importantly, hydrogenated with plasma. It is evident that a
dose of a few 10 16 cm 2 and the hydrogenation to a maximum concentration produce
the required 10 21 H/cm 3 . A saving within an order of magnitude is an
enormous gain in the production costs since the implantation is much more expensive
than the hydrogenation.
Fig. 3.22 Increase of hydrogen in the maximum position of the implantation profile by
hydrogenation. Black symbols: 1·10 15 , 1·10 16 , and 3·10 16 cm 2 hydrogen dose, E = 70 keV,
dark-gray symbols: 1·10 16 cm 2 helium dose, E = 300 keV, light-gray symbols: 1·10 15 ,
1·10 16 , and 1·10 17 cm 2 helium dose, E = 1 MeV. The concentrations were determined by
secondary ion mass spectroscopy [32].