U. Glaeser

FIGURE 2.85 Main process steps of ITOX-SIMOX.
SIMOX Substrates
For SIMOX substrates, the buried oxide (BOX) layer is formed by the implantation of a large quantity
of oxygen ions at energies of about 200 keV followed by annealing at high temperatures above 1300°C,
as shown in Fig. 2.85 [4]. Because the amount of oxygen implanted and the implantation energy are
controlled electronically with high accuracy, there is excellent control of the uniformity of the thickness
of the SOI layer and the BOX layer. A substrate obtained by high-dose oxygen implantation in the order
of 10 18 cm 2 is called a high-dose SIMOX substrate and has a BOX layer thickness of about 400–500 nm.
The presence of 10 8 cm 2 or more dislocation density in the SOI layer and the long period of time required
for the high-dose oxygen ion implantation create problems with respect to the quality of the SOI layer
and the cost and mass producibility of the substrate. On the other hand, it has been discovered that if
the oxygen ion implantation dose is lowered to about 4 × 10 17 cm 2 , there are dose regions in which the
dislocation density is reduced to below 300 cm 2 , resulting in high quality of the SOI layer and lower
substrate cost [12]. Such a substrate is referred to as a low-dose SIMOX substrate. However, the BOX
layer of this substrate is thin (about 90 nm), making it necessary to reduce the number of pinholes and
other defects in the BOX layer. In later studies, it was found that a further high-temperature oxidation
at over 1300°C after high-temperature annealing results in the formation of a thermal oxide layer at
the interface between the SOI layer and BOX layer at the same time as the oxidation of the SOI layer
surface [13]. Typically, the BOX layer thickness is increased by about 40 nm. A substrate produced with
this internal oxidation processing is referred to as an ITOX-SIMOX substrate. In this way, an SOI layer
can be formed over an oxide layer of high quality, even on SIMOX substrates formed by oxygen ion
implantation.
ELTRAN Substrates
Although thin-film SOI substrates for fine CMOS devices are categorized as either SIMOX substrates or
wafer bonded substrates, as shown in Fig. 2.84. ELTRAN substrates are classified as BESOI (Bond and
Etch-back SOI) substrates, a subdivision of the bonded substrate category. The BESOI substrate is
produced by the growth of a two-layer structure that consists of the final layer that remains on the DW
as the SOI layer and a layer that has a high etching speed by epitaxial growth followed by the formation
of a thermal oxide layer on the surface and subsequent bonding to the HW. After that, most of the
substrate is removed from the backside of the DW by grinding and polishing. Finally, the difference in
etching speed is used to leave an SOI layer of good uniformity. The fabrication process for an SOI substrate
produced by the ELTRAN method is shown in Fig. 2.86 [14]. First, a porous Si layer that comprises two
layers of different porosities is formed by anodization near the surface of the Si substrate on which the
devices are formed (DW). After smoothening of the wafer surface by annealing in hydrogen to move the
surface Si atoms, the layer that is to remain as the SOI layer is formed by epitaxial growth. After forming
the layer that is to become the buried oxide layer by oxidation, the DW is bonded to the HW. Next, a
water jet is used to separate the DW and HW at the boundary of the two-layer porous Si layer structure.
Finally, the porous Si layer is removed by selective chemical etching of the Si layer, hydrogen annealing
© 2002 by CRC Press LLC
O + ions
Si Substrate
Thermal Oxide
SOI (320 nm)
BOX (80 nm)
Si Substrate
O + Ions implantation High temperature annealing
>1300 C
Thermal Oxide
SOI (62 nm)
BOX (120 nm)
Si Substrate
Internal thermal oxidation
>1300 C