Nano-Cavity
Structures Formed by Implantation into Silicon
(by Dr. Richard Hurley)
The
history of nano-cavities formed by implantation dates back to
early observations of problems
of swelling, exfoliation and release of gas that occurred in
nuclear reactors, particle accelerators and the walls of
experimental fusion devices. Of most concern was the
implantation of insoluble inert species that formed small
nanometer bubbles through a process of vacancy accumulation,
This phenomena is of considerable current interest for
applications in the semiconductor industry; helium and
hydrogen implantation into silicon with subsequent annealing
is used in SOI processing and we are investigating this for
layer transfer applications in a wide sense.
Figs 1(a)-(c) shows the
evolution of nanocavity development caused by annealing
helium implanted silicon to various temperatures. These TEM
photos show clearly the transition from platelet structures
which appear at 4000C to lenticular and spherical
cavities for temperatures higher than about 8000C.
Platelet structures are of importance in thermal splitting
of wafers and take different forms for hydrogen and helium
implantation. This is shown in Fig 2 where hydrogen and
helium have been co-implanted to slightly differing
projected ranges. Co-implantation of these gases is of
practical importance in lowering the total fluence
requirements for thermal splitting.
Fig 1
(a) to (d)
Fig 2.
When bonded
to another wafer, studies show thermal splitting will occur
at the H platelet line providing He implant is not
first heated (before bonding) above 400°C.
If He implant is first heated at 400-500°C,
wafer splits along plane of He nanocavities. Above initial
heating at 500°C,
partial splitting occurs.
Some applications of nanocavities
Gettering of
impurities for improving lifetime in devices.
A strain
relaxation layer for thin Si/SiGe on insulator, (<30 nm)
To reduce
roughness in smart-cut with SiGe.
Enhancement of
light-emitters and Si-based opto-electronics.
A cushion for
relieving stress in thicker films.
A weakening layer
for mechanical splitting (layer transfer)
