is defined as the process of removing device-degrading impurities from
the active circuit regions of the wafer. Gettering, which can be
performed during crystal growth or in subsequent wafer fabrication
steps, is an important ingredient for enhancing the yield of VLSI
mechanism by which gettering removes impurities from device regions may
be described by the following steps: 1) the impurities to be
gettered are released into solid solution from whatever precipitate
they're in; 2) they undergo diffusion through the silicon; 3) they are
trapped by defects such as dislocations or precipitates in an area away
from device regions.
There are two
general classifications of gettering, namely, extrinsic, and intrinsic.
refers to gettering that employs external means to create the damage or
stress in the silicon lattice in such a way that extended defects
needed for trapping impurities are formed. These chemically
reactive trapping sites are usually located at the wafer backside.
methods have been used to accomplish external gettering. For
instance, the introduction of mechanical damage by abrasion, grooving,
or sandblasting can produce stresses at the backside of a wafer, which
when annealed create dislocations that tend to relieve these stresses.
These locations can then serve as gettering sites. The main drawback of
this method, of course, is its tendency to initiate and propagate wafer backside microcracks
that may compromise the mechanical strength of the wafer.
phosphorus into the wafer backside is another technique used for
external gettering. P diffusion into silicon result in phosphorus
vacancies or dislocations that serve as trapping sites for impurity
atoms, such as gold. Another effect of P diffusion is the creation
of Si-P precipitates, which have been shown to be capable of removing Ni
impurities through interactions between Si self-interstitials and Ni
atoms, nucleating NiSi2 particles in the process.
of damage by laser is another external gettering method. Scanning
a laser beam across the wafer surface induce damage that is very similar
to mechanical damage, with the exception that the laser damage is
'cleaner.' Laser subjects the irradiated areas to thermal shock,
forming dislocation nests that serve as gettering sites.
bombardment to produce wafer backside damage is another method for
external gettering, this time using high-energy ions to induce the
necessary stress within the lattices of the wafer backside. Deposition of
a polysilicon layer on the wafer backside has also been used for
external gettering. Polysilicon layers introduce grain boundaries and
lattice disorder that can act as traps for mobile impurities.
gettering that involves impurity trapping sites created by precipitating
supersaturated oxygen out of the silicon wafer. The precipitation
of supersaturated oxygen creates clusters that continuously grow,
introducing stress to the wafer as this happens.
these stresses reach the point where they need to be relieved.
Dislocation loops or stacking faults are thus formed to provide the
necessary stress relief. These dislocations and faults
subsequently serve as trapping sites for impurities.
requirement of intrinsic gettering is, of course, starting wafers
that have sufficient, but not excessive oxygen levels (15-19 ppma).
advantages of intrinsic gettering over extrinsic gettering are: 1)
it does not require subjecting the wafer to any treatment except for
heating; 2) its volume of impurity sink is significantly
larger than that of external gettering on the wafer backside; 3) its
gettering regions are much closer to the device regions.
Incoming Wafers; IC
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