Appeal No. 2006-1797
Application No. 09/866,319
thickness of a few tens of nanometers. The second
layer 81, formed by silicon/germanium, is then grown
epitaxially. It is formed by a first sublayer of Si1-x
Gex, with x for example a constant lying between 0.1
and 0.2, on top of which there is a second sublayer,
also formed by an Si1-xGex alloy (with x decreasing to 0)
and P doped with boron. The total thickness of the
layer 81 is moderate, typically from 20 to 100 nm. An
epitaxial layer 82, having a thickness of a few tens of
nanometers and made of silicon P doped using boron, is
then placed on top of the second sublayer of the layer
81.
At column 3, line 44 to column 4, line 5, Marty states the
following:
This stack of layers will make it possible to form a
silicon/germanium heterojunction base. It should be
noted here that the epitaxy for producing the
heterojunction base is nonselective epitaxy. This
silicon nitride layer also makes it possible to obtain
good thickness uniformity of the epitaxial deposition
of the base. It also makes it possible to obtain a
peak/trough level difference on the surface of the
stack on the order of 500 to 600 Å (whereas this level
difference is on the order of 1000 Å. with an initial
layer of amorphous silicon).
Next, a first layer 9 of silicon dioxide having a
thickness on the order of 200 Å is deposited on the
layer 81. A second layer 10 of silicon nitride (Si3N4)
having a thickness of 300 Å is also deposited on the
first silicon dioxide layer 9. Next (FIG. 3), a zone
100 in the nitride layer 10 corresponding to an emitter
window lying above the intrinsic collector 4 is defined
with the aid of a mask. Plasma etching of the nitride
layer 10 with termination on the silicon dioxide layer
9 is then carried out in the conventional way, with the
aid of a resin layer corresponding to the mask, so as
to expose the zone 100. Next, keeping the resin which
is present on the layer 10 and has been used in etching
the layer 10, implantation of phosphorus is carried out
through the stack. Selective overdoping of the
collector (selective implantation collector) under the
window of the emitter can be carried out in one or more
implantation steps, thus contributing to an increase in
the speed of the transistor by reducing the resistance
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