Note: Descriptions are shown in the official language in which they were submitted.
" ~Z2~33~
-- 1 --
SHALLOW-JUNCTION SEMICONDUCTOR DEVICES
Baclc~round of the Invention
.___ ___________ __________
This invention relates to shallow-junction semi-
conductor devices.
In various semiconductor devices, such as the
metal-oxide-semiconductor field-effect transistor (MOSFET),
it is desirable that a p-n junction of the device be as
close to a substrate surface as possible. The present
invention provides a means for obtaining p-n junctions
which are significantly closer to a semiconductor body
surface than was heretofore obtainable.
Summar~ of the Invention
__ ___ ____ _ _ ___ _ __. ____
A neutral species is initially implanted into a
surface region of a semiconductor body. In one example,
the neutral species is implanted to form a layer whose
maximum concentration occurs at a depth greater than that
of a p~n junction to be subsequently formed. (In another
example, the junction is subsequently established at a
depth that is greater than the depth of the peak con
centration of the neutral-species layer.) A dopant
species is then implanted into the surface region at a
depth less than the depth of the maximum-concentration
of the previously implanted neutral species. Annealing
to activate the dopant species is then ~arried out.
It is theorized that during this annealing step,
the neutral-species layer serves to getter point defects
in the body of the device. Additionally; this layer serves
as a physical barrier to diffusion of dopant species. As
a result, the diffusivity of the dopant species in the body
is significantly lowered relative to the case in which no
neutral-species layer is provided. In any event, the
result is that a p-n junction i5 formed in the body of
the device at an extremely shallow depth.
~2~ S
- la -
In accordance with an aspect of the in~ention
there is provided a shallow-~unction MOSFET device
comprising a semiconductor body having field and gate
portions on the surface of said body defining therebetween
surface areas overlying spaced-apart source and drain
regions of said device, a neutral-species layer of ions in
said body in each of said regions having a profile whose
peak concentration is at a predetermined depth below the
surface of said body, said neutral-species layer oE ions
beiny effective to limit the thermal diffusivity of active
species in said body during establishment of p-n junctions
therein, and spaced-apart p-n junctions in said body in
said respective regions at a depth below said surface that
is approximately one-tenth to two times that of said
predetermined depth.
In accordance with another aspect of the invention
there is provided a method of fabricating a MOSFET devise
in a semiconductor body, comprising the steps of forming
field and gate portions on the surface of said body to
define therebetween surface areas overlying spaced-apart
source and drain regions of said device, implanting a
neutral species through said surface areas to form a
neutral-species layer of ions having a peak concentration
at a predetermined depth below each of said surface areas
to limit the thermal diffusivity of active species in said
body during establishment of a p-n junction in each of
said regions; and introducing active species through said
surface areas into sald body to establish spaced-apart p-n
junctions in the respective regions of said body.
Brief Description of the Drawin~
FIGS. 1 through 4 are schematic representations
of a portion of a MOSFET device at successive stages of a
~Z2~33S
fabrication sequence that embodies the principles of the
present invention.
Detailed Descripti _
In accordance with this invention, shallow p-n
junctions can be formed in a variety of semiconductor
devices. These devices include, for example, p-n diodes,
bipolar transistors and MOSFET devices. By way of example,
the invention is described in connection with the provision
of shallow p-n junctions in a MOSFET device.
A portion of such a MOSFET device at an
intermediate stage of its fabrication cycle is shown in
F~G. 1, such portion comprising a known gate-and-source-and
-drain (GASAD) structure. The structure comprises a
silicon body 10 having field-oxide ~silicon dioxide)
portions 12, 14 thereon.
The structure further includes a gate-oxide
(silicon dioxide) layer 16, a doped polysilicon layer 18,
and a metallic~silicide (e.g. tantalum disilicide) layer
20. Also, the structure includes additional silicon
dioxide layers 22,24. Openings 25, 26 are defined by the
oxide layers 12, 22 and 14, 24. Source and drain regions
are later formed in the body 10 in approximate alignment
with these openings.
In accordance with the invention, a so-called
neutral species is implanted into regions of body 10
defined by the openings 25, 26. Known ion implantation
techniques can be used.
The term "neutral species" means ion species that
do not produce active carriers in the semiconductor body
and that are effective to limit the diffusivity of active
species in the body. Such neutral species include carbon,
oxygen/ argon or any other inert gas, Group IV elements
such as silicon, germanium and tin, and nitrogen (minor
activity).
Ions directed at the FIG. 2 structure are
represented by arrows 28, the ions reaching the body 10
only through the openings 25, 26. In FIG. 2, the depth of
2~33~
-- 3 --
the peak or maximum concentration of the approximately
Gaussian-shaped distribution o~ the neutral-species lmplant
in the body 10 is schematically depicted by lines 30
32 formed with x's.
~llustratively, the dosage of the neutral-species
implant represented in FIG. 2 is selected to provide
approximately one or two monolayers of the neutral species
at the peak-concentration depth. Additionally, for the
illustrative MOSFET device shown, the energy of the
incident ions is selected such that the peak concentration
of the implanted neutral species occurs approximately 2000
~ below the surface of the body 10. In some devices, the
peak concentration of the neutral-species implant is
selected to occur at a depth greater than that of the p-n
junction(s) to be subsequently formed in the body 10. In
such devices, the depth of the subsequently formed p-n
junction(s) is, for example, approximately one-tenth to
three-quarters that of the depth of the peak concentration
of the neutral species. (In other devices, described below,
the depth of the p-n junction(s) is greater than the depth
of the peak concentration of the neutral-species implant~)
Various dosages and energies can be used. One set
of dosage and energy values for the aforelisted neutral
species is as follows: carbon, 5 x 1015 ions per
~quare centimeter (i/cm~), 80 kilo-electron-volts (keV);
oxygen, 5 x 1015 i/cm2, 80 keV; silicon, 5 x
1015 i/cm2, 150 keV; germanium, 5 x 1015
i/cm2~ 300 keV; tin, 5 x 1015 i/cm2, 400 keV;
argon, 5 x 1015 i/cm2, 180 keV; and nitrogen, 5 x
1015 i/c~2, 80 keV. For these values, the
respective peak-concentration depth of each of the neutral
species is approximately 2000 A below the surface of the
body 10 shown in FIG. 2.
In some, but not necessarily all cases, the device
structure represented in FIG. 2 is next subjected to an
annealing step. ~For a subsequently introduced active
species such as arsenic, it may actually be advantageous
~2Z~3~
-- 4 --
not to anneal the implanted neutral species at this point
in the Eabrication procedure.) In this annealing step,
damage to the crystalline structure of the body 10 caused
by the neutral-species implant is reduced. Further, the
implanted species is stabilized and in effect locked in
place in the body 10. Also, some gettering of defects and
impurities in the body 10 typically occurs during this
annealing step.
The annealing is done, for example, at a
temperature in the range 700-to-~00 degrees Celsius in an
inert ambient for about one-half hour. During annealing, no
substantial vertical or lateral movement of the implanted
neutral species occurs. Nor does any substantial movement
occur later during the so-called activation annealing step
described below.
Next, an active species is introduced into the
structure by any of various known means, e.g., by ion
implantation, as indicated by the arrows 34 in FIG. 3. The
implanted active species comprisesr for example, a
pentavalent p-type impurity such as arsenic, phosphorus or
antimony, or a trivalent n-type impurity such as boron or
gallium.
The depth of the peak or maximum concentration of
the approximately Gaussian-shaped distribution of the
active-species implant in the body 10 is schematically
represented in FIG. 3 by lines 36, 38 formed with dots.
The peak concentration of the implanted active
species is selected to occur relatively close to the top
surface of the body 10, e.g., at a depth of approximately
200 to-100o ~.
More specifically, for a carbon neutral species
implant having a peak-concentration depth 30, 32 ~FIG. 3)
of about 2000 A, an arsenic implant having a peak-
concentration depth 36, 38 of approximately 200 A is
achieved by implanting 4 x 1015 i/cm2 at 30 keV. For
a carbon or nitrogen implant having a peak-concentration
depth 30, 32 (FIG. 3) of about 2000 A, a boron implant
22835i
having a peak-concentration depth 36, 38 of approximately
1000 A is achieved by implanting 4 x 1015 i/cm2 at 30
keV.
Subsequently, standard activation annealing of the
second-implanted or active dopant species is carried out.
During this step, ît is believed that gettering of point
defects and metallic impurities occurs in the body 10. It
appears also that, because of this gettering action, the
thermal diffusivity of the active species is substantially
reduced relative to what it would have been in the absence
of the neutral-species implant. Additionally, it appears
that the priorly formed neutral-species implant serves as a
physical barrier against diffusion of the active species.
However, regardless of the physical phenomena involved, the
result of the described process is that the p-n junction
formed by the active-species implant occurs at a depth far
less than it would have been if the neutral-species implant
had not been present. Extremely shallow p-n junctions are
thereby formed.
For an active species implant of arsenic,
activation annealing is carried out at, for examyle, abou~
1000 degrees Celsius for approximately three hours in a
standard mildly dry oxidi2ing atmosphere. The resulting p-n
junction is at a depth of approximately 1400 A. Without the
priorly implanted neutral-species, but with all other
processing conditions approximately the same, the p-n
junction is at a depth of about 3700 ~.
For an active-species implant of boron,
activation annealina is carried out at/ for example, about
900 degrees Celsius for approximately five hours in a
standard mildly dry oxidizing atmosphere. The p-n junction
occurs at approximately 3300 A. Without the presence of the
neutral-species implant, but with all other processing
conditions approximately the same, the p-n junction occurs
at a depth of about 6700 A.
In the example above in which the active species
comprises arsenic, the p-n junction is at a depth less than
3r~
-- 6 --
the depth of the peak concentration of an lmplanted neutral
species layer. Conversely, in the example above in which
the active species comprises boron, the p-n junction is at
a depth greater than the depth of the peak concentration of
the implanted neutral-species layer. In general, it is
feasible to form p-n junctions at a depth in the range of
approximately one-tenth to two times the depth of the peak
concentration of the implanted neutral-species layer.
It is possible to form the p-n junction at a depth
less than the depth of the peak concentration of the
implanted neutral-species layer or in a number of ways. For
example~ the active impurity species can be initially
introduced into the device structure at a shallower depth
than specified above for boron and/or by activation
annealing the structure at a lower temperature than
specified above. Or the peak concentration of the neutral-
species layer can be initially formed sufficiently deep
that, after annealing, the junction is established at a
depth less than the depth of the peak concentration of the
neutral-species layer.
