Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
The present invention is directed to a method of
forming thin films of silicon nitrides and other nitrogen-
containing compositions, such as oxynitrides, directly on a
silicon surface, and is further concerned with the application
of such films.
Thin films of silicon nitride have two significant
applications in the field of integrated circuits. Since they
exhibit a higher dielectric constant and hence a higher unit
capacitance than silicon dioxide layers, their use as capacitor
dielectrics in small dimension MOS circuits is preferred. The
increased unit capacitance that they exhibit makes it possible
to fabricate capacitors in smaller areas and hence contributes
to a denser circuit, relative to a circuit of similar complex-
ity that employ~ a silicon dioxide ilm. In addition, silicon
nitride films have greater resistance to radiation, alkali ion
and other impurity diffusion.
A second application of thin silicon nitride films
relates to the electrical isolation of integrated circuits by
means of silicon dioxide islands. Typically, these islands are
formed by oxidizing selected regions of a silicon substrate.
Silicon nitride films are used to protect areas of the subs-
trate. In current practice, a thin oxide layer is used between
the substrate and a deposited silicon nitride film to prevent
the generation of stress-induced
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faults during oxidation. The presence of this oxide layer
permits oxidization to proceed in a lateral, or horizontal,
direction as well as in the vertical direction. Oxide encroach-
ment in this lateral direction reduces the area that is
subsequently available for fabricating the integrated circuit,
and it is therefore preferable to minimize such. Accordingly,
attempts have been made to form the thin nitride layer directly
on the silicon substrate.
In the past, several different techniques have been
employed to form a silicon nitride layer directly on the surface
of a silicon substrate. One such technique is the thermal nitri-
dation of silicon, which is carried out using ammonia or nitrogen
gases at temperatures in the neighborhood of 1000~C or greater.
This approach i5 disadvantageous in that it requires treatment at
high temperatures for extended periods of time and is limited in
the types of materials which can be present on or in the
substrate. It is particularly unsuitable for use in the fabrica-
tion of VLS~ devices, since the susceptability of dopants to
diffusion at high temperatures presents problems with the smal~
geometries that are involved. In addition, the resultant films
contain a significant amount of oxygen, which hampers their
effectiveness in resisting oxidation.
A variation of this technique involves plasma-assisted
thermal nitridation with the use of inductively coupled
reactors. This technique is disclosed, for e%ample, in U.S.
~atent Nos. 4,277,320 and 4,298,629. A coil disposed around the
reaction chamber generates an electromagnetic field that induc-
tively heats the wafer to be coated and excites the gas within
the chamber to create a plasma. Such reactors have proven to be
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difficult to construct on a production-scale level, and are
therefore not in widespread use. Furthermore, they operate at
relatively high temperatures (close to lOOO~C) that are produced
by the inductive field, and hence have high power requirements.
Another technique for forming thin nitride films uses
high energy ion implantation. This technique is generally not
desirable from a commercial standpoint, since its throughput is
limited by the relatively small ion beam that is employed. In
addition, high current implantation systems are complex and
expensive.
Low energy ion bombardment is a third technique that
has been used to prepare nitride-like films on silicon. Limita-
tions associated with this technique include the fact that
processing can only be carried out on a single wafer at a time
because of the beam size generated by presently available low
energy ion bombardment ~ources, and the contamination of the
re~ulting layer with materials that are used to fabricate the ion
gun source.
A fourth technique for forming silicon nitride fi~ms is
low pressure chemical vapor deposition (LPCVD). Thickness
control of thin films is difficult with this process, and hence
the films tend to be relatively thick, in the neighborhood of 300
angstroms or more. As a result, they are not suitable for use in
trench isolation when in direct contact with silicon because they
can produce stress at the corners of the trench, which leads to
defects in adjacent regions of the siliCon substrate.
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OBJECTS AND BRIEF` STATEMPNT OF THE INVE:NTIC1N
Accordingly, it is an object of the present invention
to provide a novel means for economically converting the surfaces
of silicon and similar materials into nitride-like layers at
relatively low temperatures, i.e. at or near room temperature.
In particular, it is an object of the invention to provide a
novel method for forming silicon nitride layers, which method
does not require an ion gun source and minimizes the energy of
ions incident on metallic surfaces, to thereby keep the sput-
tering and subsequent redeposition of potential contaminants onto
the wafer surface low. It is a further object of the invention
to provide a novel method of the foregoing type which provides
effective control over the thickness of the nitride layer that is
formed.
It ls a further object of the invention to provide
novel applications for thin nitride films.
In accordance with the present invention, these objects
are achieved by carrying out a surface reaction on a substrate
layer in a vacuum cha~ber that contains an electrode which is
capacitively coupled to an rf generator. A second electrode
within the chamber, or a metal wall of the chamber itself, is
connected to ground. The silicon wafers to be treated are placed
on one of the electrodes to be in electrical and physical contact
therewith, and a reagent gas that contains nitrogen is introduced
into the chamber. An rf voltage is then applied ~etween the
electrodes to create a plasma, causing ions thereof to be accele-
rated into the silicon substrate. The nitrogen ions that are
created as a result of the sppliCation of the rf power can be
directed at the surface of a number of wafers simultaneously,
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thereby providing improved throughput over the prior art
techniques. In addition, the growth process is self limiting,
thereby providing effective control over the thickness of the
silicon nitride layer, enabling films that are in the range of
50-100 angstroms thick to be consistently produced.
According to one aspect of the present invention
there is provided a method of preparing a thin film of silicon
nitride or other nitrogen-containing composition on a silicon
substrate, comprising the steps of: placing the silicon sub-
strate in contact with one of a pair of electrodes; removingimpurities from a chamber in which said electrodes and the
substrate are located; introducing a nitrogen-containing reagent
gas into the chamber; and applying an a.c. voltage having a
frequency of about 10 KHz or greater between said electrodes to
thereby ioniæe and activate the reagent gas and accelerate ions
thereof into the substrate.
According to a further aspect of the present
invention there is provided a method for isolating active
regions from one another in an integrated circuit, comprising
the steps of: generating a plasma by introducing a nitrogen-
containing qas between a pair of spaced plate electrodes and
applying an rf frequency signal to the electrodes; placing a
silicon wafer in contact with one of said electrodes to thereby
cause a nitride film to be formed on a surface of the wafer;
removing impurities from the atmosphere in which said electrodes
and the substrate are located; removing select portions of said
film so that the remaining portions of the film correspond to
areas in which active devices are to be formed in the wafer; and
oxidizing the areas on said surface of the waer that are not
covered by the remaining portions of the nitride film.
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According to another aspect of the present invention
there is provided a method for dielectrically isolating active
regions from one another in an integrated circuit, comprising
the steps of: forming a trench in the surface of a silicon
substrate to define a boundary of at least one area in which an
active device is to be located; placing the wafer in contact
with one of a pair of spaced plate electrodes; removing impuri-
ties from the atmosphere in which said electrodes and the
substrate are located; introducing a nitrogen-containing gas
between said electrodes; applying an rf frequency signal to said
electrodes to thereby create a nitrogen plasma and cause a
nitride film to be formed on said surface of the wafer and along
the walls of said trench; and filling the remainder of said
trench with an insulatlng material.
A still further aspect of the invention is a method
of providing a dielectric film which forms a component of an
electrical device in an integrated circuit, comprising the steps
of: generating a plasma by introducing a nitrogen-containing
gas between a pair of spaced plate electrodes and applying an
rf frequency signal to the electrodes; removing impurities from
the atmosphere in which said electrodes and the substrate are
located; placing a silicon wafer in contact with one of said
electrodes to thereby cause a nitride film to be formed on said
wafer; and annealing said wafer and said film in an oxidizing
atmosphere.
According to another aspect of the present invention
there is provided a MOSFET device comprising: a silicon layer
which is doped to form a conductive channel region; source and
drain regions located on opposite sides of said channel region;
a thin nitride film disposed on said silicon layer over said
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channel region; and a polysilicon layer that is doped with
boron disposed on said nitride film to form the gate electrode
for said device.
The thin nitride films that are formed by such a
method have a variety of useful applications. Because of their
high degree of integrity and high nitrogen-to-oxygen ratio, they
are well suited for preventing oxidation during device isolation.
Furthermore, the controllability of the thickness of the films
readily facilitates the fabrication of capacitors having high
unit capacitance. For greater reliability and to reduce leakage
currents, the films can be annealed in an oxygen atmosphere
when they are to be incorporated in capacitor structures. In
addition, the electrical properties of such a film, coupled
with its immunity to impurity diffusion and radiation,
contribute to its successful application as a gate dielectric
for MOSFET devices.
Further features of the invention are described in
detail hereinafter with reference to preferred embodiments
thereof illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DR~WINGS
Figure 1 is a schematic representation of a chamber
of the type that can be used to implement the process of the
present invention;
Figure 2 is a schematic representation of an alter-
nate arrangement for the chamber;
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58.~041
Figures 3a and 3b are cross-sectional views of a
silicon wafer during steps of a LOCOS isolation process;
Figure 4 is a cross-sectional view of a trench isola-
tlon structure:
Figure 5 is a cross-sectional view of a trench
capacitor structure; and
Figure 6 is a cross-sectional view of a MOSFET device
incorporating a thin nitride film in the gate dielectric.
DESCRIPTION OF PREFERRED EMBOD'MENTS
The method of the present invention for forming a thin
film of nitride directly on the surface of a silicon substrate
can be carried out in an apparatus of the type illustrated in
Figure 1. For example, this apparatus could be a chamber of the
type that i8 typical}y used for plasma etching. This apparatus
basically comprises a sealed metal-walled chamber 10 having a
cathode 12 that is electrically insulated from the remainder of
the chamber walls, for example by means of a glass insulating
ring 14. One wall of the chamber is provided with a loading port
16 to enable silicon wafers to be placed on the cathode 12 and
removed therefrom. A vacuum source, such as a turbo-molecular
pump 1~, is in communication with the chamber to thereby evacuate
the same. In addition, the chamber is provided with a gas intro-
duction tube 20 that is connected to a source 22 of a suitable
nitrogen-containing gas through a pressure regulator 24. If
appropriate, suitable moisture and oxygen removing canni~ters
(not shown) can be connected to the tube 20 to improve the purity
of the gas that is introduced into the chamber.
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58.0041
The cathode 12 is coupled to an rf generator 26 by
means of an in-line capacitor 28, and the metallic walls of the
chamber 10 are connected to ground to form an anode. The cathode
and the anode thus form a pair of spaced plate electrodes. The
generator 26 provides an a.c. current to the cathode at an rf
frequency, and the in-line capacitor 28 maintains a d.c. bias on
the electrode. Preferably, the area of the anode is much greater
than that of the cathode, so that a relatively high self bias, in
the range of 300 to 1100 volts, is created at the cathode.
In operation, after the silicon wafers are placed on
the cathode 12, and the loading port 16 is closed to seal the
chamber, it is evacuated by means of the pump 18 to a low base
pressure so as to minimize potentially contaminating residual
gases. This base pressure is preferably about 10 6 ~orr. To
effect nitridation, gas from the source 22 is introduced continu-
ously into the evacuated chamber through the tu~e 20 to maintain
the pressure within the chamber at about 10 2 Torr. This gas
could be pure nitrogen, ammonia or a nitrogen-hydrogen mixture,
for example.
The gas within the chamber is activated by applying the
power from the rf generator 26 to the cathode. ~his power can be
in the range of about 25 to about 5C0 watts, and has a frequency
of 10 KHz to 300 MHz. As a result of the rf power, a plasma of
energized ions of the sas is created. These ions are accelerated
into the silicon substrate that is in electrical and physical
contact with the cathode, to form a silicon nitride layer.
In order to minimize heating of the silicon wafers as
well as prevent cracking of the cathode due to overheating, the
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cathode can be cooled by any suitable arrangement, such as a
water distribution system 30. To inhibit the sputtering of
oxygen-containing materials from the glass insulating ring 14
that supports the electrode, a dark space shield 32 can be
provided around the electrode. This shield can be made of a
suitable conductive material such as aluminum or stainless steel,
and can be grounded.
A capacitively coupled rf system such as that illus-
trated in Figure 1 is capable of creating nitrogen-containing
ions of sufficient energy and flux to the surface of the wafer
supported on the cathode 12. In contrast to prior systems that
employ thermal energy to induce a surface reaction, the system of
the present invention utilizes electrically excited nitrogen-
containing ions and accelerates them into the silicon surface to
promote the chemical reaction. Since the ions can be directed at
a relatively large area, a number of wafers can be processed
simultaneously, thus making the method of the present invention
an economic one for converting the surfaces of silicon into
nitride-like layers. In addition, the process is carried out at
a relatively low temperature e.g., less than 200C. (and prefer-
ably at room temperature) and does not require an ion gun source.
Furthermore, the process provides inherent control over
the thickness of the film that is formed, enabling relatively
thin films that are less than 200 angstroms thick, and preferably
about 50-100 angstroms thick, to be consistently produced. ~n
particular, the process is self-limiting in that the net growth
rate of the nitride film is inversely related to the film's
thickness for a given power density. When the film obtains a
thickness where its growth rate is equal to the constant rate at
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58.0041
which ions sputter off its surface, the process reaches a steady
state and no further net growth occurs. For example, for a power
density of 0.4 W/cm2 of the wafer, the thickness of the film only
increases by 5% as the process time is increased from 2 minutes
to 45 minutes. By adjusting the power density, films of
different desired thicknesses can be obtained.
The chamber illustrated in Figure 1 is electrically
asymmetric, in that the ratio of the anode area to the cathode
area is greater than 1, preferably at least 2:1. However,
similar results can be obtained in a more symmetric two-electrode
system, such as that illustrated in ~igure 2. In this system,
the anode 34 and the cathode 12 are approximately equal in size
and are provided on opposite walls of the chamber. In addition,
the chamber need not be made of a metal, but rather can be
composed of a suitable insulating material. With this arrange-
ment, higher pressures and different rf frequencies might be
employed to provide a relatively high d.c. bias voltage between
the two electrodes.
The control that is afforded by the process described
above and the quality of the resulting film facilitate its use in
a variety of applications. Through appropriate selection of the
density of the applied power (i.e. watts per cm2 of the wafer),
the atomic nitrogen to nitrogen-plus-oxygen ratio ([Nl/[N+Ol) f
the film can be regulated for a desired application. For
example, if the nitride film is to be used to prevent oxidation
of selected areas of the wafer during field oxidation, lN]/[N+O~
should preferably be at least 90%. When the plasma is formed
from N~, and using an anode:cathode ratio of about 20, powered by
a 13.56 MHz rf generator, the lN]/[N+O] ratio can be varied in a
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range from about 82% ~power density = 0.4 W/cm2) to ab~ut 98%
(power density = 3.2 W/cm2) for 5 minutes. If NH3 is used to
form the plasma, the lN]/lN+O] ratio goes from about 76% to about
96# for the same respective power density settings and time.
Referring to Figures 3a and 3b, one application of the
nitride film is illustrated. After a nitride film 36 has been
deposited on a silicon substrate 38 in accordance with the method
described above, a portion of the film is etched away to define a
field region. The portions of the film which remain define the
active areas where devices are to be formed. The field regions
40 are oxidized as shown in Figure 3b to provide isolation. The
nitride film 36 protects the underlying substrate from oxida-
tion When the l~l/[N+O] ratio of the film 36 is at least 90~,
it has been found that the film will resist more than 4500
angstroms of oxidation.
Subsequent to the formation of the nitride layer, it
can be annealed if desired. Such annealing can be carried out by
heating the wafer to a temperature between 900 and 1100C. In
order to prevent oxidation of the film, a chamber in which the
annealing is carried out can be filled with a non-oxidizing gas
such as nitrogen, hydrogen, or another inert gas. Alternatively,
the chamber can be evacuated to achieve the same effect.
Furthermore, the process is not limited to the forma-
tion of thin nitride films on planar surfaces. Of particular
interest in this context is the formation of the film within
trenches. It has been found that high quality thin films can be
formed along the side and bottom walls of trench structures in a
silicon substrate. Thus, the nitride film can be used as an
integral part of the insulating material that fills the trenches
for dielectric device isolation.
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Referring to Figure 4, a trench 42 is cut into the
surface of a silicon wafer to define a boundary between active
areas. If desired, the trench could completely surround an
active area. The side and bottom walls of the trench are lined
with a thin layer 44 of nitride. Preferably, this layer is only
50-70 angstroms thick. The remainder of the trench is then
filled with a suitable material 46, such as silicon dioxide or
undoped polysilicon. The nitride liner 44 on the walls of the
trench acts as a suitable barrier to the diffusi~n of the
material into the silicon substrate.
Advantage can also be taken of the relatively high
dielectric permittivity of the film formed according to the
above-described process. In particular, silicon nitride has a
higher dielectric constant than ~ilicon dioxide. Furthermore, as
the thickne~s of the film is reduced its capacitance increases,
so that the relatively thin films that can be obtained with the
present invention offer high unit capacitance. To further
increase the capacitive applications of nitride films formed
according to the process of the invention, they can be annealed
in an oxidizing atmosphere to reduce their leakage current.
~ or example, when used in a dynamic random access
memory (DRAM), an integrated capacitor should have high unit
capacitance, e.g., at least 5 f~/micron2, so as to be capable of
storing a sufficiently large charge packet that provides immunity
to noise. In addition, the capacitor 5hould exhibit low leakage
current, e.g., no greater than 10 6 A/cm2 at 2.5 V, ~o as to
afford a refresh cyc~e time of sufficient duration. The
following table illustrates how the properties of a capacitor
formed according to the present invention can be improved by
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annealing it in an oxygen atmosphere. The examples given in the
table are with respect to a nitride film formed on a silicon
wafer exposed to a plasma created from N2 gas using an
anode:cathode ratio of about 20, powered by a 13.56 MHz rf
generator at a power density of 0.4 W/cm2 for five minutes, in
accordance with the above-described process. The oxygen anneal
is carried out at 1000C.
Anneal Capacitanc~ Leakage Cur~ent at 2.5 V
Time ~min)lfF/micron ) (A/cm )
0 * ~10 5
8.8 6 x 10-6
7.7 1.~ x 10-7
Not capable of precise measurement due to high
leakage current.
As can be seen, the unit capacitance of the film
undergoes a slight decrease when annealed, most likely due to its
increased thickness and oxygen content as it undergoes oxida-
tion. However, even with an anneal for 90 minutes, the unit
capacitance still remains well above the minimum noted previously
for DRAM applications, whereas the leakage current is reduced by
two orders of magnitude over a film which has not been annealed.
The annealing temperature and time can be shortened,
for example by utilizing steam at 900C for 10-15 minutes. As
another alternative, it is possible to employ a rapid thermal
anneal ~RTA) in which high temperatures are appliéd to the wafer
for very short periods of time, e.g., tens of seconds. It is
believed that results which are equivalent to or better than
those depicted above can be obtained with these alternative
annealing processes.
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Due to the ability of the inventive process to produce
high-quality nitride films within trenches, it becomes possible
to fabricate trench capacitor structures. Such a structure is
illustrated in Pigure 5. The capacitor basically comprises a
thin nitride film 48 formed along the upper surface and within a
trench formed in an appropriately doped layer 50 of silicon or
polysilicon. The layer S0 functions as one electrode of the
capacitor, and a second electrode 52 is formed by depositing a
metal or doped polysilicon over the nitride layer 48. If the
trench has a width W and a depth D, its capacitance would be
(1 + 4D/W) times greater than that of a capacitor formed on top
of layer 50 and occupying the same amount of surface area.
Another particularly useful application for nitride
films formed according to the process of the present invention is
a5 a gate dielectric for MOSFET devices. In the past, attempts
at constructing a MOSFET using a thin oxide film in the gate
region and a doped polysilicon for the gate conductor have met
with several limitations. One of these limitations has been with
respect to the impurity that is used to dope the polysilicon.
Boron would be a desirable dopant for the polysilicon layer to
produce an enhancement type MOSFET. However, the diffusion
coefficient for boron is enhanced in the presence of hydrogen
during the deposition of polysilicon. Consequently, it tends to
break through the oxide layer and dope the underlying channel
region. ~ dopant would be another possibility but it
suffers from the fact that it would create a depletion type of
n-channel MOSFET.
However, by using a nitride film as the gate dielec-
tric, the tendency of boron to diffuse into the channel region is
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58.0041
resisted. Furthermore, even though the interface state density
of the nitride film is higher than that of silicon dioxide
layers, the nitride's contribution to the threshold voltage of
the device is minimal, due to its small thickness and high
permittivity. Therefore, it is possible to produce enhancement
type MOSFETS of higher reliability and integrity using a combina-
tion of the above-mentioned features.
Such a device is illustrated in Figure 6. The device
comprises a channel region 54 of silicon that is doped to one
conductivity, e.g. n-type. Source and drain regions 56 of
opposite conductivity are located on opposite sides of the
channel region. A thin film of silicon nitride 58 is grown over
the channel region to provide the gate dielectric. A silicon
dioxide layer 60 covers the remaining portion of the surface of
the substrate, and suitable contact h~les 62 are provided for the
source and drain electrodes. A thin layer of silicon dioxide 64
can also be disposed over the nitride film. In fact, the inter-
face between the nitride layer 58 and the silicon dioxide layer
64 may not be well-defined, but rather comprise a blend from
primarily nitride to primarily oxide over a distance of 5-20
angstroms. A polysilicon layer 66 that is doped with boron is
deposited over the thin gate dielectric films 58 and 64 to form
the gate electrode.
In summary, the nitride films that are produced accord-
ing to the process of the present invention have at least two
significant areas of application, as barriers for isolation and
during oxidation, and as elements of electrical devices such as
capacitors and MOSFETS. In the former area of application, it is
desirable that the film have a high purity, i.e., ~N]/[N+O~ is
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high. Accordingly, the process should be carried out at rela-
tively high power density levels, e.g., 1.6-3.2 W/cm2, and at
relatively low pressures. For electrical applications, where the
film is preferably annealed in oxygen, its purity is not so much
a factor as its electrical characteristics. Accordingly, the
process can be carried out with lower power densities, for
example in the range of 0.4-2.0 W/cm2. Further in this regard,
films made with N2 as the source appear to be best suited for use
as gate dielectrics, whereas those made with NH3 are preferred
for use in capacitors since they exhibit higher unit capacitance
for a given leakage current.
It will be appreciated by those of ordinary skill in
the art that the present invention can be embodied in other
specific forms without departing from the spirit or es~ential
characterigtics thereo. The presently disclosed embodiments are
therefore considered in all respects to be illustrative and not
restrictive. The scope of the invention is indicated by the
appended claims rather than the foregoing description, and all
changes that come wit~in the meaning and range of equivalents
thereof are intended to be embraced therein.
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