U.S. patent application number 09/977207 was filed with the patent office on 2002-02-14 for semiconductor device and production thereof.
Invention is credited to Hagiwara, Yasuhide, Ikeda, Shuji, Miura, Hideo, Nishimura, Asao, Ohta, Hiroyuki, Suzuki, Norio.
Application Number | 20020019146 09/977207 |
Document ID | / |
Family ID | 12794073 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020019146 |
Kind Code |
A1 |
Miura, Hideo ; et
al. |
February 14, 2002 |
Semiconductor device and production thereof
Abstract
A semiconductor device produced by forming an oxide film on a
substrate, heat treating the oxide film at a temperature of
800.degree. C. or higher in an inert atmosphere, followed by
conventional steps for formation of a transistor, is improved in
electrical reliability due to relaxation of stress generated in the
oxide film or in the surface of substrate.
Inventors: |
Miura, Hideo;
(Koshigaya-shi, JP) ; Ikeda, Shuji; (Koganei-shi,
JP) ; Suzuki, Norio; (Higashimurayama-shi, JP)
; Hagiwara, Yasuhide; (Fuchu-shi, JP) ; Ohta,
Hiroyuki; (Tsuchiura-shi, JP) ; Nishimura, Asao;
(Kokubunji-shi, JP) |
Correspondence
Address: |
ANTONELLI TERRY STOUT AND KRAUS
SUITE 1800
1300 NORTH SEVENTEENTH STREET
ARLINGTON
VA
22209
|
Family ID: |
12794073 |
Appl. No.: |
09/977207 |
Filed: |
October 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09977207 |
Oct 16, 2001 |
|
|
|
08610488 |
Mar 4, 1996 |
|
|
|
Current U.S.
Class: |
438/787 ;
257/E21.3; 257/E21.552; 438/424 |
Current CPC
Class: |
H01L 21/321 20130101;
H01L 21/76202 20130101; H01L 21/28185 20130101; H01L 21/28211
20130101; H01L 29/518 20130101; H01L 21/28202 20130101 |
Class at
Publication: |
438/787 ;
438/424 |
International
Class: |
H01L 021/76; H01L
021/31; H01L 021/469 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 8, 1995 |
JP |
07-048106 |
Claims
What is claimed is:
1. A process for producing a semiconductor device which comprises
forming a thermal oxide film on a silicon substrate, and carrying
out a heat-treatment at a temperature of not lower than 800.degree.
C. while keeping the oxide film and surface of silicon substrate in
a bare state in an inert atmosphere, followed by introduction of
impurities, formation of electrodes and wiring, and formation of an
insulating film so as to form a transistor.
2. A process for producing a semiconductor device which comprises,
after completing the selective oxidation for forming an oxide film
having a partially increased thickness on the surface of a silicon
substrate for electrically insulating and isolating the
semiconductor elements, removing the thin films other than the
oxide film, carrying out a heat-treatment at a temperature of not
lower than 950.degree. C. while keeping the oxide film or silicon
substrate in a bare state in an inert atmosphere, followed by
formation of gate oxide film, introduction of impurities, formation
of electrodes and wiring, formation of an insulating film so as to
form a transistor.
3. A process for producing a semiconductor device which comprises
forming an oxide film having a partially increased thickness on the
surface of a silicon substrate for electrically insulating and
isolating the semiconductor elements, thereafter forming a gate
oxide film of MOS type transistor and, just after completing the
gate oxidation or after forming the gate electrodes, carrying out a
heat-treatment at a temperature of not lower than 800.degree. C. in
an inert atmosphere, followed by introduction of impurities,
formation of electrodes and wiring, formation of an insulating film
so as to form a transistor.
4. A semiconductor device obtained by forming a thermal oxide film,
subsequently carrying out a heat treatment at a temperature of not
lower than 800.degree. C. while keeping the surface of the oxide
film or silicon substrate in a bare state, followed by introduction
of impurities, formation of electrodes and wiring, formation of an
insulating film so as to form a transistor.
5. A semiconductor device obtained by completing a selective
oxidation for forming on the surface of a silicon substrate an
oxide film having a partially increased thickness for electrically
insulating and isolating semiconductor elements, thereafter
removing the thin films other than the oxide film, carrying out a
heat-treatment at a temperature of not lower than 950.degree. C.
while keeping the oxide film or silicon substrate in a bare state,
followed by formation of a gate oxide film, introduction of
impurities, formation of electrodes and wiring, and formation of an
insulating film so as to form a transistor.
6. A semiconductor device obtained by forming on the surface of a
silicon substrate an oxide film having a partially increased
thickness for electrically insulating and isolating semiconductor
elements, thereafter forming a gate oxide film of MOS type
transistor and, just after completion of the gate oxidation or
after formation of gate electrodes, carrying out a heat-treatment
at a temperature of not lower than 800.degree. C., followed by
introduction of impurities, formation of electrodes and wiring,
formation of an insulating film so as to form a transistor.
7. A semiconductor device according to claim 4, wherein said
semiconductor device is a memory device selected from flash memory,
DRAM, and SRAM or a computing device.
8. A process for producing a semiconductor device according to
claim 1, wherein said thermal oxidation is carried out at least in
an atmosphere of a gaseous mixture of hydrogen and oxygen or in an
atmosphere of H.sub.2O.
9. A semiconductor device according to claim 4, wherein said
thermal oxide film is obtained by carrying out oxidation at least
in an atmosphere of a gaseous mixture of hydrogen and oxygen.
10. A process for producing a semiconductor device according to
claim 1, wherein the atmosphere of the heat-treatment is an inert
gas selected from nitrogen, hydrogen and argon, or a gaseous
mixture of these gases, said gas or gaseous mixture being able to
contain 5% or less of oxygen.
11. A semiconductor device according to claim 4, wherein the heat
treated thermal oxide film is obtained by the heat-treatment in an
inert gas selected from nitrogen, hydrogen and argon, or a gaseous
mixture of these gases, said gas or gaseous mixture being able to
contain 5% or less of oxygen.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a semiconductor device and process
for producing the same, and particularly to a process for producing
a semiconductor device suitable for forming a thermal oxide film
and a semiconductor device obtained by said process.
[0002] In the production of semiconductor elements using silicon as
a substrate, silicon oxide film formed by thermal oxidation of
silicon is used as an insulating film. In the process of forming
this thermal oxide film, a silicon-oxygen bonding is formed while
cleaving silicon-silicon bonding, due to which a great strain
(stress) appears in the vicinity of interface between silicon and
oxide film.
[0003] Since molecular volume of silicon oxide is twice or more as
great as that of silicon, the oxide film formed by oxidation
reaction tends to expand, due to which a tensile stress appears in
the silicon side and a compressive stress arises in the oxide film
side, usually. When stresses become great, crystal defects such as
dislocation and the like appear in the silicon substrate which is a
single crystal. In semiconductor elements, the presence of such
crystal defects causes leakage current and greatly deteriorates
reliability of article.
[0004] Even if no crystal defect appears in silicon substrate, the
stress arising in the oxide film can strain the atomic distance in
the oxide film and thereby lower the atomic bonding forces and, in
some extreme cases, cause injuries such as breakage of atomic
bondings. If such an injury appears, insulating characteristics of
oxide film are deteriorated, and electrical reliability of oxide
film and article decreases.
[0005] Generally speaking, the value of stress monotonously
increases as thickness of the formed oxide film increases.
Accordingly, when a thick thermal oxide film is to be formed,
relaxation of the stress is an important problem. As a method for
relaxing the stress, JP-A-3-11733 proposed a method of once
interrupting the thermal oxidation, carrying out a heat treatment
to eliminate strains, and thereafter again continuing the thermal
oxidation.
[0006] From the viewpoint of mechanism for generating stress in
oxide film, the stresses can be classified into a stress caused by
the volume expansion of oxide film in the vicinity of silicon/oxide
film interface brought about by the oxidation reaction in the oxide
film-forming process and a stress generated from the thin films
deposited on oxide film.
[0007] Although the stress caused by oxidation reaction can be
relaxed to some extent according to prior technique, there has
hitherto been no effective method for relaxing the stress generated
from the thin films deposited on oxide film. Said "stress generated
from the thin films deposited on oxide film" is generated according
to the following process.
[0008] First, as the process of formation of oxide film, there can
be referred to a process of partially forming the
elements-separating oxide film up to a thickness of about several
thousands angstroms for the purpose of electrically insulating and
isolating the elements, such as transistors, adjacently placed on a
silicon substrate. As a method for forming such an
element-separating oxide film, the selective oxidation method is
widely utilized (cf. FIG. 2). According to the selective oxidation
method, a silicon nitride film 3 (FIG. 2C) is deposited on a
silicon substrate 1 (FIG. 2A) through intermediation of a thin
thermal oxide film called "pad oxide film 2" (FIG. 2B), and then
the silicon nitride film 3 is etched off from the region on which
an element-separating oxide film is to be formed (FIG. 2D) and the
whole is oxidized to form a thick oxide film partially on the
silicon substrate (FIG. 2E).
[0009] In this selective oxidation method, the silicon nitride film
used as an oxidation-preventing film has an internal stress of
about 1,000 MPa at the time of depositing the films, in many cases,
and this stress acts upon the oxide film, too. Further, in the
process of selective oxidation, oxidizing species such as oxygen
and H.sub.2O three-dimensionally diffuse in the silicon substrate,
as a result of which oxide film 5 called "bird's beak" grows in the
vicinity of the edge of silicon nitride film.
[0010] Since volume of an oxide film expands in the growing period
of the oxide film, edge of the silicon nitride film is lifted, and
a warping deformation appears in the whole film. Reaction forces
cause by this warping deformation are concentrated into the edges
of silicon nitride, and a great stress appears in the oxide film at
the edges of silicon nitride film. This concentration of stress
takes place without fail to injure the oxide film when a silicon
nitride film exists.
[0011] Another process through which the thin film deposited on
oxide film injures the oxide film is the process of depositing a
thin film as a gate electrode on the gate oxide film of MOS (metal
oxide semiconductor) type transistor. As the gate electrode,
polycrystalline silicon thin film, high-melting metallic material
or silicide alloy thin film is used either in the form of single
layer or in a laminate structure.
[0012] Such a gate electrode material is often deposited with an
internal stress exceeding several hundreds or one thousand MPa.
Thus, when a gate electrode is fabricated, the internal stress is
concentrated into the oxide film in the vicinity of edge parts of
gate electrode, and thereby the oxide film is injured.
SUMMARY OF THE INVENTION
[0013] It is an object of this invention to provide a process for
producing a semiconductor device capable of remedying the injury of
oxide film at the edge parts of thin film partially deposited on
oxide film, and a semiconductor device obtained by said
process.
[0014] This invention provides a process for producing a
semiconductor device which comprises forming a thermal oxide film
on a silicon substrate, carrying out a heat-treatment in an inert
atmosphere at a temperature of not lower than 800.degree. C. while
keeping the surface of the oxide film or silicon substrate in a
bare state (the term "bare" means that the surface is not covered
with other film), followed by introduction of impurities, formation
of electrodes and wiring, formation of an insulating film and, if
necessary, formation of the wiring of second layer so as to form a
transistor.
[0015] This invention further provides a process for producing a
semiconductor device which comprises, after completing the
selective oxidation for forming an oxide film partially having a
partially increased thickness on the surface of a silicon substrate
for electrically insulating and isolating semiconductor elements,
removing the thin films other than the oxide film, carrying out a
heat-treatment in an inert atmosphere at a temperature of not lower
than 950.degree. C. while keeping the surface of oxide film or
silicon substrate in a bare state, followed by formation of a gate
oxide film, introduction of impurities, formation of electrodes and
wiring, formation of insulating film and, if necessary, formation
of the wiring of second layer so as to form a transistor.
[0016] This invention further provides a process for producing a
semiconductor which comprises, after forming an oxide film having a
partially increased thickness on a silicon substrate for
electrically insulating and isolating semiconductor elements,
forming a gate oxide film of MOS type transistor and, just after
completing the gate oxidation or after forming the gate electrodes,
carrying out a heat-treatment in an inert atmosphere at a
temperature not lower than 800.degree. C., followed by introduction
of impurities, formation of electrodes and wiring, formation of
insulating film and, if necessary, formation of the wiring of
second layer so as to form a transistor.
[0017] This invention further provides semiconductor devices
produced according to the above-mentioned processes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIGS. 1A to 1F are each schematic diagram illustrating the
cross-sectional structural change in the first embodiment (Example
1) of this invention.
[0019] FIGS. 2A to 2E are each schematic diagram illustrating the
cross-sectional structural change in the prior art selective
oxidation method.
[0020] FIG. 3 is a graph illustrating the oxidation
temperature-dependence of residual stress in the silicon substrate
after thermal oxidation.
[0021] FIG. 4 is a flow chart illustrating the production process
in Example 1 of this invention.
[0022] FIGS. 5A and 5B are each schematic diagram illustrating the
cross-sectional structural change in Example 2 of this
invention.
[0023] FIG. 6 is a flow chart illustrating the production process
of Example 2 of this invention.
[0024] FIGS. 7A to 7C are each schematic diagram illustrating the
cross-sectional structural change in Example 3 of this
invention.
[0025] FIG. 8 is a flow chart illustrating the production process
of Example 3 of this invention.
[0026] FIGS. 9A to 9E are each schematic diagram illustrating the
cross-sectional structural change in Example 4 of this
invention.
[0027] FIG. 10 is a flow chart illustrating the production process
of Example 4 of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] For remedying the injury in oxide film, it is effective to
carry out a heat-treatment in an inert atmosphere at a temperature
of not lower than 800.degree. C. preferably for at least 5 minutes
and further preferably for at least 20 minutes while keeping the
surface of oxide film in a bare state as possible, namely without
covering it with other films. The temperature of the heat-treatment
is 1,410.degree. C. (below the melting point of silicon substrate)
or below, and preferably 1,250.degree. C. or below, and more
specifically about 1,200.degree. C. In the process of forming an
element-separating oxide film by the selective oxidation method,
the silicon nitride films or the polycrystalline silicon thin films
used as oxidation-preventing films are exhaustively removed after
completion of the selective oxidation, and the remainder is
heat-treated at a temperature not lower than 800.degree. C. and
preferably not lower than 950.degree. C., and not higher than
1,410.degree. C. and preferably not higher than 1,200.degree. C.,
for at least 5 minutes and preferably for at least 30 minutes,
while keeping the surface of oxide film or silicon substrate in a
bare state. Further, after completing the heat-treatment succeeding
to the formation of element-separating oxide film, and after
forming the gate oxide film of MOS type transistor, additional
heat-treatments are carried out at a temperature not lower than
800.degree. C. for at least 5 minutes and preferably at least 30
minutes while keeping the surface of oxide film in a bare state.
Further, after forming electrodes on the gate oxide film
(patterning), too, a heat-treatment is carried out at a temperature
of not lower than 800.degree. C. for at least 5 minutes and
preferably at least 20 minutes while keeping the gate electrodes
and oxide film in a bare state.
[0029] Accordingly, the process for producing a semiconductor
device of this invention has any one of the following embodiments,
and the semiconductor device of this invention can be produced
according to any of these production processes.
[0030] (1) A process for producing a semiconductor device which
comprises forming a thermal oxide film on a silicon substrate,
carrying out a heat-treatment in an inert atmosphere at a
temperature of not lower than 800.degree. C. for at least 5 minutes
while keeping the surface of oxide film or silicon substrate in a
bare state, followed by introduction of impurities, formation of
electrodes and wiring, formation of an insulating film, and if
necessary, formation of the wiring of second layer, etc. so as to
form a transistor.
[0031] (2) A process for producing a semiconductor device which
comprises, after completing the selective oxidation for forming on
the surface of silicon substrate an oxide film having a partially
increased thickness for electrically insulating and isolating the
semiconductor elements, removing the thin films other than the
oxide film, carrying out a heat-treatment in an inert atmosphere at
a temperature of not lower than 950.degree. C. and not higher than
1,410.degree. C. and preferably not higher than 1,200.degree. C.
for at least 5 minutes and preferably for at least 20 minutes while
keeping the oxide film or silicon substrate in a bare state,
followed by formation of a gate oxide film, introduction of
impurities, formation of electrodes and wiring, formation of
insulating film, and if necessary, formation of the wiring of
second layer, etc. so as to form a transistor.
[0032] (3) A process for producing a semiconductor device which
comprises forming an oxide film having a partially increased
thickness on the surface of a silicon substrate for electrically
insulating and isolating the semiconductor elements, thereafter
forming a gate oxide film of MOS type transistor and, just after
completing the gate oxidation or after forming the gate electrodes,
carrying out a heat-treatment in an inert atmosphere at a
temperature of not lower than 800.degree. C. and not higher than
1,410.degree. C. and preferably not higher than 1,200.degree. C.
for at least 5 minutes and preferably for at least 20 minutes,
followed by introduction of impurities, formation of electrodes and
wiring, formation of an insulating film, and if necessary,
formation of the wiring of a second layer, etc. so as to form a
transistor.
[0033] (4) A semiconductor device obtained by forming a thermal
oxide film on a silicon substrate, subsequently carrying out a heat
treatment in an inert atmosphere at a temperature of not lower than
800.degree. C. and not higher than 1,410.degree. C. and preferably
not higher than 1,200.degree. C. for at least 5 minutes and
preferably for at least 20 minutes while keeping the surface of the
oxide film or silicon substrate in a bare state, and then carrying
out the steps necessary for formation of a transistor, for example,
introduction of impurities, formation of wiring formation of an
insulating film, formation of wiring of a second layer, etc.
[0034] (5) A semiconductor device obtained by completing the
selective oxidation for forming on the surface of a silicon
substrate an oxide film having a partially increased thickness for
electrically insulating and isolating semiconductor elements,
thereafter removing the thin films other than the oxide film,
carrying out a heat-treatment in an inert atmosphere at a
temperature of not lower than 950.degree. C. and not higher than
1,410.degree. C. and preferably not higher than 1,200.degree. C.
for at least 5 minutes and preferably for at least 20 minutes while
keeping the oxide film or silicon substrate in a bare state, and
then carrying out the steps necessary for formation of a
transistor, for example, formation of gate oxide film, introduction
of impurities, formation of electrodes and wiring, formation of
insulating film, formation of the wiring of a second layer,
etc.
[0035] (6) A semiconductor device obtained by forming on the
surface of a silicon substrate an oxide film having a partially
increased thickness for electrically insulating and isolating
semiconductor elements, thereafter forming a gate oxide film of MOS
type transistor and, just after completion of the gate oxidation or
after formation of gate electrodes, carrying out a heat-treatment
in an inert atmosphere at a temperature of not lower than
800.degree. C. and not higher than 1,410.degree. C. and preferably
not higher than 1,200.degree. C. for at least 5 minutes and
preferably for at least 20 minutes, and then carrying out the steps
necessary for formation of a transistor, for example, introduction
of impurities, formation of wiring, formation of an insulating
film, formation of the wiring of a second layer, etc.
[0036] (7) A semiconductor device according to Item (4), (5) or
(6), wherein said semiconductor device is a memory device such as
flash memory, DRAM, SRAM or the like or a processor or a computing
device.
[0037] (8) A process for producing a semiconductor device according
to Item (1), (2) or (3), wherein said thermal oxidation is carried
out at least in an atmosphere of a gaseous mixture of hydrogen and
oxygen or in an atmosphere of H.sub.2O, wherein the mixing ratio of
hydrogen/oxygen is less than 2/1 and preferably from about 1.5/1 to
about 1.8/1.
[0038] (9) A semiconductor device according to Item (4), (5), (6)
or (7), wherein said thermal oxidation is carried out at least in
an atmosphere of a gaseous mixture of hydrogen and oxygen or in an
atmosphere of H.sub.2O, wherein the mixing ratio of hydrogen/oxygen
in the gaseous mixture is less than 2/1 and preferably from about
1.5/1 to about 1.8/1.
[0039] (10) A process for producing a semiconductor device
according to Item (1), (2), (3) or (8), wherein the atmosphere of
the heat-treatment is nitrogen, hydrogen, an inert gas such as
argon or the like or a gaseous mixture of these gases, these gases
or gaseous mixtures being able to contain a few percents,
preferably 5% or less and further preferably 2% or less, of
oxygen.
[0040] (11) A semiconductor device according to Item (4), (5), (6),
(7) or (9), wherein the atmosphere of the heat-treatment is
nitrogen, hydrogen, an inert gas such as argon or the like or a
gaseous mixture of these gases, these gases or gaseous mixtures
being able to contain a few percents, preferably 5% or less and
further preferably 2% or less, of oxygen.
[0041] At the time of forming an oxide film on a silicon substrate
by the thermal oxidation method, the stress appearing in the oxide
film or on the surface of silicon substrate, namely interface with
oxide film, varies depending on the temperature of oxidation. This
is attributable to the presence of a stress-relaxation process
based on the viscoelastic behavior of the thermal oxide film.
[0042] FIG. 3 is a result of measurement of the stress appearing on
silicon substrate in the process of this thermal oxidation by
microscopic Raman spectroscopy, wherein the abscissa denotes the
temperature of oxidation and the ordinate denotes the perpendicular
stress parallel to the substrate surface remaining at room
temperature on the surface of silicon substrate, namely the
interface with oxide film. The Figure summarizes the results of
measurement in a case where a mixture of hydrogen and oxygen
(mixing ratio 1/1.5 to 1/1.8) as atmosphere of oxidation and in
another case where dry oxygen is used as the atmosphere. In the
oxidation, a single crystal of silicon ((100) surface wafer) is
used, and an oxide film having a constant thickness of 50 nm is
uniformly formed on the surface of silicon.
[0043] It is apparent from the results that the residual stress in
silicon substrate monotonously decreases as the temperature of
oxidation rises. Relaxation of stress is observed particularly
markedly when a gaseous mixture of oxygen and hydrogen is used as
atmosphere of oxidation, and the stress decreases approximately to
zero at a temperature not lower than 950.degree. C. Such a stress
relaxation process is observable not only in the process of
progress of oxidation reaction but also when heat-treatment is
carried out after completion of oxidation reaction.
[0044] That is, in an oxidation atmosphere containing a gaseous
mixture of oxygen and hydrogen, the stress of oxide film decreases
approximately to zero when an oxide film is formed at 850.degree.
C. and then an additional heat-treatment is carried out in an inert
atmosphere at 950.degree. C. for 30 minutes. Although the stress
relaxation in the process of heat-treatment exhibits an effect in
about 5 minutes, the relaxation of stress is preferably carried out
for at least 20 minutes in order to achieve a sufficient
relaxation.
[0045] FIG. 3 illustrates the results of measurement at an oxide
film thickness of 50 nm. When the oxide film thickness exceeds
about 100 nm, an oxidation at a temperature of 950.degree. C. or
above cannot always bring about a decrease of the residual stress
in silicon substrate to zero at the time of completion of the
oxidation. This is for the reason that the stress relaxation takes
a period of time. Accordingly, carrying out the above-mentioned
heat-treatment after formation of oxide film by thermal oxidation
method is effective for relaxing the stress in oxide film. The
mixing ratio of hydrogen/oxygen in the oxygen-hydrogen gaseous
mixture is less than 2/1 and preferably from about 1.5/1 to about
1.8/1.
[0046] When an oxidation is conducted by using an
oxidation-preventing film such as silicon nitride film or the like
as in the case of selective oxidation method, the stress is
concentrated into the edge parts of oxidation-preventing film as
has been mentioned above. Accordingly, the stress generated due to
the presence of thin film can also be relaxed together with the
oxidation-induced stress by removing the thin film forming a cause
of the stress (silicon nitride film in this case) after completion
of oxidation and then carrying out an additional heat-treatment in
an inert atmosphere while keeping whole surface of the oxide film
or silicon substrate in a bare state.
[0047] Further, since the gate oxide film of MOS type transistor is
mostly formed at about 850.degree. C. and a great stress often
remains in the oxide film after the film formation, practice of
such an additional heat-treatment is very effective.
[0048] When an gate electrode is deposited on the gate oxide film
or when deposition of electrode plus etching processing and the
like is carried out, too, stress is concentrated into the oxide
film of edge parts of gate electrode. In this case, too, the stress
generated in the oxide film can be relaxed by carrying out such a
heat-treatment in an inert atmosphere after completing the etching
processing of gate electrode.
[0049] The heat-treatment for the purpose of stress relaxation of
the above-mentioned thermal oxide film, oxide film, gate oxide film
or the like is preferably carried out in the presence of an inert
gas such as nitrogen, hydrogen, argon, helium or the like. Although
absence of oxygen is desirable, presence of oxygen in an amount of
5% by volume or less, preferably 2% by volume or less, is
allowable.
[0050] After the above-mentioned heat-treatment for stress
relaxation, the steps necessary for formation of a transistor, such
as introduction of impurities, formation of electrodes and wiring,
formation of an insulating film, formation of the wiring of a
second layer, etc. are carried out in the case of the
above-mentioned embodiment (1), in the usual manner, whereby the
intended semiconductor device can be produced.
[0051] In the above-mentioned embodiment (2), the heat-treatment
for stress relaxation is carried out and thereafter the steps
necessary for formation of a transistor such as formation of a gate
oxide film, introduction of impurities, formation of electrodes and
wiring, formation of an insulating film, formation of the wiring of
a second layer, etc. are carried in the usual manner, whereby the
intended semiconductor device can be produced.
[0052] In the above-mentioned embodiment (3), a heat-treatment for
stress relaxation is carried out and thereafter the steps necessary
for formation of a transistor such as introduction of impurities,
formation of electrodes and wiring, formation of an insulating
film, formation of the wiring of a second layer, etc. are carried
out in the usual manner, whereby the intended semiconductor device
can be produced.
[0053] Hereinafter, this invention is explained more concretely
with reference to Examples.
EXAMPLE 1
[0054] Example 1 is explained referring to FIG. 1, FIG. 3 and FIG.
4. FIGS. 1A to 1F are each schematic diagram illustrating the
cross-sectional change of silicon substrate in the process of
forming an element-separating oxide film using the process of this
invention for producing a semiconductor device; FIG. 3 is a graph
showing the oxidation temperature-dependence of the stress
appearing in the vicinity of silicon substrate surface (interface
with oxide film) as oxidation progresses; and FIG. 4 is a flow
chart illustrating the process of this Example.
[0055] First, according to the flow chart of FIG. 4, this Example
is explained referring to FIGS. 1A to 1F. This Example is an
application of this invention to a selective oxidation process for
forming a thick element-separating oxide film in the production
process of semiconductor device.
[0056] On silicon substrate 1 (FIG. 1A), a thin pad oxide film 2
having a film thickness of about 10 nm is formed by the thermal
oxidation method (FIG. 1B). Thereon is deposited a silicon nitride
film 3 as an oxidation-preventing film (FIG. 1C). A bare area is
formed by etching off the silicon nitride film 3 from the area on
which element-separating oxidation film is to be formed, and
thermal oxidation is carried out (FIG. 4-106) to form an
element-separating oxide film having a film thickness of about 300
to 700 nm (FIG. 1E). Then, the silicon nitride film 3 is wholly
removed, and a heat-treatment is carried out at a temperature of
not lower than 800.degree. C. for at least 5 minutes and preferably
for at least 20 minutes while keeping the oxide film 2 or 3 or
silicon substrate 1 in a bare state (FIG. 4-108).
[0057] Although the atmosphere of the heat-treatment is an inert
gas such as nitrogen, hydrogen, argon or the like or a gaseous
mixture of these gases, the atmosphere may contain about 5% or
less, preferably 2% or less, of oxygen. Further preferably,
temperature of the heat-treatment is not lower than 950.degree. C.
and not higher than 1,410.degree. C. and yet preferably not higher
than 1,200.degree. C.
[0058] The effect of the stress relaxation brought about by this
heat-treatment is explained by referring to FIG. 3, wherein the
abscissa denotes the temperature of oxidation and the ordinate
denotes the residual stress in silicon substrate after the
oxidation. The oxidation is carried out with a single crystal of
silicon ((100) surface wafer), and an oxide film having a constant
thickness of 50 nm is formed uniformly on the silicon surface.
[0059] It is apparent that the residual stress in silicon substrate
monotonously decreases as the oxidation temperature rises.
Relaxation of stress is markedly observed particularly when a
gaseous mixture of oxygen and hydrogen is used as the atmosphere of
oxidation, and the stress decreases approximately to zero at
950.degree. C. or above. The mixing ratio of hydrogen to oxygen is
less than 2/1 by volume, and preferably from about 1.5/1 to about
1.8/1 by volume. Such a stress relaxation behavior is observed not
only in the process of progress of oxidation but also when a
heat-treatment is carried out after completion of the oxidation
reaction. That is, in an oxidation atmosphere containing a mixture
of oxygen and hydrogen, the stress in the oxide film can be
decreased approximately to zero by forming an oxide film at
850.degree. C. and thereafter carrying out an additional
heat-treatment at 950.degree. C. for 30 minutes, too. Although the
stress relaxation in the process of heat-treatment exhibits an
effect in about 5 minutes, the heat-treatment is preferably carried
out for at least 20 minutes in order to attain a sufficient stress
relaxation.
[0060] FIG. 3 illustrates the results of measurement at an oxide
film thickness of 50 nm. When the oxide film thickness exceeds
about 100 nm, the residual stress in silicon substrate at
completion of oxidation cannot always reach zero, even if the
oxidation is carried out at 950.degree. C. or above. This is for
the reason that the stress relaxation takes a period of time.
Accordingly, carrying out the above-mentioned heat-treatment at
1,000.degree. C. or above and particularly at 1,200.degree. C.
after formation of the oxide film by thermal oxidation method is
effective for relaxing the stress in the oxide film. Particularly
in the selective oxidation method in which stress is concentrated
into the oxide film of the vicinity of edge parts of silicon
nitride film, the stress generated due to presence of the silicon
nitride film can also be relaxed by practicing such a
heat-treatment after removal of the silicon nitride film.
[0061] In this Example, only a silicon nitride film has been used
as the oxidation-preventing film. If desired, however, the
oxidation-preventing film may be a film of laminate structure
prepared by depositing a silicon nitride film on a polycrystalline
silicon thin film. Further, it is not always necessary to form the
oxidation-preventing film on the pad oxidation film 2, but it may
be formed directly on the silicon substrate 1. When a bare area is
formed by partially etching off the oxidation-preventing film (FIG.
4-105 or FIG. 1D), the pad oxide film 2 may also be removed to bare
the silicon substrate 1, or it is also allowable to positively etch
the silicon substrate 1 down to a depth of about 10 nm or more from
the surface to expose the silicon substrate 1 with a level
difference.
[0062] As mentioned above, this Example exhibits an effect that the
oxidation-induced stress or the stress generated in the oxide film
due to the presence of oxidation-preventing film in the selective
oxidation method can be relaxed, and structure and electrical
reliability of oxide film can be improved.
EXAMPLE 2
[0063] Example 2 is explained herein referring to FIG. 5 and FIG.
6. FIGS. 5A and 5B are each schematic diagram illustrating the
cross-sectional change of silicon substrate in the process for
forming the gate oxide film for MOS type transistor using the
process of this invention; and FIG. 6 is a flow chart illustrating
the production process of this Example.
[0064] First, this Example is explained referring to FIGS. 5A and
5B, according to the flow chart of FIG. 6. In this Example, a state
that an element-separating oxide film 4 has already been formed
(FIG. 5A) and the surface of silicon substrate 1 on which gate
oxide film is to be formed is bared is taken as an initial state
(FIG. 6-202). Preferably, the element-separating oxide film is that
formed by the production process mentioned in Example 1, though the
method of formation is not limitative.
[0065] Gate oxide film 6 is formed on the surface of silicon
substrate 1 at a temperature of, for example, 850.degree. C.
according to the usual thermal oxidation method (FIG. 5B). After
completing the oxidation of gate, a heat-treatment is carried out
at a temperature of not lower than 800.degree. C., preferably not
lower than 950.degree. C. and not higher than 1,410.degree. C.,
preferably not higher than 1,200.degree. C., for at least 5
minutes, preferably at least 20 minutes, while keeping the surface
of oxide film 4 or 6 in a bare state. Although atmosphere of the
heat-treatment is an inert gas such as nitrogen, hydrogen, argon or
the like or a mixture of these gases, the atmosphere may contain
oxygen in an amount of about 5% or less, preferably 2% or less. By
this heat-treatment, the oxidation-induced stress generated in the
oxide film in the process of forming the gate oxidation film can be
relaxed.
[0066] This Example exhibits an effect that the stress generated in
the oxide film due to the oxidation-induced stress in the gate
oxidation process can be relaxed, and structure and electrical
reliability of the oxide film can be improved.
EXAMPLE 3
[0067] Example 3 is explained herein referring to FIG. 7 and FIG.
8. FIGS. 7A to 7C are each schematic diagram illustrating the
cross-sectional change of silicon substrate in the process of
forming MOS type transistor by the production process of this
invention; and FIG. 8 is a flow chart illustrating the process of
this Example. According to the flow chart of FIG. 8, this Example
is explained referring to FIGS. 7A to 7C. The initial condition of
this Example is that gate oxide film 6 of MOS type transistor has
already been formed (FIG. 7A, FIG. 8-302).
[0068] The formation of element-separating oxide film 4 and gate
oxide film 6 in this Example is preferably according to Example 1
and Example 2 of this invention, though this is not limitative. As
gate electrode 7, a polycrystalline silicon thin film is formed,
for example, which is then processed into a shape of electrode by
an etching process (FIG. 7B). Since stress is concentrated into the
gate oxide film 6 of the vicinity of the edges of gate electrode 7
in this case, a heat-treatment is carried out at a temperature of
not lower than 800.degree. C., preferably not lower than
950.degree. C., and not higher than 1,410.degree. C., preferably
not higher than 1,200.degree. C., for at least 5 minutes and
preferably for at least 20 minutes, for the purpose of remedying
the damage which the oxide film has suffered (FIG. 8-304). Although
the atmosphere of the heat-treatment is an inert gas such as
nitrogen, hydrogen, argon or the like or a mixture of these gases,
the atmosphere may contain about 5% or less, preferably 2% or less,
of oxygen.
[0069] The material constituting the gate electrode is not limited
to polycrystalline silicon thin film, but high-melting metallic
materials such as tungsten or silicide alloys of high-melting
metals, titanium, cobalt, nickel, tungsten or the like, and
laminated structures made of thin films of the above-mentioned
materials can also be used as a gate electrode material.
[0070] Subsequently, the steps necessary for formation of a
transistor, for example, introduction of impurities (306),
formation of the wiring of first layer 12 (307), formation of
interlaminar insulating film 13 (308), formation of the wiring of
second layer 14 (309), formation of insulating film 15, etc. are
carried out to complete a MOS type transistor (311). FIG. 7C is an
example of the cross-sectional structure of a transistor formed by
the procedure mentioned herein. The procedure of transistor
formation is not limited to that expressed by this flow chart, and
the number of wiring layers is not limited to two. The MOS
transistor obtained herein may be used in a memory circuit such as
DRAM (Dynamic Random Access Memory), SRAM (Static Random Access
Memory) and the like or in a computing processor.
[0071] This Example has an effect that the stress generated in the
gate oxide film due to the internal stress of gate electrode film
in the process for forming MOS type transistor can be relaxed, and
thereby the electrical reliability of oxide film and transistor can
be improved.
EXAMPLE 4
[0072] Example 4 is explained herein referring to FIG. 9 and FIG.
10. FIGS. 9A to 9E are each schematic diagram illustrating the
cross-sectional change of silicon substrate in the process of
forming a flash memory structure by the process of this invention;
and FIG. 10 is a flow chart illustrating the process of this
Example.
[0073] First, according to the flow chart of FIG. 10, this Example
is explained referring to FIGS. 9A to 9E. The initial state of this
Example is a state that an element-separating oxide film 4 and a
tunnel oxide film 8 are formed on silicon substrate 1 (FIG. 9A,
FIG. 10-402). Said element-separating film 4 and tunnel oxide film
are preferably formed by the method mentioned in Example 1 and
Example 2 of this invention, though this is not limitative.
[0074] On the tunnel oxide film 8 is deposited a thin film for use
as a floating gate electrode, which is processed by etching into a
floating gate electrode 9 (FIG. 9B). The material constituting the
floating gate electrode 9 may be any of polycrystalline silicon,
high-melting metallic materials, silicide alloys of high-melting
metals, titanium, cobalt, nickel, tungsten and the like and
laminate structures made of thin films of the above-mentioned
materials. After forming the floating gate electrode, a
heat-treatment may be carried out, if desired, for the purpose of
relaxing the electrode-caused stress mentioned in Example 3.
[0075] Subsequently, an insulating film 10 constituted of a silicon
oxide film, a silicon nitride film or a laminate structure thereof
is formed on the floating gate electrode 9. If desired, a
heat-treatment may be carried out subsequently for the purpose of
relaxing the stress appearing at the time of forming this
insulating film (FIG. 10-405), provided that this heat-treatment is
not always necessary. Next, a controlling gate electrode 11 is
formed on the insulating film 10 (FIG. 9D). The material
constituting the controlling gate electrode may be any of
polycrystalline silicon, high-melting metallic materials, silicide
alloys of high-melting metallic materials, titanium, cobalt,
nickel, tungsten and the like and laminate structures made of thin
films of these materials.
[0076] After forming the electrode, a heat-treatment is carried out
at a temperature of not lower than 800.degree. C., preferably not
lower than 950.degree. C., and not higher than 1,410.degree. C.,
preferably not higher than 1,200.degree. C., for at least 5
minutes, preferably for at least 20 minutes (FIG. 10-407). The
atmosphere of the heat-treatment is preferably an inert gas such as
nitrogen, hydrogen, argon or the like, or a gaseous mixture of
these gases, though the atmosphere may contain about 5% or less,
preferably 2% or less, of oxygen. By this heat-treatment, the
stress appearing in the insulating film 10 due to formation of
controlling gate electrode 11, the stress appearing in the process
of forming the insulating film and the stress appearing in the
tunnel oxide film due to formation of floating gate electrode can
be relaxed.
[0077] Subsequently, the steps necessary for forming a flash memory
structure, for example, introduction of impurities (408), formation
of the wiring of a first layer 12 (409), formation of an
interlaminar insulating film 13 (410), formation of the wiring of a
second layer 14 (411), formation of an insulating film 15 (412),
etc. are carried out to complete a flash memory structure (413).
FIG. 9E illustrates an example of the cross-sectional structure of
transistor formed by this procedure, provided that the procedure
for forming the transistor is not limited to that expressed by the
flow chart and the number of wiring layers is not limited to two.
Further, the structure of electrodes constituting the flash memory
is not limited to that of this Example.
[0078] This Example has an effect that the stress appearing in the
insulating films located between the tunnel oxide film or floating
gate electrode and controlling gate electrode due to the internal
stress of controlling electrode or floating gate electrode in the
process of forming a flash memory structure can be relaxed, and
thereby the electrical reliabilities of oxide film, insulating film
and flash memory can be improved.
[0079] According to this invention, the injury of oxide film
appearing in the thermal oxide film-forming process of a
semiconductor device or the injury of oxide film due to the
concentration of stresses at the edge part of thin film partially
deposited on the oxide film or the insulating film having a
laminate structure of an oxide film and a silicon nitride film can
be remedied, and therefore this invention has an effect that
structure and reliability of oxide film or insulating film can be
improved. Further, this invention has an effect that reliability of
semiconductor memory articles such as MOS type transistor, flash
memory and the like can be improved.
* * * * *