U.S. patent application number 09/916304 was filed with the patent office on 2002-02-07 for method for forming thin film with a gas cluster ion beam.
Invention is credited to Akizuki, Makoto, Doi, Atsumasa, Harada, Mitsuaki, Yamada, Isao.
Application Number | 20020015803 09/916304 |
Document ID | / |
Family ID | 26409669 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020015803 |
Kind Code |
A1 |
Akizuki, Makoto ; et
al. |
February 7, 2002 |
Method for forming thin film with a gas cluster ion beam
Abstract
A method of forming a thin film on the surface of a substrate
such as silicon, in which a gas cluster (which is a massive atomic
or molecular group of a reactive substance taking the gaseous form
at room temperature under atmospheric pressure) is formed and then
ionized, and the cluster ions are then irradiated onto a substrate
surface under an acceleration voltage to cause a reaction. It is
possible to form a high quality ultra-thin film having a very
smooth interface, without causing any damage to the substrate, even
at room temperature.
Inventors: |
Akizuki, Makoto; (Ogaki-shi,
JP) ; Harada, Mitsuaki; (Ogaki-shi, JP) ; Doi,
Atsumasa; (Ogaki-shi, JP) ; Yamada, Isao;
(Himeji-shi, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK
Suite 800
2033 "K" Street N.W.
Washington
DC
20006
US
|
Family ID: |
26409669 |
Appl. No.: |
09/916304 |
Filed: |
July 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09916304 |
Jul 30, 2001 |
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08946663 |
Oct 7, 1997 |
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08946663 |
Oct 7, 1997 |
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08757088 |
Nov 26, 1996 |
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08757088 |
Nov 26, 1996 |
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08523922 |
Sep 6, 1995 |
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Current U.S.
Class: |
427/585 ;
257/E21.285; 257/E21.301; 427/595 |
Current CPC
Class: |
H01L 21/02238 20130101;
H01J 2237/0812 20130101; C23C 14/48 20130101; H01L 21/31662
20130101; C23C 8/36 20130101; H01L 21/32105 20130101 |
Class at
Publication: |
427/585 ;
427/595 |
International
Class: |
C23C 014/28 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 6, 1994 |
JP |
213078/1994 |
Mar 27, 1995 |
JP |
68445/1995 |
Claims
What is claimed is:
1. A method for forming a thin film on the surface of a substrate
with a gas cluster ion beam, which comprises the step of
irradiating the surface of a substrate with ions of a gas cluster
(which is a massive group of atoms or molecules of a reactive
substance taking the gseous form at room temperature under
atmospheric pressure) to cause a reaction with the surface of a
substrate and thereby form a thin film on the substrate
surface.
2. A method as claimed in claim 1, wherein the said reactive
substance is oxygen, an oxide, nitrogen, a nitride, a carbide, a
sulfide, a halide, a hydride, an organo metallic compound, a
mixture thereof or a mixture thereof with an inert gas.
3. A method as claimed in claim 1, wherein the gas cluster ions are
selected in terms of the number of component molecules.
4. A method as claimed in 1, wherein impurities present on the
substrate are removed by the irradiation with the gas cluster ion
beam, thereby forming a thin film with a low content of
impurities.
5. A method as claimed in claim 1, wherein the substrate surface is
planarized (made flat) by the irradiation with the gas cluster
ions, at the same time as the formation of the thisn film.
6. A method as claimed in claim 1, wherein the thin film formed is
an oxide.
7. A method as claimed in claim 1, wherein the substrate is
silicon.
8. A method as claimed in claim 1, wherein the reactive substance
is oxygen, an oxide of carbon, an oxide of nitrogen, a mixture
thereof, or a mixture thereof with a rare (inert) gas, and the thin
film formed is an oxide film.
9. A method as claimed in claim 1, wherein the gas cluster is
generated by supplying a pressurized gas to an expansion type
nozzle cooled by a coolant.
10. A method as claimed in claim 1, which includes the steps of (i)
forming a gas cluster from a substance which is reactive with the
surface of the substrate and is a gas at room temperature and
atmospheric pressure, (ii) ionizing the gas cluster, and (iii)
irradiating the surface of the substrate with a beam of gas cluster
ions.
11. A method for generating gas cluster ions, which comprises the
steps of generating a gas cluster with an expansion type nozzle
cooled by a coolant and ionizing the resultant cluster.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for forming a thin
film on the surface of a substrate with a gas cluster ion beam.
More particularly, the present invention relates to a method for
forming a thin film by gas cluster ion beam which is a group of
reactive substance which is gaseous at the room temperature under
the atmospheric pressure. The method is useful for the manufacture
of a semiconductor or other electronic devices and for surface
reforming of a functional material.
PRIOR ART AND PROBLEMS
[0002] A method for forming a thin film by irradiating monatomic or
monomolecular ions onto a substrate surface has conventionally been
used in practice. This method utilizes a high input energy of
several keV because low-energy ion irradiation cannot give an
adequate beam because of the space charge effect between ions.
[0003] In this conventional method, however, the use of ions having
a large input energy makes it difficult to avoid damage to the
substrate surface, and thus deterioration of semiconductor devices
has been a major problem.
[0004] As a method for forming a thin film by causing reaction
between a reactive substance, which is gaseous at the room
temperature, and a semiconductor substrate, a thermal reaction
method has been developed for forming an oxide or nitride film by
heating the substrate to a high temperature in an atmosphere of the
reactive substance. The thin film formed by this method has
excellent interface and insulation properties, and the method has
been industrialized for forming an insulating film or a capacitor
insulating film for silicon semiconductor devices.
[0005] This method requires a low temperature to reduce the
diffusion of impurities, which is desirable for integrated circuit
devices, the chemical vapor deposition (CVD) method is adopted for
this purpose.
[0006] In this case, however, it is necessary to heat the substrate
surface to a temperature of at least 400.degree. C., but this
results in defects such as a low density of the resultant film as
compared with that produced by the thermal reaction method and the
presence of many unsaturated bonds on the interface between the
substrate and the thin film.
[0007] The plasma CVD method is known as a method for forming a
thin film at a low temperature. This method however involves such
defects as a large amount of mixed impurities, occurrence of
damages to the substrate surface by ions, and difficulty in
controlling the film thickness of an ultra-thin film. Furthermore,
it is not applicable to a transistor gate insulating film or a
capacitor insulating film requiring high quality.
[0008] In the conventional techniques, as described above, the
quality of the resultant thin film deteriorates as the process
temperature becomes lower, and it is difficult to obtain an
ultra-thin film useful for a hyperfine semiconductor circuit device
such as ULSI.
[0009] There has consequently been a strong demand for a new method
which permits formation of a high-quality thin film at a lower
temperature, particularly at room temperature, without the need to
heat the substrate and without damaging the substrate surface, as a
fundamental technology for use in advanced electronics such as
ULSI.
[0010] The present invention was developed in view of the
circumstances as described above, and has an object to provide a
method for forming a high-quality ultra-thin film having a smooth
interface with the substrate free from a damage at the room
temperature, which solves the drawbacks in the conventional methods
for forming a thin film.
SUMMARY OF THE INVENTION
[0011] The present invention provides a method for forming a thin
film on the surface of substrate with a gas cluster ion beam, which
comprises the step of irradiating the surface of a substrate with
ions of a gas cluster (which is a massive group of atoms or
molecules of a reactive substance taking the gaseous form at room
temperature under the atmospheric pressure) to cause a reaction
with a substance of the substrate surface, thereby forming a thin
film on the substrate surface.
[0012] More specifically, the invention comprises the steps of
using a reactive substance, selected from the group consisting of
oxides, nitrides, carbides, mixtures thereof, and mixtures thereof
with an inert gas and causing a reaction between a gas cluster ion
beam of this substance and a substance of the substrate surface,
thereby forming a thin film on the substrate surface.
[0013] The invention provides also a method for forming a thin film
based on a gas cluster ion beam, which comprises the steps of
irradiating the surface of the substrate with the gas cluster ion
beam to form a thin film and at the same time to planarize the
surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows transmission type electron microscopic
photomicrographs of a section of a silicon substrate by irradiating
CO.sub.2 monomer and CO.sub.2 cluster ions onto the substrate
surface thereof;
[0015] FIG. 2 shows an infrared absorption spectrum of a thin film
formed by irradiating CO.sub.2 monomer ions and CO.sub.2 cluster
ions onto a silicon substrate surface;
[0016] FIG. 3 shows a chart illustrating the relationship between
the dose rate of CO.sub.2 ions to be irradiated and the
concentration of impurities in the substrate surface;
[0017] FIG. 4 shows a diagram illustrating the generation of
clusters of various gases by means of a nozzle having cooling
means;
[0018] FIG. 5 shows a graph illustrating the relationship between
the supply pressure and the cluster beam intensity at various
nozzle intensities for oxygen gas;
[0019] FIG. 6 shows a graph illustrating the relationship between
the supply pressure and the cluster beam intensity at various
nozzle intensities for nitrogen gas;
[0020] FIG. 7 shows a graph illustrating the relationship between
the supply pressure and the cluster beam intensity at various
nozzle intensities for argon gas for reference; and
[0021] FIG. 8 shows an infrared absorption spectrum of an oxide
film formed by irradiating oxygen (O.sub.2) gas cluster onto a
silicon substrate surface.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The method of the invention comprises generating a gas
cluster (which is a massive group of atoms or molecules of a
reactive substance which is a gas at room temperature under the
atmospheric pressure), irradiating electrons onto this gas cluster
to ionize same, and irradiating the thus generated gas cluster ions
onto a substrate surface by selecting a beam of a particular size
as required.
[0023] Irradiation is accomplished by accelerating the gas cluster
ions under an acceleration voltage. Because a cluster usually
consists of a group of several hundred atoms or molecules, each
atom or molecule is irradiated as an extra-low temperature ion beam
with up to several tens of eV and even under an acceleration
voltage of 10 KeV. It is therefore possible to form a high-quality
ultra-thin film with a very low degree of damage at room
temperature. A thin film with the slightest content of impurities
can be produced as the impurities are removed from the substrate
surface by the effect of the gas cluster ions.
[0024] Another effective manner of irradiation is to appropriately
select the number of component molecules in response to the
substrate, the kind of substance of the substrate surface and the
desired thin film composition.
[0025] The cluster itself can be generated by ejecting a
pressurized gas through an expansion type nozzle into a vacuum
unit, as already proposed by the present inventors. The thus
generated cluster can be ionized by irradiating electrons.
[0026] In the present invention, the gas cluster ions are
irradiated onto a substrate surface to planarize the surface and at
the same time, the irradiated ions are caused to react with the
substance of the substrate surface, thereby permitting formation of
a thin film on the substrate surface.
[0027] The gaseous reactive substances taking a gaseous form at
room temperature under the atmospheric pressure include, for
example, oxygen, oxides such as CO.sub.2, CO, N.sub.2O, NO.sub.x,
and C.sub.xH.sub.yO.sub.z, carbides, nitrides such as N.sub.2 and
NH.sub.x, sulfides, halides, hydrides such as AsH.sub.x and
SiH.sub.x, organometallic compounds such as metal carbonyls.
[0028] In the present invention, for example, the reactive
substance is preferably oxygen or a carbon oxide, or a mixture
thereof, or a mixture thereof with an inert gas substance, and the
thin film formed can be an oxide film.
[0029] In the case of any of the gases including oxygen (O.sub.2)
and nitrogen (N.sub.2), the present invention specifically proposes
a method for generating gas cluster ions, which comprises the step
of generating a gas cluster by means of a nozzle cooled by a
coolant, and ionizing the resultant cluster.
[0030] The invention will be described in further detail by the
following of examples.
EXAMPLE 1
[0031] FIG. 1 is transmission type electron microscopic
photomicrographs of a silicon substrate section when (a) CO.sub.2
monomolecular ions were irradiated onto an silicon substrate at a
dose rate of 1.times.10.sup.16 ions/cm.sup.2, and (b) CO.sub.2
cluster ions having a number of component molecules (cluster size)
of at least 500 were irradiated.
[0032] In FIG. 1(a), CO.sub.2 monomolecular ions were irradiated
under an acceleration voltage of 10 kV onto a silicon (001)
substrate. An amorphous silicon layer of a thickness of 19 nm was
formed on the substrate surface, and irregularities occurred on the
interface between the amorphous layer and the substrate. The
amorphous silicon layer is a damaged layer formed when CO.sub.2
ions ejected onto the substrate surface hit atoms of the substrate.
These irregularities of the interface and the damaged layer,
causing deterioration of semiconductor device properties, must be
converted to monocrystalline state. It is however very difficult to
achieve complete conversion even through heat treatment at a
temperature of at least 800.degree. C.
[0033] In the case of FIG. 1(b), on the other hand, CO.sub.2
cluster ions having a number of component atoms (cluster size) of
at least 500 were accelerated under conditions including a dose
rate of 1.times.10.sup.16 ions/cm2 and an acceleration voltage of
10 kV, and then, only clusters of a number of component molecules
of at least 500 were irradiated onto a silicon (001) substrate at
the room temperature by the retarding-field method. A silicon oxide
film having a thickness of 10 nm was formed on the substrate
surface, and no damaged layer was observed between the silicon
oxide film and the silicon substrate, and the interface in between
was very smooth.
[0034] The above-mentioned findings are confirmed also from FIG. 2.
FIG. 2 is an infrared absorption spectrum of a thin film formed by
irradiating CO.sub.2 monomer ions and cluster ions under conditions
including an acceleration voltage of 10 kV and a dose rate of
2.times.10.sup.16 ions/cm onto a silicon substrate surface. While a
silicon oxide film only of the order of spontaneous oxide film can
be formed by irradiation with CO.sub.2 monomer ions, irradiation
with cluster ions permits formation of an oxide film of about 70
A.
EXAMPLE 2
[0035] FIG. 3 illustrates changes in the concentration of Ni
impurities as measured with various dose rates of CO.sub.2 ions
irradiated onto a silicon substrate, ranging from 0 to
2.times.10.sup.15 ions/cm.sup.2. In this case, CO.sub.2 cluster
ions having a number of component molecules of at least 250 and
CO.sub.2 monomer ions were irradiated under an acceleration voltage
of 10 kV onto the silicon substrates to which Ni had previously
been forcedly deposited to give of 6.times.10.sup.12
atoms/cm.sup.2, and changes in the concentration of Ni impurities
was measured before and after irradiation through measurement by
the total reflection X-ray flourescence analysis method. While the
concentration of Ni impurities does not depend upon the extent of
dose rate in the irradiation of monomer ions, the concentration of
Ni impurities decreases along with the increase in the dose rate in
the irradiation of cluster ions.
[0036] As is clear from these results, in the case of irradiation
of cluster ions, it is possible to reduce the concentration of
impurities by increasing the dose rate of irradiated ions. The
higher impurities-removing effect in the irradiation of cluster
ions as compared to the irradiation of monomer (monomolecular) ions
is attributable to the fact that, while embedding of impurities
adhering to the surface into the interior of the substrate occurs
in the irradiation of monomer ions, sputtering from the irradiation
onto the substrate preferentially eliminates impurities on the
substrate surface in the irradiation of cluster ions.
[0037] As described above, the method of the present invention,
permitting formation of a clean surface free from defects on the
substrate surface, makes it possible to manufacture a high-quality
thin film with the slightest content of impurities, useful for a
semiconductor circuit device.
EXAMPLE 3
[0038] Table 1 shows, for an SiO.sub.2 film formed on a
polycrystalline silicon film irradiated with CO.sub.2 cluster ions
under conditions including an acceleration voltage of 10 kV and a
dose rate of 5.times.10.sup.15 ions/cm.sup.2, the film thickness
and the average surface roughness before and after a treatment with
fluoric acid solution. The treatment with fluoric acid solution was
applied until complete elimination of the SiO.sub.2 film. For
comparison purposes, values obtained with a polycrystalline silicon
film not irradiated with cluster ions are also shown.
1 TABLE 1 Average surface roughness (unit: .ANG.) Before After
Thickness of fluoric fluoric SiO.sub.2 acid acid Kind of sample
thin film (nm) treatment treatment Sample not irradiated -- 37 37
Sample irradiated with 8 7 9 cluster ions of size of 250 Sample
irradiated with 6 18 20 cluster ions of size of 500
[0039] The sample not irradiated with cluster ions has an average
surface roughness of 37 A, and this value does not vary with the
fluoric acid treatment. Irradiation of cluster ions of a size of
250 and 500 reduces the average surface roughness of the
polycrystalline silicon film to 7 .ANG. and 18 .ANG., respectively.
At the same time, an SiO.sub.2 thin film having a thickness of from
8 to 6 nm is formed on the surface of the polycrystalline silicon
film, thus forming an SiO.sub.2 thin film simultaneously with
planarization. A satisfactory surface planarity is kept with almost
no change by the fluoric acid treatment. Thus, the irradiation of
CO.sub.2 cluster ions permits formation of a silicon oxide film
which has a planar surface, with a smooth interface between said
film and the silicon substrate, and has a uniform thickness.
EXAMPLE 4
[0040] Conditions for generating cluster of various gases were
evaluated. As shown in FIG. 4, cooled dry nitrogen gas was passed
through a piping (2) attached to a nozzle (1) section which was
then cooled. This permitted ionization of clusters of gases which
had not been possible at room temperature.
[0041] FIG. 5 is a graph illustrating the relationship between the
intensity of oxygen (O.sub.2) gas cluster beam and the supply
pressure at various nozzle temperatures. FIG. 6 is a graph
illustrating such relationship for nitrogen (N.sub.2) gas. FIG. 7
illustrates the relationship for Ar gas for reference.
[0042] These results suggest the possibility of determining the
degree of easiness of generation of the cluster of a gas applicable
in the present invention by means of the following formula:
[0043] Easiness of Generation of Cluster
.psi.=P.sub.0.multidot.d.sub.0.multidot.(T.sub.b/T.sub.N).sup..gamma./(.ga-
mma.-1)
[0044] where,
[0045] P.sub.0: gas supply pressure,
[0046] d.sub.0: inside diameter of nozzle throat,
[0047] T.sub.b: boiling point of gas,
[0048] T.sub.N: ejecting temperature of gas,
[0049] .gamma.: ratio of specific heats of gas (specific heat at
constant pressure/isometric specific heat).
[0050] Larger P.sub.0 or T.sub.b leads to easier generation of
clusters.
[0051] Smaller T.sub.N or .gamma. corresponds to easier generation
of clusters.
[0052] In all the cases described above, various gases were
effectively utilized for the formation of thin films.
EXAMPLE 5
[0053] FIG. 8 is an infrared absorption spectrum of a thin film
formed by irradiating O.sub.2 cluster ions having a number of
component molecules (cluster size) of at least 250 onto an Si
substrate surface at room temperature under an acceleration voltage
of 4 kV and at a dose rate of 1.times.10.sup.15 ions/cm.sup.2. It
is clear from FIG. 8 that an oxide film of about 40 A was formed
with a lower acceleration voltage and a lower dose rate than the
conditions for irradiation of CO.sub.2 cluster ions.
EXAMPLE 6
[0054] Table 2 shows the results of investigation by the
photoelectron spectroscopy method of an oxide film formed by
irradiating CO.sub.2 cluster ions and O.sub.2 cluster ions under
the same irradiating conditions including an acceleration voltage
of 4 kV, a cluster size of at least 250, and a dose rate of
1.times.10.sup.15 ions/cm.sup.2. The results of investigation on a
clean Si substrate not irradiated are also shown for reference. By
using O.sub.2 cluster ions, it is possible to form a thicker oxide
film as compared with the case of CO.sub.2 cluster ions. It is also
clear that the relative intensity of emission spectrum from Cls
core level of the oxide film formed by the irradiation of O.sub.2
clusters is of the same order as that for a substrate not
irradiated and the carbon content is almost nil, even with the
largest film thickness. When presence of residual carbon in the
film is not desired, it is suggested to be effective to use O.sub.2
cluster ions to obtain a thicker oxide film.
2 TABLE 2 Raw material gas Oxide film of irradiated thickness
Relative intensity cluster ions (unit: A) of C1s signal O.sub.2 40
31 CO.sub.2 18 44 Substrate not 8 29 irradiated
[0055] According to the present invention, as described above in
detail, it is possible to form a high-quality reactive ultra-thin
film, with no damage to the substrate, having a very smooth
interface, even at the room temperature, by irradiating cluster
ions of a reactive substance onto the substrate surface to cause
reaction.
* * * * *