U.S. patent application number 10/197406 was filed with the patent office on 2002-12-19 for etching method and cleaning method of chemical vapor growth apparatus.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. Invention is credited to Eguchi, Kazuhiro, Hieda, Katsuhiko, Kiyotoshi, Masahiro, Okumura, Katsuya, Yamazaki, Soichi.
Application Number | 20020190024 10/197406 |
Document ID | / |
Family ID | 26561450 |
Filed Date | 2002-12-19 |
United States Patent
Application |
20020190024 |
Kind Code |
A1 |
Eguchi, Kazuhiro ; et
al. |
December 19, 2002 |
Etching method and cleaning method of chemical vapor growth
apparatus
Abstract
Presented is an etching method capable of easily etching an
oxide containing an alkaline-earth metal. One method is to etch the
oxide by using an etching gas containing a halogen gas except for
fluorine, an interhalogen compound consisting of only a halogen
element except for fluorine, or a halogen hydride consisting of a
halogen element except for fluorine and hydrogen. Particularly
chlorides, bromides, and iodides of alkaline-earth metals have
relatively high vapor pressures, so a thin film containing an
alkaline-earth metal can be etched by using chlorine gas, bromine
gas, or iodine gas. When a halogen gas containing fluorine is used,
damages to SiO.sub.2 portions used in a film formation apparatus
are prevented by coating these SiO.sub.2 portions with a fluoride
of an alkaline-earth metal.
Inventors: |
Eguchi, Kazuhiro;
(Chigasaki-shi, JP) ; Okumura, Katsuya;
(Yokohama-shi, JP) ; Kiyotoshi, Masahiro;
(Sagamihara-shi, JP) ; Hieda, Katsuhiko;
(Yokohama-shi, JP) ; Yamazaki, Soichi;
(Yokohama-shi, JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT &
DUNNER LLP
1300 I STREET, NW
WASHINGTON
DC
20006
US
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
|
Family ID: |
26561450 |
Appl. No.: |
10/197406 |
Filed: |
July 18, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10197406 |
Jul 18, 2002 |
|
|
|
09429074 |
Oct 29, 1999 |
|
|
|
Current U.S.
Class: |
216/37 ; 118/715;
118/724; 156/345.37; 216/63; 216/76 |
Current CPC
Class: |
C23C 16/4405
20130101 |
Class at
Publication: |
216/37 ; 216/63;
216/76; 118/715; 118/724; 156/345.37 |
International
Class: |
C23C 016/00; C23F
001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 30, 1998 |
JP |
10-311323 |
Oct 20, 1999 |
JP |
11-298271 |
Claims
1. An etching method comprising the steps of: preparing an oxide
layer containing at least one type of alkaline-earth metal; and
etching the oxide layer containing at least one type of
alkaline-earth metal by using, as an etching gas, either a halogen
gas other than fluorine gas, or a gas containing at least one
material selected from the group consisting of an interhalogen
compound consisting of halogen elements other than fluorine, and a
halogen hydride consisting of a halogen element other than fluorine
and hydrogen.
2. The etching method according to claim 1, wherein the step of
etching the oxide layer includes a step of etching using chlorine
gas as the etching gas.
3. The etching method according to claim 1, wherein the step of
etching the oxide layer includes a step of etching in an ambient at
not less than 500.degree. C.
4. The etching method according to claim 3, wherein the step of
etching the oxide layer includes a step of etching using chlorine
gas as the etching gas.
5. The etching method according to claim 1, wherein the step of
etching the oxide layer includes a step of etching the oxide layer
while changing an etching temperature.
6. The etching method according to claim 5, wherein the step of
etching the oxide layer includes a step of etching using chlorine
gas as the etching gas.
7. The etching method according to claim 5, wherein the step of
etching the oxide layer includes: a first etching step having a
first etching condition; and a second etching step having a second
etching condition, and the etching temperature in the first etching
step is different from the etching temperature in the second
etching step.
8. The etching method according to claim 7, wherein the step of
etching the oxide layer includes a step of etching using chlorine
gas as the etching gas.
9. The etching method according to claim 7, wherein the second
etching step is successively performed after the first etching
step, and the etching temperature in the first etching step is
lower than the etching temperature in the second etching step.
10. The etching method according to claim 9, wherein the step of
etching the oxide layer includes a step of etching using chlorine
gas as the etching gas.
11. The etching method according to claim 9, further comprising a
step of repeating a plurality of times the step of successively
performing the first and the second step.
12. The etching method according to claim 11, wherein the step of
etching the oxide layer includes a step of etching using chlorine
gas as the etching gas.
13. The etching method according to claim 1, wherein the step of
etching the oxide layer includes a step of using a gas activated by
plasma excitation as the etching gas.
14. The etching method according to claim 1, wherein the oxide
layer is a (Ba,Sr)TiO.sub.3 layer.
15. An etching method comprising the steps of: preparing an oxide
layer containing at least one type of alkaline-earth metal; and
etching the oxide layer containing at least one type of
alkaline-earth metal by using an etching gas composed of a gas
containing a halogen element and a gas consisting of a halide of
Ti.
16. The etching method according to claim 15, wherein the step of
etching the oxide layer includes a step of etching by using a gas
containing at least fluorine as the gas containing a halogen
element.
17. A chemical vapor growth apparatus comprising: a reaction
chamber; a heating mechanism for heating the reaction chamber; a
reaction gas supply unit connected to the reaction chamber to
supply a reaction gas; a reaction gas exhaust unit connected to the
reaction chamber to exhaust the reaction gas from the reaction
chamber; and a member installed in the reaction chamber, wherein
one of the reaction chamber and the member has a portion made of
quartz, and at least a part of a surface of the quartz portion,
which is exposed to the reaction gas, is coated with a fluoride of
an alkaline-earth metal.
18. A cleaning method of a hot wall type chemical vapor growth
apparatus which uses a quartz member and deposits an oxide
containing at least one type of alkaline-earth metal, comprising
the steps of; depositing a fluoride of alkaline-earth metal at
least on a surface of the quartz member; depositing the oxide on a
deposition substrate not less than once; and etching the oxide by
using an etching gas containing a fluorine compound.
19. The cleaning method according to claim 18, wherein the step of
etching the oxide includes a step of using an etching gas further
containing a halide of Ti as the etching gas.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an etching method of
etching an oxide containing alkaline-earth metals as constituent
elements, a chemical vapor growth apparatus for forming an oxide
film containing alkaline-earth metals as constituent elements, and
a cleaning method of the apparatus.
[0002] New materials that have not been conventionally used are
beginning to be used in new semiconductor devices represented by
very-large-scale semiconductor integrated circuits. So, chemical
vapor deposition is beginning to be demanded to deposit thin films
of these new materials. One of these new materials is barium
strontium titanate ((Ba,Sr)TiO.sub.3):BST) which is a
high-dielectric constant material used in a charge storage film
(capacitive film) of a DRAM.
[0003] In the semiconductor device fabrication processes, dry
etching superior in micro fabrication properties is widely used.
However, alkali-earth metals such as Sr and Ba constructing the
above material are not etched because the vapor pressure of a
compound formed by a conventional etching gas is low.
[0004] Meanwhile, the semiconductor device fabrication processes
extensively employ the formation of thin films by chemical vapor
deposition (CVD) having high step coverage and capable of
depositing in large areas. When a thin film is deposited by this
CVD, a deposit sticks not only to a substrate for deposition but
also to a reaction chamber exposed to a deposition gas or to jigs
installed in the chamber. This deposit peels off owing to stress or
mechanical stimulus, and the peeled dust particles fall on the
substrate during deposition or during transfer of the substrate,
thereby causing particle contamination of the formed thin film.
Therefore, it is necessary to clean and remove the deposit, other
than the objective deposit, sticking to the interior of the
apparatus.
[0005] This cleaning method is desirably performed without
disassembling the apparatus in order to raise the throughput. As
this method of cleaning without disassembling the apparatus, a
cleaning gas for changing the deposit into a substance having high
vapor pressure is supplied into the apparatus to remove the
deposit.
[0006] Unfortunately, alkaline-earth metals such as Sr and Ba
constructing the aforementioned material cannot be removed because
the vapor pressure of the compound formed by this conventional
cleaning method is low.
[0007] A thin film containing alkaline-earth metals cannot be
dry-etched and a chemical vapor deposition apparatus for depositing
a thin film containing alkaline-earth metals cannot be cleaned with
gas for the same reason: none of conventional etching gases and
cleaning gases can form an alkaline-earth metal compound having
high vapor pressure.
[0008] It is narrowly possible to etch a thin film containing
alkaline-earth metals such as Ba and Sr by using chlorine
trifluoride (ClF.sub.3) gas. However, a high temperature of
800.degree. C. or more is necessary for the etching, and yet the
etching rate of Ba and Sr is lower than that of Ti.
[0009] Additionally, under severe conditions in which ClF.sub.3 is
used at high temperatures, the corrosiveness of ClF.sub.3
increases. The most serious problem is that SiO.sub.2 is also
etched when ClF.sub.3 is used at high temperatures. SiO.sub.2 is
frequently used as interlayer dielectrics and a surface protective
film in semiconductor devices. These SiO.sub.2-based films already
formed are destroyed when a thin film containing alkaline-earth
metals is dry-etched.
[0010] Also, a reaction chamber of a chemical vapor deposition
apparatus is often made of quartz, and most jigs such as a
substrate holder and a gas supply nozzle installed in the reaction
chamber are made of quartz. Therefore, when a chemical vapor
deposition apparatus for depositing a thin film containing
alkaline-earth metals is cleaned by using a gas such as ClF.sub.3,
quartz is eroded especially at high temperatures, resulting in
destruction of the apparatus.
[0011] As described above, when a gas containing a halogen such as
fluorine is used as an etching gas or a cleaning gas, no
alkaline-earth metal compound having high vapor pressure is formed.
Consequently, etching or cleaning takes a long time and hence is
difficult to perform.
[0012] Also, when a fluorine-containing gas is used to etch or
clean a chemical vapor growth apparatus, SiO.sub.2 is corroded, and
this destroys an interlayer insulating film or the apparatus.
BRIEF SUMMARY OF THE INVENTION
[0013] It is an object of the present invention to provide an
etching method capable of easily etching an oxide of an
alkaline-earth metal.
[0014] It is another object of the present invention to provide an
etching method which does not etch SiO.sub.2-containing layers in a
semiconductor device, or an etching method which etches a thin film
containing an oxide film of an alkaline-earth metal without etching
quartz members of a chemical vapor growth apparatus, and to provide
a chemical vapor growth apparatus, and a cleaning method of the
chemical vapor growth apparatus.
[0015] To achieve the above objects, an etching method of the
present invention comprises the steps of preparing an oxide layer
containing at least one type of alkaline-earth metal; and etching
the oxide layer containing at least one type of alkaline-earth
metal by using, as an etching gas, either a halogen gas other than
fluorine gas, or a gas containing at least one material selected
from the group consisting of an interhalogen compound consisting of
halogen elements other than fluorine, and a halogen hydride
consisting of a halogen element other than fluorine and
hydrogen.
[0016] The step of etching the oxide layer desirably includes a
step of etching in an ambient at not less than 500.degree. C.
[0017] The step of etching the oxide layer desirably includes a
step of etching the oxide layer while changing an etching
temperature.
[0018] The step of etching the oxide layer may includes:
[0019] a first etching step having a first etching condition;
and
[0020] a second etching step having a second etching condition,
and
[0021] the etching temperature in the first etching step is
different from the etching temperature in the second etching
step.
[0022] The second etching step is preferably successively performed
after the first etching step, and the etching temperature in the
first etching step is lower than the etching temperature in the
second etching step.
[0023] The etching method may further comprise a step of repeating
a plurality of times the step of successively performing the first
and the second step.
[0024] The step of etching the oxide layer above-mentioned
preferably includes a step of etching using chlorine gas as the
etching gas.
[0025] The step of etching the oxide layer preferably includes a
step of using a gas activated by plasma excitation as the etching
gas.
[0026] The oxide layer above-mentioned is preferably a
(Ba,Sr)TiO.sub.3 layer.
[0027] Chlorides, bromides, and iodides of alkaline-earth metals
have relatively high vapor pressures. So, a thin film containing an
alkaline-earth metal can be etched by using chlorine, bromine, or
iodine. Also, since F highly corrosive for SiO.sub.2 is not
contained, SiO.sub.2 portions used in a semiconductor device or in
a film formation apparatus are not damaged. Therefore, dry etching
using chlorine, bromine, and iodine gases can be effectively used
as a cleaning means of a film formation apparatus. Additionally,
the etching temperature can be lowered when active halogen radicals
are formed by activating a halogen.
[0028] Particularly chlorides of alkaline-earth metals have
relatively high vapor pressures, so alkaline-earth metals contained
in a thin film can be etched at a high temperature of 700.degree.
C. or more. However, in the temperature range of 700.degree. C. or
more within which alkaline-earth metals can be etched, chlorides of
metals such as titanium and tantalum evaporate and at the same time
redeposit by thermal decomposition. This makes etching difficult to
perform. A thin film containing a plurality of metals including
alkaline-earth metals can be dry-etched by dividing the dry etching
step into: a step of preferentially dry-etching metals whose
chlorides thermally decompose to redeposit at high temperatures,
and a step of preferentially dry-etching the alkaline-earth
metals.
[0029] Also, by etching at a high temperature of about 800.degree.
C. after etching is performed using chlorine gas at a low
temperature of about 500.degree. C., metals other than
alkaline-earth metals can be previously etched. This facilitates
the etching at a high temperature of about 800.degree. C., since
the residual film contains only oxides of the alkaline-earth
metals.
[0030] In particular, metals such as titanium, tantalum, and
ruthenium whose chlorides decompose and redeposit at a temperature
of 700.degree. C. or more at which alkaline-earth metals can be
etched are etched as fluorides in this temperature range by using
an etching gas such as chlorine trifluoride. This makes two-stage
etching feasible. Since etching of alkaline-earth metals and
etching of metals other than the alkaline-earth metals can be
performed at the same etching temperature, the etching can be
performed within a short time period, and the etching gas
consumption amount can be reduced. It is also possible to suppress
film peeling caused by abrupt temperature changes during
etching.
[0031] Furthermore, a thick film containing alkaline-earth metals
can be etched by repeating an etching step of primarily etching
alkaline-earth metals and an etching step of etching metals other
than alkaline-earth metals. Since etching is done in stages, no
perfect etching needs to be performed in the individual etching
steps. This allows close temperatures such as 850.degree. C. and
700.degree. C. to be set as different etching temperatures.
Accordingly, time loss resulting from etching temperature change
can be reduced, and this shortens the etching time. By this
shortening of the etching time, it is possible to suppress
deterioration of an apparatus exposed to high-temperature exhaust
gases and reduce the etching gas consumption amount.
[0032] Also, when etching is performed while the etching
temperature is changed, a formed film containing alkaline-earth
metals can be etched even in an apparatus having a reaction chamber
whose etching temperature is difficult to abruptly change.
[0033] An etching method according to the second aspect of the
present invention comprises the steps of preparing an oxide layer
containing at least one type of alkaline-earth metal, and etching
the oxide layer containing at least one type of alkaline-earth
metal by using an etching gas composed of a gas containing a
halogen element and a gas consisting of a halide of Ti.
[0034] The etching step desirably comprises the step of etching by
using a gas containing at least fluorine as the gas containing a
halogen element.
[0035] According to the second aspect of the present invention,
even when Ti is selectively removed and no longer exists in an
object to be cleaned, Ti is supplied from a gas. This produces
halides such as Ba--Ti and Sr--Ti necessary to remove Ba and Sr.
Consequently, perfect cleaning is possible.
[0036] Also, since halides of Ti have high vapor pressures, a gas
amount containing Ti necessary for cleaning can be readily
supplied. Additionally, since a halogen is also used as an etching
gas, it does not interfere with etching.
[0037] A chemical vapor growth apparatus according to the third
aspect of the present invention comprises a reaction chamber, a
heating mechanism for heating the reaction chamber, a reaction gas
supply unit connected to the reaction chamber to supply a reaction
gas, a reaction gas exhaust unit connected to the reaction chamber
to exhaust the reaction gas from the reaction chamber, and a member
installed in the reaction chamber, wherein one of the reaction
chamber and the member has a portion made of quartz, and at least a
part of a surface of the quartz portion, which is exposed to the
reaction gas, is coated with a fluoride of an alkaline-earth
metal.
[0038] The portion of the quartz member, which is coated with a
fluoride of an alkaline-earth metal is a portion to be exposed to a
cleaning gas during cleaning of the apparatus.
[0039] Since fluorides of alkaline-earth metals have high
resistance to a highly corrosive gas at high temperatures, damages
to quartz members by a cleaning gas during cleaning can be
suppressed.
[0040] Gas cleaning done by a CVD apparatus for a thin film
containing alkaline-earth metals as constituent elements must be
performed at high temperatures by using a highly corrosive gas. A
protective film made from a fluoride of an alkaline-earth metal is
particularly effective in cleaning.
[0041] Damages to quartz caused by a cleaning gas are especially
remarkable when a cleaning gas containing fluorine is used. Hence,
protection using a fluoride of an alkaline-earth metal is
effective.
[0042] A cleaning method of a chemical vapor growth apparatus
according to the fourth aspect of the present invention is a
cleaning method of a hot wall type chemical vapor growth apparatus
which uses a quartz member and deposits an oxide containing at
least one type of alkaline-earth metal, comprising the steps of
depositing a fluoride of an alkaline-earth metal at least on a
surface of the quartz member, depositing the oxide on a substrate
for deposition once or more, and etching the oxide by using an
etching gas containing a fluorine compound.
[0043] The etching step desirably comprises the step of using a gas
further containing a halide of Ti as the etching gas.
[0044] According to the fourth aspect of the present invention, a
protective film is formed by using a gas as in normal CVD, so the
apparatus need not be disassembled in the formation of the
protective film. Since the protective film is formed, an etching
gas containing, e.g., ClF.sub.3 can be used. Even when the
protective film deteriorates by repeated BST film formation and
ClF.sub.3 cleaning, the protective film can be re-formed without
immediately disassembling the apparatus. This increases the
availability and throughput of the apparatus.
[0045] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0046] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiments of the invention, and together with the
general description given above and the detailed description of the
preferred embodiments given below, serve to explain the principles
of the invention.
[0047] FIG. 1 is a view showing an outline of the arrangement of a
chemical vapor growth apparatus according to the first embodiment
of the present invention;
[0048] FIG. 2 is a graph showing the dependence of the etching
amounts of Ba, Sr, and Ti on etching time when ClF.sub.3 gas was
used as an etching gas;
[0049] FIG. 3 is a graph showing the dependence of the etching
amounts of Ba, Sr, and Ti on etching time when ClF.sub.3 gas and
TiCl.sub.4 were used as etching gases;
[0050] FIGS. 4A to 4D are sectional views showing some of capacitor
formation steps, in order of steps, in the fabrication process of a
DRAM according to the second embodiment of the present
invention;
[0051] FIG. 5 is a view showing an outline of the arrangement of an
etching apparatus according to the second embodiment;
[0052] FIG. 6 is a view showing an outline of the arrangement of an
etching apparatus according to the third embodiment of the present
invention;
[0053] FIG. 7 is a view showing an outline of the arrangement of an
etching apparatus according to the fifth embodiment of the present
invention;
[0054] FIGS. 8A and 8B are graphs showing the dependence of the
etching rate on temperature when ClF.sub.3 and I.sub.2 were used,
respectively;
[0055] FIG. 9 is a diagram showing the relationships between the
etching temperatures and the vapor pressures of various etching
gases;
[0056] FIG. 10 is a diagram showing the relationships between
various etching gases, etching conditions, BST film etching states,
and quartz part etching (damage) results;
[0057] FIG. 11 is a view showing an outline of the arrangement of a
hot wall batch type chemical vapor growth apparatus according to
the sixth embodiment of the present invention;
[0058] FIG. 12 is a view showing an outline of the arrangement of
another hot wall batch type chemical vapor growth apparatus
according to the sixth embodiment;
[0059] FIG. 13 is a view showing an outline of the arrangement of a
hot wall batch type chemical vapor growth apparatus according to
the eighth embodiment;
[0060] FIG. 14 is a graph showing diffraction patterns of CaF.sub.2
exposed to etching processes at various temperatures;
[0061] FIG. 15 is a diagram showing the Ca and F contents and the
composition ratio of CaF.sub.2 exposed to etching processes at
various temperatures;
[0062] FIGS. 16A and 16B are graphs showing the etching rates of
Ba, Sr, and Ti of a thin (Ba,Sr)TiO.sub.3 film formed on a quartz
substrate and SiO.sub.2 when the film was and was not coated with a
CaF.sub.2 film;
[0063] FIG. 17 is a sectional view showing a semiconductor storage
device according to the 11th embodiment of the present
invention;
[0064] FIGS. 18A to 18C are sectional views of a peripheral circuit
portion in FIG. 17, for explaining an etching method of the 11th
embodiment;
[0065] FIG. 19 is a diagram showing the etching amounts of Ba, Sr,
and Ti in various etching methods of the 11th embodiment;
[0066] FIG. 20 is a schematic sectional view of a dry etching
apparatus used in the 12th to 15th embodiments of the present
invention;
[0067] FIG. 21 is a graph showing the etching temperature profile
of a cleaning method according to the 12th embodiment of the
present invention;
[0068] FIG. 22 is a graph showing the dependence of the residual
etching amounts of Ba, Sr, and Ti on etching temperature in the
12th embodiment;
[0069] FIG. 23 is a view showing the etching temperature profile of
a BST film cleaning method according to the 13th embodiment of the
present invention;
[0070] FIG. 24 is a view showing the dependence of the etching
depths of Ba, Sr, and Ti on etching temperature when a thick BST
film was etched with chlorine gas in the 13th embodiment;
[0071] FIGS. 25A to 25C are views schematically showing the
progress of etching in the 13th embodiment;
[0072] FIG. 26 is a graph showing the gas sequence of dry etching
according to the 14th embodiment of the present invention; and
[0073] FIG. 27 is a graph showing the etching temperature profile
of a dry etching method according to the 15th embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0074] Embodiments of the present invention will be described below
with reference to the accompanying drawings.
[0075] [First Embodiment]
[0076] In the first embodiment, ClF.sub.3 gas and TiCl.sub.4 gas
are used in etching a thin (Ba,Sr)TiO.sub.3 (BST) film.
[0077] FIG. 1 is a view showing an outline of the arrangement of a
hot wall batch type chemical vapor growth apparatus for forming a
thin BST film. An inner tube 102 is installed in a quartz reaction
chamber 101 placed on a reaction chamber base 103. This inner tube
102 contains a substrate boat 105 which is placed on a substrate
boat base 107 and on which substrates 115 to be processed are
placed. The substrate boat 105 can be rotated by a substrate boat
rotating mechanism 109.
[0078] A resistance heater 104 is installed outside the reaction
chamber 101. An organic metal gas for depositing a thin BST film is
supplied from an organic metal inlet port 110 into the inner tube
102 of the reaction chamber 101 through an organic metal gas supply
nozzle 108. Gases such as O.sub.2 and Ar, other than organic
metals, used when a thin BST film is deposited and ClF.sub.3 and
TiCl.sub.4 as cleaning gases are supplied from a gas inlet port
111. A vacuum exhaust pump 114 is connected to the reaction chamber
101 via a trap 112 and a pressure adjusting valve 113.
[0079] Procedures of forming a thin BST film and cleaning using
ClF.sub.3 gas and TiCl.sub.4 gas as halogen element-containing
gases by using this apparatus will be described below.
[0080] First, substrates 115 are mounted on the substrate boat 105
and loaded into the reaction chamber 101. Each substrate 115 is
composed of a silicon substrate, a 100-nm thick silicon oxide
(SiO.sub.2) film formed on the silicon substrate by thermal
oxidation, and a 50-nm thick SrRuO.sub.3 (SRO) film deposited on
this silicon oxide film by CVD using Sr(THD).sub.2 and
Ru(THD).sub.3 (THD=C.sub.11H.sub.19O.sub.2). Note that THD is
2,2,6,6-tetramethyl-3,5-heptanedione (C.sub.11H.sub.19O.sub.2).
[0081] Next, after the substrate boat 105 on which the substrates
115 are mounted is loaded into the reaction chamber 101, the
pressure adjusting valve 113 is opened to allow the vacuum exhaust
pump 114 to exhaust the reaction chamber 101. After that, the
pressure adjusting valve 113 is used to adjust the internal
pressure of the reaction chamber to 1 Torr, and the resistance
heater 104 is operated to heat the substrates 115.
[0082] When the reaction chamber 101 is well exhausted and the
temperature of the substrates 115 becomes 400.degree. C.,
Ba(THD).sub.2, Sr(THD).sub.2, Ti(i-OPr).sub.2(THD).sub.2, and
O.sub.2 (i-OPr=i-OC.sub.3H.sub.7) as gases for depositing a thin
BST film and Ar as a diluent gas are supplied into the reaction
chamber 101 to start depositing a thin BST film. Note that i-OPr is
isopropoxide (OC.sub.3H.sub.7). The flow rates of these source
gases are so adjusted that the composition of a thin BST film to be
deposited is Ba.sub.0.5SR.sub.0.5TiO.sub.3.
[0083] By film deposition for 20 min, a 20-nm thick BST film is
deposited on each substrate 115. The deposition of this thin BST
film is completed by stopping the supply of the source gases into
the reaction chamber 101. After the deposition is completed, the
reaction chamber 101 is evacuated to exhaust the residual gases.
After the residual gases are well exhausted, the internal pressure
of the reaction chamber 101 is returned to normal pressure, and the
substrates 115 are unloaded. After this thin BST film deposition,
deposition of a film containing Ba, Sr, Ti, and O was found in the
reaction chamber 101, the inner tube 102, the substrate boat 105,
and the upper portion of a heat plug 106, and outside the nozzle
108.
[0084] Subsequently, the apparatus is cleaned as follows. First,
the vacuum exhaust pump is used to exhaust the reaction chamber
101. The resistance heater 104 is then used to heat portions to be
cleaned, i.e., the reaction chamber 101, the inner tube 102, the
nozzle 108, and the substrate boat 105, to a temperature necessary
for cleaning, e.g., 850.degree. C.
[0085] Next, ClF.sub.3 and TiCl.sub.4 gases as cleaning gases of
this embodiment and Ar as a diluent gas are supplied into the
reaction chamber 101 from the gas inlet port 111. The flow rates of
ClF.sub.3, TiCl.sub.4, and Ar are 2,000, 100, and 3,000 sccm,
respectively. Also, the pressure adjusting valve 113 is used to
adjust the internal pressure of the reaction chamber during
cleaning to 1 Torr.
[0086] When the apparatus was cleaned for 5 min, the deposits
sticking to the reaction chamber 101, the inner tube 102, the
substrate boat 105, the heat plug 106, and the nozzle 108 were
completely removed. After that, thin BST films were deposited on
new substrates to be processed, and particle contamination after
the deposition was checked using a particle counter. Consequently,
no big difference was found between these substrates and the
previous substrates, and almost no particle contamination was
observed.
[0087] As described above, the present inventors have confirmed
that when ClF.sub.3 gas is used, cleaning after chemical vapor
deposition of a thin BST film according to the present invention
can be perfectly performed.
[0088] Gas cleaning of a CVD apparatus for forming a thin BST film
can be perfectly performed by using ClF.sub.3 gas and TiCl.sub.4
gas as described above because even after Ti is completely etched
and no Ti is present any longer in objects to be cleaned, the
TiCl.sub.4 gas keeps supplying Ti, so halides such as Ba--Ti and
Sr--Ti necessary for etching form.
[0089] As a control, an experiment was conducted by exposing, to
ClF.sub.3 gas, a sample formed by depositing a 100-nm BST film on a
quartz substrate by CVD. The thin BST film deposition conditions
were the same as described above. The cleaning conditions were also
the same as described above except no TiCl.sub.4 gas was used.
[0090] FIG. 2 shows the dependence of the etching amounts of Ba,
Sr, and Ti on etching time in this experiment. As shown in FIG. 2,
Ti was completely etched in about 20 sec. In contrast, although Ba
and Sr were etched until 20 sec, the etching of these Ba and Sr
showed almost no further progress even after the etching was
continued for 3 min or more. That is, Ba and Sr were not completely
etched.
[0091] From the foregoing, when ClF.sub.3 gas alone is used Ba and
Sr are etched simultaneously with Ti, and Ba and Sr cannot be
etched any longer when no more Ti exists in an object to be etched.
This implies that Ba and Sr are etched as halides Ba--Ti and
Sr--Ti.
[0092] To confirm this, the present inventors conducted the
following experiment. That is, a sample formed by depositing a
100-nm thick BST film on a quartz substrate by CVD was exposed to a
cleaning gas mixture of ClF.sub.3 gas and TiCl.sub.4 gas. The thin
BST film deposition conditions and the conditions in which the
sample was exposed to the cleaning gas mixture of ClF.sub.3 gas and
TiCl.sub.4 gas were the same as described above.
[0093] FIG. 3 shows the etching time dependence of the etching
amounts of Ba, Sr, and Ti in this experiment. As shown in FIG. 3,
Ti was completely etched in about 20 sec. After Ti was thus
completely etched, Ba and Sr were kept etched and, in about 1 min,
completely etched. This is so because even after Ti was completely
eliminated from the object to be etched, the TiCl.sub.4 gas kept
supplying Ti, so halides Ba--Ti and Sr--Ti necessary to etch Ba and
Sr formed.
[0094] The present inventors have also confirmed that a similar
effect is achieved by using Ti halides such as TiI.sub.4 and
TiBr.sub.4, instead of TiCl.sub.4 gas, and gas cleaning of the
chemical vapor growth apparatus for depositing a thin BST film can
be perfectly performed.
[0095] Furthermore, the present inventors have confirmed that the
chemical vapor growth apparatus can be similarly cleaned by using a
halogen-containing gas other than ClF.sub.3.
[0096] Note that the present invention is also applicable to
etching of a thin BST film formed on a sample, as well as to
cleaning of a chemical vapor growth apparatus.
[0097] [Second Embodiment]
[0098] In the second embodiment, dry etching of a thin BST film
used in a capacitor of a DRAM will be explained.
[0099] FIG. 4 shows some of capacitor formation steps in the
fabrication process of a DRAM having a capacitor using a thin BST
film. A hole serving for a connection plug of a storage electrode
is formed in an SiO.sub.2-based interlayer dielectric film 401, and
a barrier layer 402 is formed by sequentially depositing TiN and Ti
by CVD. After that, the hole is filled with W (tungsten) by CVD.
Excess W deposited on the interlayer insulating film 401 is removed
by CMP. The resultant structure is planarized so that the upper end
of the W is substantially flush with the surface of the interlayer
insulating film 401, and a W plug 403 is buried in the hole.
Subsequently, an SiN film 404 and an SiO.sub.2-based interlayer
dielectric film 405 are deposited by CVD. A hole for forming a
capacitor is formed in the silicon nitride film 404 and the
interlayer dielectric film 405 by using conventional lithography
and dry etching.
[0100] An SrRuO.sub.3 (SRO) film 406 serving as a storage electrode
is formed by CVD using Sr(THD).sub.2, Ru(THD).sub.3, and O.sub.2 as
materials. After this SRO film 406 is formed, the SRO film 406
deposited on top of the interlayer insulating film 405 is removed
by CMP to leave the SRO film 406 in the opening. A thin BST film
407 serving as a capacitor dielectric film is formed by CVD using
Ba(THD).sub.2, Sr(THD).sub.2, Ti(i-OPr).sub.2(THD).sub.2, and
O.sub.2 and crystallized by heat processing. FIG. 4A is a sectional
view of the element formed up to this step.
[0101] Next, an SiO.sub.2 film serving as a mask when the thin BST
film 407 is dry-etched is formed by CVD, and a mask pattern 408 is
formed by conventional lithography and dry etching. FIG. 4B shows
the element formed up to this step.
[0102] The thin BST film 407 is then dry-etched by using the mask
pattern 408. This dry etching of the thin BST film 407 is done by
using an etching apparatus shown in FIG. 5. This etching apparatus
is a single wafer equipment in which a resistance heater 504 for
heating a substrate 505 to be etched is installed together with a
substrate holder 503 in a reaction chamber 501 via a base 502. A
diluent gas can be supplied into the reaction chamber 501 via a
mass-flow controller 509 and an on-off valve 511. The material of
an etching gas is contained in a material container 507. In this
apparatus the etching gas material is assumed to be a solid or
liquid. When the material container 507 is raised to an appropriate
temperature, a vapor (gas) of the material is produced. A mass-flow
controller 508 controls the flow rate of this gas and supplies the
gas into the reaction chamber 501 via an on-off valve 510. A shower
head 506 straightens the etching gas and diluent gas supplied into
the reaction chamber and evenly sprays these gases against the
substrate 505. The reaction chamber is exhausted by a vacuum
exhaust pump 513 and is so controlled by a pressure adjusting valve
512 that the internal pressure of the reaction chamber is constant.
An exhaust gas processor 514 makes the exhaust gas harmless and
releases the gas into the air.
[0103] In the second embodiment, iodine (I.sub.2) is used as the
etching gas material, and Ar as the diluent gas. I.sub.2 gas is
produced when the temperature of the container 507 containing
I.sub.2 is raised to 70.degree. C. The flow rate of this I.sub.2
gas is controlled to 50 sccm by the mass-flow controller 508. The
flow rate of the diluent Ar gas is controlled to 50 sccm by the
mass-flow controller 509. The internal pressure of the reaction
chamber is 1 Torr. The resistance heater 504 raises the temperature
of the substrate to be processed to 650.degree. C. or more,
preferably, 700.degree. C. Under the conditions, the substrate to
be processed shown in FIG. 4B is etched for 5 min.
[0104] After this substrate was unloaded, a section of the
substrate was observed. Consequently, the thin BST film 407 was
patterned as shown in FIG. 4C. Also, the mask pattern 408 and
interlayer insulating film 405 made from SiO.sub.2 and exposed to
the I.sub.2 gas were not etched at all and were not reduced in film
thickness. When portions from which the thin BST film 407 was
removed were carefully examined, no residue of any kind was
found.
[0105] After that, the mask pattern 408 is removed. FIG. 4D is a
sectional view of the element formed as above. A DRAM capacitor is
completed by forming a plate electrode on top of the structure.
[0106] As described above, the present inventors have found that
etching of a thin BST film which is conventionally difficult to
perform can be performed by the use of I.sub.2 gas. This is so
because when a thin BST film is exposed to I.sub.2 gas at a high
temperature, iodides of Ba, Sr, and Ti as constituent elements of
the thin BST film form, and these iodides have enough vapor
pressures at which they volatize at a temperature of about
700.degree. C. The present inventors have also confirmed that
SiO.sub.2 is not etched at all even when I.sub.2 gas is heated to a
high temperature.
[0107] An experiment was conducted by using the same apparatus
under the same conditions except that bromine (Br.sub.2) gas which
is a liquid at room temperature was used in place of I.sub.2 gas,
with the result that a thin BST film was similarly etched. Note
that Br.sub.2 gas is produced by heating a material container
filled with liquid Br.sub.2 to 40.degree. C.
[0108] [Third Embodiment]
[0109] In the third embodiment, a thin BST film 407 on the
substrate to be processed shown in FIG. 4B is etched following the
same procedures as in the second embodiment by using, as etching
gases, chlorine (Cl.sub.2) as a halogen gas and hydrogen chloride
(HCl), hydrogen bromide (HBr), or hydrogen iodide (HI) as a halogen
hydride. Since Cl.sub.2, HCl, HBr, and HI are gases at room
temperature, the use of any of these gases as an etching gas makes
supply of the etching gas easier than when a liquid or solid
material is used as in the second or fourth embodiment.
[0110] An etching apparatus shown in FIG. 6 is used in dry etching
of a thin BST film using Cl.sub.2 gas because Cl.sub.2 is a gas at
room temperature. The etching apparatus shown in FIG. 6 has exactly
the same arrangement as the apparatus of FIG. 5 except for the
absence of the material container 507. So, the same reference
numerals as in FIG. 5 denote the same parts in FIG. 6. The
arrangement of the etching apparatus is simple because an etchant
is a gas. Note that Ar gas is used as a diluent gas.
[0111] To etch a thin BST film, a mass-flow controller 508 controls
the flow rate of Cl.sub.2 gas to 50 sccm. A mass-flow controller
509 controls the flow rate of Ar gas to 50 sccm. The internal
pressure of a reaction chamber 501 is 1 Torr. A resistance heater
504 raises the temperature of the substrate to be processed to
800.degree. C.
[0112] Under the conditions, the substrate to be processed shown in
FIG. 4B is etched for 5 min. After the substrate was unloaded, a
section of the substrate was observed. Consequently, the thin BST
film was patterned as shown in FIG. 4C. Also, a mask pattern 408
and an interlayer dielectric film 405 primarily made from SiO.sub.2
and exposed to the Cl.sub.2 gas were not etched at all and were not
reduced in film thickness. When portions from which the thin BST
film 407 was removed were carefully examined, no residue of any
kind was found.
[0113] Analogous experiments were conducted using HCl, HBr, and HI.
In these experiments, the etching gas flow rate was 50 sccm, and
the diluent Ar gas flow rate was 50 sccm. The etching temperature
was 800.degree. C. for HCL and 700.degree. C. for HBr and HI. When
any of HCl, HBr, and HI was used, the thin BST film 407 was
completely etched, and no residue of any kind was found.
[0114] As described above, the present inventors have found that
etching of a thin BST film which is conventionally difficult to
perform can be performed when any of HCl, HBr, and HI gases is
used. This is so because when a thin BST film is exposed to any of
these etching gases at a high temperature, chlorides, bromides, or
iodides of Ba, Sr, and Ti as constituent elements of the thin BST
film form, and these chlorides, bromides, or iodides have enough
vapor pressures at which they volatize at a temperature of about
700 to 800.degree. C. The present inventors have also confirmed
that SiO.sub.2 is not etched at all even when Cl.sub.2 gas or a
hydride of Cl, Br, or I gas is used at high temperatures.
[0115] [Fourth Embodiment]
[0116] In the fourth embodiment, a thin BST film on the substrate
to be processed shown in FIG. 4B is etched by using interhalogen
compounds such as ICl, IBr, and BrCl as etching gases. The
apparatus shown in FIG. 5 is used in etching a thin BST film
because ICl, IBr, and BrCl are a solid, solid, and liquid,
respectively, at room temperature.
[0117] First, ICl and Ar are used as an etching gas and a diluent
gas, respectively, at a flow rate of 50 sccm. The internal pressure
of a reaction chamber 501 is 1 Torr. A resistance heater 504 raises
the temperature of the substrate to be processed to 600.degree. C.
Under the conditions, the substrate to be processed shown in FIG.
4B is etched for 5 min.
[0118] After the substrate was unloaded, a section of the substrate
was observed. Consequently, a thin BST film 407 was patterned as
shown in FIG. 4C. Also, a mask pattern 408 and an interlayer
insulating film 405 primarily made from SiO.sub.2 and exposed to
the ICl gas were not etched at all and were not reduced in film
thickness. When portions from which the thin BST film 407 was
removed were carefully examined, no residue of any kind was
found.
[0119] Similar experiments were conducted using IBr and BrCl. In
these experiments, the etching gas flow rate was 50 sccm, the
diluent Ar gas flow rate was 50 sccm, and the etching temperature
was 600.degree. C. for both of them.
[0120] When either IBr or BrCl was used, BST was completely etched,
and no residue of any kind was found. As described above, the
present inventors have found that etching of a thin BST film which
is conventionally difficult to perform can be performed when IBr or
BrCl is used. This is so because when a thin BST film is exposed to
either of these etching gases at a high temperature, chlorides,
bromides, or iodides of Ba, Sr, and Ti as constituent elements of
the thin BST film form, and these chlorides, bromides, or iodides
have enough vapor pressures at which they volatize at a temperature
of about 600 to 700.degree. C. The present inventors have also
confirmed that SiO.sub.2 is not etched at all even when ICl, IBr,
or BrCl is used at high temperatures.
[0121] When an interhalogen compound such as ICl, IBr, or BrCl is
used, the etching temperature can be decreased to about 600.degree.
C. which is lower than when a halogen or a hydride of a halogen is
used as in the second or third embodiment. This is so because
halogen radicals form when an interhalogen compound is heated to
about 600.degree. C.
[0122] [Fifth Embodiment]
[0123] In the fifth embodiment, a thin BST film on the substrate to
be processed shown in FIG. 4B is dry-etched by using an etching gas
(I.sub.2) activated by a microwave plasma.
[0124] First, in FIG. 4B, a thin BST film 407 is dry-etched by
using an SiO.sub.2 mask pattern 408. An apparatus shown in FIG. 7
is used in this dry etching of the thin BST film 407. Note that the
same reference numerals as in FIG. 5 denote the same parts in FIG.
7, and a detailed description thereof will be omitted. The
characteristic feature of this apparatus is that a source gas is
activated, immediately before being supplied into a reaction
chamber 501, by a plasma which a microwave discharge tube 515 forms
by discharge when applied with a microwave from a microwave power
supply 516.
[0125] In the fifth embodiment, iodine (I.sub.2) is used as an
etching gas material, and Ar as a diluent gas. I.sub.2 gas is
produced when the temperature of a container 507 containing I.sub.2
is raised to 70.degree. C. A mass-flow controller 508 controls the
flow rate of the I.sub.2 gas to 50 sccm. The applied microwave has
a frequency of 13.6 MHz and a power of 150 W. A mass-flow
controller 509 controls the flow rate of the diluent Ar gas to 50
sccm. The internal pressure of the reaction chamber 501 is 1 Torr.
A resistance heater 504 raises the temperature of the substrate to
be processed to 600.degree. C.
[0126] Under the conditions, the substrate to be processed shown in
FIG. 4B is etched for 5 min. After the substrate was unloaded, a
section of the substrate was observed. Consequently, a thin BST
film was patterned as shown in FIG. 4C. Also, an SiO.sub.2 film 408
and an interlayer dielectric film 405 exposed to the I.sub.2 gas
were not etched at all and were not reduced in film thickness. When
portions from which the thin BST film 407 was removed were
carefully examined, no residue of any kind was found.
[0127] As described above, the present inventors have found that
etching of a thin BST film which is conventionally difficult to
perform can be performed by using I.sub.2 gas activated by a
microwave plasma. Additionally, the etching can be performed at a
lower temperature than when inactivated I.sub.2 is used as
described in the second embodiment. This is so because I radicals
form when I.sub.2 gas is activated, iodides of Ba, Sr, and Ti as
constituent elements of a thin BST film form at a lower
temperature, and these iodides have enough vapor pressures at which
they volatize. The present inventors have also confirmed that
SiO.sub.2 is not etched at all even when the activated I.sub.2 gas
is heated to a high temperature.
[0128] Similar experiments were conducted using gases obtained by
activating Cl.sub.2, Br, HCl, HBr, and HI by a microwave plasma.
Consequently, when any of these activated gases was used a thin BST
film could be etched without etching SiO.sub.2 at a lower
temperature than when these gases were not activated by a
microwave.
COMPARATIVE EXAMPLE 1
[0129] A comparative example in which ClF.sub.3 containing F is
used as an etching gas for a thin BST film will be described
below.
[0130] First, a substrate to be processed having a sectional
structure as shown in FIG. 4B is formed following the procedure
explained in the second embodiment. The apparatus shown in FIG. 6
is used in dry etching of a thin BST film using ClF.sub.3. A
mass-flow controller 608 controls the flow rate of ClF.sub.3 to 50
sccm. A mass-flow controller 609 controls the flow rate of Ar gas
as a diluent gas to 50 sccm. The internal pressure of a reaction
chamber is 1 Torr. A resistance heater 604 raises the temperature
of the substrate to be processed to 850.degree. C. Under the
conditions, the substrate to be processed shown in FIG. 4B is
etched for 5 min.
[0131] After the substrate was unloaded, a section of the substrate
was observed. Consequently, an SiO.sub.2 mask pattern 408
completely disappeared, and pattern transfer was found in an
interlayer dielectric film 405. Also, a thin BST film 407 was not
completely removed in any portion. When the remaining thin BST film
was examined, almost no Ti was detected but Ba and Sr remained.
[0132] As described above, the present inventors have found that
when ClF.sub.3 containing fluorine is used as an etching gas, Ba
and Sr are not etched even if the etching temperature is raised to
850.degree. C., so not only a thin BST film is not completely
etched but also SiO.sub.2 is damaged. This is because the etching
gas contains F. F or activated F etches SiO.sub.2 and forms
thermodynamically stable fluorides of alkaline-earth metals such as
BaF.sub.2 and SrF.sub.2. Fluorides of alkaline-earth metals remain
as residues because they have almost no vapor pressure even at high
temperatures.
[0133] FIGS. 8A and 8B show the dependence of etching rates on
temperature when ClF.sub.3 and I.sub.2 are used, respectively. When
ClF.sub.3 is used, Ba and Sr are not etched unless the temperature
is higher than 800.degree. C. Also, the etching rate of quartz
(SiO.sub.2) is higher than those of Ba, Sr, and Ti. In contrast,
when I.sub.2 is used Ba and Sr are also etched at 600.degree. C. or
more and quartz (SiO.sub.2) is hardly etched. This holds true not
only when I.sub.2 gas is used but also when the gases explained in
the second to fifth embodiments are used. Especially when an
interhalogen compound or a plasma-activated halogen gas is used,
the etching temperature can be further lowered.
[0134] FIG. 9 shows the dependence of the vapor pressures of
chlorides, bromides, and iodides of Ba and Sr on temperature. Also,
FIG. 10 collectively shows the etching results of thin BST films
and SiO.sub.2 explained in the second to fifth embodiments and
comparative example 1.
[0135] [Sixth Embodiment]
[0136] In the sixth embodiment, a cleaning method of the present
invention is applied to a chemical vapor deposition (CVD) apparatus
for forming a thin BST film.
[0137] FIG. 11 is a view showing an outline of the arrangement of a
hot wall batch type chemical vapor growth apparatus used in the
sixth embodiment. An inner tube 902 is installed in a quartz
reaction chamber 901. This inner tube 902 contains a substrate boat
904 on which substrates 905 to be processed are placed. A
resistance heater 903 is installed outside the reaction chamber
901. A material container 907 for supplying a cleaning gas is
connected to the lower portion of the inner tube 902 via a
mass-flow controller 908 and an on-off valve 910. This apparatus is
assumed to use a liquid or solid substance as a material. A diluent
gas is supplied from the lower portion of the reaction chamber 901
via a mass-flow controller 909 and an on-off valve 911. A gas
supply system required to deposit a thin BST film is omitted from
FIG. 11. A vacuum exhaust pump 913 is connected to the reaction
chamber 901 via a pressure adjusting valve 912. The exhaust side of
this vacuum exhaust pump 913 is connected to an exhaust gas
processor 914 for exhausting a cleaning gas and a thin BST film
deposition gas after making these gases harmless.
[0138] First, the substrates 905 are mounted on the substrate boat
904 and loaded into the reaction chamber 901. Each substrate 905 is
composed of a silicon substrate, a 100-nm thick silicon oxide
(SiO.sub.2) film formed on the silicon substrate by thermal
oxidation, and a 50-nm thick SRO film deposited on this silicon
oxide film by CVD using Sr(THD).sub.2 and Ru(THD).sub.3.
[0139] Next, after the substrates 905 mounted on the substrate boat
904 is loaded, the pressure adjusting valve 912 is opened to allow
the vacuum exhaust pump 913 to exhaust the reaction chamber 901.
After that, the resistance heater 903 is operated to heat the
substrates 905.
[0140] When the reaction chamber 901 is well exhausted and the
temperature of the substrates 905 becomes 400.degree. C.,
Ba(THD).sub.2, Sr(THD).sub.2, Ti(i-OPr).sub.2(THD).sub.3, and
O.sub.2 as gases for depositing a thin BST film and Ar as a diluent
gas are supplied into the reaction chamber 901 to start depositing
a thin BST film. The flow rates of these source gases are so
adjusted that the composition of a thin BST film to be deposited is
Ba.sub.0.5SR.sub.0.5TiO.sub.3.
[0141] By film deposition for 30 min, a 20-nm thick BST film is
deposited on each substrate 905. The deposition of this thin BST
film is completed by stopping the supply of the source gases into
the reaction chamber 901. After the deposition is completed, the
reaction chamber 901 is evacuated to exhaust the residual gases.
After the residual gases are well exhausted, the internal pressure
of the reaction chamber is returned to normal pressure, and the
substrates 905 are unloaded. After the thin BST film deposition,
deposition of a film containing Ba, Sr, Ti, and 0 was found in the
reaction chamber 901, the inner tube 902, and the substrate boat
904.
[0142] Next, cleaning of the apparatus will be described. First,
the vacuum exhaust pump is used to exhaust the reaction chamber
901. The resistance heater 903 is then used to heat portions to be
cleaned, i.e., the reaction chamber 901, the inner tube 902, and
the substrate boat 904, to a temperature of 650.degree. C. or more
necessary for cleaning, e.g., 700.degree. C.
[0143] I.sub.2 as a cleaning gas of this embodiment is supplied
into the reaction chamber 901 via the mass-flow controller 908 and
the on-off valve 910. Simultaneously, Ar as a diluent gas is
supplied via the on-off valve 911 while the flow rate is controlled
by the mass-flow controller 909. Since I.sub.2 is a solid at room
temperature, I.sub.2 is contained in the container 907, and this
container 907 is heated to 100.degree. C. to produce I.sub.2 gas.
The flow rates of the I.sub.2 gas and the Ar diluent gas are 500
sccm. Also, the pressure adjusting valve 912 adjusts the internal
pressure of the reaction chamber during cleaning to 1 Torr.
[0144] When the apparatus was cleaned for 5 min, the deposits
sticking to the reaction chamber 901, the inner tube 902, and the
substrate boat 904 were completely removed. Also, the quartz
reaction chamber 901, the inner tube 902, and the substrate boat
904 were not damaged at all by the cleaning gas.
[0145] After that, thin BST films were deposited on new substrates,
and particle contamination after the deposition was checked using a
particle counter. Consequently, no big difference was found between
these substrates and the previous substrates, and almost no
particle contamination was observed.
[0146] As described above, the present inventors have found that
when I.sub.2 gas is used, cleaning after chemical vapor deposition
of a thin BST film according to the present invention can be
performed without damaging quartz.
[0147] The present inventors also conducted similar cleaning
experiments by using halogen gases Cl.sub.2 and Br.sub.2 other than
I.sub.2, and HI, HBr, and HCl as hydrides of halogens. The
apparatus shown in FIG. 11 was used for Br.sub.2 which is a liquid
at room temperature, and an apparatus shown in FIG. 12 was used for
Cl.sub.2, HCl, HBr, and HI which are gases at room temperature.
Note that the same reference numerals as in FIG. 11 denote the same
parts in FIG. 12, and a detailed description thereof will be
omitted.
[0148] When substances which are gases at room temperature, such as
Cl.sub.2, HCl, HBr, and HI, are used as cleaning gases, the
cleaning gas supply means can be simplified as in the apparatus
shown in FIG. 12.
[0149] The results of the cleaning experiments showed that cleaning
after thin BST film formation by CVD could be performed at
700.degree. C. for Br.sub.2, HI, and HBr and at 800.degree. C. for
Cl.sub.2 and HCl. Also, when any gas was used, no quartz parts
exposed to the cleaning gas were damaged during cleaning.
[0150] Note that the cleaning temperature for gases containing Cl
is higher than that for gases containing Br or I because the vapor
pressures of chlorides of alkaline-earth metals such as Ba and Sr
are lower than those of bromides or iodides of the same metals.
[0151] [Seventh Embodiment]
[0152] In the seventh embodiment, interhalogen compounds BrCl, ICl,
and IBr not containing fluorine are used as cleaning gases of a
chemical vapor deposition (CVD) apparatus for forming a thin BST
film. At room temperature BrCl is a liquid and ICl and IBr are
solids, so the apparatus shown in FIG. 11 is used.
[0153] First, 20-nm BST films are deposited on substrates 905
following the same procedure as in the sixth embodiment, and the
substrates are unloaded. After this thin BST film deposition,
deposition of a film containing Ba, Sr, Ti, and 0 was found in a
reaction chamber 901, an inner tube 902, and a substrate boat
904.
[0154] Next, the apparatus is cleaned. The cleaning procedure is
the same as in the sixth embodiment except for the cleaning gases.
The temperature of the chamber 901 is 70.degree. C. for BrCl,
70.degree. C. for ICl, and 80.degree. C. for IBr. The flow rates of
these cleaning gases are 500 sccm, and the flow rate of a diluent
Ar gas is also 500 sccm. The cleaning gas temperature is
600.degree. C. for all cleaning gases.
[0155] When the apparatus was cleaned for 5 min, the deposits
sticking to the reaction chamber 901, the inner tube 902, and the
substrate boat 904 were completely removed. Also, no damages caused
by the cleaning gases were found in the quartz reaction chamber
901, the inner tube 902, and the substrate boat 904.
[0156] After that, thin BST films were deposited on new substrates
to be processed, and particle contamination after the deposition
was checked using a particle counter. Consequently, no big
difference was found between these substrates and the previous
substrates, and almost no particle contamination was observed.
[0157] As described above, the present inventors have confirmed
that when BrCl, ICl, and IBr gases are used, cleaning after
chemical vapor deposition of thin BST films according to the
present invention can be performed without damaging quartz at lower
temperatures than when halogens are singly used. Cleaning can be
performed at a low temperature of about 600.degree. C. because
active halogen radicals form when interhalogen compounds are heated
to about 600.degree. C.
[0158] [Eighth Embodiment]
[0159] In the eighth embodiment, Cl.sub.2, Br.sub.2, and I.sub.2
excited by a microwave plasma are used as cleaning gases of a
chemical vapor deposition (CVD) apparatus for forming a thin BST
film.
[0160] FIG. 13 is a schematic view of a CVD apparatus for forming a
thin BST film used in the eighth embodiment. This apparatus is
basically the same as the apparatus shown in FIG. 12 except that a
cleaning gas is plasma-excited, immediately before being supplied
into a reaction chamber 901, by a microwave discharge tube 915
applied with a microwave from a microwave power supply 916.
[0161] First, following the same procedure as in the sixth
embodiment, 20-nm thick BST films are deposited on substrates 905
to be processed, and the substrates are unloaded. After this thin
BST film deposition, deposition of a film containing Ba, Sr, Ti,
and O was found in the reaction chamber 901, an inner tube 902, and
a substrate boat 904.
[0162] Next, the apparatus is cleaned. The cleaning procedure is
the same as in the sixth embodiment except for the cleaning gas. As
the cleaning gas, Cl.sub.2, Br.sub.2, or I.sub.2 gas is used. The
flow rate of any of these cleaning gases is 500 sccm, and the flow
rate of a diluent Ar gas is also 500 sccm. Each cleaning gas is
plasma-excited by the discharge tube 915 applied with a microwave
of 13.6 MHz and 100 W from the power supply 916. The cleaning gas
temperature is 700.degree. C. for Cl.sub.2 and 600.degree. C. for
Br.sub.2 and I.sub.2.
[0163] When the apparatus was cleaned for 5 min, the deposits
sticking to the reaction chamber 901, the inner tube 902, and the
substrate boat 904 were completely removed. Also, no damages caused
by the cleaning gas were found in the quartz reaction chamber 901,
the inner tube 902, and the substrate boat 904. After that, thin
BST films were deposited on new substrates to be processed, and
particle contamination after the deposition was checked using a
particle counter. Consequently, no big difference was found between
these substrates and the previous substrates, and almost no
particle contamination was observed.
[0164] As described above, the present inventors have found that
when plasma-excited Cl.sub.2, Br.sub.2, and I.sub.2 gases are used,
cleaning after chemical vapor deposition of thin BST films
according to the present invention can be performed without
damaging quartz at lower temperatures than when halogens are singly
used. Cleaning can be performed at low temperatures because active
halogen radicals form by plasma excitation.
COMPARATIVE EXAMPLE 2
[0165] A comparative example using ClF.sub.3 containing F as a
cleaning gas will be described below. The same apparatus as shown
in FIG. 12 is used.
[0166] First, following the same procedure as in the sixth
embodiment, 20-nm thick BST films are deposited on substrates 905
to be processed, and the substrates are unloaded. After this thin
BST film deposition, deposition of a film containing Ba, Sr, Ti,
and O was found in a reaction chamber 901, an inner tube 902, and a
substrate boat 904.
[0167] Next, the apparatus is cleaned. The cleaning procedure is
the same as in the sixth embodiment except for the cleaning gas. As
the cleaning gas, ClF.sub.3 is used. The flow rate of the cleaning
gas is 500 sccm, and the flow rate of a diluent Ar gas is also 500
sccm. The cleaning temperature is 850.degree. C. Even after the
apparatus was cleaned for 10 min, the deposits sticking to the
reaction chamber 901, the inner tube 902, and the substrate boat
904 were not completely removed. The unremoved deposits were
examined and found to contain Ba, Sr, F, and O. When a halogen
compound containing F is used as described above, fluorides of
alkaline-earth metals form and remain unremoved because their vapor
pressures are very low. Also, when cleaning using ClF.sub.3 was
repeated several times, devitrification was found in the quartz
reaction chamber 901, the inner tube 902, and the substrate boat
904, indicating that SiO.sub.2 was damaged.
[0168] [Ninth Embodiment]
[0169] In the ninth embodiment, before a hot wall batch type
chemical vapor growth apparatus is cleaned with gas, all quartz
portions to be exposed to a cleaning gas at a high temperature are
coated with a 0.2-.mu.m thick CaF.sub.2 film by plasma spray
coating. The apparatus used in this ninth embodiment is the hot
wall batch type chemical vapor growth apparatus shown in FIG. 1.
The portions coated with CaF.sub.2 are a reaction chamber 101, an
inner tube 102, a substrate boat 105, a heat plug 106, and an
organic metal gas supply nozzle 108.
[0170] Procedures of forming a thin BST film and performing
cleaning using a gas mixture of ClF.sub.3 gas and TiCl.sub.4 gas by
using this apparatus will be described below. First, following the
same procedure as explained in the first embodiment, thin BST films
are formed on substrates 115. By film deposition for 20 min, a
20-nm thick BST film is deposited on each substrate 115. The
deposition of this thin BST film is completed by stopping the
supply of the source gases into the reaction chamber 101. After the
deposition is completed, the reaction chamber 101 is evacuated to
exhaust the residual gases. After the residual gases are well
exhausted, the internal pressure of the reaction chamber is
returned to normal pressure, and the substrates 115 are unloaded.
After this thin BST film deposition, deposition of a film
containing Ba, Sr, Ti, and O was found in the reaction chamber 101,
the inner tube 102, the substrate boat 105, and the upper portion
of the heat plug 106, and outside the nozzle 108.
[0171] Subsequently, the apparatus is cleaned as follows. First, a
vacuum exhaust pump is used to exhaust the reaction chamber 101. A
resistance heater 104 is then used to heat portions to be cleaned,
i.e., the reaction chamber 101, the inner tube 102, the nozzle 108,
and the substrate boat 105, to a temperature necessary for
cleaning, e.g., 850.degree. C.
[0172] Next, ClF.sub.3 and TiCl.sub.4 gases as cleaning gases of
this embodiment and Ar as a diluent gas are supplied into the
reaction chamber 101 from a gas inlet port 111. The flow rates of
ClF.sub.3, TiCl.sub.4, and Ar are 2,000, 10, and 3,000 sccm,
respectively. Also, a pressure adjusting valve 113 is used to
adjust the internal pressure of the reaction chamber during
cleaning to 1 Torr.
[0173] When the apparatus was cleaned for 5 min, the deposits
sticking to the reaction chamber 101, the inner tube 102, the
substrate boat 105, the heat plug 106, and the nozzle 108 were
completely removed. Also, no damages caused by the cleaning gases
were found in any of the quartz reaction chamber 101, the inner
tube 102, the substrate boat 105, the heat plug 106, and the nozzle
108. After that, thin BST films were deposited on new substrates to
be processed, and particle contamination after the deposition was
checked using a particle counter. Consequently, no big difference
was found between these substrates and the previous substrates, and
almost no particle contamination was observed.
[0174] As described above, the present inventors have confirmed
that when ClF.sub.3 gas is used, cleaning after chemical vapor
deposition of thin BST films according to the present invention can
be performed without damaging quartz. No damages were found in the
quartz members even when these quartz members were exposed to the
ClF.sub.3 gas ambient at a high temperature of 850.degree. C.
because the surfaces of the quartz members exposed to the ClF.sub.3
gas were coated with CaF.sub.2. That is, although quartz is
corroded by ClF.sub.3 at high temperatures, CaF.sub.2 is not
corroded by ClF.sub.3 but functions as a protective film even at
high temperatures.
[0175] Changes in the surface state when a CaF.sub.2 film was
exposed to ClF.sub.3 gas at high temperatures was checked by taking
SEM photographs. That is, the surfaces of films not treated in
ClF.sub.3 gas at high temperatures, heat-treated in a ClF.sub.3 gas
ambient at 800.degree. C., 820.degree. C., and 860.degree. C., and
heat-treated in an Ar gas ambient at 860.degree. C. were observed
on SEM photographs. Cracks were found on the surfaces of films
heat-treated in the ClF.sub.3 gas ambient. However, cracks were
also found on the samples heat-treated in the Ar gas ambient.
Therefore, these cracks were formed probably not because heat
treatment was performed in the ClF.sub.3 gas ambient but because
the thickness of the CaF.sub.2 film was as large as 100 .mu.m.
[0176] FIG. 14 shows the X-ray diffraction patterns of these
samples. As shown in FIG. 14, even when heat treatment was
performed in the ClF.sub.3 gas ambient, the same spectra before the
heat treatment were observed. This demonstrates that the
crystalline structure of CaF.sub.2 was not destroyed.
[0177] FIG. 15 shows the results of ICP (Inductively Coupled
Plasma) composition analysis of these samples. As shown in FIG. 15,
no significant changes in the weights of constituent elements were
caused by heat treatment in the ClF.sub.3 gas ambient.
[0178] The experiments using quartz substrates coated with a
fluoride of an alkaline-earth metal as described above shows that
an alkaline-earth metal fluoride has resistance against ClF.sub.3
at a high temperature of 800.degree. C. or more and hence is
effective as a quartz protecting film when cleaning using ClF.sub.3
gas is performed.
[0179] For comparison, similar experiments were conducted by using
a reaction chamber 101, an inner tube 102, a substrate boat 105, a
heat plug 106, and an organic metal gas supply nozzle 108 not
coated with CaF.sub.2. Consequently, deposits on the reaction
chamber 101, the inner tube 102, the substrate boat 105, the heat
plug 106, and the organic metal supply nozzle 108 were removed by
cleaning using ClF.sub.3 gas. However, noticeable damages were
found on the reaction chamber 101, the inner tube 102, the
substrate boat 105, the heat plug 106, and the organic metal gas
supply nozzle 108. These damages were particularly conspicuous in
portions heated to a high temperature, i.e., the reaction chamber
101, the inner tube 102, the substrate boat 105, the upper portion
of the heat plug 106, and the upper portion of the organic metal
gas supply nozzle 108. Almost no damages were found in non-heated
portions whose temperatures were not high, i.e., the lower portion
of the heat plug 106 and the lower portion of the organic metal gas
supply nozzle 108.
[0180] The present inventors also conducted similar experiments by
using a reaction chamber 101, an inner tube 102, a substrate boat
105, a heat plug 106, and an organic metal gas supply nozzle 108
coated with SrF.sub.2 or BaF.sub.2 instead of CaF.sub.2.
Consequently, the deposits were completely removed by cleaning
using ClF.sub.3 gas, as when CaF.sub.2 gas was used, and no damages
of any kind were found in the quartz reaction chamber 101, the
inner tube 102, the substrate boat 105, the heat plug 106, and the
organic metal gas supply nozzle 108. This is so because SrF.sub.2
or BaF.sub.2 also has resistance, like CaF.sub.2, against
ClF.sub.3. The reason for this is that fluorides of alkaline-earth
metals such as CaF.sub.2, SrF.sub.2, and BaF.sub.2 are
thermodynamically very stable compounds.
[0181] To confirm this, the present inventors conducted the
following experiment. A sample formed by depositing by CVD a 100-nm
thick BST film on a quartz substrate coated with a CaF.sub.2 film
was exposed to ClF.sub.3 gas. The deposition conditions of the thin
BST film and the conditions in which the sample was exposed to the
ClF.sub.3 gas were the same as the aforementioned conditions. Also,
a sample formed by depositing a thin BST film directly on a quartz
substrate not coated with a CaF.sub.2 film was used as a
comparative example.
[0182] FIGS. 16A and 16B show the etching rates of Ba, Sr, Ti, and
SiO.sub.2 (quartz). The etching rates of Ba, Sr, and Ti and their
dependence on temperature did not change regardless of whether the
CaF.sub.2 coating was present or absent. However, the etching rate
of quartz largely changed in accordance with the presence/absence
of the CaF.sub.2 coating. That is, without any CaF.sub.2 coating
quartz was etched at a higher etching rate than those of Ba, Sr,
and Ti. With the CaF.sub.2 coating, however, quartz was hardly
etched. This indicates that CaF.sub.2 functioned as a film for
protecting quartz from ClF.sub.3. Furthermore, an experiment of
exposing CaF.sub.2-coated quartz to ClF.sub.3 gas revealed that the
crystalline structure, composition, and Ca and F contents of
CaF.sub.2 were the same before and after exposure to the CaF.sub.3
gas. This implies that CaF.sub.2 has resistance against ClF.sub.3.
Additionally, this resistance of CaF.sub.2 against ClF.sub.3 did
not deteriorate even when the temperature of exposure to ClF.sub.3
was raised to 900.degree. C. This does not mean 900.degree. C. is
the critical temperature of the resistance; this means that no
further experiments have been conducted although the resistance
probably exists even at temperatures higher than 900.degree. C. The
present inventors have also experimentally confirmed that the same
effects as when the CaF.sub.2 coating film described above was used
are obtained when SrF.sub.2 and BaF.sub.2 are used as coating
films.
[0183] Note that coating the surface of quartz with CaF.sub.2
suppresses damages to the quartz. Therefore, Sr and Ba can be
etched by using a gas not containing TiCl.sub.4 (e.g., a
fluorine-containing halogen gas or an interhalogen compound gas) by
which the cleaning temperature can be further raised compared to
normal temperatures.
[0184] [10th Embodiment]
[0185] In the 10th embodiment, a method using ordinary quartz
members, rather than quartz members coated with fluorides of
alkaline-earth metals as explained in the ninth embodiment, will be
described. That is, in this 10th embodiment a fluoride of an
alkaline-earth metal is deposited by CVD before deposition of BST,
thereby depositing this alkaline-earth metal fluoride as a coating
film on quartz members.
[0186] The 10th embodiment uses the same apparatus as in FIG. 1.
However, each quartz member is not coated with CaF.sub.2. Note that
a gas for depositing a CaF.sub.2 film is supplied from a port
111.
[0187] Procedures of depositing a CaF.sub.2 protective film,
forming a thin BST film, and performing cleaning using ClF.sub.3
gas by using this apparatus will be described below. First, a
substrate boat 105 not mounting substrates 115 on it is loaded into
a reaction chamber 101. After the substrate boat 105 is loaded, a
pressure adjusting valve 113 is opened to allow a vacuum exhaust
pump 114 to exhaust the reaction chamber 101.
[0188] After that, the pressure adjusting valve 113 adjusts the
internal pressure of the reaction chamber 101 to 1 Torr. A
resistance heater 104 is operated to heat portions where a
CaF.sub.2 protective film is to be deposited. When the reaction
chamber 101 is well exhausted and the temperature of the portions
for deposition becomes 500.degree. C., Ca(HFA).sub.2
(HFA=C.sub.5HF.sub.6O.sub.2 hexafluoroacetxlaceton) as a CaF.sub.2
protective film depositing gas and Ar as a diluent gas are supplied
into the reaction chamber to start depositing a CaF.sub.2 film.
[0189] By performing this film deposition for 30 min, a 100-nm
thick CaF.sub.2 film is formed on the portions for deposition,
e.g., the reaction chamber 101, an inner tube 102, the substrate
boat 105, the upper portion of a heat plug 106, and the interior of
a nozzle 108. The deposition of this thin CaF.sub.2 film is
completed by stopping the supply of the source gases into the
reaction chamber 101.
[0190] After the deposition is completed, the reaction chamber 101
is evacuated to exhaust the residual gases. After the residual
gases are well exhausted, the internal pressure of the reaction
chamber is returned to normal pressure. After this thin CaF.sub.2
film deposition, deposition of the CaF.sub.2 film was found in the
reaction chamber 101, the inner tube 102, the substrate boat 105,
the upper portion of the heat plug 106, and the interior of the
nozzle 108.
[0191] Next, a process of depositing a thin BST film on a substrate
is performed. The substrate boat 105 mounting the substrates 115 on
it is loaded into the reaction chamber 101. Each substrate 115 is
composed of a silicon substrate, a 100-nm thick silicon oxide
(SiO.sub.2) film formed on this silicon substrate by thermal
oxidation, and a 50-nm thick SrRuO.sub.3 (SRO) film deposited on
this silicon oxide film by CVD using Sr(THD).sub.2 and
RU(THD).sub.3 (THD=C.sub.11H.sub.19O.sub.2).
[0192] Thin BST films were formed for 20 min following the same
procedure as in the first embodiment, and the substrates 115 are
unloaded. After this BST film deposition, deposition of a film
containing Ba, Sr, Ti, and 0 was found in the reaction chamber 101,
the inner tube 102, the substrate boat 105, and the upper portion
of the heat plug 106, and outside the nozzle 108.
[0193] Subsequently, the apparatus is cleaned as follows. First, a
vacuum exhaust pump is used to exhaust the reaction chamber 101.
The resistance heater 104 is then used to heat portions to be
cleaned, i.e., the reaction chamber 101, the inner tube 102, the
nozzle 108, and the substrate boat 105, to a temperature necessary
for cleaning, e.g., 850.degree. C.
[0194] Next, ClF.sub.3 and TiCl.sub.4 gases as cleaning gases of
this embodiment and Ar as a diluent gas are supplied into the
reaction chamber 101 from the gas inlet port 111. The flow rates of
ClF.sub.3, TiCl.sub.4, and Ar are 2,000, 100, and 3,000 sccm,
respectively. Also, the pressure adjusting valve 113 is used to
adjust the internal pressure of the reaction chamber during
cleaning to 1 Torr.
[0195] When the apparatus was cleaned for 5 min, the deposits
sticking to the reaction chamber 101, the inner tube 102, the
substrate boat 105, the heat plug 106, and the nozzle 108 were
completely removed. Also, no damages of any kind caused by the
cleaning gases were found in any of the quartz reaction chamber
101, the inner tube 102, the substrate boat 105, the heat plug 106,
and the nozzle 108. After that, thin BST films were deposited on
new substrates to be processed, and particle contamination after
the deposition was checked using a particle counter. Consequently,
no big difference was found between these substrates and the
previous substrates, and almost no particle contamination was
observed.
[0196] As described above, the present inventors have confirmed
that when ClF.sub.3 gas is used, cleaning after chemical vapor
deposition of thin BST films according to the present invention can
be performed without damaging quartz.
[0197] No damages were found in the quartz members even when these
quartz members were exposed to the ClF.sub.3 gas ambient at a high
temperature of 850.degree. C. because the surfaces of the quartz
members exposed to the ClF.sub.3 gas were coated with CaF.sub.2.
That is, although quartz is corroded by ClF.sub.3 at high
temperatures, CaF.sub.2 is not corroded by ClF.sub.3 but functions
as a protective film even at high temperatures.
[0198] As described above, a CaF.sub.2 film formed in the same
manner as conventional CVD by using gases also has ClF.sub.3
resistance at high temperatures and is effective as a quartz
protecting film in ClF.sub.3 cleaning.
[0199] The present inventors have also confirmed that when
SrF.sub.2 and BaF.sub.2 are used as protective films, it is
possible, as when CaF.sub.2 is used, to protect quartz when
cleaning using ClF.sub.3 gas is performed. These SrF.sub.2 film and
BaF.sub.2 film are deposited by using Sr(HFA).sub.2 and
Ba(HFA).sub.2, respectively. The present inventors have further
confirmed that quartz members are not damaged even when deposition
of thin BST films is repeated a plurality of times with respect to
one-time deposition of an alkaline-earth metal fluoride protective
film.
[0200] As described above, the method of forming a protective film
by using gases has the advantages that it is unnecessary to
disassemble the apparatus in forming a protective film since the
protective film is formed using gases as in normal CVD, and that
even if a protective film deteriorates when BST film formation and
ClF.sub.3 cleaning are performed a large number of times, the
protective film can be formed without immediately disassembling the
apparatus. This increases the availability and throughput of the
apparatus.
[0201] The above ninth and 10th embodiments are explained by taking
a BST film formation apparatus and its cleaning method as examples.
However, the present invention is not limited to these embodiments
and applicable to other cases without departing from the gist of
the invention. For example, a highly corrosive gas such as
ClF.sub.3 must be used at high temperatures in gas cleaning of a
CVD apparatus for depositing a thin film containing, as a
constituent element, an alkaline-earth metal, e.g., a ferroelectric
substance such as SrBi.sub.2Ta.sub.2O.sub.9 or an electrode
material such as SrRuO.sub.3. As a consequence, quartz members are
damaged. The present invention can be applied to a CVD apparatus
using such materials.
[0202] [11th Embodiment]
[0203] In the 11th to 15th embodiments, etching using a halogen
gas, particularly chlorine gas will be described again.
[0204] In the 11th embodiment, dry etching of a thin BST film using
chlorine gas is performed in two stages. This embodiment is an
example of thin BST film etching in a peripheral circuit portion of
a semiconductor memory device in which a thin BST film is formed as
a capacitor dielectric film.
[0205] FIG. 17 is a sectional view of a semiconductor memory device
1100 which is to undergo BST etching. In a DRAM cell portion 1102,
a 20-nm thick BST film 1103 forms a capacitor between ruthenium
films 1104 and 1105 as upper and lower electrodes. In a peripheral
circuit portion 1106, this thin BST film 1103 is formed on a
silicon oxide film 1107 of an interlayer dielectric film. The
surface of this thin BST film 1103 is coated with the ruthenium
film 1105 as an electrode.
[0206] To form interconnections and the like in the peripheral
circuit portion, it is necessary to remove the thin BST film 1103
and the ruthenium film 1105 from the peripheral circuit portion
1106. The process wills be described below with reference to FIGS.
18A to 18C. FIGS. 18A to 18C are views showing only the peripheral
circuit portion 1106 in enlarged scale for the sake of easy
understanding.
[0207] First, a 300-nm thick silicon oxide film 1108 is formed as a
hard mask on the ruthenium film 1105 by plasma CVD. This silicon
oxide film 1108 and the ruthenium film 1105 are patterned by
conventional lithography and dry etching (FIG. 18A).
[0208] The thin BST film is dry-etched as follows. That is, a
substrate 101 to be processed is supported by a transparent quartz
suscepter and heated by a heater using a halogen lamp. Nitrogen gas
as a diluent gas and chlorine gas as an etching gas are supplied at
a flow rate of 50 sccm. The etching pressure is 1 Torr. Under such
etching gas supply conditions, the thin BST film is etched under
the following different etching temperature conditions. Note that
the rate of heating is 100.degree. C./sec, and the rate of cooling,
which is natural cooling, is about 20.degree. C./sec.
[0209] The following six conditions were chosen as the etching
temperature conditions:
[0210] (1) After etching at 500.degree. C. for 2 min, etching at
820.degree. C. for 2 min.
[0211] (2) After etching at 820.degree. C. for 2 min, etching at
500.degree. C. for 2 min.
[0212] (3) Only etching at 500.degree. C. for 2 min as a
reference.
[0213] (4) Only etching at 820.degree. C. for 2 min as a
reference.
[0214] (5) Only etching at 500.degree. C. for 2 hr as a
reference.
[0215] (6) Only etching at 820.degree. C. for 2 hr as a
reference.
[0216] The results are shown in FIG. 19. FIG. 19 shows the etching
amounts, under the above temperature conditions, of metals Ba, Sr,
and Ti constructing the thin BST film.
[0217] FIG. 19 demonstrates that only Ti in the thin BST film was
etched by 500.degree. C.-etching (3), and Ba and Sr were primarily
etched by 820.degree. C.-etching (4). Comparing the results of (3)
and (4) with the results of (5) and (6) in which the etching time
was prolonged shows that even when the etching time was increased,
almost no difference was found between the etching amounts of each
metal.
[0218] In contrast, the 20-nm thick BST film was completely etched
in methods (1) and (2), indicating that a thin BST film can be
completely dry-etched by two-stage etching.
[0219] Also, sections of substrates processed under conditions (1),
(2), (3), and (4) were observed. Consequently, as shown in FIG.
18B, the thin BST film 103 was patterned under conditions (1) and
(2). However, as shown in FIG. 18C, no big difference in shape from
FIG. 18A was found in substrates processed under conditions (3) and
(4). Using samples processed under conditions (3) and (4), the BST
film 109 immediately below the opening in the silicon oxide film
was evaluated by TEM-EDX. As a consequence, only Ba and Sr were
detected as constituent elements in condition (3), and only Ti was
detected in condition (4).
[0220] That is, by combining 500.degree. C.-chlorine-etching
capable of etching only Ti and 820.degree. C.-etching almost unable
to etch Ti but capable of etching Ba and Sr, a thin BST film
containing these three metals can be completely dry-etched within a
short time period. Also, no damages were found in the SiO.sub.2
1108 or 1107 exposed to Cl.sub.2 gas, and no film thickness
reduction was found. Furthermore, the quartz suscepter on which the
substrates to be processed were mounted were examined, with the
result that none of etched marks, flaws, and deposits were
found.
[0221] Note that a thin BST film can be etched at a single etching
temperature by properly selecting an etching temperature between
820.degree. C. and 500.degree. C. However, this etching temperature
takes different values in accordance with the film composition of a
thin BST film to be etched. Accordingly, etching residues easily
form, or the actual temperature of a substrate to be processed on
which a pattern of a semiconductor device is formed changes in
accordance with this semiconductor device pattern. For this reason,
an etching temperature must be set for each pattern on a wafer, and
this decreases the process margin. In contrast, the method of the
present invention can reliably etch a thin BST film of any
composition.
[0222] By using an apparatus similar to that of this embodiment,
etching of a three-layered structure of 20-nm thick SRO
(SrRuO.sub.3)/40-nm thick SBT (Bi.sub.2SrTa.sub.2O.sub.9)/50-nm
thick SRO was tried using a silicon oxide film as a mask.
Consequently, the three-layered structure was completely etched by
performing the first etching at 500.degree. C. for 10 min and the
second etching at 80.degree. C. for 20 min under the same gas
conditions as described above.
[0223] [12th Embodiment]
[0224] In the 12th embodiment, a cleaning method of the present
invention is applied to a chemical vapor deposition (CVD) apparatus
for depositing a BST film.
[0225] In cleaning of a reaction chamber of the BST film formation
CVD apparatus of this embodiment, a thick BST film is etched by
successively performing etching by which only Ti in the BST film is
etched at a low temperature and etching by which Ba and Sr are
etched at a high temperature.
[0226] As described in the 11th embodiment, when a BST film is
etched under a single temperature condition, the etching
temperature is very difficult to choose. This makes etching at a
single temperature more difficult when a thick BST film is to be
etched as when chamber cleaning is performed. This is so because
the etching rates of Ba, Sr, and Ti constructing a BST film are
different. For example, if the etching rates of Ba and Sr are high,
Ti whose etching rate is low remains on the surface of a BST film
during etching. This residual Ti interferes with outward diffusion
of BaCl.sub.2 and SrCl.sub.2 gases from the BST film and thereby
stops the etching.
[0227] FIG. 20 shows the arrangement of a hot wall batch furnace
used in this embodiment. An inner tube 1202 is installed in a
quartz reaction chamber 1201. A substrate boat 1204 mounting
substrates 1203 to be processed on it is housed in the inner tube
1202. A heater 1205 is installed outside the reaction chamber 1201.
As this heater 1205 for the reaction chamber, a small-heat-capacity
heater suited to rapid heating and cooling is used. A quartz nozzle
1206 for cooling the heater 1205 by blowing a large amount of air
against the heater 1205 is attached to the lower portion of the
heater 1205. The reaction chamber temperature can be rapidly
decreased by blowing cooling air from the quartz nozzle 1206
against the heater to exhaust hot air from an exhaust port 1207 in
the upper portion of the quartz tube.
[0228] Chlorine of a cleaning gas and nitrogen of a diluent gas are
supplied to the lower portion of the inner tube 1202 via mass-flow
controllers 1208 and 1209 and valves 1210 and 1211. A supply system
of gases (Ba(THD).sub.2, Sr(THD).sub.2, Ti(i-OPr).sub.2(THD).sub.2,
and O.sub.2 as source gases of BST) necessary to deposit a BST film
is omitted from FIG. 20.
[0229] A vacuum pump 1213 is connected to the reaction chamber 1201
via a pressure adjusting valve 1212. The exhaust side of this
vacuum pump 1213 is connected to an exhaust gas processor 1214 for
exhausting the cleaning gas and the BST film depositing gas after
making these gases harmless.
[0230] A cleaning method will be described below. First, substrates
1203 for BST film formation are mounted on the substrate boat 1204
and loaded into the reaction chamber 1201. To evaluate etching,
each substrate 1203 was formed by forming a 100-nm thick silicon
oxide film on the two entire surfaces by thermally oxidizing a
silicon substrate.
[0231] Next, the pressure adjusting valve 1212 is opened to allow
the vacuum pump 1213 to exhaust the reaction chamber 1201. At that
time the set temperature in the reaction chamber is 300.degree.
C.
[0232] After the reaction chamber 1201 is well exhausted, the set
temperature in the reaction chamber 1201 is increased to
450.degree. C. When the temperature of the substrates 1203 becomes
450.degree. C., Ba(THD).sub.2, Sr(THD).sub.2, and
Ti(i-OPr).sub.2(THD).sub.2, and O.sub.2 as BST film depositing
gases and Ar as a diluent gas are supplied into the reaction
chamber to start depositing a BST film. The flow rate of these
source gases are so adjusted that the composition of a BST film to
be deposited is Ba.sub.0.5Sr.sub.0.5TiO.sub.3.
[0233] By film deposition for 5 min, a 20-nm thick BST film is
formed on each substrate 1203. The deposition of this thin BST film
is completed by stopping the supply of the source gases into the
reaction chamber 1201. After the deposition is completed, to
exhaust the residual gases in the reaction chamber 1201, evacuation
and nitrogen purge are repeated to completely purge the residual
gases. After the residual gases are well exhausted, the internal
pressure of the reaction chamber 1201 is returned to normal
pressure, and the substrates 1203 are unloaded. After this thin BST
film deposition, deposition of a film containing Ba, Sr, Ti, and O
was found in the reaction chamber 1201, the inner tube 1202, and
the substrate boat 1204. The composition of the film was found to
be Ba.sub.0.5Sr.sub.0.5Ti.sub.1.5O.sub.4-d (d.apprxeq.0), and the
film thickness was found to be about 20 nm. This film formation
sequence was repeated 50 times. As a consequence, a BST film with a
film thickness of about 1 .mu.m formed on the reaction chamber
1201, the inner tube 1202, the substrate 1203, and the substrate
boat 1204.
[0234] Next, the apparatus is cleaned. First, the vacuum pump is
used to exhaust the reaction chamber 1201. The heater 1203 is then
used to heat portions to be cleaned, i.e., the reaction chamber
1201, the inner tube 1202, and the substrate boat 1204, to
500.degree. C. necessary for the first etching.
[0235] Subsequently, the BST film is etched in accordance with a
temperature profile as shown in FIG. 21. Nitrogen is first supplied
into the reaction chamber at a flow rate of 3 SLM. The pressure
adjusting valve is used to hold the internal pressure of the
reaction chamber at 1.5 Torr, and the temperature in the reaction
chamber is held at 500.degree. C. After that, chlorine gas and
nitrogen gas are supplied into the reaction chamber at flow rates
of 2 SLM and 3 SLM, respectively, and the internal pressure of the
reaction chamber is controlled to 1.5 Torr. In this state, the BST
film is etched for 30 min.
[0236] Next, an etching gas is supplied. While the internal
pressure of the reaction chamber is held at 1.5 Torr, the
temperature in the reaction chamber is raised to 850.degree. C. at
a rate of 100.degree. C./min and held at 850.degree. C. for 30
min.
[0237] Then the supply of the chlorine gas is stopped. While the
temperature in the reaction chamber is lowered at a rate of
50.degree. C./min, evacuation of the reaction chamber 1201 and
nitrogen purge are repeated to purge the residual chlorine gas in
the reaction chamber. When the etching amount of the BST film on
the thermally oxidized substrate 1203 was evaluated, the 1-.mu.m
thick BST film was found to be completely etched.
[0238] A mechanism by which a BST film is completely etched in this
embodiment will be described below.
[0239] FIG. 22 shows the residual amounts of metal elements
constructing a 1-.mu.m thick BST film when a substrate on which
this BST film was formed was etched under the aforementioned
etching gas conditions while the etching temperature alone was
changed from 400.degree. C. to 900.degree. C. As shown in FIG. 22,
the etching characteristics of the constituent elements of a BST
film depend on temperature to different degrees; Ti is easily
etched at low temperatures, whereas Ba and Sr are easily etched at
high temperatures.
[0240] That is, in this embodiment etching is first performed at
500.degree. C. to selectively etch Ti in the BST film. 500.degree.
C. is selected because, as shown in FIG. 22, whether Ti in a
1-.mu.m thick BST film is completely etched greatly depends upon
the etching temperature. That is, the temperature must be lowered
to 500.degree. C. or less in order to etch a whole 1-.mu.m thick
BST film. 850.degree. C. is chosen as the etching temperature for
the second time since it is necessary to raise the reaction chamber
temperature to 700.degree. C. or more to etch Ba and Sr.
[0241] In this embodiment, a heater having a high heating rate of
100.degree. C./min is used to allow smooth switching between the
two etching temperatures.
[0242] [13th Embodiment]
[0243] In the 13th embodiment, the first etching by which Ti is
primarily etched and the second etching by which Ba and Sr are
primarily etched are repeated a plurality of times. In this method,
it is unnecessary to completely etch a specific metal in a thick
BST film by one-time etching, so the temperature of the first
etching and the temperature of the second etching can be set at
relative close values, e.g., 700.degree. C. and 850.degree. C.
Accordingly, heating and cooling need only be performed in a
relatively narrow temperature range, as compared with the 12th
embodiment, and this shortens the times necessary for heating and
cooling. As a consequence, etching can be performed within a short
time period.
[0244] Also, when a very thick BST film is to be etched, it is
difficult to completely etch only a specific metal (e.g., Ti) in
the BST film by one-time etching. Therefore, the method of this
embodiment in which a thick film is etched in stages is
suitable.
[0245] The arrangement of an apparatus for conducting an etching
experiment is the same as in FIG. 20. As in the 12th embodiment,
after a 100-nm thick thermal oxide film is formed, a 1-.mu.m thick
BST film is formed. Then, the apparatus is cleaned. First, a vacuum
pump is used to exhaust a reaction chamber 1201. A heater 1203 is
then used to heat portions to be cleaned, i.e., the reaction
chamber 1201, an inner tube 1202, and a substrate boat 1204, to
850.degree. C. necessary for the first etching.
[0246] Subsequently, the BST film is etched in accordance with a
temperature profile as shown in FIG. 23.
[0247] Nitrogen is first supplied into the reaction chamber at a
flow rate of 3 SLM. A pressure adjusting valve is used to hold the
pressure at 1.5 Torr, and the temperature in the reaction chamber
is held at 850.degree. C. Chlorine gas and nitrogen gas are then
supplied into the reaction chamber at flow rates of 2 SLM and 3
SLM, respectively. The internal pressure of the reaction chamber is
controlled to 1.5 Torr, and etching is performed for 5 min in this
state.
[0248] Next, an etching gas is supplied. While the internal
pressure of the reaction chamber is controlled to 1.5 Torr, the
temperature in the reaction chamber is lowered to 700.degree. C. at
a rate of 50.degree. C./min, and etching is performed for 5 min at
700.degree. C. After that, chlorine gas and nitrogen gas are
supplied. While the internal pressure of the chamber is kept
controlled to 1.5 Torr, the temperature in the reaction chamber
1201 is raised to 850.degree. C. at a heating rate of 100.degree.
C./min. Following the same procedure as above, 850.degree. C.-5 min
etching and 700.degree. C.-5 min etching are repeated five
times.
[0249] Next, the supply of the chlorine gas is stopped. While the
temperature in the reaction chamber is decreased to 300.degree. C.
at a rate of 20.degree. C./min, evacuation of the reaction chamber
1201 and nitrogen purge are repeated to purge the residual chlorine
gas in the reaction chamber. After the residual chlorine gas is
completely purged, the temperature in the reaction chamber 1201 is
stabilized at 300.degree. C., and the internal pressure of the
reaction chamber is returned to normal pressure. When the etching
amount of the 1-.mu.m thick BST film on the thermally oxidized
substrate 1203 was evaluated, this 1-.mu.m thick BST film was found
to be completely etched.
[0250] A mechanism by which a BST film is completely etched in this
embodiment will be described below.
[0251] FIG. 24 shows the dependence of the etching depths on
etching temperature of metals Ba, Sr, and Ti constructing a
10-.mu.m thick BST film, estimated from the etching amounts of
these metals Ba, Sr, and Ti, when a quartz substrate on which this
BST film was formed was etched in chlorine for 5 min. For example,
at 850.degree. C. the etching depth of Ti is 0 nm and the etching
depths of Ba and Sr are 200 nm. This means that at 850.degree. C.
Ti is not etched at all but Ba and Sr are completely etched to a
depth of 200 nm.
[0252] That is, in this embodiment etching is first performed at
850.degree. C. to selectively etch, as shown in FIG. 25A, Ba and Sr
in the 1-.mu.m thick BST film to a depth of 200 nm from the
surface. Next, etching is performed at 700.degree. C. to completely
etch Ti remaining on the surface (FIG. 25B). Furthermore, Ti to a
depth of 150 nm from the surface of the remaining 800-nm thick BST
film is completely etched, with the result that Ba and Sr remain on
the surface (FIG. 25C). Following the same procedure, the
high-temperature etching and the low-temperature etching are
successively repeated to completely etch the thick BST film.
[0253] In this embodiment, a furnace having a high heating rate of
100.degree. C./min and a high cooling rate of 50.degree. C./min is
used to allow the two etching temperatures to be switched within
very short time periods.
[0254] [14th Embodiment]
[0255] In the 14th embodiment, a cleaning method of the present
invention is applied to a BST film chemical vapor deposition (CVD)
apparatus. In this method, the first etching using ClF.sub.3
capable of completely etching Ti in a BST film and the second
etching using chlorine capable of completely etching Ba and Sr are
successively performed. This method etches a BST film, without ever
changing the etching temperature, by using ClF.sub.3 gas capable of
etching Ti even in a high temperature region of 800.degree. C. or
more in which Ba and Sr can be etched.
[0256] The arrangement of an apparatus for conducting an etching
experiment is basically the same as shown in FIG. 20. However, as
an etching gas supply system, a line capable of supplying ClF.sub.3
into an inner tube 1202 via a mass-flow controller and a valve is
installed, in addition to that for chlorine, and similar to a
chlorine line for supplying chlorine.
[0257] As in the 12th embodiment, substrates 1203 on each of which
a 100-nm thick thermal oxide film and a 1-.mu.m thick BST film are
formed in this order are mounted on a substrate boat 1204 and
loaded into a reaction chamber 1201. First, a vacuum pump is used
to exhaust the reaction chamber 1201. A heater 1205 is then used to
heat portions to be cleaned, i.e., the reaction chamber 1201, the
inner tube 1202, and the substrate boat 1204, to 850.degree. C.
necessary for the first etching.
[0258] Subsequently, the BST films are etched in accordance with a
gas sequence as shown in FIG. 26. Initially, nitrogen is supplied
into the reaction chamber at a flow rate of 3 SLM. The pressure is
held at 1.5 Torr by using a pressure adjusting valve, and the
temperature in the reaction chamber is held at 850.degree. C.
[0259] Next, ClF.sub.3 gas and nitrogen gas are supplied into the
reaction chamber at flow rates of 100 sccm and 3 SLM, respectively,
and the pressure is controlled to 1.5 Torr. In this state, etching
is performed for 5 min. To suppress damages to quartz, the partial
pressure of ClF.sub.3 is {fraction (1/10)} or less of the partial
pressure of chlorine in the previous embodiments. By this etching,
Ti in the 1-.mu.m thick BST film is completely etched, and about 5%
or less of the total amounts of Ba and Sr are etched. In the case
of ClF.sub.3 etching, when Ti in a BST film is completely etched,
etching of Ba or Sr stops. Therefore, the etching amounts of Ba and
Sr are at most about 5%.
[0260] The reaction chamber is then evacuated to exhaust the
residual ClF.sub.3 gas. Nitrogen gas is again supplied into the
reaction chamber 1201 at a flow rate of 3 SLM. While the internal
pressure of the reaction chamber is controlled to 1.5 Torr, purge
is performed for 5 min. Next, chlorine gas and nitrogen gas are
supplied into the reaction chamber at flow rates of 2 SLM and 3
SLM, respectively. Etching is performed for 20 min while the
temperature in the reaction chamber is held at 850.degree. C. By
this etching, Ba and Sr in the remaining BST film are completely
etched.
[0261] The supply of the chlorine gas is stopped. While the
temperature in the reaction chamber is lowered to 300.degree. C. at
a rate of 50.degree. C./min, evacuation of the reaction chamber
1201 and nitrogen purge are repeated to purge the residual chlorine
gas in the reaction chamber. After the residual chlorine gas is
completely purged, the temperature in the reaction chamber is
stabilized at 300.degree. C., and its internal pressure is returned
to normal pressure. When the etching amount of the 1-.mu.m thick
BST film on the thermally oxidized substrate 1203 was evaluated,
the 1-.mu.m thick BST film was found to be completely etched.
[0262] The etching amount of the thermal oxide film on the
substrate 1203 was also evaluated by an ellipsometer. Consequently,
the thermal oxide film was etched by about 20 nm. Additionally, the
substrate boat 1204 on which the substrates 1203 were mounted was
examined, with the result that no clear quartz etching damages such
as devitrification were observed.
[0263] [15th Embodiment]
[0264] In the 15th embodiment, a method of etching a BST film
formed in a CVD reaction chamber by continuously increasing or
decreasing the temperature in the reaction chamber will be
described. Two-stage etching (the 12th embodiment) or multistage
etching (the 13th embodiment) requires an enormous etching time
unless a furnace heater has an excellent rapid heating/cooling
characteristic. In the case of a furnace inferior in this
heating/cooling characteristic, therefore, a method of etching
which gradually changes the furnace temperature as in this
embodiment is effective.
[0265] As an experimental apparatus, an apparatus (FIG. 20) similar
to the one described in the 12th embodiment was used. As in the
12th embodiment, after a 100-nm thick thermal oxide film is formed,
a 1-.mu.m thick BST film is formed. Next, the apparatus is cleaned.
First, a vacuum pump is used to exhaust a reaction chamber 1201. A
heater 1205 is then used to heat portions to be cleaned, i.e., the
reaction chamber 1201, an inner tube 1202, and a substrate boat
1204, to 500.degree. C.
[0266] Nitrogen is supplied into the reaction chamber at a flow
rate of 3 SLM. A pressure adjusting valve is used to hold the
internal pressure of the reaction chamber at 1.5 Torr, and the
temperature in the reaction chamber is held at 500.degree. C. After
that, chlorine gas and nitrogen gas are supplied into the reaction
chamber at flow rates of 2 SLM and 3 SLM, respectively, and the
pressure is controlled to 1.5 Torr. In this state, in accordance
with a temperature profile as shown in FIG. 27, the reaction
chamber temperature is raised to 850.degree. C. and held at
850.degree. C. for 30 min, thereby etching the BST film.
[0267] The supply of the chlorine gas is then stopped. While the
temperature in the reaction chamber is lowered to 300.degree. C. at
a rate of 10.degree. C./min, evacuation of the reaction chamber
1201 and nitrogen purge are repeated to purge the residual chlorine
gas in the reaction chamber. After the residual chlorine gas is
completely purged, the temperature in the reaction chamber is
stabilized at 300.degree. C., and its internal pressure is returned
to normal pressure. When the etching amount of the 1-.mu.m thick
BST film on the thermally oxidized substrates 1203 was evaluated,
this 1-.mu.m thick BST film was found to be completely etched.
[0268] This is presumable because, in this embodiment, in the
process of heating from 500.degree. C. Ti was etched first and then
Ba and Sr were gradually etched, because only Ba and Sr remained
when the temperature was held at 850.degree. C., and because the
residual Ba and Sr were completely etched by the etching at
850.degree. C. for 30 min.
[0269] The present invention has been explained by using the
embodiments and the comparative examples. One gist of the present
invention is to use a halogen or halogen compound not containing
fluorine in dry etching of a thin film containing alkaline-earth
metals or in cleaning of a chemical vapor growth apparatus for
forming a thin film containing alkaline-earth metals. Particularly
chlorides, bromides, and iodides of alkaline-earth metals have
relatively high vapor pressures compared to other alkaline-earth
metal compounds, and the use of gases containing Cl, Br, and I is
effective in etching.
[0270] Also, an oxide containing alkaline-earth metals except for
Ti can be well etched by using an etching gas composed of a gas
containing a halogen and a gas consisting of a halide of Ti.
[0271] When a halogen gas containing fluorine is to be used,
damages to SiO.sub.2 portions used in a film formation apparatus
are prevented by coating these SiO.sub.2 portions with a fluoride
of an alkaline-earth metal.
[0272] Furthermore, the embodiments are explained by using a thin
BST film as an example. However, the present invention is, of
course, effective to thin films containing alkaline-earth metals
such as SrRuO.sub.3 and SrBi.sub.2Ta.sub.2O.sub.9.
[0273] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *