U.S. patent application number 09/804266 was filed with the patent office on 2001-10-18 for thin film production process and optical device.
Invention is credited to Ando, Kenji, Biro, Ryuji, Kanazawa, Hidehiro, Otani, Minoru, Shingu, Toshiaki, Suzuki, Yasuyuki.
Application Number | 20010031543 09/804266 |
Document ID | / |
Family ID | 26587306 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010031543 |
Kind Code |
A1 |
Ando, Kenji ; et
al. |
October 18, 2001 |
Thin film production process and optical device
Abstract
A process of producing a thin film is disclosed which comprises
the steps of: providing a vessel; placing a target such that a
surface to be sputtered of the target surrounds a discharge space;
placing a substrate on a side of an opening of the space such that
the substrate faces an anode disposed so as to close another
opening of the space surrounded by the target; supplying a
sputtering gas and a fluorine-containing gas into the vessel; and
supplying a dc power or a power obtained by superimposing pulses
with reversing polarities on the dc power, between the target and
the anode, wherein a discharge is induced in the discharge space to
sputter the target, thereby forming a fluorine-containing thin film
on the substrate.
Inventors: |
Ando, Kenji; (Kawasaki-shi,
JP) ; Otani, Minoru; (Tokyo, JP) ; Suzuki,
Yasuyuki; (Yokohama-shi, JP) ; Shingu, Toshiaki;
(Kawasaki-shi, JP) ; Biro, Ryuji; (Kawasaki-shi,
JP) ; Kanazawa, Hidehiro; (Tokyo, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
30 ROCKEFELLER PLAZA
NEW YORK
NY
10112
US
|
Family ID: |
26587306 |
Appl. No.: |
09/804266 |
Filed: |
March 13, 2001 |
Current U.S.
Class: |
438/485 |
Current CPC
Class: |
C23C 14/0694 20130101;
C23C 14/0057 20130101 |
Class at
Publication: |
438/485 |
International
Class: |
C30B 001/00; H01L
021/20; H01L 021/36 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 2000 |
JP |
2000-068513 |
May 24, 2000 |
JP |
2000-153245 |
Claims
What is claimed is:
1. A process of producing a thin film, which comprises the steps
of: providing a vessel; placing a target such that a surface to be
sputtered of the target surrounds a discharge space; placing a
substrate on a side of an opening of the space such that the
substrate faces an anode disposed so as to close another opening of
the space surrounded by the target; supplying a sputtering gas and
a fluorine-containing gas into the vessel; and supplying a dc power
or a power obtained by superimposing pulses with reversing
polarities on the dc power, between the target and the anode,
wherein a discharge is induced in the discharge space to sputter
the target, thereby forming a fluorine-containing thin film on the
substrate.
2. The process according to claim 1, wherein the sputtering gas is
supplied from the anode side into the discharge space and the
fluorine-containing gas is supplied from the exterior of the
discharge space into the vessel.
3. The process according to claim 1, further comprising the step of
supplying a reducing gas from the anode side into the discharge
space.
4. The process according to claim 1, wherein water is supplied from
the exterior of the discharge space into the vessel.
5. The process according to claim 1, wherein the sputtering gas is
a rare gas.
6. The process according to claim 1, wherein the
fluorine-containing gas is at least one selected from fluorine gas,
nitrogen fluoride gas, carbon fluoride gas, sulfur fluoride gas,
and hydrofluorocarbon gas.
7. The process according to claim 1, further comprising the step of
supplying a gas selected from hydrogen gas, deuterium gas,
hydrocarbon gas, and ammonia gas, from the anode side to the
discharge space.
8. The process according to claim 1, wherein the anode has a
surface of the same material as the target, and a number of gas
discharge holes.
9. The process according to claim 1, wherein a magnetic shield of a
magnetic material having a number of holes is placed between the
substrate and the target.
10. The process according to claim 1, wherein the target is
comprised of a metal comprising at least one selected from Mg, Al,
La, Nd, Th, Li, Y, Ca, and Gd.
11. The process according to claim 1, wherein the
fluorine-containing thin film is magnesium fluoride, aluminum
fluoride, lanthanum fluoride, neodymium fluoride, thorium fluoride,
lithium fluoride, yttrium fluoride, calcium fluoride, or gadolinium
fluoride.
12. The process according to claim 1, wherein on a surface of the
substrate, the electron temperature Te of a plasma is not more than
3 eV, the electron density is not more than 2.times.10.sup.8
electrons/cm.sup.3, and the difference between the potential of the
plasma and the floating potential of the substrate is not more than
2 V.
13. The process according to claim 1, wherein the substrate is
comprised of calcium fluoride.
14. The process according to claim 1, wherein a voltage applied to
the target is monitored and the supply rates of the gases are
controlled such that the voltage applied to the target is kept
substantially constant.
15. The process according to claim 1, wherein the frequency of the
pulses is 1 kHz to 500 kHz.
16. The process according to claim 1, wherein during the film
formation, the partial pressure of water is maintained not less
than 1.times.10.sup.-3 Pa nor more than 1.times.10.sup.-1 Pa and
the partial pressure of hydrogen gas is maintained not less than
5.times.10.sup.-2 Pa.
17. The process according to claim 1, wherein the thin film is a
film comprising magnesium fluoride as a main component and
containing oxygen in a content of not more than 5 wt % and MgO in a
content of not more than 1.5 wt %.
18. The process according to claim 1, wherein the thin film is a
film comprising magnesium fluoride as a main component and
containing a rare gas in a content of 1 wt % to 10 wt %.
19. An optical device comprising an optical system comprising an
optical part obtained by forming a fluorine-containing thin film on
a substrate by the thin film production process as set forth in
claim 1, in combination with a laser light source for generating an
ultraviolet light.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a production process of a
thin film, and an optical device and, more particularly, to a
process of forming a fluorine-containing thin film very susceptible
to a plasma damage, at a low temperature by a sputtering method of
depositing a thin film on a substrate mounted on a substrate holder
by sputtering of a target.
[0003] 2. Related Background Art
[0004] In the fields of optics heretofore, vacuum vapor deposition
methods of heating a film forming source in vacuum by an electron
beam or the like to evaporate it and deposit evaporated particles
on a substrate have mainly been used for forming optical thin films
such as antireflection films, mirrors, and so on.
[0005] In general, the antireflection films, mirrors, etc. are
comprised of either one of a material with a low refractive index
such as magnesium fluoride or the like and a material with a high
refractive index such as zirconium oxide, tantalum oxide, titanium
oxide, or the like, or a multilayer film as a combination of these
materials, or the like; layer construction, film thicknesses, etc.
are adjusted in various ways, depending upon optical performance
required.
[0006] The evaporation methods are deposition methods simple in
system configuration, capable of forming a film at a high speed on
a large-area substrate, and excellent in productivity, but they
have difficulties in control of the thickness with a high accuracy
and in development of automatic systems. Furthermore, films formed
under the condition of a low substrate temperature had problems of
insufficient strength of the film, being readily damaged, low
adhesion between the film and a substrate, and so on.
[0007] However, there have been increasing demands for more
increase in production efficiency in recent years, and thus demands
for coatings by sputtering, which is advantageous in terms of
labor-saving of production steps, stability of quality, film
quality (adhesion, film strength) and so on as compared with the
vacuum evaporation methods, have also been increasing for these
optical thin films.
[0008] When thin films of oxide dielectrics of ZrO.sub.2,
Ta.sub.2O.sub.5, TiO.sub.2, and so on are formed by use of the
sputtering method, thin films with a low absorption and a high
refractive index can easily be obtained.
[0009] The sputtering system commonly used heretofore for
production of thin films is a parallel plate magnetron sputtering
apparatus. In the apparatus, a target as a material for a thin film
and a substrate mounted on a substrate holder are placed so as to
face each other in a vacuum chamber, a plasma is generated to
sputter the target, and target particles driven out of the target
by the sputtering are deposited on the substrate. This is a simple
film forming method excellent in high-speed film formation,
large-area film formation, target lifetime, and so on.
[0010] Meanwhile, there are known sputtering systems other than the
parallel plate type systems.
[0011] Japanese Patent Application Laid-Open No. 6-17248 describes
a proposal of an off-axis type sputtering system in which the
substrate or the target is rotated by 90.degree. from the
configuration of the parallel plate sputtering system with the
target and the substrate facing each other.
[0012] Furthermore, Japanese Patent Application Laid-Open No.
5-182911 describes a proposal of a facing target sputtering system
in which surfaces to be sputtered face each other with a space
therebetween, a magnetic field is generated in a direction
perpendicular to the sputtered surfaces, and a thin film is formed
on the substrate placed beside a space between the targets.
[0013] Japanese Patent Application Laid-Open No. 8-167596 describes
a proposal of a plasma processing system and the like in which at
least one mesh plate for separation of a plasma is disposed between
a plasma generating chamber for generating a plasma and a plasma
processing chamber for housing a substrate to be subjected to film
formation, the mesh plate is provided with a plurality of
apertures, and the diameter of the apertures is not more than two
times the Debye shielding length of the plasma.
[0014] In recent years, systems using the ArF excimer laser light
source and the F.sub.2 excimer laser light source have been
developed as optical devices used for precision processing, and
there are increasing desires for fluorine-containing films with a
high quality and a high durability, for optical parts (or optical
components) constituting their optical systems.
[0015] Under such circumstances, the inventors have first conducted
an investigation on a method of forming a fluorine-containing film
of aluminum fluoride, magnesium fluoride, or the like, using the
parallel plate type sputtering system.
[0016] For example, an example of the method of forming a fluoride
thin film by the sputtering method is one as disclosed in Japanese
Patent Application Laid-Open No. 4-289165. This method is a
sputtering method for forming a film of an alkali earth metal
fluoride such as MgF.sub.2 or the like by use of a mixture of an
inert gas such as Ar or the like and a fluorine-based gas such as
CF.sub.4 or the like.
[0017] There is also a known method of DC sputtering using a metal
target and a mixture of an inert gas of Ar or the like and a
fluorine-based gas of CF.sub.4 or the like, as disclosed in
Japanese Patent Application Laid-Open No. 7-166344.
[0018] However, when a film of a fluoride material is formed on a
lens or the like by introduction of a reactive gas of NF.sub.3 or
F.sub.2 gas or the like, a cathode sheath voltage will accelerate
negative ions of fluorine and fluorine compounds sputtered from the
target and high energy particles thus generated will be incident on
the film to physically damage the film or vary the composition of
the film. Furthermore, there can be occurred a damage due to the
negative ions such as undesired etching of the substrate instead of
desired formation of a film on the substrate, depending on the
species of the target material and the sputtering conditions.
Furthermore, the ions and electrons accelerated by the ion sheath
may damage the film formed on the surface of the substrate, raise
the temperature of the substrate, or increase optical absorption of
the film. Particularly, for the ultraviolet light with a high
energy, the optical transmittance will significantly be
lowered.
[0019] The prior art thin film production methods were hardly said
to be satisfactory yet in these respects.
SUMMARY OF THE INVENTION
[0020] An object of the present invention is to provide a process
of producing a thin film having a high transmittance for the
ultraviolet light, particularly, for the vacuum ultraviolet light
and being capable of effectively utilizing the light, and also to
provide an optical device.
[0021] Another object of the present invention is to provide a
process of producing a thin film little damaged by negative ions,
positive ions, and electrons, and also to provide an optical
device.
[0022] An aspect of the present invention is a process of producing
a thin film, which comprises the steps of:
[0023] providing a vessel;
[0024] placing a target such that a surface to be sputtered of the
target surrounds a discharge space;
[0025] placing a substrate on a side of an opening of the space
such that the substrate faces an anode disposed so as to close
another opening of the space surrounded by the target;
[0026] supplying a sputtering gas and a fluorine-containing gas
into the vessel; and
[0027] supplying a dc power or a power obtained by superimposing
pulses with reversing polarities on the dc power, between the
target and the anode,
[0028] wherein a discharge is induced in the discharge space to
sputter the target, thereby forming a fluorine-containing thin film
on the substrate.
[0029] In the present invention, it is desirable to supply the
sputtering gas from the anode side into the discharge space and to
supply the fluorine-containing gas from the exterior of the
discharge space into the vessel.
[0030] In the present invention, it is desirable to supply a
reducing gas from the anode side into the discharge space.
[0031] In the present invention, it is desirable to supply water
from the exterior of the discharge space into the vessel.
[0032] In the present invention, it is desirable to use a rare gas
as the sputtering gas.
[0033] In the present invention, it is desirable to use at least
one selected from fluorine gas, nitrogen fluoride gas, carbon
fluoride gas, sulfur fluoride gas, and hydrofluorocarbon gas, as
the fluorine-containing gas.
[0034] In the present invention, it is desirable to supply a gas
selected from hydrogen gas, deuterium gas, hydrocarbon gas, and
ammonia gas, from the anode side toward the discharge space.
[0035] In the present invention, it is desirable that the anode is
one having a surface formed of the same material as the target, and
a number of gas discharge holes.
[0036] In the present invention, it is desirable to place a
magnetic shield of a magnetic material having a number of holes,
between the substrate and the target.
[0037] In the present invention, it is desirable that the target be
comprised of a metal comprising at least one selected from Mg, Al,
La, Nd, Th, Li, Y, Ca, and Gd.
[0038] In the present invention, it is desirable to produce as the
fluorine-containing thin film, a film of magnesium fluoride,
aluminum fluoride, lanthanum fluoride, neodymium fluoride, thorium
fluoride, lithium fluoride, yttrium fluoride, calcium fluoride, or
gadolinium fluoride.
[0039] In the present invention, it is desirable to produce a thin
film under such conditions that the electron temperature Te of the
plasma is not more than 3 eV, the electron density is not more than
2.times.10.sup.8 electrons/cm.sup.3, and the difference between the
potential of the plasma and the floating potential of the substrate
is not more than 2 V, on the surface of the substrate.
[0040] In the present invention, it is preferable to use calcium
fluoride as the material of the substrate.
[0041] In the present invention, it is desirable to monitor a
voltage applied to the target and to control the supply rates of
the reactive gases so that the voltage applied to the target
becomes approximately constant.
[0042] In the present invention, it is desirable to set the
frequency of the pulses within the range of 1 kHz to 500 kHz.
[0043] In the present invention, it is desirable to maintain the
partial pressure of water within the range of not less than
1.times.10.sup.-3 Pa nor more than 1.times.10.sup.-1 Pa and the
partial pressure of hydrogen gas not less than 5.times.10.sup.-2 Pa
during the film formation.
[0044] In the present invention, it is desirable to form a thin
film comprising magnesium fluoride as a main component and
containing oxygen in a content of not more than 5 wt % and MgO in a
content of not more than 1.5 wt %.
[0045] In the present invention, it is desirable to form a thin
film comprising magnesium fluoride as a main component and
containing a rare gas in a content of 1 wt % to 10 wt %.
[0046] Another aspect of the present invention is an optical device
comprising an optical system comprising an optical part obtained by
forming a fluorine-containing thin film on a substrate by the
above-stated thin film production process, in combination with a
laser light source for generating an ultraviolet light.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] The file of this patent contains at least one drawing
executed in color. Copies of this patent with color drawings will
be provided by the Patent and Trademark Office upon request and
payment of the necessary fee.
[0048] FIG. 1A is a schematic view of a sputtering apparatus used
in the present invention, and FIG. 1B is a schematic perspective
view of a target used in the present invention;
[0049] FIG. 2 is a view showing a flowchart of production steps of
a thin film according to an embodiment of the present
invention;
[0050] FIG. 3 is a view showing an example of the waveform of a
voltage applied to a cathode, used in the present invention;
[0051] FIG. 4 is a schematic view showing another sputtering
apparatus used in the present invention;
[0052] FIGS. 5A, 5B, 5C and 5D are schematic views showing the
state of a plasma in the sputtering apparatus used in the present
invention;
[0053] FIGS. 6A, 6B, 6C and 6D are schematic views showing the
state of a plasma in the sputtering apparatus used in the present
invention;
[0054] FIG. 7 is a schematic view showing a still another
sputtering apparatus used in the present invention;
[0055] FIG. 8A is a graphical representation showing a relation
between film forming time and target bias voltage, and FIG. 8B is a
graphical representation showing a relation between flow rate of
H.sub.2O gas and target bias voltage;
[0056] FIG. 9 is a schematic view showing a yet still another
sputtering apparatus used in the present invention;
[0057] FIG. 10 is a graphical representation showing optical
characteristics of an optical part;
[0058] FIG. 11 is a diagram showing a relation between film forming
parameters and characteristics of thin films;
[0059] FIG. 12 is a graphical representation showing optical
characteristics of an antireflection film;
[0060] FIG. 13 is a schematic view showing an example of the
optical device of the present invention; and
[0061] FIG. 14 is a schematic view showing a sputtering apparatus
used for formation of thin films in comparative examples.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
[0062] First, a reactive sputtering apparatus used in the
production process of a thin film according to the present
invention will be described. FIG. 1A is a vertical, cross-sectional
view of the reactive sputtering apparatus. FIG. 1B shows an example
of a target used in the present invention.
[0063] In FIG. 1A reference numeral 10 designates a vessel which
can be hermetically sealed and the interior of which can be
evacuated, and the interior of this vessel 10 is evacuated to
vacuum by an evacuation system 20.
[0064] In the vessel 10 there is a target holder 12 fixed thereto
through an insulating member 11 and a target 13 is placed on the
target holder 12.
[0065] In this example, the target is such a substantially
cylindrical target that a discharge space 18 is surrounded by its
internal surface to be sputtered, as illustrated in FIG. 1B.
[0066] An anode 41 is fixed through an insulating member 40 on the
bottom surface side of the substantially cylindrical target 13. The
anode 41 is made of the same material as the target 13 and is
isolated independently of the target 13 electrically serving as a
cathode, by the insulating member 40.
[0067] Behind this target 13, a plurality of substantially
cylindrical permanent magnets or an integral ring-shaped permanent
magnet 14 as a magnetic field generating means are disposed so as
to surround the outer surface of the target 13 and to be parallel
to the sputter surface of the internal wall of the substantially
cylindrical target 13.
[0068] In the section of the target 13 and the target holder 12
corresponding thereto, cooling water is circulated through cooling
pipes 161, 162 inside a water cooling jacket 15, so as to cool the
target 13 and permanent magnet 14.
[0069] The magnetic poles of the substantially cylindrical
permanent magnets 14 are arranged such that one end facing the
substrate 70 is the pole N while the other end facing the anode 41
is the pole S and such that the magnets can form a magnetic field
50 of a closed loop.
[0070] A sputtering gas for formation of plasma is introduced from
a sputtering gas introducing system 30 connected to the anode 41,
into the discharge space 18 in the vessel 10.
[0071] A reactive gas for reactive sputtering film formation is
introduced from the exterior of the discharge space 18 into the
vessel 10 by a reactive gas introducing system 31 connected to the
vessel 10. In the present invention, a fluorine-containing gas is
used as the reactive gas.
[0072] Numeral 16 denotes a shield plate for shielding discharge,
which functions to prevent sputtering of the other materials than
the target material and to prevent incorporation of impurities into
the thin film, which serves as a deposition preventing plate for
preventing the sputtering particles from being deposited on the
target holder 12, the insulating member 11 and so on, and which
also serves as an anode.
[0073] A substrate holder 71 for holding the substrate 70 on which
the thin film is to be formed is located above the shield plate 16.
A shutter plate 17 having an opening/closing mechanism is placed in
the vicinity of the substrate holder between the substrate holder
71 and the opening of the target, to prevent a film containing a
large amount of oxides and impurities from being deposited on the
substrate 70 during pre-sputtering.
[0074] On the other hand, a power supply means 80 for supplying a
dc power or a power obtained by superimposing pulses with reversing
polarities on the dc power, as the sputtering power for inducing
discharge, is connected to the anode 41 on the plus side and to the
target holder 12 and target 13 of the cathode on the minus
side.
[0075] Next, a production process of a thin film according to an
embodiment of the present invention will be briefly described
referring to FIG. 2.
[0076] First, the apparatus of FIG. 1A having the vessel 10 is
prepared.
[0077] The target 13 is placed on the target holder 12 such that
the surface to be sputtered of the target 13 surrounds the
discharge space 18. The substrate 70 is placed on the side of a
first (upper) opening of the space 18 such that the substrate faces
the anode 41 disposed so as to close (or block) a second (lower)
opening of the space 18 surrounded by the target 13. (step S1)
[0078] The interior of the vessel 10 is evacuated as needed. (step
S2)
[0079] After that, the sputtering gas is supplied from the
sputtering gas introducing system 30 and the fluorine-containing
gas as the reactive gas is supplied from the reactive gas
introducing system 31. (step S3)
[0080] The dc power or the power obtained by superimposing pulses
with reversing polarities on the dc power is supplied from the
power supply means 80 to between the target 13 and the anode 41.
(step S4)
[0081] A discharge is induced in the discharge space 18 in this way
to sputter the target 13 by particles of the sputtering gas,
thereby forming a fluorine-containing thin film on the substrate
70.
[0082] In the present embodiment, the sputtering gas (at least one
rare gas selected from He, Ne, Ar, Kr, Xe, and Rn) is supplied from
the anode 41 side toward the discharge space 18, and the
fluorine-containing gas (at least one selected from fluorine gas,
nitrogen fluoride gas, carbon fluoride gas, sulfur fluoride gas,
and hydrofluorocarbon gas) is supplied from the exterior of the
discharge space 18 into the vessel 10, thereby preventing the
sputtered surface of the target from being fluorinated.
[0083] Among the rare gases, He is more preferable, because it
promotes the surface reaction by incidence of metastable He in the
plasma onto the substrate surface.
[0084] Furthermore, a reducing gas (at least one gas selected from
hydrogen gas, deuterium gas, hydrocarbon gas, and ammonia gas) is
supplied from the anode 41 side toward the discharge space 18,
thereby more preventing fluorination and oxidation of the target
and suppressing light absorption of the thin film formed, in
cooperation with water supplied from the exterior of the discharge
space 18 into the vessel 10.
[0085] The reducing gas and water (H.sub.2O) terminate unbound
bonds (dangling bonds) by H and OH groups to remove the dangling
bonds from the inside of the formed film, thus reducing absorption
of light.
[0086] The anode 41 has a surface made of the same material as the
target 13 and a number of gas discharge holes and thus can
uniformly supply the sputtering gas into the discharge space.
[0087] The anode is preferably an electroconductive body having
many holes. In addition, it is preferable that at least a surface
of the anode that contacts with the plasma (i.e., the internal
surface facing the discharge space) is made of the same material as
the target. Among others, it is preferable to use an anode of a
porous member the pores of which can be utilized as gas discharge
holes. Such a porous member can be made, for example, by baking
metal particles of the same material as the target.
[0088] In the present embodiment, the dc power is supplied from the
power supply means to between the target and the anode.
Alternatively, the power obtained by superimposing pulses with
reversing polarities on the dc power may also be supplied from the
power supply means to between the target and the anode.
[0089] FIG. 3 shows the waveform of the output voltage on the
cathode side of the power supply means for applying the dc power
with pulses of different polarities superimposed thereon. In this
case, the frequency of pulses is preferably set within the range of
1 kHz to 500 kHz. The pulses with reversing polarities (pulses of a
voltage V1) can neutralize accumulation of electrons during
formation of fluorine-containing dielectric films to prevent
abnormal discharge.
[0090] When a dc power is supplied, a constant negative voltage
(for example, a voltage V2 in FIG. 3) is applied as the output
voltage on the cathode side.
[0091] In addition, as described hereinafter, it is preferable to
monitor the voltage applied to the target and to control feed rates
of the various gases such that the voltage applied to the target
becomes approximately constant.
[0092] Furthermore, as described hereinafter, it is also preferable
to place a magnetic shield of a magnetic material having many
holes, between the substrate 70 and the target 13.
[0093] The sputtering pressure during film formation is desirably
within the range of 0.3 to 3 Pa. When the sputtering pressure is
not more than the upper limit, it is possible to obtain a film with
practically acceptable uniformity and a steady refractive index in
the thickness direction. When the sputtering pressure is not less
than the lower limit, a stable discharge continues within the
practically acceptable range. More preferably, it is desirable to
set the sputtering pressure within the range of 0.4 to 1 Pa. The
most desirable range of the sputtering pressure is 0.4 to 0.8
Pa.
[0094] When sputtering is carried out under the above-stated
conditions by use of the apparatus described above, it is possible
to maintain on the surface of the substrate 70 a plasma such that
the electron temperature Te of the plasma is not more than 3 eV,
the electron density of the plasma is not more than
2.times.10.sup.8 electrons/cm.sup.3, and the potential difference
between the plasma potential and the floating potential of the
substrate is not more than 2 V.
[0095] When the plasma conditions are within these ranges, little
damage is caused in the thin film on the substrate.
[0096] The sputtering apparatus described above was actually
operated under the sputtering pressure within the range of 0.3 Pa
to 3 Pa and the electron temperature Te, the electron density Ne,
and the plasma potential Vp of the plasma were measured by the
Langmuir probe method from current-voltage characteristics obtained
by putting a metal probe into the substrate position in the vessel,
applying a dc voltage from the outside to the probe. The
measurement results verified that the electron temperature Te was
not more than 3 eV, the electron density Ne not more than
2.times.10.sup.8 electrons/cm.sup.3, and the plasma potential not
more than 2 V.
[0097] There exist positive ions substantially in the same number
as the electron density Ne in the vicinity of the substrate of the
insulting matter and the positive ions are accelerated by the
voltage corresponding to the difference between the plasma
potential Vb and the floating potential Vf to collide with the
substrate. In this case the floating potential Vf is a potential
between the plasma potential Vp and the earth potential.
[0098] In the ordinary RF or DC magnetron sputtering, though there
are slight variations depending upon gas species and conditions,
normally, the electron temperature Te is 5 eV to ten and several
eV, the electron density Ne is 1.times.10.sup.9 to
1.times.10.sup.11 electrons/cm.sup.3, the voltage of the difference
between the plasma potential Vp and the floating potential Vf is
also about several ten V, which will do great damage to the
substrate and the deposited film.
[0099] The above constitution of the present invention makes it
possible to greatly decrease the plasma damage to the substrate and
the deposited film.
[0100] The substrate 70 used in the present invention may be any
material, e.g., any one of conductors such as stainless steel,
aluminum, and so on, any one of semiconductors such as silicon and
the like, or any one of insulators such as silicon oxide, aluminum
oxide, resin, and so on. The substrate for fabrication of optical
parts for ultraviolet light is preferably one selected from silica
glass, calcium fluoride crystals, and so on.
[0101] The material of the target 13 used in the present invention
is a metal (pure metal or alloy) containing at least one selected
from Mg, Al, La, Nd, Th, Y, Li, Ca, and Gd, and is selected
depending on the thin film to be formed.
[0102] The fluorine-containing gas used in the present invention is
fluorine gas or a gas of a fluorine compound; specifically, any one
selected from F.sub.2, NF.sub.3, CF.sub.4, C.sub.2F.sub.6,
C.sub.3F.sub.8, CHF.sub.3, and SF.sub.6.
[0103] In the present invention, the fluorine-containing thin film
to be formed is a film of a fluoride containing at least one of the
above-stated metals; specifically, magnesium fluoride, aluminum
fluoride, lanthanum fluoride, neodymium fluoride, thorium fluoride,
yttrium fluoride, lithium fluoride, calcium fluoride, or gadolinium
fluoride.
Embodiment 2
[0104] FIG. 4 is a cross-sectional view of a reactive sputtering
apparatus used in the production process of a thin film according
to the present invention.
[0105] This apparatus is constructed in a structure obtained by
providing the reactive sputtering apparatus illustrated in FIG. 1A
with a magnetic shield plate 60, and the fundamental structure of
the apparatus and the basic steps of the film forming process are
the same as those described above.
[0106] The magnetic shield plate 60 peculiar to the present
embodiment will be described hereinafter.
[0107] The magnetic shield plate 60 is placed between the substrate
holder 71 and the shield plate 16 on the side opposite to the side
of the anode 41. The magnetic shield plate 60 is made of a
permalloy with a high permeability and provided in the part facing
the opening of the cylindrical target, and has such a shape that it
has many holes of a diameter of 5 mm in an aperture ratio of 50%
and covers a half part of the permanent magnet in height around the
periphery of the permanent magnets 14. It is seen that the magnetic
shield plate 60 captures the magnetic field 50, which was spread
between the target and the substrate in the absence of the magnetic
shield plate 60. This permits the apparatus to generate a plasma at
a lower temperature and in a lower density in the vicinity of the
substrate.
[0108] FIGS. 5A to 5D show the result of plasma simulation carried
out in the apparatus configuration without a magnetic shield plate
of FIG. 1A.
[0109] Similarly, FIGS. 6A to 6D show the result of plasma
simulation carried out in the apparatus configuration with the
magnetic shield plate of FIG. 4.
[0110] FIGS. 5A and 6A both show a plasma potential distribution,
FIGS. 5B and 6B both a distribution of electron temperature Te,
FIGS. 5C and 6C both a distribution of electron density Ne, and
FIGS. 5D and 6D both a distribution of ion density Ni. It is seen
from comparison between the figures that in all the items the
apparatus configuration of FIG. 4 provided with the magnetic shield
demonstrates the lower plasma density in the vicinity of the
substrate and the higher effect of confining the plasma in the
space surrounded by the substantially cylindrical target and the
anode.
[0111] The decrease of negative ion damage achieved by the
embodiments of the present invention described above is based
particularly on the following points.
[0112] (1) The target is arranged so as to surround the discharge
space such that the sputtered surfaces thereof faces each
other.
[0113] The target herein is the one of the substantially
cylindrical shape as a preferred form. This substantially
cylindrical shape means a cylinder such that a cross section of the
target in a plane perpendicular to the drawing of FIG. 1A or FIG. 4
is a circle or an ellipse, and a schematic perspective view of the
target is as illustrated in FIG. 1B.
[0114] When the target is of the form having the substantially
cylindrical inner wall as described, most of negative ions
generated at a certain point of the target impinge on the target
surface at a symmetric position on the other side of the center
line of the target, so that negative ions directly entering the
substrate can be reduced. Namely, the substrate is kept from the
negative ion damage, by employing the arrangement in which normals
to the sputter surface do not intersect with the film forming
surface of the substrate on the substrate holder.
[0115] In view of the above action, the form of the target used in
the present invention is not limited to the substantially
cylindrical shape, and it can also be contemplated that the
discharge space is surrounded continuously or intermittently by one
target or a plurality of targets.
[0116] (2) Optionally, such a structure that the reactive gas is
not directly blown against the target may be further adopted to
prevent fluorination of the target.
[0117] The target, when fluorinated by the fluorine-containing gas,
tends to generate negative ions during sputtering. In the present
embodiment, the sputtering gas is supplied through the anode that
forms the bottom surface of the substantially cylindrical target
surrounding the discharge space, thereby establishing an
environment in which the space inside the substantially cylindrical
target is filled with the sputtering gas. This makes it difficult
for the reactive gas to intrude into the substantially cylindrical
target, so that the target can be sputtered while maintaining its
metal state.
[0118] The decrease of the positive ion and electron damage can be
accomplished by making a plasma of a relatively high density for
sputtering distant from the substrate and confining the plasma in
the space created by the target and the anode as much as possible.
Specifically, it is achieved as follows.
[0119] (1) By using a dc power or a power obtained by superimposing
pulses with reversing polarities on the dc power for generating a
plasma, the plasma density in the vicinity of the substrate can be
made lower than in use of a conventional RF power.
[0120] (2) Optionally, by placing a magnetic field generator such
as permanent magnets or the like so as to surround a target, most
of a plasma can be kept in the discharge space surrounded by the
target.
[0121] (3) Optionally, by further interposing a magnetic shield
between a target and a substrate, a plasma of a high density is
furthermore prevented from spreading to near the substrate.
[0122] As more preferable deposition conditions, in order to make
the desired plasma state for suppressing the damage due to positive
ions and electrons, it is desirable to select such conditions that
when the sputtering pressure is within the range of 0.3 to 3 Pa,
the electron temperature Te of the plasma at the substrate position
is not more than 3 eV, the electron density is not more than
2.times.10.sup.8 electrons/cm.sup.3, and the difference between the
plasma potential and the floating potential of the substrate is not
more than 2 V.
[0123] A film forming process using the above reactive sputtering
apparatus will be described below in more detail.
[0124] First, because of the permanent magnet of the ring shape
arranged at the back of the target so as to surround the target, a
coaxial magnetron discharge condition in which the electric field
perpendicular to the target surface intersects with the magnetic
field approximately perpendicular to the radial direction is
achieved on the sputtered surface of the substantially cylindrical
internal wall part, thereby generating a magnetron discharge in
which electrons fly in arcs perpendicular to the target surface
(cycloid curves), collide with molecules of the sputtering gas to
ionize them, and move in the circumferential direction (closed
loop). The ionized molecules of the sputtering gas are accelerated
toward the target surface biased at the negative potential to
collide with the surface, thus starting sputtering.
[0125] If at this time the ordinary reactive sputtering method is
carried out using the target readily generating negative ions of
Al, Mg, or the like, and a fluorine-containing gas in order to form
a film of a fluoride such as aluminum fluoride, magnesium fluoride,
or the like, a thin film of a compound containing fluorine, such as
aluminum fluoride (e.g. AlF.sub.3), magnesium fluoride (e.g.
MgF.sub.2), or the like, will be formed on the target surface
because of influence of the reactive gas.
[0126] If the surface having such a fluoride formed thereon is
sputtered, negative ions will be formed in part. The negative ions
thus formed will be accelerated by the ion sheath voltage and come
to have a large kinetic energy and a directivity.
[0127] In general, negative ions or compounds coupled with negative
ions are very instable and collide with gas particles during flight
to be neutralized into neutral particles. For this reason, the
neutral particles with the directivity and large kinetic energy
will impinge upon the substrate to do great damage to the substrate
or the film formed thereon. Further, depending upon the sputtering
conditions, the etching rate of the negative ions (turning into
neutral particles during flight) may be greater than the deposition
rate, thus resulting in no film formation.
[0128] In the sputtering apparatus used in the present embodiment,
the gas supplied through the anode fills the interior of the
substantially cylindrical shape with the anode as a bottom surface
and the target as a side surface, suppresses the reactive gas
containing fluorine, supplied into the vessel, from intruding into
the cylinder, suppresses fluorination of the target and anode, and
maintaining the surfaces of the target and anode in the metal
state.
[0129] Particularly, the suppression of fluorination of the anode
surface also has the effect on stabilization of discharge
(prevention of an abnormal discharge due to charge-up), and the
suppression of fluorination of the target surface has the great
effect on suppression of formation of negative ions. It is,
therefore, more preferable to make the anode in such a structure as
to discharge the sputtering gas containing no fluorine.
[0130] In this manner, target atoms sputtered in the metal state
are fluorinated by collision with the fluorine-based gas during the
process of flight to the substrate, or even if they arrive in the
metal state at the substrate surface, they will be fluorinated by
collision with the fluorine-based gas molecules entering the
substrate surface, thereby forming the desired thin fluoride film
on the surface of the substrate.
[0131] As described above, on the sputtered surface of the
substantially cylindrical internal wall part is attained the
coaxial magnetron discharge condition in which the electric field
perpendicular to the target surface intersects with the magnetic
field substantially perpendicular to the radial direction. The
magnetron discharge is generated in which electrons fly along arcs
perpendicular to the target surface (cycloid curves) and move in
the circumferential direction (closed loop) while colliding with
the sputtering gas molecules to ionize them, so that the plasma of
a high plasma density is confined inside the substantially
cylindrical internal surface. Most of the electrons not confined by
the magnetron magnetic field fly toward the anode. As described
above, the cathode of the substantially cylindrical internal wall
part is placed near the anode on the bottom surface of the
substantially cylindrical shape to generate the strong electric
field, so that most of the electrons are captured by the anode.
[0132] However, some electrons coil around the magnetic field 50 on
the side of the opening of the target to spread into the space
between the target and the substrate. The magnetic shield can
effectively suppress this plasma existing near the substrate. The
magnetic shield captures the electrons moving as coiling around the
magnetic field on the opening side of the substantially cylindrical
target and also prevents leakage of magnetic flux from the target,
thereby enabling the magnetic flux density near the substrate to be
lowered and thus enabling decrease of the electron temperature Te
of the plasma near the substrate, the electron density Ne, and the
plasma potential Vp.
[0133] There are no specific restrictions on the material of the
magnetic shield as long as it is a magnetic material. Preferred
materials are permalloys, soft iron, and so on. The magnetic shield
is desirably one for preventing leakage of the plasma to the
vicinity of the substrate without impeding passage of the
sputtering particles emitted from the target as much as possible,
and the diameter of the holes is desirably within the range of 5 to
10 mm. The aperture rate of the magnetic shield is desirably 50 to
80%. When the aperture rate is not more than the upper limit, it
becomes feasible to confine the plasma in the substantially
cylindrical shape in the practically acceptable range while
maintaining sufficient film forming rates. When the aperture rate
is not less than the lower limit, it is feasible to confine the
plasma while realizing practically acceptable film forming
rates.
[0134] However, decreasing the plasma density in the vicinity of
the substrate has a great effect on suppression of the plasma
damage on the substrate, but in the case of the reactive sputtering
utilizing for the surface reaction the energy of ions entering from
the plasma into the substrate surface, the reaction rate on the
substrate surface might decrease to the contrary. In that case, He
gas is used as a kind of sputtering gas for promoting the reaction
on the substrate surface to be introduced into the high density
plasma, thereby efficiently making the metastable state of He gas
(positive ions of He). Since He gas in the metastable state has a
long life, the gas is transferred to the substrate to impinge on
the substrate surface, thus giving its energy to the surface to
promote the reaction.
[0135] During film formation, the temperature of the substrate
increases mainly for two reasons. The first reason is a temperature
rise due to electron impact and the other is radiation from the
target. The temperature rise due to the electron impact from the
plasma can be suppressed by lowering the electron temperature Te
and the electron density Ne in the vicinity of the substrate as
described above. Since normals to the sputtered surface of the
target do not intersect with the film forming surface of the
substrate on the substrate holder, the effective radiation amount
from the sputtered surface is small, so that the rise of the
substrate temperature can be suppressed.
[0136] As described above, it becomes feasible to implement
sputtering in the manner to minimize the negative ion damage and
the positive ion and electron damage as compared with the
conventional methods and thus obtain a fluorine-based thin film
with a high quality, which was difficult to obtain by the
conventional sputtering methods.
[0137] The production process of a thin film according to each of
the embodiments of the present invention described hereinafter is
carried out using the sputtering apparatus of the same basic
structure as in each of the embodiments described above. While
monitoring the voltage applied to the target, the introducing rate
of the reactive gas flowing between the substrate and the opening
is controlled by a reactive gas introducing rate control means so
that the voltage of the target becomes substantially constant.
[0138] In this way, without varying the flow rate of the inert gas
supplied into the hollow part and the voltage applied to the
target, by controling the introducing rate of the reactive gas such
that the surface of the target can be readily maintained in the
metal state and the target voltage is thus maintained constant, it
is possible to stabilize the sputtering rate at a high level.
Embodiment 3
[0139] FIG. 7 is a schematic cross-sectional view of a DC magnetron
sputtering apparatus used in the production process of a thin film
according to Embodiment 3 of the present invention.
[0140] The sputtering apparatus is provided with a vessel 10 for
maintaining the interior in a substantially vacuum state and an
evacuation system 20 consisting of a vacuum pump or the like for
evacuating the vessel 10.
[0141] Inside the vessel 10 there is provided a sputter electrode
section 22 with an aperture 21, in which a metal target 13 of a
hollow shape to be sputtered is mounted, and a substrate 70 to be
processed to form a thin film thereon is held on a holder 71.
Between the opening 21 and the substrate 70 there are supply ports
33, 34 of a first reactive gas supply system 31 for supplying a
reactive gas such as a fluorine-containing gas, an
oxygen-containing gas, or the like and a second reactive gas supply
system 32 for supplying a reactive gas such as H.sub.2, H.sub.2O,
or the like, and the reactive gas introducing systems and
evacuation system are arranged to prevent the reactive gases from
readily diffusing to the surface of the target 13.
[0142] The sputter electrode section 22 is provided with a
cathode-target holder 12 equipped with cooling pipes 161, 162 as
cooling means and being an electrode of a hollow shape with an
opening at one end, an anode 41 placed at the other end of the
cathode 12 with an insulator 40 therebetween, an earth
electrode-shield plate 16 insulated from the cathode 12 by an
insulator 16A, and a hollow magnetron sputter source consisting of
a target 13 placed in contact with and electrically connected to
the cathode 12, and a magnet 14 and a yoke for forming a magnetic
circuit on the surface of the target 13.
[0143] To the hollow part forming the discharge space 18A is
connected a sputter gas supply system 30 for supplying a sputtering
gas (and a reducing gas if necessary).
[0144] The potential of the anode 41 is controllable by means of a
dc power supply 81 for the anode. A voltage resulting from
superposition of a dc bias and high frequency or rectangular waves
of 1 kHz to 500 kHz can be applied from a sputter power supply 82
to the cathode 12, so that the power induces a discharge in the
discharge space 18 on the surface of the target 13, thus sputtering
the target 13. Further, the construction is adopted in whcih the
plasma is confined in the hollow part 18 and even if negative ions
are made in part of the target surface, they can be prevented from
directly entering the substrate 70.
[0145] A quickly openable/closable shutter 17 is disposed in front
of the substrate 70 located so as to face the anode 41 with the
target 13 therebetween. This substrate 70 is transferred through a
gate valve 91 to or from a load lock chamber 92, so that the
substrate 70 is carried into or out of the vessel 10 without
exposing the interior of the vessel 10 to the atmosphere. When the
substrate is set in the vessel 10, it is located at the position
where it does not intersect with the normals 23 to the surface of
the target 13.
[0146] A control unit 160 is provided to monitor and control the
flow rates of the gases introduced from the first reactive gas
supply system 31, the second reactive gas supply system 32, and the
sputter gas supply system 30 and the voltage of the target 13.
[0147] A production process of a thin film using the apparatus
illustrated in FIG. 7 will be described below.
[0148] The cylindrical target 13 of, for example, a high purity
metal Mg (99.9%) is mounted on the cathode 12.
[0149] The substrate 70 like a synthetic quartz glass plate is
cleaned and placed in the load lock chamber 92 and the interior of
the chamber is evacuated to below about 1.times.10.sup.-4 Pa. At
this time, the substrate is preferably cleaned for the purpose of
removing organic matter by heating or ultraviolet irradiation or
the like in order to remove contaminants on the surface of the
substrate 70, which is very effective in stabilizing the quality of
the film. After completion of the evacuation, the substrate 70 is
transferred through the gate valve 91 onto the holder 71 to be held
thereon.
[0150] The holder 71 is internally provided with a heater and thus
a film can be formed while heating the substrate 70 to a
temperatures up to 400.degree. C. However, because film formation
is carried out at room temperature in the present embodiment, the
heater is kept off herein.
[0151] Then, the shutter 17 is closed, and Ar and H.sub.2 are
introduced from the sputter gas supply system 30. Furthermore, the
reactive gas supply systems 31, 32 supply a mixed gas of F.sub.2
and Ar diluted in about 5 vol % and H.sub.2O so as to attain a
total pressure of about 0.3 Pa to 3 Pa. A sputter power of about
300 W is applied to the cathode 12 to generate a magnetron plasma
on the surface of the target 13. At the same time as it, a
rectangular voltage of 1 kHz to invert the polarities of the target
surface is superimposed to cancel the charge on the target surface
and others, thereby maintaining the discharge stable. It is noted
that the introducing rates of the respective gases, the pressure,
the sputter power, and the rectangular voltage are not limited to
the above specific values.
[0152] The gas introducing part of the anode 41 is as described
above and is preferably a member having many holes made therein for
permitting the gas to be uniformly introduced to the sputtered
surface of the target 13 of the hollow shape, or a porous member
made of the same material as the target 13. Here, the gas
introduced is accurately controlled in the flow rate, purity, and
pressure.
[0153] On the sputtered surface of the target 13, the electric
field is formed perpendicularly to the magnetic field of the
maximum flux density of 250.times.10.sup.-4 T formed parallel to
the sputtered surface by the aforementioned magnets 14. When the
magnetic field and the electric field are formed perpendicularly to
each other in this way, electrons moving by the electric field
applied to the target 13 are turned by the magnetic field and move
in cycloid motion to be trapped by the surface of the target 13 of
the flat plate shape. The electrons in cycloid motion have long
flight distances, so that their probability of collision with the
gas molecules becomes higher. The gas molecules colliding with the
electrons are ionized to generate the magnetron discharge. Since
the negative voltage is applied from the sputter power supply 82 to
the target 13, the ionized gas molecules are accelerated toward the
sputtered surface of the target 13 and then collide with the target
13 to sputter the target material. At this time the color of
discharge varies from pale blue green (discharge color of argon) to
green (discharge color of Mg) depending on the applied power and
the partial pressure of F.sub.2 gas. For high-speed sputtering, it
is preferable to implement the sputtering under the green discharge
conditions in which the surface of the target 13 is kept in the
metal state.
[0154] Although not illustrated in FIG. 7, by installing an
emission spectrophotometer, spectroscopically measuring the
emission from the surface of the target 13, and always controlling
the partial pressures of the reactive gases so as to maintain the
intensity and wavelength of the emission color of the target metal,
it becomes feasible to implement stable film formation while
maintaining the surface of the target 13 in the metal state.
[0155] It is also possible to maintain this state by use of a mass
spectrometric analyzer or the like instead of the emission
spectrophotometer.
[0156] Since the F.sub.2 gas reacts with H.sub.2O to form HF, they
are introduced from the separate introducing systems. The H.sub.2
gas and Ar are introduced together and this is not only for the
purpose of introducing them simply as sputtering gases, but also
for the purpose of carrying out the reaction with F.sub.2 in the
deposition atmosphere as much as possible and for the purpose of
introducing the H.sub.2 gas into the plasma formed on the surface
of the target 13 to generating active H atoms and H.sub.2
molecules, thereby enhancing the reactivity.
[0157] The discharge is maintained for a while, and when the
discharge has been stabilized, the shutter is opened to form a film
of MgF.sub.2 on the substrate 70.
[0158] When the target voltage during the film formation is
monitored in situ (to effect in-situ monitor) by the control unit
160, a phenomenon in which the absolute value of the target bias
voltage gradually decrease is observed. The inventors have found
that this is caused by change in the state of the surface of the
target 13 because of variation in the partial pressures of F.sub.2
and H.sub.2O in the vacuum vessel or the like. An example of such
variation in the target bias voltage is presented in FIG. 8A.
[0159] The film formation in this state will result in a
heterogeneous film having its refractive index varied in the
direction of thickness. Further, the sputtering rate is instable,
so that the quality of the film is instable.
[0160] Furthermore, in the case where the partial pressure of
F.sub.2 is high and the power applied to the target is low, the
sputtering rate decreases extremely. This is because a fluoride
film such as a MgF.sub.2 film is formed on the target surface.
[0161] Namely, in order to form a fluorine-containing film at a
higher speed, with better homogeneity, and with good repeatability,
the target surface needs to be always kept in the metal state.
However, it is very difficult to always monitor the target surface
and maintain it at a constant state.
[0162] Therefore, the film formation is carried out while
controlling the partial pressures of H.sub.2O and F.sub.2 to
suppress the variation with the lapse of time of the target bias
voltage. Specifically, the control unit 160 monitors the target
bias voltage and controls the gas introducing rates so as to keep
the voltage value constant, thereby forming a fluorine-containing
film such as an MgF.sub.2 film. FIG. 8B shows dependence of the
target bias voltage (in the negative sign) on the supply rate of
H.sub.2O gas. In such a case, the control can be done in a tendency
to reduce the supply rate of H.sub.2O gas with the lapse of
time.
[0163] Further, maintaining the target potential constant in this
way makes the sputtering rate stable, so that the thickness of the
film can be controlled with a very high accuracy by controlling the
film forming time.
[0164] As described above, the present embodiment makes it possible
to form a fluorine-containing film such as a MgF.sub.2 film at room
temperature. Accordingly, it is also possible to use a plastic
material or the like as the substrate 70. Further, since the
sputtering rate is stable, it is also feasible to control the film
thickness with higher accuracy than by the conventional evaporation
methods, thereby forming optical thin films with a high quality and
fabricating optical parts with as-designed characteristics,
including antireflection films and mirrors by stacking such optical
thin films.
[0165] The present embodiment employed Mg as the target 13 and Ar,
H.sub.2, F.sub.2, and H.sub.2O as the gases, but Ar as the
sputtering gas may be replaced by a rare gas such as He, Ne, Kr,
Xe, or the like.
[0166] The reducing gas may be any gas that can supply H by
dissociation in the plasma, such as CH.sub.4, NH.sub.3, and so on,
instead of H.sub.2.
[0167] The fluorine-containing gas may be a gas such as CF.sub.4,
NF.sub.3, SF.sub.6, and so on, instead of F.sub.2.
[0168] Furthermore, H.sub.2O gas may be replaced by H.sub.2O.sub.2
or the like.
[0169] When the rectangular voltage of not less than 50 kHz is
superimposed for prevention of an abnormal discharge, there is a
possibility that the discharge may spread and the absorption in the
ultraviolet region may increase in the case of thin fluoride films
susceptible to plasma damage. Thus, when a thin fluoride film with
a low absorption in the ultraviolet region is necessary, it is
preferable to use a frequency of not more than 50 kHz.
[0170] Using the apparatus, it is also feasible to form a thin
oxide film of Al.sub.2O.sub.3 or the like that forms a multilayer
film together with a fluorine-containing film. In this case, an
optical thin film with a low absorption can also be obtained by
superimposing high frequency or rectangular waves of 1 kHz to 500
kHz for prevention of an abnormal discharge. When such high
frequency waves are not superimposed, many particles are
incorporated in the film because of the abnormal discharge to
provide a film with large scattering.
[0171] In the present embodiment, by forming the hollow section 18
as a substantially closed target space, and by adopting the
constitution such that gases of Ar, H.sub.2, etc. are flown into
the hollow section 18 to reduce the partial pressures of the
reactive gases of F.sub.2, H.sub.2O, etc. in the vicinity of the
target 13, and further by keeping the target voltage constant, it
is possible to form a thin film with a high quality.
[0172] The target voltage is strongly dependent on the surface
condition of the target 13. For realizing high-speed sputtering in
the present embodiment, the surface condition of the target needs
to be kept in the metal state. For this reason, it is necessary to
make the partial pressures of the reactive gases near the surface
of the target 13 as low as possible.
[0173] Therefore, in the present embodiment, the target bias
voltage is regulated, particularly, by controlling with the control
unit 160 the supply flow rates of F.sub.2 or H.sub.2O supplied from
the first reactive gas supply system 31 and from the second
reactive gas supply system 32 to between the substrate 70 and the
target 13 such that their partial pressures decrease with the lapse
of time.
[0174] In contrast with it, if the flow rate of the sputtering gas
of Ar or the like introduced into the hollow section 18 or the
power applied to the target 13 is varied, the partial pressure of
the reactive gas in the vicinity of the surface of the target 13
becomes variable, which results in failing to stabilize the film
quality and the sputtering rate.
[0175] As described above, according to the production process of a
thin film of the present embodiment, by controlling with the
control unit 160 the introducing rates of the reactive gases
flowing between the substrate 70 and the target 13, i.e., to the
exterior of the sputter electrode section 22 and between the
substrate 70 and the opening 21, it is possible to reduce the
partial pressures of the reactive gases in the vicinity of the
target 13 disposed in the hollow section 18 of the sputter
electrode section 22 and to maintain the target voltage constant.
This makes it easier to keep the target 13 in the metal state and
permits the sputtering rate to be kept high and stable by
controlling the target voltage at a constant level, thereby forming
a non-heterogeneous, high-quality optical thin film. Since the
sputtering rate can be stabilized, the film thickness can be
controlled with a high accuracy by controlling the film forming
time.
[0176] Further, since the plasma is confined in the hollow section
18, and since negative ions formed in part of the target surface
are also kept from directly entering the substrate 70, the
substrate 70 is prevented from being damaged by charged
particles.
[0177] By introducing the gas of H.sub.2, CH.sub.4, NH.sub.3, or
the like into the hollow section 18 to be dissociated in the plasma
to supply H atoms and active H.sub.2 molecules to the vicinity of
the substrate 70, unnecessary oxygen in the film is reduced and
removed and dangling bonds are terminated, thereby making it
feasible to form an optical thin film with a low absorption in the
ultraviolet region. Particularly, by reducing MgO taken into the
film during formation of an MgF.sub.2 film, it is feasible to
obtain an optical thin film with a low absorption even in the
vacuum ultraviolet region.
[0178] Further, the superposition of the high frequency waves of 1
kHz to 500 kHz can prevent the abnormal discharge due to the charge
accumulation and also prevent contaminations from being
incorporated into the film.
[0179] Then, by forming such an optical thin film with a high
quality at a high speed in a monolayer or multilayer structure on a
substrate to form an antireflection film, a mirror, a beam
splitter, or the like, it becomes feasible to provide optical
members with a high quality at a low cost.
Embodiment 4
[0180] Next, FIG. 9 is a schematic cross-sectional view of a
sputtering apparatus used in the production process of a thin film
according to Embodiment 4 of the present invention.
[0181] The sputter power supply 82 for supplying a sputter power to
the cathode 12 is perfectly floating from the earth potential and
the sputter power supply 82 is configured to apply a voltage
between the anode 41 and the cathode 12. Further, the anode
potential can be set either positive or negative relative to the
earth potential, by using an anode dc power supply 81.
[0182] When the anode potential is set positive relative to the
earth, the plasma potential becomes higher than the earth
potential, so that positive ions generated in the plasma are
released to the substrate 70 side.
[0183] When the anode potential is set negative relative to the
earth on the other hand, the plasma potential becomes lower than
the earth potential, so that electrons in the plasma are released
to the substrate 70 side.
[0184] In this way, controlling the anode potential makes it
feasible to form a film while changing the plasma density in the
vicinity of the substrate 70.
[0185] Since the structure except for the above is fundamentally
similar to that of the thin film forming apparatus described in the
third embodiment, the detailed description thereof is omitted
herein.
[0186] As described previously, the film can be damaged when the
plasma density is high and the electron temperature is also high in
the vicinity of the substrate 70 during film formation. However,
when this apparatus is applied to formation of a film of an oxide
such as A1.sub.2O.sub.3 or the like, it is rather necessary to
positively utilize the assist by a plasma in order to increase the
density and refractive index of the film.
[0187] In such cases, it is effective to control the anode
potential to the positive or negative side relative to the earth,
thus positively increasing the plasma density in the vicinity of
the substrate 70.
[0188] The present embodiment can also improve the thin fluoride
films in density, adhesion, and hardness of the film by increasing
the plasma density to some extent.
[0189] As described above, the control of the voltage of the anode
41 by the control unit 160 can control the plasma density in the
vicinity of the substrate 70 to increase the concentration of
reactive, active species depending on the kind of the formed film
thereby enhancing the reactivity, or to increase the incident
energy and density of Ar ions to the substrate 70 during film
formation to improve the denseness and adhesion of film.
Furthermore, controlling the Ar ion energy can reduce the
concentration of Ar taken into the film to provide an optical thin
film with a low absorption.
[0190] Further, in the present embodiment, similarly to Embodiment
3, by controlling with the control unit 160 the introducing rates
of the reactive gases flowing between the substrate 70 and the
target 13, it is also feasible to reduce the partial pressures of
the reactive gases in the vicinity of the target 13 disposed in the
hollow section 18 of the sputter electrode section 22 and to
maintain the target voltage constant. This makes it easier to keep
the target 13 in the metal state and the control of the target
voltage constant permits the sputtering rate to be increased and
stabilized, whereby an optical thin film can be formed without
heterogeneity and with high quality. Since the sputtering rate can
be stabilized, high-accuracy film thickness control can be
implemented using the film forming time as a parameter.
[0191] By introducing the gas of H.sub.2, CH.sub.4, NH.sub.3, or
the like into the hollow section 18 to be dissociated in the plasma
to supply H atoms and active H.sub.2 molecules to the vicinity of
the substrate 70, unnecessary oxygen in the film is reduced and
removed to terminate the dangling bonds, thereby enabling formation
of an optical thin film with a low absorption in the ultraviolet
region. Particularly, during formation of an MgF.sub.2 film, by
reducing MgO taken into the film during the film formation to
terminate dangling bonds of Mg and MgO to form MgH and MgOH, it is
feasible to obtain an optical thin film with a low absorption even
in the vacuum ultraviolet region.
[0192] Then, by thus forming a high-quality optical thin film at a
high speed in a monolayer or multilayer structure on a substrate to
form an antireflection film or a reflection enhancing film, it is
feasible to provide such high-quality, inexpensive optical parts as
lenses, mirrors, beam splitters, and the like.
EXAMPLES
Example 1
[0193] The present invention will be described hereinafter in
further detail with reference to the drawings. The following will
describe a process of forming an MgF.sub.2 film adaptable for the
range from the visible region to the vacuum ultraviolet region.
[0194] First, the reactive sputtering apparatus of the same
structure as illustrated in FIG. 1A was prepared. The sputter gas
introducing system 30 connected to the anode was arranged such that
the bottom surface of the anode had a number of holes so as to be
able to uniformly introduce the gas to the cylindrical target
sputter surface. The ringlike permanent magnets 14 used was one
capable of forming a magnetic field of a maximum magnetic flux
density of 250 Gauss parallel to the sputter surface and the
electric field perpendicular thereto, in the vicinity of the
sputter surface of the target 13.
[0195] The target 13 used was a 99.99% purity metal Mg target
having an opening of about 4 inches in diameter and 70 mm in depth,
and the substrate 70 used was a substrate of quartz glass with a
low absorption at the ArF laser wavelength of 193 nm (trade name
EDH; available from Nihon Sekiei).
[0196] After the substrate was held on the substrate holder 71 and
at a position distant by 80 mm from the opening of the target and
the interior of the vessel 10 was evacuated to a high vacuum
condition by the vacuum pump 20, the sputter gas introducing system
30 as connected to the bottom surface of the anode started
introducing a mixed gas of helium (He) gas at 100 sccm for
assisting the surface reaction, argon (Ar) gas at 100 sccm as a
main component of the sputtering gas, and hydrogen gas at 100 sccm
as a reducing gas for removing dangling bonds from the interior of
the film on the substrate, through the anode 41 into the space
surrounded by the cylindrical target and the anode. At the same
time, the reactive gas introducing system 31 started introducing a
mixture of fluorine (F.sub.2) gas diluted in 5 vol % with argon
(Ar) at 200 sccm and water (H.sub.2O) as a reducing gas for
removing dangling bonds from the interior of the film on the
substrate, at 5 sccm whereby the sputtering pressure was controlled
within the range of 0.3 Pa to 3 Pa.
[0197] Then, in the closed state of the shutter 17, the DC power
supply 80 applied the power of 0.5 kW to the anode 41 as a positive
electrode and to the cathode 12 as a negative electrode, thereby
initiating a discharge.
[0198] In this state, pre-sputtering was continued for ten minutes
to remove an oxide layer and a contaminant layer from the target
surface. After the discharge color turned into pale green to
indicate that the metal was exposed in the target surface, the
shutter plate 17 was opened to implement film formation for thirty
minutes. This process resulted in formation a film of MgF.sub.2 in
a thickness of 300 nm on the substrate.
[0199] After that, the quartz substrate was taken out of the vessel
and subjected to measurement with a spectroscope, and it was found
from the measurement that the formed film was an excellent film
having a refractive index of 1.44 at the wavelength of 193 nm and
demonstrating a low light absorption down to near 180 nm.
[0200] For investigating the plasma state in the vicinity of the
substrate, the Langmuir probe (available from ISA JOBIN YVON SOFIE)
for plasma measurement was set at the substrate position instead of
the substrate and the plasma was generated under the same
conditions as above to measure the plasma state, and it was found
from the measurement that the electron temperature Te=2 eV, the
electron density Ne=8.times.10.sup.7 electrons/cm.sup.3, and the
plasma potential Vp=1 V.
Example 2
[0201] A polycarbonate substrate was held instead of the quartz
substrate on the substrate holder 71 and film formation was carried
out under the same conditions as in above Example 1. As a result, a
transparent thin film of MgF.sub.2 with excellent adhesion was also
obtained on the resin substrate.
[0202] Further, a thermolabel for indicating the temperature on the
substrate was attached to the substrate and film formation was
conducted under the same conditions as above. After that, the
polycarbonate substrate was taken out of the vessel to check the
temperature of the thermolabel, and it was verified that the
temperature was low, not more than 40.degree. C.
Example 3
[0203] Using the reactive sputtering apparatus illustrated in FIG.
4, an MgF.sub.2 film was formed under the same conditions as in
Example 1. As a result, the obtained film showed optical film
characteristics in the same level as those of the film obtained in
Example 1.
Comparative Example 1
[0204] As a comparative example, a film formation experiment
similar to the above was conducted using a high frequency power
supply of 13.56 MHz instead of the DC power supply in the apparatus
configuration of FIG. 1A. The high frequency discharge failed to
confine the plasma in the space surrounded by the cylindrical
target and the anode as the bottom surface of the substantially
cylindrical shape, so that the plasma spread throughout the vacuum
chamber. As a result, the formed MgF.sub.2 film was brown and had a
strong absorption for a light of a wavelengths of 180 to 400 nm.
The Langmuir probe for plasma measurement was set at the substrate
position to measure the plasma state in the same manner as above,
and it was verified therefrom that the electron temperature Te=6
eV, the electron density Ne=5.times.10.sup.9 electrons/cm.sup.3,
and the plasma potential Vp=20 V.
[0205] It is speculated from the result that in Comparative Example
1 the plasma spread throughout the entire vacuum chamber caused
excess electrons and positive ions to enter the film during the
film formation, thereby degrading the film quality.
Comparative Example 2
[0206] A parallel plate magnetron sputtering film forming apparatus
illustrated in FIG. 14 was prepared.
[0207] In the structure of this apparatus, the portions denoted by
the same reference symbols as in FIGS. 1A and 1B are of the same
structure and the description thereof is omitted herein.
[0208] The sputtering gas for formation of plasma is introduced
from the sputter gas introducing system 30 into the vessel. The
fluorine-containing gas is introduced from the reactive gas
introducing system 31 into the vessel 10.
[0209] The target holder 12 is fixed through the insulating member
11 to the vacuum chamber 10 and is mounted therein, and a flat
plate type target T is set on the target holder 12 parallel to the
substrate 70. A water cooling jacket is provided in contact with
the back surface of this flat plate type target T and a cooling
fluid is supplied through the cooling pipes 161, 162. The permanent
magnets 14 are buried in the target holder 12 at locations
corresponding to the central part and peripheral part of the flat
plate type target, the permanent magnet in the central part being
positioned with its pole S facing the substrate, and the permanent
magnets in the peripheral part being positioned with their pole N
facing the substrate, so as to make the magnetic field 50.
[0210] Numeral 16 designates the shield plate, which shields a
discharge and also serves as an anode.
[0211] The RF power supply 80 applies a high frequency power to the
target, whereby an RF plasma is generated in the vicinity of the
flat plate type target.
[0212] A film formation experiment was conducted under the same
film forming conditions as in Example 1, using a flat plate target
of the shape having a diameter of 4 inches and a thickness of 6
mm.
[0213] As a consequence, no film formation was recognized in a
region of the substrate which is in opposition to the sputter
surface of the target. This is because the high-energy neutral
particles originating in negative ions enter the substrate, as
described previously.
[0214] Furthermore, a brown film was formed in a region of the
substrate which is not in opposition to the sputter surface of the
target. This is conceivably because the formed film was damaged by
electrons and positive ions in the plasma in the vicinity of the
substrate. The Langmuir probe was set on the substrate holder 71 to
check the plasma state in the vicinity of the substrate, and it was
found that at a location in opposition to the target the electron
temperature Te was 8 eV and the electron density Ne was
2.times.10.sup.9 electrons/cm.sup.3 and that in a region not in
opposition to the target the electron temperature Te was 6 eV and
the electron density Ne 8.times.10.sup.8 electrons/cm.sup.3.
Comparative Example 3
[0215] Furthermore, film formation was carried out under the above
film forming conditions on the polycarbonate substrate using the
conventional parallel plate magnetron system, and it seemed that
there occurred a difference in thermal expansion in the substrate
because of the locations of the substrate and the target, and there
appeared peeling off of the MgF.sub.2 film considered due to the
thermal expansion difference.
[0216] A thermolabel was attached to the polycarbonate substrate in
the same manner as above to measure the temperature during the film
formation, and the temperature was 100.degree. C.
Example 4
[0217] Next, a process of forming a thin film using the apparatus
illustrated in FIG. 7 will be described below.
[0218] The target material used was high-purity Mg metal (99.9%)
and it was mounted on the cathode.
[0219] The substrate to be processed like a synthetic quartz glass
plate was cleaned and set in the load lock chamber, and the
interior thereof was evacuated down to below 1.times.10.sup.-4 Pa.
After completion of the evacuation, the substrate was transported
through the gate valve onto the holder to be held thereon.
[0220] Then, the shutter was closed, Ar and H.sub.2 were introduced
each at 150 ml/min, a mixed gas of F.sub.2 (5 vol %) and Ar was
introduced at 100 ml/min, and H.sub.2O at 5 ml/min was introduced
to set the total pressure to 0.3 Pa to 3 Pa. A sputter power of 300
W was then applied to the cathode to generate a magnetron plasma in
the vicinity of the surface of the target. At the same time as it,
a rectangular voltage of 1 kHz to invert the polarities of the
target surface was superimposed on the sputter power.
[0221] The discharge was continued for a while, and the shutter was
opened after the discharge has become stable.
[0222] Since such a variation with the lapse of time of the target
bias voltage as illustrated in FIG. 8A would occur with
continuation of the film formation at the constant supply rate of
H.sub.2O gas, the data of FIG. 8B was preliminarily inputted in the
control unit and the flow rate of the gas was controlled based
thereon in the present example.
[0223] Specifically, in order to avoid the decrease of the target
voltage with the lapse of time during the film formation, the film
formation was carried out so as to maintain the target voltage
approximately constant by monitoring the target voltage and
performing such control as to decrease the supply rate of H.sub.2O
gas to adjust the partial pressures of the respective gases by the
control unit 160.
[0224] The spectral characteristics of the MgF.sub.2 film formed in
this way are presented in FIG. 10. As is seen from FIG. 10, an
MgF.sub.2 film was formed without heterogeneity and with a low
absorption in the range from the visible region to the ultraviolet
region.
[0225] Although this film was an MgF.sub.2 film formed on the
substrate 70 approximately at room temperature (not more than
40.degree. C.), the film had good adhesion and a hardness
equivalent to that of an evaporated hardcoat (heating at
300.degree. C.). Further, the film had a packing density close to
100% and showed little variation in the spectral characteristics
with the lapse of time.
Example 5
[0226] In the present example, film formation was carried following
the same procedures as in the above example, using the apparatus
illustrated in FIG. 7. Furthermore, in order to reduce absorption
in the ultraviolet region, evaluation was conducted by carrying out
various analyses for relationship among the partial pressures of
Ar, F.sub.2, H.sub.2, H.sub.2O, etc. during the film formation,
film composition, and absorption.
[0227] FIG. 11 presents the analysis results of absorption at the
wavelength 193 nm and XPS (X-ray photoelectron spectroscopy) of
MgF.sub.2 films formed with adjustment of the partial pressures.
The films of MgF.sub.2 contained several % of oxygen and binding
states of this oxygen in the MgF.sub.2 films were evaluated based
on the relation between film absorption and the ratio of MgOH
linkage and MgO linkage by waveform separation of the binding
energy of is orbit of oxygen. In Table 1, Loss (193 nm) film
thickness 100 nm means a loss including absorption and scattering
at the wavelength 193 nm, per thickness 100 nm. The same also
applies to Loss (248 nm) film thickness 100 nm.
[0228] The oxygen incorporated into the MgF.sub.2 films is
considered to result from oxygen and H.sub.2O existing as residual
gases after the film formation or to be introduced at the time of
communication with the atmosphere after the film formation, and it
is impossible to eliminate the oxygen perfectly. Therefore, the
inventors have investigated a way of reducing it by H.sub.2 or a
way of changing it into a state posing no problem in absorption
even if it is incorporated into the film.
[0229] In Table 1, among the samples No. 1 to No. 6, films to
demonstrate low Loss of 0.1% at 193 nm, i.e., low-absorption films
are No. 1 and No. 2. Films with low Loss of 0.1% at 248 nm are No.
1, No. 2, and No. 5. Namely, the samples No. 1 and No. 2 are the
films with the lowest absorption in the both wavelength regions of
193 nm and 248 nm, and the sample No. 5 is the film with low
absorption equivalent to No. 1 and No. 2 in the wavelength region
of 248 nm.
[0230] Now let us discuss factors contributing to the
low-absorption films.
[0231] In No. 1, a quarter of the oxygen content of 4.0 wt % of the
film is oxygen participating with MgO. Namely, in No. 1, the oxygen
concentration in the binding state of MgO is calculated as follows:
4.0 wt %.times.0.25=1.0 wt %.
[0232] In No. 2, 30% of the in-film oxygen content of 4.5 wt % is
oxygen participating with MgO. Namely, in No. 2, the oxygen
concentration in the binding state of MgO is calculated as follows:
4.5 wt %33 0.3=1.35 wt %.
[0233] In No. 5, 35% of the in-film oxygen content of 6.5 wt % is
oxygen participating with MgO. Namely, in No. 5, the oxygen
concentration in the binding state of MgO is calculated as follows:
6.5 wt %.times.0.35=2.28 wt %.
[0234] In No. 1 and No. 2, the in-film oxygen content thereof are
low, 4.0 wt % and 4.5 wt % respectively, and the ratios of MgO are
also low. On the other hand, in No. 5, the in-film oxygen content
is high, 6.5 wt %, but the ratio of MgO is low.
[0235] It became apparent from the above that the absorption of the
film can be decreased when the oxygen content of MgF.sub.2 is small
and when the oxygen incorporated into the film exists not in the
state of MgO but in the state of MgOH (i.e., when the ratio of MgO
is low). The oxygen content of the film could be reduced depending
upon the partial pressure of H.sub.2 but could not made perfectly
null.
[0236] It was therefore clarified that an MgF.sub.2 film with a
particularly low absorption was obtained when the in-film oxygen
content was not more than 5 wt % and when the concentration of
oxygen in the binding state of MgO was not more than 1.5 wt %.
[0237] It was also clarified that when the partial pressure of
H.sub.2O was not less than 1.times.10.sup.-1 Pa, oxidation of
MgF.sub.2 proceeded to increase absorption in the short wavelength
region and make the stabilization of the target voltage difficult
and as a result it became difficult to stabilize the film quality
and sputtering rate. It was also found on the other hand that when
the partial pressure of H.sub.2O was not more than
1.times.10.sup.-3 Pa, the in-film oxygen content and the MgO/MgOH
ratio increased to provide a film with a large absorption.
[0238] Furthermore, it was also found that when the partial
pressure of H.sub.2 was not more than 5.times.10.sup.-2 Pa, because
the oxygen in the film was not reduced sufficiently and a part of
H.sub.2 contributed to the termination of dangling bonds of MgO and
the like, the resultant film demonstrated a large absorption,
particularly, on the short wavelength side.
[0239] Namely, it was clarified that, in order to form an MgF.sub.2
film with a low absorption in which the in-film oxygen content is
not more than 5 wt % and in which the concentration of oxygen in
the binding state of MgO is not more than 1.5 wt %, it is necessary
that during the film formation the partial pressure of H.sub.2O is
not less than 1.times.10.sup.-3 nor more than 1.times.10.sup.-1 Pa
and that the partial pressure of H.sub.2 is not less than
5.times.10.sup.-2 Pa.
Example 6
[0240] Next, an Al.sub.2O.sub.3/MgF.sub.2 multilayer antireflection
film was formed on a quartz substrate, using the apparatus
illustrated in FIG. 7, and the characteristics thereof were
evaluated.
[0241] As in Example 4, an MgF.sub.2 film was formed by using a
high-purity Mg metal (99.9%) as the first target and introducing
Ar, F.sub.2, H.sub.2, and H.sub.2O as gases.
[0242] An Al.sub.2O.sub.3 film was formed by using a high-purity Al
metal (99.99%) as the second target and introducing Ar, He,
O.sub.2, F.sub.2, H.sub.2, and H.sub.2O as gases.
[0243] The operations of alternately forming the MgF.sub.2 film and
the Al.sub.2O.sub.3 film were carried out repeatedly, thereby
obtaining the antireflection film.
[0244] The characteristics of the antireflection film thus obtained
are presented in FIG. 12. Thus, an optical element could be
fabricated which had a wide wavelength region of low reflectance
and good reflection characteristics.
[0245] The transmittances of this element were high, not less than
99.8%, and the absorption was also very small. Furthermore, in the
present example, since both Al.sub.2O.sub.3 and MgF.sub.2 could be
sputtered at a high speed and their sputtering rates were also
stable, the antireflection characteristics as designed could be
achieved at a high speed.
Example 7
[0246] In the present example, a low-absorption MgF.sub.2 film was
formed on a surface of a PC (polycarbonate) substrate, using the
apparatus illustrated in FIG. 9, while setting the anode potential
to +50 V to -50 V relative to the earth.
[0247] This MgF.sub.2 film was analyzed by RBS (Rutherford
backscattering), with the result that the film contained several %
of Ar. Furthermore, the film was analyzed by XRD (X-ray
diffraction) and it was verified therefrom that the thin film
obtained was crystallized, the lattice spacing was 1 to 2% wider
than that of the single crystal of MgF.sub.2, and thus the
MgF.sub.2 crystal was distorted because of intrusion of Ar in the
lattice. This Ar correlated with Ar ions entering the substrate and
the result was that a film containing a large amount of Ar because
of a large ion incidence was superior in adhesion, denseness, and
hardness. It was clarified that the MgF.sub.2 film was particularly
superior when the Ar content of the thin film of MgF.sub.2 was
about 1 to 10 wt %.
[0248] The amount of Ar introduced into the film varied depending
upon various parameters including the Ar partial pressure of the
film formation atmosphere, the film forming pressure, the
sputtering rate, and so on, but it was verified that the plasma
assist effect could be readily controlled by controlling the anode
potential as in the apparatus illustrated in FIG. 9 and used in the
present example and that MgF.sub.2 thin films with a high quality
could be obtained readily.
[0249] When the assist effect of Ar ions was further enhanced, the
denseness and hardness of film increased, but absorption also
increased. Also, distortion of film became too large and peeling
off of film also occurred. At this time Ar in film was over
15%.
[0250] From the above results, by controlling Ar in the MgF.sub.2
film within the range of 1 to 10 wt %, a dense, hard MgF.sub.2 film
is obtained with a low absorption within the range from the visible
region to the ultraviolet region and with excellent adhesion to the
substrate even at a low substrate temperature. Although the present
example was described as to Ar, similar effect can also be attained
with inert gases (He, Ne, Xe, Kr) which do not react with Mg or
oxygen normally used in sputtering to bring about no
absorption.
[0251] In the present example, an antireflection film was obtained
with very good characteristics. In the conventional sputtering, the
substrate surface was damaged because of the affection of
.gamma.-electrons from the target and films obtained were inferior
in adhesion. In contrast, the process of the present example
permits control of damage and enables optical thin films with a
high quality to be formed at a low substrate temperature.
Example 8
[0252] Next, an antireflection film for the F.sub.2 excimer laser
(the wavelength 157 nm) consisting of LaF.sub.3/MgF.sub.2 multiple
layers was formed on a fluorite substrate, using the apparatus
illustrated in FIG. 7. LaF.sub.3 can be replaced by SiO.sub.2,
GdF.sub.3, or NdF.sub.3 having a similar function.
[0253] An optical device of the present invention will be described
below.
[0254] The thin films formed by the production process of a thin
film according to the present invention, as described above, can be
suitably utilized as antireflection films or reflection-enhancing
films of optical parts.
[0255] Optical devices such as laser oscillators, aligners, and so
on can be fabricated by properly combining the optical members
having the thin films thus obtained.
[0256] FIG. 13 shows an aligner for photolithography having a laser
oscillator, as an example of the optical device.
[0257] The laser oscillator 200 emits the vacuum ultraviolet light
such as ArF excimer laser, F.sub.2 excimer laser, or the like and
has a laser gas chamber 21, and a pair of windows 22 and resonators
23 made of calcium fluoride. The aligner is constructed by assembly
of an illumination optical system 25, an imaging optical system 26,
a stage 28, and so on, and each of the optical systems 25, 26 is
composed of a plurality of lens units.
[0258] The optical parts with the thin films according to the
present invention are used to make the windows 22, resonators 23,
illumination optical system 25, imaging optical system 26, and so
on.
[0259] The light having passed through an optical mask (reticle) 27
travels through the imaging optical system 26 to form a light image
of the reticle 27 on a body to be exposed (work) W, which is
mounted on a stage 28 as a holding means. Typical examples of the
work W are substrates of Si wafers and glass plates or the like
having a photoresist thereon.
[0260] In the case of the step-and-repeat system, the apparatus
repeatedly carries out such an operation that after exposure of an
area of a first section the stage is stepped and an area of a next
section is exposed by single-shot exposure or scanning exposure. In
the case of the scanning exposure, the exposure is carried out
while moving the reticle relative to the stage.
[0261] This exposure makes a latent image in the photoresist and
then the photoresist is developed to become a mask pattern for
etching or ion implantation.
[0262] After that, using the mask pattern, ions are implanted in
the substrate, or the substrate surface is etched.
[0263] In this way devices can be fabricated in high integration
density, using the optical device of the present invention.
[0264] According to the present invention, it is feasible to
implement the film formation using target materials readily
generating negative ions or the film formation of fluoride films
susceptible to damage, which were impossible by the conventional
parallel plate type magnetron sputtering, and also to form thin
films with little plasma damage at a low temperature and at a high
speed on plastic substrates and others vulnerable to heat.
[0265] Furthermore, when the target voltage is controlled to a
substantially constant level by controlling the introducing rate of
the reactive gas flowing between the substrate and the aperture
while monitoring the voltage applied to the target, it is easy to
keep the surface of the target in the metal state, the sputtering
rate can be kept stably high, and thin films having improved film
characteristics and being transparent without absorption within the
range from the visible region to the ultraviolet region can be
formed at a low cost and at a high speed.
* * * * *