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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming a thin film which is
used as conductor in a semiconductor integrated circuit and more
particularly to a method for forming a planarized thin film over an
undercoat film having a rough surface which is not planarized.
2. Description of the Prior Art
So far, a vacuum evaporation or sputtering process has been widely used to
form a thin metal film which is used as conductor in a semiconductor
integrated circuit. However, the surfaces of a substrate over which thin
metal films are formed are generally not planar, but are rough. That is,
the surfaces have small ridges and valleys or are convex and concave. As a
result, when a thin film is formed over the rough surface of such a
substrate by the vacuum evaporation or sputtering process, a shape of a
thin film covering the rough surface is deteriorated. For instance, an
overhanging thin film is formed at a ridge so that a micro-crack occurs at
a step of the ridge to result in a wiring breakdown such as disconnection.
In order to solve the above-described problems, it is required that a thin
metal film completely fills ridges and valleys of the substrate to
planarize the rough surface of the substrate as much as possible. In order
to satisfy the above-described requirement, we can consider the following
two processes. One process is such that after a thin metal film has been
formed over a rough surface of a substrate to poorly cover the rough
surface, defective portions of the thin metal film thus poorly covering
the rough surface are remedied by some methods. The other process is such
that a planarized thin film is so formed that its planarized surface
covers uniformly the rough surface.
However, there has not been proposed yet the former process which is
satisfactory in practice. As the latter process, there is a technique
which is a so-called "bias sputtering method" such as a proposal by Homma
et al. in Journal of Electrochemical Society, Vol. 132, (1985), pp.
1466-1472, "Planar Deposition of Aluminum by RF/DC Sputtering with RF
Bias". In the bias sputtering method, a material of a thin film is
sputtered to form a thin film over the rough surface of a substrate under
a condition that a DC or AC bias voltage such as a voltage in a range of
-100 V through -500 V is applied to the substrate According to the
conventional bias sputtering method, the formation of a thin film proceeds
while a part of the thin film deposited over the substrate is being etched
out.
That is, as shown in FIG. 1, the bias sputtering method utilizes the fact
that an etching rate of a thin film is dependent upon an ion incident
angle in order to planarize the surface of the deposited thin film, as
will be described in detail hereinafter.
The planarization process will be explained in detail with referring to
FIGS. 2A-2E. While a thin film formation period is short, an
unsatisfactory Al film 101 with poor planarization is formed over a
substrate 100 having convex portions 102 as shown in FIG. 2A. Thereafter,
in a little while, the inclined surfaces of the Al thin film 101 are
etched faster than the horizontal surfaces thereof, and at the same time,
Al is deposited over the inclined and horizontal surfaces, so that the
covering shape of the thin film being deposited varies as shown in FIGS.
2B and 2C. When the formation of the thin film is further carried out, the
etching rate at the inclined surfaces is faster than that at the
horizontal surfaces parallel to the surface of the substrate 100, so that
the inclined surfaces recede toward the middle portions of the projections
102, as shown in FIG. 2D. In this case, while the deposited thin film is
etched, a new thin film is formed by the deposition of sputtered atoms
flying from a target, so that the thickness of the thin film 101 is not so
extremely thin. As the time further elapses, the inclined surfaces over
the projections 102 further recede toward the middle portions of the
projections 102 and finally the inclined surfaces which are receding from
both sides disappear at the midddle portions of the projections 102, so
that the thin film 101 has a planarized surface, as shown in FIG. 2E.
The above-described bias sputtering method, however, has the following
fatal defects.
Firstly, the rate at which a new thin film is formed by the deposition of
sputtered atoms from the target must be lower than the rate at which the
inclined surfaces recede or disappear, so that a planarized surface is
obtained. As a result, there arises the problem that it takes a long time
before the surface of the thin film is fully planarized. According to a
conventional magnetron sputtering process, the deposition rate of aluminum
is about 1 .mu.m/min, while according to the above described bias
sputtering process, the deposition rate is about 20 nm/min. That is, the
deposition rate of the bias sputtering process is 50 times as slow as the
deposition rate of the magnetron sputtering process.
Secondly, as is clear from the above-described explanation with reference
to FIGS. 2A-2E, when the width of a projection is narrow, the surface of a
thin film can be planarized within a relatively short time, but when the
width of a projection is wide, it takes a longer time before the surface
of a thin film deposited is planarized. As a result, when the surface
pattern of an undercoat or substrate has projections in various sizes, the
thickness and the shape of a thin film deposited over the undercoat or
substrate are dependent upon the surface pattern of the undercoat or
substrate. More particularly, the thickness of the thin film varies from
one pattern projection to another and the surface on a large pattern
projection is not sufficiently planarized. Due to this dependency of the
surface of the thin film on the surface pattern of the substrate, the thin
film remains partially unetched in the succeeding etching step, so that a
yield of the finished product becomes low.
In addition, according to the bias sputtering process, accelerated ions are
made to impinge against a surface of a substrate, so that sputter etching
proceeds at the same time that a thin film is deposited over the surface
of the substrate. As a result, during the initial time that a thin film is
not sufficiently deposited over the surface of the substrate, the elements
of the substrate which have been sputter-etched are mixed into the thin
film being deposited and accelerated ions are also injected into the thin
film, so that a purity of the deposited thin film is lowered. When ions
are impinged against the substrate surface with a higher acceleration
energy, the structure of the deposited thin film is varied and accordingly
a high-quality thin film cannot be formed continuously.
FIG. 3 is a photograph taken by a scanning type electron microscope,
illustrating the surface condition when a thin aluminum film is formed
over the surface of an SiO.sub.2 film by the bias sputtering process with
a higher acceleration energy. It is seen that the crystal grain growth of
aluminum is adversely restrained by oxygen and silicon emitted from the
substrate so that column crystals in which crystal grains are spaced apart
from each other are grown. As a result, the aluminum film thus deposited
exhibits infinite resistance.
Because of the defects described above, the bias sputtering process has not
been satisfactorily used in practice to form a thin metal film.
SUMMARY OF THE INVENTION
In view of the above, one of the objects of the present invention is to
provided a novel method for forming a planarized thin film which covers a
convex and concave surface of a substrate with a good covering shape and
has a planarized surface at a high rate.
Another object of the present invention is to provide a method for forming
a well planarized thin film with a good covering shape over the patterned
surface of the substrate independently of the surface pattern thereof.
A further object of the present invention is to provide a method for
forming a planarized thin film which is high both in quality and purity.
A yet further object of the present invention is to provide a method for
forming a thin metal film which has a low resistance, excellent crystal
properties and a high degree of mirror surface properties.
A still further object of the present invention is to provide a method for
forming a planarized thin film without causing any damage on the surface
of a substrate.
A still further object of the present invention is to provide a method for
forming a planarized thin film preferably adapted to define a conduction
layer in a VLSI of the order of less than one micron meter in width.
To the above and other ends, according to the present invention, charged
particles are irradiated over a thin film formed on a convex and concave
surface of a substrate or over a thin film being formed on a convex and
concave surface of a substrate. During the irradiation, raise in
temperature of the thin film and impingement of charged particles cause
the fludization of the thin film, so that a planarized thin film is formed
within a short period of time.
The present invention is based upon the fludization phenomenon of aluminum
or Al discovered when the inventors conducted extensive studies and
experiments of the aluminum sputtering process and is, therefore, quite
different in principle from the prior art bias sputtering process
utilizing the etching phenomenon.
According to another aspect of the present invention, after elements of a
target are deposited without irradiating charged particles to a substrate
until a continuous film is formed by the deposition, a sputter deposition
is carried out while charged particles are being irradiated to the
substrate.
More particularly, first elements of the target are deposited on the
substrate without irradiating charged particles. The film thus deposited
consists of high purity target elements without contamination of the
elements of the substrate. Thereafter, while charged particles are being
irradiated to the substrate, the sputter deposition is carried out. In
this case, the deposited film is subjected to the etching process, so that
there seems to be the possibility that the composition materials are mixed
into a newly formed film, but the purity of a newly deposited film is not
decreased, since the deposited film itself consists of a high purity
target material. In addition, in the case of the above-described method,
the surface of the substrate is prevented from being directly impinged by
the ions, so that no damage is caused on the substrate.
In the first aspect of the present invention, a method for forming a
planarized thin film, comprises the steps of:
forming a thin film on a substrate having a non-planarized surface; and
irradiating charged particles over the thin film so that the thin film is
fluidized by a temperature rise of the thin film and bombardment of the
thin film with the charged particles.
Here, the thin film may be an aluminum film. The charged particles
irradiating the thin film may be accelerated by a bias voltage whose
absolute value is higher than .vertline.-850.vertline. volts. A
temperature of the thin film may be lower than a melting point of the thin
film.
After the formation of the thin film on the substrate having the
non-planarized surface, the charged particles may be irradiated on the
thin film after the substrate has been heated from the exterior or while
the substrate is being heated from the exterior.
A temperature of the thin film in case of the irradiation with the charged
particles and the heating from the exterior may be lower than a melting
point of the thin film.
In the second aspect of the present invention, a method for forming a
planarized thin film on a substrate having a non-planarized surface, the
method comprises the steps of:
irradiating charged particles on the thin film which is being formed; and
forming the thin film while fluidizing the thin film by a temperature rise
of the thin film and bombardment of the thin film with the charged
particles.
Here, the thin film may be an aluminum film. The charged particles
irradiating the thin film may be accelerated by a bias voltage whose
absolute value is higher than .vertline.-700.vertline. volts. A
temperature of the thin film may be lower than a melting point of the thin
film.
The substrate may be heated from the exterior when the thin film is formed
on the substrate having the non-planarized surface.
A temperature of the thin film in case of the irradiation with the charged
particles and the heating from the exterior may be lower than a melting
point of the thin film.
In the third aspect of the present invention, a method for forming a
planarized thin film, comprises:
a first step of forming a first thin film on a substrate having a
non-planarized surface; and
a second step of forming a second thin film on the first thin film while
irradiating on the second thin film being formed with charged particles
during the formation of the second thin film.
Here, a thickness of the first thin film deposited in the first step may be
so determined that the first thin film is not deposited in the form of
islands but is deposited as a continuous film.
The charged particles may be irradiated against the second thin film being
formed when the second thin film is formed in the second step, and the
second thin film may be formed while the second thin film is fluidized by
a temperature rise of the thin film and bombardment of the thin film with
the charged particles.
The thin film may be an aluminum film. The charged particles irradiating
the thin film may be accelerated by a bias voltage whose absolute value is
higher than .vertline.-700.vertline. volts. A temperature of the thin film
may be lower than a melting point of the thin film.
The substrate may be heated from the exterior in the second step.
A temperature of the thin film in case of the irradiation with the charged
particles and the heating from the exterior may be lower than a melting
point of the thin film.
The above and other objects, effects, features and advantages of the
present invention will become more apparent from the following description
of preferred embodiments thereof taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram used to explain the underlying principle of a
conventional bias sputtering process;
FIGS. 2A-2E are cross sectional views showing sequential steps of the
conventional bias sputtering process;
FIG. 3 is a photograph taken by a scanning type electron microscope and
illustrating the surface of an aluminum film formed on an SiO.sub.2
substrate by the conventional bias sputtering process;
FIG. 4 is a schematic view showing an apparatus used to carry out the
present invention;
FIG. 5 is a sectional view showing a detail of a target and a substrate
thereof;
FIG. 6 is a cross sectional view showing a modification of a sputtering gun
shown in FIG. 5;
FIGS. 7A and 7B are cross sectional views illustrating sequential steps of
a first embodiment of the present invention;
FIGS. 8A-8C and FIGS. 9A-9E are photographs illustrating bias voltage and
irradiation time dependence of coating conditions of aluminum films formed
in accordance with a first embodiment of the present invention;
FIG. 10A illustrates a characteristic curve of a dependence of the degree
of planarization on a bias voltage in the first embodiment of the present
invention;
FIG. 10B is an explanatory diagram used to explain the definition of the
term "degree of planarization";
FIG. 11 is a photograph illustrating the coating conditions of an aluminum
film deposited over the top surface of a substrate when charged particles
are irradiated to the bottom surface of the substrate;
FIGS. 12A-12D are photographs illustrating the coating conditions of the
aluminum films deposited in accordance with a second embodiment of the
present invention;
FIG. 13 is a diagram illustrating the dependence of a degree of
planarization of an aluminum film upon a bias voltage in the second
embodiment of the present invention;
FIG. 14 is a diagram illustrating the dependence of a film deposition rate
and an etching rate upon a bias voltage in the second embodiment of the
present invention;
FIG. 15 is a timing chart illustrating a relationship of power inputs
between a target power supply and a substrate power supply;
FIG. 16 is a timing chart used to explain the mode of operation of a
sequence controller;
FIG. 17 illustrates a relationship of a resistivity of an aluminum film
with bias voltage;
FIG. 18 illustrates a time-resistivity characteristic curve of a variation
in resistivity of an aluminum film with respect to time t.sub.1 ;
FIG. 19 is a photograph taken by a scanning type electron microscope and
illustrating the surface of an aluminum film formed by the bias sputtering
process when time t.sub.1 =60 sec;
FIG. 20 shows a characteristic curve illustrating the relationship between
the initial film thickness and the step coverage when the whole film
thickness is 1.5 .mu.m;
FIG. 21 is a photograph taken by a scanning type electron microscope and
illustrating the cross section of an aluminum film formed by bias
sputtering on a substrate having grooves with a depth of 0.8 .mu.m and a
line-and-space of 1.0 .mu.m-1.5 .mu.m; and
FIG. 22 illustrates characteristic curves representative of the
relationship between the thickness of a deposited film and its resistivity
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Prior to the description of embodiments of the present invention, an
example of an apparatus adapted to carry out the present invention will be
described with reference to FIG. 4.
In FIG. 4, a substrate 2 and a target 3 are disposed in opposed
relationship with each other within a vacuum chamber 1. A target power
supply 4 electrically connected to the target 3 may be a DC or RF power
supply. A substrate power supply 5 electrically connected to the substrate
2 may be a DC or RF voltage generator. The target power supply 4, the
substrate power supply 5, a vacuum exhausting unit 6 for evacuating the
vacuum chamber 1, a gas controller 9 for controlling the flow rate of a
gas to be supplied into the vacuum chamber 1 and a shutter 10 interposed
between the substrate 2 and the target 3 are controlled to be energized or
interrupted by the control signals from a sequence controller 8. Valves 11
and 12 are provided between the vacuum exhausting unit 6 and the vacuum
chamber 1 and between the gas flow controller 9 and the vacuum chamber 1
to control vacuum condition of the vacuum chamber 1 and the gas flow into
the vacuum chamber 1, respectively.
FIG. 5 shows in detail the substrate 2, the target 3 and their associated
parts, but does not show the detail of the vacuum chamber 1 and a gas
supply line and an exhaust line communicating therewith. A cylindrical
magnet 21A is fitted into a sputter gun 21 and a magnet 21B is disposed in
the cylindrical magnet 21A in coaxial relationship therewith in such a
manner that the magnetic poles of the cylindrical magnets 21A and 21B are
opposite to each other as shown in FIG. 5. An electrode 21C is attached to
the inner end of the magnet 21B within the vacuum chamber 1 and is
grounded. The target 3 is attached to the inner end of the magnet 21A
within the vacuum chamber 1. The target 3 is, for example, made of
aluminum and is tapered radially inwardly. For instance, the outer
diameter of the target 3 is about 18 cm and the area of the target surface
is about 200 cm.sup.2. The target 3 is connected to the power supply 4 in
such a way that a negative potential is applied to the target 3 from the
power supply 4. Since the sputter gun 21 has the coaxial magnets 21A and
21B as described above, the plasma for sputtering the target 3 is produced
only around the target 3. A susceptor 22 supported by a substrate
electrode 23 is disposed in opposed relationship with the sputter gun 21
and is spaced apart therefrom by about 8 cm and the surface area of the
susceptor 22 is, for example, about 80 cm.sup.2. The susceptor 22 has a
peripheral flange about 1 mm in height which is extended inwardly to be
made contact with the bottom surface of the substrate 2. The substrate
electrode 23 is connected to the DC or AC bias power supply 5. Retainers
24 for retaining the substrate 2 in position are disposed around the
susceptor 22 in such a manner that the substrate 2 is electrically
isolated from the vacuum chamber 1. The sputter gun 21 and the substrate
electrode 23 are gas-tightly attached to the vacuum chamber 1 by means of
an O ring 25 and a sealing member 26, respectively. It should be noted
that the sputter gun 21 may be a sputter gun structure having a flat
target 3' as shown in FIG. 6.
When a thin film is deposited by using the apparatus shown in FIGS. 4 or 5,
the substrate 2 is first mounted in the vacuum chamber 1 which is then
evacuated by the vacuum exhausting unit 6. Thereafter a sputtering gas is
introduced into the vacuum chamber 1 through the gas controller 9 so that
the pressure in the vacuum chamber 1 is maintained at a predetermined
level. As the sputtering gas an inert gas such as argon can be used in
most cases. Next, the target power supply 4 is turned on. When the target
3 is of the two pole, parallel plate non-magnetron type sputter, a plasma
7 is generated between the target electrode 3 and the substrate 2. When
magnets 21a and 21b are used in combination with the target electrode 3, a
plasma 7' is generated in the vicinity of the target electrode 3. Due to
the sputtering action caused by a bias generated between the target
electrode 3 and the plasma 7 or 7', the element consisting the target
electrode 3 is is sputtered. Under a condition that the shutter 10 is
opened, the element of the target is deposited on the substrate 2.
Here, explanation will be made of effects when a bias voltage is applied to
the substrate 2 from the bias power supply 5. If a negative DC or RF bias
voltage is applied to the substrate 2, charged particles in the plasma 7
or a plasma newly generated by the bias voltage applied to the substrate 2
are accelerated by the bias voltage to impinge against the substrate 2. In
this case, an acceleration energy of the charged particles irradiated to
the substrate 2 is determined by the DC bias voltage in case of DC bias or
a self bias voltage generated between the substrate 2 and the plasma in
case of RF bias.
Next, a first embodiment of the present invention will be explained.
For example, as shown in FIG. 7A, a silicon oxide film is used as the
substrate 2 having projections 31. An aluminum film 32 is formed on the
substrate 2 by the sputtering process without applying any bias voltage to
the substrate 2. Subsequently, an RF, for instance, 13.56 MHz voltage is
applied from the power supply 5 to the substrate 2, so that the plasma is
produced in the vacuum chamber 1. The charged particles (mainly ions) such
as argon ions are caused to impinge against the aluminum film 32 on the
substrate 32 to which the bias voltage is applied. As a result, the
aluminum film 32 is fluidized, so that a planarized film 32' is obtained
as shown in FIG. 7B.
Next referring to FIGS. 8A-8C, actual variations in the coverage condition
of the aluminum film in this case will be described. FIG. 8A is a
perspective view corresponding to FIG. 7A and illustrating the coverage
shape of the aluminum film 32 formed on the silicon oxide substrate having
the projections 31 which were 2.5 .mu.m in width, 1.0 .mu.m in height and
4.5 .mu.m in pitch, without applying a bias voltage to the substrate. The
portion of the aluminum film 32 corresponding to the projection 31 had a
thickness of 1.5 .mu.m. The film formation time was 7.5 minutes.
The aluminum film 32a deposited on the side walls 33 of the projection 31
was extremely thin as compared with the aluminum film 32b deposited on the
top of the projection 31 and the aluminum film 32c deposited on the bottom
of the groove 34 between the projections 31 converged upwardly. As
described above, the aluminum film 32 formed by the sputtering process
without applying a bias voltage to the substrate 2 exhibited extremely bad
coverage, because of reflecting the shadowing effect of the sputtering
process.
Thereafter, a bias voltage was applied to the substrate 2 and the charged
particles were irradiated over the surface of the aluminum film 32. Then,
due to the application of the bias voltage, the aluminum film 32 was
fluidized so that its coverage was considerably improved and consequently
a planarized aluminum film 32' was formed. This process is shown in FIGS.
8A-8C.
In this case, the bias voltage means a self-bias voltage produced in the
substrate electrode 23 (See FIG. 5) due to the generation of the plasma.
The higher the power of the power supply 5, the higher the bias voltage
becomes.
FIG. 8B shows the coverage of the aluminum film 32' formed by the
processing for ten minutes when the bias voltage was -850 V (corresponding
to 100 W power). When FIG. 8B is compared with FIG. 8A, it is seen that
FIG. 8B shows the transition step in which both the edges of the aluminum
film 32b deposited on the top of the projection 31 are rounded and the
cross section of the aluminum film 32c deposited on the bottom of the
groove 34 is varied due to fluidization, whereby the overall aluminum film
is gradually planarized.
FIG. 8C shows the coverage of the aluminum film 32' formed by the
processing for ten minutes when the bias voltage was -1200 V
(corresponding to 220 W power). It is seen that the aluminum film 32' is
fluidized to fill in the groove 34, so that the side walls 33 of the
groove 34 are satisfactorily coated by the aluminum film 32' and
consequently a degree of planarization of the deposited aluminum film is
improved further.
FIGS. 9A-9E illustrate irradiation time dependence of the coating condition
when charged particles are irradiated to the aluminum film with a bias
voltage of -1400 V. In this case, the projection of the silicon oxide film
substrate had a height of 1 .mu.m, a width of 1 .mu.m and a pitch of 3
.mu.m. The thickness of the aluminum film on the projection was the same
as in FIG. 8A.
FIG. 9A shows a condition before charged particles are irradiated and has
the same shape as in FIG. 8A.
FIG. 9B shows a covering condition of the aluminum film 32' when charged
particles were irradiated for two minutes. The aluminum film 32c on the
bottom of the groove 34 which had a converging shape was planarized and
the both edges of the aluminum film 32b on the projection 31 were slightly
rounded.
FIG. 9C shows a shape after the irradiation for three minutes. The shape
was not remarkably varied compared with the shape shown in FIG. 9B after
the irradiation for two minutes.
FIG. 9D shows a shape after the irradiation for four minutes. The height of
the aluminum film 32b on the projection 31 was decreased and the height of
the aluminum film 32c on the bottom of the groove 34 was increased with an
inclined surface. This inclination indicates that the aluminum film 32b on
the projection 31 was fluidized and ran into the groove 34.
FIG. 9E shows a shape after the irradiation for five minutes. The surface
of the aluminum film was satisfactorily planarized.
As clear from the above examples, the aluminum film on the projection
suddenly flows into the groove for the first time after the radiation for
a predetermined time. This indicates that a temperature of the substrate 2
rises due to the impingement of charged particles, so that the aluminum
film was fluidized.
FIG. 10A shows the dependence of a degree of planarization on the bias
voltage. The degree of planarization is defined by .gamma.=1-(d.sub.1
/d.sub.0), where d.sub.0 is the height of a step-like projection, i.e.,
the depth of a groove of an undercoat (substrate) and d.sub.1 is the depth
of a recess formed on a surface of an aluminum film deposited on the
substrate. When the surface of the aluminum film is completely planarized,
.gamma.=1 and when the surface pattern of the aluminum film corresponds to
the surface pattern of the undercoat or substrate, .gamma.=0. Furthermore,
when d.sub.1 is greater than d.sub.0, the value of .gamma. becomes
negative.
It is seen from FIG. 10A that when the absolute value of the bias voltage
is substantially larger than .vertline.-850.vertline. V (for example,
.vertline.-1200.vertline. V) when the apparatus of the type described
above with reference to FIG. 5 is used, it becomes possible to obtain a
highly planarized aluminum film which does not reflect the step portions
of the surface pattern of the undercoat or substrate. The upper limit of
the bias voltage is a bias voltage value immediately before an aluminum
film reaches a temperature (melting point) at which an aluminum film
starts to melt.
Next, substantial differences between the present invention and the
conventional bias sputtering process will be briefly described. As
described above, the conventional bias sputtering process utilizes etching
phenomenon. On the other hand, according to the present invention, even
though the aluminum film 32 is more or less etched when a bias voltage is
applied to the substrate, the etching rate is extremely low. For instance,
with the bias voltage of -850 V, the etching rate is about 200 .ANG./min
and with the bias voltage of -1200 V, the etching rate is about 300
.ANG./min. Thus, within such a short time interval as described above, it
is impossible to recede to planaraze the aluminum film 32b on the top of
the projection 31 having a width of 2.5 .mu.m. For instance, even if the
aluminum film 32b is receded, no aluminum is supplied by the sputtering
process, so that it cannot be explained why the aluminum film 32c which
converges upwardly in the groove 34 as shown in FIG. 8A becomes uniformly
planarized as shown in FIG. 8B.
According to the present invention, the aluminum film 32 is irradiated with
charged particles so that the temperature of the aluminum film rises. In
addition, due to the bombardment of the surface of the aluminum film with
the accelerated charged particles, the aluminum film is fluidized to fill
in the groove so as to planarize the surface of the aluminum film as shown
in FIGS. 8B and 8C.
It should be noted that according to the present invention, the temperature
rise of the aluminum film caused by the irradiation of the charged
particles must be less than a melting point of the aluminum film. For
instance, at the bias voltage of -850 V, the temperature rise was about
380.degree. C. and at the bias voltage of -1200 V, the temperature rise
was about 410.degree. C.
As described above, the bombardment of the surface of the aluminum film
with the charged particles plays an important role in the present
invention and the reason was cleared as a result of extensive studies and
experiments conducted by the inventors as follows.
FIG. 11 shows conditions of the aluminum film when the specimen as shown in
FIG. 8A is turned upside down and the rear surface thereof is irradiated
with the charged particles under the same condition as in FIG. 8C. When
the rear surface of the substrate is irradiated with the charged particles
at the bias voltage of -1200 V, the temperature rises substantially to the
same level as the temperature as shown in FIG. 8C, but the coverage of the
aluminum film is substantially similar to that as shown in FIG. 8A and is
not improved.
When heat dissipation is caused by thermal radiation from the substrate,
there exists no temperature difference between the top and rear surfaces
of the substrate as disclosed in Journal of Vacuum Science Technology Vol.
11 (1974), pp. 1177-1185. In view of the above, the temperature per se is
not an essential requirement to cause the fludization of the aluminum film
and the above-described fact shows that the bombardment of the surface of
the thin film with the charged particles is also required in order to
realize the effects of the present invention.
In the above-described first embodiment of the present invention, the
substrate is not positively heated from the exterior. It is of course
possible to irradiate the surface of a thin film with the charged
particles while the substrate is heated from the exterior by means of a
heater or an infrared lamp. As compared with the case in which the
substrate is not heated from the exterior, the aluminum film is easily
fluidized when the substrate is heated from the exterior, so that the bias
voltage required for obtaining the corresponding degree of planarization
can be remarkably decreased. However, in this case, it is required that
the temperature rise due to the irradiation of the charged particles and
the external heating does not exceed a melting point of the aluminum film.
While in the first embodiment, the step for depositing an aluminum film and
the step for irradiating charged particles to the aluminum film are
sequentially and continuously processed, the steps can be processed
separately, since the steps are independent of each other. For instance,
the depositing step of the aluminum film can be processed in a different
vacuum chamber or by employing a different deposition method such as
vacuum evaporation method, chemical vapor deposition method. In addition,
a further step such as a patterning step of the aluminum film can be
processed between the above-described two steps, without any hindrance to
embody the present invention.
Next, a second embodiment of the present invention will be described.
According to the second embodiment, while an aluminum film is being
formed, its surface is planarized. A substrate having a convex and concave
surface is mounted in the vacuum chamber 1 and an argon gas or the like is
introduced into the vacuum chamber 1. While a high frequency power of, for
instance, 13.56 MHz is applied to the substrate electrode 23 from the
power supply 5 and charged particles are irradiated to the substrate 2,
the sputter gun 21 is energized to sputter the aluminum target 3, so that
an aluminum film is formed on the rough surface of the substrate 2.
FIGS. 12A-12D show the coverage shape of the aluminum thin film which was
formed upon the surface of a silicon dioxide film having projections 40
having a width of 1.0 .mu.m, a height of 1.0 .mu.m and a pitch of 3.5
.mu.m while the bias voltage of -850 V was applied to the substrate 2.
While the film formation period is short, the temperature rise is not
sufficient so that the coverage of the aluminum film 41 is not
satisfactory, even though the coverage is not so worse as shown in FIG.
8A.
Thereafter, the deposition of the aluminum film 41 is further proceeds,
while the same bias voltage is maintained. Then, the aluminum film 41 is
gradually fluidized because of the temperature rise of the substrate 2 and
the bombardment of the surface of the growing aluminum film 41 with the
charged particles as described above in the first embodiment. As a result,
as shown in FIGS. 12B-12D, the aluminum film 41 completely fills in the
grooves 42 and the surface of the aluminum film 41 is planarized.
As described above, according to the second embodiment of the present
invention, even though the absolute value of the bias voltage is as low as
-850 V as compared with the first embodiment, the surface of the aluminum
film is planarized because the temperature of the substrate rises due to
the impingement of the sputtered atoms thereon. In this case, the
temperature of the substrate was about 450.degree. C.
FIGS. 12A-12D show how the degree of planarization of the aluminum film
formed under the same bias voltage condition is improved according to
processing time period. It is confirmed that similar improvement can be
attained at different bias voltages according to the results of the
studies and experiments conducted by the inventors.
FIG. 13 shows the dependence of the degree of planarization upon the bias
voltage. It is seen that when the bias voltage has an absolute value
greater than .vertline.-700.vertline. V, the degree of planarization is
improved and especially when the bias voltage has an absolute value
greater than .vertline.-800.vertline. V, the surface of the deposited
aluminum film is so planarized that the convex and concave portions of the
surface are almost zero. In this case, the upper limit of the bias voltage
must be so determined that the temprature of the aluminum film does not
exceed a level at which the aluminum film is melted.
One of the greatest differences between the second embodiment of the
present invention and the prior art bias sputtering process resides in the
fact that a ratio R of (etching rate)/(film formation rate) is
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