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BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
The present invention relates to a method of and apparatus for forming a
thin film through ion impact, and to circuit devices having such a thin
film. The invention is particularly suitable for application to barrier
layers or wiring films of large scale integrated circuits.
2. DESCRIPTION OF THE PRIOR ART
As the density of integration in large scale integrated circuits (LSIs) or
very large scale integrated circuits (VLSIs) becomes high, the diameters
of contact holes between a silicon substrate and an aluminum wiring film
and of through holes between aluminum wiring layers become small,
resulting in a larger aspect ratio as shown in FIG. 21. With a magnetron
sputter method commonly used in the art, as the aspect ratio becomes near
1, the step coverage shown in FIG. 21 becomes poor due to the shadowing
effect during sputter deposition of wiring material, so that wiring
failure due to an increase in wiring resistance or electromigration may
occur. To eliminate such disadvantages, a bias sputter method, as
disclosed in Japanese Patent Unexamined Publication JP-A-61-261472
(corresponding to EP-A-202572) and Japanese Patent Unexamined Publication
JP-A-61-214174 (no corresponding foreign application), has been developed
whereby a film is formed while applying negative voltages to a target and
a substrate at all times. The principle of a DC magnetron bias sputter
method is exemplarily shown in FIG. 16. A sputter DC power source 12 is
connected to a target 6, while a bias (reverse sputter) DC power source 11
is connected to a substrate 8. These negative electrodes are always
supplied with DC negative voltages as shown in FIG. 17 during film
formation so that a film is formed while the substrate 8 is being
subjected to Ar ion impact (reverse sputter). Therefore, the step coverage
is improved as compared with the sputter method employing no bias voltage.
In FIG. 16, reference numeral 7 denotes magnets, 10 insulators, 17 a
vacuum chamber.
However, it has been found that the crystal orientation (111) of an
aluminum wiring film formed by the bias sputter method becomes
considerably degraded. It is known that the orientation of crystalline
grains is related to electromigration and stressmigration, and the more
the orientation becomes excellent, the more the tolerance against
electromigration and stressmigration is improved. It has also been found
that the bias sputter method may cause cracks in a barrier layer and an
inferior barrier effect.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a thin film forming
method and apparatus capable of improving the step coverage and quality of
an aluminum wiring film of integrated circuit devices.
It is another object of the present invention to provide a sputtering or
vapor deposition method for practicing the thin film forming method as
above.
It is a further object of the present invention to provide an integrated
circuit device having a wiring film with an improved film quality and step
coverage.
It is still a further object of the present invention to provide an
aluminum wiring film for integrated circuit devices.
According to the film forming method of this invention, a negative voltage
is applied alternately to a target and a substrate to perform film
formation and reverse sputter alternately.
The factors affecting the orientation of crystalline grains are impurity
gasses such as O.sub.2, N.sub.2, H.sub.2 O and the like residual in a
vacuum chamber. Admixture of impurity gasses into sputter particles
produces crystalline nuclei with different crystal orientations and
deposition defects during a growth process, to thereby lower the film
quality.
FIG. 18 diagrammatically shows a film forming mechanism of a conventional
DC magnetron bias sputter method. Since the substrate 8 and the target 6
are always energized by negative voltages as shown in FIGS. 16 and 17,
parts of aluminum and impurities deposited on the substrate are subjected
to reverse sputter. Reverse sputtered impurities are dissociated and
activated in atomic state, which impurities are admixed again with
aluminum sputter particles from the target, thereby further degrading the
film quality. According to the present invention, negative voltages whose
waveforms are schematically shown in FIG. 2 are applied alternately to the
target and the substrate to solve the above problems. FIGS. 4 and 5
illustrate a film forming mechanism of this invention. Aluminum sputter
particles are deposited onto the substrate with a negative voltage being
applied to the target. At this process, residual impurity gasses in the
vacuum chamber are introduced into the inside of the film on the substrate
surface. Next, the voltage applied to the target is turned off and a
negative voltage is applied to the substrate, whereby a fraction of
aluminum and impurities deposited onto the substrate are subjected to
reverse sputter by Ar ion impacts to thereby clean and shape the film. The
above processes are alternately and continuously repeated so that a film
with a good orientation and step coverage is formed. According to the
present invention, a step coverage 0.3 or more is possible. When a film is
formed on a substrate with a recess, the film is substantially parallel to
the underneath surface of the substrate at the bottom and side walls of
the recess and the upper side surrounding the recess and inclined at the
edge of the recess. The principle idea of this invention has been given
above. However, if sputter and bias voltages are alternately turned off to
0 voltage, the film forming discharge range is limited. The reason is as
follows: As illustrated in the apparatus of this invention shown in FIG.
1, magnets 7 are provided on the target for focussing plasma and enabling
sputter discharge at a pressure in the order of 10.sup.-3 Torr. However,
since no magnets are provided on the substrate 8, discharge does not occur
even if a negative voltage is applied to the substrate. Thus, the
substrate is not subjected to reverse sputter. However, if a coil 5 is
mounted between the target 6 and the substrate 8 and a high frequency
current is made to flow therethrough to generate plasma, then it becomes
possible to hold stable discharge and reverse sputter at high vacuum
region. According to the present invention, the pressure of Ar atmosphere
may be lowered to 10.sup.-3 Torr or less. Accordingly, discharge between
the target 6 and the electrically grounded vacuum chamber 17 completely
diminishes, and discharge between the substrate and the grounded vacuum
chamber can be effected if the substrate is energized to some voltage
level. Under certain film forming conditions, the discharge (bias current)
may become insufficient. In such case, according to the present invention,
as seen from the waveforms shown in FIG. 3, after a negative voltage is
switched from the target to the substrate, sputter discharge is caused to
continue by applying a negative base voltage (power) smaller in absolute
magnitude than that during sputter (during film forming), to the substrate
without making it zero. As a result, parts of Ar ions flow into the
substrate which is then subjected to reverse sputter. If the absolute
value of this base voltage is made too high, the amount of impurities
introduced again into the film becomes large so that an optimum absolute
value of the base voltage must be determined. In addition to the above
factors, other factors such as switching period, conduction ratio of bias
to sputter and the like are selected properly to greatly improve the film
characteristic and step coverage.
It is preferable to use a DC bias switching sputter in case of a conductive
metal film, and to use a high frequency bias switching sputter in case of
an insulating film.
According to a sputter method of this invention, it is possible to set the
pressure of Ar gas within a vacuum chamber at 10.sup.-3 Torr or less.
Therefore, admixture of impurities contained in the atmosphere such as
O.sub.2, N.sub.2, H.sub.2 O or the like into the film can be suppressed to
accordingly obtain a good film quality. If the film forming method of this
invention is applied to forming an aluminum wiring film of an integrated
circuit device, it is possible to obtain a film whose peak value of x-ray
diffraction strength in (111) plane is 150 Kcps or more, and whose step
coverage is 0.3 or more. By virtue of these, it becomes possible to make a
wiring breakage due to electromigration difficult to occur.
The effect of enabling to prevent occurrence of electromigration of a
wiring film can be enjoyed not only for pure aluminum wiring films but
also for all Al alloy wiring films such as Al-Cu-Si alloy wiring films,
Al-Pd-Si alloy wiring films, and Al-Si alloy wiring films. Not only pure
aluminum but also Al alloy may be used accordingly as a wiring film of an
integrated circuit device.
If the film forming method of this invention is applied to a barrier layer
of an integrated circuit device, the resultant barrier layer has a layered
structure of granular and columnar crystals or a mixed structure thereof
so that it has an efficient barrier effect and a large specific
resistance.
A material of a barrier layer is preferably TiN and TiW, however the
invention is not limited thereto.
The film forming method of this invention is applicable not only to an
integrated circuit device but also to all the devices where a conductive
film or an insulating film is formed on a substrate.
According to the film forming method of this invention, it is preferable to
ground a vacuum chamber and apply negative pulse voltages alternately to
both a target and a substrate.
It is also preferable to set the peak value of a pulse applied to the
target higher than the peak value of a pulse applied to the substrate.
It is preferable to set the time during which a negative pulse voltage is
applied to the target longer than that during which a negative pulse
voltage is applied to the substrate.
It is further preferable to set the base voltage to the target higher than
that to the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram showing the structure of an embodiment
of a thin film forming apparatus according to the present invention;
FIGS. 2 and 3 show examples of waveforms used in the apparatus shown in
FIG. 1;
FIGS. 4 and 5 illustrate a film forming mechanism according to the present
invention;
FIG. 6 is a characteristic curve illustrating a discharge limit pressure;
FIGS. 7 and 8 are characteristic curves of an x-ray diffraction strength
relative to switching factors;
FIG. 9 is a characteristic line of a breakage time relative to an x-ray
diffraction strength;
FIG. 10 is a characteristic curve of step coverage relative to a bias
ratio;
FIG. 11 shows examples of high frequency waveforms used for sputter and
bias;
FIG. 12 schematically illustrates the structure of a TiN film formed
according to this invention;
FIG. 13 is a schematic block diagram showing the structure of another
embodiment of a thin film forming apparatus according to the present
invention;
FIG. 14 shows examples of waveforms used in the apparatus shown in FIG. 13;
FIG. 15 is a schematic block diagram showing the structure of a still
further embodiment of a thin film forming apparatus according to the
present invention;
FIG. 16 shows the structure of a conventional DC magnetron bias sputter
apparatus;
FIG. 17 shows examples of waveforms used in the apparatus shown in FIG. 16;
FIG. 18 illustrates a film forming mechanism associated with FIGS. 16 and
17;
FIG. 19 is a diagrammatical view of the structure of a TiN film formed by a
conventional DC magnetron bias sputter method;
FIG. 20 is a diagrammatical view of the structure of a TiN film formed by a
conventional DC magnetron sputter method without a bias;
FIG. 21 is a view used for explaining an aspect ratio and step coverage;
FIG. 22 shows characteristic curve and lines of an Ar atmospheric pressure
relative to an x-ray diffraction strength during forming an Al film; and
FIG. 23 is a partial, sectional view of an integrated circuit device which
shows one of applications of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
EXAMPLE 1
Referring to FIGS. 1 to 3, an embodiment of a thin film forming apparatus
according to the present invention will be described. FIG. 3
diagrammatically shows waveforms used in the apparatus. In FIG. 3, PW
denotes a sputter peak power (voltage), BW a sputter base power (voltage),
PV a bias peak voltage, BV a bias base voltage, BW/PW a sputter base power
ratio, T2/(T1+T2) a bias ratio, and (T1+T2) a switching period. Referring
to FIG. 1, the thin film forming apparatus comprises a waveform-controlled
sputter power source 2, a constant-voltage waveform-controlled reverse
sputter power source 1, a bias current detecting sensor 4, a high
frequency coil 5 for generating plasma, a bias current controlling high
frequency power source 3 for controlling a bias current and enabling a
stable discharge at a high vacuum region, a vacuum chamber 17 within which
film is formed, a substrate (e.g., Si substrate) 8, a target 6, insulators
10, and an optional waveform generator 9 made of a CPU and the like for
setting sputter power, bias voltage and current waveforms under program
control.
With the thin film forming apparatus constructed as above, first the
optional waveform generator 9 sets the sputter power waveform, bias
(reverse sputter) waveform and bias current. These signals set by the
optional waveform generator 9 are supplied to the waveform controlled
sputter power source 2, constant-voltage waveform controlled reverse
sputter power source 1 and bias current controlling high frequency power
source 3. These power sources have feedback functions to maintain the
waveforms as set, even under variation of loads or the like. Therefore, a
change in any one set value will not affect the other values. A bias
current control method which is one of the features of this invention will
be described in detail. A bias current with the bias voltage PV set, e.g.,
at 150 V changes if for example a sputter power is changed. To avoid this,
a bias current is detected by the bias current detecting sensor 4 and
compared with a signal set by the optional waveform generator 9. Based on
this comparison, the bias current is maintained at the set value by
controlling a high frequency power supplied to the high frequency coil 5
by means of the bias current controlling high frequency power source 3.
These operations serve to maintain stable discharge at a high vacuum
region.
Next, the explanation will be made of forming a thin film by the thin film
forming apparatus shown in FIG. 1 with reference to FIGS. 6 to 10. Thin
film formation described below was made under the same conditions that a
target was an Al-1 weight % Si alloy, a substrate was a Si wafer, an
attained vacuum pressure was 3.times.10.sup.-7 Torr, and a discharge
pressure was 5.times.10.sup.-4 Torr under an Ar atmosphere. The relation
between a high frequency power (to be supplied to the coil) and an Ar
atmospheric pressure, with respect to continuation of sputter discharge,
is shown in FIG. 6 wherein a sputter power was set constant at 500 Watts
and a high frequency power supplied to the coil was varied in the range of
0 to 200 Watts. The discharge limit pressure at a high frequency power of
0 Watt was 9.times.10.sup.-4 Torr. However, as the high frequency power
increased, the discharge limit pressure lowered. With the high frequency
power at about 100 Watts, the discharge limit pressure lowered little and
maintained substantially constant. With the high frequency power over 100
Watts, the discharge limit pressure remained 8.times.10.sup.-5 Torr. As
seen from the above, discharge according to this invention can be
performed at a vacuum region ten times or more higher than that of a
conventional DC magnetron sputter method, thus improving a film quality.
The high frequency power is preferably in a range of 100 to 200 Watts as
seen from FIG. 6.
FIG. 7 shows the experimental result of studying influence of a sputter
base power ratio on an x-ray diffraction strength at (111) orientation,
with the conditions that a switching period (T1+T2) was 1 second, a bias
peak voltage PV was 150 V, a bias base voltage BV was 50 V, a bias current
was 0.3 A, and a sputter base power ratio (BW/PW) was changed from 0 to 1.
The x-ray diffraction strength of 370 Kcps was obtained at about 0 to 0.3
of the sputter base power ratio. At a larger sputter base power ratio, the
diffraction strength considerably lowered to the extent that the strength
was 130 Kcps at a sputter base power ratio of 1. The reason why the x-ray
diffraction strength is lowered as the sputter base power ratio approaches
1 is that since a sputter power undergoes continuous discharge in the
similar manner as conventional, impurity gases (N.sub.2, O.sub.2, H.sub.2
O) are introduced into the inside of a film. The diffraction strength was
8 Kcps with a conventional method under the same conditions as above. It
should be noted that the diffraction strength according to this invention
was about fifty times as high as a conventional strength. The sputter base
power ratio is preferably 0.8 or less, and more preferably 0.3 or less.
FIG. 8 shows the experimental result of studying influence of a switching
period upon a diffraction strength, with the conditions that a sputter
base power ratio was 0.3 which could obtain the maximum diffraction
strength as shown in FIG. 7, and a switching period (T1+T2) was changed
from 0.1 to 100 seconds. The x-ray diffraction strength was maintained
substantially constant within the range of 0.1 to 10 seconds of the
switching period, and considerably lowered at the period of 100 seconds.
As appreciated, the switching period also becomes one of the important
film formation factors so that it is necessary to change it in accordance
with material and application field.
FIG. 9 shows the experimental result of studying influence of an x-ray
diffraction strength at (111) orientation plane upon a wiring breakage due
to electromigration. An Al-1 weight % Si alloy with the thickness of 0.5
microns was formed on a Si substrate and thereafter, a 2 mm long, 0.8
microns wide stripe pattern was formed thereon, and annealed at
450.degree. C. for 60 minutes. 1 .mu.m thick passivation SiO.sub.2 film
was made thereon by a CVD method. A time was measured till the wiring was
broken at a current density of 2.times.10.sup.6 A/cm.sup.2 and under a
temperature of 150.degree. C. As seen from FIG. 9, it can be understood
that as the x-ray diffraction strength increased to the range of 8 to 370
Kcps, the breakage time considerably increased to the range of 40 to 510
hours, thus presenting a certain correlation between the x-ray diffraction
strength (orientation) and the electromigration resistance. The
diffraction strength of a film formed by means of a conventional DC
magnetron sputter method was 8 Kcps as shown in FIG. 9. The wiring
breakage time of this film was 40 hours, whereas the film formed by this
invention with a maximum diffraction strength of 370 Kcps exhibits a
breakage time of 510 hours. Accordingly, the migration resistance of this
invention was improved about 14 times as conventional.
FIG. 10 shows the result of studying influence of a bias ratio (T2/(T1+T2))
upon step coverage. Under the conditions that a switching period (T1+T2)
was 1 second, a sputter base power ratio (BW/PW) was 0.3, and a bias peak
voltage (PV) was 150 V, a pattern having a through hole diameter of 0.8
micron and an aspect ratio of 1 was formed on an Si substrate, and a film
of Al-1 weight % Si alloy was formed thereon to study the step coverage
state by using a scanning microwave spectrometer. The step coverage
presented a maximum value 60% at a bias ratio 0.3, and it lowered at a
ratio over and under 0.3. As seen from the film configuration at the
through hole section shown in FIG. 10, the film configuration could vary
with the bias ratio. For instance, it was possible to further improve the
step coverage by setting the bias ratio large at the start of film
formation and lowering it with time. The step coverage of a film formed by
a conventional DC magnetron bias sputter method under the same bias
voltage 150 V as of the present invention was 22%. The film configuration
can be varied with the bias ratio according to the present invention.
However, the conventional method fixes the bias voltage and no other
factors remain so that the step coverage cannot be improved. With the
conventional method, if the bias voltage is increased, the kinetic energy
of ions increases so that the step coverage can be improved more or less.
However, in this case, ion impact may damage the substrate and the film
quality is degraded so much.
FIG. 22 shows the comparison results of conventional various methods with
the method of this invention with respect to influence of an Ar
atmospheric pressure upon the x-ray diffraction strength at (111)
orientation of an Al film formed on an Si substrate, wherein the apparatus
shown in FIG. 1 was used for film formation of this invention.
Film formation of this invention was made under the conditions that a
switching period of sputtering relative to reverse sputtering was 1
second, a bias peak voltage was 150 V, a bias ratio was 0.3, and a sputter
base power ratio was 0.3.
For those conventional methods shown in FIG. 22, the sputter method is such
a method that does not undergo the reverse sputter, the bias sputter
method is such a method that sputtering is carried out while a negative
voltage is applied to a substrate, and the waveform controlling AC sputter
method, as described in Japanese Patent Application No. 61-169590
(unexamined publication JP-A-63-26361), is such a method that sputtering
is carried out using an AC voltage whose waveform is being controlled.
With the method of this invention, film formation can be carried out at a
pressure lower than 10.sup.-3 Torr so that the x-ray diffraction strength
can be made considerably higher than conventional.
In contrast, the conventional methods can not be used at the Ar atmospheric
pressure lower than 10.sup.-3 Torr and the x-ray diffraction strength is
limited to 100 Xcps at most. The method of this invention has first
succeeded in making the x-ray diffraction strength of an Al film at (111)
plane higher than 150 Xcps.
EXAMPLE 2
The DC sputter of Example 1 is effective for conductive targets, but a
target of insulating material cannot be discharged. In Example 2, the
waveform controlled sputter power source 2 and the constant-voltage
waveform controlled reverse sputter power source 1 shown in FIG. 1 were
replaced with high frequency power sources at about 13.5 MHz having
waveforms as shown in the schematic diagram of FIG. 11, thereby allowing
discharge of a target of insulating material. An Al film was formed and
patterned in the manner described in Example 1, and thereafter an
SiO.sub.2 film as an inter-layer insulating film of a multi-layered wiring
was formed thereon. Contact between step portions was excellent, and the
surface unevenness was considerably improved.
EXAMPLE 3
Contact resistance increases largely for contact holes smaller than 1
micron, because the diameter of Si grains crystallized from an Al wiring
film may sometimes exceed 1 micron. One of the methods for preventing this
is to use a barrier metal. Various types of barrier metal are known. TiN
is most prominent to prevent reaction between Al and Si. However, a TiN
barrier film formed under an (Ar+N.sub.2 +O.sub.2) atmosphere with a Ti
target is grown to have columnar crystals as shown by a film cross section
of FIG. 20. According to the characteristics of this film, it has less
residual stress, no defect such as cracks, and excellent barrier effect.
However, it has a disadvantage of large specific resistance in the order
of 1000 to 2000 .mu..OMEGA..cm.
On the other hand, a film formed by applying a negative bias (reverse
sputter) to a substrate has fine and granular crystals as shown in FIG.
19. As compared with the above specific resistance, this film has a very
low resistance in the order of 50 to 200 .mu..OMEGA..cm. However, it has
large residual stress and is likely to generate cracks, and also has poor
barrier effect.
Material having a small specific resistance and presenting excellent
barrier effect is suitable for the barrie | | |
