Plasma process method and apparatus

Plasma processing gas is introduced into an upper portion of a processing vessel and a film-formation gas is simultaneously introduced into the vicinity of a substrate to be processed. The plasma processing gas is ionized to form a first plasma and any of the plasma processing gas that has temporarily recombined in locations close to the substrate to be processed is re-ionized as a second plasma. As a result, the density of etchant ions used for cutting away overhangs around the openings of grooves can be increased. In other words, the number of etchant ions can be increased. This makes it possible to reduce the bias voltage applied to the substrate to be processed, preventing damage thereto.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
The present invention relates to a plasma processing method and apparatus 
for subjecting a substrate to be processed to a predetermined plasma 
process in a plasma atmosphere. More specifically, it relates to a plasma 
processing method and apparatus for forming a predetermined thin film, 
such as an insulating film, on a substrate to be processed in a plasma. 
2. Related Technical Information 
During the process of fabricating a semiconductor device, processing called 
plasma chemical vapor deposition (CVD) is performed to form a film such as 
a predetermined thin conductive film or an insulating film such as one of 
silicon dioxide (SiO.sub.2) on the surface of a substrate to be processed 
such as a semiconductor wafer (hereinafter called "wafer") W, in a plasma 
atmosphere. 
Various different types of plasma CVD apparatus are used in the art for 
this plasma CVD processing. However, the degree of integration of 
circuitry has increased so far nowadays that extremely precise processing 
is necessary, and thus apparatuses that can generate high-density plasmas 
are used, such as inductively coupled plasma (ICP) CVD apparatuses, 
microwave plasma CVD apparatuses, and electron cyclotron resonance (ECR) 
plasma CVD apparatuses, whereby the film is formed under the downflow of a 
high-density plasma. 
If one of these conventional apparatuses is used to embed very fine, 
high-aspect-ratio grooves such as those of a width of 0.2 .mu.m and a 
depth of 1 .mu.m in a film of silicon dioxide (SiO.sub.2), the film tends 
to extend laterally over the top of the grooves and obstruct the opening 
portions of the grooves (this is called "overhang"). As a result, there is 
a danger of voids being generated within the grooves. 
For that reason, the conventional process of embedding the groove includes 
the application of a high-frequency bias voltage of approximately 100 kHz 
to the wafer to cause the overhanging portions to be cut away by etching 
with ions such as argon ions (this process is called "shoulder cutting"). 
However, in a conventional apparatus in which a downflow method is used, 
the long distance between the generated plasma and the wafer causes 
problems such as the argon ions recombining before they reach the wafer, 
so that there is not a sufficiently high density of etchant ions such as 
argon ions in the vicinity of the wafer. In other words, the absolute 
number of these etchant ions is small. 
Thus, in order to achieve the desired cutting effect with such a small 
absolute number of etchant ions, there is no option but to increase the 
bias voltage so that it is now necessary in the art to use a bias voltage 
of as high as 1.5 kV. But this leads to damage to the wafer from ion 
impacts. 
SUMMARY OF THE INVENTION 
An objective of the present invention is to provide a plasma processing 
method and apparatus together with a plasma film-formation method and 
apparatus that can be used to form a predetermined film that does not have 
any defects such as voids over tiny, high-aspect-ratio grooves and holes, 
without applying an excessively large bias voltage to the substrate to be 
processed (such as a wafer), which would cause damage. 
A plasma processing method in accordance with this invention subjects a 
substrate to be processed to a predetermined plasma process in a plasma 
atmosphere and comprises the steps of: 
introducing a plasma processing gas into a processing vessel and ionizing 
the plasma processing gas to form a first plasma within the processing 
vessel; and 
re-ionizing the plasma processing gas that has recombined in locations 
close to the substrate to be processed, to form a second plasma. 
A plasma film-formation method in accordance with this invention causes a 
plasma to be generated within a processing vessel and also introduces a 
processing gas into the processing vessel and forms a film on a substrate 
to be processed to which is applied a bias voltage, and comprises the 
steps of: 
introducing a plasma processing gas into an upper portion of the processing 
vessel and also introducing a film-formation processing gas into the 
vicinity of a substrate to be processed; 
ionizing the plasma processing gas to cause a first plasma to be generated 
within the processing vessel; and 
re-ionizing the plasma processing gas that has recombined in locations 
close to the substrate to be processed, to form a second plasma. 
Note that the purpose of the plasma processing gas of this invention is to 
create etchant ions such as argon ions by disassociating them in the 
plasma, to perform the previously described "shoulder cutting". 
In addition, the first plasma generation means could be any of the various 
currently available plasma generating mechanisms such as an inductively 
coupled plasma (ICP) generation mechanism, a mechanism that generates a 
plasma by microwaves, or a mechanism that causes the generation of a 
plasma by ECR. Alternatively, it could be a mechanism that causes the 
generation of a plasma between opposing electrodes by the supply of 
radio-frequency electrical power, or a mechanism that generates a plasma 
by helicon waves. 
A suitable second plasma generation means would be a mechanism for 
supplying radio-frequency electrical power to an antenna such as a looped 
antenna, to generate a plasma. In such a case, a configuration in which 
the radio-frequency electrical power is supplied to a single-turn loop 
antenna would greatly simplify the overall structure of the apparatus. 
In accordance with the plasma film-formation method of this invention, a 
plasma processing gas is introduced into an upper part of the interior of 
a processing vessel and a film-formation processing gas is simultaneously 
introduced into the vicinity of the substrate to be processed. The plasma 
processing gas is ionized to form a first plasma and any of the plasma 
processing gas that has temporarily recombined in locations close to the 
substrate to be processed is re-ionized by a second plasma. As a result, 
the density of etchant ions used for cutting away overhangs around the 
openings of grooves can be increased. In other words, the number of 
etchant ions can be increased. This makes it possible to reduce the bias 
voltage applied to the substrate to be processed, preventing damage 
thereto. 
Since the film-formation processing gas is introduced into the vicinity of 
the substrate to be processed, a desired film can be formed by halting the 
generation of the second plasma or reducing the power applied to the 
second plasma to such an extent that the film-formation processing gas 
does not disassociate. 
The actual ionization voltage varies with the type of processing gas, but 
it is necessary to control the energy of the second plasma source to 
generate certain ions at a predetermined density. Therefore, an optimal 
degree of cutting of the overhang portions can be obtained if the 
configuration is such that the ion density of the second plasma is 
controlled. This could be done in such a manner that the recombined plasma 
processing gas is left alone for a fixed interval then re-ionized (by, for 
example, repeatedly generating and halting the plasma at fixed intervals), 
or by varying the output of the radio-frequency power that causes the 
generation of the plasma during the re-ionization of the recombined plasma 
processing gas. 
A plasma film-formation apparatus in accordance with this invention enables 
time modulation of the radio-frequency electrical power that causes the 
generation of the plasma. Thus it can cause the plasma to be generated for 
a constant period then halt the generation, so that the ion density within 
the second plasma can be controlled. This makes it possible to implement 
the plasma film-formation method of embodiments of this invention as 
required. In other words, the ion density within the plasma is related to 
the time during which the maximum voltage is discharged at the instant of 
the ionization. Therefore, if some mechanism for time-modulating the 
radio-frequency electrical power is provided, as in the plasma 
film-formation apparatus described as embodiments of this invention, the 
ion density of the plasma can be controlled both simply and efficiently. 
Such a configuration enables "shoulder cutting" with etchant ions while 
the second plasma is being formed, alternating with the film-formation 
process while the second plasma is halted. 
The second plasma generation means in the plasma film-formation apparatus 
in accordance with this invention is configured to receive radio-frequency 
electrical power and thereby generate a plasma. Since it is also provided 
with an output-varying mechanism for this radio-frequency electrical 
power, the radio-frequency output can be varied to enable control of the 
ion density in the second plasma.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
A first embodiment of the present invention, applied to a plasma CVD 
apparatus, is described below with reference to the accompanying drawings. 
A cross-sectional view of an ICP plasma CVD apparatus 1 in accordance with 
a first embodiment of this invention is shown schematically in FIG. 1. A 
processing vessel 2 of this plasma CVD apparatus 1 is configured in 
substantially circular cylindrical form of a material such as aluminum 
with a surface that has been subjected to processing to form a material 
such as alumite oxide. The interior of this processing vessel 2 can be 
freely sealed hermetically and it is grounded. Note that this processing 
vessel 2 could also be formed of titanium. 
A mounting stand 4 for holding a 12-inch semiconductor wafer (hereinafter 
called "wafer") W, which is the substrate to be processed, is accommodated 
within the processing vessel 2 on an insulating support member 3 formed of 
a material such as a ceramic or quartz. This mounting stand 4 is of a 
material such as aluminum with a surface that has been subjected to 
processing to form a material such as alumite oxide, and cooling passages 
5 are formed in the interior thereof. Thus coolant such as water that is 
supplied from the outside can be passed through these cooling passages 5 
to cool the wafer W to a predetermined temperature. 
Note that the provision of a heating means such as a ceramic heater, a 
suitable temperature sensor, and a temperature control mechanism would 
make it possible to maintain the wafer W both accurately and automatically 
at that predetermined temperature. 
An electrostatic chuck 6 is provided on the mounting stand 4 to attract and 
hold the wafer W. This electrostatic chuck 6 could have a configuration 
consisting of a thin conductive film that is sandwiched from above and 
below by a polyimide resin. When a predetermined DC voltage is applied 
thereto from a high-voltage DC power source (not shown) which is located 
outside the processing vessel 2, the Coulomb force generated thereby 
attracts and holds the wafer W. 
Note that, if an annular focussing ring of a conductive material such as 
monocrystalline silicon is provided around the periphery above the 
mounting stand 4 so as to surround the electrostatic chuck 6, the plasma 
density around the wafer W can be adjusted to ensure that the plasma 
density over the wafer W can be made more uniform. 
A high-frequency bias voltage of, for example, 100 kHz is applied to the 
mounting stand 4 through a regulator 11 from a high-frequency power source 
12 for applying a bias voltage. 
An exhaust port 13 is formed in a lower portion of the side wall of the 
processing vessel 2 and an exhaust pipe 15 is connected to this exhaust 
port 13 via an evacuation means 14 such as a vacuum pump or a 
turbo-molecular pump. The operation of the evacuation means 14 enables the 
interior of the processing vessel 2 to be evacuated to a predetermined low 
pressure such as any desired pressure between 10 mTorr and 100 mTorr. In 
such a case, the configuration could be such that a pressure sensor is 
provided in the processing vessel 2 and the predetermined low pressure is 
maintained on the basis of a detection signal from this pressure sensor. 
The exhaust port 13 could also be formed in the base of the processing 
vessel 2. 
A plate 21 of an insulating material such as quartz is provided sealed into 
an upper portion of the processing vessel 2, in other words, a portion 
equivalent to the ceiling thereof. An antenna 22 in the form of a 
single-turn loop is disposed above this quartz plate 21 as means for 
generating a first plasma. The configuration is such that radio-frequency 
electrical power of a strength sufficient to cause the generation of an 
inductively coupled plasma, such as 13.56 MHz, is supplied to this antenna 
22 from a radio-frequency power source 24 via a matching box 23. 
An annular gas distribution member 25 is provided below the quartz plate 21 
within the processing vessel 2 as a member that configures a first gas 
introduction means for introducing a plasma processing gas. A large number 
of distribution vents are formed in this gas distribution member 25 with 
the configuration being such that, when a predetermined processing gas 
(such as oxygen and argon) is supplied from a first gas introduction pipe 
26, that processing gas is dispersed from these distribution vents and is 
introduced into the upper portion of the processing vessel 2. This gas 
distribution member 25 is made of a material such as quartz. 
Similarly, an annular gas distribution member 27 is disposed close above 
the mounting stand 4 as a second gas introduction means for introducing a 
film-formation processing gas. The configuration is such that, when a 
predetermined film-formation processing gas (such as SiH.sub.2 Cl.sub.2) 
is introduced from the second gas introduction pipe 28 into this gas 
distribution member 27, the film-formation processing gas is distributed 
close above the wafer W from a large number of distribution vents formed 
in the gas distribution member 27. This configuration ensures that 
precursor gases SiH.sub.2 Cl.sub.2 O, SiH.sub.2 Cl.sub.2 O.sub.2, 
SiH.sub.2 Cl.sub.2 O.sub.3, and SiH.sub.2 Cl.sub.2 O.sub.4 are created 
from this SiH.sub.2 Cl.sub.2 introduced into the processing vessel 2. 
It should be noted that this embodiment of the present invention concerns a 
configuration in which the second gas introduction pipe 28 is provided in 
a side wall of the processing vessel 2, as the second gas introduction 
means for introducing the film-formation processing gas, and the gas 
distribution member 27 that is connected to the second gas introduction 
pipe 28 is disposed close above the wafer W. However, the configuration 
could equally well be one in which the film-formation processing gas is 
introduced below the periphery of the wafer W, that is, from below the 
periphery of the mounting stand 4. In such a case, it would not be 
necessary to dispose the gas distribution member above the wafer W, which 
means that sputtering of the materials by the plasma is reduced, thus 
reducing the amount of contamination within the processing vessel 2. 
An antenna 31 in the form of a single-turn loop is disposed close above the 
upper surface of the gas distribution member 27 as means for generating a 
second plasma. The configuration is such that radio-frequency electrical 
power of a strength sufficient to cause the generation of an inductively 
coupled plasma, such as 13.56 MHz, is supplied to this antenna 31 from a 
radio-frequency power source 33 via a matching box 32. 
The radio-frequency electrical power from the radio-frequency power source 
33 is modulated by a time modulation device 34 in such a manner that the 
power is output for 5 microseconds then halted for a further 5 
microseconds, as shown in (b) of FIG. 4. This time modulation can be 
varied so that the output time and output-halted time are controlled. Thus 
it can be set so that the power is output for 8 microseconds then halted 
for 4 microseconds, for example. 
The second plasma generation means could equally well be configured in such 
a manner that it is provided with a mechanism for varying the output of 
the radio-frequency electrical power, with effects as shown in (c) of FIG. 
4. 
The ion density within the second plasma can be controlled by leaving the 
recombined plasma processing gas for a fixed interval then re-ionizing it 
(by, for example, repeatedly generating and halting the plasma at fixed 
intervals). 
The main components of the plasma CVD apparatus 1 of this first embodiment 
are configured as described above. The description now turns to the 
processing performed when a film of an oxide (e.g., SiO.sub.2) is to be 
formed on a wafer W of silicon, to embed fine pattern lines within this 
oxide film. First of all, after the wafer W has been attracted to and held 
by the electrostatic chuck 6, the pressure within the processing vessel 2 
is reduced by the evacuation means 14. When the predetermined pressure has 
been achieved, gases such as oxygen and argon are introduced into the top 
of the processing vessel 2 from the first gas introduction pipe 26, a gas 
such as SiH.sub.2 Cl.sub.2 is introduced to close above the wafer W from 
the second gas introduction pipe 28, and the pressure within the 
processing vessel 2 is set and maintained at a suitable level such as 20 
mTorr. 
When radio-frequency electrical power at 13.56 MHz and 1.2 kW is supplied 
from the radio-frequency power source 24 to the antenna 22, a first plasma 
P1 is created in the upper portion of the processing vessel 2, as shown in 
FIG. 2, so that the introduced oxygen and argon gases are ionized to 
produce oxygen and argon ions. 
At the same time, radio-frequency electrical power at 13.56 MHz and 400 W 
is supplied from the radio-frequency power source 33 to the antenna 31, to 
form a second plasma P2 close above the wafer W, as shown in FIG. 2. This 
second plasma P2 makes it possible to increase the number of argon ions, 
even if they have recombined and their number has decreased. 
A high-frequency bias voltage, of, for example, -500 V at 100 kHz, is also 
applied to the mounting stand 4 from the high-frequency power source 12 to 
ensure that the argon ions are incident on the wafer W. This makes it 
possible for the argon ions to etch the overhang portions covering the 
openings of the grooves, removing them. 
When the supply of radio-frequency electrical power from the 
radio-frequency power source 33 is halted, the deposition seeds amongst 
the precursors are deposited on the surface of the wafer W, so that an 
oxide film (SiO.sub.2) is formed on the surface of the wafer W. Repeating 
this film-formation process and the process of etching the overhang 
portions with the argon ions ensures that the grooves in the wafer W can 
be buried efficiently without defects such as voids forming in the 
grooves. 
During this film-formation process, the first plasma P1 that generates the 
argon ions is induced in the upper part of the processing vessel 2. A 
number of these argon ions recombine before they reach the wafer W and, as 
a result, the absolute number of the argon ions decreases. However, since 
the second plasma P2 is induced close to the wafer W, the argon gas is 
re-ionized by this second plasma P2, so that a sufficiently large number 
appear for etching the overhang portions of the groove openings. Thus the 
overhang portions can be efficiently and sufficiently etched during the 
embedding process, with no likelihood of defects such as voids forming 
within the grooves, even with a bias voltage of -500 V, which is less than 
half that in a conventional inductively coupled plasma CVD apparatus. 
Since the argon ions are made incident on the wafer W by such a low bias 
voltage, there is no danger of the wafer W becoming damaged by impacts. 
Since the radio-frequency electrical power that induces the second plasma 
P2 can be modulated in a temporal fashion by the time modulation device 
34, the times during which the argon is ionized can be controlled so that 
the number of argon ions can be controlled. Thus the number of etchant 
ions can be controlled efficiently in correspondence with factors such as 
the dimensions of the grooves and the type of processing gas introduced 
during the film-formation process. These processes of forming a film and 
etching away overhang portions with argon ions can be alternated as 
appropriate. 
It should be noted that the oxygen and argon gases that form the plasma 
processing gas of this embodiment are introduced into the processing 
vessel 2 by the first gas introduction pipe 26 provided in the side wall 
of the processing vessel 2 and the gas distribution member 25 provided in 
the top of the processing vessel 2. However, a roughly flat showerhead 
having a large number of distribution vents could equally well be provided 
on the lower surface of the quartz plate 21 instead. 
To induce the first plasma P1 with this embodiment, an inductively coupled 
method is used in which radio-frequency electrical power is supplied to 
the antenna 22 provided above the quartz plate 21. Alternatively, the 
configuration could be that of a plasma CVD apparatus 50 relating to a 
second embodiment of this invention as shown in FIG. 3, wherein an upper 
electrode 52 is provided facing a mounting stand 4 in an upper portion of 
a processing vessel 51, radio-frequency electrical power is supplied to 
the upper electrode 52 which is insulated from the grounded processing 
vessel 51, and the first plasma P1 is induced by glow discharge. Note that 
components in FIG. 3 denoted by the same reference numbers as those in 
FIG. 1 have the same configuration. 
The plasma CVD apparatus 50 relating to this second embodiment uses a 
horizontal, flat-plate type of configuration which makes it possible to 
have a comparatively simple apparatus structure and which is suitable for 
large-diameter wafers. In the same manner as the plasma CVD apparatus 1 of 
the first embodiment, the second plasma P2 in the plasma CVD apparatus 50 
relating to this second embodiment can naturally generate a sufficiently 
large number of ions for etching the overhang portions, so that 
film-formation processing can be implemented without any voids forming in 
the grooves, even with a low bias voltage. Note that the upper electrode 
52 in the plasma CVD apparatus 50 of this second embodiment could have a 
showerhead configuration, as described previously, so that the plasma 
processing gas can be introduced into the processing vessel 51 through the 
upper electrode 52. 
An electron cyclotron resonance (ECR) plasma film-formation apparatus 60 
relating to a third embodiment of this invention is shown in FIG. 5. The 
interior of a vacuum vessel 2 is divided into a plasma chamber 65 
positioned at the top and a reaction chamber 66 communicating therewith at 
the bottom. A waveguide 62 is provided outside a transparent window 21 and 
this waveguide 62 is connected to a radio-frequency source 61 as means for 
supplying radio-frequency power for generating a plasma at, for example, 
2.45 GHz. This configuration enables radio-frequency power generated by 
the radio-frequency source 61 to be guided by the waveguide 62 through the 
transparent window 21 and into the plasma chamber 65. 
Means such as an annular electromagnetic coil 64 is disposed close around 
the periphery of the inner wall that divides the plasma chamber 65, as 
means for applying a magnetic field. This forms a magnetic field B of, for 
example, 875 Gauss directed from the top of the interior of the plasma 
chamber 65 downward, to provide suitable conditions for an ECR plasma. 
Thus radio-frequency power of 2.45 GHz from the radio-frequency source 61 
for plasma generation is transferred through the waveguide 62 to the 
ceiling portion of the vacuum vessel 2 and passes through the transparent 
window 21 there, so that microwaves 63 are introduced into the plasma 
chamber 65. The magnetic field B is applied by the electromagnetic coil 64 
to the interior of the plasma chamber 65 from the top to the bottom 
thereof, and the mutual reactions between the magnetic field B and the 
microwaves 63 induces a field that is E (electric field) times B (magnetic 
field) to create electron cyclotron resonance. This resonance ionizes the 
argon and oxygen ions and increases their density. 
The high-density plasma generated in the plasma chamber 65 is sucked 
towards the mounting stand 4 to which is applied a negative high-density 
bias voltage, so that it flows downward into the reaction chamber 66. 
In this embodiment too, the second plasma P2 is formed close above the 
wafer W when radio-frequency electrical power of a frequency of 13.56 MHz 
and, for example, 400 W is supplied from the radio-frequency power source 
33. Therefore, the number of argon ions can be increased again by the 
second plasma P2, even if they have been depleted by recombination, and 
the overhang portions can be etched and removed by these argon ions. This 
makes it possible to implement the formation of a film with no voids in 
the grooves and with no damage, even with a small bias voltage. 
It should be noted that, although the above embodiments were described with 
reference to a wafer as the substrate to be processed, the present 
invention is not limited thereto and it can equally well be applied to the 
formation of a film on other substrates such as an LCD substrate.