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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a plasma etching apparatus for subjecting
a substrate to be processed, such as a semiconductor wafer, to an etching
process.
2. Description of the Related Art
A plasma etching apparatus is used to pattern an electrically conductive
film for wiring, which is formed on a substrate such as a semiconductor
wafer, an LCD substrate, etc. The plasma etching apparatus has a vacuum
processing chamber (process chamber) for storing the substrate to be
processed and defining a processing space. A pair of opposed upper and
lower electrodes are provided within the process chamber, and a substrate
to be processed, such as a semiconductor wafer, is placed on the lower
electrode functioning as susceptor. A process gas (etching gas) is
introduced into the process chamber and a high-frequency power is applied
across the upper and lower electrodes. Then, the process gas is made into
a plasma. Reactive ions in the plasma are pulled by a self-bias potential
of the wafer, and an electrically conductive film formed on the wafer is
etched and patterned.
A focus ring (electric field compensating ring) is provided to surround the
wafer on the lower electrode, thereby to effectively direct the reactive
ions onto the wafer. It is necessary that the focus ring have
anti-corrosion properties (anti-chemical properties with high resistance
to etching gas), anti-plasma properties, heat resistance and electrical
conductivity. From this standpoint, a ring formed integrally of amorphous
carbon is generally used as a focus ring.
In the plasma etching apparatus using the above focus ring, however, an
etching rate and in-plane uniformity of etching anisotropy may deteriorate
in some cases, depending on process conditions. Specifically, the etching
rate is high at the peripheral portion of the wafer and low at the central
portion thereof. In particular, this tendency is conspicuous when the
temperature of the major surface of the wafer is set at high value, and
the etching rate at the peripheral portion of the wafer is very high. In
addition, under the circumstances, the etching anisotropy at the
peripheral portion of the wafer deteriorates and side etching occurs in
the patterning of the electrically conductive film. As a result, the width
of the formed wiring becomes less than a set value.
The main factor of in-plane uniformity of the etching rate and etching
anisotropy is considered to be the influence of a gas stream caused by
exhaust in the process chamber during etching. The gas stream is led
downward from above the lower electrode through a region surrounding the
lower electrode. Thus, a great deal of fresh process gas is led to the
peripheral portion of the wafer, whereas the gas stream stagnates at the
central portion thereof. A less quantity of fresh process gas reaches the
central portion of the wafer. As a result, an etching mechanism differs
between the peripheral portion and central portion of the wafer, and the
etching rate and etching anisotropy become non-uniform.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a plasma etching apparatus
capable of obtaining a uniform etching rate and a uniform anisotropy over
the entire surface of a substrate to be processed.
According to a first aspect of the invention, there is provided an
apparatus for etching an etching target on a major surface of a substrate
by using a plasma, comprising:
a process chamber defining a vacuum process space for containing and
processing the substrate;
a supply system for introducing an etching gas to be made into a plasma
into the process chamber;
an exhaust system for exhausting the process chamber;
a pair of counter electrodes provided within the process chamber and
opposed to each other;
a support member, provided within the process chamber, for supporting the
substrate such that the major surface is exposed within the process space;
a power supply for applying a voltage between the counter electrodes so as
to generate an electric field for making the etching gas into a plasma;
and
a source member having a surrounding surface exposed to the process space
and surrounding the major surface of the substrate, the source member
being formed of a material containing a component which is a main
component of the etching target and produces such a reaction product as to
be substantially adsorbed on the etching target by contact with the
etching gas, so that the reaction product generated from the surrounding
surface diffuses to the major surface of the substrate while the plasma is
being generated, thereby correcting a distribution of the amount of the
reaction product on the major surface.
According to a second aspect of the invention, there is provided an
apparatus for etching an etching target on a major surface of a substrate
by using a plasma, comprising:
a process chamber defining a vacuum process space for containing and
processing the substrate;
a supply system for introducing an etching gas to be made into a plasma
into the process chamber;
an exhaust system for exhausting the process chamber;
a pair of counter electrodes provided within the process chamber and
opposed to each other;
a support member, provided within the process chamber, for supporting the
substrate such that the major surface is exposed within the process space;
a power supply for applying a voltage between the counter electrodes so as
to generate an electric field for making the etching gas into a plasma;
and
a focus ring having a surface surrounding the major surface of the
substrate, the surface of the focus ring comprising first and second
surrounding surfaces exposed to the process space and located inside and
outside, respectively, the first surrounding surface being formed of an
electrically conductive material which produces no such a reaction product
as to be substantially adsorbed on the etching target by contact with the
etching gas, the second surrounding surface being formed of a material
containing a component which is a main component of the etching target and
produces such a reaction product as to be substantially adsorbed on the
etching target by contact with the etching gas, so that the reaction
product generated from the second surrounding surface diffuses to the
major surface of the substrate while the plasma is being generated,
thereby correcting a distribution of the amount of the reaction product on
the major surface.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view schematically showing a plasma etching
apparatus according to a first embodiment of the present invention;
FIG. 2 is a plan view showing the relationship between a wafer and a focus
ring of the apparatus shown in FIG. 1;
FIG. 3 is a cross-sectional view schematically showing an etching system
including the apparatus shown in FIG. 1;
FIG. 4 is a partly enlarged cross-sectional view of the susceptor and focus
ring of the apparatus shown in FIG. 1;
FIG. 5 is a graph showing experimental results relating to etching
processes performed by using various focus ring samples;
FIG. 6 is a partly enlarged cross-sectional view showing a modification of
the focus ring;
FIG. 7 is a partly enlarged cross-sectional view showing another
modification of the focus ring;
FIG. 8 is a cross-sectional view schematically showing a plasma etching
apparatus according to a second embodiment of the present invention;
FIGS. 9A to 9C show etching configurations obtained by a conventional
process and the process relating to the second embodiment;
FIG. 10 shows the in-plane uniformity of the etching rate in the
conventional process and the process relating to the second embodiment;
FIG. 11 is a graph showing the etching rate and etching selection ratio in
the process relating to the second embodiment;
FIGS. 12A and 12B illustrate an undercut amount and a measuring method of a
CD loss;
FIG. 13 is a graph showing the relationship among a variation in N.sub.2
ratio, an undercut amount and a CD loss in the process of the second
embodiment; and
FIG. 14 is a graph showing the relationship between a variation in N.sub.2
ratio and the in-plane uniformity of an etching rate in the process of the
second embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a cross-sectional view schematically showing a plasma etching
apparatus according to the first embodiment of the present invention.
The plasma etching apparatus 10 includes an electrically conductive,
airtight vacuum processing chamber (process chamber) 12 for storing a
wafer S or a substrate to be processed and defining a processing space.
The wall of the process chamber 12 is formed of an electrically conductive
material such as an aluminum material having an alumite surface. A
susceptor 14 for supporting the wafer S, which is formed of an
electrically conductive material such as an aluminum material having an
alumite surface, is disposed at a center region of the process chamber 12.
The susceptor 14 has a circular, flat shape and includes a peripheral
flange portion 16 and a columnar table 18 projecting upwards at the
central area of the susceptor 14. The upper surface of the table 18 is
flat. An electrostatic chuck 22 for attracting and holding the wafer S by
Coulomb force is provided on the upper surface of the table 18. The
electrostatic chuck 22 has a structure wherein an electrically conductive
layer formed of, e.g. an electrolytic copper foil is sandwiched by upper
and lower insulating layers of a polyimide film. The electrically
conductive layer is connected to a DC power supply 24 provided outside the
process chamber 12, and a DC voltage of, e.g. 2.0 kV is applied to the
electrically conductive layer. As is shown in FIG. 2, a focus ring 102
surrounding the wafer S substantially concentrically in a complementary
manner is placed on the flange portion 16 of the susceptor 14. The focus
ring 102 will be described later in greater detail.
A heat exchange source 26 for setting the temperature of the wafer S is
provided within the susceptor 14. The heat exchange source 26 is connected
to a controller 28 provided outside the process chamber 12. The controller
28 controls the temperature of the wafer by means of the heat exchange
source 26. As will be described later, the heat exchange source 26 may be
constituted by using a combination of a cooler for supplying a refrigerant
such as liquid nitrogen to a space within the susceptor 14 and a heater
such as a ceramic heater.
An inert gas such as helium can be selectively supplied from a gas source
32 provided outside the process chamber 12 into a gap between the bottom
surface of the wafer S and the upper surface of the electrostatic chuck
22. The inert gas functions as heat transfer medium and contributes to
heat transmission between the susceptor 14 and wafer S during etching
performed within a vacuum atmosphere.
A gas supply head 34 formed of an electrically conductive material such as
amorphous carbon, SiC or an aluminum material having an alumite surface is
provided above the susceptor 14 within the process chamber 12. The head 34
includes a space 36 for temporarily storing a process gas such as an
etching gas. The space 36 is made to communicate with gas sources via a
supply pipe 38. A lower part of the head 34 is formed as a diffusion plate
42 with many diffusion holes 44. The process gas within the space 36 is
supplied uniformly to the wafer S via the diffusion holes 44.
The diffusion plate 42 of the head 34 and the susceptor 14 function as
parallel, flat plate-type upper and lower electrodes. The susceptor 14 or
the lower electrode is connected to a high frequency power supply 46 via a
capacitor and a matching circuit. The diffusion plate 42 or the upper
electrode is grounded. At the time of etching, the distance between the
upper and lower electrodes 42 and 14 is set at about 15 to 20 mm. A
high-frequency voltage of, e.g. 13.56 MHz is applied from the power supply
46 between the upper and lower electrodes 42 and 14. Thereby, an electric
field is generated between the upper and lower electrodes 42 and 14. 10
FIG. 3 is a vertical cross-sectional side view schematically showing an
etching system 50 including the plasma etching apparatus 10 as shown in
FIG. 1.
The etching system 50 includes a transfer chamber 52, the pressure of the
atmosphere within which can be set at a negative level. The transfer
chamber 52 is connected to the process chamber 12 of the abovedescribed
etching apparatus 10 and to a cassette chamber 56 for storing wafers in
units of a cassette 54. A connection path between the transfer chamber 52
and the process chamber 12 is opened and closed by a gate valve 58, and a
connection path between the transfer chamber 52 and the cassette chamber
56 is opened and closed by a gate valve 62.
When the gate valves 58 and 62 are closed, the transfer chamber 52, process
chamber 12 and cassette chamber 56 have independent airtight spaces. The
transfer chamber 52, process chamber 12 and cassette chamber 56 are
evacuated independently by a common exhaust device 64 such as a turbo
molecular pump or a dry pump, and the pressure of the atmosphere within
each chamber can be set at a negative level, e.g. 10.sup.-5 Torr to
10.sup.-1 Torr. An exhaust line 66 of the transfer chamber 52 is opened
and closed by a valve 68. An exhaust line 72 of the process chamber 12 is
opened and closed by a valve 74. An exhaust line 76 of the cassette
chamber 56 is opened and closed by a valve 78. 10 A transfer device 82 for
transferring a wafer is provided within the transfer chamber 52. In the
present embodiment, the transfer device 82 comprises a multi-joint arm
device including a vertically movable and rotatable base 84 and an
extendible transfer arm 86 attached on the base 84. The surface of the
transfer arm 86 is coated with electrically conductive Teflon, etc. as an
electrostatic countermeasure.
The cassette chamber 56 includes an opening, formed in a side wall opposed
to the connection path to the transfer chamber 52, for taking in and out
the wafer cassette 54, and a gate valve 88 for airtightly closing the
opening. A turntable 92 for supporting the cassette 54 is disposed within
the cassette chamber 56. This structure is suitable for conveying the
cassette 54 into the cassette chamber 56 by means of a transfer robot (not
shown). The cassette 54 contains a predetermined number of wafers S or
substrates to be processed, e.g. 25 wafers S, at vertically regular
intervals. The wafers S are taken out of the cassette 54 and inserted into
the cassette 54 one by one by the transfer device 82 within the transfer
chamber 52.
A description will now be given of a process of etching a tungsten film on
the wafer S by means of the etching system shown in FIG. 3.
The cassette 54 storing 25 wafers S is introduced into the cassette chamber
56 and the gate valve 88 is 10 closed. Then, the exhaust valve 78 of the
cassette chamber 56 is opened and the internal pressure of the cassette
chamber 56 is reduced by the exhaust device 64 to, e.g. 10.sup.-1 Torr.
Subsequently, the gate valve 62 of the cassette chamber 56 is opened and a
predetermined number of wafers S are taken out of the cassette 54 one by
one by means of the transfer device 82 and transferred into the transfer
chamber 52. Thereafter, the exhaust valve 68 of the transfer chamber 52 is
opened and the internal pressure of the transfer cassette chamber 52 is
reduced by the exhaust device 64 to, e.g. 10.sup.-2 Torr.
The gate valve 58 of the process chamber 12 is then opened and one wafer S
is introduced into the process chamber 12 by the transfer device 82. The
focus ring 102 is set at a predetermined position prior to introduction of
the wafer S. The wafer S is placed on the electrostatic chuck 22 and,
after the transfer device 82 is retreated into the transfer chamber 52,
the gate valve 58 is closed. A DC voltage is applied from the power supply
24 and the wafer S is attracted and held on the electrostatic chuck 22.
Subsequently, the exhaust valve 74 of the process chamber 12 is opened and
the internal pressure of the process chamber 12 is reduced by the exhaust
device 64. At the same time, a process gas, e.g. Cl.sub.2 /SF.sub.6 or an
etching gas is supplied into the process chamber 12 from the introducing
pipe 38 via the head space 36 and diffusion holes 44. Thereby, the
internal pressure of the process chamber 12 is set and kept at, e.g.
10.sup.-3 Torr.
Following the above, a high-frequency power of 13.56 MHz is applied from
the power supply 46 between the upper and lower electrodes 34 and 14.
Thereby, the process gas is made into a plasma between the upper and lower
electrodes 34 and 14 and the reactive ions in the plasma are accelerated
and made to impinge upon the tungsten film on the wafer S. Thus,
anisotropic etching is effected.
After the etching process for a predetermined time period, the generation
of the plasma is halted and the internal atmosphere of the process chamber
12 is replaced with an inert gas. The processed wafer S is taken out of
the process chamber 12 by the transfer device 82 in the order of steps
reverse to that described above. The processed wafer S is then inserted
into the cassette 54 within the cassette chamber 56.
The relationship among the susceptor 14, wafer S and focus ring 102 will
now be described in detail.
As is shown in FIG. 4, the diameter of the upper surface of the table 18 of
the susceptor 14 and the diameter of the electrostatic chuck 22 are each
less than the diameter of the wafer S. Accordingly, in the state in which
the wafer S is placed on a predetermined position on the table 18, the
edge of the wafer S projects outside the periphery of the upper surface of
the table 18. For example, if the size of the wafer S is 8 inches
(diameter: 200 mm), the diameter of the table 18 and that of the upper
surface thereof is 195 mm to 198 mm.
The focusing ring 102 is placed on the flange 16 of the susceptor 14 and
surrounds the wafer S in a substantially complementary manner. The
diameter or inside diameter D1 of the opening 108 of the focusing ring 102
is slightly greater than the diameter of the table 18. For example, if the
diameter of the wafer S is 8 inches, the diameter D1 is 196 mm to 199 mm.
The outside diameter D2 of the focusing ring 102 is set at 230 mm to 300
mm in accordance with a process. One of the functions of the focus ring
102 is to prevent diffusion of plasma and make reactive ions of the
process gas effectively incident on the wafer S. If the wafer S has an
orientation flat, as shown in FIG. 2, it is desirable that the inner edge
of the upper surface of the focus ring 102 has a shape similar to that of
the wafer S.
The focus ring 102 has a construction of a combination of inner and outer
annular parts 104 and 106. The inside diameter D1 and outside diameter D2
of the focus ring 102 are defined by the annular inner and outer parts 104
and 106. As is shown in FIG. 4, a gap G of 10 about 1 mm in the radial
direction is provided between the inner and outer parts 104 and 106, in
consideration of mutual thermal expansion. The inner and outer parts 104
and 106 have complementary L-cross sections, so that no plasma reaches the
susceptor 12 through the gap G, and the inner and outer parts 104 and 106
are superposed on each other and connected to each other. The inner and
outer parts 104 and 106 have upper surfaces which are flush with the major
surface of the wafer S. The surface roughness of the upper surfaces of the
inner and outer parts 104 and 106 is set at 1.6 .mu.m or less on average.
The surface roughness is greater than this value, dust may easily adhere
and the adhered dust is difficult to remove.
The inner part 104 is made of a carbon-based material such as amorphous
carbon or SiC, for example, like the conventional focus ring. The inner
part 104 has a stepped upper edge portion of the circular opening 108.
THUS, the inner part 104 has a first portion 112 having an upper surface
flush with the major surface of the wafer S and a second portion 114
having an upper surface lower than the bottom surface of the wafer S, for
example, by about 1 to 1 mm.
The inside diameter D3 of the first portion 112 of the inner part 104 is
greater than the diameter of the wafer S by about 1 to 2 mm. For example,
when the size of the wafer S is 8 inches, the diameter D3 is set at 201 mm
to 202 mm. On the other hand, the second portion 114 of the inner part 104
is situated in a gap between the wafer S projecting radially outside the
upper surface of the table 18 and the flange 16 of the susceptor 14. The
projection of the second part 114 can prevent a local charge-up phenomenon
at the peripheral portion of the wafer S.
The outer part 106 is formed of a material having a main component, which
is at least partially common to the material of a specific etching target.
For example, when an electrically conductive film of W (tungsten) or WSi
(tungsten silicide) for wiring is to be etched, it is desirable that the
outer part 106 be formed of tungsten. Thus, the amount of a remaining
reaction product per unit area is substantially equalized between a
peripheral portion and a central portion of the wafer S during the etching
process, and the in-plane uniformity of etching characteristics such as an
etching rate, etching anisotropy, etc. is enhanced.
As has been state above, the reaction product elimination rate differs
between the peripheral portion and central portion of the wafer S due to
the influence of a gas stream occurring within the process chamber 12
during the etching process. The reaction product is adsorbed on the
surface of the etching object and functions as a temporary protection film
or etching prevention film. Thus, the amount of the remaining reaction
product per unit area is an important factor which determines the etching
rate and etching anisotropy.
From the standpoint of the above, experiments were conducted on various
focus ring samples F1 to F4, with a WSi film being used as an etching
target, which was formed on an 8-inch silicon wafer (diameter: 200 mm).
Each of all samples F1 to F4 as used has an inside diameter of 196 mm, an
outside diameter of 260 mm and a width of 32 mm. The upper surface thereof
is slightly below the bottom surface of the wafer. The sample F1 is
entirely formed of amorphous carbon, and the sample F2 is entirely formed
of tungsten. The samples F3 and F4 relate to this embodiment of the
present invention. Each of the samples F3 and F4 comprises an inner part
formed of an amorphous carbon ring and an outer part formed of a tungsten
ring. The outer parts of the samples F3 and F4 have inside diameters of
230 mm and 246 mm, respectively.
The samples F1 to F4 were used, and the WSi film on the entire surface of
the wafer S was etched with the wafer set temperature of 60.degree. C.,
the process pressure of 9 mTorr and RF power of 250 W, while the process
gas of Cl.sub.2 /SF.sub.6 being supplied at a flow rate of 55/13 SCCM.
FIG. 5 shows the results of the experiments.
As is shown in FIG. 5, when the sample F1 formed of a conventional typical
material alone was used, the etching rate was very high at a peripheral
portion of the wafer S. By contrast, when the sample F2 was used, the
etching rate was low at the peripheral portion of the wafer S but the
in-plane uniformity of the etching rate was enhanced. On the other hand,
when the samples F3 and F4 according to the present invention were used,
the problem with the sample F2 was solved and the inplane uniformity of
the etching rate was further improved.
It is considered that the in-plane uniformity of the etching rate in the
samples F2, F3 and F4 was enhanced for the following reason. When a W film
or a WSi film is etched with a process gas such as NF.sub.3, SF.sub.6,
Cl.sub.2 each including a halogen element, a halide of tungsten is
produced as a reaction product. Similarly, an exposed W portion on the
surface of the focus ring reacts with the process gas and a halide of
tungsten is produced. These reaction products are adsorbed on the surface
of the etching target and function to lower the etching rate and enhance
the etching anisotropy. In other words, the peripheral portion of the
wafer S, at which the rate of elimination of the reaction product is high
due to the influence of the gas stream, is provided with a similar
reaction product from the focus ring by diffusion, the in-plane uniformity
of the etching rate and etching anisotropy between the peripheral portion
and central portion of the wafer can be enhanced.
However, the in-plane uniformity of the etching rate differs among the
samples F2, F3 and F4. To find the reason for this, further experiments
were conducted. As a result, it has been found that in the etching
apparatuses as shown in FIGS. 1 and 4, a difference appears in temperature
between the wafer S and focus ring 102 during etching, and this difference
affects the mechanism of the etching. After generation of the plasma, the
wafer S is cooled by the heat exchange source 26 controlled by the
controller 28 and the rise in temperature is prevented, whereas the focus
ring 102 is not substantially cooled by the heat exchange source 26 and
the temperature thereof rises steeply due to the influence of the plasma.
More specifically, as shown in FIG. 4, a heat transmission path HTP1 is
maintained even in the vacuum atmosphere between the heat exchange source
26 within the susceptor 14 and the wafer S. This is because, during the
etching process, an inert gas functioning as a heat transmission medium is
supplied from the gas source 32 into the gap between the electrostatic
chuck 22 and the bottom surface of the wafer S. In addition, even in the
case where the susceptor 14 comprises a plurality of parts and gaps are
present among the parts, a heat transmission medium is supplied to these
gaps.
By contrast, a heat transmission path HTP2 between the heat exchange source
26 within the susceptor 14 and the focus ring 102 is substantially cut off
in the vacuum atmosphere in which the etching process is performed.
Specifically, a gap is present between the flange 16 of the susceptor 14
and the focus ring 102 placed thereon and this gap is set in substantially
the same reduced pressure state as in the vacuum atmosphere in which the
etching process is performed. In other words, the heat transmission path
HTP2 is cut off between the flange 16 and the focus ring 102 during the
etching process, except at point contact portions.
It is considered that the etching rate at the peripheral portion of the
wafer S of the focus ring sample F2 formed entirely of tungsten decreased
because of overheating of the above-mentioned focus ring sample F2. In
other words, a great deal of reaction product was produced from the
overheated focus ring sample F2, and the reaction product excessively
curbed the etching at the adjacent peripheral portion of the wafer.
By contrast, in the samples F3 and F4, there is a distance between the
wafer S and the outer part of the tungsten ring. Thus, part of the
reaction product produced from the overheated outer part is exhausted
without reaching the peripheral portion of the wafer S. In addition, no
reaction product, which may adhere to the etching target is produced from
the inner part of amorphous carbon. For these reasons, the amount of
remaining reaction product per unit area is close between the peripheral
portion and central portion of the wafer in the samples F3 and F4 and the
in-plane uniformity of etching characteristics is enhanced.
From the above standpoint, experiments were conducted, as shown in Table 1,
on various processes of processing electrically conductive metal films for
wiring with etching gases containing halogen elements, with use of the
etching apparatus shown in FIG. 1, and desirable modes of the focus ring
were examined. Taking the actual process into account, the set temperature
of the major surface of the wafer S used in the experiments was room
temperature (e.g. 25.degree. C.) to 150.degree. C. In this temperature
range, a temperature difference .DELTA.T(.degree.C.) between the major
surface of the wafer S and the upper surface of the focus ring 102 was
50.degree. C. to 100.degree. C.
TABLE 1
______________________________________
TARGET ETCHING GAS OUTER PART
______________________________________
W, WSi NF.sub.3, SF.sub.6, Cl.sub.2
W
Ti, TiSi, TiN
Cl.sub.2, HBr
Ti
Al, Al--Si--Cu
Cl.sub.2, BCl.sub.3
Al
______________________________________
The conditions for obtaining good in-plane uniformity of etching
characteristics under the above temperature condition were that the
distance L1 between the edge of the wafer S and the inner edge of the
outer part 106 along the upper surface or exposed surface of the focus
ring 102 is 5 mm to 30 mm, preferably 15 mm to 25 mm, and that the width
E2 of the exposed surface of the outer part is 5 mm or more. It was also
found that the focus ring 102 of the present invention was particularly
effective when the etching target was uniformly distributed over the
substantially entire surface of the wafer S.
The distance L1 varied, depending on the temperature difference
.DELTA.T(.degree.C.). The greater the difference .DELTA.T, the greater the
desired value of the distance L1. However, the distance L1 was not
substantially influenced by the size of the wafer. The upper limit of the
width E2 was determined in consideration of the sizes of the process
chamber 12 and the susceptor 14, rather than the etching characteristics.
For example, in the case of an 8-inch wafer, the width E2 should desirably
be 50 mm or less.
Furthermore, an examination was made on the ratio E2/E1 of the width E2 of
the outer part 106 to the width E1 of the focus ring 102 on the upper
surface or exposed surface of the focus ring 102. As a result, it has been
found that in the case of the 8-inch wafer, the ratio E2/E1=0.15 to 0.75,
preferably, E2/E1=0.25 to 0.75, may be used.
As has been described above, the in-plane uniformity of etching
characteristics such as etching rate and etching anisotropy can be
improved by employing the focus ring 102 comprising a compound structure
of inner and outer parts 104 and 106 and selecting the specific materials
of the inner and outer parts. The inner part is formed of an electrically
conductive material, e.g. amorphous carbon, which causes substantially no
reaction product by contact with an etching gas, or an electrically
conductive material which does not cause, at least, any reaction product
which is substantially adsorbed on an etching target, by contact with an
etching gas. The outer part is formed of a material containing a component
which is a main component of the etching target and causes such a reaction
product as to be substantially adsorbed on the etching target by contact
with an etching gas, preferably, a metallic material.
FIG. 6 shows a modification of the focus ring 102. In a focus ring 112
shown in FIG. 6, an annular base part 114 formed of amorphous carbon,
etc., which corresponds to the inner part 104 of the focus ring 102 shown
in FIG. 4, extends over the entire width of the focus ring 112. An annular
thin plate part 116 formed of tungsten, etc., which corresponds to the
outer part 106 of the focus ring 102 shown in FIG. 4, is placed and
attached on an outer circular surface of the base part 114. The upper
surface of the base part 114 is flush with the major surface of the wafer
S. An inside upper edge of the base part 114 is stepped downward, and an
integral extension portion 115 underlies the wafer S to prevent a
charge-up phenomenon of the peripheral portion of the wafer S.
FIG. 7 shows another modification of the focus ring 102. In a focus ring
122 shown in FIG. 7, an annular base part 126 formed of tungsten, etc.,
which corresponds to the outer part 106 of the focus ring 102 shown in
FIG. 4, extends over the entire width of the focus ring 122. An annular
thin plate part 124 formed of amorphous carbon, etc., which corresponds to
the inner part 104 of the focus ring 102 shown in FIG. 4, is placed and
attached on an inner circular surface of the base part 126. The upper
surface of the thin plate part 124 is flush with the major surface of the
wafer S. An inner edge portion 127 of the base part 126 underlies the
wafer S to prevent a charge-up phenomenon of a peripheral portion of the
wafer S.
In FIGS. 4, 6 and 7, the outer part of the focus ring has an annular shape.
However, the outer part may be constituted by a plurality of segments,
arranged circumferentially at intervals.
FIG. 8 is a cross-sectional view showing a plasma etching apparatus
according to a second embodiment of the present invention. In the first
embodiment, attention is paid to the process in which the temperature
range is set between room temperature, e.g. 25.degree. C., and 150.degree.
C. In the second embodiment, attention is paid to a process in which the
temperature range is set between -100.degree. C. and 60.degree. C.
A plasma etching apparatus 201 shown in FIG. 8 includes a process chamber
204 comprising an inner frame 202 and an outer frame 203 formed of
aluminum, etc. The inner frame 202 comprises a cylindrical wall portion
202a, a bottom portion 202b provided at a little distance upward from the
lower end of the cylindrical wall portion 202a, and an outwardly bent
flange portion 202c provided on an outer periphery of the lower end of the
cylindrical wall portion 202a. On the other hand, the outer frame 203
comprises a cylindrical wall portion 203a and a top portion 203b, and is
mounted on the outwardly bent flange portion 202c so as to airtightly
cover the inner frame 202.
An upper portion of the cylindrical wall portion 203a of the outer frame 4
is connected to a gas supply system 205 from which a mixture gas of
SF.sub.6 and N.sub.2 can be introduced from process gas sources, i.e. an
SF.sub.6 gas source 205a and an N.sub.2 gas source 205b, through a mass
flow controller 205c into the process chamber 204. A gas exhaust system
206 is provided on a lower part of the opposite side portion of the
cylindrical wall portion 203a. The process chamber 204 can be evacuated by
a vacuum pump (not shown).
A magnetic field generating device, e.g. a permanent magnet 7, for
generating a horizontal magnetic field on a semiconductor wafer S or a
substrate to be processed is rotatably provided above the top portion 203b
of the outer frame 203. A magnetron discharge can be generated by the
horizontal magnetic field generated by the magnet and an electric field
produced to intersect at right angles with the horizontal magnetic field.
As is shown in FIG. 8, a susceptor assembly 208 for supporting the wafer S
is situated within the process chamber 204. The susceptor assembly 208 is
placed on the bottom portion 202b of the inner frame 202 via a plurality
of insulating members 209. An insulating member 210 of, e.g. an O-ring
shape, is interposed between the side surface of the susceptor assembly
208 and the cylindrical wall portion 202a of the inner frame 202. Thus,
the susceptor assembly 208 is kept in an insulated state from the inner
frame 202 and outer frame 203 which are grounded outside.
The susceptor assembly 208 has a three-layer structure. An electrostatic
chuck sheet 212 is placed on a first sub-susceptor 208a. The wafer S is
supported on the electrostatic chuck sheet 212. A focus ring 208d of, e.g.
amorphous carbon is situated on the support surface of the first susceptor
208a so as to surround the electrostatic chuck sheet 212 and wafer S,
thereby efficiently radiating a generated plasma on the surface to be
processed. The electrostatic chuck sheet comprises a pair of polyimide
resin films 213 and 214, between which a thin conductive film 215 such as
a copper foil is sealed. The conductive film 215 is connected to a DC
power supply 217 via a conductive wire. A current is supplied from the DC
power supply 217 so that the wafer S can be fixed on the susceptor by
Coulomb force.
A second sub-susceptor 208b or an intermediate layer of the susceptor
assembly 208 is provided with a heater 222 for controlling the temperature
of the wafer S. The heater 222 is connected to a heat controller (not
shown), and temperature control is effected in response to a signal from a
temperature monitor (not shown) for monitoring the temperature of the
susceptor assembly 208.
The first sub-susceptor 208a is removably fixed on the second sub-susceptor
208b by means of a coupling member such as a bolt 223. Thus, the first
subsusceptor 208a alone can be exchanged, separately from the second
sub-susceptor 208b connected to a high-frequency power supply 224, if the
first sub-susceptor 208a is contaminated. The maintenance of the apparatus
is made easier.
Since the insulating member 210 such as an O-ring is inte | | |
