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Claims  |
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What is claimed is:
1. A plasma processing apparatus for processing a substrate having a main
surface to be processed, and a rear surface opposite to said main surface,
by use of plasma of a process gas, while attracting and holding said
substrate on a stage having an electrostatic chuck by an electrostatic
attractive force, comprising:
(a) a process chamber for enclosing said substrate;
(b) means for supplying said process gas to said process chamber;
(c) means for evacuating said process chamber and creating a vacuum state;
(d) means for making said process gas into plasma in said process chamber;
and
(e) said stage provided in said process chamber, said stage comprising:
a main body having a supporting surface for supporting said substrate via
said rear surface thereof;
the electrostatic chuck having a first electrode and a second electrode,
said first and second electrodes of said electrostatic chuck arranged to
face said supporting surface, said first and second electrodes being
insulated from each other;
first power supply means for selectively applying a first potential to said
first and second electrodes;
second power supply means for selectively applying a second potential
different from said first potential to said second electrode;
switch means for selectively connecting said first and second electrodes;
and
first and second resistive layers for covering said first and second
electrodes, respectively, said first and second resistive layers being
insulated from each other, each having a surface brought into contact with
said rear surface of said substrate when said substrate is supported by
said supporting surface, and each exhibiting an electric resistivity of
1.times.10.sup.10 .OMEGA..multidot.cm to 1.times.10.sup.12
.OMEGA..multidot.cm in a temperature range when attracting and holding
said substrate,
wherein while said first and second potentials are applied to said first
and second electrodes, respectively, a closed loop from said first power
supply means via said substrate to said second power supply means is
formed, a contact potential difference is created between said surface of
each of said first and second resistive layers and said rear surface of
said substrate, and said substrate is attracted and held by said first and
second resistive layers, and
while said first potential is applied to said first and second electrodes,
and a third potential different from said first potential is applied to
said substrate via said plasma, a contact potential difference is created
between said surface of each of said first and second resistive layers and
said rear surface of said substrate, and said substrate is attracted and
held by said first and second resistive layers;
the apparatus further including control means for controlling said first
and second power supply means such that said first potential is applied to
said first and second electrodes when plasma is generated in said process
chamber.
2. An apparatus according to claim 1, wherein said surface of each of said
first and second resistive layers is formed to have such a surface
roughness that a center line average height falls within a range of 0.1 to
1.5 .mu.m.
3. An apparatus according to claim 1, wherein said temperature range is set
within -50.degree. C. to 120.degree. C., and each of said first and second
resistive layers is formed of a material selected from the group
consisting of SiC and Al.sub.2 O.sub.3 containing conductive impurities
adjusted.
4. An apparatus according to claim 1, wherein said supporting surface
comprises a surface for supporting a semiconductor wafer.
5. A plasma processing apparatus for processing a substrate having a main
surface to be processed, and a rear surface opposite to said main surface,
by use of plasma of a process gas, while attracting and holding said
substrate on a stage having an electrostatic chuck by an electrostatic
attractive force, comprising:
(a) a process chamber for enclosing said substrate;
(b) means for supplying said process gas to said process chamber;
(c) means for evacuating said process chamber and creating a vacuum state;
(d) means for making said process gas into plasma in said process chamber;
and
(e) said stage provided in said process chamber, said stage comprising:
a main body having a supporting surface for supporting said substrate via
said rear surface thereof,
first and second electrodes for attracting said substrate, arranged in said
main body to face said supporting surface, said first and second
electrodes being insulated from each other,
first power supply means for selectively applying a first potential to said
first and second electrodes,
second power supply means for selectively applying a second potential,
different from said first potential, to said second electrode, and
switch means for selectively connecting said first and second electrodes,
wherein said first and second potentials are applied to said first and
second electrodes, respectively, from said first and second power supply
means so as to attract and hold said substrate on said stage, while no
plasma is generated in said process chamber, and
said first potential is applied to both of said first and second electrodes
from said first power supply means and a third potential different from
said first potential is applied to said substrate via said plasma so as to
attract and hold said substrate on said stage, while said plasma is
generated in said process chamber;
the apparatus further including control means for controlling said first
and second power supply means such that said first potential is applied to
said first and second electrodes when plasma is generated in said process
chamber.
6. The apparatus according to claim 5, further comprising first and second
resistive layers for covering said first and second electrodes,
respectively, each having a surface facing said rear surface of said
substrate when said substrate is supported by said supporting surface.
7. The apparatus according to claim 6, wherein said first and second
resistive layers each exhibit an electric resistivity of 1.times.10.sup.10
.OMEGA..multidot.cm to 1.times.10.sup.12 .OMEGA..multidot.cm in a
temperature range when attracting and holding said substrate, and are
electrically insulated from each other.
8. The apparatus according to claim 7, wherein said temperature range is
set within -50.degree. C. to 120.degree. C., and said resistive layer is
formed of a material selected from the group consisting of SiC, and
Al.sub.2 O.sub.3 containing conductive impurities.
9. The apparatus according to claim 7, wherein said temperature range is
set within 300.degree. C. to 600.degree. C., and said resistive layer is
formed of a material selected from the group consisting of pyrolytic boron
nitride, Si.sub.3 N.sub.4, and Al.sub.2 O.sub.3 containing conductive
impurities.
10. The apparatus according to claim 5, wherein said supporting surface
comprises a surface for supporting a semiconductor wafer.
11. The apparatus according to claim 5, wherein said process gas comprises
an etching gas.
12. The apparatus according to claim 6, wherein said surface of each of
said first and second resistive layers is formed to have a surface
roughness such that a center line average height falls within a range of
0.1 to 1.5 .mu.m, and is brought into direct contact with said rear
surface of said substrate.
13. The apparatus according to claim 6, wherein said surface of said
resistive layer is coated with a coating of an insulating material, and a
thickness of said coating is 1 .mu.m or less. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a stage having an electrostatic chuck,
used for attracting and holding a to-be-processed substrate such as a
semiconductor wafer during a process such as etching or film formation for
the substrate, and the plasma processing apparatus employing such a stage.
2. Description of the Related Art
In a processing apparatus such as a plasma etching apparatus for processing
semiconductor wafer one by one, a susceptor which serves as a lower
electrode is provided in its process chamber which can be set to a vacuum
state. A wafer, to-be-processed substrate, is placed on and fixed to the
susceptor, and then subjected to the process. There are mainly two
commonly-used ways of fixing a wafer to a susceptor, i.e. the mechanical
supporting means such as clamp, and the electrostatic chuck for attracting
a wafer by means of an electrostatic attractive force.
U.S. Pat. No. 4,771,730, issued on Sep. 20, 1988, discloses an
electrostatic chuck provided on a susceptor or a table for holding a
to-be-processed substrate. The electrostatic chuck includes two dielectric
layers, and an electrode interposed therebetween. The electrode is
connected to the positive terminal of an external direct current (DC)
power supply, and the negative terminal of the direct current power supply
is grounded. A heat conductive gas is supplied between the wafer and the
upper dielectric layer. The US Patent discloses a structure in which an
Al.sub.2 O.sub.3 layer is used as a dielectric layer, in FIG. 2, and a
structure in which a polyimide sheet is used as a dielectric layer, in
FIG. 3.
When plasma is generated in the process chamber, the semiconductor wafer
placed on the electrostatic chuck is grounded via the plasma and the upper
electrode. With this structure, if a positive potential is applied to the
electrode of the electrostatic chuck from the DC power supply, the wafer
will have a negative potential, and the upper dielectric layer is
polarized, with its upper surface having a positive potential. Therefore,
an electrostatic attractive force is generated between the wafer and the
dielectric layer, and due to the attractive force, the wafer is attracted
and held on the susceptor.
However, with such an electrostatic chuck, attractive force remains (to be
called as "residual attractive force" hereinafter) between the wafer and
the chuck even if the electrode of the electrostatic chuck and the power
supply are disconnected after the process. In this state, when the wafer
is lifted with pusher pins built in the susceptor, a large force is
applied to the wafer locally, thereby damaging or displacing the wafer. To
solve this problem, there have been proposed several methods of removing
residual electric charge on the rear surface of the wafer or the surface
of the chuck, as shown in application Ser. No. 08/017,379 now abandoned,
filed on Feb. 12, 1993.
In addition, since a dielectric sheet of polyimide is likely to have a
dielectric breakdown when it is too thin, the sheet is generally formed to
have a thickness of about 50 .mu.m. In accordance with such a thickness,
the voltage of the DC power supply should be set high in order to obtain a
strong attractive force. However, if a high voltage is involved, an
unnecessary discharge is likely to occur, or the cost of the power supply
is raised.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a stage having an
electrostatic chuck which does not generate residual remaining attractive
force.
According to an aspect of the present invention, there is provided a stage
having an electrostatic chuck for attracting and holding a substrate
having a main surface to be processed, and a rear surface opposite to the
main surface, by an electrostatic attractive force, comprising:
a main body having a supporting surface for supporting the substrate via
the rear surface thereof;
a chuck electrode provided on the holding surface;
power supply means for selectively applying a first potential to the chuck
electrode; and
a resistive layer for covering the chuck electrode, the resistive layer
having a surface facing the rear surface of the surface when the substrate
is supported by the supporting surface, and the resistive layer exhibiting
an electric resistivity of 1.times.10.sup.10 .OMEGA..multidot.cm to
1.times.10.sup.12 .OMEGA..multidot.cm in a temperature range when
attracting the substrate,
wherein while the first potential is applied to the chuck electrode and a
second potential different from the first potential is applied to the
substrate, a contact potential difference is created between the surface
of the resistive layer and the rear surface of the substrate, and the
substrate is attracted and held by the resistive layer.
According to another aspect of the present invention, there is provided a
stage having an electrostatic chuck for attracting and holding a substrate
having a main surface to be processed, and a rear surface opposite to the
main surface, by an electrostatic attractive force, comprising:
a main body having a supporting surface for supporting the substrate via
the rear surface thereof;
first and second electrode arranged on the holding surface, the first and
second electrodes being insulated from each other;
first power supply means for selectively applying a first potential to the
first electrode;
second power supply means for selectively applying a second potential
different from the first potential to the second electrode;
switch means, located closer to the first and second electrodes than to the
first and second power supply means, for selectively connecting and
separating the first and second electrodes; and
first and second resistive layers for covering the first and second
electrodes, respectively, the first and second resistive layers being
insulated from each other, each having a surface brought into contact with
the rear surface of the substrate when the substrate is supported by the
supporting surface, and each exhibiting an electric resistivity of
1.times.10.sup.10 .OMEGA..multidot.cm to 1.times.10.sup.12
.OMEGA..multidot.cm in a temperature range when attracting the substrate,
wherein while the first and second potentials are applied to the first and
second electrodes, respectively, a closed loop from the first power supply
means via the substrate to the second power supply means is formed, a
contact potential difference is created between the surface of each of the
first and second resistive layers and the rear surface of the substrate,
and the substrate is attracted and held by the first and second resistive
layers, and
while the first potential is applied to the first and second electrodes,
and a third potential different from the first potential is applied to the
substrate, a contact potential difference is created between the surface
of each of the first and second resistive layers and the rear surface of
the substrate, and the substrate is attracted and held by the first and
second resistive layers.
Additional objects and advantages of the invention will be set forth in the
description which follows, and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized and obtained by means of
the instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate presently preferred embodiments of the
invention, and together with the general description given above and the
detailed description of the preferred embodiments given below, serve to
explain the principles of the invention.
FIG. 1 is a cross sectional view showing a plasma etching apparatus
according to a first embodiment of the present invention;
FIG. 2 is an enlarged cross sectional view showing an interface between the
resistive layer of an electrostatic chuck of the present invention, and a
wafer;
FIG. 3 is an enlarged cross sectional view showing an interface between the
dielectric layer of an electrostatic chuck of the conventional technique,
and a wafer;
FIG. 4 is a cross sectional view showing a plasma etching apparatus
according to a second embodiment of the present invention;
FIG. 5 is a cross sectional view showing a plasma etching apparatus
according to a third embodiment of the present invention;
FIG. 6 is a plan view of the electrostatic chuck of the apparatus shown in
FIG. 5;
FIG. 7 is a diagram showing the usable range of center line average heights
and electric resistivities;
FIG. 8 is a timing chart for the operations of the switches of the
apparatus shown in FIG. 5;
FIG. 9 is a plan view showing a modification of the electrostatic chuck;
and
FIG. 10 is a plan view showing another modification of the electrostatic
chuck.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The plasma etching apparatus according to the first embodiment of the
present invention, which is shown in FIG. 1, includes a process chamber 1
made of a conductive metal such as aluminum having an anodic oxide
surface. On a side wall of the process chamber 1, gate 11 and 12 are
provided for closing passage ways to the first and second load lock
chambers 7 and 8, respectively. A vacuum pump 14 is connected to the lower
portion of the process chamber 1 via an exhaustion pipe 13. By means of an
exhaustion pump 14, the pressure in the process chamber 1 can be reduced
to a desired vacuum level.
A cooling block 21 is provided at the center of the bottom portion of the
process chamber 1 via an insulator 16. The cooling block 21 is formed of a
conductive metal such as aluminum having an anodic oxide surface, into,
for example, a column shape. In the cooling block 21, a bore 22 is formed
for the purpose of circulating a coolant such as liquid nitrogen. An
introduction tube 22a and an exhaustion tube 22b are connected to the bore
22, and the cooling liquid is supplied into the bore 22 via the
introduction tube 22a, and is exhausted to the outside of the process
chamber 1 via the exhaustion tube 22b.
On the cooling block 21, the susceptor 2 made of a conductive material such
as aluminum having an anodic oxide surface is detachably fixed by means
of, for example, bolts (not shown). The susceptor 2 serves as a lower
electrode, and is connected to a high-frequency power supply 23 provided
outside the process chamber 1 via a capacitor 28. The power supply 23 is
grounded via a switch 29.
The upper surface of the susceptor 2 serves as a wafer-holding surface, and
an insulative layer 31 made of, e.g., polyimide is adhered to the upper
surface. On the insulative layer 31, a conductive chuck electrode 32 and a
resistive layer 3 made of a material having a high value of resistance are
arranged in this order. A semiconductor wafer W is placed on the resistive
layer 3. In this embodiment, the chuck electrode 32 is made of silver
paste, and applied on the lower surface of the resistive layer 3. The
chuck electrode 32 may be prepared by forming a silver or palladium film
on the lower surface of the resistive layer 3 by screen printing in place
of the application of the silver paste.
A conductive line 33 covered by an insulation cable is provided in the
suceptor 2, one end of the line 33 being connected to the chuck electrode
32. A through-hole 24 is formed in the cooling block 21 at a position
corresponding to the conductive line 33. In the through-hole 24, an
electric supplying rod 34 is provided such that the rod is connected to
the other end of the conductive line 33. The electric supplying rod 34 is
connected to the DC power supply 36 provided outside the apparatus via a
switch 35. With the mentioned structure, the chuck electrode 32 is
connected to the DC power supply 36 via the conductive line 33, the
electric supplying rod 34, and the switch 35. The switch 35 is a relay
switch, and can be switched to a terminal 38 of the grounding line. The
power supply 36 is grounded via the switch 39.
From the resistive layer 3 to the cooling block 21, there is formed a gas
supplying passage 25 for supplying a heat conductive gas to the rear
surface of the wafer W placed on the resistive layer 3. The supplying
passage 25 is connected to a heat conductive gas source (not shown) such
as of He via a valve 25a. Further, from the resistive layer 3 to the
cooling block 21, there are a plurality of, for example, three pusher pins
27 for moving the wafer up and down with respect to the upper surface of
the resistive layer 3. The pusher pins 27 are actuated by driving means
26.
Above the susceptor 2, there is provided an upper electrode 41 made of a
conductive material such as aluminum having an anodic oxide surface. The
upper electrode 41 is arranged so as to face the susceptor 2 serving as a
lower electrode. The upper electrode 41 is grounded, and constitutes a
pair of counter electrodes of the parallel-plate type, together with the
susceptor 2, which is the lower electrode, connected to the high-frequency
power supply 23.
The upper electrode 41 includes a bore 44 inside, and serves as a header
for supplying a process gas. To the bore 44, there is connected a gas
introduction tube 42, through which the process gas, for example, a
mixture of a reactive gas, CF.sub.4 and a carrier gas, At, is supplied. At
the lower portion of the upper electrode 41, there is provided a gas
diffusion plate 43 for supplying the process gas into the process chamber
1, for example, in a shower-like manner.
The operation of each of the switch 35 of the DC power supply 36, the
switch 29 of the high-frequency power supply 23, and the valve 25a for
switching the heat conductive gas, is controlled by a control unit 50. A
predetermined program is stored in the control unit 50 in advance, and
each of these members is switched in accordance with an instruction output
on the basis of the program.
The susceptor 2 and the cooling block 21 are electrically insulated from
the members outside the process chamber 1 (except for the high-frequency
power supply 23). Such a version is disclosed in application Ser. No.
08/104,475, filed on Jul. 18, 1993 now abandoned, the teachings of which
are hereby incorporated by reference.
The process of a semiconductor wafer W is carried out in the following
steps. First, the wafer W is loaded from the load lock chamber 7 through
the gate 11 into the process chamber by means of a transfer arm (not
shown) provided in the first load lock chamber 7. The wafer W is then
placed on the susceptor 2 by the transfer arm and the pusher pins 27,
which move in cooperation with each other.
Next, the process gas is supplied into the process chamber 1 from the gas
introduction tube 42 via the bore 44 of the upper electrode 41, and the
gas diffusion plate 42. At the same time, the process chamber 1 is
exhausted by the pump 14 via the exhaustion tube 13 so as to maintain the
pressure of the inside of the process chamber at a certain vacuum level.
Further, a high-frequency voltage, for example, of 380 kHz and 1.5 kW is
applied from the high frequency power supply 23 between the susceptor 2
and the upper electrode 41, and thus, the process gas is made into plasma
on the wafer W.
Further, the positive terminal of the DC power supply 36 having a voltage
of 300 V is connected to the chuck electrode 32. Thus, there is generated
an electrostatic attractive force, by which the wafer W is held on the
susceptor 2 via the resistive layer 3 of the electrostatic chuck. The heat
conductive gas is supplied between the wafer and the resistive layer 3,
and cold is transferred from the cooling block 21 to the wafer W so as to
set a temperature of the wafer W.
Ions contained in the plasma are irradiated vertically onto the surface of
the wafer W so as to physically etch the target substance located on the
surface. Active species contained in the plasma react with the substance,
and thus the surface of the wafer is chemically etched.
After the completion of etching of the wafer W, the supply of the process
gas and the heat conductive gas is stopped, and the high-frequency power
supply 23 is turned off. Consequently, the inside of the process chamber 1
is replaced by an inert gas to a certain degree. Also, the switch 35 is
operated to switch from the power supply 36 to the ground terminal 38.
Then, the pusher pins 27 project from the resistive layer 3 so as to push
up the wafer from the susceptor 2. After that, the wafer w is unloaded
from the process chamber 1 to the load lock chamber 8 via the gate 12 by
means of a transfer arm (not shown) provided in the second load lock
chamber 8.
The thickness of the resistive layer 3 of the electrostatic chuck is set at
5 mm or less. The resistive layer 3 is made of a material having an
electric resistivity Re of 1.times.10.sup.10 .OMEGA..multidot.cm to
1.times.10.sup.12 .OMEGA..multidot.cm. An example of the material is
commercially available SiC. Another example is commercially available
Al.sub.2 O.sub.3, which is prepared to have an electric resistivity
falling in the above range by adjusting the contents of the conductive
impurities.
The electric resistivity of the material decreases usually as the
temperature of use increases. Therefore, it is necessary to select such a
material for the resistive layer 3 as to exhibit an electric resistivity
within the above range at a temperature, at which the electrostatic chuck
is to be used. In the plasma etching apparatus shown in FIG. 1, the
temperature of the electrostatic chuck during etching is -50.degree. C. to
120.degree. C., and each of the above examples exhibits an electric
resistivity within the above range with respect to the temperature range.
For example, in the case where the electrostatic chuck is used in a
temperature range of 300.degree. C. to 600.degree. C. as in the thermal
CVD, it is required that a material exhibiting an electric resistivity of
1.times.10.sup.10 .OMEGA..multidot.cm to 1.times.10.sup.12
.OMEGA..multidot.cm in this temperature range, be selected. For example,
pyrolytic boron nitride and Si.sub.3 N.sub.4 are commercially available
products which exhibits an electric resistivity falling within the above
range when they are heated to this high temperature range. Again, Al.sub.2
O.sub.3 is another example, which is prepared to exhibit an electric
resistivity falling in the above range as for the high temperature range,
by adjusting the contents of the conductive impurities.
The surface of the resistive layer 3 is finished so as to have a non-mirror
state. More specifically, the surface of the resistive layer 3 is formed
to have such surface roughness that the center line average height Ra
falls within a range of 0.1 to 1.5.
It should be noted that the center line average height Ra is a value
expressed in .mu.m obtained by the following equation, where portions
having a length L measured from a roughness curve in the direction towards
its center line, are extracted; the center line of the extracted portions
is taken as an X axis; the direction of the longitudinal magnification is
taken as a Y axis; and the roughness curve is expressed in y=f(x).
##EQU1##
The center line of the roughness curve is a straight line which is drawn in
parallel with the average line of the roughness curve and divides the area
defined between the straight line and the roughness curve itself, into two
regions having the same area one on either side of the straight line. The
average line of the roughness curve is a straight line or a curve which
has a geometrical shape of a measured surface in the extracted portions of
the roughness curve, and is set such that the sum of squares of the
deviation from the line to the roughness curve has the minimum value.
As shown in FIG. 7, an examination was conducted by varying the center line
average hight Ra (.mu.m) and the electric resistivity Re
(.OMEGA..multidot.cm) of the resist layer 3 so as to find the range of
conditions within which the etching apparatus shown in FIG. 1 can be used.
As a wafer W, a Si wafer was used. The target value of an attractive force
was set at 10 kg/cm.sup.2 or more during chucking, and the target value of
a leak current from the resistive layer 3 was set at 1 .mu.A/cm.sup.2 or
less.
In FIG. 7, log Re is indicated by the X axis, and Ra by the Y axis. The
lines L1, L2, L3, and L4 are expressed by the functions of x=10, y=0.1,
y=1.5 and y=2x-23.7, respectively. The region defined by the four lines L1
to L4 satisfies the conditions which can be used by the present invention.
In the left side of L1, i.e., x<10, the leak current is in excess, whereas
in the lower side of L2, i.e., y<0.1, a sufficient potential difference
between the resistive layer 3 and the wafer W cannot be obtained. Above
L3, i.e. y>1.5, the distance between resistive layer 3 and the wafer W is
too large, whereas in the right side of L4, the residual attractive force
is too large.
As shown in FIG. 2, due to the roughness of the surface of the resistive
layer 3, there exists an interstice BI between the wafer W and the
resistive layer 3. When the potential of the positive terminal of the DC
power supply 36 is applied to the chuck electrode 32, and the wafer w is
grounded via the plasma, a large potential difference is created between a
potential V.sub.3 of the lower surface of the wafer W and a potential
V.sub.2 to the upper surface of the resistive layer 3 due to a small
voltage drop of the resistive layer 3. As a result, due to the potential
difference, i.e. the contact potential difference, a large electrostatic
attractive force is generated, and the wafer W is attracted to the
resistive layer 3. It should be noted that v.sub.1 and v.sub.4 represent
the potentials of the lower surface of the resistive layer 3 and the upper
surface of the wafer W, respectively.
As described, in the present invention, a wafer W is attracted to the
resistive layer by utilizing an electrostatic attractive force created by
the contact potential difference between the resistive layer 3 and the
wafer W. With this structure, a strong attractive force can be obtained by
a DC power supply of a low voltage. Further, the problem of the residual
attractive force generated in the conventional electrostatic chuck can be
solved. The following is a description of the reason for such a solution.
In the conventional electrostatic chuck such as disclosed in U.S. Pat. No.
4,771,730, polarization occurs in the dielectric layer or sheet on the
wafer side, generating a positive charge on the dielectric layer on the
side in contact with the wafer. In contrast, the wafer is negatively
charged, creating an electrostatic attractive force between the wafer and
the dielectric layer, whereby the wafer is attracted to the layer. As
regards such a conventional electrostatic chuck, the residual potential of
the surface of the dielectric layer after unloading a semiconductor wafer
was measured. The positive terminal of a direct current power supply of
1.5 kV was connected to the electrode of the electrostatic chuck.
According to the results of the measurement, the potential on the surface
of the dielectric layer was -700 V. As mentioned above, since a positive
charge is created on the surface of the dielectric layer, it was expected
that the residual potential on the surface of the dielectric layer was to
be a positive potential. However, the actual results of the measurement
indicated that the residual potential was negative. From this fact, the
following explanation can be provided as to the residual attractive force
of the conventional electrostatic chuck. As shown in FIG. 3, there exists
an interstice in the interface between the wafer w and the dielectric
layer, for example, polyimide sheet PS, of the electrostatic chuck. Into
the interstice, a heat conductive gas, He, is supplied. The He gas
contains a little amount of moisture, and the moisture is influenced by
high voltages from a DC power supply for the electrostatic chuck and a
high-frequency power supply for generating plasma, and ionized into
hydrogen ion (H.sup.+), hydronium ion (H.sub.3 O.sup.+), and hydroxyl ion
(OH.sup.-). H.sup.+ and H.sub.3 O.sup.+ are attracted to the wafer W which
is negatively charged, whereas OH.sup.- is attracted to the dielectric
sheet PS which is positively charged. The charge of OH.sup.- cannot move
through the dielectric sheet PS since the sheet PS is highly insulative,
and therefore remains on the surface of the sheet PS even after the
electrostatic chuck and the DC power supply are disconnected.
Consequently, the residual potential on the surface of the sheet PS
becomes negative because of this OH.sup.-, and the attracting force is
generated between OH.sup.- and H.sup.+ or H.sub.3O.sup.+, thereby creating
a residual attractive force.
Similarly, in the present invention, there exists an interstice in the
interface between the wafer W and the resistive layer 3, and it is
considered that in the interstice, moisture is ionized into H.sup.+,
H.sub.3 O.sup.+, which are attracted to the wafer W, and OH.sup.-, which
is attracted to the resistive layer 3. However, since the resistive layer
3 is conductive while being highly resistive, the charge of OH.sup.-
attracted to the surface of the resistive layer 3 goes to the power supply
36 while the power supply 36 is in connection, whereas to the ground via
the switch 35 and the terminal 38 after the power supply 36 is switched
off by the switch 35. With this structure, OH.sup.- does not remain on the
surface of the resistive layer 3, and therefore a residual attractive
force is not generated unlike in the case of the conventional
electrostatic chuck.
In the conventional electrostatic chuck, an insulation breakdown is likely
to occur to the dielectric sheet if the sheet is too thin, and it is
difficult to manufacture a thin sheet. Therefore, the thickness of the
dielectric sheet must be increased to a certain degree. Further,
conventionally, a high voltage power supply must be used in order to
polarize a thick dielectric sheet, and to obtain a sufficient attractive
force. In the present invention, the resistive layer 3 attracts the wafer
W by means of the contact potential difference at the interface between
the resistive layer 3 and the wafer W, and therefore the application of a
low voltage can create a sufficient attractive force. Consequently, the
undesirable influence on the periphery device caused by the application of
a high voltage can be removed, and the power supply device can have a
simple structure. In the case where the absolute value of the negative
potential charged on the wafer W by generation of plasma is large, a
contact potential difference sufficient to chuck the wafer W can be
obtained simply by grounding the chuck electrode 32 during the generation
of the plasma. In this case, the DC power supply 36 can be omitted.
FIG. 4 shows the electrostatic chuck of a plasma etching apparatus
according to the second embodiment of the present invention. The apparatus
of the second embodiment is identical to that shown in FIG. 1 except for
what is so indicated in FIG. 4. In FIG. 4, the structural elements
corresponding to those of the apparatus shown in FIG. 1 are designated by
the same reference symbols used in FIG. 1, and the description of each of
such elements will be omitted.
In this embodiment, an insulative coating 5 is provided on the surface of a
resistive layer 3. In the case where the insulative coating 5 is formed so
on the resistive layer 3, the contact resistance between the surface of
the coating 5 and the wafer W is high, and therefore the resistance as a
whole is high. However, since the contact potential difference is not
reduced, a sufficient attractive force can be obtained. If the insulation
coating 5 is made too thick, OH.sup.- created by ionization of moisture
remains on the surface of the coating 5, and may generate a remaining
attractive force. Therefore, it is preferable that the thickness of the
coating 5 shoul | | |
