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BACKGROUND OF THE INVENTION
The present invention relates to a ceramic electrostatic chuck with
built-in heater or, more particularly, to a ceramic-made device used for
heating a flat substrate material such as a semiconductor silicon wafer as
a workpiece under chucking with an electrostatic attracting force in the
manufacturing process of, for example, semiconductor devices which
involves a step of working on the substrate at an elevated temperature.
It is sometimes the case in the manufacturing process of semiconductor
devices that working on a substrate such as a semiconductor silicon wafer
is performed under heating of the substrate at a high temperature by
mounting the wafer on a heater. Conventional heaters used for this purpose
utilize electric resistance heating with a coiled resistance wire as the
resistance heater element. Such an electric heater, however, is
disadvantageous in respect of the possibility of contamination of the
semiconductor silicon wafer with the metallic elements as the constituents
of the resistance wires. In this regard, proposals have been made for a
so-called ceramic heater which is an integral device consisting of an
electrically insulating ceramic base body and a thin film of an
electroconductive ceramic material such as graphite formed on one surface
of the ceramic base body to serve as a heater element (see, for example,
Japanese Patent Kokai No. 4-124076).
It is also required in the above mentioned process of a semiconductor
silicon wafer with a ceramic heater that the silicon wafer is fixed and
immobilized on the heater in order to ensure accuracy of working on the
silicon wafer by the use of a chucking means. Since the working process of
a silicon wafer in many cases is conducted under a reduced pressure or in
vacuum, traditional vacuum chucks can no longer work there so that
electrostatic chucks are currently under use in an atmosphere under
reduced pressure. Along with the trend in the semiconductor processes that
the working temperature on a silicon substrate is increased higher and
higher, ceramics are used as the material of the electrostatic chucks
(see, for example, Japanese Patent Kokai No. 52-67353 and No. 59-124140).
It is also a trend in recent years to use a ceramic electrostatic chuck
with built-in heater, which is an integral device as a combination of a
ceramic electrostatic chuck and a ceramic heater. Such a ceramic
electrostatic chuck with built-in heater has a structure in which a base
body of a ceramic material is provided, on one surface, with a first
electroconductive layer for electric resistance heating and, on the other
surface, with a second electroconductive layer to serve as the electrodes
for generating an electrostatic attracting force, each of the
electroconductive layers being overlaid with an insulating layer for
protection. When the working temperature of such a ceramic electrostatic
chuck with built-in heater is relatively low as in the etching process,
the material of the insulating layer thereof can be alumina and the like
(see, for example, Japanese Patent Kokai No. 59-124140) while, when the
working temperature is very high as in the CVD process, a highly
refractory material such as pyrolytic boron nitride and the like is used
for the insulating layer thereof (see, for example, Japanese Patent Kokai
No. 4-358074, No. 5-109676 and No. 5-129210).
Although, as is described in several literatures, the electrostatic
attracting force of a ceramic electrostatic chuck is increased as the
volume resistivity of the insulating layer is decreased, the insulating
layer in an electrostatic chucking device must have a volume resistivity
in the range from 10.sup.10 to 10.sup.13 ohm.multidot.cm or, preferably,
around 10.sup.11 ohm.multidot.cm because, when the volume resistivity of
the insulating layer is too low, eventual break of the device is sometimes
caused due to a leak current.
When alumina is used as the material of the insulating layer of a ceramic
electrostatic chuck working in an intermediately high temperature range of
500.degree. C. to 650.degree. C., for example, the resistivity of the
insulating layer is so low that break of the device is sometimes
unavoidable due to the leak current while, when pyrolytic boron nitride is
used for the insulating layer, the resistivity of the insulating layer is
too high to give a sufficiently high electrostatic attractive force of,
for example, 100 to 500 gf/cm.sup.2.
SUMMARY OF THE INVENTION
The present invention accordingly has an object to provide, by solving the
above described problems in the prior art, a novel ceramic-based
electrostatic chuck with built-in heater capable of satisfactorily working
at an intermediately high temperature in the range from 500.degree. to
650.degree. C.
Thus, the ceramic-based electrostatic chuck with built-in heater of the
invention is an integral body which comprises:
(a) a base body of a sintered blend of boron nitride and aluminum nitride
having flat surfaces;
(b) a first electroconductive layer of pyrolytic graphite formed on one of
the flat surfaces of the base body to serve as an electric resistance
heater element;
(c) a first insulating layer of pyrolytic boron nitride formed on the first
electroconductive layer to serve as an insulating layer;
(d) a second electroconductive layer of pyrolytic graphite on the surface
of the base body opposite to the surface on which the first
electroconductive layer is formed; and
(e) a second insulating layer of a pyrolytic composite nitride of boron and
silicon, of which the content of silicon is in the range from 1 to 10% by
weight or, preferably, from 5 to 9% by weight formed on the second
electroconductive layer to serve as an insulating layer.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic vertical cross sectional view of the inventive
ceramic electrostatic chuck with built-in heater.
FIG. 2 is a graph showing the volume resistivity of pyrolytic boron nitride
(curve I), pyrolytic composite nitride of boron and silicon containing 1%
by weight of silicon (curve II), pyrolytic composite nitride of boron and
silicon containing 10% by weight of silicon (curve III) and alumina (curve
IV) as a function of temperature up to 1000.degree. C.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As is described above, the present invention provides a ceramic
electrostatic chuck with built-in heater, which is an integral body
comprising a substrate plate made from a sintered body of a mixture of
boron nitride and aluminum nitride provided on one flat surface with a
first electroconductive layer of pyrolytic graphite to serve as a
heat-generating layer and a first insulating layer of pyrolytic boron
nitride formed on the first electroconductive layer and also provided on
the other flat surface with a second electroconductive layer of pyrolytic
graphite to serve as the electrodes for electrostatic chucking and a
second insulating layer made from pyrolytic composite nitride of boron and
silicon containing 1 to 10% by weight of silicon as formed on the second
electroconductive layer.
To explain it in more detail, the inventors have conducted extensive
investigations, in connection with a ceramic electrostatic chuck with
built-in heater having insulating layers of pyrolytic boron nitride, to
develop a means for preventing the decrease in the electrostatic
attracting force within an intermediate temperature range and, as a
result, have arrived at an unexpected discovery that this problem can be
solved by providing a substrate or a sintered ceramic base body of a
mixture of boron nitride and aluminum nitride, on one of the flat
surfaces, with a first electroconductive layer of pyrolytic graphite to
serve as a resistance heat-generating layer and a first insulating layer
of pyrolytic boron nitride and, on the other of the flat surfaces, with a
second electroconductive layer of pyrolytic graphite to serve as the
electrodes to generate the electrostatic attracting force and a second
insulating layer of a composite pyrolytic nitride of boron and silicon in
which the content of silicon is in a specified range by weight.
FIG. 1 of the accompanying drawing schematically illustrates a typical
example of the inventive electrostatic chuck with built-in heater by a
vertical cross sectional view. In this figure, the base body 1 made from a
sintered body of a mixture of boron nitride and aluminum nitride is
provided on one of the flat surfaces with a first electroconductive layer
2 of pyrolytic graphite to serve as a heat-generating layer by electric
resistance and, further thereon, with a first insulating layer 3 of
pyrolytic boron nitride and, on the other of the flat surfaces opposite to
the first electroconductive layer 2, with a second electroconductive layer
4 of pyrolytic graphite to serve as the electrodes for the generation of
an electrostatic attracting force and, further thereon, with a second
insulating layer 5 which is made from pyrolytic composite nitride of boron
and silicon containing 1 to 10% by weight of silicon.
In the conventional ceramic electrostatic chucks with built-in heater, both
of the insulating layers, one, on the first electroconductive layer of
pyrolytic graphite to serve as an electric resistance layer for heat
generation formed on a sintered base body of a mixture of boron nitride
and aluminum nitride and, the other, on the second electroconductive layer
of pyrolytic graphite to serve as electrodes for electrostatic chucking on
the other surface of the base body are formed each from pyrolytic boron
nitride alone.
A problem in the above mentioned conventional structure of electrostatic
chucks with built-in heater is that, as a consequence of the high volume
resistivity of the pyrolytic boron nitride forming the insulating layers
at a temperature in the range from 500.degree. to 650.degree. C., a
sufficiently high electrostatic attracting force can hardly be obtained in
this temperature range so that the workpiece such as a silicon wafer
cannot be attracted to and held on the chucking surface with a high
contacting force so that the temperature distribution in the workpiece is
consequently sometimes not uniform enough resulting in low reproducibility
in the performance of the semiconductor devices prepared thereby.
In contrast thereto, the most characteristic feature of the inventive
electrostatic chuck with built-in heater consists in that one of the
insulating layers on the second electroconductive layer of pyrolytic
graphite to serve as the electrostatic electrodes is formed from a
pyrolytic composite nitride of boron and silicon containing from 1 to 10%
by weight of silicon. By the use of this very unique material for the
insulating layer, a fully high electrostatic attracting force can be
obtained even at a temperature in the range from 500.degree. to
650.degree. C. to give a very uniform temperature distribution in the
workpiece thereon without any risk of damage on the workpiece due to a
leak current through the insulating layer so that the yield of acceptable
products can be greatly increased.
When the content of silicon is less than 1% by weight in the second
insulating layer, the above mentioned improvements in the performance of
the electrostatic chuck can hardly be obtained as a matter of course
while, when the content of silicon is too large, an undue decrease is
caused in the electric resistivity of the insulating layer within the
above mentioned temperature range resulting in an eventual damage on the
workpiece held by the electrostatic chuck due to a leak current.
FIG. 2 of the accompanying drawing is a graph showing the volume
resistivity of various materials as a function of temperature in the range
from 100.degree. to 1000.degree. C., of which the curve I is for pyrolytic
boron nitride, curve II is for a pyrolytic composite nitride of boron and
silicon containing 1% by weight of silicon, curve III is for a pyrolytic
composite nitride of boron and silicon containing 10% by weight of silicon
and curve IV is for aluminum oxide. It is understood from this graph that
a volume resistivity of 10.sup.10 to 10.sup.13 ohm.multidot.cm can be
obtained in the temperature range from 500.degree. to 650.degree. C. when
the content of silicon is from 1 to 10% by weight in a pyrolytic composite
nitride of boron and silicon.
The base body of the inventive electrostatic chuck with built-in heater can
be rather conventional and the material thereof is a sintered body of a
mixture of boron nitride and aluminum nitride as disclosed in Japanese
Patent Kokai 7-10665 though not particularly limitative thereto. The
mixing ratio of boron nitride and aluminum nitride is usually in the range
from 1:0.05 to 1:1 by weight. When the proportion of aluminum nitride is
too large, the thermal expansion coefficient of the sintered body is too
high as compared with pyrolytic graphite while, when the proportion of
aluminum nitride is too small, the thermal expansion coefficient of the
sintered body is too low as compared with pyrolytic graphite.
The electroconductive layers of pyrolytic graphite to serve as the
resistance heater element and electrostatic electrodes for chucking can be
formed by the deposition of graphite produced by pyrolysis of, for
example, methane at a temperature of 1900.degree. to 2200.degree. C. under
a pressure of around 5 Torr on the respective flat surfaces of the base
body. The thickness of the electroconductive layers of pyrolytic graphite
is, preferably, in the range from 10 to 300 .mu.m. When the thickness is
too small, the layers have only insufficient mechanical strengths while,
when the thickness is too large, the layer eventually causes falling from
the substrate surface by exfoliation.
The first insulating layer on the first electroconductive layer of
pyrolytic graphite for the resistance heater element is formed from
pyrolytic boron nitride according to a known procedure in which, for
example, a gaseous mixture of ammonia and boron trichloride in a mixing
ratio of 4:1 is subjected to pyrolysis at a temperature of 1800.degree. to
2000.degree. C. under a pressure of 10 Torr to deposit boron nitride on
the pyrolytic graphite layer. The thickness of the first insulating layer
is, preferably, in the range from 50 to 500 .mu.m the same reasons as in
the electroconductive layers of pyrolytic graphite.
The second insulating layer on the second electroconductive layer of
pyrolytic graphite for the electrostatic electrodes is necessarily formed
from a pyrolytic composite nitride of boron and silicon in which the
content of silicon is in the range from 1 to 10% by weight. Such a
composite nitride layer can be formed by the pyrolysis of a gaseous
mixture of ammonia, boron trichloride and silicon tetrachloride in a
volume ratio of 40:9:1 to 24:5:1 at a temperature in the range from
1600.degree. to 2000.degree. C. under a pressure of 5 to 10 Torr to
deposit the pyrolytic nitride on the surface of the pyrolytic graphite
layer. The thickness of this second insulating layer is, preferably, in
the range from 50 to 500 .mu.m. When the thickness thereof is too small,
troubles are sometimes caused due to dielectric breakdown in the
insulating layer while, when the thickness is too large, an undue decrease
is caused in the electrostatic attracting force for holding the workpiece
mounted on the insulating layer due to the increase in the distance from
the electrostatic electrodes to the workpiece.
It is of course optional that the first insulating layer on the first
electroconductive layer of pyrolytic graphite is formed not from
substantially pure pyrolytic boron nitride as is described above but from
the same composite pyrolytic nitride of boron and silicon as the second
insulating layer so that an advantage is obtained by the simplification
and by the improvement in the productivity of the manufacturing process of
the inventive electrostatic chucks because both of the two insulating
layers can be formed at one time by using the same pyrolysis system.
In the following, the present invention is described in more detail by way
of an example.
EXAMPLE
A disc-formed base body for an electrostatic chuck with built-in heater
having a diameter of 200 mm and a thickness of 10 mm was prepared by
sintering a powder blend of boron nitride and aluminum nitride in a weight
proportion of 3:1 at a temperature of 1900.degree. C. under a compressive
force of 200 kgf/mm.sup.2.
The above prepared base body was brought into a pyrolysis chamber into
which methane was introduced and pyrolyzed therein at a temperature of
2200.degree. C. under a pressure of 5 Torr to deposit pyrolytic graphite
on both of the flat surfaces of the base body in a thickness of 100 .mu.m.
The pyrolytic graphite layer on one of the flat surfaces of the base body
was mechanically worked by shaving into the pattern of an electric
resistance heater dement to serve as the built-in heater and the pyrolytic
graphite layer on the other flat surface was mechanically worked by
shaving into the pattern of electrodes for electrostatic chucking.
Further, the base body provided with the pyrolytic graphite layers was
brought into a pyrolysis chamber into which a gaseous mixture of ammonia,
boron trichloride and silicon tetrachloride in a mixing ratio of 32:7:1 by
volume was introduced and subjected to pyrolysis therein at a temperature
of 1600.degree. C. under a pressure of 5 Torr to deposit pyrolytic
composite nitride of boron and silicon on the surface of the respective
pyrolytic graphite layers in a thickness of 200 .mu.m to serve as an
insulating layer. The content of silicon in the insulating layers was
found by analysis to be 7% by weight.
A semiconductor silicon wafer having a diameter of 200 mm and a thickness
of 0.6 mm was mounted on the second insulating layer of the thus prepared
electrostatic chuck with built-in heater and subjected to a process for
the deposition of polysilicon layer thereon under heating at around
600.degree. C. The temperature distribution on the surface of the silicon
wafer during the process was very narrow with a difference of 10.degree.
C. between the spots of the highest and lowest temperatures. The thickness
of the polysilicon layer thus formed was also uniform to be within a range
of 0.10 to 0.11 .mu.m.
COMPARATIVE EXAMPLE
Another ceramic electrostatic chuck with built-in heater was prepared in
just the same way as in the Example described above excepting omission of
silicon tetrachloride in the gaseous mixture for the pyrolytic deposition
of the insulating layers.
The evaluation test of this electrostatic chuck was undertaken also in the
same manner as in the Example to find that the difference in the
temperature of the silicon wafer mounted thereon was 26.degree. C. between
the spots of the highest and lowest temperatures and the thickness of the
polysilicon layer was distributed in the range from 0.08 to 0.12 .mu.m.
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