U.S. patent application number 10/876939 was filed with the patent office on 2005-08-25 for thermal treatment equipment.
Invention is credited to Endo, Tomoyoshi, Fukuda, Kenji, Nishizawa, Shinichi, Senzaki, Junji, Yashima, Teruyuki.
Application Number | 20050183820 10/876939 |
Document ID | / |
Family ID | 34836460 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050183820 |
Kind Code |
A1 |
Fukuda, Kenji ; et
al. |
August 25, 2005 |
Thermal treatment equipment
Abstract
Thermal treatment equipment for rapidly heating a SiC substrate
having a diameter of several inches or larger to a temperature as
high as 1200.degree. C. or higher with a high in-plane evenness by
heating a peripheral zone of a substrate using high frequency
induction and by heating a central zone of the substrate using
infrared lamps while the substrate and a stage thereof are covered
with a shield plate.
Inventors: |
Fukuda, Kenji; (Tsukuba-shi,
JP) ; Senzaki, Junji; (Tsukuba-shi, JP) ;
Nishizawa, Shinichi; (Tsukuba-shi, JP) ; Endo,
Tomoyoshi; (Tokyo, JP) ; Yashima, Teruyuki;
(Sagamihara-shi, JP) |
Correspondence
Address: |
PRICE HENEVELD COOPER DEWITT & LITTON, LLP
695 KENMOOR, S.E.
P O BOX 2567
GRAND RAPIDS
MI
49501
US
|
Family ID: |
34836460 |
Appl. No.: |
10/876939 |
Filed: |
June 24, 2004 |
Current U.S.
Class: |
156/345.27 ;
118/724; 156/345.37 |
Current CPC
Class: |
F27B 5/04 20130101; F27B
17/0025 20130101; C04B 35/565 20130101; C04B 35/64 20130101; H01L
21/67115 20130101 |
Class at
Publication: |
156/345.27 ;
118/724; 156/345.37 |
International
Class: |
C23F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 24, 2004 |
JP |
2004-47295 |
Claims
The invention claimed is:
1. A thermal treatment equipment comprising: a chamber for allowing
thermal treatment to be carried out in vacuum or various gas
atmospheres; an electrically conductive sample stage positioned
within said chamber for receiving a sample to be treated; a high
frequency coil surrounding said sample stage; an infrared generator
consisting of at least one infrared waveguide quartz column placed
above or below said sample; an infrared lamp and a rotary
elliptical reflector placed on one end of said infrared waveguide
quartz column; a coaxial double-wall quartz tube placed inside said
high frequency coil for receiving cooling water in said coaxial
double-wall quartz tube; and wherein said infrared lamp is water or
air cooled by cooling water or air flowing outside said infrared
lamp.
2. The thermal treatment equipment as defined in claim 1, wherein a
quartz plate is interposed between a sample and said infrared
waveguide quartz column.
3. The thermal treatment equipment as defined in any one of claims
1 or 2, wherein a sample and said sample stage are covered with a
shield plate.
4. The thermal treatment equipment as defined in any one of claims
1 through 3, wherein said sample stage is covered with an
electrically conductive shield plate provided with a gap having a
dimension in a range of from about 1 mm to about 30 mm.
5. The thermal treatment equipment as defined in any one of claims
1 through 4, wherein said sample stage is made of one of tungsten,
molybdenum or tantalum.
6. The thermal treatment equipment as defined in any one of claims
1 through 4, wherein said shield plate is made of one of tungsten,
molybdenum or tantalum.
7. The thermal treatment equipment as defined in any one of claims
1 through 4, wherein said sample stage is made of one of carbon or
SiC coated carbon.
8. The thermal treatment equipment as defined in any one of claims
1 through 4, wherein said shield plate is made of one of carbon or
SiC coated carbon.
9. The thermal treatment equipment as defined in any one of claims
1 through 8, wherein the high frequency wave has a frequency of
less than about 50 kHz.
10. The thermal treatment equipment as defined in any one of claims
1 through 9 and further comprising a mechanism adapted to adjust a
distance between one end surface of said quartz column and a sample
in a range of from about 0.5 mm to about 20 mm.
11. The thermal treatment equipment as defined in any one of claims
1 through 10 and further comprising a sample temperature control
means adapted to measure a temperature of one of the sample stage
or a sample using one of a pyrometer or a thermocouple to thereby
control the value of voltage or current applied to the infrared
lamp or the high frequency coil.
12. The thermal treatment equipment as defined in any one of claims
1 through 11, wherein said quartz column is tilted.
13. The thermal treatment equipment as defined in any one of claims
1 through 12, wherein said equipment is programmed so that a SiC
substrate is heated from a room temperature to about 1200.degree.
C. or higher in from about 10 seconds to about 5 minutes, then
maintained at such temperature for about 10 seconds to about 10
minutes and thereafter the SiC substrate is cooled to a temperature
lower than about 1200.degree. C. in about 10 seconds to about 30
minutes.
14. The thermal treatment equipment as defined in claim 13, wherein
said equipment is programmed so that the SiC substrate is
previously heated to a temperature lower than about 1200.degree.
C., then heated from a room temperature to about 1200.degree. C. or
higher in about 10 seconds to about 5 minutes and thereafter cooled
to a temperature lower than about 1200.degree. C. in about 10
seconds to about 30 minutes.
15. Thermal treatment equipment comprising: a chamber for allowing
thermal treatment to be carried out in vacuum or various gas
atmospheres; an electrically conductive sample stage positioned
within said chamber for receiving a sample to be treated; a high
frequency coil surrounding said sample stage; an infrared generator
consisting of at least one infrared waveguide quartz column
positioned to direct infrared energy toward said sample; and a
temperature control circuit coupled to said coil and said infrared
waveguide quartz column and programmed to heat a SiC substrate from
a room temperature to about 1200.degree. C. or higher in from about
10 seconds to about 5 minutes, then maintained at such temperature
for about 10 seconds to about 10 minutes and thereafter the SiC
substrate is cooled to a temperature lower than about 1200.degree.
C. in about 10 seconds to about 30 minutes.
16. Thermal treatment equipment comprising: a chamber for allowing
thermal treatment to be carried out in vacuum or various gas
atmospheres; an electrically conductive sample stage positioned
within said chamber for receiving a sample to be treated; a high
frequency coil surrounding said sample stage; an infrared generator
consisting of at least one infrared waveguide quartz column
positioned to direct infrared energy toward said sample; and a
temperature control circuit coupled to said coil and said infrared
waveguide quartz column and programmed so that said SiC substrate
is previously heated to a temperature lower than about 1200.degree.
C., then heated from a room temperature to about 1200.degree. C. or
higher in about 10 seconds to about 5 minutes and thereafter cooled
to a temperature lower than about 1200.degree. C. in about 10
seconds to about 30 minutes.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to thermal treatment equipment
used in a manufacturing process in which a thermal treatment must
be achieved in as short a time as possible, for example, by the
process for activating thermal treatment after ion implantation of
impurities into SiC.
[0002] After impurities such as phosphor or nitrogen have been ion
implanted into SiC substrate, a thermal treatment at a temperature
as high as 1500.degree. C. or higher is necessary in order to
generate an impurities activating carrier. For such thermal
treatment, it has already been reported to use a resistive heating
oven, however, such resistive heating ovens inconveniently take an
unacceptably long time until a temperature rises to about
1500.degree. C. or higher. Furthermore, a duration of approximately
30 minutes is required for an effective thermal treatment and
inevitably Si evaporates from the SiC substrate surface, resulting
in irregularities on the substrate surface. In addition, not only
Si but also the impurities are evaporated, so the impurities ion
implanted region exhibits an unacceptably high resistance value,
and it is not possible to fabricate a normal SiC element. Thermal
treatment using high frequency heating has also been reported,
however, such method may lead to uneven temperature distribution
since the substrate is heated from its peripheral zone. High-speed
thermal treatment equipment and a method using an infrared lamp
have also been employed. According to this method, the temperature
can rise to about 1700.degree. C. in one minute and evaporation of
Si from SiC substrate surface restrained. While it is possible for
this method to achieve the temperature rise in a desired short time
by convergence of infrared rays for heating, application of this
method is limited to thermal treatment of the SiC substrate having
a size on the order of 1 cm.sup.2. In other words, this method is
not suitable for mass production of SiC elements. In view of such
problem encountered by the method and equipment of well known art,
there is a serious need for a thermal treatment equipment so
improved that even a SiC substrate having a diameter of about 2
inches or larger can be heated to a desired high temperature in a
short time with a practically even temperature distribution.
[0003] Kazuo Arai and Sadafumi Yoshida: "Principle and Application
of SiC element" published by Ohmsha, p. 110.
SUMMARY OF THE INVENTION
[0004] As has been described above, the thermal treatment equipment
relying on the infrared lamp of prior art cannot be used for mass
production of the SiC element from the SiC substrate having a
diameter of several centimeters or larger. While the thermal
treatment method using the high frequency oven has already been
proposed, this method results in that the temperature in the
peripheral zone is relatively high while the temperature in the
central zone is relatively low. Therefore, the temperature
distribution becomes significant and, in the SiC substrate, an area
in which the impurities are adequately activated and an area in
which the impurities are not adequately activated appear in the SiC
substrate. Eventually, the in-plane unevenness of the electric
properties of the SiC element becomes so serious that such
equipment cannot be used for mass production of or for
industrialization of SiC elements.
[0005] In view of the problem as has been described above, it is a
principal object of the present invention to provide a thermal
treatment equipment improved so that a SiC substrate having a
diameter of several inches or larger can be rapidly heated to a
temperature as high as about 1200.degree. C. or higher with a high
in-plane evenness by heating a peripheral zone of a substrate using
high frequency induction and by heating a central zone of the
sample using infrared lamps while the substrate and a stage thereof
are covered with a shield plate.
[0006] In the case of heating the substrate at a high temperature
by infrared lamps used with a quartz column, the conventional
equipment has been accompanied also with another problem such that
various impurities generated from the substrate stage may cling to
the quartz column and obstruct transmission of the infrared rays.
In view of this problem, it is also an object of the present
invention to provide a thermal treatment equipment improved so that
a quartz plate is interposed between the substrate or the stage
thereof and the quartz column and thereby various impurities
generated from the stage of the substrate are prevented from
clinging to the quartz column.
[0007] In the case of heating by the infrared lamp, the temperature
of the substrate becomes higher in the central zone than in the
peripheral zone, as illustrated by FIG. 1(A). In the case of
heating by the high frequency wave heating, on the other hand, the
temperature of the substrate becomes higher in the peripheral zone
than in the central zone because the peripheral zone of the
substrate is primarily heated as illustrated by FIG. 1(B). The
temperature distribution can be substantially uniformized first by
heating the substrate by simultaneously using both the infrared
lamp and the high frequency wave as illustrated by FIG. 1(C).
[0008] The object set forth above is achieved, according to the
present invention, by thermal treatment equipment comprising a
vacuum chamber allowing thermal treatment to be carried out in
vacuum or various gas atmospheres, an electrically conductive
sample stage provided within the vacuum chamber, and a sample
placed on the sample stage. A high frequency coil surrounds the
sample stage, and an infrared generator, consisting of a single or
plural infrared waveguide quartz column(s), is placed above and/or
below the sample. An infrared lamp and a rotary elliptical
reflector both placed on one end of the infrared waveguide quartz
column, and a coaxial double-wall quartz tube is placed inside the
high frequency coil so that cooling water may flow between this
coaxial double wall quartz tube, wherein the infrared lamp is water
or air cooled by cooling water or air flowing outside the infrared
lamp in order to prevent the sample from being heated by the
infrared lamp.
[0009] The present invention may be implemented also in various
preferred manners. In the thermal treatment equipment according to
claim 1, a quartz plate is interposed between the sample and the
infrared waveguide quartz column as also described in claim 2. In
the thermal treatment equipment according to claim 1 or 2, the
sample and the sample stage are covered with a shield plate as
described in claim 3.
[0010] In the thermal treatment equipment according to any one of
claims 1 through 3, the sample stage is covered with an
electrically conductive shield plate provided with a gap having a
dimension in a range of about 1 mm to about 30 mm as described in
claim 4.
[0011] In the thermal treatment equipment according to any one of
claims 1 through 4, one or both of the sample stage and the shield
plate is or are made of tungsten, molybdenum or tantalum as
described in claim 5.
[0012] In the thermal treatment equipment according to any one of
claims 1 through 4, one or both of the sample stage and the shield
plate is or are made of carbon or SiC coated carbon as described in
claim 6.
[0013] In the thermal treatment equipment according to any one of
claims 1 through 6, the high frequency wave has a frequency less
than about 50 kHz as described in claim 7.
[0014] In the thermal treatment equipment according to any one of
claims 1 through 7, further comprising a mechanism adapted to
adjust a distance between one end surface of the quartz column and
the sample in a range from about 0.5 mm to about 20 mm as described
in claim 8.
[0015] In the thermal treatment equipment according to any one of
claims 1 through 8, further comprising a sample temperature control
means adapted to measure a temperature of the sample stage or the
sample itself by a pyrometer or a thermocouple and thereby control
the voltage or current applied to the infrared lamp or the high
frequency coil as described in claim 9.
[0016] In the thermal treatment equipment according to any one of
claims 1 through 9, the quartz column is arranged in a tilted
posture as described in claim 10.
[0017] In the thermal treatment equipment according to any one of
claims 1 through 10, wherein said equipment is programmed so that
the SiC substrate is heated from a room temperature to about
1200.degree. C. or higher in from about 10 seconds to about 5
minutes, maintained at such temperature for about 10 seconds to
about 10 minutes and thereafter the SiC substrate is cooled to a
temperature lower than about 1200.degree. C. in about 10 seconds to
about 30 minutes as described in claim 11.
[0018] In, the thermal treatment equipment according to claim 11,
said equipment is programmed so that the SiC substrate is
previously heated to a temperature lower than about 1200.degree.
C., then heated from a room temperature to about 1200.degree. C. or
higher in about 10 seconds to about 5 minutes and thereafter cooled
to a temperature lower than about 1200.degree. C. in about 10
seconds to about 30 minutes as described in claim 12.
[0019] The present invention has a unique construction as has been
described above and provides an effect as follows.
[0020] The thermal treatment equipment as defined in claim 1
comprises a vacuum chamber allowing thermal treatment to be carried
out in vacuum or various gas atmospheres, an electrically
conductive sample stage provided within the vacuum chamber, and a
sample placed on the sample stage. A high frequency coil surrounds
the sample stage, and an infrared generator consisting of a single
or plural infrared waveguide quartz column(s) is placed above
and/or below the sample. An infrared lamp and a rotary elliptical
reflector are both placed on one end of the infrared waveguide
quartz column, and a coaxial double-wall quartz tube placed inside
the high frequency coil so that cooling water may flow between this
coaxial double wall quartz tube. The infrared lamp is water or air
cooled by cooling water or air flowing outside the infrared lamp in
order to prevent the sample from being heated by the infrared lamp.
Such equipment allows a temperature to rise from a room temperature
up to about 1800.degree. C. as rapidly as in about 1 minute and to
ensure a temperature distribution having unevenness as negligible
as .+-.50.degree. C. without any anxiety of equipment
destruction.
[0021] The thermal treatment equipment defined in claim 2
corresponding to the equipment according to claim 1, wherein a
quartz plate is interposed between the sample and the infrared
waveguide quartz column wherein the quartz plate prevents any
impurities from clinging to the end surface of the infrared
waveguide quartz column, so the infrared irradiation can be carried
out for a long period without exchanging the infrared waveguide
quartz column. The quartz plate is exchanged when it is
desired.
[0022] The thermal treatment equipment defined in claim 3
corresponding the equipment according to claim 1 or 2, wherein the
sample and the sample stage are covered with a shield plate which
allows a temperature to rise up to about 1200.degree. C. or higher
rapidly.
[0023] The thermal treatment equipment defined in claim 4
corresponding to the equipment according to any one of claims 1
through 3, wherein the sample stage is covered with an electrically
conductive shield plate provided with a gap having a dimension in a
range of about 1 mm to about 30 mm is effective to prevent an
induction heating by the high frequency wave and to restrain a
temperature rise of the coaxial double quartz tube due to a
temperature rise of the electrically conductive shield plate.
[0024] The thermal treatment equipment defined in claim 5
corresponding to the equipment according to any one of claims 1
through 4, wherein one or both of the sample stage and the shield
plate are made of tungsten, molybdenum or tantalum is effective to
prevent the shield plate from being molten even at a high
temperature and thereby prevents the shield plate from changing
from its initial shape.
[0025] The thermal treatment equipment defined in claim 6
corresponding to the equipment according to any one of claims 1
through 4, wherein one or both of the sample stage and the shield
plate are made of carbon or SiC coated carbon which allows the
thermal treatment to be stabilized even at a high temperature.
[0026] The thermal treatment equipment defined in claim 7
corresponding to the equipment according to any one of claims 1
through 6, wherein the high frequency wave has a frequency less
than about 50 kHz promotes the high frequency wave to propagate
into the sample so that a zone of the sample in the vicinity of its
center can be sufficiently heated so as to minimize unevenness of
the temperature distribution.
[0027] The thermal treatment equipment defined in claim 8
corresponding to the equipment according to any one of claims 1
through 7, further comprising a mechanism adapted to adjust a
distance between one end surface of the quartz column and the
sample in a range from about 0.5 mm to about 20 mm improve a
heating effect of the infrared rays.
[0028] The thermal treatment equipment defined in claim 9
corresponding to the equipment according to any one of claims 1
through 8, further comprising a sample temperature control means
adapted to measure a temperature of the sample stage or the sample
itself by a pyrometer or a thermocouple and control the voltage or
current applied to the infrared lamp or the high frequency coil to
allow outputs of the infrared lamp and the high frequency wave to
be controlled and, thereby, the temperature of the substrate to be
controlled.
[0029] The thermal treatment equipment defined in claim 10
corresponding to the equipment according to any one of claims 1
through 9, wherein the quartz column is tilted to allow many quartz
columns to be used and, thereby, the infrared irradiation area to
be enlarged.
[0030] The thermal treatment equipment defined in claim 11
corresponds to the equipment according to any one of claims 1
through 10, wherein said equipment is programmed so that the SiC
substrate is heated from a room temperature to about 1200.degree.
C. or higher in about 10 seconds to about 5 minutes, then
maintained at such temperature for about 10 second to about 10
minutes and thereafter the SiC substrate is cooled to a temperature
lower than about 1200.degree. C. in about 10 seconds to about 30
minutes. With such an arrangement, a resistance value of the SiC
substrate ion-implanted with impurities such as phosphor, nitrogen,
aluminum or boron can be adequately lowered and, at the same time,
evaporation of Si from the SiC substrate leading to the undesirable
surface irregularities can be prevented. In this way, a high
quality SiC element can be manufactured.
[0031] The thermal treatment equipment defined in claim 12
corresponds to the equipment according to claim 11, wherein said
equipment is programmed so that the SiC substrate is previously
heated to a temperature lower than about 1200.degree. C., then
heated from a room temperature to about 1200.degree. C. or higher
in about 10 seconds to about 5 minutes and thereafter cooled to a
temperature lower than about 1200.degree. C. in about 10 seconds to
about 30 minutes. With such an arrangement also, a resistance value
of the SiC substrate ion-implanted with impurities such as
phosphor, nitrogen, aluminum and boron can be adequately lowered
and, at the same time, evaporation of Si from the SiC substrate
leading to the undesirable surface irregularities can be prevented.
In this way, high quality SiC element can be manufactured.
[0032] These and other features, objects and advantages of the
present invention will become apparent upon reading the following
description thereof together with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 and FIGS. 1(A)-1(C) are a schematic drawing and
aligned temperature graphs which illustrate the temperature
distribution through a substrate using the system of the present
invention;
[0034] FIG. 2 is a vertical cross-sectional view of the thermal
treatment equipment according to the invention;
[0035] FIG. 3A is an enlarged vertical cross-sectional view of the
shielding structure;
[0036] FIG. 3B is a top plan view of the shield shown in FIG.
3A;
[0037] FIGS. 4(1)-4(6) are schematic views which illustrate various
placements of the infrared lamps;
[0038] FIG. 5 is a graph plotting the measured progression of
temperature rise in a substrate having a diameter of about 2 inches
with use of the high frequency heating alone; and
[0039] FIG. 6 is a graph plotting the measured progression of
temperature rise in a substrate having a diameter of about 2 inches
using both high frequency heating and infrared heating.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0040] FIGS. 1 and 1(A)-1(C) illustrate the temperature
distribution appearing as a result of heating by the infrared
heating and/or the high frequency heating. FIG. 1(A) illustrates
the temperature distribution appearing as a result of the infrared
heating alone. FIG. 1(B) illustrates the temperature distribution
appearing as a result of the high frequency heating alone, and FIG.
1(C) illustrates the temperature distribution as a result of both
the infrared heating and the high frequency heating. FIG. 2 is a
sectional view of a thermal treatment equipment according to the
invention. FIG. 3 illustrates an example of a shielding structure.
While the illustrated shielding structure is made of tantalum, this
shielding structure may be made also of tungsten, molybdenum,
carbon or SiC coated carbon. FIG. 4 illustrates placement of the
infrared lamps. As illustrated, a plurality of infrared lamps are
placed above and/or below a sample stage. FIG. 5 is a graph
plotting a measured progression of temperature rise in the
substrate having a diameter of 2 inches with use of the high
frequency heating alone, in which a solid line indicates the
temperature in the central zone and the broken line indicates the
temperature in the peripheral zone. FIG. 6 is a graph plotting a
measured progression of temperature rise in the substrate having a
diameter of 2 inches with use of both the high frequency heating
and the infrared heating. In this graph, the progression of
temperature rise having been measured until first approximately 50
seconds elapse indicates the result obtained by use of the infrared
heating alone, and the progression of temperature rise having been
measured thereafter indicates the result obtained by use of both
the infrared heating and the high frequency heating. The solid line
indicates the temperature in the central zone and the broken line
indicates the temperature in the peripheral zone of the
substrate.
[0041] In the following description, structural elements are
identified by the following reference numerals:
REFERENCE NUMERALS USED IN THE DRAWINGS
[0042] 1 infrared lamp
[0043] 2 rotary elliptical reflector
[0044] 3 infrared waveguide quartz column
[0045] 3A end surface
[0046] 4 vacuum chamber
[0047] 5 cooling water canal
[0048] 6 quartz tube
[0049] 7 high frequency coil
[0050] 8 vacuum pumping exhaust port
[0051] 9 infrared waveguide quartz column rise and fall
mechanism
[0052] 10 sample stage
[0053] 11 shield plate
[0054] 12 sample
[0055] 13 quartz plate
[0056] 14 temperature sensor pickup port
[0057] 15 infrared temperature sensor insertion port
[0058] 16 gas introducing port
[0059] 17 gas exhausting port
[0060] 18 inlet
[0061] 19 outlet
[0062] 20 gap
[0063] 21 opening
[0064] 22 lid
[0065] 23 temperature control
[0066] Referring to FIG. 2, infrared rays emitted from an infrared
lamp 1 are collected by a rotary elliptical reflector 2, then
guided through an infrared waveguide quartz column 3 to an end
surface 3a of the quartz column 3. A sample 12 and a sample stage
10 are irradiated with the infrared rays coming from the end
surface of the quartz column 3. Void space is defined around the
rotary elliptical reflector 2 and this void space is provided with
an inlet 18 and an outlet of cooling water so that the reflector 2
may be water cooled. Alternatively, it is possible to construct the
void space so that the water cooling may be replaced by air
cooling. The sample stage 10, shown also in FIG. 1, must be formed
of an electrically conductive material. According to the invention,
this requirement is met by forming the sample stage 10 of a
metallic material having a sufficiently high melting point to
resist a predetermined high temperature, e.g., tungsten, molybdenum
and tantalum. Alternatively, it is possible to form the sample
stage 10 by high purity carbon substantially free from metallic
impurities, e.g., titanium, vanadium, chromium, manganese, iron,
cobalt, nickel or copper, or by high purity carbon from which
nitrogen, boron, aluminum or phosphor each being apt to become
N-type or P-type impurities was removed as perfectly as possible,
or by carbon having its surface coated with SiC.
[0067] Heat conduction from the sample stage 10 heated by infrared
rays causes the temperature of sample 12, such as the SiC
substrate, to rise. On the other hand, inductive heating by high
frequency wave is applied to a high frequency coil 7 from a high
frequency oscillator and heats by induction the sample stage 10,
thereby causing the temperature of sample 12 to rise through
thermal conduction. In the case of the sample 12 which is
electrically conductive, the sample 12 itself also is heated by an
inductive heating effect by high frequency RF energy. A shield
plate 11 is provided in order to restrain heat dissipation. If this
shield plate 11 is made of electrically conductive material, the
temperature of shield plate 11 itself would rise similarly to the
sample stage 10 and heat the members surrounding the sample stage
10, such as the quartz tube 6 for cooling water, eventually leading
to destruction by melting or fissure. To avoid such an undesirable
result, shield plate 11 is made similarly to the sample stage 10 of
a high melting point metallic material, such as tungsten,
molybdenum or tantalum or high purity carbon or SiC coated carbon.
A gap 20, as illustrated in FIGS. 3A and 3B, is provided to
interrupt an induced current. According to the invention, such gap
has a dimension of about 4 mm. While the gap may be enlarged to
enhance the effect of interrupting the induced current, the
shielding effect is correspondingly deteriorated. From this
viewpoint, the gap should be dimensioned to be on the order of
about 1 mm or larger but selected to maintain the desired shielding
effect. The shield plate 11 may be provided with a lid 22 having a
circular opening 21 through which the cylindrical infrared
waveguide quartz column 3 is inserted.
[0068] To maximize an effect of infrared irradiation, a raising and
lowering mechanism 9 vertically moves the infrared waveguide quartz
column 3 so that the temperature may rise as rapidly as possible
and the temperature may be distributed as evenly as possible. If
the end surface 3a of the infrared waveguide quartz column 3
smears, transmission of the infrared rays as well as rise of the
temperature will be obstructed. To solve this problem, a quartz
plate 13 is placed on the sample stage 10 and thereby prevents any
impurities coming from the sample stage 10 to contaminate the end
surface 3a of the infrared waveguide quartz column 3. Taking into
account the fact that the thermal treatment is carried out in
various atmospheres, for example, vacuum, argon, nitrogen, helium
or hydrogen, the components of the equipment directly serving for
the thermal treatment are arranged within a vacuum chamber 4 which
is provided with a vacuum pumping exhaust port 8 communicating with
a vacuum pump and a gas inlet port 16.
[0069] Outside the shield plate 11, the coaxial double-walled
cylindrical quartz tube 6 and cooling water tube 5 are provided to
prevent the temperature in the vicinity of the sample stage 10 from
rising excessively and destroying the equipment. The temperature of
the sample can be measured by a thermocouple and an infrared
temperature sensor, and the temperature can be controlled. There is
provided a temperature sensor pickup port 14 for a lead wire of the
thermocouple and a temperature sensor port 15 for the infrared
temperature sensor. A temperature control circuit 23 is coupled to
each of the quartz lamps 1 to the temperature sensors and to the RF
coils 7 to control the temperature profile to which the sample is
exposed, as shown, for example, in FIG. 6. The circuit 23 includes
a suitable microprocessor programmed to control the temperature
using conventional interface circuits between the microprocessor
and the heating elements (1 and 7) and the temperature sensors.
While a pair of the infrared lamps are provided one above other in
the particular embodiment illustrated in FIG. 2, the placement of
the infrared lamps is not limited to this embodiment. For example,
the sample may be irradiated with infrared rays from two or three
infrared lamps placed below the sample as illustrated by FIGS. 4(1)
and 4(2); or from three infrared lamps placed above the sample as
illustrated by FIG. 4(3); or from two pairs of infrared lamps
placed above and below the sample, respectively, as illustrated by
FIG. 4(4). Alternatively, the sample can be irradiated by a single
infrared lamp placed above the sample and two infrared lamps placed
below the sample as illustrated by FIG. 4(5); or from three
infrared lamps placed above the sample and three infrared lamps
placed below the sample as illustrated by FIG. 4(6). By tilting the
quartz columns (i.e., mounting them off the vertical axis of FIG.
2), the number of the infrared waveguide quartz columns may be
increased and thereby the area irradiated with infrared rays may be
enlarged depending on a size of the SiC substrate to be thermally
treated. Also for convenience of operation, particularly for the
purpose of facilitating the sample 12 to be taken out, it may be
selected whether the infrared lamps should be placed exclusively
above the sample or exclusively below the sample.
[0070] FIGS. 5 and 6 plot a result of experimentally heating a 2
inch diameter SiC substrate. The experiments were conducted under
conditions as follow: output of the infrared lamp: 100 V, 30 A (3
KW); output of high frequency: 14688 W; and frequency; 25.5 KHz. As
for the atmosphere under which the thermal treatment was conducted,
air pressure within the vacuum chamber was reduced to approximately
0.1 Pascal by a thermal-molecular pump and then argon gas was
introduced into the chamber at a rate in the other of 1 L/min. The
temperature was measured by an infrared temperature sensor mounted
on the sample stage 12. In the case of heating by the high
frequency wave alone (FIG. 5), it was found that the temperature of
the sample is higher in its peripheral zone than in its central
zone and the maximum temperature is approximately 1750.degree. C.
The thermal treatment temperature necessary for activation of the
impurities being apt to become P-type impurities in SiC is normally
in the order of 1800.degree. C. and the desired activation can not
be achieved at the temperature of approximately 1750.degree. C. In
addition, a differential temperature between the peripheral zone
and the central zone of the sample was as remarkable as about
300.degree. C. Such uneven temperature distribution inevitably
makes electric properties of the SiC element uneven. Based on this
observation, such method is practically unsuitable for mass
production.
[0071] In the case of heating by the infrared lamps alone during
less than 30 sec (FIG. 6), the maximum temperature was
approximately 1000.degree. C., which is insufficient to activate
the impurities ion implanted into the SiC substrate. As opposed to
the case of heating by the high frequency wave alone, the
temperature of the sample was determined to be higher in the
central zone than in the peripheral zone of the sample.
Specifically, the differential temperature therebetween was
approximately 600.degree. C., which inevitably makes the electric
properties of the SiC element and makes such method of thermal
treatment unsuitable for mass production. After the sample had been
maintained at a temperature lower than 1000.degree. C. for 30
seconds using infrared heating, the high frequency heating was
started. Thereupon, the temperature rapidly rose to a temperature
of 1800.degree. C. or higher after 40 seconds. In this way, the
temperature reached a sufficiently high level to activate the
impurities implanted into the SiC substrate. Approximately 10
seconds after completion of the about 40 second thermal treatment,
the temperature was reduced to about 1200.degree. C. or lower
without any significant evaporation of Si from the surface of SiC.
Consequently, appearing of irregularities on the surface of SiC was
effectively restrained. Irregularities on the surface of the SiC
substrate measured by an atom force microscope were substantially
same as before the thermal treatment, i.e., the surface was
adequately smooth. Significant irregularities would deteriorate
electric properties of the SiC, such as pressure-resistance. The
product obtained by use of the equipment according to the invention
was observed to be free from such deterioration. The differential
temperature between the peripheral zone and the central zone
occurring in the thermal treatment at a temperature of 1800.degree.
C. was as minor as 44.degree. C. and such minor differential
temperature did not affect an in-plane evenness of the electric
properties of the SiC element thermally treated by the equipment
according to the invention. Based on the result of experimental
measurement, the thermal treatment equipment according to the
invention is suitable for mass production.
[0072] While the SiC substrate has been described and illustrated
as an example of the sample, the sample is not limited to the
SiC.
[0073] It will become apparent to those skilled in the art that
various modifications to the preferred embodiment of the invention
as described herein can be made without departing from the spirit
or scope of the invention as defined by the appended claims.
* * * * *