U.S. patent application number 09/836272 was filed with the patent office on 2001-10-25 for making method of optical waveguide substrate.
Invention is credited to Aoi, Hiroshi, Ejima, Seiki, Makikawa, Shinji, Shirota, Masaaki.
Application Number | 20010034074 09/836272 |
Document ID | / |
Family ID | 18631128 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010034074 |
Kind Code |
A1 |
Makikawa, Shinji ; et
al. |
October 25, 2001 |
Making method of optical waveguide substrate
Abstract
A method of making a high quality optical waveguide substrate is
provided, in which the surface of a silicon substrate is oxidized
through relatively large thickness and no foreign matter particles
are adhered on the surface thereof. The silicon substrate to form a
quartz film for the optical waveguide is mounted on a carbon
contained ceramics sample base and is inserted into a carbon
contained ceramics furnace core tube of which its external
circumference is arranged in a heating furnace. When the inside of
the furnace core tube is heated to 200 to 600.degree. C. by the
heating furnace, an oxidant gas for the silicon substrate surface
is introduced, then by further heating up to 1200 to 1350.degree.
C., the silicon surface is thus oxidized.
Inventors: |
Makikawa, Shinji;
(Annaka-shi, JP) ; Aoi, Hiroshi; (Annaka-shi,
JP) ; Shirota, Masaaki; (Annaka-shi, JP) ;
Ejima, Seiki; (Kita-Gun, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
277 S. WASHINGTON STREET, SUITE 500
ALEXANDRIA
VA
22314
US
|
Family ID: |
18631128 |
Appl. No.: |
09/836272 |
Filed: |
April 18, 2001 |
Current U.S.
Class: |
438/31 |
Current CPC
Class: |
C30B 29/06 20130101;
C30B 33/005 20130101 |
Class at
Publication: |
438/31 |
International
Class: |
H01L 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 21, 2000 |
JP |
2000-120291 |
Claims
What is claimed is:
1. A making method of an optical waveguide substrate comprising a
step of oxidizing a silicon substrate by an oxidant gas which is
mounted on a sample base of carbon contained ceramics and are
inserted into a core tube of carbon contained ceramics, of which
external circumference is arranged in a heating furnace, wherein
said oxidizing step includes that the silicon substrate is heated
up to a temperature of 1200 to 1350.degree. C. by the furnace and
introducing the oxidant gas into the core tube begins on the way of
heating up to the temperature.
2. The making method of the optical waveguide substrate according
to claim 1, characterized in that the introducing the oxidant gas
begins at when heated at 200 to 600.degree. C.
3. The making method of the optical waveguide substrate according
to claim 1, characterized in that the introducing the oxidant gas
continues until the temperature of the core tube is cooled to 200
to 600.degree. C. on termination of the oxidation of the said
silicon surface.
4. The making method of the optical waveguide substrate according
to claim 1, characterized in that the said oxidant gas comprises
steam or oxygen.
5. The making method of the optical waveguide substrate according
to claim 1, characterized in that the carbon contained ceramics is
a silicon carbide.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method of making a high
quality optical waveguide substrate of which the surface is
oxidized from a silicon substrate through relatively large
thickness and has no foreign matter particles adhered to the
surface.
[0002] A waveguide device for optical communication is constituted
of an optical waveguide and a semiconductor integrated circuit. The
optical waveguide is formed on a quartz substrate. The quartz
substrate is such that a quartz film, i.e. silicon dioxide, is
formed on the surface of a silicon substrate. Since this film
functions optically as an under-clad of the optical waveguide, it
is required to be as thick as 5-30 .mu.m.
[0003] On the other hand, the semiconductor integrated circuit is
formed on a silicon substrate. A thickness of an oxidation film
required for the semiconductor integrated circuit is 0.2-3 .mu.m,
which is much thinner than the quartz film for the optical
waveguide.
[0004] Since silicon constituting a substrate has strong affinity
with oxygen and is easily oxidized, a technique to form a quartz
film by oxidation the on the silicon surface of the substrate is
adopted as a method for forming such oxidation film of the
semiconductor integrated circuit and quartz film of the optical
waveguide.
[0005] Such techniques are, for example, dry oxygen oxidation
method, wet oxidation method, steam oxidation method, hydrogen
burning oxidation method and hydrochloride oxidation method. The
wet oxidation method, the steam oxidation method and the hydrogen
burning oxidation method are able to form a quartz film through
oxidation of the silicon surface by applying high reaction property
oxygen generated by thermal decomposition of steam, in which the
silicon substrate placed on a carbon contained ceramics sample base
inside a high thermal resistance carbon contained ceramics furnace
core tube is heated to a high temperature while in contact with
steam. Oxidation rates of those methods are relatively high.
[0006] FIG. 2 shows the relationship between the thickness of the
oxidation film and the oxidation time when oxidizing the silicon
surface in the steam oxidation method, relative to various
oxidation temperatures, i.e. heating temperatures of the silicon
surface. As seen in FIG. 2, to form an quartz film of 0.2 to 3
.mu.m using for a semiconductor integrated circuit, it takes 10 to
1000 minutes at 1200.degree. C. Usually, to obtain such homogeneous
thin quartz film formed in a short time, the silicon substrate is
exposed to inert gas atmosphere while heating temperature is risen
to the oxidation temperature. Once it reaches the oxidation
temperature the silicon substrate is in contact with steam.
[0007] If the quartz film of 10 to 25 .mu.m generally used for
optical waveguide is to be formed in the same manner, it takes
20000 to 125000 minutes at 1200.degree. C., which is a considerable
amount of time. It is at this point that foreign matter particles
adhere to the surface of the quartz substrate. Such foreign matter
particles amplified through an electron microscope can be observed
as a brown needle like foreign matter.
[0008] The development mechanism of the particle is assumed as
follows. For instance, if a sample base or a furnace core tube made
of silicon carbide is used, the silicon carbide on the surface of
the sample base exposed to high temperature for a long time may be
partially oxidized and form a quartz film having a thickness of
several micron meters. When the sample base, for instance, is
repeatedly used and is exposed to inert gas atmosphere at a high
temperature, the silicon carbide inside operates as a reducer to
the quartz film and SiO having a subliming property will generate
as shown in the following chemical formula:
3SiO.sub.2+SiC .fwdarw. 4SiO+CO.sub.2
[0009] It is assumed that the SiO, after being sublimated and
scattered, adhered to the surface of the silicon substrate and was
gradually grown to become foreign matters. Since such consequence
attributes to the reductive action by carbon, same results may be
obtained in general when using ceramics containing carbon as a
sample base or a furnace core tube.
[0010] The adhered foreign matter particles may become optical
diffusion spots, or cause optical loss, and consequently, may not
only degrade the performance of the optical waveguide substrate but
also make poor yield.
SUMMARY OF THE INVENTION
[0011] The object of the present invention developed to solve the
foregoing problem, is to provide a method of making a high quality
optical waveguide substrate, in which the surface of the silicon
substrate is oxidized through relatively large thickness and is
free from any foreign matter particles adhering on its surface.
[0012] The method of making an optical waveguide substrate
according to the present invention, developed to achieve the
foregoing object, is such that a silicon substrate to form a quartz
film for optical waveguide is placed on a carbon contained ceramics
sample base and is inserted into a furnace core tube made of carbon
contained ceramics, of which a heating furnace is arranged to its
exterior, and an oxidant gas for the silicon surface of the silicon
substrate introduce into the furnace core tube which is heated up
to 1200 to 1350.degree. C. by the heating furnace to thus oxidize
the silicon surface. It begins to introduce the oxidant gas into
the furnace core tube on the way of heating up to the temperature
of 1200 to 1350.degree. C.
[0013] The beginning to introduce the oxidant gas into the furnace
core tube prefer when the inside of the furnace core tube is heated
to 200 to 600.degree. C. on the way of heating up to the
temperature.
[0014] During the introduction of the oxidant gas, if the
temperature of the furnace core tube is less than 200.degree. C.,
foreign matter particles may easily adhere to the silicon substrate
by, for instance, static electricity, while if the temperature is
higher than 600.degree. C., the reaction as in the afore mentioned
chemical formula may occur.
[0015] In order to oxidize the silicon surface, if the temperature
of the furnace core tube is less than 1200.degree. C., it takes
long to form a thick quartz film, thus is inefficient. If the
temperature is higher than 1350.degree. C., defects such as slips
tend to occur in the silicon substrate.
[0016] The termination of the oxidation of the silicon surface is
preferably implemented by continuously introducing the oxidant gas
until the temperature inside the furnace core tube is dropped to
200 to 600.degree. C. The introduction of the gas is terminated
after being cooled down to the above temperature.
[0017] It is preferred that the oxidant gas comprises steam or
oxygen. It is possible to introduce a simple substance of steam
into the furnace core tube, or generate steam by introducing oxygen
gas and hydrogen gas into the furnace core tube. Moist oxygen gas
or high-pressure oxygen that has gone under high partial pressure
may be introduced as well.
[0018] It is preferably implemented that the ceramics containing
carbon is a silicon carbide.
[0019] According to this method, when the furnace core tube is at
high temperature, the inside thereof is filled with oxidant gas.
Under this oxidant gas atmosphere, carbon contained in the ceramics
constituting the furnace core tube and the sample base is
restrained from acting as a reducer. As a result, the reduction
reaction shown in the foregoing chemical formula hardly happens,
therefore, no SiO is produced, and consequently, no foreign matter
particles adhere to the surface of the silicon substrate.
BRIEF EXPLANATION OF THE DRAWINGS
[0020] FIG. 1 is a cross sectional view showing a making device
used to carry out the making method of the optical waveguide
substrate according to the present invention.
[0021] FIG. 2 is a graph showing the relationship between the
thickness of the oxidation film and the oxidation time regarding
various oxidation temperatures in the steam oxidation method.
DETAILED EXPLANATION OF THE INVENTION
[0022] Preferred embodiments applied the present invention will be
explained below with reference to the drawings but the scope of the
claimed invention is not limited the embodiments.
[0023] FIG. 1 shows a schematic diagram of a making device 1, which
is used when carrying out the making method of the optical
waveguide substrate according to the present invention.
[0024] The making device 1 for the optical waveguide substrate
oxidizes the silicon surface of the silicon substrate 15, which
forms a quartz film for the optical waveguide. This making device 1
consists of a furnace core tube 18 made of silicon carbide and its
outer surface arranged with a heating furnace 17. It is possible to
apply a silicon carbide film having a thickness of some .mu.m by
chemical vapor deposition to the inner surface of the furnace core
tube 18. One end of the furnace core tube 18 is roughly sealed up
and is connected to a gas introduction tube 11, which oxidizes the
silicon surface of the silicon substrate 15. A switch valve 12 to
control the introduction of gas is arranged en route to the gas
introduction tube 11. A lid 19, which is attached to an air release
pipe 20, covers the other end of the furnace core tube 18. A
temperature sensor 14 made of thermo-couple is arranged inside the
furnace core tube 18. The temperature sensor 14 is connected to a
temperature indication device (not shown in the drawings). A sample
base 16 made of silicon carbide has many slitting grooves. The
silicon substrate 15, which is inserted in the slitting grooves of
the sample base 16, is inserted in the furnace core tube 18. The
furnace core tube 18 is inserted in a container 13.
[0025] By using the device 1, the optical waveguide substrate is
made as follows.
[0026] A number of silicon substrates 15, made of silicon and
shaped in circular plates, are placed in equal spacing on the
sample base 16. The sample base 16 is inserted into the furnace
core tube 18 from its opened end, and is covered by the lid 19.
[0027] The furnace core tube 18 is slowly heated by the heating
furnace 17. The switch valve 12 is closed until the inside
temperature of the furnace core tube 18 reaches a fixed temperature
in the range of 200 to 600.degree. C. When the inside temperature
of the furnace core tube 18 detected by the temperature sensor 14
reaches the fixed temperature, the switch valve 12 opens and
oxidant gas is introduced into the furnace core tube 18. Heating
continues until the inside temperature of the furnace core tube 18
reaches 1200 to 1350.degree. C. It is kept at this temperature
while the introduction of the oxidant gas continues. The silicon
surface of the silicon substrate 15 is oxidized by an oxidant gas,
such as oxygen, to form a quartz film shown in the chemical
formula; Si+O.sub.2 .fwdarw. SiO.sub.2.
[0028] When terminating the oxidation, the furnace core tube 18 is
slowly cooled. As the inside temperature of the furnace core tube
18 is dropped to the fixed temperature within the range of 200 to
600.degree. C., the introduction of the oxidant gas is stopped.
Sample board 16 is drawn out, and an optical waveguide substrate,
which both surfaces of the silicon substrate are formed with quartz
films, is obtained.
[0029] The experimental embodiments of the optical waveguide
substrate made by the method according to the present invention
will be described below in embodiments 1 and 2. Experimental
embodiments of the optical waveguide substrate made by a method
other than the present invention is described in the comparative
example.
EMBODIMENT 1
[0030] The external circumference of a circular plate shaped
silicon substrate 15, which is 4 inches in diameter and is 0.6 mm
thick, is partially scraped flat. The flat portions are faced
downwards and three plates of the silicon substrate 15 thereof are
inserted in the groove on sample base 16 in equal spacing. The
sample base 16 made of silicon carbide is inserted in the furnace
core tube 18, which has a diameter of 200 mm and is made of silicon
carbide, then is covered by a lid 19, which has an air release pipe
20. The external circumference of the furnace core tube 18 is
arranged with a heating furnace 17, which assumes a tube shaped
furnace of a kanthal heater, Kanthal AMP240.phi. (Trade name code
by Kanthal Corporation). A switch valve 12 arranged en route to the
gas introduction tube 11, which is connected to a quartz container
(not shown in diagram) and generates steam by boiling pure water,
is closed.
[0031] The furnace core tube 18 is heated gradually by a heating
rate of 3.degree. C. per minute to prevent the furnace core tube
from breaking due to sudden change in temperature. As the inside
temperature of the furnace core tube 18 reaches 500.degree. C., the
switch valve 12 is opened and 3 L per minute of steam is introduced
to the furnace core tube 18. Heating continues until the inside
temperature of the furnace core tube 18 reaches 1250.degree. C. It
is kept at this temperature for 5000 minutes while the introduction
of the steam continues, and the silicon surface of the silicon
substrate 15 is oxidized.
[0032] After 5000 minutes, the furnace core tube 18 is cooled at a
cooling rate of 3.degree. C. per minute. As the temperature inside
the furnace core tube 18 is dropped to 500.degree. C., the
introduction of the oxidant gas is stopped. Sample board 16 is
drawn out, and an optical waveguide substrate, which both surfaces
of the silicon substrate 15 are formed with quartz films having the
desired thickness of 5.1 .mu.m, is obtained.
[0033] When the surface was measured by a foreign matter examining
device (provided from Hitachi Engineering Corporation), it was
found that the number of foreign matter particles larger than 0.3
.mu.m was as small as less than 100 pieces on average per
substrate.
EMBODIMENT 2
[0034] An optical waveguide substrate was experimentally made with
the same device as in embodiment 1, except for that a gas
introduction tube connected to an oxygen gas bomb and a hydrogen
gas bomb was used instead of a gas introduction tube connected to a
quartz container, which generates stream.
[0035] First, the furnace core tube 18 is heated gradually by a
heating rate of 3.degree. C. per minute. As the temperature inside
the furnace core tube 18 reaches 500.degree. C., the switch valve
12 is opened and, by adjusting the regulator of the oxygen gas
bomb, 1 L per minute of oxygen gas is introduced to the furnace
core tube 18. As the inner tube temperature reaches 800.degree. C.,
the regulator of the hydrogen gas bomb is adjusted and 1.8 L per
minute of hydrogen gas is introduced. At temperatures higher than
800.degree. C., oxygen gas and hydrogen gas react together and
generate steam. Further heating continues until the temperature
inside the furnace core tube 18 reaches 1250.degree. C. It is kept
at this temperature for 5000 minutes while steam continues to
generate, and the silicon surface of the silicon substrate 15 is
oxidized.
[0036] After 5000 minutes, the introduction of hydrogen gas is
stopped and while 1 L per minute of oxygen gas is introduced, the
furnace core tube 18 is cooled at a cooling rate of 3.degree. C.
per minute. When the temperature inside the furnace core tube 18 is
dropped to 500.degree. C., the introduction of the oxidant gas is
stopped.
[0037] An optical waveguide substrate, which both surfaces of the
silicon substrate 15 are formed with quartz films having the
desired thickness of 5.0 .mu.m, is obtained. When the surfaces were
measured by the foreign matter examining device, it was found that
the number of foreign matter particles larger than 0.3 .mu.m was as
small as less than 50 pieces on average per substrate.
COMPARATIVE EXAMPLE
[0038] An optical waveguide substrate was experimentally made in
the same way as in embodiment 1, except for that during the process
of heating or cooling where the inner tube temperature of the
furnace core tube ranges between 500 to 1250.degree. C., 1 L per
minute of nitrogen gas, which is an inert gas, was introduced
instead of the introduction of steam. An optical waveguide
substrate, which has a thickness of 4.8 .mu.m of quartz film formed
on both surfaces of the silicon substrate, was obtained. When the
surfaces were measured by a foreign matter examining device, it was
found that the number of foreign matter particles larger than 0.3
.mu.m was as many as 400 pieces on average per substrate.
[0039] As has been described in detail, according to the method of
making an optical waveguide substrate of the present invention, it
is possible to obtain a high quality optical waveguide substrate,
which the surface of its silicon substrate is oxidized through
relatively large thickness and has no foreign matter particles
adhered on its surface.
[0040] Usage of the present optical waveguide substrate allows to
form an optical waveguide device of excellent performance.
* * * * *