U.S. patent application number 10/819472 was filed with the patent office on 2004-12-23 for lithium tantalate substrate and method of manufacturing same.
Invention is credited to Kajigaya, Tomio, Kakuta, Takashi.
Application Number | 20040255842 10/819472 |
Document ID | / |
Family ID | 33519653 |
Filed Date | 2004-12-23 |
United States Patent
Application |
20040255842 |
Kind Code |
A1 |
Kajigaya, Tomio ; et
al. |
December 23, 2004 |
Lithium tantalate substrate and method of manufacturing same
Abstract
In a process for manufacturing a LT substrate from a LT crystal,
after growing the crystal, a LT substrate in ingot form is imbedded
in carbon power, or is place in a carbon vessel, and heat treated
is conducted at a maintained temperature of between 650.degree. C.
and 1650.degree. C. for at least 4 hours, whereby in a lithium
tantalate (LT) substrate, sparks are prevented from being generated
by the charge up of an electric charge on the substrate surface,
and thereby destruction of a comb pattern formed on the substrate
surface and breaks or the like in the LT substrate are
prevented.
Inventors: |
Kajigaya, Tomio; (Hokkaido,
JP) ; Kakuta, Takashi; (Hokkaido, JP) |
Correspondence
Address: |
KATTEN MUCHIN ZAVIS ROSENMAN
575 MADISON AVENUE
NEW YORK
NY
10022-2585
US
|
Family ID: |
33519653 |
Appl. No.: |
10/819472 |
Filed: |
April 6, 2004 |
Current U.S.
Class: |
117/2 |
Current CPC
Class: |
C30B 33/00 20130101;
C30B 29/30 20130101; G02F 1/3551 20130101; C30B 33/02 20130101;
C30B 15/00 20130101; G02F 2202/20 20130101 |
Class at
Publication: |
117/002 |
International
Class: |
C30B 001/00; G02F
001/35; G02F 002/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 8, 2003 |
JP |
2003-104176 |
Dec 26, 2003 |
JP |
2003-432472 |
Mar 3, 2004 |
JP |
2004-061862 |
Claims
What is claimed is:
1. A lithium tantalate substrate having a thermal history of heat
treatment wherein lithium tantalate in ingot form is maintained at
a temperature of between 650.degree. C. and 1650.degree. C., while
embedded in a carbon powder, or in a carbon vessel.
2. A lithium tantalate substrate having a thermal history of heat
treatment wherein lithium tantalate in ingot form is maintained at
a temperature of between 650.degree. C. and 1400.degree. C., while
embedded in a Si powder, or in a Si vessel.
3. A lithium tantalate substrate having a thermal history of heat
treatment wherein lithium tantalite in wafer form is maintained at
a temperature of between 350.degree. C. and 600.degree. C., while
embedded in a metal powder selected from a group consisting of Ca,
Al, Ti, and Si.
4. A lithium tantalate substrate having a thermal history of heat
treatment wherein lithium tantalite in wafer form is maintained at
a temperature of 350.degree. C. or higher and below the melting
point of Zn, while embedded in a Zn powder.
5. A lithium tantalate substrate according to any one of claim 1
through claim 4, wherein the heat treatment lasts 4 hours or
longer.
6. A lithium tantalate substrate which is blackened by the heat
treatment according to any one of claim 1 through claim 5.
7. A method of manufacturing a lithium tantalate substrate using a
lithium tantalate crystal grown using the Czochralski method,
comprising the step of heat treating the lithium tantalate crystal
in ingot form, which is embedded in carbon powder or in a carbon
vessel, at a maintained temperature of between 650.degree. C. and
1650.degree. C.
8. A method of manufacturing a lithium tantalate substrate using a
lithium tantalate crystal grown using the Czochralski method,
comprising the step of heat treating the lithium tantalate crystal
in ingot form, which is embedded in Si powder or in a Si vessel, at
a maintained temperature of between 650.degree. C. and 1400.degree.
C.
9. A method of manufacturing a lithium tantalate substrate using a
lithium tantalate crystal grown using the Czochralski method,
comprising the step of heat treating the lithium tantalate crystal
in wafer form, which is embedded in a metal powder selected from a
group consisting of Ca, Al, Ti, and Si, at a maintained temperature
of between 350.degree. C. and 600.degree. C.
10. A method of manufacturing a lithium tantalate substrate using a
lithium tantalate crystal grown using the Czochralski method,
comprising the step of heat treating the lithium tantalate crystal
in wafer form, which is embedded in a Zn powder, at a maintained
temperature of 350.degree. C. or higher and below the melting point
of Zn.
11. A method of manufacturing a lithium tantalate substrate
according to any one of claim 7 through claim 10, wherein the heat
treatment lasts 4 hours or longer.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a lithium tantalate ((L)
substrate used in surface-acoustic wave elements and the like, and
a manufacturing method thereof.
BACKGROUND OF THE INVENTION
[0002] A lithium tantalate crystal is a ferroelectric substance,
with a melting point of approximately 1650.degree. C. and a Curie
point of approximately 600.degree. C., which has piezoelectric
properties. Lithium tantalate substrates manufactured from lithium
tantalate crystals are used primarily as a material for
surface-acoustic wave (SAW) filters used to accomplish signal noise
rejection in mobile phones. Factors such as the use of higher
frequencies with mobile phones, and the proliferation of Bluetooth
(2.45 GHz) as a wireless LAN for a variety of electronic equipment,
mean that from now on a rapid increase in demand is anticipated for
SAW filters in and around the 2 GHz frequency domain.
[0003] In the construction of a SAW filter, a pair of comb
electrodes made of a metallic thin film produced from an AlCu alloy
or the like, arc formed on a substrate made of a piezoelectric
material, for example a LT substrate. These comb electrodes have
the important function of controlling the polarity of the device.
The comb electrodes are formed by depositing a metallic thin film
on the piezoelectric material by a sputtering method, and leaving a
pair of comb shaped patterns while etching away the unwanted
portions using photolithographic techniques.
[0004] To be compatible with even higher frequencies, it is
necessary for the comb shaped pattern to be fine, as well as thin.
Compared to devices operating in and around the 800 MHz frequency
domain, which is currently the mainstream, devices operating in and
around the 2 GHz frequency domain require a distance between
electrodes approximately one third as wide, that is between 0.3
.mu.m and 0.4 .mu.m, and a film thickness less than one fifth as
thick, that is below 200 nm or thereabouts.
[0005] Industrially, LT crystals are grown using the Czochralski
method, normally using a high-melting iridium crucible inside an
electric furnace, under a nitrogen-oxygen mixed gas atmosphere with
an oxygen concentration between a few percent and 10% or
thereabouts, and the crystals are removed from the electric furnace
after being cooled at a predetermined cooling rate inside the
electric furnace (Albert A. Ballman: Journal of American Ceramic
Society, Vol. 48 (1965)).
[0006] The process of manufacturing a wafer from a LT crystal
involves crystal growing (ingot), poling, cylindrical grinding,
slicing, lapping, polishing and wafer completion, in that
order.
[0007] Specifically, a grown LT crystal is either colorless and
transparent, or exhibits a highly transparent pale yellow color.
After the LT crystal is grown, heat treatment of the crystal is
performed under an even temperature near the melting point to
remove residual strain in the crystal caused by thermal stress. In
addition, poling is performed to obtain a single polarization. In
other words a series of processes is performed involving; heating
the LT crystal from room temperature to a predetermined temperature
above the Curie point (approximately 600.degree. C.), applying a
voltage to the LT crystal, and while applying this voltage,
lowering the temperature of the LT crystal to a predetermined
temperature below the Curie point, and subsequently stopping
voltage application and lowering the temperature of the LT crystal
to room temperature. After poling, the LT crystal ingot, which has
undergone cylindrical grinding to prepare its external form, is
sliced to form wafers, and these wafers undergo machining including
lapping and polishing and the like, to obtain LT substrates. The
thus obtained LT substrates are nearly colorless and transparent,
and have extremely low electrical conductivity, at approximately
10.sup.-13 S/m (volume resistivity 10.sup.15 .OMEGA.cm).
[0008] A LT crystal, which is a ferroelectric substance, also has
pyroelectric properties. Accordingly, when LT substrates are
obtained by conventional methods, the temperature variations
sustained during the surface acoustic wave device manufacturing
process can cause an electric charge to accumulate on the surface
of the LT substrate due to the pyroelectric properties of the LT
crystal, and this charge can generate sparks. These sparks can
destroy the comb pattern formed on the surface of the LT substrate,
and cause cracking or the like of the LT substrate, which reduces
yield during the surface acoustic wave device manufacturing
process. Moreover, because the electrical conductivity of the LT
substrate is extremely low as mentioned above, the charge-build-up
state is maintained, and a state in which sparking can occur easily
continues for extended periods of time.
[0009] Furthermore, because the LT substrate has high light
transmittance, a problem occurs in that the light which passes into
the LT substrate during the photolithographic process, which is one
part of the surface acoustic wave device manufacturing process, is
reflected from the rear surface of the LT substrate back onto the
front surface, causing deterioration of the resolution of the comb
pattern formed on the substrate.
[0010] In order to solve these problems, in Japanese Patent
Publication No. Tokukai Hei 11-92147 and Japanese Patent
Publication No. Tokukai Hei 11-236298, a solution is proposed
whereby lithium niobate (LN) crystals are exposed to a reducing
atmosphere of argon, water, hydrogen, nitrogen, carbon dioxide,
carbon monoxide or a mixture of gases selected from this group, at
a temperature range of between 500 and 1140.degree. C., thereby
blackening the LN crystal wafers and thus controlling the high
light transmittance of the LN substrate, while increasing the
electrical conductivity, so that light reflected back from the rear
surface of the LN substrate is suppressed, and at the same time the
pyroelectric properties of the LN substrate are reduced.
[0011] Although these publications refer to LT crystals as well as
LN crystals, there are no substantial disclosures relating to LT
crystals.
[0012] Furthermore, according to experiments carried out by the
inventors of the present invention, it was discovered that the
methods disclosed in these publications were effective with LN
crystals that had a low melting point of approximately 1250.degree.
C., but had no effect with LT crystals that had a high melting
point of approximately 1650.degree. C.
[0013] In Japanese Patent Publication No. Tokukai 2004-35396
(WO2004/002891A1), it is disclosed that LT crystals are susceptible
to a charge-build-up state caused by an electric charge produced by
heat or mechanical stress, and for devices which use LT crystals,
from a stability viewpoint it is necessary to dissipate this
charge, and that with LN crystals, the heat treatment under a
reducing atmosphere causes electrical conductivity to increase,
allowing charge-build-up to be prevented, and also that the same
effects cannot be obtained for LT crystals as were obtained for LN
crystals using the same methods.
SUMMARY OF THE INVENTION
[0014] An object of the present invention is to provide a LT
substrate in which sparks generated by the charge-build-up of an
electric charge on the surface of the lithium tantalate (LT)
substrate, due to temperature variation sustained during the
surface acoustic wave element manufacturing process are prevented,
whereby destruction of the comb pattern formed on the surface of
the LT substrate and breaks or the like in the LT substrate caused
by these sparks is prevented from occurring.
[0015] Furthermore, another object of the present invention is to
provide a method of manufacturing a LT substrate in which during
the photolithographic process, light which passes into the LT
substrate does not reflect back from the rear surface of the LT
substrate onto the front surface, whereby the resolution of the
comb pattern is prevented from being deteriorated.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0016] In order to achieve the above objects, a lithium tantalate
(LT) substrate according to the present invention, is made in a
process for manufacturing the LT substrate from a lithium tantalate
(LT) crystal grown using the Czochralski method, and has a thermal
history undergoing at least one heat treatment selected from the
following:
[0017] in other words;
[0018] (1) heat treatment performed on a LT crystal ingot together
with carbon powder or in a carbon vessel, under an inert or
reducing atmosphere, at a maintained temperature of between
650.degree. C. and 1650.degree. C.,
[0019] (2) heat treatment performed on a LT crystal ingot together
with Si powder or in an Si vessel, under an inert or reducing
atmosphere, at a maintained temperature of between 650.degree. C.
and 1400.degree. C.;
[0020] (3) heat treatment performed on a LT crystal wafer, together
with a metal powder selected from a group consisting of Ca, Al, Ti
and Si, under an inert or reducing atmosphere, at a maintained
temperature of between 350.degree. C. and 600.degree. C.; and
[0021] (4) heat treatment performed on a LT crystal wafer together
with Zn powder, under an inert or reducing atmosphere, at a
maintained temperature of 350.degree. C. or higher and below the
melting point of Zn.
[0022] Heat treatments (1) and (2) are performed on LT crystals in
ingot form before poling is performed, and heat treatments (3) and
(4) are performed on wafers, after poling is performed.
[0023] With these heat treatments, the wafer or ingot is preferably
embedded in the powder, and the treatment preferably lasts 4 hours
or longer. After undergoing such heat treatment, the lithium
tantalate (LT) substrate of the present invention is blackened.
[0024] The present invention achieves an increase in yield for the
surface-acoustic wave element manufacturing processes. In other
words, sparks resulting from the charge-build-up of an electric
charge on the surface of the lithium tantalate (LT) substrate due
to temperature variation sustained during the surface acoustic wave
element manufacturing process can be prevented, and destruction of
the comb pattern formed on the surface of the LT substrate and
breaks or the like in the LT substrate caused by these sparks can
also be prevented. Furthermore, deterioration in the resolution of
the comb pattern, because of light passing into the LT substrate
during the photolithographic process being reflected from the rear
surface of the LT substrate back onto the front surface, does not
occur.
[0025] Now, the present invention is further detailed.
[0026] The inventors of the present invention found that if
electrical conductivity is induced in a LT crystal by a specific
method, then even if an electric charge accumulates on the surface
of the LT crystal due to pyroelectricity, the charge is neutralized
immediately, charge-build-up does not occur, and sparks are not
produced, and thus arrived at the present invention related to the
aforementioned specific method.
[0027] The volume resistivity (electrical conductivity) and color
of lithium tantalate (LT) crystals varies according to the oxygen
vacancy concentration within the LT crystal. Specifically, if
oxygen is taken by Al, for example, and oxygen vacancies are
introduced into the LT crystal, this results in excess electrons.
Accordingly, because of the need to maintain a charge balance, the
excess electrons are trapped by Ta ions, so that the valence value
of some of the Ta ions changes from 5+ to 4+, causing electrical
conductivity, and at the same time photoabsorption, to occur.
[0028] In other words, it is thought that electric conduction is
induced because carrier electrons move between Ta.sup.5+ ions and
Ta.sup.4+ ions. Accordingly, the number of Ta.sup.4+ ions, which
contribute to electric conduction, is proportionate to the amount
of oxygen vacancy that is introduced. The electrical conductivity
of the crystal is determined by the product of the number of
carriers per unit volume and the carrier mobility. Given the same
mobility, the electrical conductivity is proportionate to the
number of oxygen vacancies. It is thought that color variation due
to photoabsorption is caused by the electron level introduced by
the oxygen vacancies.
[0029] Control of the number of oxygen vacancies can be performed
by so-called "treatment under controlled atmosphere", which
utilizes the equilibrium between a solid and a gas. The oxygen
vacancy concentration of a crystal placed under a specific
temperature varies so as to maintain equilibrium with the oxygen
potential (oxygen concentration) of the atmosphere under which the
crystal is placed. When the oxygen concentration becomes low, the
oxygen vacancy concentration increases. Furthermore, even under
high temperatures the oxygen vacancy concentration generally
increases. Accordingly, in order to increase oxygen vacancy
concentration, and increase opacity, preferably a high temperature
is used, and the oxygen concentration of the atmosphere is
lowered.
[0030] Controlling the number of oxygen vacancies can be performed
by heat treatment using a so-called reducing agent, utilizing the
equilibrium between solids. Preferred reducing agents for LT
crystals are C, Zn, Ca, Al, Ti and Si. Furthermore, it is also
effective to use a compound of these elements in powder form or
oxides of these elements as the reducing agent. Specifically, heat
treatment is performed by placing a LT crystal ingot or wafer in a
vessel made from these elements, or embedding the ingot or wafer in
a powder of these elements. However, the application conditions are
very different for ingots and wafers.
[0031] With heat treatment performed in a vessel or embedded in a
powder, in order to prevent deterioration due to excess oxidation
of the elements which form the vessel or powder, the heat treatment
is preferably performed under a reducing atmosphere including weak
reducibility such as nitrogen gas, inert gas such as Ar gas,
nitrogen-hydrogen forming gas, or under a vacuum. At this time, the
gas pressure of the atmosphere is preferably at or below
atmospheric pressure.
[0032] As described above, with the present invention, it is
thought that because LT is reduced using a solid reducing agent
such as Al, thereby introducing oxygen deficiency, the oxygen is
completely consumed by the solid reducing agent such as Al,
resulting in an inert gas atmosphere, and subsequently, the LT is
losing oxygen due to the unreacted solid reducing agent. However,
it is conceivable that oxygen deficiency can occur even when a
small amount of oxygen is present in the atmosphere, because the
solid reducing agent takes oxygen from the LT by a solid-solid
reaction.
[0033] Furthermore, the heat treatment temperature is preferably
high, but naturally the upper temperature limit is set to the
melting point of the LT crystals or the melting point of the
element which forms the vessel or the powder, whichever is
lower.
[0034] Because LT crystals have strong ion-binding properties, the
vacancy diffusion speed of LT crystals is relatively fast. However,
because changes to the oxygen vacancy concentration require the
intra-crystalline diffusion of oxygen, it is necessary to keep the
LT crystal under the atmosphere for a fixed time (more than four
hours). This dispersion speed depends greatly on temperature, and
at room temperature or thereabouts, changes to oxygen vacancy
concentration do not occur within a realistic amount of time.
Accordingly, to obtain an opaque LT crystal in a short period of
time, it is necessary to keep the LT crystal under a low oxygen
concentration atmosphere at temperatures high enough to allow
sufficiently fast oxygen diffusion.
[0035] The heat treatment temperature differs for LT crystal ingots
and wafers. This is because the heat treatment temperature applied
to ingots and wafers is different, above or below the Curie point.
In other words, before poling is performed, heat treatment can be
performed at temperatures above the Curie point, but after poling
is performed, heat treatment must be performed at temperatures
below the Curie point.
[0036] If LT crystals in wafer form are treated at temperatures
between 650.degree. C. and 1650.degree. C., the Curie point will be
exceeded, negating the effects of the poling process performed to
obtain a single polarization. Even assuming that it is possible to
perform poling of crystals in wafer form, it is an extremely time
consuming process, and therefore heat treatment at temperatures
between 650.degree. C. and 1650.degree. C. must be applied to
ingots and not to the wafers.
[0037] On the other hand, although it is not impossible to control
pyroelectricity by performing heat treatment of LT crystals in
ingot form at temperatures between 350.degree. C. and 600.degree.
C., it will take an extremely long heat treatment time to introduce
oxygen vacancies all the way to the center of the ingot, and to
obtain an even oxygen vacancy concentration throughout the ingot.
Accordingly, realistically, heat treatment at temperatures between
350.degree. C. and 600.degree. C. must be applied to wafers and not
to the ingots.
[0038] After performing treatment at high temperatures, if the LT
crystal is cooled quickly, a LT crystal which retains the oxygen
vacancy concentration introduced at high temperatures can be
maintained at room temperature. The lower limit of processing time
can be determined easily by experimentation based on the processing
temperatures in the heat treatment methods described above, taking
economic efficiency into consideration.
[0039] As the optimum conditions, taking into consideration the
controllability of the treatment process, the characteristics of
the finished substrate, uniformity, and reproducibility of the
characteristics, it is effective to use as the sample a wafer cut
from an ingot after poling is performed, embed the wafer in a mixed
powder of Al and Al.sub.2O.sub.3, and perform heat treatment under
a nitrogen gas atmosphere or an inert gas atmosphere such as Ar
gas, at a temperature below the Curie point of the LT crystal.
[0040] The pyroelectric effect is caused by deformation of the
lattice which occurs with changes in the crystal temperature. In
crystals which have electric dipoles, it is understood that this
occurs because the distance between the dipoles varies with the
temperature. The pyroelectric effect only occurs with materials
that have high electrical resistance. Ion displacement causes a
charge to occur on the crystal surface in the dipole direction (the
Z direction in a LT crystal), but in a material which has low
electrical resistance, this charge is neutralized by the electrical
conductivity of the crystal itself. In the case of a normal
transparent LT crystal, because the electrical conductivity is at
the 10.sup.-13 S/m level, the pyroelectric effect is very
noticeable. However, in the case of an opaque LT crystal, because
the electrical conductivity increases to approximately 10.sup.-8
S/m (volume resistivity 10.sup.10 .OMEGA.cm), pyroelectricity is no
longer apparent.
[0041] With the present invention, even pale yellow LT crystals and
LT crystals that are almost fully colorless and transparent are
colorized and opacified (known as blackening), and their electrical
conductivity is improved. Because the color tone after colorization
and opacification looks a reddish brown color under transmitted
light and black under reflected light, this colorization and
opacification phenomenon is referred to here as "blackening".
[0042] A practical method for determining whether or not the
pyroelectricity of the LT crystal is gone after the heat treatment
of the present invention is a thermal cycle test, performed so as
to imitate the temperature variation sustained by the LT substrate
during the actual surface acoustic wave device manufacturing
process. When a thermal cycle involving heating from room
temperature to 200.degree. C. at a rate of 10.degree. C. per
minute, and then cooling to room temperature at a rate of
10.degree. C. per minute is applied to the LT substrate, with LT
substrates produced by conventional techniques, sparks are observed
on the substrate surface. If the thermal cycle test is performed on
a LT crystal which is just grown, it becomes a destructive test in
that the generated sparks cause the crystal to crack, and it is
therefore difficult to carry out the thermal cycle test during the
manufacturing process. On the other hand, with a blackened LT
substrate, no sparks are observed at the substrate surface.
Accordingly, determining the presence or lack of blackening is
useful as a practical method for determining pyroelectricity for LT
crystals.
[0043] Moreover, blackening can be clearly observed by performing
heat treatment for over four hours.
EXAMPLES
Example 1
[0044] A LT crystal with a diameter of 4 inches (101.6 mm) was
grown using the Czochralski method, from raw materials of congruent
composition. The growth atmosphere was a nitrogen-oxygen mixed gas
with an oxygen concentration of approximately 3%. The obtained LT
crystal ingot was a transparent pale yellow color.
[0045] The LT crystal ingot was then embedded in carbon powder, and
heat treatment was performed. The heat treatment conditions were
1000.degree. C., for 10 hours, under a nitrogen gas atmosphere.
[0046] Subsequently, while maintaining the nitrogen gas atmosphere,
cooling to room temperature was performed, and the LT substrate was
removed. The obtained LT crystal ingot was an opaque reddish-brown
color.
[0047] After subjecting the LT crystal ingot to heat treatment to
remove thermal strain (nitrogen atmosphere, 1400.degree. C., 40
hours) and poling to obtain a single polarization (nitrogen
atmosphere, 650.degree. C., 2 hours), peripheral grinding, slicing,
and polishing were performed to obtain a 36.degree. RY (Rotated Y
axis) LT substrate. The 36.degree. RY LT substrate was an opaque
reddish-brown color. Furthermore, volume resistivity was 10.sup.8
.OMEGA.cm.
[0048] A thermal cycle test was then performed in which the
obtained 36.degree. RY LT substrate was heated from room
temperature to 200.degree. C. at a rate of 10.degree. C. per
minute, and then cooled to room temperature at a rate of 10.degree.
C. per minute.
[0049] As a result, surface potential was not generated, and
absolutely no sparking phenomenon was observed. In addition, the
Curie point of the 36.degree. RY LT substrate was 603.degree. C.,
the surface acoustic wave velocity was 4150 m/sec, and the physical
properties having an influence on surface acoustic wave element
properties were substantially the same as for a conventional
36.degree. RY LT. substrate.
Example 2
[0050] Treatment and testing was performed substantially in the
same manner as in example 1, with the exception that the heat
treatment temperature was 650.degree. C. The obtained 36.degree. RY
LT substrate was an opaque reddish brown color, with volume
resistivity of 10.sup.10 .OMEGA.cm. Other characteristics were
substantially the same as example 1.
Example 3
[0051] Treatment and testing was performed substantially in the
same manner as in example 1, with the exception that the heat
treatment temperature was 1600.degree. C. The obtained 36.degree.
RY LT substrate was an opaque reddish brown color, with volume
resistivity of 10.sup.6 .OMEGA.cm. Other characteristics were
substantially the same as example 1.
Example 4
[0052] Treatment and testing was performed substantially in the
same manner as in example 1, with the exception that the LT crystal
ingot was placed in a carbon crucible. The obtained 36.degree. RY
LT substrate was an opaque reddish brown color, with volume
resistivity of 10.sup.8 .OMEGA.cm. Other characteristics were
substantially the same as example 1.
Example 5
[0053] Treatment and testing was performed substantially in the
same manner as in example 1, with the exception that the LT crystal
ingot was placed in Si powder, and the heat treatment temperature
was 650.degree. C. The obtained 36.degree. RY LT substrate was an
opaque reddish brown color, with volume resistivity of 10.sup.12
.OMEGA.cm. Other characteristics were substantially the same as
example 1.
Example 6
[0054] Treatment and testing was performed substantially in the
same manner as in example 1, with the exception that the LT crystal
ingot was placed in Si powder, and the heat treatment temperature
was 1000.degree. C. The obtained 36.degree. RY LT substrate was an
opaque reddish brown color, with volume resistivity of 10.sup.10
.OMEGA.cm. Other characteristics were substantially the same as
example 1.
Example 7
[0055] Treatment and testing was performed substantially in the
same manner as in example 1, with the exception that the LT crystal
ingot was placed in Si powder, and the heat treatment temperature
was 1400.degree. C. The obtained 36.degree. RY LT substrate was an
opaque reddish brown color, with volume resistivity of 10.sup.8
.OMEGA.cm. Other characteristics were substantially the same as
example 1.
Example 8
[0056] A LT crystal with a diameter of 4 inches (101.6 mm) was
grown using the Czochralski method, from raw materials of congruent
composition. The growth atmosphere was a nitrogen-oxygen mixed gas
with an oxygen concentration of approximately 3%. The obtained LT
crystal ingot was a transparent pale yellow color.
[0057] After subjecting the LT crystal ingot to heat treatment to
remove thermal strain (air atmosphere, 1400.degree. C., 40 hours)
and poling to obtain a single polarization (air atmosphere,
650.degree. C., 2 hours), peripheral grinding, slicing, and
polishing were performed to obtain a 36.degree. RY LT substrate.
The obtained 36.degree. RY LT substrate was colorless and
transparent, with a Curie point of 603.degree. C. and a surface
acoustic wave velocity of 4150 m/sec.
[0058] The obtained 36.degree. RY LT substrate was then embedded in
Al powder, and heat treatment was performed under a nitrogen gas
atmosphere at 550.degree. C. for 10 hours. After heat treatment the
36.degree. RY LT substrate was an opaque reddish-brown color.
Furthermore, volume resistivity was 10.sup.10 .OMEGA.cm.
[0059] A thermal cycle test was then performed in which the
36.degree. RY LT substrate after heat treatment was heated from
room temperature to 200.degree. C. at a rate of 10.degree. C. per
minute, and then cooled to room temperature at a rate of 10.degree.
C. per minute.
[0060] As a result, surface potential was not generated, and
absolutely no sparking phenomenon was observed. In addition, the
Curie point of the 36.degree. RY LT substrate was 603.degree. C.,
the surface acoustic wave velocity was 4150 m/sec, and the physical
properties having an influence on surface acoustic wave element
properties were substantially the same as for a conventional
36.degree. RY LT substrate.
Example 9
[0061] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the heat
treatment temperature was 350.degree. C. The obtained 36.degree. RY
LT substrate was an opaque reddish brown color, with volume
resistivity of 10.sup.12 .OMEGA.cm. Other characteristics were
substantially the same as example 8.
Example 10
[0062] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the heat
treatment temperature was 600.degree. C. The obtained 36.degree. RY
LT substrate was an opaque reddish brown color, with volume
resistivity of 10.sup.9 .OMEGA.cm. Other characteristics were
substantially the same as example 8.
Example 11
[0063] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a mixed powder of Al 10%
and Al.sub.2O.sub.3 90%, and the heat treatment temperature was
350.degree. C. The obtained 36.degree. RY LT substrate was an
opaque reddish brown color, with volume resistivity of 10.sup.12
.OMEGA.cm. Other characteristics were substantially the same as
example 8.
Example 12
[0064] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a mixed powder of Al 10%
and Al.sub.2O.sub.3 90%, and the heat treatment temperature was
550.degree. C. The obtained 36.degree. RY LT substrate was an
opaque reddish brown color, with volume resistivity of 10.sup.11
.OMEGA.cm. Other characteristics were substantially the same as
example 8.
Example 13
[0065] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a mixed powder of Al 10%
and Al.sub.2O.sub.3 90%, and the heat treatment temperature was
600.degree. C. The obtained 36.degree. RY LT substrate was an
opaque reddish brown color, with volume resistivity of 10.sup.10
.OMEGA.cm. Other characteristics were substantially the same as
example 8.
Example 14
[0066] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a mixed powder of Al 90%
and Al.sub.2O.sub.3 10%, and the heat treatment temperature was
350.degree. C. The obtained 36.degree. RY LT substrate was an
opaque reddish brown color, with volume resistivity of 10.sup.12
.OMEGA.cm. Other characteristics were substantially the same as
example 8.
Example 15
[0067] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a mixed powder of Al 90%
and Al.sub.2O.sub.3 10%, and the heat treatment temperature was
550.degree. C. The obtained 36.degree. RY LT substrate was an
opaque reddish brown color, with volume resistivity of 10.sup.11
.OMEGA.cm. Other characteristics were substantially the same as
example 8.
Example 16
[0068] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a mixed powder of Al 90%
and Al.sub.2O.sub.3 10%, and the heat treatment temperature was
600.degree. C. The obtained 36.degree. RY LT substrate was an
opaque reddish brown color, with volume resistivity of 10.sup.10
.OMEGA.cm. Other characteristics were substantially the same as
example 8.
Example 17
[0069] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a powder of Ca, and the
heat treatment temperature was 350.degree. C. The obtained
36.degree. RY LT substrate was an opaque reddish brown color, with
volume resistivity of 10.sup.12 .OMEGA.cm. Other characteristics
were substantially the same as example 8.
Example 18
[0070] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a powder of Ca, and the
heat treatment temperature was 600.degree. C. The obtained
36.degree. RY LT substrate was an opaque reddish brown color, with
volume resistivity of 10.sup.10 .OMEGA.cm. Other characteristics
were substantially the same as example 8.
Example 19
[0071] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a powder of Ti, and the
heat treatment temperature was 350.degree. C. The obtained
36.degree. RY LT substrate was an opaque reddish brown color, with
volume resistivity of 10.sup.12 .OMEGA.cm. Other characteristics
were substantially the same as example 8.
Example 20
[0072] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a powder of Ti, and the
heat treatment temperature was 600.degree. C. The obtained
36.degree. RY LT substrate was an opaque reddish brown color, with
volume resistivity of 10.sup.10 .OMEGA.cm. Other characteristics
were substantially the same as example 8.
Example 21
[0073] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a powder of Si, and the
heat treatment temperature was 350.degree. C. The obtained
36.degree. RY LT substrate was an opaque reddish brown color, with
volume resistivity of 10.sup.12 .OMEGA.cm. Other characteristics
were substantially the same as example 8.
Example 22
[0074] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a powder of Si, and the
heat treatment temperature was 600.degree. C. The obtained
36.degree. RY LT substrate was an opaque reddish brown color, with
volume resistivity of 10.sup.10 .OMEGA.cm. Other characteristics
were substantially the same as example 8.
Example 23
[0075] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a powder of Zn, and the
heat treatment temperature was 350.degree. C. The obtained
36.degree. RY LT substrate was an opaque reddish brown color, with
volume resistivity of 10.sup.12 .OMEGA.cm. Other characteristics
were substantially the same as example 8.
Example 24
[0076] Treatment and testing was performed substantially in the
same manner as in example 8, with the exception that the obtained
36.degree. RY LT substrate was placed in a powder of Zn, and the
heat treatment temperature was 410.degree. C. The obtained
36.degree. RY LT substrate was an opaque reddish brown color, with
volume resistivity of 10.sup.10 .OMEGA.cm. Other characteristics
were substantially the same as example 8.
Example 25
[0077] Treatment and testing was performed substantially in the
same manner as in examples 1 through 24, with the exception that
the heat treatment time was 4 hours. The obtained 36.degree. RY LT
substrates were all opaque reddish brown color. The substrate
characteristics were respectively substantially the same as for
examples 1 through 24.
Comparative Example 1
[0078] Processing and testing was performed substantially in the
same manner as in examples 1 through 25, with the exception that
heat treatment according to the present invention was not performed
in the manufacturing process. The obtained crystal ingot was a
transparent pale yellow color after each process. The obtained
36.degree. RY LT substrate was colorless and transparent, with a
Curie point of 603.degree. C. and a surface acoustic wave velocity
of 4150 m/sec.
[0079] The obtained 36.degree. RY LT substrate was placed in an SUS
vessel, and heat treatment was performed under a nitrogen gas
atmosphere, at a temperature of 1000.degree. C., for 8 hours. The
36.degree. RY LT substrate after heat treatment was a pale yellow
color, and no blackening was observed. Volume resistivity was
10.sup.15 .OMEGA.cm.
[0080] A thermal cycle test was then performed in which the
36.degree. RY LT substrate after heat treatment was heated from
room temperature to 200.degree. C. at a rate of 10.degree. C. per
minute, and then cooled to room temperature at a rate of 10.degree.
C. per minute. As a result, an intense sparking phenomenon was
observed at the substrate surface.
Comparative Example 2
[0081] Processing and testing was performed substantially in the
same manner as in comparative example 1, with the exception that
the heat treatment temperature was 800.degree. C. The 36.degree. RY
LT substrate after heat treatment was a pale yellow color, and no
blackening was observed. During the thermal cycle test, an intense
sparking phenomenon was observed at the substrate surface.
Comparative Example 3
[0082] Processing and testing was performed substantially in the
same manner as in comparative example 1, with the exception that
the heat treatment temperature was 480.degree. C. The 36.degree. RY
LT substrate after heat treatment was a pale yellow color, and no
blackening was observed. During the thermal cycle test, an intense
sparking phenomenon was observed at the substrate surface.
* * * * *