U.S. patent application number 11/124845 was filed with the patent office on 2005-09-15 for method and apparatus for increasing bulk conductivity of a ferroelectric material.
Invention is credited to Alexandrovski, Alexei L., Caudillo, David, Foulon, Gisele L., Galambos, Ludwig L., Lazar, Janos J., McRae, Joseph M., Miles, Ronald O., Miller, Gregory D., Risk, Gabriel C..
Application Number | 20050201926 11/124845 |
Document ID | / |
Family ID | 29999359 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050201926 |
Kind Code |
A1 |
Miles, Ronald O. ; et
al. |
September 15, 2005 |
Method and apparatus for increasing bulk conductivity of a
ferroelectric material
Abstract
In one embodiment, a ferroelectric material is processed by
placing the material in an environment including metal vapor and
heating the material to a temperature below the Curie temperature
of the material. This allows the bulk conductivity of the
ferroelectric material to be increased without substantially
degrading its ferroelectric domain properties. In one embodiment,
the ferroelectric material comprises lithium tantalate and the
metal vapor comprises zinc.
Inventors: |
Miles, Ronald O.; (Menlo
Park, CA) ; Galambos, Ludwig L.; (Menlo Park, CA)
; Lazar, Janos J.; (Redwood City, CA) ; Risk,
Gabriel C.; (Burlingame, CA) ; Alexandrovski, Alexei
L.; (Los Gatos, CA) ; Miller, Gregory D.;
(Foster City, CA) ; Caudillo, David; (Saratoga,
CA) ; McRae, Joseph M.; (San Jose, CA) ;
Foulon, Gisele L.; (Cupertino, CA) |
Correspondence
Address: |
OKAMOTO & BENEDICTO, LLP
P.O. BOX 641330
SAN JOSE
CA
95164
US
|
Family ID: |
29999359 |
Appl. No.: |
11/124845 |
Filed: |
May 9, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11124845 |
May 9, 2005 |
|
|
|
10187330 |
Jun 28, 2002 |
|
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Current U.S.
Class: |
423/594.8 |
Current CPC
Class: |
C01G 33/00 20130101;
C01G 35/00 20130101; C01P 2006/42 20130101 |
Class at
Publication: |
423/594.8 |
International
Class: |
C01G 033/00 |
Claims
1-11. (canceled)
12. A system for processing a lithium tantalate material, the
system comprising: a sealed container containing a lithium
tantalate material and a metal; and a process tube configured to
heat the container at a temperature below the Curie temperature of
the lithium tantalate material such that vapor of the metal reacts
with the lithium tantalate material to increase the bulk
conductivity of the lithium tantalate material.
13. The system of claim 12 wherein the lithium tantalate material
is in wafer form.
14. The system of claim 12 wherein the lithium tantalate material
is in a wafer cage inside the sealed container.
15. The system of claim 12 further comprising a housing enclosing
the sealed container.
16. The system of claim 12 wherein the metal comprises zinc.
17-20. (canceled)
21. A system for processing a ferroelectric material, the system
comprising: a container containing a metal and a plurality of
wafers, each of the plurality of wafers comprising a ferroelectric
material; and a heater configured to heat the container such that
the plurality of wafers are heated to a temperature less than a
Curie temperature of the wafers and the metal is converted to metal
vapor and reacts with the wafers to increase the bulk conductivity
of the wafers.
22. The system of claim 21 wherein the ferroelectric material
comprises lithium tantalate and the metal comprises zinc.
23. The system of claim 21 wherein the metal comprises zinc.
24. The system of claim 21 wherein the ferroelectric material
comprises lithium tantalate.
25. The system of claim 21 wherein the container is a sealed
container.
26. The system of claim 21 wherein the temperature is less than or
equal to about 595.degree. C.
27. The system of claim 21 wherein the plurality of wafers are
placed in a wafer cage inside the container.
28. The system of claim 21 wherein the heater is part of a process
tube where the container is placed to be heated.
29. The system of claim 28 further comprising: a cantilever on
which the container rests in the process tube, wherein the
cantilever is programmed to move the container towards a door of
the process tube at a particular rate during a ramp down of a
heater temperature at an end of a processing of the wafers.
30. The system of claim 21 further comprising: a housing
surrounding the container while the container is heated.
31. The system of claim 30 wherein the housing has a closed-end and
an open-end, and wherein the housing is placed in a process tube
with the open-end facing a door of the process tube to facilitate
creation of a thermal gradient in the container during a
temperature ramp down of the container.
32. A system for processing a ferroelectric material, the system
comprising: a sealed container containing a metal and a plurality
of wafers in a wafer cage, each of the plurality of wafers
comprising a ferroelectric material; a housing containing the
sealed container; and a process tube containing the housing that
contains the sealed container, the process tube being configured to
heat the housing such that the wafers are heated to a temperature
below a Curie temperature of the wafers and the metal is converted
to metal vapor that interacts with the wafers to increase the bulk
conductivity of the wafers.
33. The system of claim 32 wherein the ferroelectric material
comprises lithium tantalate.
34. The system of claim 32 wherein the metal comprises zinc.
35. The system of claim 32 wherein the housing has a closed-end and
an open-end, and wherein the open-end is facing a door of the
process tube to facilitate creation of a thermal gradient in the
container during a temperature ramp down of the container.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to material
processing, and more particularly, but not exclusively, to methods
and apparatus for processing a ferroelectric material.
[0003] 2. Description of the Background Art
[0004] Lithium tantalate (LiTaO.sub.3) and lithium niobate
(LiNbO.sub.3) are widely used materials for fabricating nonlinear
optical devices because of their relatively large electro-optic and
nonlinear optical coefficients. These nonlinear optical devices
include wavelength converters, amplifiers, tunable sources,
dispersion compensators, and optical gated mixers, for example.
Lithium tantalate and lithium niobate are also known as
ferroelectric materials because their crystals exhibit spontaneous
electric polarization.
[0005] Because lithium tantalate and lithium niobate materials have
relatively low bulk conductivity, electric charge tends to build up
in these materials. Charge may build up when the materials are
heated or mechanically stressed. Because the charge may short and
thereby cause a device to fail or become unreliable, device
manufacturers have to take special (and typically costly)
precautions to minimize charge build up or to dissipate the
charge.
[0006] The bulk conductivity of a lithium niobate material may be
increased by heating the lithium niobate material in an environment
including a reducing gas. The reducing gas causes oxygen ions to
escape from the surface of the lithium niobate material. The
lithium niobate material is thus left with excess electrons,
resulting in an increase in its bulk conductivity. The increased
bulk conductivity prevents charge build up.
[0007] Although the just described technique may increase the bulk
conductivity of a lithium niobate material under certain
conditions, the technique is not particularly effective with
lithium tantalate. A technique for increasing the bulk conductivity
of a lithium tantalate material is desirable because lithium
tantalate is more suitable than lithium niobate for some
high-frequency surface acoustic wave (SAW) filter applications, for
example.
SUMMARY
[0008] In one embodiment, a ferroelectric material is processed by
placing the material in an environment including metal vapor and
heating the material to a temperature below the Curie temperature
of the material. This allows the bulk conductivity of the
ferroelectric material to be increased without substantially
degrading its ferroelectric domain properties. In one embodiment,
the ferroelectric material comprises lithium tantalate and the
metal vapor comprises zinc.
[0009] These and other features of the present invention will be
readily apparent to persons of ordinary skill in the art upon
reading the entirety of this disclosure, which includes the
accompanying drawings and claims.
DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows a schematic diagram of a container in
accordance with an embodiment of the present invention.
[0011] FIG. 2 shows a schematic diagram of a housing in accordance
with an embodiment of the present invention.
[0012] FIG. 3 shows a system for increasing the bulk conductivity
of a ferroelectric material in accordance with an embodiment of the
present invention.
[0013] FIG. 4 shows a flow diagram of a method of increasing the
bulk conductivity of a ferroelectric material in accordance with an
embodiment of the present invention.
[0014] The use of the same reference label in different drawings
indicates the same or like components. Drawings are not necessarily
to scale unless otherwise noted.
DETAILED DESCRIPTION
[0015] In the present disclosure, numerous specific details are
provided such as examples of apparatus, process parameters, process
steps, and materials to provide a thorough understanding of
embodiments of the invention. Persons of ordinary skill in the art
will recognize, however, that the invention can be practiced
without one or more of the specific details. In other instances,
well-known details are not shown or described to avoid obscuring
aspects of the invention.
[0016] Moreover, it should be understood that although embodiments
of the present invention will be described in the context of
lithium tantalate, the present invention is not so limited. Those
of ordinary skill in the art can adapt the teachings of the present
invention to increase the bulk conductivity of other ferroelectric
materials such as lithium niobate, for example.
[0017] In accordance with an embodiment of the present invention,
the bulk conductivity of a ferroelectric material may be increased
by placing the material in an environment including metal vapor and
heating the material to a temperature up to the Curie temperature
of the material. Generally speaking, the Curie temperature of a
ferroelectric material is the temperature above which the material
loses its ferroelectric properties. By heating a single domain
ferroelectric material to a temperature below its Curie temperature
in the presence of a metal vapor with relatively high diffusivity,
the ferroelectric domain state of the ferroelectric material is not
appreciably degraded.
[0018] Preferably, the metal to be converted to vapor has
relatively high diffusivity and has the potential to reduce the
oxidation state of the ferroelectric material. The inventors
believe that these properties will allow ions of the metal to
diffuse a few microns into the surface of the ferroelectric
material to fill lattice site vacancies, reducing the state of
oxidation and thereby liberating electrons from the ferroelectric
material and beginning a process of filling negative ion site
vacancies throughout the bulk of the material. The electrons that
fill these negative ion site vacancies are believed to be bound to
point defect sites. These bound electrons, in general, will have a
spectrum of energy levels that leave the ferroelectric material
with a distinctive broad coloration. With the filling of lattice
site vacancies and supplying neutralizing electrons to point defect
sites, excess charge can be rapidly neutralized or conducted away
perhaps as a polaron. When excess charge (electron) is introduced
into the lattice, it is energetically favorable for the electron to
move as an entity within the polarization of the lattice. Such an
entity, referred to as a "polaron", results in increased electron
mobility. Since the electron charge is screened by the lattice,
polarons may move unobstructed by electrostatic forces along the
lattice.
[0019] In one embodiment, the metal to be converted to vapor
comprises zinc and the ferroelectric material comprises lithium
tantalate in wafer form. Zinc vapor may be created by heating zinc
to a temperature slightly below the Curie temperature of the
lithium tantalate wafer. To obtain a vapor pressure that is high
enough for efficient diffusion at a temperature below the Curie
temperature, the metal and lithium tantalate wafer may be heated in
a sealed container that has a predetermined volume. The inventors
believe that heating a lithium tantalate wafer in zinc vapor causes
zinc to diffuse into the surface of the lithium tantalate wafer and
fill lithium site vacancies. It is believed that this results in
the release of extra electrons according to equation 1:
Zn+VLi.sup.-=Zn.sup.+2Li+2e.sup.- EQ. 1
[0020] It is believed that the extra electrons are trapped in
negative ion site vacancies in the bulk of the lithium tantalate
wafer. Increased electron mobility in the bulk of the lithium
tantalate wafer results when excess charge build up due to
pyroelectric or piezoelectric effects are conducted away as
polarons. That is, the inventors believe that the increased
conductivity of the lithium tantalate wafer appears to be polaron
in nature.
[0021] Referring now to FIG. 1, there is shown a schematic diagram
of a container 210 in accordance with an embodiment of the present
invention. Container 210 may be used to hold one or more wafers 201
to be processed and a metal 202 to be converted to vapor. Container
210 includes a body 211 and an end-cap 212. End-cap 212 may be
welded onto body 211 using an oxygen-hydrogen torch, for
example.
[0022] Body 211 includes a tube section 213 and a tube section 214.
Container 210 may be sealed by capping tube sections 213 and 214,
and welding end-cap 212 onto body 211. Tube section 214 may be
capped by inserting a plug 215 into tube section 214 and welding
the wall of plug 215 to that of tube section 214. Tube section 213
may be a sealed capillary tube. A vacuum pump may be coupled to
tube section 214 to evacuate container 210. A sealed tube section
213 may be cracked open at the end of a process run to increase the
pressure in container 210 (e.g., to bring the pressure in container
210 to atmospheric pressure).
[0023] Still referring to FIG. 1, one or more wafers 201 may be
placed in a wafer cage 203, which may then be inserted into
container 210. A metal 202 may be placed inside wafer cage 203
along with wafers 201. Wafer cage 203 may be a commercially
available wafer cage such as of those available from LP Glass, Inc.
of Santa Clara, Calif. Wafer cage 203 may be made of quartz, for
example.
[0024] Table 1 shows the dimensions of a container 210 in one
embodiment. It is to be noted that container 210 may be scaled to
accommodate a different number of wafers.
1TABLE 1 (REFER TO FIG. 1) Dimension Value (mm) D1 Inside Diameter
120.00 D2 Outside Diameter 125.00 D3 217.00 D4 279.24 D5 76.20 D6
80.00 D7 40.00 D8 60.00 D9 25.40 D10 Inside Diameter 4.00 Outside
Diameter 6.00 D11 Inside Diameter 7.00 Outside Diameter 9.00
[0025] FIG. 2 shows a schematic diagram of a housing 220 in
accordance with an embodiment of the present invention. Housing 220
may be a cylindrical container made of alumina. Container 210 may
be inserted in housing 220, as shown in FIG. 2, and then heated in
a process tube, as shown in FIG. 3. Housing 220 surrounds container
210 to allow for uniform heating of container 210. Additionally,
housing 220 serves as a physical barrier to protect container 210
from breaking.
[0026] As shown in FIG. 2, housing 220 may have a closed-end 224
and an open-end 221. Container 210 is preferably placed inside
housing 220 such that end-cap 212 is towards open-end 221. Open-end
221 allows for convenient removal of container 210 from housing
220. Open-end 221 also facilitates creation of a thermal gradient
in container 210 during a temperature ramp down. The thermal
gradient results in a cold spot in end-cap 212 that attracts
precipitating metal vapor away from the wafers inside container
210. This minimizes the amount of precipitates that have to be
removed from the surface of the wafers. This aspect of the present
invention will be further described below.
[0027] FIG. 3 shows a system 300 for increasing the bulk
conductivity of a ferroelectric material in accordance with an
embodiment of the present invention. System 300 includes a process
tube 310 containing housing 220. As mentioned, housing 220 houses
container 210, which in turn holds metal 202 and wafers 201.
Process tube 310 may be a commercially available furnace generally
used in the semiconductor industry. Process tube 310 includes
heaters 303 (i.e., 303A, 303B, 303C) for heating housing 220 and
all components in it. Process tube 310 may be 72 inches long, and
divided into three 24-inch heating zones with the middle heating
zone being the "hot zone". Process tube 310 may include a first
heating zone heated by a heater 303A, a second heating zone heated
by a heater 303B, and a third heating zone heated by a heater 303C.
Process tube 310 also includes a cantilever 302 for moving housing
220, and a door 301 through which housing 220 enters and leaves the
process tube. Housing 220 may be placed in the middle of process
tube 310 with open-end 221 facing door 301.
[0028] FIG. 4 shows a flow diagram of a method 400 for processing a
ferroelectric material in accordance with an embodiment of the
present invention. Method 400 will be described using container
210, housing 220, and system 300 as an example. It should be
understood, however, that flow diagram 400, container 210, housing
220, and system 300 are provided herein for illustration purposes
and are not limiting.
[0029] In step 402 of FIG. 4, metal 202 and one or more wafers 201
are placed in wafer cage 203. Wafer cage 203 is then placed inside
container 210. In one embodiment, wafers 201 are 42 degree
rotated-Y lithium tantalate wafers that are 100 mm in diameter,
while metal 202 comprises zinc that is 99.999% pure. In one
embodiment, five wafers 201 are placed in wafer cage 203 along with
about 8 grams of zinc. The zinc may be in pellet form. Zinc pellets
that are 99.999% pure are commercially available from Johnson
Matthey, Inc. of Wayne, Pa. Note that the amount of zinc per wafer
may be varied to suit specific applications.
[0030] In step 404, container 210 is pumped down to about 10.sup.-7
Torr and then heated to about 200.degree. C. for about five hours.
Step 404 may be performed by welding end-cap 212 onto body 211,
capping tube section 213, coupling a vacuum pump to tube section
214, and heating container 210 with a heating tape wrapped around
container 210. Step 404 helps remove oxygen sources, water, and
other contaminants out of container 210 before metal 202 is
melted.
[0031] In step 406, container 210 is back-filled so that the
pressure in container 210 at slightly below Curie temperature is
approximately 760 Torr. In one embodiment, container 210 is
back-filled to about 190 Torr. This increases the pressure inside
container 210, thus making it safer to heat container 210 to
elevated temperatures for long periods of time. Container 210 may
be back-filled with an inert gas such as Argon. Optionally,
container 210 may be back-filled with forming gas comprising 95%
nitrogen and 5% hydrogen. Note that the forming gas alone is not
sufficient to reduce a lithium tantalate material so that its bulk
conductivity is increased. However, in the present example, forming
gas helps in trapping oxygen that may have remained in container
210 after step 404. Back-filling container 210 with forming gas may
not be needed in applications where container 210 has been
completely purged of contaminants. Container 210 may be back-filled
by welding plug 215 to tube section 214, breaking the cap off tube
section 213, and then flowing back-fill gas through tube section
213.
[0032] In step 408, container 210 is sealed. At this point,
container 210 may be sealed by removing the source of the back-fill
gas and capping tube section 213. (Note that end-cap 212 has
already been welded onto body 211 and tube section 214 has already
been capped in previous steps.)
[0033] In step 410, container 210 is inserted in housing 220.
[0034] In step 412, housing 220 is heated in process tube 310 at a
temperature below the Curie temperature of wafers 201. Heating
housing 220 at a temperature below the Curie temperature of wafers
201 melts metal 202 without substantially degrading the
ferroelectric properties of wafers 201. Melting metal 202 results
in metal vapor surrounding wafers 201. In this example, the metal
vapor comprises zinc vapor and wafers 201 are of lithium tantalate.
The interaction between zinc vapor and lithium tantalate that the
inventors believe causes the bulk conductivity of wafers 201 to
increase has been previously described above.
[0035] In one embodiment, housing 220 is heated in the middle of a
process tube 310 that is 72 cm long. Also, as shown in FIG. 3,
housing 220 may be placed in process tube 310 such that open-end
221 is facing door 301. Container 210 is preferably placed inside
housing 220 such that end-cap 212 is towards open-end 221 (see FIG.
2).
[0036] In one embodiment, housing 220 is heated in process tube 310
at a ramp up rate of about 150.degree. C./hour to a maximum
temperature of about 595.degree. C., for about 240 hours.
Preferably, housing 220 is heated to a maximum temperature just a
few degrees below the Curie temperature of wafers 201. Because the
Curie temperature of wafers may vary depending on their
manufacturer, the maximum heating temperature may have to be
adjusted for specific wafers. The heating time of housing 220 in
process tube 310 may also be adjusted to ensure adequate
indiffusion of the metal vapor. Note that because method 400 is
performed on bare wafers 201 (i.e., before devices are fabricated
on wafers 201), the total process time of method 400 does not
appreciably add to the amount of time needed to fabricate a
device.
[0037] Continuing in step 414, the temperature inside process tube
310 is ramped down to prevent the just processed wafers 201 from
being degraded by thermal shock. In one embodiment, the temperature
inside process tube 310 is ramped down by setting its temperature
set point to 400.degree. C. Thereafter, cantilever 302 (see FIG. 3)
may be programmed to move housing 220 towards door 301 at a rate of
about 2 cm/minute for 3 minutes, with a 1.5 (one and a half) minute
pause time between movements. That is, housing 220 may move at a
rate of 3 cm/minute for 3 minutes, then pause for 1.5 minutes, then
move at a rate of 3 cm/minute for 3 minutes, then pause for 1.5
minutes, and so on for a total of 40 minutes until housing 220
reaches door 301.
[0038] As housing 220 is moved towards door 301, open-end 221 of
housing 220 becomes cooler than closed-end 224. This results in a
thermal gradient inside container 210, with end-cap 212 (which
along with open-end 221 is facing door 301) becoming colder than
the rest of container 210. The creation of a thermal gradient in
container 210 may also be facilitated by adjusting the heaters of
process tube 310 such that the temperature is lower towards door
301. The thermal gradient inside container 210 results in end-cap
212 becoming a cold spot that attracts precipitating metal vapor
away from wafers 201.
[0039] In step 416, housing 220 is removed from process tube 310.
Container 210 is then removed from housing 220.
[0040] In step 418, wafers 201 are removed from container 210. Step
418 may be performed by first cracking open tube section 213 (see
FIG. 1) to slowly expose container 210 to atmosphere. Container 210
may also be back-filled with an inert gas. Thereafter, end-cap 212
may be cut away from body 211 using a diamond-blade saw, for
example.
[0041] In step 420, wafers 201 are polished to remove precipitates
from their surface and to expose their bulk. In one embodiment,
both sides of a wafer 201 are polished by chemical-mechanical
polishing to remove about 50 microns from each side.
[0042] In an experiment, five 42 degree rotated-Y lithium tantalate
wafers that are 100 mm in diameter, hereinafter referred to as
"experimental wafers", were processed in accordance with the just
described method 400. The experimental wafers were placed in a
container 210 along with 8 grams of zinc, and then heated in a
process tube 310 to 595.degree. C. for 240 hours. Thereafter, the
temperature of the process tube 310 was ramped down and the
experimental wafers were removed from the container 210. The
experimental wafers were then polished on both sides and visually
inspected. The experimental wafers looked homogenous and grayish in
color. The bulk conductivity of the experimental wafers was then
tested by placing them one at a time on a hot plate, raising the
temperature of the hot plate from 80.degree. C. to 120.degree. C.
at a rate of 3.degree. C./min, and measuring the resulting electric
field near the surface of the wafers. The electric field was
measured using an electrometer from Keithley Instruments of
Cleveland, Ohio under the model name Model 617. The experimental
wafers did not produce any measurable electric field near their
surface, indicating that their bulk conductivity has increased.
[0043] For comparison purposes, an unprocessed 42 degree rotated-Y
lithium tantalate wafer that is 100 mm in diameter, referred to
herein as a "control wafer", was placed on a hot plate. The
temperature of the hot plate was then increased from 80.degree. C.
to 120.degree. C. at a rate of 3.degree. C./min. Measuring the
electric field near the surface of the control wafer indicated a
400V increase for every 20.degree. C. change in temperature. This
indicates that the bulk conductivity of the control wafer is
relatively low.
[0044] While specific embodiments of the present invention have
been provided, it is to be understood that these embodiments are
for illustration purposes and not limiting. Many additional
embodiments will be apparent to persons of ordinary skill in the
art reading this disclosure. Thus, the present invention is limited
only by the following claims.
* * * * *