Electrically-Reduced Lithium Tantalate Compositions
According to some embodiments, a pre-poled, single-domain body of a ferroelectric crystalline material such as lithium tantalate or lithium niobate is electrically reduced by applying a voltage across the body in a non-oxidizing environment while the body is heated to a process temperature below its Curie temperature. The voltage generates an electric field along the polar axis of the body. Electrodes may be formed on the body surface by applying an acetate-based silver paint. Exemplary methods allow achieving lithium tantalate electrical conductivity values of 10−11 to 10−9 Siemens/cm and average optical absorption values greater than 50% per 0.4 mm thickness at wavelengths of 300 nm and 460 nm.
This application is a divisional of U.S. patent application Ser. No. 11/681,948, filed Mar. 5, 2007, scheduled to issue on Jun. 1, 2010 as U.S. Pat. No. 7,728,697, which claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/847,483, filed Sep. 26, 2006. Both applications referenced above are herein incorporated by reference.BACKGROUND
The present invention relates generally to systems and methods for increasing the electrical conductivity of ferroelectric materials such as lithium niobate and lithium tantalate.
Lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) are well known ferroelectric materials. Their single polar properties coupled with their transparency from 350 nm to 4000 nm make them useful for many non-linear optical applications. The piezoelectric properties of these crystals have found wide application as substrates for Surface Acoustic Wave (SAW) devices. A thin film inter digital transducer (IDT) deposited on the free surface of a piezoelectric crystal may be used for RF filters and IF filters in cellular phones and base stations, among others. Various rotated-axis cuts of LiTaO3 have also been used to realize new and unique SAW devices for a number of applications. The trend for SAW devices using this material is to smaller area footprints and thinner wafer material.
Substantial progress has been made in producing exceptionally flat LiTaO3 material with improved techniques in polishing and grinding. At the same time, many of the lithium tantalate treatment processes expose the material to potentially damaging high electrical fields due to pyroelectric effects produced within the crystal during various process heat treatments. High electric fields can be developed across semi-insulating pyroelectric crystals with relatively small changes in temperature. For a crystal such as LiTaO3, whose pyroelectric coefficient is 10−8 Coul/cm2 deg K and dielectric constant is 46, a 25° C. change can produce an electric field on the order of 50 kV, easily enough to create a discharge within the crystal or breakdown the air surrounding it. Pyroelectric effects can cause device fabrication problems including eventual device performance degradation, and in some cases catastrophic crystal fractures. To work around the inconvenience of these effects makes the fabrication process more costly and time consuming.
Charge build up during processing can be reduced by creating a more conductive crystal substrate. Increasing crystal conductivity may be achieved by using a reducing agent in a heat treatment close to the Curie temperature of the crystal. In U.S. Pat. No. 6,932,957, Miles et al. describe a method of increasing the bulk conductivity of lithium tantalate by placing the lithium tantalate in an environment including zinc vapor, and heating the lithium tantalate to below its Curie temperature.SUMMARY
According to one aspect of the present invention, an electrical reduction method comprises heating a substantially single-domain body of a ferroelectric material selected from lithium niobate and lithium tantalate to a temperature below a Curie temperature of the body; and while maintaining the heated body below the Curie temperature in a non-oxidizing environment, applying a voltage across the body to increase a bulk conductivity of the body.
According to another aspect, a composition comprises a substantially single-domain, electrically-reduced body of lithium tantalate having an average optical absorption per 0.4 mm thickness greater than 50% at 300 nm and greater than 50% at 460 nm, and in particular between 60% and 80% at 300 nm and between 60% and 80% at 460 nm.
According to another aspect, a composition comprises a substantially single-domain, electrically-reduced body of lithium tantalate having an electrical conductivity higher than about 10−11 Siemens/cm (10−11 Ω−1cm−1) and in particular between 10−9 Siemens/cm and 10−11 Siemens/cm.
According to another aspect, an apparatus comprises: a vacuum furnace comprising an internal chamber having a gas inlet for receiving a gas to establish a non-oxidizing environment in the internal chamber, a heater for heating the internal chamber, and a set of electrical contacts situated inside the internal chamber; a substantially single-domain body of a ferroelectric material selected from lithium niobate and lithium tantalate, wherein the body is connected to the electrical contacts; and a controller connected to the vacuum furnace, for controlling a heating of the furnace to heat the body to a temperature below a Curie temperature of the body, and controlling an applying the voltage across the contacts to increase a bulk conductivity of the body while the body is maintained in the non-oxidizing environment below the Curie temperature of the body.
The foregoing aspects and advantages of the present invention will become better understood upon reading the following detailed description and upon reference to the drawings where:
The following description illustrates the present invention by way of example and not necessarily by way of limitation. Any reference to an element is understood to refer to at least one element. A set of elements is understood to include one or more elements. A plurality of elements includes at least two elements. Unless otherwise specified, any described method steps need not be necessarily performed in a particular illustrated order. Unless otherwise specified, applying a voltage across a body is not limited to applying the voltage using a constant-voltage DC source. Applying a voltage across a body encompasses applying a time-varying voltage, as well as connecting a constant-current or controlled-current source to the body to generate the applied voltage. Unless otherwise specified, any recited material (e.g. lithium tantalate) encompasses without limitation doped as well as undoped forms of the material. Unless otherwise specified, a DC voltage/current encompasses constant as well as time-varying DC voltages/currents. Such a time-varying voltage/current may include, for example, a smoothly-ramping DC voltage/current, or a rectified alternating voltage/current. The term boule refers to a single-crystal ingot. Unless otherwise specified, any recitation of an average optical absorption per a set length/depth (e.g. 0.4 mm thickness) refers to an average absorption scaled with the length/depth, regardless of any internal variations in optical absorption. Thus a wafer 0.3 mm-thick displaying an optical absorption of 40% has an average optical absorption of about 50% per 0.4 mm thickness, even if such a wafer exhibits variations in its optical absorption with depth that may result in different localized optical absorption characteristics at different depths.
According to some embodiments, the present invention provides systems and methods for the production of conductive ferroelectric materials, particularly lithium tantalate and/or lithium niobate via a reduction technique using direct charge transport by applying an external DC electric field along the polar axis of the ferroelectric material in a non-oxidizing environment (e.g. vacuum or inert or reducing environment).
Body 22 may be a pre-poled, single-domain crystal of a ferroelectric material such as lithium niobate or lithium tantalate.
A pair of silver-paint electrodes are formed on opposite surfaces of boule 120 as described above with reference to
A hollow stainless steel or alumina cylinder 132 opened on one end is situated over boule 120, to add thermal stability and maintain a more uniform temperature during the reduction process. A gas inlet 134 is situated along the bottom of vacuum furnace 104. Gas inlet 134 is connected to a vacuum pump through a port 136, and to a source of non-oxidizing (inert or reducing) gas through a port 138. Controller 130 is connected to ports 136, 138 and/or the vacuum pump and the source of non-oxidizing gas, for controlling the atmospheric environment within vacuum furnace 104.
The current flow through the crystal(s) and its time-integral are monitored (step 520). It was observed that even after the applied voltage is stabilized, the current through the crystal(s) continues to spontaneously grow in the presence of electronic charge filling charge vacancies within the lattice. The charge transport through the crystal(s) can be determined by monitoring the current and integrating the area under the current curve on a point-by-point basis. The applied voltage is controlled (e.g. stopped or reduced) according to the determined current and current integral values. In particular, it was observed empirically that when the charge density reaches about 1×10+3/cm3, the conductivity of the reduced lithium tantalate is about 1×10−11 Siemens/cm. The reduction process is stopped when the crystal(s) have been sufficiently reduced (step 530). The process run may be terminated according to the acquired charge density, or after a set period of time sufficient to achieve a desired charge density to deliver a desired conductivity at room temperature. The reduction process may be terminated slowly as the furnace temperature is ramped down slowly (e.g. at 60° C./hour) for ingots, or faster for wafers. The applied voltage is allowed to remain constant, and the current begins to drop as the temperature drops. Maintaining a constant applied voltage throughout the temperature ramp-down prevents charge reversal during the long cool down period. After room temperature is reached, the crystal(s) are removed from the furnace (step 532). The reduced crystals may undergo further processing known in the art and be assembled in electronic devices such as surface acoustic wave (SAW) filter devices.
Applying an electric field along the polar axis of the crystal, which creates charge transport, facilitates the electrical reduction process. In the presence of excess free electrons, existing electronic vacancy sites are filled, which makes the material more conductive. Relevant process parameters include: applied voltage, charge transport (current), temperature, and process gas pressure.
In some embodiments, a process timeline may be characterized by a voltage ramp and/or a temperature ramp.
In a temperature-ramp sequence shown in
The sequence of
A dwell phase 614 follows onset phase 612 (
Finally, a cool-down phase 616 follows dwell phase 614 (
The approaches illustrated in
The exemplary electrical reduction processes described above result in a number of changes to the electrical and optical properties of the lithium tantalate crystals.
A number of tests were run to evaluate electrically reducing boule and wafer lithium tantalate samples. Exemplary results of such tests are described below. The following examples illustrate aspects of particular embodiments of the present invention, and do not limit the scope of the invention.
In one process run, the electrically reduced body was a LiTaO3 ingot 25.6 mm thick, 101.6 mm in diameter, and having an area of 81 cm2, a volume of 210.79 cm3, and a Y-axis rotated angle of 42°. The material was prepared and processed as a single ingot in an apparatus as shown in
In another process run, the electrically reduced body was a LiTaO3 wafer 0.41 mm-thick, having a 101.6 mm diameter, an area of 81 cm2, volume of 3.32 cm3, and a Y-axis rotated angle of 38°. The wafer material was prepared and processed as a single wafer in apparatus similar to that shown in
The Y-faces were coated with two layers of acetate-based silver paint. The electrodes resulting from the silver paint application were electrically contacted with Pt foils. The wafer was placed in a vacuum furnace fitted with a six-inch diameter hearth, electrical feedthroughs and platinum leads for connecting to the crystal electrodes. The positive electrode of a power supply was connected to the positive crystal pole, and the negative electrode of the power supply was connected to the negative crystal pole. The vacuum furnace chamber was pumped down to 2.5×10−6 Torr using a turbo pump. The furnace temperature was then ramped at 120° C./hour to a process temperature of 595° C. The chamber was continuously pumped during the thermal ramp to avoid interference from out-gassing acetate by-products. After the furnace was held at 595° C. for 3 hours, the chamber was backfilled to 700 Torr of forming gas (N2\H2 85\15%) at the process temperature, and the voltage applied to the wafer was ramped at a rate of 0.25 V/minute to a voltage set point of 2.5 V. The electrical reduction process was allowed to run for about 14 hours.
High resistance measurements were made on the wafer to determine the volume conductivity of the material. Measurements were made using a Keithley® 801 high resistance probe and Keithley® 3517A electrometer. An alternating 50 V 15-second DC pulse was applied, and the current response measured. A weighted average of the measured current for each successive pulse was taken with the resultant used to determine the material resistivity. The measured current response is shown in
The following discussion reflects the inventor's beliefs regarding mechanisms through which electrical reduction proceeds and other aspects of electrical reduction, and is not intended to limit the scope of the invention. As the ferroelectric material is heated close to the Curie temperature in a partial vacuum of inert gas, reduction begins to take place in the material as oxygen atoms deplete the crystal creating excess electrons.
An additional external voltage applied via electrodes sets up a uniform electrical field having at least a component along the polar axis of the material. As electrons are accelerated through the material they are captured in the available oxygen vacancies. Unlike in a purely chemically-induced reduction process, increasing the voltage of the external field allows increasing the number of available free electrons, thus increasing the Fermi level in the energy bands and allowing electrons to occupy higher energy-state sites. As the presence of the electronic charge builds, the conductivity of the material increases, and an accompanying spontaneous growth in current for a set voltage is observed. Monitoring the current growth under these conditions provides a real time measurement of the reduction in the crystal. Integrating the area under the current growth curve on a point-by-point basis gives a real time measure of the charge transport through the material. The charge transport density for a given volume can be used as a metric for the reduction process in terms of material conductivity, where, for instance, a charge transport density of 1.0×10+3 Coul/cm3 indicates the room temperature volume conductivity of the material is on the order of 1.0×10−11 Siemens/cm.
The value of voltage applied to the sample may be determined empirically according to the thickness of the sample (for example, to achieve a given unit voltage along the length of the polar axis) and the rotational orientation of the Y-axis. The applied voltage may also be chosen according to the resistive drop across the electrode material, which is influenced by the coating conductivity and how tightly the conducting electrode layer bonds to the surface of the crystal at the process temperature. Process uniformity is enhanced if the conductive electrode layer forms a uniform bond to the crystal surface. The acquired volume charge density, which is directly related to the resultant material conductivity, may be used as a process metric. The applied voltage may be selected so as to generate enough charge density per unit volume to achieve a desired conductivity.
Using an external voltage to facilitate the reduction process provides a flexible mechanism to optimize and control the reduction process to achieve desired results, such as increased conductive uniformity over a greater depth. This process can be made more efficient by scaling the parameters up from a wafer process to bulk ingots.
An exemplary electrical reduction process as described above was observed to allow producing lithium tantalate crystals with superior conductivity characteristics, in particular conductivity values on the order of 1.0×10−11 to 1.0×10−9 Siemens/cm. Measurements performed on lithium tantalate chemically reduced by a zinc reduction process as described in the above-referenced U.S. Pat. No. 6,932,957 yielded conductivity values on the order of 1.0×10−13 to 1.0×10−12 Siemens/cm.
It is believed that a chemical reduction process as described in the above-referenced U.S. Pat. No. 6,932,957 strips oxygen atoms from the surfaces of lithium tantalate wafers, creating an excess of free electrons and creating an electrical imbalance in the crystal. Oxygen atoms begin migrating toward the surface and electrons are pulled into their vacancies to equilibrate the charge imbalance. As the number of electrons increases within the crystal lattice, color centers form, leaving the material less transparent with a gray-black cast and more conductive with the presence of the electrons. The process is driven by the potential energy acquired from the interaction of the reducing material with the ionic bonds of the oxygen in the tantalate-octahedral. This energy also determines where in the lattice the electrons will reside and how tightly they are bound. The available vacancy sites are associated with energies corresponding to the ionization state of the defect lattice. If the electron doesn't possess sufficient energy it cannot occupy sites with a higher ionization potential. Such sites are also more difficult to re-oxidize when exposed to air at high temperatures.
Presently available chemical reduction processes designed to increase volume conductivity in LiTaO3 and LiNbO3 may be limited in practice to reducing agents which can be applied to the surface uniformly at a process temperature below the Curie temperature of the material, which can effectively form an intimate bond with the surface near the Curie temperature of the crystal, and still produce the amount of charge required to achieve a desired level of conductivity. The options for reducing LiNbO3 are generally broader than for LiTaO3, since the Curie temperature for LiNbO3 is above 1000° C. At these high temperatures, almost any inert atmosphere can create the required conditions for reduction in LiNbO3. Lithium tantalate, however, is generally more difficult to reduce. Lithium tantalate has a low Curie temperature of about 604° C. At this temperature the oxygen bonds within the crystal lattice are stronger, requiring a greater reduction potential. These constraints limit the candidates for a reducing medium that can generate the adequate reduction potential to create highly conductive material. Chemical reduction of the crystal using reactive metal vapor or high-temperature alkali metal coatings may limit the effectiveness of the process to a relatively narrow range of reduction potentials. For example, due to the relatively low reduction potentials of metal vapors, higher conductivities desirable for emerging technologies may be difficult to achieve with a chemical reduction process using metal vapors. Variations in the crystal orientation, which occurs due to some Y-rotational cuts for various SAW device applications, may also pose problems for a chemical reduction process, due to the geometric orientation of oxygen octahedra to the polar axis of the crystal, which forms a path for electronic transport. Some rotated Y-axis configurations may require higher energies for occupation of the depleted sites. The required electronic energy may increase beyond the capability of the reduction potential to effectively reduce the material. Therefore the process may be effective for one orientation, but not have enough energy to sufficiently reduce another.
It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. For example, the present disclosure provides numerous specific details 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 these specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention. Moreover, although embodiments of the present invention are described mainly in the context of lithium tantalate, an electrical reduction process according to embodiments of the present invention need not be so limited. Those of ordinary skill in the art can use the teachings of the present invention to increase the bulk conductivity of other ferroelectric materials such as lithium niobate, for example. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.
1. A composition comprising a substantially single-domain, electrically-reduced body of lithium tantalate, wherein:
- an average optical absorption of the body is greater than 50% per 0.4 mm thickness at a wavelength of 300 nm; and
- the average optical absorption of the body is greater than 50% per 0.4 mm thickness at a wavelength of 460 nm.
2. The composition of claim 1, wherein the optical absorption of the body is substantially greater at 460 nm than at 350 nm.
3. The composition of claim 1, wherein the average optical absorption of the body has a value between 60% and 80% per 0.4 mm thickness at the wavelength of 300 nm.
4. The composition of claim 3, wherein the average optical absorption of the body has a value between 60% and 80% per 0.4 mm thickness at the wavelength of 460 nm.
5. A composition comprising a substantially single-domain, electrically-reduced body of lithium tantalate having an average electrical conductivity higher than about 10−11 Siemens/cm.
6. The composition of claim 5, wherein the electrical conductivity is lower than about 10−9 Siemens/cm.
International Classification: C01G 35/00 (20060101);