Ferroelectric device with integrated heater for polarization

Abstract

An electro-optical method and device that allows for effective polarization of a ferroelectric device. An electro-optical device includes a waveguide, a barium titanate (BTO) layer adjacent to the waveguide, a heater electrode for heating the BTO layer in a region adjacent to the waveguide, and at least one polarization electrode adjacent to the waveguide for establishing an electric field in the BTO layer heated by the heater electrode.

Description

[0001] Related Applications

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 431,448, filed on Dec. 9, 2022, under 35 U.S.C. § 119(e), which is incorporated herein by reference in its entirety. Background of the Invention

[0003] Silicon photonics has emerged as a platform for dense and low-cost photonic integrated circuits (PICs) for a wide range of applications that require fast and energy-efficient electro-optic (EO) switches in many cases.

[0004] However, silicon modulators have major limitations. Fast modulation of only the optical phase is not possible because the changes in the real and imaginary parts of the refractive index are correlated. Additionally, the operating speed is limited by the charge carrier lifetime in forward- or reverse-biased devices. Existing silicon-based modulators typically rely on different doped regions in waveguides. Higher speed operation requires higher doping, but higher doping increases absorption. Another option is heater-based devices. Here, a heater (usually a metal wire) changes the temperature via Joule heating (current) and thus changes the temperature in a waveguide that is in close proximity but far enough away such that the optical mode does not see strong optical absorption from the heater conductor. Such heater-based devices tend to be slow, have high power consumption, and may suffer from crosstalk.

[0005] Pockels materials avoid these problems. In such materials, the refractive index is changed by an electric field. However, the Pockels effect does not exist in centrosymmetric crystals such as silicon. Therefore, materials with a significant Pockels coefficient must be integrated onto silicon photonics structures to combine the benefits of bulk Pockels modulators with the low manufacturing cost of integrated silicon photonics. Thus, photonic integrated circuits based on Pockels materials are often multi-domain and often need to be aligned to achieve a large effective Pockels effect.

[0006] There are several methods for integrating materials with a large effective Pockels effect in silicon-based modulators. For example, the Pockels effect exists in single-crystalline lithium niobate (LiNbO3, LN). Also, lithium niobate has been integrated with silicon waveguides, for example, via wafer bonding techniques. However, the size mismatch between LN wafers and silicon wafers makes it difficult to scale the integration process to large substrate sizes, which results in relatively high chip costs.

[0007] For several reasons, barium titanate (BaTiO3, BTO) has emerged enabling the implementation of Pockels effect-based devices on silicon. First, BTO has one of the largest Pockels coefficients. Second, it has been previously used in thin-film EO modulators on small-sized oxide substrates. Third, BTO can be grown on silicon substrates with large wafer sizes and excellent crystal quality, among other things. Photonic electro-optic components based on BTO on silicon wafers have been demonstrated. However, two issues strongly affect the EO response. The first is the alignment of crystalline domains, and the second is the alignment of ferroelectric domains.

[0008] In crystalline alignment, the domains are typically determined by the epitaxial process and are fixed after chip fabrication. The effective Pockels effect in waveguide (WG)-based devices depends on the orientation of the crystalline domains with respect to the light propagation direction and the orientation / direction of the applied electric field.

[0009] In ferroelectric alignment, the as-grown film typically has ferroelectric domains with spontaneous polarization. The polarization is randomly oriented along a specific crystalline direction. Thus, zero net polarization is produced in most films, which results in a zero effective Pockels coefficient. The application of an external electric field aligns the ferroelectric domains. This process is called "ferroelectric poling". However, despite the electric field, some domains may not be polarized, for example due to defect pinning or domain size effects. The lower the temperature of the film, the more significant the defect pinning. SUMMARY OF THE INVENTION

[0010] Modifying the material properties of electro-optic devices is complex and expensive and often does not result in an enhancement of the material properties of the substrate or chip. If the temperature required for effective polarization / domain alignment is applied to the entire chip, other components may be damaged.

[0011] The present invention allows for the alignment of ferroelectric domains and / or crystalline domains at achievable voltages and can be supported in a more efficient manner by a control electronics device.

[0012] One aspect of the present invention relates to an electro-optic device structure that includes a waveguide, a Pockels material structure (such as a barium titanate (BTO) layer) having the waveguide, a heater electrode, and an electrode formed together with the waveguide. According to one configuration, the Pockels material layer is disposed within and / or adjacent to the waveguide, and a heater is provided for heating the Pockels material layer in a region near the waveguide. An electrode is further provided near the waveguide for establishing an electric field in the Pockels material layer heated by the heater electrode. This arrangement allows for the alignment of the ferroelectric domains and / or crystalline domains of the Pockels material layer after the fabrication of the Pockels material structure.

[0013] These methods can offer many advantages. The material properties, crystallization effect, and ferroelectric effect in the electro-optic device can be altered by temporarily heating the electro-optic device (to change the crystalline phase and / or ferroelectric phase, coercive field) when an electric field is applied to the electrode.

[0014] One or more heater electrodes are arranged above or below the waveguide and / or adjacent to the waveguide in different configurations. Heat flow to the vicinity of the waveguide can be isolated by using etched air grooves, suspended waveguides, and / or thermal insulation with low thermal conductivity materials, and / or heat transfer from the heater electrode to the waveguide can be optimized by using high thermal conductivity materials to effectively transfer heat from the heater electrode to the Pockels layer.

[0015] A second aspect of the present invention relates to a method of manufacturing an electro-optic device. The method includes the steps of depositing and patterning or otherwise forming a Pockels material structure, such as a barium titanate (BTO) layer; depositing and patterning or otherwise forming a waveguide associated with the Pockels material layer; forming one or more heater electrodes adjacent to the Pockels material layer (e.g., above, adjacent, and / or below); forming one or more polarization electrodes and / or device drive electrodes on either side of the waveguide, although the same electrode can be used as a polarization electrode during the process of aligning ferroelectric domains and / or crystalline domains and as a drive electrode during the operation of electro-optic modulation or switching; and depositing a cladding layer around the waveguide.

[0016] These manufacturing steps can generally be performed in different orders depending on the configuration of the device and manufacturing efficiency.

[0017] Then, a polarization voltage (such as a step or ramp or any other modulation function) is applied to the polarization electrode, such as energizing the heater electrode by heating the Pockels material layer adjacent to the waveguide through a drive current to reach, for example, an elevated temperature, its Curie temperature, or the next crystalline phase transition temperature, or above / below its Curie temperature or the next crystalline phase transition temperature. This can occur in any order. Then, the heater electrode is de-energized, allowing the BTO layer to cool while maintaining the polarization voltage.

[0018] In this implementation, when the effective Pockels response drops below the threshold limit, or during the initialization of the device before operation, the polarization effect can also be restored at periodic intervals or based on a feedback loop. The polarization voltage and temperature are applied at periodic intervals of seconds, minutes, hours, days, or months in the deployed device. In the same implementation, even shorter or longer polarization intervals can be applied. The polarization events can also be applied based on irregular intervals (e.g., based on an open-loop or closed-loop control circuit).

[0019] The method can offer many advantages, such as being able to use a lower voltage compared to polarization at ambient temperature. With the proposed method, polarization can be effectively accomplished during the cooling process without heating the entire chip or wafer and maintaining a planar electric field localized to the active device region. It should also be noted that it is difficult to obtain a sufficiently large in-plane field at the wafer scale because the gaps between electrodes are typically large, on the order of the wafer diameter. This method can have a field with a pitch of a few micrometers. In-plane domains require an in-plane field or converting crystalline domains into such in-plane domains. Out-of-plane fields and polarization are possible in principle.

[0020] Furthermore, compared to the multi-domain crystalline structure imposed by thermodynamic / kinetic properties during film fabrication, crystalline alignment can occur at the electro-optic device level, which results in a single, or a preferred crystalline orientation of the Pockels material to maximize the Pockels response or optimize other film parameters such as propagation loss, dielectric constant, etc.

[0021] In some exemplary embodiments, the electro-optic device is slowly heated for several seconds to several minutes and held at a high temperature for a specific period of time. The electro-optic device is heated and cooled at a rate between microseconds to milliseconds to seconds. A voltage is continuously applied, such as between 3 volts (V) or less to 100 V or more.

[0022] Alternative methods for aligning ferroelectric domains are subject to different challenges, such as the need for a very large voltage when performing such alignment at the full wafer level, specifically for aligning domains in the wafer plane. When aligning ferroelectric domains in a device without increasing the temperature, it typically results in a relatively low degree of domain alignment due to domain pinning and other effects that prevent individual domain switching.

[0023] The method of aligning crystalline domains requires specialized engineering and matching of the crystalline lattice, as well as engineering of the coefficient of thermal expansion. Due to symmetry mismatch, it is particularly difficult to align a tetragonal crystal to a cubic crystal in a specialized manner, such as orienting the c-axis of the tetragonal crystal parallel to the substrate surface. To align crystalline domains, it is advantageous to undergo a crystalline phase change, for example from cubic symmetry to tetragonal symmetry, while applying an electric field. The electric field can break the symmetry during this phase change. For example, in the same situation, it can distort the lattice via the piezoelectric effect. However, similar to the above, applying an electric field at the wafer level while changing the temperature requires a very large voltage. The method disclosed in the present invention can offer many advantages, such as being able to use a lower voltage.

[0024] The above and other features of the present invention, including various novel details of the construction and combination of components, and other advantages, will now be described more specifically with reference to the accompanying drawings and pointed out in the claims. It will be understood that the specific methods and apparatus embodying the present invention are shown by way of illustration and not as a limitation of the present invention. The principles and features of the present invention can be used in various and numerous embodiments without departing from the scope of the present invention. Description of the Drawings

[0025] In the drawings, reference numerals refer to the same components in different views. The drawings are not necessarily to scale, but emphasis is placed on illustrating the principles of the present invention. In the drawings:

[0026] Figure 1A 、 Figure 1B and Figure 1C are cross-sectional views of implementations of electro-optical devices according to different embodiments of the present invention;

[0027] Figure 2A and Figure 2B are top views of the electro-optical devices;

[0028] Figure 3 is a hysteresis loop showing the change in coercive behavior of a ferroelectric material at different temperatures;

[0029] Figures 4A - 4D shows a process of annealing a substrate at an elevated temperature and in an electric field to polarize BTO and align its crystal domains;

[0030] Figure 5 shows a crystal domain alignment procedure for a substrate according to an embodiment of the present invention;

[0031] Figures 6A - 6D is a graph showing the polarization voltage and heater current as a function of time for a phase change of a substrate during crystal domain alignment and / or ferroelectric domain alignment according to an embodiment of the present invention; and

[0032] Figure 7A is a flowchart of a method for manufacturing an electro-optical device according to an embodiment of the present invention;

[0033] Figure 7B is a flowchart of a method for modifying the material properties of an electro-optical device according to an embodiment of the present invention. Detailed Description

[0034] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which illustrative embodiments of the present invention are shown. However, the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art.

[0035] As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. Moreover, all conjunctions used should be understood in the most inclusive sense possible. Therefore, the word "or" should be understood to have a logical "or" definition, rather than a logical "exclusive or" definition, unless the context clearly requires otherwise. Further, the singular form and the articles "a", "an" and "the" are intended to also include plural forms, unless otherwise explicitly stated. It should also be understood that when used in this specification, the terms: include, comprise, include and / or include specify the presence of the features, wholes, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, parts and / or their groups. Further, it will be understood that when an element including a component or subsystem is mentioned and / or shown as being connected or coupled to another element, it may be directly connected or coupled to another element, or there may be an intermediate element.

[0036] It will be understood that although terms such as "first" and "second" are used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Therefore, without departing from the teachings of the present invention, the element discussed below can be referred to as a second element, and similarly, the second element can be referred to as a first element.

[0037] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs. It will be further understood that terms (such as those defined in commonly used dictionaries) should be interpreted as having a meaning consistent with their meaning in the context of the relevant art, and will not be interpreted in an idealized or overly formal sense, unless explicitly defined as such herein.

[0038] Figure 1A , Figure 1B and Figure 1C are cross-sectional views of three implementations of the electro-optical device according to the present invention.

[0039] The electro-optical device 100 includes a Pockels material structure adjacent to the waveguide 104, such as a barium titanate (BaTiO3, BTO) layer 102.

[0040] In other examples, the Pockels material can be at least one of lithium niobate, barium strontium titanate ((Ba,Sr)TiO3), lead zirconate titanate (Pb(Zr,Ti)O3), and / or a polymer.

[0041] The waveguide 104 is positioned near the Pockels material structure 102 such that the optical mode in the waveguide extends into the BTO or other material. Note that in other embodiments, other materials are used as the Pockels material structure instead of BTO. Additionally, the Pockels material structure can be a combination of different layers, some of which are Pockels and some are not, such as BTO and strontium titanate (STO) or any other combination with oxide materials, for example.

[0042] Note that Figure 1C A rib waveguide configuration is shown where the waveguide 104 is part of the Pockels material structure 102.

[0043] In other examples, the waveguide 104 is arranged on one side of the Pockels material layer 102.

[0044] Further, the electro-optical device 100 has a heater electrode 106 for heating the Pockels material layer 102 near the waveguide 104. Further, the electro-optical device 100 has at least one polarization electrode or a combination of polarization and drive electrodes 108 near the waveguide 104 for establishing an electric field in the Pockels material layer 102 heated by the heater electrode 106. In some examples ( Figure 1A ) a separate polarization electrode 108 and drive electrode 112 are used. Generally, these electrodes are made of aluminum, tungsten, tin, gold, titanium, platinum, a conductive oxide, or copper, or any other conductive layer or any combination of these materials.

[0045] The Pockels material structure 102 is typically transferred to a semiconductor substrate W (such as a silicon wafer, germanium wafer, germanium-on-silicon wafer, silicon-on-insulator (SOI) wafer, etc.) via wafer bonding or deposited on the semiconductor substrate. There are typically several layers and devices C between the actual silicon wafer material W and the Pockels material layer 102. The heater 106 can typically be fabricated in these layers C between the wafer substrate W and the BTO layer 102.

[0046] The semiconductor wafer W can have various sizes, such as 100 mm, 150 mm, 200 mm, 300 mm, or larger. In one embodiment, a large 300 mm silicon wafer or a larger silicon wafer is used to increase productivity.

[0047] The waveguide 104 can be a SiN, SiON, Ge, or Si waveguide fabricated on the Pockels material layer 102. Other materials with a refractive index higher than that of the cladding layer 110 can also be used. It can also be constructed mainly of the Pockels material 102 patterned by partial etching (see Figure 1C ). It can be noted that the waveguide 104 (such as a SiN waveguide) can be tuned to have a high nonlinear refractive index. The electro-optic device 100 also includes a cladding layer 110 of silica (SiO2), Si3N4, SiO2, Al2O3, MgO, SiCN, SiON, SiCO, SiOCN, and HfO that surrounds the waveguide 104 and the Pockels material structure 102. In an exemplary embodiment, the thickness of the cladding layer 110 generally has a uniform thickness of about 1 micrometer (um) to 5 micrometers (um) or greater.

[0048] Furthermore, the cladding layer 110 is generally disposed on top of the Pockels material structure 102, the waveguide 104, the heater electrode 106, and the polarization electrodes 108 and the drive electrode 112, thereby encapsulating them. The cladding layer 110 can be provided to enhance the temperature in the Pockels material layer 102 and improve the stability of the heater electrode 106. Further, the heater electrode 106 is driven by a current and is positioned close to the waveguide 104 to obtain a good thermal connection. However, the heater electrode 106 should be far enough from the waveguide to avoid significantly attenuating the waveguide optical mode.

[0049] At least one pair of polarization electrodes 108 is provided to establish an electric field in the BTO layer 102 across the electrodes after supplying current to the heater electrode 106. This pair of polarization electrodes 108 is used to establish the alignment of the Pockels material structure 102. Then, the drive electrode 112 or the combined drive and polarization electrodes 108 (as Figure 1B and Figure 1C shown) is used to modulate the effective refractive index of the waveguide 104 during device operation, thereby modulating the phase of the optical signal propagating in the waveguide 104 via the Pockels effect in the Pockels material structure 102.

[0050] In addition, it is important to mention that if there are multiple sets of electrodes, i.e., drive electrodes and polarization electrodes, they can share a physical electrode (e.g., a common ground electrode), thereby producing only three conductors instead of four conductors.

[0051] Careful fabrication of the Pockels material structure 102 is important. To align the crystalline domains, the device can be heated and cooled while applying an electric field, similar to the polarization of ferroelectric domains described above. The annealing temperature can be higher than the temperature of the crystalline phase transition (e.g., from tetragonal to cubic in the case of BTO), for example, having a range of 120 °C to 750 °C or higher. Specifically, annealing at a higher temperature can help release the stress at the interface of the Pockels material structure 102. It should be noted that annealing to a high temperature can reduce the mechanical coupling between the Pockels material layer 102 and the substrate / interface. The stress can be eliminated in the Pockels material layer. If the Pockels material is patterned in a mesa structure (as shown in Figure 1C ), the strain relaxation can be uniaxial, resulting in a reduction in symmetry and thus alignment of the extraordinary crystalline direction and reduction of domains with different orientations. The mesa structure means that the Pockels layer is at least partially removed adjacent to the waveguide, for example, at the location of the electrode 108 and / or adjacent to the electrode, typically at or near the side of the electrode away from the waveguide.

[0052] After fabricating the Pockels structure 102, it can be biaxially strained. If the film is patterned in a mesa structure, uniaxial strain relaxation may occur, especially at elevated temperatures. When the material is heated and then cooled through the phase transition temperature, this can lead to preferential alignment of the crystalline domains, reducing the density of domain boundaries and potentially improving the electro-optic device performance. The heater / electrode can be entirely (a) or partially (b) on the BTO mesa.

[0053] It can be noted that there may be a large difference in the coefficient of thermal expansion (CTE) between the substrate W and the Pockels material structure 102 and / or any adjacent layers near the waveguide. For example, the CTE of BTO depends on temperature and is approximately (10 - 15)E - 6 / °C, and the CTE of silicon is approximately 2.6E - 6 / °C.

[0054] The large CTE difference between silicon and BTO can cause the stress in the Pockels material structure 102 to experience relaxation from when the film is deposited (which can be in the range of 300 °C to 1200 °C) to tensile stress at ambient temperature. Thus, at room temperature, the net stress or the principal stress in the Pockels material structure 102 can be tensile, which can be energetically favorable for in-plane orientation. Therefore, the annealing temperature can be selected such that the Pockels material layer 102 can be under tensile stress at the ferroelectric phase transition temperature and can stabilize the polarization after cooling in the plane of the film. Applying an electric field during cooling is an additional option to break the symmetry between the two orthogonal directions (parallel and perpendicular to the waveguide) and can be favorable for forming a specific type of crystalline domain.

[0055] Referring to Figure 1A, the thickness T of the Pockels layer is typically 100 nanometers to 500 nanometers (nm), but it can also be from 10 nm to 2000 nm (but not limited to this). The electrode gap G between the polarization electrodes is usually 4 micrometers to 20 micrometers, and can also be 1 micrometer to 100 micrometers, or even as small as 20 nm in the case of, for example, a plasmonic device. The cladding layer thickness can range from 200 nm to 10 micrometers, but in some cases it can be even thicker. In some other cases, when the heater is placed, for example, under the Pockels material, there may be no cladding layer located above and enclosing the waveguide 104.

[0056] The heater spacing H is typically in the range from 500 nm to 4 micrometers from the waveguide 104, but it can also be in the range from 100 nm to 10 micrometers.

[0057] In some cases, heating can be carried out via the polarization electrode 108 or the drive electrode 112 instead of the dedicated heater electrode 106. In this case, the polarization electrode or the drive electrode is used to apply an electric field to the Pockels layer 102 and heat the layer 102 via Joule heating.

[0058] Figure 2A is a top view 200 of the electro - optical device 100. Figure 2B is a top view showing a number of devices 200A - 200C.

[0059] As discussed, the polarization electrode 108 is connected to the polarization voltage source 222 via the contact 202. The at least one heater electrode 106 is connected to the heater current source 220 via the contact 204. These sources are typically controlled by a control circuit. In one example, multiple polarization electrodes can be provided to align the ferroelectric domains and / or crystalline domains in the Pockels material layer 102. The second metal connection or conductive connection 205 is provided with vias to cross the cladding layer 110 or other layers to cross the waveguide 104. The second metal connection 205 is connected to the left - hand polarization electrode 108 at one end and to the contact 202 for the driver or control circuit at the other end. In some other implementations, the contacts 202 or 204 can also be on different sides of the waveguide without the need for the metal connection 205.

[0060] As Figure 2B shown, multiple polarization electrodes are implemented to allow different directions of the electric field, and thus allow the polarization direction and the polarization amplitude along the waveguide direction. This figure shows a number of devices 200A, 200B, 200C along a common waveguide 104. This implementation will be used for periodic polarization, which enables a completely different set of applications in non - linear optical devices. In some other implementations, the heating electrodes can be shared across several devices, i.e., (for example) only one heater source can be used for several devices.

[0061] In some cases, the polarization electrode 108 or the drive electrode 112 may be used as a heater instead of a dedicated heater electrode 110. In this case, one or several polarization electrodes are in contact at least at two locations, preferably at the ends of the electrodes, to allow current to be applied along one or both of these electrodes to generate heat via Joule heating.

[0062] Figure 3 is a graph showing the polarization (μC / cm) of the hysteresis loop 300. 2 ) as a function of electric field (kV / mm), illustrating the change in the coercive behavior of a ferroelectric material at different temperatures. This shows only two specific examples: Process 1, Process 2.

[0063] The hysteresis loop 300 changes with the temperature change caused by the heater electrode 106. In polarization strategy 1 "Process 1", the polarization field is generally higher than the field applied during device operation (operating voltage). At the polarization field E_po, a certain proportion of ferroelectric domains will be polarized, for example 90%. When the field is reduced to the operating field E_op, this proportion of polarized domains can be reduced. As polarization strategy 2 "Process 2", heating the device at a higher temperature (Temp_2) changes the shape of the hysteresis curve. At the same polarization field E_po, a higher proportion of polarized domains, for example about 100%, can be achieved. Reducing the voltage of the operating field E_op can slightly attenuate this proportion. After cooling from Temp_2 to the initial temperature Temp_1, the proportion of aligned ferroelectric domains can be higher than polarization strategy 1 (for example, about 100%).

[0064] Figure 4A The domain structure of the ferroelectric film for different poling strategies is shown. This is the process of annealing the substrate above the Curie temperature and in an electric field in order to polarize the ferroelectric domains in the Pockels material layer and align its crystalline domains.

[0065] First, at room temperature (RT), the polarization of the crystal of the Pockels material layer 102 is random on the tetragonal axis. When the sample is heated above the phase transition, the film changes the crystal structure. For example, for BTO, the spontaneous polarization disappears in the crystal, and when heated above the Curie temperature, the film becomes a non-ferroelectric. The transition temperature may not be a specific temperature, but a temperature range, such as 120C to 250C for the BTO film. The energy barrier between the two polarization states decreases with temperature. The Pockels material layer 102 can then be cooled under the action of an applied constant electric field (i.e., a voltage between 3V and 100V) to invoke a net polarization.

[0066] Figure 4B , Figure 4C and Figure 4D Different alignments between electrodes and different domain patterns are shown.

[0067] In Figure 4B it, the domains are simplified by squares. In fact, these domains have irregular shapes. The arrows indicate the direction of the ferroelectric polarization. In a specific example like BTO, this can be flipped by 180 degrees. Above the phase transition temperature, the domains have a different structure, e.g., the cubic structure of BTO, which allows for a new crystalline domain pattern upon effective cooling that permits a 90-degree rotation. The orientation of the waveguides in the electrodes relative to the orientation of the ferroelectric domains can be varied and can be used to optimize properties such as the effective Pockels effect and the refractive index depending on the geometry of the device. It should be noted that the electrodes do not need to be aligned with the long axis of the crystal as long as one of the two crystalline directions is preferred. The alignment of the electric field is not 45 degrees relative to the crystal long axis. Since the energy difference between the two alignments decreases, the yield of the alignment can vary.

[0068] In some materials (such as BTO), the application of such an electric field enhances the two-fold degeneracy of the tetragonally distorted crystal, but the polarization state of the antiparallel alignment is metastable until the critical (i.e., coercive) field. The coercive field depends on the height of the energy barrier between different polarization states and on the temperature.

[0069] Furthermore, the rotation of the crystalline domains requires more energy and is generally not feasible under an electric field below the Curie temperature. For example, if BTO is annealed above the Curie temperature and an electric field is applied, the crystal stabilizes and the polarization axis (parallel to the crystalline c-axis) aligns with the electric field. It can be noted that in some materials such as BTO, the electric field needs to enhance the two-fold degeneracy. All or some of the domains will align along the electric field. For example, layers can exist at either interface where the domain orientation is different from the bulk of the film, e.g., mostly deviated by 90 degrees.

[0070] The atomic displacements below the Curie temperature result in ferroelectric polarization and a tetragonal crystal structure. The energy barrier between different polarization states decreases as the temperature increases from RT and vanishes above Tc, and the electric field required for polarization decreases.

[0071] The energy barrier decreases with increasing temperature as described above, and when an external electric field is increased (E>0) and the crystal is polarized, one polarization state is preferred. The field (E>0) is applied while cooling, and the crystal remains in the polarized state, which is stable at lower temperatures.

[0072] Figure 5FIG. 500 shows a graph of the applied polarization voltage generated by the polarization voltage source 222 and the heater current from the heater source 220 as a function of time to achieve crystal domain and / or ferroelectric domain alignment of the Pockels structure 102. Specifically, the heater current is initially turned off (a), and then the temperature of the Pockels structure is ramped up (b). In the case of crystalline alignment (c), the temperature is increased to a higher temperature and even above the Curie temperature. Once at the target temperature, an electric field is applied by raising the voltage 612. With the electric field held constant, the temperature is ramped down (d).

[0073] Figure 5 The same procedure shown in FIG. 500 can be used only for ferroelectric domain alignment. In that case, a temperature below the Curie temperature can be employed.

[0074] Figures 6A - 6B FIGS. 600 and 602 are graphs of the voltage 612 and the heater current 610 as a function of time for the phase change of the substrate during crystalline domain and ferroelectric domain alignment according to other embodiments of the polarization process.

[0075] Ferroelectric polarization occurs at elevated temperatures and at a voltage (V 极 ), and crystalline domain alignment typically occurs above the Curie temperature (T>Tc). Polarization is performed as part of an initial fabrication process (such as as an inline alignment procedure, such as after completion of fabrication at the wafer level, such as during testing of the final device or later during operation / initialization). This concept can also be used repeatedly or periodically as a refresh operation to compensate for any unwanted domain flipping processes during operation or storage. In refresh polarization, for example, heating and polarization are performed at periodic intervals (such as minutes, hours, days, months, etc.) or based on a feedback loop, such as when the effective Pockels response drops below a threshold.

[0076] In operation, the heater electrode 106 slowly supplies heat to the Pockels material structure 102. Heating can be accomplished by slowly heating (such as from a few seconds to a few minutes) and maintaining a high temperature for a specific period of time (such as a few seconds, a few minutes), and then slowly cooling. The heating operation can also be completed quickly (e.g., between a few microseconds and a few milliseconds for heating and cooling). Such a rapid operation can reduce, for example, the effects of leakage / ion migration in the Pockels layer.

[0077] A voltage 612 is constantly applied, for example, between 3V and 100V, thereby generating an electric field of approximately 3 kV / cm to 1 MV / cm between the polarization electrodes. Depending on the coercive field, device geometry and size, and other material properties such as the piezoelectric coefficient, the value of the applied voltage can also be smaller or larger. Refer to the polarization voltage graph 612. In other examples, a pulsed electric field is applied between a few nanoseconds and a few milliseconds, which can reduce ion migration and other non-polarization effects or effects unrelated to the alignment of crystalline domains, as Figure 6B shown. In another embodiment, a pulsed heater current 610 with a largely steady-state voltage 612 is applied between a few milliseconds and a few seconds, as Figure 6C and Figure 6D shown. In other examples, the electric field can be non-constant, such as oscillating with a constant or varying amplitude and with increasing and decreasing values. The time constant of this variation of the electric field can be in the range of, for example, nanoseconds to milliseconds. In some cases, longer time constants can also be used. Applying an electric field or a ramp enhances the polarization effect during the heating and cooling processes. In other examples, the heating operation and / or the application of the electric field are cycled with a constant or varying amplitude. For example, a constant electric field is applied, and the heater current and thus the temperature of the Pockels material are cycled between a low temperature (e.g., room temperature) and a varying higher temperature (e.g., above the Curie temperature). The heater current can also enhance the alignment of ferroelectric domains and / or crystalline domains by cycling around the Curie temperature or any phase transition temperature.

[0078] After the polarization program, the voltage can be reduced from V 极 to 0, or from V 极 to V 操作 , as Figure 6A shown. V 操作 refers to the voltage used during operation, which can be greater than 0 and equal to or less than V 极 . In the same case, it can be greater than V 极 . Generally, there is no interruption in the voltage supply between the polarization and the operation of the device, i.e., at any time during or after polarization, the voltage does not drop below any value lower than V 操作 .

[0079] The graph shows how the Pockels material layer 102 is annealed above the Curie temperature and in an electric field. The crystal can be stabilized as the polarization axis (parallel to the crystalline c-axis in the case of BTO) aligns with the electric field, as discussed above.

[0080] Figure 7A shows a method 800 for manufacturing an electro-optic device 100 according to an embodiment of the present invention.

[0081] In a first step, a Pockels material layer 102 is formed. At step 802, it can be deposited and patterned, or layer transfer can be performed. The substrate may include structures such as heaters. Further, at step 804, a waveguide 104 is deposited and patterned on the Pockels material structure 102. The patterning of the waveguide on the Pockels material structure 102 or as part of the Pockels material structure 102 can be accomplished by patterning BTO or by integrating other layers to be patterned (such as SiO2, SiN).

[0082] At step 806, one or more polarization electrodes 108 and device drive electrodes 112 are fabricated on either side of the waveguide 104.

[0083] At step 808, a first cladding layer 110 is deposited over the waveguide 104.

[0084] At step 810, a heater electrode is formed above the waveguide.

[0085] Then at step 811, a second cladding layer is deposited over the heater electrode.

[0086] Figure 7B is a flow chart of a method 801 for modifying the material properties of an electro-optic device 100 in accordance with an embodiment of the present invention.

[0087] At step 812, a polarization voltage is applied to at least one polarization electrode 108. Simultaneously, at step 814, the heater electrode 106 is energized to heat the Pockels material structure (such as the BTO layer 102) adjacent to the waveguide 104 to an elevated temperature, such as the Curie temperature or higher or lower temperature. However, in some examples, the BTO layer is first heated or possibly preheated. Subsequently, at step 816, the heater electrode 106 is typically then de-energized, allowing the BTO layer 102 to cool while maintaining the polarization voltage. The polarization voltage during cooling can be different from the polarization voltage at peak temperature. After the polarization step 816, at step 818, the voltage at the polarization electrode or drive electrode is typically set to the operating voltage or storage voltage.

[0088] It should be noted that prior to this process (especially when it is implemented as a "refresh" procedure during operation), there may already be a voltage applied to the polarization electrodes. It should also be noted that the polarization electrode and the drive electrode can be the same pair of electrodes.

[0089] For example, a standard BTO phase shifter is employed, and an integrated heater is arranged close to the BTO phase shifter (such as above or at the side of the BTO phase shifter drive electrode), or even the drive electrode is used as the heater and / or polarization electrode as described above.

[0090] Then, the BTO phase shifter is heated to a temperature closer to or even higher than the Curie temperature or the phase transition temperature. The coercive voltage decreases or even disappears, and / or the crystal structure changes. A voltage is applied to the electric field, and the heater is ramped down. Polarization will occur at a lower voltage and / or the crystal alignment will change within the region of the electric field.

[0091] In one aspect, a lower voltage can be used. When operating the chip at a lower temperature (e.g., below room temperature or close to 0K), the material properties of the Pockels or any other layer can vary. For example, the coercive field and / or the crystal structure can vary compared to operation at a higher temperature (e.g., room temperature). In the same situation, the coercive field can become very large making ferroelectric polarization difficult and heating the entire chip / system may be required. By using the integrated heater discussed previously and not heating the entire chip and thus the entire BTO layer 102 and maintaining the electric field during cooling, polarization at such lower temperatures can be effectively carried out.

[0092] Crystal alignment can occur at the device level, allowing other crystal orientations (to maximize the Pockels response) compared to the crystal orientation imposed by the thermodynamic / kinetic properties during the fabrication of the BTO layer 102.

[0093] Although the invention has been specifically shown and described with reference to its preferred embodiments, those skilled in the art will understand that various changes in form and detail may be made therein without departing from the scope of the invention as covered by the appended claims.

Claims

1. An electro-optical device, comprising: A waveguide; A Pockels material structure, the Pockels material structure being adjacent to and / or including the waveguide; A heater electrode for heating the Pockels material structure; and At least one polarization electrode near the waveguide for establishing an electric field in the Pockels material structure heated by the heater electrode.

2. The electro-optical device according to claim 1, wherein, The material of the Pockels material structure includes at least one of lithium niobate, barium titanate (BaTiO3), strontium barium titanate ((Ba,Sr)TiO3), lead zirconate titanate (Pb(Zr,Ti)O3), and / or polymers.

3. The electro-optical device according to claim 1 or 2, wherein, The Pockels material structure can be one or more layers.

4. The electro-optical device according to any one of claims 1 to 3, further comprising a cladding layer.

5. The electro-optical device according to any one of claims 1 to 4, wherein, The heater electrode is disposed above or below the waveguide or adjacent to the waveguide.

6. The electro-optical device according to any one of claims 1 to 5, wherein, At least one polarization electrode is configured to function as a heater electrode.

7. The electro-optical device according to any one of claims 1 to 6, wherein, The heater electrode is shared among multiple devices.

8. The electro-optical device according to any one of claims 1 to 7, wherein, The heat transfer from the heater electrode to the waveguide is optimized by using etched air grooves, suspended waveguides, or thermal insulation of materials with low thermal conductivity.

9. A method for changing the material properties of an electro-optical device, the method comprising: Applying a polarization voltage to the polarization electrode and energizing the heater electrode to heat the Pockels material structure associated with the waveguide; And De-energizing the heater electrode, thereby allowing the Pockels material structure to cool while maintaining the polarization voltage.

10. The method according to claim 9, wherein, When the effective Pockels response drops below a threshold limit, perform the polarization effect at periodic intervals or based on a feedback loop.

11. The method according to claim 9 or 10, wherein Apply the polarization voltage and temperature at periodic intervals of seconds, minutes, hours, days, or months.

12. The method according to any one of claims 9 to 11, wherein, Slowly heat the electro-optical device for several seconds to several minutes and maintain it at a high temperature for a specific period of time.

13. The method according to any one of claims 9 to 12, wherein, Heat and cool the electro-optical device at a rate between microseconds and milliseconds.

14. The method according to any one of claims 9 to 13, wherein The electric field is continuously applied or modulated between 3 volts and 100 volts.

15. The method according to any one of claims 9 to 14, wherein Operate the heater to modulate the temperature of the Pockels material during the polarization process.

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